Interlaboratory Comparison of Quantification of PAH in Atmospheric Aerosols by gas and Liquid Chromatographies and by Shpol'Skii Fluorometry

133

INTERLABORATORY COMPARISON OF QUANTIFICATION OF PAH IN ATMOSPHERIC AEROSOLS BY GAS AND LIQUID CHROMATOGRAPHIES AND BY SHPOL’SKII FLUOROMETRY

M. LAMOTTE

Lab. de Chimie Physique A - Universitk de Bordeaux I - LA 348 351, cours de la libkration - 33405 Talence ckdex, France P. MASCLET

Lab. de Physico-chimie Instrumentale - Universitk Paris VII 2, place Jussieu - 75251 Paris ckdex 05, France

ABSTRACT An intercomparison exercise on polynuclear aromatic hydrocarbons (PAH) analysis in atmospheric aerosols is reported, The sample was taken from an underground parking lot in Jussieu (Paris University) using glass filter and HIVOL Sampler. After extraction (cyclohexane/dichloromethane, 60/40 in volume), the dried sample was dissolved in methanoL Three techniques have been used for the analysis: - HPLC on reversed phase and fluorimetric detection (4 laboratories) - Shpol’skii fluorimetry at 15 K (2 laboratories) - Capillary gas chromatohraphy (3 laboratories). 16 aromatic including the ones recommended by the EPA has been quantified. There respective concentrations were found to be about 2 to 6 ng/m’ of air. With respect to the mean values, the deviations ranged from 20 to loo%, they are however more reduced when only results obtained by HPLC are compared. The results obtained from Shpol’skii fluonmetry at low temperature are in good agreement with the HPLC values. However, one laboratory gave some values which are slighly lower. Values obtained by capillary gas chromatography appears, for most PAH, substantially overestimated. As no previous separation was performed, these results must be caused by th,e complexity of the chromatograms. Whereas the determination of relative concentrations and/or their variations have a widely accepted significant character, the present intercomparison points to the difficulties in determining absolute concentrations. The discrepancies in the reported results are for the main part due to intrinsic defects of each technique but must be also attributed to the difficulties in preparing homogeneous reference materials and samples: evaporation loss occuring when changing the solvent, photochemical degxadation, etc., are problems which are currently encountered in environmental monitoring programs.

134 1. INTRODUCI'ION

Eight french laboratories took part in an intercomparison exercise on the analysis of PAH in an extract of atmospheric aerosols. PAH are compounds, most often highly carcinogenic, whose origin is the combustion of fossil fuels. Three criteria have been considered for this study: - the reliability of the method - the maximum sensitivity for the assay of traces in the atmosphere - the rapidly of the measurement, in order to approach a routine technique. The sample was taken in the underground car-park of Jussieu on a Whatman GF/A glass filter by a HIVOL sampler. The amount collectec corresponds to about 200 m3 of heavity polluted air where the PAH come from motor vehicles. The sample was prepared by only one laboratory, by extraction with a cyclohexane/dichloromethane mixture (60/40), then diluted in methanol after evaporation to dryness. It is known that the sample preparation method affect the results markedly. This amount concerns only the discussion of the advantages and disadvantages of the analytical methods: - reverse phase High Performance Liquid Chromatography (HPLC) with fluorometric or W detection (4 laboratories) - low temperature Shpol'skii fluorimetry (2 laboratories) - capillary gas phase chromatography (GPC) with flame ionisation detection or coupled with mass spectroscopy (2 laboratories). The analysis concerned 16 PAH, including those listed by the Environmental Protection Agency (EPA). The results are given in Tab. 1. The names of the laboratories do not appear; only the technique employed is indicated. 2. HPLC ANALYSIS

The chromatographic conditions and those of identification and assay differ for each laboratory. In the laboratory I the analysis was performed in two stages under isocratic conditions with an acetonitrile - water mixture. The column used was a Merck Lichrosorb (5 m). In laboratory I1 the analysis was carried out in one step using a particulary economical methanol/water gradient. In laboratory I11 the separation required two chromatograms on a Whatman 10 ODS column of 9 mm internal diameter and 50 cm long. These are rather susprising conditions fot the analysis of traces and more appropriate to preparative chromatography. Nevertheless the resolution is very good. The laboratory IV used a Vydac column reputed to be very suitable for the separation of PAH. In all the cases it appears that the chromatographic conditions present no particular problem, at least for the tetracyclic compounds and bigger. All peaks are resolved and their retention times always differ by at least 30 s. This separation allows the peaks to be integrated under good conditions.

135 Tab. 1. Concentrations of PAH (ng in the sample

-

20 m 3 air)

Lab o ra t o irie s Compound Fluorthe PhBnanthrhe Anthracene Fluo ranthene Pyrhe BaAnthrache Chryskne B bFluoranth6ne BkFluoranthkne BePyrhe BaPyrkne DibzahAnthracSne Peryline BghiF'6rylkne Ind6noPyrhe Coronene

I

25 25 25 135 110 270 330 430 185 260 295 15 65 460 245 180

I1

111

220 6 390 40 60 10 230 150 170 110 150 200 270 270 270 450 130 180 170 270 230 240 35 100 50 50 220 400 320 290 700 80

IV -

950 580 180 300 600 210 430 40 590 550 530 330 -

v

VI

VII

VIII

Meanvalue

600 1970

-

-

2 7 40 20 standard 170 265 130 80 125 90 40

-

-

-

-

-

390 180 -

230 440 270 -

140 110 70

-

380

5 10 -

390 -

1200 -

-

240 900

-

850 1350 -

1000 1000 600 170 400 200

-

-

-

230 240 200 320 380 160 250 380 240 80 370 250 -

It must be noted, however, that the background intensity depends on the method chosen. The detection conditions are as follows: all the laboratories used variable wavelength fluorometric detection Laboratory IV used fluorometricand W detection. Laboratory I used 7 pairs of wavelengths making it possible be measure the PAH under the most sensitives conditions; only 3 or 4 PAH can be measured from each chromatogram, with the consequent disadvantage that analysis is too long. Laboratory I1 used 4 pairs of wavelengths chosen so that they could be changed during the chromatogram without stopping it; the analysis is shorter but the highest sensitivity is not always achieved. An example of a chromatogram is given in Fig. 1. Laboratory I11 used a different pair of wavelengths for each PAH which was identified by stopping the chromatogram just before the top of the peak and recording the fluorescence spectrum. The assay was perfomed on the spectrum itself, usually by adjusting the wavelengths and the slits. The obvious advantage is in the unambignous identification of each compound eluted, but its complexity and the excessive duration of the analysis make this method unsuitable for routine measurements. Laboratory IV used fluorometewith a fEed wavelength of 3 7 0 4 1 0 nm; only a few PAH can be measured under these conditions. The others were analysed by W; in this case the aspect of the chromatogram changes and since the sample has not been retreated, many compounds can be coeluted, hindering peak assignment and, in particular, introducing errors into the peak area calculations. An example of a chromatogram is given in Fig. 2. This method can still be applied to a very polluted sample but is no longer applicable when the samples come from little polluted sites. The calibration methods are various, either by measurements of the fluorescence spectrum with a calibration curve, or by means of external or internal standards. The

136

0

P

Mttt

1 t t t t t -?l

ii

10mn

20mn

30mn

t

co

'II

Fig. 1. HPLC of PAH in airborne particulates with variable fluorimetric detection

first is the most reliable but obviously the longest. The second is easy and give good results but does not allow for the correction of possible variations in the luminous intensity in the fluorometei. This problem arises frequently and the method is somewhat risky. The internal calibration method gives good results, but requires a supplementary chromatogram if the standard chosen is present in the sample (as it happens, fluoranthene for laboratory 11). If the site is only slightly polluted a concentration ratio of 100 between the standard and the sample avoids the use of corrections. The results are of good quality for laboratories I, I1 and 111, the values being close to the average and always in the range of experimental error. Some low values are however obtained by laboratory 11. For coronene, nevertheless, the values are not consistent. The results of laboratory IV are generally bad; the errors rising to as much as a factor of 10. This laboratory carried out the analysis 3 months after the samples were taken, in contrast to the others laboratories. This can explain in part the very low value for BaP (40 ng for an average of 380 ng) since it is known that BaP is rapidly degraded in solution. For fluoranthene (950 for an average of 230) only an error of calihation or identification can explain this difference. We have already seen the causes of error for compoundsassayedby W. 2. ANALYSIS BY LOW TEMPERATURE SHPOL'SKII SPECTROSCOPY

Laboratories V and VI used this method at 10°K or 16°K in an n-octane matrix. One

137

UV

,254nm

FLU0 Xex = 370nm

Fig. 2. HPLC of PAH in airborne particulates with UV detection and fixed wavelength fluorimetric detection

interest of the method is obvious; it requires no chromatographic separation since each PAH appears in the fluorescence spectrum as one or more peaks (multiplets due to different insertion of the PAH in the n-octane matrix). The lines are very precise and characteristic. The analysis itself is short but is requires calibration curves set up by the technique of adding known amounts (Fig. 3). This clearly requires time and many experiments. The appearance of the spectrum given in Fig. 4 shows that the background noise is considerable. This does not hinder peak assignment but makes the method less sensitive (except for some PAH such as BaP) than HPLC coupled with fluorescence. The two laboratories using Shpol’skii spectroscopy did not employ the same calibration, and for this reason there may be differences in the results. Laboratory V obtains excellent results (always very close to the average) but few measurements were performed. Laboratory VI often obtains values somewhat less than the average; note however that the ratios betyeen the concentrations of the PAH stay the same. This is no doubt due to a small systematic error which difficult to explain (perhaps pyrene was not a good choice for the internal standard). 3. ANALYSIS BY CAPILLARY GAS CHROMATOGRAPHY

Laboratories VII and VIII employed this method. Laboratory VII coupled the chro-

Extract SO

BkF

1

398

392

. . . . . . . . . . I

392

390 nm

nC8 15K

Fig. 3. Calibration curves in Sholp’skii fluorometryat 15 K (B k Fluoranthene in n-octane)

1

1

I

400

1

I

I

I

425

Fig. 4. Sholp’skii spectrometry of an extract of PAH in airborne particulates

1

450

1

hhm)

139

EM :ester m&hylique Pht: phtalate A119 6 A130: n alcanes

P ht

P

:M 36 EM

I

2A:

Pht

Fig. 5. Ion chromatogram of airborne particulates (GC/MS)

matograph to a mass spectrometer. The chromatogram obtained (Fig. 5 ) shows that the PAH are not the most important compounds but that esters, phtalates and aliphatic compounds (C19 to C, alkanes) predominate. Some PAH were identified by their molecular weight in particular small peaks difficult to assay. This laboratory concludes that it is impossible to use the sample in the state it was supplied. In several cases, when GPC is used, the authors carriedout a pre-separation, either by liquid-liquid extraction with DMSO (for example) or by chromatography on a column or on a silica gel plate. The two methods can be used together and lead to a marked decrease in the non-aromatic compounds but also to (small) losses of PAH. The chromatogram is simplified and can be exploited. Laboratory VIII used GPC with a capillary column (CP SIL 5) taking care to correct the base line as the temperature varied; conventional flame ionisation detection was used and the PAH are assayed by the addition ot two internal standards. Despite these precautions, since the sample had not been preseparated, the results are frankly poor. Only B g h ~perylene, indenopyrene and fluoranthene are correctly evaluated. All the others are seriously overestimated by factors 2 to 10 (BeP : 1000 ng for an average of 250 ng). The cause is obviously the complexity of the chromatogram (no selective detection) and the systematic overestimation of concentration due to the superposition of coeluted components.

140 4. THE PARTICULAR CASE OF TRICYCLIC COMPOUNDS

The results which are on the whole consistent for tetracyclic or bigger PAH are not for the tricyclic compounds: phenanthrene anthracene and fluorene. Several causes can be suggested: the most likely is due to the preparation of the sample by evaporation of the solvent, leading to considerable losses (up to 60% for phenanthrene). Next, the time between the preparation of the sample and the analysis seems always to be too long. The only laboratory (11) which performed the analysis immediately after the preparation (the same day) finds high values. The low concentrations observed in some cases can also be explained by evaporation during a change of solvent, in Shpol’skii spectroscopy. An other, equally plausible, reason lies in the possibility of interference of the peaks of the tricyclic compounds. In reverse HPLC, these compounds are rapidly eluted in a region where polar aromatic compounds appear, identification becomes difficult and peaks are likely to be superposed. 5. CONCLUSIONS

h Conolusion, three methods have been used. Generally speaking, it appears that the results are satisfactory for 12 of the 16 PAH studies (20 to 100% deviation on average), but that each method is shown to have particular difficulties. HPLC with fluirimetric detection is a classical, relatively long method, and one it obliged to limit the number of wavelengths used for the assay. The internal calibration method seems preferable, the analysis time can now be reduced by employing ”high speed analysis” column; in this case it constitues a routine method, but the sensitivity is not very good. In our study the sensitivity is excellent (threshold detection for BaP is 10 pg/m3 air). Shpol’skii spectroscopy also gives very good results and promise to see wider use in the future because it is well adapted to PAH measurements. The sensitivity of the Shpol’skii is comparable to that obtained in HLPC with fluorimetric detection for many PAH; however, since the apparatus is more specialised it cannot be used for the analysis of others types of compounds. In conclusion, on the three criteria considered two are satisfied: 1 - the methods are reliable (HLPC and Shpol’skii; perhaps GPC would be also after pretreatment of the sample), 2 - the methods are sensitive; the detection threshold allows short sampling times in rural area or even in marine aerosols, 3 - on the other hand it appears that, at present, no method is really adapted to routine analysis; in the most favorable case the sampling, the preparation and the analysis of the sample require a day’s work. It is in this direction that the studies must be furthered. It appeared that the problem of assaying the light PAH was not resolved at the level of the analysis itself. Since moreover, the concentrations of these volatile compounds depends markedly on the sampling conditions (temperature, etc.) three are serious doubts about the validity of measurements on these compounds.

141 LABORATORIES TAKING PART IN THE INTERCOMPARISON EXERCISE: UNIVERSITE BORDEAUX I(33); laboratoire de Chimie Physique A; Lamotte M. et Joussot-Dubien J . UNIVERSITE PARIS 7 (75); laboratoire de Physico-Chimie Instrumentale; Masclet P. et Mouvier G. UNIVERSITE DE SAVOIE - CHAMBERRY (73), service de Chimie; Martin Bouyer M., Jarosz J., Paturel L. et Wittenberg M. INSTITUT CURIE (91), ORSAY; section de biologie; Muel B. UNIVERSITE D’AIX-MARSEILLE (13); centre de Spectroscopie Moleculaire; Mille G. LABORATOIRE D’HYGIENE DE LA VILLE DE PARIS (75); Person A. et Festy B. UNIVERSITE DE METZ (57), service d’analyse commun LAMMA; MulIer J. F. INSTITUT FRANCAIS DU PETROLE DE RUEIL (92); analyses; Petroff N.