Ink analysis may be an important part of the investigation of questioned documents , including forged checks, wills, or altered records. Although all blue or black inks may look the same, there can be some important differences in their chemical composition. These can be revealed by laboratory analysis and the results can help assess whether there have been any additions or alterations to a document.
Analysis of documents under a microscope can be informative as a first step. The investigator may be able to see slight changes in ink color, not visible to the naked eye, that could be indicative of alterations, or there may be suggestions of obliteration and overwriting. The ink itself may be analyzed by non-destructive or destructive testing, depending on whether a sample needs to be taken from the document, a process that would alter it. It is preferable to try the non-destructive approach first, so that the document is left intact.
The main method of non-destructive ink analysis is micro-spectrophotometry . This involves scanning the ink with ultraviolet or infrared light to record its spectrum, that is, the wavelengths of light it absorbs. Some inks fluoresce, or emit light, on exposure to ultraviolet, while others disappear. Each ink should give a distinct pattern or spectrum on exposure to ultraviolet or visible light. Put simply, this is a way of discovering the true "color" of the ink. The spectrum of the ink on the document can therefore be compared with the spectra of standard inks. Other non-destructive or minimally destructive methods, such as Raman spectroscopy , can be used to supplement micro-spectrophotometry. It can be very informative to scan the document with infrared light because, at high frequencies, ink is invisible but pencil marks which may lie underneath will show up.
The main method of destructive testing of ink is known as thin layer chromatography (TLC). In reality, it is not very destructive to the document if done with care. However, a photographic record of the original document is taken before the procedure is started. A tiny sample of the inked paper is punched out using a thin, hollow needle; a hypodermic syringe is ideal. The investigator avoids places where the pen has changed direction or where ink lines meet. This avoids any interference with subsequent handwriting analysis . The sample is placed in a test tube with a solvent that dissolves the ink. Next, a tiny spot of the sample solution is placed onto a strip of paper, alongside spots from various reference ink samples. The paper is placed in a beaker containing a small amount of another solvent. It is positioned so that the paper dips into the solvent but the spots of sample remain dry. The solvent is drawn up the paper through capillary action and the sample spots move up with it. Chromatography means "writing with colors" and the chemical components of the ink, which are, of course, colored, travel with the solvent at a speed that depends upon their composition.
The end result with TLC is a pattern of colored spots, known as a chromatogram, for each ink. Different inks will have different chromatograms. If the sample ink has the same chromatogram as one of the reference inks, it suggests they are the same, and so identification can be made. The United States Secret Service has a reference ink database and the U.S. Treasury has a database of ink thin layer chromatograms which can be very informative.
Another technique called high performance liquid chromatography (hplc), which can be used as an alternative to tlc. Hplc involves injecting the ink sample onto a long thin metal column that is then washed over with a mixture of solvents, carrying the ink components one at a time to an electronic detector. Between them, non-destructive and destructive methods of analysis can identify more than 90% of ballpoint pen inks.
Just because a document appears to have been written throughout in the same ink does not mean it has not been altered or added to. The same ink fills many different writing implements. Pens, usually ballpoint pens, can be distinguished from one another by looking at the non-inked areas known as striae within a line of writing with a microscope. Striae arise from imperfections within the ball or ball housing of the writing pen.
However, it can be difficult to age an ink, unless it is known for sure that the ink did not exist when the document was said to have been prepared. From the 1990s, some ink manufacturers started to add a chemical tag to their products to indicate the year of manufacture. This enables chemical analysis to age the ink, but the tests to do this remain rather expensive.
see also Document forgery; Questioned documents; Secret writing.