where \(\delta_{\text{raw}\left(\text{std}\right)}\) is the isotope
composition of the standard relative to the reference gas (Eq. 1) and\(\delta_{\text{true}\left(\text{std}\right)}\) is the certified
isotope composition of the standard. When two or more standards are used
for normalization as recommended by the International Atomic Energy
Agency (IAEA)11,27,\(\delta_{\text{true}\left(\text{sample}\right)}\) can be calculated
using a least squares regression between\(\delta_{\text{true}\left(\text{std}\right)}\) and\(\delta_{\text{raw}\left(\text{std}\right)}\) according to the
following equations: