Alternative Ion Sources

An HCD is the ion source used in the majority of PTR-MS instruments. However, other ion sources have been explored and exploited in PTR-MS and we briefiy illustrate some of these in this section. For full details the reader is referred to the original publications. 

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Olson, L.K. Caruso, J.A. The helium microwave-induced plasma—An alternative ion-source for plasma-mass spectrometry. Spectrochim. Acta, Part B 1994, 49, 7-30. 

A single instrument — a hybrid of a quadrupole and a TOF analyzer — can measure a full mass spectrum of ions produced in an ion source. If these are molecular ions, their relative molecular mass is obtained. Alternatively, precursor ions can be selected for MS/MS to give a fragment-ion spectrum characteristic of the precursor ions chosen, which gives structural information about the original molecule. 

MS-MS is a term that covers a number of techniques in which two stages of mass spectrometry are used to investigate the relationship between ions found in a mass spectrum. In particular, the product-ion scan is used to derive structural information from a molecular ion generated by a soft ionization technique such as electrospray and, as such, is an alternative to CVF. The advantage of the product-ion scan over CVF is that it allows a specific ion to be selected and its fragmentation to be studied in isolation, while CVF bring about the fragmentation of all species in the ion source and this may hinder interpretation of the data obtained. 

Another issue with metals is availability. For example, cobalt is not produced in the United States, but it used extensively in a wide variety of alloys and in the production of one of the most common types of lithium batteries. The availability of cobalt is crucial to several segments of American industry. For example, batteries being developed for use in automobiles powered by alternate energy sources are currently envisioned to use a lithium ion battery that also contains cobalt. However, cobalt is not the only strategic metal, and there is concern about the availability of several metals that are vital to industries in the United States, China, and Japan. There will be competition and stockpiling of strategic metals as the reserves become less accessible. 

The alternative measurement technique [5] is to select the ions of interest from the ion source and sequentially inject these into the accelerator. As only one beam is in the system at any given time, there is no possibility of inter-beam interferences. With such a system, it will be necessary to ensure that the cycle time between beams is short compared to the time-span over which any efficiency changes could occur. As well, the normal method for stabilizing the accelerator will not be available, and other methods must be developed. 

In an attempt to produce TS-1 at low cost, alternative, cheaper sources of Ti and Si and other bases such as binary mixtures of (tetrabutylammonium and tetraethylammonium hydroxides), (tetrabutylphosphonium and tetraethylpho-sphonium hydroxides), (tetrapropylammonium bromide and ammonia, water, hexanediamine, n-butylamine, diethylamine, ethylenediamine, or triethanolamine) in place of TPAOH have been used (284—294). TS-1 was synthesized in the presence of fluoride ions but the material thus formed contained extraframework Ti species (295-297). 

Fig. 1. Schematics of an electrospray ion trap mass spectrometer. The ions produced from the electrospray ion source are transferred into the vacuum system of the mass spectrometer and guided into the trapping region where they are stored. After storage they can be scanned toward the detector to generate a mass spectrum. Alternatively, ions of a single m/z can be stored inside the trap, accelerated so that they fragment on collision with restgas molecules, and the fragments can be scanned toward the detector to determine their mass.

If an analyte is definitely insoluble or only soluble in solvents that are not acceptable for the standard MALDI sample preparation technique, it can alternatively be ground together with the solid matrix, preferably in a vibrating ball mill. The resulting fine powder is then spread onto the target. To avoid contamination, nonadherent material should be gently blown away from the target before insertion into the ion source. [103,108,109]... 

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