Study on the Variation of Three Solvents in Black Gel Pen Handwriting with Writing Time Based on Gas Chromatography Technology

Based on the principle of solvent volatilization, the solvent components of glycerol, diethylene glycol and triethylene glycol in diachronic handwriting samples of black gel pen were determined by GC-MS and GC-FID, and the curves (aging curves of handwriting) of the contents of 3 solvents with writing time were drawn. Nine black gel inks with glycerol, diethylene glycol and triethylene glycol as the main solvents were selected from 196 kinds of black gel pens purchased in the market by GC-MS. The pen was used to draw lines on A4 paper against a ruler. Three samples were made for each pen once a month, and the samples for 21 consecutive months were collected and stored in a closed drawer at room temperature. Ten ink spots were taken from each A4 paper with a punch at a diameter of 0.5 mm, and the solvent was extracted by methanol solution containing 5 mg·L^-1 ethyl benzoate (internal standard). The extract was introduced into GC-FID, and the solvents adsorbed on the DB-FFAP chromatographic column were separated under the temperature programmed condition and detected by FID. 50 mg·L^-1 mixed standard solution was prepared for quantification by single point method. Aging curves of handwriting were drawn with writing time as abscissa and the mass of solvent in the corresponding unit writing length as ordinate. The results showed that the initial contents of solvents in each black gel pen handwriting were different, but the inflection points of each solvent content changing with writing time were basically the same. The contents of glycerol, diethylene glycol and triethylene glycol showed inflection points around the seventh month, the third month and the ninth month, respectively. After the inflection point, the solvent contents tended to 13, 3, 7 ng·mm^-1. The proposed method was used to analyze a real case (authenticity identification of the receipt), and 20, 34 ng·mm^-1 glycerol was detected in the two receipts, which turned out that both receipts were recently forged.