Solid-phase microextraction and headspace solid-phase microextraction for the determination of polychlorinated biphenyls in water samples

A solid-phase microextraction (SPME) method has been developed for the quantification of polychlorinated biphenyls (PCBs) in water samples. Parameters such as sampling time, volume of water, volume of headspace, temperature, addition of salts, and agitation of the sample were studied. Because the time for reaching equilibrium between phases takes several hours or days, depending on the experimental conditions, it was necessary to work in nonequilibrium conditions to keep the sample analysis to a reasonable time. The possibility of sampling the headspace over the water sample (HSSPME), instead of immersing the fiber into the water (SPME), was also investigated, and despite the low partition of PCB into the headspace, HSSPME offered higher sensitivity than SPME at 100 °C. The adsorption kinetics for SPME at room temperature, SPME at 100 °C, and HSSPME at 100 °C were investigated and compared. The proposed HSSPME method exhibits excellent linearity and sensitivity. The detection limit was in the sub-ng/L level. This method has been applied to a real industrial harbor water and compared with liquid-liquid extraction. Both techniques offered similar results, but HSSPME was much more sensitive and considerably faster, by eliminating all the manual process intensive sample workup, and reduces solvent consumption entirely. The only drawback was that matrix effects were observed, but with the addition of deuterated surrogates to the sample or the use of a standard addition calibration, accurate quantification can be achieved.     

Nonequilibrium solid-phase microextraction for determination of the freely dissolved concentration of hydrophobic organic compounds: matrix effects and limitations

Solid-phase microextraction (SPME) has recently been applied to measure the freely dissolved concentration, as opposed to the total concentration, of hydrophobic substances in aqueous solutions. This requires that only the freely dissolved analytes contribute to the concentration in the SPME fiber coating. However, for nonequilibrium SPME the sorbed analytes that diffuse into the unstirred water layer (UWL) adjacent to the SPME fiber can desorb from the matrix and contribute to the flux into the fiber. These processes were described as a model. Experimentally, an equilibrated and disconnected headspace was used as a reference for the freely dissolved concentration. The expected contribution of desorbed analytes to the uptake flux was measured for PCB no. 52 in a protein-rich solution, while it was not measured in a matrix containing artificial soil. The latter was possibly due to slow desorption of the analyte from the artificial soil. On the basis of the present study, a contribution of desorbed analytes to the uptake flux is expected only if(1) the rate-limiting step of the uptake process is diffusion through the UWL, (2) the concentration of the sorbed analyte is high, and (3) desorption from the matrix is fast.     

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.     

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.     

Limitations to the use of solid-phase microextraction for quantitation of mixtures of volatile organic sulfur compounds

A study of the range of volatile organic sulfur compounds produced by brassica plants has highlighted limitations to the use of Carboxen/PDMS fibers for their analysis by solid-phase microextraction (SPME). These fibers are sometimes advocated for the analysis of sulfur gases, but a quantitative comparison of analytical data derived by SPME and by direct gas sampling of standard mixtures of volatile low molecular weight sulfur compounds at 0.01-10 mg/L has identified potential errors associated with their use. Higher molecular compounds displace lower molecular weight compounds as a consequence of competition for active sites on the fiber, and the relative proportions of the components adsorbed onto the fiber depend on their ratio in the headspace. As their relative concentrations change from sample to sample, the varying interactions result in irregular analytical responses, reflected in erratic calibration curves. Standards containing single components are not valid; only a standard containing all components found in the sample to be analyzed, and at the same relative concentrations, is appropriate. In practice, this may preclude the use of the fibers for quantitative analysis of multicomponent mixtures.     

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.     

Kinetics and the on-site application of standards in a solid-phase microextraction fiber

The kinetics of the desorption of analytes from a SPME fiber into an agitated sample matrix was studied, and a theoretical model was proposed to describe the dynamic desorption process, based on the steady-state diffusion of analytes in the extraction phase and in the boundary layer. It was found that the desorption of analytes from a SPME fiber into an agitated sampling matrix is isotropic to the absorption of the analytes onto the SPME fiber from the sample matrix under the same agitation conditions, and this allows for the calibration of absorption using desorption. The calibration was accomplished by exposing a SPME fiber, preloaded with a standard, to an agitated sample matrix, during which desorption of the standard and absorption of analytes occurred simultaneously. When the standard was the isotopically labeled analogue of the target analyte, the information from the desorption process, i.e., time constant a, could be directly used for estimating the concentration of the target analyte. When the standard varied from the target analyte, the mass-transfer coefficient of the analyte could be extrapolated from that of the standard. These predictions agree well with experimental results. This approach facilitates the full integration of sampling, sample preparation, and sample introduction, especially for on-site or in vivo investigations, where the addition of standards to the sample matrix, or control of the velocity of the sample matrix, is very difficult.     

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.     

Solid-phase microextraction field sampler

To facilitate the use of solid-phase microextraction (SPME) for field sampling, a new field sampler was designed and tested. The sampler was versatile and user-friendly. The SPME fiber can be positioned precisely inside the needle for time-weighted average sampling or exposed completely outside the needle for grab sampling. The needle is protected within a shield at all times, hereby eliminating the risk of operator injury and fiber damage. A replaceable Teflon cap is used to seal the needle to preserve sample integrity. Factors that affect the preservation of sample integrity (sorbent efficiency, temperature, sealing materials) were studied. The use of a highly efficient sorbent for the fiber is recommended as the first choice for the preservation of sample integrity. Teflon was a good material for sealing the fiber needle, had little memory effect, and could be used repeatedly. To address adsorption of high boiling point compounds on fiber needles, several kinds of deactivated needles were evaluated. RSC-2 fiber needles were the more effective. A preliminary field sampling investigation demonstrated the validity of the new SPME device for field applications.     

Comparison of Gas-Sampled and SPME-Sampled Static Headspace for the Determination of Volatile Flavor Components

Traditional static headspace and headspace solid-phase microextraction (SPME) techniques were compared for their effectiveness in the extraction of volatile flavor compounds from the headspace of various juice samples. Each method was used to evaluate the responses of certain analytes from real samples and calibration standards in order to provide sensitivity comparisons between the two techniques. Experimental results showed traditional static headspace lacked the sensitivity needed to evaluate certain flavor volatiles, such as α-terpinene and linalool, and that further concentration of the headspace was necessary. Dramatic improvements in the extraction abilities of the SPME fibers over the traditional static headspace method were noted. Different SPME fibers were investigated to determine the selectivities of the various fibers to the different flavor compounds present in the juice samples. Of the various fibers investigated, the PDMS/DVB fiber proved to be the most useful for these analyses. Aging studies of juice samples were also performed which verified that degradation could be observed and quantified.     

Time-weighted average sampling with solid-phase microextraction device: implications for enhanced personal exposure monitoring to airborne pollutants

The solid-phase microextraction (SPME) device is used as a time-weighted average (TWA) sampler for gas-phase analytes by retracting the coated fiber a known distance into its needle housing during the sampling period. Unlike in conventional spot sampling with SPME, the TWA sampling approach does not allow the analytes to reach equilibrium with the fiber coating, but rather they diffuse through the opening in the needle to the location of the sorbent. The amount of analytes accumulated over time gives the measurement of the average concentration to which the device was exposed to. Depending on the sorbent used as the sink, TWA sampling for various analytes is possible with times ranging from 15 min to at least 16 h. Both the poly(dimethylsiloxane) (PDMS) and poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB) fiber coating phases were tested, with the latter employing on-fiber derivatization for reactive carbonyl compounds, e.g., formaldehyde. Described herein are the theoretical and practical considerations for using the SPME device as a TWA sampler.     

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.     

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%.     

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.     

Time-weighted average passive sampling with a solid-phase microextraction device

A modified Solid-Phase Microextraction (SPME) device has been used as a passive sampler to determine the time-weighted average (TWA) concentration of volatile organic compounds (VOCs) in air. Unlike conventional sampling with SPME, in which the fiber is extended outside its needle housing, during TWA passive sampling, the fiber is retracted a known distance into its needle housing. The SPME passive sampler collects the VOCs by the mechanism of molecular diffusion and sorption on to a coated fiber as collection medium. This process has been shown to be described by Fick's first law of diffusion, whereby determination of the amounts of analytes accumulated over time enable measurement of the TWA concentration to which the sampler was exposed. A series of fibers, 100-microm poly(dimethylsiloxane), 65-microm poly(dimethylsiloxane)/divinylbenzene, and 75-microm Carboxen/poly(dimethylsiloxane), were tested for their "zero sink", face velocity, and response time behavior. Of the fibers tested, that coated with 75-microm Carboxen/poly(dimethylsiloxane) was found to be an excellent passive sampler for VOCs. TWA passive sampling with a SPME device was shown to be almost independent of face velocity and to be more tolerant of high and low analyte concentrations and long and short sampling times, because of the ease with which the diffusion path length could be changed. It was found that environmental conditions, e.g., temperature, pressure, relative humidity, and ozone, have little or no effect on sampling. The 75-microm Carboxen/poly(dimethylsiloxane) fiber can retain VOCs for up to two weeks without significant loss. When the SPME device was tested in the field and the results were compared with those from National Institute of Occupational Health and Safety method 1501, good agreement was obtained.     

Fiber introduction mass spectrometry: fully direct coupling of solid-phase microextraction with mass spectrometry

This work describes the first fully direct coupling of solid-phase microextraction (SPME) with mass spectrometry. An inlet system using a septum as the only interface between the ambient and the high-vacuum mass spectrometer was constructed to allow the introduction of the SPME needle directly into the ionization region of a mass spectrometer. The PDMS-coated fiber was then placed and exposed exactly between the two ionization filaments. Uniform heating of the fiber, efficient thermal desorption, and electron ionization of the analytes were achieved. Using this new analytical technique, here termed fiber introduction mass spectrometry (FIMS), we have been able to detect and quantitate several volatile (VOC) and semivolatile (SVOC) organic chemicals (carbon tetrachloride, benzene, toluene, xylenes, gamma-terpinene, diisoamyl ether, chlorobenzene, and many PAHs) and two herbicides (Sylvex and its methyl ether) from aqueous solutions at low-ppb to ppt levels using either SPME headspace or solution extraction. FIMS shows high sensitivity (ng/L), good reproducibility, and accuracy, providing therefore a simple and effective approach to rapid analysis of VOC and SVOC in various matrixes.     

Sampling and Raman confocal microspectroscopic analysis of airborne particulate matter using poly(dimethylsiloxane) solid-phase microextraction fibers

Commercial poly(dimethylsiloxane) (PDMS) 7-microm solid-phase microextraction (SPME) fibers were used for sampling and Raman spectroscopic analysis of a tailpipe diesel exhaust, candle smoke, cigarette smoke, and asbestos dust. Samples were collected via direct exposure of the SPME fiber to contaminated air. The mass loading for SPME fibers was varied by changing the sampling time. Results indicate that PDMS-coated fibers provide a simple, fast, reusable, and cost-effective air sampling tool for airborne particulates. The PDMS coating was stable; Raman bands of the PDMS coating were observed exactly at the same wavenumber positions before and after air sampling. Raman spectroscopic analysis resulted in identification of several characteristic bands allowing chemical speciation of particulates. The advantage of the SPME fiber is the open bed geometry allowing for application of various spectroscopic methods of particulate analysis. This paper describes the first-ever combined application of SPME technology with Raman confocal microspectroscopy for sampling and analysis of airborne particulates. Advantages of the combination of solid-phase microextraction and Raman microspectroscopy for airborne particulate analysis are discussed. Challenges associated with combined SPME sampling and Raman analysis of single particles are also described.     

Theory and validation of solid-phase microextraction and needle trap devices for aerosol sample

Previous aerosol studies utilizing solid-phase microextraction (SPME) predominantly focused on volatile and semivolatile compounds in the gaseous phase. Difficulties were associated with quantitative analysis of these compounds when they were associated with atmospheric particles. The present study combines SPME technology with that of carboxen packed needles (needle trap, NT) for analysis of gaseous and particle-bound compounds in atmospheric samples. The NT device is constructed as a micro trap by placing some small sorbents in a needle. Aerosol samples are collected by drawing air through the NT device with a pump. The trapped components contain both gaseous chemical compounds as well as particulate matter present in the sample. The total concentration of analytes in an aerosol sample can be obtained on the basis of the exhaustive sampling mode of the NT device. Direct SPME is simultaneously used to determine gaseous compound in the aerosol sample. As a result, the SPME and NT devices, when used together, can provide a complete solution to highly efficient and accurate aerosol studies. The theoretical considerations of SPME and NT devices for aerosol sampling are validated by sampling seasalt aerosol, barbecue, and cigarette smoke. The concentrations of PAHs in the different phases of the samples are few ng/L. Result analysis shows that SPME and the NT device demonstrate several important advantages such as simplicity, convenience, and low costs under laboratory and on-site field sampling conditions.     

Calibration for on-site analysis of hydrocarbons in aqueous and gaseous samples using solid-phase microextraction

Rapid sampling and sample preparation methodology was investigated using adsorptive poly(dimethylsiloxane)/divinylbenzene and Carboxen/poly(dimethylsiloxane) solid-phase microextraction (SPME) fiber coatings and volatile aromatic hydrocarbons (BTEX: benzene, toluene, ethylbenzene, and o-xylene). A flow-through system was used to generate a standard aqueous solution of BTEX as model sample with known linear velocity. Parameters that affect the extraction process, including sampling time, concentration, water velocity, and temperature, were investigated. Very short sampling times from 10 s and sorbents with strong affinity and large capacity were used to ensure the effect of '"zero sink" and to calibrate the extraction process in the initial linear extraction region. Several different concentrations were investigated, and it was found that mass uptake changes with concentration linearly. The increase of water velocity increases mass uptake, though the increase is not linear. Temperature does not affect mass uptake significantly under typical field sampling conditions. To further accurately describe rapid SPME analysis of aqueous samples, a new model translated from heat transfer to a circular cylinder in cross-flow was used. An empirical correlation to this model was used to predict the mass-transfer coefficient. Findings indicate that predicted mass uptake compares well with experimental mass uptake. The new model was tested for rapid air sampling, and it was found that this new model also predicted rapid air sampling accurately. Findings presented in this study extend the existing fundamental knowledge related to rapid sampling/sample preparation with SPME.