Adsorption SPME methods

Recent studies showed that the wine matrix strongly influences the apparent partition between the liquid phase and the SPME fiber coatings (Fedrizzi et al., 2007a 2007b). Wine components may potentially affect the sampling remarkable sugar content may induce the signals to rise, and polyphenols and esters can participate in competitive adsorption processes (Murray, 2001 Lestremau et al.,2004) and redox reactions (Murat et al., 2003 Blanchard et al., 2004). Consequently, a reliable HS-SPME method needs the use of a suitable internal standard, such as 6-mercaptohexan-l-ol (6-MH). 

The performance of SPME is based on the adsorption of analytes onto a fused silica or polymeric fiber — uncoated or coated with various materials (polydimethylsiloxane, polyacrylate, Carbowax or its mixture with graphite, etc.) [210, 211] - which is housed inside the needle of a microsyringe. To conduct the analysis, the needle is placed into a contaminated water sample and the fiber is exposed to the solution for a controlled time. Since the adsorption equilibrium is established quickly on the thin layer of the absorbing material, long exposure of the fiber is not required. Then, the needle is placed into the injector of a gas chromatograph and the adsorbed compounds thermally desorb for subsequent separation and quantification. The method is simple and fast and does not require the use of solvents and so SPME is very popular in the analysis of volatile and semi-volatile compounds in food. However, sometimes poor reproducibifity and linearity as well as the possibifity to analyze only volatile and thermally stable compounds Limit the application of the SPME method. 

Solid-phase microextraction (SPME) was developed as an alternative to many other sample preparation methods because it uses virtually no solvents or complicated equipment. It is an adsorption/desorption method where the compounds of interest are adsorbed onto a coated fused silica fiber. After a given time, the fiber is placed in a gas chromatography (GC), where the compounds are thermally desorbed. SPME has recently been adapted for use in HPLC, where compounds that are adsorbed are desorbed using an appropriate solvent. 

The in-tube SPME method is suitable for the extraction of less volatile or thermally labile HCAs compounds [89]. The food sample is treated with HCl followed by centrifugation, the sample supernatant is neutralized with NaOH, and the HCAs are extracted by the Blue Rayon adsorption method. This method can selectively adsorb compounds having polycyclic planar molecular structures, such as HCAs, in order to concentrate them from the aqueous solution. The extract is passed through a syringe micro filter, and a capillary colunm is used as a SPME device. This column is placed between the injection loop and the injection needle of the auto sampler. The method is simple, rapid, automatic, and gives 3-20 times higher sensitivity in comparison with the direct liquid injection method [89]. 

Because the SPME method is characterized by a sufficiently linear range, it can also be used successfully to quantily flavor compounds, although an accurate quantitative determination of individual components requires considerable effort in terms of calibration. The concentration of the analytes concentrated onto the SPME fiber is proportional to the concentration in aqueous solution or in the gas phase, so that quantification can be performed by attaining equilibrium, or by always using the same fiber exposure times. The time required to establish equilibrium depends on the distribution coefficients of the analytes and the film thickness of the phase. Practical experience indicates that maintaining identical times is more critical than establishing complete equilibrium. With this approach, even kinetically less favorable substances can be precisely quantified even before the equilibrium concentration has been reached. For routine analysis, therefore, it is not absolutely necessary to reach complete adsorption equilibrium, provided the fiber exposure time is always kept exactly constant. 

Marsili compared SPME and dynamic headspace (DH) GC/MS techniques for the analysis of light-induced lipid oxidation products in milk (4). In the SPME method, 3 g of milk (2% milkfat) and 4-methyl-2-pentanone internal standard (10 jL of a 20 ppm solution in methanol) were placed in a 9-mL vial and capped. A 75- j,m Carboxen-1006/PDMS fiber was inserted into the headspace above the milk sample. Hie Carboxen/PDMS fiber has a combination of micro-, meso-, and macro-pores ranging from 6 to 50A. The volatile flavor compounds that are the best indicators of light-induced oxidation in milk are pentanal, hexanal, and dimethyl disulfide. The Carboxen/PDMS fiber was selected for this study because it is well suited for the analysis of low-molecular-weight volatiles. Adsorption of volatiles from the milk onto the SPME fiber was conducted at 45 C for 15 min with stirring. The sealed vial was allowed to equilibrate for 2 min at 45 C before the SPME fiber was inserted. 

Typical areas of application are identification of trace (ppm or ppb level) volatile organics in complex mixtures (e.g. olfactory principles) and monitoring of residual monomers in polymeric materials. Apart from HS-GC, analysis of volatiles can also be carried out by a variety of other methods, including hydrodistillation, SFE, US, adsorption trapping and SPME. 

Classical sample preparation methods such as distillation, soxhlet extraction are still used [839, 840], but specific techniques such as supercritical fluid extraction (SFE) [841], and increasingly in recent years, adsorption techniques such as solid phase micro-extraction (SPME) [841a] are also being used for isolation, separation, and identification of flavor and fragrance materials. 

This method is based on the partitioning of compounds between a sample and a coated fibre immersed in it [16-18]. The volatiles and other compounds are first adsorbed onto the fibre immersed in a liquid sample, an extract, or in the headspace above a sample for a certain period of time. After adsorption is complete, the compounds are thermally desorbed into a GC injector block for further analysis. Particularly in food applications, headspace SPME is preferred to avoid possible contamination of the headspace system by non-volatile food components [16]. 

One of these methods is called kinetic calibration, in which analyte absorption from the sample to the liquid coating (PDMS) on the fiber is related to analyte desorption from the coating to the sample. The isotropy of absorption and desorption in the kinetic calibration has been described by Chen et al.31 In kinetic calibration, also called in-fiber standardization, desorption of a radio-labeled standard (preloaded on the fiber coating) into the sample is used to calibrate the extraction (absorption/adsorption in the case of a liquid/solid coating) of analyte from the sample into the fiber. This calibration approach considerably facilitates the use of SPME for the on-site field sampling of water, where the control of flow velocity or addition of a standard to the matrix is very difficult. 

Solid Phase MicroExtraction (SPME) is a solvent-free sample preparation method based on the adsorption of analytes directly from an aqueous sample onto a coated fused-silica fiber. Headspace SPME was used in combination with gas chromatography-mass spectrometry/ selective ion monitoring (GC/MS-SIM) to analyze for TCA in wine. 

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. 

Solid-phase microextraction (SPME) is a technique that was first reported by Louch et al. in 1991 (35). This is a sample preparation technique that has been applied to trace analysis methods such as the analysis of flavor components, residual solvents, pesticides, leaching packaging components, or any other volatile organic compounds. It is limited to gas chromatography methods because the sample must be desorbed by thermal means. A fused silica fiber that was previously coated with a liquid polymer film is exposed to an aqueous sample. After adsorption of the analyte onto the coated fiber is allowed to come to equilibrium, the fiber is withdrawn from the sample and placed directly into the heated injection port of a gas chromatograph. The heat causes desorption of the analyte and other components from the fiber and the mixture is quantitatively or qualitatively analyzed by GC. This preparation technique allows for selective and solventless GC injections. Selectivity and time to equilibration can be altered by changing the characteristics of the film coat. 

The search of adequate extraction techniques allowing the identification and quantification of wine volatile compounds has attracted the attention of many scientists. This has resulted in the availability of a wide range of analytical tools for the extraction of these compounds from wine. These methodologies are mainly based on the solubility of the compounds in organic solvents (liquid-liquid extraction LLE, simultaneous distillation liquid extraction SDE), on their volatility (static and dynamic headspace techniques), or based on their sorptive/adsorptive capacity on polymeric phases (solid phase extraction SPE, solid phase microextraction SPME, stir bar sorptive extraction SBSE). In addition, volatile compounds can be extracted by methods based on combinations of some of these properties (headspace solid phase microextraction HS-SPME, solid phase dynamic extraction SPDE). 

Similar to all of the other methods described thus far, SPME affords a certain view of the volatile composition of a food. This view is determined by factors common to headspace techniques as well as the unique affects contributed by the adsorption process. To use the method effectively, one has to be very familiar with the factors that influence volatile recovery. These factors have been discussed in detail in the literature (references cited above). If the method provides an isolate that has the component(s) one wishes to measure and it is adequately reproducible, the method is quite attractive. There are no solvents for contamination it is simple, automated, moderately sensitive and rapid. 

Precision of the method is controlled by several factors, first of all, the status of the SPME fiber. The mean fiber lifetime is 100 runs when desorbing into an injector heated to 220°C or immersing into water saturated with salt and at pH 2. Direct sampling from dirty matrices may cause faster degradation of the fiber, due to the adsorption of high-molecular-weight species such as proteins, salt crystals, or humic materials. If the process is reversible, the fiber can be soaked with a proper solution [8j. When a fiber used for quantitative analysis starts to degrade. 

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