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A statistical methodology for the comparison of blue gel pen inks analyzed by laser desorption/ionization mass spectrometry

Authors
Journal
Science & Justice
1355-0306
Publisher
Elsevier
Publication Date
Volume
51
Issue
3
Identifiers
DOI: 10.1016/j.scijus.2010.10.008
Keywords
  • Questioned Document
  • Gel Pens
  • Mass Spectrometry
  • Laser Desorption/Ionization
  • Discriminating Power
  • Pearson Correlation Coefficient

Abstract

A statistical methodology for the objective comparison of LDI-MS mass spectra of blue gel pen inks was evaluated. Thirty-three blue gel pen inks previously studied by RAMAN were analyzed directly on the paper using both positive and negative mode. The obtained mass spectra were first compared using relative areas of selected peaks using the Pearson correlation coefficient and the Euclidean distance. Intra-variability among results from one ink and inter-variability between results from different inks were compared in order to choose a differentiation threshold minimizing the rate of false negative (i.e. avoiding false differentiation of the inks). This yielded a discriminating power of up to 77% for analysis made in the negative mode. The whole mass spectra were then compared using the same methodology, allowing for a better DP in the negative mode of 92% using the Pearson correlation on standardized data. The positive mode results generally yielded a lower differential power (DP) than the negative mode due to a higher intra-variability compared to the inter-variability in the mass spectra of the ink samples.

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