Direct SPME

FIGURE 1.46 Effects of major experimental variables (A) extraction time, (B) pH, (C) sodium chloride concentration, and (D) temperature on the efficiency of direct SPME of anticonvulsants in plasma sample.1 7 (Reproduced with permission from the authors.)... 

Arthur and Pawliszyn introduced solid-phase microextraction (SPME) in 1990 as a solvent-free sampling technique that reduces the steps of extraction, cleanup, and concentration to a unique step. SPME utilizes a small segment of fused-silica fiber coated with a polymeric phase to extract the analytes from the sample and to introduce them into a chromatographic system. Initially, SPME was used to analyze pollutants in water - via direct extraction. Subsequently, SPME was applied to more complex matrixes, such as solid samples or biological fluids. With these types of samples, direct SPME is not recommended nevertheless, the headspace mode (HSSPME) is an effective alternative to extracting volatile and semivolatile compounds from complex matrixes. (Adapted from Llompart et ah, 2001)... 

The speed of extraction is controlled by the mass transport of the analytes from the sample matrix to the coating. This process involves convective transport in an air or liquid sample, desorption of the analytes from the solid surface when particulate matter is present, and diffusion of the analytes in the coating [205], In direct SPME sampling, the mass transfer rate is determined by the diffusion of analytes in the coating provided the sample matrix is thoroughly agitated. 

In aqueous matrices, most compounds have quite small values (< 0.25), so the headspace has a low analyte trapping capacity. As a result, the sensitivity of headspace SPME is almost the same as that of direct SPME. The sensitivity loss is only significant when the target analytes partition well into the headspace (i.e. when they possess large... 

Direct SPME Routine In situ chemical derivatization In situ redox reaction Most compounds Polar compounds Inorganic ions Gaseous and liquid... 

Figure 8.13 Analysis of o-xylene and BTEX (in water) using solid-phase microextraction (a) direct SPME fibre mode (b) headspace SPME fibre mode (c) results obtained for o-xylene using mode (a) (d) results obtained for BTEX using mode (b) , no stirring IH, with stirring , with stirring, plus salt , benzene , toluene a, ethylbenzene , m-, p-xylene(s) x, o-xylene [4] (cf. DQ 8.11).

Air pollution is a important problem for public health. For air analysis, the National Institute for Occupational Safety and Health specifies exposure limits for amines in industrial air (10 to 30 ng/ml) and for amines in indoor air (10 to 300 pg/ml). The direct SPME could be used to monitor the amounts of the amines extracted from air in the following order Carbowax divinylbenzene CAX(DAB) > poly(acrylate) PA > poly(dimethyl-siloxane) PDMS." °... 

Direct SPME, headspace and gas phase SPME with PDAM derivatization... 

There are many differences between samphng from the liquid phase (direct SPME) and from the headspace (HS-SPME). The factors afifechng direct SPME and HS-SPME, and the conditions that lead to the optimum performance of the analytical method, are different due to the nature of each process. In direct SPME the mass transfer rate of analytes is limited by the diffusion in the liquid phase, while in HS-SPME the limiting rate is the transport of analytes from the sample to the headspace. Because diffusion in the liquid phase is much slower than in the headspace and transport of analytes from the hquid to the vapor phase can be accelerated by proper conditions, the time taken to reach equilibrium by HS-SPME is shorter than in direct SPME. A comparative study showed, how for the optimal conditions of each method, the time taken to reach equilibrium in HS-SPME was shorter than for direct SPME (see Table 14.3). Limits of detection were also slightly better for HS-SPME than for direct SPME. 

The hnal selection between direct and HS-SPME depends on the nature of the water matrix and the target compounds being analyzed. HS-SPME is recommended over direct SPME in order to avoid contamination of the hber when dirty samples are being analyzed. Sampling from the headspace also involves a selechve separation of the most volatile compounds like BTEX from the liquid matrix and protects the GC column from high-molecular-mass nonvolatile compounds. Direct SPME is recommended only for clean water samples and for the analysis of high boihng point analytes. 

The amount of analyte extracted in direct SPME, N from a volume V2 of a sample with an initial concentration Co, can be calculated by the following equation ... 

Addition of salt to aqueous samples is often used to drive polar compounds into the headspace. Therefore addition of salt should not increase the recovery of BTEX compounds when direct SPME is applied. 

SPME was developed by Pawlisz)m and coworkers in 1987 [161-163]. The reader may find further information on the historical evolution, principles, and commercially available devices of SPME in an excellent review by the pioneer of the technique [164]. SPME is based on a partitioning equilibrium of the solutes between the sorbent phase and the aqueous and/or gas matrix. A small amount of sorbent phase is dispersed on a solid support, which will be exposed to the sample for a predetermined time. Different implementations were developed such as suspended particles, coated-stirrer, vessel walls, disks, stirrers, or membranes, although the fiber and in-tube are explored theoretically and experimentally in depth. The former consists of a thin, fused-silica fiber-coated with sorbent on its surface and mounted in a modified GC syringe, which protects the fiber and allows handling. The latter in-tube implementation consists of an internally coated tube or capillary. The analytes are extracted by sorption when either coated fiber or tube are immersed in the water sample (direct SPME) or in the headspace above the sample (HS-SPME). 

The absorption process is the most important step. In direct-SPME, where the fiber is introduced directly into the sample and so analytes are retained in the fiber (Figure 23.3). This extraction mode is specially suited for separating low volatile analytes. In the case of HS-SPME, a fiber in the needle tip of a microsyringe is exposed to the headspace above the sample (Figure 23.3). Next, the fiber is retracted into the microsyringe and... 

Sample volume This is directly related to the sensitivity. As the sample volume increases also the extracted amoimt increases to a certain degree. Generally, typical 2 mL GC autosampler vials are used for direct SPME. 

Although SPME can be coupled with HPLC and CE, analyses of VOCs are performed by GC. Once analytes have been extracted, either by direct-SPME, HS-SPME, or membrane-SPME, the polymeric fiber is desorbed in the GC injector. The fiber is withdrawn into the needle, and after piercing the GC septum, the fiber is exposed inside the glass insert where thermal desorption occurs. 

Interlaboratory studies were carried out for the validation of SPME quantitative analysis of VOCs in aqueous samples [180]. Comparable repeatability, reproducibility, and accm-acy were obtained for both SPME and reference methods (P T and HS). Better precision was foimd for HS-SPME than direct SPME. 

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