EPA-600/2-78-060
March 1978
Environmental Protection Technology Series
DEVELOPMENT OF A PORTABLE DEVICE
TO COLLECT SULFURIC ACID AEROSOL
Second Interim Report
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-060
March 1978
DEVELOPMENT OF A PORTABLE DEVICE TO
COLLECT SULFURIC ACID AEROSOL
Second Interim Report
by
Herbert C. Miller, David W. Mason
and William J. Barrett
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-2468
Project Officer
Kenneth J. Krost
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences
Research Laboratory, U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recom-
mendation for use.
ii
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ABSTRACT
Progress is reported on research to develop a quantitative,
interference-free method for collecting airborne sulfuric acid
aerosol on a filter. Since previous research found that severe
losses of sulfuric acid were caused by ammonia, ambient par-
ticulate material, and other interferents, a method was needed
that converts sulfuric acid to a stable derivative for subse-
quent analysis. Methods evaluated for direct fixation of
sulfuric acid aerosol were not found to be selective. There-
fore, a sampling method was investigated that uses a combination
of selective volatilization of the sulfuric acid, prefiltration
of particulate interferents, and derivatization of the vaporized
acid on an alkali-impregnated filter. The initial stages of
this current research on the volatilization-derivatization
technique are described.
This report was submitted in fulfillment of Contract No.
68-02-2468 by Southern Research Institute under the sponsorship
of the U.S. Environmental Protection Agency. This report covers
a period from September 1976 to September 1977, and work was
completed as of September 1977.
111
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\>
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CONTENTS
Abstract iii
Figures .- vii
Tables viii
1. Introduction 1
2. Conclusions and Recommendations 3
3. Summary of Results 5
Results of Experiments Collecting Sulfuric Acid as a
Derivative of Perimidylammonium Bromide (PDA-Br).. 6
Results of Experiments in which Sulfuric Acid Was
Collected After Volatilization and Prefiltration.. 6
4 . Experimental Apparatus and Procedures 7
Sulfuric Acid Aerosol Generator 7
Operating Parameters 7
Background Contamination 10
Particle Distribution 12
Analytical Methods 16
Barium Chloranilate Method 16
Flame Photometric Detection of Volatilized
Sulfuric Acid 16
Filter Materials 22
Filter Evaluation with FPD Determinations. 22
Filter Evaluation with BCA Determinations 26
5. Results of Interference Studies 27
Collection of Sulfuric Acid Aerosol 27
Interference from Fe20s 27
Interference from PbO 29
Interference from Ambient Particulate Material.. 29
Interference from Ammonium Salts of Sulfuric
Acid 33
Interference from Ambient Particulate Material—
Analysis Corrected for Interference from
Ammonium Salts 33
6. Derivatization of Sulfuric Acid Aerosol 38
Derivatization with Perimidylammonium Bromide 38
Calibration of the FPD with PDA-SO^ 38
Interference from Ambient Particulate Material.. 39
Interference from Ammonium Sulfate Salts 42
Fixation of Sulfuric Acid Aerosol with Versapor
Filters .2
Volatilization, Prefiltration, and Derivatization of
Sulfuric Acid Aerosol 43
v
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Volatilization and Prefiltration Apparatus 43
Interference from Ammonium Sulfate Salts 47
Derivatization of Sulfuric Acid Vapor with
PDA-Br -..-. 47
Derivatization of Volatilized Sulfuric Acid
Aerosol with Alkali •. 48
New Sample Probe 49
Derivatization of Volatilized Sulfuric Acid
Aerosol at Increased Flow Rates 50
VI
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FIGURES
Number Page
1 Schematic of sulfuric acid aerosol generator 8
2 Plot of slope of cumulative number distribution vs.
particle diameter for H2S(H generator 13
3 Plot of cumulative volume (mass) distribution vs.
particle diameter for H2SOit generator 14
4 Plot of slope of cumulative volume distribution vs.
particle diameter for H2SOit generator 15
5 Flame photometric detector apparatus for H2S(H
analysis 18
6 FPD calibration for H2SOn deposited on filters 20
7 Modified flame photometric detector apparatus for
H2SOit and PDA-SO^ analysis 21
8 Calibration for H2SOi» deposited on Mitex filters and
volatilized for analysis at 200°C 23
9 Calibration for H2SO^ deposited on Mitex filters and
volatilized for analysis at 150°C 24
10 FPD calibration for PDA-SO^ decomposed at 450°C 41
11 Apparatus for volatilization, prefiltration, and
derivatization of sulfuric acid aerosol 46
VII
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TABLES
Number Page
1 Aerosol Generator Standard Operating Parameters ...... 9
2 Sulfuric Acid Collected from the Generator as a
Function of Sampling Time .......................... 9
3 Determination of SOa Concentration in the
Aerosol Generator with ^he West-Gaeke Method ....... 11
4 Data for the Calibration of the FPD Apparatus ........ 19
5 Comparison of Parallel ^luoropore and Mitex Filters
in Sampling Sulfuric Acid Aerosol ....... ............ 25
6 Effect of Predeposited FeaOa upon Sulfuric Acid
Collected on Mitex LS Filters ...................... 28
7 Effect of Predeposited PbO upon Sulfuric Acid
Collected on Mitex LS Filters ...................... 29
8 Effect of Predeposited Ambient Particulate Material
upon Sulfuric Acid Collected on Mitex LS Filters ... 31
9 Effect of Ambient Particulate Material on FPD
Measurement of Sulfuric Acid Predeposited on Mitex
LS Filters ......................................... 32
10 Interference from Ammonium Salts of Sulfuric Acid in
FPD Analysis ....................................... 34
11 Results of FPD Measurement of Sulfuric Acid Collected
on Mitex LS Filters Following Collection of Ambient
Particulate Material ............................... 35
12 Results of FPD Measurements of Sulfuric Acid Collected
on Mitex LS F Alters Prior to Collection of Ambient
Particulate Material ............................... 37
13 Deri vat ization of Sulfuric Acid Aerosol with PDA-Br
in the Presence of Ambient Particulate Material.... 39
viii
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Number Page
14 Evaluation of Selectivity of PDA-Br with Aerosols
of Sulfate Salts 43
15 Passage of Sulfuric Acid through the Heated Glass
Sampling Probe 45
16 Passage of Sulfuric Acid through the Heated Teflon-
Lined Sampling Probe 45
17 Passage of Volatilized Sulfuric Acid Aerosol through
Mitex Filters as a Function of Temperature 47
18 Retention of Sulfate Salt Aerosols by a Heated Mitex
Filter 48
19 Derivatization Efficiency of Alkali-Impregnated
Filters for Sulfuric Acid Vapor at 110°C 49
20 Volatilization Efficiency of the Heated Probe at a
Temperature of 120°C and a Flow Rate of 14 1/min... 51
21 Prefiltration Efficiency of Mitex Filters at a
Temperature of 130°C and a Flow Rate of 14 1/min... 51
22 Efficiency of Sulfuric Acid Derivatization by
Alkali-Impregnated Teflon Filters at a Flow Rate
of 14 1/min 52
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SECTION 1
INTRODUCTION
The increasing use of sulfur-bearing coal for the produc-
tion of electrical energy, the generation of significant
quantities of sulfuric acid aerosol by catalytic converters in
automobiles, and the appearance of new information on the irri-
tant effects of sulfuric acid and sulfates, are factors that
have combined to intensify interest in the measurement of ambi-
ent sulfuric acid aerosol. The emission of sulfuric acid by
automobiles is a problem of special interest because of the
fairly high concentrations found in the immediate vicinity of
roadways and the small size of the sulfuric acid particles
generated.
In a series of measurements made with catalyst-equipped
automobiles at the General Motors Test Track in October, 1975,
it was found that most of the sulfur in gasoline was oxidized
to sulfur trioxide and appeared as sulfuric acid aerosol near
the test site. Concentrations in the range of 3 to 6 yg/m3 were
found. The sulfuric acid was apparently neutralized or partial-
ly neutralized by ambient ammonia within a fairly short time and
within a fairly short distance of travel. Nevertheless, the
possible harmful effect on the health of persons in the vicinity
of heavily traveled roadways continues to be a matter of serious
concern.
The potentially harmful health effects stem from certain
unique properties of sulfuric acid aerosol. Sulfuric acid is
the most severe bronchial and lung irritant among the various
species of sulfur compounds that can occur in the atmosphere.
The particles derived from automobiles have a mass mean diameter
of only a few hundredths of a micrometer and therefore can pene-
trate deeply into the lung and be deposited in the alveoli.
These small particles are relatively highly concentrated in
terms of the molarity of the sulfuric acid and are therefore
stronger irritants than particles that have grown in size over
a period of time by coagulation and absorption of water vapor.
In order to assess the health effects potential of sulfuric
acid under these circumstances, it is necessary to have sampling
and analytical methods that will measure sulfuric acid concen-
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trations without interference or artifacts arising from the
presence of other substances in the sampled atmosphere or from
various environmental factors such as relative humidity or
temperature. Because many interfering reactions are possible,
especially during the collection of a sample, the accurate
determination of sulfuric acid aerosol is a difficult problem
that has so far not been adequately solved.
Although sampling cannot be arbitrarily isolated from the
method of analysis subsequently used to determine the species
sought, there are certain difficulties peculiar to sampling
sulfuric acid aerosol. In general terms, these difficulties may
be said to result from interactions of sulfuric acid with other
particulate and gaseous constituents of the ambient atmosphere
or the sample and from the simultaneous presence in the atmo-
sphere of sulfate salts and sulfur dioxide. Reactions of sul-
furic acid with copollutants have been shown to occur on filters,
the collection medium that is generally best suited for sampling
atmospheric particulate material. The oxidation of sulfur
dioxide to sulfate also occur^ on filters under certain condi-
tions. These phenomena—and ^thers that may be expected to
occur—often result in failure > f attempts to measure ambient
concentrations of sulfuric acid because some or all of the acid
is lost during the sampling process. Furthermore, methods of
analysis that involve dissolving the sulfuric acid along with
other soluble constituents of the sample introduce errors
resulting from reactions that occur in the solvent medium; a
sampling method that will permit subsequent processing and
analysis of the sample without loss (or addition) of sulfuric
acid is therefore desirable.
Work on this contract has been concerned principally with
providing a sampling and analytical method that will avoid errors
resulting from degradation of sulfuric acid during or after its
collection on a filter. During the early months of the contract
period, efforts were made to evaluate the collection of sulfuric
acid aerosol and effects of interferents using flame photometric
detection. However, it was later concluded that the fixation of
sulfuric acid as a stable, nonreactive product is clearly a
desirable approach, and may well provide the only means of
avoiding interference effects.
This Interim Report describes the techniques used to gener-
ate the sulfuric acid aerosol; the analytical methods used to
measure sulfuric acid; and the apparatus and procedures for
derivatization of sulfuric acid both as the aerosol and as the
prefiltered vapor. Further, it gives the results of experiments
designed to determine the effects of various potential inter-
ferents on the collection of sulfuric acid on filters, and
summarizes the conclusions reached.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
The collection of sulfuric acid aerosol on Teflon filters,
as a first step in the measurement of ambient sulfuric acid con-
centrations, is subject to interference by alkaline gases and by
particulate material in the air sample. Substantial losses of
sulfuric acid are caused by gaseous ammonia and ambient particu-
late material from an urban environment. Since the determination
of atmospheric sulfuric acid aerosol collected on a filter over
a period of time is subject to error from interfering reactions
that may occur during sampling, the need is evident for stabili-
zation of sulfuric acid during sampling. Direct methods of
fixation of sulfuric acid aerosol were not found to be selective,
suggesting that a better method of sampling would involve
selective volatilization of the sulfuric acid and filtration of
the ambient particulate interferents prior to derivatization of
the vapor.
Some progress has been made in the study of derivatization
of sulfuric acid during this project. The following recommenda-
tions are a logical extension of this work and, as such, will be
part of the experimental study to be conducted during the
remainder of this contract:
• Optimize the conditions and apparatus for volatili-
zation and prefiltration of sulfuric acid, emphasizing
the effects of volatilization temperature, sampling
flow rate, and nature of exposed surfaces within the
sampling device.
• Conduct a systematic evaluation of alkaline deriva-
tizing agents and filter media impregnated with the
derivatizing agents to achieve optimal fixation of
the volatilized sulfuric acid so that the derivative
can be readily identified and quantitated.
• Investigate the use of ion chromatography as an inter-
ferent-free analytical method to quantitate the
derivatized sulfuric acid.
• Investigate in depth the potential for interference
from ambient particulate material and ambient gaseous
compounds, especially ammonia and sulfur dioxide,
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during sampling of sulfuric acid aerosol with the
combined volatilization, prefiltration, and derivati-
zation methodology.
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SECTION 3
SUMMARY OF RESULTS
The most significant results of the experimental studies
conducted during the first year of this contract are summarized
in the following paragraphs. Details are given in the later
sections.
RESULTS OF INTERFERENCE STUDIES WITH SULFURIC ACID
• Sulfuric acid aerosol was collected on Teflon filters
with predeposited FeaOa. Average recoveries of sul-
furic acid samples were 18% for "high" levels and 79%
for "low" levels of FezOi, as measured by FPD at 200°C
• Sulfuric acid aerosol was collected on Teflon filters
with predeposited PbO. Average recoveries of sulfuric
acid samples were 19% for "high" levels and 66% for
"low" levels of PbO as measured by FPD at 200 °C.
• Ambient particulate material was collected on Teflon
filters for periods of 3 and 8 hr at 6 1/min; the
filters were then used to collect sulfuric acid
aerosol. For both levels of particulate loading the
losses of the acid approached 90% as measured by FPD
with volatilization at 200°C.
• Sulfuric acid aerosol was collected on Teflon filters;
the filters were then used to sample ambient particu-
late material for one hour at 14 1/min. The average
loss of sulfuric acid was 93% as measured by FPD at
150°C.
• Aerosols of ammonium sulfate salts were found to be a
positive interference in the determination of sulfuric
acid by FPD with volatilization at 200 °C. Lowering the
FPD volatilization temperature to 150°C reduced the
contribution from the ammonium salts but also adversely
affected the sensitivity of detection of sulfuric ncid.
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RESULTS OF EXPERIMENTS COLLECTING SULFURIC ACID AS A DERIVATIVE
OF PERIMIDYLAMMONIUM BROMIDE (PDA-Br)
• Approximately 100 yg of ambient particulate material
was collected on Teflon filters impregnated with the
PDA-Br reagent. The filters were used to collect
sulfuric acid aerosol and then analyzed by FPD with
thermal decomposition at 450°C. The results suggested
that sulfuric acid was completely recovered.
« Aerosols of the ammonium sulfate salts were collected
on Teflon filters impregnated with PDA-Br and analyzed
by FPD at 450°C and 200°C. No response was found at
200°C, but the quantitative response found at 450°C
indicated that PDA-Br was not selective for sulfuric
acid since it appeared that the ammonium salts were
also derivatized.
RESULTS OF EXPERIMENTS IN WHIJH SULFURIC ACID WAS COLLECTED
AFTER VOLATILIZATION AND PREFTT/TRATION
® Sulfuric acid aerosol was volatilized at. a flow rate
of 3 1/min in a heated sampling probe at 110°C. The
vapors were then passed through a Teflon filter heated
to 120°C. At least 90% of the resulting vapor passed
the heated filter. The barium chloranilate (BCA)
method was used for determination of sulfuric acid
as sulfate.
• Ammonium sulfate aerosols were sampled under the same
conditions with the heated sampling probe. BCA analysis
showed that an average of 85% of the ammonium salts
were retained by the heated (120°C) prefilter.
• Volatilized and prefiltered sulfuric acid was passed
through filters impregnated with PDA-Br, sodium carbonate,
or sodium hydroxide at 3 1/min. Very little of the sul
furic acid vapor was collected by the PDA Br filters but
an average of 60% was fixed by the sodium carbonate
filters and 80% by the sodium hydroxide fi]:.cr:>.
• An increase in the flow rate to 14 1/min was achieved
with a new probe, but the overall efficiency of the
volatilization and prefiltration was lowered to 70%
from a previous overall efficiency of 80% at 3 1/min.
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SECTION 4
EXPERIMENTAL APPARATUS AND PROCEDURES
SULFURIC ACID AEROSOL GENERATOR
Operating Parameters
A modified version of the generator described by Thomas,
e_t al, * was used for laboratory studies with sulfuric acid
aerosol. The use of this system, which was constructed and
used in earlier work at Southern Research Institute (under EPA
Contract No. 68-02-2234), was continued because it was shown to
have certain attributes that were considered to be desirable for
the purposes of this project. In particular, no other generator
system could be expected to produce predominantly submicron
particles, while simultaneously permitting flexibility for the
addition of potential gaseous and particulate interferents
directly to the aerosol. It also allowed easy adjustment of
concentrations, particle size, flow rates, and other parameters.
The generator is illustrated in Figure 1. Its principal
component was a Beckman No. 4020 atomizer-burner situated at the
base of a 1.22-m by 99-mm i.d. Pyrex chimney. The fuel gas was
hydrogen and the oxidizer was a mixture of oxygen and argon. A
dilute solution of sulfuric acid was aspirated into the flame
from a beaker placed underneath the burner. The ends of two
13-mm i.d. glass probes were located close together about 10 cm
below the top of the chimney. One of these probes had a side
arm closed with a silicone septum through which gases could be
added at known flow rates. The probes were connected to stain-
less steel holders for 47-mm filters or impinger-bubblers which
were followed by 6-1/min critical orifices and a carbon-vane
vacuum pump. One side of this dual sampling system was always
used as the reference for determination of the concentration of
the sulfuric acid aerosol, while the other side was used for the
introduction of interferent gases or for holding an impregnated
filter or a filter containing predeposited particulate material.
The excess aerosol from the top of the chimney was exhausted
through a hood.
*Thomas, R. L., V. Dharmarajan, and P. W. West. Convenient
Method for Generation of Sulfuric Acid Aerosol, Environ. Sci.
Technol., 8(10): 930-935, 1974.
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I
EXHAUST
FILTER
HOLDER
OPTIONAL
CONFIGURATIONS
SAMPLE PROBE
y
SEPTUM ' |
a wLw
FILTER HOLDERS
REFERENCE H-
PROBE
I
db
CRITICAL
ORIFICE
80%
!§. — ISOPROPANOL
fe IMPINGER
CRITICAL
ORIFICE
SAME
OPTIONAL
CONFIGURATIONS
PYREX
CHIMNEY
FLAME7 I , , I
SULFURIC
ACID
SOLUTION
HYDROGEN
OXYGEN + A.^GON
Figure 1. Schematic of sulfuric acid aerosol generator
(not to scale).
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The basic operating parameters for the aerosol generator
and their typical values are shown in Table 1. All of these
parameters were varied experimentally in order to find the
optimum set of conditions„ Data indicating typical amounts of
sulfuric acid aerosol collected during operation of the labora-
tory generator are given in Table 2»
TABLE 1. AEROSOL GENERATOR STANDARD
OPERATING PARAMETERS
a Feed solution—4 x 10~3 N H2SO,»
® Positive burner pressures—H2y 1.8 kPa (13 mmHg)
02, 18 kPa (130 mmHg)
Ar, 120 kPa (880 mmHg)
• Aspiration rate—ca. 2 ml/min
» Aerosol concentration—ca. 500 yg/m3
• Sampling rate—6 1/min
« Sampling time—10 to 20 min
• Typical sample size—30 to 60 yg H2SO.,
• Probe temperature—30 to 40°C
TABLE 2. SULFURIC ACID COLLECTED FROM THE
GENERATOR AS A FUNCTION OF SAMPLING TIME
H2SOit found on parallel
Sampling 47-mm filters, yg*
time, min A B
20 54 59
20 60 52
10 32 26
5 88
* Each value is the average of triplicate analyses with the
FPD technique using the square roots of the measured peak
heights on a relative scale to determine the amoi t ->f
H2SO.,.
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Aerosol samples usually containing 20 to 60 jig of suifuric
acid were collected on 47-mm Mitex LS filter disks in the ex-
perimental work. Smaller disks (ca. 6.3 mm) were punched from
the experimental filters for analysis in the useful range of the
flame photometric detector (FPD)'. Each small disk represented
about 3.3% of the effective area of the sample that is collected
on a 47-mm disk. Each FPD analysis reported for the filters was
calculated to give the total on the 47-mm filter. Triplicate
samples were taken from each filter by punching three 6.3-mm
disks equidistantly from the center. The "A" and "B" samples in
Table 2 were obtained simultaneously with a parallel sampling
arrangement. These data showed that while a 10-min sample
collected approximately one-half of the suifuric acid that was
obtained with the 20-min sample, the 5-min sample collected
somewhat less than one-fourth. These results suggested that a
small, relatively constant loss of suifuric acid probably
occurred in the handling of samples prior to analysis. This
loss was probably less than 10% of the 20-min samples but may
have been as much as 50% of the 5-min samples. The nature of
this loss of suifuric acid is uncertain but may be due to reac-
tion of the acid with the amt i"nt ammonia present in the
laboratory.
Background Contamination
Nitrogen Dioxide Concentration—
- Measurements of the concentration of nitrogen dioxide in
the effluent of the suifuric acid aerosol generator were made
previously under different operating parameters for the gener-
ator. At that time, argon was not used for dilution of the
hydrogen-oxygen flame and effluent temperatures were substan-
tially higher (ca. 100°C). Levels of nitrogen dioxide were
found to be as' hTgh as 6 mg/m3 under these conditions and
attempts were subsequently made to minimize this concentration.
Addition of argon to the flame in conjunction with lower
pressures of hydrogen and oxygen resulted in lower effluent
temperatures of approximately 30 to 35°C and a lower nitrogen
dioxide concentration of 0.11 mg/m3.
The conditions for the operation of the aerosol n'-nerator
during these measurements were the same as those given in
Table 1. The nitrogen dioxide was determined by a method based
on the Griess-Saltzman reaction as described in Intersociety
Committee Method 42602-01-68T.
The ambient level of nitrogen dioxide was measured simul-
taneously with that of the generator and was determined to be
about 0.05 mg/m3 or approximately one-half of the level presenh
in the effluent of the generator. Therefore, the nitrogen
dioxide concentration in the generator effluent was judged to
be low enough under current operating parameters for our work.
10
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Sulfur Dioxide Content—
Previous attempts to use the West-Gaeke method (Inter-
society Committee Method 42401-01-69T) to determine the concen-
tration of sulfur dioxide in the effluent of the aerosol
generator were not successful.* However, because different
operating parameters were later used for the aerosol generator,
the measurements were repeated. The results of these experi-
ments are shown in Table 3. Essentially, these results were
identical to those obtained earlier; that is, no sulfur dioxide
could be detected. However, as before, spikes of sulfur dioxide
added to the tetrachloromercurate absorbing solution both before
and after sampling the generator effluent could not be recovered,
Thus, it appeared that the tetracholormercurate-sulfur dioxide
complex was decomposed by some component of the effluent pro-
duced by the aerosol generator. Due to the fact that sulfuric
acid was eliminated as the interference (second entry in Table
3), no further explanation for this phenomenon could be offered.
However, based on these results and those of previous work, it
can be concluded that sulfur dioxide is not stable in the
presence of the gaseous combustion products of the flame and is
probably oxidized rapidly to sulfate.
TABLE 3. DETERMINATION OF S02 CONCENTRATION IN THE
AEROSOL GENERATOR WITH THE WEST-GAEKE METHOD
SO 2 found , yg
Standard, 20 yg of S02 (as Na2SO3)
Standard, 20 yg of S02 (as Na2S03) and 59 yg
of HaSOij (standard aqueous solution)
Sample collected from H2SOit aerosol generator t
Sample collected from H2SOit aerosol generator
with spike of 20 yg of SO2 (as Na2SO3) added
to bubbler solution after samplingt
Sample collected from H2SOi, aerosol generator
with spike of 20 yg of SO2 (as Na2SO3) added
to bubbler solution before samplingt
20
23
0.4
0.4
0.8
t Samples were collected for 20 min at a flow rate of 6 1/min.
*Barrett, W. J., H. C. Miller, J. E. Smith, Sr., and C. H. Gwin,
Southern Research Institute. Development of a Portable Device
to Collect Sulfuric Acid Aerosol. EPA-600/2-77-027, U.S.
Environmental Protection Agency, Research Triangle Park, North
Carolina, 1977. pp 15-16.
11
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Particle Distribution
Estimates of the particle-size and particle-mass distribu-
tions of the aerosol produced by the sulfuric acid generator
were made with a Model 3030 Thermo-Systems Electrical Aerosol
Size Analyzer (EAA) and a modified Climet Particle Analyzer.
These measurements were made with the generator operating under
the conditions given in Table 1. The results of these measure-
ments are presented graphically in Figures 2 through 4.
Figure 2, which is a plot of the slope of the cumulative
number distribution as a function of particle diameter, indicates
that the majority of the aerosol particles had a diameter of
less than 0.1 ym. On the other hand, it can be ascertained from
Figures 3 and 4, which are plots of the cumulative volume dis-
tribution and the slope of the cumulative volume distribution,
respectively, both as a function of particle diameter, that
approximately 80% of the mass oy aerosol particles was accounted
for by those having a diameter between 0.2 and 1.5 ym.
It was found in an earlier evaluation of the particle dis-
tribution that more than 98% c both total mass and number of
particles was associated with those smaller tha.i. 0.3 ym. These
measurements were made during the time when operating conditions
for the generator were such that the effluent temperature was in
excess of 80°C. A comparison of the more recent results with
those obtained earlier suggested that the overall particle dis-
tribution of the output of the aerosol generator was a sensitive
function of the flame temperature. As a result, steps ware
taken to regulate accurately the fuel (hydrogen) pressure of the
atomizer-burner in order to control the characteristics of the
effluent aerosol.
Although the sulfuric acid generator does not produce an
"ultrafine" aerosol with the current operating parameters, it
appears that with appropriate adjustment of fuel, oxidant, and
diluent gas pressures the output can be varied to produce an
aerosol of a given distribution over a reasonably broad range
of particle sizes. However, the generator should be operated
within constraints necessary to avoid high concentrations of
potential interferents, such as nitrogen dioxide, and excessive
effluent temperatures, which can promote vaporization of the
sulfuric acid aerosol.
It is probable th-t the relative humidity (RH) of the gen-
erator effluent is an important factor in establishing a given
particle distribution. The RH of the effluent is, of course,
dependent on the partial pressure of the water vapor, which is
relatively constant, and the temperature, which can be varied
over a relatively wide range (e.g., 30 to 150°C) by changing the
pressure of the burner fuel. At low RH (high effluent tempera-
tures) the particle distribution appears to favor the smaller
12
-------
Q
o>
o
-------
600
500
u
?! 400
o
111
5
D
_l
O
01
D
D
U
300
200
100
O EAA
A CLIMET
0.01
0.1
1.0
10
DIAMETER,
Figure 3. Plot of cumulative volume (mass) distribution vs. particle
diameter for H^SOq generator.
-------
1000
CO
E
^o
en"
u
CM
« 100
o>
o
10
1.0
OEAA
6CLIMET
0.01
0.1
1.0
10.0
DIAMETER, fim
Figure 4. Plot of slope of cumulative volume distribution vs. panHe
diameter for H^SO^ generator.
15
-------
particles; however, at higher RH (lower effluent temperatures)
the distribution is dominated by larger particles. Thus, it
appears that the sulfuric acid particles grow more rapidly in
the high RH environment during their brief residence time
(<1 sec) in the generator than under lower RH conditions.
ANALYTICAL METHODS
The principal analytical techniques used in this work were
based on the barium chloranilate (BCA) method for the determina-
tion of total sulfate and the flame photometric detection (FPD)
of a volatilized sulfur species.
The procedures described in the literature were modified
somewhat to meet the needs of this project. Detailed protocols
for these methods are described in the following paragraphs.
Barium Chloranilate Method*
For the determination of sulfate by the barium chloranilate
method, a filter sample was p. .^ed in a 125-ml screw-capped
Erlenmeyer flask containing about 25 mg of bariora chloranilate
and 10.00 ml of 80% isopropanol. The sample was then mixed for
30 min on a rotary shaker, centrifuged, and the absorbance of
the resulting supernatent solution measured at 310 nm in a 1-cm
fused-silica cell with a Beckman Model DU spectrophotometer.
An 80% isopropanol solution was used in the reference cell. A
linear calibration curve Was prepared with known quantities of
standard sulfuric acid solutions that were treated in the same
manner as experimental samples. The useful range of the method
was 10 to 50 yg of sulfuric acid per filter without dilution of
the sample.
Flame Photometric Detection of Volatilized Sulfuric Acid
Much of the work under the previous EPA contract (No. 68-
02-2234) was intended to study the effect of selected inter-
ferent materials on the stability of sulfuric acid collected on
filter media. Early in that study, analyses for sulfuric acid
were conducted by selective extraction of the acid with
benzaldehyde followed by a barium-Thorin microtitration of
sulfate. However, in the later stages of the investigation, an
FPD apparatus was constructed and successfully used to measure
sulfuric acid. With this methodology the sulfuric acid was
*Schafer, H. N. S. An Improved Spectrophotometric Method for
the Determination of Sulfate with Barium Chloranilate as
Applied to Coal Ash and Related Materials. Anal. Chenu ,
39(14): 1719-1726, 1967.
16
-------
thermally volatilized from the filter medium and quantitated
with the FPD, Analyses were performed more rapidly with this
technique and the limit of detection was significantly lower
than that of the '"wet01 methods employed earlier. One disadvan-
tage of the FPD apparatus,, however, was that the results
obtained with it often possessed poor reproducibility.
Apparatus—•
Early in the current contract (No, 68-02-2468) efforts were
directed toward optimization of the FPD technique as it is
applied to our program„ First, in order to simplify the labora-
tory apparatus, the Model FPD 100AT flame photometric detector
was removed from the Meloy Laboratories Model SA 160-2 Sulfur
Gas Analyzer., This move facilitated the use of much shorter
Teflon transfer lines from the volatilization chamber to the
heated detector block„ High voltage for the photomultiplier
tube (PMT) of the FPD was provided by a Kepco Model ABC1500-M
power supply,, and the PMT current was measured with a Keithley
Model 414A Picoammeter and an appropriate strip-chart recorder.
The sample chamber for thermal volatilization was also mod-
ified to improve reproducibility and to simplify the necessary
manipulations during analysis,, The glass chamber that was used
previously was replaced with a Teflon-lined glass tube with
significantly smaller surface area and radead volume"» A shorter
Teflon transfer line was also made possible by replacing the
aluminum heater block with a small tube furnace for maintaining
the temperature of the sample chamber<> The sample purging sys-
tem was also modified to provide better control of the airflow
necessary to sweep the sulfuric acid vapors from the sample
chamber to the FPD, A schematic diagram of the FPD apparatus
is shown in Figure 5,,
Calibration with Known Amounts of Sulfuric Acid—
Several experiments were conducted to determine the practi-
cal working range for the quantitation of sulfuric acid with the
FPD apparatus. Data for a calibration curve were obtained by
measuring the peak height of the detector response as a function
of the amount of sulfuric acid added to Mitex LS filters from
standard solutions in isopropyl alcohol„ In theory, the re-
sponse of the FPD to a volatile sulfur species is proportional
to the square of the sulfur mass concentration in the hydrogen-
rich flame» As a test of this relationship and for purposes of
calibration„ the logarithms of the peak heights were plotted
versus the logarithms of the corresponding amounts of sulfuric
acid present in the standard samples. The theoretical slope of
this plot should be 2, and a value of 2,09 was found experi-
mentally o The linearity of this log-log calibration covered a
useful range of approximately 0,1 to 2 yg of sulfuric acid per
sample. Also, a relative standard deviation of 5,6% was ob-
tained for 16 replicate analyses that were performed over a
period of 2 weeks on standard samples at the 2-yg level. These
17
-------
CRITICAL
ORIFICE
VENT
TEFLON
SAMPLE
CHAMBER
200° C
FILTER
DISC
FPD
BLOCK
200° C
HEATING DEVICE
SAMPLE
PURGE VALVE
AIR
PMT
FLOWMETERS
HV SOURCE
PICOAMMETER
RECORDER
Figure 5. Flame photometric detector apparatus for H2SO4 analysis.
18
-------
results demonstrated that adequate sensitivity and reproduci-
bility could be obtained with the FPD device.
For practical purposes, calibration curves were constructed
from plots of the square root of detector response (peak height)
versus micrograms of sulfuric acid. The advantage of this plot
was that the calibration was linear in the amount of sulfuric
acid. Typical calibration data are given in Table 4. These
data are also shown graphically in Figure 6.
TABLE 4. DATA FOR THE CALIBRATION OF THE FPD APPARATUS
H2SOi, added,* Square root of FPD response,t
yg arbitrary units
2.05 17.0
17.6
17.9
1.23 10.4
10.8
10.2
0.82 7.1
7.0
6.9
0.33 2.0
2.2
2.3
* Standard solutions of sulfuric acid in isopropyl alcohol were
applied to Mitex LS filters (6.3-mm disks) with a microburet.
t These data are the square roots of the measured peak heights
on a relative scale.
Modifications to the FPD Apparatus—
Later, further modifications were made to the FPD appara-
tus. The major change was the addition to the apparatus of the
second volatilization chamber. Figure 7 depicts the apparatus
in its final state of development. The use of two separate
chambers allowed the determination of sulfuric acid at olatil-
ization temperatures from 150 to 200°C and the perimidviammonium
sulfate (PDA-SCM derivative at its decomposition temperature
of 450°C. Teflon tubing was used to replace all metal transfer
lines from the volatilization chambers to the FPD block.
19
-------
20
ffi
cc
o
o
ffi 10
cc
Q
&
o
o
cc
tii 5
cc
O
CO
I I I I
I I I !l
I I I I
I I I I
I I I I
I I I I
1 I I I
0.5 1.0 1.5
AMOUNT OF H2S04, fJLg
2.0
2.5
Figure 6. FPD calibration for
deposited on filters.
20
-------
SAMPLE
PURGE
VALVE
CRITICAL
ORIFICE,
5 ml/min
TEFLON
TRANSFER
LINES
FPD
BLOCK,
200°C
TEFLON
SAMPLE
CHAMBER, 150°C
RECORDER
INTEGRATOR
GLASS
SAMPLE
CHAMBER
450°C
FLOWMETERS
f\ CRITICAL
ORIFICE,
L-r-l 220 ml/min
AIR
NEEDLE VALVE
Figure 7. Modified flame photometric detector apparatus for
arid PDA-S04 analysis.
21
-------
For purposes of calibration in previous work the square
root of the peak height of the FPD response provided a satis-
factory linear correlation with the amount of sulfuric acid.
However, a more rigorous treatment was performed by lineariza-
tion and integration of the sulfur-FPD1 peaks, The linearizer
incorporated into the FPD apparatus electronically obtained the
square root of the FPD signal and manual planimetry provided
integration of the linearized peaks.
The method of calibration for the FPD varied during differ-
ent phases of the project. The method used for calibration will
be noted in the tables that give data from FPD determinations.
Calibration of the improved FPD apparatus with known amounts
of sulfuric acid on Mitex filters produced the curves shown in
Figures 8 and 9. The volatilization temperature used to obtain
the data in Figure 7 was 200°C. The linear range of the cali-
bration was found to be 2 to 20 yj" of sulfuric acid per 47-mm
filter disk (an accurately cut portion of the 47-mm filter was
used for the analysis). The volatilization temperature for the
data in Figure 9 was 150°C, and the linear range at this temper-
ature was 4 to 15 yg of sulfuric icid per 47-mm filter. The
slight reduction in sensitivity was probably due to less
efficient volatilization of the sulfuric acid from the Mitex
filter and some adsorption on surfaces at the lower temperature.
Poor reproducibility was also observed in the data for the
calibration curve at 150°C.
FILTER MATERIALS
Mitex and Fluoropore are two types of Teflon (PTFE) filters
that have been used for the collection of sulfuric acid aerosol.
During previous contract work several measurements demonstrated
that 5-ym Mitex filters were 99% efficient in the collection of
sulfuric acid aerosols with particle sizes in the range of. 0.3
to 0.005 ym. However, because Fluoropore filters are frequently
used by other workers for the collection of sulfuric acid
aerosol, a comparison was made by sampling the laboratory qener-
ator simultaneously with Mitex and Fluoropore filters in
parallel.
Filter Evaluation with FPD Determinations
Mitex filters with a 5-ym pore size rating and a type of
Fluoropore filter (1-ym pore size) that is manufactured without
the polyethylene backing were used simultaneously to collect
sulfuric acid from the generator. Analyses of both filters were
conducted with the FPD apparatus. The results of the comparison
are given in Table 5. The greater pressure drop across the
Fluoropore filter resulted in a lower flow rate; consequently,
the amount of sulfuric acid collected was less for .t'e
22
-------
19
c
3
15
o
co
O
Q.
co
O
a,
u.
10
a
z
<
O
u
N
OC
<
HI
0.0
1
1
0.1 0.2 0.3 0.4
AMOUNT OF H2S04 . US
0.5
0.6
5. Calibration for H^SQ/i deposited on Mitex filters and volatilized
for analysis at 200°C.
23
-------
15
c
ID
uT
V)
H
O
a.
t/>
HI
cc
Q
a.
LL
8
10
O
O
LU
N
ill
Z
/
1
1
0.0 0.1 0.2 0.3
AMOUNT OF H2S04, U9
0.4
0.5
Figure 9. Calibration for H^SO^ deposited on Mitex filters and volatilized
for analysis at 150°C.
24
-------
TABLE 5. COMPARISON OF PARALLEL FLUOROPORE AND MITEX
FILTERS IN SAMPLING SULFURIC ACID AEROSOL*
Filter
material
Mitex
Fluoropore
Flow rate,
1/min
6.0
5.2
HaSOii found, t
yg
61.4
55.4
H 2 SO 4 concn ,
yg/m3
513
533.
* Sampling time was 20 min.
t Each result is the average of triplicate analyses.
than for Mitex. However, the calculated results for the sul-
furic acid concentration in the generator effluent were
essentially identical for the two filters within experimental
error .
These results suggested that neither filter offers any
particular advantage over the other for the collection of sul-
uric acid from the aerosol generator. However, it is conceiv-
able that Fluoropore filters may be more efficient in the
collection of the aerosol if the operating conditions of the
generator were adjusted to give a higher proportion of "ultra-
fine" particles.
-These results were based on FPD analysis of filters
with volatilization at 200°C. Also, these filters had collected
about 60 yg of sulfuric acid from the aerosol generator, and
at that sulfuric acid level the results obtained with both
filters were essentially the same. On the other hand, it had
been observed earlier for smaller amounts of sulfuric acid
aerosol (e.g., less than about 20 yg per 47-mm disk) that the
recovery upon analysis was somewhat less than expected for
Mitex filters. It appeared that a small, constant amount of
sulfuric acid (ca. 5 yg) was not recovered from the Mitex
filters. This amount was not significant at the 60-yg level
and fell within the uncertainty of the FPD analyses, but at the
10- to 20-yg level the loss was substantial (25 to 50%) .
In subsequent work this phenomenon was found to be more
pronounced for Fluoropore filters than for Mitex. For example,
collection of about 15 yg of sulfuric acid aerosol on Mitex
filters (measured independently on a parallel filter by the BCA
method for total sulfate) followed immediately by FPD analysis
with volatilization at 200°C gave a result of only 8 yg or about
50% recovery. On the other hand, only about 6 yg of the acid
was found by FPD analysis of a Fluoropore filter that had col-
lected about 33 yg of aerosol (approximately 20% recovery) .
25
-------
TKes§ fPD' results repSiftM f6'f -fefie cdfitfMfisah' Sf Mitex arid
Flubrbp'bre filters were" based on volatillzlti-o'n' it 26'6'°G;
however/ the recoveries were eve'ri ibwer at iSff^G-^— t:-hej vbMtiii-
zatibri temp'erature riecessary to miftamize FPD iriterfSrgrLegs ffb'm
the\ amm'bhltift salts bf sulfuric £cid.< Fdir gxaiflple1/ FP'D deterM-
natibn of 36 yg of sulfufic acid aerosol collected on f'luorbpbre
gave 12 yg at 26o°C ('about 35% recovery)' and' 3 yg at 150'dG
(about 10% recovery)' . With Mitex the' re'sults were somewhat-
better : for 19 yg as the aerosol, FPD determiftattibfl at 2'60°C
gave 14 yg (about 70% recovery) and at i56°C the' result was
12 yg . (about 65% recovery)'; Further/ it sHo'uld be1 rioted that
the determinations for each filter a't both: teinperatures we're
referred to individual calibration' curves . Th;ese cu'rves' wef el
prepared from FPD responses, a't the; correspbhdirig temperatufes ,
to known spikes of sulfufic acid added to the appropriate
filters from a solution in isbprbpat-hol.
Filter Evaluation with'1 BCA beterMr'.ftatibn.s'
It Was interesting also tKat^ the same pliehb'men'bh a's dds-
cribed for t'PD occurred with th BCA determination's of sulfate'
when the filters were not agitatwi* ultfa"s6riica-llyr during ex-
traction into 80% isbpf op'anol i That is> a't the 70'-yg level the
recoveries were about 90% without uitfasoriic extractibn for
both Mitex and Fluorbpbre' f liters- spiked with" Jcnbwn amburits 6'f
sulfuric acid, £ut at the lb;-yg level the recoveries we're b'rily
about 70l for Mitex a'ri'd 5'0% foir Fluofopbre.- However/ witfe
ultrasonic, extrac'tioii the low level fecbveries lor both filters
approached 100%; These results sugges'te'd that the suifuric a^cid
may have diffused ifitb the1 iriterib'r pores 6'f the fliters inhere'
it was difficult to remove by extracti6n or, therm'al vdlatili-
zatibn. The phenomenon appears to irtvolve a ebh§tant amount of
sulfuric acid arid the situation seems to be wbfse with
Flubropore filters.
Iri cbriclusibri, neither the M'itex ribf Flu'brbp'dfe' filte'r
appears to be eritif^ly adequate1 for the tKerm'al- vblatilizatibn-
FPD' detefminatibri bf suifuric acid; Howfev^f; the results tend
tb favor Mitex filters' because of the su'b'starit-i ally higher .re-
cover ie's that were fburid fe*ia^^ive' to Fluiorcifp'bf e .- ifi afiy event;
these results further indicate problem's in t-K§ c'6'rive'fitib'nal
vbla'tiiizatib'n'-FPb arialy'sis' of sulfuric
-------
SECTION 5
RESULTS OF INTERFERENCE STUDIES
COLLECTION OF SULFURIC ACID AEROSOL
The effect of selected interferent materials upon the sta-
bility of sulfuric acid collected on Teflon filter disks was
studied with quantitation of the acid with the FPD apparatus.
The interferents were chosen from those representative of a
roadside environment and those which have the potential to react
with sulfuric acid. This study essentially concluded the inter-
ference studies that were begun under the previous EPA contract
(No. 68-02-2234) concerning sulfuric acid aerosol.
Interference from Fe203
Experiments were conducted to measure the effect of Fe203
on the stability of sulfuric acid collected on a Mitex LS filter
with analysis by the FPD method. A similar study was performed
in previous work with analyses by the benzaldehyde extraction
technique (specific for sulfuric acid) and insignificant losses
of sulfuric acid were found. However, results of experiments
with analyses by FPD indicated that substantial losses of acid
occurred for the "high" levels of predeposited Fe203 as opposed
to somewhat more moderate losses for the "low" level. These
data are given in Table 6. The average recoveries of sulfuric
acid sampled as aerosol following deposition of Fe203 were 18%
for the "high" level and 79% for the "low" level. The Fe203
particulate was sampled from an aerosol produced by a DeVilbiss
powder blower; no effort was made to characterize the size
distribution of this aerosol. The two levels of Fe203 were
obtained by sampling the aerosol ten times longer for the "high"
level than for the "low" level.
Recoveries of spikes of sulfuric acid (in isopropanol solu-
tion) added to filters also depended upon the level of predepos-
ited Fe203. Average recoveries of the spikes were 54% for the
"high" level and 106% for the "low" level. Although the
recoveries of spikes were adequate at the "low" level of Fe203,
the greater losses of sulfuric acid for both spiked and aerosol
samples at the "high" level could have resulted from further
reaction of the residual acid with the additional Fe203 on the
filter at the elevated temperature (ca. 200°C) of the FPD
27
-------
TABLE 6, EFFECT OF PREDEfOSJTED
ACID COLLECTED OK MlEEX
FILLERS
Fe203
added, mo
added,*
tig
spike,t
H2SOt,
found,
Recovery ^
Hj.gh levels
0.3
0.6
0,3 (0.3)*
0.5
0.3
Low levels
none
none
40 (47)
45
52
none
63
none
none
none
34
?.2 (44)
6.3
10*8
54
18 (96)
14
21
<0.1 none
<0*1 hone
<0.1 62
<6.1 64
<0.1 64
none
63
none
hone
none
<:0,1
67
44
54
53
106
71
84
82
* The amount 6f sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol .
t The spike Was added to the filter- exposed to FesOj from a
standard solution of sulfuric acid in isOpropyl alcohol.
§ Each result is the average "t£ triplicate FPD determinations
calculated from the square roots of the peak heights.
# The numbers in parentheses are the results of similar experi-
ments in which the analyses for sulfuric acid were conducted
with the benzaldehyde extraction technique.
volatilization chamber. Accordingly, it is possible that the
lower recoveries £ound with F£d analyses as opposed to those
obtained earlier with the benzaldehyde extraction technique
resulted from reaction of sulfuric acid with FeaOa during
volatilization in the FPD. However, the fact that the re-
coveries for the aerosol Samples were lower than those of the
spiked samples suggested that additional losses of acid might
have occurred during aerosol sampling,
28
-------
Interference from PbQ
The evaluation of PbO as an interference during the collec-
tion of sulfuric acid aerosol was conducted in the same manner
as described earlier for Fe20a. The results of these experi-
ments are given in Table 7. Although there was a relatively
large variability in replicate values for recovery of acid in
the presence of PbO at the "high" level, it was evident that
substantial losses of acid occurred. The same trend was ob-
served with PbO as with Fe203 in that the losses of acid at the
"low" level of interferent were significantly less than at the
"high" level. These results cannot be compared with previous
data because PbO was not included in interference studies con-
ducted with the benzaldehyde extraction technique.
TABLE 7. EFFECT OF PREDEPOSITED PbO UPON SULFURIC
ACID COLLECTED ON MITEX LS FILTERS
PbO added, H2SOi, t^SO., Recovery,
mg added, yg* found, ygt %
High levels
none 0.1
0.5 59 3.9 6
0.7 62 20 32
0.7 60 12 20
Low levels
0.1 none 0.1
0.1
0.1
0.1
60
58
57
40
38
37
67
66
65
* The amount of sulfuric acid added was determined by ar.?.1- <;ts
of a parallel filter exposed only to the sulfuric acid
aerosol.
t Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights.
Interference from Ambient Particulate Material
The effect of ambient particulate material on the stabi'ity
of sulfuric acid collected on a filter was studied in previous
work with analyses by the benzaldehyde extraction technique.
29
-------
At that time, losses of sulfuric acid approaching 70% were
found for collection of the acid aerosol on filters previously
exposed to ambient particulate. Because the effect of ambient
particulate is important and was studied by us with FPD analyses
only semiquantitatively in our earlier work, this potential
interferent was evaluated again with the improved, more nearly
quantitative FPD technique.
The ambient particulate was collected on 47-mm Mitex LS
filters beside a busy roadway in Birmingham. The results of the
FPD determinations of sulfuric acid added to the filters from
the aerosol generator are given in Table 8. The recoveries of
sulfuric acid were determined for two levels of atmospheric
particulate material—the "high" level representing an 8-hr
sampling period and the "low" level, a 3-hr sample. These data
demonstrated, again, the severe losses of sulfuric acid that
can occur in the presence of ambient particulate on a filter
disk. For both the "high" and "low" levels of particulate, the
losses of the acid approached ?0%, which was somewhat more than
determined by the wet methods of analysis that were used earlier.
The greater losses of sulfuric - -.id that were determined by FPD
analyses could have resulted frc.u further reaction of the
residual acid with the particulate on the filter at the elevated
temperature (ca. 200°C) of the FPD volatilization chamber. In
any event, the addition of a spike of sulfuric acid in isopropyl
alcohol to a filter exposed to ambient particulate also resulted
in a low recovery of the acid (ca. 18%).
On the other hand, detectable FPD responses were routinely
observed for filters containing only the ambient particulate
material. Although the response could possibly be due to a
sulfur species other than sulfuric acid, the concentration of
ambient sulfuric acid calculated from these data was approxi-
mately 1 to 2 yg/m3. These data are also in approximate agree-
ment with earlier results obtained with wet methods.
Experiments were also conducted in which the order of col-
lection of ambient particulate material and sulfuric acid was
reversed. First, known amounts of sulfuric acid from the
aerosol generator were deposited on Mitex filters, and then
ambient particulate material was collected, either in the labor-
atory or near a street with heavy traffic. The experimental
conditions and the results of the FPD analyses are presented in
Table 9. The recoveries of sulfuric acid were much greater than
when ambient particulate material was deposited first.
Essentially all of the sulfuric acid was recovered from the
filters exposed at the roadside and about two-thirds was re-
covered from the filters exposed in the laboratory.
The two most important factors likely to influence the
results of these experiments are neutralization of the acid by
ambient ammonia and reaction of the collected sulfuric acid
30
-------
TABLE 8. EFFECT OF PREDEPOSITED AMBIENT PARTICULATE
MATERIAL UPON SULFURIC ACID COLLECTED
ON MITEX LS FILTERS
Ambient H^SO^
particulate added,*
added , mg yg
H2SO,
spike , t
pg
H2SO,
found , §
ug
Recovery ,
%
0.3 (0.3)# none none 5.3 (4)
0.3 none 63 11.1 18.
0.1 (2.0) 68 (29) none 7.1 (10) 10 (34)
0.2 54 none 5.2 10
0.3 65 none 7.6 12
Low
< 0.1 none none 3.9
<0.1 60 none 6.4 11
<0.1 58 none 8.5 15
<0.1 57 none 5.4 10
* The amount of sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol. .
t The spike was added to the filter exposed to ambient partic-
ulate material from a standard solution of sulfuric acid in
isopropyl alcohol.
§ Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak height.
# The numbers in parentheses are the results of similar ex-
periments in which the analyses for sulfuric acid were
conducted with the benzaldehyde extraction technique.
31
-------
TABLE 9. EFFECT OF AMBIENT PARTICULATE MATERIAL
ON FPD MEASUREMENT OF SULFURIC ACID
PREDEPOSITED ON MITEX LS FILTERS '.
Sampling
time,*
min
added,t
vg
found,§
11 g
Recovery,
%
Roadside atmosphere
5
10
20
40
60
180
45
45
46
41
41
0
45
44
46
40
34
4
100
98
100
98
83
Laboratory atmosphere
40
180
300
360
300
55
58
57
57
0
36
39
39
42
6
65
67
68
74
^
* Sampling rate was 6 1/min at a level of APM of about 100
yg/m3 .
t The amount of sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol.
§ Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. The
temperature of the volatilization chamber was 200°C.
with alkaline particulate materials on the filter. It was ex-
pected that when the sulfuric acid was deposited first,
neutralization by ambient ammonia during subsequent collection
of the APM would cause loss of the acid. Based on an estimate
of the amount of ammonia sampled simultaneously with the APM,
partial losses of sulfuric acid were expected for sampling
times under 80 min and total losses for sampling times sub-
stantially over 80 min. The results, however, were not
consistent with this expectation. If reaction of the prede-
posited acid with ammonia did occur, then it appeared that posi-
tive interference was caused by decomposition of the resulting
ammonium salts at 200°C, Another conceivable explanation of
the high recoveries is that neutralization by ammonia did
occur, at least not completely, and that the predeposited
32
-------
sulfuric acid diffused into the interior pores of the filter
where it could not react with alkaline particulate material.
Interference from Ammonium Salts of Sulfuric Acid
Several uncertainties resulted from the experiments de-
scribed above in which sulfuric acid deposited on Mitex filters
was exposed to ambient particulate material (APM). The principal
uncertainties involved recoveries of sulfuric acid that were
higher than expected based on the measured concentration of
ambient ammonia that was sampled simultaneously with the ambient
particulate material. These results suggested the possibility
of positive interference from ammonium salts in the FPD analysis
at 200°C. Subsequently, a study of the interferences from
ammonium sulfate and ammonium bisulfate was performed by apply-
ing either aqueous or alcoholic solutions of the salts to Mitex
filters followed by FPD analysis. The results of these experi-
ments are given in Table 10. From these data it appeared that
the FPD response was dependent on the temperature of the
volatilization chamber. Both ammonium salts gave positive
responses at the 200°C volatilization temperature but no sig-
nificant response was observed at 150°C with either. Because
of these findings, the evaluation of APM as a possible inter-
ferent in the collection of sulfuric acid aerosol was repeated
with attention to the temperature of volatilization in the FPD
analysis.
Interference from Ambient Particulate Material—Analysis Cor-
rected for Interference from Ammonium Salts
The evaluation of ambient particulate material as an inter-
ference in the collection of sulfuric acid aerosol was continued
with experiments in which a cyclone was used in ambient sampling
to eliminate particles greater than about 2 ym in diameter. A
description of the cyclone is given in a report prepared by
Southern Research Institute for the Environmental Protection
Agency (EPA 650/2-74-102-a, August 1975}. Our work in the past
had not included size fractionation of ambient particulate
material collected for interference studies. However, by
eliminating the larger particles the evaluation of interference
due to ambient particulate material becomes more realistic in
relation to some of the state-of-the-art devices that have been
developed for sampling and analysis of ambient aerosols of
sulfuric acid. Also, in these experiments the temperature of
the volatilization chamber was adjusted to 150°C to reduce
interference from ammonium salts.
Predeposition of Particulate Material—
Table 11 gives the results of FPD measurement of sulfuric
acid aerosol collected from the laboratory generator on Mitex
LS filters after sampling ambient particulate material. From
the data obtained with the cyclone it appeared that the extent
33
-------
10. IfofESfEBINCE fROM AMMONIUM SALVS
OF SOMUJMC ACID IN FP6 AMA^YSIS
Amfnnn i lifrt ^A 1 f-
eictdjed, yg
tfH.HSO,
0.32t
>100#
0.34§
PPD respons
200°c
0.12
0,08
1.14
0.09
0.09
0.34
e as H^SOi, , yg*
150°d
0
0
0
0
0
0.1
* The values are the averages of triplicate FPD determinations
calculated from the square roots of the peak heights.
f A standard solution of the salt in 801 isopropyl alcohol was
applied to Mitex filters.
§ A standard solution of the salt in water was applied to Mitex
filters.
# A s'Riall crystal of the ammonium salt was placed on the filter.
34
-------
TABLE 11. RESULTS OF FPD MEASUREMENT OF SULFURIC ACID
COLLECTED ON MITEX LS FILTERS FOLLOWING COLLECTION
ON AMBIENT PARTICULATE MATERIAL
Ambient particulate
added, yg*
Without cyclone
%90 (109 yg/m3)**
^90
^90
^90
With cyclonett
<50 (68 yg/m3)
<50
<50
<50
<50
<50
<90 (109 yg/m3)
<90
<90
<90
<90
<90
A! 2. o\_/ ii
aerosol
added,
ygt
none
none
none
10.1
7.0
9.2
none
none
none
15.4
11.2
12.2
none
none
none
8.3
11.5 .
9.4
spike ,
yg§
none
13.7
13.7
none
none
none
none
13.7
13.7
none
none
none
none
13.7
13.7
none
none
none
found ,
yg#
0.6
4.0
4.0
2.0
1.6
2.2
0
11.4
8.3
5.9
4.8
6.4
0.5
7.1
5.6
1.3
1.9
1.6
Recovery ,
%
.
29
29
20
23
24
83
61
38
43
52
-
52
41
16
9
17
* The sampling rate was 14.1 1/min for 1 hr.
t Analysis of a parallel filter exposed only to the sulfuric
acid aerosol was used to determine the amount of sulfuric
acid added.
§ The spike was added from a standard solution of sulfuric
acid in isopropyl alcohol.
# Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. The
temperature of the volatilization chamber was 150°C.
** The numbers in the parentheses are the total ambient partic-
ulate levels during sampling.
tt An uncertain fraction of the particulate material in the air
sampled was removed by the cyclone.
35
-------
of reaction of sulfuric acid with ambient particulate material
was a function of the concentration of total ambient particu-
lates. In these experiments, higher recoveries of sulfuric
acid resulted when ambient sampling was done at the lower con-
centration of ambient particulate. Sampling of ambient
particulate material without the cyclone did not appear to
reduce the recovery of sulfuric acid substantially, but did
reduce the recovery of the spike of sulfuric acid solution com-
pared with that observed for experiments with the cyclone. In
any event, these results indicated that trends observed in
previous studies of interference from ambient particulate
material with the FPD determination and the benzaldehyde ex-
traction technique are valid.
Predeposition of Sulfuric Acid—
Table 12 gives the results of FPD measurements of sulfuric
acid when the acid aerosol was collected before sampling APM.
Calculations based on a measured -joncentration of ambient
ammonia of 3.6 yg/m3 indicated *-.hat sufficient ammonia was
present in the 0.83 m3 of sampled air to neutralize all of the
predeposited sulfuric acid. Th •*. results of the FPD analyses
for this experiment agreed with \iis assumption due to the low
recoveries of sulfuric acid found with the 150°C volatilization
temperature. Thus, the higher recoveries reported earlier from
identical experiments, except for volatilization 200°C, probably
resulted from interference by ammonium salts that were formed
on the filter during the sampling of APM. Furthermore, the use
of the cyclone for size discrimination of the APM would not be
expected to alter these results.
Although the severity of the interference from APM in the
collection of sulfuric acid aerosol appeared to be dependent
upon the level of ambient particulate, the interference w. ..
generally sufficient in any event to complicate the accu ate
measurement of the concentration of sulfuric acid aerosi . Of
equal importance is the additional interference due to amb.ient
ammonia that is sampled simultaneously with the sulfuri aci-i.
36
-------
TABLE 12. RESULTS OF FPD MEASUREMENTS OF SULFURIC ACID
COLLECTED ON MITEX LS FILTERS PRIOR TO COLLECTION
OF AMBIENT PARTICULATE MATERIAL*
Ambient particulate
added, ygt
35 (45 yg/m3)tt
35
35
35
35
35
35
aerosol
added ,
yg§
none
none
none
13.1
8.9
9.6
10.7
HzSO*
spike,
yg#
none
13.7
13.7
none
none
none
none
H2SO,>
found,
yg**
0
6.5
6.2
0.6
0.8
0.6
0.8
Recovery ,
-
47
45
5
9
6
7
* Size discrimination of APM was not used for these experi-
ments .
t The sampling rate was 14.1 1/min for 1 hr. A cyclone in
front of the filter eliminated particulate greater than 2 ym
in diameter.
§ Analysis of a parallel filter exposed only to the sulfuric
acid aerosol was used to determine the amount of sulfuric
acid added.
# The spike was added from a standard solution of sulfuric
acid in isopropyl alcohol.
** Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. The
temperature of the volatilization chamber was 150°C.
tt The number in parentheses is the total ambient particulate
level during sampling.
37
-------
SECTION 6
DERIVATIZATION OF SULFURIC ACID AEROSOL
DERIVATIZATION WITH PERIMIDYLAMMONIUM BROMIDE
In light of the results of interference studies, it became
apparent that the stabilization of sulfuric acid during sampling
would be necessary. This approach was investigated with a
derivatization technique based or the reaction of sulfuric acid
with perimidylammonium bromide (PDA-Br) first adapted to this
problem by P. W. West and his associates.* With this method-
ology samples are collected on filters impregnated with PDA-Br.
Ideally, the sulfuric acid aerosol reacts with the PDA-Br salt
to form the insoluble sulfate iafore the acid c\n be neutralized
or otherwise destroyed by other reactions. For quantitation the
PDA derivative (PDA-SOiJ is decomposed at 450°C for FPD measure-
ment of a volatilized sulfur species.
Calibration of the FPD with
Following modifications of the FPD apparatus to allow
volatilization at 450°C, calibration data were obtained for
thermal decomposition of the perimidylammonium sulfate (PDA-SOM. )
derivative of sulfuric acid. To obtain these data standard
solutions of sulfuric acid in isopropanol were added to Mitcx
filters impregnated with PDA-Br and the resulting derivative
(PDA-SOn) was decomposed at 450°C in the FPD. In sub" -pent
experiments, samples were collected from the sulfn j.<- acid
aerosol generator with PDA-Br filters; the results o, ...... FFD
analyses of the filters gave the expected concentiati " ot
sulfuric acid in the generator effluent. The pnnsit-" ity jf tfco
derivative method was found to be essentially the r^-ua as that
resulting from conventional volatilization of ?' 'furic acid at
200°C.
With regard to the speed of the derivatization reaction,
preliminary work indicated that the fixation process was very
* Thomas, R. L., V. Dharmarajan, G. L, Lundquist, and P. W.
West. Measurements of Sulfuric Acid Aerosol, Sulfur Trioxide,
and the Total Sulfate Content of the Ambient Air. Anal. Chenu
48(4) : 639-641, 1976
38
-------
rapid. For example, when Mitex filters impregnated with PDA-Br
were used to collect sulfuric acid aerosol and were quickly
analyzed (within 1 min of sampling) by FPD with volatilization
at either 200 or 450°C, the expected FPD response was obtained
at 450°C, but no response was observed at 200°C. This result
indicated that the sulfuric acid aerosol reacted quickly and
completely to form the more refractory PDA-SO^.
Interference from Ambient Particulate Material
In further experiments ambient particulate material was
evaluated as an interference in the derivatization process during
sample collection. In these experiments, Mitex filters impreg-
nated with the PDA-Br reagent and plain filters (not impregnated)
were used to sample the ambient atmosphere. Determination of
the derivatized sulfuric acid was performed with the FPD
apparatus with decomposition of the derivative at 450°C. The
results of these experiments are given in Table 13.
TABLE 13. DERIVATIZATION OF SULFURIC ACID AEROSOL WITH PDA-Br
IN THE PRESENCE OF AMBIENT PARTICULATE MATERIAL
H2SOij Equivalent FPD response,
aerosol HzSO, arbitrary
added, yig* found, yigt units §
Mitex filters
impregnated with
PDA-Br + APM#
None
28
6
32
75
350
Plain Mitex None 10 150
filters + APM# 32 36 600
* FPD analysis of the appropriate parallel filter exposed only
to sulfuric acid aerosol served to determine the amount
added.
t Each result is the average of triplicate determinations
calculated from linearized and integrated FPD responses.
The FPD volatilization temperature was 450°C.
§ Separate calibration curves were prepared for impregnated
filters.
# The amount of APM on each filter disk was about 100 u.j
(particle size, <2 vim) .
39
-------
Approximately 100 pg of ambient particulate material
(<2 ym) were collected on each filter. Known amounts of sul-
furic acid aerosol from the generator were then deposited on
the filters and analyzed by the FPD method with volatilization
at 450°C. The amounts of sulfuric acid found were estimated
from separate calibration curves (one for PDA-SCK and one for
H2SOit) . The calibration curve for PDA-SO^ is shown in Figure
10. Because the slope of the calibration curve for sulfuric
acid at 450°C is greater than that for PDA-SO.,, it appeared that
a given amount of acid yields a smaller FPD signal when it is
converted to the PDA-S04 derivative.
Although earlier experiments had shown that no FPD response
is obtained when unimpregnated filters exposed to ambient
particulate material and sulfuric acid are heated at 150 or
200°C, these results indicated complete recovery of sulfuric
acid at 450°C (thermal decomposition of the reaction product of
sulfuric acid and ambient particr.late material) . Complete re-
covery was also obtained from the PDA-impregnated filters. Thus,
it could not be concluded from these results that PDA-SOi, was
formed selectively in the presence of APM. The results merely
showed that, whatever the produ • :s of the reactions on the
filters may be, the acid is recoverable from treated or untreated
filters at 450°C, but not at lower temperatures. However, the
smaller FPD responses for PDA-Br filters (see FPD responses
listed in Table 13) were initially taken as a favorable indi-
cation that the PDA-SOi» derivative may have been formed prefer-
entially on the impregnated filters,
It should be pointed out that the ammonium sulfate salts
present in APM would also contribute to the FPD response at
450°C regardless of whether they were fixed by the PDA-Br
reagent or not. The contribution from sulfate salts in APM can
be seen in the FPD responses listed in Table 13 for the filters
to which no sulfuric acid aerosol was added. Also, th^ data for
the filters with added sulfuric acid reflect this interference
in that the analytical results for equivalent sulfuric acid
exceeded the amount added.
On the other hand, if the sulfate salts were not fixed by
the derivatizing reagent, it is possible that they coi;'-d be
selectively removed by volatilization at temperatures below the
decomposition temperature of PDA-SCX - Several experiments w *:3
conducted to investigate the possibility of prevolatilization
of interfering sulfate ..-.alts, but the preliminary results were
inconclusive. However, before proceeding further v;ith'this
approach it was decided that the most important experiments
were those that would establish whether or not the PDA-Br
reagent fixed the ammonium salts of sulfuric acid.
40
-------
15
C/5
Z
D
CC.
<
CC
H
00
CC.
LU
CO
Z
o
O.
C/3
LU
CC
Q
n_
u.
Q
LU
CC
LU
I-
z
Q
z
Q
LU
N
CC
LU
10
0.0
0.2 0.4 0.6
AMOUNT OF H2S04, fig
0.8
Figure 10. FPD calibration for PDA-S04 decomposed at 450°C.
41
-------
Interference from Ammonium Sulfate Salts
Simultaneous fixation of ammonium sulfate salts and sulfuric
acid constitutes a severe limitation on any methodology that
depends upon selective derivatization of sulfuric acid aerosol.
The selectivity of the PDA-Br reagent for sulfuric acid was
evaluated in a relatively simple series of experiments with
aerosols of ammonium bisulfate and ammonium sulfate.
Aerosols of these sulfate salts were collected simultaneous-
ly on plain and reagent-impregnated Mitex filters in a manner
that was identical to experiments with sulfuric acid aerosols.
The sulfate aerosols were generated in a laboratory apparatus
by nebulizing dilute aqueous solutions of the appropriate
ammonium salt with a Beckman No. 4020 atomizer-burner. This is
the same device that is used for generation of the sulfuric acid
aerosol; however, only argon (at a pressure of 140 kPa (20 psig))
was used to aspirate the feed solutions and the flame was not
employed for generation of the suifate salt aerosols.
In order to determine if the sulfate salts were fxxed on
the PDA-Br filters, FPD determi: .tions were made on both plain
and impregnated filters with a volatilization ten.perature of
200°C. At this temperature large FPD responses were observed
for ammonium sulfate and ammonium bisulfate aerosols collected
on plain Mitex filters. Thus, if the salts were not fixed
on the PDA-Br filters, large FPD responses would be expected.
On the other hand, if the sulfate salts were completely fixed
and formed PDA-SO^ in the same manner as sulfuric acid aerosol,
then no FPD response would be observed at 200°C due to the higher
decomposition temperature of the PDA-SO^ derivative. The results
of these experiments are shown in Table 14. These data indicated
that both ammonium salts of sulfuric acid were efficiently
derivatized (>80%) by topochemical reaction with PDA- :3r on the
impregnated filters. Therefore, it was concluded that the
reaction of PDA-Br to form the PDA-SCU derivative is not a^le
tive for sulfuric acid.
FIXATION OF SULFURIC ACID AEROSOL WITH VERSAPOR FILTERS
In earlier work at the Institute it was shown that
acid aerosol reacted with Versapor filters (Gelman Instrument
Company) to form an insoluble product that could not be do or-
mined by methods depending upon extraction and which was
probably a sulfate. The filter is characterized by the manu-
facturer as a glass-fiber medium with an epoxy binder.
In experiments with sulfuric acid aerosol it was f^unc? • .i
the reaction product formed on Versapor filters was r-->-
decomposed to a volatile sulfur species at either TOO . 1
-------
TABLE 14. EVALUATION OF SELECTIVITY OF PDA-Br
WITH AEROSOLS OF SULFATE SALTS
NH^HSOi,. (NH^)2SO^ FPD response, Sulfate
aerosol aerosol arbitrary derivatized,
dded, ucr* added, uq* uni.tst,§ %
Mitex with
PDA-Br
Plain Mitex
^
81
_
81
94
- '
94
~
275
250
1400
4700
80
95
_
"•
* The output of the aerosol generator was measured by deter-
mining sulfate with the barium chloranilate method•and
ammonium ion with the indophenol method.
t Volatilization temperature of the FPD apparatus was 200°C.
§ FPD responses were linearized and integrated.
exposed to sulfuric acid and analyzed in the FPD apparatus.
Although not amenable to thermal volatilization-FPD analysis,
the filter could be useful if it could be shown not to react
with sulfate salts.
However, in experiments identical to those described in the
previous section for PDA-Br filters it was demonstrated that the
Versapor filters also fixed the ammonium sulfate salts as well
as sulfuric acid. Because Versapor filters did not appear to
offer any advantage over PDA-Br filters for fixing sulfuric
acid aerosol, work with these filters was discontinued.
VOLATILIZATION, PREFILTRATION, AND DERIVATIZATION OF SULFURIC
ACID AEROSOL
After it was demonstrated that PDA-Br filters fixed
aerosols of sulfate salts as well as sulfuric acid aerosols,
experiments were performed to determine if sulfuric acid aerosol
could be volatilized during sampling and sulfates of lower
volatility removed by filtration of the heated air stream.
Volatilization and Prefiltration Apparatus
To facilitate the thermal volatilization of sulfuric acid
aerosol, a 30-cm section of a 12-mm (i.d.) glass sampling
probe was wrapped with a resistance heating tape. At a flow
rate of 3 1/min, the aerosol sample from the laboratory
43
-------
generator could be heated in this manner from about 35°C to as
high as 200°C.
In preliminary experiments, the heated probe and a heated
filter holder with a Mitex filter were run simultaneously with
an identical reference sampling line that was not heated. The
airstream was cooled in a 35-cm section of tubing following the
filter. It then passed into an impinger-bubbler containing 80%
isopropanol to collect vapors and recondensed sulfuric acid
aerosol. Following the sampling period, any sulfuric acid con-
densed on internal surfaces behind the filters was rinsed into
the bubblers. Because of the nature of the bubbler samples and
the higher levels of sulfuric acid used in these preliminary
experiments, it proved to be practical to use the barium
chloranilate method for these analyses.
Preliminary results indicated that although only 30% of the
total sample of sulfuric acid aerosol (relative to the reference
side) could be accounted for, oT~er 70% of this amount passed
through the filter at 130°C. The recovery was only slightly
increased by changing from a st 'nless steel filter holder to
one constructed of Teflon.
These results suggested that the losses of sulfuric acid
probably occurred in the heated zone of the glass sampling
probe. This possibility was investigated further by determining
the amount of sulfuric acid passing through the heated probe as
a function of temperature. In these experiments the sulfuric
acid was collected in impinger-bubblers following both the
heated and reference probes. The results of these experiments
are shown in Table 15. These data showed that for temperatures
sufficient to volatilize an appreciable portion of the sulfuric
acid aerosol (ca. 90 to 130°C), most of the resulting vapor
then lost to tEe" walls of the heated glass probe.
In an effort to improve the passage of sulfuric acid in
the heated probe, a Teflon lining was inserted in the heated
zone to reduce the reactivity of the inner surface. This
thermal volatilization apparatus is shown schematically ;.n
Figure 11. An identical series of experiments was ther L,t-..-
formed to evaluate this modification. The results are .jiven i
Table 16. In contrast, these data showed that sulfm ic acid
vapor was passed efficiently through the heated zone wj i.h the
Teflon lining. In further experiments with this apparatus \:h
amount of sulfuric acid vapor passing through the Mitex r>r --
filter was determined as a function of the volatilization
temperature. The results of these experiments are given in
Table 17. These data indicated that sulfuric acid aerosol
was volatilized efficiently at 110°C and that at "east 90£ of
the resulting vapor passed the initial Mitex filter (h*
to about 120°C).
44
-------
TABLE 15. PASSAGE OF SULFURIC ACID THROUGH
THE HEATED GLASS SAMPLING PROBE
Temperature of
heated sample
•airstream, °C
76
82
84
108
129
133
200
HzSOit
sampled ,
yg*
122
106
103
111
117
100
115
HzSO^
found ,
ygt
129
108
105
68
31
29
33
Recovery ,
%
106
102
102
61
26
29
29
* These are the amounts of sulfuric acid that passed through
the unheated reference probe (ca. 30°C) run simultaneously
with the heated probe. Analyses for HaSOi* were performed
by the barium chloranilate method.
t These are the amounts of sulfuric acid that passed through
the heated glass probe.
TABLE 16. PASSAGE OF SULFURIC ACID THROUGH THE
HEATED TEFLON-LINED SAMPLING PROBE
Temperature of
heated sample
airstream, °C
82
102
184
204
H2SO^
sampled,
yg*
90
78
69
47
HzSO^
found ,
ygt .
96
71
56
35
Recovery ,
%
107
91
81
75
* These are the amounts of sulfuric acid that passed through
the unheated reference probe (ca. 30°C) run simultaneously
with the heated probe; analyses for HzSOit were performed by
the barium chloranilate method.
t These are the amounts of sulfuric acid that passed through
the heated Teflon-lined probe.
45
-------
TEFLON
FILTER
HOLDER
HEATING
MANTLE
SECOND BACK-UP
FILTER
\
TO PUMP
3 l/min
STAINLESS STEEL
FILTER HOLDER
47-MM REAGENT-IMPREGNATED
BACK-UP FILTER
TEFLON SCREEN
.47-MM PREFILTER
10 MM I.D. TEFLON LINE
17 CM
HEATING ZONE
30 CM
H2S04
AEROSOL
Figure 11. Apparatus for volatilization, prefiltration, and derivatization
of sulfuric acid aerosol.
-------
TABLE 17. PASSAGE OF VOLATILIZED SULFURIC ACID AEROSOL
THROUGH MITEX FILTERS AS A FUNCTION OF TEMPERATURE
Volatiliza- Amount of • -_,„, ,_ Portion of
TT r^^ lifcbUit round , uq .. __
tion H2SO^ - - - - — : - ' H^ ' — H2SOi,
temperature, aerosol on passing passing
sampled, yg* prefilter pref liter t prefilter, %
85
100
105
110
83
45
94
141
43
12
10
10
40
33
84
131
48
73
89
93
* The amount of aerosol sampled was determined with a parallel
unheated sampling probe; all analyses for H2SOH were per-
formed by the barium chloranilate method.
t Sulfuric acid vapor passing the prefilter was determined with
a back-up impinger-bubbler containing 80% isopropanol.
Interference from Ammonium Sulfate Salts
After establishing that sulfuric acid aerosol could be
efficiently volatilized and prefiltered at 110°C, experiments
were conducted to determine the extent of interference, if any,
from aerosols of ammonium sulfate salts under identical condi •-
tions. The results of these experiments are given in Table 18.
These data indicated that prefiltration. of the heated sample
air stream effectively eliminated the sulfate salts. Therefore,
it appeared to be feasible with this technique to eliminate
interference from these particulates in the determination of
sulfuric acid aerosol.
Derivatization of Sulfuric Acid Vapor with PDA-Br
In order to stabilize a sample collected in tho field, wa
originally attempted the use of a back-up filter impregnated
with PDA-Br (in a separate, unheated filter holder) to fix
sulfuric acid aerosol that recondensed following prefiltration
of the vapor. However, in a series of experiments it was con-
sistently observed that most of the vapor condensed on cool
surfaces after prefiltration without forming an aerosol that
could be collected on the impregnated filter. Therefore, it
was of interest to determine if the vapor itself could be fi;.,.u
by a PDA-Br filter. If so, the reagent-impregnated filter could
be mounted behind the prefilter (with an appropriate spacer)
in the same heated filter holder.
47
-------
TABLE 18. RETENTION, OF SULFATE SALT AEROSOLS
BY A HEATED MITEX FILTER
Aerosol*
NHuHSOi*
(NHJ 2 SO,,
Amount
sampled ,
ygt
24
48
Amount found, yg
on
pref liter
16
45
passing
prefilter§
5
<4
Portion of
salt retained,
%
76
>90
* Temperature of the volatilization zone was 110°C.
t The amount of aerosol sampled was determined with a parallel,
unheated sampling probe; analyses were performed by the
barium chloranilate method for culfate.
§ The amount of interferent siipping the prefilter was deter-
mined with a back-up filter followed by an impinger-
bubbler containing 80% isopi'^anol.
Accordingly, PDA-Br filters were used to sample sulfuric
acid vapor at about 110°C and were analyzed by FPD to determine
their retention of the vapor as PDA-SOit. In these experiments
it was found that less than 5% of the vapor was derivatized.
These results suggested that any fixation reaction relying upon
the formation of an insoluble sulfate derivative (e.g_;, PDA-SO*
or BaSOO may not occur rapidly enough with sulfuric acid vapor
to be useful. It appeared to be'likely that the only suitable
type of fixation scheme for sulfuric acid vapor would involve
an acid-base reaction. Therefore, we began to evaluate the
effectiveness of back-up filters impregnated with alkaline
substances, such as NaaCOa and NaOH, for reaction with the
vapor at elevated temperatures. The preliminary results were
favorable.
Derivatization of Volatilized Sulfuric Acid Aerosol with Alkali
After it was demonstrated that sulfuric acid vapor was not
efficiently derivatized by filters impregnated with perimidyl-
ammonium bromide (PDA-Br), several experiments were conducted
with Mitex filters impregnated with alkaline solids, such as
sodium carbonate or sodiom hydroxide. It was anticipated that
the reaction of sulfuric acid vapor passing the prefilter would
be very rapid on the back-up filter impregnated with the
strongly alkaline material. Results of experiments in which
sulfuric acid aerosol was sampled with the combined volatiliza-
tion, prefiltration, and derivatization technique are given in
Table 19. These data showed that sulfuric acid vapor was
48
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TABLE 19. DERIVATIZATION EFFICIENCY OF
ALKALI-IMPREGNATED FILTERS FOR
SULFURIC ACID VAPOR AT 110°C
yg
on im-
Impregnated
alkali*
aerosol
sampled,
ygt
pregnated slipping
back-up back-up
prefilter filter filter§
on
Vapor
collection
efficiency
of back-up
filter, %#
Na2C03
low level of 58
impregnation
high level of 76
impregnation
12
14
57
32
10
30
85
NaOH
low level of
impregnation
60
53
106
12
<4
23
40
43
66
9
10
17
82
81
80
* The alkali was impregnated onto Mitex filters from saturated
alcoholic solutions.
t The amount of aerosol sampled was determined with a parallel,
unheated sample probe; all analyses were performed by the
barium chloranilate method for sulfate and reported as equiva-
lent H2SO.».
§ Slippage was determined with a secondary back-up filter
(reagent-impregnated) followed by an impinger-bubbler con-
taining 80% isopropanol.
# The collection efficiency is relative to the amount of H2SOi»
vapor to which the impregnated filter was actually exposed.
adequately fixed on Mitex filters impregnated with sodium
hydroxide. A comparable level of sodium carbonate was somewhat
less efficient in fixing the vapor. Although the alkali-
impregnated filters were much more efficient than PDA-Br filters
for derivatization of sulfuric acid vapor, a significant amount
of the acid did slip the impregnated filters.
New Sample Probe
After it was established that sulfuric acid aerosol could
be efficiently volatilized and then prefiltered at approximately
49
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ilbdc: and that ffibrf than 7:S% bf thfceTFerlni §ut£afee salts
removed by the pSrfefiltet aftef- pas§ag£ ©f fcht ;fi"§fbstlf through
the heated probe at temperatures as high as 136faC> modifications
were made so that the sampling rate of the apparatus ve"'dul€ fee
increased from 3 tb 14 1/min to allow sh&rter eollebtiLoh period's
for ambient levels of sulfuric acid •aer&sbl-.
The larger probe was constructed from a L;2-m by 12/5-mm
i.d. brass outer tube with a longer 1.3'5-m by §-mm i.d-. Teflon
liner. Both ends of the Teflon tube were threaded so that
filter holders or other devices could be directly attached* To
effect the thermal volatilization of sulfuric acid aerosol, an
80-cm section of the Teflon-lined brass tube was wrapped with a
resistance heating tape and then insulated* A thermocouple was
attached to the surface of the probe to monitor the temperature
of the heated zone so that overheating of the Teflon liner could
be avoided. A second thermocouple was inserted into the Teflon
filter holder behind the impregnated collection filter. This
thermocouple was used to record the actual temperature of the
heated airstream during sample collection.
To evaluate the modif icati*. :-s to the sampling apparatus,
the heated probe (120°C) and the heated filter holder (130°C)
were operated simultaneously with a reference sampling line that
was not heated. Results of preliminary experiments with sulfuric
acid aerosol are given in Table 20. These data indicated that
an aVerage Of about 85% of the volatilized sulfuric acid
successfully passed through the heated zone of the new probe.
The data presented in Table 21 show further than an average of
about 84% of the sulfuric acid vapor from the heated zone
passed through the Mitex prefilter. On the basis of the data
from both tables an overall efficiency of about 70% was indi-
cated for the combined volatilization and prefiltratibh process*
In earlier experiments the overall efficiency for the volatili-
zation and prefiltration at a flow rate of 3 1/mih was found to
be in excess of 80%. Nevertheless, the present system appeared
to be adequate.
Derivatization of Volatilized Sulfuric Acid Aerosol at .Increased
Flow Rates
With regard to the fixation or derivatizatioh of sulfuric
acid on ah alkali-impregnated filter (following the prefilter),
preliminary results with the new probe, shown in Table 22,
suggested that the efficiency was significantly lower at this
Increased flow rate. Low efficiency was indicated by slippage
of a substantial portion bf the acid vapor past the impregnated
filter into a back-up impinger-bubblers Initial effbrts tb
improve the efficiency by increasing the lev^I of imp'feghdtsd
alkali were only partially successful * Furthermore, inter-
ferences with the analytical method precluded ccificlusive' (evalu-
ation of the effect of ihcfea's'egl impfggriati8h bi alkali *
§8
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TABLE 20. VOLATILIZATION EFFICIENCY OF THE HEATED PROBE
AT A TEMPERATURE OF 120°C AND A FLOW RATE OF 14 1/MIN
Amount
aerosol
of HzSOu
sampled, yg*
152
141
143
117
Amount of HZ SO i*
found, yg-f.
130
125
99
112
Recovery
%
86
89
70
96
* Determined by barium chloranilate analysis of the filter
from the reference probe.
t Determined by barium chloranilate analysis of the impinger-
bubbler catch following the heated probe.
TABLE 21. PREFILTRATION EFFICIENCY OF MITEX FILTERS
AT A TEMPERATURE OF 130°C AND
A FLOW RATE OF 14 1/MIN
Amount of H2SO.j
vapor sampled ,
yg*
94
104
Amount of
on
prefilter
17
16
H2SOit found, yg
passing
prefiltert
75
88
Portion of
H2SOi, passing
prefilter, %
82
85
* This is the amount of volatilized sulfuric acid aerosol from
the heated zone of the sampling probe; all HaSOi* analyses
were performed by the barium chloranilate method.
t The sulfuric acid vapor that passed through the prefilter
was determined by analysis of an alkali-impregnated filter
and a back-up impinger-bubbler.
51
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TABLE 22, EFFICIENCY OF SULFURIC ACID DERI VAT IZAT ION
BY ALKALI-IMPREGNATED TEFLON FILTERS AT A
FLOW RATE OF 14 I/MIR
Amount of
EzSO*
aerosol
sampled , yg
29
104
60
50
Amount of B2SO\ found,* yg
on
pref liter
8
16
20
13
on
impregnated
filter
9
28'
<2
25
passing
impregnated!
filter
12
60
38
12
Efficiency
of im-
pregnated
filter, %
43
32
<5
68
* Determinations of sulfate wers made by the barium
chloranilate method; HzSOit parsing the impregnated filters
was collected in an impinge-r^-bubbler containing 80%
isopropanol.
It appeared that the face velocity through the impregnated
47-mm filters may have been too great at the 14-1/min flow rate
to allow efficient reaction of the acid vapors with the impreg-
nated alkali. In other experiments it was found that higher
volatilization temperatures (ca. 150°C) also decreased the
efficiency of derivatization with the impregnated filters.
Because the higher flow rate is~ desirable for shorter
sampling periods with ambient sulfuric acid aerosolst an effort
was made to increase the efficiency by first increasing the
level of impregnation of alkali and then by using thicker Teflon
filters for impregnation. Unfortunately, evaluation of the
results of these experiments was complicated by a substantial
degree o£ interference with the barium chloranilate (BCA) method
for sulfate. The interference was apparently due to increased
levels of chloride that resulted from the neutralization with
hydrochloric acid of excess alkali extracted from the impreg-
nated filters. However, preliminary results based on BCA
analyses of the back-up bubblers containing 80% isopropanol
indicated that somewhat less sulfuric acid slipped the thicker
alkali-impregnated filters.
A more meaningful evaluation of current modifications to
the sampling system will be made with the ion chromatographic
(1C) method for sulfate that will be included in future work on
this project. In addition to offering greater sensitivity for
sulfate than the present BCA method, the 1C method should elimi-
nate the interference problems that have complicated previous
experiments *
52
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TECHNICAL REPORT
(Please read Instructions on the reverse
DATA
before completing)
1. REPORT NO.
EPA-600/2-78-060
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
DEVELOPMENT OF A PORTABLE DEVICE TO COLLECT
SULFURIC ACID AEROSOL
Second Interim Report
5. REPORT DATE
March 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Herbert C. Miller, David W. Mason and William 0. Barrett
8. PERFORMING ORGANIZATION REPORT NO.
3778-XII
SORI-EAS-77-731
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
1AA601 CA-28 (FY-78)
11. CONTRACT/GRANT NO.
68-02-2468
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTP,NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim 9/76-9/77
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
Previous Report: EPA-600/2-77-027, February 1977
Progress is reported on research to develop a quantitative, interference-free
method for collecting airborne sulfuric acid aerosol on a filter. Since previous
research found that severe losses of sulfuric acid were caused by ammonia, ambient
participate material, and other interferents, a method was needed that converts
sulfuric acid to a stable derivative for subsequent analysis. Methods evaluated for
direct fixation of sulfuric acid aerosol were not found to be selective. Therefore,
a sampling method was investigated that uses a combination of selective volatilization
of the sulfuric acid, prefiltration of particulate interferents, and derivation of the
vaporized acid on an alkali-impregnated filter. The initial stages of this current
research on the volatilization-derivatization technique are described.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
* Air Pollution
* Sulfuric Acid
* Aerosol
* Collecting Methods
* Filters
* Tests
13B
07B
07 D
14B
8. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report/
UNCLASSIFIED
21. NO. OF PAGES
63
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
53
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