Hi Silvio,

did you try pulsed measurements? By integrating 2-second pulses I once
could reduce the spot size from 10 to 4 µm on albite compared to standard
5-second-measurements and still get the same results, Na loss started at 3
µm.

Cheers, Ralf


> 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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