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Operation
7.7 Surface ions
Under the electron bombardment in the ion source, adsorbed contaminations are desorbed as so-called
EID ions, which then appear in the spectrum, e.g. with masses 16 (O+), 19 (F+), 23 (Na+), 35/37 (Cl+)
and 39/41 (K+). EID ions mainly become apparent during UHV measurements. They can be reduced by
degassing (degas or temporary operation with high emission). In order to distinguish between EID ions
and ions from the volume, reduce the value of the field axis voltage at the "Field Axis" parameter. This
significantly reduces the peak height of normal ions. That of EID ions is less affected as they are formed
on the highest potential. Therefore, to prevent suppression of the normal ions, do not select a too low
"Field Axis" value.
7.8 Using degas function
The degas function is intended in particular for UHV measurements with grid ion source.
Procedure
► Observe the information on the individual ion sources.
► Do not switch on the degassing at pressures >10
► Optimize the "Protection Current" filament protection for degas operation.
► If you don't want to use the degas function, or if it is impermissible, set the "Protection Current"
7.9 Determining optimal parameter values
For some applications, it is necessary to deviate from the factory setting. As the contamination increas-
es, or after overhauls, it is recommended to optimize the settings as described in the following sections.
For a schematic overview of the potentials and their designations, refer to the "Technical Data" section.
With the standard values for the various ion sources that are integrated into the device, a spectrum
should be measurable – except for the "SPEC(ial)" ion source type. However, the values always have to
be optimized for the analyzer used. The goal of optimizing the ion source parameters is to achieve a
high sensitivity, good peak shape and low mass discrimination. Sometimes other conditions also have
to be met. This chapter by and large applies to all ion sources; some potentials are not needed with for
all ion sources.
Procedure
► Observe the detailed information on the individual ion sources.
38/52
the ion source.
to 0 A, to block degas operation.
-7
hPa, as this can cause severe contamination of
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