Instant Notes: Analytical Chemistry

very small volumes of sample and solvents rapidly, and having the advan-

tage of allowing flow in both directions if required.

● Well plates containing upwards of 96 individual miniature sample-

containers in a rectangular array and fitted with miniature SPE packed beds

or disks. Well plates are used in xyz liquid handlers for processing large

numbers of samples prior to the transfer of aliquots to analytical instruments,

particularly gas and liquid chromatographs and mass spectrometers.

● Solid-phase microextraction (SPME)is an important variation of SPE that

allows trace and ultra-trace levels of analytes in liquid or gaseous samples to

be concentrated. The sorbent is a thin layer of a polymeric substance such as

polydimethylsiloxane (PDMS) coated onto a fused-silica optical fibre about

1 cm long and attached to a modified microsyringe (Fig. 3). The fibre is

exposed to the sample and then inserted directly into the injection port of a

gas or liquid chromatograph to complete the analysis. An advantage of

SPME over SPE is the avoidance of solvents, but good precision for quantita-

tive determinations is more difficult to achieve, and automated systems are

only just being developed. SPME is finding particular use in water analysis,

the analysis of fragrances and volatiles in foodstuffs by headspace sampling

(Topic D5), the detection of drugs and their metabolites in urine, blood and

breath samples, and the monitoring of air quality in working environments.

118 Section D – Separation techniques

Polymer coated
fused silica fiber

Septum piercing

needle

Stainless steel

tube

Viewing

Adjustable needle window

guide/depth gauge

Z-slot

Barrel Plunger

Fig. 3. Diagram of a solid phase microextraction device.