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Technical Description: System
Mass Spectrometer Parameters
Mass
Spectrometer
Parameters
Sensitivity
Whenever the instrument completes a calibration, a calibration report
is produced that details the sensitivity and relative sensitivity of each
analyte. The sensitivity indicates the ion current as measured at the
collector and is proportional to the percentage of a particular analyte
in the gas used for calibration. Equal pressures of different gases in
the ion source do not necessarily produce an equal ion current. This
is due partly to differences in molecular ionisation cross-sectional
area, those with a larger area being more easily ionized, and in part
due to the different transmission efficiency of the analyser for ions of
different mass. Consequently a sensitivity factor, relative to a
reference analyte, is calculated. This factor is known as the relative
sensitivity.
For the calibration report, relative sensitivity is calculated as being
the sensitivity of the analyte relative to the base gas defined in the
gas database. For example, if the relative sensitivity of nitrogen
measured by the instrument is 1.0 and helium measured by the
instrument is 0.2, for equal quantities of nitrogen and helium in the
ion source the ion current measured for nitrogen will be five times
that of helium.
Resolution
The resolving power or resolution indicates the highest adjacent
mass numbers that a particular instrument can separate. The
definition used is known as the 10% Valley definition. This states
that the resolving power of a mass spectrometer is the highest mass
number at which peaks of adjacent molecular weight and equal
heights have a valley between them of 10% of the peak height.
Figure B–11 shows the appearance of two such mass peaks. The
intensity of the two peaks is additive, and if each contribute 5% at
the midpoint between them, then the net effect is a 10% valley.
For example: If an instrument has a resolving power of 100 by this
definition then, if peaks of equal height could be formed at mass 100
and mass 101, the valley between them would be 10% of the height
of either peak. At lower masses, the valley between adjacent masses
is reduced, and at higher mass numbers, the valley between adjacent
masses increases.
Thermo Fisher Scientific
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
B-15
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