The relative natural abundance of the main heavy stable isotopes in bioelements (2H, 13C, 15N, 18O) is generally considered as fixed. However, it varies in subtle limits (± 10-4 - 10-2 atom %; AP), due to various isotope fractionation processes in nature. The specialized instrument dedicated to measure such small differences precisely is a gas isotope ratio mass spectrometer (GIRMS), which requires the introduction of the sample as pure gas.