Isotope ratio mass spectrometry of iron, nickel, chromium and palladium using their volatile phosphorus trifluoride complexes: Application of a microscale cocondensation method to sample preparation

This work describes the setting up of a microscale cocondensation synthesis method for production of Fe(PF₃)₅, Ni(PF₃) ₄, Cr(PF₃)₆ and Pd(PF₃)₄, to be used for sample introduction in gas source isotope mass spectrometry. Apparatus for down-scaled cocondensation synthesis was built at the Institute for Reference Materials and Measurements (IRMM) and the synthesis procedures were developed. Yields obtained for the four M(PF₃)_x were: Fe(PF₃)₅ 1–5% (29% in one experiment), Ni(PF ₃)₄ 4–20%, Cr(PF₃)₆ 4% and Pd(PF ₃)₄ 5–66%. The amount of metal needed for the synthesis is 5 mg. The synthesis procedure takes about 5–6 h per sample in total. Yields were sufficient for isotopic analysis with a magnet sector instrument.; The potential of combining the microscale synthesis method with high precision gas mass spectrometry, for isotopic analysis was assessed by characterising samples with both quadrupole and magnet sector instruments. All samples synthesised on microscale suffered from organic contamination, and studies on bulk Fe(PF ₃)₅ revealed also an interference by F₃ + and HF₃+ on the minor isotopes 57Fe and 58Fe. 62Ni+ peak was interfered by P₂+, 50Cr + by PF+, and the iPd+ by hydrolysis products of PF₃. Reproducibilities of the isotope amount ratios of iron and nickel in their M(PF₃)x were about 10−3 as measured with a magnetic sector gas mass spectrometer. Accuracies varied between 0.6–11%, with reference to values published by the International Union of Pure and Applied Chemistry (IUPAC). Better precision and accuracy was expected but not reached because of impurities and complications in the measurement. Also, isotopic fractionation was suspected to distort the isotope amount ratios of nickel. The isotope amount ratios of chromium and palladium in their M(PF₃) _x were measured with the quadrupole instrument. A reproducibility of only 5·10−2 was obtained with the quadrupole instrument due to non-optimal measuring conditions for the small amounts of sample available, and the full potential of the method could not be evaluated. The measured ratios were in good agreement with the IUPAC reference values, deviations from the reference values ranging from 0.03% to 5.6%.; If problems in synthesising the complexes in a pure enough form were solved, all four M(PF₃)_x could be used for isotopic analysis, using either their iM+, iMF + or iM(PF₃)+ ions to record the isotope amount ratios.