Assign MAN Fits
This menu allows the user to display or modify the MAN (mean atomic number) background assignments and fits used for the background correction of elements in the currently open Probe for EPMA database. The user has the option of using absorption corrected intensities and constant, linear or polynomial fits. An interactive graphical display is provided to facilitate the assignment process.
These MAN assignments are based on the element, x-ray, spectrometer number and crystal (and kilovolts and take-off angle) conditions and are applied globally to all quantitative samples in the run. If an element is analyzed on more than one set of x-ray, spectrometer number or crystal conditions, then the user is required to assign MAN fits for each additional set of conditions.
Be sure to click the Update Fit button if any changes are made to the MAN assignments. This must be done after the changes to the MAN assignments to each element are made.
MAN background corrections can be utilized when the P/B is sufficiently high (accuracy limit is ~100-200 PPM in most oxides and silicates) unless the “blank” correction is also applied.
The MAN Fits menu allows the user to enter which MAN (mean atomic number) background samples will be used to calculate the background fit for the polynomial MAN iteration of an analyzed sample. The MAN background correction scheme is valid, because the primary factor of background or continuum variation (aside from trace contamination and peak over-lap interferences) is the average atomic number of the sample. If a number of pure samples of known atomic number are measured, then a function can be fit to the data and applied to the unknown sample. However, since the atomic number depends on the composition of the unknown and that is not known, then the MAN background must be applied during the ZAF iteration calculation.
Again, remember that 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. When measuring trace quantities be sure to use the off-peak measurements although the MAN correction can be utilized if a suitable matrix matched “blank: standard is available.
Selecting Suitable Standards for the MAN Calibration
Correction for Absorption of the Continuum
Graphical Plot of Background vs. Z-bar