At a crime scene there are many different methods for gaining information about the crime and the perpetrator. Most commonly, finger prints are used as a method for identifying the offender, this process is known as dactyloscopy. There are three main types of finger prints: latent, patent and plastic finger prints. Latent finger prints are those that have been created by the oils of the body and sweat. Latent finger prints cannot be seen with the naked human eye and will require chemical techniques (later discussed) to help reveal them. Patent finger prints are those that have been left on hard surfaces and do not require any chemical enhancement as they can be seen by the human eye and are therefore simply photographed. The third type of finger prints are plastic prints which are 3D and have the ridges of the true finger, whey can be made using wax, clay or paint, although perpetrators try to generally avoid these.
Image from: Forensicsciencelaw12
To this day, crime scene investigators still use a well known technique known as dusting for finger prints. It has been in use since 1891. A very fine powder is gently spread across the area where the finger prints may be present. The fine particles adhere to the oils in the finger print impression and revealing it, this can then be removed with tape or photographed. The fine particles usually have a pigment which visualises the print and gives greater contrast. There is also a binder present that helps the powder stick to the impression, iron powder is broadly used binder. Well known pigments are carbon particles and flakes of metal can even be used, such as aluminium and copper. Sometimes fluorescent powders are used but this can transfer DNA and change the impression.
Image from: Emaze
A more recent method for revealing finger prints was discovered in 1982 by mistake by Japanese scientists. This technique is known as cyanoacrylate fuming, cyanoacrylate is a molecule which is found in superglue and so people often call this method "revealing fingerprints using superglue". This method works because the oils left from the fingers causes the cyanoacrylates to form polymer chains when the two come into contact. For this reason cyanoacrylate fuming is able to reveal latent finger prints.
Another well known device used by the police force is the breathalyser. There are different types of breathalyzers, most commonly known is the alcosensor and the intoxilyzer. The alcosensor makes use of hydrogen fuel cells with a pair of platinum electrodes. The breath sample of the suspect flows through one side of the fuel cell and ethanol present will be oxidised by the platinum, the products of which are electrons, protons and ethanoic acid (a carboxylic acid). If there is lots of ethanol present in the breath sample, there will be more electrons produced and hence a greater electrical current.
The intoxilyzer uses infrared spectroscopy which is based on the fact that each bond in molecules absorbs a certain wavelength, causing the bond to stretch or bend. Then when analysing the results of infrared spectroscopy, you can determine whether the OH bond is present in the molecule and find other evidence of the presence of ethanol.
Image from: Leoaffairs
The image below shows what the results of Infrared spectroscopy of ethanol may look like:
Image from: Chemguide
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