0.2 Forensics lab  (Page 2/4)

The active ingredient in superglue is cyanoacrylate. To cure for adhesive applications, it only needs hydroxyl ions, which are found in any trace amounts of water making superglue applicable to most materials. With the help of water, cyanoacrylate molecules undergo anionic polymerization to make a very strong plastic mesh. In crime scene investigations, superglue reacts with the traces of amino acids, glucose, sweat, fatty acids, and proteins in the latent fingerprint and the moisture in the air to produce a visible, sticky white material that forms along the ridges of the fingerprint. The final result is an image of the entire latent fingerprint.

In order for cyanoacrylate to react with amino acids and sweat, it must be in the gaseous state. It has a boiling point between 49°C and 65°C so low heating can vaporize it. The presence of moisture in the air can expedite the reaction with the latent print. The main components of the vaporizing chamber will be shown under procedure; a) glass dish to contain fumes b) vial with water to provide moisture c) hot plate on low heat and d) bottom of aluminum can to hold superglue.

Silver nitrate: Silver nitrate reacts with chlorides in the fingerprints, to give the insoluble salt, silver chloride, which rapidly turns black on exposure to light. This method is not suitable for fabrics or rough surfaces.

After developing the latent impression it is photographed and lifted with a clear tape to be placed on a backing card with a contrasting background. It can then be entered into a computer, which allows it to be quickly and easily recalled and compared to the fingerprint of a suspect. Identification depends on showing a minimum of twelve matching characteristics in the ridge pattern. When these points of comparison are shown, it is considered that the proof of identity has been established.In this lab you will be developing your fingerprints using 2 methods: the first uses iodine vapor whereas the second uses fuming cyanoacrylate

Part 2. identification of inks

In document examination, the examination of inks often plays an important part. As a rule, the examination centers on the question as to whether the ink of certain passages or of alternations in the text is identical with the ink found in the possession of the suspect. For this reason the examination of questioned documents seldom consists of a complete determination of the inks in question but is usually restricted to a comparative examination of certain properties of these inks.Many different nondestructive techniques of the examination of inks are available: reflected infrared radiation, reflectance microspectrophotometry, lasers and scanning electron microscopy. Unfortunately, the reflectance methods are often subject to interference effects from“bronzing”or“sheering”of the ink.Semi-destructive methods involve high-performance liquid chromatography (HPLC) and thin layer chromatography (TLC). Most chromatographic techniques are based on the minute sampling of a single written character representative of the questioned text. Small samples of ink bearing paper are removed from the document, they are then extracted with a suitable solvent, and the components of the solution are separated using HPLC and TLC. If the inks being compared show different composition, they did not come from the same pen.In this lab you will be separating dyestuffs of several ballpoint pens using thin layer chromatography. Comparison of the dye composition will allow you to find out which pen was used by your TA to spot the TLC plate.