Nth Point Off-Peak Acquisition
Another acquisition option method utilized to save time, is the Nth Point background method which simply acquires an off-peak background measurement every “N” points. That is, one performs an off-peak background measurement on the first point of a set of data and then re-uses that off-peak measurement for subsequent data points, instead of explicitly measuring the off-peak background for all points in the data set.
Typically this would be utilized on major and minor elements, with the trace elements continuing to be measured using normal off-peak background measurements.
The downside to this method is that if the composition changes, the average Z of the material will change and according to Kramer's Law, the continuum intensity will change as well. Although the change will be proportional to the degree to which the average Z changes and therefore will be small for a small difference in composition. However if the change in background intensity is greater than the precision of the measurement, it will affect the accuracy of the analysis.
A new option for the Nth Point off-peak background acquisition to handle samples with variable composition is the “smart” monitor element feature. Based on a user specified monitor element the software will automatically re-measure the off-peak backgrounds if the on-peak intensity of the monitor element changes by more than the user specified percentage.
Note that the monitor element should normally be a major element (with a statistical significance that is appropriate for the specified monitor element intensity change percent specified and also an element that finishes its on-peak count integration BEFORE the Nth Point off-peak ground elements in order to function properly.