Literature Cited (1) Federal Aviation Administration, Office of Aviation Medicine, Carbon Monoxide and Cyanide Hazards in Air Transport Accidents Accompanied by Fire, 1973. (2) National Transportation Safety Board, Safety Recommendations A-73-67-70, September 5, 1973. (3) Gross, D., Loftus, J. J., Lee, T. G., Gray, V. E., US.Dept. of Commerce, Building Sci. Ser. 18, Building Res. Div., Smoke and Gases Produced by Burning Aircraft Interior Materials, 1969. (4) Coleman, E. H., Thomas, C. H., J . Appl. Chern., 4, 506 (1957). ( 5 ) Fish, A., Franklin, N. H., Pollard, R. T., ibid., 13,506 (1963). (6) Dufour, R. E., Underwriters Laboratories, Chicago, Ill., Bull. No. 53, “Survey of available information on toxicity of combustion products of certain building materials under fire conditions,’’ 1963.
(7) Ausbosky, S., VFDBZeitschrift, 16, 58 (1967). (8) Woolley, W. D., Brit. PolyrnerJ., 4,27 (1972). (9) Murray, D., Mod. Plast., p 69 (September 1973). (10) Federal Trade Commission News, “Complaint challenging knowing marketing of plastics presenting a serious fire hazard,”
May 30,1973.
(11) Kline, G. M., Tech. Ed., Mod. Plast., p 94 (February 1973).
(12) Asti, F. J., Rizzo, F. J., paper presented before the 97th Meeting of the Division of Rubber Chemistry of the American Chemical Society, Washington, D.C., May 8, 1970. (13) American Chemical Society, Abstracts of papers presented at the 166th National Meeting, Chicago, Ill., Papers Nos. 5361, Division of Polymer Chemistry, August 26-31, 1973. (14) Kusnetz, H. L., Saltzman, B. E., Lanier, M. E., J. Arner. Ind: Hyg. Ass., 21, 361 (1960).
Received for review October 1, 1973. Accepted August 28, 1974. Work partially supported by Firetect, Inc., Los Angela, Calif.
Response of Commercial ChemiluminescentNO-NO, Analyzers to Other Nitrogen-Containing Compounds Arthur M. Winer, John W. Peters, Jerome P. Smith, and James N. Pitts, Jr.* Statewide Air Pollution Research Center, University of California, Riverside, Calif. 92502
Commercial chemiluminescent oxides of nitrogen analyzers employing carbon or molybdenum converters are nonspecific for determination of nitrogen dioxide (N02). The instruments not only measure NO2, but also simultaneously respond nearly quantitatively to peroxyacetyl nitrate (PAN), and a variety of other organic nitrates and nitrites. Furthermore, they respond nonquantitatively to compounds such as nitroethane and nitric acid. The implications of these observations are not serious for most ambient air analyses where the concentrations of the interfering nitrogenous compounds are low relative to NO2 levels. However, for highly quantitative ambient air or smog chamber measurements under circumstances where relatively low concentrations of NO2 occur simultaneously with high concentrations of PAN and other nitrogen-containing compounds, corrections for interference by these compounds can be significant. In the absence of such corrections, the NO, mode of commercial chemiluminescent analyzers must be viewed to a good approximation as measuring total gas phase “oxides of nitrogen,” not simply the sum of NO and NO2.
Atmospheric concentrations of nitric oxide (NO) and nitrogen dioxide (N02) are now routinely measured by instruments employing the gas phase chemiluminescent reaction of ozone with nitric oxide:
The light emitted by the electronically excited NO2 is monitored to obtain a quantitative measure of the NO concentration. Major advantages of this technique are high precision, selective response to NO, sensitivity in the part per billion range, linearity over a factor of 106 in concentration, and relatively short response times (1-4). Con1118
Environmental Science 8, Technology
sequently, increasing use is being made of commercial chemiluminescent NO-NO2 analyzers, and in many applications they are replacing the less expensive but more laborious and less accurate Saltzman technique (5) and phenyl disulfonic acid procedure (6). A critical evaluation of the Saltzman technique has been presented recently (7). In all recent commercial chemiluminescent analyzers, nitrogen dioxide is determined as nitric oxide after passage of the sample stream through a heated carbon, stainless steel, or molybdenum converter which quantitatively reduces NO2 to NO. The total NO (i.e., the original NO plus the reduced NO2) is then reacted with ozone, and the chemiluminescent emission recorded as total oxides of nitrogen (NO,). Electronic subtraction of the NO signal from the NO, signal yields the amount of NO2 originally present in the sample. Interference from any of the common air pollutants such as NOz, CO, hydrocarbons, “3, and SO, are apparently negligible in the chemiluminescent detection of NO with ozone (1-3). The same specificity, as well as the other advantages cited above for NO determination, is implied for the NO2 determination by commercial instrument manufacturers. However, when the instruments are operated in the NO, mode (i.e., passage of the gas stream through the converter), potential interferences include nitrogen-containing compounds which may be reduced to NO in the thermal converter, thereby causing anomalously high signals for NO, and hence correspondingly high values for NOz. Although this problem was pointed out in the discussion of chemiluminescent detection as a reference method (8) it has not received widespread attention. The possibility of interferences in the determination of NO2 when using chemiluminescent analyzers became of particular concern to us in the course of smog chamber studies in which higher-than-ambient levels of peroxyacetyl nitrate (PAN) were formed. Preliminary experiments did, in fact, lead to the observation that PAN was being read as NO2 by the commercial chemiluminescent NO-NO2 analyzers in our laboratory (9). It then became
of interest to determine whether other compounds such as organic nitrates, organic nitrites, and nitrogenous acids also decompose on the converter to produce NO with resulting interferences in the determination of NOz. If so, in cases of highly quantitative chamber or ambient air studies these interferences could be of sufficient magnitude to necessitate correction of the NO, and NO2 recorded signal response. Such corrections would require quantitative determination of the instrument response to each species of interest, as well as determination of the species concentration by a n independent method. We report here the signal response characteristics of PAN, ethyl nitrate, ethyl nitrite, nitroethane, and nitric acid resulting from passage of the appropriate gaseous stream through the molybdenum catalytic converter in the NO, and NO2 modes of a commercial chemiluminescent analyzer. [n addition we report similar data for PAN and n-propyl nitrate for a chemiluminescent oxides of nitrogen analyzer employing a carbon converter. Experimental The chemiluminescent NO-NO2 instruments employed in this study were commercial instruments whose only modification was a reduction in sample flow rate to about 174 ml/min arid 600 ml/min for the analyzers equipped with carbon and molybdenum converters, respectively. The lower flow rates were required to minimize volumes sampled during chamber experiments and did not affect the sensitivity, precision, or accuracy of measurement. Both instruments were used a t the converter temperatures set a t the factory. All other instrumental conditions were those recommended by the manufacturer. Peroxyacetyl nitrate was prepared according to the procedures of Stephens et al. (10) a t -1000 ppm concentrations pressurized to 115 psi with nitrogen in 34.4-liter lowpressure oxygen (LPO) tanks. The PAN concentration in the tank was determined from its infrared absorption band at 1163 c m - l ( I 1 ) . Ethyl nitrate (1000 ppm a t 115 psi), ethyl nitrite (1140 ppm a t 115 psi), and nitroethane (13 ppm at 115 psi) samples were i~ndividuallyprepared by accurately injecting the liquids into the evacuated LPO tanks with microliter syringes and then pressurizing the tanks with prepurified (Liquid Carbonic High Pure) nitrogen. The ethyl nitrate, ethyl nitrite, and nitroethane were obtained from Eastman Kodak, Mallinckrodt, and Matheson, Coleman, and Bell (Practical), respectively. NO from a standard cylinder (Matheson Gas Products, CP, >99.0% filtered through Ascarite) was injected into an LPO tank and pressurized to 115 psi with prepurified nitrogen for use as a control sample. Gas phase nitric acid ("03) was prepared by bubbling prepurified Na through a solution of 45% "03 and diluting the resulting mixture with Nz to the desired concentration. The concentrated, pressurized gases were individually diluted in a nitrogen stream using a series of calibrated flow meters. After dilution through the consecutive flow meters, the gases were analyzed for signal response on the NO, and NO2 mode of the chemiluminescent analyzers (i.e., after passage through the converter), Background signal response to the diluent nitrogen a t the appropriate flow rates was measured on the NO,, NO*, and NO modes of the chemiluminescent analyzer and any necessary corrections were applied to the signal response observed for the nitrogen-containing compounds. Before all measurements, the chemiluminescent analyzers were Calibrated with a standard calibration tank of NO (Scott Research Laboratories, Inc., certified and rechecked to 1 2 % ) the concentration of which (19.5 ppm)
was further authenticated by analysis using the Saltzman method. The efficiency for NO2 to NO conversion for each instrument was checked by employing the gas phase titration of NO with 0 3 to produce NO2 (12). Specifically, as the NO sample stream was titrated with increasing amounts of ozone (up to equal parts 0 3 to NO), the NO, response was monitored as a measure of the converter efficiency. Thus, for 100% efficiency no change in NO, signal level was observed during the progressive titration, since all of the NO2 produced was reconverted quantitatively to NO in the instrument. However, for efficiencies less than 100% the NO, reading decreased during the titration since some NOz was not being read (i.e., converted to NO) by the instrument. During this study the molybdenum converter efficiency was 100 2%, while that for the carbon converter was 95 f 3% over the concentration range studied.
*
Results and Discussion Most of our studies were carried out on the NO-NO2 analyzer equipped with a molybdenum converter since it had 10 times more sensitivity and demonstrated a more rapid converter response (i.e., reached maximum efficiency most rapidly for the compounds studied), both desirable traits for the purposes of this study. The determination of the chemiluminescent signal response to peroxyacetyl nitrate was considered of greatest importance in terms of the relative concentrations of the nitrogenous compounds investigated in ambient air or synthetic smog systems. Figure 1 shows the response observed for an NO sample stream employed in an identical manner to that for the studies of PAN and other compounds. The 2% difference between the observed NO response (solid line) and the calculated NO concentrations (dashed line) is taken (within a 1% standard deviation) as a measure of the experimental uncertainties in the sample preparation and dilution flow procedures employed. The solid line in Figure 2 is a least squares fit of the observed response of the molybdenum converter instrument vs. the corresponding concentrations of PAN in the diluted sample stream. Identical data were obtained from both the NO2 and NO, modes since there was no NO present in the sample stream. The dashed line in Figure 2 again corresponds to a 100% instrument response. The least squares analysis of the data in Figure 2 shows that for this commercial instrument PAN is reduced to NO during passage over the molybdenum converter with an efficiency of 92% as shown by the concomitant signal
t
501 25
NITRIC OXIDE (PPBI
Figure 1. Response to nitric oxide for NO (and NOx) modes with molybdenum converter
Volume8, Number 13, December 1974
1119
response in either the NO, or NO2 mode. The response is linear in the concentration range from 0-400 ppb, with a linear correlation coefficient of 0.99. Thus, nearly 100% corrections to NO, and NO2 data obtained with this instrument are required at all PAN concentration levels typically observed in our smog chamber experiments, as well as for concentrations observed in ambient air. The PAN study was repeated for the carbon converter chemiluminescent analyzer. Whereas in the case of the molybdenum converter the response to PAN was essentially instantaneous (i.e., limited by the inherent time constant of the instrument), for the carbon converter the response to PAN increased gradually with time (over a period of minutes) to a maximum value. This behavior was observed for both nitrogen- and air-diluted sample streams. The least squares fit of response of the carbon converter analyzer to PAN (Figure 3) yielded an efficiency of NO production, and hence measurement as NOz, of 98% with a standard deviation of i3%. Results from the reference experiment, using NO to determine uncertainty introduced in the experimental procedure, are shown for the carbon converter instrument in Figure 4 and correspond to a difference between observed and calculated values of 5-10% over the concentration range studied. Thus, within the experimental uncertainty and except for the difference in time to reach maximum response, the same quantitative conversion of PAN is observed for both the molybde-
num and carbon converters. A summary of the data for the two types of converters is given in Table I. The response of the molybdenum converter NO-NO2 analyzer to ethyl nitrate and ethyl nitrite is shown in Figures 5 and 6, respectively, and the results are summarized in Table I. Within an experimental uncertainty of approximately 5%, a 100% linear response is observed in the NO2 mode of the analyzer for these compounds over a wide range of concentration. In contrast to these results and those for PAN, an examination of nitroethane (Figure 7 ) , a thermodynamically more stable isomer of ethyl nitrite produced a signal response of only 6-7% of the nominal nitroethane concentration in the sample stream. The response of the molybdenum converter analyzer to H N 0 3 was difficult to determine quantitatively. The Nzdiluted H N 0 3 stream gave a substantial response in both the NO, and NO2 modes, but the response did not appear to be linear with "03 concentration, and the possibility of damage to both the sample lines and reaction chamber of the analyzer caused curtailment of the experiment. However, it can be said that gas phase H N 0 3 did result in an instrument response, although the form of the response function could not be determined. In a single determination of the response of the carbon converter analyzer to n-propyl nitrate, a signal equal to 92% of the nominal n-propyl nitrate concentration was ob-
Response to PAN for NO;! (and NO,) lybdenum converter
Figure 4.
modes with mo-
Figure 2.
549,
I PAh cAaBcbi
I
I
Response to nitric oxide for NO (and NO,)
modes
with carbon converter
I
1
450
540
cmvCa-Ea
a 3600 z
'0
Figure 3.
90
I80
270 360 PAN (PPB)
Response to PAN for NO2 (and NOx) modes with car-
bon converter 1120
Environmental Science & Technology
Figure 5. Response to ethyl nitrate for NO2 (and NOX) modes .with molybdenum converter
~
7
2
1
~
1-
I
i
I
1
ETHYL NITRITE
/‘
11
MOLYBDENUM CONVERTER
Table I. Response of Two Commercial Chemiluminescent NO-NOZ-NO, Analyzers to Nitrogen-Containing Compounds in NOzand NO, Modes ReCon- sponse,* verter“ %
Compound
-
I ETHYL
NITRITE (PPB)
Response to ethyl nitrite for NO2 (and N O x ) modes with molybdenum converter Figure 6.
1
hlTROETHbNE MOLYBDENUM C O N V E R T E 9
,,,,,‘‘
i i I
4
NITROETHANE
(PPB)
Response to nitroethane for with molybdenum converter Figure 7.
NO2
(and NOx) modes
served. Thus, within experimental uncertainty, the conversion of this nitrate to NO on the carbon catalyst appears to be essentially quantitative; however, additional measurements over a range of concentrations are necessary to confirm this.
Conclusions Several widely used chemiluminescent NO-NO2 analyzers are nonspecific in the NO2 and NO, (i.e., NO + NO2) modes. Thus, they quantitatively determine the concentration of not ,just NO2, but actually the sum of NO2, PAN, and a variety of other organic nitrates as well as organic nitrites. I n addition, they respond nonquantitatively to compounds such as nitroethane and nitric acid. The implications of these observations are not serious in the case of most ambient air analyses, where concentrations of PAN seldom rise above 40 ppb, and the concentrations of organic nitrates and nitrites and nitric acid are expected to be somewhat lower (-0-10 ppb). However, for highly quantitative ambient air studies, under circumstances where relatively low concentrations of NO2 occur simultaneously with high concentrations of PAN and other nitrogen-containing compounds, corrections for interference by these compounds in NO2 determinations by the chemilumiriescence method may be nonnegligible. In the case of smog chamber experiments in which higherthan-ambient concentrations of PAN and nitrates are often produced and very accurate data are required, for example for modeling studies, it will be necessary to de-
Peroxyacetyl nitrate Nitric oxidec Peroxyacetyl nitrate Nitric oxidec Ethyl nitrate Ethyl nitrite Nitroethaned Nitroethanee n-Propyl nitrate
Std dev
%
’
Linear correlation coeff
Range of concn, ppb
92 1.0 1.00 0-410 102 1.0 1.00 0-140 C 98 3.0 1.00 0-500 C 100 1.0 1.00 0-145 M 103 6.0 0.99 0-355 M 92 1.3 1.00 0-66 M 6 0.4 0.96 0-340 M 7 0.3 1.00 0-240 C 92’ a M = molybdenum. C = carbon Least squares analysis over indi. cated range of concehtration. CMeasured i n the NOz mode with no NOn in the sample stream-i.e., NO = NO.. Air diluent. e NZ diluent. f Determined for a single concentration. M M
...
...
...
termine independently the concentrations of interfering species by gas chromatography, infrared spectroscopy, or other methods and to then apply quantitative corrections to the chemiluminescent NO2 determinations. Finally, in the absence of corrections for interferences, the data obtained in many smog chamber experiments using the NO, mode of commercial chemiluminescent analyzers must be viewed as more nearly a total gas phase oxides of nitrogen measurement, not simply the sum of NO and N02.
Acknowledgment We gratefully acknowledge the technical assistance of Monty Price in preparation of samples, and helpful discussions with Drs. John McAfee and Karen Darnall. Literature Cited (1) Fontijn, A , , Sabadall, A . J., Ronco, R. J., Anal. Chem., 42, 575 (1970). (2) Stuhl, F., Niki, H., Scientific Research Staff Report, Ford Motor Co., March 1970. (3) Stedman, D. H., Daby, E. E., Stuhl, F., Niki, H., J Air Pollut. Contr. Ass., 22, 260 (1972). (4) Sigsby, . J. E., Black, F . M., Bellar. T. A , . Klosterman. 0. L.. Enuiron. Sei. Technol., 7, 51 (1973). ( 5 ) Saltzman, B. E., Anal. Chem., 32, 135 (1960). (6) USA Standard Z 116 M (1966), “Phenyl disulfonic acid procedure for determination of oxides of nitrogen,” ASTM Designation D 1608-60 (1967). (7) Fine, D. H., Enuiron. Sci. Technol., 6, 348 (1972). (8) Fed. Regist., 38, No. 110, 15177 (1973). (9) Pitts, J. N. Jr., Winer, A . M., Darnall, K . R., Doyle, G., McAfee, J., Peters, J., Smith, J., “Chemical Consequences of Air Quality Standards and of Control Implementation Programs: Roles of Hydrocarbons, Oxides of Nitrogen, and Aged Smog in the Production of Photochemical Oxidant,” Quarterly Progress Report, California Air Resources Board Contract =3-017, September 30, 1973. (10) Stephens, E. R., Burleson, F. R., Cardiff, E. A,, J . Air Pollut. Contr. Ass., 15, 87 (1965). (11) Stephens, E. R., Anal. Chem., 36,928 (1964). (12) Rehme, K . A , , Martin, B. E., Hodgeson, J. A , , “The Application of Gas-Phase Titration in the Simultaneous Calibration of NO, NOz, NO, and 0 3 Atmospheric Monitors,” presented at the 164th Kational ACS Meeting, New York, S.Y., September 1972. Received for review June 6, 1974. Accepted September 3, 1974. This work was supported in part by the California Air Resources Board (Contract N o . 3-017) and the Environmental Protection Agency (Grant N o . R-800649). The contents do not necessarily reflect the uieus and policies of the Environmental Protection Agency or the California Air Resources Board, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
Volume8, N u m b e r 13, December
1974
1121
