Isotopic Measurement of Biosulfur in Environmental Samples by Secondary Ion Mass Sp ectrometry

Abstract

Recently, it has been significant that the sulfur isotopic composition of biological samples reflects their living environment. A rapid method for the determination of the sulfur isotopic ratio (³²S/³⁴S) in biological samples was investigated by means of secondary ion mass spectro metry (SIMS). A human nail sample of 50-100 mg is ignited in a Parr 1108 oxygen com bustion bomb filled with oxygen to 30 atm and added with 1 cm³ of water. The sulfur trioxide formed dissolves in water as 5O₄^2- to which one drop of 1 M (mol⋅dm^-3) hydrochl oric acid and 5 cm³ of 10% barium chloride solution are added to form barium sulfate. The dried barium sulfate is then heated with 20 cm³ of tin(II)-strong phosphoric acid at 280°C for 20min under a flow of nitrogen. The hydrogen sulfide formed is absorbed in a zinc acetate solution to fix as the zinc sulfide which is in turn converted to silver sulfide by adding 10 cm³ of O.1 M silver nitrate solution. Fifteen milligrams of the silver sulfide is ground and pressed into a thin film. A pair of thin films of silver sulfide, the sample to test and the standard, is set in the same hole of a sample holder. The sulfur isotopic ratio of the standard silver sulfide was deteremined by the mass spectrometric (MS) method reported previously. The ³²S/³⁴S ratio determined was 22.300 which was based on the value (³²s/³⁴S=22.220) of the international standard, Canyon Diablo troilite. Cs⁺ is used as a primary ion and the emitted ³²S^- and ³⁴S^- secondary ions are measured. Analyses are performed under computer c ontrol. Typical operating conditions are as follows: The accelerating potential, beam current and beam diameter of the primary ion are 10 kV, 0.1-0.5 μA and 165 μm, respectively, The sulfur isotopic ratio of samples measured by SIMS is corrected against that of the standard silver sulfide. RelativestandarddeviationswereO.39, 0.17 and 0.47% for the sulfur isotopic ratio(³²S/³⁴S)of 21.986 (cystine), 22.281(methionine) and 22.026 (nail), respectivdy. The δ³⁴S permil values of cystine, methionine and a kind of human nail were +10.64, -2.74 and +8.81‰.