A 30 cm radius 90° sector double collecting mass spectrometer with a capacitor integrating detector for high precision isotopic analysis of carbon dioxide

Abstract

The long term performance of a 30.48 cm radius 90° sector double collecting mass spectrometer using a novel automatic capacitor integrating detector for high precision isotopic analysis of carbon dioxide is described. The incorporation of a new design of changeover valve in the dual inlet system and other aspects of the design of the mass spectrometer have resulted in the reduction of all the instrumental corrections applicable to measured isotopic differences between sample and standard gases to a negligible level. Isotopic differences in the ratio of ratios ▪ can be determined routinely with a reproducibility (expressed as the 1σ standard deviation about the mean value obtained by replicate analysis) of better than ±0.002%. The efficiency of the ion source is about 5×10 −4 ions molecule −1. The leak correction has averaged 0.004±0.016‰ for the 46/(44+45) ratio and −0.025±0.008‰ for the 45/44 ratio. The sums of all the other instrumental corrections (background, valve mixing, and abundance sensitivity) are 1.00038 for δ46/(44+45) values and 1.00078 for δ45/44 values. These values compare favourably with published sums of correction factors which average about 1.03. The capacitor integrating detector has the advantages that the ion beam intensities are objectively averaged, it runs the mass spectrometer automatically once set up, and provides a digital output.