Distinguishing Different Kinds of Blue-Black Ink from Handwriting and the Regularity of Artificial Aging of the Ink Used in Handwriting by HPLC

Several sets of parallel straight lines were drawn on sampling papers separately with the blue-black ink samples and the standard dye solutions prepared individually from acid ink blue G and direct sky blue 5B,which are the dyes used commonly for preparation of the blue-black ink.A portion of 1 cm in length was cut from the paper with the writing,and the ink on the paper was extracted with 0.1 mL of extraction agent(a mixture of methanol,H2O and glacial acetic acid mixed in the ratio of 50 to 50 to 0.5 by volume).An aliquot of 10 μL was taken and introduced into the instrument for HPLC analysis.C18 chromatographic column and mobile phase solution containing methanol and 0.08 mol·L-1 tetrabutylammonium hydroxide solution(mixed in the ratio of 50 to 50 by volume) were used in the reverse phase-ion-pair chromatographic separation.Based on the results of HPLC analysis,kind of the blue-black ink was identified.Artificial aging treatments of the ink-writing were performed by methods of heating and irradiation with ultra violet light or with natural light,and laws of the change in relative contents of the respective dyes under various aging treatments were found,which furnished the scientific basis for the determination of time of formation of the writing.