Ensuring stability in method performance. Parameters under control.

Linearity as a function of the tube current

The tube current for secondary target and direct X ray tube excitation spectrometers will have linear influence on the spectrum emitted by the sample. Do measurements of a calibration standard with increasing tube current as parameter, from the lowest to the highest setting. Keep record on the stability of these parameters, and make corrections whenever the observed variations exceed the maximum acceptable boundaries.

Detector resolution (FWHM) as a function of time.

The better the resolution the better overlapping peaks can be apportioned. A value for the FWHM close to specification of the detector should be reached. The manufacturer have specified his detector under certain conditions, usually at a low count rate for a Mn Ka peak (originating from an Fe-55 source).

Stability of the energy calibration

For the Rh secondary target spectrometer a drift of the energy calibration is observed. In order to control the situation you can measure a Zr foil repeatedly.

Calibration.

For each element under consideration (in principle for each element found in an unknown sample) an adequate number of very well defined standards, with a range of concentration values bracketing the sample concentration, must be prepared. For calibration, use measurements of either pure standards with 100 % concentration (metals), or simple compounds (of highest possible concentration) pressed as pellets.