Hi John,
Yes I am asking for analytical strategies for measuring trace elements in sulfides with a JEOL JXA-8530F microprobe. The configuration now used counting time on the peak 300 seconds for Au, 180 seconds for As, Se, Mg, Al, and 60 seconds for the rest of the elements. Background count times are 30 seconds at high and low positions for all elements. Are these conditions common for sulfides?
I also want to ask how to use the multiple backgrounds. Is there an automatic procedure to extract the backgrounds and to use the data, without having to the corrections afterwards?
Nicole
Hi Nicole,
It is difficult to be specific without more information on your spectrometer/crystal configuration because the choice of emission lines will affect the sensitivities, and this depends on which crystals are available.
But in general one should increase counting time and/or beam current (and beam energy) to improve sensitivity. Accuracy at the trace level is also important, hence the need for a "blank" sample. That is a sulfide with zero or known trace elements from another technique. I have a very pure natural iron sulfide (pyrite) that I have had ICP-MS performed on, so I can measure that as an unknown along with my other unknowns (using the same conditions) to check trace accuracy. The question is: can one accurately measure zero? See here for more info:
http://probesoftware.com/smf/index.php?topic=928.msg5958#msg5958A paper I wrote on this topic looks at SiO2 but the strategies are applicable to any material:
http://epmalab.uoregon.edu/pdfs/3631Donovan.pdfBack to improving sensitivity, you should realize that at zero concentration the background measurement is statistically as important as the on-peak measurement. That is, when measuring concentrations close to zero (say, below 500 PPM), we generally count as long off peak as we do on-peak. So for example, 300 seconds on-peak, 150 second on the high off-peak and 150 seconds on the low off-peak. This is because the variances add in quadrature when calculating the net intensities. As the expected concentrations increase one can decrease the amount of time integrating the off-peak intensities.
You ask about "multiple backgrounds" but it sounds to me that you are using the JEOL software, correct? I think that only Probe Software's Probe for EPMA software can acquire and regress multiple off-peak backgrounds as described here:
http://probesoftware.com/smf/index.php?topic=701.msg4283#msg4283Of course one could perform these multiple off-peak background corrections manually (as many labs used to do, until they obtained Probe for EPMA for their JEOL instruments), but it's painful and slow.
I suggest you start choosing emission lines that provide the best intensity (for sensitivity), then increase the on and off-peak counting times, and increase the beam energy to increase to interaction volume and run at least 100 or 200 or more nA of beam current and run some test measurements on some "blank" sulfides and see what sort of detection limits you can obtain.
Maybe someone reading this has a JEOL probe run measuring traces in sulfides that they can share with us... I have a Cameca instrument which is a different animal when looking at trace elements, e.g., the Cameca is better at low energy element sensitivities, while the JEOL is better at high energy emission lines due to its Xe detectors. Of course this may all change in the next few years as the instrument vendors utilize SDD detectors in place of WDS gas detectors.
john
PS Do not forget about spectral interferences (another nice thing is that Probe for EPMA corrects these easily):
http://probesoftware.com/smf/index.php?topic=803.0And to add to your woes(!), be careful measuring trace elements near phases with high concentrations of those elements, due to secondary fluorescence from boundary phases:
http://probesoftware.com/smf/index.php?topic=58.0By the way, if you input standard and unknown count rates into this dialog in CalcZAF (Run | Model Detection Limits menu), you can obtain estimates of your sensitivity as shown here:
http://probesoftware.com/smf/index.php?topic=121.0john
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