Single/multi hit quantification
London, Andy
Hi all,
Somewhat random, but I think it should come into our discussion of file formats and standards.
I was reading [https://doi.org/10.1016/j.ultramic.2020.113018] which reports the following:
“Specifically, we found using the unsampled single-hit corrected TOF spectrum exported by IVAS (CRW [spectrum exported from the Cal/Recon Wizard]) provided the most accurate analysis results. Generating a single-hit corrected TOF spectrum from the *.ePOS file did not, due, at least in part, to a difference in how the multi-hit events are defined in the file. Currently, this dramatically limits the application of our approach to single phase samples or easily parsed data sets, since the 3-D reconstructed data (*.ePOS data) cannot be used to perform the most accurate analyses.”
Obviously most users can’t just ignore the multiple hits (imagine a system where most of one component arrives as part of a multiple hit, C in Fe for example), but if there is a difference in the “raw” and reconstructed data causing this much difference, then in the standard we need to be very clear about what the file actually contains.
And if all of our down-stream processing is on the EPOS file, then users need to be aware of potential limitations/differences.
Maybe David can comment on the differences between how single and multiple hits are defined differently between the Recon wizard and what is exported in the EPOS file.
Andy
David Reinhard
Hello everyone,
The origin of the difference between single and multiple hits in the recon wizard vs the EPOS file lies in the fact that the events aren’t classified in the EPOS file, rather their classification (single or multi) is inferred by reading the number of pulses since the last hit for each event.
Part of the underlying hit-finding process involves cutting artifacts. One such artifact is involves a real hit being detected, and an additional hit being generated between the MCP plates (likely due to a gas ion being liberated and detected in the second plate) that is detected at a characteristic time after, and distance from, the first hit. This event is classified as a multi-hit event, but the second ion in that event is not a real hit from the sample. The hit-finding algorithm cuts out this second ion, so it appears in neither the reconstruction wizard nor the EPOS file. However, in the reconstruction wizard, the first ion from this event is still classified as a multi-hit ion because it was from a multi-hit event on the detector. The EPOS file does not know about the hit classification, it only knows that the first ion was the only ion reconstructed for a given pulse (because the second ion was cut by the prompt-ion feedback filter) so any EPOS processing tool can’t distinguish this type of ion from a normal single hit event ion.
David Reinhard
APT Product Manager | CAMECA Instruments, Inc
5470 Nobel Drive | Madison, WI 53711 | USA
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London, Andy
So potentially in the reference given below, a lot of actual ions are getting caught by the prompt ion filter? That would explain why the CRW and EPOS data are so different. And I wouldn’t expect different materials to produce different amounts of secondaries (although I’m no expert on that).
Potentially they could have changed the voltage to push whatever multiples were arriving during the ion filter to a different delta-TOF. Personally, I would like the option to turn off the secondary ion filter, maybe then there’s option of users to apply their own filtering.
Andy
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