Inorganic Ionization Sources

Mass spectrometry is not only an indispensable tool in organic and biochemical analysis, but also a powerful technique for inorganic analysis [89-91]. Indeed, over the last 20 years the application of mass spectrometry to inorganic and organometallic compounds has revolutionized the analysis of these compounds. Important advances have been made in the diversification of ionization sources, in the commercial availability of the instruments and in the fields of applications. 

In addition, quantitative and qualitative elemental analysis of inorganic compounds with high accuracy and high sensitivity can be effected by mass spectrometry. For elemental analysis, atomization of the analysed sample that corresponds to the transformation of solid matter in atomic vapour and ionization of these atoms occur in the source. These atoms are then sorted and counted with the help of mass spectrometry. The complete decomposition of the sample in the ionization source into its constituent atoms is necessary because incomplete decomposition results in complex mass spectra in which isobaric overlap might cause unsuspected spectral interferences. Furthermore, the distribution of any element in different species leads to a decrease in sensitivity for this element. 

Four techniques based on mass spectrometry are widely used for multi-elemental trace analysis of inorganic compounds in a wide range of sample types. These techniques are thermal ionization (TI), spark source (SS), glow discharge (GD) and inductively coupled plasma (ICP) mass spectrometry. In these techniques, atomization and ionization of the analysed sample are accomplished by volatilization from a heated surface, attack by electrical discharge, rare-gas ion sputtering and vaporization in a hot flame produced by inductive coupling. 

All of these ionization sources are classical sources used also in optical spectroscopy. The only fundamental difference is that these sources are not used for atomization/excitation processes to generate photons but to generate ions. 

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This chapter does not intend to be a comprehensive coverage of all the inorganic chemistry occurring in mass spectrometers nor does it intend to have much experimental detail of the operation of either the mass spectrometers or the ionization sources. That said, there is a need for a cursory overview of some of the mass spec-trometric techniques (including limitations) used in gas-phase chemistry. 

These methods require that the sample is either a gas or, at least, a volatile substance which can be easily converted into a gas (this explains the utility of mass spectrometry in the field of organic chemistry). In inorganic chemistry it is often more difficult to obtain a gaseous sample, and so other ionization sources have been developed. If the sample is thermally stable, it may be volatilized by depositing it on a filament and heating the filament (thermal ionization mass spectrometry - see below). In restricted cases (e.g., organometallic chemistry), chemical treatment of the sample may give a more volatile sample. 

Most analytical studies using FT-ICR mass spectrometry, where ions have been produced inside (or just outside) the analyzer cell, have used lasers as ionization sources. Other than some very limited Cs secondary ion mass spectrometry (SIMS) studies [77], most research utilized direct laser desorption to form various organic [78] and inorganic [79] ions, including metal [80] and semiconductor [81] (including carbon) clusters. More recently matrix assisted laser desorption ionization (MALDI) has been used to form ions of high molecular weight from polymers [82] and many classes of biomolecules [83]. 

Much of the work in the early development of the preceding techniques incorporated pulsed electron-impact ionization sources or any of several types of laser ionization techniques. In almost all of these cases the ions were created in a pulsed fashion in vacuum and formed in or sent into the acceleration region of the mass spectrometer, where a static acceleration field present there injected them into the mass spectrometer. Such ion sources use the TOF-MS very efficiently because the repetition rate of the spectrometer is limited by the frequency of the ionization event itself. This arrangement allows the TOF-MS to mass analyze of all of the ions formed completely. However, many of the most popular ionization techniques being used in inorganic analysis today are continuous in nature. 

This book describes the fundamental operating characteristics of the most common inorganic mass spectrometers. At the heart of this discussion is a description of the various ionization sources that generate a representative analyte population for mass analysis. The initial chapters introduce the mass spectrometric hardware that separates the ionized fractions of analytes, one mass from another. The detection schemes used to measure this ion population, and the data processing systems that permit this information to be of value to the chemical analyst, are also discussed. 

Mass spectrometry is now widely used for inorganic characterization and microsurface analysis. El is the preferred ionization source for volatile inorganic compounds, whereas the others which are non-volatile may be analysed using ionization sources already described such as SIMS, FD, FAB, LD or ESI [92-94], The example in Figure 1.40 shows the analysis of orthorhombic sulfur (Sg ring) and Cr(CO)2(dpe)2, respectively obtained with an El source and an ESI source. 

The following ionization sources are used mainly in inorganic (atomic) MS, where the elemental composition of the sample is desired. The glow discharge (GD) and spark sources are used for solid samples, while the inductively coupled plasma (ICP) is used for solutions. All three sources are also used as atomic emission spectroscopy sources they are described in more detail with diagrams in Chapter 7. 

Efficient but mild ionization source (produces mainly singly charged ions) Sample introduction for solutions of inorganic salts is rapid and convenient... 

Source J. J. Christensen, L. D. Hansen, and R. M. Izatt, Handbook ofProton Ionization Heats and Related Thermodynamic Quantities, Wiley-Interscience, New York, 1976 D. D. Perrin, Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution, 2d ed., Pergamon Press, 1982. 

Spark source mass spectrometry is used for the examination of non-volatile inorganic samples and residues to determine elemental composition. An RF spark of about 30 kV is passed between two electrodes, one of which may be the sample itself, causing vaporization and ionization. Powdered samples or residues from ashed organic materials can be formed into an electrode after mixing with pure graphite powder. 

There are several methods of producing gas-phase inorganic ions, the starting materials in mass spectrometric studies. The properties of the source of the ions required for study are important in the choice of ionization method. The production of bare metal ions from an involatile nonmolecular source requires a large amount of energy deposited on the surface of the material. The processes that occur after the initial ionization process may also affect the ions finally observed (e.g., clustering). At the other end of the ionization energy spectrum, gas-phase ions of a complexity similar to those observed in the condensed phases require a soft ionization process. A brief description of some of the ionization methods follows. 

Bare metal cations can be prepared from almost any inorganic source as long as enough energy is given to the sample to allow dissociation, vaporization, and ionization. Metal anions are less well studied due to the low electron affinities of most transition metals. Where M+ and M ions are compared, the M ions are generally less reactive. 

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