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2013 02 12_matthew s. mac_lennan_ink ageing chemistry

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2013 02 12_matthew s. mac_lennan_ink ageing chemistry

  1. 1. The Analytical Chemistry of Ink Ageing Group Meeting Monday, Dec 02, 2013
  2. 2. Ink • Many different ink compositions • Solvent(s) • 2-Phenoxyethanol • Resin(s) • Pigment(s)
  3. 3. Paper • Cellulose • Treated cellulose: • Glossy • Made from recycled material • Coloured • “Hairy” or “smooth”
  4. 4. Pen stroke • Straight line versus curved line • Pressure applied
  5. 5. Analysis – Database scouring • Curating a database of pen inks from various companies and years • Create drying curves for each ink • Match the unknown ink to a curve(s) in the database based on a sampling
  6. 6. Analysis – Dynamic Ink Ageing methods • Three methods covered • Rate of decrease of volatile components of ink (R threshhold value) • Rate of decrease of solvent extractability of ink (D threshhold value) • Determining the ink age factor (V% threshhold value)
  7. 7. Dynamic ink ageing methods
  8. 8. Method 1: Rate of decrease of volatile components R% • Hole punch sampling of ink • One sample is dissolved (P); the other sample is heated then dissolved (PT) • R% = [(P-PT)/P]*100
  9. 9. Method 1: Rate of decrease of volatile components R% • Expect less solvent to be present after heating • Small solvent present = small loss after heat = Old sample • Large solvent present = large loss after heat = New sample • If the difference between before and after heating is large, then (P-PT) will be large. • (P-PT)/P will be closer to = 1
  10. 10. Method 2: Rate of decrease of solvent extractability (D) • Hole punch sampling (duplicates) • One sample duplicate is not heated. One dissolved in weak solvent, the other in strong solvent • The other sample duplicate is heated. One dissolved in weak solvent, the other in strong solvent
  11. 11. Method 2: Rate of decrease of solvent extractability D • Expect less solvent to be present after heating • Uses two solvents: slow-extracting weak solvent and fast-extracting strong solvent • Achieves the same effect as Method 1, but has a two-solvent descriptor of phenoxyethanol differences • D = P – PT; • D = 100*[(Mw,u/(Mw,u+Ms,u)) - (Mw,h/(Mw,h+Ms,h))]
  12. 12. Method 3: Determining the ink age factor V% • “Headspace GC-MS”: two temperatures for desorption • No solvents
  13. 13. Method 3: Determining the ink age factor V% • Statistical Neumann’s trend test • n = # measurements (x) chronologically ordered; s = standard deviation • Given a probability level (Conf. Interval), what is Q in relation to the threshhold value
  14. 14. Method Validation • Interlaboratory validation is a huge problem • Needed for application to real cases • Forensic science tends to be a bit secretive • Current ink dating methods need to be tested randomly by other labs for their robustness etc...
  15. 15. Interpretation of ink ageing analysis • Likelihood ratio • Accepting normal distribution • Two or more hypotheses • d = a given value of D • Likelihood of one hypothesis being more true than the other
  16. 16. Conclusion • Because there is not an extensive body of research supporting and validating these methods, forensic scientists need to present their findings with humility and full awareness of the advantages and disadvantages. • Confine reports to within the empirically tested bounds of the field, intelligently understood (not relying on a single study or conference proceedings)

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