Abstract: A method for tagging an ink with an isotopic tagging composition is provided. The method includes tagging the ink with a tagging composition having an artificial abundance of at least one isotope of an element to produce an authentic ink having an artificial abundance of the at least one isotope of the element which exceeds a natural abundance thereof in the authentic ink. A method for authenticating unidentified ink samples by comparing isotope abundances and/or abundance ratios in the unidentified ink samples with isotope abundances and/or abundance ratios in authentic inks tagged with isotopic tagging compositions is also provided. The method also includes tagging a pigment-based ink with trityl-protected thiol additives and methods for identifying inks containing the same.