Ne and Ar isotope analysis of samples with high abundance ratios of Ar/Ne and low abundance of Ne by MMS and QMS

Abstract

Analysis of isotopes of neon and argon using the same mass spectrometer is somewhat difficult because the ⁴⁰Ar background after argon measurement can interfere with the ²⁰Ne determination due to the very similar mass-to-charge ratios of ⁴⁰Ar⁺⁺ and ²⁰Ne⁺. Analysis becomes even more difficult if the sample has a high abundance ratio of Ar/Ne and an extremely low amount of Ne. Here, we present a method for precise measurement of neon isotopes on a noble gas mass spectrometer (MMS) and argon isotopes on a quadruple mass spectrometer (QMS). The desorption curves of neon and argon on a charcoal trap and stainless steel trap were respectively determined, and the results demonstrated that the stainless steel trap was more suitable for the absolute separation of neon and argon despite requiring a lower temperature. The argon background effect of the MMS on the ²⁰Ne determination was discussed emphatically; it was found that the charcoal trap immersed in liquid nitrogen (LN) near the ion source could effectively decrease the argon background. Furthermore, the liquid level of the LN should not only be located as close to the ion source as possible but should be kept constant, which is very important for the measurement of ²⁰Ne with a small signal size. The ratio of CO₂⁺⁺/CO⁺ for MMS was 1.2 ± 0.3%, which enabled the correction of CO₂⁺⁺ interference in the ²²Ne determination. An artificial mixture of standard gases with an abundance ratio of argon to neon as high as 1000 was prepared. 0.1 cm³ standard gas mixture with 5.50 × 10⁻¹³ mol argon and 5.53 × 10⁻¹⁶ mol neon was measured each time. The precisions of the isotopic ratios of neon measured by MMS were ±0.15% for ²⁰Ne/²²Ne and ±1.6% for ²¹Ne/²²Ne. The precisions of the isotopic ratios of argon measured by QMS were ±0.5% for ⁴⁰Ar/³⁶Ar and 4.0% for ³⁸Ar/³⁶Ar. Although the precision of the argon isotope analysis obtained by QMS was not as good as that obtained by MMS, it fully meets the requirements for application in geochemical tracers. Moreover, this method avoids the use of the same mass spectrometer for analysis of both argon and neon, thus guaranteeing precision of the neon isotope measurements.