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The implementation of a non-linear combination of two reference voltages to control the anode voltage in the previously described biasing system of an electron source with a hot cathode allows elimination of the correlation between the emission current and the accelerating voltage. The presented system is highly suitable for applications in electron-impact mass spectrometers, ionization gauges and other instruments (for example, electron microscopes)., (Copyright © 2011 John Wiley & Sons, Ltd.)

Stable oxygen isotope compositions (δ(18)O values) of two commercial and one synthesized silver orthophosphate reagents have been determined on the VSMOW scale. The analyses were carried out in three different laboratories: lab (1) applying off-line oxygen extraction in the form of CO(2) which was analyzed on a dual inlet and triple collector isotope ratio mass spectrometer, while labs (2) and (3) employed an isotope ratio mass spectrometer coupled to a high-temperature conversion/elemental analyzer (TC/EA) where Ag(3)PO(4) samples were analyzed as CO in continuous flow mode. The δ(18)O values for the proposed new comparison materials were linked to the generally accepted δ(18)O values for Vennemann's TU-1 and TU-2 standards as well as for Ag(3)PO(4) extracted from NBS120c. The weighted average δ(18)O(VSMOW) values for the new comparison materials UMCS-1, UMCS-2 and AGPO-SCRI were determined to be + 32.60 (± 0.12), + 19.40 (± 0.12) and + 14.58 (± 0.13)‰, respectively., (Copyright © 2011 John Wiley & Sons, Ltd.)

The reduction of silver phosphate with graphite under vacuum conditions was studied at final reaction temperatures varying from 430 to 915°C to determine: (i) the CO(2) extraction yield, and (ii) the oxygen isotopic composition of CO(2). The CO(2) yield and oxygen isotopic composition were determined on a calibrated dual inlet and triple collector isotope ratio mass spectrometer. We observed the following three stages of the reduction process. (1) At temperatures below 590°C only CO(2) is formed, while silver orthophosphate decays to pyrophosphate. (2) At higher temperatures, 590-830°C, predominantly CO is formed from silver pyrophosphate which decays to metaphosphate; this CO was always converted into CO(2) by the glow discharge method. (3) At temperatures above 830°C the noticeable sublimation of silver orthophosphate occurs. This observation was accompanied by the oxygen isotope analysis of the obtained CO(2). The measured δ(18)O value varied from -11.93‰ (at the lowest temperature) to -20.32‰ (at the highest temperature). The optimum reduction temperature range was found to be 780-830°C. In this temperature range the oxygen isotopic composition of CO(2) is nearly constant and the reaction efficiency is relatively high. The determined difference between the δ(18)O value of oxygen in silver phosphate and that in CO(2) extracted from this phosphate is +0.70‰., (Copyright © 2010 John Wiley & Sons, Ltd.)

The most precise method of chlorine isotope analysis described to date is based on the isotope ratio mass spectrometry (IRMS) of chlorine quantitatively converted into chloromethane, CH(3)Cl. This gas can be produced from several chlorine-containing compounds and analyzed by IRMS. However, the mass spectrum of chloromethane is rather complicated and the ratio of the most abundant ions (mass-52/mass-50) differs from the (37)Cl/(35)Cl isotope ratio. This difference becomes significant when the delta37 Cl exceeds 10 per thousand. Moreover, the electron ionization source yields approximately 80% of all the ionic species at the useful masses 50 and 52. To overcome these drawbacks, we have devised a negative ion mass spectrometer which retains all the best features of IRMS, including a dual-inlet system with changeover valve, dual collector assembly and CH(3)Cl gas as analyte. In the modified ion source we have replaced the ionization chamber with an electron beam by a metal tube with a hot metal filament inside it. Within this tube the (35)Cl(-) and (37)Cl(-) ions are produced with an efficiency dependent on the filament material and its temperature. No other ionic species were found in the mass spectrum except of traces at masses 26 and 28 at ppm levels, probably due to the formation of CN(-) and CO(-). The minimal amount of Cl used in our method is of the order of 5 micromol (3 mg AgCl) and the precision is better than 0.005 per thousand (1sigma)., (Copyright (c) 2009 John Wiley & Sons, Ltd.)

A negative chlorine ion source has been designed and constructed. The source utilizes direct surface ionization of chloromethane gas on a hot metal filament. Four different alloys for the filament material were tested: W99Th1, W75Re25, Hf97.5Zr2.5 and Mo52.5Re47.5. We conclude that the best filament material is the MoRe alloy, for which the signal-to-noise ratio is optimal. The ion source is used for chlorine isotope ratio measurements with higher precision and sensitivity than the positive ionization source used previously. Inasmuch as only negative ions of the two isotopes of interest are observed, no corrections to the measured isotope ratio are necessary, and less rigously purified samples may be analyzed. The negative ion currents are considerably larger than positive ion currents obtained with an electron ionization source. This implies higher analytical precision (typically 0.005 permil) and sensitivity., (Copyright 2008 John Wiley & Sons, Ltd.)

We describe the use of a simple voltage stabilizer that controls the filament temperature (T(f)) in the ion source of a thermal ionization mass spectrometer. The filament voltage (V(f)) is measured by means of a separate pair of wires connected inside of the ion source in parallel to the wires supplying power. It has been demonstrated that V(f) is directly proportional to T(f) in a wide range of filament temperature. The T(f) value is solely controlled by the reference voltage (V(r)) that can be manually selected from a voltage divider or by means of a computer. Digital signals from the computer in the form of a series of pulses are transmitted opto-electronically and subsequently converted to analog signals. The temperature controller described here was successfully applied for analysis of potassium concentration by the isotope dilution method., (Copyright 2001 John Wiley & Sons, Ltd.)