Thin-film microextraction

The properties of a thin sheet of poly(dimethylsiloxane) (PDMS) membrane as an extraction phase were examined and compared to solid-phase microextraction (SPME) PDMS-coated fiber for application to semivolatile analytes in direct and headspace modes. This new PDMS extraction approach showed much higher extraction rates because of the larger surface area to extraction-phase volume ratio of the thin film. Unlike the coated rod formats of SPME using thick coatings, the high extraction rate of the membrane SPME technique allows larger amounts of analytes to be extracted within a short period of time. Therefore, higher extraction efficiency and sensitivity can be achieved without sacrificing analysis time. In direct membrane SPME extraction, a linear relationship was found between the initial rate of extraction and the surface area of the extraction phase. However, for headspace extraction, the rates were somewhat lower because of the resistance to analyte transport at the sample matrix/headspace barrier. It was found that the effect of this barrier could be reduced by increasing either agitation, temperature, or surface area of the sample matrix/headspace interface. A method for the determination of PAHs in spiked lake water samples was developed based on the membrane PDMS extraction coupled with GC/MS. A linearity of 0.9960 and detection limits in the low-ppt level were found. The reproducibility was found to vary from 2.8% to 10.7%.     

Air sampling with porous solid-phase microextraction fibers

A new, rapid air sampling/sample preparation methodology was investigated using adsorptive solid-phase microextraction (SPME) fiber coatings and nonequilibrium conditions for volatile organic compounds (VOCs). This method is the fastest extraction technique for air sampling at typical airborne VOC concentrations. A theoretical model for the extraction was formulated based on the diffusion through the interface between the sampled (bulk) air and the SPME coating. Parameters that affect the extraction process including sampling time, air velocity, air temperature, and relative humidity were investigated with the porous (solid) PDMS/DVB and Carboxen/PDMS coatings. Very short sampling times from 5 s to 1 min were used to minimize the effects of competitive adsorption and to calibrate the extraction process in the initial linear extraction region. The predicted amounts of extracted mass compared well with the measured amounts of target VOCs. Findings presented in this study extend the existing fundamental knowledge related to sampling/sample preparation with SPME, thereby enabling the development of new sampling devices for the rapid sampling of air, headspace, water, and soil.     

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.     

Determining chemical activity of (semi)volatile compounds by headspace solid-phase microextraction

This research introduces a new analytical methodology for measuring chemical activity of nonpolar (semi)volatile organic compounds in different sample matrices using automated solid-phase microextraction (SPME). The chemical activity of an analyte is known to determine its equilibrium concentration in the SPME fiber coating. On this basis, SPME was utilized for the analytical determination of chemical activity, fugacity, and freely dissolved concentration using these steps: (1) a sample is brought into a vial, (2) the SPME fiber is introduced into the headspace and equilibrated with the sample, (3) the SPME fiber is injected into the GC for thermal desorption and analysis, and (4) the method is calibrated by SPME above partitioning standards in methanol. Model substances were BTEX, naphthalene, and alkanes, which were measured in a variety of sample types: liquid polydimethylsiloxane (PDMS), wood, soil, and nonaqueous phase liquid (NAPL). Variable sample types (i.e., matrices) had no influence on sampling kinetics because diffusion through the headspace was rate limiting for the overall sampling process. Sampling time was 30 min, and relative standard deviations were generally below 5% for homogeneous solutions and somewhat higher for soil and NAPL. This type of activity measurement is fast, reliable, almost solvent free, and applicable for mixed-media sampling.     

Solid-phase microextraction in combination with GC/MS for quantification of the major volatile free fatty acids in ewe cheese

This work describes a method for quantification of the major free fatty acids of ewe cheese that contribute to its distinct and strongly marked flavor. A headspace SPME method in combination with GC/MS was used for the extraction, identification, and quantification of butanoic, hexanoic, octanoic and decanoic acids in ewe cheeses. The method used for sample preparation was simple. A fiber coated with 85-microm polyacrylate film was chosen to extract the free fatty acids. To perform a reliable quantification, several factors were taken into consideration for reliable quantification, namely, (i) the influence of addition of water, of an electrolyte or of a hygroscopic salt, on the release of free fatty acids from the matrix; (ii) the linear relationship between the amount of analyte adsorbed by the SPME polymer film and the initial concentration of the analyte in the cheese sample; and (iii) the competition for adsorption by fiber. Water removal with sodium sulfate promoted a more efficient extraction of volatile free fatty acids; biases due to competition or linear range excesses were controlled by choosing the appropriate amount of sample for each ewe cheese. The method of standard additions was used with success for the quantification of free fatty acids. Calibration curves that were constructed for the major short-chain free fatty acids (butanoic, hexanoic, octanoic, and decanoic acids) spiked into cheese followed linear relationships with highly significant (p < 0.001) correlation coefficients (r > 0.999). Coefficients of variation of <7.9% indicated that the technique was reproducible. A marked increase in concentration of short-chain free fatty acids was observed during cheese ripening, ranging from 0.35 to 9.33 mg/100 g for butanoic acid, 0.363 to 4.34 mg/100 g for hexanoic acid, 0.343 to 2.0 mg/100 g for octanoic acid, and 1.291 to 3.85 mg/100 g for decanoic acid. The limits of quantification were registered at levels of parts per million. The absolute quantification of butanoic acid was also carried out by using isotope dilution assays (IDA). The levels of acid obtained with this method were similar to those obtained by the standard additions method.     

In situ derivatization/solid-phase microextraction for the determination of haloacetic acids in water

An in situ derivatization solid-phase microextraction method has been developed for the determination of haloacetic acids (HAAs) in water. The analytical procedure involves derivatization of HAAs to their methyl esters with dimethyl sulfate, headspace sampling using solid-phase microextraction (SPME), and gas chromatography-ion trap mass spectrometry (GC/ITMS) determination. Parameters affecting both derivatization efficiency and head-space SPME procedure, such as the selection of the SPME coating, derivatization-extraction time and temperature, and ionic strength, were optimized. The commercially available Carboxen-poly(dimethylsiloxane) (CAR-PDMS) fiber appears to be the most suitable for the determination of HAAs. Moreover, the formation of HAA methyl esters was dramatically improved (up to 90-fold) by the addition of tetrabutylammonium hydrogen sulfate (4.7 micromol) to the sample as ion-pairing agent in the derivatization step. The precision of the in situ derivatization/HS-SPME/GC/ITMS method evaluated using an internal standard gave relative standard deviations (RSDs) between 6.3 and 11.4%. The method was linear over 2 orders of magnitude, and detection limits were compound-dependent, but ranged from 10 to 450 ng/L. The method was compared with the EPA method 552.2 for the analysis of HAAs in various water samples, and good agreement was obtained. Consequently, in situ derivatization/HS-SPME/GC/ITMS is proposed for the analysis of HAAs in water.     

Quantitative in vivo microsampling for pharmacokinetic studies based on an integrated solid-phase microextraction system

An integrated microsampling approach based on solid-phase microextraction (SPME) was developed to provide a complete solution to highly efficient and accurate pharmacokinetic studies. The microsampling system included SPME probes that are made of poly(ethylene glycol) (PEG) and C18-bonded silica, a fast and efficient sampling strategy with accurate kinetic calibration, and a high-throughput desorption device based on a modified 96-well plate. The sampling system greatly improved the quantitative capability of SPME in two ways. First, the use of the C18-bonded silica/PEG fibers minimized the competition effect from analogues of the target analytes in a complicated sample matrix such as blood or plasma samples, which is a common problem associated with solid coating SPME fibers for quantitative analysis. Moreover, the C18-bonded silica/PEG fibers provide high sensitivity and a large dynamic range that covers the possible sample concentration range during diazepam administration and elimination. Second, the kinetic calibration method offers more accurate quantitation than the calibration curve method for in vivo SPME, because it compensates for convection and matrix effects during sampling. Therefore, it is especially suitable as a fast sampling technique for pre-equilibrium SPME. Furthermore, with the high-throughput desorption device, the integrated system offers compactness and high efficiency. Its feasibility for in vivo sampling was demonstrated by monitoring diazepam pharmacokinetics and validated by conventional chemical assays and equilibrium SPME. In addition, we propose a simple method to determine the apparent distribution constant between an SPME fiber and a blood matix (Kfs) and the distribution constant between an SPME fiber and a pure PBS buffer sample matrix (Kfb). As a result, both total and free concentrations of the drug and its metabolites can be detected simultaneously. Accordingly, the binding constants to the blood matrix can be obtained, which are of special significance for clinical diagnosis and drug discovery.     

In-tube solid-phase microextraction coupled to capillary LC for carbamate analysis in water samples

Recently, the on-line sample preparation technique, intube solid-phase microextraction (SPME), was successfully implemented with a Hewlett-Packard 1100 HPLC system for analysis of carbamates in water samples. This paper describes the coupling of in-tube SPME to capillary LC and explores its utility as a sample preparation method in that format, relative to conventional LC. The Hewlett-Packard HPLC system was upgraded to a capillary LC system using commercially available accessories from LC Packings. The combination of in-tube SPME with a capillary LC system was expected to build on the merits of both in-tube SPME and the capillary LC to generate a sensitive method with an easy, effective, and efficient sample preparation. Due to the relatively large effective injection volume of the in-tube SPME technique (30-45 microL), on-column focusing was employed in order to achieve good chromatographic efficiency. Excellent sensitivity was achieved with very good method precision. For all carbamates studied, the RSD of retention time was between 0.5 and 0.8% under 4 microL/min microgradient conditions. The RSD of peak area counts was between 1.5 and 4.6%. The detection limits for all carbamates studied were less than 0.3 microg/L and, for carbaryl, just 0.02 microg/L (20 ppt). Compared with the conventional in-tube SPME/LC method, the LODs were lowered for carbaryl, propham, methiocarb, promecarb, chlorpropham, and barban, by factors of 24, 45, 42, 81, 62, and 56, respectively. The optimized method was successfully applied to the analysis of carbamates in surface water samples.     

Quantitative analysis of fuel-related hydrocarbons in surface water and wastewater samples by solid-phase microextraction

Solid-phase microextraction (SPME) parameters were examined on water contaminated with hydrocarbons including benzene and alkylbenzenes, n-alkanes, and polycyclic aromatic hydrocarbons (PAHs). Absorption equilibration times ranged from several minutes for low molecular weight compounds such as benzene to 5 h for high molecular weight compounds such as benzo[a]pyrene. Under equilibrium conditions, SPME analysis with GC/FID was linear over 3-6 orders of magnitude, with linear correlation coefficients (r(2)) greater than 0.96. Experimentally determined FID detection limits ranged from ～30 ppt (w/w hydrocarbon/sample water) for high molecular weight PAHs (e.g., MW > 202) to ～1 ppb for low molecular weight aromatic hydrocarbons. Experimental distribution constants (K) were different with 100- and 7-μm poly(dimethylsiloxane) fibers, and poor correlations with previously published values suggest that K depends on the fiber coating thickness and the sorbent preparation method. The sensitivity of SPME analysis is not significantly enhanced by larger sample volumes, since increasing the water volume (e.g., from 1 to 100 mL) has little effect on the number of analyte molecules absorbed by the fiber, especially for compounds with K < 500. Water sample storage should utilize silanized glassware, since hydrocarbon losses up to 70% could be attributed to unsilanized glassware walls when samples were stored for 48 h. Hydrocarbon losses at part-per-billion concentrations also occurred with surface waters due to partitioning onto part-per-thousand concentrations of suspended solids. Quantitative determinations of aromatic and aliphatic hydrocarbons (e.g., in gasoline-contaminated water) can be performed using GC/MS with deuterated internal standard or standard addition calibration as long as the target components or standards had unique ions for quantitation or sufficient chromatographic resolution from interferences. SPME analysis gave good quantitative performance with surface waters having high suspended sediment contents, as well as with coal gasification wastewater which contained matrix organics at 10(6)-fold higher concentrations than the target aromatic hydrocarbons. Good agreement was obtained between a 45-min SPME and methylene chloride extraction for the determination of PAH concentrations in creosote-contaminated water, demonstrating that SPME is a useful technique for the rapid determination of hydrocarbons in complex water matrices.     

Combining drop-to-drop solvent microextraction with gas chromatography/mass spectrometry using electronic ionization and self-ion/molecule reaction method to determine methoxyacetophenone isomers in one drop of water

A novel analytical technique termed drop-to-drop solvent microextraction (DDSME) was developed to determine three methoxyacetophenone isomers in one drop of water, which were then detected by gas chromatography/mass spectrometry using electronic ionization mass spectrometry for quantification analysis and self-ion/molecule reaction/tandem mass spectrometry for isomer differentiation. The best optimum parameters for the DDSME technique were as follows: extraction time, 5 min; using toluene as the extraction solvent; volume of extraction solvent, 0.5 microL and no salt addition. The advantages of this method are rapidity, convenience, ease of operation, simplicity of the device, and extremely little solvent and sample consumption. The limit of detection (LOD) for this technique was 1 ng/mL. The relative standard deviation was less than 2.6% (n = 5). The linear range of the calibration curve of DDSME is from 0.01 to 5 microg/mL with correlation coefficient (r2) of >0.954. In the comparison of the LOD of DDSME with other sample pretreatment methods including liquid/liquid extraction (LLE), single-drop microextraction (SDME), solid-phase microextraction (SPME), and liquid-phase microextraction (LPME) using a dual gauge microsyringe with hollow fiber methods, this method shows much better in sensitivity than the LLE (25 ng/mL) and it is compatible with SDME (0.5 ng/mL), SPME (0.5 ng/mL), and LPME using a dual gauge microsyringe with a hollow fiber (1 ng/mL). However, DDSME was more convenient than the LPME using a dual gauge microsyringe with a hollow fiber method and much lower cost than the SPME technique.     

Calibration of a commercial solid-phase microextraction device for measuring headspace concentrations of organic volatiles

Solid-phase microextraction (SPME) is a versatile new technique for collecting headspace volatiles prior to GC analysis. The commercial availability of uniform SPME fibers makes routine, practical quantitation of headspace concentrations possible, but straightforward information for relating GC peak areas from SPME analyses to headspace concentrations has not been available. The calibration factors (amount absorbed by the fiber divided by headspace concentration) were determined for 71 compounds using SPME fibers with a 100 μm poly(dimethylsiloxane) coating. The compounds ranged from 1 to 16 carbons in size and included a variety of functional groups. Calibration factors varied widely, being 7000 times higher for tetradecane than for acetaldehyde. Most compounds with a Kovats retention index of <1300 on a nonpolar GC column (DB-1) equilibrated with the fiber in 30 min or less. A regression model is presented for predicting the calibration factor from GC retention index, temperature, and analyte functional class. The calibration factor increased with retention index but decreased with increasing sampling temperature. For a given retention index, polar compounds such as amines and alcohols were absorbed by the fibers in greater amounts than were hydrocarbons. Henry's law constants determined using SPME were in general agreement with literature values, which supported the accuracy of the measured calibration factors. An unexpected concentration dependence of calibration factors was noted, especially for nitrogen-containing and hydroxy compounds; calibration factors were relatively higher (the SPME fiber was more sensitive) at the lower analyte concentrations.     

Simultaneous determination of estrogenic short ethoxy chain nonylphenols and their acidic metabolites in water by an in-sample derivatization/solid-phase microextraction method

An in-sample derivatization headspace solid-phase microextraction method has been developed for the simultaneous determination of nonylphenol, nonylphenol mono- and diethoxylates (NP, NP1EO, NP2EO), and their acidic metabolites (NPlEC, NP2EC) in water. The analytical procedure involves derivatization of NPEOs and NPECs to their methyl ethers--esters with dimethyl sulfate/NaOH and further headspace (HS) solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) determination. Parameters affecting both derivatization efficiency and headspace SPME procedure, such as the selection of the SPME coating, derivatizationextraction time, temperature and ionic strength were optimized. The commercially available Carbowax-divinylbenzene (CW-DVB) fiber appears to be the most suitable for the simultaneous determination of both NPEOs-NPECs. Run-to-run precision of the in-sample derivatization/HS-SPME-GC/MS method gave relative standard deviations between 8 and 18%. The method was linear for NP over 2 orders of magnitude, and detection limits were compound dependent but ranged from 20 to 1500 ng/L. The SPME procedure was compared with a solid-phase extraction SPE-GC/MS method for the analysis of NPEOs-NPECs in water samples and good agreement was obtained. Therefore, in-sample derivatization HS-SPME-GC/MS can be used as a method for the simultaneous determination of short ethoxy chain nonylphenols and their acidic metabolites in water.     

Temporal resolution of solid-phase microextraction: measurement of real-time concentrations within a dynamic system

To address the challenge of measuring real-time analyte concentrations within dynamic systems, the temporal resolution of the solid-phase microextraction (SPME) approach has been investigated. A mass-uptake model for SPME within a dynamic system was developed and validated, with experimental factors affecting the temporal resolution (sampling time, agitation, SPME fiber dimensions, sample concentration and change rate, and instrument sensitivity) characterized. Calibration methods for time-resolved sampling in a dynamic system were compared. To demonstrate the efficacy of time-resolved SPME, this approach was successfully applied to investigate the binding kinetics between plasma proteins and pharmaceuticals, which verified a decrease in free pharmaceutical concentrations over time in the presence of bovine serum albumin. The current study provides the theoretical and logistical framework for applying SPME to the real-time measurement of dynamic systems, facilitating future SPME applications such as in vivo metabolomic studies.     

Kinetic calibration for automated headspace liquid-phase microextraction

The kinetics of the absorption and desorption of analytes for headspace liquid-phase microextraction (HS-LPME) were studied. It was found that the desorption of analytes from the extraction phase into the sample matrix is isotropic to the absorption of the analytes from the sample matrix into the extraction phase under the same conditions. This therefore allows for the calibration of absorption using desorption. Calibration was accomplished by exposing the extraction phase, which contained a standard, to the sample matrix. The information from the desorption of the standard, such as time constant a, could be directly used to estimate the concentration of the target analyte in the sample matrix. This new kinetic calibration method for headspace LPME was successfully used to correct the matrix effects in the BTEX analysis of an orange juice sample. In this study, the headspace LPME techniques were successfully fully automated, for both static and dynamic methods, with the CTC CombiPal autosampler. All operations of headspace LPME, including sample transfer and agitation, filling of extraction solvent, exposing the solvent in the headspace, withdrawing the solvent to syringe and introducing the extraction phase into injector, were autoperformed by the CTC autosampler. The fully automated headspace LPME technique is more convenient and improved the precision and sensitivity of the method. This automated dynamic headspace LPME technique can be also used to obtain the distribution coefficient between the sample matrix (aqueous or another solution) and the extraction phase (1-octanol or another solvent). The distribution coefficient between 1-octanol and orange juice, at 25 degrees C, was obtained with this technique.     

Sol-gel coating technology for the preparation of solid-phase microextraction fibers of enhanced thermal stability

A novel sol-gel method is described for the preparation of solid-phase microextraction (SPME) fibers. The protective polyimide coating was removed from a 1-cm end segment of a 200 μm o.d. fused-silica fiber, and the exposed outer surface was coated with a bonded sol-gel layer of poly(dimethylsiloxane) (PDMS). The chemistry behind this coating technique is presented. Efficient SPME-GC analyses of polycyclic aromatic hydrocarbons, alkanes, aniline derivatives, alcohols, and phenolic compounds in dilute aqueous solutions were achieved using sol-gel-coated PDMS fibers. The extracted analytes were transferred to a GC injector using an in-house-designed SPME syringe that also allowed for easy change of SPME fibers. Electron microscopy experiments suggested a porous structure for the sol-gel coating with a thickness of ～10 μm. The coating porosity provided higher surface area and allowed for the use of thinner coatings (compared with 100-μm-thick coatings for conventional SPME fibers) to achieve acceptable stationary-phase loadings and sample capacities. Enhanced surface area of sol-gel coatings, in turn, provided efficient analyte extraction rates from solution. Experimental results on thermal stability of sol-gel PDMS fibers were compared with those for commercial 100-μm PDMS fibers. Our findings suggest that sol-gel PDMS fibers possess significantly higher thermal stability (>320 °C) than conventionally coated PDMS fibers that often start bleeding at 200 °C. This is due, in part, to the strong chemical bonding between the sol-gel-generated organic-inorganic composite coating and the silica surface. Enhanced thermal stability allowed the use of higher injection port temperatures for efficient desorption of less-volatile analytes and should translate into extended range of analytes that can be handled by SPME-GC techniques. Experimental evidence is provided that supports the operational advantages of sol-gel coatings in SPME-GC analysis.     

In situ hydrothermal grown silicalite-1 coating for solid-phase microextraction

A novel fiber coated with silicalite-1 for solid-phase microextraction (SPME) was prepared by in situ hydrothermal growth method. Six substituted benzenes (nitrobenzene, p-dichlorobenzene, m-dichlorobenzene, 1,3,5-trichlorobenzene, p-chloronitrobenzene, and m-chloronitrobenzene) were employed as model analytes. The fiber exhibited high thermal stability (little weight loss up to 600 °C) and high chemical stability (no loss of function after sequential immersion in 0.1 M HCl, 0.01 M NaOH, methanol, and n-hexane each for at least 4 h). Compared with commercial fibers, 3-6 times higher extraction efficiencies were shown on the fiber for mono- and p-substituted benzenes. Under the preoptimized conditions, the fiber afforded satisfactory enhancement factors (517-1292), wide linear ranges (more than 2 orders of magnitude), low limits of detection (0.001-0.130 μg/L), and acceptable repeatability (<9.6%) and reproducibility (<8.8%). Furthermore, the fiber offered distinct shape-selectivity attributed to the uniform molecular-scale pore structure of silicalite-1. The ratios of extraction were approximately 70 between p-dichlorobenzene and 1,3,5-trichlorobenzene, 30 between p-chloronitrobenzene and m-chloronitrobenzene, and 3 between p-dichlorobenzene and m-dichlorobenzene. After pore narrowing by surface modification with SiCl(4), the selectivity for p-dichlorobenzene over m-dichlorobenzene was further enhanced by another 10 times. Finally, the fiber was successfully applied to analysis of a real water sample.     

Kinetic calibration for automated hollow fiber-protected liquid-phase microextraction

Recently, a kinetic calibration method was developed for the quantification of microextraction. In this study, we proved that the sample volume and sampling time do not affect the feasibility of the calibration method, theoretically. The new theoretical considerations of the kinetic calibration method were validated through the investigation of the kinetics of the absorption and desorption processes of hollow fiber-protected liquid-phase microextractrion (HF-LPME). The kinetic calibration method for HF-LPME was successfully used to correct the matrix effects in the carbaryl analysis of a red wine sample. This research extends the kinetic calibration approach to fast sampling and some in-vial analyses, whereby the sample volume is not much larger than the product of the distribution coefficient and the volume of the extraction phase. HF-LPME technique was successfully automated with a CTC CombiPal autosampler, and a new device was designed for the automation of HF-LPME in this study. All steps of the HF-LPME technique, including the filling of the extraction solvent, sample transfer and agitation, withdrawing the solvent to a syringe, and introducing the extraction phase into the injector, were automated by a CTC autosampler. The fully automated HF-LPME technique is more convenient and more accurate. The good reproducibility of the fully automated HF-LPME technique eliminates the need for an internal standard to improve the analytical precision. The automated HF-LPME technique can be also used to obtain the distribution coefficient between the sample matrix and the extraction phase. The distribution coefficients of carbaryl and (13)C-carbaryl between 1-octanol and red wine, at 25 degrees C, were obtained with this technique.     

Sampling and determination of formaldehyde using solid-phase microextraction with on-fiber derivatization

Gaseous formaldehyde is sampled by derivatization with o-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) adsorbed onto poly (dimethylsiloxane)/divinylbenzene solid-phase microextraction fibers. The product of the reaction is an oxime which is thermally very stable and insensitive to light. The oxime can be analyzed by gas chromatography with flame ionization detection and other detectors. Loading PFBHA on the fiber is by room-temperature headspace extraction from aqueous solutions of PFBHA. The process of loading and desorption of unreacted PFBHA, and oxime formed, is both highly reproducible and reversible, with more than 200 loading, sampling, and analysis steps possible with one fiber. The standard formaldehyde gas concentrations studied ranged from 15 to 3200 ppbv with sampling times from 10 s to 12 min. Quantification can be achieved via interpolation from calibration curves of area counts as a function of formaldehyde concentration for a fixed sampling time. Sampling for 10 s yields a method detection limit of 40 ppbv and at 300 s the method detection limit is 4.6 ppbv. This is equal to or better than all other conventional grab sampling methods for gaseous formaldehyde employing sampling trains or passive sampling techniques. Alternatively, gaseous formaldehyde can be quantified with an empirically established apparent first-order rate constant (0.0030 ng/(ppbv s) at 25 °C) for the reaction between sorbed PFBHA and gaseous formaldehyde. This first-order rate constant allows for quantitative analyses without a calibration curve, only requiring detector calibration with the oxime. This new method was used for the headspace sampling of air known to contain formaldehyde, as well as other carbonyl compounds, and from various matrixes such as cosmetics and building products.     

Pre-equilibrium solid-phase microextraction of free analyte in complex samples: correction for mass transfer variation from protein binding and matrix tortuosity

The accurate measurement of free analyte concentrations within complex sample matrixes by pre-equilibrium solid-phase microextraction (SPME) has proven challenging due to variations in mass uptake kinetics. For the first time, the effects of the sample binding matrix and tortuosity on the kinetics of analyte extraction (from the sample to the SPME fiber) are demonstrated to be quantitatively symmetrical with those of the desorption of preloaded deuterated standards (from the fiber to the sample matrix). Consequently, kinetic calibration methods can be employed to correct for variation in SPME sampling kinetics, facilitating the application of pre-equilibrium SPME within complex sample systems. This approach was applied ex vivo to measure pharmaceuticals in fish muscle tissues, with results consistent with those obtained from equilibrium SPME and microdialysis. The developed method has the inherent advantages of being more accurate, precise, and reproducible, thus providing the framework for applications where rapid measurement of free analyte concentrations (within complicated sample matrixes such as biological tissues, sediment, and surface water) are required.