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The difference between the nominal extraction potential (= 10 k V, equivalent to 0 V offset) and the maximum of the observed energy distribution measured on a standard zircon determines the offset voltages to be applied during analysis.
For the example to the right, offset values for Centering the secondary ion image in the center of the field aperture (FA) is necessary for each new analysis spot because primary beam paths and conditions of secondary ion extraction vary over the area of the sample surface,in particular when spot-to-spot movements over large distances (few mm) are required.
In particular it is generally possible to find original magmatic domains yielding concordant ages.
In contrast, destructive techniques such as LA-ICP-MS consume a large volume, leave a deep crater in the target grain, and often sample heterogeneous domains that are not visible and thus often yield discordant results which are difficult to interpret.
The sequence is as follows: In order to stabilize the secondary ion signal and to remove surface contamination, the primary beam is focused on the sample surface for a certain time period prior to data acquisition.