Sample Introduction

As IMS is a vapour-phase analysis technique, it is most commonly used with gaseous or volatile

Figure 2 A typical mobility spectrum, showing the reactant ion peak (RIP) from clean air and a product ion peak (PIP) from 2,4-lutidine.

analytes, and analytes are introduced into the reaction region in a carrier gas. This is generally the same gas that is used for the drift flow, and consequently the sample inlet and drift gas flow rates are balanced so that analyte cannot be blown into the drift region.

The direct introduction of ambient air is not normally an effective sample introduction technique as the result is high levels of water and traces of ammonia in the reaction region. The high levels of water in ambient air can lead to formation of cluster ions and thus a loss of resolution in the mobility spectra, while ammonia can dominate the spectrum and prevent analyte response owing to its high proton affinity. Consequently a heated dimethylsilicone membrane is frequently used in the inlet. This excludes excessive amounts of water and ammonia from the reaction region but allows analytes to be sampled. This method works effectively with the military IMS units used for CAM. However, the use of a membrane in the inlet increases the response times of the instrument and reduces the sensitivity.

Not surprisingly, IMS has frequently been used with gas chromatography (GC). In fact the first reports of IMS described GC-IMS systems, and some workers still maintain that IMS cannot be effectively used without chromatography for the sample input. GC has been recognized as intrinsically compatible with IMS, as the carrier gas will not generate a response, samples are typically small enough to avoid saturation of the instrument, and pre-separation of analytes simplifies ionization procedures and responses. Interfacing the techniques is also relatively straightforward, although memory effects were initially found to be a problem. This was overcome by introducing the column effluent either laterally or axially after the ionization source (Figure 3), and allowing it to be carried back through the ionization region by the drift flow. These unidirectional flow configurations reduce IMS cell clearance times and significantly enhance the response of the instrument.

IMS has also been used for liquid samples. Membranes between the liquid sample and a flowing gas stream have enabled IMS to be used to detect chlorinated hydrocarbons and ammonia in water, for example. However, a recent and important development is the coupling of electrospray to IMS. This has been successfully used with IMS to analyse a wide variety of nonvolatile analytes and liquid samples. The electrospray needle is connected into the ioniz-ation region instead of the 63Ni source, and the voltages are applied by a power supply independent of the drift voltage supply. Optimization is required in terms of cell and needle temperatures to improve resolution and avoid vaporization of samples before ionization. The electrospray needle is also insulated

Figure 3 Methods for interfacing IMS to gas chromatography. Both these arrangements ensure that clear down times in the reaction region are rapid, while at the same time enabling the efficient production of product ions.

to avoid corona discharges. This coupling has been found to provide stable molecular ions and reproducible well-resolved ion mobility spectra.

Several major applications of IMS involve the detection of nonvolatile analytes at trace levels for example narcotics and explosives. In these applications the analysis of the analytes' headspace would not give a satisfactory response. However, thermal desorption of microparticulates of the analytes into a carrier gas stream and analysis with a heated IMS cell provides a highly sensitive and effective alternative that is the basis of several instrument systems used all over the world in support of police, customs, forensic and airline security applications.

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