Analysis of C1- and C2-Halocarbons in Ambient Air from Remote Areas using Stainless Steel Canister Sampling, Cold Trap Injection HRGC, and a Static Calibration Technique S. Muller and M. Oehme* Norwegian Institute for Air Research, PO. Box 64, N-2001 Lillestrram, Norway

Key Words: Capillary gas chromatography Cold trapping Sampling in stainless steel canisters Calibration Halocarbons in remote areas

Summary 5 ml air samples from 1.6 I stainless steel canisters were cryogenically preconcentrated before gas chromatographic separation on a thick-film capillary using a home-made cryo- trap. Quantification in the 10-500 ppt range was carried out with an electron capture detector. Problems during the cryotrap step were caused by carrier gas contamination, water and COP content of the sample, and a non-optimal heat desorption rate. Their elimination is described in detail. Compound losses by wall adsorption on stainless steel surfaces were observed when dry gases were used for standard generation. A static multipoint calibration technique using humidified He was developed allow- ing a precise standard generation down to 100 ppt. The concentration changes in the stainless steel canisters could be neglected over a storage period of at least 1 month as long as a sufficient degree of humidity was present.The overall reprodu- cibility of the quantification and calibration technique is in the order of 1-2 YO for concentrations between 100-400 ppt when daily recalibrations are carried out.

1 Introduction In the past few years several methods have been published for the quantitative analysis of selected C1- and Cz-halocarbons in the atmosphere [l-41. Most of these compounds have relatively long atmospheric lifetimes (up to 150 years) and are involved in reactions which may lead to a depletion of the stratospheric ozone layer IS ] . High resolution gas chromatography has been proposed as the method of choice for the quantitative analysis of halocarbons in ambient air [Z, 4, 61. It allows the separation of all important compounds within a single analysis without subambient cooling of the gas chromatograph when capillaries with stationary phases of at least 1-2 pm film thickness are used.

A list of the most important halocarbons in the atmosphere which are present at concentrations above 10 ppt is given in Table 1. A minimum sampling volume of 5 ml is necessary to obtain a sufficient detection limit using an electron capture detector. An exception is CHzClz which has a considerably lower response factor, requiring a much higher volume. Since only a fraction of a ml can be injected into a capillary column without band broad-

Table 1

List of the analyzed haloaarbons present in the atmosphere at typical background levels above 10 ppt.

Compound name Abbreviation Remote area levels at Spitzbergen 79"N, 12"E

Difluorodichloromethane Fluorotrichloromethane 1, 1, 2-Trifluoro-1, 2 , 2-tri- chloroethane Dichloromethane Trichloromethane 1, 1, 1-Trichloroethane Tetrachloromethane Trichloroethylene Tetrachloroethylene

F12 F11 F113

CHzC12 CHC13 CH3CC13 cc14 C2HC13 cZc14

-355 -200 = 30

= 80 = 20 -140 =110 - 10 S= 20-40

ening, cryogenic refocusing of the injected air sample has to be employed. However, this step requires some measures to avoid severe contamination problems caused by impurities in the carrier gas, connection lines, and seals.

Stainless steel canisters with electropolished internal surfaces have also been used for sampling of volatile hydro- and halocar- bons for many years [ l , 2, 7, 81. Both excellent long-term stability of the collected samples over several months 11, 8, 91 and problems due to wall adsorption have been reported [ lo] . Steel canister sampling has been used at our institute for C 2 - c ~ hydrocarbon analysis in Arctic air on a routine basis without any problems [ 11, 121. The aim of the presented work was to use the same samples for halocarbon quantification. The following points were studied during method development:

Investigation of the suitability of stainless steel canisters for halocarbon sampling.

- Adaptation and modification of a HRGC method with cryogenic enrichment for steel canister sampling.

- Study of the influence of sample and standard gas humidity on wall adsorption problems.

34 Journal of High Resolution Chromatography 8 1990 Dr. Alfred Huethig Publishers

Analysis of C1- and C2-Halocarbons in Ambient Air

- Development of a method for preparation of accurate ppt-

- Detection and elimination of contamination sources

Solid adsorbent sampling using e.g. Tenax has also been pro- posed for the collection of volatile halocarbons. The minimum sample volume of 5 ml is close to the break-through volumes for Freon F12 and F11. Therefore, recently a sampling procedure has been proposed using a mixture of Tenax TA and activated charcoal coated with SP 2100 which increases the retention for these compounds to about 1-10 1 [4, 131. Our own experiments with similar tubes showed that it was very difficult to obtain sufficiently low blank values for the analysis of Arctic air. The main problem was a steady increase of the blank during storage, probably due to a slow migration of halocarbons from the adsorbent. The applied cleaning method 1111 is obviously not able to remove these compounds completely. Similar problems with blank values have also been reported by others [ 141. Therefore, no further efforts had been made to apply solid adsorbent sampling to remote areas.

The applied sampling technique and the developed analysis method are described in detail, including the construction of a cryotrap. Furthermore, practical problems such as contamination sources, compound losses by wall adsorption, influence of humid- ity on sample recovery, as well as calibration procedures are discussed.

calibration standards

2 Experimental

2.1 Instrumentation

A Carlo Erba Vega 6000 series gas chromatograph equipped with an electron capture detector (ECD 400) was employed for sepa- ration and quantification of the halocarbons. Initial experiments using cryogenic preconcentration were carried out with a Chrom- pack thermodesorption unit (Middelburg, The Netherlands). In addition, a home-made cold trap was constructed with a separate power supply consisting of a 12V car accumulator (220 Ah) and a 0.6 Q shunt resistance. Switching of the heating current (20 A) was performed by the external event controller of the gas chromatograph. Liquid nitrogen was supplied by the original Chrompack control unit. A schematic diagram of the preconcen- tration unit is shown in Figure 1. All tubes (1/~6" or ?/sf ') were made of SS 304 stainless steel (SS) pretreated for gas chromatographic use. Connections were made either by Swagelok (SS 316) or by Valco fittings (stainless steel). A Valco ten-port valve (ClOW, Valco Instruments, Houston, TX 77255, USA) was used for sample loop and gas flow switching. It was dismantled before use, cleaned in an ultrasonic bath with pentane, and dried at 100 "C overnight. The sample loop of 5 ml volume was made from stainless steel. In front of the sample inlet a drying tube (V4" tube, 75 mm length) was mounted filled with potassium carbonate (E. Merck, no. 4928) or NaOH on an asbestos support (E. Merck, no. 1576) and equipped with a stainless steel frit at each end (Sulpelco Inc., Bellefonte, PA, USA, no. 5-8264, 2 pm). The drying capacity was sufficient for about 50 samples. A restriction between sample bottle and trap inlet was used to regulate the flow through the loop (%6" tube, 30 cm length, hammered flat, giving 3 mlimin at Ap = 1 bar).

The cold trap consisted of a 40 cm long deactivated fused silica capillary of 0.32 mm i d . (Chrompack, no. 4075) in a '116'' stainless steel capillary which was placed into a V4" Teflon tube. Liquid N2

9 -1

I I I I I I I

Figure 1

Schematic set-up of the preconcentration unit. 1: Sample bottle; 2 flow restriction; 3 drying tube (K2C03); 4 sample loop; 5: 10-port valve (sampling position); 6: fused silica trap; 7: stainless steel capillary; 8 liquid Np-inlet with cooling jacket; 9: thermocouple, 10 Np-outlet; 11: needle valve restriction.

was metered into the cooling jacket by a magnetic valve placed opposite the sample inlet, giving a less steep temperature decrease. This measure and the bent form of the trap reduce the risk of break-through caused by aerosol formation during the freeze-out step [15. 161. All capillary connections to the Valco valve were made with polyimide ferrules (Valco Instruments). The valve was mounted in an aluminum box kept at 65 1 "C using a light bulb as heat source. This measure eliminated adsorption problems. The reproducibility of the injected sample volume was k 0.5 %.

Sample transfer to the separation column was carried out with a deactivated non-coated fused silica capillary which was guided through the GC-injection port (120 "C). It was connected with a butt connector (Supelco, no. 2-3796).

2.2 Chemicals and Gases

He of 99.995 % purity was used as carrier gas. It contained traces of halocarbons which were removed by a tandem cold trap immersed in a 10 1 Dewar filled with liquid N2. The first part was made from a V4" SS tube of 70 cm length filled with molecular sieve (E. Merck, no. 5704, 3A, spherical, 2 mm) pretreated at 200 "C for 2 h with a He flow of 40 mlimin. It was connected to a second coiled but empty cryotrap of 150 cm length made from %"SS tube. The traps could be used for at least 8 weeks without break- through or clogging at a flow rate of 5-6 ml/min.

Synthetic air (Norsk Hydro) was purified by a molecular sieve (see before) and an activated charcoal cartridge (E. Merck, no. 2514, granulated, 1.5 mm) of 20 mm i.d. and 22 cm length. N2 (99.99 %) was employed as make-up gas for the electron capture detec- tor.

Ultrapure pentane (Promochem, D-4230 Wesel) was used for cleaning the Valco valves.

2.3 Gas Chromatography

Separation of the halocarbons was carried out on a 50 m x 0.32 mm fused silica capillary coated with 5 pm CP Sil 5 CB (Chrompack)

Journal of High Resolution Chromatography VOL. 13, JANUARY 1990 35

Analysis of CT- and C2-Halocarbons in Ambient Air

using the following detector conditions: Detector temperature, 300 "C; N2 as make-up gas 30 ml/min; constant current mode with 40V pulse amplitude and 1 ps pulse width. He at a flow rate of 1.5 ml/min was used as carrier gas. The temperature program was as follows: 45 "C for 3 min, 45-180 "C at 10 "/min, 180 "C for 30 min .

2.4 Sampling and Preconcentration Procedure

Stainless steel bottles of 1.6 1 volume with electropolished internal surfaces and equipped with Nupro SS-4-H-4 shut-off valves were obtained from Biospheric Research Corporation, Hillsboro, OR 97123, USA. A T-piece connector with 2 valves facilitated purging and filling 191. Contaminated bottles were cleaned by connecting them to a hydrocarbon-free high-vacuum system (see ref. [ 111 for more details). Samples were collected using a metal bellows pump with a Teflon-covered viton diaphragm (type M41, Metal Bellows Corp., Sharon, MA 02067, USA). No adsorption effects or sample contamination could be observed when the pump was flushed for about 10 min before sampling. The outlet flow of the pump was free for traces of halocarbons when purified He was pumped.

The pump was coupled to the bottles by a V4" stainless steel tube. After a purging period of 10 min the bottles were filled to a final pressure of 2 bar which took about 8 min. The pressure was read by a clean manometer.

The sample bottle was then connected to the restriction at the inlet of the preconcentration unit, and the sample loop was flushed and conditioned for 15 min at a flow rate of ca. 6 ml/min. In the meantime the cold trap was cooled down to -140 "C with liquid N2 from a 2.5 1 Dewar. The content of the sample loop was then transferred to the trapping capillary (flow rate of 4 mlimin) by switching the ten-port valve. The transfer was completed by flushing both loop and trap with 25 ml of He (6 min) and resetting the valve into sample position. After 10 s the temperature program of the gas chromatograph was started and the heating current of the trap switched on. The resulting steep temperature rise from -140 "C to 100 "C within 30 s desorbed the trapped compounds and transferred them to the separation column. After 30 s the trap heating was switched off

150mllmin 015-2 5 bar

5 4 -IOrnl/rnin

Figure 2

Set-up for standard generation. 1: molecular sieve and activated charcoal filter; 2: primary standard generator (Thermo Environment, model 360); 3: 6-port valve with 0.5 ml loop mounted in GC-oven; 4: flow restriction, 5: metal bellows pump; 6: pressure gauge; 7: standard bottle; 8: He humidifier; 9: Dewar with liquid N2 for He cleaning.

2.5 Calibration

The set-up of the calibration equipment is shown in Figure 2. The permeation rate of home-made diffusion vials (see ref. [17] for preparation) and Permeation tubes (Thermo Environment, Hop- kinton, MA, USA) at 30 "C was gravimetrically controlled. A standard gas generator from the same company (Model 360) was used to generate primary standards of 100-300 ppb employing an air dilution flow rate of 150 ml/min. A T-piece was mounted at the generator outlet allowing further dilution by drawing the stand- ards with a pump through a 500 pl loop (1/8"stainless steel tube). It was connected to a Valco six-port valve (Type 6 UWP). The loop volume was transferred to a stainless steel bottle (1.6 1 volume) using precleaned He. A restriction made from a I/d' stainless steel tube which was hammered flat, kept the He flow rate at about 50 ml min-l bar.

The finalbottle pressure of ca. 2.5 bar was measured exactly using a precision manometer (WIKA, Klingenberg/M, FRG, Series 3, 0-2.5 bar, k 0.5 %, resolution 0.005 bar), and the concentrations were calculated. By varying the air flow rate, the number of transferred loop volumes and the bottle pressure, standard concentrations of ca. 10-400 ppt could be generated. The dilution valve was mounted in a GC-oven and kept at 50°C to avoid adsorption. The dilution apparatus produced He blank chromato- grams without any background signals.

Humidified standards were generated by bubbling the dilution He through a l/z" stainless steel tube of 60 cm length l/3 filled with distilled and de-ionized water [18] which had been boiled for 30 min. Stainless steel frits were mounted on both sides of the tube. The He was free from any background signals after a 12 h bubbling period. A multipoint calibration was carried out in the following way:

1. Blank check of the dilution He. 2. Blank check of the stainless steel bottle by filling it with

purified and humidified He to 2.5 bar (20 min waiting time). 3. Pressure release to 1 bar, precise pressure determination with

a manometer, transfer of one or several loop volumes (maxi- mum one per min, pump switched off) with a He-pressure of 2.5 bar. The final pressure of 2.5 bar was reached after ca. 20-30 min.

4. After a 10 min equilibration time and a pressure check, the standard was ready for analysis.

5. Multipoint calibration with increasing concentrations was carried out by reducing the bottle pressure to 1 bar again (pressure check) and by adding more loop volumes to the bottle (see step 3). Decreasing concentrations were obtained by dilution with He.

3 Results and Discussion Stainless steel canister sampling is used at our institute for the determination of C2-C6 hydrocarbons in air from remote areas. It was therefore of interest to evaluate whether the same samples could be used for the analysis of halocarbons.

As a first step separation of the selected halocarbons (see Table 1) on different capillary columns was studied. The AI2O3/KC1-coated capillary used for the analysis of light hydrocarbons [12] was not suitable due to partial adsorption of some compounds such as trichloromethane at levels below 100 ppt. As found by other authors [6, 131 a thick-film capillary coated with a non-polar stationary phase such as polymethylsilicone was able to separate

36 VOL. 13, JANUARY 1990 Journal of High Resolution Chromatography

Analysis of C1- and C2-Halocarbons in Ambient Air

Table 2

Detection limits and comparison of the intra-day reproducibility of K2C03 and NaOH drying using two samples from a remote area (Birkenes, Southern Norway).

Compound KzCO3 drying NaOH drying Detection limit Reproducibility (n=5) Reproducibility (n=4) [ppt], S/N 10: 1 fi [PPtl s, l % I x [PPtl s, [ % I

F12 16 405 f 3.6 0.9 386 t 4.2 1.1

F113 7 73 f 2.5 3.4 69 + 3.2 4.7

CH3CC13 3 123 f 2.0 1.6 119 4 2.5 2.1 cc14 1 84 f 1.2 1.4 82 k 1.6 1.9

F11 1 261 ? 3.1 1.2 253 t 3.8 1.5

CHC13 10 15.2 t 1.2 7.8 not suitable

CZHC13 5 18.2 f 0.5 2.7 19 f 2.6 14.1 cZc14 1 17.2 f 0.4 2.3 19 4 0.3 1.6

x average, s, relative standard deviation, S/N signal-to-noise ratio

-_

h a n k He

' 45 ' 60 sb 100 120 l i0 160 I80 l80 T( "C) A

Ambient air L I lle s t r a m

L a U. c N ct

+ 5:

L

a a N N

* N

- U

U

L

B 0 t* 8 12 16 20

t ( m i d

I 1 min ' I

,- --""1/- C V

1 Influence of drying tube w i thou t

0 2 4 6 0 2 4 6 t (m in1 D

Figure 3

A. Blank chromatogram of a sample bottle filled with He and the preconcentration unit (5 ml sample). B. Ambient air sample from Lillestrarm. C. Comparison of the Freon F11 peak shape obtained by the Chrompack and the home-made cryotrap. D. Water interference, 1: unknown, 2: water, 3: Freon F12.

all halocarbons without subambient cooling of the gas chromato- graph. A rather long capillary of 50 m and slow temperature program was necessary to obtain a sufficient separation of F12 from the occasionally interfering negative COz-signal (see below).

A sample volume of 5 ml was necessary to obtain a sufficiently low detection limit for all compounds of interest. This required cryogenic preconcentration. During this step contaminants pres- ent in the carrier gas and polymeric sealings were also enriched and caused interference. Cleaning of the employed He with a liquid Nz-trap (see experimental) and sole use of stainless steel tubes, connectors, and ferrules was necessary to solve this problem. Graphite-filled vespel sealings can be tolerated at a few sites of the preconcentration system (e. y. the ten-port valve).

Another serious problem was caused by traces of water present in the sample volume. It was also concentrated in the trap and caused a serious baseline disturbance and detector signal quenching in the elution range of Freon F12 (see Figure 3). A drying tube was therefore placed between sample bottle and trap inlet. Of all evaluated drying agents such as Mg(C10&, PzOb (granulated), and KzC03 only the last named showed no com- pound adsorption and removed the water completely (see Figure 3). Occasionally a small signal remained, interfering partly with Freon F12. It was identified as C 0 2 by mass spectrometry. Similar interferences from either H20 or COz can also be observed in chromatograms from other publications (see e.g. Fig. 7 in reference [4]). If desired, C 0 2 can be removed by replacing K2C03 with NaOH on an asbestos support. However, NaOH as drying medium adsorbed chloroform completely. The losses of trichloro- ethylene and -ethane could normally be neglected (below 2-3 %). Table 2 surveys the usefulness of K2C03 and NaOH drying including detection limits and intra-day reproducibilities.

Depending on the operation conditions and cleanness of the ECD, negative signals from, e.g. , large excesses of hydrocarbons could be observed in the chromatogram. First experiments with the thermodesorption unit from Chrompack showed that too slow a heating rate of the concentration capillary and too large a dead volume in the T-piece of the split outlet caused general peak broadening. This resulted in an insufficient separation between the Freon F11 signal and an interfering negative peak. Therefore, a home-made preconcentration unit was constructed with a faster heating rate and a longer cold trap employing backflush desorp-

Journal of High Resolution Chromatography VOL. 13, JANUARY 1990 37

Analysis of C1- and C2-Halocarbons in Ambient Air

tion. A band width reduction of about 25 % could be achieved. As can be seen from Figure 3, reasonable separation of both signals could be obtained.

Some information has been published about long-term sample stability, wall adsorption, and contamination problems using stainless steel bottle sampling for volatile halocarbons. Oliver et al. [81 measured a concentration decrease of about 6-7 % over a 30-day storage period for the halocarbons shown in Table 1. This was close to the quantification reproducibility (5 %). Concentra- tion changes of less than 1-2 % over several years have been reported from an international project studying the atmospheric life time of halocarbons [9]. Our initial experiments showed concentration decreases in the order of 2-5 % during 1 month for the more polar compounds. A more detailed study over a storage period of several months is now on-going.

First tests of the calibration procedure indicated that wall adsorption effects are dependent on the humidity content of the sample. The generation of ppt-standards with dry He confirmed these observations. As can be seen from Figure 4 less volatile and more polar compounds such as CCI4 showed strong wall adsorption giving non-linear calibration graphs. Humidification of the dilution He (see experimental) produced reasonably linear calibration curves down to 10 ppt for all compounds including CC14. Compared to dry standards the response factors for the less volatile compounds increased considerably (e. g., 25 fold for CC14). A pronounced temperature influence was observed for non- humidified standards. Higher concentrations ( ~ 3 0 %) were found when the bottle was heated from 20 to 80°C indicating wall adsorption. No signal enhancement was observed for bottles filled with humidified standards. The non-polar Freons showed no wall adsorption at all and gave linear calibration curves for both dry and humid He. The calibration standards can be stored for at least 1 week but were normally freshly prepared before calibration.

Wall adsorption problems during real sampling could be easily avoided by flushing the bottles for 10 min with sample air. No concentration difference was found between bottles which were stored with dry He and He with 70 % relative humidity (RH) before sampling. Even when samples of relatively dry air were taken (e.g., 75 % RH at -2 “C giving 18 % RH at 20 “C) a 10 min purging period was sufficient to moisten the walls.

Similar effects have also been described by Yokohata et al. [ lo] . The CC14 concentration in stainless steel bottles did not change as long as the humidity in the sample canisters was high enough. No evidence for compound decomposition was reported in this work or could be found in the results of our experiments.

Table 2 gives an overview over the detection limits obtainable with 5 ml air samples. The blank values of the sample bottles corresponded to the detection limits of the respective compounds and sample contamination was never a problem. Compounds which are present at concentrations below 10 ppt or have low response factors such as dichloromethane can also be determined by increasing the sample volume by a factor of ten. However, this requires rather long sampling intervals and adsorption problems may become a limiting factor.

The reproducibility of the method is dependent on compound concentration and polarity and was of the order of 1-2 % for levels well above the detection limit (see Table 2). The calibration procedure allows generation of a gas mixture with an average precision of better than t 2 ppt for concentrations above 100 ppt.

Figure 4

Influence of gas humidity on wall adsorption effects in a stainless steel bottle. Calibration graphs of a non-adsorbing (Freon F12) and strongly adsorbing compound (CCL,) are shown.

At the moment, a 10 ppt level of more polar compounds can be reproduced within f 3 ppt.

The developed method was tested at our institute by analyzing air from a remote area in southern Norway which is influenced by long range transport. Stainless steel canister sampling is employed for the determination of c 2 - C ~ hydrocarbons in a current project [ 191. The same samples were used for halocarbon analysis. The results for two real samples are given in Table 2. They are in good agreement with other measurements in similar areas. A larger-scale measuring campaign is now in progress.

4 Conclusions The above method offers the following advantages:

- Blank chromatograms of the whole procedure show no halocar- bon background.

- A multiple analysis of the same sample is possible. Other compounds such as NzO, CH4, and volatile hydrocarbons can also be determined using additional methods.

- No risk for contamination or change in sample composition during storage periods of at least 1 month.

- Standards with concentrations comparable to real sample levels can be generated and stored in stainless steel bottles of the same type as used for sample collection. Increasing or decreasing concentration levels can be prepared using only one bottle.

Acknowledgment

St MuUer acknowledges a postdoctorate fellowship from the Swiss National Science Foundation. The authors thank Norbert Schmidbauer for valuable advices and help.

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38 VOL. 13, JANUARY 1990 Journal of High Resolution Chromatography

Analysis of C1- and C2-Halocarbons in Ambient Air

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14.

Ms received: April 20, 1989 Accepted: November 30, 1989

Journal of High Resolution Chromatography VOL. 13, JANUARY 1990 39