Tadeusz Górecki

Theory of analyte extraction by selected porous polymer SPME fibres

Extraction of analytes by the new porous polymer solid phase microextraction (SPME) fibres is based on adsorption rather than absorption. The equilibrium theory developed for the liquid poly(dimethylsiloxane) (PDMS) coating does not apply to these coatings. The paper presents theoretical description of the extraction process for adsorption-type fibres, including PDMS–DVB (divinyl benzene), Carbowax–DVB and Carbowax–TR (template resin). The model is based on Langmuir adsorption isotherm. Expressions describing the amount of analyte extracted by the fibre in two- and three-phase systems are presented and discussed. The effect of selected experimental variables is discussed. In general, there is a non-linear dependence between the amount of an analyte extracted by the fibre and its concentration in a sample. The dependence can be approximated by a straight line for low concentrations only. Matrix composition can significantly affect the amount extracted. Interferences co-extracted with the analyte of interest may reduce the amount extracted and the quasi-linear range of the response. Great care should be exercised therefore when performing quantitative analysis with porous polymer SPME fibres. The phenomena discussed are illustrated on an example of benzene and 4-methyl-2-pentanone extraction from water by PDMS–DVB and Carbowax–DVB fibres.

Protocol for solid-phase microextraction method development.

Solid-phase microextraction (SPME) is a sample preparation method developed to solve some of the analytical challenges of sample preparation as well as sample introduction and integration of different analytical steps into one system. Since its development, the utilization of SPME has addressed the need to facilitate rapid sample preparation and integrate sampling, extraction, concentration and sample introduction to an analytical instrument into one solvent-free step. This achievement resulted in fast adoption of the technique in many fields of analytical chemistry and successful hyphenation to continuously developing sophisticated separation and detection systems. However, the facilitation of high-quality analytical methods in combination with SPME requires optimization of the parameters that affect the extraction efficiency of this sample preparation method. Therefore, the objective of the current protocol is to provide a detailed sequence of SPME optimization steps that can be applied toward development of SPME methods for a wide range of analytical applications.

Effect of Sample Volume on Quantitative Analysis by Solid-phase MicroextractionPart 1. Theoretical Considerations

This paper discusses the effect of sample volume on the amount of analyte extracted from a sample by solid-phase microextraction (SPME) in two-phase (sample-fiber coating) and three-phase (sample-headspace-fiber coating) systems. Up-to-date knowledge is summarized, and new concepts are introduced. The effect of sample volume on quantification and precision of results can be neglected only in rare cases. The minimum sample volume which ensures that the amount extracted, n, is lower than 1% of the initial amount of the analyte present in the sample, as well as the volume for which exactly half of the initial amount of the analyte is extracted, have been calculated for both two- and three-phase systems. It is critical that the volumes of samples and standards are the same during analysis by SPME. Extraction kinetics in headspace analysis is dependent on the headspace capacity. If it is sufficiently large, the analyte is extracted almost exclusively from the gaseous phase, and equilibration can be very fast. On the other hand, this causes a significant loss of sensitivity. The effect of sample volume on the determination of the value of the partition coefficient, K, is also discussed. If the change in concentration of the analyte in the sample at equilibrium is not taken into account, erroneous results are obtained. Even when a proper procedure is used, there are practical limitations to the accuracy of the K value determination. Large sample volumes should always be used for K value determination, as they enable broader ranges of K values to be covered with good accuracy.

Strategies for the analysis of polar solvents in liquid matrixes.

Various approaches to the analysis of polar compounds in different matrixes by solid-phase microextraction (SPME) were studied. The analysis of polar analytes in nonpolar matrixes was performed with custom-made SPME fibers coated with Nafion perfluorinated resin. The sensitivity of this fiber in this type of analysis was better by 1 order of magnitude on average as compared to those of any of the commercially available fibers. The fiber was the most sensitive for the most polar of the compounds studied, i.e., methanol. Determination of methanol, ethanol, and 2-propanol in unleaded gasoline was illustrated. Except for methanol, the fiber did not perform very well in the analysis of alcohols in water. The fiber was capable of extracting water from benzene. SPME analysis of polar compounds in water was studied using aqueous solutions of acetone, methyl ethyl ketone, methyl isobutyl ketone (MIBK), 2-propanol, 2-methyl-2-propanol, and tetrahydrofuran. Fibers coated with poly(dimethylsiloxane)/divinylbenzene yielded the highest sensitivity in this type of analysis. Low- or sub-ppb detection limits were obtained for all the analytes with FID detection when the samples were saturated with NaCl. Since fibers of this type extract analytes by adsorption rather than absorption, nonlinear responses were observed when all the analytes were allowed to equilibrate because of the limited number of adsorption sites on the surface of the coating and displacement of compounds with low distribution ratios by compounds with high distribution ratios (mainly MIBK). Two approaches allowed a significant improvement in linearity:  extraction of a vigorously stirred sample for a short time, or extraction under static conditions for a time much shorter than that required for equilibration of all the analytes. In both cases the amount of MIBK extracted was significantly reduced, while the remaining analytes were affected to a much lesser degree. The sensitivity of acetone determination was greatly improved by in-solution derivatization with o-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride and extraction of the oxime formed.

Applications of polydimethylsiloxane in analytical chemistry: a review.

Silicones have innumerable applications in many areas of life. Polydimethylsiloxane (PDMS), which belongs to the class of silicones, has been extensively used in the field of analytical chemistry owing to its favourable physicochemical properties. The use of PDMS in analytical chemistry gained importance with its application as a stationary phase in gas chromatographic separations. Since then it has been used in many sample preparation techniques such as solid phase microextraction (SPME), stir bar sorptive extraction (SBSE), thin-film extraction, permeation passive sampling, etc. Further, it is gaining importance in the manufacturing of lab-on-a-chip devices, which have revolutionized bio-analysis. Applications of devices containing PDMS and used in the field of analytical chemistry are reviewed in this paper.

Determination of tetraethyllead and inorganic lead in water by solid phase microextraction/gas chromatography

A new method for the determination of tetraethyllead (TEL) and ionic lead in water by SPME has been developed. TEL is extracted from the headspace over the sample. Inorganic lead is first derivatized with sodium tetraethylborate to form TEL, which is extracted in the same way as pure TEL samples. The analytical procedure was optimized with respect to pH, amount of derivatizing reagent added, stirring conditions, and extraction time. The detection limit obtained for TEL was found to be 100 ppt when using FID and 5 ppt when using ion trap MS (ITMS). The detection limit for Pb(2+), limited by the nonzero blank, was found to be 200 ppt. Linear calibration curves were obtained for both analytes when FID was used for detection. For lead they spanned over 4 orders of magnitude. ITMS offered excellent sensitivity and selectivity, but the calibration curves were nonlinear when the m/z = 295 ion was used for quantitation. The method has been verified on spiked tap water samples. An excellent agreement was found between the results obtained for standard solutions prepared using NANOpure water and spiked tap water samples.

Aromatic intermediate formation during oxidative degradation of Bisphenol A by homogeneous sub-stoichiometric Fenton reaction.

The elimination of Bisphenol A (BPA) from contaminated waters is an urgent challenge. This contribution focuses on BPA degradation by homogeneous Fenton reagent based on reactive ()OH radicals. Pronounced sub-stoichiometric amounts of H(2)O(2) oxidant were used to simulate economically viable processes and operation under not fully controlled conditions, as for example in in situ groundwater remediation. Aside from the most abundant benzenediols and the monohydroxylated BPA intermediate (which were detected as stable intermediates in earlier contributions), a wide array of aromatic products in the molecular weight range between 94 Da (phenol) and approximately 500 Da could be detected, the overwhelming majority of which have not been reported thus far. The identification was carried out by GC/MS analysis of trimethylsilyl ethers. The structural assignments were confirmed through the use of fully deuterated [(2)H(16)] BPA as the substrate, as well as using retention indices calculated on the basis of the increment system. The occurrence of aromatic intermediates larger than BPA, which typically share either a biphenyl- or a diphenylether structure, can be explained by oxidative coupling reactions of stabilized free radicals or by the addition of organoradicals (organocations) onto BPA molecules or benzenediols. The hydroxycyclohexadienyl radical of BPA was recognized to play central role in the degradation pathways. Ring opening products, including lactic, acetic and dicarboxylic acids, could be detected in addition to aromatic intermediates. Since some of those intermediates and products are recalcitrant to further oxidation under the conditions of sub-stoichiometric Fenton reaction, they should be carefully considered when designing and optimizing Fenton-driven remediation systems.

Isolation and preconcentration of volatile organic compounds from water

Abstract Methods for the isolation and/or concentration of volatile organic compounds from water samples for trace organic analysis by gas chromatography are reviewed. The following basic groups of methods are discussed: liquid-liquid extraction, adsorption on solid sorbents, extraction with gas (gas stripping and static and dynamic headspace techniques) and membrane processes. The theoretical bases of these methods are discussed. Experimental arrangements for the isolation and/or concentration of volatile compounds from water are presented and discussed with respect to their efficiency. The applicability of the described methods to the isolation and/or concentration of various organic compounds from waters of various origins is discussed.

Development of membrane extraction with a sorbent interface–micro gas chromatography system for field analysis

The commercially available portable gas chromatographs have a rather limited scope of applications, typically allowing analysis of gaseous samples only, and having relatively poor sensitivity. Combination of those instruments with modern sampling/sample preparation techniques can remedy these problems. A Chrompack micro-GC system equipped with a thermal conductivity detector has been coupled to membrane extraction with a sorbent interface (MESI). The sorbent trap has replaced the GC injector. The design of the trap was also modified in order to enhance the preconcentration of analytes. The use of a thin flat sheet membrane reduces the response time, and decreases the memory effect of the system. Rapid separation times were achieved, and the sensitivity was significantly improved. MESI enables semi-continuous monitoring of both gaseous and aqueous samples, owing to the selectivity of the membrane material. The system does not use moving parts, therefore being reliable. The sensitivity of the micro-GC system was increased by a factor of more than 100 by the addition of the MESI system, even with a preconcentration time as short as 1 min. Chloroform, having a concentration lower than 1 ppb, was detected in tap water. A cup system was used to allow headspace sampling of volatile organic compounds from aqueous matrices, keeping the membrane away from interfering species that could be present in water, and improving the mass transfer. A linear calibration line was obtained, and the estimated limit of detection was 60 ppt. This represents a great improvement for the sensitivity of the micro-GC system.

The effect of sample volume on quantitative analysis by solid phase microextraction Part 2.† Experimental verification

The sample volume plays a very important role in solid phase microextraction (SPME) analysis. Its effect on the results of analysis can be neglected only when it is much larger than the fibre capacity KVf (K = fibre/sample partition coefficient, Vf = fibre volume). Good agreement was obtained between theoretical predictions and experimental results for analyte extraction from two- and three-phase systems. The effect of headspace capacity on SPME extraction results and kinetics was illustrated on an example of amphetamine and methamphetamine determination in water. A dramatic improvement in extraction speed was achieved by increasing the extraction temperature, and thus also the headspace–sample partition coefficients. Difficulties with the accurate determination of large partition coefficients are discussed on an example of the extraction of C8–C12 hydrocarbons from air. Analyte sorption on the container walls led to significant losses of less volatile compounds, especially when vials of large surface-to-volume ratio were used. A discussion of problems encountered when trying to determine accurately partition coefficients of semi-volatile compounds in water is also presented.