Fingerprint Powders

University of Queensland scientists have just completed a study revealing that the human factor involved in the process of identifying a set of fingerprints could lead to errors and false convictions of innocent people.
In the study 37 qualified fingerprint experts and 37 novices were given pairs of fingerprints to examine and decide whether a simulated crime scene matched a potential suspect or not. Some of the print pairs belonged to the "criminal" while others were highly similar but actually belonged to an "innocent" person.
The experts correctly matched just over 92 percent of the prints to the criminal. But, they mistakenly matched 0.68 percent of the prints to the innocent person.

Fingerprint powders are fine powders used in dusting for fingerprints by crime scene investigators. Fingerprint powders are often white or black in colour. White powders would be used on dark surfaces while black powders would be used on light coloured surfaces.

Examples of powders that have been used to dust for fingerprints:

White Powders Black Powders Calcium oxide Chalk Titanium dioxide White tempera (starch + titanium dioxide) Haddonite white (titanium dioxide + kaolin + chalk) Lanconide (zinc sulfide + zinc oxide + barium sulfate + titanium dioxide + bismuth oxychloride + calcium carbonate) Charcoal Graphite Lampblack Dragon's blood (Daemonorops draco plant resin) Haddonite black (lampblack + graphite + powdered acacia) Dactyl black (graphite + lampblack + gum acacia)

The fingerprint powder must be fine enough to show fingerprint details. The finer the powder is the better it should be.
To be a good fingerprint powder the powder must adhere to, or stick to, the fingerprint but not to the surface the fingerprint is on.


Reference:
Association for Psychological Science (2011, May 11). Dusting for fingerprints -- It ain't CSI. ScienceDaily. Retrieved May 15, 2011, from http://www.sciencedaily.com­ /releases/2011/05/110511162536.htm

Further Reading:
Pure Substances and Mixtures
Elements and Compounds
Allotropes
Writing Ionic Formula
Calculating Percentage Composition

Study Questions

1. What are the main chemical constituents of:
   
   • chalk
   • lampblack
   • charcoal
   • graphite
2. Draw up a table dividing the fingerprint powders into pure substances and mixtures.
3. For the pure substances listed in question 2, draw up another table dividing these up into elements and compounds.
   
4. Write the chemical formula for each of the following:
   
   • calcium oxide
   • titanium dioxide
   • zinc sulfide
   • zinc oxide
   • barium sulfate
   • calcium carbonate
5. Calculate the percentage composition of each of the following compounds:
   • calcium oxide
   • titanium dioxide
   • zinc sulfide
   • zinc oxide
   • barium sulfate
   • calcium carbonate
6. Titanium dioxide features in many of the recipes for making white fingerprint powders. What properties of titanium dioxide make it useful for this purpose?
7. Graphite is a common constituent in many black fingerprint powders. What properties of graphite might make it particularly useful as a fingerprint powder?
8. For the University of Queensland study, give examples of the following types of variables:
   
   • independent variable
   • dependent variable
   • controlled variables
9. Based on the information provided in the article, do you think the University of Queensland study was a fair test? Explain your answer.
10. Design an experiment to test the statement that graphite is a better powder than charcoal to use for detecting fingerprints on the surface of tiles.
    
    

VMD for Fingerprinting

Forensic experts at the University of Abertay Dundee and the Scottish Police Services Authority (SPSA) are researching vacuum metal deposition (VMD) to recover fingerprint ridge detail and impressions from fabrics.

Vacuum metal deposition, VMD, is a common method of depositing a thin film on a substrate.
The source metal is evaporated in a vacuum which allows the vapor particles to travel directly to the target object, the substrate, where they condense back to the solid state. Evaporated materials deposit non-uniformly if the substrate has a rough surface, and, because the evaporated material attacks the substrate mostly from one direction, protruding features block the evaporated material from some areas which is called "shadowing" or " step coverage".

The Scottish scientists have been using gold and zinc in a VMD process to recover fingerprint marks on fabrics. The fabrics are placed in a vacuum chamber then gold is heated up to evaporate it. The gold particles spread out in a thin film over the fabric. Zinc is then heated up, and the zinc particles attach to the gold particles where there are no fingerprint residues. The fingerprint ridges show up as clear fabric, but where there are no fingerprint ridges the distinctive grey colour of the zinc metal is seen.

While only 20% of the public are classed as "good donors" for leaving fingerprints, the researchers have had great success in revealing the shape of a handprint on a number of fabric types. Handprints could help the police piece together a timeline of events which could be used to provide evidence in cases where someone was pushed, or grabbed, in a particular area of their clothing. For example, an impression of a palm print on the back of someone's shirt might indicate they were pushed off a balcony, rather than jumping.

Reference
Joanna Fraser, Keith Sturrock, Paul Deacon, Stephen Bleay, David H. Bremner. Visualisation of fingermarks and grab impressions on fabrics. Part 1: Gold/zinc vacuum metal deposition. Forensic Science International, 2010; DOI: 10.1016/j.forsciint.2010.11.003

Further Reading
Physical and Chemical Changes

Study Questions

1. What do Chemists mean when they refer to evaporation?
2. What do Chemists mean when they refer to condensation?
3. Are evaporation and condensation examples of chemical or physical changes? Explain your answer.
   
4. Write a chemical equation to represent the evaporation of solid gold as described above in the process of vacuum metal deposition.
5. Do you think the equation you wrote above is an example of an evaporation process? Explain your answer.
6. Write a chemical equation to describe the process of gold vapor condensing on a fabric as described in the process of vacuum metal deposition.
7. Give the name for the change of state being described in each of the following:
   
   • heating solid gold until it forms a liquid
   • heating solid gold in a vacuum so that it forms a vapor
   • cooling gold vapor in a vacuum so that if forms solid gold
   • cooling gold liquid until it forms solid gold
   • heating liquid gold until it forms gold vapor
   • cooling gold vapor until it forms liquid gold