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Detection of Multiply Charged Dyes Using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry for the Forensic Examination of Pen Ink Dyes Directly From Paper

NCJ Number
Jamie D. Dunn M.S., John Allison Ph.D.
Date Published
September 2007
7 pages
This study examined whether matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) could detect multiply charged dyes of pen inks directly from paper.
The study successfully detected more than 20 polyionic dyes that could be used in manufacturing inks by using MALDI MS direct from paper. This was done with the matrix 2-(4-hydroxyphenylazo)benzoic acid and the additive diammonium hydrogen citrate. This method expands document examiners' ability to detect ink dyes. In 2001, it was first demonstrated that the organic dye crystal violet from a blue ballpoint pen ink could be detected directly from paper by using laser desorption mass spectrometry (LDMS). LDMS is appealing for questioned document examination, since no extraction step is required. Molecules are desorbed and ionized directly from the paper's surface, and many mass spectra can be generated from the ink in a single pen stroke. LDMS has been used to analyze dyes from blue, black, and red ballpoint pen ink dyes; however, hundreds of dyes that can be used in pen inks contain multiply charged groups. If there is no chemical mechanism through which the dye could be desorbed and ionized with a single charge, then it cannot be detected by LDMS. When it is known that dyes are present but no signals are detected, MALDI target crystals can be grown on a pen stroke on paper, incorporating polyionic soluble dyes, and positive/negative MALDI mass spectra can be obtained. The authors have successfully detected dyes that have contained a maximum of eight sulfate groups. With LDMS and MALDI MS, neural, anionic, cationic, and polyionic dyes, as well as insoluble pigments, can be analyzed. The descriptions of materials and methods address MS, LDMS, MALDI MS, the placing of dyes on paper, the making of pen strokes on paper, and thin-layer chromatography. 10 figures and 20 references

Date Published: September 1, 2007