Ion Optics and Lenses Instrument Tuning

Details of instrument tuning vary widely among instrument types and recommended tuning procedures are highly manufacturer dependent. Most modern mass spectrometers provide the facihty to store values of potentials apphed to the various ion optical elements in a Tune File in the computer, which can he called up for 

It should also be noted that the RF-only quadrupoles (as well as the related RF-only hexapoles and octapoles) can be operated as ion lenses, often referred to as RF-only ion guides or simply ion guides. RF-only quadrupoles (denoted as lower case q , nonitalicized, to distinguish them from the closely related quadrupole mass filters Q) also act as efficient collision cells for collision induced dissociation in triple quadrupole analyzers (QqQ, Section 6.4.3) and hybrid tandem quadrupole-time of flight instruments (QqTOF, Section 6.4.7). Discussion of these devices must be postponed imtil the principles of all hnear quadrupoles have been considered (Section 6.4.2). 

In this section the mass analyzers that are important in current practice of trace level quantitation are described in some detail. Before starting the descriptions of specific analyzers, a topic of general applicability (calibration of the m z axis) is discussed. In addition, it should be mentioned here that an extremely important component of any mass spectrometer is the vacuum system this is discussed separately (Section 6.6). Another topic that is important for tandem mass spectrometry is that of colli-sional activation of ions, and discussion is also deferred till later (Section 6.5). 

As for any physical quantity (Section 1.1), measurement of mJz values of ions is ultimately a comparison of the unknown with a standard of some kind. In the case of a mass spectrometer, the raw data transferred to the computer do not involve tnlz values at all but are specifications of the times at which electrical signals (arising from arrival of ions at the detector) are transferred to the computer during a scan of the electric and/or magnetic fields that the particular analyzer employs to separate the ions. (In the case of time of flight analyzers the relevant quantity is the time elapsed since the ions were pulsed into the flight mhe, see Section 6.4.7.) To transform these times of detection into mlz values, it is necessary to obtain a cahhration of time vs mlz obtained using mass cahbration standards that yield ions of known mlz calculated from their molecular formulae and values of atomic masses (Table 1.4). It can be mentioned that 

Finally in this section, it is worthwhile to emphasize again that careful mass calibration is an essential feature of validation of any analytical method. Automated calibration routines are convenient and easy to use, but should be checked before use. 

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