Saturday, March 19, 2016

Lipstick Evidence

Lipstick is sticky stuff!
Kissing someone on the cheek with your lusciously lipsticked lips will invariably leave a colourful impression. And, after you've had your sip of coffee your "lips" are left behind in vivid colour on the cup. Lipstick can even end up on tissues after  a momentary touch as you blow nose, or wipe tears from your eyes. And we've all seen movies in which a wife discovers lipstick (not her own) on her husband's collar. Needless to say then that lipstick can be found at a crime scene and is considered to be an example of "trace evidence".
Researchers at  Western Illinois University have been investigating better ways to lift and analyse this lipstick evidence.
In general, lipstick is composed of

  •     65% castor oil
  •     15% beeswax
  •     10% other waxes
  •     5% lanolin (also known as wool wax or wool grease)
  •     5% dyes, pigments and perfume
In other words, most of the mass of a lipstick is made up of lipids (fats, oils and waxes).
To lift the lipstick from the material, the researchers developed a two part process:
  1.   Add an organic solvent to remove most of the oils and waxes.
  2.   Add a basic organic solvent to extract the remaining residue.
The components of the lipstick are now present in solution.
In order to determine the chemical composition of the solutions, they will need to undergo separation and analysis. Three common methods of doing this are:
Different brands of lipsticks have different chemical compositions so they produce different chromatographs.
Using known brands and colours of lipsticks, the researchers can produce a database of chromatographs. When lipstick evidence is found at the scene of a crime, forensic scientists can produce a chromatogram of it and compare this with the database of known brands and colours in order to find a match. In this way forensic scientists can determine the brand and colour of the lipstick. Law enforcement officials could then investigate whether a suspect uses that particular lipstick.
The researchers are still performing analyses of lipsticks, but at this stage they have reported that the best results are achieved with gas chromatography (GC).

American Chemical Society. "Tying lipstick smears from crime scenes to specific brands." ScienceDaily. ScienceDaily, 14 March 2016.

Further Reading:
Percentage composition 
w/w % concentration
Parts per million (ppm) concentration
Lipids (oils, fats and waxes) 
Properties of Carboxylic Acids 
Preparation and Naming of Simple Esters

Suggested Study Questions:
  1. A tube of lipstick contains 4.0 grams of lipstick. Calculate the mass of each of the following components of the lipstick:
    • castor oil
    • beeswax
    • lanolin
  2.  The castor oil used to make the 4.0 grams of lipstick is itself made up of a number of fatty acids notably about 90% ricinoleic acid, 4% oleic acid and 3% linoleic acid. Calculate the mass of each of these fatty acids present in the lipstick.
  3. Why do you think the concentrations of chemical compounds found in lipstick are given as % w/w (percentage by weight or percentage by mass) rather than in units of mol L-1 or ppm?
  4.  What is meant by the term "fatty acid" in chemistry?
  5.  Draw and name the functional group that is present in both carboxylic acids and fatty acids.
  6.  Acetic acid (ethanoic acid) is miscible (soluble in all proportions) in water,  whereas the solubility of pentanoic acid is 3.4 g mL-1, and of hexanoic acid is 1.0 g mL-1. Would you expect oleic acid (C17H34O2) to be soluble in water? Explain your answer.
  7.  What is meant by the term "triglyceride" in chemistry?
  8.  Draw the functional group that is common to both triglycerides and esters.
  9.  Esters are immiscible in water so an organic solvent is used to extract the triglycerides from the lipstick marks. Imagine you have been given samples of cyclohexane, ethanol, and acetone. Which of these do you think would be the best solvent to use on the lipstick mark, and explain your answer.
  10.  Design an experiment that you could perform to test your hypothesis in question 9 above regarding which of the solvents would be best to use on the lipstick mark.

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