Too high rutile totals

[Dominik]

I am measuring small rutile inclusions in mostly titanite. I first did with 20 kV, as we are interested in trace elements. The totals were a little high, but almost OK, but we often got mixed analyses from the adjacent titanite. So I switched to 10 kV, less influence by the titanite, but the TiO2 is up between 2-4 wt% (net Ti cps are up 4-6%) compared to the standard, which always gives a fine ~100 wt% TiO2. The rutile is in the µm range, no idea what the problem is. Some effect due to the grain size?
I use 120 nA current (for both 10 and 20 kV), again to get a good signal of trace elements.
I use a JEOL 8530F-Plus and the standard JEOL software.

Thanks, Dominik

[Ben Buse]

Hi Dominik

What standard are you using? What's the count rate for Ti on your spectrometer, is it running into deadtime >60k on either standard or TiO2

[Ben Buse]

If it's worse at 10kV also need to think about are both standard and unknown coated with same thickness of carbon. If a pure metal standard does it have an oxide layer at surface.

[Dominik]

I use a pure rutile as a standard, and then also measure rutile. I am using LIFL and Ti Kb, so I don't have dead time issues. Net counts are around 1650 cps. I thought about coating, but even between grains on the sample, the variation is up to 2-3 wt%. And I use spot mode because of the small grains.