United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/016 Apr. 1985
&EPA Project Summary
Chemical Transformations in
Acid Rain: Volume I.
New Methodologies for
Sampling and Analysis of
Gas-Phase Peroxide
Roger L. Tanner
New methodologies for sampling
and analysis of gas-phase peroxides
(H2O2 and organic peroxides) using (a)
diffusion denuder tubes and (b) gas-
to-liquid transfer with prior removal of
ozone have been investigated. The
purpose was the development of an
interference-free method for deter-
mining H2O2(g) in ambient air. A
denuder approach using ferrous (1,
10-phenanthroline)-coated tubes was
unsuccessful for, although H2O2 was
removed, the capacity was low and
ozone was also removed, possibly
through surface decomposition to
H2O2 and its radical precursors.
Gaseous peroxide in compressed
airstreams could be collected in im-
pingers without artifact formation
from surface ozone decomposition if
O3 was first removed by gas-phase
titration with nitric oxide.
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
Research Triangle Park, NC, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The purpose of this research task was
to develop fundamentally new methods
for sampling and analysis of gas-phase
hydrogen peroxide (H202) and organic
peroxides, if possible, through the use of
diffusion-denuder tubes. In addition,
sampling methods for H202 using gas-to-
liquid transfer and capable of avoiding in
situ production of H202 from ozone (03)
decomposition and other processes were
studied. The goal of the research was an
interference-free method for gas-phase
peroxides with 0.1 ppb limit of detection
and 15 min time resolution.
Much analytical effort has been expend-
ed in the past few years in measuring
gaseous and aqueous H202 following the
recognition that H202 could oxidize
dissolved S(IV) rapidly throughout the pH
range of rain, cloud and fog waters. Fur-
thermore, the high solubility of H202 in
water led to significant H202 concentra-
tions in cloudwater. Methods for deter-
mining liquid-phase H202 have been
developed using several approaches:
luminol chemiluminescence, p-hydroxy-
phenylacetic acid dimer fluorescence,
scopoletin fluorescence quenching and
peroxyoxalate chemiluminescence. At-
tempts to measure gas-phase hydrogen
peroxide by collection in impingers or by
other dissolution techniques have been
shown to be generally unreliable due to
the in situ formation of hydrogen peroxide
from low-solubility constituents of am-
bient air during collection by impingers. It
is suspected that surface-initiated ozone
decomposition via H02~ and 02~ in-
termediates is the likely mechanism of
"artifact" H202 formation.
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Two possible research approaches for
artifact-free sampling of H202(g) were in-
vestigated in this project: selective, reac-
tive sampling onto a coated denuder tube,
employing H202 redox chemistry and sup-
pression of in situ H202 formation by
selective removal of ozone.
This study describes a denuder ap-
proach using ferrous (1,10-phenanthroline)
coated tubes that was successful: the
capacity for H202 was low and ozone was
also removed, possibly through surface
decomposition for H202 and its radical
precursors. However, the study also
documents that gaseous peroxide in com-
pressed airstreams can be collected in im-
pingers without artifact formation from
surface ozone decomposition if 03 is first
removed by gas-phase titration with nitric
oxide.
Experimental
Experiments in this task were con-
ducted by using a system in which gas-
phase hydrogen peroxide was reproduci-
bly generated in the 1-500 ppbv range by
multiple dilutions with compressed air,
with facilities to subsequently humidify
the air and add ozone and nitric oxide to
the diluted airstream (see Figure 1).
Parallel airstreams were then formed with
appropriate mixing chambers, and perox-
ide was collected in parallel series of 2 or
3 midget impingers that were followed by
flow meters, charcoal traps for ozone and
H202, and a common, selectable inlet line
to the ozone analyzer. Diffusion denuder
tubes containing coatings of Fe(ll)-1,
10-phenanthroline were tested in ex-
periments by placing them in the flow
streams after division at points A and B,
just prior to impingers 1-1 and 2-1,
respectively. Aqueous H202 in each of the
six impingers was analyzed after each ex-
periment using the POHPAA fluorescence
method by sample injection into a flow in-
jection system.
For the most of the experiments con-
ducted in this task, an alternate, manually
operated "stopped-flow" approach to
aqueous H202 analysis was used. In this
approach, three volumes of sample were
mixed, with one volume of peroxidase/
POHPAA/EDTA solution in pH 8.5 TRIS
buffer. The premixed sample was allowed
to standard for 2-3 min, and then an ali-
quot was injected directly into the
fluorimeter flow cell for analysis. The flow
cell was rinsed out thoroughly with TRIS
buffer between injections. Extensive
washing of the sample lines in contact
with catalase solution was required before
reuse.
Results and Discussion
Considerable effort was expended to
improve the POHPAA fluorescence
technique for aqueous-phase peroxide.
These efforts were required because low
levels of peroxide would have to be
analyzed for passive denuder-collection
Mixing
Chamber
NO/
A/2
H202 samples or for artifact-free impinger
collections (if such could be achieved).
Without the improved analytical sensitivity
attained by these efforts, the stated goals
of this task (0.1 ppbv, 15 min time resolu-
tion) would not have been possible. The
final limit of detection achieved during
this effort was 0.3 ppb aqueous (~ 0.01
/Jvl), This sensitivity was just barely ade-
quate to attain the task goal for the
artifact-suppression or denuder ap-
proaches.
Denuder Tube Sampling
Pyrex denuder tubes, 0.6 cm OD by 30
cm long, were coated with ferrous-! 1,
10-phenanthroline)-sulfate solution in
methanol. They were then placed in the
parallel airstreams downstream from the
mixing chamber and, in the case of line 2,
after the addition of NO in N2 to the
airstream. Various admixtures of com-
pressed air and ozone and/or H202 were
passed through the denuders. Nitric oxide
(6.2 ppm after dilution) was added to line
2 and the apparent H202 in each of im-
pingers 2-1 through -3 and 1-1 through -3
(see Figure 1) was determined using the
POHPAA fluorescence technique. Four-
teen experiments were conducted to test
the denuder sampling technique: The
results obtained suggested the following:
(1) Ferroin-coated denuder tubes of the
type tested have limited capacity for
H202 removal, but they also remove
F/M
2
2-1
2-2
Impingers
2-3
Vent
Excess
Thru
Charcoal
8410
03
Monitor
F/M
1
1-1
1-2
Impingers
1-3
I 1
Optional
Humidifier
Figure 1. Modified apparatus for W2O2 generation and impinger collection.
2
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03 at continuously decreasing efficien-
cy.
(2) Ferroin-coated denuders do not pre-
vent artifact H202 formation in im-
pingers downstream of the denuder.
(3) It appears that ozone reacting on the
denuder surface may generate artifact
H202 on partially exhausted denuder
tubes.
We do not wish to assert that all
iron(ll) complexes would exhibit the same
behavior when used as denuder coatings
as did ferroin, but only that the denuder
approach as defined was heretofore un-
successful in removing H202 quantitative-
ly. In addition, complications are intro-
duced because of co-removal of ozone
and the likely co-reaction with the ferroin.
Gas-to-Liquid Sampling
After O3 Removal
As noted above, previous observations
have suggested that artifact H202 formed
in gas-to-liquid sampling using impingers
or other approaches seems to be related
to levels of ozone and one or more other
air constituents. As a result, sampling ap-
proaches in which ozone is selectively
removed from the sampled air may be
successful in eliminating the artifact
formation of H202 while simultaneously
transmitting H202 at high, reproducible ef-
ficiency to an aqueous solution for
POHPAA analysis of H202. Unreported
data suggest that the amount of H202
formed in bubblers is non-linearly related
to 03 concentration in sampled air, but
since 03 reaction on the bubbler surfaces
appears to be the initial and limiting step
in artifact H202 formation, removal of 03
prior to sampling should effectively
eliminate the process.
The evidence that titration with excess
NO removes artifact H202 formation in im-
pinger collection of H202 is shown in
Table 1. Hydrogen peroxide (calculated to
be 28 + 2 ppb in stream 1 and 22 ± 4
ppb in stream 2) was admitted without O3
or NO in Expts. 1 and 2; peroxide was
found in roughly equal amounts in bub-
blers 1-1 and 2-1. No peroxide was found
in subsequent bubblers. Ozone at 327 ±
13 ppb was admitted to the system in
Expts. 3 through 9, with NO (6.2 ppm)
present in stream 2 only for Expts. 4
through 9. In Expt. 3, addition of ozone
alone to both streams produced H202 in
all impingers with most being found in im-
pinger 1 of each stream. Addition of
ozone + H2O2 mixtures to stream 1 and
03/H202/NO to stream 2 (Expts. 4, 5, and
9) produced additional peroxide in all
stream 1 bubblers consistent with the
results of Expt. 3, but no peroxide was
formed in stream 2 bubblers sampling NO-
containing air.
The amount of artifact H202 sampled
from 03-containing airstreams was
variable, and appears to be reduced in
Expts. 5 to 9 in comparison to Expts. 1 to
4. Indeed, no artifact H202 was formed in
impingers 2-2 and 2-3 during Expts. 6 to
8. In contrast, only in Expt. 6 was a small
amount of H202 found in the 03/NO/air-
stream. This could be due to inadequate
mixing of NO with the ozone/airstream,
but is more likely the result of desorption
of H202 retained in the mixing chamber
from the previous experiment. Never-
theless, the preponderance of evidence
suggests that ozone is removed sufficient-
ly fast that no measurable artifact H202 is
formed.
Artifact H202 is formed in variable
amounts when an airstream containing
about 300 ppb 03 is sampled. Collected
amounts correspond to about 3-15 ppb of
gaseous H202 in the first bubbler and
roughly an order of magnitude lower in
subsequent bubblers. This differs
somewhat from results reported by others
in which roughly equal amounts of perox-
ide were formed in subsequent bubblers,
and indeed, it is different from our own
early results, suggesting that the extent of
the process in which artifact H202 is form-
ed is quite dependent on the presence of
other atmospheric constituents in addition
to 03. That these constituents may differ
widely in their aqueous solubility is sug-
gested by the low production rate of ar-
tifact H202 in impingers 2 and 3 compared
to impinger 1 for the experiments sum-
marized in Table 1.
Conclusions
The research approaches investigated in
this task for artifact-free sampling of
H202(g) included selective, reactive
sampling onto a coated denuder tube,
employing H202 redox chemistry and sup-
pression of in situ H202 formation by
selective removal of ozone.
A denuder approach was attempted
employing Fe(ll)-1,10-phenanthroline-
coated glass tubes. Hydrogen peroxide
was removed by such tubes but collection
efficiencies less than calculated values
were observed even with relatively fresh
tubes. This suggested that surface deple-
tion of sorption sites was reducing the
capacity of dry coating on the diffusion
tube. In addition, ozone was removed to
a significant extent by the Fe(ll)-phenan-
throline denuder tubes, which indicated a
lack of specificity for H202 and raised the
spectre of surface decomposition of 03,
possibly to gas-phase H02 and/or H202.
Table 1. Collection of H2O2 from Ozone-Containing and Ozone-Free Air Streams
Experiment
No.
;
2
3
4
5
6
7
8
9
H202 (fJ\
Composition
H202/Air
"
O3/Air
03/H2O2/Air
"
03/A/r
"
"
03/H202/Air
VI) in Sampled
Bubbler 1
0.58
0.55
0.36
0.96
0.63
0.15
0.044
0.047
0.69
Gas Stream 1
Ave,
Bubbler 2 + 3
ND*
••
0.022
0.018
0.016
ND
ND
ND
0.060
H202 (fuM
Composition
H2O2/Air
"
O3/Air
03/H2O2/NO/Air
"
03/NO/Air
"
"
03/H202/NO/Air
') in Sampled Gas
Bubbler 1
0.56
0.39
0.31
0.52
0.45
0.026
ND
ND
0.50
Stream 2
Ave,
Bubbler 2 + 3
ND
ND
0.014
ND
ND
ND
ND
ND
ND
*ND = none detected {•< Blank)
Gas Phase Concentrations:
[Ozone] = 327 ± 13 ppb (Expts. 3-9)
[H2O2] = 28.0 ± 2.0 ppb I Stream 1); 22.2 ± 3.6 ppb IStnvn 2)
[NO) = 6.2 ppm
Sampling Conditions:
Air sampled for 30 min 0.50 L/min in each One.
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Thus, removal of H202 onto denuders by
nominally H202-specific chemisorption
does not appear to offer significant ad-
vantages over gas-to-liquid sampling for
gaseous H202 analysis.
Suppression of in situ production of
H202 in gas-to-liquid sampling (bubblers,
impingers) by upstream titration of the
ozone in the sampled airstream has been
successfully demonstrated for cases in
which compressed air is used. Gaseous
hydrogen peroxide was collected com-
pletely (>99%) in the first bubbler
whereas confluent ozone produced
measureable peroxides in the second and
third bubblers; no peroxide was observed
in the second and third bubblers for those
experiments in which 03 (100-300 ppb)
was titrated by 6 ppm NO prior to bubbler
collection. The addition of 6 ppm NO
does not significantly interfere with
POHPAA analysis of collected aqueous
H202.
Roger L Tanner is with Brookhaven National Laboratory, Upton, NY 11973.
Marc/a C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Chemical Transformations in A cid Rain: Volume I.
New Methodologies for Sampling and Analysis of Gas-Phase Peroxide, "(Order
No. PB 85-174 425/AS; Cost: $8.50, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at'
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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