Comments on the multiple headspace-solid phase microextraction (MHS-SPME) technique for dating inks

Abstract

A recent article by San Román et al. (2015) treats the problem of determining the age of ink writing using the MHS-SPME technique. Ballpoint pen inks are studied since they tend to age more slowly than other writing inks (i.e., they contain solvents that evaporate more slowly than those in other writing inks). The MHS-SPME technique basically determines, via multiple extraction steps, the fraction β of ink solvent molecules that remain after an extraction from a closed vial. During the extraction process, the molecules in the closed vial must be in a state of equilibrium. Using this fraction and the number of solvent molecules extracted during the first extraction, the total number of solvent molecules in the ink sample can be computed. The authors determine the age of ink via the behavior of β with ink age rather than the behavior of total amount of solvent molecules in the ink with ink age. They found that lnβ increases linearly with lnt where t represents the ink age. The authors also found that β is dependent on the amount of ink examined. The purpose of this presentation is to briefly review the MHS-SPME technique and to address these latter two findings.