Optimization strategies of in-tube extraction (ITEX) methods

Analytical and Bioanalytical Chemistry, Jul 2015

Microextraction techniques, especially dynamic techniques like in-tube extraction (ITEX), can require an extensive method optimization procedure. This work summarizes the experiences from several methods and gives recommendations for the setting of proper extraction conditions to minimize experimental effort. Therefore, the governing parameters of the extraction and injection stages are discussed. This includes the relative extraction efficiencies of 11 kinds of sorbent tubes, either commercially available or custom made, regarding 53 analytes from different classes of compounds. They cover aromatics, heterocyclic aromatics, halogenated hydrocarbons, fuel oxygenates, alcohols, esters, and aldehydes. The number of extraction strokes and the corresponding extraction flow, also in dependence of the expected analyte concentrations, are discussed as well as the interactions between sample and extraction phase temperature. The injection parameters cover two different injection methods. The first is intended for the analysis of highly volatile analytes and the second either for the analysis of lower volatile analytes or when the analytes can be re-focused by a cold trap. The desorption volume, the desorption temperature, and the desorption flow are compared, together with the suitability of both methods for analytes of varying volatilities. The results are summarized in a flow chart, which can be used to select favorable starting conditions for further method optimization.

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Optimization strategies of in-tube extraction (ITEX) methods

Optimization strategies of in-tube extraction (ITEX) methods Jens Laaks 0 1 2 Maik A. Jochmann 0 1 2 Beat Schilling 0 1 2 Torsten C. Schmidt 0 1 2 0 BGB Analytik AG , Lettenstrasse 97, 8134 Adliswil , Switzerland 1 Instrumental Analytical Chemistry, University Duisburg-Essen , Universitätsstrasse 5, 45141 Essen , Germany 2 Maik A. Jochmann Microextraction techniques, especially dynamic techniques like in-tube extraction (ITEX), can require an extensive method optimization procedure. This work summarizes the experiences from several methods and gives recommendations for the setting of proper extraction conditions to minimize experimental effort. Therefore, the governing parameters of the extraction and injection stages are discussed. This includes the relative extraction efficiencies of 11 kinds of sorbent tubes, either commercially available or custom made, regarding 53 analytes from different classes of compounds. They cover aromatics, heterocyclic aromatics, halogenated hydrocarbons, fuel oxygenates, alcohols, esters, and aldehydes. The number of extraction strokes and the corresponding extraction flow, also in dependence of the expected analyte concentrations, are discussed as well as the interactions between sample and extraction phase temperature. The injection parameters cover two different injection methods. The first is intended for the analysis of highly volatile analytes and the second either for the analysis of lower volatile analytes or when the analytes can be re-focused by a cold trap. The desorption volume, the desorption temperature, and the desorption flow are compared, together with the suitability of both methods for analytes of varying volatilities. The results are summarized in a flow chart, which can be used to select favorable starting conditions for further method optimization. In-tube extraction; ITEX; ITEX DHS; Method development; Parameter optimization - Method development for microextraction techniques can be a very time-consuming task, because a multitude of different parameters influence the efficiency of extraction. Even in the simplest system, where only a coated fiber (solid-phase microextraction or SPME) is immersed in a liquid sample, the extraction can be influenced by (i) the choice of the polymeric coating, (ii) the extraction time together with (iii) shaking or stirring, (iv) the extraction temperature, (v) the pH for ionizable compounds, (vi) the ionic strength, and (vii) the presence of organic solvents or matrix compounds such as humic substances [1]. Dynamic microextraction techniques, where the sample is actively passed over the sorbent material or through a sorbent bed, are more complex and thus have even more parameters to optimize during the steps of the extraction and thermal desorption procedure, e.g., the volume and the corresponding flows that are applied during extraction and desorption [2–4]. I n - t u b e e x t r a c t i o n ( I T E X ) i s a f u l l y a u t o m a t e d microextraction technique for CTC PAL series autosamplers and uses a gastight syringe to pump the sample headspace repeatedly through an attached tube, filled with a sorbent material for analyte enrichment. The syringe, as well as the sorbent tube, is enclosed by an electric heater to avoid sample condensation in the syringe and to facilitate thermal desorption to the inlet system of the gas chromatograph, respectively. The syringe also features a side-port hole in the glass body, which allows the flushing of the syringe and the sorbent tube with a pure, inert gas for trap conditioning to avoid carryover between analyses. The four stages of the ITEX procedure (sample conditioning, analyte extraction/sorption, desorption/injection, and trap conditioning), together with the main parameters governing the performance of each stage, are depicted in Fig. 1 [4]. The aim of this work is to summarize the experiences gained in the ITEX method development and to present a guideline that allows future user to minimize the number of experiments, which are required to find the appropriate parameters for their analytical task. The target compounds used in the developed methods can be sorted into two categories: volatile organic compounds (VOCs) as water contaminants and aroma compounds in food matrices. The VOCs are comprised of halogenated hydrocarbons, BTEX compounds (benzene, toluene, ethylbenzene, xylenes), and gasoline oxygenates [ethyl tert-butyl ether (ETBE), methyl tert-butyl ether (MTBE), and tert-amyl methyl ether (TAME)]. The volatile compounds include several alcohols, aldehydes, esters, terpenes, and 2,3-butanedione, pyridine, methylpyrazine, and 2-furanmethanol. A complete list, together with the sample matrix and the used sorbent material, is given in Table 1. The experiments were performed on two instruments. The first instrument was a Thermo Trace GC Ultra (S+H Analytik, Fig. 1 Stages of the ITEX procedure with the corresponding parameters for optimization, adapted from [4] Mö (...truncated)


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Jens Laaks, Maik A. Jochmann, Beat Schilling, Torsten C. Schmidt. Optimization strategies of in-tube extraction (ITEX) methods, Analytical and Bioanalytical Chemistry, 2015, pp. 6827-6838, Volume 407, Issue 22, DOI: 10.1007/s00216-015-8854-4