Standard Sample Bracketing

Isotope compositions are always given as ratios, e.g. 56Fe/54Fe. Further, the compositions – i.e., isotope ratios – of unknown samples are given relative to a standard. This is, for example, the IRMM-014 standard in case of Fe. The composition of a sample is then calculated in the isotope notation, e.g., the delta- or epsilon-notation. However, isotope ratios are not directly measured in a mass-spectrometer. Rather, the amount of a specific isotope – e.g., 56Fe – is measured as the number of this isotope per time interval, which is typically output as Volt. In standard-sample bracketing, standards and samples are measured alternately. Possible variations in the intensities when measuring many samples over long periods of time are then unproblematic, as standards are measured before and after each individual sample. The two standards bracketing each sample are then averaged and used to calculate the isotope compassion in the respective notation. Samples and standards should ideally have identical concentrations of the element of which the isotopes are measured.