IsoPrime User’s Guide
Version 1.01 IsoPrime100 User’s Guide Page 83 of 219
At the end of the scan, the Peak Centre Results window opens automatically. Accept the Peak
Centre, by clicking on the Accept Peak Centre button.
Save the new value for the Accelerating Voltage by clicking on the Save option of the Edit menu
from the Tune Page window.
By this stage, each of the three ion beams from the three isotopes of CO
2
is focussed into each
of the three collectors. There remains to perform a fine tuning for the other parameters.
It is important to note three features from the Scan window, shown above:
(i) The Peak shape for mass 45 in the central collector is symmetrical, with
steeply rising sides.
(ii) It has a flat top with at least 10 Volts of flat.
(iii) The flat tops for the three ion beams coincide over a range of several Volts.
The focus result on the peak centre is a good indication of acceptable peak shape. A value above
0.550 would be considered to be acceptable. If this cannot be achieved it is an indication that
the ion beams are not focussing correctly into the collectors and the mass spectrometer
requires attention. It is an indication that the source may be badly tuned, dirty, or that the
magnet is not in the correct position.
FINE TUNING OF SOURCE PARAMETERS
NOTES ON TUNING
Instrument linearity and peak shape are directly affected by instrument tuning. Increasing the
source sensitivity may cause the instrument peak shape to degrade; there may also be an
adverse effect on instrument linearity. In this respect a compromise is normally reached where
peak shape, linearity and sensitivity are all acceptable.
As all instruments behave slightly differently it is worth becoming familiar with the source scans
of each individual instrument. A significant change in a source scan may be an early indication of
a problem with the instrument. Fine tuning of the source parameters is necessary in order to
obtain a linear source response with the maximum sensitivity.
The recommended source parameters given in the table in the section entitled Coarse tuning
the Ion Source gives ranges for parameters where it is most likely to obtain linearity; the
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