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Laboratory examination: search, recovery, and analysislocked

  • Jim Fraser


‘Laboratory investigation: search, recovery and analysis’ outlines the various stages of recovering, documenting, and analysing evidence. The first stage of a laboratory investigation is the recovery of samples. As many samples from different investigations share the same laboratory space, care must be taken to prevent contamination. The sample can then undergo a range of tests, and all interactions with the sample are recorded in case notes. A number of biochemical assays exist which test for the presence and source of bodily fluids. It is important when performing and recording such analyses to separate fact (for example from a test result) from the forensic scientist's own opinion.

We now move from the crime scene to the laboratory and the various stages of recovering, documenting, and analysing evidence. Some of the principles and processes will now be familiar to us as they reflect those applied at crime scenes. The new dimension is the specific application of scientific testing of case items and the range of scientific disciplines involved. In this chapter, we will cover the types of examinations carried out in particular case types and the specific scientific and legal procedures required to meet the standards of criminal law.

Recovery of evidence

The modern forensic science laboratory contains a bewildering array of science and technology that is focused on providing answers and indicating valuable lines of inquiry in criminal cases. In addition to meeting formal scientific standards, it has to comply with legislation and legal procedures to ensure that the criminal justice system is best served. This chapter outlines how items are examined, the techniques that are used to recover evidence, and the potential range of methods available for analysis. The importance of minimizing contamination, maintaining continuity (chain of custody), and quality assurance are also explained. There is no standard structure that all laboratories follow as this will depend on the type and amount of examinations they carry out.

p. 34Some labs employ only a small number of people (say 15), others can be very large, containing many hundreds of staff. Some labs are part of police organizations, others are independent public sector or state organizations, and some are private commercial enterprises. In England and Wales, all the major forensic science laboratories are private companies. The privatization of these laboratories remains a matter of considerable debate in terms of whether this is in the interest of justice. Detractors of this arrangement suggest that it will inevitably lead to erosion of standards and a focus on profits to the detriment of justice. Supporters point to other private industries with very high technical standards, such as the airline industry, as evidence that private enterprise does not mean lower standards of science and technology. Only time will tell how effective these arrangements are. In most other countries around the world, including the USA and Australia, commercialization of forensic science is rare and tends to be in the form of small companies which provide one specialist type of evidence such as DNA profiling.

Whatever their funding basis, most medium-sized labs (say around 100 staff) will have broadly similar structures based on the types of investigation they are involved in and the scientific disciplines that they need to do this work. Very few labs in the world, perhaps none, have the capability to carry out every forensic examination, and most seek to balance the skills they have with their users’ needs. Table 6 provides an overview of the scientific disciplines, departments, and case types found in a medium-sized laboratory. In terms of case numbers, the bulk of the work will be volume crime (burglary, car crime) and drugs analysis, where small numbers of items will be examined in individual cases. Although serious offences will be small in number, the workload in these cases is likely to involve many more examinations, sometimes of hundreds of items.

Irrespective of how the laboratory is structured, the first stage of any examination is the recovery of materials. In small laboratories, p. 35

Table 6. Disciplines, departments, and case types in a typical forensic science laboratory

Section/Evidence type



Evidence recovery unit

Routine initial examination of a wide range of items such as clothing, weapons, etc. to recover evidence for further analysis using standard techniques: visual examinations, taping, and sweeping.

This is the first stage in most examinations and is carried out by an assistant under the supervision of a case-reporting scientist.

General chemistry

Analysis, comparison, and identification of a wide range of materials and chemical substances such as greases, waxes, plastics.

Involves a wide range of cases, such as burglary, thefts of material, major crime, and road traffic crashes.

Chemistry trace evidence

Analysis, comparison, and identification of minute quantities of paint, glass, soil, and other chemical traces.

Involves cases similar to those above. In a small laboratory, may be part of General chemistry.


Comparison of shoe marks, tool marks, tyre marks, and manufacturing marks of various kinds. Will also include the provision of intelligence and linkage of scenes.

Widely used in volume crime, especially burglary, and major crime such as homicide.


Analysis and identification of drugs of abuse including synthetic and natural products and prescribed drugs.

Includes seizures from individuals as well as bulk importations in trafficking cases and materials from clandestine laboratories.


Identification and quantification of alcohol, drugs, and poisons in body samples in suspicious or sudden deaths and drink driving cases.

Poisoning is rare, but toxicology screening is a routine part of homicide investigation (and other serious cases) as drug use may have a bearing on the behaviour of victims and suspects.


Routine examination and comparison of a wide range of biological evidence (e.g. blood, semen, saliva) not dealt with in specialist sections.

Cases are typically violent or sexual assaults and homicide.


Genetic analysis of biological fluids, tissues, and stains by a range of techniques.

Includes examinations of mixtures of body fluids and paternity/maternity testing.

Fibres and hairs

Comparison and identification of natural and synthetic textiles, human hair, and animal hair.

Typically confined to serious cases such as sexual assaults and homicide, but increasingly used in wildlife crime.

Botanical evidence

Examination and identification of a wide range of infrequently encountered materials such as plant fragments, seeds, wood, pollen.

In many cases, external experts will be consulted due to the specialist skills involved.


Examination of questioned documents in fraud and counterfeit cases ‐ contracts, wills, letters, passports, currency ‐ to establish ownership or authenticity.

Includes examination of documents from computer printers and faxes, their method of production and the analysis of ink.


Examination of handwriting to attribute or eliminate a putative author or connect documents that may have been written by the same author.

Involved in a wide range of cases including fraud, robbery, homicide.

Fire investigation

Examination of fire scenes and analysis of debris from fires or flammable liquids (‘accelerants’).

A great deal of this work is done at scenes, with the main laboratory work being the identification of flammable liquids.


Examination and test firing of pistols, rifles, military weapons, and related devices. Identification of firearms discharge residues (FDR).

Examination of firearms and the identification of FDR is usually done in separate sections due to the potential contamination issues.


Comparison, identification, and enhancement of finger marks. In most cases, the laboratory work is confined to enhancement of marks, with comparison being carried out in a separate fingerprint department.

Many laboratories do not have a fingerprint section but most will have some capacity to enhance or visualize marks for examination by fingerprint experts.

Crash investigation

Investigation and reconstruction of road traffic incidents. Much of this work is done at scenes, but there may be follow-up analysis required.

Examination of tachographs and damaged vehicle parts, calculation of speeds and trajectories of vehicles to reconstruct crashes.

Digital evidence

Examination of computers, networks, and mobile devices (phones, PDAs, SATNAV, etc.).

Probably the most rapidly expanding area of forensic science. Digital devices are now so widespread that they are involved in many different investigations.

Photography and imaging

Routine record photography and analysis of imaging devices such as media from CCTV and still cameras.

Extensively used in a wide range of investigations, for example to identify individuals at or near crime scenes and for presentation of evidence in court.

p. 36p. 37p. 38p. 39individual scientists, perhaps working with an assistant, will do this. In larger laboratories, there will be an evidence recovery unit staffed with individuals trained to recover all of the potential evidence from items using many recovery techniques. Before the examination commences there are some basics steps to take. The first is to ensure that you have all the available information to carry out an examination, including relevant witness statements and police reports. This usually means a phone call to the investigating officer to check any facts which may have changed (this can happen overnight in major cases). Secondly, some basic planning needs to take place to ensure you have identified an appropriate sequence of examinations (you can’t look for the red fibres from the jumper of the victim until you know what these look like).

This of course raises an issue of potential contamination, so you need to make sure that all relevant examinations are separated in space (different benches in different labs) and time (different days) and wearing different protective clothing. If there are three suspects, two victims, and a scene involved in a case, this will take some careful thought and planning. The risks of contamination are higher given that all of the materials from the incident are now in one place (the laboratory) and will probably be examined by a single scientist. However, the items are now in controlled conditions and can be managed more easily and effectively than at a crime scene. From the outset, systematic, stringent procedures are taken to prevent contamination and records that demonstrate compliance with these procedures are made and retained. What constitutes contamination and the steps taken to avoid it varies in the different disciplines of forensic science, and to an extent in different laboratories. Trace evidence ‐ glass, paint, soil, hairs, fibres, and other particulate materials ‐ are particularly prone to contamination, and the following steps are commonly used to minimize contamination:

Items from different sources, e.g. the scene, suspect, and victim, are stored from the outset of the examination in separate places.p. 40

The sequence of the examination should minimize the risk. Where possible the trace evidence is recovered before the control sample (the potential source) is examined. Once the traces are recovered, the opportunities for contamination are considerably lower.

Items from which trace evidence is to be recovered are examined in different locations and at different times, generally a minimum of a working day apart.

Different lab coats, examination benches, and instruments are used for each related set of items, e.g. a set of clothing from one individual. Extensive use is made of disposable instruments and protective clothing.

The instruments assigned to a search bench are dedicated to it and do not leave it. The lab coat used is stored there until the case is finished.

All of the above details are noted in the case file.

Examination of items

In this section, we will deal mainly with biological evidence, since aspects of chemistry (trace evidence and drugs analysis) are covered in later chapters. The examination of clothing for body fluids, such as blood, and trace evidence would commence by choosing an appropriate bench which will be cleaned, disinfected, and protected by a layer of clean paper. The examiner will wear a freshly laundered lab coat, new gloves, face mask, and disposable cap. The tools used in the examination (pens, forceps, etc.) will be located at that bench and will not be removed from there. The examination of an individual item proceeds as follows:


The label of the item is checked and compared with the relevant paperwork. There should be no significant discrepancies. If there are, this will have to be explored to eliminate any problems with the integrity or continuity of the item. The item number (or description in some jurisdictions) acts as a unique identifier and will be referred to in reports and in court so must be recorded exactly in the case notes.p. 41


The integrity of the packaging is reviewed. The item should be sealed and the packaging intact. Any deficiencies, such as damage or poor seals, must be noted. Where there is a significant problem, such as an unsealed item or breached packaging, the item may not be examined. Detailed notes of the packaging and sealing are recorded in the case notes.


The package is then opened in a different part from the original seals (which must be kept intact) and gently the item is laid out on the bench. The item is then briefly scanned for visible material of interest that might be easily dislodged and lost. This should be removed and retained in a separate labelled package (typically a small polythene bag). Minimal handling should be used at this stage.


If required, the surface of the item is then taped using transparent adhesive tape to systematically and completely recover extraneous trace evidence. Tapes from each item are packaged separately and labelled with the item details (description or number) and the location where the tape was taken from (e.g. front right sleeve, left rear, etc.).


Alternatively, when recovering particulate evidence such as glass and paint (and when there is no need to recover fibres), the item can be brushed to remove any microscopic particles after a careful and thorough visual and low-power microscopic examination.


Following recovery of the trace evidence, the item is examined visually, slowly and systematically, for other relevant evidence. Specialist lighting, e.g. from fibre-optic lamps, is often used for this process.


The item is then described—a shoe, jumper, or knife—in sufficient detail for it to be identified readily in future, e.g. in court. Its condition (old, new, worn), and any other significant or distinguishing features such as stains, marks, or damage, should also be noted. In cases involving fibres, the composition of the garment given on the garment label is noted.


In cases where items subsequently may be searched for fibres matching the item being examined, a control sample should be removed. This must be representative of the item and include all fibre types and colours.p. 42


The case notes should reflect what the item was examined for, its description, and any findings and interpretations, accurately and concisely, supported by measurements, diagrams, and photographs where relevant.

The purpose of examining items in this manner is to ensure that all evidence is recovered; to identify any relevant materials, such as body fluids, present; to produce accurate, detailed notes about the nature of the item; and to determine which analyses will be carried out next (such as the individual stain for further attention). These notes will be continually updated throughout the examination of the case and act as a detailed history of events and information received as well as analytical results. For example, if a case briefing is held, a record of the meeting will be stored in the case file. The case file will subsequently be used as the basis for drafting reports and to act as an aide-memoire when giving evidence in court. It should also be a transparent record of how the item was examined, by whom, for what purpose, at what time, and in what sequence. All of these matters are of potential interest during a trial.

Blood and body fluids

In physical and sexual assaults, body fluids can be shed and transferred to clothing, objects, and weapons. In addition to identifying the source, that is the individual from whom the stain has come, the location, amount, and pattern of staining can be important in interpreting findings. A common explanation for bloodstaining found on the clothing of individuals accused of assault is that they were near the attack but did not take part in it. The staining therefore must be examined to determine if there is any evidence to support or refute this statement. Saliva staining inside a mask can indicate that it has been worn and by whom. Semen and saliva are also routinely encountered in sexual offences on body swabs (e.g. from the vagina or mouth) and clothing, particularly underwear. Locating and identifying blood, semen, p. 43saliva, and other biological materials form a routine aspect of forensic biology.

The first stage of the examination uses a number of simple tests (so-called presumptive tests) that can give an initial indication of the type of stain which is then further analysed by confirmatory tests. Dried bloodstains have a characteristic red-brown appearance and are usually readily recognizable. A number of presumptive tests can be used to indicate the presence of blood, all of which rely on the catalytic activity of haemoglobin, the protein found in red blood cells. The Kastle Meyer (KM) test is commonly used to identify bloodstains. This is based on the oxidation by peroxidase of the colourless form of phenolphthalein to give a bright pink colour. For the test to be reliable, the distinctive pink colour must appear almost instantly, since the phenolphthalein will gradually oxidize and turn pink in the air anyway. The combination of the visual appearance of the bloodstain followed by a satisfactory KM test is generally regarded as sufficient to establish that a stain is blood. Stains that give a KM positive test but do not resemble blood may be mixtures of blood with another body fluid, such as saliva due to bleeding from the mouth. Alternatively, the stain may not contain blood, as a range of other biological materials and some chemical oxidants can give false positive reactions, although these are rare. The test is applied by taking a small piece of filter paper and rubbing it gently against the stain. Since DNA profiles can be successfully obtained from extremely small stains, one must ensure that the stain is not lost or destroyed in this process.

Semen consists of a fluid (seminal plasma) containing many millions of sperm as well as proteins, salts, sugars, and ions. Seminal staining on fabrics is usually whitish in colour but this can vary, especially if it is mixed with other body fluids. It can also form deposits that are colourless and can therefore be difficult to locate on some items. Establishing the presence of seminal staining relies on detecting seminal fluid and sperm. Seminal fluid contains a high p. 44concentration of the enzyme acid phosphatase that can be detected using a presumptive test known as the acid phosphatase (AP), or brentamine, test. This test relies on the formation of a purple azo dye from brentamine due to the catalytic action of acid phosphatase. The deeper the colour and the faster it appears (usually a few seconds), the more confident one can be that the reactions are due to seminal fluid. Other body fluids, in particular vaginal fluid, can also react to this test but the colour is different (more pinkish) and the reaction takes longer (over 30 seconds). However, given that most stains encountered in sexual offences are mixtures of semen and vaginal fluid, the difficulties in interpreting such a test will be evident. Following a strong AP test, the presence of sperm can identify semen. This is done by microscopic examination of a small amount of material extracted from the stain. Sperm have a characteristic appearance (see Figure 5), consisting of a head and tail section, and can be stained using histological dyes. The amount of semen found on vaginal swabs can be used to estimate time since intercourse, although this method is fairly crude.

Saliva is encountered in sexual offences and a wide range of other cases, such as robberies (on masks and gags) and homicides. Saliva is secreted by the salivary glands and contains water,

5. Sperm and vaginal cells stained with haematoxylin and eosin

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p. 45mucus, proteins, salts, and enzymes. One of the enzymes present ‐ amylase ‐ is found in very high concentrations in saliva compared to other body fluids and its detection can indicate the presence of saliva. Saliva deposits usually form colourless stains, therefore identification must take into account the circumstances and location of the stain. The presence of epithelial cells typical of those from the mouth can sometimes be used as a confirmatory test, but these cells are similar to those found in the vagina (and elsewhere in the body) and are therefore of limited value.

The tests described above usually constitute the first steps in cases involving biological evidence, but the scientific procedures in a major case will require extensive examinations by other scientific disciplines with the aim of establishing answers to

Table 7. Samples and examinations from a male suspect in a sexual homicide



Buccal (mouth) swab

DNA reference sample


Therapeutic drugs, drugs of abuse


Alcohol analysis


Alcohol, drugs

Genital/Anal swabs

Body fluids – blood, saliva, vaginal material


Trace evidence, e.g. fibres, hairs, DNA from body fluids

Head hair

Hair reference sample

Pubic hair

Hair reference sample


Trace evidence, body fluids, contact DNA


Blood, body fluids, trace evidence

p. 46investigative inquiries. Table 7 gives an indication of the typical samples submitted to the laboratory from a suspect in a sexual homicide and the purpose of their examination or analysis. Blood samples of different types are required depending on the purpose of the examination. For example, the DNA reference sample contains an anti-clotting agent which enables extraction of DNA, and the sample for alcohol analysis contains a preservative that prevents infection by bacteria that can produce or metabolize alcohol and therefore cause misleading results. Urine samples are also required for alcohol and toxicology analysis, since alcohol and metabolites from drug breakdown can be detected in urine even when none is present in the blood. Depending on the detailed nature of the case, swabs from parts of the body including the genitalia and anus will be taken from the victim (by forensic medical examiners) to be examined for body fluids. Head hair and pubic hair samples are used for reference purposes should any hairs be found on the victim that require microscopic comparison, although this is increasingly rare given that DNA profiles can be obtained from hairs. Combed head and pubic hair samples may also be examined for fibre transfer. Finally, clothing known to be worn at the time would be submitted for examination for any evidence of relevance.

Table 8 provides a summary of the types of examination carried out in laboratories and the range of analytical techniques involved. Although many specialist techniques are used, we can say that in almost all instances the process begins with a visual examination of the items involved, usually to recover the evidence. The most commonly used range of techniques is microscopy, of which there are a number of specialist types that have particular application for the examination and comparison of trace evidence. Presumptive testing is also widely used to screen biological and chemical substances. The main analytical method which underpins most forensic biology work is DNA profiling, since this can identify the donor of almost any type of biological fluids or tissue. A large number of analytical techniques is used to identify the diverse range of substances encountered in forensic chemistry.p. 47

Table 8. Analytical methods for different types of evidence

Key: Black, regular or routine; dark grey, occasionally or where relevant; light grey, rarely; blank, not applicable

HPLC: high-performance liquid chromatography; CE: capillary gel electrophoresis; FTIR: Fourier transform infrared spectrocopy; SEM: scanning electron microscopy

p. 48The particular method used will depend on the nature of the substance involved, whether it is organic or inorganic, solid or liquid, or present in trace quantities or large amounts. A number of the techniques mentioned in Table 8 are further described in other chapters: Chapter 5 (DNA), Chapter 7 (trace evidence), and Chapter 8 (identification of drugs). It is not possible to cover all of the techniques in the table, but for those readers who wish to explore this area further, suitable references are provided.

Physical fits

This chapter is about the recovery and analysis of evidence, but on occasions both of these processes can be remarkably simple. A physical fit occurs when two separate items fit together in such a manner that it becomes instantly recognizable that they were originally one item. This rarely requires any analytical equipment other than occasionally a low-power microscope, nor does it require any scientific interpretation. For example, a 24-hour service station in London was robbed and the assistant was handed a McDonald’s takeaway bag to put the cash in. As the robber took the bag, the assistant held on to it, gently tearing a small piece off. The fragment contained parts of the multi-coloured McDonald’s logo and fitted perfectly with the torn section of the bag recovered from the robber’s address. A photograph of the items fitting together as one was taken to accompany the short statement and to illustrate the nature and significance of the evidence. A similar example to this is illustrated in Figure 6. In a second example, fragments of glass from a broken container and a piece of burnt fabric were recovered from the scene of a fire and were suspected to be parts of a petrol bomb. The glass fragments pieced together to form most of a milk bottle and the fabric was a badly burnt fragment from a white vest which bore traces of a flammable liquid. The remains of a white vest were recovered from a suspect’s premises and were compared with the burnt fragment. In this case, the match was less clear-cut and required careful microscopic examination of both the vest and burnt fragment to establish a fit.

p. 49Nonetheless, the burnt fragment fitted together with part of the left shoulder strap of the vest, showing that they were originally one item.

Until now, we have been considering the process of examination, the physical actions and logical steps by which it is achieved. There is a great deal more to an effective examination than following a process. In searching, there is a need to be alert to potential evidence that may be unforeseen. The examiner must take an intelligent, inquiring approach which is suitably detached and dispassionate. It is reasonable to have general expectations ‐ we know that individuals involved in violence where there has been bloodshed may get blood on their clothing ‐ but we should not be motivated by such expectations in individual cases to achieve any particular outcome. We proceed from observation to analysis and then by inference to interpretation. Statements and reports should also follow this pattern as we move from observation (fact), analysis (fact), interpretation (a mixture of fact and opinion), to conclusion (usually opinion). A report should separate fact and opinion so that this is clear to the reader. The discipline of this procedure is valuable; otherwise we can be prone to errors that arise from statements which are opinion but appear to be fact. In other words, the sequence is not only procedural but cognitive. Considerable care is needed to prevent the synthesis of fact and interpretation in our minds because we are so used to handling particular types of analyses and interpretations. For example we categorize a blood pattern as ‘impact’ on the basis of an inference from our observation of certain characteristics. This is nonetheless a contestable assertion and not a fact, an assertion we need to explain and justify if necessary. Much of the value of forensic science derives from the combination of scientific testing and rational transparent process which can be judged by others as objective (or not). It is the scientific approach that is objective, not the scientists: scientists are no more objective than anyone else.

This chapter has covered the fundamental processes of forensic examination: search, recovery, and analysis. It has also described p. 50

6. Physical fit of two parts of a petrol receipt

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the range of methods involved in such analyses, how to prevent contamination and maintain the integrity of the evidence, and some of the issues that arise in the interpretation of evidence. In the following chapters, we will consider these issues and other specific areas of forensic science in more detail.