Simultaneous determination of saturated and aromatic hydrocarbons in soil by on-line high performance liquid chromatography-gas chromatography

doi:10.3724/SP.J.1123.2021.02011

Abstract

Abstract:

To strengthen regulation for mitigating the risk posed by petroleum contaminants in soil, the Ministry of Ecology and Environment of the People’s Republic of China has classified petroleum hydrocarbons as a key monitoring item for regulatory contamination monitoring. Petroleum is principally derived from petroleum and synthetic fuels, which contain an extremely high content of hydrocarbon compounds that have varied boiling points. These compounds are chemically classified primarily as saturated and aromatic. Aromatic hydrocarbons are typically highly alkylated monocyclic, bicyclic, and polycyclic, which are more toxic to human and animal life than saturated hydrocarbons. Because of the significant toxicological differences among the various hydrocarbons, it is difficult to accurately assess their environmental toxicity by only determining the total content of petroleum in soil. However, there are no analytical methods for the simultaneous determination of saturated hydrocarbons and aromatic hydrocarbons in soil according to Chinese standards.
In this study, extraction and purification procedures were completely optimized depending on the matrix of the soil samples. The advanced analytical technique of on-line high performance liquid chromatography-gas chromatography (HPLC-GC) was performed after sample preparation for the simultaneous determination of saturated hydrocarbons and aromatic hydrocarbons in soil. For the extraction, n-hexane/ethanol (1∶1, v/v) was chosen as the extraction solvent. The ratio of solid sample (soil) to the solvent was chosen as 1∶4, and extraction was performed once at room temperature, for 1 h. Water was then added and mixed to remove ethanol from the extracts, and the upper n-hexane layer was separated; thus, the petroleum hydrocarbons in the samples were completely extracted. However, the oils and fats in the matrix of the soil sample were also simultaneously extracted. Because of the limited retention capacity of the HPLC column, the presence of oils, fats, and other interferents would affect the subsequent determination of saturated hydrocarbons and aromatic hydrocarbons. Therefore, an additional purification step is required before sample injection into the HPLC-GC equipment. In this study, purification was performed using a lab-made silica gel column, which is commonly used for the determination of saturated hydrocarbons and aromatic hydrocarbons in food. The purification column was conditioned and eluted with an 8∶2 ratio (v/v) of n-hexane to dichloromethane after sample loading. Subsequently, the eluent was concentrated and injected into the HPLC-GC equipment for analysis. The flame ionization detector (FID) is ideal for petroleum hydrocarbons quantification because of its nearly identical responses to all hydrocarbons; hence, with the FID, reference standards are not required for quantification, and internal standards are typically used for quantifying the total hydrocarbon content. In this study, cyclohexylcyclohexane (Cycy) and 2-methylnaphthalene (2-MN) were used as internal standards for determining the saturated and aromatic hydrocarbons, respectively. The limit of quantification (LOQ) of this proposed method was 0.4 mg/kg. Moreover, the suitability of the method was verified by comparing the obtained content against the soil petroleum hydrocarbon standard (SQC-116); the measured value was found to be within the credible interval provided by the standard. The relative error (RE) was 10.6% with a relative standard deviation (RSD) of 1.4%, which indicates that the proposed method is accurate and reliable, and the precision meets analytical requirements. Finally, the method was applied to the determination of hydrocarbons in five soil samples from the Beijing area. Saturated hydrocarbons (C₁₀-C₄₀) were detected in all five samples, with contents ranging from 3.3 to 32.1 mg/kg, while aromatic hydrocarbons (C₁₀-C₄₀) were detected in four samples, with contents ranging from 0.8 to 4.3 mg/kg.
HPLC-GC combines the high selectivity of HPLC with the high separation efficiency of GC, and as demonstrated in this study, can be used for the simultaneous determination of saturated and aromatic hydrocarbons in soil. The source of hydrocarbon contamination can also be preliminarily identified by chromatographic analysis.

Key words: on-line high performance liquid chromatography-gas chromatography (HPLC-GC), saturated hydrocarbons, aromatic hydrocarbons, soil

CLC Number:

O658

LIU Lingling, LI Bingning, WU Yanwen. Simultaneous determination of saturated and aromatic hydrocarbons in soil by on-line high performance liquid chromatography-gas chromatography[J]. Chinese Journal of Chromatography, 2021, 39(8): 905-912.

Figures/Tables 7

Fig. 1 HPLC-GC chromatograms of saturated hydrocarbons (SH) in the same soil sample extracted using different solvents Cycy: cyclohexylcyclohexane; Cho: 5α-cholestane.

Table 1 Influence of extraction time on the determination of SH (C10-C40) in the same soil sample

Extraction time	Contents/(mg/kg)				RSD/%
Extraction time	1	2	3	Average	RSD/%
1	14.0	13.8	13.8	13.9	0.8
2	14.3	15.1	14.2	14.5	3.4

Fig. 2 Elution order of mixed standard solution containing SH and aromatic hydrocarbons (AH) through the SPE column packed with silica gel and a solvent mixture of n-hexane and dichloromethane (8∶2, v/v) 5B: pentylbenzene; MN: methylnaphthalene; TBB: tri-tert-butylbenzene; Per: perylene.

Fig. 3 HPLC-GC chromatograms of mixed standard solution of SH/AH purified by the SPE column packed with silica gel

Fig. 4 HPLC-GC chromatogram of n-alkane (C10-C40) standards

Table 2 Contents of SH and AH (C10-C40) in soil samples

Sample No.	SH/(mg/kg)	AH/(mg/kg)	Aromatics ratio/%
1	6.6	1.0	13.2
2	32.1	4.3	11.8
3	3.3	<0.4	-
4	13.8	1.1	7.4
5	11.8	0.8	6.3

Fig. 5 HPLC-GC chromatograms of SHs and AHs in two soil samples

References

[1]	Ossai I C, Ahmed A, Hassan A, et al. Environ Technol Inno, 2020,17:100526
[2]	Caetano M O, Schneider I A H, Gomes L P, et al. Environ Technol, 2017,38(11):1408
[3]	Lv M, Luan X L, Liao C Y, et al. Nat Sustain, 2020,3:878
[4]	European Committee for Standardization. Soil Quality-Determination of Content of Hydrocarbon in the Range C₁₀ to C₄₀ by Gas Chromatography. London, Britain: British Standards Institution, 2011
[5]	Massachusetts Department of Environmental Protection. Method for the Determination of Extractable Petroleum Hydrocarbons (EPH). Massachusetts, U. S.: Commonwealth of Massachusetts, 2004
[6]	Pikovskii Y I, Korotkov L A, Smirnova M A, et al. Eurasian Soil Sci, 2017,50(10):1125
[7]	Okparanma R N, Mouazen A M. Appl Spectrosc Rev, 2013,48(6):458
[8]	Zhang L F, Liu F, Ren Y, et al. Chemical Reagents, 2017,39(12):1289
[8]	张利飞, 刘昉, 任玥, 等. 化学试剂, 2017,39(12):1289
[9]	Liu D Q. Environmental Monitoring in China, 2020,36(1):138
[9]	刘丹青. 中国环境监测, 2020,36(1):138
[10]	Wu Y W, Li B N, Liu L L, et al. Chinese J Anal Chem, 2021,49(3):341
[11]	Nespeca M G, Sequinel R, Oliveira J E D. Microchem J, 2019,150:104163
[12]	Li T Y, Cao X C, He S H, et al. Environ Sci Pollut Res, 2020,27:37444
[13]	Cheng Z Y, Gao X Y, Zhang L, et al. Chemical Analysis and Meterage, 2020,29(5):112
[13]	程志炎, 高翔宇, 张利, 等. 化学分析计量, 2020,29(5):112
[14]	Zhao C P, Feng X K, Zhu Q. Physical Testing and Chemical Analysis (Part B:Chemical Analysis), 2020,56(7):827
[14]	赵昌平, 冯小康, 朱强. 理化检验-化学分册, 2020,56(7):827
[15]	Ministry of Ecology and Environment of the People’s Republic of China. Soil Environmental Quality Risk Control Standard for Soil Contamination of Development Land. Beijing: China Environment Publishing Group, 2018
[15]	生态环境部. 土壤环境质量建设用地土壤污染风险管控标准(试行). 北京: 中国环境出版集团, 2018
[16]	Ministry of Ecology and Environment of the People’s Republic of China. Soil and Sediment-Determination of Petroleum Hydrocarbons (C10-C40)-Gas Chromatography. Beijing: China Environment Publishing Group, 2019
[16]	生态环境部. 土壤和沉积物石油烃(C10~C40)的测定气相色谱法. 北京: 中国环境出版集团, 2019
[17]	European Food Safety Authority (EFSA). EFSA Journal, 2012,10(6):2704
[18]	Biedermann M, Grob K. J Chromatogr A, 2012,1255:56
[19]	Zhang S C, Liu L L, Li B N, et al. Food Addit Contam A, 2019,36(8):1261
[20]	Xie Y Q, Li B N, Liu L L, et al. Food Chem, 2019,294:458
[21]	Xie Y Q, Liu L L, Li B N, et al. Food Science, 2020, http://kns.cnki.net/kcms/detail/11.2206.TS.20200514.1025.026.html
[21]	谢尧卿, 刘玲玲, 李冰宁, 等. 食品科学, 2020, http://kns.cnki.net/kcms/detail/11.2206.TS.20200514.1025.026.html
[22]	Liu L L, Li B N, Ouyang J, et al. Food Chem, 2020,333:127488
[23]	United States Environmental Protection Agency. SW-846 Test Method 3540C: Soxhlet Extraction. [2021-02-07]. https://www.epa.gov/sites/production/files/2015-12/documents/3540c.pdf
[24]	Wei D, Guo M, Wu H Z, et al. Chinese Journal of Chromatography, 2020,38(8):945
[24]	魏丹, 国明, 吴慧珍, 等. 色谱, 2020,38(8):945
[25]	Ding M Y, Yin W, He H J, et al. Pretreatment Technology and Application of Analytical Samples. Beijing: Tsinghua University Press, 2017: 162
[25]	丁明玉, 尹洧, 何洪巨, 等. 分析样品前处理技术与应用. 北京: 清华大学出版社, 2017: 162
[26]	Liu L L, Wu Y W, Li B N, et al. Chinese Journal of Analytical Chemistry, 2016,44(9):1419
[26]	刘玲玲, 武彦文, 李冰宁, 等. 分析化学, 2016,44(9):1419
[27]	Li B N, Liu L L, Zhang Z X, et al. Chinese Journal of Analytical Chemistry, 2017,45(4):514
[27]	李冰宁, 刘玲玲, 张贞霞, 等. 分析化学, 2017,45(4):514
[28]	Li B N, Wu Y W, Liu L L, et al. J Am Oil Chem Soc, 2017,94(2):215
[29]	Liu L L, Huang H, Wu Y W, et al. J Oleo Sci, 2017,66(9):981
[30]	Liu L L, Zhang Z X, Li B N, et al. Food Science, 2017,38(22):303
[30]	刘玲玲, 张贞霞, 李冰宁, 等. 食品科学, 2017,38(22):303
[31]	Biedermann M, Munoz C, Grob K. J Chromatogr A, 2017,1521:140
[32]	Liu L L, Li B N, Ouyang J, et al. Food Science, 2019,40(16):302
[32]	刘玲玲, 李冰宁, 欧阳杰, 等. 食品科学, 2019,40(16):302