Depletion solid-phase microextraction for the evaluation of fiber-sample partition coefficients of pesticides

A depletion solid-phase microextraction (SPME) method for the characterisation of SPME sorption for 13 pesticides selected as probe compounds is described. The sample is extracted and analysed multiple times by SPME-GC/MS. The observed depletion in peak areas is used for the calculation of extraction ratios that varied between 3 and 28% for a PDMS fiber with confidence intervals between 0.7 and 5.4%. Apparent fiber-sample partition coefficients can be calculated and extrapolated to equilibrium conditions if specific sorption kinetics are known. Under the chosen conditions, problems were encountered for more polar compounds (logK(ow)<3) due to inefficient extraction. The extracted amount was found to be the decisive parameter for depletion SPME and the extraction conditions therefore need to be adapted to the polarity of the analyte. The importance of the initial analyte concentration especially for mixed-mode fibers is demonstrated. Compared with conventional external calibration using liquid injection, depletion SPME eliminates uncertainties due to solvent effects during injection. Furthermore, it does neither require authentic reference compounds nor knowledge of the initial analyte concentration, and thus can even be used for unknowns.     

Use of solid-phase microextraction for measuring oil-water partition coefficients and correlation with high-performance liquid chromatographic methods for lipophilicity

For flavour compounds, lipophilicity is often estimated by the partition coefficient between oil and water (log Koil-water), which is highly relevant to food. A modification of the shake-flask method is reported here where compounds are quantified in the two phases using solid-phase microextraction (SPME). SPME's highly sensitivity to non-polar compounds facilitates quantification in the water phase. Twelve flavour compounds representing a broad range of lipophilicities and functional groups were analysed by two methods. Their log Koil-water was determined using SPME quantitation and their log k(w) using a reversed-phase HPLC methodology. The isocratic capacity factor at 60% methanol and predicted log P value also showed high correlation factors with other methods. The octadecyl silylated surface of the HPLC column provides a matrix that interacts with lipophilic compounds where the retention time is the indication of lipophilicity. Both methods gave reproducible results (median 3% and 4% RSD) and similar but not identical values for lipophilicity. The relationship between the two methods is log k(w) =0.85 log Koil-water +0.48 with a correlation coefficient of 0.94. The new SPME detection method, with the ability to quantify limonene and 2-pentylfuran at 1 ppm in the water phase, is preferred for flavour compound analysis due to the applicability of oil-water partitioning in food.     

Determination of liposome/water partition coefficients of organic acids and bases by solid-phase microextraction

The extraction of two methylated anilines and three chlorinated phenols by solid-phase microextraction (SPME) fibers coated with polyacrylate was investigated as a function of pH. Only the neutral species of the acids and bases partitioned into the polymer. Extraction kinetics were accelerated for the hydrophobic phenols at pH values around their acidity constant. This is presumably due to a reconstitution of the neutral species in the unstirred aqueous layer adjacent to the polymer surface by the charged species through the fast acid-base equilibrium. Although the charged species is not taken up into the polymer, liposome/water distribution ratios could be measured up to a pH value, where 99% of the compounds were present as charged species. The partition coefficients of the neutral and charged species were extrapolated from the pH profiles of the liposome/water distribution ratios. The resulting values were slightly lower than those measured with equilibrium dialysis. The discrepancies are discussed with respect to differences in the experimental conditions and the possibility of matrix effects during SPME measurements.     

Solid-Phase microextraction with Whatman 1PS paper and direct room-temperature solid-matrix luminescence analysis

Solid-phase microextraction has been combined with solid-matrix luminescence for the detection of a variety of compounds at sub-nanogram levels for the first time. Whatman 1PS paper was placed in water solutions of polar and nonpolar compounds for the selective removal of the nonpolar compounds such as benzo(a)pyrene. Distribution constants were obtained for 4-phenylphenol, benzo(f)quinoline, benzo(h)quinoline, phenanthrene, and benzo(a)pyrene. The distribution constants showed that phenanthrene and benzo(a)pyrene in water had a very strong affinity for the 1PS paper. Once the solutes were extracted for a fixed period of time, the 1PS paper was dried, and either the solid-matrix fluorescence or solid-matrix phosphorescence was detected from the adsorbed lumiphors by using the appropriate excitation wavelengths. It was a simple matter to detect at least three adsorbed compounds on the 1PS paper by solid-matrix luminescence. Benzo(a)pyrene was easily detected at a level of 0.02 ng ml⁻¹ in water.     

Equilibrium and kinetic solid-phase microextraction determination of the partition coefficients between polychlorinated biphenyl congeners and dissolved humic acid

A conventional solid-phase microextraction (SPME) method combined with liquid–liquid extraction was applied under equilibrium and nonequilibrium conditions to determine the partition coefficients (K_doc) of 25 polychlorinated biphenyl congeners (PCBs) between Sigma–Aldrich humic acid (HA) and water. The values of log K_doc determined with equilibrium SPME were linearly correlated with the logarithm of the octanol–water partition coefficients (K_ow) for PCB congeners at log K_ow < ∼7.2, but the trends were disrupted for log K_ow from ∼7.2 to 8.18. In addition, short-term (5 min to 4 days) and long-term (5–44 days) uptake profiles of PCBs were established, from which a pseudo-equilibrium for sorption of PCBs was revealed at ∼4 days of extraction. To understand this phenomenon, the uptake profiles were fitted with two equations (one equation is often used for pure water samples and the other one is applicable for samples containing complex matrices) derived from a first-order kinetics model. Subsequently, K_doc values obtained through kinetic approaches were compared with those acquired from equilibrium SPME. The comparison of K_doc values indicated that the pseudo-equilibrium was caused by the slow desorption of PCBs from HA rather than the biphasic desorption mechanism.     

碳纳米管固相微萃取涂层吸附性能研究

利用空气氧化和稀酸回流纯化单壁碳纳米管,用高分辨透射电镜、拉曼光谱对碳纳米管进行了表征.在分子模拟中,非极性氢气、甲烷分子采用单点Lennard-Jones球形分子模型,流体分子与C原子之间相互作用采用虚拟原子模型.以液氮温度下碳纳米管对氮气的吸附等温线实验数据为依据,利用巨正则蒙特卡罗方法模拟得到了碳纳米管的孔径分布,主要集中在6nm.计算了常温常压下碳纳米管中甲烷及氢气的吸附等温线,298K及0.1MPa压力下,氢气的吸附量达到0.015%(质量分数),甲烷在样品中的吸附量可以达到0.5%(质量分数).模拟研究结果表明碳纳米管可以用作固相微萃取涂层材料.     

Determination of haloanisoles in paper samples for food packaging by solid-phase microextraction and gas chromatography

A method was developed for the determination of trichloroanisole, tribromoanisole and pentachloroanisol by solid-phase microextraction and gas chromatography in paper samples (Kraft liner, Test liner and Miolo). Four commercial SPME fibers were evaluated: Polydimethylsiloxane (PDMS), Polyacrylate (PA), Carbowax/Divinylbenzene (CW/DVB) and Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS). DVB/CAR/PDMS gave the best results and was therefore selected. Other variables involved in the extraction procedure were studied and optimized, such as: sample volume, salting-out effect, temperature and extraction time, effect of organic solvent and previous sample preparation. Optimum conditions were obtained using 20 mL of sample with 5 mol L⁻¹ NaCl in a 40 mL vial, extraction temperature of 70 °C and sonication and extraction time of 30 and 40 min, respectively. Detection limits ranged from 0.43 to 1.32 ng g⁻¹ for all analytes. Recoveries between 70 and 100% were obtained and relative standard deviation was below 10% for all compounds.     

Solid-phase microextraction for the evaluation of partition coefficients of a chlorinated dioxin and hexachlorobenzene into humic substances.

We developed a method for the evaluation of the partition coefficients (K(oc)) of hexachlorobenzene (HCB) and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) into humic substances (HSs) by using solid-phase microextraction (SPME). In the aqueous solution containing HCB or HpCDD and HS, the unbound species of HCB or HpCDD were accumulated on the SPME fiber. Subsequently, HCB or HpCDD on the SPME fiber was directly analyzed by GC-ECD. When the concentration of organic carbon in HS ([OC]) was plotted against the ratio of [HCB] or [HpCDD] in the absence of HS to that in the presence of HS, linear relationships were observed. The slope of the line corresponded to the K(oc) value. The log K(oc) values for HCB and HpCDD evaluated were in the ranges of 3.9 - 4.9 and 5.9 - 7.2, respectively. These values were the same order as those in the literature, which were evaluated by other methods (e.g., solubility enhancement, solid-phase extraction and dialysis). The relative standard deviations of the log K(oc) values evaluated in this study were within 5%.     

Characterization of the sorption of gaseous and organic solutes onto polydimethyl siloxane solid-phase microextraction surfaces using the Abraham model

Water-to-polydimethylsiloxane (PDMS) and gas-to-PDMS sorption coefficients have been compiled for 170 gaseous and organic solutes. Both sets of sorption coefficients were analyzed using the Abraham solvation parameter model. Correlations were obtained for both "dry" headspace solid-phase microextraction and conventional "wet" PDMS coated surfaces. The derived equations correlated the experimental water-to-PDMS and gas-to-PDMS data to better than 0.17 and 0.18 log units, respectively. In the case of the gas-to-PDMS sorption coefficients, the experimental values spanned a range of approximately 11 log units.     

Development of a solid-phase microextraction method with micellar desorption for the determination of chlorophenols in water samples. Comparison with conventional solid-phase microextraction method

A novel analytical method is presented for the determination of chlorophenols in water. This method involves pre-concentration by solid-phase microextraction (SPME) and an external desorption using a micellar medium as desorbing agent. Final analysis of the selected chlorophenols compounds was carried out by high-performance liquid chromatography (HPLC) with diode array detection (DAD). Optimum conditions for desorption, using the non-ionic surfactant polyoxyethylene 10 lauryl ether (POLE), such as surfactant concentration and time were studied. A satisfactory reproducibility for the extraction of target compounds, between 6 and 15%, was obtained, and detection limits were in the range of 1.1-5.9ngmL(-1). The developed method is evaluated and compared with the conventional one using organic solvent as a desorbing agent. The method was successfully applied to the determination of chlorophenols in water samples from different origin. This study has demonstrated that solid-phase microextraction with micellar desorption (SPME-MD) can be used as an alternative to conventional SPME method for the extraction of chlorophenols in water samples.     

Combination of COSMOmic and molecular dynamics simulations for the calculation of membrane–water partition coefficients

The importance of membrane‐water partition coefficients led to the recent extension of the conductor‐like screening model for realistic solvation (COSMO‐RS) to micelles and biomembranes termed COSMOmic. Compared to COSMO‐RS, this new approach needs structural information to account for the anisotropy of colloidal systems. This information can be obtained from molecular dynamics (MD) simulations. In this work, we show that this combination of molecular methods can efficiently be used to predict partition coefficients with good agreement to experimental data and enables screening studies. However, there is a discrepancy between the amount of data generated by MD simulations and the structural information needed for COSMOmic. Therefore, a new scheme is presented to extract data from MD trajectories for COSMOmic calculations. In particular, we show how to calculate the system structure from MD, the influence of lipid conformers, the relation to the COSMOmic layer size, and the water/lipid ratio impact. For a 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphocholine (DMPC) bilayer, 66 partition coefficients for various solutes were calculated. Further, 52 partition coefficients for a 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC) bilayer system were calculated. All these calculations were compared to experimental data. © 2013 Wiley Periodicals, Inc.     

Application of single-drop microextraction and comparison with solid-phase microextraction and solid-phase extraction for the determination of alpha- and beta-endosulfan in water samples by gas chromatography-electron-capture detection

Water contamination due to the wide variety of pesticides used in agriculture practices is a global environmental pollution problem. The 98/83 European Directive requires the measurement of pesticides residues at a target concentration of 1.0 microg/l in surface water and 0.1 microg/l in drinking water. In order to reach the level of detection required, efficient extraction techniques are necessary. The application of a new extraction technique: single-drop microextraction (SDME), followed by gas chromatography with electron-capture detection, was assessed for determining alpha-endosulfan and beta-endosulfan in water samples. Experimental parameters which control the performance of SDME, such as selection of microextraction solvent and internal standard, optimization of organic drop volume, effects of sample stirring, temperature and salt addition, and sorption time profiles were studied. Once SDME was optimized, analytical parameters such as linearity, precision, detection and quantitation limits, plus matrix effects were evaluated. The SDME method was compared with solid-phase microextraction and solid-phase extraction with the aim of selecting the most appropriate method for a certain application.     

Determination of Henry''s law coefficients by combination of the equilibrium partitioning in closed systems and solid-phase microextraction techniques

This paper describes the determination of Henry's law coefficients by means of the EPICS (equilibrium partitioning in closed systems) technique in combination with SPME (solid-phase microextraction). The use of solid-phase microextraction-sampling allowed us to extend the possibilities of the equilibrium partitioning in closed systems technique with respect to the range of Henry's law coefficients which can be measured. Whereas the equilibrium partitioning in closed systems technique is limited to determine air–water equilibrium partitioning of volatile compounds with Henry's law coefficients of at least 0.06 (dimensionless), the current method allowed to measure coefficients between 0.0023 and 13.5. In this way Henry's law coefficients of 20 compounds, being in a range covering five orders of magnitude, were measured with relative standard deviations between 1.0 and 19.8% (mean standard deviation: 5.7%; median of standard deviations: 4.8%, n=99). Several types of compounds were examined i.e. aliphatic hydrocarbons, monocyclic and polycyclic aromatic hydrocarbons, chlorinated and fluorinated compounds, ethers and esters, biphenyl and N-containing compounds, including compounds for which availability of experimental Henry's law coefficients is limited. Measurement of the equilibrium partitioning in the 2 to 25°C range allowed to establish relations of Henry's law coefficient as a function of temperature.     

Determination of polydimethylsiloxane-seawater distribution coefficients for polychlorinated biphenyls and chlorinated pesticides by solid-phase microextraction and gas chromatography-mass spectrometry

Applications of solid-phase microextraction (SPME) in the measurement of very hydrophobic organic compounds (VHOCs) are limited, partly due to the difficulty of calibrating SPME fibers for VHOCs. This study used a static SPME strategy with a large sample volume (1.6 L) and a five-point calibration procedure to determine the distribution coefficients for a large suite of polychlorinated biphenyls (PCBs) and chlorinated pesticides between a polydimethylsiloxane (PDMS) phase (100 microm thickness) coated on a glass fiber and seawater. An extraction time of 12 days was deemed adequate for equilibrium calibration from kinetic experiments. Two groups of randomly selected fibers divided into three batches (up to nine fibers in each batch) were processed separately with two gas chromatography-mass spectrometry (GC-MS) systems. Matrix effects arising from losses of the analytes to glass container walls and stirring bars were corrected. Relative standard deviations within the same batch were generally smaller than those for the entire group. Furthermore, KfVf (Kf and Vf are the distribution coefficient of an analyte between the polymer-coated fiber and aqueous phase and the fiber volume, respectively) values determined with two GC-MS systems were statistically different. These results indicate the calibrated KfVf values were less affected by the random selection of SPME fibers than by other experimental conditions, and therefore average KfVf values may be used for the same type of commercially available SPME fibers. The relative accuracy of our calibration method was similar to that of a previous study [P. Mayer. W.H.J. Vaes, J.L.M. Hermens, Anal. Chem. 72 (2000) 459] employing different coating thickness and calibration procedure. The present study also obtained a bell-shaped relationship between log Kf and log Kow (octanol-water partition coefficient) for PCB congeners with the maximum log Kf corresponding to log Kow approximately 6.5. This bell-shaped relationship was attributed mainly to steric effects arising from the interplay between the PDMS thickness and molecular sizes of the target analytes.     

Development of a solid-phase microextraction method for direct determination of pentachlorophenol in paper and board samples: Comparison with conventional extraction method

A solid-phase microextraction (SPME) method has been developed for the determination of pentachlorophenol (PCP) in paper and board samples. The analytical procedure involves direct extraction of PCP from paper and board samples and determination by gas chromatography with electron capture detection (GC-ECD). Two kinds of commercially available fibres; 100 microm polydimethylsiloxane (PDMS), apolar, and 85 microm polyacrylate (PA), quite polar, were evaluated to determine the extraction efficiency of pentachlorophenol. Parameters affecting the extraction process, such as temperature and time, were studied. Moreover, time of desorption and the effect of addition of salt were also investigated. The optimized procedure was applied to the analysis of pentachlorophenol (PCP) in five samples of virgin and recycled paper and board. The PCP content was determined by GC-ECD. To evaluate the effectiveness of the proposed method, it was compared with conventional extraction method with liquid-liquid extraction and derivatization. Detection limit of 0.015 microg/g for PCP in paper was achieved with a RSD of 14%.     

Infinite dilution activity coefficients and gas-to-liquid partition coefficients of organic solutes dissolved in 1-sec-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and in 1-tert-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

Inverse gas chromatography was used to measure infinite dilution activity coefficients and gas-to-liquid partition coefficients for 48 organic solute probes in either 1-sec-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-tert-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide in the temperature range from 323.15 to 373.15 K. Partial molar excess enthalpies of solution were calculated from the variation of the infinite dilution activity coefficients with temperature. Abraham model correlations were also derived from the experimental partition coefficient data. The derived Abraham model correlations describe the observed partition coefficients to within 0.11 log units.     

Processing gas chromatographic data and confidence interval calculation for partition coefficients determined by the phase ratio variation method

The phase ratio variation (PRV) method is widely used for the determination of partition coefficient values (dimensionless Henry's law constants) by headspace gas chromatography. Traditional data processing by linear regression has several drawbacks: potential bias introduced by linearization, absence of quality indicator of the resulting value and, in case of replicate determinations, poor utilisation of the existing measurements leading to unnecessarily large confidence intervals. The paper compares existing PRV data processing methods (linear and nonlinear regression, parametric) and derives confidence intervals for the resulting partition coefficient values. The possibility of using several series of measurements to derive a single partition coefficient value with tighter and more reliable confidence intervals is presented for all three processing methods. The methods are tested on published literature data and new experimental data for 12 volatile organic compounds in water at 25 degrees C. The nonlinear regression based on several series of measurements appears to be the method of choice.