Anal. Chem. 1993, 65, 2472-2477
2472
Development of a Gas Chromatograph for Trace Level Measurement of Peroxyacetyl Nitrate Using Chemical Amplification Pierrette Blanchard,*p+Paul B. Shepson,?Harold I. Schiff,tgt and John W. Drummondt Department of Chemistry and Centre for Atmospheric Chemistry, York University, North York, Ontario, M3J lP3, Canada, and Unisearch Associates, 222 Snidercroft Road, Concord, Ontario, Canada
A gas chromatographic technique for trace level detection of peroxyacetyl nitrate (PAN) that relies on postcolumn chemical amplification as a means of improving the detection limit has been developed. The GC detector is based on luminol chemiluminescence detection of Not. PAN eluting from the chromatographic column is thermally decomposed, resulting in a free-radical chain reaction in which NO is converted to NO2, using CO as a chain carrier. The system operates at optimized carrier gas NO and CO concentrations of 6 ppm and 8%,respectively. Under these conditions, the experimentally determined chain length (NO2 detected per PAN injected) for [PAN] < 1 ppb is 180 f 20. The imtrumeat is found to exhibit a slightly nonlinelir response to PAN concentrations, in the raage 0.05-3 ppb, based on comparison with a GC-ECD. The chemical amplification GC was compared against a series of ambient air PAN measurements conducted using a GC-ECD instrument in December 1992 at a rural Ontario site. For ranging between
method for trace level ambient PAN measurement without sample preconcentration.
INTRODUCTION Peroxytrcetyl W W W PA&, C&+(5(0)00N02]is a secondary photochemical )ml!utant and constitutes, due to its highly temperature dependent thermal decomposition,' an important temporary reservoir of N o x a 2Vertical profiles of PAN show that there is a free tropospheric reservoir of PAN3 which, through downward transport and subsequent decomposition, could lead to source of NO, and radicals a t the surface. PAN transport and chemistry can therefore have a significant impact on the global distribution of 03. In the free troposphere, PAN can constitute as much as 40% of odd n i t r ~ g e n .An ~ assessment of the importance of PAN in global oxidant chemistry requires measurement of its vertical profiles, particularly in the tropics, where most of the global
hydrocarbon oxidation occurs due to the relatively higher NMHC and OH concentration^.^ Although PAN can be transported from the free tropospheric reservoir to the boundary layer, because of its short thermal lifetime ita surface concentration can be very low. In fact, Muller and Rudolph6 measured PAN over the tropical Atlantic using a gas chromatograph equipped with a cryogenic sample preconcentration system. Although the detection limit of their instrument was 0.4 ppt, for a latitude band between 30° N and 30° S their observed PAN concentrations were consistently below the detection limit. Though the cryogenic preconcentration technique is viable, it is somewhat cumbersome and can limit the achievable time resolution. In order to evaluate the global importance of PAN, improvements in ultratrace measurements in remote areas are necessary. I t has recently been shown that the chromatographic separation of PAN, followed by thermal conversion of PAN to NO2 and subsequent detection of NOz, is an attractive alternative approach for ambient air measurements.' Although this method proved successful for continental air masses, the detection limit was reported to be of the order of -25 ppt, insufficient for trace level measurements. However, a substantial improvement in this method can be achieved by taking advantage of the fact that PAN decomposition yields peroxy radicals. Cantrell et al.8have developed a technique for the measurement of atmospheric peroxy radicals (ROz and HOz) using chemical amplification, involving peroxy radical chain oxidation of NO to NOz. In this paper we describe the development of a PAN gas chromatograph [chemical amplification GC, (CA-GC)] that employs postcolumn chemical amplification of the conversion of PAN to NOz, followed by luminol-based chemiluminescence detection to achieve ultratrace detection without preconcentration.
EXPERIMENTAL SECTION Instrument Description. The principal of chemical amplification involving PAN, NO, and CO has been described by Hastie et a1.,9whousedPANasa peroxy radicalsource to calibrate a radical amplification H02/R02 detector. Thermal decomposition of PAN, in the presence of large concentrations of NO and CO, leads to peroxyacetyl radicals, which (in the presence of NO) ultimately produce an OH radical, as shown in reactions 1-6 below. Then, through reactions 6 and 7, a chain is established
(5) Kanakidou, M.; Singh, H. B.; Valentin, K. M.; Crutzen, P. J. J. Geophys. Res. 1991, 96,15395-15413. (6) Muller, K. P.; Rudolph, J. J. Atmos. Chem. 1992, 15, 361-367. (7) Blanchard, P.; Shepson, P. B.; So, K. W.; Schiff, H. I.; Bottenheim, J. W.: Gallant, A. J.: Drummond. J. W.:. Wone, -. P. Atmos. Enuiron. 1990. 24A, 2839-2846. (8) Cantrell, C. A.; Stedman, D. H.; Wendel, G. J. Anal. Chem. 1984, H.I.;Mackay,G.I.;Karechi,D.R.;Davis,D.D.;Bradshaw,J.D.;Rodgers, 56, 1496-1502. M. 0.; Sandholm, S. T.;Torres, A. L.; Condon, E. P.; Gregory, G. L.; Beck, (9) Hastie, D. R.; Weissenmayer, M.; Burrows, J. P.; Harris, G. W. S. M. J. Geophys. Res. 1990, 95, 10179-10192. Anal. Chem. 1991, 63, 2048-2057. (4) Ridley, B. A Atmos. Enuron. 1991, 25A, 1905-1926. t York University.
Unisearch Associates. (1) Hendry, D. G.; Kenley, R. A. J . Am. Chem. SOC.1977, 99, 3198. (2) Singh, H. B.; Hanst, P. L. Geophys. Res. Lett. 1981, 8, 941-944. (3) Ridley, B. A,; Shetter, J. D.; Gandrud, B. W.; Salas, L. J.; Singh, H. B.; Carroll, M. A.; Hubler, G.; Albritton, D._L.; Hastie, D. R.; Schiff, t
0003-2700/93/0365-2472$04.00/0
0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993 1000
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Solution A Solution B
-
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ii
/
Post-Column Ampiincatlon Chemicat Ampiincatlor
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[ N O Z I . PPb
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Flgure 2. Detector response for two different luminoi solutions as a function of NOp concentration (in the presence of 3 ppm NO).
Lumlnoi
Permeation Source
Exhaust
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Fburo 1. Chemical amplification QC schematic.
producing several NO, for each HO2 produced in reaction 5. The principal radical termination steps are reactions 8-10. CH,C(O)OONO,
F?
CH,C(O)OO'
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CH,C(O)OO'+NO-CH,C(O)O'+NO,
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CH30' + 0, HCHO + HO, HO, + NO OH + NO,
---
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01
OH + CO HO, + CO, (7) OH + NO HONO (8) HO, + wall products (9) HO, + NO, HO,NO, (10) The instrument described here relies on use of a postchromatographic column reactor in which the PAN thermally decomposes to initiate the process described above. In Figure 1,we present the main componentsof the system. The instrument was designed to use ambient air as the carrier gas, thereby simplifying use for aircraft-based measurements. In the load position the sample air is drawn in through a 6.3-cm3sample loop at a rate of -600 cm3 min-l. A portion of the compressed air is used to pressurize the luminol solution delivery system (not shown). The sample air is pumped through an activated charcoal trap and a drying trap (CaSO,), followed by an oven heated to 110 OC where the remaining PAN in the carrier is converted to NOz. After the oven, NO from a 50 ppm standard mixture in NZ and pure CO (Canadian Liquid Air) are metered into the carrier gas stream using mass flow controllers (Tylan). From the addition point, the carrier gas (air/NO/CO) is passed to a FeS04 trap which reduces any NO2 present to NO,l0thus producing a stable detector baselineindependentof variable ambient concentrations of NO, and PAN (the detector responds to PAN as well as NO,). The carrier gas then passes at a rate of 125 mL min-l to a glass column (3.8 mm i.d. X 50 cm) packed with 5% Carbowax 400 on Chromosorb G-AW, heated to 37 OC. In the inject position, the (10)Ridley, B. A.; Carroll, M. A.; Torres, A. L.; Condon, E. P.; Sachse, G. W.; Hill, G . F.; Gregory, G . L. J. Geophys.Res. 1988,93,15803-15811.
contents of the sample loop are swept directly to the column, where PAN is separated from other potential interferences. The PAN retention time is -3.2 min. The eluting PAN peak then passes to the chemical amplification reactor. The reactor is a glass cylinder (2 cm i.d. X 11cm length), 9 cm of which is heated to 120 OC. At this temperature, the PAN lifetime (l/kl) is 3 X lo-, s.ll The amplification occurs in the heated section. The reaction time (defined as time to produce 90% of the final NO,) of -0.75 s corresponds to -0.5 cm of reactor length. The carrier gas that contains the amplified NO2 peak then passes to an NO, permeation source where a constant amount of NO2 (-30 ppb) is added in order to improve the linearity of the detector. It is then transferred to a Scintrex/ Unisearch LMA-3 Luminox NO2 detector, which detects NO, through the chemiluminescent reaction between luminol (3aminophthalhydrazide)and The luminolflow is controlled with a liquid flow controller, necessaryfor reduced baseline noise. The signals from the detector were integrated using an HP3393 integrator. Because this system responds only to species that elute from the columnand that can thermally decomposeto yield either radicals or NO2, there are few potential interferences. Therefore only a small degree of chromatographic separation is necessary. Thus, with the short retention time for PAN, injections are possible every 6 min. It is important to note that proper system exhaust and leak tests are essential safety considerations when working with these levels of CO. For large concentrations of NO (>-0.5 ppm), a decrease in sensitivity for the LMA-3 detector has been reported.8 This results in a limitation to the amount of NO that should be added to the system (see below). Moreover,a nonlinear behavior of the LMA-3 detector for NO, calibrations in the presence of large concentrations of NO has been reported.l3 This was indeed observed in our system using the "luminol" solution provided by Unisearch (proprietary; named solution A here). The detector linearity can be improvedthrough adjustment of the composition of the luminol solution. The solution we used (B)is that recommended by Cantrell et al.,13containing 0.2 M Na~SOa,0.05 M NaOH, 1 X 1W M luminol, and 1.5 X lo-' M EDTA. In addition, we have added a surfactant (at 0.1% 1, which causes the solution to flow homogeneously over the wick surface and thus reducesthe baselinenoise. NO2 calibration curvesin the presence of 3 ppm NO for the LMA-3 detector using formulations A and B are presented in Figure 2. The NO2 standards were sent directly to the LMA-3at a flow rate of 125mL min-l. Although solution B yielded a decrease in sensitivity of the detector, it resulted in an improvement of the detector linearity. This soiution was adopted for all experiments. Laboratory Evaluation. A series of experiments was conducted to evaluate the optimum geometry of the reactor with
-
(11)Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson,R. F., Jr.; Ken, J. A.; Troe, J. Znt. J. Chem. Kinet. 1989,21,115-120. (12)Schiff, H. I.; Mackay, G . I.; Castledine, C.; Harris, G. W.; Tran, Q. Water, Air, Soil Pollut. 1986,30,104-114. (13)Cantrell,C.A.;Shetter,R.E.;Lind,J.A.;McDaniel,A.H.;Calvert, J. G.;Parrish,D.D.;Fehsenfeld,F.C.;Buhr,M.S.;Trainer,M.J. Geophys. Res. 1993,98,2897-2909.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
respect to maximizingthe chain length. In this paper we define the chain length as the number of NO2 produced per PAN injected (the PAN injected is relatively rapidly converted to HO2 in the reactor). Under optimizedconditions, the dominant termination reaction should be the loss of HOz radicals at the reactor walls (see Results and Discussion and reaction 9). Thus, in principal, greater chain lengths would be achievable by minimizing the surface/volume ratio, leading to a prediction that a spherical reactor is optimum. Tests were conducted using a 10-cm3 spherical glass reactor, where the surface/volume ratio was 2.2 cm-1. However,this geometryled to lower chain lengths compared to an open tube reactor (0.38 cm i.d. X 117cm length), for which the effective surface/volume ratio was 10 cm-I. Presumably laminar flow conditions (achievedin low-flowopen tube reactors) result in a significantly lower HO2 wall loss rate, relative to the turbulent conditionsthat exist in the sphericalreactor. Therefore all further systemtesting was conducted using open tube reactors. The wall loss of HOz is dependent on the transport time to the wall, which increases with increasing diameter, and on the wall affinity for HO2. To evaluate the dependence of chain length on reactor diameter, a series of experiments was conducted using a 0.38-cm-i.d. tube reactor and a 2 cm i.d. X 11 cm cylindrical Pyrex reactor. The 0.38-cm4.d. reactor was operated in a fashion similar to the chemical amplifier of Hastie et al.,B where the first 2 cm of the tube was heated to 200 OC and the chemical amplification occurred downstream of the heated section. For the 2-cm-i.d. reactor a 9-cm length was heated to 120 OC, corresponding to all the reaction zone. It is desirable for the entire reaction zone to be heated so that the HO2 + NO2 reaction becomes unimportant. Although an even larger diameter would lead to larger chain lengths, there is a limitation due to loss of chromatographic resolution. Regardingthe wall affinity for HOz, one would expect the HOz wall loss rate to depend on the nature of the surface. However, we found no significantdependence of chain length on the reactor surface, using Teflon, and Pyrex treated with Silon CT, with H3BOa,or with HsPO4. These results are in agreement with the HOz wall loss experiments reported by Cantrell et aL8 Thus for all experiments reported here untreated Pyrex was used. Experiments were conducted to determine the dependence of the chain length on the CO and NO concentrations. For these experiments gas-phase PAN standards were prepared using solutionsof PAN in dodecane,synthesizedas described by Nielsen et al.,14 followed by extraction of the PAN into dodecane.16 To prepare dynamic gas-phase PAN standards, the PAN solution, maintained at 0 "C, is purged with Nz, using a small fritted impinger bottle. The PAN/N2 mixture is then diluted to low ppb levels with clean air metered with a mass flow controller. Clean air is provided by compressed air passed through an AADCO Model 737-12A clean air generator. For a fixed PAN sample concentration (- 1.5 ppb),the chain length was measured for both the 0.38-cm-i.d. and the 2-cm4.d. reactors as a function of [CO], for different NO concentrations. During these experiments, the responseof the CA-GCluminoldetector was calibrated using standard mixtures of NO2 in air (bypassing the column). The NO2 was prepared using a standard cylinder of NO in Nz (Scott) which was continuously diluted in dry air to low ppb levels, followed by conversion of the NO to NO2 using a CrOa converter.16 Two different methods were used to calculate the experimentally determined chain length for the reactors. For evaluation of the performance of the 0.38-cm4.d. reactor, the chain length was obtained by division of the NO2 peak area obtained from injections of PAN in the presence of NO and CO, by the peak area obtained for the same PAN sample in the absence of CO/ NO. It was then necessary to make a correction for the detector sensitivity change in the absence of NO, determined from independent experiments (factors of -2.3 and -3.3 for 2 and 3 ppm [NO], respectively). For the 2-cm4.d. reactor, a direct NO2 calibration in the presence of NO was done. The chain
length was calculated by dividing the NO2 peak area obtained in the presence of NO and CO by an NO9 calibration factor obtained in the presence of NO. The chain length was then obtained by dividingthe NO2concentration obtained by the actual PAN concentration determined using a calibrated gas chromatograph equipped with an electron capture detector (GCECD). A set of experiments was designed in order to evaluate the linearity of response of the chemical amplification GC (usingthe 2-cm-i.d. reactor) by comparison to the response of a GC-ECD to varying concentration PAN samples. GC-ECD instruments are widely used for atmospheric PAN measurements and are known to exhibit a linear response as a function of [PAN], for concentrations between 0 and 40 ppb.17 The PAN concentrations in flowing PAN/air mixtures (prepared as described above), ranging from 0.08 to 2.5 ppb, were determined using a previously calibrated GC-ECD. The GC-ECD was equipped with a 3.8 mm i.d. X 105 cm packed column (10% Carbowax400on Chromosorb G-AW),maintained at 35 OC, and the ECD (ValcoModel 140BN) temperature was 50 OC. The carrier gas was 5% CHJ95% Ar at a flow rate of 75 mL min-l. The GC-ECD was calibrated using PAN standards synthesized as described above. For calibration of the GC-ECD, the PAN concentration in the flowing air stream (i.e., the standard) was determined using a Monitor Labs Model 8840 chemiluminescenceNO, monitor.la The NO, monitor was calibrated with the same NO/N2 standards used to calibrate the CA-GC luminol detector. An effect of humidity on the chromatographicanalysis of PAN has been reported before.lem A series of tests was conducted in order to assessthe nature of any humidity problem in our system. Potential humidity effects on the chromatography were investigated by sending humidified PAN samples to the instrument in the absence of NO and CO, while effects on the chain length were investigated with NO and CO present. Humidified PAN/ air mixtures were prepared in a dynamic (flowing)mode, as well as in Teflon bags, where water waa injected to the bag (-1-2 mL) in order to produce a humidified sample. In both cases the humidity was measured at the sampling point using a dew point hygrometer (E.G.&G. Model 911). Ambient Measurements. To evaluate the performance of the CA-GC for actual atmospheric samples, ambient air measurements were conducted in which the CA-GC PAN determinations were comparedto those from a GC-ECD. For the ambient air measurements both GCs were calibrated using the following procedure. A high-concentration (- 100ppm) PAN/Nz sample was injected into a preparative GC-ECD. The eluted PAN peak was then collected,using a three-way valve upstream of the ECD, in a Teflon bag (80-200 L) previously fiiled with dry air. The PAN concentration in the bag was monitored using a Monitor Labs NO, monitor as the PAN mixed into the bag. After -2-3 min when the NO, level became constant, 2-4 ppm NO2 was injected to the bag to prevent the thermal decay of PAN. Under these conditions the PAN concentration was very stable. Several bags of different PAN concentrations were obtained to generate a calibration curve, and for each bag, multiple injections to both GCs were done. The CA-GC was operated at NO and CO concentrations of 6 ppm and 8%, respectively. The ambient air measurements were conducted in Hastings, ON (a rural area 150 km northeast of Toronto) for the period November 26 to December 10, 1992. Injections were conducted every hour for the GC-ECD, and every 6 min for the CA-GC. Air samples were taken from a glass manifold which waa continuously flushed at high flow rate with ambient air. From the manifold, the sample air for each GC was drawn through conditioned '/E-in.-o.d. stainless steel lines equipped with PFA Teflon filters (5 Km).
(14)Nielsen, T.; Hansen, A. M.; Thomsen, E. L. Atmos. Enuiron. 1982, 16,2447-2450. (15)Gaffney, J. S.;Fajer, R.; Senum, G. I. Atmos. Enuiron. 1984,18, 216-218. (16)Levaggi, D.;Kothny, E. L.; Belsky, T.; de Vera, E.; Mueller, P. K. Enuiron. Sci. Technol. 1974,8,348-350.
D.J.; Tuck, A. F.; Vaughan, G. Atmos. Enuiron. 1984,18, 2691-2702. (18)Winer, A. M.;Peters, J. W.; Smith, J.P.;Pitts, J. N., Jr. Enuiron.
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RESULTS AND DISCUSSION
To determine the optimum operating conditions,the chain length was measured as a function of [COI and [NO],for (17)Brice,K.A.;Penkett,S.A.;Atkins,D.H.F.;San&,F. J.;Bamber,
Sci. Technol. 1974,8,1118-1121. (19)Holdren, M. W.; Rasmussen, R. A. Enuiron. Sci. Technol. 1976, 10,185-187. (20)Lonneman, W. A. Enuiron. Sei. Technol. 1977,11,194-196.
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in kg. For the 0.38-cm4.d. reactor, the results presented in Figure 3 (top) indicate an increase in chain length of -30% as NO increases from 2 to 3 ppm. Furthermore, a model simulation, discussed below, using k9 = 5 s-1 but for [NO] = 6 ppm, yields a computed upper limit chain length of 110, as shown in Figure 3. However, as mentioned above, an increase in [NO] also reduces the detector sensitivity, and thus a trade-off is required. Therefore for fixed [NO], eq I implies that the chain length is best increased by decreasing kg, the HOz wall loss rate constant. As mentioned previously, the wall loss rate constant is partly dependent on the transport time to the walls, which increases with increasing reactor tube diameter. We find that, for the 2-cm-i.d. reactor for [NO] = 3 ppm and [COI = 6%, the chain length obtained is 130 f 20 (Figure 3, bottom) compared with 75 f 25 for the 0.38-cm4.d. reactor at the same [NO] and [COI (Figure 3, top). To conduct an explicit analysis of the data, model simulations were done, using the Acuchem program21(mechanism in Appendix) for [PAN] = 1.5 ppb. The simulation results are also shown in Figure 3. For the simulations of the 0.38cm-i.d. reactor, the rate coefficient for the decomposition of PAN is assigned for 200 "C, that of the heated section of the reactor, whereas the other reaction rate coefficients are set for room temperature. This stems from the fact that the amplification occurs within a volume of 2 cm3, corresponding to a reactor length of -18 cm. Thus the bulk of the radical chemistry occurs downstream of the heated section (only 2-cm length), where room temperature can be assumed. Although this is an approximation, it is a reasonable one, since none of reactions 2-10 are highly temperature dependent. The simulations for this reactor indicated a best fit to the data for an HOz wall loss rate coefficient of 5 s-1 (Figure 3, top). For the 2-cm-i.d. reactor, the chemistry occurs a t 120 OC and the rate constants were adjusted accordingly. A best fit to the data was obtained for kg = 2.5 s-1 (Figure 3, bottom). For related simulations of an ROz radical amplifier with 0.38cm4.d. reactor geometry, Hastie et al.9 reported a value for k g of 2.5 s-1 and a chain length of -240 (for radical concentrations of -50 ppt, [NO] and [COI of 6 ppm and 874, respectively). For a PAN concentration of 50 ppt, we calculated a chain length of 206. Our model included the reaction CH30 + NO CH30N0, which resulted in a slightly lower chain length. Cantrell et al.13 have also reported a computedchain length, using Acuchem, of 220 where [NO] = 3 ppm, [CO] = l o % , kg = 2.5 s-l, and [HOzl = 50 ppt. For similar conditions we obtained a chain length of 180. All the above considerations require a compromise between need for higher [NO] vs loss of detector sensitivity, need for high [COl/ [NO] vs the need to stay below the explosion limit for [COI, and the need for low surface/volume ratio vs need to maintain chromatographic resolution. The combined results of these considerationsled us to select use of the 2-cm-diameterreactor at 8% CO and 6 ppm NO for detailed investigation of the viability of this method. The linearity of response of the CA-GC was evaluated by comparison to a GC-ECD. In Figure 4 we present the results of a PAN calibration curve where the CA-GC area counts were plotted as a function of the concentration of PAN injected, as determined from simultaneous GC-ECD injections. The concentrations of NO and CO in the CA-GC carrier gas were 6 ppm and 8%,respectively. Replicate injections of PAN samples for these operating conditions indicated a precision of -f4 % (one standard deviation). As can be seen from Figure 4, there is a slight nonlinear behavior observed. The data were fitted with a second-order regression, with an
-
r!,
,
,
,
,
,
,
0 Exp. ReauIta, [NO]= 2 Ppm
,
0 Exp. R a r u l t r , [NO]= 3 Ppm . - - M o d e l Sirnulotion. [NO]- 2 Ppm. k p = 5 --Modal Sirnulotion. [NO]= 3 ppm. k p = 5 s _ I . Model Sirnulotion. [NO]= 6 PPm. kn- 5 S
.'
20
-
250 225
r
7
200
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175
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150
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125
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100
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75
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0 C
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0 Exp. Results. [NO]-6
Simulotion. [NO!-6
--Model
0
1
2
3
4
5
6
kB- 2 . 5 pprn. kn* 2.5
7
-
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a
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[CO], %
Flgurr 3. Experlmental results and model simulatlons of the chain lenm for [NO] = 2,3,and 6 ppm as a functlon of CO concentratlon, for ke = 5 s-l (top) and k8 = 2.5 s-1 (bottom).
both reactors. In the limit of very low [PAN], the important chain termination steps are reactions 8 and 9. Under these conditions, a steady-state analysis of reactions 1-9 (for a reaction time long enough to allow reactions to go to completion) leads to the following equation for the chain length, Q:
(1) The number 3 in eq I results from the NO2 produced in reactions 1,2, and 4. In Figure 3 (top), we present the chain length calculated from eq I, as a function of [COI, for NO concentrations of 2 and 3 ppm and kg = 5 s-l. The first term of eq I describes the initial part of the curve, where there is a competition between OH reaction with CO (reaction 7) and termination by reaction with NO (reaction 8), and thus the chain length is sensitive to the [COI/[NOl ratio. At sufficiently high [COI/[NOl, the [H021/[OHl ratio is large, reaction 9 becomes the dominant termination step, and the first term becomes unimportant. Under such conditions the chain length approaches (ks[NO]/kg) 3 and is independent of [CO]. The results of the chain length measurements are presented in Figure 3 for the 0.38-cm-i.d. open tube reactor (top) and the 2-cm-i.d. reactor (bottom) for injected PAN concentrations of -1.5 ppb. The observed chain length is slightly lower than predicted by eq I because, for 1.5 ppb of PAN, the reaction HOz + NO2 (reaction 10) contributes (-11%) to radical termination. However, the overall dependence of chain length on [CO] ,as described by eq I, agrees well with the observations. According to eq I, a t high [COI/[NOl [where Q = (kdNOI/ kg) + 31, an increase in chain length could be achieved either through an increase in NO concentration or through adecrease
+
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(21) Braun, W.; Herron, J. T.; Kahaner, D. K. Znt. J. Chem. Kinet. 1988,20, 51-62.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
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0.5
t
o.8
0.0
0.0
A
0.5
1 .o
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I
2.5
150
5 125 m
:
75
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0.0 I 25
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Flgurr 4. Signal obtained from the chemical amplification =-as a functlon of PAN concentratlons measured by a QGECD. 200
,
I
I
29
i
1 3 December Date
I
,
5
7
9
11
Flgurr 6. Comparison of ambient alr PAN measurements conducted using the CA-GC and a QGECD. Calibrationsare Indicated by vertical
lines.
I
I
t1
I
to recall that the carrier gas was dried using a CaSO, trap. When NO and CO were absent, the PAN signal observed did not vary significantly with increasing humidity, indicating no humidity-dependent chromatographic losses. This is in agreement with the findings of Lonneman.20 However, when NO and CO were present, a decrease in chain length of the order of 30-40% was seen for the flowing humidified PAN samples. For the static samples, a decrease in chain length of 20 % was seen. Although there is a discrepancy between the dynamic and the static sample results, a definite decrease in chain length was seen for humidified PAN samples. Above a relative humidity of 2 % ,the signalwas invariant to relative humidity. It is conceivable that changing heterogeneous reaction rates in the reactor (at 120 "C) could possibly explain these results. This humidity dependence is potentially important for the radical amplification RO, detectors and should be further investigated. To evaluate the performance of the instrument under ambient conditions, a series of measurements with the CAGC was made for a 2-week period and compared to the GCECD data. Simultaneous calibrations of both instruments, as described above (using humidified "bag" standards), were conducted periodically during the study. In Figure 6, we plot the resulting PAN concentrationsdetermined by the CA-GC and the GC-ECD. Since the injection times did not correspond exactly, the CA-GC PAN concentrationswere obtained from the average of three consecutive injections that bracket the hourly GC-ECD injections. Points in time when PAN calibrations were conducted are indicated by vertical lines. Overall, over the observed concentration range of 0.04-0.5 ppb, the two determinations were fairly well correlated. Peroxypropionyl nitrate (PPN) has been reported to be on the order of -8% of ambient air PAN concentrations.23The CA-GC is expected to measure PPN as PAN, and the PPN is not chromatographically separated from the PAN. Hence, the CA-GC should overestimatethe PAN by 8 % compared to the GC-ECD. To evaluate the correlation between these two instruments determinations, we plotted the ratio [PANI~A.~J[PAN]GCLE~D vs [PANIGC-EDin Figure 7. The average ratio was 1.12 f 0.02 at the 95% confidence limit. The fact that the CA-GC system yields PAN concentrations that are, on average, 12% higher than those from the GCECD is consistent with simultaneous detection of PPN by the CA-GC. There is considerable scatter at the low concentration end, i.e., for [PAN] < 0.15 ppb. For these measurements the detection limit for the CA-GC was -0.015
-
-
-Model
1
I
3 s-'
Simulation, k,= I
,
I
-
-
(22) humelis, N.;Glavas, S. Anal. Chem. 1989,61, 2731-2734. (23) Shepeon, P.B.;Hastie, D.R.;So,K. W.;Schiff, H. I.; Wong, P. Atmos. Ewiron. 1992,26A,1259-1270.
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 18, SEPTEMBER 15, 1993
1.5
c
0.0 1 0.0
ACKNOWLEDGMENT
. ..
I
* .
The authors thank Dr. D. R. Hastie and Dr. M. Mozurkewich for helpful discussions, and K.w. So for technical assistance. This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. The work described herein was undertaken as part of the Canadian Institute for Research in Atmospheric Chemistry (CIRAC) project "Atmospheric Chemistry and Toxicology of Peroxyacyl Nitrates" and is Scientific Contribution 92-5.
APPENDIX 0.1
0.2
0.3
I 0.4
0.5
Chemical reaction mechanism for model results:
[PANZ;C-ECD1 ppb
Flguro 7. Plot of the ratio [PANICmI[PAN][PAN]aoEm for ambient measurements.
as a function of
k at 298 Ka
reactions
PAN- CHsC(0)Oz+ NO2
ppb (defined as 3 times the standard deviation of the noise), while that of the GC-ECD was -0.02 ppb. Therefore, there is contribution to the observed scatter in this ratio from both instruments a t the low concentration end, which approaches the detection limits for both. Above 0.25 ppb, the observed ratio was 1.09 f 0.02. The detection limit obtained for our CA-GC is insufficient for sub-ppt PAN measurements. However we estimate that the improvement of S/N ratio resulting from the amplification is -50. This is less than the effective chain length because of loss of detector sensitivity from high concentrationsof NO. The detection limit currently obtained for a commercially available luminol-based instrument (Unisearch PAN LPA-4) is -25 ppt. This detection limit could be further improved by applying the chemical amplification process to such an instrument.
CH&(0)02 + NO2 PAN CHaC(0)Oz+ NO CHsCOO + NOz CHsCOO CHs02 + COz CHsOz + NO CHsO + NO2 CHsO + 02 HCHO + HOz HOz + NO OH + NO2 OH + CO HOz + COz OH + NO HONO HO2 + wall products HOz + NO2 HO2NO2 HOzNOz HOz + NO2 OH + NO2 HONOz OH + HCHO H2O + HCO HCO + 0 2 HOz + CO CHaC(0)02 + CHsC(0)02 CH&OO + CHsCOO CHsC(0)Oz+ HOz CHsC(0)OzH+ 0 2 CHsC(0)Oz+ CHsOz CHsCOOH + HCHO CHsC(0)02+ CHsOzCHsCOO + CHsO CHsOz + CHsOz CHsOH + HCHO CHs02 + CHs02 CHs0 + CHgO CHsOz + HOz CHaOOH + 02 CHsOz + NO2 CHsOzNOz CHsOzN02 CHsOz + NO2 HOz + H0z H2Oz + 02 OH + OH H2Oz OH + HOz HzO + 02 OH + Hz02 -+ HzO + HOz CHsO + NO CHsONO CHsC(0)Oz + wall products C&OZ + wall products OH + wall products +
+
+
+
+
-+
+
+
+
+
+
+
-.L
+
CONCLUSIONS
+
In this study we have demonstrated that the principle of gas chromatography using postcolumn chemicalamplification as a means of improving the detection limit is a viable technique for trace level measurement of PAN. For NO and CO concentrations of 6 ppm and 8%, respectively, and a postcolumn reactor diameter of 2 cm, the chain length obtained for [PAN] < 1 ppb was 180 f 20. The CA-GC exhibits a slightly nonlinear behavior attributed to a nonlinearity of the luminol detector in the presence of 6 ppm NO. This nonlinearity, which requires routine detailed calibrations of the instrument, could be eliminated through further adjustment of the compositionof the luminol solution. To achieve S/N ratios sufficient for detection limits below 1 ppt will require improvements in the baseline detector noise. We believe this can be achieved through a stable carrier gas system (containingNO and CO), use of a stable and relatively constant NOz permeation source, and through further improvements in the stability of the luminol solution delivery system. However, sub-ppt PAN measurements will require that the PAN can be quantitatively transmitted from sample inlet through the chromatographic system. This instrument could potentially enable trace level PAN measurements, with 6-min time resolution, without sample preconcentration.
-
+
+
+
+
+
+
+
-
-C
+
+
k at 393 KO 11
1.02 x l(r 31.3 (at 200 OC) 5.12 X 10-12 9.3 x 10-12 1.4 X 10-11 1.4 X 10-" 2.19 X 7.68 X 1.92 X 10-16 8.28 X 10-1z 2.4 X 10-1s 6.2 X 10-12 variable 1.1 x 10-12 8.5 X 1V2 1.1 x 10-11 1.11 x 10-11 6.6 X 1.66 x 10-11
6.02 X 10-18 6.64 X 10-12 4.61 X 1P16 6.81 X 10-12 2.4 X 10-IS 3.12 X 10-lz variable 8 X le13 2.3 X 102 5.46 x 10-12 1.21 x 10-11 5 x 10-12 1.08 X 10-11
3x
6.07 X
10-12
4.7 x
10-12
8.6 X
4.3 x
10-12
1.86 X
10-19
2.31 X leis
1.93 X 10-18
1.24 X
1.04 X
10-18
10-13
4.87 X
2.2 x
4 x 10-12 1.9 1.66 X 1 x 10-11 1.11 x 10-10 1.7 X 10-12 2 x 10-11 variable variable variable
1.68 X 6.6 X l(r 1.01 x 10-12 6.4 X 9.1 x 10-11 1.93 X 1.88 x 10-11 variable variable variable
10-12
First order in 8-1; second order in cm3 molecule-' 8-1.
RECEIVEDfor review April 7, 1993. Accepted June 23,
1993.' *Abstract published in Advance ACS Abstracts, August 15,1993.
