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
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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)