Introduction
Fingerprints are still one of the most important contact traces left during the commission of a crime as their uniqueness puts an individual at the scene. Therefore, it is vital that fingerprint recovery bureaus have methods that are effective in the recovery of latent fingerprints. To this end a variety of chemical reagents and associated methods have been successfully developed for visualizing latent fingerprints, some of which are routine and others that are more specialized. Over the years there have been many useful reviews of latent fingerprint recovery techniques and current knowledge of chemical reaction mechanisms and reaction conditions are typically included in these reviews, providing a good starting point for those interested in the science of latent fingerprint recovery. Thus, in order to avoid repetition within the literature this article takes a slightly different slant on the subject by tackling the scientific knowledge from the standpoint of the practical issues involved in the recovery of fingerprints in operational forensic casework. In this way the day to day scientific limitations and difficulties are put into context providing a clear vision of those areas of study that need further investigation.
Before progressing further, it should be noted that fingerprints are also recovered after being deposited in a number of substances, most commonly blood, but these methods are not discussed in this article. In addition, it should be emphasized that the successful identification of an individual from a latent fingerprint relies not only on the quality of recovery but also on the expertise of many disciplines working effectively together. Therefore, as this article concentrates on the chemistry of the process it must be remembered that without high quality photography and pattern recognition the whole recovery process would be fruitless.
This article begins with the current understanding of the chemical composition of natural latent fingerprint residue, followed by the chemistry involved in the recovery of latent fingerprints and finally, the most pressing scientific problems where increased knowledge would be most useful are highlighted.
Chemical Composition of Latent Fingerprint Residue
Intriguingly very little is actually known about the precise chemical constituents of a latent fingerprint at the moment a forensic examiner attempts its recovery. This is due partly to the high number of parameters that affect the final composition but, more importantly, because there has never been any concerted scientific effort towards a greater understanding of the problem.
Traditionally the starting point for a chemical understanding of fingerprint residue composition has been the more rigorously derived composition of those sources deemed responsible for the major compounds present on a fingertip prior to transfer onto a surface namely: sebaceous secretions, eccrine sweat and apocrine sweat. Although, these sources undoubtedly contribute to fingerprint residue they do not provide either a comprehensive list or acknowledge potential chemical activity over the time between deposition and visualization. Since most chemical reagents used in fingerprint recovery procedures are developed on this theoretical model of latent fingerprint residue there is an incentive to pursue a more accurate understanding of the true composition if researchers are to develop new and more effective reagents. Nevertheless, these sources of natural material remain the basis of current knowledge of latent fingerprint residue.
Factors contributing to latent fingerprint residue
Skin surface residue From Table 1 it might be reasonable to assume that only eccrine sweat and keratins would be found on fingertips, however, through touching the face sebaceous material is also present. These, thus, represent the three main sources of natural material that contribute to the residue found on the surface of a fingertip. Together these natural sources produce a surface film on human skin that is an emulsion of an aqueous and lipid phase (the main emulsifiers being cholesterol and wax esters). Another possible source of material, the apocrine glands, is often included in descriptions of fingerprint residue but is not considered in detail further as their location on the body makes their contribution unlikely or small.
Table 1 The main potential contributing sources of natural material found on the fingertips of humans
| Source | Bodily distribution |
| Eccrine | All skin surfaces, especially palms |
| and soles | |
| Apocrine | Armpits, chest, abdomen, genitals |
| and soles | |
| Keratinizing epidermis | All skin surfaces |
| Sebaceous | Forehead, chest, back, abdomen |
In conclusion there are no definitive data on the chemical composition of a fingerprint and clearly there are some obvious difficulties with ever obtaining such data, not least because every fingertip will have a slightly different surface film mixture. Therefore, the best data available are from the assumption that the film will be a mixture of material from sebaceous glands, eccrine glands and keratinizing epidermis sources as discussed. Tables 2 and 3 provide a summary of the main chemical constituents in each of these sources.
There have been numerous studies of the individual sources especially sebaceous and eccrine glands. However, care must be taken in interpreting any individual analysis of the glandular material as the means of material collection vary considerably from study to study and some methods discriminate against certain material. Also, it should be questioned how representative these studies are of the actual material deposited as residue from a fingertip. Furthermore, it is not possible from these data to determine the exact ratio of constituents within a fingerprint. Nevertheless, it is a reasonable assumption that at the moment of deposition some if not all of the compounds in Tables 2 and 3 are present in various amounts. Studies show that a latent fingerprint weighs less than 10 ug and is around 0.1 um thick.
Table 2 Concentrations of the main constituents of sweat
| Compound/electrolyte | Approximate |
| concentration | |
| (vgmr1) | |
| Water | |
| Lactic acid | 2000 |
| cr | 1750 |
| Na+ | 1350 |
| Amino acids | 900 |
| Urea | 850 |
| K+ | 550 |
| Ammonia | 125 |
| Protein | 80 |
| Glucose | 40 |
| Ca2+ | 30 |
| Creatinine | 10 |
| Mg2+ | 9 |
| Acetic acid | 8 |
| Uric acid | 6 |
| Zn2+ | 1 |
Table 3 The major lipids present on the skin surface
| Lipid | Surface (%) |
| Squalene | 10 |
| Sterol esters | 2.5 |
| Sterols | 1.5 |
| Wax esters | 22 |
| Triglycerides | 25 |
| Mono- and diglycerides | 10 |
| Fatty acids | 25 |
| Unidentified | 4 |
Deposition The physical contact of the fingertip on a surface is clearly important in determining the initial composition of fingerprint residue. The most obvious parameters involved in this process are time, contact angle and pressure, although it is difficult to determine the precise influence of these parameters in isolation from the nature of the surface itself. Studies have indicated that both the quantity and type of material transferred are dependent on the nature of the surface. Currently, it is not clear whether this transfer process is due to physical or chemical interactions and represents an important scientifically unexplored area of study.
Donor The chemical composition of material found on the fingertips of individuals is known to differ between people but also varies over time with a single individual. There are numerous factors that affect perspiration rates and material secreted. As an example, recent work has indicated that there is a difference in the chemical composition of fingerprints between children and adults. Other factors may include gender, health, diet and medication. Whether these factors have a genuine and important contribution to initial composition has not been investigated scientifically. More excitingly, if gender, age, etc. could be deduced from the chemical analysis of latent fingerprints this could provide important information within the context of a criminal investigation.
As indicated, a major problem in analyzing the chemical composition of a latent fingerprint is in overcoming the problem of sampling. Studies to date usually sample through either washing or wiping the fingertips with a solvent or solvent mixture. Clearly associated information such as donor age, gender, sample area preparation and size of sample group should be documented if meaningful comparisons between studies are to be made. Furthermore, consideration of the solubilities of the various compounds in certain solvents is another important factor. Otherwise there could be difficulties in quantifying results or not collecting certain compounds at all. Only a couple of attempts have been made to sample from a latent fingerprint directly. After sampling a variety of analytical methods have been used to analyze the constituent components including high performance liquid chromatography (HPLC), thin-layer chromatography (TLC), gas chromatography-mass spectrometry (GC-MS), gas chromatography, liquid chromatography and spectroscopy. To date the results are consistent with those expected from the analysis of sweat and sebum. This is not a great surprise as most of the work has relied on samples collected directly from the fingertip. What is needed is to deposit fingerprints and allow them to stand under a variety of conditions commonly encountered in operational forensic casework prior to sampling.
Substrate The substrate onto which the fingerprint is deposited is extremely important for determining the process of visualization. Typically substrates are characterized by two broad types: porous and non-porous. Paper and cardboard typify the former and plastics and metals the latter. Fingerprint residue is usually absorbed into porous surfaces but remains on the surface of nonporous surfaces. These physical (and possibly chemical) effects are still poorly understood. Sweat is more readily absorbed than lipid material therefore the former is more efficiently transferred onto porous surfaces and the latter onto non-porous surfaces. For instance, it has been shown that up to three times more amino acid material is found on absorbent material than similar prints on nonab-sorbent surfaces.
Categorization of surface types into porous and nonporous is obviously an oversimplification and in reality there is a range of surface types with varying degrees of porosity. Since the method of enhancement is often highly dependent on surface nature a method for easily and effectively determining physical parameters such as porosity and surface energy would be extremely useful. Such information would allow recovery techniques to be aligned against these substrate physical properties.
Another aspect of surface is its chemical nature. It is reasonable to assume that there may be direct chemical reaction with certain surfaces and also diffusion of chemicals into the fingerprint from the substrate, such as plasticizers. An understanding of this requires further work in the area of surface versus bulk chemical properties. In particular, knowledge on the diffusion depth of different residue components into a substrate is vital in developing reagents targeting surface and/or diffused fingerprint material.
Contaminants Touching involves a two-way transfer of material and everyday activities bring the fingertips into contact with a huge range of materials that could potentially transfer chemical compounds onto fingertips that, importantly for fingerprint recovery, are trapped in the residue of a latent fingerprint. This is an aspect of fingerprint recovery that has been neglected by scientific study.
Bacteria Skin contains many bacterial species that have largely been ignored by examiners of latent fingerprints, although there have been some attempts to utilize their presence for visualization. It is unknown how important bacteria are in existing recovery mechanisms, in particular their involvement in the subsequent chemistry of the residue after deposition. Inherent fluorescence from latent fingerprints represents a powerful method and the fluorescence mechanism is unknown despite knowledge from other studies that porphyrins produced by bacteria are the origin of the characteristic orange fluorescence of skin.
Ambient conditions and time As indicated earlier there are many potential parameters that could have an effect on the chemical nature of both the substrate and the fingerprint residue after deposition. Time, light, temperature and humidity are some of the more obvious factors but others such as immersion in water or fire damage need to be addressed. This represents an important area for more scientific progress since a more detailed study of these effects would provide a more accurate understanding of the chemicals encountered in latent fingerprints under operational circumstances than is currently available. This is also important in the international arena as climates vary considerably. Since new recovery methods are developed worldwide it is not uncommon for the effectiveness of the methods to vary from country to country presumably due to differences in environmental conditions.
Reagent Chemistry
This section describes the chemical interaction between the latent fingerprint and the methods used for developing a visible print suitable for subsequent pattern analysis.
The photophysics and reagent chemistry of latent fingerprint visualization is largely unknown. Almost all current scientific knowledge is from experiments performed under atypical environments compared to those encountered during the recovery of latent fingerprints. For instance the pathway for the reaction between ninhydrin and amino acids was derived from experiments carried out in solution. There is a requirement for experimental studies on the reaction mechanisms and kinetics of chemical reagents under the typical conditions of latent fingerprint examination, that is, using typical reagents and formulations on real fingerprint residue deposited on both porous and nonporous surfaces. Without this work latent fingerprint recovery will continue to rely on empirical observations and experience in order to optimize reaction conditions. Nevertheless, knowledge from these ‘atypical’ experiments is vital in providing a lead into this problem.
It is the aim of all latent fingerprint recovery processes to maximize the contrast between the fingerprint and its background. As the variety of methods developed to achieve this goal are too numerous to discuss here, the discussion will concentrate on the factors that govern the success of any chosen method and where current knowledge is lacking. These factors are particularly important when considering the development of new enhancement methods since ideas and techniques developed in the research laboratory need to be developed with an understanding of the practical realities of operational latent fingerprint recovery.
Surface
Since latent fingerprints are inherently invisible it is the surface to be searched that provides the first decision point in choosing a suitable recovery process. Surfaces are traditionally categorized by their perceived porosity based on empirical observation since very little work has been done to establish a method for quickly and reliably characterizing surface properties. It is the properties of the surface that determine the procedures chosen for latent fingerprint recovery as reagents and their formulations have been developed not only for their specificity to certain physical/ chemical characteristics of latent fingerprint residue but also for the surface type involved.
As a note of caution the knowledge of the bulk properties of an item may not be indicative of the surface of the item. By way of an example drink cans may be made of aluminum but the surface properties are those of a plastic due to the characteristics of the label printing process.
Ongoing research into surface properties and the development of surface specific reagents is necessary due to the rapid growth of new materials. New surfaces are appearing all the time, such as the development of heat-sensitive papers used in fax machines and tills.
Reagent
As already mentioned there are many chemical reagents used for latent fingerprint recovery (see Table 4). In many cases reagents were discovered by accident rather than design. It is most common to categorize reagents in terms of either the surface type or the residue component thought to be the target. Very little is understood about the actual chemical, physical or photophysical mechanisms involved. Strategies for recovering latent fingerprints, therefore, rely on systems derived from practical experience gained over many years. However, it is still important that a deeper understanding of the precise chemical mechanisms involved is obtained for existing methods to be improved. Such knowledge could be vital in answering many observed phenomena, for instance, why do some fingerprints react with ninhydrin and not with DFO (1,8-diazafluoren-9-one), despite the understanding that they both react with primary amino groups? Although it is known that ninhydrin reacts with keratins whether DFO reacts is unknown. Mechanisms for these reactions and others have been described many times and can be found in the literature. A precise understanding of reaction mechanisms would also provide a clear guide to those components within a latent fingerprint that are currently untargeted by recovery methods.
Table 4 Example reagents, formulations, delivery methods and reaction conditions for some common latent fingerprint recovery methods
| Reagent/method | Formulation | Delivery method | Reaction conditions |
| Laser | 514 nm and 488 nm | Light guide | |
| DFO | Freon:methanol:acetic acid | Dip | 75 C 15% relative |
| (47:2:1) | humidity; 30 min | ||
| Cyanoacrylate | Cyanoacrylate liquid monomer | Fume | Room temp.; |
| 80% RH; 30 min | |||
| Vacuum metal deposition | Gold then zinc | Evaporation | Approx. 10~4 Ton- |
| Physical developer | A complex mixture of surfactant, | Dip | Ambient conditions |
| iron and silver salts |
Experience has shown that in order to provide the maximum chance of latent fingerprint recovery from a particular surface more than one reagent is required. This has led to sequences of reagents being set up for the most commonly encountered exhibit surface types. The treatment sequences allow for the recovery of the maximum possible number of latent fingerprints. The various treatments are chosen as they complement each other but they do need to be applied in strict order as certain treatments have an adverse effect on others. Such series of treatments are many and varied but examples of those used within the Forensic Science Service are given in Table 5, others can be found elsewhere. In the main, these treatment series have been established empirically.
The chemical reasoning behind such sequences can be best introduced by following one typical sequence of reagents. Table 5 shows that semigloss paper is first examined by light that may either induce fluorescence in the visible and/or ultraviolet. In addition the difference in reflective/absorption properties between the fingerprint and the surface can sometimes be exploited to provide a high contrast image. Cya-noacrylate fuming is applied next as the acetic acid in the DFO and ninhydrin formulations to be used next interferes with the formation of the cyanoacrylate polymer on the fingerprint ridge detail. The precise mechanism of the formation of the polymer from the vapor phase is not fully understood although it is believed that the polymerization is catalyzed by alkaline moieties in the latent fingerprint. Cyanoacrylate fuming is followed by DFO and ninhydrin that react with any amino groups present to produce, normally, a fluorescent and purple product, respectively. Finally, physical developer is applied as it is known to react/interact with the lipid fraction although this again is not fully understood.
Table 5 Examples of chemical treatments applied sequentially to surfaces of exhibits at the Forensic Science Service
| Paper | Semigloss paper | Glossy paper | Plastic | |
| Treatment order 1 | Lighta | Light | Light | Light |
| Treatment order 2 | DFO | Cyanoacrylate | Vacuum metal deposition | Vacuum metal deposition |
| Treatment order 3 | Ninhydrin | DFO | Cyanoacrylate | Cyanoacrylate |
| Treatment order 4 | Physical developer | Ninhydrin | DFO | Rhodamine 6G in water |
| Treatment order 5 | Physical developer | Ninhydrin | Crystal violet | |
| Treatment order 6 | Modified physical | Gentian violet | ||
| developer | ||||
| Treatment order 7 | Rhodamine 6G in | |||
| methanol |
It can be seen from Table 5 that observation of inherent fluorescence from latent fingerprints is the first treatment in most instances. Although it has been known for some time that fingerprint residue emits fluorescence both in the visible and in the ultraviolet the fluorophore responsible has so far eluded chemical detection. Investigation of possible inherent infrared fluorescence from latent fingerprints has not been attempted.
Reagent formulation
Each reagent has many chemical formulations. These formulations have evolved primarily through empirical observations and experience. Other factors such as cost and to a lesser or greater extent health and safety considerations can also influence formulation. More recently, legislation over chlorofluorohydro-carbons has had a dramatic effect on formulation since Freon (l,l,2,trifluorotrichloroethane) is an important solvent for several reagents currently in use throughout the world. Some examples of reagent formulation are given in Table 4.
A reagent formulation has three important roles. (l) a medium for the reagent itself; (2) to transport the reagent onto/into the surface, and (3) to provide suitable reaction conditions for the reagent. In addition, it is preferable that any formulation does not interfere with subsequent forensic examination of the exhibit. For instance, Freon as a solvent carrier for ninhydrin allows the development of latent fingerprints on paper without causing any handwriting ink to run.
In the case of porous substrates, such as paper, it is necessary to have a formulation that will allow the reagent to penetrate the surface but has a sufficiently rapid evaporation rate to prevent diffusion of the latent fingerprint residue. These issues have to be addressed whilst remembering the health and safety and forensic compatibility constraints mentioned earlier.
Certain methods have been developed to transport the reagent in the vapor phase such as in the cyanoacrylate, DMAC (dimethylaminocinnamaldehyde), fuming and metal deposition techniques.
Reaction conditions
Each reagent and formulation has associated reaction conditions for optimum performance. In some instances these conditions have been optimized for routine work and others for more specific exhibit types. Again, these optimization procedures have been found largely through trial and error and there is a need for more fundamental scientific knowledge on the kinetics of these reactions in order to optimize the reaction conditions.
Table 4 gives some typical reaction conditions for a few of the most common treatments. Between fingerprint recovery bureaus there are subtle variations to these conditions. For instance, most bureaus use cyanoacrylate as a stage in the recovery of latent fingerprints from nonporous exhibits with the majority applying cyanoacrylate in humidified cabinets. Despite its widespread use there has been little work on the flow effects involved in mixing the water vapor and monomer (or oligomers?) in the gas phase. Hence, cabinet design may potentially have a significant effect on the success rate of the method.
It should be noted that the nature of certain casework exhibits, such as vehicles, puts limits on the amount of control that can be placed on the reaction conditions. However, increased understanding through experience has pushed the development of cyanoacrylate fuming tents where vehicles can be fumed under more preferable conditions. Despite these advancements the extremes in ambient temperature determine the relative humidity (hence absolute humidity) that can be achieved which could cause both over- or underfuming.
Forensic compatibility
It must never be overlooked that items that have been treated for fingerprints may be required for examination by other forensic techniques such as DNA or document examination. Therefore, whenever a new reagent is developed its implications for subsequent forensic examination need to be considered. In certain instances reagent formulations have been developed specifically with this in mind, for example Freon as a transport solvent for DFO does not make inks run on paper.
New reagents would also preferably fit into existing treatment series without being detrimental to other techniques in the series. New complementary methods should be able to fit between current treatments or, if they are improved/similar treatments, they could replace existing ones.
Reagent performance
Measuring the relative success of latent fingerprint reagents is one of the most difficult problems facing those developing new reagents, formulations, delivery systems and the application to new surfaces. What is required is a standard against which new methods can be judged. This is not as simple a problem as it may at first appear. To begin with how do you obtain a ‘standard latent fingerprint’ that can be used in the tests? Clearly, surfaces could be defined for test comparison purposes but again the properties of deposition of a ‘test fingerprint’ would need to be overcome. Currently the best indicator of a reagent’s effectiveness in the recovery of latent fingerprints is by testing it on a range of ‘naturally’ handled items such as bank checks or similar objects.
Issues concerning the quality control of reagents and delivery systems also need to be more widely addressed. For instance, some reagents have a shelf-life and its determination is an important aspect of reagent development. Cheap chemical (or other) tests are needed to provide an indication of the quality of a reagent prior to its use to ensure that should no fingerprints be recovered it was not because of a faulty reagent mixture.
The whole issue of quality management therefore has to be addressed; methodologies must be written down and strictly followed, apparatus tested and calibrated regularly, and there should be meaningful quality assurance tests. There are already some systems available but without a more complete understanding of the chemistry/physics some methods may not be operating at their optimum.
Some important unresolved chemical problems in latent fingerprint chemistry
• Recovery of latent fingerprints from skin: to date there is no reliable method for visualization of latent fingerprints from cadavers. Many methods have been attempted but none have led to a routine procedure.
• DNA friendly formulations: as the ability to recover DNA profiles from ever smaller quantities of body fluids or cells increases, the need to ensure compatibility of the recovery method with subsequent DNA techniques is vital. This may not only require the back testing of current systems but could necessitate the development of new reagents/formulations.
• Testing regime for novel reagents/methods: new reagents/methods for latent fingerprint detection are being published constantly but there is no systematic method for their testing. Ideally any test would provide information on sensitivity, residue specificity, and compatibility relative to the best existing techniques.
• Substrate characterization: a simple, robust, reliable and cheap method for characterizing surface properties such as porosity would be a valuable tool in deciding treatment sequences to be applied.
• Fingerprint residue composition: a thorough understanding of latent fingerprint chemical composition would be desirable in order to determine new target compounds for reagents and for optimization of current techniques.
• Aging fingerprints: a method allowing determination of the age of a latent fingerprint could provide vital information in the course of an investigation.
• Reaction mechanisms and kinetics of reagents with latent fingerprints: knowledge of reaction mechanisms and kinetics of the reactions of current reagents with latent fingerprints under typical operational conditions would allow for the optimization of techniques.
• Quality assurance tests: realistic methods for testing the quality of reagents and systems for latent fingerprint recovery would provide confidence and maintain quality between bureaus.
• Event order: methods for establishing the order of events, such as whether the fingerprint was present before/after the handwriting or which of the superimposed fingerprints was deposited first would be of forensic value.
Conclusion
Fingerprints are still and will continue to be a vital evidence type available to law enforcement agencies and therefore the need for continuing research is essential in order to improve on existing methods and to meet the challenges of future needs.
Hopefully, it has been made clear that the recovery of latent fingerprints is not a straightforward chemistry problem but is a complex scenario requiring the consideration of many interrelated factors. This provides a rich and interesting scientific field of endeavor with many questions still to be answered. It was the aim of this article to establish where these questions lie currently and to stimulate activity to provide the answers.
