MAN (mean atomic number) background corrections are utilized for major and minor elements when the P/B is sufficiently high (accuracy limit is ~100-200 PPM in oxide and silicates) and can also be used for trace element analysis if a suitable “blank” standard is available.
The user can specify the standards used to calibrate the background fit for the polynomial MAN iteration of an analyzed sample. The MAN background correction method works because the primary cause of background (aside from trace contamination and peak over-lap interferences) is the average atomic number of the sample. Therefore a function can be fit to appropriate standard data and applied to the analysis of unknown samples. Since the atomic number depends on the composition and that is unknown to begin with, the MAN background is not applied until the analytical calculation iteration. Remember :
- MAN backgrounds can be used whenever the ratio of x-ray peak to background is high (>10:1), and this is usually the case with WDS microanalysis
- Trace element concentrations generally require the use of off-peak measurements but MAN background corrections can be utilized if a suitable matrix matched “blank” standard is available (e.g., most pure elements, oxides, many simple sulfides and silicates.
There are a number of options to specify for the multi-point background acquisition. First one must define the number of positions to acquire on each side of the peak (maximum = MAXMULTI%) and the spectrometer position of each background.
Note that the adjustment of multi-point backgrounds is usually performed graphically using the wavescan plot Model Background dialog.
The number of points to iterate to and the background fit type (Linear or Polynomial) can be specified before or after the background intensity data has been acquired. Note that the sum of all multi-point intensities is automatically calculated and saved to the normal off-peak intensity arrays to so that normal off-peak background calculations can also be performed.