<p>The stable HCNOS isotope compositions can be reported in various ways depending on scientific domain and needs. The most common notations are 1) the isotope ratio of two stable isotopes; 2) isotope delta value, and 3) atom fraction of one or more of the isotopes. Frequently recalculations between these notations are required for certain applications, particularly when merging different data sets. All these recalculations require using the absolute isotope ratio for the zero points of the stable isotope delta scales (<em>R<sub>std</sub></em>). However, several <em>R<sub>std</sub></em> with very contrasting values have been proposed over time and there is no common agreement on which values should be used word-wide (Skrzypek and Dunn, 2020a).</p><p>Differences in the selection of <em>R<sub>std</sub></em>value may lead to significant differences between different data sets recalculated from delta value to other notations. These differences in R<sub>std</sub> have a significant influence also on the normalization of raw values but only when the normalization is conducted versus the working standard gas value. We proposed a user-friendly EasyIsoCalculator (http://easyisocalculator.gskrzypek.com) that allows recalculation between the main expressions of isotope compositions using various <em>R<sub>std</sub></em> and aids for identification of potential inconsistencies in recalculations (Skrzypek and Dunn, 2020b).</p><p>&#160;</p><p>Skrzypek G., Dunn P. 2020a. Absolute isotope ratios defining isotope scales used in isotope ratio mass spectrometers and optical isotope instruments. Rapid Communications in Mass Spectrometry 34: e8890.</p><p>Skrzypek G., Dunn P., 2020b. The recalculation of the stable isotope expressions for HCNOS &#8211; EasyIsoCalculator. Rapid Communications in Mass Spectrometry 34: e8892.</p>
Selection of Collision Energies in Proteomics Mass Spectrometry Experiments for Best Peptide Identification: Study of Mascot Score Energy Dependence Reveals Double Optimum