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Re: Pressure dependence of isotope ratios

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Mon, 17 Sep 2018 15:01:38 +0000

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 ```In an electron impact source the ion current i is proportional to the gas pressure p, the size s and length l of the ionising electron beam   i = c(i) x p x s x l where c(i) is the ionization constant which accounts for all ionization specific parameters which depend on the particular isotopic species. Another way of putting this is:   i = E(q) x p(q) where E(q) is the source pressure sensitivity in A/mbar and p(q) is the pressure inside the ion source. Meeting the condition given by this equation is a prerequisite for accurate measurement of isotope ratios. That being said, because the term p(q) is impossible to measure and thus not known, the following equation is used.   i = E(m) x [Ds/Dt] where Ds/Dt is the sample consumption rate and E(m) is the molar sensitivity which is measured in As/mol. E(m) is related to the system pressure sensitivity E(p) by: E(m) = RTL(qp)E(p) L(qp) is the molecular gas conductance and R is the universal gas constant. This relationship can be approximated by: E(m) in As/mol is approx equal to 400E(p) in As/mbar. The term E(m) is the basis of (or is being converted into) the term E(mm) that gives the number of gas molecules required in the ion source to detect one ion. By rule of thumb an E(mm) of 1 molecule/ion corresponds to an E(m) value of 10,000 As/mol. ****************************************************** Prof. Dr W Meier-Augenstein, CChem, FRSC Stable Isotope Forensics & Analytical Sciences Robert Gordon University School of Pharmacy and Life Sciences The Ian Wood Building Garthdee Road Aberdeen AB10 7GJ E-mail: [log in to unmask] http://scholar.google.co.uk/citations?user=ty6c5IMAAAAJ ________________________________ From: Stable Isotope Geochemistry <[log in to unmask]> on behalf of Lukas Flierl <[log in to unmask]> Sent: 17 September 2018 14:12:27 To: [log in to unmask] Subject: Re: [ISOGEOCHEM] Pressure dependence of isotope ratios Hi guys, Thank you very much for your suggestions. Unfortunately, this was not the thing I meant, I have to admit my question was not very precise.  I thought about the pressure dependence while measuring it using a gas mass spectrometer. I found some literature about noble gases, like. [1] M. Honda, I. McDougall, D. B. Patterson, A. Doulgeris, and D. A. Clague, “Noble gases in submarine pillow basalt glasses from Loihi and Kilauea, Hawaii: A solar component in the Earth,” Geochimica et Cosmochimica Acta, vol. 57, no. 4, pp. 859‐874, 1993. [2] F. K. A. Burnard P. G., “Calibration of pressure‐dependent sensitivity and discrimination in Nier‐type noble gas ion sources,” Geochemistry, Geophysics, Geosystems, vol. 1, no. 7. In these the authors describe, that the ionization efficiency dependes on the gas pressure. Unfortunately, there are no reasons given why it depends on the pressure. Maybe some of you knows why or where to find a reasnoble explanation. Cheers, Lukas ----------------------------------------------------------------- Lukas Flierl Physikalisch-Technische Bundesanstalt PTB Metrology in Chemistry, 3.11 Bundesallee 100 38116 Braunschweig, Germany Tel. +49-531-592-3318 e-mail: [log in to unmask] web: www.ptb.de ________________________________ This email has been scanned for spam and viruses by Proofpoint Essentials. Click here to report this email as spam. ________________________________ Robert Gordon University has been awarded a TEF Gold award for the quality of its undergraduate teaching and learning, placing it in the top 20% of Universities in the UK Robert Gordon University, a Scottish charity registered under charity number SC 013781. This e-mail and any attachment is for authorised use by the intended recipient(s) only. It may contain proprietary material, confidential information and/or be subject to legal privilege. It should not be copied, disclosed to, retained or used by, any other party. If you are not an intended recipient then please promptly delete this e-mail and any attachment and all copies and inform the sender. Please note that any views or opinions presented in this email are solely those of the author and do not necessarily represent those of Robert Gordon University. Thank you. ```