Forensic Ink Analysis

The term "line crossing" in forensic document analysis refers to an instance where two lines, prepared by either writing or printing, intersect. It is often crucial to determine the sequence of application, i.e. which line was printed or written at a later time. Surface analysis and especially ToF-SIMS can be of tremendous help in deciding so.

Printer Ink
Signature 1
Signature 2

Inks have usually ingredients which give characteristic signals in static SIMS. When mapping their lateral distribution, one can often determine the sequence of application based on patterns like attenuation (when the ink of the later line masks the other) or smearing (when the ink of the later line dissolves the other). The example shows a macroscan image (size 15 x 15 mm2) from a document with print and two crossing signatures. An optical image of the corresponding signature is given on the right side. All three inks have characteristic secondary ion signals. The lateral distribution of these signals clearly shows that signature 1 was written later than signature 2.