GAS CHROMATOGRAPHIC DETERMINATION OF AROMATIC HYDROCARBONS IN AIR USING A SEMI-AUTOMATIC PRECONCENTRATION METHOD* E. BLRGHARDT and R. JELTES T.N.0. Research Institute for Environmental
Hygiene, Postbus Delft. The 4etherlands
(Firs
rewired
28 .Vocrmber
1974 and in jinnlforrn
214. Schoemakerstraat
97.
10 April 1975)
Abstract--;\ of aromatic
practically useful and relatively simple technique for sensitive and selective measurement hydrocarbons in air has been developed. It is based on a modification of the method tlsed bv Grob and Grob (1971) for concentrating organic compounds. combined with the semi-automatic sampler described by Guicherit et al. (1972) with gas chromatographic analysis of the extract solutions on a highly polar column. Results of applications of the technique in the Netherlands are described and compared with those of other countries.
ISTRODCCUON Besides the measurement of lower aliphatics (Sawicki t’r al.. 1970; Jeltes and Burghardt, 1972) that of other hydrocarbons in air is also important for several reasons.
I. The emission of important quantities of many hydrocarbons by various sources, such as traffic, oil refineries. (petro)chemical industry, and domestic fuel. 2. The occurrence of toxic (benzene, some polynuclear aromatics) and odorous (styrene) components. 3. The photochemical reactivity of, notably. di, and trialkyl-benzenes, and branched aliphatics. There are differences in the reactivities of these components (Lonneman et al., 1965). 4 The interest and importance of determining the source of these hydrocarbons in the atmosphere (e.g. Altshuller et al.. 1971; Pilar and Graydon, 1973; Grob and Grob, 1971; Jeltes, 1971) to prevent air pollution. It follows that. for the measurement of such components gas chromatographic (gc) techniques are obligatory. Simultaneous measurement of C,-C5 and higher hydrocarbons is still difficult to realize (Grob and Grob, 1971: Sawicki et al., 1970; Jeltes and Burghardt. 1972). therefore as a rule measurement is focussed on certain fractions. Gas chromatographic determination of higher hydrocarbons (> C,) makes higher demands on the separation, this because of the occurrence together of paraffins, olefins, and aromatics and the many possibilities for isomerism. Therefore wall-coated or support-coated open tubular-columns are desirable for separation. But even then, unambiguous identification may not be attained. blass spectrometric identification is obligatory then, but this technique also cannot solve all problems in * Publication So. 191 of the T.N.O. for Environmental
Hygiene.
Research
Institute
this respect (Raymond and Guiochon. 1973). It is questionable whether such techniques can be combined and automated economically in the near future. We restricted ourselves therefore to the development of a relatively simple method for measuring the important fraction of the lower aromatics (C6-C,& As with C,-Cs aliphatics (Jeltes and Burghardt, 1972) a very good separation of the components of this fraction can still be obtained with packed columns. This fraction can be separated from the aliphatics in the same boiling-range by means of various modern polar stationary phases such as described by e.g. Esposito and Swann (1966), Dmitriev and Pributkov (1972) and Boer and van Arkel (1971). Still higher boiling aliphatics could interfere, but these will generally occur in much lower concentration (Chovin and Lebbe, 1965). Many of the C6-C,,, aromatics occur in such low concentrations, that direct measurement in air is not feasible as yet. Therefore, preconcentration techniques are obligatory. For this purpose we started with an absorption technique (Bergshoeff, 1966; Jeltes, 1974). using nitrobenzene or N-methylpyrolidone as absorbing agent but then at best only concentrations from several ppb could be measured. To measure lower concentrations (down to 0.1 ppb). preconcentration on an adsorbent was used. For this active carbon was selected, the concentration method of Grob and Grob (1971) was investigated and. after the introduction of several modifications. applied. Application of other adsorbents, like silica gel and polymer gc stationary phases does have some important disadvantages. Silica gel is sensitive to water vapour, polymers may bleed off interfering compounds. After some investigations the concentration method has been applied in a semi-automatic mode. making use of the apparatus for aidehydes described before (Guicherit et al.. 1972). The concentrated samples can be analyzed by means of
E.
936
BCRGHARDT and R. JELTES
gc. Ths possibilities for storage of the packed columns bsfors and aftsr sampling
have been investigated.
EXPERIYIENT.AL
Storage experiments were made to stud> how asil the filters could be kept. both after preparation and after sampling. Furthermore some tests were performed with calibration gases of differing relative humidities. Semi-auromaric
For
calibration
the open diflusion method was used (Forruin. 1956: XlcKelvey and Hoelscher. 1957; Altshuller and Cohen. 1960).TIIO calibration gas mixtures bvere used. One had concentrstlons of benzene. toluene and ethylbenzrne respectiveI> of 50, SO and 60 ppb in air (I). In one case this gas was accidentally used uithout benzene. The second contained p-uylens. o-sylens and IX-trimethylbenzene rsspzctivel) at 3. 3 and 4 ppb (II). The relative humidity of the calibration gas could be regulated b\ means of a round-bottomed-flask with water. positioned in the calibration gas stream. which could be kept at different temperatures by means of a thermostat. The relative humidity of the calibration gas was measured with a simple hygrometer. For the preconcentration the method of Grob and Grob (19711 \ins investigated. Active carbon from Merck was sieved b! us to obtain the YO-IO0mesh fraction. For purilication the carbon was tirst heated for 3 h at 3OO’C in the dryins-kiln. After packing the carbon in small columns. as prescribed, it was pretreated by elution bvith carbon disulphide. To chsck the concentration efficiency of the sampling tubes. two tubes in series were tested. For dcsorption of aromatics adsorbed on the carbon filters the extraction method of Crob and Grob (1971) was Investigated and compared with an elution method. performed b) us b) trickling @75 cm3 of carbon dislrlphidr out of a burette on the carbon filter. To investigate if complete desorption is obtained in this ~vay filter tubes used for sampling were eluted twice with 0.75 cm’ carbon disulphide.
sampling
For semi-automatic sampling a method \$as used as described (Guicherit er al.. 1972). However. in place of the usual gas bubblers. our active carbon filter tubes were used (Fig. 1). By means of critical How capillaries the air sampling rate can be selected. We used a capillary that permitted How rates of470 cm’ min- ‘. This semi-automatic sampling method was investigated for reliability and reproduclbllity. With the time-programmer built in. I-h average samples can be obtained. In some experiments IS min averages were collected. Gas
chromarogmphic
anal_vsis
For the gc measurements an Aerograph 1100 (VARIAN) was used. The operating conditions were as follows. Column-3m x Qin. o.d. S.S. Packing-lo“, 1.2.j-rris-(2’-cqanoethoz) )-propane. Fractonitril III, Merck. Temperatures-Injection port. 160 C. Column oven, 73’C. Detector. I5O’C. Gas flows--Carrier gas (Yvl). 25 cm’ min- ‘, Hydrogen. 30 cm’ min- ‘. Air, XXI cm’ min- ‘. Detector-f.i.d. Electrometer-Range, IO- “A. Attenuation. > 2 x. The peak heights measured for the aromatics have been given at an attenuation value 1. The noise level under these conditions is O,j_1 mm. Sample e.xtract volumes of 1jtl were analysed. except when otherwise stated. For peak identification the retention times were used. With this column parafiins with I I or less carbon atoms elute before benzene. RESULTS
When the aromatics in 101. of calibration gas were concentrated exactly according to Grob and Grob we
found the reproducibility was poor. When we used 0.75 cm3 carbon disulphide however, better reproducibilities (4 20 per cent) were found. Reproducibility was still better. when the 5 cm3 extraction flask was cooled, after extraction, for 15 min in ice water. Table 1 compares results obtained by extracting the filter, and by our (simpler) elution method. In this case 201. of calibration gas I was sampled. Table 2 shows the results of the experiments with calibration gas II and atmospheric air to test the concentration efficiency of the method in concentration and in desorption by elution. 601. volumes were sampled. Table 3 shows results of further tests of the elutiondesorption method. In this case 901. of calibration gas I was sampled. Table I. Comparison of extraction and elution for desorption of sampled aromatics gc peak heights (mm) Extraction
Fig. I. Semi-automatic samplin? apparatus. At the tc‘P of the sampling tubes IS shown enlarged.
Benzene Toluene Ethylbenzene
80 al 43
Elution 75 29 45
Gaschromatographic
93’
determinations
Table 2. Results of investigations of the sficiency of the active carbon concentration o-x~lttnc p-uylsne’ gc peak heights (mm)
Toluene
Calibration ~1s
1st filter
Atmospheric air
2nd filter 1st tilter
1st 2nd 1st 1st 2nd 1st
2nd filter
eluate eluare eluate eluate eluate eluate
method
3S 0 0 I5 0 0
5S 0 0 130 0 0
ZO?l 0 0
40 0 0 Ii) 0 0
* The retention time of nl-uylene. also present in atmospheric air. is equal to that of p-uklene
Table 3. Dssorption cticirnc~ of elution procedure gc peak heights (mm) for j/II extract quantities Benzene Toluene Ethylbenzene 1st eluate 2nd
I536
eluatc
2
1093
556
7
4
Table I summarizes the results respectively of storage experiments with freshly prepared filters. both with and without prr-elution, and of those with preeluted filters used for sampling of toluene and ethylbenzene in 491. of calibration gas. Table 5 shows gc results of sampling at different relative humidities. The semi-automatic sampling apparatus was tested for reproducibility and reliability. The calibration gas I was drawn at 470 cm3 min- ’ for I5 min through each of the 12. filter tubes in the sampler. The tubes were eluted here with 1.5 cm3 CS2, 1p1 quantities of the extracts were analysed (Table 6). When one filter was used I2 times in succession in the sampler for sampling identical volumes of calibration gas I, similar data were found.
The modified Grob and Grob preconcentration method (main modification: elution of concentrated sample in place of Soshlet extraction), combined with our semi-automatic sampler. with gc-analysis as described was used for measurements in the field. Table 7 summarizes the results of series of measurements
at various sites in the Netherlands in 1974. Figure 3 shows thz concentration-time relationship for several hydrocarbons found in Dclft. also the \\ind direction and velocity. In Fig. 3 an example of the daily trend of some aromatIcs in the atmosphere of the Hague is indicated. Figure 1 shows a chromatogram recorded in the Hague. DISCL’SSlOh
For the (prekoncentration of lower aromatic hydrocarbons the Grob and Grob method was selected Table
5. Results of the investipation of sampling eKicienc> at different relative humidities: gc peak heights (mm)
<10
Moisture content (“,) Samplmg volume (I)
4.5
Benzene Toluens Ethylbenzene
63 31 IS
53
>90
4.5
>9O
4.5
6s 29 17
1’7
59 2s I6
295 1-7 II0
Table 6. Investigation of the reproducibility and reliability of the semi-automatic sampler. 7.05 I. of calibration gas I was drawn through each of the I2 filters present gc peak height (mm) Range Average Benzene Toluene Ethylbenzene
62-7-l 5464 j-$39
%“,,I
67 -3 Ib 37
4 3 2
Table 1. Results of storage experiments for freshly prepared filters (I without pre-elution. II pre-eluted) and for pre-eluted filters used for sampling of toluene and ethylbenzene in 4.YI. of calibration gas: gc peak-heights (mm) Freshly prepared filters Peak’ I 2 3 Direct analysis of rluate 3 days in the atmosphere 3 dabs in desiccator 3 days in closed vessel in N, atmosphere
I II I II I II
I3 IO 5 3 3 1
8 1 7 0 1 0
4 z 2 I , 0
Filters used for sampling Component Toluene Ethhlbenzene 36
25
40
27
* Peaks I. 2 and 3 respecti\ely have retention times of benzene. toluene and ethylbenzene.
E.
9313
and R.
BLRGHARDT
JELTES
Table 7. Results of two-hour average measurements for various sites m the Netherlands and anai)tical for petrol
rssults found
* Average of four commercially important petrols in the Netherlands.
because of a number of partially self-evident advantages. Traps with a small quantity of active carbon
ivhich have been used for sampling. Grob (1973) now advises storing the fresh filters in CS:: they can be dried uith clean nitrogen before use. The relative humidity does have a small effect on the sampling procedure. onI! in case of very high values it can have some effect on the sampling efficiency of benzene (Table 5). The analytical results have to be corrscted for the carbon disulphide impurit!. I\ hen prewnt. with the retention time of benzene. Our semi-automatic sampler functioned properly ivhen used \vith these active carbon filters. Our reproducibility measurements produced standard deviations < 10 per cent Lvhen calibration gas I was sampled repetitively for 15 min. Such sampling is comparable to 2 h of sampling of j-10 ppb atmospheric concentrations. The sampler has provisions for collecting 2-h average samples,
of small particle size allow maximal recovery with minimal solvent volume, so a high concentration factor can be reached. The method was somewhat modified. because otherwise insufficient reproducibility was obtained: one and a half times as much carbon disulphide was used: elution was used instead of extraction for desorption. the former technique being simpler and quicker. One elution with 0.75 cm3 CS2 uas found to be sufficient. Before use, the active carbon is kept for some hours at 3OO’C.after which pre-elution with CS, is advantageous for purification. The fresh active carbon filters have to be stored sealed-off in a well-closed vessel in a clean gas atmosphere. This also holds for filters
36-11
I I
I -TOl”A Propone
‘G-- ----32-
---..
_- . .
/ I
I I
r :
I I I I I I I I I li” -4
Acetylene
Benzene
i
3i-
a-x-
%-
,
I
:
;
1
:
:
: I
zzm-
3
28
May -\ 45
1974
35”j
29 . i
I / / , 4 4525454::3~35696 Wind
Fig.
2.
May I
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1974 /
d d
30 May 1974 / )
I j
\ \ ? \ 1 / 555:5s6)6655si5745
I
I
I
/
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, Wind 352
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Patterns for some hydrocarbons averaged for two days. at Delft. Sprins 1974
Gaschromatographic
9.v
determinattons
i
Fig. 4. Chromatographic
Tine Fig.
3.
result of a measurement Hague.
---e
Example of the daily trend of some aromatics in the atmosphere of the Hague.
So the modified concentration method of Gob and Grob can be applied in our institute’s semi-automatic
sampler and then offers. in combination with a polar column gc method. a practically useful and relZt[vely simple technique for selective and sensitive measurement of lower aromatics in the atmosphere. The efftciency of separation of isomeric C,-C,0 aromatics has not been investigated here. The main source of aromatics in the atmosphere is traffic. the principal aromatic constituents of exhaust gases are in the more volatile C&Z8 range. Though the method is useful for measurement of this category, it does not guarantee separation of all aromatics in the C,-C,, range. Besides the sampling the gc-analysis can also be performed in a semi-automatic mode. However. up until now we only apply an electronic integrator for the quantitative determination of the aromatics in the &rates. The usefulness of the technique. especially for source recognition studies, is increased by combining it with measurement techniques for other components. e.g. C,-Cj hydrocarbons and carbon monoxide. In principle the semi-automatic concentration method described can also be applied for
other organic pollutants, like halogenated hydrocarbons, with the appropriate gc techniques. About the applications (Table 7) the following can be summarized, see also Table S for a comparison of our results with those of others. The results of the Roelofarendsveen measurement station. lying just North of an important highway. have &n separated according to wind directions. The maximum and average benzene and toluene concentrations measured in Delft, TNO. at the border of this relatively small city. and in the Hague are lower than in Los Angeles (Lonneman e’t nl., 1965) and Toronto (Pilar and Graydon. 1973). The peak concentrations of Fig. 2 may be attributed respectively to contributions of Delft-city and the highway Rotterdam-Delft. according to wind direction. Our results for the Hague (Fig.3) show that the benzene and toluene concentrations vary vvith time in the same way. linked to the traffic rush hours, which suggests a common source for both components According to Pilar and Graydon (1973) the reason for higher toluene: benzene ratio in air than in exhaust gas remains in doubt. They think this fact suggests direct evaporation of gasoline as a source. In the Hague we find about the same toluene: benzene ratio as was found in Toronto. We also believe the influence of evaporation of gasoline should not be neglected. However. it is not clear to us that evapo-
Table S. Comparison of measurement results of several sites. concentrations Benzene max. ave. Los Angeles Toronto Zurich Delft The Hague Tunnel, Neth.
57 9s 74 s 79 33.2
in the
15 I3 35 I 9 25
Toluene max. ave. ISS I29 I21 ‘0 5-l 63
30 37 50 3 IS 35
[C-l: [C,] ratio range ave. I+4~1 I .?A.0 06-2@5 I G-6.0 1.1-2.2
2.5 74 I4 4.5 24 1.5
in ppb
Reference Lonneman et ul. (1968) Pilar and Graydon (1973) Grob and Grob (1972) This work This work This work
E. BLRGHARDTand R. JELTES
340
ration should lead to lower toluene:bsnzene
ratio,
b2cauj2 :
It is gsnzrally kno\bn that the composition of the aromatics in exhaust gases correspond to that of thz petrols of origin: Furthermore gasoline
vapour
will
have
lower
rolwne:benzsne ratio than the gasoline of origin becauseof the higher vapour pressure of benzene. Perhaps one sink for benzene in the atmosphere is dissolution in (rain) water. benzene does dissolve much better in water than the other aromatics. In Table 8 among others the toluene:benzene ratio found in North America and Europe have been compared. The ratio of the toluene and 1 xylene concentrations found by Lonneman rt al. (1965) range I,?-I.5 Our values from the areas most heavily polluted by traffic are in the same range.
REFERE.UCES .\itshuller A. P. and Cohen I. R. (1960) Application of diffusion cells to the production of known concentrations of gaseous hydrocarbons. &~a[. Chen~. 32. 502-8 IO. -\ltsh~~ller A. P., Lonneman W. A.. Sutterfield F. D. and Kowczvnski S. Z. (1971) Hvdrocarbon com0osition of . , the atmosphere of the Los Angeles Basin--1967. Ewirorl. Sci. fichrrol. 5. IOO9-IO 16. Bergshoeff G. (1966) Improved absorbers for sampling air contaminants. 111t.J. .Air Wnr. Polht. 10, 629-631. Boer H. and van .Arkel P. (1971) An automatic PNA anaIyser for (heavy) naphtha. Chromnrographia 1, 300-308. Chovin P. and Lebbe J. (1965) Le dosage par chromatographie en phase gazeuse des hydrocarbures aromatiques dans I’air des locauv de travail. C~/I. notes tfocwnrrltuirrs 39, 63-69. Dimitriev M. T. and Pributkov L. D. (1973) Ionizationchromatographic determination of aromatic hydrocar-
bons in atmospheric BITfin Russia). Gy. j Sm~r.37. ‘-1YEsAdsito G. G. and S\h-arm 11. K. (19661 Determination of aromatic content of hydrocarbon pamt solvents b> gas chromatograph!. J. P~i/lr &i~wi. 38. 377-330. Fortuin J. Xl. H. (19561 Lou- constant vapour concentrations obtained b) a dinamx method based on diffusion, ;Irlcll. Cirem. .-IcCiz15. 521-533. Grob K. and Grob L. I 1371)Gas-liquid chromatographicmass spectrometric inlsstigstion of C,-CL, urganic compounds in an urban atmosphere. J Cl~romtrto~. 62. I-13. Grab K. and Grob G. ( 1972) .\