Modern methods of detecting fingerprints.

Modern methods for identifying handprints..

Shelkov Vadim Antoninovich

MODERN METHODS FOR DETECTING HAND TRACES.

Fumigation with cyanoacrylates and the effectiveness of this method.

The main task during the inspection of the crime scene is to identify and record traces as the results of any material impact on the initial state of the environment (situation) that occurred during the commission of the crime. A trace is a materially recorded reflection of the external structure of one object (trace-forming) on ​​another (trace-receiving). In some cases, a trace does not convey the external structure of the influencing object, but is formed by its peeling or layering as a result of mechanical, chemical, thermal or other impact.

A special place in forensic practice is occupied by the identification and recording of fingerprints. This is explained by the unique properties of human palm skin: individuality, relative stability and restorability of the papillary pattern.

A person can sometimes be identified by the imprint of a small part of the palm, although mainly traces of the nail phalanges of the fingers are used.

Fingerprints are usually divided into three types:

• visible, formed by imprints of some foreign substance that was on the palm (ink, paint, oils, dirt or blood);
• barely visible, formed by sweat and fat secretions of the skin on smooth, hard, non-absorbent surfaces and not forming a noticeable contrast with the surface that receives the stain;
• invisible, formed by sweat and fat secretions of the skin on absorbent surfaces (paper, cardboard, plywood, etc.).

The most difficult task is to identify and record barely visible and invisible traces consisting of natural secretions of human skin glands. These secretions are produced by glands of external secretion, sebaceous and sweat glands, and consist of inorganic and organic components.

The former include water, ammonia, chlorides, sulfates and phosphates, as well as metal ions, primarily potassium, sodium and iron. Organic inclusions are represented by amino acids, proteins, fatty acids, glycerol, and other metabolic products: urea, lactic and uric acid, and various sugars.

Table 1 Systematization of secretions of skin glands.

Selecting the optimal means and methods for identifying subtle and invisible traces is a key moment in the work of a forensic expert. To do this, it is necessary to have a clear understanding of the mechanism of the ongoing processes.

Factors such as the age of the trace and the impact of the environment on it, as well as the substances of the trace-receiving surface, should be taken into account. Water and alcohols are the first to leave the trace substance. Thus, methods using substances that react with the water component of the trace are of little use for processing old traces. And, on the contrary, substances that interact with fatty components are more effective in these cases. Of course, the surface on which the trace is left can absorb and disperse the components contained in the trace, thus worsening the distinguishability of the pattern of papillary lines.

In some cases, successful detection of fingerprints requires the consistent use of various methods.

Since many “developers” have a destructive effect on traces, the expert must clearly understand the sequence of application of various methods. The fact is that during the reaction the chemical composition of the trace substance changes, after which there are no more components left in it suitable for further use of the selected method.

Fingerprint powders.

Powders adhere to both water and fat components of the trace. They are successfully used to detect relatively fresh traces. The powder is selected based on the idea of ​​achieving maximum contrast with the trace-receiving surface.

Powders are used on any dry, relatively smooth, non-sticky surface. They are used after laser fluorescent methods and cyanoacrylates before using ninhydrin. The results obtained largely depend on the qualifications of the expert.

The detected traces are photographed or removed using trace-copying materials.

Fluorescent powders.

These powders contain luminophores and have a certain advantage over conventional fingerprint powders. Fluorescent powders provide increased contrast to the background and are used to detect traces on complex surfaces when conventional powders are ineffective.

The method of application is similar to the previous one. Traces are photographed or removed using trace copying materials.

Ultraviolet illuminators and expert light sources are used to effectively detect traces treated with fluorescent powders.

Iodine method.

Iodine vapors, reacting with the oil-fat components of traces, form a short-lived yellow-brown image formed by the reaction products. The iodine method is used to detect fresh traces on porous and slightly porous non-metallic surfaces.

The detected traces should be photographed immediately, as they quickly fade. This method is used before using ninhydrin and silver nitrate.

DFO (1.8 Diazafluorene — 9-one)

DFO interacts with amino acids and is a fluorescent analogue of ninhydrin. As practice has shown, it detects 2.5 times more traces than ninhydrin itself. DFO is used to treat porous surfaces and is especially effective on paper.

The revealing effect of DFO can be enhanced by controlled heating of the surface to be treated. It is used before applying ninhydrin. DFO can be useful in revealing weak blood traces. For effective detection of traces, it is recommended to use an expert light source with l = 530 mm when viewing behind a threshold filter with l = 610 — 630 mm.

Ninhydrin

Ninhydrin reacts with amino acids to form purple stains. Works well on porous surfaces, especially paper. Development time can be up to 10 days, but heat and water vapor can speed up the process. Ninhydrin is used after iodine, before silver nitrate. However, it is useless for detecting stains on surfaces that have been exposed to water.

Silver Nitrate.

Silver nitrate reacts with chlorides contained in the secretions of the skin glands and forms silver chloride, a light-sensitive substance that turns grey when illuminated. The detected traces should be photographed immediately, since the reaction inevitably causes a «tightening» of the background. Silver nitrate can be successfully used on paper, cardboard, plastic, unpolished light wood. It is used after ninhydrin and iodine. Also, like ninhydrin, it should not be used on surfaces that have been exposed to water.

Physical developer.

Physical developer is another example of a silver-containing reagent that reveals traces in dark gray tones. Like silver nitrate, it can be used to reveal traces on paper, cardboard, and unpolished light wood. Physical developer is used after ninhydrin and iodine.

Unlike the previous reagents, physical developer can be successfully used on surfaces that have been exposed to water.

Small particle reagent (SPR)

The finely dispersed reagent is an aqueous suspension of the smallest particles of molybdenum disulfide (MoS2). They adhere to the fatty components of fingerprints and reveal them in gray tones. The finely dispersed reagent is used to reveal fingerprints on smooth, non-porous surfaces, including wet ones. This reagent is used instead of or after a physical developer. The detected fingerprints should be photographed immediately, since the reaction products adhere very weakly to the surface. In addition, the fingerprints can be carefully removed using traditional fingerprinting materials.

Cyanoacrylates (fumigation with superglue)

Cyanoacrylate (super glue) vapors polymerize in the presence of water vapor and some other components of sweat and sebum secretions, forming relatively hard whitish traces. To increase contrast, the detected traces can be additionally treated with fingerprint powders or fluorescent dyes.

Black amide

Black amide reacts with proteins contained in the blood, forming traces stained black.

This reagent is used only for the development of traces stained with blood and is characterized by high sensitivity (developing ability). It is used on non-porous and slightly porous surfaces. In some cases, this reagent made it possible to detect blood-stained traces of the criminal's hands on the body of a corpse.

Powder for Scotch-type adhesive tape

This powder settles on the traces left on the adhesive side of the adhesive tape and gives good results on transparent and lightly colored tapes: packaging, protective, surgical, tapes for repairing pipelines, adhesive microporous for isolating seams, as well as adhesive labels. The powder is also applicable for detecting traces on wet tapes.

Fluorescent preparations.

Traces can be treated with special fluorescent dyes that react with individual components of the trace substance. Some of them significantly increase the contrast of the trace when illuminated by expert light sources. Others, often used to color traces detected by cyanoacrylates, require the use of an ultraviolet light source.

The properties of the trace-receiving surface have a significant impact on the efficiency of detecting traces using various methods. Despite all the diversity, these surfaces can be combined into nine main groups.

1. SMOOTH, NON-POROUS SURFACES.

These include glass, hard plastic, and surfaces coated with a layer of paint or varnish (excluding clean metal surfaces). Fingerprinting powders, iodine, finely dispersed reagent, and cyanoacrylates with fluorescent dyes are used for processing.

2. ROUGH NON-POROUS SURFACES.

Rough, rough, or textured surfaces, granulated plastic. Finely dispersed reagent and cyanoacrylates are used. Conventional fingerprinting powders are not suitable for working on such surfaces.

3. PAPER AND CARDBOARD.

In addition to paper and cardboard, these surfaces should also include gypsum plasterboards not coated with a layer of plastic, enamel or mastic (e.g. wax). Iodine, ninhydrin, DFO, silver nitrate or physical developer are used. At the same time, fingerprint powders generally have a weak detecting ability when processing old traces.

4. PLASTIC PACKAGING MATERIAL.

This category includes: polyethylene, polypropylene, cellulose acetate and laminated paper surfaces. Iodine, finely dispersed reagent, cyanoacrylate and powders are used. Cyanoacrylate is especially suitable for processing foamed plastic.

5. SOFT VINYL (PVC), RUBBER AND LEATHER.

These surfaces also include artificial leather and tight-fitting plastic materials (heat-shrink films).

Iodine, finely dispersed reagent, cyanoacrylate and powders are used.

6. METAL (UNPROTECTED)

Metal surfaces not protected by a layer of paint, varnish or enamel. Finely dispersed reagent, powders, cyanoacrylate with fluorescent dyes or powders are used.

7. UNPOLISHED WOOD.

This category includes clean wooden surfaces not covered with a layer of varnish, paint, polish, etc. Ninhydrin is used, and on smooth wooden surfaces, fingerprint powders. Silver nitrate and physical developer are used for processing light-colored wood.

8. WAX AND WAXED SURFACES.

These include candles themselves, as well as paper, cardboard, and wood impregnated with wax and similar substances. Conventional (non-magnetic) powders and cyanoacrylate with fluorescent dyes are used to detect traces.

9. SURFACES COATED WITH A LAYER OF ADHESIVE.

The adhesive side of adhesive tapes, labels with an adhesive layer, and other similar surfaces that are insoluble in water. Special powders for adhesive tapes are used.

Table 2. Recommended reagents for detecting fingerprints depending on the type of surface.

SLOW CATALYTIC REACTION WHEN SMOKING WITH SUPERGLUE

There are three ways to use cyanoacrylate.

The first method uses only cyanoacrylate. About three grams of super glue is applied to an aluminum plate or just a piece of foil, which is placed in a closed container along with the evidence being examined. Compared to others, this method requires the greatest investment of time. Fumigation in such a container can last for several hours or even days. A significant development of this method of detecting traces was the use of cyanoacrylate gel in convenient plastic packaging («envelopes»), containing concentrated cyanoacrylate ester. The most widespread are «envelopes» of the American company LOCTITE under the brand name HARD EVIDENCE, which in English means “strong evidence” (Fig. 2).

Fig. 2. “Envelopes” of cyanoacrylate gel from the American company LOCTITE with the brand name HARD EVIDENCE

Similar “envelopes” under the name FINDER, literally — “the one who finds”, are produced by the largest supplier of materials and equipment for forensic scientists, the American company SIRCHIE.

When using LOCTITE “envelopes”, the efficiency of fumigation with cyanoacrylate increases significantly. Such “envelopes” are extremely convenient for processing small closed spaces: car interiors, office and residential premises. The time required for fumigation and the number of envelopes with gel are determined by simple arithmetical calculation. In this case, two important points should not be overlooked: a cup of warm water should be placed in the container during the processing and your control fingerprint should be placed to monitor the fumigation process and prevent the characteristic tightening” of the background when it develops.

For this reason, and also because superglue in its normal state can cause irritation of the respiratory system and mucous membranes of the eyes, all work with cyanoacrylates should be carried out in a fume hood or, at least, with the use of personal protective equipment for the eyes and respiratory tract.

The third method of using cyanoacrylates is associated with chemical acceleration of the fumigation process. In principle, many substances react with cyanoacrylate, but not all provide the expected results. To ensure spontaneous combustion with the release of the required heat, flat «napkins» are prepared from 100% cotton wool, pre-soaked in a solution of caustic soda. Dried napkins are stored for subsequent use.

If necessary, a portion of such a «napkin» is placed in an improvised cuvette made of aluminum foil, which is placed in a container for fumigation. When liquid cyanoacrylate is applied to the soaked cotton napkin, fumigation begins immediately. Similar results are obtained when using caustic potash.

When cotton wool is impregnated with alkali metal salts, such as sodium chloride, the reaction proceeds more slowly. This provides some time delay in the start of fumigation, which allows the staff not only to apply superglue to the impregnated cotton wool, but also to carefully close the container. At the same time, the safety rules for working with cyanoacrylates are best followed.

ON THE USE OF VACUUM CHAMBERS

HARD EVIDENCE CYANOACRYLATE GEL FROM THE AMERICAN COMPANY LOCTITE.

Superglue — cyanoacrylate is supplied not only in liquid form, but also in the form of cyanoacrylate ester, packed in special plastic envelopes. Such packaging with gel contains concentrated cyanoacrylate, which itself spreads in the surrounding space, increasing the efficiency of fumigation. At the same time, according to the assurances of the manufacturer, the time of detection of traces depends on the size of the container used: 10 l — 20 — 30 minutes, 20 l — 30 — 40 minutes, 40 l — 40 — 50 minutes.

The quality of the traces revealed depends on the humidity in the container. The optimum is 30 — 60%. Reduced humidity slows down the quality of traces development and worsens the process of their detection. Therefore, it is recommended to place a cup of warm water in the container together with cyanoacrylate.

When fumigating small spaces: a car interior, an office or a living room, the time it takes for traces to appear depends on the temperature and the movement of cyanoacrylate vapors in this closed space. Lowering the temperature slows down the process. To achieve the best results when fumigating large spaces, you can use a portable hair dryer to speed up the movement of vapors.

ABOUT TESTING CYANOACRYLATE ETHER.

In April 1996, specialists from the Forensic Science Center of the Ministry of Internal Affairs of the Russian Federation, at the request of the author, tested envelopes with cyanoacrylate ether.

The surfaces most commonly used in practical fingerprinting were used as trace-receiving surfaces: glass, smooth plastic, porcelain, metal painted with a glossy surface, and polyethylene.

For testing, good-quality traces on degreased surfaces with an age of application from one to ten days were used.

Fumigation was carried out for 8 hours in a closed container, which also contained a cup of warm water and a HARD EVIDENCE> plate.

Tests have shown that fingerprints 10 days old were fully revealed on all surfaces, papillary lines are painted white, there is no background, the edges of the lines are clear, pores are visible.

Thus, envelopes with cyanoacrylate ether HARD EVIDENCE can be recommended for use in the practical activities of forensic units both in laboratory conditions and during inspection of crime scenes (cars, small enclosed spaces).

But not everything is so simple. We will talk about some unsuccessful ways of using cyanoacrylates next time…

APPENDIX 1.

INSTRUCTIONS FOR USING CYANOCRYLATE ETHER.

SMUGGING SMALL OBJECTS.

Place the object in the container. Place a cup of warm water nearby. Remove the protective layer and attach the envelope to the side of the container.

Close the container. Fumigation begins immediately. After 20 — 30 minutes, remove the object being treated and check for traces.

If additional treatment is required, return the item to the container and continue fumigation. Periodically monitor the progress of trace detection until satisfactory results are obtained. Treat the detected traces with fingerprint powders and remove them onto fingerprint film.

The envelope with cyanoacrylate gel can be used to fumigate other items with a total of up to 12 hours of operation.

FUMING CAR INTERIORS. ​​

Open the car interior and place 3-4 envelopes with cyanoacrylate gel inside. For best results, place the car in a heated area (in the sun or in a closed garage).

Open the envelopes with cyanoacrylate gel and attach them to areas where fingerprints are most likely: windows, dashboard, etc. The cyanoacrylate vapors emitted by the envelope are heavier than air, so it is advisable to place them above the surface being treated.

Close the car doors and fumigate it for 45 minutes to 2 hours.

Monitor the progress of traces periodically. The maximum fumigation time can be up to 12 hours.

After reaching maximum contrast, remove the envelopes with cyanoacrylate gel and ventilate the car interior.

After this, treat the traces with powders and transfer them to photocopying film.

TREATMENT OF SMALL SPACES.

Close the windows, doors and ventilation ducts in the room being examined. Place several envelopes with cyanol acrylate gel in it at the rate of: 1 envelope per 1.4 square meters of floor, with a ceiling height of 2.5 meters.

Place the envelope with cyanoacrylate gel near objects where fingerprints are most likely to be found: switches, lamps, telephone, windows, placing them slightly higher than the specified objects.

Fumigate for 1 — 2 hours. Extend the treatment time if necessary.

After fumigation, ventilate the room, treat the detected traces with powders and remove them with trace-copying film.

APPENDIX #2.

A FEW WORDS ABOUT SUPERGLUE

Cyanoacrylate glue (superglue), used to reveal fingerprints, is produced by the American company LOCTITE. This is a one-component, quick-hardening adhesive that does not contain solvents and is supplied with stabilizing components. It glues metals, most plastics, rubber both to each other and to each other. A small amount of glue is applied to pre-cleaned and degreased surfaces in the form of dots or stripes, and then the objects to be glued are tightly pressed. The action time is from 10 to 120 seconds. The final strength of the connection is achieved in 12 hours.

When using cyanoacrylate glue, special care should be taken: it glues human skin in a matter of seconds. If this does happen, the glued areas of skin should be carefully separated using a flat, blunt object soaked in warm soapy water. The glue that has dried on the skin falls off on its own after a few days during the skin renewal process.