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.