A~iyti~t~ Chimica Acta. 124 (1981) l-18 o Eisevier Scientific Pubiishing Company, Amsterdam - Printed in The Netherlands

DETERMINATION OF TETRAMETIIYLLEAD ANDTETRAETHYLLEAD IN THE ATMOSPHERE BY A TWO-STEP ENRICHMENT METHOD AND GAS CHIXOMATOGR.APHIC-MASS SPECTROMETRIC ISOTOPE DILUTION ANALYSIS

TGRBEN NIELSEN*, HELGE EGSGAARD and ELPINN LARSEN

Chemistry Department. Rid National Labomtory. DK-4000 Roskilde (Denmark)

GUSTAV SCHROLL

Department of Geneml and Grganic Chemistry. Uniuersity of Copenhagen, Uniuersitets- parken 5. DK-2100 Copenhagen d (Denmark)

(Received 1st September 1980)

SUMMARY

The development of a specific and sensitive technique for determining tetramethyl- lead (Th4L) in air is described. The method has been tested in different areas under differing meteorological conditions to determine the atmospheric content of TML and tetraetbyllead (TEL). The advantages and limitations of the method are critically die- cussed. The tetraaikyilead compounds are collected on Porapak QS or N at ambient temperature, desorbed, re-coiiected at -80°C on a small column containing 4% Apiezon M on Chromosorb PAW-DCMS, and analysecl by gas chromatography-mam spectrometry with single ion monitoring. The isotope dilution technique is used by adding known amounts of d,,-TML and d,,,-TEL to the sampling columns in advance. This makes it possible to correct for decomposition during the sampling and/or during the analytical procedure. The detection limit for T&IL is 20 pg m-‘_

Inorganic lead seems to be present in all kinds of environmental samples.

A large part of the amount in the biosphere probably comes from the human uses of lead [l]. It is known that inorganic mercury compounds may be converted to metbylmercury compounds under natural environmental con- ditions [2, 31. Laboratory studies have shown that micro-organisms in the aquatic environment can methylate inorganic lead compounds to tetra- methyllead (TML) [4, 51. Trimethyllead compounds and TML are more toxic than inorganic lead compounds [6]. It is of importance, therefore, to explore the possible biomethylation of lead and the distribution of organo- lead compounds from man-made sources in the environment. Recently, Chau et al. [7] demonstrated that fish caught in remote areas may contain detectable amounts of tetraalkyllead. Furthermore, Harrison and Laxen [8] observed abnormally high ratios of organolead vapour to total lead in atmos-

pheric samples from a rural area when the air masses had passed over open sea and ccastal regions adjacent to the sampling sites.

*Resent address: Swedish Water and Air Pollution Research Institute, Box 5207, S-402 24 Gijteborg, Sweden.

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Several methods have been developed for the determination of organo- lead compounds in the urban atmosphere [S, 9-141. Only a few of these are selective for TML and at the same time are sufficiently sensitive to apply to rural areas 111, 141. However, they are inconvenient for long-term sampling in the field requiring extensive cooling during collection.

Laveskog Cl53 collected TML and tetraethyllead (TEL) from urban air in a small gas chromatographic column at -8O”C, desorbed them by heating and determined them by gas chromatographic-mass spectrometric (g-c.- m-s.) fragmentography in his pioneering work in Stockholm in 1969. The limitations of this method are that the formation of ice-plugs in the sampling tube restricts the sampling volume to 1 1 of air, and the detection limits for TML and TEL are about 10 ng rnm3.

This paper describes a two-step enrichment method based upon the isotope dilution technique. The sampling is performed at ambient temperatures, and the sampling volume can be increased to more than 100 1. By this method it is possible to detect 20 pg TML m”.

&I important advantage of this method compared to other g-c. methods utilizing either atomic absorption spectrometric detection [ 11, 131 or micro- wave plasma detection [14], is that the use of isotope dilution m-s. makes it possible to correct for decomposition of TML during the collection and/or the subsequent analytical procedure. The method has been tested in different modifications for the determination of atmospheric TML and TEL in urban and suburban areas under varying meteorological conditions and for the determination of TML in a rural area.

EXPERIMENTAL

Apparatus and reagents The g.c.-m.s. measurements were performed on a Perk&Elmer F 11 gas

chromatograph interfaced by a two-stage all-glass jet separator to a Varian MAT mass spectrometer, model CH 5D. The m-s_ fragmentographic signal was amplified and reccrded on a twochannel recorder.

TML, TEL, d12-TML and dzO-TEL were synthesized from the respective Grignard reagents (alkylmagnesium iodides) and lead nitrate [16, 171. The mass spectra of the tetraalkyllead compounds are shown in Fig. 1. The standards used were freshly prepared solutions in analytical grade n-pen&me. The carrier gas for the desorption of tetraalkyllead from the sampling columns and for the g.c.-m.s. analysis was dried and purified helium.

Samphgprocedure and concentmtbn of tetraalkyllead in small columns

The adsorption tubes for air samples were stainless steel tubes (50 cm X l/4 in.) packed with Porapak N or QS (ca. 3.2 g). The columns were conditioned for 4-8 h at 190°C with purified nitrogen. Known amounts of d12-TML and d,,-TEL (ca. 3 ng of each) were added to the columns kept at room tempera- ture with the injection port at 90°C The columns were then kept at -20°C

ml.2

Fig. I. The 70-eV mass spectra of tetramethyllead, d,,-tetramethyllead, tetraethyllead, and d,,-tetraethyllead.

until sampling. This was performed at about 5°C above the ambient tempera- ture (-15 to + 25°C). The temperature increase was caused by the membrane pump (Thomas Ind. Inc., Model 107CD18 3) used for the sampling. On the inlet side the column was connected to a stainless steel tube doubled over to prevent snow or rain from enteriig the sampling tube. The sampling time varied in the different investigations from 2 to 26 h and the sampling volume from 13 to $90 1. In most cases a 24-h sampling time and 80-90-l sampling volumes were used. The collected samples were kept at -20°C.

Recollection of tetraalkyllead in a small column. The g-c.-ms. analysis of TML and TEL demands that the sample be concentrated in a small volume. Tetraalkyllead was desorbed from the sampling tube at 90°C with dried and purified helium at 50 ml min-’ (Fig. 2). The flow direction was opposite to that of the sampling. The recollection of tetraalkyllead was done at -80°C in a glass U-tube (40 cm X 6 mm o-d.) filled with Chromosorb P-AW-DCMS (60-80 mesh) with 4% Apiezon M, and furnished with two tenon valves. They were closed at the end of this step and the Apiezon M column was heated to about 90°C to vapourize TML and TEL.

4

He g.c.- m.s

--

U

U-tube with Apiezon M

Fig. 2. Regulation of the flowswitching pattern with a six-port rotary valve. (-_) Re- collection of tetraalkyllead; (---) g.c.-m.s. of R,Pb.

G.c.-ms. procedure The connections between the sampling tube, the U-tube with Apiezon M,

and the gas chromatograph, as well as the flow direction in the U-tube are regulated with a six-port rotary valve as shown in Fig. 2. The parameters for the g.c.-m.s. are given in Table 1.

After heating the sample in the U-tube to about 90°C the six-port rotary valve is turned, and +he two t&Ion valves on the U-tube are opened. In this way the sample is introduced into the semi-polar g-c. column. The different methylethyllead compounds, Me,,Et.,_,$b (n = O-4), are separated (Fig. 3) and detected by single ion monitoring. It was possible to monitor at only two masses with the instrument used. Therefore, to detect TML, TEL and the two deuterated standards m/z = 237 [(CH3)2207Pb+*, CZH520sPb+] and

TAB-U 1

Parameters for the g.c.-m.s. system

G.c. column

Column temperature Carrier gas Flow rate Separator Ionizing energy Ion source temperature Detector, electron multiplier

10% EGSS-X on SO-100 mesh Chromosorb P-AW (2 m X 0.4 cm i-d., glass column) 100°C Helium 50 ml min-’ 100°C 70 eV (100 PA) 200°C Optimum conditions

5

.wQ-,cn,l,

\

i_

osv I-----! osv ------------I ._ ._:

‘*. IV : -.

..-.___. ,’ -.

--__ I . .___.

1 I 1 I h 1 I ~----------___________ _______

I I t I

cl 1 2 3 L 5 0 1 2 3 L 5

min ml”

Fig. 3. Gas chromatogram of O-l ~1 of sample of leaded petrol (97 octane) detected at m/z = 237.

Fig. 4. Gas chromatogram of an air sample collected in a suburban area- TML and TEL are detected at m/z = 237 ( -) and the internal standards, d,.-TML and d,,-TEL, at m/z = 242 (---).

m/z = 242 [(CDg)2206Pb+-, CiDs 2osPb* ] were used (see Figs. 1 and 4). In those cases where ~ only TML was investigated m/z = 253 [(CH,),208Pb+] and m/z = 262 I’(CD ) 3 3208Pb+] were used, leading to enhanced sensitivity.

RESULTS AND DISCUSSION

Sampling adsorbent The adsorbent was selected in order to comply with the following require-

ments: a low breakthrough volume of water vapour at ambient temperatures; high adsorption energies for TML and TEL, i.e., high breakthrough volumes at ambient temperatures and low ones at 90°C; no induced decomposition of the adsorbed TML and TEL.

The dependence of the retention volume of TML on the temperature was determined for the following six materials: Tenax GC (60-80 mesh). Porapak QS (SO-100 mesh), Porapak N (SO-100 mesh), and 30% Apiezon M, 20% OV-11 or 20% OV-101 on Chromosorb P-AW-DCMS (60-80 mesh). The tested materials have a low breakthrough volume for waler vapour. The breakthrough volume of TML at ambient temperature was obtained by extrapolation of the linear graph of ln(retention volume) vs. l/T, reducing the possible candidates to Porapak QS and N (Fig. 5). The breakthrough volumes for TML at 30°C were 170 1 and 64 1 per g of adsorbent. The adsorp tion energies calculated from the slopes of the linear plots were 14.6 and 14.4 kcal mol-‘, respectively_

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20 22 2L i6 2.8 30 22 IL I IO 100 1000

10’11 pg Pb

Fig. 5. The temperature dependence (K) of the retention volume V, (ml) of TML on a Porapak QS (A) and a Porapak N (0) column (both, 100 cm X 0.29 cm o.d., 1.4 g of adsorbent).

Fig. 6. A calibration graph for TML. The ordinate shows the ratio between the peak heights of TML and d,,-TML at, respectively, m/z 253 [(CH,),“‘“Pb+] and m/z 262 [(CD,),z08Pb+]. The abscissa shows the injected amount of TML (as Pb). In all cases the injected amount of d,,-TML was 3800 pg Pb.

The sample recovery was evaluated by injecting standards of TML and TEL into the Porapak QS and N columns and sampling 4-60 1 of purified air. During the desorption and recollection step, standards of d12-TML and d,,TEL were injected (see Fig. 2) and the recoveries of TML and TEL were determined by single ion monitoring at m/z = 237 and 242. The recoveries (mean i standard deviation for 5 measurements) were 102 + 4% for TML and 52 + 11% for TEL for Porapak QS, and 92 + 3% for TML and 73 +- 5% for TEL for Porapak N. The recoveries were independent of the volume of purified air, excluding the possibility that adsorbents catalyzed oxidation of TML and. TEL by oxygen, as does activated carbon [S] . High recoveries for the collection of TML and TEL in Apiezon M U-tube at -80°C had been found earlier [ 153.

Te& of the sampling method with purified air did not indicate whether the oxidants in the atmosphere (e.g., ozone, HO’ or O(‘P)) decompose the collected TML and TEL during sampling. The Porapak materials are co- polymers of ethylvinyl- and divinyl-benzene. The relative reactivities of TML: TEL:m-xylene in the vapour phase at room temperature towards ozone are 60:20000:1; towards 0(3P) they are 3:5:1, and towards HO’, O-4:3:1 1181. The low selectivities of HO’ and O(‘P) indicate little possi- bility of causing decomposition of the collected TML and TEL during the sampling. The high selectivity of ozone and the high rate constants for the reaction between ozone and either TML or TEL indicate that during

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sampling the presence of a large ozone concentration in the air may cause a considerable loss, particularly of TEL.

In other methods where sampling is done at temperatures of -80°C or lower 111, 13-15, 191, the possibility of decomposition during sampling is low. But reactive photochemical oxidants (e.g., ozone and peroxyacetyl nitrate) may also be collected, and may give rise to decomposition of TML and TEL during the subsequent analytical procedure. These aspects appear to have been disregarded except in one investigation [ 191.

G.c.-m.s. .analysis Discrepancies in the results for the contents of TML and TEL in urban

atmospheres in different investigations hav e caused some discussion [6, 9, 111 about whether the method of Laveskog 1151 is selective for TML and TEL or not. In the case in which a semi-polar column is used at lOO”C, the retention times of TML and TEL are short compared with those of hydrocarbons with similar molecular weights. The difference between the retention time of TML and that of n-pentane is less than 30 s. The chromato- gram of a sample of petrol detected at m/z = 237 (Fig. 3) contained five peaks with the same retention times as those peaks observable at m/z = 237 in a mixture of TML, TEL, Me,PbCl and Et,PbCl. The two latter catalyze alkyl exchange reactions of tetraalkyllead compounds [ 201; therefore, it is concluded that the five peaks in Fig. 3 belong to Me,Et,_,Pb (n = O-4). The amount of TML and TEL in the petrol was about 0.4 g Pb l-l, in accord- ance with the maximum permissible content of lead in petrol in Denmark. Thus, it appears that no compounds in the petrol interfere with the deter- mination of TML and TEL. Furthermore, two experiments with car exhaust samples were performed.. The ratio between the mass spectrometric peaks of TML measured at m/z = 253 and 237 was identical with that of a TML standard.

Apparently, the only possible compounds that may interfere with the TML and TEL determinations are certain organometallic compounds and freons. However, none of the organometallic compounds which might inter- fere are suspected of being present in environmental samples. Freons present in detectable amounts in the atmosphere have all molecular weights lower than 200 1211. This also implies that the method of Laveskog [15] is selective for TML and TEL.

The detection limits for TML and TEL defined as a signal equal to twice the average noise, are 60 and 10 pg of lead respectively, measured at m/z = 237. Concerning TML separately, a mass spectrometric detection limit of about 3 pg can be obtained by measuring TML at m/z = 253 (cf. Figs. 1 and 6). Blank values for TML measured at m/z = 253 on four conditioned sampling columns did not exceed the detection limit. For a 150-l sample, 20 pg TML mm3 can be detected; this is 25 times lower than was attainable with previous methods [ 11,141.

The ratio between the peak height of TML measured at m/z = 253 and that of d12-TML measured at m/z = 262 was linearly dependent on the

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injected amount of TML, keeping that of dZ2-TlML constant. The same dependence was observed when TML was measured at m/z = 237 and dlz- TML at m/z = 242, or TEL at m/z = 237 and dZO-TEL at m/z = 242.

The relative standard deviation for the preparation of standard solutions and the determination of the calibration graph was 2%; for the injection of a known amount of internal standards into the sampling columns it was 6%; and for the determination of the sampling volumes, it was 8%. Thus, if no decomposition occurs during the sampling, the coefficient of variation for atmospheric analyses would be about 10%.

Determination of TML and TEL in the atmosphere A survey of the results is shown in Table 2. Different modifications of the

sampling method were used in these investigations. In January/February 1979 the sampling was performed using internal standards (d,,-TML and d,,-TEL). Known amounts of these were added to the samples in the U-tube with Apiezon M during the re-collection step. The results of TEL were multi- plied by 2 in accordance with the laboratory recovery test.

During the summer, high levels of photochemical oxidants are common, especially in suburban and rural areas [ 22-241. The tests of the method in January/February, and May/June 1979 were done in connection with an investigation by the Danish Air Pollution Laboratory of the atmospheric pollution of lead in an industrialized suburban area [25]. This implied that the columns were installed in the sampling boxes on the locations 12 h before the sampling started and were removed 12 h after the sampling stopped. It was considered, therefore, that the test in May/June 1979 represents the “worst case circumstances” for field measurements in Denmark. To correct for possible degradation of the TML and TEL collected, known amounts of d12-TML and dZO-TEL were added to the sampling columns in advance. Furthermcre, twc samples were collected in each case on Porapak QS and Porapak N, respectively. The sampling volumes also differed; they were 80-90 1 and about 20 1, respectively. In the samples for Sept. 1979, and

TABLE 2

Atmospheric content of TML and TEL at different locations

Location No. of Date Sampling Adsorbenta TML <rag Pb m-‘) TEL (ng Pb m-‘)

SamPkS time (h) Mt3lll Range Ml?= Rage

Suburban.9sites 26 Jan./Fcb. 24 QS 19 2-54 7 <l-36 1979

Petroistation 1 Feb.1979 10 ::.

1100 60

Suburban.9 sites 15 May/June 24 N 20 0.9-58 8 <l-36 1979

Copenhagen. bury 2 ,streelsb Sept. 1979 2 QS 150 140-150 45

Stockholm. busy street 6 March 1980 10 QS. N 61 47-77 Stockholm. Quiet street 6 March1980 10 QS. N 16 11-22 RurdaIea 4 April1980 16-26 N 1.5 0.5-2.5

aQS = Porapak QS; N = F’orapak, N. bCoUected inside a bus (diesel oil).

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March and April 1980 (TabIe 2) internal standards were added to the sampling columns in advance.

Some general conclusions may be drawn from these investigations_ There does not seem to be any observable different e between the use of Porapak QS or Porapak N as adsorbent, even though the laboratory recovery experi- ments may indicate a minor difference. The dZo-TEL added to the column may decompose totally during sampling in summer as in winter, while there was no observable decomposition of d12-TML in winter samples. In the test in May/June 1979 the recovery of di2-TML varied considerably. Only in those cases where the recovery of d12-TML was high were the measurements considered to be reliable. Thus, in the test in May/June 1979 in 30% of the 27 samples with Porapak QS as adsorbent and in 37% of the Porapak N samples, were the measurements considered satisfactory. The relative standard deviations between those results obtained with the two adsorbents were on the order of 40%. The meteorological conditions on the sampling sites indicate that episodes with photochemical oxidants were possible_ But, unfortunately, no measurements of ozone or other photochemical oxidants were performed in connection with the test in May/June 1979. In RSrvik at G’iteborg, Sweden, 200 km north of the investigated area, the 24-h mean values of ozone were between 62 and 106 ppb, and the maximum mean values for 1 h were between 80 and 155 ppb in this period [26].

When the results of the atmospheric content of organolead compounds are compared with those obtained in other investigations in Europe, it should be remembered that in some of these the atmospheric content of TML and TEL was determi_ned [15, 271, in some the total content of tetra- alkyllead compounds 113,191, and in others the total content of organolead compounds passing a filter 18, 10, 121. Furthermore, there are great varia- tions in the relative compositions of tetraalkyllead compounds in petrol samples from different manufacturers [28]. In Denmark the average ratio of TML to TEL in petrol is 1.9 [29].

The order of the observed levels of TML (Table 2: petrol station > busy streets in urban area > quiet street in urban area = suburban area > rural area) is in accordance with the observations of others. Furthermore, the levels of TML and TEL are at the same magnitude as those reported by others [6, 8-10,12,13, 15,19, 271.

The mean ratio of TML to TEL in the samples January/February 1979 was 2.7 (Table 2). This implies that losses of TEL during sampling had not been of general importance in these measurements, as the ratio would be expected to be slightly higher than 1.9 [S] . Although none of the detcrmina- tions of TEL in May/June 1979 is considered to be very accurate, as the recovery of d*,,-TEL in all cases appeared to be low, the observed mean ratio of 2.5 of TML to TEL is reasonable_

In Stockholm the ratio of 3.8 between TML in a busy street and in a street without traffic (Table 2) agrees with the 4-5 times lower levels of particulate lead found at locations remote from streets in Copenhagen com-

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pared with that found in busy streets, as the dominant part of the particulate lead comes from car exhausts [30].

There are differing opinions as to whether the relative content of tetra- alkylIead compounds to the total content of lead is lower or higher than 5% in urban air 16, 9, 311. In some cases in the industrialized suburban area (Table 3) the relative content of TML and TEL was lower than 4%. However, in those cases the relative levels of particulate antimony and tin were higher than normal. The locations of the sampling sites and a lead smelter in relation to the wind direction confirmed that some of the particulate lead came from the lead smelter 1251. In those cases where the contribution from this point source was small or negligible, the mean relative content of TML and TEL in the winter samples was 6.6 f 0.5% (n = 18) and in the summer samples 7 r 1% (n = 8). In agreement with those values, Laveskog 1151 found a relative content of TML and TEL of 6-10’S in most of his measurements, and Rohbock et al. 1131 reported that 4-10’S of the total lead in city air consists of tetraalkyllead compounds. In contrast, the measurements of the relative content of tetraalkyllead of Harrison et al. [19] ranged from 0.9 to 3.3%. It should be added that TML and TEL seem to constitute the main part of the content of tetraalkyllead compounds in petrol [28]. It is, there- fore, reasonable to assume that tetraalkyllead compounds make up 5-10% of the total lead content in the air of European cities. As discussed pre-

TDLE 3

Relative atmospheric content of TML and TEL on different sites in a suburban area in winter and summer

TML TEL Particulate Pba % Sb rel. to 5% Sn rel. to 5% (TML + TEL) (ng Pb (ng Pb (ng Pb particulate particulate of total Pb m-‘) m-‘) m-‘) Pba Pb=

Winter 30 11 420 0.6 1.2 9 15 6 290 CO.4 1.0 7

4.3 1 78 <1.3 1.8 6 54 11 19oon 7.9 3.0 3 14 2 13oob 17 8.3 1 14 <l 3100b 41 24 0.4

Summer 58 - 480 1.3 3.5 11 49 20 720 1.1 1.3 8 14 5 320 <o-3 2.3 6 10 <4 4aob 6.5 10 <3 24 <3 1400b 4.6 13 2

=Fron [25], airborne particulate matter was collected on Nucleopore (N 1200) and cellulose acetate (Selectron ST 69) filters in series, and analyzed by proton-induced X-ray emission [ 32 1. bContribution from a lead smelter [ 25]_

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viously [6] the proportion of tetraalkyllead in urban air in the U.S.A. and Canada is probably lower, on the order of 2-5s [ 11,141.

Recent investigations have shown that the dominant amount of several organic compounds with boiling points up to 400°C e.g. anthracene and fluoranthene, are in the vapour phase in cooled diluted exhaust gases and in urban atmospheres [33-353. As trimethyllead chloride sublimes at 190°C 1361, it cannot be excluded that if trialkyllead compounds are present in the atmosphere, the dominant proportion will be in the vapour phase. This assumption is supported by the presence of most methylmercury compounds in the gas phase in the atmosphere 137, 381. As trialkyllead compounds are probably more chemically stable than tetraalkyllead compounds [6], their existence in the atmosphere is plausible.

As this problem is unsolved, it is difficult to assess results obtained with techniques measuring organolead compounds in relation to those obtained with those measuring tetraalkyllead compounds_ Using similar methods, one group found a relative content of vapour-phase organolead compounds in urban air of ll-21% [lo] and another 3-13s [12].

It was not possible in this investigation to make measurements of TML in areas where levels of the order of 0.02-0.2 ng Pb me3 could be expected. Four measurements (Table 2) were made of atmospheric TML in a rural area under circumstances where the pollution with TML from the use of leaded petrol was as low as possible. Two of those samples had to be collected under strong gale conditions; the sampling site was situated more than 10 km downwind from the nearest road, and more than 25 km from the nearest town (about 20000 inhabitants), and more than 100 km from larger cities (>lOO,OOO inhabita&). The levels of TML were 0.5 and 0.7 ng Pb ma3. It seems reasonable to assume that levels of TML below these values are very uncommon on Zealand. During part of the sampling time the air masses in the two other samples had passed over areas with a higher population density as well as a road with moderate traffic situated 0.5 km from the sampling site. Thus the levels of TML were higher, 2.2 and 2.5 ng Pb mm3, respectively. In all four samples the recoveries of d12-TML were high. A few measurements of ozone indicated that the levels of ozone had not exceeded 50 ppb during the sampling periods [39]. Therefore, the method is considered to be very well suited to investigations of atmospheric TML in rural and remote areas under circumstances in which the levels of photochemical oxidants are low.

Conclusions The method presented is useful for the determination of TlML in air in

areas with concentrations down to 20 pg mm3. The method is the only one available for determining TML in air in remote areas in order to explore the possible biological formation of TML and its distribution from man-made sources in the environment. It is suitable for field measurements as TML is collected at ambient temperatures, and also because it is possible to correct for any decomposition of TML taking place during sampling, and the col- lected samples can easily be transported over long distances_

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It would be desirable to diminish the decomposition problems, especially of TEL, during sampling in the summer. As a sensitive method for de&mining TML and TEL in water samples is needed [7, 403, this will be tested in the near future, if a simple modification of the present method is capable of solving this problem.

The two-step enrichment method described here may be useful for investi- gating other volatile organometallic compounds, e.g. tetramethyltin [41], in the atmosphere.

This work was supported by the National Agency of Environmental Protection, Denmark. Finn Palmgren Jensen and Knud Hansen of the Danish Air Pollution Laboratory are acknowledged for making the sampling boxes, assisting with the collection of samples, and for stimulating discussions. We also thank Anders Jonsson and Sven Berg, Stockholm University, for collect- ing the samples in Stockholm, and Bente Christensen, Tomas Fernquist, Jytte Funch, Ole J&gensen and Tove Thomsen for skilful technical assistance.

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