To expose forgeries, the Document Laboratory of the Zurich Cantonal Police successfully applies Thermal Desorption coupled with GC/MS.
Forensic science has contributed greatly to exposing forgeries. Today, the forensic scientist is in a position to determine if handwritten or printed lines are original or if they have been manipulated, if documents have been altered or if they have been forged entirely, according to the forensic scientists Andreas Rippert, Ph.D. and Rolf Hofer, Ph.D. of the Department of Forensic Sciences of the Zurich Cantonal Police. Based on their method, it is possible to also pinpoint the time when a document was forged, which can help solve open cases. Cut-outs of less than 5 mm diameter from the document are sufficient for analysis and conclusive findings.

Pyrolysis-GC/MS complicates the picture
Pyrolysis GC-MS is often used to examine paper and documents. The sample is pyrolyzed under anaerobic conditions, i.e. under a flow of oxygen-free inert gas. While Pyrolysis GC/MS provides a lot of information, the technique does pose some challenges. Attempts to reveal the document’s material composition using Pyrolysis GC/MS result in a large number of peaks, which could originate either from the ink or from the paper. In addition, the high temperatures used give rise to decomposition products that further complicate data interpretation.

Thermal Desorption – the method of choice
To reach a clear conclusion about the authenticity of a document, a GC introduction method is needed that provides the possibility of varying, i.e. programming the temperature over the course of the Thermal Desorption / Thermal Extraction step. Organic compounds are extracted from the sample in successive steps at different temperatures. As carrier gas sweeps across the paper sample at increasing temperatures, the relevant analytes ranging from volatile to semi-volatile, are successively desorbed and cryofocused prior to introduction to the GC/MS system.

Analysis and results
At temperatures below 100 °C, volatiles are extracted, especially phenol and benzene derivatives - as well as other hydrocarbons up to heptadecane. At temperatures above 100 °C less volatile compounds such as fatty acids, phthalates, and higher-boiling hydrocarbons are extracted. If ink has been applied to the document during the past weeks, i.e. in case of "fresh tracks", volatile hydrocarbons are emitted in clearly detectable quantities, as are semi-volatile compounds like phenoxyethanol and phenoxyethoxyethanol. At 210 °C, final residues of volatile substances are desorbed even from older ink samples. While the complete range of detected substances is required for conclusive classification and differentiation of written material, phenoxyethanol and phenoxyethoxyethanol are the main indicators when it comes to age determination.

TDS/GC/MS system
The document laboratory of the Zurich Cantonal Police uses a GERSTEL Thermal Desorption System TDS in combination with a GERSTEL-Cooled Injection System CIS and an Agilent Technologies GC 6890 GC. The system is used to differentiate between ink samples.