Anal. Chem. 1000. 6 0 . 816-819
016
vessels to measure gases dissolved in seawater is to use a seawater equilibrator. The equilibrator, a variant of the headspace technique, sprays seawater through a fixed volume of air from which small samples can be periodically removed and analyzed. With typical ambient seawater concentrations and solubilities (161, the concentration of DMS in the equilibrated air is 1-5 ppbv. Untile now all methods of analysis have employed sample preconcentration, either by a cryogenic freeze-out loop or by adsorption on gold wool. The ECD sulfur system in ita present form is sufficiently sensitive to measure the equilibrated DMS concentrations by direct injection.
ACKNOWLEDGMENT We thank Richard Gammon and Tim Bates for their support and John Birks and Brian Lamb for helpful discussions. Registry No. HzS,7783-06-4;OCS, 463-58-1;CH,SH, 75-18-3; DMS, 75-18-3; CSp, 75-15-0.
LITERATURE CITED (1) Zehner, J. M.; Simonatis, R. A. J . Chromatogr. Sci. 1978, 74. 348-350. (2) D'Ottavio, T.; Garber, R.; Tanner, R. L.; Newman, L. Atmos. Environ. 1981, 75, 197-203. (3) Farwell, S. 0.; Barinaga. C. J. J . Chromatogr. Sci. 1986, 24, 483-494.
Gluck, S. J . Chromatogr. Sci. 1982, 20,103-108. Nelson, J. K., Getty, R. H. Birks, J. W. Anal. Chem. 1983, 55, 1767-1770. Ledweil, J. R.; Watson, A. J.; Broecker, W. S. Nature (London) 1888, 323,322-324. Watson, A. J.; Liddicoat, M. I.Atmos. Environ. 1985, 19, 1477-1484. Benner, R. L.; Lamb, B. J . Atmos. Oceanlc Techno/. 1985, 2 , 582-589. Rasmussen, R. A.; Khalil, M. A. K.;Dalluge, R. W. Science 1981, 21 1 , 285-287. Singh, H. B.; Salas, L. J.; Stiles, R. E. J . Geophys. Res. 1983. 8 8 , 3675-3683. Meshri, D. T. J . Fluorine Chem. 1988, 33, 195-226. Lovelock, J. E. United Stales Patent 3 725 009, 1973. O'Brien, M. J. I n Modern Practice of Gas Chromatography, 2nd ed.; Grobe, R. L., Ed.; Wiley: New York, 1985; p 265. Bates, T. S.: Cline, J. D.; Gammon, R. H.; Kelly-Hansen, S. R. J , Geophys. Res. 1987, 92, 2930-2938. Andreae, M. 0. I n The Role of Air-Sea Exchange in Geochemical Cycling; Bwt-Menard, P., Ed.; Reidei: Dordrect, 1987. Dacey, J. W. H.; Wakeham, S. G.; Howes, 8. L. Geophys. Res. Lett. 1984, 7 1 , 991-994.
RECEIVED for review August 24, 1987. Accepted December 9, 1987. This work was funded by NOAA's National Acid Precipitation Assessment Program and by NOAAs Radiatively Important Trace Species (RITS) program. This work was done while J.E.J held a National Research Council-NOAA Research Associateship. This is Contribution Number 923 from the NOAA/Pacific Marine Environmental Laboratory.
Gas Chromatographic Method for Measuring Nitrogen Dioxide and Peroxyacetyl Nitrate in Air without Compressed Gas Cylinders Mark R. Burkhardt, Nyengenya I. Maniga, and Donald H. Stedman'
Department of Chemistry, University of Denver, Denver, Colorado 80208 Richard J. P a w
Environmental Monitoring Systems Laboratory, United States Environmental Protection Agency, Research Triangle Park, North Carolina 27711
A gas clwomatographlc technique that measures atmospheric concentratlons of peroxyacetyi nitrate (PAN) and NOp has been developed that uses lumlndbased chemiluminescence for detection. The carrier gas Is air that has been scrubbed by passlng il over FeSO,, whlch eilmlnates the need for any compressed gas cylhders. A novel gas sampling system and timer enable variable sample volumes of contaminated air to be InJected. Ambient PAN and NOz measurements can be made every 40 s wtth detectkn lhnfls of 0.12 ppb for PAN and 0.2 ppb for NO,. Seven other atmogpherlc species, lncludlng ozone, gave no interference. Linear response was observed for NO, from 0.2 to 170 ppb and for PAN from 1 to 70 ppb.
Nitrogen oxides (NO,) are an important component in the understanding and modeling of air pollution processes. NO, (NO + NOz) is a source of ozone in the troposphere (1)and NO, reactions produce a variety of inorganic and organic nitrates (2). Nitrates of recent concern are nitric acid, and its role in acidic deposition (3,4 ) , and peroxyacetyl nitrate 0003-2700/88/0360-0816501.50/0
(PAN), and ita role in long-range transport (5-7). Definition of the atmospheric chemistry of NO, requires a detector that can measure individual NO, components and their secondary products and one that will allow no interference from any other atmospheric constituent (8). A widely used NO, monitoring approach employs the chemiluminescence monitor based on the NO + O3 reaction (9, 10). A difficulty with this chemiluminescent method is that the monitor does not distinguish between NOz and interfering species such as HN03, HNO,, and PAN (11-13). The reduction of NOz to NO on various catalysts is not specific for NO,; that is, other nitrogen species can be reduced to NO. Gas chromatographic analysis of NOz has also been reported in the literature (14). This technique used cryogenic preconcentration and a thermal conductivity detector to measure ambient concentrations of NOz. We report on the first gas chromatographic analysis of atmospheric concentrations of NOz which used neither compressed carrier gas nor cryogenic preconcentration. Other methods for the direct measurement of NOz include the photodissociation of NO2 to NO and then reaction with O3 (15),the absorption of infrared radiation for 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 80, NO. 8, APRIL 15, 1988
817
Table I. Physical and Chemical Parameters Used by the Cylinderless Gas Chromatograph NO
I1
ir
Figure 1. Block diagram of the PAN/NO, gas chromatograph.
detection (16),and the luminol-based chemiluminescence method (17). PAN is a product of photochemical reactions between hydrocarbons and NO,. It is an eye irritant and a phytotoxicant (18-20) occurring in concentrations as high as 210 ppbv (21) in urban air masses. The most common method for measuring PAN has been a gas chromatograph equipped with an electron-capture detector (EC-GC) (13,19,20). The drawbacks to this system are (1) the detector is radioactive, causing both licensing and transportation problems, (2) the system requires cylinders of compressed carrier gas to achieve adequate separation from the major Oz peak and the other electron-capturing substances such as organic nitrates and highly halogenated species, (3) the retention time for the PAN peak is approximately 15 min, and (4) the detectors tend to lose sensitivity because of the plating effect that hydrocarbons have on the nickel foil. Another drawback with conventional gas chromatography, including EC-GC, is that the systems use fixed-volume, relatively expensive gas sampling valves. Wendel et al. (17) have described the development of an NO2detector based on the chemiluminescent reaction of NOz with a luminol-based solution. In that work, PAN gave an interfering response that was the same as or greater than that given by the same concentration of NOz. Making use of the PAN interference reported by Wendel et al. as a detection tool, we have developed this gas chromatographic method for measuring PAN and NO2 in air. We incorporated the separating potential of gas chromatography and the selectivity of the luminol-based detector in our instrument for measuring both PAN and NO2. "he method, as developed, offers several advantages over the above-described EC-GC method for PAN and the NO O3chemiluminescent method for NOz, including selectivity, portability, and the absence of compressed gas cylinders. Potiential disadvantages of the system include temperature dependence and other sources of sensitivity drift of the luminol-based detector and nonlinearity a t low concentrations of NOz and PAN. These aspects are discussed herein.
+
EXPERIMENTAL SECTION Apparatus and Procedure A block diagram of the PAN/N02 GC is shown in Figure 1. Ambient air was drawn in through the Teflon valved pressure pump (Air Cadet 7530-40) at a flow of 275 mL/min. The sampling system was maintained at a measured 17 psi (gauge) at all times by means of an adjustable leak. The air then passed through a Teflon three-way solenoid valve (General Valve 1-43-900)in the normally open (NO) mode for 52 s. In this mode it passed through a ferrous sulfate trap which converted NO2to NO and removed PAN. The ferrous sulfate trap contained approximately 100 g of oven-dried ferrous sulfate and had gn effective lifetime of 1 year. Any NO2that did pass through the trap was interpreted as part of the base-line signal for the chromatograms. This "clean" air was used as the carrier gas for the system. When the solenoid valve was changed to the normally closed (NC) mode, a 6.4-splug of "dirty" sample air was allowed directly onto the column. The sample volume could be changed by varying the amount of time the solenoid valve remained in the NC position. NO2 and PAN were separated on the column, and detection of each was by a luminol-based instrument. The results described herein were obtained with the commercially
LMA-3 detector parameters high voltage zero setting time constant pump parameters system pressure flow rate from column sample valve parameters time in the NC position time in the NO position column packing column support stationary phase trap parameters materials
-642.3 V
4.9 0.3 s 17.0 psi
120.0 mL/min 52.0 s 6.4 s
Chomosorb W DMCS/AW, 80-100 mesh 10% Carbwax 400 oven-dried FeSO,,
-100
g
available Scintrex LMA-3 Luminox instrument. The column packing was made by loading Chromosorb W DMCS/AW 80/100 mesh with 10% Carbowax 400. The column material was placed in a piece of 0.125 in. 0.d. Teflon tubing, 15 in. long, and packed. One end of the column was connected to the common (C) outlet of the solenoid valve and the other end was connected directly into the detection cell of the LMA-3 detector. The connection to the detection cell was made with a 0.25 in. to 0.125 in. Teflon reducing ferrule and 0.25 in. Teflon Swagelok nut. All surfaces exposed to the sample gas were made of stainless steel or Teflon. The instrument was conditioned by operating it for 24 h in ambient air and then sampling > 1 ppm of PAN for 1 h. Once the column had been conditioned, no loss ~ even after 10 days with the instrument of sensitivity w a observed unused. The luminol-based solution that the detector used was 0.05 M NaOH, 0.1 M Na2S03,2.0 X lo4 M luminol, and 0.05% (v/v) tert-butyl alcohol in deionized water. The composition of the solution differed from that used previously by Wendel et al. (I 7). Recent experiments show that solutions containing tert-butyl alcohol give a linear response to a lower concentration range than the methanol-containing solutions. PAN samples were synthesized by the method presented by Gaffney et al. (22) and stored as solids (pansicles) at -80 "C. Table I lists the operational parameters used for the instrument. Reagents. Luminol was obtained from Aldrich Chemical Co. and was recrystalized twice before use. NaOH, Na2S03, and tert-butyl alcohol were all obtained from the J. T. Baker Chemical Co. The Chromosorb W DMCS AW 80-100 mesh (lot no. 205) was obtained from the Johns-Manville Corp. and the Carbowax 400 from the Supelco Co. RESULTS AND DISCUSSION Parameter Optimization. We investigated the response of the system to changes in a number of parameters, including column packing, column diameter and length, sample volume, flow rates, and pressure. The column packings reported in the literature for measuring PAN (23,24)were tested and gave nonlinear responses to both PAN and NOz by luminol-based detection. The optimum column packing found in a series of tests was 10% Carbowax 400 on Chromosorb W DMCS AW, which gave a linear response for PAN over a concentration range from 1 to 65 ppb and over 0.2 to 170 ppb for NOz. We tested various diameters and lengths of Teflon tubing; and 0.125 in. Teflon tubing, 15 in. long, packed with the above-mentioned column material gave the best separation in under 1 min. The sample time, chosen as a compromise between resolution and sensitivity, was 6.4 s. Longer sample times resulted in increased overlapping of the PAN and NOz peaks. Shorter sample times resulted in better separation but lower PAN and NOz signals. In the laboratory and field studies, a sample time of 6.4 s corresponds to a sample volume of approximately 13 mL. We tested inlet pressures ranging from 10 to 18 psi. For pressures less than 12 psi, the PAN peak retention time was too close to that for NO2and we could
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
818
Table 11. Interference Tests
I
0 Y
! 1
a
w
a
B
A
0
re1 response as NOz a
a a a a a a
1 PPbb
"Response was less than the detection limit of 0.2 ppb. * Resuonse due to 1 Dub NO, generated in the ozone source.
w
X
10 10 1000 1 1 10 1 2
03
n
c 3
U
acetaldehyde
so2
-r
concn tested, ppm
C1Z COZ HCO HNOB CHSNOB
NO2
-D
chemical species
100
200
0
100
200
T I M E /Seconds1
Flgure 2. Direct copies of typical chromatograms obtained in ambient air using the PANINO, gas chromatograph. Chromatogram (A) from 4 / 3 / 8 7 shows 35 ppb of NO, with no detectable PAN peak. Chromatogram (B) from 4/6/87 shows 76 ppb of NO, and 3.5 ppb of PAN. Chromatograms shown are not to scale.
detect no separation. For inlet pressures ranging between 13 and 18 psi, the PAN peak could be resolved from the NOp peak. Deviations between duplicate injections of NOz and PAN standards were less than 2%. The flow rate from the columns into the detection cell depended on the column packing and pressure. For the experiments presented, a flow rate of 120 mL/min at a pressure of 17 psi gave retention times of 11.4 s for NOzand 36.4 s for PAN (Figure 2). The retention times were measured from the start of the injection period. Columns that gave a flow rate of 60 mL/min or less into the detection cell, at high inlet pressures, exhibited poor resolution and were repacked. Calibration curves for PAN and NOz were obtained by placing a Thermo Electron Model 14B/E NO, monitor in parallel with the luminol-based instrument and connecting the inlets of both to an exponential decay flask. The 14B/E monitor was equipped with a heated molybdenum catalyst that measures NOpand PAN (24,25). NO,-free (
