Author Topic: Time Dependent Intensity (TDI) Corrections  (Read 11800 times)

BenH

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #45 on: May 15, 2019, 09:37:43 am »
Hi John.
I'm not sure you are calculating the uncertainty for data collected with the TDI correction switched on.  Are you taking in to account the uncertainty of each point on the TDI curve and propagating uncertainty through the extrapolation to time=0 seconds?  We ran 30 points on a beam sensitive glass and compared the standard deviation to the uncertainty reported by the software.  With TDI off the standard deviations compare very well to the reported "% ERR".  The absolute uncertainties for these analyses with TDI switched off are shown below (they agree quite nicely):

TDI off                   ZnO      Al2O3       SiO2      
Standard deviation   0.09       0.07       0.16           
Reported "% ERR"   0.09       0.05       0.18           

With the TDI switched on the reported standard deviation values are quite a bit larger than the %ERR as expected if the uncertainty on each TDI point isn't propagated through the extrapolation to time zero:

TDI on                   ZnO      Al2O3       SiO2
Standard deviation   0.46       0.25       0.62
Reported "% ERR"   0.09       0.05       0.18

In this data set we used the log-quadratic fit to the data.  The uncertainty calculated by the software might be misleading if I have this right.


John Donovan

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #46 on: May 15, 2019, 10:47:10 am »
Hi Ben,
Which TDI output are we discussing here?  The Analyze! window/log window output, the Output | Save Time Dependent Intensities menu, or the Output | User Specified output menu, or some other place?
john
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BenH

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #47 on: May 15, 2019, 10:54:34 am »
I used the user specified output and selected relative error.

John Donovan

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #48 on: May 15, 2019, 11:02:05 am »
Hi Ben,
I don't see relative error in that dialog.  Did you mean sample standard deviation and/or sample standard error?
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BenH

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #49 on: May 15, 2019, 11:11:24 am »
"analytical errors in relative percents" in the user specified format output.


John Donovan

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #50 on: May 15, 2019, 12:15:15 pm »
Hi Ben,
Ah, OK.

So as you know, these are the analytical sensitivities calculated using the method of Love/Scott, as seen here from the PFE Reference manual:



These analytical sensitivity errors are basically a sort of peak to background estimation which according to Love/Scott are predictive as to knowing that two numbers are statistically different from each other, as opposed to simply variances in the counting statistics.

In our implementation of this equation we utilize any corrections that have been applied to the raw data that changes the net intensities, in order to be somewhat more accurate in the analytical sensitivity estimates. This includes changes to the net intensities due to the spectral interference, APF, MAN and TDI corrections (and probably a few other corrections).

Now if I pick a (very) beam sensitive sample, specifically the NIST K-375 glass, and output the results *without* the TDI correction I get these results:



Note that the Na and Si average analytical sensitivity errors are 1.550108 and 0.463727 respectively. Now with the TDI correction turned on I get:



Now note that the average analytical sensitivity errors are now 0.724499 and 0.474985 respectively. They are smaller for Na because due to the slope of the TDI corrections, so the net intensity was increased and the analytical sensitivity improved, because the peak to bgd improved. And for Si, the net intensities were instead decreased, because of the different slope for the Si TDI corrections.  That is, due to the TDI corrections, the Na P/B increased (a lot), but the Si P/B decreased (slightly).

So I cannot explain why you are not seeing much of a change in your analytical sensitivities from the TDI corrections, unless your TDI corrections are very small.

You asked about the TDI fit deviation and the answer is that the variance due to the regression scatter in the TDI plot is not applied in this calculation. There are however several different options for outputing the TDI correction magnitude and fit variance as seen here:



I would have to think if there is a way the TDI fit variance could be applied to the analytical sensitivity calculation, but in the meantime you might want to look at the output from the Output | Save Time Dependent Intensities menu. This output option saves a lot of parameters utilized in the TDI corrections to a tab delimited file.  I know several users have leveraged this information in their own TDI studies.

By the way, this variation in the TDI correction from point to point is exactly why the "assigned TDI" (as opposed to the "self TDI") correction was created. Basically Ian Carmichael at UC Berkeley wanted a TDI correction that utilized a constant slope TDI correction for each data point in a sample, so that he could be sure that the variation between data points was due to the composition changing, as opposed to the statistics bouncing around from one data point to the next.

The reason almost nobody uses the "assigned" TDI correction is that due to all kinds of different ion migration physics, even a fairly homogeneous sample can show statistically significant variation from one acquisition to the next.

https://probesoftware.com/smf/index.php?topic=116.msg454#msg454

john
« Last Edit: May 15, 2019, 03:16:52 pm by John Donovan »
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Probeman

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Re: Time Dependent Intensity (TDI) Corrections
« Reply #51 on: May 17, 2019, 01:10:55 pm »
I would sum up the above by noting that the analytical sensitivity calculation by Love/Scott is essentially a description of the P/B ratio for an individual data point, which is affected by the slopes of the TDI corrections for each element, since the TDI correction can change the net intensity, but not the bgd intensity. So the TDI correction changes the analytical sensitivity of each data point and is calculated for each data point separately.

While the standard deviation is a description of the point to point variation for all data points in a sample. These individual data points are affected by the TDI correction of each point, but not in a systematic manner. That is, the TDI correction might amplify the statistical variance in the TDI curves if the variation in the TDI calibration curves are essentially random variation, but the TDI correction might also not change the variance of the data points much at all, if the TDI curves are very consistent from point to point.

Hope that helps.
« Last Edit: May 17, 2019, 03:18:19 pm by Probeman »
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