Probe current reduction, but still good totals

[dawncruth]

Hey all,
I've been running some standard pyroxene analyses. Our FEG is on its way out and the probe current has been unstable.  In my latest analytical run, Probe for Windows is sometimes recording currents that are significantly less than the set current (e.g. 7 nA for a 20 nA run). Thing is: the totals are ok and the counts are reasonable.  Any thoughts about this?

Cheers,
Dawn

[Probeman]

I've been running some standard pyroxene analyses. Our FEG is on its way out and the probe current has been unstable.  In my latest analytical run, Probe for Windows is sometimes recording currents that are significantly less than the set current (e.g. 7 nA for a 20 nA run). Thing is: the totals are ok and the counts are reasonable.  Any thoughts about this?

Hi Dawn,
This makes perfect sense. 

Since Probe for EPMA measures the beam current for each data point and performs a beam current normalization on the unknown x-ray intensities with that measured beam current, as long as the beam drift isn't too excessive within the duration of a single data point acquisition, the software will generally be able to correct for any beam current drift.  Note this correction is applied to all elements equally...  also note that if you are using "combined condition" samples (those where different elements have different beam conditions), the beam current is measured separately for each beam condition, so using an even finer scale beam drift correction.

Most software packages (to my knowledge) already something similar to this.  What PFE does that is unique is that it also performs a *standard* intensity drift correction on an element by element basis, so long as the standard is acquired both before *and* after the unknown acquisition. In fact Probe for EPMA can handle up to 40 standardizations per probe run, which allows the software to correct for x-ray intensity drift on an individual spectrometer and also individual standard "basis", for each element separately.

One of my favorite stories is a user calling me one morning saying their totals from an overnight run were around 103%. So I suggested rerunning the standards to take advantage of the aforementioned standard intensity drift correction, and about 30 minutes later they called back and said "thank-you so much, our totals are now all around 100% now"

This standard intensity drift correction is very helpful in labs where the HVAC isn't quite up to par and the resulting temperature variation causes each spectrometer to drift in an entirely different manner from every other spectrometer, mostly depending on the crystal type...
john

[dawncruth]

Hi John,
Neato!  I've been running the MAN correction.  Should I run all standards after for the *standard* intensity drift correction?

Dawn

[Probeman]

Neato!  I've been running the MAN correction.  Should I run all standards after for the *standard* intensity drift correction?

Hi Dawn,
It certainly won't hurt and could help accuracy if any of your spectrometer crystals have drifted.  Basically we always run standards before and after our unknowns and sometime on long runs, every "n" hours using the re-standardization feature in the Automate! window in Probe for EPMA.  See attachment below (one must be logged in to see attachments)
john