The mass spectrum of a given compound depends on instrumental conditions
to the degree that the instrument must be tuned to achieve an agreed-upon
spectrum for a target compound. There are several such calibration
compounds, such as perflurokerosine (PFK) and the perflurotributylamine
(PFTBA) used in this laboratory. Tuning involves adjusting the source
elements until the mass spectrum of the calibration compound ``looks
right.'' This generally involves balancing overall sensitivity, mass
discrimination effects, resolution and other characteristics.
Though a typical tune involves adjusting the instrument parameters
to achieve an agreed-upon 'standard' spectrum for the calibration
compound, there is nothing magical about this spectrum. Indeed, to
obtain this standard spectrum, low mass ultimate sensitivity must
generally be reduced; the instrument can likewise be tuned for specific
purposes, such as maximum low mass sensitivity. There in nothing wrong
with doing this (and it is often desirable), but one must not attempt
to compare unkown spectra thus measured with library or standard spectra
obtained by an instrument tuned to produce 'normal' mass spectra.
The calibration compound is also used to establish the mass axis for the instrument. The reactant and product ion signals for the calibration compound lie at specific masses, so if the spectrum is measured, the mass channels can be assigned to real, measured data.