ISOTOPIC MEASUREMENT OF BIOSULFUR IN ENVIRONMENTAL-SAMPLES BY SECONDARY ION MASS-SPECTROMETRY

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 (S-32/S-34) in biological samples was investigated by means of secondary ion mass spectrometry (SIMS). A human nail sample of 50 approximately 100 mg is ignited in a Parr 1108 oxygen combustion bomb filled with oxygen to 30 atm and added with 1 cm3 of water. The sulfur trioxide formed dissolves in water as SO42- to which one drop of 1 M (mol.dm-3) hydrochloric 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-degrees-C for 20 min 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 0.1 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 determined by the mass spectrometric (MS) method reported previously. The S-32/S-34 ratio determined was 22.300 which was based on the value (S-32/S-34 = 22.220) of the international standard, Canyon Diablo troilite. Cs+ is used as a primary ion and the emitted S-32- and S-34- secondary ions are measured. Analyses are performed under computer control. Typical operating conditions are as follows: The accelerating potential, beam current and beam diameter of the primary ion are 10 kV, 0.1 approximately 0.5-mu-A and 165-mu-m, respectively. The sulfur isotopic ratio of samples measured by SIMS is corrected against that of the standard silver sulfide. Relative standard deviations were 0.39, 0.17 and 0.47% for the sulfur isotopic ratio (S-32/S-34) of 21.986 (cystine), 22.281 (methionine) and 22.026 (nail), respectively. The delta-S-34 permil values of cystine, methionine and a kind of human nail were +10.64, -2.74 and +8.81 parts per thousand.