The Analytical Chemistry of Ink Ageing

Group MeetingMonday, Dec 02, 2013

Ink

• Many different ink compositions• Solvent(s)• 2-Phenoxyethanol• Resin(s)• Pigment(s)

Paper

• Cellulose• Treated cellulose:• Glossy• Made from recycled material• Coloured• “Hairy” or “smooth”

Pen stroke

• Straight line versus curved line• Pressure applied

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

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)

Dynamic ink ageing methods

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

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

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

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))]

Method 3: Determining the ink age factor V%

• “Headspace GC-MS”: two temperatures for desorption

• No solvents

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

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...

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

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)