Module 5 - Polygraph Testing & Forensic Document Analysis

Chromatography

Chromatography is a scientific technique used by forensic experts to determine the type of pen used to create a suspect document. During this technique, mixtures are separated to analyze, identify, or quantify their individual components. Chromatography is used by forensic document analysis experts to separate the chemical components within ink samples to identify the source of the ink. 

The simplest way to perform this technique is the paper chromatography method that involves applying a small amount of ink to an absorbent material such as paper (such as ordinary paper or filter paper).  The molecules making up the ink are absorbed into the paper and then begin to migrate upwards and separate.  To speed this upward migration, a solution of acetone or ethanol is added. As this movement occurs, the chemical components are deposited at certain points along the paper according to their relative mass. 

This movement of ink molecules through paper creates a chromatographic pattern.  Every compound (including ink) creates a different chromatographic pattern according to the unique components.  Thus, the blue ink in two different pens might look the same, but these samples can be differentiated from one another according to the distinct chromatographic patterns they create.

The chromatographic techniques that forensic experts use are more elaborate and precise than paper chromatography; however, they are based on the same principle.  Forensic document experts use equipment that is automated and the results are transferred into a computer for analysis.  Using a computerized system speeds the process while at the same time increases the accuracy of the results.

Did you know?

Since 1968, the United States Treasury Department has maintained the International Ink Database. This database contains the chemical composition and chromatographic patterns of over 6 000 different types of ink for forensic comparison purposes. Every year this database is updated as major manufacturers of inks tag their inks or add unique trace dyes to their inks. This tagging program allows inks to be dated to the exact year they were made.

Continue to try a chromotography lab...