SPR — developer for detecting traces on wet surfaces.

SPR — developer for detecting traces on wet surfaces..

SPR — DEVELOPER FOR REVEALING TRACES ON WET SURFACES

In the catalogues of many foreign companies supplying materials and equipment for forensic scientists, as a means of detecting traces, so to speak, of the «second echelon» (after traditional powders, cyanoacrylates, ninhydrin and iodine), there is always a preparation with the mysterious abbreviation SPR (small particle reagent) — the so-called finely dispersed reagent. And in literal translation «some substance in a state of very fine grinding».

In essence, it is a bluish-gray or even black powder of small shiny crystals of molybdenum disulfide MoS2, used in the form of an aqueous suspension.

By the way, MoS2 is indirectly familiar to many car enthusiasts who use additives to motor oil. As a rule, they include the specified molybdenum compound. It is noted that MoS2 actively «sits» on oil and fat inclusions and helps to improve the mechanical characteristics of rubbing pairs: reducing friction and increasing their wear resistance.

This property is used to detect fingerprints: like conventional fingerprint powders, MoS2 precipitates on the fatty components contained in fingerprints left on material evidence. Only this occurs in an aqueous suspension, which is a mechanical suspension of small MoS2 particles in the thickness of the carrier liquid (not a solvent!).

There are two known ways to use the SPR preparation:

1. The surface to be treated is sprayed with a hand sprayer. Since SPR causes heavy contamination of the surface, areas not to be treated should be covered with paper or a napkin.
2. Small evidence can be treated by immersing it in the working solution for 2-3 minutes. A suitable container can be used for this.

For obvious reasons, the results depend largely on the amount of substance contained in the trace.

Catalogues and advertising materials particularly emphasize the «ability of SPR to reveal traces on wet surfaces, such as glass display cases in rainy weather or even cars pulled out of water after accidents. When using a sufficiently powerful sprayer, the SPR developer can even be used underwater (!) with a correspondingly increased consumption of the useful substance.»

In addition, the SPR trace developer is also applicable in cases where the use of conventional powders and brushes can spoil the trace.»

Based on the mechanism of this method, it can be assumed that it will have a lower detection capacity: mechanical (physical) detection using an aqueous suspension will naturally be worse than when using traditional fingerprint powders, including in terms of the brightness and contrast achieved. But the possibility of detecting traces on a still wet surface or literally in the rain is extremely tempting.

As for the trace-receiving surfaces for which SPR is suitable, there is a clear discrepancy among different suppliers.

In the instructions for use of SPR from the American company Lightning Powder Co., Inc. we read: «SPR can be successfully used on surfaces such as paper, cardboard, clean and rusty metal, brick, concrete, stone, plastic, wood, galvanized metal and glass». The well-known European supplier — the company BVDA (Netherlands), on the contrary, warns against using SPR on porous surfaces and, above all, paper and cardboard,while citing a completely obvious argument: a suspension with small black particles of the working substance simply pollutes the surface being treated.

But the most convincing is the experience of practical use of such an original means. An example of such use of a finely dispersed reagent suggested itself, and is demonstrated by the author in the photographs provided. Like the two-color Bi-Chromatic powders, fingerprints detected by the SPR preparation appear in dark gray strokes on light-colored surfaces, and in light gray — on dark ones.

But the most significant is the review of such an authoritative institution as the Expert-Criminalistic Center of the Ministry of Internal Affairs of the Russian Federation, which in March 1996, at the request of the author, conducted a test of the said means.

Only good quality fingerprints formed on clean surfaces were used for testing. Fingerprints were detected on both dry and wet surfaces. The age of formation and detection of fingerprints was within three days. The fingerprint powder «malachite» was used as a control.

The following were used as a fingerprint-receiving surface: metal with smooth painted and rough surfaces, painted chipboard, plastic, as well as paper with a glossy surface and cardboard.

The criteria were: ease of use, quality and completeness of fingerprint detection, as well as the possibility of high-quality extraction of fingerprints on fingerprint film.

As expected, it was found that the brightness and contrast of traces detected on smooth surfaces were somewhat worse than those detected by traditional powders. On cardboard, traces were detected as spots without displaying papillary lines, and on rough metal, traces did not appear.

Fingerprints were copied qualitatively onto the dactyl film after the object's surface had completely dried.

It was noted that after the traces were detected, a large amount of waste liquid remained.

Conclusions:

The solution can be recommended for detecting fingerprints when inspecting crime scenes in bad weather conditions: inspecting cars, shop windows, etc.

Convincing, although some conclusions seem quite obvious. As for the use of SPR indoors, other than the treatment of wet objects in bathrooms, tested by the author of these lines, another use of the black MoS2 suspension does not even come to mind.

In conclusion, we will give some general recommendations for the use of finely dispersed reagent.

TREATING LARGE OBJECTS BY SPRAYING.

1. Since heavy MoS2 particles quickly settle to the bottom, the working composition should be shaken vigorously before each use.
2. The composition is applied to the surface to be treated using a sprayer. The direction of treatment is from top to bottom. The surface can be either wet or dry.
3. If some traces are noticed, the treatment continues by spraying the area located slightly above them, but in no case on any traces that are not detected!
4. Excessive amount of developing agent is removed by spraying with clean water from the second sprayer (it is advisable to use «soft water»). In order to avoid damaging the traces, washing with water should be done very carefully, spraying the area above the revealed traces.
5. Traces revealed by the SPR preparation can be removed to the trace copying film only after drying. It cannot be ruled out that when attempting to remove the revealed traces from a wet surface, they will be spoiled by drops of water that have ended up under the working surface of the trace copying film. It is not recommended to use a hair dryer to speed up the drying process.
6. In some cases, due to the insignificant contrast with the surface, traces are poorly visible on it until they are transferred to the trace copying film.
7. In all cases, it is strongly recommended to photograph the revealed traces before you try to transfer them to the trace copying film.

PROCESSING SMALL OBJECTS BY THE IMMERSION METHOD.

1. For processing small pieces of evidence, the immersion method can be used using an ordinary photographic cuvette.
2. The item should be immersed immediately to ensure uniform distribution of the working substance over the entire surface being processed, for which the cuvette should be shaken sharply, as is done when developing large-format photographic prints.
3. The working composition should not be stirred during processing.
4. After processing, the object is carefully removed from the cuvette and rinsed under a stream of clean water (or by immersing it in another cuvette with water, with the working surface facing down).

In all cases, a certain amount of skill is required to obtain satisfactory results.

After a long period of time, on the order of several months, the SPR working substance may leave behind difficult-to-remove dirty marks on the surface being examined.

PREPARATION OF THE WORKING SOLUTION.

In many cases, SPR comes as a ready-made composition (a certain amount of MoS2 in a weak solution of some detergent — washing agent). But the working composition can be easily prepared at home.

1. Measure out 1 liter of water into a mixing container. Although distilled water is recommended for making the working composition, in many areas it is permissible to use ordinary tap water. This should be verified empirically.
2. With continuous stirring, add the contents of one package (30 g of SPR) to the mixing container with water and continue to vigorously stir the composition for 3-5 minutes.
3. Add 2-3 drops of wetting agent (e.g. baby shampoo) to improve suspension formation. But not in excess! A sign of this will be the appearance of foam on the surface of the working solution. At the same time, there should be no unmixed lumps of MoS2 floating on the surface.
4. After preparation, the working solution is poured into a hand sprayer using a funnel.
5. The second sprayer is filled with clean water.

When working with the SPR trace developer, it is recommended to observe appropriate safety precautions: use rubber gloves and safety glasses. Ordinary soap can be used to remove dirt.

The shelf life of the finished solution is about 4 weeks. With longer storage, deterioration in the performance of the SPR trace developer cannot be ruled out. In this case, it must be replaced with a new one.

We can only wish success to everyone who does not sit idly by, but constantly seeks and masters new, more effective working methods.