Mass ionization source

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Liquid chromatography is a separation method that is often applied to nonvolatile, thermally labile materials such as peptides, and, if their mass spectra are required after the separation step, then a mild method of ionization is needed. Since FAB/LSIMS is mild and works with a liquid matrix, it is not surprising that attempts were made to utilize this ionization source as both an inlet... 

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

There are two common occasions when rapid measurement is preferable. The first is with ionization sources using laser desorption or radionuclides. A pulse of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will be no ions left by the time the scan has completed more than a few nanoseconds (ion traps excluded). If a point ion detector were to be used for this type of pulsed ionization, then after the beginning of the scan no more ions would reach the collector because there would not be any left The array collector overcomes this difficulty by detecting the ions produced all at the same instant. 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

When mass spectrometry was first used as a routine analytical tool, El was the only commercial ion source. As needs have increased, more ionization methods have appeared. Many different types of ionization source have been described, and several of these have been produced commercially. The present situation is such that there is now only a limited range of ion sources. For vacuum ion sources, El is still widely used, frequently in conjunction with Cl. For atmospheric pressure ion sources, the most frequently used are ES, APCI, MALDI (lasers), and plasma torches. 

Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS). 

Electrospray is both an atmospheric-pressure (API) liquid inlet system for a mass spectrometer, and, at the same time, it is an ionization source. 

Although the conventional mass spectra of the five C- nitro derivatives of indazole are nearly identical, the corresponding metastable peak shapes associated with the loss of NO-can be used to differentiate the five isomers (790MS114). The protonation and ethylation occurring in a methane chemical ionization source have been studied for a variety of aromatic amines, including indazoles (80OMS144). As in solution (Section 4.04.2.1.3), the N-2 atom is the more basic and the more nucleophilic (Scheme 5). 

Analytical information taken from a chromatogram has almost exclusively involved either retention data (retention times, capacity factors, etc.) for peak identification or peak heights and peak areas for quantitative assessment. The width of the peak has been rarely used for analytical purposes, except occasionally to obtain approximate values for peak areas. Nevertheless, as seen from the Rate Theory, the peak width is inversely proportional to the solute diffusivity which, in turn, is a function of the solute molecular weight. It follows that for high molecular weight materials, particularly those that cannot be volatalized in the ionization source of a mass spectrometer, peak width measurement offers an approximate source of molecular weight data for very intractable solutes. 

More than 20 different kinds of commercial mass spectrometers are available depending on the intended application, but all have three basic parts an ionization source in which sample molecules are given an electrical charge, a tnass analyzer in which ions are separated by their mass-to-charge ratio, and a detector in which the separated ions are observed and counted. 

Infrared, ultraviolet, and nuclear magnetic resonance spectroscopies differ from mass spectrometry in that they are nondestructive and involve the interaction of molecules with electromagnetic energy rather than with an ionizing source. Before beginning a study of these techniques, however, let s briefly review the nature of radiant energy and the electromagnetic spectrum. 

Inghram and Corner showed that the mass spectra of molecules were much simpler using a field ionization source than with an electron bombardment ion source. Mainly parent ions are formed, unlike under electron impact which gives rise to considerable fragmentation. The simplicity of the mass spectra offers obvious applications in analysis of complex organic mixtures and their use is likely to become widespread... 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

The particles then enter a conventional mass spectrometer source where they are vaporized prior to being ionized using electron impact or chemical ionization. As with other interfaces, this may cause problems during the analysis of thermally labile and highly in volatile compounds. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

Dacheux N, Aupiais J (1997) Determination of uranium, thorium, plutonium, americium, and curium ultratraces by photon electron rejecting alpha liquid scintillation. Anal Chem 69 2275-2282 Duan YX, Chamberlin EP, Olivares JA (1997) Development of a new high-efficiency thermal ionization source for mass spectrometry. Inti JMass Spectrom IonProcessesl61 27-39 Edwards RL, Chen JH, Wasserburg GJ (1987) systematics and the precise... 

Which directs them toweurds the analyzer slits. Alternatively, they may be extracted by the field penetration of the high voltage on the focusing electrodes. In both instances the ion beam is usually focused, collimated and accelerated to provide a beam of narrow energy dispersion that is capable of traversing the analyzer section of the mass spectrometer. In modern mass spectrometers the ionization source and analyzer sections are usually differentially pumped, allowing the source to operate at a distinctly higher... 

Figure 9.4 Soaematic diagram of a thexmospray interface ionization source connected to a quadrupole mass analyzer and Insert indicating the mechanism of vapor production by the heated thermospray.

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