EPA-650/2-74-096
MAY 1974
Environmental Protection Technology Series
'. V
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EPA-650/2-74-096
EFFECT OF FUEL ADDITIVES STUDY
by
D. M. Steffenson, D. H. Stedman, and D. J. Patterson
University of Michigan
College of Literature, Science, and the Arts,
DC, and College of Engineering, DME
Ann Arbor, Michigan 48104
Grant No. R-802419-01
ROAP No. 26AAE-24
Program Element No. 1AA002
EPA Project Officer: Ronald Bradow
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
May 1974
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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SUMMARY
We have carried out studies to be published in Analytical
Chemistry on the optimization of a chemilumlnescent NO detector.
This detector has been used to study the effect of N containing
fuel additive combustion using a gas chromatographic technic.
Progress so far indicates N selective GC detection down to 1.6 ng
of diethyl nitrosamine (^0.5 ppm in a 1 ml gas sample). Similar
data are obtained for amines. The detector has at least two orders
of magnitude more sensitivity currently not realised due to pro-
blems with the GC columns.
Preliminary studies have been made of rapid determination of
total N in gasoline, and of combustion products in a flat flame
propane-air burner.
111
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Introduction
The overall goal of this research, as outlined In the original
proposal, was to develop and apply analytical techniques to deter-
mine combustion products arising from two gasoline fuel additives:
disalicylidene propane diamine (Universal Oil Products 55) and NC
alkenyl succimide (sic)(Lubrisol 580). Specifically, this meant
developing and adapting a chemiluminescent NO detector as a nitrogen
specific gas chromatography detector sensitive enough to measure ppm
concentrations of either additive or combustion products, and using
the detector to look for these products when these additives were
added to the fuel of either a flat flame burner or an automobile
engine.
The sequence of experiments were to be as follows: 1.) Carry
out some physical and chemical tests on the two fuel additives
supplied by EPA. 2.) Build and optimize a chemiluminescent NO de-
tector. 3.) Develop the detector as a N specific G.C. detector.
4.) Establish the limits of detectibility of the instrument for
several N containing compounds. 5.) Develop techniques for adding
ppm quantities of fuel additive to the propane fuel of a flat flame
burner and sampling the possible combustion products for analysis
with the G.C. and the new detector. 6.) Repeat 5.) with a propane
fueled single cylinder engine. 7.) If warranted by the results of
5.) and 6.), extend the measurements to a multicylinder gasoline
fueled engine. As will be shown in the report, we have successfully
accomplished steps 1.) - 3.), and have done a great deal towards
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finishing i».) and 5.), but certain difficulties remain, and some
new approaches are being tried to improve the detector and the
burner experiments.
Fuel Additives
As it turned out, the two fuel additives supplied for this
project were chemically bad choices for analytical research using
gas chromatography. Their particular disadvantages will be dis-
cussed separately.
Lubrizol 580, originally identified to us as NC,,- alkenyl
succimide, is a product of a condensation reaction of maleic an-
hydride and polybutylene amine, where the polybutylene amine is a
C50 hydrocarbon with a single amine group on the end (1). The pro-
duct polymer undoubtedly is a mixture of several adducts with the
final formulation containing 0.8$ M by weight (1). The additive
is supplied commercially as a mixture of polymer, xylenes, a long
chain alcohol, and an emulsifier (1).
The above information was confirmed by a simple column chroma-
tography separation of Lubrizol 580 and taking nmr spectra of the
separated constituents. The polymer accounts for 70% by weight of
the mixture, while the alcohol is 15% by weight.
The polymer has not been found to pass through any gas chroma-
tograph, and would not volatilize below 3^0°C. Therefore, with
Dr. Ronald Bradow's permission, we abandoned any further work with
Lubrizol 580 and concentrated our effort on the other additive.
N,N -Disalicylidene-l,2-propanediamine, also known as salicyl
propanediamine, or a,a'-(propylenedinitrilo)-di-o-cresol was
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originally represented to us as a sample of a Universal Oil Products
fuel additive, UOP-55, but a request for more of the additive is found
in a number of applications, for instance DuPont Metal Deactivator. It
seems the compound is a well-known metal chelating agent which is supplied
dissolved in an appropriate organic solvent. We have recrystallized
it from cold heptane and used the pure crystals to make up known
solutions in benzene. Our purified sample had a melting point of
38-40°C, and the nmr spectrum agreed generally with the expected
structure, but there was an absorbance at VL3 ppm, presumably due
to the double bond hydrogen, that looks more like it is bonded
directly to the nitrogen. A high resolution mass spectrum of the
compound had a large parent peak at about mass 280 (expected 284)
and the mass fractions were consistent with the expected structure.
The problem with this additive stems from its chelating ability
and subsequent strong affinity for metal surfaces. On glass or cera-
mic material the compound vaporized at an appreciable rate at approxi-
mately 85°C. On stainless steel, temperatures on the order of 220-
250°C were required before any appreciable amount disappeared. Since
the burner, the inlet system of the G.C., the G.C. columns, and
part of the gas sampling valve are all metal, considerable difficulty
has been encountered in finding a gas chromatographic system that
will pass the additive.
On the basis of the gas chromatography literature four matched
pairs of six foot stainless steel G.C. columns were made for separa-
ting amines: 5% polyethylene imine on Chromsorb G, 5% Versamid 900
on Chromsorb G, 5* Triton X-100 on Chromsorb G, and Chromsorb 103.
Using several primary and secondary amines from Polysclence Corpora-
tion Qual-Kits (98% H- purity) it was found that all of these columns
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separate amines with negligible tailing. The Chromsorb 103 column
was good with low molecular weight amines, while the others seem
to be better with higher molecular weights.
The only column that would pass pure disalicylidene propanedi-
amine was the polyethylene imine column at 240°C with the injector
at 300°C. Under these conditions, however, there is considerable
background bleed of the liquid phase imine that is picked up by the
N specific detector, and this column is not suitable to detect the
additive or its combustion products at ppm levels. In fact no
column with a liquid phase molecule which contains one or more N
atoms can be used unless column temperatures are kept well below
maximum levels. Thus the Versamid 900 column was also unsatisfactory.
The Chromsorb 103 polymer contains no nitrogen, but it only
passed low molecular weight amines easily. Furthermore 50 ppm NO
in nitrogen when injected as a Ice gas sample disappeared on
this column. Therefore it did not seem to be promising as an analy-
tical system dealing with nitrosoamines and large molecular weight
fragments from the fuel additive.
This left only the Triton X-100 column,which did separate low
molecular weight amines an<3 nitrosoamines satisfactorily, and by itself
caused quite low background levels on the detector. The problem here is
that the temperature limit for the column is 200°C, and the additive wil
not go through a stainless steel column at that temperature. We
tried to saturate the metal surfaces by continuously injecting a 1%
solution of the additive in benzene into the G.C. at about 3 p1/min
for l|-5 hours and under these conditions the additive was detected,
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but once continuous injection was stopped 1 y.l injections of the
1% solution were not getting through to the detector.
Two columns which are both 5% Triton X-100 on Chromsorb G
have been made. In addition one column also has 5% KOH on it and
the other 5% "Desicote" (a Beckman Instruments, Inc. silicone solu-
tion), and the stainless steel tubing for the column was thoroughly
washed on the inside with a 1% benzene solution of the disalicyli-
dene propanediamine additive. Unfortunately, instrument problems
have prevented testing the ability of these new columns to pass the
additive by the time this report was written.
The inability of finding a column that will pass the parent
additive to the detector has been frustrating, and makes development
of injection and sampling techniques with the flat flame burner very
difficult. What might work well for lower molecular weight amines
and nitrosoamines might be unsatisfactory for the high molecular
weight additives and their combustion products.
Chemlluminescent NO Detector
The construction and optimization of the chemiluminescent NO
detector with a limit of NO detectibility of at least 10 ppb has
been covered in a preliminary report to EPA last fall. The experi-
ments that examined the ideal operating parameter and reactor design
led to a paper, which was submitted to Analytical Chemistry, and has
now been accepted for publication. A preprint of that paper has
been included with this report, and one can refer to it for complete
descriptions of the detector, reactor, and operating parameters
utilized in these experiments. (Appendix I)
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A__Ni^rogen Speciflc__G._C_._Detector
Perhaps the most Important contribution of this research has
been the successful development of the NO detector as an N specific
detector for gas chromatography. The experimental design is as
follows.
For these experiments,the G.C. was a Perkin-Elmer model 800 with
dual flame ionization detectors with output to both a 10 milivolt
strip chart recorder and an Infotronics Corp. CRS-10H digital
integrator. Samples could be introduced with either a Carle gas
sampling valve or Hamilton liquid or gas syringes. The normal entrance
to the flame ionization detector was blocked. As shown in Figure 1, all
of the effluent from the column, in 1/8 inch stainless steel tubing,
passed into a 1/8 inch Swagelok tee where it met a stream of pure oxy-
gen and passed immediately into the conversion furnace.
The conversion furnace was 6-7 inches of 1/8 inch stainless
steel tubing which was wrapped with about six feet of 32 gauge
nichrome wire (10.58 ohms/ft resistance) over a thin layer of asbes-
tos tape. Another layer of asbestos was wrapped over the wire and
a thin layer of Sauereisen was applied over the whole furnace. A
chromel-alumel thermocouple had been placed near the middle of the
furnace between the tubing and the first asbestos layer. The nichrome
wire was connected to a Variac, and the furnace produced a linear
range of temperatures from 450 to 950°C for Varia^ settings of 58 to
108 volts. About two feet of platinized (from chloroplatonic acid)
platinum wire, was folded up and inserted into the stainless steel tubing
to serve as a catalyst for the conversion of all the nitrogen atoms in
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recorder
gas sampling
valve
furnace
H
e
1
i
u
m
—O-
O
0
column oven
—\ column
• column 1
— 1
1
^ 1
/
Ju
v
1
plcoammeter
Bourdon
guage
«J- needle valve
'reactor
WJMV
i^_r«
0
X
y
g
e
n
z
O
n
i
z
e
r
integrator
P,M. tube
power supply
Figure 1. Block diagram of the gas chromatograph, conversion furnace,
and chemiluminescent NO detector.
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the effluent molecules to NO, the thermodynamically stable oxide
of nitrogen at 800-900°C.
The actual location of the furnace in the column oven is shown
in Figure 1. The products from the furnace are carried in the 1/8
inch stainless steel tubing through a Bourdon tube vacuum-pressure
gauge (Marsh Instrument Company), a needle valve, and then connected
to the black Tygon inlet tubing of the reactor of the NO detector.
In addition to being displayed on a 10 millvolt recorder, the output
signal from the NO detector (now the G.C. detector) was also put
into the digital integrator.
The system worked beautifully as a G. C. detector. For a
variety of primary amines, secondary amines, and several nitrosoamines,
the detector produced nice Gaussian peaks over a wide range of
sample concentrations. For example, Figure 2 shows a sample chrom-
atogram for a mixture of three primary amines, and Figure 3 shows
two chromatograms of the same mixture of diethylamine and N-nitro-
sodlethylamine where the amine concentrations differs by at least 10^
in the sample injected into the G. C. Clearly the detector ignores
the benzene solvent, but responds as one would expect of a G. C.
detector for the amine compounds.
As soon as it was clear that the NO detector and conversion
furnace would make a satisfactory G. C. detector a number of ex-
periments were carried out to determine the critical operating
parameters for the detector and hence the optimum operating con-
ditions. From the previous experiments dealing with the optimi-
zation of the NO detector, it seemed that the crucial parameters
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Figure 2. A chromatogram of a l-(il sample of a mixture of n-propyl amine
n-butyl amine (21.V/0), and n-hexyl amine (63.9%) diluted by 1% by volume in ben
zene. The sample was run on a Ghromsorb 103 column with column oven at 220°C,
ozone flow to the detector of 30 cc/min, furnace at 795°C, and oxygen flow to
the furnace of 2 cc/min. 70% of full scale is shown.
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Both A -and B are for a 5#
on Chromsorb 0 column.
Ozone flow - 30 cc/mln.
02 to furnace - 2 cc/mln.
Furnace temp. - 800 "C.
Column oven - 140 °C
Triton X.-1QO
Figure 3
A chromatogram of 1 jal of a mixture of diethyl amine (8.9$), N-nitroso-
diethyl amine (9.2%), and benzene (8l.9%) by volume. 6k% of full scale is
shown. B. A chromatogram for 1 cc of a gas sample of the mixture in JA
after 250 min in the dilution bulb. The mixture in JA was injected into the
bulb to initially form 100 ppm of each component in the bulb. 90% of full
scale is shown and both chromatograms have the same zero on the recorder.
]
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would be 1) rate of ozone flow into the reactor of the NO detector,
2) rate of 02 flow into the furnace and hence into the reactor,
3) furnace temperature, and 4) rate of sample flow into the reactor
as controlled by the needle valve and measured by the Bourdon
gauge.
Experiments showed that the first three were important para-
meters, but the 4th was unimportant, as long as the pressure in
the furnace was atmospheric or less. If the needle valve was
turned down until the pressure built up outside the reactor inlet,
the signal dropped. The amount of vacuum was not critical, however,
so the needle valve was left wide open for all of the experiments
reported here and the vacuum varied from 17-22 inches of Hg.
The effects of the other three parameters has been indicated
by presenting Just a few of the data in Table I. All of these
results are for injection of 1 yl of \% diethylamine in benzene
on a 5$ polyetheylene imine on Chromsorb 0. column. By comparing
runs, 1,2,3, and 4, one can see that the 02 flow into the.furnace
could have a large effect on signal. A minimum amount was required
to efficiently oxidize the amine, but further flow raised the
pressure in the reactor without increasing the NO flow. Since the
NO detectbr signal varies directly with NO flow/pressure (2), the
signal drops with increased 02 flow. By comparing runs 4,5, and 6,
one can see that the signal was completely insensitive to furnace
temperature between 600 and 930°C. However, this might not hold
true for larger or more thermally stable molecules, so the furnace
o
has been kept at about 800 C or higher for all of the experiments
reported on here. Finally, by comparing runs 7*8, and 9* one can
11
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TABLE I. The Effect of Operating Parametric on the Detector
Response to One Ul Sample of One Percent (by Volume)
Ditheylamine in Benzene uj-ume;
lc
2
3
4
5
6
7
8
9
807
807
807
807
931
600
745
745
807
^2*- J-»-'w j_ a i_
cc/min
2.0
•x*0
7.0
5.0
5.0
5.0
5.0
5.0
2.0
« u now rat
cc/min
35
35
35
35
35
35
85
8
35
:e" Peak Area"
UX 10 6 <5P»3 1 0^
__ •"• J-|J &t;cij.e i
20,112
6,087
13,650
17,361
17,504
17,180
15,258
16,280
18,965
a. Rate of flow of 03 + 02 from the ozonizer into the reactor.
" 5* POly.thyl.ne
Data taken five days apart without calibration.
12
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see that the signal was somewhat sensitive to ozone flow, more
precisely oxygen flow rate, through the ozonizer into the reactor.
The detector was less sensitive to this parameter and a change of
a factor of two only produced only a change of + 25$ in detector
signal.
At the end of these experiments, it was determined that the
optimum operating conditions for the detector for diethylamine
were those shown in runs 1) and 9) in Table I, and they have been
used for all of the other data reported here.
At this point there was some concern as to the stability and
reproducibility of the detector. Experiments showed that on any
given day, samples could be reproducibly detected to better than
+ 2# and the limiting factor seemed tobe reproducibility of sample
injection with the 10 /il syringe. On a day-to-day basis, however,
the stability and reproducibility of the detector is still open
to question. The initial results, shown in Table I, show about a
5% change over a five day period for diethamine, and this did not
seem important at the time. However, the results shown in Table II
over a longer time period show that the detectrr sensitivity has
changed characteristically for both diethyl amine and N-nitrosodi-
eithylamine over an interval of several months.
This change in detector signal was discovered in conjunction
with experiments with smaller sample when it was noted thafc the
response ratio of N-nitrosodiethylamlne to diethylamine had changed.
Originally we had been quite please that the nitrosoamine response
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Table II. Detector Response for 1 Ul sample of 1% Diethylamine and 1%
N-Nitrosodielhylamine in Benzene. Peak Areas (1 x 1(T6 scale)
Column Diethylamine N-Nitrosodiethvlamine
Dec, 1973 Polyethylene imine 18,965 36,346
March, 1974 Triton X-100 23,907 85,439
May, 1974 Polyethylene imine 26,047 118,302
May, 1974 Triton X-100 85a a
a. Integrator range used for earlier data was not working, so a
different range was used. The relationship between the two is
unknown.
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(2 N atom per molecule) had been about twice the amine response
(1 N atom per molecule), and were dismayed to discover that this
ratio had changed. We satified ourselves that the absolute sen-
sitivity had increased, and since this change did not effect more
important experiments, we have not had time to experiment to find
out why the sensitivity changed.
It could be the conversion furnace, the interaction and loss
of the sample on the column walls of the G. C., or the integrator
is the source of the change. Recently, either because of experiments
where large amounts of UOP-55 were continuously injected into the
G. C. or because of age, the conversion furnace has stopped function-
ing. Therefore, we pian to build a new one and to systematically
measure sensitivity or a function of time, column material, and
intergratior, which has also developed problems. In particular
we want to establish absolute conversion percentages by calibrating
the detector with a calibration gas of NO in N2.
Once it had been established that the NO detector was indeed
a N specific G. C. detector, the first task was to establish
its limit of detectibility for diethylamine and N-nitrosodiethyl-
amine. This turned out to be a harder task than anticipated.
The first approach was to pump out used 12 liter freon tanks
and fill them with enough sample and nitrogen to have a tank of
100 ppm each of diethylamine and N-nitrosdiethylamine at three
atmospheres pressure. The intent was to pass this calibration
mixture through a flow meter and mix it with varying known flows
nitrogen to produce known concentration of the two samples in the
final mixture, which would then be injected into the G. C. However,
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both compounds showed great affinity for the walls of the can,
and even if they were used right after filling, only trace
amounts of the sample could be found.
The second approach was to construct a 5.^5 liter exponential
dilution bulb with a magnetically driven vane stirrer inside the
bulb for rapid mixing. By establishing a constant and measured flow
of nitrogen through the bulb and then injecting enough diethylamine
and N-nitrodiethylamine into the bulb to produce 100 ppm concen-
tration at time zero, one could then take samples from the bulb as
a function of time and get a decreasing concentration of the two
compounds. A plot of log (response) versus time could be extrapolated d
to time zero whefce concentration was known and used to establish
the concentration of the limiting response of the detector after
a period of constant dilution.
This approach worked better,but not as well as we had hoped.
The problem is the loss of the amine compounds on the walls of
the vessel and gas handling system. It turns out that amines are very /
difficult to work with at the nanagram or ppm level. Indded we
had noticed this problem in trying to calibrate the detector with
dilute benzene solution of diethylamine and observed loss of sample
on the walls of the flask and subsequent irreproducibillty for o.l*
(1 mic.ogram in one microliterof solution and o.OlJg (100 nanograms
in one microliter of solution) solution. With gas phase samples
in the dilution bulb, initial results indicated that most of the
sample disappeared onto the walls of the bulb and even mildly
heating the bulb would not bring them off. Attempts to dry the walls
to coat the walls with a silicone solution (Deslcote-), and to pass
NH3 through the bulb to deactivate the walls did not stop the loss
of sample in the bulb.
16
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What eventually produced the best results was to leave
samples of th. amines In the vessel to saturate the walls and
then pass N2 through the bulb .until the background was negligible
and then do the dilution experiment. The best results obtained
are shown in Figure M, and they are not completely satisfactory.
The initial concentration of the two compounds should have been
about 100 ppm, but the G. C. response was similar to the response
obtained for 5^.8 ppm NO in N2. The dilution rate of the bulb
was fixed so the half-life of sample in the bulb was 55 minutes.
On this basis the results for N-nitrosodiethyl amine appears in
good agreement, but that for diethylamine is clearly out of line.
We hope to obtain better results in subsequent experiments..
We can use the results in Figure 4 to establish an upper limit
for the detectibility of N-nitrosodiethylamine with our detector.
Figure 2 B shows the chromotogram for the last point ploted in
Figure 4 at 250 minutes. Where the initial detector response for
the 100 ppm (or less) sample has been reduced by a factor of 20
(1.3 log units). This would indicate that the response in Figure
2B is for a sample that is now 5 ppm (or less) in the nltrosamine,
and the peak in Figure 2fi could certainly be reduced by a factor of
10 more and be readily detected. Thus a sample oT the nitro-
soamine of 1 to 0.5 ppm in a 1 cc gas sample (l to 0.5 nanograms
of sample) or less should be detectible with this detector.
Clearly the current calibration system is not satisfactory,
and recent conversations with Dr. D. F. S. Natusch of the University
of Illinois has put us onto a better approach for these calibrations.
.17
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100 120 140
TIME, Nin.
160 180 200 220 240
Figure h. Plot of log (peak area) vs. time for an initial concentration of 100 ppm diethyl amine
and 100 ppm N-nitroso-diethyl amine in the 5-45-liter exponential dilution bulb.
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He had encountered similar problems in working with sulfur com-
pounds, and has suggested an approach using a diffusion cell
for continously adding small amounts of the test gases to a
flow system from: a detachable sample tube that can be weigh-
ed. This system, much like a permeation tube can be run con-
tinously until the walls are saturated, and it is similar to
several described in Nelson (3) aad references therein, and
need not be discussed further here. We are currently building
a similar piece of apparatus for repeating these sensitivity
studies for several compounds with our detector and with the
flare lonization detector for comparison.
We are in position, however, to make some Judgment as
to the quality of this detector as a N specific NO detector.
For high concentration sample of o.ljg o* better, the de-
tector clearly does a beautiful Job. In Table III, the results
of analysis of two amine mixtures supplied with the Polysclence
Qual-kits of amines is shown, and agreement, without any
calibration of the detector is quite satisfying.
The detector also was used to make a rough measurement
of the total N in gasoline. 1 ml samples of gasoline were
injected into a Triton x-100 column, and the results are shown
in Table IV. Since gasoline fuel nitrogen has been' determined
19
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Table III. Detector Response for two
A. Secondary Amines
Compound % Given % Found
Diethylamine 15.2 15.5
Di-n-propylamine 20.6 21.5
Di-n-butylamine 26.1 25.7
Cyclohexylamine 38.2 37.3
B. Primary Aminesa
n-Propylamine 21.6*
n-Butylamine 24.8£
f
n-Hexylamine 53.6°
n-Propylamine 2l.6£
n-Butylamine 24.8
f
n-Hexylamine 53.6
,a
16.6
23.2
60.2
14.7
21.4
63.9
Amine Mixtures.
Column
Polyethyleneimine
Polyethyleneimine
Chromsorb 103
The bottle containing the primary amine had
tinued to leak. These two analyses were run
and the sample continued to lose the lighter
others.
This data is shown in Figure 1.
leaked, and con-
several weeks apart
amine relative the
20
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TABLE IV Detector Response for 1 mi aamples of gagoline
^~ Peak Area (10 x 10^ Sca1fl| Column
gasollnea 31,775* Triton X-100
5^.8 ppm NO in N2 ' 39,79^ Triton X-100
5^.8 ppm NO in N2 31,163 Polyethyleneimine
a Can that had
stored
Average of three determinations
to be from 5-30 ppm by mlcro-KJeldahl techniques, the agree-
ment with the 54.8 calibration gas sample is encouraging, m
fact, with proper calibration or addition of an internal
standard, this detector shows real promise as a means of rapid
and accurate analysis of total fuel nitrogen or other nitrogen
analysis at the nanogram level.
Finally we can make some comparison of over N specific
detector with other N specific 0. C. detectors. A recent re-
view of element selective detectors for gas chromatography (4)
puts the sensitivity of the Ooulson conductivity detector (such
as the tracer electrolytic conductivity detector) at a limit
of 100 picograms of N which would require at least 0.5 to 1 0
nanograms of parent sample. This Is the upper level of sen-
sitivity we have estimated for our detector for N-nitrosodlethyl-
amine. However, we have almost two orders of ragnltude Qf
Bitivlty left on our NO detector, which ls currently
21
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background from the column and conversion furnace. As yet we
have not done anything to reduce this background, but our de-
tector does have the potential to be two orders of magnitude
better than any other currently available. One of our future
projects is to increase this sensitivity in conjunction with
better calibration techniques discussed above.
Flat Flame Burner Studies
In spite of the difficulties in clearly establishing the
sensitivity limits of our detector, we felt that we had estab-
lished the fact that we could see at least 1 ppm quantities
of nitrogen containing molecules, and thus we moved on to pre-
liminary experiments with the flat flame burner.
The burner was borrowed from Dr. W. Mirsky of the Automotive
Engineering Department of the University of Michigan, where
it had been constructed and used by Dr. A. Gad El-Mawler in
his Ph.D. research (5). The burner itself was constructed of
a 2 in. diameter, 6 in. long copper tube having at the upper
and lower ends 1/4 in. thick porous discs of sintered brass,
90$ copper and 10$ tin. The space between these discs was
filled with 3 mm diameter glass beads to provide mixing of
the propane fuel and air, and to produce an even flow to the
top of the burner. The burner was housed in a thick-walled,
cylindrical iron-chamber provided with four 3-in diameter view-
ing and access parts. The burner could be raised or lowered
22
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vertically without rotation through a distance of about 1/2 in
by a screw mechanism at the bottom of the chamber. The propane
from a liquid propane gas cylinder and air from a cylinder of
dry compressed air were put into the burner by a tee at the
bottom of the burner. The burner was lit by sending a spark
through a copper wire to the burner with a tesla coil.
A reproducible flame was maintained by adjusting the re-
gulator made valve and second stage pressure until a slightly
fuel lean flame of adequate size was established and then fix-
ing the valves and the regulator pressure screws at these
positions. The flame was then turned on and off by opening
and closing the valves between the tank and the first stage of
the regulator. The flow of air was measured to be about 15
liters/min with a wet test meter and the flow of propane was
measured to be about 0.6 liter/min.
The sample of additive was added to the flame by prepar-
ing an appropriate solution of the additive in benzene (in
the neighborhood of log to minimize the Injection of benzene)
and injecting it through an 8 mm quartz tube, drawn to a fine
tip, at a constant rate with a continuous infusion -- with-
drawal syringe pump (Harvard Apparatus Model 901). The rate
of infusion and additive concentration were chosen to provide
a certain ppm concentration in the 15.6 1/min of air/propane
passing through the burner.
Currently the additive is injected directly into the
23
-------
burner flame. There 18 anopenlng lnto fche copper tufee Qf ^
burner body where samples could be Injected directly Into the
-burned gases before they reach the top plate. The tempera-
ture here was measured tote only 65«0, and the UOP-55 would
not be readily vaporized at this te^erature. For future work
we plan to put heating tape around the burner so all additions
to the flame can be made Into the unburned gas.
Samples are withdrawn from the flame by another 8 „ quartz
probe drawn down to a tip which is mounted vertically over the
flame and can be moved to several positions in the flame A
30 cc syringe is attached to the probe at a tee with a rubber
septum at the third position. Sampies are drawn into the syringe
and a sample is obtained with a 2 cc gas syringe through the
septum for direct injection into the gas chromatograph.
At this point in time, all our results with the burner are
negative as far as finding- any combustion products for the UOP-55
additive, we have shown that with 1000 and 100 ppm concentration
of diethylamine, we can see the amine in the unburnt gases, but after
the flame is lit, all peaks disappear ^ ^ ^ ^ ^^ ^
off by turning off the propane does not cause the diethylamine to re-
appear as long as the burner and probe remain hot. with similar sam-
ples of the disalycylidene propane diamine, nothing was detected
either before or after the flame was turned on. since we do not have
a 6. C. column that will pass this compound at low concentrations,
this result was not surprising.
-------
These burner experiments are far from completed. We
have Just begun to think about trying different probe
positions or sampling techniques and different flame temper-
tures (the burner has cooling coils Just under the top plate).
Earlier work with the burner clearly found unburned hydro-
carbons in the propane flame (5), and we have yet to find the
right conditions to find amine fragments from simple amines.
Once these are established, we can then have more meaningful
attempts to find combustion fragments for the fuel additive.
Conclusions and Future Plans
Because this research is not yet completed this report
included some discussion of incomplete experiments and negative
results. However, we are planning to continue this research
for more than another year, in which time we will try some
of the new, and hopefully better, techniques discussed above.
We have developed a new and promising N specific detector for
gas chromatography that is at least as sensitive to N-nitroso-
diethylamine as any similar N specific detector. We hope to
improve and establish the sensitivity and reliability of this
detector for a variety of amine compounds including the fuel
additive. We have begun a series of flat flame burner studies
that need further experimental adjustments before they can be
adequately used to look for fuel additive combustion products.
At this point we remain optimistic that we can deal with these
problems before fall and may be able to move on to the engine studies
portion of this project.
-------
Some particular steps to be taken are:
a) Use of glass or teflon columns for fuel additive.
b) Upgrading and writing a complete report on the N-selectlve
GC method.
c) Provision to E.P.A. of a converter furnace and specifica-
tions for use with their chemilumlnescent detector.
d) Future flame studies as described earlier, leading to
engine studies if successful.
26
-------
References
1. Dr. C. Miller, Lubrizol Corporation, Cleveland, Ohio
personal communication (1973).
2. D. M. Steffenson and D. H. Stedman, Optimization of the
Operating Parameters of Chemilumlnescent NO Detectors,
Analytical Chemistry, accepted for publication, May 15,
197^.
3. G. 0. Nelson, "Controlled Test Atmosphers", Ann Arbor
Science Publishers, Inc., Ann Arbor, Michigan, 1971,
p. 130.
4. D. F. S. Natusch and F. M. Thorpe, Anal. Chem. 4£, 1973,
5. A. Gad El-Mawla and W. Mlrsky, College of Engineering,
Department of Mechanical Engineering, Progress Report
No. 4, 1965.
27
-------
OPTIMIZATION OF THE OPERATING PARAMETERS
OF CHEMILUMINESCENT NO DETECTORS
Analytical Chemistry In Press
By
D.M. Steffenson and D.H. Stedman
Department of Chemistry
University of Michigan
Ann Arbor, Michigan 48104
Abstract
s
The parameters that effect the sensitivity of a typical
chemiluminescent NO detector have been investigated. Using a 54.8
ppm in NO sample and an N0:0-, flow ratio of 2:1, the detector signal
was measured as a function of reactor pressure at several different
pumping speeds in different reactors. The results are consistent
with an analysis of the kinetic limitations on the chemiluminescent
intensity, which shows that
Detector Signal=(Reactor Gas Flow/Reactor Pressure)(G)(l-exp(-T ,/
J t-9 C. L*
TNQ)) where G is the geometry of photon collection of the reactor
and exP(-Treact/TMo) is tne fraction of NO molecules whose residence
lifetime in the reactor (T ,) is short compared their reactive
lifetime with 0.~ (TNn)' From this equation, and results using
different reactor designs and different photomultipliers, it is
shown that the relevant parameters for optimizing the detector are
pumping speed, reactor size, 0., flux, reactor design, and choice
of photomultiplier. The choice of these parameters Is discussed and
they are somewhat different for an atmospheric NO monitor from those
for a laboratory detector for limited samples.
Present address, Department of Chemistry, Albion College, Albion,
Mich. 49224
28
-------
The use of chemiluminescent nitric oxide detectors has grown
rapidly in the past few years, particularly as air pollution mon-
itoring instruments for atmospheric concentrations of NO . A
yx
recent review of chemiluminescent detectors used in the measurement
of air pollutants (l) lists nine different manufacturers of com-
mercial N0x detectors even though the feasibility of such detectors
was only established by Fontijn et al (2) in 1970,and the prototype
s
for a. number of these commercial instruments was developed in 1972
(3). Besides their application as monitors for atmospheric NO, these
detectors have been used as laboratory analytical instruments for
measuring NO as a product or reactant in standard kinetic or photo-
chemical experiments (4).
In spite of the growth and rapid development of commercial
instruments, there has been little systematic study of the physical
and kinetic parameters that determine the optimum sensitivity of the
NO detector. The basic design and the components of the detec-
tor have been established, but there remains a wide latitude of choice
of the operating capabilities of each component.. These choices must
also be compatible with the basic kinetic limitations of the chemi-
luminescent NO-0-, reaction. Furthermore, . operating conditions for
use in atmospheric gas monitoring where the supply of sample is
virtually unlimited are not "necessarily ideal for use in the labor-
atory to detect NO in a limited amount of.sample.
In this work we have systematically studied the operating para-
meters and reactor design that effect the intensity of the chemilum-
inescence that reaches the photomultiplier tube of the detector.
29
-------
This intensity was found to be a function of pumping speed and the
flow parameters of NO and 0^ into the reactor In a way that 1s
quite consistent with the proposed kinetics and mechanism of the
•reaction. The intensity is also a function of physical parameters,
particularly reactor design, that facilitate getting the light to
the photocathode of the photomultiplier. Finally we made a few
experiments with a different photomultiplier to demonstrate how
important the choice of photomultiplier is to the ultimate sensi-
tivity of the detector.
Experimental
The schematic design of the NO/CU chemiluminescent detector
used in these experiments is identical to those published earlier
(1, 3)> and is similar to those of commercial instruments. The
photomultiplier used i.n most of the experiments was an RCA 8852
with an ERMA III response that extended its range to 900 nm. It
was dry ice cooled and nitrogen gas from a tank was slowly passed
through a small space between the reactor window and the photo-
multiplier window to prevent frost formation. A Kodak Wratten
gelatin filter was used to remove any emission below 600 nm. A
few experiments were made using an Amperex 150 CVP photomultiplier
with an S-l response and nitrogen gas vaporized from a Dewar of
liquid nitrogen was used for cooling. The power supply for the
photomultiplier was a Heath EU-42A variable high voltage power
supply with a maximum output of 1500 volts. The photocurrent was
measured with a Keithly 4l4s picoammeter whose output was displayed
on a Honeywell 10 mv strip-chart recorder.
A Cenco Hyvac 14 pump was used to maintain the flow of NO
and 0~ through the reactor, and a bellows valve was installed in
30
-------
front of the pump in order to vary the pumping speed. The flows
of NO and 0- into the reactor were controlled with teflon needle
valves and the flow rates were measured with GiImont No. 11 or
No. 12 flow meters. The flow meters were calibrated with a wet
test meter or by displacing water from a volumetric flask.
For these experiments, the instrument was used as an NO
detector with a tank of Linde 54.8 ppm N0»in N^ as the NO source
for all of the experiments. Oxygen was passed through an
ozonizer operated at 9000 volts. The pressure in the reactor was
measured with either a mercury or oil manometer using a catheto-
meter to read the manometer levels to .05 mm.
Four different reactors were used in these experiments in
order to test design features that might increase the sensitivity
of the detector. The design and dimensions of the two reactors
used for most of the experiments are shown in Figure 1. The
smallest reactor, referred to hereafter as-the "brass reactor",
effective volume about 36 cc, was designed to facilitate rapid
and effective mixing of NO and 0- with the gases emerging from
small jets interleaved with one another and surrounded by a glass
cylinder to guide the reactants toward the front window before
being pumped down the sides and out the back. The other reactor,
referred to hereafter as the "glass reactor", volume about 300 cc,
was designed to mix the reactant gases as close to the window in
front of the photomultiplier tube as possible. The other two
reactors were also made of Pyrex glass, and were similar to the
glass reactor in Figure 1. One was identical except that' the
outlet to the pump was two 10 mm tubes exiting at the front of
31
-------
k-
16 cm-
5 cm
o.d.
1
'DHOLES
TO PUMP
I
WINDOW
GLASS REACTOR
6cm
o.d.
— 6cnv
•5cm-
WINDOW vJ.
H.8 cm
8.2 cm
SIDE VIEW
r>»- 8mm
MIXING
TUBES
cm
FRONT VIEW
BRASS REACTOR
Figure 1. Cross-sectional designs of the two main reactors used in the NO
detector. Both the window and the internal glass cylinder of the brass re-
actor had a diameter of 3.8 cm, which limited the effective reaction volume
open to the photomultiplier.
32
-------
the reactor near the window. The other was larger, about 20
cm long and about 400 cc volume, but the gas inlets had only
one hole in the end and they pointed straight forward with a.
'separation approximately four times that of the glass reactor.
This reactor provided the poorest mixing of NO and 0-,.
With both the glass reactor and the brass reactor, at the
highest pumping speed, the maximum photocurrent was obtained with
an N0:0~ flow ration of 2:1 (where 0. refers to the Op/Oo mixture
from the ozonizer). This flow ratio was used for all of the experi-
ments. The pumping speed was varied by a factor of ten, so that
the total gas flow in the reactor as measured by the calibrated flowme-
ters and corrected for the reactor pressure, varied from around
170,000 ml/min to 17,000 ml/min. Thus the residence time of the
gases in the reactor ranged from about 0.11 sec to 1.1 sec for the
glass reactor and from about 0.012 sec to 0.12 sec for the brass
reactor. In almost all of these experiments the high voltage power
supply for the photomultiplier was set at 1140 volts. A few measure-
ments were made at 1500 volts. Usually the photomultiplier was
cooled with dry ice, but cooling was not critical since the cooled
and uncooled signals were identical and only the dark current was
reduced.
Results
The experiments were designed to measure the intensity, I,
of the chemiluminescence for the 54.3 ppm sample as a function of
reactor pressure. This pressure was varied by changing the total
flow of reactants through the reactor. The results are shown in
Figures 2 and 3 for the glass and brass reactors, each at three
different pumping speeds. The pumping speeds were determined from
33
-------
6 -
OX
o
x
o
Z
LU
o:
a:
o
o
X
Q.
0
40 60
REACTOR PRESSURE, -forr
80
100
Figure 2. NO detector signal as a function of reactor pressure at three different pumping speeds for
the glass reactor. A = data at 136 1/min, O = data at 63 1/min, and Q = data at 30.5 1/min.
-------
vx
vn
0
10 20 30
REACTOR PRESSURE, +orr
Figure ~5. NO detector signal as a function of reactor pressure at three different pumping speeds
for the brass reactor. A = data at 136 1/min, O = data at 40.5 1/min, and Q = data at 16 1/min.
The last Q at the l6 1/min pumping speed represents experimental points at 71 and 108 torr with
that ordinate value.
-------
the total flow and the pressure in the reactor, and they are plotted
in Figure 4 as a function of pressure.
The effect of reactor designs on the sensitivity of the
•detector is shown in Table I along with data for the dark current
and noise of the photomultiplier. Table I also shows the change
in signal and dark current achieved by an S-l response photomulti-
plier.
Discussion
The main parameters that can effect the signal intensity of
the detector can be understood through the kinetics and mechanism
of the NO-0, reaction, and the relationship of the residence life-
time of the reactants in the reactor to their kinetic lifetime. The
mechanism of the reaction between NO and 0^ has been previously
established (5, 6)
NO + 03 > N02* + 02 (1)
NO + 03 ^ N02 + 02 (2)
N02* - ^ N02 + hv (3)
N02*+ M - > N02 + M (4)
*
where N02 is an excited electronic state which emits radiation
between 600 and 3000 nm with a maximum at 1200 nm. The rate con-
stants for (1) and (2) were measured to be k-L= 1.1- 0.6 X 1C""1-*
cnr5 molecules' sec" and k2 = 1.4- 0.6 X 10~ cnr molecule"1sec~1
If the only important reactions were (1) and (3), then all
of the NO molecules entering the reactor would emit photons, thus
= fNO (5)
where I is the intensi.ty or' the signal in photons sec^and f is
the mass flow of NO into the reactor in molecules sec."1 In fact,
36
-------
Table I. The Effect of Reactor Design and Photomultiplier Voltage
on the Signal, Dark Current, and Noise of a Chemiluminescent
NO Detector.
RCA 3852 ERMA III Response
1140 volts
Dark Current &
Reactor Signal, amps Noise, amps
1. brass (36cc) 6.0 X 10~9 2- X 10"12
(see Fig. 1)
2. glass (300cc) 18.0 X 10'9
(see Fig. 1)
-9
3. glass (300cc) 3.2 X 10
(same as 2 with
front pumping)
4. glass (40Qcc) 6.5 X 10~9
(same as 2 with
poor gas mixing)
5. glass (300cc) 60.0 X 10'9
(same as 2 with
reflective coating)
6. brass 70 X 10"9
(same as above)
n .m.
n .m.
x 10
" 13
2-1 X
1500 volts
Dark Current
Signal, amps Noise, amps
6.2 X 10'8 4-4 X 10"11
3.2 X 10"7
n.m.
n.m.
Amperex 150CVP, S-l Response
9-0.4 X 10'11*
n .m.
n.m.
n .m.
1.4 X 10'7 3-2 X 10"11
n .m
n.m,
* The dark current is very dependent on the cooling efficiency and this
represents the lowest value found over a four day period. On other
days it was as much as 20 times higher.
- 37
-------
the above kinetics impose three constraints on equation (5).
1) Only a fraction, ^/(^ + kg) = .073, of NO + 03 encounters
leads to excited N02 . 2) Quenching of N0g* allows only a frac-
.tiori, k3/(k3 + k^[M]) = 8.3 X 10"3 at 1 torr, of the possible
photons to be emitted. 3) The rate of reaction of NO + 0^ is
relatively slow which allows a fraction of the entering molecules,
e*P ("^react^NO^ to leave the reactor untouched, or allows
only a fraction, (1-exp (-TNO/Treact^ ' of the enterinS NO mole-
cules to react inside the reactor. Here TNQ, the reactive lifetime
of NO, is the inverse of the pseudo-first order rate constant for
the reaction of NO in the presence of excess 0,
+ k2°3]' S6C
and Treact' the residence lifetime of the reactants in the reactor,
is determined by reactor size and the flow rate in the reactor,
Treact= (A)(dMp)/F> sec (7)
where A is the cross sectional area of the reactor in cm2, d is the
reactor length in cm. P is the reactor pressure in molecules cm'3,
and F is the total flow in molecules sec." The above expression
for the fraction of unreacted NO molecules derives from the pseudo-
first order kinetics of equation (1) and (2). The fraction of un-
reacted NO molecules present at the end of the reaction Is
reac t
where t = Treact. Therefore, with the above three constraints on
equation (5), the light emitted from the reaction zone becomes
38
-------
photons per second.
In practice we cannot collect all of the photons emitted, so
the signal from the photomultiplier is not ltot, but has been atten-
uated by a further factor G which includes the geometry of photon
V
collection and photomultiplier characteristics. Since MM] }} k
and [M] is the pressure in molecules cm'3, equation (9) can be
(l-exp(-T
V PV react
^
NO
This is the signal measured by the picoammeter and has been plotted
as a function of reactor pressure in Figures 2 and 3.
For the moment assume Tregct » TNQ, so that most of the NO
molecules react inside the reactor, and we can neglect the exponential
portion of equation (10). In this case, the signal is directly pro-
portional to fNQ/P or Ftot/P since the NO flow was a fixed fraction
(2/3) of the total flow. One can enhance the signal by increasing
fNO, decreasing P, or increasing fNO/P. For NO detectors with a
constant volume vacuum pump, fNQ and P are not independent of one
another. If one doubles the flow of NO molecules into the reactor,
one simultaneously doubles the pressure. One can only increase the
signal by increasing f^/P. Since fNO/P is in units of cm3 sec'1,
it is a measure of pumping speed and it can be increased by increasing
the speed of the pump.
Figure k shOws the different pumping speeds utilized in these
experiments. Comparison of this data with the signal intensities
39
-------
-p-
o
20
40 60
PRESSURE, -forr
80
100
Figure 1*0. Pumping speed as a function of reactor pressure. Q = data collected
with the glass reactor and A = data collected with the brass reactor. Pumping
speed can be coverted to F/P in units of cm5 sec'1 torr'1 by multiplying the num-
bers along the ordinate by 7.85.
-------
in Figures 2 and 3 gives excellent qualitative agreement as pre-
dicted by the above analysis. Decreasing pumping speed does
decrease the photomultiplier signal.
The fall-off in pumping speed at low pressures, which is
matched by an identical fall-off in signal, deserves some comment.
The speed pressure curve for the Cenco Hyvac 14 (140 1/min maximum
pumping speed) indicates that the pumping, speed should be almost
constant in this pressure region, and such a fall-off should not
occur until 10"-* torr. However, at low pressures, pumping speed
is often limited by the "resistance" of smaller tubing in part
of the pumping system (7), and here the tubing from the reactor to
the pump, including the ozone-killer section, filled with wire, in
front of the pump, causes the early onset of the fall-off in pumping
speed at low pressures.
Quantitatively, on the basis of equation (10), one would
expect a plot of signal vs pumping speed to be linear with a zero
intercept and a slope that varies with G. the geometry of photon
collection for a particular reactor. Such a plot, using the data
from Figures 2 and 4 for the glass reactor, was indeed linear with
a zero intercept. For the brass reactor, using the data in Figures
3 and 4 only the data for the lowest two pumping speeds falls on
a straight line through the origin. As predicted, the slope of this
line for the brass reactor was different from that of the glass reactor
but the signal for the highest pumping speed was 2.5 times lower than
that predicted by this plot. Clearly a large fraction of the poss-
ible signal had been lost.
-------
In order to understand the strong attenuatjon in the signal
in the brass reactor at high pumping speeds, we must examine the
(l-exp(-Tregct/TNO)) term prev'ously neglected In equation (10).
.If the residence lifetime of NO and 0, in the reactor gets short,
compared to the reactive lifetime of NO, as one might expect at
high pumping speeds in a small reactor, then some fraction of the
photons are emitted outside the reactor, and the signal is atten-
\
uated by the fact that only a fraction of NO molecules, (1-exp
^~Treact/'TNO^ WD'-11 actually emit light within sight of the photo-
multiplier. Thus, one cannot increase the sensitivity of an NO
detector without limit by just increasing the pumping speed. One
must consider the limitations of Treact and t in optimizing the
NO detector signal.
From Equation (7) one can see that Treact is inversely pro-
portional to F/P and thus is a constant for a given pumping speed
except at low pressures where it increases as pumping speed falls
off. In the experimental section it was noted that T , was
reac t
0.11 sec for the glass reactor and 0.012 sec for the brass reactor
at the highest pumping speeds in the pressure region where F/P
is constant.
Equation (6) gives the expression for TNQ, which can be put
in a more useful form by expressing [0-,] as
[03] = (P)(#03)fo2/F (11)
where P is the reactor pressure in molecules/cm , % 0^ is the mole
percent of 0~ produced in the oxygen streaming through the ozom'zer,
and fo2/F is the fraction of the total flow in the reactor that
is 0.3+ 02> Substitution of equation (11) into equation (6) gives
-------
Since (k^ + k^) and F/P are constant, the important variable is
the product of the oxygen flow in the reactor and the percentage
of 0_ produced in the oxygen. This however is also constant
'rather than variable.
Using an ozonizer similar to the one we used in our NO
detector, Jack Hor'vath at the Space Physics Research Laboratory
at the University of Michigan measured the mole percent 0, in Cu
as a function of fo2 at several ozonizer voltages. He found that
the mole % of 0_ decreased as the flow of oxygen through the
ozonizer increased, and the ozonizer produced a constant flux of
0, (8). At 9000 volts (the operational voltage of our ozonizer),
the (% Cu)(fo2) product was effectively constant at 1.25 + .0£cnrVmin,
over an 02 flow range of 60 to 2160 cc/min as measured on the external
flow meter. This covers all but the lowest flows of oxygen used in
our experiments.
Thus Tj^Q should be constant for a given pumping speed except
at low pressures where it decreases as the pumping speed falls off.
For the highest pumping speeds in the pressure region where F/P
is constant in either the brass or glass reactor, TNQ is 0.073 sec
using the values of (% 0^)(fo2) from the calibrated ozonizer:
Since both Tregct and TNQ are constant when F/P is constant,
their ratio is also constant:
Treact/TNO =_(Ad)(* <>3)(fo2) P2/F2 (13)
the NO detector signal, equation (10), will be optimized if T ./
iG3 C t
TNO 1s lerge £0 that exp(~Treact/TNo) is negligible. This fraction
i.s negligible in these results except for the brass reactor at the
1*3
-------
highest pumping speed where the signal is severly attenuated
compared to the glass reactor. According to equations(10) and
(13), this signal loss is due to the smaller volume of the brass
reactor and the subsequent escape of unreacted NO from the
reactor.
The easiest way to keep Treact/TNO large in an NO detector
is to make the reactor volume (Ad) large. This will help compen-
sate for the fact that an increased pumping speed, F/P, which has
been shown to directly enhance the signal, can also decrease the
signal by lowering this ratio by a, factor of (P/F)2. One might
further jncrease this ratio by increasing the efficiency of 0
production which would increase the % 0^ produced for a given
02 flow. This would only be important if the detector is operated
in a region where (13) is important and the signal thus gained is
significant.
At this point one might be tempted to optimize an NO detector
by using a very high speed pump and a large volumn reactor, but
there is a. third important factor, G in equation (10), that must
also be considered. This factor represents the geometry of the
photon collection system and the photomultiplier characteristics.
These have been studied by varying the reactor design and by
changing photomultiplier tubes.
The goal of good reactor design is to mix the entering flows
of NO and 03 very quickly and efficiently and then allow them to
react as near the window to the photomultiplier as possible. This
is because chemiluminescence is a diffuse source and cannot be
focused by lenses or mirrors onto the photocathode. Also, as the
-------
reactants flow through the reactor, the intensity of the light
reaching the window falls off with the square of the distance.
This is the reason one cannot continue to compensate for high
pumping speed with large reactors. The ratio of T ct^NO ma^
be kept large but some fraction of the reaction occurs far
enough from the window to be effectively lost to'detection.
The results in Table I with four different reactors support
the above analysis. The best reactor was the glass reactor that
mixed the reactants well in front of the window and allowed them to
flow down the length of the reactor. Pumping them forward and
out the side was 5-6 times less effective, and using a larger
reactor with less efficient mixing gave only a third of the glass
reactor signal. As already indicated, the brass reactor was too
small, but its efficient mixing system and forward pumping might
work well in a larger reactor. The best improvement in signal
was produced by coating the glass rea.ctor with Eastman Kodak re-
flectance paint which increased the signal by a factor of three.
The choice of photomultiplier tube is very important, but we
did not have the resources to undertake any systematic study of
this parameter. Ideally one would like to have a photomultiplier
with a spectral response curve that maximally overlaps the chemj-
luminescence spectrum and one with a low dark current and noise.
On the basis of spectral overlap, most photomultipliers fall short
*
of the NO^ emission which peaks at 1200 nm. The ERMA-III response
of the RCA 8852 photomultiplier extends about as far into the
-------
Infrared as any (900 nm) except the S-l response of the Amperex 150
CVP, which extends to 1080 nm. This is shown in Table I by an
order of magnitude increase in signal using this latter tube with
the brass reactor. However, at its best, the S-l photomultiplier
has a dark current 45 times larger than the ERMA-III.
For maximum detector sensitivity for NO samples at ppb concen-
s
trations, one must be concerned about the background noise of the
photomultiplier, and the important characteristic is the signal/
noise ratio of the tube. The noise was such a strong function
of cooling efficiency for the S-l response photomultiplier that
h -2
its signal/noise ratio varied from 1-7 X 10 to less than 10 .
The ERMA-III response photomultiplier has the best signal/
noise ratio for the reflectance painted glass reactor of 6 X 10 ,
and it seems to be the better of the two for use in an NO detector.
It should be noted in Table I that one can increase the signal by
increasing the operating voltage of the photomultiplier, but the
noise is increased by the same factor and the signal/noise ratio
remains about the same.
We can now summarize the choice of parameters for the optimi-
zation of the signal in an NO detector. They will be somewhat
different for a detector designed to measure atmospheric NO where
there is an infinite sample of gas available than for a detector
designed to measure the -NO concentration in a kinetics experiment
or in a smog chamber where the amount of sample is finite and one
may want to disturb the gases as little as possible.
First one should maximize the pumping speed within the con-
straints of being able to supply _enough reactants to keep the
-------
reactor pressure in the plateau region such as in Figures 2 and 3.
For atmospheric sampling one does not care if the fall-off in
•pumping efficiency of the system comes at high pressures because
one can supply enough sample to keep the pressure above this
point. For laboratory sampling, however, one wants to have a
high speed pumping system that falls off at a very low pressure
so only a minimum of sample is required to keep the reactor
pressure in the plateau region.
Secondly, one should choose a reactor that is large enough
to keep the residence lifetime, Treact> large compared to the
reactive lifetime, TNQ. There is a compromise between maximizing
pumping speed and minimizing Treact/TNO because the signal inten-
sity decreases by a factor of the inverse of the distance squared
of the excited molecules from the photomultiplier, and there may
be an upper limit to the usable pumping speed unless one can lower
TNQ by increasing the ozone flux into the reactor.
Thirdly, reactor design should provide rapid efficient mixing
as close to the window as possible. Furthermore, the walls of the
reactor should be made as reflective as possible by coating with
reflectance paint. It is difficult to put a mirror deposit on the
inside of the reactor because of the large amount of ozone passing
through it.
Finally, choose a photomultiplier with an extended response
into the infrared that has a low dark current and low noise. One
might be tempted to choose a photomultiplier with a larger diameter
photocathode, in order to collect more light, but the noise increases
as the square of the radius and one does not necessarily improve the
signal/noise ratio. Ultimately the quality and characteristics of
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the photomultiplier may be the most important factor in extending
the sensitivity of an NO detector to lower NO concentrations.
Credit
The financial support of Grant 802418 from the Environmental
Protection Agency 1s gratefully acknowledged.
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References
1. R.K. Stevens and J.A. Hodgeson, Anal. Chem., 45, 443A (1973).
2. A. Fontijn, A.J. Sabadell, and R.J. Ronco, Anal. Chem., 42,
575 (1970).
3. D.H. Stedman, E.E. Daby, F. Stuhl, and H. Niki, J. Air Pollut.
Contr. Ass., 22, 260 (1972).
4. D.H. Stedman and H. Niki, Environ, Sci. Technol., 7, 735 (1973).
5. M.A.A. Clyne, B.A. Thrush, and R.P. Wayne, Trans. Faraday Soc.
60, 359 (1964).
6. P.N. Clough and E.A. Thrush, ibid., 63, 915 (1967).
7. H.W. Melville and E.G. Gowenlock, "Experimental Methods in Gas
Reactions," McMillan Sc Co. Ltd., London, 1964, p. 4l.
8. J. Horvath, Space Physics Research Lab., University of Michigan,
Ann Arbor, Michigan, personal communication, (1974).
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO. 2.
EPA-650/2-74-096 |
"TITLE AND SUBTITLE
Kt'lVcl ol' Fuel Additives Study
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
May 1971| _._
6. PERFORMING ORGANIZATION COOL
7. AUTHOR(S)
D.M. Steffenson, D.H. Stedman, and D.J. Patterson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The University of Michigan
College of Literature, Science, and the Arts, DC, and
College of Engineering, DME
Ann Arbor, Michigan
10. PROGRAM ELEMENT NO.
1AA002;ROAP 26AAE-2U
NO.
R-802U19-01
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, National Environmental Research Center
Chemistry and Physics Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Studies were carried out on the optimization of a chemiluminescent NO detector.
This detector has been used to study the effect of N containing fuel additive
combustion using a gas chromatographic technique. Progress so far indicates
N selective GC detection down to 1.6 ng of diethyl nitrosamine (^0.5 ppm in a 1
ml gas sample). Similar data are obtained for amines. The detector has at least
two orders of magnitude more sensitivity currently not realized due to problems
with the GC columns.
Preliminary studies have been made of rapid determination of total N in gasoline,
and of combustion products in a flat flame propane-air burner. (Author summary
modified).
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI I:icld/Croup
Air Pollution
Fuel Additives
Chemiluminescence
Gas Chromatography
Combustion products
Nitrogen Oxide ^0
Nitrogen Oxide (NO)
disalicylidene—propane-
diamine
alkenyl-succimide
21D, 07D
213
07A
ike
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Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
50
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
50
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