Anal. Chem. 1994,66, 3137-3143
Gas Chromatographic Detector for Selective and Sensitive Detection of Atmospheric Organic Nitrates Cunsheng Hao,? Paul B. Shepeon,'*+John W. Drummond,* and Kayambu Muthuramut Department of Chemistry and Centre for Atmospheric Chemistry, York University, 4700 Keele Street, North York, Ontario, Canada M3J 1P3, and Unisearch Associates, 222 Snidercroft Road, Concord, Ontario, Canada
A highly selective and sensitive detector for measuring organic nitrates in atmospheric air samples has been developed. This detector is based on the luminol chemiluminescence-based detection of NO2 produced by postcolumn thermal decomposition of organic nitrates. Determination of the relative detector sensitivities to C3-Ca alkyl nitrates, a,@-hydroxynitrates, a,@dinitrates, and several mono- and multinitro aromatic compounds as well as N-nih.osodimethylamine has been conducted. The detector is shown to produce one NO2 per molecule at a pyrolysis temperature of 350 "C, yielding identical sensitivities for alkyl and hydroxy nitrates, consistent with the known thermal decomposition pathway and thermal lifetimes at this temperature. As expected, the alkyl dinitrate studied yielded a calibration slope equal to twice those of the mononitrates. At sufficiently high temperatures, the detector also responds to a variety of nitroaromatics and to N-nitrosodimethylamine. However, a yield of 50% NO2 was obtained from the thermal decomposition of N-nitrosodimethylamine and all mononitroaromatic compounds, except 1-nitronaphthalene, which yielded a slope similar to those of the alkyl nitrates. NO2 yields of 0.70 and 0.90 were obtained for 2,CDNT and 2,4,6TNT, respectively. The detection system is found to exhibit a linear response for all N-containing compounds studied. The significant selectivity advantage of the chemiluminescence detector is demonstrated through comparison with the ECD for both laboratory standard and ambient samples. The detection limit for determination of organic nitrates was found to be -0.05 pmol. In recent years it has been shown that global background ozone concentrations have increased by roughly a factor of 3 over the past century' and appear to be increasing at a rate of about 1% per year.2 Because of the continuing problem of air quality exceeding local ozone standards, both field and laboratory investigations are needed to improve our understanding of the processes related to tropospheric ozone formation. Of particular importance are detailed studies of odd nitrogen (NO,) chemistry, as the nitrogen oxides are often the controlling factor in oxidant formation.3 Our understanding of nitrogen oxide chemistry has improved recently as a result of the development and use of ' Address correspondence to this author. Present address: 1393 Herbert C. Brown Laboratory of Chemistry, Purdue University, West Lafayette, IN 47907-
1393. f
8
York University. Unisearch Associates.
(1) Sandroni, S.; Anfossi, D. ViarengoS. J . Geophys. Res. 1992,97,2535-2539. (2) Low, P. S.; Davies, T. D.; Kelly P. M.; Farmer, G. J . Geophys. Res. 1990.95,
22441-22453.
0003-2700/94/0366-3737$04.50/0 0 1994 American Chemical Soclety
various chromatographic and chemiluminescence-based techniques for measuring N-containing air pollutants. However, recent measurements of NO, and its individual speciated components indicate that for well-aged air masses the measured sum of the concentrations of the individual components (i.e. EYJ is often significantly less than the measured total NO,, Le. cYJNOy < 1. The NO, "shortfall" can be as large as 25% and as high in effective concentration as any individual ~ o m p o n e n t .The ~ shortfall species appear to originate from photochemical processes, and it has been suggested5 that organic nitrates could contribute to the missing NOy fraction. Alkyl nitrates are formed in the atmosphere during the OH radical initiated oxidation of saturated hydrocarbons in the presence of NO, as described by Atkinson et a1.,6 while more complex multifunctional nitrates such as hydroxy nitrates can also be produced during the oxidation of unsaturated hydrocarbon^.^ Because of the relatively long atmospheric lifetime of many of the organic nitrate species, they can represent an important reservoir for NOy in the atmosphere.8.9 Since their original detection from laboratory smog chamber studies,1° the alkyl nitrates have been measured at several urban, rural, and remote locations.11-13 The organic nitrates have most often been detected using capillary chromatography with electron capture detection (GC/ECD). Although the ECD is very sensitive to these nitrate species, the existence of a large number of various halogenated hydrocarbon species in the atmosphere, which are detected with high sensitivity by ECD,14 leads to very complex chromatograms for ambient samples. Thus unambiguous (3) Chameides, W. L.; Fehsenfeld, F.; Rodgers, M. 0.;Cardelino, C.; Martinez, J.;Parrish,D.;Lonnenman,W.;Lawson,D. R.;Rasmussen,R.A.;Zimmerman, P.; Greenberf. J.; Middleton, P.; Wang, T. J. Geophys. Res. 199597, 60316055. (4) Ridley, B. A. Amos. Enuiron. 1991, 25A, 1905-1926. ( 5 ) Fahey, D. W.; Huebler, G.; Parrish, D. D.: Williams, E.G.; Norton, R. B.; Ridley, B. A.; Singh, H. B.; Liu, S. C.; Fehsenfeld, F. C. J . Geophys. Res. 1986, 91, 9181-9193. ( 6 ) Atkinson, R.; Aschmann, S.M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J . Phys. Chem. 1982,86,4563-4569. (7) Shepson, P. B.; Edney, E. 0.;Kleindienst, T. E.; Pittman, J. H.; Namie, G. R.; Cupitt, L. T. Enuiron. Sci. Technol. 1985, 19, 849-854. (8) Roberts, J. M.; Fajar. R. W. Enuiron. Sei. Technol. 1989, 23. 945-951. (9) Zhu, T.; Bahnes, I.; Becker, K. H. J . Atmos. Chem. 1991, 13, 301-311. (10) Carter, W. P. L.; Lloyd, A. C.; Sprung, J. L.; Pitts, J. N., Jr. Inr. J . Chem. Kinef. 1979, 1 J . 45-101. (11) Atlas, E.; Schauffler. S. Enuiron. Sci. Technol. 1991, 25, 61-67. (12) Shepson, P. B.; Anlauf, K. G.; Bottcnheim, J. W.; Wiebe, A.; Gao, N.; Muthuramu, K.; Mackay, G. I. Amos. Enuiron. 1993, 27A, 149-151. (13) Atlas, E.; Schauffler, S. M.; Merrill, J. T.; Hahn, C. J.; Ridley, B.; Walega, J.;Greenberg,J.;Heidt,L.;Zimmerman,P. J . Geophys.Res. 1992,97,1033110348. (14) Schomberg Gerhard. Gas Chromafography;VCH Verlagsgwellschaft mbH:
Weinheim, 1990; Chapter 2.
Analytical Chemistty, Vol. 66, No. 2 1, November I, 1994 3737
"Yl
w Compressor
Luminol Solution
Carrier Gas Out
I
I
NO, Perm. Source
Mass Flow Controller
- L
Flgwe 1. Schematic diagram of the organic nitrate-specific detector.
identificationof the substantial variety of atmospheric monoand multifunctional organic nitrates is difficult. We describe here the development of a chromatographic detector based on postcolumn thermal decomposition of organic nitrates in a quartz tube pyrolyzer to yield N02, which is subsequently detected using a highly sensitive and selective chemiluminescence-based NO2 detector, using luminol as the chemiluminescencereagent. The detector has been thoroughly evaluated using purified standards of a number of alkyl and multifunctional nitrates, as well as N-nitrosodimethylamine and a series of nitroaromatics. In addition, the chemiluminescence detector is evaluated by comparison with the ECD for real atmospheric samples, obtained from the Canadian Arctic, and from a laboratory photochemical reactor. EXPERIMENTAL SECTION The newly developed organic nitrate detector is composed of a quartz tube pyrolyzer and a chemiluminescence NO2 detector. The detector was evaluated using a capillary GC/ ECD (Varian 3300), which is equipped with a septum-cooled programable injector (SPI) with on-column injection and used Varian STAR software for peak integration. The luminol detector is connected in series with the ECD, which is used as the basis for comparison with the new detector. The column used for organic nitrate analysis is a DB-1701 megabore column (30-m length, 0.53-mm o.d., 1.O-pm film thickness, Restek). The following SPI injector temperature program was used: 50 OC for 0.1 min, followed by heating at 180 OC/ min to 150 OC, for alkyl nitrates, hydroxy nitrates, and N-nitrosodimethylamine, and to 240 OC for nitroaromatic compounds. For alkyl and hydroxy nitrates, the column was maintained at an initial temperature of 50 OC for 7 min, followed by heating at 5 OC/min toa final constant temperature of 150 OC. For nitroaromatic compounds the final column temperature was 240 OC. 3738
Ana&tial ChemWy, Vol. 66, No. 21, November 1, 1994
The carrier gas used for the GC/pyrolyzer/chemiluminescence detector was extra dry nitrogen (5 mL/min carrier, 30 mL/min makeup). As shown in Figure 1, after exiting the ECD the carrier gas stream is mixed at the tee part of the quartz convertor with a small flow of NO2 in 0 2 (6 mL/min) to yield an NO2 baseline concentration of 15 ppb. The luminol-NO2 reaction chemiluminescence is known to be nonlinear for concentrations below 3 ppb.15 In the present system, this nonlinearity is removed by adding a constant mixing ratio of NO2 to the carrier gas stream. The 0 2 is necessary for the chemiluminescence chemistry and is also used to prevent reduction of the NO2 produced in the pyrolyzer to NO. The organic nitrate components in the carrier gas stream elute from the capillary column, mix with the N02/ 0 2 flow, and then pass through the pyrolyzer, where the thermal decomposition reaction (to yield NO2) takes place. The NO2 produced by the pyrolysis reaction then enters the chemiluminescence NO2 detection cell where the chemiluminescence reaction occurs. The photons emitted from the reaction are detected using a photomultiplier. The signals were recorded using an HP3395 integrator. The luminol chemiluminescence GC detector is composed of a cylindrical quartz pyrolyzer (1.0-mm i.d. X 180-mm length; 0.1 4-cm3volume) which was connected directly to the ECD outlet with a l/16 in. zero dead volume Teflon union, as shown in Figure 1. The pyrolyzer tube was heated using a ceramic fiber heater (model VC401A060A, 220 W, Watlow), which was wrapped uniformly around the outside of the quartz tube. The tube was then wrapped with fiberglass insulation and enclosed in a cylindrical metal casing. Optimum temperatures for quantitative pyrolysis and absence of solvent peaks were investigated for organic nitrates, N-nitrosodimethylamine, and nitroaromatics. The detector is based on (IS) Kelly, T.J.; Spier, C.W.;Ward, G.F. Amos. Emiron. 1990,24,4, 23972403.
the postcolumn thermal decomposition of the organic nitrates (and other N-containing compounds) to yield N02. In the case of organic nitrate pyrolysis, the reaction proceeds by 0-N bond homolysis as follows:16
A
RONO,
RO'
+ NO,
2.5
(1)
2.0
The pyrolyzer temperature was controlled using an Omega temperature controller (model CN132) and measured with a Gordon thermocouple sensor (model ABEOOQ120EK100), which was placed at the midpoint of the pyrolyzer tube. The tee part of the inlet to the pyrolyzer was heated to 180 OC using a Watlow firerod heater (model ElJ39,60 W) for alkyl and hydroxy nitrates and 240 OC for nitroaromatics, in order to avoid possible adsorptive loss of the components in the tee. The temperature of the quartz tee part was controlled using the same Omega temperature controller and measured with a thermocouple. The luminol detector is based on the sensitive chemiluminescent reaction between NO2 and luminol(3-aminophthal hydrazide) in solution, as first described by Maeda et ala1' and the commercial model LMA-3 NO2 analyzer described by Schiff et a1.18 The detector cell itself is a 15-cm X 8-cm X 2-cm rectangular block, with inlet and outlets for the carrier gas and luminol flows. The reaction cell contains a fabric wick that is wetted with the luminol solution (1 X l W M luminol, 0.2 M NazSO3,0.05 M NaOH, 1.5 X 1 W M EDTA, and 0.1% surfactant). The wick is viewed by a photomultiplier tube through an acrylic window, which is transparent to the chemiluminescent light at 425 nm. The internal volume of the reaction cell is 65pL. The response time of the detector is < O S s. When an NO2 peak enters the cell, a fraction of the NO2 dissolves in the solution on the surface of the wick (according to the Henry's Law constant for NOz), which then reacts with luminol to ultimately yield a strong chemiluminescence centered around 425 nm17. The luminol solution is metered by a servo system involving a pressurized feed bottle and a liquid mass flow sensor and then introduced directly behind the top of the wick to yield a homogeneous flow to the drain at the bottom of the wick. The luminol flow rate was 10 pL/min for all measurements. The detector was evaluated principally with organic nitrate standard samples, synthesized and purified as described by Muthuramu et aLi9 A variety of C3-C6 alkyl nitrates, hydroxy nitrates, and dinitrates were used. Other commercially obtained compounds, such as 2-propyl nitrate, isobutyl nitrate, N-nitrosodimethylamine, 1-nitronaphthalene, nitrobenzene, 0-, m-and p-mononitrotoluene, were from Aldrich and were used without further purification. Dinitrotoluene (DNT, 97%) and trinitrotoluene (TNT, >97%) were obtained from Scintrex, Inc. (Concord, Ontario) and used as received.
1.5
-
RESULTS AND DISCUSSION The detector shown in Figure 1 was evaluated using purified standards of an array of organic nitrates: c 3 4 6 alkyl nitrates, (16) Roberts, J. M. Atmos. Enuiron. 1990, 24A, 243-287. (17) Maeda, Y.;Aohi, K.; Munemori, M. Anal. Chem. 1980,52, 307-311. (18) Schiff, H. I.; Mackay G. I.; Castledine, C.; Harris G . W.; Tran, Q..Water, Air. Soil Pollut. 1986, 30, 105-1 14. (19) Muthuramu, K.; Shepon, P. B.; OBrien, J. M. Enuiron. Sci. Technol. 1993, 27, 1117-1124.
1.o
0.5 0.0 175
225
275
325
375
425
475
525
575
Temperature("C)
Fhre 2. Effect of pyrolyzer temperature on detector signal for several organic nitrates: (W) 2-pentyl nitrate, (0)3-pentyf nitrate, (A)1,2bk(nitrmxy)butane,(0)3-(nitrwxy)2-butanol, (0)14nItrooxy)2-butanol
C Z - C ~hydroxy nitrates, and C2-C4 dinitrates. Standard samples, diluted in benzene or cyclohexane, were used to evaluate the optimum converter conditions, detection limit, linearity of response, and relative responses for this set of organic nitrates. PyrolyzerStudies. A series of experiments was conducted in which the chemiluminescence signal intensity was measured as a function of the pyrolyzer temperature for repeated injections of a mixed organic nitrate standard sample. The results are shown in Figure 2, as the observed signal intensity for several compounds as a function of the pyrolyzer temperature. As shown in the figure, there is essentially no signal at temperatures lower than about 200 OC. From the kinetic data summarized by Roberts,2owe calculate a thermal decomposition lifetime for alkyl nitrates at 200 OC of -160 s, while the residence time for gases in the pyrolyzer was 0.24 s. There is a sharp increase in the signal intensity with increasing pyrolyzer temperature between 200-225 OC, at which point the signal reaches a maximum level, assumed to be equivalent to quantitative conversion of the alkyl nitrates to NO2 (see discussion below). In the temperature region between 225 and 525 "C, the intensity of the signal remained constant. At 350 OC the thermal lifetime of thealkyl nitrates is -0.005 s, much lower than the residence time of gases in the pyrolyzer tube. Above 525 OC, the signal drops off dramatically. For high pyrolyzer temperatures, i.e. above 525 OC, pyrolysis and combustion of the solvent molecules occurs, which produces substantial quantities of partially oxidized products, e.g. alcohols, aldehydes, acids, CO, and C02, which decrease the detector sensitivity. As the solvent peak eluted from the column, the detector signal dropped off dramatically due to interference from the products with the chemiluminescence chemistry. One possible explanation is the dissolution of large quantities of C02 into the wick solution, which would lower the pH. The chemiluminescence chemistry is known to be highly pH dependent, as OH- is one of the (20) Roberts, J. M. Atmos. Enuiron. 1990, 24A, 243-287.
AnaMical Chemistry, Vol. 00, No. 21, November 1, 1994
3739
ECD
Luminol Detector 1
1
2
1314 1516
3
18
3
2
4 5 6 1 0 9 101112
19
,
,
,
I
,
I
I
I
I
I
I
I
5
10
15
20
25
30
5
10
15
20
25
30
Retention Time, min. Figure 9. Typical capillary GC chromatograms of a standard organic nitrate sample using the ECD and the luminol detector. Peaks numbered 1-12 correspond to 2-propyl, 1-propyl, 2-butyl, 1-butyl, d-methyI-%-butyl,3-penty1, 2-penty1, 2-methyl-1-butyl, 3-methyl-l-butyl, 1-pentyl, Shexyl, and 2-hexyl nitrate, respectively. Peaks numbered 13-18 correspond to l-(nitrooxy)-2-propanol, 2-(nltrooxy)-l-ethanol, 2-(nihooxy)-l-propanol, 2-(nitrooxy)-3-butanoi, l-(nitrooxy)-2-butanoi, and 2-(nitrooxy)-l-butanoI, respectlvely. 1,2-Bis(nltrooxy)butane eluted as the last peak.
reactants.” Injection of comparable amounts of COS (Le. assuming quantitative conversion of the solvent to COZ) directly into the column indicated a substantial negative peak. We find that the magnitude of the interference depends strongly on the nature of the solvent, e.g. the interference is significantly worse for benzene compared to cyclohexane, presumably the result of differing rates of oxidation for these solvents. The results obtained in the pyrolyzer temperature studies shown in Figure 2 suggest that, for organic nitrate analysis, the ECD temperature should be maintained at a temperature high enough to ensure high sensitivity detection and to prevent adsorptive loss of the analytes, but also lower than 225 OC, to prevent their decomposition in the ECD (which is much less sensitive toNO2 than to the organic nitrates). In addition, if the organic nitrates decompose in the ECD region, the following reduction reaction will occur catalytically on the metal surface of the ECD cell:
A
NO,
NO
(2)
It is also for this reason that the most suitable pyrolyzer material is quartz tubing rather than metals such as stainless steel. For further evaluation of the prototype detector for determination of organic nitrates, a pyrolyzer temperature of 350 OC was chosen, as it is in the region of quantitative conversion to NOz, yet is cool enough that the negative solvent peak is minimized. The ECD temperature was maintained at 180 OC for all experiments. Standard Sample/Sensitivity Studies. A mixed standard of 19 organic nitrates was analyzed by capillary GC using the ECD and luminol-based detectors operated in series, as shown in Figure 1, under the optimum GC, ECD, and pyrolyzer conditions as described above. Figure 3 shows a typical 3740
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
chromatogram of the standard obtained using the ECD (left) and the luminol-based detector (right). The compounds in the standard sample were present at concentrations of 15 pmol/pL (2-pL injections). The peaks numbered 1-12 in the chromatograms correspond to c3-C~ alkyl nitrates, while numbers 13-18 correspond to C Z - C ~hydroxy nitrates. 1,2Bis(nitrooxy)butane eluted as the last peak. As shown in Figure 3, the chromatogram (right) obtained using the pyrolysis/luminol chemiluminescence detector shows that this detector responds with good sensitivity to all organic nitrates and that only a small loss of resolution occurs during processing of the peak in the pyrolyzer and chemiluminescence cell, in comparison with the ECD chromatogram. For example, the resolution (R,) for 3- and 2-pentyl nitrate (at -15 min) is 1.35 for the ECD and 1.24 for the luminol detector. The peak widths are -5-7 s for the ECD detection, while for the Luminol Detector, they are -6-8 s. A substantial negative solvent peak (at -6 min) is apparent in Figure 3 for the luminol detector. As indicated above, the magnitude of this solvent peak depends on the nature of the solvent, e.g. low molecular weight solvents such as methanol produce more severe negative peaks than does benzene. However, a much smaller negative solvent peak is apparent for cyclohexane with the thermal converter at 350 OC. This is likely a result of the variability of the nature and yields of oxidation products such as COZ,HCOOH, etc. that will affect the luminol solution conditions, particularly pH. The impact of the solvent peak can be most easily removed by dumping the solvent peak with use of a 4-port valve just upstream of the detector. However, if a low molecular weight solvent is used that has a relatively short retention time, the detector baseline will recover before elution of most analytes of interest. By using a series of standards containing species 1-19, calibration curves for the luminol detector were obtained. The results presented in Figure 4 indicate that, over the range of
-
Figure 4. Typical calibrationcurves for selected organic nlrates using the lumlnoi detector: (0)1,2-bk(nRrooxy)butane, (M) 3-(nitrooxy)-2butanol, (V)%pentyl nitrate, (0)2-(nitrooxy~l-butanoI,(V)l-(nitrooxy)2-propanol, (A)2-(nlrooxy)-l-propanol, and (0) I-propyl nitrate. Table 1. Relatlve Response Factors for Alkyl and Hydroxy Nitrates for the Pyrolyzer/Lumlnol Detector (Pyrolyzer Temperature = 350 ‘C)
compound
retention time (min)
Alkyl Nitrates (RON02) 9.05 2-butyl nitrate 3-methyl-2-butyl nitrate 11.47 12.83 3-pentyl nitrate 2-pentyl nitrate 13.17 1-pentyl nitrate 15.83 16.51 3-hexyl nitrate 2-hexyl nitrate 17.27 Hydroxy and Dinitrates 1-(nitrooxy)-2-propanol 20.03 2-(nitrooxy)- 1-propanol 21.11 3-(nitrooxy)-2-butanol 21.44 1-(nitrooxy)-2-butanol 23.10 24.26 2-(nitrooxy)- 1-butanol 26.31 1,2-bis(nitrooxy)butane
relative response’ 1.oo 0.99 (0.02) 1.01 (0.02) 1.26 (0.03) 1.08 (0.02) 1.14 (0.03) 1.20 (0.02) 1.07 (0.03) 1.03 (0.02) 1.03 (0.02) 0.97 (0.03) 1.12 (0.02) 2.12 (0.02)
a Relative to 2-butyl nitrate, from relative calibration curve slopes. Combined standard deviation of the slopes in parentheses.
about 1-360 pmol injected, the luminol detector exhibits a very linear response to the organic nitrates tested. For example, a linear least squares best fit for the 3-(nitrooxy)2-butanol calibration curve yielded a 1.0% standard error of the slope, and r2= 0.998. The detection limit for the prototype system was found to be -0.05 pmol for organic nitrates (defined as 2.58X standard deviation of the noise, i.e. at the 99% confidence level), which is 10 times higher than that found for organic nitrates using the ECD (-0.005 pmol), in reasonable agreement with the ECD detection limit for alkyl nitrates of -0.2 pg, reported by Atlas and Schauffler.” It is expected that a comparable detection limit could be achieved through use of an improved luminol flow delivery system, as fluctuations in the luminol flow were found to be the dominant source of noise in the baseline signal. The relative slopes (relative to 2-butyl nitrate) for all alkyl and hydroxy nitrates tested were obtained from the calibration curves, and are presented in Table 1. As indicated, this detector is found to respond nearly identically to all the alkyl and hydroxy nitrates studied. The differences in the slopes probably are largely a reflection of minor differences in the standard sample purities. These data are interpreted as an indication that for these compounds there is quantitative
conversion to NO2, at the pyrolyzer temperature of 350 OC. Similarly, the dinitrate compound yielded a slope equal to twice those of the mononitrates. These relative sensitivities have been found to be consistent over a 2-year period. Therefore, this detector offers the significant advantage that a known concentration standard is needed for only one alkyl nitrate, hydroxy nitrate, or dinitrate, in order to obtain the instrument calibration factor for others, and that authentic samples are only necessary for retention time determination. Ambient and Smogchamber Sample/Selectivity Studies. As part of the evaluation of the detector, ambient air samples were obtained at Alert, N. W. T. in the Canadian Arctic (82.5’ N) using charcoal trap sampling” and analyzed using the new detector. The system was operated using the same conditions as those used for standard sample analysis. Chromatograms from both the ECD and pyrolyzer/chemiluminescence detector were recorded for the same sample injection and demonstrate clearly the selectivity benefit for analysis of organic nitrates. An example is shown in Figure 5 for a sample obtained at Alert in April 1992, where the ECD signal is shown at left and the luminol detector signal shown at right. With the use of organic nitrate standard samples synthesized in our laborat~ries’~ and GC/MS analysis, the majority of the peaks from the luminol detector chromatogram were unambiguously identified as alkyl nitrates. For the ECD chromatogram, a number of chlorinated hydrocarbons were additionally present, e.g. freons, tri- and tetrachloroethylene, CC14, and CHC13, and a variety of halomethanes derived from sea water. For example, CH2Br2, CH2BrC1, C2Cl4, CHClBr2, and CHBr3 were identified in the ECD chromatogram at retention times of 6.67, 6.90, 8.48, 10.62, and 14.80 min, respectively (see Figure 5 , left). Thus unambiguous identification and quantitation of organic nitrates using the ECD may be difficult for some environments. This is particularly true for the low molecular weight multifunctional nitrates that elute from the column at 20-30 min, where the ECD chromatogram is very complex. The hydroxy nitrates, which elute in this region of the chromatogram, are currently being considered as potentially important components of NO,,, particularly for air masses impacted by fresh anthropogenic emissions (rich in alkenes) or those impacted by natural hydrocarbons (e.g. isoprene) emitted by vegetation.*’ The study by O’Brien et al. (21) indicated an average difference between the ECD and luminol detector determinations for various organic nitrates for rural air samples to be --f22%. To evaluate the detector performance for a laboratorygenerated sample, we conducted an irradiation of a mixture of isoprene/NO/isopropyl nitrite in a 10-m3 Teflon “smog chamber”. The photolysis of isopropyl nitrite, in the presence of NO, generates O H radicals, that in turn react with isoprene to generate a variety of reaction products, incluing various carbonyl compounds and organic nitrates.22 In Figure 6 we show ECD (top) and luminol (bottom) detector chromatograms for a smog chamber sample of a mixture of the OH/ isoprene/NO reaction products. Again, the chromatogram for the luminol detector demonstrates clearly the selectivity (21) O’Brien, J. M.; Shepson, P. B.; Muthuramu, K.; Hao, C.; Taylor, R.; Nib, H.; Roussel, P. J. Geophys. Res., submitted for publication. (22) Tuazon, E. C.; Atkinson, R. Int. J . Chem. Kinet. 1990, 22, 1221-1236.
Analyfcal Chemistty, Vol. 66,No. 21, November 1, 1994
3741
Luminol Detector
ECD
a
)1 I :
L
5
0
10
15
20
25
5 Retention T i e , min. 30 0
10
15
20
25
30
Flgure 5. Typical capillary GC chromatograms for an Arctic air sample using the ECD (left)and the luminol detector (right).
ECD
I 0
10
20
30 40 Retention Time, min.
50
60
Figure 6. Typical capillary GC chromatogramsfrom a smog chamber sample of OH/isoprene/NO reaction products using the ECD (top) and the luminol detector (bottom).
advantage relative to the ECD. Using GC/MS analysis we found that a large number of non-nitro compounds, mostly aldehydes and ketones (e.g. methyl vinyl ketone and methacrolein) were detected by the ECD (Figure 6, top), while only nitrate peaks were observed with the luminol detector. As these organic nitrates have yet to be identified and individually quantified, this detector may also prove useful in mechanistic studies of reactive atmospheric hydrocarbons. 3742
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
Potential Applications. The results described above indicate that this detector may be successfully utilized for the sensitive and selective detection of any N-containing species that may thermally decompose to yield N02. To investigate this possibility, further evaluations of the new detection system were performed using various N-containing species, specifically aromatic nitrocompounds including nitrobenzene, isomers of mononitrotoluene, nitronaphthalene, 2,4dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT). The nitrocompounds were chosen largely because of the potential use of this detector for detection of explosives. Nitro-PAH, such as nitronaphthalene, are products of the atmosphericoxidation of PAHs, and many of these species have been found to be mutagenic.23 We also investigated applications to determination of one of the most important nitrosamine species, N-nitrosodimethylamine (NNDA), another potential environmental mutagen. A series of experiments was carried out to optimize the chromatographic conditions for each of these species in order to yield Gaussian peak shapes and linear responses for each compound. Significantly different pyrolyzer temperature and GC temperature programs were used for the different species. Once optimum conditions were established, calibration curves were obtained and the slopes relative to that found for 2-butyl nitrate were determined. Assuming that for temperatures above 350 "C 2-butyl nitrate quantitatively decomposes to yield NO2, the relative slopes are equivalent to the NO2 yield for the test compound at that temperature. The results are presented in Table 2. The signal for N-nitrosodimethylamine reached a constant value at 350 "C(corresponding to an NO2 yield of 0.5). Higher converter temperatures were needed for C-NOz bond cleavage for the mononitroaromatic compounds: nitrobenzene, 0-,m-,and p-nitrotoluene. For pyrolyzer temperatures of 800 "C, the NO1 yields were only 0.74 and 0.90 for DNT and TNT, respectively. Of the nitroaromatic compounds, only 1-nitronaphthalene was found (23) Zielinska, B.;Arey, J.; Atkinson, R.;Winer, A. M.; A m " Enuiron. 1989, 23, 223-229.
Table 2. Relatlve Response Factors for Aromatlc Nltrocompounds and Nltrosamlne for the Pyrolyzer/Lumlnol Detector temperature of the quartz tube relative compound for pyrolysis ("C) response"
N-nitrosodimethy lamine nitrobenzene o-mononitrotoluene m-mononitrotoluene p-mononitrotoluene 1 -nitronaphthalene 2,4-dinitrotoluene (2,4-DNT) 2,4,6-trinitrotoluene (2,4,6-TNT)
350 650 650 650 650 650
800 800
0.51
0.53 0.48 0.51 0.44 0.99 0.74
0.90
Relative to 2-butyl nitrate, obtained at the same temperature as for the related species in the table.
to be 100% thermally converted to NO2, at 650 OC. A possible interpretation of the incomplete conversion of the nitroaromatics to NO2 is that competitive reaction channels, such as isomerization, at the relatively low temperatures (below 900 "C) are relatively more important, especially for the case of dinitrotoluene and trinitrotoluene pyrolysis.2426 On the basis of the studies cited, simple C-NO2 bond scission is favored only at the much higher temperatures encountered in combustion. Our pyrolyzer components limited us to a maximum temperature of about 800 OC. It is likely that close to quantitative NO2 yields for the nitro-PAH are possible, perhaps due to increased resonance stabilization of the radical intermediates produced from C-NO2 bond scission. However, we found good linearity and stability of the calibration curves for these compounds and the nitrosoamine. For these compounds standard samples are readily available, making quantitative conversion less important, and thus this detector provides a highly sensitive and selective alternative to the use of ECD or FID detection. The dection limit achievable for nitro-PAH using this detector (i.e. 0.05 pmol) compares favorably to that reported for the thermal energy analyzer (TEA) of -0.3 ng (Le. -1 pmo12' ). As both detectors are based on pyrolysis of the nitro-PAH to yield NO2, the difference in detection limits is a reflection of the inherent sensitivity of the luminol chemiluminescence detector. The TEA detector has also been used for detection of N-nitrosamines, with detection limits on the order of 3 pmo1,28 i.e. over 1 order of magnitude greater than for the luminol detector. (24) Adams, G. F. Annu. Rev. Phys. Chem. 1992, 43, 311-340. (25) Tsang, W.; Robaugh, D.; Mallard, G. W. J. Pfiy. Chem. 1986,90,5968-5973. (26) Tsang, W. Annu. Rev. Phys. Chem. 1990,41, 559-599. (27) Tomkins, B. A.; Brazell, R. S.;Roth,M. E.; Ostrum, V. H. Anal. Chem. 1984, 56, 781-786. (28) Billedeau. S.M.; Thompson, H. C., Jr. J. Cfiromafogr.1987,393,367-376.
The detection of N-nitrosodimethylamine by the pyrolysis/ chemiluminescence process raises an interesting question as to the mechanism for NO2 production for this compound. On the basis of several studies with a similar compound, dimethylnitroamine (DMNA), as discussed in the review by ad am^,^^ we expect the dominant thermal decomposition process for NNDA to be N-NO bond scission to yield NO, which is not detected by the chemiluminescence detector. Injection of pure N O confirmed that N O is not detected, nor converted in the pyrolyzer to N02. Therefore there must be other decomposition pathways possible, perhaps heterogeneous, for NNDA. In any case, this detector may be very useful for this particular species, as the nitrosamines are often present in very complex environmental samples, and we find that the ECD showed no response to this compound.
CONCLUSIONS We have developed a new detector that responds selectively to organic nitrates, believed to be important atmospheric components of active nitrogen. Since it has been shown that not all of the measured atmospheric odd nitrogen is accounted for and that this detector responds identically to all organic nitrates (i.e. mononitrates), this detector may aid in identification of the unknown N-containing compounds. The detector is particularly useful for determination of organic nitrates for which standards are not available. We find that the sensitivity for the prototype instrument is 1OX lower than for the ECD, but could be significantly improved through improvements in the solution delivery system, and/or the composition of the luminol solution. With appropriate selection of the pyrolyzer temperature, the detector can also respond with high sensitivity to a variety of nitrocompounds that are used as explosives and to mutagenic nitro-PAHs that are prevalent in the very chemically complex atmospheric particulate matter extracts. For both gas-phase and particulate-phase atmospheric samples, the luminol detector provides a distinct selectivity advantage for detection of N-containing compounds, in the presence of the substantial variety of halogenated and oxygenated compounds found in such samples. ACKNOWLEDGMENT This project was supported by the Environmental Technologies Program (project no. ET0066AN) of the Ontario Ministry of the Environment. We thank Liz Sahsuvar for her assistance with this project. Recelved for review August 4, 1993. Accepted June 18, 1994.' *Abstract published in Advance ACS Absfracrs, August 1, 1994.
Analytical Chemistry, Vol. 00, No. 21, November 1, 1994
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