While committing a crime, the culprit, even after taking precautions, leaves fingerprints according to Locard’s principle of exchange. Those left fingerprints are hidden. We cannot see them with our naked eyes. To visualize, collect, and preserve them, we have to use some developing techniques we will discuss here.
To develop latent fingerprints at the scene of a crime, we can use mechanical methods that include the use of fluorescent powders or chemical methods, including iodine fuming, ninhydrin, cyanoacrylate, and silver nitrate method.
In addition to the methods to develop latent prints, we will be discussing the collection and preservation of the latent prints collected to send them to the labs for its analysis.
Physical Methods to Develop Latent Fingerprint
The process of transforming the latent Fingerprints into visible prints by the use of powders or chemicals is known as development. We can use many scientific methods to develop latent prints. Most of the latent prints are developed by the use of colored powder.
Prints on non-absorbent surfaces like glass, mirror, tile, printed wood or visualized by the application of powders.
The subsequent points are to be noted before the development of latent impressions by the powder method: –
- Take a brush that should be feathery in touch.
- Choose a contrasting powder.
- Take a little quantity of powder on a plate and dip the brush in the powder.
- Lightly dust the brush over the latent print.
- Avoid excess friction while applying the powder on the surface; otherwise, it may easily damage the ridges.
- Small particles of the powder will adhere to the oil deposit of the finger.
- As soon as the ridges become visible, the brush should be moved in the direction of the ridgeline to remove the extra powder on the print.
Chemical Methods to Develop Latent Prints
Chemical methods are used for developing chance prints because they do not rely on moisture remaining in print but rather on a chemical reaction taking place in between the developer and sweat pores. Prints on absorbent, soft pores materials such as paper, cardboard, clothes are generally treated with a chemical. The subsequent methods are describing the ways to develop prints.
- Iodine Fuming Process – Iodine is a solid crystal that, upon being heated, is transformed into vapors without passing through a liquid phase. This process is called sublimation. Iodine fumes are sprayed on the suspected surface. Fumes are absorbed by fatty or oily matter present in the latent residue. Prints appear in yellow-brownish color. Unfortunately, iodine prints are not permanent and begin to fade out once the fuming process is stopped. Therefore it is necessary to get the photograph of the print to make a permanent record.
- Ninhydrin Method – The development of the latent prints with ninhydrin depends on the traces of amino acid present in the sweat residue. A solution is prepared by mixing the ninhydrin powder with a suitable solvent like acetone or ethanol. The solution is applied to the suspected surface by spraying prints that begin to appear within an hour at room temperature. The development can be extended by the application of heat in which the treated specimen is heated on a hot tray up to 100 degrees Celsius. The prints appear as purple red color. This method has developed latent print on paper as old as 15 years.
- Cyano-Acrylate – In this process, on heating super glue, the glue vapors adhere to the surface and builds up the ridge detail. An air-tight container made of glass or plastic material is used to prevent the escape of glue vapors. The specimen is placed in that container in such a way as to expose all areas to the vapors. All visible latent prints are then photographed. To accelerate the process, the glue is placed on a hot plate and heated approx and heated up to 980 degrees Celsius.
- Silver Nitrate Method – It develops prints from several weeks to several years. A 3-5% silver nitrate solution in distilled water (3-5 grams of silver nitrate is dissolved in 100 ml distilled water) is brushed lightly onto the print can b dipped in the solution. Silver nitrate solution reacts with the sodium chloride present in sweat residue. After this reaction, an insoluble silver chloride precipitate along the ridges removes the excess solution with the water and then allows it to dry in darkness. Later the print is exposed to light. Latent prints appear as black or reddish-brown in color.
Collection and Preservation of Latent Fingerprints
- Prints should be lifted after photography.
- Transparent cello tape should be pressed evenly and firmly to the surface on which latent print is developed.
- It is then peeled off gently from the surface and then affixed to a smooth opaque background o a white or black color contrasting the powder.
- Every effort should be made to avoid air bubbles under such lifts.