SPME sorbents

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

For structural integrity, SPME sorbents are most commonly immobilized by coating onto the outside of fused silica fibers or on the internal surface of a capillary tube. The phases are not bonded to the silica fiber core except when the polydimethylsiloxane coating is 7 pm thick. Other coatings are cross-linked to improve stability in organic solvents [135], De Fatima Alpendurada [136] has reviewed SPME sorbents. 

Analyte size, concentration levels, and detection limits must all be taken into consideration when selecting SPME sorbents [55], Physical characteristics, including molecular weight, boiling point, vapor pressure, polarity, and presence of functional groups, of the analytes of interest must be considered [135], Analyte size is important because it is related to the diffusion coefficient of the analyte in the sample matrix and in the sorbent. 

Extraction recovery can be optimized by changing sample conditions such as pH, salt concentration, sample volume, temperature, and extraction time [130,132,133,136]. Currently, all commercially available SPME sorbents are neutral, such that the sample pH should be adjusted to ensure that the analyte of interest is also neutral [131]. 

Determination of the optimum time for which the SPME sorbent will be in direct contact with the sample is made by constructing an extraction-time profile of each analyte(s) of interest. The sorption and desorption times are greater for semi volatile compounds than for volatile compounds. To prepare the extraction-time profile, samples composed of a pure matrix spiked with the analyte(s) of interest are extracted for progressively longer times. Constant temperature and sample convection must be controlled. Stirring the... 

Pesticides Online SPME Sorbents a PPY-coated capillary, a PMPY-coated capillary,... 

The use of SPME for CE has not (yet) been studied widely. Li and Weber (170) reported an off-line SPME-CE approach for the determination of barbiturates in urine and serum, utilizing a sorbent of plasticized PVC coated around a stainless steel rod. Eor extraction, the coated rod was inserted for 4 min in a Teflon tube containing 50 p.1 of sample, and next the rod was repeatedly desorbed in another Teflon tube which each time contained 5 p.1 of desorption solution. This solution was transferred to an injection vial and an aliquot was injected into the CE system (Eigure 11.19). The extraction procedure appeared to be selective and effectively allowed the handling of very small samples. 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

Another recently developed technique is headspace sorptive extraction (HSSE) with PDMS stir bars [552]. HSSE-GC was compared with SHS and HS-SPME. SBSE and HSSE extract organic analytes from aqueous or vapour samples. In SBSE, the stir bar is inserted into the aqueous sample and extraction takes place during stirring whereas in HSSE the glass rod is suspended within the headspace volume and sampling takes place during headspace equilibration. New trends are the development of selective sorbents. 

Solid Phase Sorbents. Solid Phase Extraction Formats. Automated Solid Phase Extraction. Solid Phase Microextraction. Applications of SPE and SPME. 

The following table provides a listing of the major types of sorbents used in sampling, concentrating, odor profiling, and air and water pollution research.16 These materials are useful in a wide variety of research and control applications. Many can be obtained commercially in different sizes, depending on the application involved. The purpose of this table is to aid in the choice of a sorbent for a given analysis. Information that is specific for solid phase microextraction (SPME) is provided elsewhere in this chapter. 

TWA-SPME Glass fibers coated with a thin layer of various sorbent materials Water boundary layer BTEX, PAHs, organometallic compounds Integrative Up to several days Thermal desorption in GC inlet 22,38,77... 

Sorbent/ solid phase Solid phase extraction (SPE) Solid phase microextraction (SPME) Stir bar sorptive extraction (SBSE) INCAT/OTT/in-tube-SPME SPDE Headspace-solid phase microextraction (HS-SPME) Headspace stir-bar sorptive extraction (HS-SBSE) Purge-and-sorbent trapping Spray-and-sorbent trapping... 

Another important mode of operation in SPME is in-tube SPME.65 In this system, usually coupled on-line to HPLC, a finite portion of sample is drawn through an internally coated capillary tube and then ejected into the sample vial. This technique requires more complex instrumentation than that used for standard SPME, but a greater sensitivity is obtainable with a longer tube (and consequently more sorbent). Two solvent desorption modes—are usually applied for introducing species into HPLC off-line desorption and on-line desorption. In the latter, the HPLC mobile phase is used for desorbing the analytes. 

Is analyte recovery using a solid-supported liquid phase classified as LLE or LSE In Section 2.2.4, a process described as solid-supported LLE [49,50] was discussed in which the liquid sorbent phase was distributed on the surfaces of individual particles (Figure 2.18). The solid-supported phases in the LSE section have been arbitrarily distinguished as liquids mechanically supported on solid devices, such as the liquid-coated fused silica fibers used for SPME or the liquid-coated glass sheath of a stirring bar in used SBSE, rather than liquids supported on finely divided solid particles. 

PDMS-DVB is a multiple-component bipolar sorbent coating. PDMS-DVB is commercially available in a film thickness of 65 pm for SPME of volatile, amine, or nitroaromatic analytes for GC analyses or in a film thickness of 60 pm for SPME of amines and polar compounds for final determination by HPLC [137], DVB is suspended in the PDMS phase [135],... 

Although various ways to implement SPME are proposed and are being developed (Figure 2.46), there are two primary approaches to conducting SPME (Figure 2.47) with the sorbent coated on the outer surface of fibers... 

SPME (Figure 2.48) can be conducted as a direct extraction in which the coated fiber is immersed in the aqueous sample in a headspace configuration for sampling air or the volatiles from the headspace above an aqueous sample in a vial (headspace SPME analyses are discussed elsewhere) or by a membrane protection approach, which protects the fiber coating, for analyses of analytes in very polluted samples [136]. The SPME process consists of two steps (Figure 2.49) (a) the sorbent, either an externally coated fiber or an internally coated tube, is exposed to the sample for a specified period of time (b) the sorbent is transferred to a device that interfaces with an ana-... 

In the fiber mode, the sorbent coated fiber is housed in a microsyringelike protective holder. With the fiber retracted inside the syringe needle, the needle is used to pierce the septum of the sample vial. The plunger is depressed to expose the sorbent-coated fiber to the sample. After equilibrium is reached or at a specified time prior to reaching equilibrium, the fiber is retracted into the protection of the microsyringe needle and the needle is withdrawn from the sample. The sorbent is then interfaced with an analytical instrument where the analyte is desorbed thermally for GC or by solvents for HPLC or capillary electrophoresis. For the in-tube mode, a sample aliquot is repeatedly aspirated and dispensed into an internally coated capillary. An organic solvent desorbs the analyte and sweeps it into the injector [68,130,133]. An SPME autosampler has been introduced by Varian, Inc., that automates the entire process for GC analyses. 

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