1032
Anal. Chem. 1985, 57, 1032-1035
columns can be operated simultaneously; automation is also possible. Since no dilution occurs prior to the separation of iron, no changes occur in the ratio of arsenic(II1) to arsenic(V). The oxidation of As(II1) by FeCl, and the reduction of As(V) by FeCl, in the leaching solution effectively stop after iron removal on the resin. The analytical method is useful for the study of the oxidation of arsenic(II1) to arsenic(V) during the ferric chloride leaching of arsenic-bearing minerals at moderate temperatures.
Registry No. As, 7440-38-2;FeC13,7705-08-0;HCl, 7647-01-0. LITERATURE CITED (1) Dutrizac, J. E.; Morrison, R. M. I n “Hydrometallurgical Process Fundamentals”; Plenum: New York, 1984 pp 77-1 12. (2) Yates, J. S.; Thomas, H. C. J. Am. Chem. SOC. 1956, 78, 3950-3953. (3) de Smecht, L. M.; Berube, Y. “Chemlcal Stability of Arseniferous Waste”; Arctic Land Use Research Program, Publlcatlon No. 8018000-EE-A 1; Department of Indian Affarls and Northern Development Canada, Ottawa, 1975.
(4) Morrison, R. M. “The Analysis of Arsenic(II1) by Differential Pulse Polarography in Hydrochloric Acid Hydroxylamine Hydrochloride Electrolyte”; Division Report MRP/MSL 84-56 (TR); CANMET, Energy, Mlnes and Resources Canada. Ottawa. 1984. (5) Morrison, R. M. “Application of Voltammetric Techniques to the Analysis of Arsenic-Containing Solutions”; Division Report MRP/MSL 83-59 (TR); CANMET, Energy, Mines and Resources Canada, Ottawa, 1983. (6) Hansen, L. D.; Richter, B. E.; Rollins, D. K.; Lamb, J. D.; Eatough, D. J. Anal. Chem. 1979, 51, 633-637. (7) Ricci, G. R.; Shepard, L. S.; Colovos, G.; Hester, N. E. Anal. Chem. 1981, 53, 810-613. (8) Williams, R. J. Anal. Chem. 1983, 55, 851-854. (9) Donaldson, E. M. Talanta 1977, 24, 105-110. (10) Siemer, D. D. Anal. Chem. 1880, 52, 1874-1877. (11) Bynum, M. A. 0.; Tyree, S. Y., Jr.; Weiser, W. E. Anal. Chem. 1981, 53, 1935-1936. (12) Nelson, F.; Murase, T.; Kraus. K. A. J. Chromatogr. 1964, 13, 503-535 - - - - - -. (13) Kireeva, G. N.; Nam, L. S.; Ryzhkova, V. N.;Savel’eva, V. I . ; Seieznev, V. P.; Sudarlkov, B. N. Chem. Abstr. 1976, 85, 9 9 7 0 3 ~ .
-
RECEIVED for review November 26,1984. Accepted January 9, 1985. The receipt of a Visiting Fellowship from the Natural SCknces and Engineering Research Council of Canada is greatly acknowledged (L.K.T.).
Determination of Sub-Part-per-Million Levels of Formaldehyde in Air Using Active or Passive Sampling on 2,4-Dinitrophenylhydrazine-Coated Glass Fiber Filters and High-Performance Liquid Chromatography Jan-Olof Levin,* Kurt Andersson, Roger Lindahl, and Carl-Axel Nilsson
National Board of Occupational Safety and Health, Research Department in Umeb, Box 6104, S-900 06 Umeb, Sweden
Formaldehyde Is sampled from air wlth the use of a standard mlnlature glass flber fllter Impregnated wlth 2,4-dlnltrophenylhydrazine and phosphorlc acid. The formaldehyde hydrazone Is desorbed from the fllter with acetonitrlle and determined by high-performance llquld chromatography using UV detection at 365 nm. Recovery of gas-phasegenerated formaldehyde as hydrazone from a 13-mm Impregnated fllter Is 80-100% In the range 0.3-30 kg of formaldehyde. Thls corresponds to 0.1-10 mg/m3 In a 3-L alr sample. When the fllter sampling system Is used In the active mode, air can be sampled at a rate of up to 1 L/mln, affording an overall sensttlvlty of about 1 pg/m3 based on a 60-L alr sample. Results are given from measurements of formaldehyde In Indoor alr. The DNP-coated fllters were also evaluated for passive sampling. I n thls case 37-mm standard glass fibers were used, and the sampling rate was 55-65 mL/mln in two types of dosimeters. The diffusion samplers are especially useful for personal exposure monitoring in the work environment.
Formaldehyde is an extremely important industrial chemical. Approximately 5.2 billion pounds of formaldehyde were consumed in 1983 in the manufacturing of more than 29 types of products and in the performance of medical services. About 50% of the formaldehyde produced is used to make resins for adhesives in the manufacture of particleboard, fiberboard, and plywood. More than 1.5 million workers are potentially exposed to formaldehyde (1). The threshold limit value for occupational exposure in the U.S.is 1 ppm (2). Also, large groups of the general public are exposed to low levels of
formaldehyde, since it is a ubiquitous contaminant of indoor air. A number of analytical methods for the determination of formaldehyde have been published. The most widely used methods are spectrophotometric (3, 4 ) . These have been reported to suffer from a number of negative interferences, however (5), and the sensitivity is rarely sufficient for measuring indoor air levels. More recently, chromatographic methods have been used for determining formaldehyde or formaldehyde derivatives, such as hydrazones. One of the most rapid and sensitive methods is high-performance liquid chromatographic determination of the 2,4-dinitrophenylhydrazone of formaldehyde. This method has been used by several investigators (6-8). Various devices have been developed for sampling formaldehyde from air. Most methods employ impinger collection ( 3 , 4 ,6,8), but bubblers or impingers are not convenient in field investigations, especially not in personal monitoring of worker exposure. In personal monitoring, solid adsorbents are preferable for sampling. The first method for sampling formaldehyde using a reagent-coated solid adsorbent was reported by us in 1979. The method utilized chemosorption of formaldehyde on 2,4dinitrophenylhydrazine (DNP)-coated Amberlite XAD-2 (9). The method was later evaluated for acrolein and glutaraldehyde (10) and for simultaneous sampling of formaldehyde, phenol, furfural, and furfuryl alcohol (7). Other workers have subsequently used DNP-coated solid sorbents with various solid supports, such as silica (11),glass beads (8, 121, and octadecylsilane-bonded silica (13). Other methods utilizing solid sorbent sampling of formaldehyde include the use of a 13X molecular sieve (14), impregnated active charcoal (151,
0 1985 American Chemical Society 0003-2700/85/0357-1032$01.50~0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
stainless steel grid glass fiber
Table I. Dosimeter Constants and Calculated Formaldehyde Sampling Rate for Filter Cassette (FC) and Organic Vapor Monitor (OVM)
filter,
DNP-coated glass fiber filter support pad
Iv
/
1033
sampler
area, cm2
diffusion length, cm
calcd sampling rate, cm3/min
FC OVM
7.1 5.1
1.0 0.9
68
U
/
54
Flgure 1. Filter cassette for 37-mmdiameter filters used for passive
sampling. A
N-benzylethanolamine-coatedChromosorb 102 (16), and Chromosorb W coated with 7-amino-5-hydroxy-2naphthalenesulfonic acid (I7). However, these methods suffer from the drawbacks of colorimetric analysis (14,In,instability of samples (I@, or large blanks (16). A few methods have been reported for passive sampling of formaldehyde (19,20). These methods are designed for colorimetric analysis and hence lack the specificity and sensitivity required for measurement of low formaldehyde levels (5). We now wish to report a rapid and very sensitive method for formaldehyde monitoring, which is a further development of our previous method using DNP-coated XAD-2 (7). The method utilizes DNP-coated glass fiber filters, which are simple to prepare and easy to handle and can be used for both active and passive sampling of formaldehyde in the range 1-10000 pg/m3.
EXPERIMENTAL SECTION Chemicals. Solvents used were methanol (Merck, p.a.) and acetonitrile (Rathburn, HPLC Grade). Formaldehyde 2,4-dinitrophenylhydrazone was prepared from formaldehyde (Merck 37%, p.a.), 2,4-dinitrophenylhydrazine (DNP) (Fluka, p.a.), and concentrated HCl and recrystallizedtwice from ethanol, mp 166 OC. Filters for Active Sampling. To 300 mg of DNP, recrystallized twice from 4 M HCl, was added 5 mL of 10% phosphoric acid (Merck, p.a.) and 5 mL of formaldehyde-free acetonitrile. Since some batches of Rathburn HPLC Grade acetonitrile contain considerable amounts of formaldehyde, the batch to be used has to be checked by running a chromatogram on the coating solution. The mixture is heated with stirring and 13-mm-diameter glass filters, organic- and binder-free (Type AE,0.3-pm pore size, SKC, Inc.), are immersed in the hot solution for a few seconds. The filters are then allowed to dry in a closed desiccator over saturated sodium chloride solution to assure a constant relative humidity. Ten filters coated in this manner showed an average weight increase of 8.3 mg, of which approximately 3 mg is DNP. To avoid breakthrough at high sampling rates (0.5-1.0 L/min), it is important that the coating should not be less than 8 mg. It is also recommended that double filters be used when sampling more than 60 L of air at 1.0 L/min. The filters can be stored for at least 1 month before use. Filters for Passive Sampling. Since the sampling rate in the passive mode is lower, a lower coating was used. To 300 mg of DNP-HC1was added 5 mL of 10% phosphoric acid and 45 mL of acetonitrile. A 0.5-mL portion of this solution (3 mg of DNP) was added to each 37-mm-diameter glass fiber filter, organic- and binder-free (Type AE, 0.3-pm pore size, SKC, Inc.). The filters were conditioned in the same way as the filters for active sampling. Recovery Experiments. For active sampling, the 13-mmdiameter coated filters were used in two-section polypropylene filter holders (No. 225-32, SKC, Inc.). The filter cassette was fitted to a glass tube, 80 X 4 mm i.d., provided with an injection port. Air of a certain relative humidity was drawn through the glass tube at a flow rate of 0.20 or 1.0 L/min. A known amount of formaldehyde in acetonitrile was injected into the glass tube, and 3-30 L of air was drawn through the filter. The apparatus used to produce humidified air has previously been described in detail (21).
Passive Sampling. In the passive sampling experiments, the 37-mm-diameter DNP-coated filters were used. The filter was mounted with a support pad in a standard polystyrene 37-mm
B
0.010AUFS
.2 I
0 min
5
i o 0 min
1
5
10
Flgure 2. Liquid chromatogram of coated filter blank (A) and desorbed 60-L air sample containing 3.6 pglm3 formaldehyde (B).
three-section filter holder (Millipore M0037AO), modified according to Figure 1, or in an Organic Vapor Monitor 3500 (OVM) (3M Co.) passive sampler, from which the active charcoal adsorbent has been removed. For the latter sampler, the filters had to be cut down to 30 mm diameter. In the filter cassette sampler, an uncoated filter was used as diffusion barrier. In the OVM cassette, the original barrier was used. Sampling rates for the passive samplers were calculated, according to Fick's law (22), from the cross sectional area ( A )in cm2and diffusion path length ( L ) in cm sampling rate in mL/min = D ( A / L ) X 60 The diffusion coefficient (D) for formaldehyde was set to 0.16 cm2/s (23). Dosimeter constants and calculated sampling rates are given in Table I. The passive samplers were evaluated in a 15-m3test chamber with a known formaldehyde concentration. The standard atmosphere was generated from formaldehyde solution via the ventilation system of the test chamber. The concentration of formaldehyde in the test chamber was monitored with parallel air samples continuously taken in the active sampling mode with a personal sample pump (Model 223-3, SKC, Inc.) at 200 mL/min. The temperature in the chamber was 23 "C, relative humidity was 55%, and an air velocity of 0.2 m/s was maintained with a small fan. Liquid Chromatography. The formaldehyde 2,4-dinitrophenylhydrazone was eluted from the filter by shaking for 1 min with 3.0 mL of acetonitrile. The solution was filtered through a Millex-SR filter and 10 pL was injected into the liquid chromatograph. A Waters HPLC system consisting of one M-6000 A pump, an M-710 B autosampler, an M-730 printer/plotter, and an M-440 absorbance detector was used. The column was a Waters Radial-PAK A (100 X 5 mm i.d., octadecyl silane, 10-pm particles). The mobile phase was 40% water in methanol, and the flow rate was 0.8 mL/min. Under these conditions, k'was 2.2 for formaldehyde 2,4-dinitrophenylhydrazone.The hydrazone was detected at 365 nm with a detection limit of approximately 0.5 ng (signal to noise ratio 3:1), corresponding to 0.07 ng of formaldehyde. Figure 2 shows chromatograms of desorbed filter blank and air sample. The formaldehyde blank is less than 0.1 pg/filter (RSD approximately 20%), corresponding to less than
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 0, MAY 1985
Table 11. Recovery of Formaldehyde 2,4-Dinitrophenylhydrazonefrom DNP-Coated 13-mm Glass Fiber Filters Using Active Sampling (Sample Flow Rate 0.20 L/min, Sample Volume 3.0 L) amt of formaldehyde added,pg 0.3 3.0d 30 3.0e
3.d
recovery at 20% recovery at 85% RH” RH rec, RSD,b rec, RSD, % % n c % % n 81 78 87 84 86
3 2 2 11 5
6 6 5
103 9 8
6 6
8 7 99
9
0
’Relative humidity. *Relative standard deviation.
6 2 3 2 4
5 5 7 6 6
Table 111. Levels of Formaldehyde in Indoor Air (Sample Flow Rate 1.0 L/min) filter no.
location
1 2 3 4 5 6 7
conference room
chem lab
0.5 wg/m3 in a 60-L air sample. Quantitation was performed using synthesized formaldehyde 2,4-dinitrophenylhydrazoneas external standard. There was no deviation from linearity in the response of the hydrazone in the range 0.5-50 pg/mL.
RESULTS AND DISCUSSION The DNP-impregnated filters were evaluated for formaldehyde sampling in active and passive sampling systems using standard amounts of gas-phase-generated formaldehyde. Active Sampling. Table I1 shows the recovery of formaldehyde as 2,4-dinitrophenylhydrazonefrom 13-mm DNPimpregnated filters at various relative humidity (RH) values, with an air flow of 200 mL/min for 15 min. As the table shows, recoveries range from 81 to 103%. Recoveries are somewhat lower at 20% relative humidity; still they are commensurate with other active sampling methods. The amounts of formaldehyde correspond to 0.1-10 mg/m3 in a 3-L air sample. When low levels of formaldehyde are monitored, larger flow rates and sample volumes can be used. The filters were evaluated for sampling up to 60 L of air with a flow of 1.0 L/min. Table I1 shows the recovery of formaldehyde 2,4-dinitrophenylhydrazoneat the 3.0-pg formaldehyde level, with a flow of 1.0 L/min and a total sample volume of 30 L. The recovery of formaldehyde is still in the same range, which shows that a sampling rate of a t least 1 L/min can be used. As Table I1 shows, storing the exposed filters for 2 weeks does not decrease recovery. The method was used to determine low formaldehyde levels in indoor air. Air samples were taken in a conference room (floor space 20 m2) with particle-board wall paneling in a building 5 years old. Samples were also taken in a well-ventilated chemical laboratory in the same building. The results are shown in Table 111. In all cases back-up filters were used. All filters had a collection efficiency greater than 95%. Formaldehyde levels were in the range 15-20 pg/m3 in the conference room, and about 4 pg/m3 in the chemical laboratory. A number of investigations show that indoor formaldehyde concentrations in new dwellings and offices often exceed 100 pg/m3 (24). However, most of the methods used in the studies reported cannot detect formaldehyde levels below 20-30 pg/m3. DNP-coated 13-mm glass fiber filters used in the active mode permit determination of formaldehyde down to about 1 pg/m3 in a 60-L air sample. Passive Sampling. DNP-coated glass fiber filters were evaluated for passive sampling of formaldehyde from air. A commercially available standard sampling cassette for 37-mm filters and the cassette of the Organic Vapor Monitor 3500 (OVM) (3M Co.) passive sampler were used. The standard 37-mm sampling cassette was slightly modified according to Figure 1 and loaded with a DNP-coated filter. The OVM passive sampler was used in its original form after replacing
air concn,
20 40 40 30 30 60 60
16 16 18 4.2 4.0 3.6 3.6
wg/m3
Table IV. Sampling Rates for Passive Samplers
Number of
experiments. dCorrespondsto 1mg/m3 in a 3-L air sample. OFlow rate 1.0 L/min for 30 min. /Stored for 2 weeks in the dark at room temperature.
sampling time, min
sampler
FC
sampling time, min
formaldehyde concn,O
sampling rate,
mg/m3
mL/min
0.65 0.57 0.57 0.67 0.60 0.60 0.65 0.61 0.61
61 65 58 60 57 65 59 59 56
93
100
OVM
100 148 261 261 93 261 261
” Determined with active sampling. the charcoal adsorbent with the DNP-coated filter. The passive samplers were exposed to a standard formaldehyde concentration of 0.57-0.67 mg/m3 in a test chamber for various periods of time. The filters were analyzed and the sampling rates determined. The results are shown in Table IV. The mean sampling rate for the filter cassette sampler is 61 mL/min, with a standard deviation of 3 mL/min. The sampling rates are independent of the sampling time for the sampling times investigated. These experimentally determined sampling rates are in agreement with the calculated values stated in Table I. With a sampling rate of around 60 mL/min, the passive samplers are well suited for measuring the 8-h time-weighted average (TLV-TWA) for formaldehyde. The minimum sampling time for detecting 0.2 ppm (20% of the TLV-TWA) is about 15 min, based on the 60 mL/min sampling rate. At present, studies are being made to determine the sampling rate over a wide range of sampling times and to evaluate the passive samplers for personal monitoring of formaldehyde in worker environments. Registry No. DNP, 119-26-6;formaldehyde, 50-00-0.
LITERATURE CITED Bernsteln, R. S.; Stayner, L. T.; Elllot, L. J.; Kimbrough, R.; Folk, H.; Blade, L. Am. Ind. Hyg. Assoc. J . 1984,45, 778-785. “Threshold Limit Values for Chemical Substances and Physical Agents In the Work Environment with Intended Changes for 1983-84”; Amerlcan Conference of Governmental Industrial Hygienists: Clncinnati, OH, 1983. “NIOSH Manual of Analytical Methods, Vol I”, 2nd ed.; National Instltute of Occupatlonal Safety and Health: Cincinnati, OH, 1978; P& CAM 125. Miksch, R. R.; Anthon, D. W.; Fanning, L. Z.; Holiowell, G. D.; Revzan, K.; Glanville, J. Anal. Chem. 1981,53. 2118-2123. Ahonen, I.;Priha, E.; AIJaIa, M.-L. Chemosphere 1984, 73,521-525. Kuwata. K.; Ueborl, M.; Yamasakl, H. J . Chromatogr. Sci. 1979, 77, 264-268. Andersson, K.; Hallgren, C.; Levln, J.-0.;Nilsson, C.-A. Scand. J . Work Envlron. Health 1981, 7 , 282-289. Fung, K.; Grosjean, D. Anal. Chem. lg81,53, 168-171. Andersson, K.; Andersson. G.; Nilsson, L A . ; Levln, J.-0. Chemosphere 1979,8 , 823-827. Andersson. K.; Hallgren, C.; Levin, J.-0.;Nilsson, C.-A. Chemosphere 1981, 10, 275-280. Beaslev. R. K.; Hoffman, C. E.; Rueppel, M. L.; Woriey, J. W. Anal. Chem 1980,52, I 1 10-1 114. Grosjean, D.; D.; Fung, K. Anal. Chem. 1982,54, 1221-1224. Kuwata, K.; Uebori, M.; Yamasakl, H.; Kuge, Y. Anal. Chem. 1983, 55, 2013-2016.
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Anal. Chem. 1985, 57, 1035-1039 (14) Matthews, T. 0.; Howell, T. C. Anal. Chem. 1982, 5 4 , 1495-1498. (15) Kim, W. S.; Geracl. C. L.; Kupel. R. E. Am. Ind. Hyg. Assoc. J . 1980, 4 7 , 334-339. (16) Kennedy, E. R.; Hill, R. H., Jr. Anal. Chem. 1982, 54, 1739-1742. (17) Blsgaard, P.; Molhave, L.; Rletz, B.; Wllhardt, P. A m . Ind. Hyg. AsSOC. J . 1984, 4 5 , 425-429. (16) Smlth, D. L.; Bolyard, M.; Kennedy, E. R. Am. Ind. Hyg. Assoc. J . 1983, 4 4 , 97-99. (19) Krlng, E. V.; Ansul, G. R.; Baslllo, A. N., Jr.; McGibney, P. D.; Stephens, J. S.; O’Dell, H. L. Am. Ind. Hyg. Assoc. J . 1984, 45, 318-324. (20) Gelsling, K. L.; Tashima, M. K.; Glrman, J. R.; Mlksch, R. R.; Rappaport, S. M. Envlron. Int. 1982, 8 , 153-158.
1035
(21) Andersson, K.; Levln, J.-0.; Llndahl, R.; Nilsson, C.-A. Chemosphere, 1084, 13. 437-444. (22) Rose, V. E.; Perklns, J. L. Am. Ind. Hyg Assoc J . 1982, 4 3 , 605-621. (23) Krlng, E. V.; Thornley, G. D.; Dessenberger, C.; Lautenberger, W. J.; Ansul, 0. R. Am. Ind. Hyg. Assoc. J . 1982, 43, 786-795. (24) “Indoor Air, Vol. 3, Sensory and Hyperreactivity Reactions to Sick Buildings”; Swedish Council for Building Research: Stockholm, Sweden, 1964.
RECEIVED for review December 3, 1984. Accepted January 22, 1985.
Determination of Methane in Ambient Air by Multiplex Gas Chromatography Jose R. Valentin,* Glenn C. Carle, and John B. Phillips’ National Aeronautics and Space Administration, Ames Research Center, Moffett Field, California 94035
A multiplex gas chromatographic technique for the determination of methane in ambient air over extended periods is reported. A modest gas chromatographwhich uses air as the carrier gas was modlfied by adding a silver oxide sample modulator for multiplex operation. The modulator selectively catalyzes the decomposition of methane in air. The resulting analytical system requires no consumables beyond power. A profile of the methane concentration In this laboratory was obtalned for an 8-day period. During this period, methane concentration varied with an approximately daily period from a low of 1.53 0.60 ppm to a high of 4.63 0.59 ppm over the entire 8 days. Some of the measured concentrations are higher than those reported elsewhere indicating the presence of some local source or sources for methane. This work has demonstrated the utility of a relatively simple multiplex gas chromatograph for the analysis of environmental samples. The technique should be applicable to other trace components in alr through use of other selective modulators.
*
*
Methane is of interest in the study of earth’s atmosphere because of ita implication in the possible future global warming of the earth’s surface (I, 2). This warming or “greenhouse effect” is produced by the absorption of infrared energy by trace gases (3). Methane has been of particular interest because it has both natural and anthropogenic sources. Ehhalt and Schmidt (4) have estimated that between 10 and 30% of the annual global production of methane is from enteric fermentation associated with animals, mostly cattle; 25 to 45% is emitted from rice paddy fields; and only 2 to 5% is produced directly from anthropogenic activites such as industrial processes, coal and lignite mining, fuel oil production, and automobile exhaust. Increases in these methane emissions have been linked to growth in the human population (5,6). It has been estimated that in the next 40 to 50 years, methane could contribute 20 to 25% as much atmospheric warming as that expected from carbon dioxide increases (3, 7). With the expected increase in atmsopheric methane, it will be important to accurately monitor the concentration over Permanent address: D e p a r t m e n t of Chemistry a n d Biochemistry, Southern I l l i n o i s University, Carbondale, IL 62901. 0003-2700/85/0357-1035$01.50/0
long and contiguous periods of time. Therefore, studies to investigate sources, and cycles of methane will require analytical methods capable of continuous unattended measurement with temporal resolution of an hour or less for weeks at a time. Various methods have been used for the measurement of methane in ambient air in addition to other constituents in the earth’s atmosphere. Smith (8) compiled atmospheric gas concentration profiles from 0 to 50 km by IR absorption spectrometry. Dianov-Klokov et al. (9) monitored variations of methane abundance in the atmosphere using the method of absorption spectrometry with the sun as the light source and found seasonal variations in the concentration of methane. Gas chromatography has proven to be one of the most powerful h l s available for the determination of trace amounts of organic substances in air (5). Although it has been widely used in trace atmospheric analysis, conventional gas chromatography has some serious limitations. First, detection sensitivity is constrained by the sample volume limitations of the column. Only relatively small volumes of sample may be introduced into the column and then these samples are further diluted by the carrier gas during the chromatographic process. Therefore, in order to make trace-level measurements, sensitive and usually sophisticated detectors must be employed. Second, only discrete sample injections, widely separated in time, may be introduced since usually all sample components must elute before a subsequent analysis may begin. Thus, only slowly changing systems may be monitored. Finally, a consumable carrier gas is usually required even when the sample itself is a gas. This requirement limits usefulness of the conventional gas chromatograph in long-term, unattended operation. Multiplex gas chromatography provides an alternative method to deal with these limitations (IO). In multiplex gas chromatography many samples are pseudorandomly introduced to the chromatograph without regard to the length of time required for an analysis. At the head of the column, each sample component is modulated, Le., changed, in concentration to produce a chemical concentration signal. In multiplex gas chromatography, there is no need to wait for a chromatogram to be completed before introducing additional samples. A new sampling cycle may begin as often as every few seconds. This effectively increases the sample 0 1985 American Chemical Soclety
