Mass analyzers instrument tuning

Quadrapole mass analyzers are by far the most common type of mass spectrometer in use today and the literature on these type of analyzers is extensive. Quadrapole mass analyzers are often thought of as mass filters because they can be tuned to transmit ions of a narrow range of mass/charge (w/z) ratios. Fig. 6 shows a generalized block schematic of a quadrapole mass spectrometer. A typical quadrapole instrument separates ions with different masses by application of a combination of static and radio frequency electric fields to four cylindrical rods. A headspace gas sample is introduced at an inlet and fed into an ion source where electrons are emitted from a filament and ionize the sample gas. The sample ions are then accelerated in an electrical field and are injected into the opening at the center of the rods. In the simplest systems, one pair of rods is connected and attached to the positive... 

This is particularly helpful when analyzing a mixture of tryptic peptides covering a wide mass range. Table 13.1 shows the molecular ion peak widths for a mixture of tryptic peptides from BSA in the normal delayed extraction mode, and in the MCA mode. In both cases, the instrument was tuned optimally for the mass of insulin (m/z 5733) to around 4 ns. In the normal delayed extraction mode, peak widths range from 8.6 ns at m/z 2832 to 11.4 ns at m/z 928. In the MCA mode, peak widths are in the range of 2.8 to 4.5 ns. 

No rule exists in the relationship of the absolute intensity (i.e., counts) of a molecular ion as determined by ESI-MS with the concentration in solution of the analyte that yields the ion. The ion intensity of an analyte determined by ESI-MS could be affected by many factors related to sample preparation, ionization conditions, tuning conditions, the analyzer and detector used in the mass spectrometer, etc. Minor changes of these factors could lead to significant changes of ion intensity from one state to another. As MS instruments become more and more sensitive, the influences of these factors on determination of ion intensities by ESI-MS become more and more evident. Thus, it would be difficult for scientists to virtually repeat a measurement of absolute ion counts for an analyte in a biological sample from time to time, from an instiument to another instrument, or from one laboratory to the others. 

The usability of the calibration depends on the type of instrument and can last for a period of up to several days or weeks. All tuning parameters, in particular the adjustment of the ion source, affect the calibration as described above. In particular the analyzer scan speed has a strong impact on the mass calibration with many instruments. Special attention should also be paid to a constant temperature of the ion source. Regular mass calibration using analysis conditions is recommended to comply with the lab internal QA/QC (quality assurance/quality control) procedures. 

---