In our lab we usually have to analyze geological samples for fluorine. The normal situation is:
High F contents in Fe-poor samples (e.g. 3.5% F and 0.5% FeO, as in apatite) is less of a problem, although still rather common since this mineral is widespread.
The TAP crystal gives a very low count rate, which is a problem for beam-sensitive samples which need rather short count times (around 10 s, and it would be better to shorten this time); in addition, these materials cannot withstand very high currents without F migration, which results either in an increase or decrease of the count rate (probably depending on the crystal orientation with respect to the beam). Moreover, higher-order P and Ca lines interfere with F (they can be filtered with PHA, but that´s an additional complication and leads to even lower counts). The LDE1 crystal, whose count-rate is with several orders of magnitude higher, has severe F-Fe overlap and troublesome background positions (reasonably close to the Fe+F peak) in samples that have Mn and Al.
I was wondering if you would mind to share with the list how you cope with this problem in your lab. Has anybody adopted the approach of Witter and Kuehner (2004, American Mineralogist 89: 57-63)? If so, are you satisfied with the results?
Thanks in advance for your input,
Fernando Colombo
LAMARX-Universidad Nacional de Córdoba
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