Determination of Chlorophenols and their Degradation Metabolites in Environmental Samples by SPME and on-fibre Silylation with GC–MS

Dec 01, 2010
By LCGC Editors
Volume 23, Issue 11

A simple, rapid and sensitive method based on solid-phase microextraction (SPME) and gas chromatography–mass spectroscopy (GC–MS) was developed to determine five chlorophenols and their two major metabolites in aqueous samples. To improve the peak shape, silylation was performed on the SPME fibre to get less polar forms, which are more favourable for GC–MS analysis. All parameters influencing SPME–GC–MS performance, including pH, salinity, extraction time, derivatization time, as well as desorption time, were carefully optimized. The method was successfully applied to the analysis of chlorophenols and their metabolites, excluding tetrachlorohydroquinone in pond water with recovery percentages ranging from 80.4% to 91.4%.

Chlorophenols (CPs), are known to contaminate the environment, and are highly toxic, poorly biodegradable and potentially carcinogenic.1,2 Within the list of 11 phenolic compounds considered as major pollutants, drawn up by the US Environmental Protection Agency (EPA), CPs are among the most toxic and carcinogenic.3 They are produced commercially for use as preservative agents, pesticides, antiseptics, disinfectants and are also used as intermediates in many industries including the pharmaceutical industry.4 In addition, some chlorophenols, especially 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP), could be produced when water is disinfected with chlorine, resulting in unpleasant and persistent organoleptic properties in the disinfected water.5,6

CPs enter the environment either directly from industrial waste or indirectly as important metabolites of some other chlorinated pesticides, such as PCBs.7 As well as CPs, their two major metabolites, dichlorohydroquinone and tetrachlorohydroquinone, also show high toxicities. They were reported to be more toxic because they could cause DNA damage by breaking a single strand of DNA chain.8,9 This makes the simultaneous determination of CPs and their metabolites very important when accessing their environmental risks. However, until now, most publications have put the emphasis on the analysis of one specific CP, or a group of CPs, but have ignored their metabolites.

Different techniques have been used to determine chlorophenols. Most of them used chromatographic-based instrumentation such as gas chromatography (GC)4,10–20 and high performance liquid chromatography (HPLC), particularly reversed-phase liquid chromatography (RPLC)21–31 in combination with ultraviolet detection (UV),22–25 fluorescence (F) detection,26,27 electrochemical (EC) detection28 or mass spectrometry (MS).13–20,29,30 Apart from when coupled with fluorescence or an MS detector, HPLC coupled with other detection methods suffers from interference from matrix compounds such as humic substances naturally occurring in environmental samples. GC is therefore preferable because of its high resolution, high sensitivity and selectivity when coupled with a versatile, selective detector such as an electron-capture detector (ECD), atomic emission detector (AED) or mass spectrometer11–20 During the GC separation process, peaks obtained from free chlorophenols show strong tailings. In this case, derivatization methods were suggested to get less polar forms, which could improve the peak shape.12,14,16 Silylation is the technique of choice because phenolic functional groups can be readily derivatized, and the reaction mixture can be directly injected into the GC without further sample pretreatment. This technique, however, requires prior separation of the analytes from the sample to avoid quenching of the silylating agent by the large amount of water or ethanol present in the matrix. Therefore, two of the major drawbacks of these technologies are laborious sample pretreatment and poor recoveries because of the volatile nature of the derivatives.

Due to the low concentration levels of CPs and the complicated matrices of environmental samples, different preconcentration methods have been used prior to the analysis. Liquid-liquid extraction (LLE)29 and solid-phase extraction (SPE)11,12,23,24,30 have been widely used. However, these traditional methods have the well-known disadvantages of being solvent- and sample consuming, time- and labour consuming and also have the risk of analyte losses. Therefore, liquid phase microextraction (LPME),13,15,16,22,25 solid phase microextraction (SPME)19–21 and head-space solid phase microextraction (HS-SPME)10 have been proposed as alternative methods to avoid the use of large volumes of sample and organic solvent. However, the precision of LPME suffers from the instability of the droplet when using single droplet LPME as a sample preparation method. Recently, research concerning the determination of chlorophenols by SPME has focused on the evaluation of new fibre coatings.10,19,20 These research projects have broadened the range of SPME applications. However, the new fibre coatings are not commercially available and are instable from fibre to fibre, which is the main problem to be solved.

The aim of this paper is to develop a simple, rapid and sensitive method for the determination of chlorophenols, as well as their degradation metabolites, in aqueous samples. To achieve this, SPME in combination with GC–MS was used. In addition, the silylation method was used to improve the peak shape. This is different from the traditional silylation procedure. The silylation in this paper was carried out on the SPME fibre based on head-space concept (i.e., the fibre was inserted into the headspace of silylating agent), after the extraction of chlorophenols from aqueous matrix. On-fibre silylation has the advantages of SPME but also makes up for the drawbacks of traditional silylation because the analytes have been separated by the fibre from the matrix.31–33

The small volume of water remaining on the fibre can react with the excess vapour of the silylating agent, whereas the silylating agent would be quenched by the large amount of water or ethanol present in the matrix during the traditional silylation procedure. Moreover, the derivatives generated on-fibre could be introduced directly and immediately into GC, which avoids the losses of analytes.

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