You can calculate the MDC for an instrument by considering the background counts during a typical measurement, total detection efficiency, conversion factors, and the probe area.
Variability in the calculated MDC reflects natural variability in the detection efficiency and conversion factors. This variability may or may not be significant. For the MDC to be applicable, the sample or field measurement conditions must match the conditions under which the background was measured.
See, Chapter 6: Field Measurement Methods and Instrumentation on the MARSSIM Manual download page.