Before samples can be analyzed by they must be converted into the simple gases N2 or CO2. Poor precision can often be traced to inadequate grinding that leaves fibrous matter or visible granules in the sample. Wiley mills do not do an acceptable job of grinding for this application.
All mass spectrometers show a confounding effect of sample size on determined isotope ratio.
Samples are therefore weighed after grinding to achieve a uniform sample size. For isotopic analysis, sample size and total element content have a major effect on the analysis. The size of the subsample weighed out depends upon the density of the material as well as its N and C content. It may not, of course, be possible to provide both these ideal amounts in one sample. In addition, for element-poor soil samples the sheer bulk of the sample becomes significant.