Introduction

Solid phase microextraction (SPME) was developed to address the need for rapid sampling/sample preparation, both in the laboratory and on-site (in the field where the investigated system is located) [1]. It presents many advantages over conventional analytical methods by combining sampling, sample preparation, and direct transfer of the analytes into a standard gas chromatograph (GC), thus minimizing analyte losses due to multi-step processes. Since its introduction in the early 1990s [2], SPME has been applied successfully to the sampling and analysis of environmental, food, pharmaceutical, and forensic samples [3]. More recently it has been used in passive sampling of air and water. Figure 1.1 shows the schematic of the first SPME device, which was implemented by incorporating coated fibres into a microsyringe [2]. The metal rod, which serves as the piston in a microsyringe, is replaced with stainless steel microtubing with an inside diameter (i.d.) slightly larger than the outside diameter (o.d.) of the fused silica rod. Typically, the first 5 mm of the coating is removed from a 1.5 cm long fibre, which is then inserted into the microtubing. High temperature epoxy glue is used to mount the fibre permanently. The coated fibre can be moved into and out of a stainless steel needle that serves the purposes of protecting the fibre when not in use and guiding it into the injector. The needle can also serve as a time-weighted average (TWA) passive sampling device in which the coating is kept inside the needle during sampling. This contrasts with conventional SPME, in which the coating is extended outside the needle and exposed directly to target analytes

Comprehensive Analytical Chemistry 48

R. Greenwood, G. Mills and B. Vrana (Editors)

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48001-6

© 2007 Elsevier B.V. All rights reserved. 3

Y. Chen and J. Pawliszyn Syringe barrel

Syringe needle

Y. Chen and J. Pawliszyn Syringe barrel

Syringe needle

Plunger cap

Stainless steel microtubing

Epoxy glue

Fig. 1.1. The custom-made SPME device based on the Hamilton 7000 series syringe.

Plunger cap

Coating

Stainless steel microtubing

Epoxy glue

Fig. 1.1. The custom-made SPME device based on the Hamilton 7000 series syringe.

from a number of matrices, and the analytes then reach equilibrium with the coating.

Several different coatings are commercially available, including poly-dimethylsiloxane (PDMS), polyacrylate (PA), PDMS/divinylbenzene (PDMS/DVB), and Carboxen. The PDMS and PA coatings are a non-porous, amorphous polymeric phase whereas the PDMS/DVB and Carboxen are predominantly porous polymeric phases. Analyte uptake on PDMS and PA is by absorption whereas it is adsorptive for PDMS/ DVB and Carboxen.

The use of SPME devices is very simple. When the plunger is depressed, the fibre is extended outside the needle and exposed to the sample matrix. After a certain amount of extraction time, the fibre is withdrawn into the needle. The needle is then introduced into the hot injector of a GC, where the analytes are thermally desorbed from the coating (Fig. 1.2). The analytes then pass into the GC column for separation and quantification.

At this point, it should be emphasized that one of the major advantages of SPME is that all of the sorbed analytes are analysed. In addition, no solvent vehicle is used with SPME; background noise from the solvent is therefore absent. Narrower peak widths are also obtained, thus increasing the overall analytical efficiency. Other quite important advantages are that the SPME sampling system is fully reusable and that when an SPME coating is analysed it is immediately available for a subsequent sampling session (the coating is clean). SPME is also readily amenable to field portability and automation [4].

0 0

Post a comment