NewsWise has an interesting article about the U.S. Secret Service using Thin Layer Chromatography to analyze inks - here's some excerpts:
Inks in one form or another have been around for at least 5,000 years. Scientists can analyze an ink’s components and determine when it was first manufactured, its brand, its composition, and other information by comparing the analysis results to the more than 10,000 inks and matching analyses stored in the International Ink Library managed by the U.S. Secret Service.
To analyze an ink sample, forensic scientists separate its components using planar thin layer chromatography. This process uses solvents to separate the ink into bands of color on a page. Each ink creates its own color bands, thus forming a “fingerprint.” Scientists compare the unknown sample’s bands to known samples to find a match.
And here's that great clip from CSI showing ink analysis with Thin Layer Chromatography: