Negative cps Ratio Values in Imaging Results

Nana

Hello everyone.

Thank you very much for your kind response to my previous question. Your advice was extremely helpful.

I would like to ask a new question. I am currently performing imaging again, and I noticed something unexpected. In the Result window after running the DRS, the cps values for each element are positive; however, the cps ratio values sometimes become negative. Could you please explain why this happens?
I have tried reselecting the baseline several times, making sure not to include the irradiated part of the sample but only the true baseline. Nevertheless, when I check the imaging results (where the color scale bar shows positive values) and use an ROI, the values still turn out negative.
I have consulted the manual and searched this forum, but I could not find a clear explanation. I am sorry if this is a very basic question, but I would greatly appreciate your guidance.
Thank you very much for your time and support.

Bence

Hi @Nana
I think the issue here is that you're looking at the Results view, which shows the averages of selections. If the average is positive, you won't see the effect of few negative values. But if you divide by a very small negative number, you get a very large negative number, and this might be enough to move the selection average enough that you'll see it in the Results View.

My recommendation is to view your data in the Time Series View, and make sure that all data points are above the baselines. If they're not, you might adjust your selections, or accept that negative values represent values are below/indistinguishable from baselines.

Kind regards,
Bence

Nana

Bence

Dear Bence,
Thank you very much for your reply.

When I checked the CPS values in the Time Series View, both were positive.I am having trouble understanding how the ratio can become negative when taking a ratio of two positive values.
Does this mean that the CPS ratio is being calculated point-by-point (per integration time)?

My understanding is that the Intermediate CPS ratio is calculated as follows:
(average raw counts of the selection for an element – average raw counts of the baseline) / (average raw counts of the selection for the IS element – average raw counts of the baseline)
Is that correct?

I don’t fully understand the calculation method in this part. If possible, could you please let me know where this is documented?

Best regards,Nana

Bence

Hi Nana,

Every part of the data reduction process works on a time-point by time-point basis. This includes baseline subtraction, normalisation to an int std, and calibration. It is not documented by itself, but is discussed at the start of the 3D Trace Elements Webinar:

https://iolite.xyz/support/webinars/webinar_recording_3dte.php

If you would like to send your experiment to the iolite support email, we can take a look and show you were the negative values are coming from, but it's very likely to be in the time resolved values (rather than the selection averages).

Kind regards,
Bence