Pyrolysis GC/MS, in which the specimen disintegrates in the absence of atmospheric oxygen, is a standard technique for investigating paper and written documents. There is, however, a drawback to the process. As Dr Jürgen Bügler from the Bavarian criminal investigation authorities in Munich explains, it is possible to convert relevant molecular components of ink and paper to the gas phase using this technique. "Viable conclusions can only be drawn to a limited extent, however. Attempts to discover the secret of the material composition of written documents via pyrolysis GC/MS led initially to numerous peaks that resulted mainly from the paper investigated and only to a minor extent from the writing media on the paper; the high pyrolysis temperature causes a very large number of low-molecular decomposition products to be created that make it difficult or impossible to interpret the results."
Thermal desorption as the process of choice
In order to be able to draw definite conclusions, a GC injection technology was needed in which the desorption temperature of the specimen could be varied in accordance with the requirements, so that the escaping gases could be determined in several stages, i.e. at different temperatures. Dr Bügler remembers that the basic idea in development of the method was to convert the analytically interesting components of the writing medium to the gas phase and, at the same time, to prevent disintegration of the paper substrate material.
The proof of the pudding is in the eating: the chemist found the solution in thermal desorption. "By having the substrate gas flow over the paper specimen at different temperatures that are in turn below the disintegration temperature of the paper material, we desorb all the relevant volatile and non-volatile analytes consecutively, freezing them out at extremely low temperatures before injection into the separation column", says Dr Jürgen Bügler. Disintegration of the matrix is prevented, so that it is easy to detect the analytes that are present in traces.
And this is how the scientist operates:
The TDS is heated at 280°C for 10 minutes. After a brief cooling phase, the specimen - just a few millimetres of written paper are enough - is put in the desorption tube and is flushed with substrate gas at 40°C in the thermal desorption unit, in order to remove adsorbed volatile substances from the surroundings.
The actual test begins after completion of the equilibrisation phase: at temperatures below 100 degrees, we primarily detect volatile compounds like phenol and benzene derivatives as well as hydrocarbons up to heptadecane. At temperatures above 100°C, non-volatile substances like long-chain carboxylic acids, phthalates and hydrocarbons with higher boiling points can be detected too.
If the suspicious writing has been added to the paper relatively recently, most of the solvents in the writing medium already evaporate at a low desorption temperature. The final residue of volatile substances desorbs from older ink specimens too at 200°C.
While the total amount of the substances detected provides sound evidence for distinguishing between writing media, the focus in determination of age is on investigation of the temperature dependence of phenoxyethanol gas release. The newly developed process has been tested on a large number of the writing media available on the market in extensive comparative studies. Dr Bügler says that it has been a major advantage in this context that a comprehensive collection of writing media with samples from before 1970 up to the present day has been kept in the document laboratory run by the Bavarian criminal investigation authorities for decades now, on the basis of which new methods and processes can be developed and tested intensively in practical environments. GDeußing