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To get usable mass spectra from a system, it must be operational and set up correctly. In this section, the following topics will be presented:
TuningIn a mass spectrometer, the ion source, mass filter, and detector operational parameters influence the mass spectra produced.
Tuning is performed to check the mass spectrometer or to ensure that the spectra produced resemble a previously determined standard.
Electronically tuning the mass spectrometer involves adjusting the voltages of the ion source, mass filter, and detector. The values of these voltages affect the mass spectra obtained.
One compound used to tune a mass spectrometer to a "standard" set of conditions is perfluorotributylamine (PFTBA, also known as FC-43).
How you tune will affect the quality of your data and your ability to match mass spectra against a spectral data base.
The ion source parameters affect how many ions are produced, how many ions are directed toward the mass filter, and the relative amount of a given mass ion that is directed into the mass filter.
The mass filter parameters in a quadrupole instrument affect the peak widths, mass assignments, mass resolution, and sensitivity.
The detector parameters affect the magnitude of the signal and the sensitivity of the system. All of the tuning parameters affect the quality of the mass spectra and the ability of a given mass spectrum to be matched against a spectral data base.
Setting the Proper Mass Range
The mass range determines how much information one can obtain from a given sample. The choice of the mass range is dictated by the sample.
If the mass range is too small, one loses information in an irretrievable fashion. For example, in the spectrum below, the scan range is too narrow so it does not include the molecular ion of caffeine at m/z=194.
If the mass range is too large, data files will be larger than necessary and you will be able to perform a reduced number of scans per time.
Setting the Proper Threshold
The threshold setting determines the minimum abundance that is recognized as a real signal. Mass peaks that are smaller than the abundance specified by the threshold are thrown away as noise.
If the threshold is set too high, then important mass abundance data can be missed, simply never acquired, and never stored. The analyst never has access to them again.
Verification of System Performance
A mass spectrometer is a complex piece of equipment with many parts working independently. To verify the proper performance of all parts, one must acquire data from samples and compare these results to accepted results.
In addition to parameters set during tuning, one should check the following:
Background signal
Chromatographic performance
System sensitivity
Maintaining System Performance
To keep an MS system operating at peak performance, the following should be done:
Maintain a good vacuum
Maintain an elevated temperature in the MS
Maintain continuous carrier-gas flow into the MS
Replace consumables when required
Method Development
In many analyses, the type of sample (or a target analyte) specifies the method. The method specifies such things as mass range, temperature, and threshold.
In cases where the identity of the components of a sample are unknown, the method should include the following:
Large mass range
Low threshold
Full range of chromatographic parameters
Summary
Mass spectrometers (especially quadrupole instruments) can be tuned in a variety of ways. The analyst must choose a set of parameters that are consistent with the sample requirements.