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
E-mail: [log in to unmask]
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
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.
 M. Honda, I. McDougall, D. B. Patterson, A. Doulgeris, and D. A. Clague, $B!H(BNoble gases in
submarine pillow basalt glasses from Loihi and Kilauea, Hawaii: A solar component in the
Earth,$B!I(B Geochimica et Cosmochimica Acta, vol. 57, no. 4, pp. 859$B!>(B874, 1993.
 F. K. A. Burnard P. G., $B!H(BCalibration of pressure$B!>(Bdependent sensitivity and discrimination in
Nier$B!>(Btype noble gas ion sources,$B!I(B 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.
Physikalisch-Technische Bundesanstalt PTB
Metrology in Chemistry, 3.11
38116 Braunschweig, Germany
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