The following study describes the experimental results obtained by energy dispersive x-ray analysis on a series of representative samples, concentrated by precipitation, to spots 1.5 mm in diameter on Millipore (R) membranes.
Using modified detector collimation, the contributions of air scatter and membrane (Substrate) scatter to the background under a peak, can be minimized to achieve limits of detection approaching a nanogram of the element present in the precipitate.
The equipment used here is composed of an 80 mm2 Si(Li) detector connected to a computer based data handling unit. Excitation was achieved with a Rhodium target x-ray tube having a maximum output of 50 kV at 5.0 ma. Even with the high detection efficiency of the Si(Li) detector and the capability of minimizing the source to sample and sample to detector pauh lengths, it was found with these samples that at least 3.0 ma of tube current is needed in the excitation source.
A mechanism has been developed for this work that allows reproducible sample positioning for standards and unknowns. This mechanism is used to produce element profiles on larger precipitated standards to measure radial uniformity.