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

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 inonization mass spectrometry. An external precision of 0.014% was obtained from the 235U:238U measurement of six samples of a National Bureau of Standards (NBS) 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. (author)