Nineteen paper samples representing eleven different types of paper from seven manufacturers were analyzed for Cu, Mn, Sb, Cd, Cr, Co, Ag, Pb, Mg, and Fe using electrothermal atomization atomic absorption spectrometry. Unsupervised learning pattern recognition techniques were utilized for data analysis. Only six features were needed to identify 16 of 19 papers completely. The three not completely separated came from different manufacturing runs of the same type of paper. The five elements found to be most powerful in classifying papers were Cu, Mn, Sb, Cr, and Co. For Pb, high correlation with Mn, Cr, and Co was found. Mg, Fe, and Cd exhibited high intra-sheet scatter. The sixth feature used was density. These results indicate that papers can be characterized using trace metal data.The identity and source of paper is quite important to the forensic scientist in judicial problems encountered in forgery, counterfeiting, and felonious use of certificates, securities, wills, etc. It is, therefore, important to be able to state conclusively whether an unknown paper sample does or does not come from a particular batch.The primary source of trace metals in paper is contamination of the chemicals used as additives such as alumina, Ti02, talc, clay, ZnO, and others (1,2). Trace metals may also originate in the wood pulp, process water, or process equipment. Characterization of papers has utilized colorimetric methods for certain metals (3), often in conjunction with paper or thin-layer chromatographic separation procedures. These techniques are time consuming and semiquantitative at best.The potential for characterizing paper by neutron activation analysis has been shown by two independent laboratories (2, 4). In one study (2) where 600 paper samples were investigated for 23 elements, it was found that certain elements such as Na, Mn, Ag, and Cu have a high frequency of occurrence whereas Ta, As, and Sb have a qualitative significance because of their low frequency of occurrence.These results indicated that papers were uniform in regards to their trace element concentration and papers from different manufacturers could be distinguished by qualitative and quantitative tests.Barnard et al.(5, 6) used the scanning electron microscope (SEM) for elemental analysis. The SEM x-ray microanalysis technique examined individual particles near the paper surface. Studies on the variation within a sheet and within a box were performed. Better results were obtained if the samples were ashed before examining them under the SEM. These results on 54 different samples for major elements seem to indicate that SEM data can be used for identification purposes. All elemental amounts determined are relative amounts and not absolute concentrations making these results difficult to compare directly to other techniques which give actual concentrations.1 Present address, Food and Drug Administration District Laboratory, 585 Commercial Street, Boston, Mass.
