RUBIDIUM SULFATE AMMONIUM-SULFATE SOLID-SOLUTION - A STANDARD FOR USE DURING THE DETERMINATION OF NITROGEN ABUNDANCE AND ISOTOPIC COMPOSITION AT THE PPM LEVEL BY STATIC-VACUUM MASS-SPECTROMETRY

Ammonium sulfates N-1 (delta-N-15 = +0.45 parts-per-thousand) and N-2 (delta-N-15 = +20.35 parts-per-thousand), which are currnet N isotope reference materials, have been dissolved in rubidium sulfate to produce submilligram-sized reference materials containing ppm amounts of N, of known isotope composition. Three batches of material were prepared with target N concentrations between 728 and 1660 ppm. Within a given batch, the variation in concentration was found to be +/- 4% (or less) around that of the target, with the mean value for six analyses being within +/- 2.5% of the target. The delta-N-15 values were all within +/- 1.0 parts-per-thousand of the expected, however, within a given batch the intersample variation was +/- 0.5 parts-per-thousand (or less), which is close to the limit of the analytical precision. Compared to existing reference materials containing trace quantities of N, these solid solutions have the advantages that the N is of known isotopic composition, the uncertainty in N concentration (< +/- 5%) is less by a factor of 2 and the amount of material required in order to obtain this level of uncertainty is less by more than 3 orders of magnitude. Such standards are well suited for use with static-vacuum mass spectrometers.