Microextraction, solid-phase

SPME is a multiphase equilibrium technique and, therefore, the analytes are not completely extracted from the matrix. Nevertheless, the method is useful for quantitative work and excellent precision and Unearity have been demonstrated. An extraction is complete when the concentration of analytes has reached distribution equilibrium between the sample and coating. This means that once the equihbrium is achieved, the amount extracted is independent of further increase in extraction time. If extraction is terminated before the equihbrium is reached, good precision and reproducibihty is still obtained if incubation temperature, sample agitation, sample pH and ionic strength, sample and headspace volume, extraction and desorption time are kept constant. The theory of the thermodynamic, kinetic and mass transfer processes underlying direct immersion and HS-SPME has been extensively discussed by Pawhszyn [2]. The sensitivity and time required to reach adsorption equilibriiun depends on the partition coefficients between the fiber and the analytes, and the thickness of the phase. Limits of detection and quantitation are often below 1 ppb. 

SoHd-phase microextraction (SPME) is currently under investigation in many laboratories for its usefulness for a large variety of bioanalytical applications SPME involves extraction and pre-concentration with a fused silica fibre or tubing coated with a polymeric stationary phase. SPME can be performed in two-phase (sample-fibre coating) and three-phase (sample-headspace-fibre coating) systems [58]. 

Desorption for GC-analysis is performed directly in the GC-injector by increasing the temperature. For HPLC analysis an interface has been constmcted for solvent extraction of the analyte from the fibre, followed by introduction of the solvent into the LC injector [59]. 

Besides applications to volatiles from solid samples, liquids and gaseous samples, polar and less volatile compounds are increasingly under study as analytical targets and difficulties with small partition coefficients and long equilibration 

Automated in-tube solid-phase microextraction (SPME) has recently been coupled with liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS), e. g. for the determination of drugs in urine [60, 62]. In-tube SPME is an extraction technique in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. The analyte is then desorbed with methanol and transferred to an analytical HPLC-column. 

Solid-phase microextraction (SPME) is effectively a miniamrised version of SPE. Instead of using a packed cartridge, a rod is typically used, which is coated with the stationary phase. This is dipped into a solution of the analyte and allowed to extract for a pre-determined period of time. After this incubation period, the rod is removed from the solution and may be inserted directly into the injection system of the GC or HPLC. All of these operations can be automated on an autosampler. Clearly, the success of this technique depends intimately on the affinity of the analyte for the stationary phase. Frost, Hussain and Raghani [34] used SPME with GC-FID to measure benzyl chloride and chloroethylmethyl ether (amongst other process impurities) in pharmaceutical preparations. 

Thermal desorption from a solid phase microextraction (SPME) fiber has shown considerable potential for selectively introducing semivolatile chemicals into an IMS. ° The SPME approach is a simple design patterned after the early platinum wire introduction thermal desorption system described. With SPME, semivolatile compounds are extracted by either absorption or adsorption onto a nonvolatile polymeric coating or solid sorbent phase that has been coated onto a small fiber. Normally, the adsorption liber is housed in the needle of a syringe to permit puncture of a sample bottle septum and to protect the fiber from contamination during transfer of the fiber from the sample to the IMS instrument. After the analytes are adsorbed onto the SPME fiber, the fiber is retracted into the needle and then injected in a normal syringe technique such that the fiber is extended into the heated region of the IMS and the analytes are desorbed from the fiber into the clean carrier gas of the IMS. 

SPME affords a means of enhancing selectivity and specificity as well as increasing sensitivity by preconcentration of vapors or dissolved compounds. SPME can also serve to transport samples that were collected in the field to the laboratory, where the analysis may take place. 

Other extraction and preconcentration methods patterned after the SPME approach include stir-bar sorptive extractors in which sorptive materials are coated onto a stir bar, and as the bar is stirred in solution, selected organics or other analytes adsorb onto the surface. As with the previous methods, these stir bars are heated to release their adsorbed analytes into the IMS. 

Principles and Characteristics Solid-phase microextraction (SPME) is a patented microscale adsorp-tion/desorption technique developed by Pawliszyn et al. [525-531], which represents a recent development in sample preparation and sample concentration. In SPME analytes partition from a sample into a polymeric stationary phase that is thin-coated on a fused-silica rod (typically 1 cm x 100 p,m). Several configurations of SPME have been proposed including fibre, tubing, stirrer/fan, etc. SPME was introduced as a solvent-free sample preparation technique for GC. 

HPLC solvents (PDMS-coated fibres are incompatible with hexane). PDMS fibres are more selective towards nonpolar compounds and polyacrylate fibres towards polar compounds such as acids, alcohols, phenols and aldehydes. Another feature of SPME fibre selectivity is discrimination towards high-MW volatiles. SPME has successfully been applied to the analysis of both polar and nonpolar analytes from solid, liquid or gas phases. Li and Weber [533] have addressed the issue of selectivity in SPME. 

Two basic methods are used for SPME direct immersion of the fibre into the sample and headspace sampling. Experimental parameters comprise the polarity of the sample matrix and coating material, solvent and salting-out. Other parameters for optimisation of SPME conditions include desorption time, injector port temperature and initial oven temperature. 

Desorption of an analyte from the SPME fibre depends on the boiling point of the analyte, the thickness of the coating on the fibre, and the temperature of the injection port. The fibre can immediately be used for a successive analysis. Some modifications of the GC injector or addition of a desorption module are required. It is possible to automate SPME for routine analysis of many compounds by either GC-MS or HPLC. A significant advantage of SPME over LLE is the absence of the solvent peak in SPME chromatograms. SPME eliminates the separate concentration step from the SPE and LLE methods because the analytes diffuse directly into the coating of the SPME device and are concentrated there. 

Derivatisation/SPME of polar analytes in the sample matrix is the simplest way to improve an analyte s partition coefficient and enhance SPME performance [539]. With the continued introduction of new SPME fibres, the 

An effective combination of focused microwave-assisted extraction with solid-phase microextraction (FMAE-SPME) was carried out for the extraction of cocaine 

MEPS has so far been applied mainly to the analysis of drugs in biological samples only one application for the extraction of PAHs in water has been published.26 One of the major advantages of the MEPS design is that the packed syringe can be used many times over, for example, more than 400 times for water samples. Moreover, the technique permits a fast handling time in the analysis of PAHs in water, the speed enhancement being 15 and 100 times compared to the literature procedures of solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE), respectively see Sections 4.2.3 and 4.2.4. 

SPME was developed by Arthur and Pawliszyn in 1990 as a viable alternative to LLE and SPE techniques that are labor-intensive and solvent demanding, although SPE requires significantly smaller quantities of solvents than LLE. The SPME device, commercially marketed by Supelco, 

During the last 20 years or so, the SPME technique has probably reached the culmination of its development in terms of mode of operation, automation, miniaturization and interfacing to other instruments, innovation of new coating materials, calibration procedures, and fields of application. As a result of these developments, SPME has become the currently most commonly used microextraction technique in held and laboratory experiments of a multidisciplinary nature.29 Accordingly, the following sections will merely record progress in SPME from different perspectives. 

Microfluidics and miniaturization hold great promise in terms of sample throughput advantages [100]. Miniaturization of analytical processes into microchip platforms designed for micro total analytical systems (/i-TASs) is a new and rapidly developing field. For SPE, Yu et al. [123] developed a microfabricated analytical microchip device that uses a porous monolith sorbent with two different surface chemistries. The monolithic porous polymer was prepared by in situ photoinitiated polymerization within the channels of the microfluidic device and used for on-chip SPE. The sorbent was prepared to have both hydrophobic and ionizable surface chemistries. Use of the device for sorption and desorption of various analytes was demonstrated [123]. 

As analytical capabilities improve, multiple procedures are linked together in series to effect analyses. Procedures combined in this manner are called hyphenated techniques. Ferrer and Furlong [124] combined multiple techniques—accelerated solvent extraction (ASE) followed by online SPE coupled to ion trap HPLC/MS/MS—to determine benzalkonium chlorides in sediment samples. Online SPE, especially coupled to HPLC, is being used more routinely. This approach allowed online cleanup of the ASE extract prior to introduction to the analytical column. 

The distribution constant, Kfs, between the coated fiber SPME sorbent and the aqueous sample matrix is given by 

When equilibrium conditions are reached, the number of moles, , of analyte extracted by the fiber coating is independent of increases in extraction time, such that 

Examination of equation (2.35) leads to the conclusion that when the sample volume is very large (i.e., K Vf Vs), the amount of extracted analyte is independent of the volume of the sample, such that 

Koziel and Novak [37 ] recently reviewed the application of this combined sampling and sample preconcentration procedure to indoor air VOC measurement. Typically, a SPME sampler consists of a fused silica fibre that is coated by a suitable polymer (e.g. PDMS, PDMS/divinylbenzene, carboxen/PDMS) and housed inside a needle [37]. The fibre is exposed to indoor air and after sampling is complete, it is retracted into the needle until the sample is analysed. Compared with other sampling methods, it is simple to use and reasonably sensitive. However, samples collected by the procedure are markedly affected by environmental factors such as temperature. Therefore such samples cannot be stored for extended periods of time without refrigeration [36]. 

Unlike solvent extraction, the entire amount of analyte is not extracted. The amount of analyte extracted by the coated fiber is proportional to the concentration of analyte in the sample. This will be true if equilibrium between the fiber and the sample is achieved or before equilibrium is achieved 

SPME sampling is nsed for a wide variety of analytes, including environmental pollutants, volatiles from botanical samples (e.g., nsed to identify tobacco species), explosives, and chemical 

Double-tapered ferrule SPME fiber Solvent desorption chamber 

A recent and very successful approach to sample preparation is SPME invented by Pawliszyn and coworkers [15], and reviewed recently [16]. SPME integrates sampling, extraction, concentration, and sample introduction into a single, solvent-free step. It is excellent as a sampling tool for GC and gas chromatography-mass spectrometry (GC-MS). It is routinely used for extraction of volatile and semivolatile organics, mostly as headspace (HS) analysis. 

The SPME apparatus looks like a modified syringe (see Fig. 5) consisting of a fiber holder needle and a fiber assembly, the latter equipped with a 1-2 cm long retractable SPME fiber. The fiber itself is a thin fused-silica optical fiber, coated with a thin polymer film (such as polydimethylsiloxane, PDMS), as shown 

The most widespread SPME applications utilize injection to a GC (or GC-MS) system. Thermal desorption in the GC injection port depends on the temperature, exposure time, analyte volatility, and the type and thickness of the fiber coating. To ensure a high linear flow, a narrow-bore GC injector insert is required. The fiber needs to be inserted to a depth corresponding to the hot injector zone. This is important because the temperature varies along the length of the injector, and desorption of analytes is very sensitive to the temperature. 

Desorption time is generally in the 10-100 s range, but it needs to be optimized. To ensure high sensitivity, the injector is usually operated in splitless mode— this is possible as no solvent is used in SPME. A frequent practical problem using SPME is that GC septa are easily damaged with the wide (24-gauge) SPME needles. To avoid septum coring, predrilled GC septa or septum-less injector valves may be used. 

The main advantages of SPME are good analytical performance combined with simpUcity and low cost [18]. It is well adapted to most compounds, which can be studied by GC. SPME produces clean and concentrated extracts and is ideal for GC-MS appUcations [17,19,20]. SPME is suitable for automation, which not oidy reduces labor costs but also often improves accuracy and precision. The main disadvantage of SPME is that it is less well adapted for quantitative analysis. Accurate measurements (in terms of quantitation) require careful control of a number of experimental variables, which is elaborate and not always feasible. 

The solvent-free micro extraction technique SPME is an important step towards the instrumentation and automation of the SPE technique for online sample preparation and introduction to GC-MS (Zhang et al., 1994 Eisert and Pawliszyn, 1997 Lord and Pawliszyn, 1998). It involves exposing a fused silica fibre coated with a liquid polymeric material to a sample containing the analyte. As an extraction and enrichment technique it compares to P T methods (MacGillivray et al., 1994). Also derviatization steps can be coupled to the extraction process for polar compounds and improved efficiencies (Pan et al., 1997). The typical dimensions of the active fibre surface are 1 cm X100 pm. 

The analyte diffuses from the gaseous, headspace or liquid sample phase into the fibre surface and partitions into the coating material according to the first partition coefficient. The agitated sample is typically incubated at a constant temperature before and during the sampling process to achieve maximum recovery and precision for quantitative assays (Gorecki and Pawliszyn, 1997). 

The application of internal standards or the use of the standard addition method for quantitation is strongly recommended for achieving low relative standard deviations (Nielsson etal., 1994 Poerschmann etal., 1997). After the equilibrium is established, the fibre with the collected analytes is withdrawn from the sample and transferred into a GC injector, either manually or more convenient via an autosampler. The analyte is desorbed thermally in the hot injector from the coating. The fibre material is used for a large number of samples in automated serial analysis. Modern autosampler are capable to exchange the fibre holder to provide automated access to different fibre characteristics according to the analyte requirements. 

SPME offers distinct advantages for automated sample preparation including reduced time per sample, less manual sample manipulation resulting in an increased sample throughput and, in addition, a significant reduction of organic solvent use in the routine laboratory (Pawhszyn, 1997 Berlardi and Pawliszyn, 1989 Arthur et al., 1992). 

Fibre sheath (pierces septum of sample vial and GC injector) 

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Solid-phase microextractions also have been developed. In one approach, a fused silica fiber is placed inside a syringe needle. The fiber, which is coated with a thin organic film, such as poly(dimethyl siloxane), is lowered into the sample by depressing a plunger and exposed to the sample for a predetermined time. The fiber is then withdrawn into the needle and transferred to a gas chromatograph for analysis. 

Caffeine is extracted from beverages by a solid-phase microextraction using an uncoated fused silica fiber. The fiber is suspended in the sample for 5 min and the sample stirred to assist the mass transfer of analyte to the fiber. Immediately after removing the fiber from the sample it is transferred to the gas chromatograph s injection port where the analyte is thermally desorbed. Quantitation is accomplished by using a C3 caffeine solution as an internal standard. 

Pawliszyn, J. Solid-Phase Microextraction Theory and Practice, Wiley NewYork, 1997. 

Schematic diagram of a device for solid-phase microextractions.

Caffeine in coffee, tea, and soda is determined by a solid-phase microextraction using an uncoated silica fiber, followed by a GC analysis using a capillary SPB-5 column with an MS detector. Standard solutions are spiked with G3 caffeine as an internal standard. 

Solid-phase microextraction (SPME) was used for headspace sampling. The FFA were extracted from the headspace with PA, Car/PDMS, and CW/DVB fibers. It was examined whether addition of salt (NaCl) and decreasing the pH by addition of sulphuric acid (H SO ) increased the sensitivity. FFA were analyzed using gas chromatography coupled to mass spectrometry in selected ion monitoring. 

In recent decades the development of preconcentration steps to be implemented prior to analytical determinations of trace level compounds has been explored in considerable depth. With a view to eliminating or at least minimising the use of organic solvents used in conventional liquid-liquid extraction, other methodologies have been developed, such as membrane extraction, solid-phase extraction, solid-phase microextraction, etc. 

SOLID-PHASE MICROEXTRACTION COUPLED WITH GAS OR LIQUID CHROMATOGRAPHY... 

Although solid-phase microextraction (SPME) has only been introduced comparatively recently (134), it has already generated much interest and popularity. SPME is based on the equilibrium between an aqueous sample and a stationary phase coated on a fibre that is mounted in a syringe-like protective holder. Eor extraction, the fibre... 

Figure 11.14 Analysis of amphetamines by GC-NPD following HS-SPME exti action from human hair (a) Normal hair (b) normal hair after addition of amphetamine (1.5 ng) and methamphetamine (16.1 ng) (c) hair of an amphetamine abuser. Peak identification is as follows 1, a-phenethylamine (internal standard) 2, amphetamine 3, methamphetamine 4, N-propyl-/3-phenethyamine (internal standard). Reprinted from Journal of Chronatography, B 707,1. Koide et ai, Determination of amphetamine and methamphetamine in human hair by headspace solid-phase microextraction and gas cliromatography with niti ogen-phosphoms detection, pp. 99 -104, copyright 1998, with permission from Elsevier Science.

S. Ulrich and J. Maitens, Solid-phase microextraction with capillai y gas-liquid cliro-matography and niti ogen-phosphoi us selective detection foi the assay of antidepressant drugs in human plasma , J. Chromatogr. B 696 217-234 (1997). 

J. Pawliszyn, Applications of Solid Phase Microextraction, Royal Society of Chemistry, Cambridge, UK (1999). 

A recent method, still in development, for determining total 4-nitrophenol in the urine of persons exposed to methyl parathion is based on solid phase microextraction (SPME) and GC/MS previously, the method... 

A recent method, still in development, for determining total 4-nitrophenol in the urine of persons exposed to methyl parathion is based on solid phase microextraction (SPME) and GC/MS previously, the method has been used in the analysis of food and environmental samples (Guidotti et al. 1999). The method uses a solid phase microextraction fiber, is inserted into the urine sample that has been hydrolyzed with HCl at 50° C prior to mixing with distilled water and NaCl and then stirred (1,000 rpm). The fiber is left in the liquid for 30 minutes until a partitioning equilibrium is achieved, and then placed into the GC injector port to desorb. The method shows promise for use in determining exposures at low doses, as it is very sensitive. There is a need for additional development of this method, as the measurement of acetylcholinesterase, the enzyme inhibited by exposure to organophosphates such as methyl parathion, is not an effective indicator of low-dose exposures. 

Magdic S, Pawliszyn JB. 1996. Analysis of organochlorine pesticides using solid-phase microextraction. J Chromatogr A723(l) lll-122. 

Lu, G. et al.. Quantitative determination of geosmin in red beets (Beta vulgaris L.) using headspace solid-phase microextraction, J. Agric. Food Chem., 51, 1021, 2003. 

Shen, S. et al.. Comparison of solid-phase microextraction, supercritical fluid extraction, steam distillation, and solvent extraction techniques for analysis of volatile consituents in Fructus amomi, J. AOAC Int., 88, 418, 2005. 

Pretreatment of hair samples also includes an extraction, usually with an alkaline sodium hydroxide solution, followed by cleaning up with LLE with n-hexane/ethyl acetate. Instead of LLE, the employment of SPE is also possible. Furthermore, the solid phase microextraction (SPME) in combination with head-space analysis is usable [104-106]. In the case of using hair samples, possible external contamination (e.g., by passive smoking of Cannabis) has to be considered as false positive result. False positive results can be avoided by washing of the hair samples previous to extraction [107]. Storage of collected samples is another important fact that can cause false results in their content of A9-THC and metabolites [108-110]. 

Alissandrakis, E., Tarantalis, P. A., Harizanis, P. C., and Polissiou, M. (2007). Aroma investigation of untfloral Greek citrus honey using solid-phase microextraction coupled to gas chromatographic-mass spectrometric analysis. Food Chem. 100, 396-404. 

Soria, A. C., Martmez-Castro, I., and Sanz, J. (2003). Analysis of volatile composition of honey by solid phase microextraction and gas chromatography-mass spectrometry. /. Sep. Sci. 26, 793-801. 

Several extraction methods for water samples are applicable, such as solvent extraction, SPE using a cartridge and disk and solid-phase microextraction (SPME). 

The most widely employed techniques for the extraction of water samples for triazine compounds include liquid-liquid extraction (LLE), solid-phase extraction (SPE), and liquid-solid extraction (LSE). Although most reports involving SPE are off-line procedures, there is increasing interest and subsequently increasing numbers of reports regarding on-line SPE, the goal of which is to improve overall productivity and safety. To a lesser extent, solid-phase microextraction (SPME), supercritical fluid extraction (SEE), semi-permeable membrane device (SPMD), and molecularly imprinted polymer (MIP) techniques have been reported. 

GCB = graphitized carbon black SPME = solid-phase microextraction PDMS = polydimethylsiloxane PS = polystyrene DVB = divinylbenzene SDB = styrene-divinylbenzene. 

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

The need to understand the fate of pesticides in the environment has necessitated the development of analytical methods for the determination of residues in environmental media. Adoption of methods utilizing instrumentation such as gas chro-matography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), liquid chromatography/tandem mass spectrometry (LC/MS/MS), or enzyme-linked immunosorbent assay (ELISA) has allowed the detection of minute amounts of pesticides and their degradation products in environmental samples. Sample preparation techniques such as solid-phase extraction (SPE), accelerated solvent extraction (ASE), or solid-phase microextraction (SPME) have also been important in the development of more reliable and sensitive analytical methods. 

During the last few years, miniaturization has become a dominant trend in the analysis of low-level contaminants in food and environmental samples. This has resulted in a significant reduction in the volume of hazardous and expensive solvents. Typical examples of miniaturization in sample preparation techniques are micro liquid/liquid extractions (in-vial) and solvent-free techniques such as solid-phase microextraction (SPME). Combined with state-of-the-art analytical instrumentation, this trend has resulted in faster analyses, higher sample throughputs and lower solvent consumption, whilst maintaining or even increasing assay sensitivity. 

Tong W., Link A., Eng J.K., and Yates, J.R. Ill, Identification of proteins in complexes by solid-phase microextraction/multitep elution/capillary electrophoresis/ tandem mass spectrometry, Anal. Chem. 71, 2270, 1999. 

Supercritical fluid technology. . . 81 3.5.2 Solid-phase microextraction. 129... 

Figure 3.20 Solid-phase microextraction apparatus. Reproduced by permission of Supelco Inc. (Sigma-Aldrich)...

Table 3.45 Characteristics of solid-phase microextraction Advantages...

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