Sequential Treatment and Enhancement

Introduction

Although there are many chemical and physical techniques referred to in the literature for revealing and enhancing latent fingerprints there are only a relatively small number which are used widely and are most productive for operational use. The chemical methods for latent fingerprint detection all rely on reactions or interactions that occur between the applied reagent and components of latent fingerprints. Studies have shown that the distribution of chemical constituents varies widely between individuals, and for the same individual from day to day and hour to hour. The use of techniques in sequence will often therefore reveal more fingerprints than any single technique. Such sequential processing can be labor intensive and is sometimes only used in more serious crime investigations. Exhibits from minor crimes will often be treated with only the most cost-effective and straightforward process. The careful choice of processes and their sequence

Factors Affecting the Choice of Development Technique

As well as considering the types of surfaces that are to be treated, it is important that the following points are considered when selecting a sequence of treatments.
1. The importance of the case and resources available.
2. Techniques should be used if possible in an order in which the effectiveness of methods used later in the sequence are not adversely affected by those used at the beginning of the sequence.
3. Any contamination on the surface that could help or hinder fingerprint development, e.g. blood, grease etc.
4. Sampling of biological material prior to treatment for fingerprints. If this is not possible due to insufficient deposit of material, specialized advice on the effects of fingerprint development techniques on DNA profiling should be obtained.
5. Samples of other trace evidence such as hairs or fiber should usually be taken prior to treatment for fingerprints but in a way that does not compromise the fingerprint evidence. ESDA for developing impressions in paper must also be performed before chemical treatments for fingerprint development are applied.
6. The effects of exposure to the elements, especially exposure of the surface to rain or moisture may affect the choice of treatment sequence.
7. Full use is made of optimized reagent formulations and specialized equipment at every stage of examination for fingerprints.
Detailed information on the selection of latent fingerprint development techniques can be found in the Manual of Fingerprint Development Techniques listed in the bibliography. Flow charts in this publication have been developed to graphically represent the selection of processes for specific surfaces, an example of which is given in Fig. 1.
What follows here is a synopsis of the basic concepts that underpin the selection of techniques. Summaries of the compatibility of fingerprint development techniques with surface type, and with contamination from water, blood and grease etc. are given in Table 1

Figure 1 Process selection chart for smooth nonporous surfaces.

Fingerprint Development: Nonporous Surfaces

Background

Many articles at scenes of crime fall into this category including glass, gloss-painted surfaces, metal and plastic. Fingerprints on nonporous surfaces can be very easily damaged as most of the trace residues deposited from the fingerprint ridges usually remain on the surface.

Development techniques

Visual examination Visual examination of any article may sometimes reveal latent fingerprints, particularly if the surface is smooth and clean. A variety of light sources should be used, e.g. oblique illumination. Any fingerprints should be photographed before proceeding with other techniques.
Fluorescence examination Examination of surfaces with a high intensity light source or laser, usually working in the near UV, blue or green parts of the spectrum and using barrier filters in front of the eyes, will sometimes reveal fingerprints either by the rarely observed fluorescence of naturally occurring components of the latent fingerprint, or more often by the fluorescence of some contamination which may have been on the finger such as oil or grease. Fingerprints can also sometimes be visualized by a combination of light absorption by the fingerprint ridges and fluorescence of the background.

Table 1 Summary of the use of fingerprint development techniques

Vacuum metal deposition (VMD) Of the methods that are used to develop latent fingerprints on non-porous surfaces, this is the most sensitive, being capable of detecting monolayers of fat by sequential deposition of a very thin coating of thermally evaporated gold followed by zinc. Plastic wrapping materials up to about 1000 mm x 1000 mm and other small solid objects may be treated in specially modified industrial vacuum coating systems. VMD has the great advantage that it can develop fingerprints on surfaces that have previously been wet or even submerged in water for extended periods of time. If fingerprints are not revealed by VMD, superglue, powders or other techniques may be used subsequently.
Fingerprint powders Powdering, one of the oldest techniques for detecting fingerprints on smooth non-porous articles, is quite effective and widely used; many latent fingerprints are still found by this simple method. Although there are many powders described in the literature, those with a flat, flake like structure such as milled aluminum or brass, or molybdenum disulfide are significantly more sensitive and generally more effective at developing fingerprints on smooth, clean surfaces than the more traditional black or white powders which are of a more granular nature. The metallicflake powders are most effectively applied with a fine glass fiber brush. Other powders are often applied with soft animal hair brushes.
Surfaces which are dirty or contaminated will sometimes take up too much of the flake powders all over the surface so that granular black or white powders may be more suitable. Rough or grained surfaces may sometimes be treated using iron, cobalt or nickel-based powders used with a magneticapplicator although superglue fuming is probably to be preferred.
There are many fluorescent powders which may also be used in conjunction with a suitable light source. Powders may be used on surfaces which have been wet, but only after the articles have been thoroughly dried at temperatures not exceeding 30°C.
Superglue fuming Superglue fuming can be used on many nonporous surfaces and is particularly useful on surfaces such as rough or grained plastic surfaces which cannot be easily treated using VMD. This technique is particularly effective when performed under optimum controlled conditions (normally 80% relative humidity, ambient temperature and pressure). When superglue fuming is performed under these high humidity conditions fibrous polymer growth occurs across the ridge deposit and is believed to be initiated by water droplets present on chloride deposits in the fingerprint deposit. This polymer growth increases mechanical strength and robustness of the developed fingerprint and produces a very large surface area, this appears white but can be very effectively enhanced by means of a solution of fluorescent dye which is absorbed and may be visualized using fluorescence examination. Use of fluorescent staining techniques instead of direct photography of the white superglue deposit can double the number of fingerprints detected. Many dyes have been used but Basic Yellow 40 in ethanol is very effective and is believed to be of lower toxicity than some others.
Superglue fuming may be carried out at reduced pressure but fingerprints developed under these conditions have a smooth surface with less surface area, are more difficult to visualize and absorb less dye.
Superglue fuming can be used after VMD and powders but is ineffective if used after a water-based development technique, or on articles that have been wet.
Small particle reagent (SPR) SPR consists of a suspension of molybdenum disulfide suspended in an aqueous detergent solution. This is used by agitating the reagent mixture so that the particles are temporarily suspended in the liquid and holding the article to be treated under the liquid allowing the molybdenum disulfide particles to deposit onto the surface. The article is gently removed and drawn through a container of clean water. The molybdenum disulfide particles adhere to fats deposited in the fingerprint, producing a gray-black image. SPR is used in dishes or tanks for portable articles. SPR can also be used by spraying directly onto larger surfaces, however used in this fashion it is considerably less sensitive than the submersion method.
Treatments for nonporous surfaces which are contaminated with blood or grease are discussed in a later section.
Preferred sequence of techniques The order in which techniques are used will be determined by the following factors:
1. the importance of the case;
2. the size of the article;
3. whether the surface is rough or smooth;
4. whether the surface has been wet;
5. surface contamination.
When these factors are taken into account, sequences of treatments can easily be determined. For example, a plasticbag from a serious crime which is not contaminated with blood etc. might be treated by visual examination and fluorescence examination followed by VMD and possibly super-glue fuming or SPR.

Fingerprint Development: Porous Surfaces

Background

The most common porous surfaces examined for fingerprints include paper, wall paper, cardboard and matt emulsion painted surfaces. The reagents used for these surfaces react either with amino acids, traces of fats and lipids or chlorides absorbed into the surface.

Development techniques

Visual examination Visual examination is less likely to reveal fingerprints on porous surfaces unless they have been contaminated with dirt, blood, grease or similar materials.
Fluorescence examination Examination of such surfaces will sometimes detect fingerprints either by the rarely observed fluorescence of naturally occurring components of the latent fingerprint or fluorescence of some contamination which may have been on the finger such as oil or grease. Fingerprints can also be visualized on fluorescent surfaces when the fingerprint deposit absorbs light; again such fingerprints should be photographed before proceeding.
l,8-Diazafluoren-9-one (DFO) The most sensitive reagent currently available for detecting fingerprints on porous surfaces is DFO. This compound reacts with amino acids deposited in the fingerprints to produce a faintly colored but intensely fluorescent compound which can be easily photographed. Since amino acids are soluble in water, neither DFO nor ninhydrin can be used to treat porous surfaces which have been wet.
Articles are dipped into a solution containing DFO, acetic acid and an inert carrier solvent and then heated in a dry oven at 100°C for 20 min. If humidified conditions are used for DFO-treated articles, no fingerprints are detected. DFO is not routinely used at scenes of crime because it is difficult to generate sufficient heat in a room to produce an acceptable rate of fingerprint development although local heating can be used for small areas.
Ninhydrin This is a widely used process which also reacts with amino acids and produces a purple colored product. Articles are dipped into a solution of ninhydrin and acetic acid in an inert carrier solvent and then heated in a humid atmosphere for rapid fingerprint development, the optimum conditions are 65% relative humidity and 80°C for 3-4minutes. (If these same humidification conditions are used for DFO-treated articles, no fingerprints are detected.) The developed purple-colored fingerprints can be easily recorded using conventional photography, a green filter can improve contrast.
Ninhydrin can be very effectively used at scenes of crime with the same formulation being brushed onto the surfaces. Sealing the room and raising the temperature with domesticheaters will assist fingerprint development but development will take several days. The time taken for this technique varies widely because humidity and temperature cannot be optimized.
In large-scale comparisons it has been shown that DFO develops about 60% more fingerprints than are found using ninhydrin. However, when ninhydrin is used after DFO, a further 10% of fingerprints are developed compared to instances where DFO is used alone. DFO is ineffective in developing latent fingerprints if it is used after ninhydrin.
When ninhydrin-developed fingerprints are developed on dark nonfluorescent backgrounds or when the background is a very similar color to the fingerprint zinc toning can be used to enhance visualization. The fingerprint is treated with zincchloride solution and gently heated to produce a fluorescent fingerprint which can then be photographed.
Powders Smooth papers may be treated with some types of black or magnetic powder although these will usually only detect heavier or more recent fingerprints.
Superglue fuming This may be used on some smooth surfaces such as cigarette packets but enhancement with fluorescent dyes may be difficult due to absorption of dye by the background.
Physical developer (PD) In cases when a porous surface has been wet, the only available technique for detecting fingerprints is PD. This reagent is an aqueous solution of silver nitrate containing a Fe(II)/ Fe(III) redox couple and two detergents. The exact reaction mechanism is unknown although it may detect intercalated sodium chloride molecules present in the small amounts of fat deposited in the absorbed fingerprints. The developed fingerprints are gray-black in color and can be recorded using conventional photography. PD is impractical to use at scenes of crime. Fingerprints developed by the process may be enhanced by radioactive labeling or mapping of the silver deposit using X-ray radiographictechniques.
Treatments for porous surfaces which have been contaminated with blood or grease are discussed in a later section.
Preferred sequence of techniques The preferred sequence of fingerprint treatments on porous surfaces would be: visual examination; fluorescence examination; powders (where applicable); superglue fuming (where applicable); DFO; ninhydrin; physical developer. The actual order to be used in a particular case will be determined by:
1. the importance of the case;
2. whether or not the article has been wet;
3. whether the paper surface is very smooth.
If the article has been wet, DFO and ninhydrin are omitted and physical developer used by itself.
If the paper has a glossy finish, the use of powders or superglue fuming described in the section on non-porous surfaces may be more appropriate. Good examples of this sort of surface are cigarette packets and glossy magazines.

Fingerprint Development: Adhesive Surfaces

Background

Surfaces in this category include the adhesive sides of sticky tapes and labels. There are three general techniques that are used to develop latent fingerprints on these surfaces: gentian violet, superglue fuming and powder suspensions. No significant comparative trials are described in the literature. The gentian violet technique has been used for many years but other techniques are becoming widely used. Physical developer may also be used on certain types of paper tapes.

Development techniques

Gentian violet (crystal violet) The most effective gentian violet formulation (also known as crystal violet) is a solution of crystal violet (CI 42555), phenol and ethanol in water. Adhesive tapes are drawn across the surface of the solution two or three times so that the dye is absorbed by the fingerprint ridges. Other adhesive surfaces to be treated with gentian violet can be floated on top of, or held in contact with the solution for ease of processing. Once treated, the surfaces are washed in slowly running water. Fingerprints on lightly colored surfaces will be visible to the naked eye and can be recorded using conventional photography. Fingerprints on black or dark-colored surfaces will not be immediately visible. This problem can be easily overcome by transferring the developed fingerprints onto photographicpaper. Once this has been done it is important to remember that the fingerprints will have been laterally reversed. It is important to bear in mind that fingerprints on the adhesive side of tapes may already be laterally reversed before transfer onto photographicpaper.
Superglue fuming Superglue fuming of adhesive surfaces is accomplished in the same way as on non-porous surfaces. Again, fluorescent dyeing and subsequent fluorescent examination may provide significant enhancement although background uptake can be a problem.
Powder suspensions Powder suspensions such as ‘Sticky-side Powder’ can also be used to develop fingerprints on adhesive tapes. A concentrated suspension of the powder is applied to the adhesive side of the tape, either by dipping or careful brushing. The solution is left in contact with the tape for 10-15 s and then washed off using slowly running water. Fingerprints are seen as gray-black images which can be recorded using conventional photography. Precise treatment times and reagent concentrations have not been determined or reported to date.
Preferred sequence of techniques Little is known about the relative effectiveness of the three techniques above but the use of powder suspensions or GV will render superglue ineffective as an after-treatment as both of the reagents are water based.

Fingerprint Development: Blood Contamination

Background

It is generally advisable that samples of blood are taken for DNA and other analyses before fingerprint treatments. If this is not possible, specialized advice should be sought to determine which fingerprint development techniques are compatible with DNA profiling; the latter is a rapidly developing area of technology and compatibility with fingerprint development techniques is likely to need reviewing.
None of the techniques that are commonly used to enhance bloody fingerprints are specific for blood and therefore cannot be used as presumptive tests for blood.
There are many techniques used for blood splatter imaging that rely on the iron centers in hemoglobin to catalyze chemical reactions which result in color changes or chemiluminescence. Some of these techniques can be adapted for use in enhancing fingerprints in blood although few have been fully evaluated for this purpose.

Development techniques

Fluorescence examination An effective method for detecting bloody fingerprints is by using fluorescence examination to excite fluorescence of the substrate. Blood itself is not fluorescent but absorbs light. Consequently, if the background surface can be made to fluoresce, the fingerprint can be viewed as black ridges on a white background. Fluorescence examination should be the first technique used to enhance latent fingerprints as it is simple to use and nondestructive provided significant heating is avoided.
Amido black The most commonly used dye for enhancing bloody fingerprints is Acid Black 1 (CI 20470) which is generally referred to as amido black. This is a general protein stain which binds to the proteins in blood plasma and to blood corpuscles; amido black is not a specific blood stain.
Amido black is commonly used as a solution in methanol. However, if a surface has to be treated that would be adversely affected by methanol such as some types of plastic, a water-based formulation may be substituted. Fingerprints developed by amido black are dark blue/black in color and can be recorded using conventional photography.
DFO and ninhydrin DFO and ninhydrin can also be used to develop bloody fingerprints on porous surfaces. These reagents react with amino acids in the blood residue to give fluorescent or colored fingerprints, respectively. As stated in the section on porous surfaces, DFO must be used before ninhydrin for fingerprint development to be effective.
Physical developer (PD) Very little information is available on the relative effectiveness of this technique for enhancing bloody fingerprints compared to amido black, DFO or ninhydrin.
Powders Powders can also be used to visualize bloody fingerprints when only part of the ridge detail is in blood; carefully applying powder will develop the ridges which are not contaminated with blood.
Preferred sequence of techniques The preferred sequence for enhancing bloody fingerprints on porous surfaces is as follows: fluorescence examination, DFO, ninhydrin, amido black then physical developer.
With one exception, the sequence for bloody fingerprints on nonporous surfaces is simpler as neither of the amino acid reagents can be used: fluorescence examination, amido black and PD. When a fingerprint is only partially in blood, fingerprint powders may also be used.
The exception is where a bloody fingerprint or partially bloody fingerprint is on a black, nonfluor-escent surface. Under these circumstances both fluorescence examination and amido black are ineffective, so that the only effective process is ninhydrin followed by zinctoning. In this case the ninhydrin reacts with amino acid and/or protein residues in the blood to produce Ruhemann’s purple, which may not be visible. The Ruhemann’s purple is then converted into a fluorescent product by further treatment with zincchloride rendering the fingerprint recordable using fluorescence examination techniques. Other fluorogenicreagents that detect amino acid residues in fingerprints such as DFO have not proven effective in enhancing fingerprints heavily contaminated with blood on nonporous, nonfluorescent surfaces.

Fingerprint Development: Grease Contamination

Introduction

Fingerprints in grease and other types of surface contamination can be enhanced using a number of techniques such as sudan black, gentian violet and occasionally physical developer. Other types of contamination can also be examined using these techniques including sticky deposits on the outside of drink cans and milk cartons etc.
Sudan black Sudan black uses an ethanolic solution of a dye called Solvent Black 3 (CI 26150) to stain the greasy fingerprint ridges. The surface under examination is treated with the solution for 2 min either by immersing the article in the solution or by floating the article on top of the solution. The surface is then gently rinsed under slowly running tap water and allowed to dry. Fingerprints are recorded using conventional photography.
Physical developer and gentian violet When enhancing greasy fingerprints on porous surfaces physical developer can also be used. On nonporous surfaces, gentian violet can be used to effectively develop fingerprints. Little information is available on the relative effectiveness of either of these techniques.