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n-hexane

Fig. 1.15. Relationship between face velocity and sampling rate (Z — 0.45 cm).

n-hexane

Fig. 1.15. Relationship between face velocity and sampling rate (Z — 0.45 cm).

achieved either by increasing the volume of the coating or by changing its affinity for the analyte. Because increasing the coating volume would require an increase in the size of the device, the optimum approach to increasing the integration time is to use sorbents characterized by large distribution constants. If the matrix filling the needle is something other than the sample matrix, an appropriate diffusion coefficient should be used in Eqs. (1.29) and (1.30).

In the system described, the length of the diffusion channel can be adjusted to ensure that mass transfer in the narrow channel of the needle controls overall mass transfer to the extraction phase, irrespective of the convection conditions (Fig. 1.15) [32]. This is a very desirable feature of TWA sampling, because the performance of this device is independent of the flow conditions in the system investigated. This is difficult to ensure for high surface area membrane permeation-based TWA devices, such as, for example, a passive diffusive badge [33] and semipermeable membrane devices [34]. For analytes characterized by moderate-to-high distribution constants, mass transport is controlled by the diffusive transport in the boundary layer. The performance of these devices therefore depends on the convection conditions in the investigated system [35].

1.2.5.4 SPME field sampler

To facilitate the use of SPME for field sampling, a new field sampler was designed and tested [36]. The structure incorporated a commercialised fibre assembly to make the sampler more universal and to achieve inter-fibre reproducibility. The first requirement for a field sampler is that the fibre needle must be sealed. Teflon is soft and provides good sealing of the needle. It also is an inert material that minimizes adsorption of analytes released from the fibres and contamination from the environment. The Teflon cap should be attached to the SPME field sampler because a loose cap could be easily lost and would be difficult to find in the field. The cap should be easily replaceable if it becomes worn or is heavily contaminated. The next requirement is that the fibre needle must be protected. The needle shields the fibre, allows for introduction of the fibre into an injection port, and provides a diffusion channel for TWA sampling. Fibre protection is necessary throughout the sampling/sample preparation, storage, and transportation period due to the risk of operator injury and fibre damage. The third requirement is that the field sampler should be user-friendly, for acceptance in the industry as an alternative to existing methodologies. For example, a pen-like device would be easy to deploy and transport. The last requirement is that the field sampler should be amenable to automation, which requires that the physical dimensions of the field sampler be small, and the use of the device involves only a few simple movements.

Figure 1.16 shows the schematic of the new field sampler. Parts (a) and (b) are two cylinders with matching male and female screws. The needle of the fibre assembly (c) can be put through the central hole of part (a), whose inner diameter is slightly larger than the outside diameter, until the fibre assembly sits on part (a). Holding part (a) with the fibre assembly on it, the hub of the fibre assembly passes through the central hole of part (b) from the female screw end. Tightly screwing part (a) and (b), the fibre assembly is fixed to the holder. The hub of the

Fig. 1.16. Schematic of the new SPME field sampler (g). Parts (a) and (b) are the fibre holders. Part (c) is a commercialised fibre assembly. Part (d-1) is the cross view of the adjustable cylinder, and Part (d-2) is the side view of the adjustable cylinder. Part (e) is the protecting shield. Part (f) is a replaceable teflon cap.

Fig. 1.16. Schematic of the new SPME field sampler (g). Parts (a) and (b) are the fibre holders. Part (c) is a commercialised fibre assembly. Part (d-1) is the cross view of the adjustable cylinder, and Part (d-2) is the side view of the adjustable cylinder. Part (e) is the protecting shield. Part (f) is a replaceable teflon cap.

fibre assembly can be connected to the inner pistol of part (d-1) by a screw. Part (d) can move along the fibre holder consisting of part (a) and (b). By controlling the position of part (d-2), the fibre can be positioned inside the needle for storage, transportation, or TWA sampling, or outside the needle for fibre injection, or rapid/short-term sampling. Part (e) is a protecting shield. The upper part of the protecting shield can hold and move along the fibre holder. Three side-holes are milled in the middle part of the shield, providing windows for analytes to access the fibre coating. The lower part of the shield is used to support the Teflon sealing cap (f). The Teflon cap can be easily replaced in the case of bad sealing or heavy contamination. Configuration (g) is the schematic of the final SPME field sampler that resembles a large pen. The overall dimensions of the field sampler are 137 mm x 13 mm. The prototype field sampler is larger than the final goal for the design, because this field sampler is designed for commercialised fibres. Since the dimensions of the SPME fibres can be decreased significantly, it can be expected that future SPME field samplers will be smaller.

The field sampler is very easy to use and the operation is schematically shown in Fig. 1.17. Configuration (a) is the field sampler in the status of standby, storage, or transportation. To use the sampler, first, unlock the protecting shield (part (e) in Fig. 1.16), pull the shield outward until it stops and is locked at the sampling position (b). Second, unlock the adjustable cylinder (part (d) in Fig. 1.16), adjust and lock the adjustable cylinder so that the fibre can be positioned further inside the

Fig. 1.17. Operation of the new SPME field sampler. (a) The status of standby, storage, or transportation. (b) The status when the protecting shield is pulled outward and locked at the sampling position. (c-1) The model for TWA sampling, and (c-2) the model for grab sampling.

Fig. 1.17. Operation of the new SPME field sampler. (a) The status of standby, storage, or transportation. (b) The status when the protecting shield is pulled outward and locked at the sampling position. (c-1) The model for TWA sampling, and (c-2) the model for grab sampling.

Fig. 1.18. Introduction of the fibre into a GC injector. (a) The fibre is protected, (b) the protecting shield is removed, (c) exposure of the fibre.

needle for TWA sampling (c-1), or exposed completely outside the needle for rapid/short-term sampling (c-2). After sampling, restore and lock the position of the adjustable cylinder (b), then restore and lock the protecting shield (a). When the sampler is transported to a laboratory, unlock the protection shield, and remove it from the sampler (Fig. 1.18b). The needle can then be introduced into the injection port of a GC for desorption (Fig. 1.18c).

The sampler is versatile and user-friendly. The SPME fibre can be positioned precisely inside the needle for TWA sampling, or exposed completely outside the needle for rapid sampling. The needle is protected within a shield at all times hereby eliminating the risk of operator injury and fibre damage. A replaceable Teflon cap is used to seal the needle to preserve sample integrity. A preliminary field sampling investigation has demonstrated the validity of the new SPME device for field applications.

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