Classification and dating of black gel pen ink by ion-pairing high-performance liquid chromatography.

A novel approach for classification and dating of the black gel pen ink entries on document was developed based on ion-pairing high-performance liquid chromatography (IP-HPLC). Ninety-three black gel pens were collected and divided into two groups, dye-based and pigment-based, by preliminary solubility test. The chromatographic conditions for separation of the dye-based black gel pen inks were optimized and the dye components in inks were satisfactorily separated by using 40 mmol/L tetrabutylammonium bromide as ion-pairing reagent. According to the number and the chromatographic retention times of the main dye components, the 50 dye-based inks were categorized into four classes. The inks within a class can be further identified by the percentage of each dye component. The compositional changes of the dye components in the black gel pen ink entries on paper were investigated in light and natural aging conditions and it has been found that the dye components in the ink entries underwent obvious decomposition, and the decomposing extent of the dye components was related to the aging time. The results can provide scientific evidences for dating of the suspicious black gel pen ink entries on documents.