Mass
Spectrometer
PlasmaQuant MS Series
132
The resolution is altered by changing the ratio of the DC potential
U
to the maximum
RF potential
V
applied to the quadrupole. As the resolution improves, the efficiency of
ion transmission decreases, so in practice resolution has to be traded for sensitivity.
For any given quadrupole system, there is a resolution limit. Once this has been
reached any attempt to improve the resolution by further changes in the DC to RF ratio
results in a decrease in ion transmission without any corresponding improvement in
resolution.
The resolution of the ICP-MS is variable under computer control and is typically set to
0.80 AMU calculated at 5% of the peak maximum. The peak resolution is set using the
Resolution and Trim page in the instrument module of the Aspect MS software. The
software calculates the required ratio of DC to RF maximum potential and the
quadrupole controller unit delivers the appropriate voltages to the quadrupole.
Figure 8-27 - Aspect MS software: Resolution and Trim page in the instrument module
The automated software routine will set the resolution of the Be9 and Th232 peaks to
0.80 AMU. The trim function will create a two point mass calibration at these masses.
As the calibration is only performed at two masses, a complete mass calibration is
required after performing this function. The mass calibration routine will determine
the software values for the RF and DC Scale Factors and Offsets required for the
quadrupole controller to correctly distribute the isotopes.
For instructions on how to perform a resolution adjustment refer to the Aspect MS
Help.
The mass calibration routine determines the appropriate values in the software to
send to the quadrupole controller. The mass calibration page is located in the
instrument module of the Aspect MS software. It is an automated routine included in
the automated instrument start-up sequence.
Mass calibration