Here is another example of forensic application of HPLC that allows to classify and date the black gel pen ink entries on documents.

Gel pens first came out in 1984 in Japan and quickly became popular throughout the world due to their solid line, bold colors, and low cost. They are also regarded as environmentally friendly since the ink gel is water-based. The general design of a gel pen is similar to that of a ballpoint pen, however, the gel ink is different and consists of pigment suspended in a water-based gel. The pigments are typically copper phthalocyanine and iron oxides, and the gel is made up of water and biopolymers, such as Xanthan gum and tragacanth gum, as well as some types of polyacrylate thickeners.

The increasing popularity of gel pens has seen them used to sign many legal documents and as a direct result, they have also become the subject of forensic scrutiny in cases of suspected fraud. Several well-established methods exist for analyzing inks from ballpoint and fountain pens, but not for inks from gel pens.

Scientist from Analytical and Testing Center of Beijing Normal University and Beijing Forensic Science Institute have published an extensive study on the black inks used in gel pens. They collected 93 gel pens and divided them in two groups: 50 dye-based and 43 pigment-based inks. Straight lines of ink were drawn on paper which was stored for various lengths of time to age under natural conditions in the dark at room temperature and some samples were aged under UV light. Aging is an important forensic aspect for ink analysis. If an old document has been altered recently, the newer ink will have a different composition to the old, aged ink and should be detectable by the appropriate technique.

The aged ink was extracted from paper by a mixture of organic solvent acetonitrile and tetrabutylammonium bromide (TBAB) and then analyzed by ion-pairing (IP) HPLC with UV detection. The chromatography showed a clear distinction between non-aged and aged samples – it has been found that the dye components in the ink entries underwent decomposition, and the decomposing extent of the dye components was related to the aging time.

Overall, the results provide evidence for dating suspicious documents that have been prepared using black gel pens. The next stage will be to establish a library of data for the inks from different manufacturers for more accurate characterization.

Reference:

J. Chromatogry. A 2006, 1135, 57:
“Classification and dating of black gel pen ink by ion-pairing high-performance liquid chromatography”
Yi-Zi Liua, Jing Yub, Meng-Xia Xie
Analytical & Testing Center of Beijing Normal University, Beijing 100875, China
Beijing Forensic Science Institute, Beijing 100054, China

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