Introduction

The principle of mass spectrometry (MS) is the separation of ions in a vacuum, using an electrical or magnetic field or a combination of both. The ions may be formed through a variety of processes, but it is perhaps the fragmentation of the molecular ion that produces much of the analytical power of the technique. Mass-to-charge ratios are recorded and the structure of the parent ion may be determined from the ion molecular mass and the pattern of the fragment ions recorded. Experienced mass spectrometrists can recognize typical fragment ion patterns, however, although there are libraries available for the automated identification of mass spectra, careful judgement must be used in the final assignment of the compound's identity. The theory and uses of MS have been well documented as an analytical technique both as a standalone and a hyphenated technique, for example coupled with gas chromatography (GC-MS).

Less is known about the chemistry within ion mobility spectrometers, which are used in the field to monitor for contraband substances such as explosives, drugs, and on the battlefield to detect chemical warfare agents. Originally referred to as plasma chromatography, ion mobility spectrometry (IMS) is a technique concerned with the formation of ion-molecule clusters in air and their movement in an electric field, at or close to atmospheric pressure. The average ion velocity of an ion species in an electric field, vd, is the product of that electric field, E, and a constant of proportionality, K, i.e. vd = KE. K is called the mobility of the ions, and is characteristic of a particular ion species in a specified drift gas. K may be calculated indirectly from drift time, td, from the equation td = ld/vd, where ld is the drift length. The theory of ion mobility and reaction chemistry is covered in two monographs listed in the Further Reading section, and need not be reproduced here. Notably, the Mason-Schamp equation for mobility (an equation that attempts to reconcile fundamental properties of ions with their mobility) includes a term containing a collision integral, to which mobility is inversely proportional. The value of the collision integral is determined by the cross-section. Therefore, the mobility, and consequently the ion drift velocity, is dependent upon mass, size, shape, and polarizability. The mobilities observed for ions are weighted averages of the mobilities of all the cluster ions participating in a localized equilibrium between the ion swarm and the neutral molecules they encounter as they traverse the drift region. If the drift gas, electric field gradient, temperature, pressure, and therefore the molecular number density remain constant, mobility depends only on ion charge, reduced mass, and collision cross-section. The collision processes undergone by ions during their drift time are very complicated, and are much too complicated to go into here. However, it must be noted that these processes are affected by variations in temperature and pressure in the drift region. Ion cluster formation and fragmentation are also governed by temperature. Therefore, to simplify the situation, and to allow easy comparison between different systems, mobility of an ion is normalized for temperature and pressure, the corrected term being referred to as reduced mobility, K0 (u0 in some texts).

The initial distribution of ions immediately following ionization is modified by various chemical reactions, forming more stable ions. In clean air, these ions form what is called a reactant ion peak (RIP). Positive ion chemistry can involve proton transfer, nucleophilic attachment, hydride or hydroxide extraction, and oxidation; negative ion chemistry involves electron capture, charge transfer, dissociative capture, proton abstraction, and electrophilic attachment; both positive and negative chemistries can be subject to complex rearrangements.

When a sample atmosphere enters the ion mobility spectrometer, many competitive reactions occur and to a first approximation proton or electron affinities may define the reaction pathways. These competing species may be target or possible interference compounds. Ion mobility spectrometers respond to a broad range of compounds with various functional groups. Therefore, complicated spectra are common in ion-molecule systems based upon water chemistry, due to the relatively low proton affinity of the water molecule. Selectivity may be improved with the introduction of trace quantities of an appropriate dopant chemical into the detector carrier gas, thereby altering the degree of affinity required for reaction. This can have an effect on resolution, sensitivity, response and recovery times.

Whilst ion mobility spectrometers respond to many compounds, in the field the operator is only able to identify the compound being detected, by an ion mobility spectrometer, from its display. The efficacy of the instrument display depends upon calibration and software programming. However, as the observed peaks represent cluster ions participating in a localized equilibrium, even in the laboratory, with instrumentation capable of displaying the mobility spectra, the accurate identification of species may be difficult.

Although identification of unknowns by IMS alone is problematic, the coupling together of IMS and MS (IMS-MS) produces a powerful technique. The masses of ion-molecule clusters forming the RIP and product ion peaks are recorded either in positive or negative mode mass spectra, depending on the polarity of the ions being studied. When tuned ion analysis is performed on a specific mass in the mass spectrum, the mobility of the ion mass can be determined, i.e. its position in the mobility spectrum. With the technique enhanced, further by coupling IMS to tandem MS, the composition of ion-molecule clusters can be identified from the results of collision-induced dissociation (CID).

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Solar Panel Basics

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