Date: Wed, 28 Sep 94 09:00:43 cst
Subject: Re: Quant EDS and Window Contamination
From: christoffersen@snmail.jsc.nasa.gov

Given the recent discussion on EDS window contamination and cleaning I
thought I might raise a point that is probably obvious but is nevertheless
worth throwing open for discussion. The point is that for those of us using the
Cliff-Lorimer ratio technique for quantifying EDS analyses in the TEM, window
contamination would seem to be a worrysome problem. Quite obviously if you
calibrate the K-factors for your microscope/detector system with a clean and
dry detector these same K-factors will not obtain as the detector ices up or
contaminates because the ice does not absorb all energies equally. The thing
that bothers me is that icing/contamination is progressive and on any given day
you don't know if it has progressed far enough to invalidate your present set
of K-factors. Plus, even if you de-ice the detector like we can do on our Link
system, how do you know the K-factors have returned to their previous values,
especially if the initial K-factor measurements weren't necessarily made with a
dry/clean detector.
I suppose the answer is to run a standard with light and heavy elements
every day you analyze, but what a pain! So my questions are: 1) How much should
I worry about this problem? 2) Do others of you worry about the problem? 3) How
do I handle the problem?
Please validate my paranoia. Thanks.

Roy Christoffersen
NASA Johnson Space Center
Houston, TX
christoffersen@snmail.jsc.nasa.gov