Alternating On/Off intensities

[BenjaminWade]

Hi John and all
I have recently been doing some Ti in quartz, and am using the alternating intensities option (used an unknown count time factor of eight). I am going through the plots and as expected am seeing the "On-(H+L)/2" decreasing over time. As such I started playing around with linear/quad/logarithimic fits to see how it might affect the data which is where I have run into a problem, which is more than likely me not understanding something.

Correct me if I am wrong, but for analyses that have used the "alternating intensities option", the software is using the y-intercept on the plots to calculate the P-B cps/nA value for a specific analysis?

Firstly I noticed that the cps/na value in the analyse window for a specific analysis does not match the y-intercept value for the same analysis when plotted on the graph? Its close but doesn't match. I have attached a capture displaying sp1 and you will see the y intercept is ~0.591, but in the second image you will see for sp1 it is 0.61?

Also if I change the fit and close the window, it doesn't seem to "save" the change. When I go back into the graphing window its back to linear fit.

Is there something I am doing wrong...?

Cheers

[John Donovan]

Correct me if I am wrong, but for analyses that have used the "alternating intensities option", the software is using the y-intercept on the plots to calculate the P-B cps/nA value for a specific analysis?

Hi Ben,
No, the software only displays the data for the alternating on-off-peak measurements. Only the TDI acquired intensities are utilized to correct for change in intensity over time during the quantification.  The alternating on/off intensity display is just to document changes for both the on and off-peak counts over time.

The reason for the alternating on/off peak acquisition is that it compensates for changes between the on and off-peak intensities over time by tracking the delta between on and off peak measurements.  It is generally only utilized for trace elements where sample damage can occur.  The idea being that the sample damage is spread over *both* the on and off-peak measurements.

In other words, the extrapolation of the on-peak intensities to zero time during the quantification only occurs with the TDI acquired intensities. Yes, extrapolation of the alternating on/off intensities to zero time could be added to the quantification, but it would take a bit of work...

Also if I change the fit and close the window, it doesn't seem to "save" the change. When I go back into the graphing window its back to linear fit.

Is there something I am doing wrong...?

Nope, you are fine. I wasn't saving one of the dialog variables quite right.  This is fixed now and will be available for download tonight.

Thanks!
john

[BenjaminWade]

Hi John
Thanks for the quick reply, my mistake. For some reason I thought it was going to work like the TDI but sounds like that isn't implemented. Let me go add it to the wishlist now...

Cheers

[John Donovan]

Hi Ben,
You can download the PFE update (v. 11.6.6) now.
john

[Julien]

Hi John,

I was wondering how exactly is the Alternating ON-and-OFF acquisition implemented in your software. I had some (more or less) successful attempt today at measuring trace amounts in glass. The individual regression on either the peak or each individual background yield very nice log-quadratic regression. Yet, the P-B results do not show such a nice correlation. I am hoping that what the software is actually doing is actually regressing individually the peak and the backgrounds, and then only determining the "true" and "before damage" measurement of the regressed peak measurement minus the regressed background interpolation obtained from the interpolation of the two individual background regression. Is this the case?

It is my opinion that this would be the way to go. If you consider determining the P-B for each individual measurement, you add a certain layer of uncertainty as each alternating background measurement is acquired after the peak...

Attached is a image showing a quite dramatic example.



Thanks in advance for your reply!

Julien

[John Donovan]

Hi Julien,
I merged your topic with an existing topic on the alternating on/off background method. The answer to your regression question is in the posts above.

That said, I agree that it is a little misleading to show the regression of the alternating on and off-peak (and P-B) measurements, since the regression is not actually utilized in the background correction. It's just displayed as a aid for visualization.

But, I think your alternating on/off measurements look to be doing exactly what they should be doing.  That is tracking the relative changes in the on and off-peak intensities over time. And as shown in your on and high and low intensity plots, we see significant drift in the x-ray counts as the glass is damaged.   This is a very appropriate application for this acquisition method.

The beauty of the alternating on/off method is that it tracks these intensity drifts through the entire acquisition, therefore characterizing the thing we actually care about, which is the delta between the on and the off-peak intensities (i.e., the net intensity). Look at the P-B plot. The fact that the P-B plot shows more scatter is exactly what one would expect when subtracting two noisy signals from each other.  That is, the noise increases in quadrature.  In fact what is particularly wonderful about your particular P-B plot example is that it shows that in spite of the large changes over time in the raw data, the P-B values are relatively constant, albeit a little noisy. So I think this is working exactly as intended.

Now, as a reminder, these P-B values are recorded (for documentation purposes) at acquisition time, but the sum of all the sub intervals for the on and high and low off-peak intensities are also summed and stored at acquisition time for subsequent quantification. The basic idea is that one is spreading the beam damage effects on intensity over both the on and off-peak measurements as the sample is damaged, thus producing a more accurate trace element measurement.

You are a coder, so if you are interested in the gory details, I can send you the acquisition code module (it is over 30K characters so I can't post it here).