Hi Ioannis,

I always measured it as first element...

Cheers

Silvio

On 30-Jan-19 4:16 PM, Ioannis Baziotis wrote:
> Just an additional simple thought: at which sequence do you measure Na? Other than first may produce severe lossing. Measure it first and the rest of the elements.
> ----------------------------------------------------------------
> Ioannis Baziotis [log in to unmask]
> Assistant Professor of Mineralogy and Petrology
> Agricultural University of Athens
> Laboratory of Mineralogy and Geology
> Athens, 11855 Greece
> (+30)210-529-4155
> Twitter: https://twitter.com/EGU_GMPV
> Personal site: http://ibaziotis.wix.com/ioannis-baziotis
> ----------------------------------------------------------------
>
> ----- Original Message -----
> Από: Allaz Julien <[log in to unmask]>
> Ημερομηνία: Wednesday, January 30, 2019 8:57 am
> Θέμα: Re: severe alkali-loss during microprobe analyses of low alkali andesitic glasses
> Προς: [log in to unmask]
>
>
>> Hi Silvio,
>>
>> Yes, Na (or K) in hydrous glass can be tricky. Using the EDS and a low
>> current is probably the safest way to get decent analysis (we do this
>> regularly on our 30 mm2 Thermo Ultradry detector at 1.5 to 2 nA and 15
>> keV), but if you work on the electron microprobe here are a couple
>> suggestions (in addition to the one already suggested, notably the use
>> of a cryostage which is your best chance):
>>
>> 1) Lower the electron beam even more - 2 nA and 5 um is our normal
>> routine for very sensitive and small-sized material, especially
>> hydrous alkali glass, and it is acceptable in most cases, even highly
>> hydrous andesitic glass.
>>
>> 2) Lower the counting time on Na. I had in the past analysed even more
>> challenging minerals (jarosite and alunite, which are alkali-rich
>> hydrosulfate), and the most accurate analyses were obtained with a 3
>> second counting time on Na at 2 to 3 nA with an almost focused beam
>> (we couldn’t even afford a 5 um beam size due to strong zoning in
>> these 10 um sized minerals). Beyond this, the analyses were
>> inaccurate… Such results are very low precision, but you have here to
>> choose between accuracy and precision, and once again, if you can get
>> multiple analysis point, then the average will be more acceptable in
>> term of precision.
>>
>> 3) If you are using a Cameca instrument, and if you have access to the
>> PC0 crystal (2d = 45 Å), then go for it! You will have a much higher
>> count rate, and can get very good results even with just 3 seconds
>> counting time.
>>
>> 4) Use a Time Dependent Intensity correction. I believe JEOL and
>> Cameca now offers this option (on the newest instrument at least, and
>> often as an additional software package), otherwise it is available on
>> the software "Probe for EPMA” from John Donovan.
>>
>> 5) If none of the above works, lower even more the beam current. I
>> would strongly advise to monitor the count rate of Na using the Chart
>> Recorder and determine at which point the Na intensity is decreasing.
>>
>> Note that I am not certain that lowering the acceleration voltage
>> would help: doing so, you will strongly reduce the analytical volume,
>> and the beam damage will be increase. The damage is a function not
>> just of the counting time and the current, but also the impacted
>> volume (electron density). If you increase the electron density, you
>> will increase the heat effect and thus the damage…
>>
>> Best,
>>
>> Julien
>>
>>
>> P.S. You can also address your question on one of the following
>> probe-specific email list which regroup many microprobe users:
>> - Cameca: [log in to unmask]<
>> - JEOL: [log in to unmask]<
>>
>>
>> ###########################
>> Dr. Julien Allaz
>> Head assistant for SEM/EPMA
>> Inst. für Geochemie und Petrologie
>> ETH Zürich
>> NW E 84
>> Clausiusstrasse 25
>> 8092 Zürich
>> Switzerland
>>
>> Tel: +41 44 632 37 20
>> Fax: +41 44 632 16 36
>> Email: [log in to unmask]<
>> ###########################
>>
>>
>> On 30 Jan 2019, at 13:16, Clemens, JD, Prof [[log in to unmask]< wrote:
>>
>> Hi Silvio,
>>
>> I am guessing that the granitic glasses you were referring to may not
>> have been hydrous or perhaps they had much lower H2O contents than
>> your andesitic sample. Na counting losses correlate positively with Na
>> abundance and with H2O content of the glass.
>>
>> I assume you are using EDS. You can virtually eliminate the problem if
>> you:
>>
>> lower the beam voltage (10 kV?)
>> move the nose piece of the detector closer to the sample to get more
>> counts and shorten the time for analysis
>> most critically, use a cooling stage to lower the sample temperature
>> to that of liquid N2.
>>
>> I have tested an albite-composition glass with 9 wt% H2O in it under
>> these conditions and easily got stoichiometrically perfect Ab analyses
>> out of it.
>>
>> So, you may have to invest in the freezing stage if you want to
>> continue using a finely focused beam. There really is very little
>> choice about that.
>>
>> I don’t know who might make these things, but people do have them.
>> Manchester and Stellenbosch both have (or had) such facilities, for example.
>>
>>
>> Prof. John D. Clemens
>> Dept of Earth Sciences
>> University of Stellenbosch
>> +27 (0)21 808 3159
>>
>> On 30 Jan 2019, at 13:44, Silvio Ferrero
>> <[log in to unmask]< wrote:
>>
>> Dear All,
>>
>> We are experiencing almost complete alkali loss during microprobe
>> analyses of andesitic glasses with narrow beam (diameter around 1
>> micron), at 9 nA and 15 kV. The glass under investigation is hydrous
>> (around 5 wt% H2O), and based on other data we were expecting 1-2 wt%
>> Na2O. Our analyses show instead a remarkably constant amount of
>> 0.05-0.1 wt% Na2O over 20 analyses. Analyses on granitic standards
>> during exactly the same session show a very limited loss for Na
>> (around 20% relative), and in both cases the Na is only measured for 6
>> seconds. Few analyses were conducted with diameter of 3-4 microns, and
>> still the Na was in the same range of values. Larger beams cannot be
>> used because we are analyzing small melt inclusions in garnet.
>>
>> I´m fully aware of the alkali loss problem in alkali-rich glasses, but
>> I never experienced a almost complete (around 90% relative I would
>> say) loss of Na in any previous case study. Anyone has experienced
>> such a problem analyzing hydrous andesitic glasses with very narrow beams?
>>
>> Any insights, suggestion or comment is welcome!
>>
>> Silvio
>>
>> --
>> Silvio Ferrero
>> tel. 0049(0)3319775705
>> Universität Potsdam
>> Institut für Erd- und Umweltwissenschaften
>> - Dept. of Earth and Environmental Sciences -
>> Haus 27, Karl-Liebknecht-Straße 24-25
>> 14476 Potsdam
>>
>> "In those early days people laughed at me. They quoted Saussure who had
>> said that it was not a proper thing to examine mountains with microscopes,
>> and ridiculed my action in every way. Most luckily I took no notice of
>> them"
>> (Henry Clifton Sorby)
>>
>> To many petrologists a volatile component is  exactly like a Maxwell
>> daemon; it does just what one may wish it to do.
>> (The evolution of the igneous rocks, N.L. Bowen, 1928)
>>
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-- 
Silvio Ferrero
tel. 0049(0)3319775705
Universität Potsdam
Institut für Erd- und Umweltwissenschaften
- Dept. of Earth and Environmental Sciences -
Haus 27, Karl-Liebknecht-Straße 24-25
14476 Potsdam

"In those early days people laughed at me. They quoted Saussure who had
said that it was not a proper thing to examine mountains with microscopes,
and ridiculed my action in every way. Most luckily I took no notice of
them"
(Henry Clifton Sorby)

To many petrologists a volatile component is  exactly like a Maxwell
daemon; it does just what one may wish it to do.
(The evolution of the igneous rocks, N.L. Bowen, 1928)

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