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Dear Silvio, I have never met so severe Na loss during analysis. However I would recommend to use EDS for Na estimate in such a case - if you have a detector with large area (50 mm2 or more), you should be able to get data with reasonable standard deviation after relatively short acquisition time and probe current as low as 1 nA or less. Together with use of a defocussed beam, I would expect the Na loss negligible... Best regards Martin st 30. 1. 2019 v 12:44 odesílatel Silvio Ferrero < [log in to unmask]> napsal: > 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) > > ######################################################################## > > To unsubscribe from the GEO-METAMORPHISM list, click the following link: > https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=GEO-METAMORPHISM&A=1 > -- Mgr. Martin Racek, Ph.D. Tel.: +420 221 95 1437 E-mail: [log in to unmask] Ústav petrologie a strukturní geologie Univerzita Karlova v Praze Přírodovědecká fakulta Albertov 6, 128 43 Praha 2 www.natur.cuni.cz Institute of Petrology and Structural Geology Charles University in Prague Faculty of Science Albertov 6, 128 43 Praha 2 www.natur.cuni.cz/en ######################################################################## To unsubscribe from the GEO-METAMORPHISM list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=GEO-METAMORPHISM&A=1