Background: Platinum (Pt), palladium (Pd), rhodium (Rh) and iridium (Ir) are platinum
group elements (PGEs) and also important elements of geochemistry and environmental chemistry
with the similar physic-chemical properties, which have been widely used in industry and laboratory.
However, due to the low abundance and inhomogeneous distribution in natural ore as well as the
nugget effect, the accurate determination of PGEs has been a challenge to analytical chemistry.
Methods: In this work, a novel fire assay method was reported for the determination of ultra-trace Pt,
Pd, Rh and Ir in geochemical samples. Tin powder (Sn) instead of stannic oxide (SnO2) was used as a
fire assay collector to reduce the melting temperature from 1250°C to 1050°C, the escape of molten
material caused by high temperature was successfully avoided. Tin bead was compressed into thin
slice and dissolved by HCl. For the target Pt, Pd, Rh and Ir, HCl insoluble substance such as PtSn4,
PdSn4, RhSn4 and Ir3Sn7 were formed and separated from matrix by filtering. The metal compounds
precipitate together with filter paper were microwave-assisted completely digested by aqua regia
(50%, v/v), thence the sample solution was determined by inductively coupled plasma mass spectrometry
Results: Compared with nickel oxide and lead oxide in nickel sulfide /lead fire assay, the reagent
blank of tin powder were relatively low and could be directly employed in tin fire assay to collect Pt,
Pd, Rh and Ir without purifying. Moreover, the harm of nickel oxide and lead oxide to the analyst
and environment was avoided by using the non-toxic tin powder. The decomposition method of
chromite and black shale was investigated as well as the amount of tin powder and flour, microwave
digestion program for the determination of Pt, Pd, Rh and Ir were optimized. Besides, the influence
of mass spectrum interference of co-existing elements was discussed and the standard mode and kinetic
energy discrimination collision pool mode were compared. Under the optimal conditions, excellent
curve fitting of Pt, Pd, Rh and Ir were obtained between 0.01~100 ng mL-1, with the correlation
coefficients exceeding 0.9996. The detection limits were from 0.003 ng g-1 to 0.057 ng g-1.
Conclusion: The developed method was applied to analyze the Chinese Certified Reference Materials
and the determined values were in good agreement with the certified values.