Fluorine: what are the best analytical practices for quantification?

[dawncruth]

What a timely post!

[Edit by John: Dawn is referring to this topic: https://smf.probesoftware.com/index.php?topic=1854.0]

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]

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.

For example, here's a topic on correcting for excess oxygen when calculating oxygen by stoichiometry when halogens replace some of the stoichiometric oxygen:

https://smf.probesoftware.com/index.php?topic=1247.0

This calculation doesn't have much effect for chlorine (e.g., chlor-apatites), but for fluorine in fluor-apatites or fluor-phlogopites, it can have a very large effect (up to ~15% relative).

[Radek_MM]

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?

The JEOL LDE1 has a 2d of 60A and LDE2 100A (compared to PC2 with 95A). While still working at the Geological Survey of Finland I tried to reproduce oxygen measurements on the iHP200F (LDE1) compared to the CAMECA SX100 (PC0) but failed constantly. Then I realized that the peak is wider on the JEOL so some more interferences to take into account in synthetic slag samples with crazy compositions not occuring in natural minerlas as such...   But oxygen analysis on the probe is a separate topic 

Thank you for the PHA tuning topic! I will definitely try it on my JEOL instruments at the UWA once I have time with F measurements on LDE1 and TAP!

[JonF]

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?

JEOL does not. I quite tried hard to get JEOL to put an "LDE45" in to our new iHP200F, as I used PC0 a lot (F, O, Na!). My requests went through JEOL UK and all the way back to the factory, and nobody had heard of a 45A LDE crystal, which is a shame as there's a whopping great gap between TAP and LDE1. I ended up with TAPL and LDE1L to try and cover off that energy region (and an SXES-ER, as both the JS300N and JS2000 can see F Ka 1st order).

PS sem-geologist, that's a cracking post, thanks for writing it all up. And you did it all without any pictures!