In the Western world, the use of seals is largely limited to legal documents but in China, seals are in common everyday use. They are used by artists and scholars, as well as the legal profession, but many members of the public also own personal seals which they use for official purposes and on cheques, in lieu of a signature. Strictly speaking, although they tend to be called seals, they are really a type of stamp.

The stamps themselves can be made from several materials such as stone, wood and ivory. Individuals can also design their own by carving the design on a block of soapstone. They are generally used in conjunction with a red ink paste which is pressed onto the document.

As with most forms of signature, the seals/stamps are often the subject of forensic examination when the validity of documents is called into question. For instance, matching two or more seals on a given document can prove that no underhand alterations have taken place. One of the best ways to compare seals is to examine the ink paste that was used and Chinese researchers have adopted a novel approach based on the combination of HPLC and GC/MS.

Meng-Xia Xie and colleagues from Beijing Normal University and the Institute of Beijing Criminal Science and Technology collected 69 red ink pastes or oils that are commercially available in China to test their methods. The materials were impressed on printing paper and stored under different conditions.

Following extraction with dimethylformamide over 12 h, the inks were analysed by HPLC with a programmable multi-wavelength detector and a C₁₈ column. Since most ink dyes used in the seal inks are anionic compounds, they were resolved under ion pairing conditions. The optimum ion pairing agent was tetrabutylammonium bromide and it was used in a gradient elution program with acetonitrile as organic modifier.

With detection at 500 nm, which is close to the absorption maxima of the red dyes, the inks could be grouped into four classes. Class I contained 32 ink pastes with one major chromatographic peak at 8.4 min, class II had 13 pastes with one major peak at 18.1 min and class II comprised 10 inks with two peaks at 8.4 and 18.1 min. The fourth class had two or more peaks with dissimilar retention times. The dyes were identified from their retention times as Bronze Red C (8.4 min), Pigment Fast Red R (15.4) and Scarlet Powder (18.1 min).

Since this procedure alone could not differentiate the inks, it was augmented by HPLC with detection at 254 nm, which detects some of the ink additives. In practice, detection at more than one wavelength is a routine task in the analytical lab thanks to multi-wavelength detectors. The combination of two wavelengths did provide better differentiation but too many inks were still grouped together.

The researchers turned to GC/MS to provide a third level of analysis. Good separation revealed the presence of phthalates, alkylphenols, hydrocarbons, poly(ethylene glycol), and unsaturated acids. Different combinations of these helped to classify the inks to some extent. However, a combination of HPLC and GC/MS gave the best discriminatory power, allowing almost 90% of the inks to be classified.

The teams also used HPLC and GC/MS to study degradation of the dyes over time, under fluorescent light, UV light, or natural aging conditions. They found that the decomposition rates depended on the structures of the dyes, the aging conditions and the other additives present. However, there was a good relationship between the changes in composition of the inks and the aging time, which would help document examiners to estimate the date at which a document had been stamped.

So, most of the inks in the 69 seal pastes could be differentiated by the joint HPLC and GC/MS procedures and an estimate of the age of the paste could also be attempted. The main disadvantage of the method is the destructive nature of the sampling, since evidence, or some part of it, will be destroyed during ink extraction.

Related links:

• Journal of Separation Science 2009, 32, 2919-2927: "Differentiation and dating of red ink entries of seals on documents by HPLC and GC/MS"

Article by Steve Down

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