Proper PHA tuning for high accuracy quantitative analysis

[Probeman]

Starting a new topic to focus on the correct method to tune one's PHA settings because proper tuning of PHA is a "black box" to most analysts!  Including myself until quite recently! 

This link is to the "Limits of EPMA Accuracy" which explores this topic in broader detail ("peak shift" matching, dead time calibration, and better standards):

https://smf.probesoftware.com/index.php?topic=1831.0

Let's start by analyzing Ti Ka in synthetic SrTiO3 using TiO2 as a primary standard (without "count rate" matching our primary standard to our secondary standard or unknown!).

Running a PHA scan using the default PHA settings we obtain this scan on the TiO2 standard (remember, we always want to adjust our PHA settings on a material with the highest concentration (generally the primary standard) at the highest beam current we expect to utilize (that is, at the highest count rate we expect to see, that is, tuning our PHA to avoid pulse height depression effects which can cause non-linear responses)



Note first that we are in "integral" PHA mode. In integral PHA all photons to the right are still counted, even those that plot (invisibly!) to the right of the maximum x axis displayed!

Next we clearly see that the baseline level is too high and is intersecting the PHA peak. Why is that a problem?  Because if the peak shifts to the right or left, the baseline level cuts off more or less of the PHA peak and we obtain a non-linear response in our counting electronics!

However, we can make two adjustments to avoid this: first by lowering our baseline level below the PHA peak, and second, increasing our gain (for Cameca instruments) or our bias (for JEOL instruments) we can shift the PHA peak to the right *completely above* the baseline level, thus ensuring a linear response in our counting electrics. 

Here is the PHA scan after adjusting our baseline level and our gain (or bias):



Better, but the baseline level still intersects the PHA peak tail a bit on the left.  After raising our PHA gain still further, we obtain this PHA scan:



This is what we will utilize for our quantitative analyses in the next post.  The same PHA tuning procedure was utilized for Fe Ka as seen here:

[Probeman]

OK, so how does this rather unintuitive PHA tuning method, shown in the previous post actually perform?

Let's start with Ti Ka in SrTiO3 using TiO2 as the primary standard (showing three different matrix correction models) The raw k-ratio for this pairing is around 0.43 so over a 100% difference in count rates:



All three models perform quite well and are all within ~1% relative accuracy, though the DAM BSE correction does somewhat better.

Now let's look at Fe Ka in SRM K-411 glass using Fe3O4 as a primary standard:



Again, all three models do very well using the integral baseline PHA tuning method, though again, the DAM BSE models is somewhat more accurate. This pair has a raw k-ratio of 0.1405, so an even more extreme extrapolation than the TiO2 example above.

Do you care about EPMA accuracy? Why don't you try this PHA tuning method and do some intensity extrapolations from your primary standard to a secondary standard and see what you learn... send me a private message (or email) if you are too shy to share your results.  I'm here to help!   

[sem-geologist]

Starting a new topic to focus on the correct method to tune one's PHA settings because proper tuning of PHA is a "black box" to most analysts!  Including myself until quite recently! 

I think one of primary things which should be very much bolded, underlined, SHOUTED, SCREAMED, printed in large font and send all over the earth with paper mails... and I don't know what else... is this:

DO NOT RELY ON OEM AUTO PHA FUNCTION - THOSE ARE MADE BASED ON HISTORICALLY WRONG ASSUMPTIONS AND IT LEADS TO VERY HUGE ERROR!

We could ask EMAS, AMAS, MAS and other EPMA-related societies to update their mouse pads, wall-cheat-sheets/posters with this disclamer – maybe it would start changing something...

[Probeman]

Starting a new topic to focus on the correct method to tune one's PHA settings because proper tuning of PHA is a "black box" to most analysts!  Including myself until quite recently! 

I think one of primary things which should be very much bolded, underlined, SHOUTED, SCREAMED, printed in large font and send all over the earth with paper mails... and I don't know what else... is this:

DO NOT RELY ON OEM AUTO PHA FUNCTION - THOSE ARE MADE BASED ON HISTORICALLY WRONG ASSUMPTIONS AND IT LEADS TO VERY HUGE ERROR!

We could ask EMAS, AMAS, MAS and other EPMA-related societies to update their mouse pads, wall-cheat-sheets/posters with this disclamer – maybe it would start changing something...

I could not agree more, though I think only the Cameca has such an "auto PHA" button. The JEOL might have it but apparently it's only accessible by the service engineer for running tests.

But regardless, the auto OEM or traditional manual PHA tuning methods basically *force* the user into utilizing "matrix matched" or as I call more properly: "count rate matched" natural standards (with all their documented heterogeneity and inclusions). Because if there is a difference in the count rate between the standard and the unknown, the traditional PHA tuning will cause a non-linear response, due to the baseline/window filtering.

Yup, at this point I think we mostly have a sociological, or at least an educational endeavor (or for some stubborn souls, perhaps a psychological hurdle) to overcome traditional PHA tuning methods.  Once people overcome their initial reluctance they will find that they can use high purity synthetic end member mineral standards and still achieve ~1% relative accuracy on their unknown samples.

By the way, if anyone with a JEOL instrument is interested in running PHA tests similar to those in first post above:

https://smf.probesoftware.com/index.php?topic=1854.0

I would be very interested in seeing a series screen captures of PHA adjustments where they attempt the integral-baseline PHA tuning method described above.

Then see what quantitative results they get when extrapolating Fe Ka from say Fe3O4 to the NIST glasses or Ti Ka extrapolating from TiO2 to SrTiO3...

[Probeman]

By the way, if anyone with a JEOL instrument is interested in running PHA tests similar to those in first post above:

https://smf.probesoftware.com/index.php?topic=1854.0

I would be very interested in seeing a series screen captures of PHA adjustments where they attempt the integral-baseline PHA tuning method described above.

Then see what quantitative results they get when extrapolating Fe Ka from say Fe3O4 to the NIST glasses or Ti Ka extrapolating from TiO2 to SrTiO3...

Not to put too fine a point on this integral-baseline PHA tuning method, but this next example is pretty graphic. Here is Fe Ka measured at multiple keVs in NIST SRM K-412 mineral glass using Fe3O4 as a primary standard:



This measurement yields a raw k-ratio of ~0.97 at 20 at keV, which means that this is a count rate extrapolation of 10x from the primary standard to the secondary standard!

St  160 Set  12 NBS K-412 mineral glass, Results in Elemental Weight Percents
 
ELEM:       Ti      Fe      Sr      Rb      Mn      Cr      Si      Mg      Ca      Al       O
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    60.00   60.00     ---     ---     ---     ---     ---     ---     ---     ---     ---
BEAM:    19.94   19.94     ---     ---     ---     ---     ---     ---     ---     ---     ---

ELEM:       Ti      Fe      Sr      Rb      Mn      Cr      Si      Mg      Ca      Al       O   SUM  
  1169    .003   7.756    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.094
  1170    .000   7.732    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.067
  1171    .002   7.713    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.050
  1172    .002   7.727    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.064
  1173   -.002   7.761    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.094
  1174   -.001   7.750    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.085

AVER:     .000   7.740    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.076
SDEV:     .002    .019    .000    .000    .000    .000    .000    .000    .000    .000    .000    .018
SERR:     .001    .008    .000    .000    .000    .000    .000    .000    .000    .000    .000
%RSD:   402.07     .24     .00     .00     .00     .00     .00     .00     .00     .00     .00

PUBL:     n.a.   7.742    n.a.    n.a.    .077    n.a.  21.199  11.657  10.899   4.906  43.597 100.077
%VAR:      ---    -.03     ---     ---     ---     ---     ---     ---     ---     ---     ---
DIFF:      ---   -.002     ---     ---     ---     ---     ---     ---     ---     ---     ---
STDS:       22     395     ---     ---     ---     ---     ---     ---     ---     ---     ---

STKF:    .5611   .6860     ---     ---     ---     ---     ---     ---     ---     ---     ---
STCT:  1833.94 1035.67     ---     ---     ---     ---     ---     ---     ---     ---     ---

UNKF:    .0000   .0666     ---     ---     ---     ---     ---     ---     ---     ---     ---
UNCT:      .01  100.60     ---     ---     ---     ---     ---     ---     ---     ---     ---
UNBG:     2.64    1.38     ---     ---     ---     ---     ---     ---     ---     ---     ---

ZCOR:   1.1885  1.1615     ---     ---     ---     ---     ---     ---     ---     ---     ---
KRAW:    .0000   .0971     ---     ---     ---     ---     ---     ---     ---     ---     ---
PKBG:     1.00   73.96     ---     ---     ---     ---     ---     ---     ---     ---     ---

And here even at 10 keV we are well within 1% relative accuracy:

St  160 Set   1 NBS K-412 mineral glass, Results in Elemental Weight Percents
 
ELEM:       Ti      Fe      Sr      Rb      Mn      Cr      Si      Mg      Ca      Al       O
TYPE:     ANAL    ANAL    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC    SPEC
BGDS:      LIN     LIN
TIME:    60.00   60.00     ---     ---     ---     ---     ---     ---     ---     ---     ---
BEAM:    19.99   19.99     ---     ---     ---     ---     ---     ---     ---     ---     ---

ELEM:       Ti      Fe      Sr      Rb      Mn      Cr      Si      Mg      Ca      Al       O   SUM  
   707    .008   7.781    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.124
   708   -.015   7.945    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.265
   709    .015   7.659    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.009
   710    .009   7.778    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.122
   711   -.011   7.827    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.152
   712    .011   7.800    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.147

AVER:     .003   7.798    .000    .000    .077    .000  21.199  11.657  10.899   4.906  43.597 100.136
SDEV:     .013    .092    .000    .000    .000    .000    .000    .000    .000    .000    .000    .082
SERR:     .005    .038    .000    .000    .000    .000    .000    .000    .000    .000    .000
%RSD:   438.46    1.18     .00     .00     .00     .00     .00     .00     .00     .00     .00

PUBL:     n.a.   7.742    n.a.    n.a.    .077    n.a.  21.199  11.657  10.899   4.906  43.597 100.077
%VAR:      ---     .73     ---     ---     ---     ---     ---     ---     ---     ---     ---
DIFF:      ---    .056     ---     ---     ---     ---     ---     ---     ---     ---     ---
STDS:       22     395     ---     ---     ---     ---     ---     ---     ---     ---     ---

STKF:    .5446   .6648     ---     ---     ---     ---     ---     ---     ---     ---     ---
STCT:   317.84   88.25     ---     ---     ---     ---     ---     ---     ---     ---     ---

UNKF:    .0000   .0635     ---     ---     ---     ---     ---     ---     ---     ---     ---
UNCT:      .01    8.42     ---     ---     ---     ---     ---     ---     ---     ---     ---
UNBG:      .96     .34     ---     ---     ---     ---     ---     ---     ---     ---     ---

ZCOR:   1.1948  1.2290     ---     ---     ---     ---     ---     ---     ---     ---     ---
KRAW:    .0000   .0954     ---     ---     ---     ---     ---     ---     ---     ---     ---
PKBG:     1.02   25.77     ---     ---     ---     ---     ---     ---     ---     ---     ---

I encourage you to try this new PHA tuning method and convince yourself.  See attached pdf.

[dawncruth]

What a timely post!

I have a user VERY interested in high precision F and Cl in amphiboles and I am diving headfirst into the PHA settings, a topic which I still haven't had a good explanation of despite running a lab (looks around sheepishly). I think some of the confusion stems from the way PHA is discussed in Cameca vs JEOL software discussions...

I have two questions:
1. Can't we correct for PHA related interferences with interference corrections rather than changing the PHA settings? Not opposed to changing, just wondering
2. What are folks' favorite settings for F in amphibole?

[Probeman]

What a timely post!

I have a user VERY interested in high precision F and Cl in amphiboles and I am diving headfirst into the PHA settings, a topic which I still haven't had a good explanation of despite running a lab (looks around sheepishly). I think some of the confusion stems from the way PHA is discussed in Cameca vs JEOL software discussions...

Yes, it's confusing because JEOL only has 2x gain steps, so to fine tune one's PHA, one has to use the bias voltage on JEOL instruments. Whereas Cameca instruments set the bias to the optimum level and then adjust the (fine) gain as needed.

I have two questions:
1. Can't we correct for PHA related interferences with interference corrections rather than changing the PHA settings? Not opposed to changing, just wondering

Absolutely yes!  As I mentioned in this post:

https://smf.probesoftware.com/index.php?topic=1831.msg13974#msg13974

Yes, differential mode can help with some higher Bragg order interferences, but it doesn't help at all with *same* Bragg order interferences, and only partially with higher Bragg order interferences. In fact there are only a few rare spectral interference situations I can think of where differential mode might help, such as Na Ka 2nd Bragg order interfering when measuring trace oxygen, because it's difficult to find a standard for the interference correction that contains sodium but no oxygen.

Otherwise it makes much more sense to tune your PHAs to obtain a linear response in count rate over a large range of count rate, and then correct for any spectral interferences using the quantitative interference correction in software:

Donovan, John J., Donald A. Snyder, and Mark L. Rivers. "An improved interference correction for trace element analysis." Proceedings of the Annual Meeting-Electron Microscopy Society of America. San Francisco Press, 1992.

2. What are folks' favorite settings for F in amphibole?

There is the debate between using TAP which has low sensitivity, and using PC0 (45A 2d) which has excellent sensitivity, but lower spectral resolution. I assume JEOL has a 45A 2d LDE?

[KerstinGruender]

What a timely post!

I have a user VERY interested in high precision F and Cl in amphiboles and I am diving headfirst into the PHA settings, a topic which I still haven't had a good explanation of despite running a lab (looks around sheepishly). I think some of the confusion stems from the way PHA is discussed in Cameca vs JEOL software discussions...

I couldn't agree more, maybe we need a good discussion around PHA to make us all understand this a bit better. You're not alone

2. What are folks' favorite settings for F in amphibole?

I haven't measured F in amphibole, but for Fe-bearing volcanic glasses (<1000 ppm F) we use the method of Zhang (2016) https://doi.org/10.1111/j.1751-908X.2015.00390.x, with LDE1 crystal & their F-free glasses. Should be the same for amphiboles. The previous lab manager set this up and fine-tuned PHA etc. successfully on our JEOL system. I have not been able to 'translate' the method into PfE yet, I suspect partially because not fully understanding PHA. It's also not been requested often, so using the 'old' JEOL system if users do want fluorine I did try TAP crystals at some point but even if using two simultaneously it does not seem to capture F. This remains on my to-do-list and I wonder whether it is worth a separate post to gather and compare methods for fluorine? I don't think there is an existing discussion, as I've searched before and not found anything 'concrete'? In theory, I think it would be a mix of utilising interference/blank standards and treating it as a trace element...?

[Probeman]

What a timely post!

I have a user VERY interested in high precision F and Cl in amphiboles and I am diving headfirst into the PHA settings, a topic which I still haven't had a good explanation of despite running a lab (looks around sheepishly). I think some of the confusion stems from the way PHA is discussed in Cameca vs JEOL software discussions...

I couldn't agree more, maybe we need a good discussion around PHA to make us all understand this a bit better. You're not alone

I've been running a lab for ~40 years and it's only been in the last few years that I think I've finally figured this out!   

The good news is that now anyone can properly tune their PHA with a few steps, but before I list them let's consider PHA pulse height depression effects, where the PHA shifts to the left with increasing count rates as seen here:

https://smf.probesoftware.com/index.php?topic=1831.msg13978#msg13978

Now imagine that one has tuned their Fe ka PHA on a unknown mineral with under 10 wt% Fe and gets it nice and centered in the PHA voltage range.  When they try to use a primary standard such as magnetite, which has a roughly 10 times the count rate, the peak on the standard shifts to the left, getting vut off by the baseline level, thus yielding a non-linear response from the system. And also obtaining a very inaccurate result.

This in short, is why people have felt compelled to use so called "matrix matched" or what I call "count rate" matched standards for so many years. Thus they rely on problematic natural standards with all their natural variability.  But if they instead:

1. Set their PHA to *integral* mode. This way, all photons to the right of the baseline level are counted. Even those photons that no longer plot in the normal PHA range to the right.

2. Tune their PHA on a primary standard with a high concentration of the element, at the highest beam current they expect to use for that element as seen here:



Then increasing their gain (Cameca) or bias (JEOL) such that the PHA peak on the primary standard is *completely* above the baseline level. This way, all photons will still be counted in integral mode. And they obtain a PHA peak that will provide a linear response at all lower count rates (unknowns, etc.)

3. Of course since your standard and unknown will have different count rates, we also want to be sure that our dead time calibrations are accurate, and that is why I suggest using the constant k-ratio method here:

https://smf.probesoftware.com/index.php?topic=1466.msg11173#msg11173

along with the logarithmic dead time expression which works up to count rates of ~300 kcps to 400 kcps which are not uncommon on modern large area crystals (the traditional dead time correction fails at count rates above ~30 to 40 kcps).

4. And finally, use high purity synthetic end member standard materials as demonstrated in this topic and also the topic "The Limits of EPMA Accuracy":

https://smf.probesoftware.com/index.php?topic=1831.0

Try it., you'll like it!

2. What are folks' favorite settings for F in amphibole?

I haven't measured F in amphibole, but for Fe-bearing volcanic glasses (<1000 ppm F) we use the method of Zhang (2016) https://doi.org/10.1111/j.1751-908X.2015.00390.x, with LDE1 crystal & their F-free glasses. Should be the same for amphiboles. The previous lab manager set this up and fine-tuned PHA etc. successfully on our JEOL system. I have not been able to 'translate' the method into PfE yet, I suspect partially because not fully understanding PHA. It's also not been requested often, so using the 'old' JEOL system if users do want fluorine I did try TAP crystals at some point but even if using two simultaneously it does not seem to capture F. This remains on my to-do-list and I wonder whether it is worth a separate post to gather and compare methods for fluorine? I don't think there is an existing discussion, as I've searched before and not found anything 'concrete'? In theory, I think it would be a mix of utilizing interference/blank standards and treating it as a trace element...?

To be fair, trace elements such as F and Cl are subject to different accuracy concerns than major elements as I describe here:

https://smf.probesoftware.com/index.php?topic=1535.msg12121#msg12121

I would also add secondary fluorescence from boundary phases to the trace element slide in the link above.

Another thing to consider for trace background correction is the MAN background correction (Donovan et al., 2016), which when utilized with a blank material can give excellent accuracy. Consider that the variance measured on your blank, when applied to your unknown is now equal to your accuracy, if the blank is truly a blank. Pretty cool. 

Remember, you cannot have off-peak interferences with the MAN background correction, because you don't measure off-peak backgrounds at all!

Of course one does also need to consider whether the trace inaccuracy is due to the instrument (trace Ti in SiO2, secondary Bragg diffraction) or the material (trace Au in pyrite, nearby absorption edge) and choose a suitable blank material. 

And of course we must also utilize the quantitative interference correction in Probe for EPMA with Cl in apatites and for F in Fe bearing minerals!  It works great!

There are several topics on these questions in this forum. Try the search function at the top of the main forum page.  Feel free to post to those topics.

[Les Moore]

Regardless of the approach, looking at the PHA at different count rates is an important activity. The peak shift and doubling and even tripling peaks show up at higher count rates. To include or exclude - that is the question. If you run over a pure material you may even saturate the detector and it is blinded and gives no counts for an appreciable time afterwards.   

[Probeman]

Regardless of the approach, looking at the PHA at different count rates is an important activity.

I found it to be very educational!

But the most surprising aspect for me, was discovering that in integral mode all photons to the right of the baseline are counted.  Even when they appear to be "cutoff" in the graphical display.  That was the "a ha!" moment for me.

The peak shift and doubling and even tripling peaks show up at higher count rates. To include or exclude - that is the question. If you run over a pure material you may even saturate the detector and it is blinded and gives no counts for an appreciable time afterwards.

These PHA distributions were acquired at fairly "normal" beam currents, 20, 30 nA or so which makes sense for major and minor elements. Under these conditions I only see the normal peak and sometimes the escape peak if the gas physics permits it.

For trace elements of course one can acquire the primary standard at a lower beam current than the unknown, but then one must be sure to check their picoammeter linearity using the constant k-ratio method:

https://smf.probesoftware.com/index.php?topic=1466.msg11124#msg11124

[Ben Buse]

Starting a new topic to focus on the correct method to tune one's PHA settings because proper tuning of PHA is a "black box" to most analysts!  Including myself until quite recently! 

This link is to the "Limits of EPMA Accuracy" which explores this topic in broader detail ("peak shift" matching, dead time calibration, and better standards):

https://smf.probesoftware.com/index.php?topic=1831.0

Let's start by analyzing Ti Ka in synthetic SrTiO3 using TiO2 as a primary standard (without "count rate" matching our primary standard to our secondary standard or unknown!).

Running a PHA scan using the default PHA settings we obtain this scan on the TiO2 standard (remember, we always want to adjust our PHA settings on a material with the highest concentration (generally the primary standard) at the highest beam current we expect to utilize (that is, at the highest count rate we expect to see, that is, tuning our PHA to avoid pulse height depression effects which can cause non-linear responses)



Note first that we are in "integral" PHA mode. In integral PHA all photons to the right are still counted, even those that plot (invisibly!) to the right of the maximum x axis displayed!

Next we clearly see that the baseline level is too high and is intersecting the PHA peak. Why is that a problem?  Because if the peak shifts to the right or left, the baseline level cuts off more or less of the PHA peak and we obtain a non-linear response in our counting electronics!

However, we can make two adjustments to avoid this: first by lowering our baseline level below the PHA peak, and second, increasing our gain (for Cameca instruments) or our bias (for JEOL instruments) we can shift the PHA peak to the right *completely above* the baseline level, thus ensuring a linear response in our counting electrics. 

Here is the PHA scan after adjusting our baseline level and our gain (or bias):



Better, but the baseline level still intersects the PHA peak tail a bit on the left.  After raising our PHA gain still further, we obtain this PHA scan:



This is what we will utilize for our quantitative analyses in the next post.  The same PHA tuning procedure was utilized for Fe Ka as seen here:

Might be missing something but this is how JEOL have always recommended PHA - set to 4 volts. And idealy want to include escape peak, if not exclude escape peak, but ensure baseline is not on peak, it's what Paul Carpenter also has been talking about for decades.

[Probeman]

Might be missing something but this is how JEOL have always recommended PHA - set to 4 volts. And idealy want to include escape peak, if not exclude escape peak, but ensure baseline is not on peak, it's what Paul Carpenter also has been talking about for decades.

Yes, you are missing something.   

For one thing these are Cameca plots and the Cameca PHA only goes to 5 volts. So on a JEOL instrument, you would be setting the PHA peak to 8 volts!  Do you get it now?   

In addition, with this integral-baseline method you *always* want to include the escape peak no matter how high one has to amplify the PHA.  Not having the escape peak above the baseline means that at some low enough count rate the escape peak could shift to the right, and negating your counting linearity.

The more important point is that we now know that photons that appear "cut off" graphically are still all counted in integral PHA mode. I did not know that until a few years ago, did you?

The final point is that we no longer need to "count rate match" our unknowns to our standards and therefore we can finally begin using globally distributed synthetic end member composition standards.

Combining all these points together will allow us as a global community to finally attain ~1% (or better) relative accuracy throughout the community.

I believe that doing all this (better PHA tuning, better dead time calibrations, better matrix corrections and better globally distributed high purity synthetic standards), as a community, will finally allow us to produce compositions that are in agreement with each other. Because that is certainly not the case today:

Wieser, P. E., Kent, A. J., Till, C. B., Donovan, J., Neave, D. A., Blatter, D. L., & Krawczynski, M. J. (2023). Barometers behaving badly I: assessing the influence of analytical and experimental uncertainty on clinopyroxene thermobarometry calculations at crustal conditions. Journal of Petrology, 64(2), egac126.

[Ben Buse]

Thanks John for explaining. By "count rate match" you mean changing the beam current so count rate same on standards and unknowns? Might be tricky for some multi element phases.

[Probeman]

Thanks John for explaining. By "count rate match" you mean changing the beam current so count rate same on standards and unknowns? Might be tricky for some multi element phases.

No, I do not.

I mean the widely utilized "practice" of selecting a natural material (no matter how poorly characterized or heterogeneous), to use as a standard to match the count rate observed in an unknown.

The only benefit being that then the dead time and PHA tuning (and matrix corrections) have small effects, but now the accuracy of the standard becomes dominant. But we also know that these natural "standard" materials are not good enough as has been documented by Vicenzi, Fournelle and Nachlas and others.

Of course one can also utilize different beam currents for the standard and unknown as one commonly does for trace element analysis, though accuracy in trace elements then depends on the picoammeter linearity.