Determination of polynuclear aromatic hydrocarbons in aerosol by solid-phase extraction and gas chromatography–mass spectrum

Talanta 60 (2003) 1245
/1257 www.elsevier.com/locate/talanta

Determination of polynuclear aromatic hydrocarbons in aerosol by solid-phase extraction and gas chromatography
mass spectrum

/

Meng-Xia Xie *, Fang Xie, Zhi-Wei Deng, Guo-Shun Zhuang Analytical and Testing Center, Beijing Normal University, Beijing 100875, China Received 19 January 2003; received in revised form 28 March 2003; accepted 4 April 2003

Abstract The optimum procedures for clean up aerosols in silica solid-phase extraction (SPE) cartridge prior to the analysis of polynuclear aromatic hydrocarbons (PAHs) by gas chromatography
/mass spectrum (GC
/MS) in selected ion monitoring detection mode have been investigated. The silica SPE cartridge is activated by dichloromethane and hexane, and then dried up by vacuum in SPE instrument for about 5 min. The sample volume loaded on the cartridge is 3 ml and after loading the sample the cartridge was dried in vacuum for 5 min. The PAHs were eluted from the cartridge by 3 ml 20% dichloromethane in hexane. The flow-rate of sample loading and eluting was controlled in about 1 ml min 1. The aerosols collected on the campus of Beijing Normal University from August 2001 to July 2002 have been processed with these optimum procedures and 16 EPA priority pollution PAHs in the aerosols have been quantitatively determined. # 2003 Elsevier B.V. All rights reserved. Keywords: Polynuclear aromatic hydrocarbons; Solid-phase extraction; Aerosol; GC
/MS

1. Introduction The analysis of organic contaminations in aerosols is an important aspect of environmental research. Since the middle of last century, with the rapid development of industry, a large amount

* Corresponding author. Tel.: /86-10-6220-7981; fax: /8610-6220-0076. E-mail address: [email protected] (M.-X. Xie).

of pollutants have been released into the air incessantly. Many big cities with large populations located in northern China are heavily contaminated. Aerosol is a complex matrix that contains different kinds of organic compounds such as alkanes, fatty acids, alcohols, aldehydes, ketones, esters, phenols, nitrogen, oxygen and sulfur heterocompounds, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls etc. PAHs have mutagenic and carcinogenic properties and consequently have been included in US EPA and

0039-9140/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0039-9140(03)00224-8

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European Union priority lists of pollutants [1]. PAHs are mainly emitted from incomplete combustion of organic materials, in particular coal burning and petroleum fuels. Because of their widespread presence in air, soil, and water, their identification and determination still remains an important analytical problem [2]. Due to the complexity of aerosol and low concentration levels of PAHs, enrichment and clean-up procedures are usually required prior to the final chromatographic analysis. Column chromatography [3
/9] has often been used to clean up the aerosol; however, it requires large volume of solvents and is usually time consuming. Solidphase extraction (SPE) can significantly reduce the solvent consuming and simplify the sample pretreatment. It has been successfully used for the clean up and pre-concentration of PAHs from drinking water, precipitation and sediment samples [10
/16]. There are several reports mentioning the application of SPE on the pretreatment of aerosols [10,15
/17]. Gyula Kiss et al. cleaned the extract of aerosols on a C18
/SPE cartridge to protect the HPLC column from long chain aliphatic compounds for the determination of organic pollutants in aerosol [15]. SPE
/CN and SPE
/Si combinations were also applied to separate and determine 15 PAHs in extracts of airborne particulate matter [16]. Hydroxy-polycyclic aromatic hydrocarbons in aerosol have been cleaned up on the silica cartridge and their derivatives were determined in GC [10]. In this paper, the determination method of PAHs in aerosol is systematically investigated by silica SPE and gas chromatography
/mass spectrum in selected ion monitoring detection mode (GC
/MS-SIM). Several factors that would influence on the recovery of PAHs, e.g. conditioning of SPE cartridge, rinsing, the strength of eluting solvent, drying the SPE cartridge after conditioning and sample loading, sample volume, the flow rates of sample loading and eluting, have been studied and discussed. The optimized SPE procedures were applied successfully to the analysis of 16 EPA priority pollution PAHs in aerosols collected from August 2001 to July 2002 on the campus of Beijing normal university which is

situated in the north of Beijing and about 100 m distance from main streets in three directions.

2. Experiment 2.1. Instrument and analysis conditions The analysis was performed on a Trace GC
/MS spectrometer (ThermoQuest, Finnigan San Jose, CA, USA). Data was collected with a Xcalib software data process system. Separation was carried out on a DB-5MS fused-silica capillary column (J & W Scientific, Folsom, CA, USA), 30 m/0.25 mm I.D., 0.25 mm film thickness. High purity helium (99.999%) was used as carrier gas (1.0 ml min 1). The temperature program was: initial 65 8C, hold 1 min, rate 7 8C min 1 to 200 8C, hold 5 min, rate 4 8C min 1 to 300 8C, hold 5 min. The split/splitless injector was set to 275 8C and 1 ml was injected with the split vent closed for 5 min. The mass spectrometer was operated in the electron impact mode (EI) using 70 eV ionization voltage. The ion source temperature was 220 8C and the GC
/MS-interface was set to 250 8C. The analyses were performed by SIM detection mode (molecular weight of the PAHs analyzed). SPE instrument, 10-port vacuum manifold (Agilent Technologies, P.A., CA, USA). Air sampler TSP samples were collected using a medium volume sampler (Laoshan Electronic Instrument Factory, Qingdao, China). Sonic bath (AS3120A, Automatic Science Instrument Co. Ltd., Tianjin, China). 2.2. Chemicals and materials Hexane and dichloromethane were HPLC grade (Fisher, USA), all other solvents were analytical grade and were re-distilled before use. Naphthalene (99.2%, NAPH), acenaphthylene (99.5%, ACY), acenaphthene (91.4%, ACE), fluorene (99%, FLU), phenanthrene (99%, PHEN), anthracene (98.1%, ANTH), fluoranthene (98.0%, FLT), pyrene (98%, PYR), benzo[a]anthracene (98%, BaA), chrysene (98%, CHRY), benzo[b]fluoranthene (99%, BbF), benzo[k]fluoranthene

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(97.5%, BkF), benzo[a]pyrene (99.2%, BaP), dibenz[a,h]anthracene (98.3%, DIBahA), benzo[ghi]perylene (99.2%, BghiP), indeno[1,2,3cd]pyrene (99.5%, INPY) and hexamethylbenzene (99.5%) were from Chem. Service (West Chester, PA, USA). SPE Strata Si-1 cartridges (500 mg, 3 ml) (Phenomenex, Torrance, CA, USA). The aerosols were collected on the 8 in. /11 in. glass fiber filters (Whatman Company, Maidstone, UK) which were heated at 350 8C for 10 h before use. 2.3. Preparation of the stock solutions Eleven PAHs (NAPH, ACY, PHEN, ANTH, FLT, PYR, BaA, CHRY, BkF, BaP, BghiP) that span the 2
/6-ring size range were selected for the optimizing SPE procedure experiments. Hexamethylbenzene (100 mg l 1) was used as internal standard [18,19]. A stock solution of 11 selected PAHs was prepared in hexane in the concentration range of 30
/60 mg l 1. The concentration of the 11 PAHs solution used for the determination of the recovery in the SPE cartridge is about 100 mg l 1 of each compound. 2.4. SPE procedures The silica SPE cartridge was conditioned with 10 ml dichloromethane and 10 ml hexane, respectively. Then the cartridge was dried in vacuum for 5 min. 3 ml sample solution contained 11 PAHs (about 100 mg l1 of each PAHs) was introduced at a flow-rate of 1 ml min1. After drying in vacuum for about 5 min, elution was performed using 3 ml 20% dichloromethane in hexane (v/v) at a flow rate of 1 ml min 1. The eluate was evaporated to about 500 ml with nitrogen and 100 ml internal standard solution (about 100 mg l 1) was added and then diluting the solution to 1 ml with hexane for GC
/MS analysis in SIM mode. 2.5. Determination of the optimum SPE conditions Different volume ratios (v/v, from 0 to 100%) of dichloromethane in hexane, which were prepared

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by mixing dichloromethane with hexane, were used as eluting solvent individually in above SPE procedures to determine the optimum composition of eluting solvent. Using 3 ml petroleum and 3 ml hexane, respectively, as the rinsing solvent after the sample loading to study the effect of rinsing on the recoveries of PAHs, other steps were identical with the SPE procedures described in Section 2.4. Effects of drying the silica SPE cartridges on recovery of PAHs were studied by designing three experiments. Experiment A (Exp. A): After conditioning the cartridge with 10 ml of dichloromethane and 10 ml hexane, respectively, 3 ml solution of 11 PAHs was introduced directly without drying the cartridge. Elution was performed by 3 ml 20% dichloromethane in hexane at a flow rate of 1 ml min 1. Other procedures were same as that of Section 2.4. Experiment B (Exp. B): the SPE cartridge was dried up in vacuum for about 5 min after the conditioning step, and then the sample was loaded and eluted as the procedures of Exp. A. Experiment C (Exp. C): after loading the sample, the SPE cartridge was dried in vacuum for 5 min, all other procedures were the same as that of the Exp. B. The effect of loading volume on recovery was examined as follows: 10, 25, 50 ml sample solutions were prepared by diluting 3 ml solutions of 11 PAHs (about 100 mg l1 of each PAHs) with hexane, respectively. The SPE procedures were as described in Section 2.4 except that the sample volumes were 3, 10, 25 and 50 ml individually and the flow-rate of sample loading was controlled to 5 ml min 1. When studying the effect of the flow rates of sample loading and eluting on the recovery of PAHs, a SPE instrument controlled the flow-rate of sample loading and eluting to about 1, 10 and 20 ml, and the other procedures were the same as that of Section 2.4. 2.6. Sample collection TSP Samples were collected using a mediumvolume air sampler. Particulate-associated PAHs were trapped on glass fiber filters. Sampling was conducted at the flow-rates of about 120 l min1

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for 24 h yielding air volumes of 173 m3. Sampling site was on the top of a 40 m-high campus building of Beijing Normal University, which is located in the north of Beijing and is about 100 m away from main streets in three directions. Sample filters were sealed in the polyethylene plastic bag immediately after sampling and stored in refrigerator at / 30 8C in the dark until analysis. Six sheets of aerosols in duplicate were collected each month from August 2001 to July 2002. 2.7. Sample pretreatment and analysis Aerosol was cut into small pieces and extracted ultrasonically four times, each time with 25 ml dichloromethane for 10 min. Combined the extraction solution and evaporated at 25 8C in a rotary evaporator under reduced pressure to near dryness and re-dissolved in 3 ml hexane. After the pretreatment by above optimal SPE procedures, the eluate was evaporated to about 500 ml by a gentle flow of nitrogen, then 100 ml internal standard solution was added and diluted to 1 ml with hexane. Dividing the solution into two parts, one part for GC/MS analysis directly, another part (500 ml) was added 950 ml internal standard, and then diluted to 10 ml with hexane for GC/MS analysis. The blank glass fiber filters was treated as above procedures and analyzed as control. There have not been detected any PAHs in the blank glass fiber filters.

than that of sample solution. Because the sample solution of PAHs was prepared in hexane, the choice of secondary solvent was hexane. 3.1. Determination the optimum elution solvents After sample loading, the analytes eluted from the cartridge with an appropriate solvent. The choice of eluting solvent should be carefully considered. If the solvent is too powerful, more interference will be eluted out. If the elution strength of the solvent is not enough, a larger elution volume will be needed, and then it will dilute the sample and lower the detection sensitivity. In our experiments, the volume of eluting solvent was 3 ml, and the suitable strength of eluting solvent was determined by examining the recoveries of PAHs with different percentages of dichloromethane in hexane. For the reason of clarity, Fig. 1 only presented the results of ACY, FLT, BaA and BaP out of the 11 selected PAHs. From Fig. 1, it can be seen that 20% dichloromethane in hexane can elute most PAHs from the silica SPE cartridge. The recoveries of other PAHs were also between 80 and 106% when the percentage of dichloromethane in hexane was above 20%. The recovery of PAHs can not be apparently improved by increasing the polarity of elution solvent. The result showed 20% dichloromethane in hexane was enough for the efficient elution. 3.2. Effect of rinsing on the recovery of PAHs

3. Results and discussion The conditioning step is important for the SPE procedures. The first step of conditioning was to wet the silica SPE cartridge with dichloromethane. Wetting of the cartridge can open up the groups on the sorbent surface and thus, increases the surface area available for interaction with the analyte. In addition, it can remove the residues from the packing material that might interfere with the analysis. The second step was to wash the sorbent bed with a solvent to prepare the suitable surface for the adsorption of analyte. The second solvent will have weaker or equal eluting strength

After the sample loading, rinsing step is usually needed to selectively remove endogenous compounds from the SPE cartridge that might interfere with the subsequent analysis. The rinsing step can help clean the sample and protect GC or HPLC column. The rinsing solvent should elute as much interference substance as possible, but cannot decrease the recovery of the analyte significantly. In our experiments, 3 ml petroleum ether or 3 ml hexane was used as the rinsing solvent, respectively, before the elution step. Their effects on recovery of PAHs were summarized in Table 1.

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Fig. 1. Correlation between the recoveries of ACY, FLT, BaA, BaP and the percentage of dichloromethane in hexane (v/v).

It can be seen from the data of Table 1 that petroleum ether or hexane leads a remarkable loss of the 2
/3 ring PAHs. Above 90 percent of NAPH, ACY and PHEN has been lost during the rinsing step. The recoveries of ANTH, FLT, PYR and BaA were also below 50%. It demonstrated that silica SPE cartridge has a weak retention of these PAHs due to their lower polarity. Actually, for aerosol, the main purpose of the rinsing step is to diminish the interference of alkanes. Alkanes, which have less polarity than

Table 1 Effect of rinsing on recoveries (%) of PAHs PAHs

NAPH ACY PHEN ANTH FLT PYR BaA CHRY BkF BaP BghiP

Petroleum ether

Hexane

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

4 4 4 12 14 23 25 57 66 93 95

23 20 22 15 18 13 26 11 6 4 6

6 7 5 15 13 32 32 85 87 94 92

19 16 19 15 17 10 14 4 5 7 5

Conditions: n/4; 3 ml sample solution; 3 ml rinsing solvent; 3 ml 20% dichloromethane in hexane as eluting solution.

PAHs do, have little retention on the SPE
/Si cartridge, and most of them will elute from the cartridge during the sample loading process. Their residues in the cartridge have little interference with the analysis of PAHs, especially in the selected ions monitoring detection mode. Therefore, this step can be omitted. 3.3. Effects of drying of the SPE cartridge after conditioning and after sample loading on the recovery of PAHs Generally, the SPE cartridge cannot be dried up because it would produce crack on sorbent bed and result in low recovery and repeatability. But sometimes, especially for the analytes that have low retention on the SPE cartridge, the drying process is important. Gyula Kiss et al. [20] have reported that low recoveries and higher standard deviations (S.D.) of PAHs on the C18
/SPE cartridge were observed without drying the cartridge after sample loading when they determined the PAHs in water samples. The effects of drying the silica SPE cartridge after conditioning and drying it both after conditioning and sample loading on the recovery of PAHs was investigated and compared with that of without drying the cartridge during the SPE process. The three experiments were conducted and the results are listed in Table 2.

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Table 2 Effect of drying on recoveries (%) of PAHs PAHs

NAPH ACY PHEN ANTH FLT PYR BaA CHRY BkF BaP BghiP

Experiment A

Experiment B

Experiment C

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

79 87 92 68 83 53 60 70 67 73 77

6 4 5 7 5 9 9 5 6 11 14

69 90 91 98 95 89 86 69 72 80 80

14 3 5 3 6 7 11 14 18 13 14

79 90 92 96 95 89 92 89 90 92 94

2 3 3 7 4 5 3 10 10 7 4

Conditions: n /4; 3 ml sample solution; elution with 3 ml 20% dichloromethane in hexane.

It can be seen from the Table 2 that drying process has significant influence on the recoveries of most PAHs. 91% of average recovery of 11 PAHs and 5.3% of average relative standard deviation (R.S.D.) were obtained by both drying out cartridge after conditioning and sample loading. Without drying the cartridge during the SPE cartridge, the average recovery of 11 PAHs was only 74%, and if only drying the cartridge after conditioning, the average recovery was 84% and the average R.S.D. was 9.8%. Therefore, drying the silica SPE cartridge both after conditioning and after sample loading can obtain higher recovery of PAHs and lower R.S.D. Drying the SPE cartridge after conditioning can ensure contact of analytes with the pores of sorbent fully and leads to a better retention, otherwise, it will cost a longer time for retaining the analytes and decrease the recovery of the analytes. The process of elution can explain the difference in recoveries that resulted from drying after the sample loading. If the cartridge is dried, the organic solvent is forced through it by gravity, thus enables complete elution of PAHs from all the pores of the stationary phase. However, if the drying process is neglected, the pores are filled with solvent, and the eluting solvent cannot or can only slowly penetrate into the pores because of miscibility and/or viscosity reasons.

3.4. Effect of sample volume loaded on the SPE cartridge on the recovery of PAHs In order to enrich the trace analytes, sample volume varied from few milliliters to hundreds of milliliters, and in some cases, even liters were introduced into the SPE cartridge [14,21
/23]. The sample volume is a key factor that will influence on the recovery of analytes in some instance [24], especially for those analytes that have low retention on the SPE cartridge. It is important to optimize the sample volume to gain high recovery of analytes. Various sample volumes of PAHs were prepared by diluting 3 ml standard sample solution with hexane individually to guarantee the same quantity of PAHs loaded on the silica SPE cartridge and to avoid the variations of recovery caused by different quantities of analytes. Table 3 listed the effect of sample volume on the recovery of PAHs. The average recovery of 11 PAHs was decreased with the increase of sample volume. When the sample volume loaded on the cartridge was 3 ml, the average recovery was 91%, while the average recovery decreased to 17% for 50 ml sample. The recovery of 2
/3 ring PAHs lost significantly when the sample volume was 10 ml. This effect is actually the result of a shift in the adsorption/ desorption equilibrium favoring increased deso-

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Table 3 The influence of sample volume on the recoveries (%) of PAHs PAHs

NAPH ACY PHEN ANTH FLT PYR BaA CHRY BkF BaP BghiP

3 ml

10 ml

25 ml

50 ml

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

79 90 92 96 95 89 92 89 90 92 94

2 3 3 7 4 5 3 10 10 7 4

37 48 42 69 57 76 58 93 90 81 84

11 6 9 6 7 6 12 8 11 10 13

17 23 17 39 25 36 26 53 53 81 80

13 17 16 11 14 12 15 10 9 9 10

6 11 6 21 11 16 13 20 20 32 26

22 22 27 16 18 19 21 19 15 14 16

Conditions: n /4; the 10, 25 and 50 ml sample solutions were obtained by diluting 3 ml standard solution of PAHs (about 100 mg l 1 of each PAH) by hexane, respectively. The flow-rate of sample loading is 5 ml min1.

rption from the packings and causing a net loss of adsorbate from the silica SPE cartridge with increased sample volume. The loss is especially apparent for the low molecular PAHs with relatively small k? value [25]. Therefore, the sample volume loaded on the silica SPE cartridge should be limited to about 3 ml.

3.5. Effect of flow-rate of sample loading on the recovery of PAHs Various flow-rates of sample loading that range from 2 to 100 ml min 1 depending on the sample processed have been reported [14,25
/29]. However, at higher flow-rate, non-equilibrium process can lead to lower retention volumes of the analytes [30,31]. This may result in a loss of recovery, particularly for the compounds that have a retention volume close to the sample volume. Nevertheless, in some experiments the results indicate that flow-rate does not have to be closely controlled since the combinations of the small particle size and high surface area ensures contact of the analytes with the adsorbent even when very rapid flow rates are employed [29]. Three flow-rates of sample loading were examined to observe their influence on the recovery of PAHs. The results were shown in Table 4. The recovery of most PAHs was slightly decreased with

the increase of the sample loading flow-rates. The average recoveries were 91, 87 and 84% at the flow-rates of 1, 10 and 20 ml, respectively. The average R.S.D. has no significant changes during variation of the flow-rate, ranging from 5.3 to 6.5%. Because of the flow-rate of sample loading has little influence on the recovery of PAHs, it can be adjusted in certain range according to the sample volume. The sample volume in our experiments is relatively small and sample preparation time is not a problem. 1 ml min 1 of flow rate of sample loading can be suggested to achieve a better recovery.

3.6. Effect of eluting flow-rate on the recovery of PAHs The eluting step is also important for the recoveries of analyte. Three different eluting flow rates were performed to determine their effects on the recovery of PAHs. Results are summarized in Table 5. The results showed that the eluting flow-rate has remarkable influence on the recovery of PAHs. In high eluting flow-rates (10 and 20 ml min 1), the average recovery of PAHs was below 50% (42 and 31%) and the average S.D. was above 15% (19 and 16%). Therefore, the eluting flow-rate should be

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Table 4 Effect of flow-rate of sample loading on the recoveries (%) of PAHs PAHs

NAPH ACY PHEN ANTH FLT PYR BaA CHRY BkF BaP BghiP

1 ml min 1

10 ml min 1

20 ml min 1

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

79 90 92 96 95 89 92 89 90 92 94

2 3 3 7 4 5 3 10 10 7 4

80 93 79 88 93 87 92 87 87 88 88

3 3 2 8 5 3 3 10 4 13 9

72 84 79 85 92 90 88 82 83 85 84

7 2 2 2 9 2 4 10 12 11 10

Conditions: n /4; 3 ml sample solution (about 100 mg l 1 of each PAH); elution with 3 ml 20% dichloromethane in hexane.

controlled carefully, and 1 ml min 1 is a suitable eluting flow-rate. In summary, the optimum conditions for clean up aerosol in silica gel SPE cartridge for analysis of PAHs by GC
/MS in selected ion monitoring detection mode is as follows. The cartridge is activated by 10 ml dichloromethane and 10 ml hexane, respectively, and then dried up by vacuum in SPE instrument for about 5 min. The sample volume is about 3 ml and after loading the sample the cartridge was dried in vacuum for 5 min. The PAHs were eluted from the cartridge by 3 ml 20% dichloromethane in hexane. The loading and

eluting flow-rate was controlled in about 1 ml min1.

3.7. Quantitative determination of the 16 PAHs in aerosols The quantitative determination of 16 kinds of PAHs was conducted by GC/MS in SIM mode. The selected ions are their molecular weights. Fig. 2 is the separation chromatogram of the cleanedup aerosol and the mixture of 16 standard PAHs samples. The 16 PAHs to be detected in aerosols

Table 5 Effect of flow-rate of elution on the recoveries (%) of PAHs PAHs

NAPH ACY PHEN ANTH FLT PYR BaA CHRY BkF BaP BghiP

1 ml min 1

10 ml min 1

20 ml min 1

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

Recovery

R.S.D. (%)

79 90 92 96 95 89 92 89 90 92 94

2 3 3 7 4 5 3 10 10 7 4

27 37 34 54 53 46 44 46 44 40 36

19 16 15 21 10 13 19 17 12 16 18

24 31 23 39 28 29 25 29 34 36 41

20 20 17 25 16 14 18 21 19 15 12

Conditions: n /4; 3 ml sample solution (about 100 mg l 1 of each PAH); elution with 3 ml of 20% CH2Cl2/C6H14.

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Fig. 2. Selected ion chromatogram of cleaned up aerosol sample (A) and the mixture of 16 standard PAHs (B). The ions selected were the molecular ions of the 16 PAHs: 128, 152, 154, 166, 178, 202, 228, 252, 276, 278. Other chromatographic conditions see Section 2. 1. Naphthalene, 2. acenaphthylene, 3. acenaphthene, 4. fluorene, 5. phenanthrene, 6. anthracene, 7. fluoranthene, 8. pyrene, 9. benzo(a)anthracene, 10. chrysene, 11. benzo(b)fluoranthene, 12. benzo(k)fluoranthene, 13. benzo-(a)pyrene, 14. indeno(1,2,3cd)pyrene, 15. dibenzo(a,h)anthracene, 16. benzo(g,h,i)perylene.

were satisfactorily separated and there is no interference peaks except several of their isomers. The SPE treatment can purify the extraction of aerosols, and can protect the chromatographic column in subsequent GC
/MS analysis. Meanwhile, it can also reduce the interference peaks in the analysis of PAHs. Fig. 3 shows the GC
/MS chromatograms of aerosol samples cleaned-up with (Bottom) and without (upper) SPE treatment in full scan mode. The external standard calibration curves of 16 PAHs in selected ion mode were obtained in the concentration range from 5 to 1000 mg l 1 and hexamethylbenzene was selected as internal standard. Their calibration coefficients were all above 0.999. 3.7.1. Recovery of PAHs from aerosol matrix With the optimum SPE pretreatment procedures, aerosol was spiked with standard PAHs solution from the extraction step to observe the influence of matrix on the recovery of PAHs. To achieve this purpose, several sheets of aerosols

were cut into small pieces, and then divided into four parts that have the same weight. Two parts of the aerosol were processed with above optimum SPE procedure to determine the concentration of PAHs in the aerosols. Another two parts that were spiked with a solution containing the 16 EPA priority pollution PAHs were treated in the same procedure to examine the recoveries of PAHs. Recoveries of PAHs from aerosol matrix obtained in four parallel experiments are calculated and shown in Table 6. The recoveries of PAHs ranged from 83 to 97% and the R.S.D. are below 10%. These data are consistent with the recoveries of PAHs without matrix influences. The overall R.S.D. (%) are little bigger due to the various concentrations of PAHs in the matrix. It demonstrated that the pretreatment procedures are efficient for the cleaned-up and enrichment of PAHs in the aerosols. 3.7.2. Determination of PAHs in aerosols Analysis was carried out with aerosols of 6 different days per month from August 2001 to July

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Fig. 3. Full scan chromatograms of aerosol sample cleaned-up with (Bottom) and without (upper) SPE treatment. Chromatographic conditions see Section 2.1.

2002. With the optimal sample processing method established above, the contents of the 16 EPA priority pollution PAHs were determined and converted to ng per cubic meter in the air. Because the contents of PAHs in the aerosols varied in a

wide concentration range from day to day and some of them were beyond the linear range, the final sample solutions must be divided into two parts, one for the analysis of PAHs in low concentration levels and the another, after dilut-

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Table 6 Recovery of PAHs from aerosols matrix PAHs

Recovery (%)

R.S.D. (%)

NAPH ACY ACE FLU PHEN ANTH FLT PYR BaA CHRY BbF BkF BaP INPY DIBahA BghiP

85 91 89 88 95 97 83 91 95 88 86 91 94 88 97 95

6 7 6 5 4 4 5 6 4 10 5 8 4 4 6 5

Conditions: n /4.

ing, for determining the PAHs in high concentration levels. The average contents of PAHs in each season are listed in Table 7.

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The recoveries of PAHs during trapping on glass fiber filters are different for various PAHs [12]. The recoveries of NAPH, ACY, ACE and FLU are below 10%, and this is the one reason that they have low concentrations in aerosols. For PHEN and ANTH, the recoveries are about 10%. Most five- and six-ring PAHs (BkF, BaP, CHRY, INPY, DIBahA and BghiP) have higher recoveries (90
/95%), but the recovery of BbF is only about 60%. The recoveries of four-ring PAHs FLT, PYR, BaA are 20, 35 and 75%, respectively. There are few reports determining the PAHs contents of aerosols collected from Beijing District [32,33]. The ranges of PAHs contents (converted to ng per cubic meter) in recent publications are summarized and listed in Table 7 (the data in bracket). Because the concentration of PAHs differed significantly from day to day, the average contents of PAHs in each season depend largely on the day that the aerosols collected and the numbers of aerosols analyzed. The PAHs contents published have a wide range of variations. The results obtained by our determination are within

Table 7 Average contents of EPA priority pollution PAHs (ng m 3) in air determined from the aerosols

NAPH ACY ACE FLU PHEN ANTH FLT PYR BaA CHRY BbF BkF BaP INPY DIBahA BghiP Total

Winter (November 2001
/ February 2002)

Spring (March 2001
/ April 2002)

Summer (May 2002
/ July 2002)

Autumn (August 2001
/ October 2001)

0.85 0.85 0.02 3.25 (0.21
/2.68)a 24.45 (10
/77) 14.25 (0.50
/22) 81.50 (19
/159) 70.05 (18
/145) 26.65 (21
/114) 25.90 (16
/89) 73.65 (19
/109)b 24.65 48.55 (13
/74) 51.25 (7
/71) 4.95 (0.91
/6) 55.90 (13
/79) 505.72 (153
/873)

0.04 0.11 0 0.12 (0.01
/0.12) 1.43 (0.74
/22) 0.52 (0.05
/12) 3.17 (2
/17) 2.27 (2
/26) 1.60 (1
/18) 1.83 (1
/8) 3.87 (2
/62) 0.96 5.57 (1
/28) 6.67 (0.20
/18) 0.71 (0.18
/2) 4.13 (2
/14) 34.00 (21
/220)

0.13 0.18 0 0.38 (0.03
/0.05) 0.96 (0.08
/22) 1.10 (0.02
/16) 2.70 (0.30
/71) 2.73 (0.24
/45) 2.19 (0.28
/13) 0.89 (0.13
/1.72) 1.45 (0.14
/61) 0.54 0.57 (0.13
/26) 1.20 (0.84
/20) 0.70 (0.02
/1) 5.03 (0.19
/20) 20.75 (5
/396)

0.13 0.26 0.11 0.59 (0.09
/0.85) 5.07 (0.70
/77) 1.43 (0.05
/40) 9.97 (1.44
/238) 8.03 (0.60
/134) 3.27 (1
/52) 1.88 (0.20
/10) 5.83 (2
/204) 2.01 5.83 (0.82
/93) 4.30 (1
/55) 0.27 (0.25
/0.88) 12.20 (0.36
/13) 61.07 (14
/315)

a Data in brackets are the range of PAHs contents published in reference [32,33], including the high pollution urban areas and less pollution suburb areas. b Data in brackets are the sum of BbF and BkF.

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the range of publication for each season and each PAH. From Table 7, it can be seen that NAPH, ACY, ACE have relative low concentration in air, below 1 ng m3 in most case, and the content of ACE was lower than the detection limit. These PAHs have high volatility and low retention ability in column. The publications [32,33] have not detected these PAHs due to their pretreatment methods (column chromatography were used for the cleaned-up and enrichment of PAHs in aerosols). Although some studies showed that the changes in emission patterns (stationary and vehicular) and meteorological conditions (including less daylight hours, reduced ambient temperatures, and lower volatilization and photochemical activity) contributed to the higher PAHs levels during winter [34,35]. In northern China, the increased amount of coal burn for heating in winter will contribute more. In Beijing, heating began from November and ended in middle of March. In these periods, the PAHs contents are far higher than those of other seasons. The origin of the PAHs in other seasons need further study and will be discussed in next publications.

4. Conclusion A SPE method has been developed for the quantitative determination of PAHs in aerosols. Since petroleum ether or hexane leads a remarkable loss of the low molecular weight of PAHs, the rinsing step was omitted and a clean gas chromatogram can be got when mass spectrometer was operated in SIM mode. It was shown 20% dichloromethane in hexane (v/v) was enough to elute most PAHs from the silica SPE cartridge. Higher recoveries and lower S.D. were obtained with drying up the cartridge in vacuum after conditioning and after the sample loading. It was also shown that the sample volume could considerably influence the efficiency of sample preparation. No significant changes of recoveries were observed with the variation of the sample loading flow rates, but higher eluting flow-rate can reduce recoveries of PAHs apparently. The recovery of PAHs at the optimized SPE procedures ranged

from 79 to 96%, and the R.S.D. ranged from 2 to 10%. The method was applied for the analysis of aerosols collected from August 2001 to July 2002 in Beijing. Higher PAHs concentrations in air were found in winter than that in other season owing to the increased amount of coal burn for heating in winter in northern China.

Acknowledgements The authors would like to thank the financial support of National Science Foundation of China (Grant No. 29837190, 20077004) and Beijing Natural Science Fund (Grant No. 8991002).

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