Sometimes, the key to determining authenticity of a document lies in the ink that the writer used. Inks that appear the same, physically, may be much different, chemically. This distinction helps the examiner determine whether the same ink was used for each page or word of a document and may even help reveal whether a particular ink existed at the time the document supposedly was prepared.
One nondestructive method of ink comparison is called microspectrophotometry. This process enables the examiner to accurately determine whether the colors of the two inks match by comparing their light transmission, absorption, and reflection characteristics.
Another method for comparing ink samples is thin-layer chromatography (TLC), which includes following four steps:
Very small samples of the inked paper are punched from the written lines using a thin hollow needle.
The tiny pieces of paper are placed in a test tube, and a solvent that dissolves the ink is added.
A drop of the solvent solution, which now carries the ink, is placed on a paper strip along with drops of several known control inks.
The strip is dried and then dipped into another solvent that migrates up the paper strip, dragging the inks along with it.
The distances that all of the inks migrate along the strip are determined by the respective sizes of their molecules. This process separates the inks into bands. Whenever inks from two pages of a questioned document are tested and they yield different bands, the writing on the two pages was done with two distinctly different inks.
An extensive ink reference database is located at the Ink Library, a part of the U.S. Secret Service Forensic Services Division's Questioned Document Branch. In addition, the U.S. Treasury Department maintains an extensive database of the TLC patterns of commercial inks.
In a more recent development, many manufacturers began adding fluorescent-dye tags to their products so that they are easier to identify. And because the tags are changed annually, examiners can readily determine the year that the product was manufactured.
