Direct immersion SPME

An SPME fiber can be exposed in two modes—by immersing it directly in the liquid sample to be analyzed (direct immersion SPME—DI-SPME), or by exposing it to the headspace (HS-SPME). In the latter case, the fiber is inserted into the headspace, above a liquid or solid sample. 

Alternatively, a direct-immersion SPME method with DVB/CAR/ PDMS fiber (50/30 pm, 2 cm length) has been proposed. An aliquot of 15 mL of wine is transferred into a 20-mL vial and equilibrated at 30 °C for 5 min, then the fiber is immerged into the solution for 30 min under stirring. The fiber is then desorbed into the GC injection port at 250 °C (Fan et al., 2007). 

Because liquid and headspace sampUng methods differ in kinetics, the two approaches are complementary. Equilibrium is attained more rapidly in headspace SPME than in direct-immersion SPME, because there is no liquid to hinder diffusion of the analyte onto the stationary phase. For a given sampling time, immersion SPME is more sensitive than HS-SPME for analytes predominantly present in the liquid. The reverse is true for analytes that are primarily in the headspace. Several additional factors can affect SPME and do influence the choice between immersion and headspace sampling [997]. Overall extraction with HS-SPME is apt to be lower than in direct-immersion because transfer of analytes from the sample to the gas phase seldom is quantitative. HS-SPME was compared with PT [998] and HS-GC-MS [954,999]. Application of HS-SPME eliminates many problems of other headspace techniques and extends headspace sampling to less volatile compounds due to the concentration effect at the fibre coating. Thermal desorption... 

Figure 4 Graph showing the relative extraction differences for carboxylic acids between headspace and direct immersion SPME techniques. The acids were dissolved in water at a concentration of 10 ppm each and the solutions were extracted for 5 minutes at 40°C, without stirring, using a 85-pm polyacrylate fiber. Both aqueous and 25% NaCl solutions were examined and the extractions were monitored by capillary GC as in Fig. 3.

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. 

Valproic acid Direct immersion PDMS (100) GC-FID (LOD l mg/mL) Equilibrium dialysis followed SPME Krogh et al., 1995 (7)... 

Koster et al. [140] conducted on-fiber derivatization for SPME to increase the detectability and extractability of drugs in biological samples. Amphetamine was used as a model compound. The extraction was performed by direct immersion of a 100-pm polydimethylsiloxane-coated fiber into buffered human urine. On-fiber derivatization was performed with pentafluorobenzoyl chloride either after or simultaneously with extraction. 

Fiber coating thickness is a second consideration in selecting a fiber for both direct immersion and headspace SPME. The PDMS coating is avail-... 

The direct coupling of SPME with 1CP-MS was described for the species-specibc determination of methyl-Hg [19]. A E>ber was inserted into a splitless-type GC injector, which was placed directly at the base of the torch. Immersion SPME was severely inBuenced by the matrix and led to a 70-fold decrease in sensitivity [19]. Analytical results showed good agreement between certified and measured values for the analysis of the NRCC seafood CRMs DORM-2 Dogbsh muscle and DOLT-2 Dogbsh liver [19]. 

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. 

SPME can either be performed by head-space extraction (HS-SPME) by placing the fiber in the vapour above a gaseous, liquid or solid sample, or by direct immersion extraction (DI-SPME), by immersing the fiber in a liquid sample. After a certain extraction time, the SPME needle is removed from the septum and inserted into the injection port of the GC or into the desorption chamber of the SPME-HPLC interface. The desorption is performed by heating the fiber in the GC inlet, or by pumping a solvent through the desorption chamber of the SPME-HPLC interface. The main advantages of SPME compared to LLE and solid phase extraction (SPE) are that no or little solvent is... 

Coupling MAE and SPME (the SPME fiber is directly immersed into the aqueous media or exposed to the headspace of the sample followed by MAE)... 

In bio-analytical methods, both direct-immersion (DI-SPME) and head-space fiber (HS-SPME) have been applied with or without a derivatization step. Using the direct immersion approach, which means exposure of the fiber to the sample in solution, clenbuterol in urine and serum as well as citalopram, fluoxetine, and their main metabolites in urine, were determined without a derivatization step by HPLC analysis. 

Ultimately, molecular selectivity is desirable. Calix-[4]-arenes improved the extraction of propranolol. Antibodies can be used for affinity separations. Li et al." have demonstrated selectivity for barbiturates using a specially designed receptor molecule. The need for more selectivity is made clear by the following common observation. Recall that SPME extractions can take a long time to reach equilibrium. Is it worthwhile to wait for equilibrium It can be as mentioned above, it should be more reproducible. But interestingly, for many samples extracted by direct immersion of the probe into the sample there is no detection limit advantage to longer extractions because interferences... 

More than one hundred papers reporting diverse applications to analyse wines were published until know. The majority of the referred methodologies use the headspace mode (HS-SPME) instead of the direct immersion mode (DI-SPME). In terms of performance, SPME showed comparable results to LLE or SPE. However, SPME is simpler and solvent-free, and uses smaller volumes of sample nevertheless, on the other hand, LLE had the possibility of carrying out simultaneously the extraction of several samples (Bohlscheid et al., 2006 Castro et al, 2008). When the interest is to obtain the maximum information about the volatile fraction of a wine, the coating DVB/CAR/PDMS seem to be the most suitable (Tat et al., 2005). On the other hand, for specific applications, the choice of a suitable solid-phase, depends on the class of compounds be analyzed, e.g. CAR/PDMS for volatile sulphides and disulphides (Mestres et al., 1999), on-fibre derivatization (PA) for the determination of haloanisoles and halophenols (Pizarro et al, 2007). 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

Solid-phase microextraction (SPME) is a modified SPE procedure based on the use of a fiber, usually made of fused silica, coated with a suitable stationary phase such as poly(methylsiloxane) [13,77,148]. The fiber can be directly immersed in the water sample and maintained under stirring during the preconcentration step. Alternatively SPME fiber can be exposed to the headspace vapor over the water sample [122]. 

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