Communication. Isotopic ratio measurement using a double focusing magnetic sector mass analyser with an inductively coupled plasma as an ion source

Abstract

An inductively coupled plasma source was coupled to a magnetic sector mass analyser equipped with seven Faraday detectors. An electrostatic filter located between the plasma source and the magnetic sector was used to create a double focusing system. Isotopic ratio measurements of uranium and lead standards revealed levels of internal and external precision comparable to those obtained using thermal ionization mass spectrometry. An external precision [relative standard deviation (RSD)] of 0.014% was obtained from the ²³⁵U:²³⁸U measurement of six samples of a National Bureau of Standards (NBS)[now National Institute of Standards and Technology (NIST)] Standard Reference Material (SRM) U-500, while an RSD of 0.022% was obtained from the ²⁰⁶Pb:²⁰⁴Pb measurement of six samples of NBS SRM Pb-981. Measured isotopic ratios deviated from the NBS value by approximately 0.9% per atomic mass unit. This deviation approximates to a linear function of mass bias and can therefore be corrected for by the analysis of standards. The analysis of NBS SRM Sr-987 revealed superior levels of internal and external precision. The normalization of the ⁸⁷Sr:⁸⁶Sr ratio to the ⁸⁶Sr:⁸⁸Sr ratio reduced the RSD to approximately 0.008%. The measured ratio was within 0.01% of the NBS value and the day-to-day reproducibility was consistent within one standard deviation.