Instrumentation mass analyser

An MS-MS instrument only relatively recently made available commercially for LC-MS applications is the Q-ToF system, i.e. the combination of a quadrupole mass analyser for precursor-ion selection and a time-of-fiight analyser for production detection. As described earlier in Section 3.4.1.4, this instrument has the... 

Hybrid mass spectrometer An MS-MS instrument combining magnetic sector and quadrupole mass analysers. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

The quadrupole mass filter is the most abundant mass analyser today and RF-only multipoles are used as transmission devices/collision regions in various instrumental configurations. The mass filter is used extensively as a stand-alone mass analyser and as an analyser in multistage mass spectrometers. 

Principles and Characteristics Analytical multistage mass spectrometry (MSn) relies on the ability to activate and dissociate ions generated in the ion source in order to identify or obtain structural information about an unknown compound and to analyse mixtures by exploiting two or more mass-separating steps. A basic instrument for the currently most used form, tandem mass spectrometry (MS/MS), consists of a combination of two mass analysers with a reaction region between them. While a variety of instrument set-ups can be used in MS/MS, there is a single basic concept involved the measurement of the m/z of ions before and after a reaction in the mass spectrometer the reaction involves a change in mass and can be represented as ... 

Many excellent reviews on the development, instrumentation and applications of LC-MS can be found in the literature [560-563]. Niessen [440] has recently reviewed interface technology and application of mass analysers in LC-MS. Column selection and operating conditions for LC-MS have been reviewed [564]. A guide to LC-MS has recently appeared [565]. Voress [535] has described electrospray instrumentation, Niessen [562] reviewed API, and others [566,567] have reviewed LC-PB-MS. For thermospray ionisation in MS, see refs [568,569]. Nielen and Buytenhuys [570] have discussed the potentials of LC-ESI-ToFMS and LC-MALDI-ToFMS. Miniaturisation (reduction of column i.d.) in LC-MS was recently critically evaluated [571]. LC-MS/MS was also reviewed [572]. Various books on LC-MS have appeared [164,433,434,573-575], some dealing specifically with selected ionisation modes, such as CF-FAB-MS [576] or API-MS [577],... 

In addition to the diversity of ionisation techniques available, mass spectrometers offer a selection of mass analyser configurations. Of note are single (MS) and triple quadrupole (MS—MS) instruments, ion trap analysers (MS)n, time-of-flight (ToF) analysers, sector field analysers, and Fourier transform-ion cyclotron resonance (FTICR) instruments. 

Mass resolution generally increases for instruments in the listed order. Hybrid variants utilising various composites of mass analysers are likewise available and have become commonplace (e.g. quadrupole-ToF). 

In this instrument, the final stage of the triple quadrupole is replaced by an orthogonal time-of-flight (ToF) mass analyser, as shown in Figure 3.10. The configuration is typical of the latest generation of ToF instruments in which a number of reflectrons, in this case two, are used to increase the flight path of the ions and thus increase the resolution that may be achieved. 

The difference between this and other MS-MS instruments is the way in which the MS2 unit operates, as discussed in Section 3.3.4 above. To reiterate, in contrast to other mass analysers which are scanned sequentially through the... 

Reflectron An ion lens used in the time-of-flight mass analyser to increase the distance travelled by an ion and thereby increase the resolution of the instrument. 

Sample-standard comparison is more applicable in MC-ICP-MS, in which instrument mass fractionation is fundamentally a steady state phenomenon (Marechal et al. 1999). This method has been used successfully for some non-traditional stable isotopes, particularly involving Fe, in which analyses of samples are bracketed by standards to cope with systematic instrumental drift (e.g., Zhu et al. 2002 Beard et al. 2003). However, other methods have been used for Mo stable isotope work published to date because of concerns about non-systematic changes in instrument mass fractionation, particularly arising from differences in matrices, between samples and standards. Such concerns are more acute for Mo than for Fe and many other elements because Mo is a trace constituent of most samples, increasing the challenge of rigorous, high-yield sample purification. 

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