Ionization

Currently, the standard ionization source in IMS instrumentation is 63Ni, favoured because it is stable and places no power demands on a portable instrument. A foil is typically used, often electroplated onto gold or platinum. Ionization principles based on this source are well understood, but there are powerful operational incentives to remove radioactive sources from what are primarily designed as portable instruments. Alternative methods include:

• electrospray ionization;

• photoionization using a UV lamp, which produces no reactant ions and limits the analytes which can be studied;

• laser ionization; and

• corona, with latterly, pulsed corona discharge sources that may be configured to behave in a similar manner to 63Ni.

Currently the development of the pulsed corona discharge source appears to be the most promising alternative to radioactivity for general applications as it consumes little power, lasts a long time, operates in both positive and negative mode, and is already being incorporated into the next generation of miniaturized IMS instruments.

Ionization of the gases in the reaction region is perhaps usefully described in terms of 63Ni, as this is the most widely studied and best-understood ioniz-ation source. The first step is direct ionization of the carrier gas by the ft-particles emitted by the source, which triggers off a multistage reaction leading to the formation of stable reactant ion species. The following example is for the positive-mode ions in air, or nitrogen with low moisture content:

The dominant positive-mode reactant ions in clean air will thus be (H2O)nH +, where n will be dependent on the moisture content of the gas as well its pressure and temperature. Minor contributions are also generally seen from (H2O)nNH# and (H2O)nNO +.

An analogous reaction scheme occurs for the negative ions, resulting in (H2O)BO;T and some (HaOLW.

When analyte vapours are present in the source region, they may undergo collisional charge transfer reactions with the reactant ions to form product ions in atmospheric pressure chemical ionization (APCI) processes. In almost every case molecular ions are formed in IMS, as APCI causes little fragmentation. Some analyte fragmentation has been observed in IMS, although this has been attributed to thermal decomposition. Typical reaction pathways may be summarized as:

Proton transfer

Cluster formation

Electron capture

Dissociative electron capture

Cluster formation

Proton abstraction

where R is the reactant ion species, P is the product ion species and M is a neutral fragment.

When any of these reactions occur, the mobility spectrum changes, the size of the reactant ion peak (RIP) reduces as the charge reservoir is depleted, and a new peak, the product ion peak (PIP), appears corresponding to the analyte ion (see Figure 2). These peaks will rise and fall in a synergistic relationship, and as charge is conserved in IMS, the summed peak areas of the mobility spectrum should remain constant throughout these changes.

As analyte concentration increases some polar compounds (e.g. esters and alcohols) form a second PIP, observed at longer drift time, which is due to an ion containing two analyte molecules. This is essentially a clustering reaction forming a proton bound dimer:

Dimer formation

A detailed discussion of the formation of proton bound dimers is beyond the scope of this article. Their appearance in a mobility spectrum is a function of the thermodynamics and kinetics associated with their formation along with their stability in the drift tube. The development of proton-bound dimers is usually associated with highly nonlinear responses associated with the monomer form of the PIP (Figure 4).

The formation of product ions occurs rapidly with one or two simple reactions, while the formation of reactant ions is a comparatively slow multi-step process. Once the analyte concentration rises above a critical level the rate of removal of the reactant ions will be faster than their production. This leads to rapid depletion of the charge reservoir in the reactant region, and no further increase in instrument response will be seen. This is referred to as saturation of the instrument, and sets a limit on the response behaviour.

Typical IMS response behaviour with analyte level is shown in Figure 4. The relationship between the instrument response and analyte level for a single-step reaction leading to a product ion may be simply expressed in terms of:

where RIP0 is the size of the charge reservoir (RIP peak area in the absence of analyte), RIPx is the RIP area at analyte concentration x, PIPx is the PIP area at analyte concentration x, ft is the 'reactivity coefficient', a function of reaction time and rate constant and x is the analyte concentration.

Analyte concentration (g m 3)

Figure 4 Schematic representation of the relationship between analyte concentration and three IMS peaks: R, reactant ions; M, monomer product ion; and D, dimer product ion.

Analyte concentration (g m 3)

Figure 4 Schematic representation of the relationship between analyte concentration and three IMS peaks: R, reactant ions; M, monomer product ion; and D, dimer product ion.

Attempts to fit linear functions to IMS response trends have shown that linearity can only be approximated (with less than 5% errors) over the first 30—40% of the response range. Quantitative work in the literature suggests that the linear dynamic range of IMS is typically between 1 and 2 orders of magnitude of concentration. Beyond this range quantitation is undertaken on the basis of logarithmic relationships. Eventually, once the reactant ions are depleted, the instrument saturates to a population of product ions.

Working at or near saturation should be avoided. Peaks are frequently seen to broaden and/or their mobility vary as excess neutral analyte molecules cluster around the ions (forming new peaks), or further reactions occur in the drift region (broadening and smearing peaks). An excess of neutral analyte within the instrument also often leads to adsorption onto internal surfaces, such that spurious analyte peaks may be seen for a long time after the original analysis.

Proton or electron transfer can only take place if the proton, or electron, affinity of the neutral molecule is greater than that of the reactant ion. In the default case for positive mode air, the proton is held by water, which has a relatively low proton affinity. This suggests a method by which selectivity can be introduced into the ionization process. If a constant supply of suitably high concentration vapour is provided to the reaction region, then all the protons will be captured to form a new population of reactant ions. This is known as 'doping' the instrument. A dopant can be chosen to have a proton affinity just below that of the target analyte, so that the required response will still be generated but interferences from all compounds with proton affinity lower than the dopant will be prevented. This method has been successfully applied in many laboratory and field applications and has been found to reduce interferences and simplify responses to mixtures, and in some cases to enhance separation and sensitivity. For example acetone is used to dope CAM units, while nicotinam-ide is the dopant used for narcotics detection and chlorinated volatile organic compounds are used for explosives doping. However, not all compounds are suitable as dopants, for example pyridine-doped systems respond to all compounds (despite pyridine's high proton affinity) and give distorted peaks. This is because clustering reactions rather than charge exchange reactions are occurring.

The concentration of dopant has also been found to be important, as too little does not impart full selectivity (i.e. some 'old' RIP still remains to react), while too much causes cluster formation due to excess neutrals. When doping conditions are optimized then often no changes in PIP position or quantitative behaviour are observed between systems with different dopants.

IMS responses to mixtures can become complicated as components in a mixture compete for charge. The distribution of charge between them tends to be on the basis of concentration and 'reactivity' (e.g. proton or electron affinity). Thus peak areas for analytes in a mixture will not necessarily quantitatively reflect the proportions of each species present in the reaction region. A further problem with mixtures is that 'mixed dimer' ions can be formed, where molecules of two different analytes cluster together around a charge centre. This leads to the appearance of new peaks and more complicated spectra. In these terms the competitive ionization processes can be considered as a source of interferents, in much the same way as overlapping peaks in column chromatography.

In summary, the ionization processes are the cause of some of the major problems of IMS, for example the complicated and congested spectra obtained from mixtures and the limited linear range for many applications. However, they also provide some of the most useful features of the technique such as the spectacular detection limits due to the large number of collisions that occur at atmospheric pressure. Trace levels of analyte are ionized efficiently and the technique is able to respond to a large number of analytes.

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