Jdv

Copper Mesh

Fig. 17.2. SPME TWA water sampler.

illustrates a SPME diffusive sampler for water sampling designed by G. Ouyang et al. [31]. This type of SPME device can also be used for measuring organic pollutants in soils and sediments.

Recently, more sensitive, convenient and low-cost TWA passive samplers based on the SPME technique have been developed. These new samplers are referred to as PDMS-rod [10] and PDMS-membrane [11] passive samplers (Fig. 17.3). The device on the left is a PDMS-rod passive sampler, composed of a pure PDMS rod, 1-mm wide and 1-cm long, with a volume of 7.85 mL. The device on the right is a PDMS-membrane passive sampler. It is a piece of pure PDMS membrane, 125-mm thick with a volume of 62.5 mL. The volumes of the PDMS-rod and the PDMS-membrane samplers are much larger than commercial PDMS fibres (0.61 mL). This increases the sensitivity of the passive samplers. The simple use of a PDMS rod and membrane as a TWA passive sampler is based on the newly developed kinetic calibration method for SPME [32]. This kinetic calibration method, also called the on-fibre standardization technique, uses the desorption of the standards, which are pre-loaded in the extraction phase, to calibrate the extraction of analytes.

The kinetic process for the absorption of analytes into a PDMS rod or membrane from a medium with constant analyte concentration can be described by [33] n

Fig. 17.3. PDMS-rod and membrane samplers.

where n is the amount of analyte in the extraction phase at time t, a is a first-order exchange rate constant that is dependent on the volumes of the extraction phase, headspace and sample, mass transfer coefficients, distribution coefficients and the surface area of the extraction phase. The kinetic process of the desorption of the internal standard from the extraction phase to a medium with negligibly low internal standard concentration is defined by [32]

K es V e + V s where Ve and Vs are the volumes of the extraction phase and the sample, respectively. K'es is the distribution coefficient of the internal standard between the extraction phase and the sample, q is the amount of standard lost from the extraction phase at time t and q0 is the amount of pre-loaded standard in the extraction phase. Let Q — q0 — q, and Q is the amount of the standard remaining in the extraction phase after exposure of the extraction phase to the sample matrix for the sampling time, t. Thus, for the desorption process, Eq. (17.2) can be expressed by [34]

Q0 - 9e where qe is the amount of standard remaining in the extraction phase at equilibrium. If the desorption and absorption processes occur simultaneously, the constant a should be similar for the analytes and the internal standard, if their physicochemical properties are similar. Then, Eqs. (17.1) and (17.3) can be combined to form

As qe and ne can be calculated with the distribution coefficient between extraction phase and sample:

KesV eq0 (175)

where Kes is the distribution coefficient of the analyte between extraction phase and sample. As K^ ~ Kes, then Eq. (17.4) can be expressed by [10]

0 0

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