Forensic Science HQ

Investigate the forensic science world

Forensic DNA Analysis

DNA analysis was first introduced to the U.S. courts in the early 1990’s. It is without a doubt the biggest advancement in Forensic Science since the discovery of fingerprints.

Forensic DNA STR analysis

Why is DNA analysis so important in criminal investigations?

  • A DNA profile can positively include or exclude a person as source of evidence.
  • DNA results can be obtained from a very small, old or degraded sample, and is found in almost all biological material.
  • With the exception of identical twins, no two people have the same DNA. It is unique.

What are the disadvantages of DNA analysis?

  • Cost. DNA equipment and trained personnel are not cheap, although advances in science and technology are making it more affordable every day.
  • Sensitivity. Because DNA is so prevalent and the techniques so sensitive, extreme care must be taken not to contaminate the evidence.
  • Complexity. While the interpretation of a single source of DNA is usually fairly straight forward, it can be very difficult to correctly interpret samples with multiple sources and concentrations. Forensic scientist must undergo a long, continual training program to stay current with the technology.
  • Time. Because of the complexity, DNA analysis usually takes several days at best. Also due to the demand, there is often a backlog of cases to be analyzed and a limited number of resources.


Forensic DNA Yield Gel

DNA Yield Gel

What types of tissue can be analyzed for DNA? There are many, but the most common are:

  • Blood
  • Saliva
  • Hair (with a root)
  • Bone
  • Teeth
  • Semen
  • Vaginal secretions

What types of evidence have been used for DNA analysis?

  • Bloodstains
  • Toothbrush
  • Beverage containers
  • Hats
  • Cheek swabs
  • Sexual assault stains
  • Gloves
  • Chewing gum
  • Cigarettes

Almost anything that comes in contact with the human body will contain traces of DNA and is a potential source of evidence.

Forensic Trace Analysis

Forensic Trace Analysis the analysis of paint, accelerants, glass or chemical debris.  This type of evidence is most commonly involved in arson and vehicular homicides.  Some forensic laboratories also consider the analysis of microscopic hair and fiber evidence part of the trace chemistry discipline.

Air samples or fire debris are often brought to the laboratory and analyzed for the presence of an accelerant, indicating an intentionally set fire.   Trace chemical analysis can also be performed on debris from an explosion to determine which type of explosive may have been used.  The forensic scientist will use a variety of instruments, including microscopes, x-ray diffraction, gas chromatography, infrared spectroscopy and energy dispersive x-ray micro-analysis.

Hit and run accidents often leave behind a transfer of paint or other chemical materials from one surface to the next.   The forensic trace analyst is often asked to analyze the paint to determine the possible origin, including the make and  model of the originating vehicle.   If a suspect is identified, the analyst may need to compare the evidence sample to a “known” sample from the suspect vehicle to determine if the two samples “match”, and could share a common origin.

In addition to fire, explosion and paint debris, forensic trace analysis could include the chemical analysis of virtually any other type of evidence.  This could include items such as duct tape, cosmetics, concrete, fabric, soil, and gunshot residue.

Forensic Polygraph Analysis

Forensic polygraph examiners often play a very important role in criminal investigations for local and national law enforcement. Polygraph analysis is part art, part science. In addition to maintaining and operating the polygraph machines, the forensic polygraph examiner is also responsible for administering the actual polygraph exam, and then interpreting the results to come to a reasonable conclusion on the truthfulness of the test subject.

Polygraph exams are often not considered sufficient proof on their own to warrant a conviction, or even to charge a person with a crime. Even in jurisdictions where they are not, the results can still point investigators in the right direction, and also to help them narrow down the list of potential suspects. There have been some criticism regarding the usefulness of forensic polygraph analysis. Considering that suspects and witnesses are only subjected to the test voluntarily, because it is illegal to force someone to submit to such a test against their will, investigators do not always get to examine all the suspects they would like in this manner. Despite the legal ability of suspects and witnesses to avoid a polygraph examination, many people view taking one as an opportunity to clear their name, while others will submit because they either think they can beat it or the results cannot be used against them.

Polygraph examiners must be trained in several specialties. Like forensic scientists, they must understand the principles that govern their analysis. But unlike other forensic disciplines, which often have straight forward results, the data resulting from a polygraph examination ca be much more open to interpretation. So in addition to knowing the general stress ranges of people when they are lying, the polygraph examiner must also establish each test subject’s individual range. This is the purpose of the “control” questions at the beginning and end of each polygraph test, because due to the uniqueness of each person’s physiology, the readings that indicate regular stress such as the heart rate, body temperature, and blood pressure changes are compared with those while being untruthful. Setting test subjects at ease in an environment where they are very much “on the spot” can be quite difficult, especially if the subject is a suspect in a serious crime. Most examiners draw out the control period for an extended period of time, to establish what the patient’s resting state.

The examiner must also be trained in how to spot the methods people can use to “beat” the polygraph test. It is not uncommon for test subject to try to control their breathing or attempt other disruptive behavior whenever they are asked a question. The idea behind this is to alter the patterns of the readings to make analysis impossible. It is the knowledge of how to spot such behavior and still get meaningful data that separates a forensic polygraph examiner from being a technician.

Another skill the forensic polygraph examiner must learn is how to interpret body language and behavioral clues. Before the subject is even “hooked up” to the polygraph machine, a successful polygraph examiner begins the analysis by observing the subject and reviewing with them the questions that will be asked during the exam. It is during this part of the process that a skilled examiner can actually gain the trust of a subject and get them to reveal clues or even confess to their actions.

Questioned Documents

A forensic scientist specializing in questioned documents usually work cases involving alleged forgery, blackmail, kidnapping and death threats.

Although questioned documents examiners are often referred to in the media as “handwriting experts”, they frequently examine evidence with no hand writing at all.  These items may include printed materials, typewritten documents, photographs, and duplicated productions (copies).   It is often the task of the document examiner to determine if the item in question has been altered, forged, or obliterated.  This requires the use of special light techniques, most commonly infrared or ultraviolet light.  Inks and printing appear differently under certain light depending on their chemical composition and the amount of force applied to the medium.  It is therefore possible to determine if more than one utensil has been used to produce a document and if more than one technique or source has been used.

forensic questioned document

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For example, a common tactic among forgers is to alter a legitimate check and increase the amount of the draft.  A check for $100 may appear to have changed to read $10,000.  By examining the check under alternate light sources, it can sometimes become very obvious that a two different pens were used to add the additional zeros to the amount.  This type of analysis is photographed and can easily be demonstrated to a jury should the case go to trial.

In the case of handwriting analysis, it is possible to prove that a specific individual did or did not write the document in question.   This is done by obtaining “known” samples of handwriting from the person of interest and comparing those samples to the piece of evidence.   The examiner will look at individual strokes, loops, spacing , and slant.   Determining that someone is NOT the source of a document is often times easier and quicker than concluding that an individual is the source.  In addition to using alternate light sources in their analysis, the question document examiner will also often use various microscopes, including comparison scopes and even electron microscopes.

In addition to handwriting and printed materials, the document examiner may also be asked to perform impression analysis.  This may include evidence involving rubber stamps, gloves, footwear and tire impressions,  indented writing and tool marks, glove impressions, and footwear and tire impressions.
Training to become a question document examiner takes several months and involves case studies and laboratory exercises.   The forensic document examiner must frequently work in close conjunction with latent print examiners, as there is often times a need for fingerprint analysis on the evidence as well as document analysis.

forensic document

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Individuals interested in a career as a document expert should expect slow, methodical work and must be able to explain their analysis to a jury that often confuses the science behind this type of work with the erroneous perception that is a subjective art.   Forensic Questioned Document examiners are not the same as handwriting hobbyists who assign personality traits to various writing styles or to profiling experts who analyze the content and psychological meanings of criminal evidence.

Forensic document examiners in a criminal laboratory determine if a piece of evidence has been altered, obliterated, forged, or otherwise tampered with and who may or may not be responsible for the document based on careful comparison between it and known samples.

Famous cases in the United States that involved question document analysis include the “Unibomber”, the “Zodiac Killer”, and the Jonbonet Ramsey homicide.


Forensic Toxicology

Forensic toxicology is the use of scientific techniques to aid medical or legal investigation of death, poisoning, and drug use.

Forensic Toxicology

Forensic Toxicology

A Forensic Toxicologist may use a variety of analytical instrumentation and techniques to isolate and detect intoxicating substances in various kinds of biological samples.
Isolating the toxic substance from the sample is often the most difficult, as forensic tissues are often old, contaminated or degraded. The sample has to be concentrated and purified as much as possible before being analyzed for identification.

Because many poisons are changed by human metabolism after ingestion, it is important for the forensic toxicologists to gather as much background information from the investigating agency as possible. This information will be used to consider all possibilities of a toxilogical source when presented the data from the analytical results of testing.

The most common types of samples the forensic toxicologists works includes blood, hair, saliva, urine, eye tissue and skin. They may be asked to examine the samples for organic compounds, metals or drugs and present their finding in court as an expert witness.

Latent Prints

“Latent prints” is a term used to describe the forensic discipline of identifying fingerprints, palm prints, toe and sometimes even lip prints to a single individual. The term “latent” actually means hidden, which of course, fingerprints at a crime scene usually are. Forensic scientist also examine “patent” prints, which means obvious, or clear , i.e. not hidden. An example of a patent print may be one made in blood and easily visible.

Fingerprint examination has been in use as an identification technique for over a hundred years.

Human fingers have friction ridges in the skin which makes up a unique pattern. No two people, and in fact, no two fingers have the same pattern. Therefore, matching a fingerprint to the finger that it originated from is an absolute identity.

latent print

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A fingerprint is made when the debris from the friction ridge of the skin comes in contact with an object. The debris is usually dirt, oil, perspiration, grease or ink. The print is often “lifted” from the surface, which simply means transferred to something that can be moved. It is then brought to the lab and the forensic scientist may use a vast array of procedures, such as the application of powders, chemicals, fumes and dyes to make it visible. Special devices, including powerful ultraviolet lamps and lasers, may also be utilized.

Once it can be seen, the print is photographed, scanned and examined. The Latent Print examiner spends tedious hours studying the patterns of the print, and identifying specific points of the ridge pattern that make it unique. If enough detail can be found through the analysis, the pattern can be compared to known prints of suspects and victims. People are then included or eliminated as a possible source of the print. Many times, upon completion of their analysis, the latent print examiner is able to positively identify or exclude a person of interest. A positive identity is the most powerful form of forensic analysis and therefore the most sought after in the legal system.

Latent prints may also be searched against a nationwide database of prints known as AFIS (Automated Fingerprint Identification System). AFIS contains the fingerprints of millions of persons, as well as unknown prints of other crime scenes. The computer analysis will provide a list of possible matches to the forensic scientist who will then manually compare the results to determine if a complete match really exists. Thousands of cases are solved or linked together as a result of the AFIS system every year.

Educational Requirements for a Forensic Scientist

Forensic Scientists are considered experts in their field and often must testify in court to explain their analysis and the significance of their findings. To become a forensic scientist requires continual learning, however for those just starting a career in forensic science, a Bachelor’s Degree in any field is often acceptable. An advanced degree or an emphasis in science classes is beneficial to the initial hiring process.

Forensic Science Educational Requirements

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Due to the rising popularity of forensic science brought about by several successful television series, several colleges and universities are now offering courses specializing in forensics and legal studies. These courses can certainly help you when you look for a job as a forensic scientist but they are not required. Certain specialties in forensic science require specific classes, for example, Genetics, Biochemistry and Organic Chemistry are required for DNA analyis. If you are interested in a becoming a forensic scientist, it is best to check with nearby forensic laboratories before applying to see if they have any advice for you that may be unique. Laboratory experience is also helpful in addition to a degree, even if it’s not in a forensic lab. Some agencies offer internships which is an excellent way to get started.

A “discipline” refers to a specialized area of expertise in forensic science. Examples of disciplines are question documents, fingerprint analysis, DNA analysis, and drug chemistry. Most forensic laboratories enroll scientist in their own training program designed for their discipline immediately after hiring. Everything about the forensic specialties is taught to the trainee after they are hired. Laboratories strive to have a certain number of scientists with advance degrees. For example, laboratories accredited with ASCLD require that a lead DNA analyst have a minimum of a master’s degree. So while it’s possible to be hired with only a Bachelor’s degree, those with a Master’s or phD do have an advantage.

If you are interested in becoming a forensic scientist, the first thing you need to do is learn about the various disciplines and descide which one intests you. Contact the forensic laborartory in your area and ask about their requirements and hiring opportunities. Forensic labs are found in nearly all states and privately owned laboratories exist as well. On a national level in the United States, the Federal Bureau of Investigation (FBI) operates a forensic science laboratory and hires trainees as well.

A good education is required to become a forensic scientist and it’s crucial to take your studies seriously if you want to succeed. A career in Forensic Science is highly competitive and very demanding. Not only are good grades important, but you must also be willing to continue your education throughout your career.

Forensic Chemistry

Forensic Chemistry, or “Drug Chemistry” is the forensic discipline of identifying evidence thought to be a controlled substance. Most often, the types of evidence the forensic drug chemist examines are pills, capsules, plant materials, tables, powder, drug paraphernalia and residues.

drug chemistry

Drug Chemistry

When an individual is prosecuted for possession or distribution of a controlled substance, the degree of the punishment and offense is often directly related to the amount of drugs identified in the case. It is the job of the drug chemist to accurately weigh the amount of substance, using a balance and then positively identify the drug in question.

The evidence is inventoried, often times photographed, and tested with a one or more techniques, including traditional spot tests, thin layer chromatography (TLC) and extraction, as well as instrumental techniques such as ultraviolet (UV) spectroscopy and gas chromatography (GC).

Forensic Drug Chemistry

Marijuana is frequently examined.

Advanced techniques such as Infrared spectroscopy (IR) or mass spectrometry (MS) are used to confirm the identity of questioned substances. Microscopy is also available for examining case material, such as marijuana. A report is issued to the investigative agency identifying any controlled substances amount detected. The Forensic Chemist is often required to testify as an expert in court to describe the methods used and the conclusions that were reached.


A Microscopist is a forensic scientist who specializes in the analysis of hairs, fibers and debris. Most commonly, a microscopist will examine hairs and fibers to determine their possible origin.

When a hair is examined, the scientist is often asked to answer the following questions:

  • Is it human?
  • Is it scalp hair or pubic hair?
  • What is the race of the individual it originated from?
  • If it is not human, what species is it from?
  • Was the hair dyed or treated?
  • Is genetic material present for DNA testing?
Hair under a microscope

Microscopic view of a hair

Fibers are also commonly examined, with the goal of answering similar questions:

  • What type of fiber is it?
  • Is it man made or natural?
  • What kind of material is the fiber from?  (Carpet, blanket, sweater, etc.)

In addition to hairs and fibers, the microscopist may be asked to examine such things as:

  • Building materials
  • Safe insulation
  • Wood
  • Soil and rock

Microscopy is a very labor intensive and lengthy process.  Forensic microscopists must be patient and enjoy looking in a microscope and working with very tiny materials.

Forensic Biology

Forensic Biology is the analysis of body fluids, stains and other bodily materials to help solve a crime. Typically, this involves the positive identification of blood, semen, or saliva and further genetic testing (DNA) to determine who the material may have originated from, typically the alleged victim, suspect or other involved party.

Some laboratory systems expect the Forensic Biologist to handle all evidence in their area from a given case from “cradle to grave”, i.e from initial detection to identification and DNA testing and prosecution. Some use an “assembly line” approach, where one scientist may do only evidence screening, which means they only look for the body fluids, another would do tests to determine what the fluids are, and yet another would do DNA analysis. This approach allows the scientist to focus on a single area of expertise, although the “cradle to grave” approach gives a better overall understanding of the case and what may or may not be important based on related factors.

Depending on the agency or company you work for, you therefore may be trained in a variety of biological techniques or possibly just a single specialty, like DNA analysis. Some agencies also expect their scientists to go to the crime scene and collect evidence Most, however have crime Scene Technicians dedicated to this task. The evidence is then transported to the laboratory for analysis.

A Forensic Biologist usually begins their analysis by examining a piece of evidence for the presence of hairs, fibers and stains. Any collected hairs or fibers of evidential value are most often transferred to a Forensic Microscopist for further analysis. However, it is usually the job of the Forensic Biologist to collect and preserve the hairs and fibers initially.

Many times an alternate light source, such as a laser is used to find stains on articles such as bed sheets or clothing. A portion of the stain is removed and tested for identification. The most common types of test are those for blood and semen, although at times it is necessary to test for other body fluids or tissues such as urine or saliva. A forensic scientist must know not only how to perform the appropriate test, but also how to interpret the results. Some tests can provide absolute answers, for instance, if a stain is in fact blood. Others can only provide a “likely” answer, like in the case of saliva, where there is no absolute conclusion. A positive analysis for this type of test means only that a substance is indicated (probable), not conclusively identified. Overstating conclusions is always a risk for the poorly trained or over confident Forensic Biologist.

Once a stain is identified, the scientist is often asked to determine who it could or could not have originated from. For example, if blood is found in a car, it may be beneficial to the case to find out if it originated from a people thought to be involved in the case, usually a victim and suspect. In this scenario, DNA analysis would be conducted on the blood in the “questioned stain” to develop a DNA profile. The questioned profile would then be compared to the known DNA profiles of the victim and suspect. If the profiles are the same, the are considered a “match” and the next step is to figure out what that match means. How many other people could be expected to have the same profile? Depending on amount of detail discovered in the DNA profile, this could range from a lot of people, to one in several billion, or essentially only a single person. With this type of strong match, DNA analysis if very similar to a fingerprint, and the analyst is able to testify in court that the blood originated from specific individual.