Optimized cleanup method for the determination of short chain polychlorinated n-alkanes in sediments by high resolution gas chromatography/electron capture negative ion–low resolution mass spectrometry
Graphical abstract
The sediment sample could be purified by the optimized cleanup method, and satisfying cleanup efficiency was obtained.
Highlights
► The elution characters of sPCAs and interfering substances were evaluated on three adsorbents. ► An optimized cleanup method was developed for sPCAs with satisfying cleanup efficiency. ► The cleanup method combined with HRGC/ECNI–LRMS was applied for sPCAs analysis. ► The sPCAs levels range from 53.6 ng g−1 to 289.3 ng g−1 in tested sediment samples.
Introduction
Polychlorinated n-alkanes (PCAs) are commercial mixtures which have been synthesized for use as lubricant additives, cutting fluids, plasticizers and flame retardants since the 1930s [1], [2]. They have been detected in most of the environmental compartments including air, water, sediments and biota in the past ten years [1], [3]. Of particular interests are the short chain polychlorinated n-alkanes (sPCAs) with carbon chain lengths from 10 to 13 and a chlorine content between 30% and 72% by weight [4], which have the greatest potential for environmental release and the highest toxicity [5], [6], [7]. In 2007, European Community and its member states proposed sPCAs as persistent organic pollutants (POPs) candidate in document UNEP/POPS/COP.3/12 [8].
The environmental analysis of sPCAs is very challenging due to their complex composition, variation in congener patterns in the environment, and the lack of suitable reference standards [9]. So far, there is no standard routine analysis technique for reliable determination of sPCAs [10]. In the last two decades, numerous analytical approaches have been developed for the analysis of sPCAs in environmental matrix, which has been reviewed in detail [1], [11], [12]. Among them, high resolution gas chromatography (HRGC) coupled with mass spectrometry (MS) in electron capture negative ion (ECNI) mode is the most widely used approach, due to its high selectivity and sensitivity [10], [13]. However, some organochlorine compounds, such as polychlorinated biphenyls (PCBs) and some organochlorine pesticides, can interfere with the quantitative analysis of sPCAs, even using high resolution mass spectrometry (HRMS) [14]. Therefore, it is very necessary to establish a highly efficient cleanup method in order to separate sPCAs from these interfering substances [10], [15].
Column chromatography is mostly used for the cleanup of sPCAs in environmental matrix [3], [11]. Adsorbents such as silica gel, Florisil, and alumina are frequently employed. Several researchers adopted Florisil fully activated or partially deactivated with water to separate sPCAs from other contaminants in sediments using hexane and dichloromethane (DCM) as elution solvents [14], [16]. Using this method, sPCAs could be separated from all the PCBs, chlorinated benzenes, DDT and its metabolites. But more polar organochlorine compounds such as heptachlor epoxide and dieldrin are eluted with sPCAs in the same fraction [3]. Silica gel column deactivated with water (≤5%) have also been used for the separation of sPCAs from sediment, sludge and biota [17], [18], [19], [20]. With the same elution solvents on Florisil, the sPCAs fraction contained benzenehexachloride (HCH), heptachlor epoxide, and toxaphene, etc. [3]. Steven et al. [21] proposed a combination of silica gel with 3% water and alumina in a single cleanup column in the analysis of sPCAs in sludge, but no recovery data were reported. Marvin et al. [22] proposed a successive cleanup method using Florisil with 1.2% water and activated alumina, and reported that the internal standards recoveries were generally higher than 75%. However, Rieger and Ballschmiter [23] did not recommend the use of activated alumina considering the likelihood of dehydrochlorination of chlorinated paraffins during their adsorption on activated alumina. Besides, Parera et al. [16] found it difficult to elute sPCAs on activated alumina using pure DCM. Hence, the use of activated alumina as adsorbent of column chromatography for sPCAs analysis still needs further study.
As shown from the results of the laboratory intercomparison study organized by Pellizzato et al. [10], the analysis of sPCAs is far from being satisfactory. The lack of sufficient cleanup of the extract is one of the important reasons for the discrepancy in the results of sPCAs analysis among different laboratories. It is essential for the routine and reliable determination of sPCAs to establish a critical cleanup procedure which is selective enough to avoid potential interferences from the other organochlorine compounds. In this study, two-step column chromatography using silica and alumina (neutral or basic) as adsorbents were evaluated by comparing the recoveries and selective separation of sPCAs from their potential interfering substances such as PCBs, 17 different organochlorine pesticides and toxaphene. Quality parameters for the proposed cleanup method combined with HRGC/ECNI–low resolution mass spectrometry (HRGC/ECNI–LRMS) were established. China is the largest producer of PCAs worldwide [1], however, the studies of sPCAs in China are very limited. In this study, the analytical method established was applied for the determination of sPCAs mass fractions and congener patterns in sediment samples from the mouth of the Daliao River, which run through the largest chemical industry areas in Liaoning Province, China. It is hoped to provide insight into the level of sPCAs in China.
Section snippets
Reagents and materials
Three stock standard solutions of sPCAs (51%, 55.5% and 63% chlorine content, 100 ng μL−1 in cyclohexane) and four stock standard solutions of chloroparaffin C10-, C11-, C12- and C13-PCAs (65.0%, 55.2%, 55.0% and 55.0% chlorine content, 10 ng μL−1 in cyclohexane) were obtained from Dr. Ehrenstorfer (Augsburg, Germany). The sPCAs stock solutions with 53.5% and 59% chlorine content were achieved by 1:1 (v/v) mixing of the sPCAs stock standard solutions with 51% and 55.5% chlorine content as well as
Selection of m/z values for SIM mode
Preliminary studies were carried out to select representative ion m/z values for quantification of sPCAs in sediment samples purified by column chromatography. The response factor of sPCAs and the patterns of generated characteristic ions in the ECNI mode were demonstrated to be sensitive to the chlorine content of individual polychloroalkanes and ion source conditions [14], [27]. The ECNI mass spectra of 1,2,5,6,9,10-hexachlorodecane at the ion source temperature of 150 °C was shown in Fig. 1.
Conclusion
The analysis of sPCAs is complicated by the achievement of an efficient cleanup method because of the complex composition and the strong interferences from other organochlorine compounds. The cleanup efficiencies of three adsorbents, i.e. silica gel, neutral and basic alumina, were compared. The results indicate that a two-step cleanup method using silica gel column and subsequent basic alumina column following the Soxhlet extraction and GPC could completely eliminate the interferences from all
Acknowledgements
The authors thank the National S&T Major Project for water pollution control and management (grant no. 2009ZX07527), the National Natural Science Foundation of China (grant no. 21077102), and the National Basic Research Program of China (973 Program, grant no. 2009CB421602) for financial support.
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2020, Comprehensive Analytical ChemistryCitation Excerpt :PCB, HCB, o,p- and p,p'-DDT, o,p- and p,p'-DDE, octachlorostyrol (OCS), cis- and trans-chlordane, heptachlor, trans-nonachlor, mirex, and part of the toxaphene could be separated from SCCPs while heptachlor epoxide, hexachlorocyclohexanes, endosulfan, endosulfan sulphate, the greatest part of the toxaphene, and o,p- and p,p'-DDD were eluted with SCCPs. Further purification treatment, such as alumina chromatography column [36] or GPC [51], is required. Parera et al. [39] compared the elution recovery of SCCPs on completely activated and partially activated Florisil and silica gel.