Negative ion source for chlorine isotope ratio measurements

Abstract

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.

Studies on Isotope Ratio Measurement of Cl by Inductively Coupled Plasma Triple-quad Mass Spectrometry

“…The effects from polyatomic ion interferences that could not be removed completely were also considered. In the present study, ICP-MS/ MS in single-MS mode with H2 as collision reaction gas was not demonstrated because huge polyatomic ions of 36 …”

“…Table 3 summarizes analytical sensitivities such as slope, blank, limits of detection (LODs, 3σ) and background equivalent concentration (BEC) observed for ICP-SFMS and ICP-MS/MS examined in the present study. The ICP-SFMS, ICP-MS/MS in MS/MS mode with H2 gas and ICP-MS/MS in single-MS mode without collision reaction gas showed similar values of slope and BEC for each Cl isotope, except for the BEC obtained by ICP-MS/MS in single-MS mode for 37 Cl which was attributed to the huge background signal from 36 Ar 1 H polyatomic ion interference. The lower LODs could be obtained by the ICP-MS/MS with H2 gas for both Cl isotopes as listed in Table 3.…”

“…The results observed are representative for those obtained by ICPMS with single quadrupole MS without collision reaction gas. Though the linear correlations between the signals for both Cl isotopes and their concentrations of 5 -100 mg kg -1 could be obtained, the high background signal of 37 Cl attributed to the 36 …”

“…Though the present study is out of scope for the isotope ratio measurements with high precision (0.005 -0.0001% of relative standard deviation, RSD) which is expected by IRMS 35,36 and TIMS, 37,38 it is expected to be applicable to the studies for stable isotope tracers, 1 H and 36 …”

Improved three‐filament thermal ionization mass spectrometry ion source for isotope ratio determination of two samples

“…This is a reason why thermal ionization mass spectrometry (TIMS) is a convenient method for the precise measurement of Li and K isotope ratios. The TIMS method can also be applied to the ionization of several other compounds and atoms in both, positive and negative modes …”