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Crime Scene




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Trace Evidence

Forensic science is the application of the scientific method to help the judicial system, or common good of society. As a forensic scientist one must be impartial, and only draw conclusions based on what the evidence shows. A forensic scientist might be asked if other scenarios are "possible", and even if other scenarios are as strongly supported by the evidence as the scientist's opinion, the scientist must admit it when other scenarios are possible.

This is does not mean "anything is possible". My computer had to be manufactured before I could buy it. In many instances in science, scientific certainty simply means that probability of any other option is so remote it is a practical impossibility. For example, I am scientifically certain the sun will rise tomorrow morning. While in theory hostile aliens from a distant galaxy could blow up the sun overnight, the chances of that happening are so remote, it is a practical impossibility - so much so that as a scientist, it would be imprudent to entertain such a notion.

Today forensic scientists are called upon by law enforcement, courts, defendants, and plaintiffs to analyze evidence and report conclusions. Sometimes forensic scientists are asked to be an expert witnesses in court and report those findings to a trier of fact.

Forensic Alcohol

It might come as a shock to some, but in alcohol - DUI cases - the forensic scientist will be facing the toughest cross-examination type of their career. When thinking of a "tough case" a lot of people think of high-profile homicides or the like. The truth is that DUI cases are some of the toughest, because the defense attorneys are highly skilled and specialized in such cases.

And why not? They automatically know their clients will have some money, they had a car and insurance right? On top of that, a lot of defense attorneys are ex-prosecutors. So they had years of training on alcohol analysis from the state's forensic scientist. In many cases the defense attorney might be the most knowledgeable "expert" in alcohol analysis in the courtroom (or maybe the judge).

There are two major types of analysis types for DUI cases in the United States. The first is breath alcohol analysis. That is an instrument is used to measure the alveolar (or deep lung) air of the subject. This air ethanol concentration is either used in an attempt to relate back to a blood alcohol concentration, or local laws might have limits for breath alcohol levels as well.

The second type of analysis is blood alcohol analysis. This is when a blood sample is drawn from the subject (sometimes forcibly by search warrant) and an ethanol concentration is determined thru various methods. The blood draw can either take place at a hospital, or by trained police officers, depending on the applicable local laws. The blood samples are then analyzed in a laboratory and the results obtained. If a hospital does the analysis, before using the reported value, one has to know if the hospital used "whole blood" or just "blood serum/plasma". Most per se laws are written for whole blood alcohol concentrations, and if a hospital did not analyze whole blood a conversion factor must be applied.

If the analysis is carried out in a forensic laboratory, whole blood is analyzed. Usually the blood isn't analyzed directly, but rather the head-space above the blood. Following Henry's Law, the concentration of ethanol in the original blood sample can be obtained.

If the forensic scientist wants to really get some good court experience, forensic alcohol is the specialty to seek out.

The best place for training on forensic alcohol analysis is the Indiana University Center for Law in Action Borkenstein Course. Here forensic scientists, defense attorneys, police officers, and prosecutors can all learn about forensic alcohol analysis and impairment.

Arson Analysis

Arson analysis involves testing for ignitable liquid residues. At a suspected arson scene, investigators collect samples they suspect contain ignitable liquid residues – the remains of a liquid propellant used to start or propagate the fire. The investigator chooses where to sample based on different criteria including visual clues at the scene, training and experience of the investigator, and any possible indications from a specially trained canine.

The samples should be stored in an airtight container if they are collected debris from the scene. Generally unused/clean paint cans or glass jars are used. If the sample is a liquid, a sample of the entire liquid can be collected in a small glass vial with an appropriate cap/lip. The samples are submitted to the laboratory for analysis. Once at the lab, the samples should be analyzed in a reasonably short period of time to prevent loss of the volatile evidence.

There are various sampling techniques, see ASTM methods listed below. A liquid can be diluted by a factor of about 1000:1, or it can have direct headspace, analyzed by GC/MS analysis.

A debris sample in an airtight container can be extracted/concentrated by passive adsorption. A charcoal strip can be introduced into the airtight container in such a way as to not come into contact with the debris (like hanging from a string). The container is then put in an oven at low heat (60 –80 degrees C for example) for several hours. Any volatile chemicals in the debris will adsorb onto the charcoal strip. The strip is then treated with an organic solvent (pentane, carbon disulfide for example), and that solvent is analyzed by GC/MS analysis.

The results of the GC/MS analysis are compared to known standards run on the same GC/MS. The forensic chemist will not try to identify what specific ignitable liquid residue was present, but rather what ASTM classification it belongs to. The classification system is comprised of Classes 0-5:

Class 0 – Miscellaneous. This classification is broken down further based upon what type of chemicals are present. There are oxygenated solvents, isoparaffinic, normal alkane, and aromatic products.
Class 1 – Light Petroleum Distillates. This classification includes some lighter fluids and camping fuels.
Class 2 – Gasoline
Class 3 – Mid-Range Petroleum Distillates. This classification includes some paint thinners and charcoal lighter fluids.
Class 4 – Kerosene
Class 5 – Heavy Petroleum Distillates. This classification includes some jet fuels, diesel and fuel oils.

ASTM E 1386, Standard Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Solvent Extraction, American Society for Testing and Materials, Philadelphia, PA, 1995.

ASTM E 1388, Standard Practice for Sampling of Headspace Vapors from Fire Debris, American Society for Testing and Materials, Philadelphia, PA, 1990.

ASTM 1412, Standard Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Passive Headspace Concentration, American Society for Testing and Materials, Philadelphia, PA, 1995.

ASTM 1387-01, Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography, American Society for Testing and Materials, Philadelphia, PA, 1995.

ASTM 1618-01, Standard Guide for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry, American Society for Testing and Materials, Philadelphia, PA, 1994.

Newman, R., M. Gilbert, and K. Lothridge, GC-MS Guide to Ignitable Liquids, CRC Press, New York, 1998.

Forensic Biology

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Crime Scene

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Forensic Document Examination

Typically, a forensic document examiner will be asked to determine authorship or authenticity of a document. In either case, the examiner may also detect "latent" or "hidden" images on a document.

Authorship - Usually, the examiner will compare the handwriting or hand printing from a questioned or disputed document to known handwriting or hand printing from an individual in order to identify or eliminate that individual as the author of the questioned document(s). Comparison of the style, line quality, connecting strokes, idiosyncrasies, character formations and other features are used to determine authorship.

Sometimes examiners will be asked to identify the brand or even the specific office machine such as a typewriter, copier, or printer that created the document. Identification of the print process and analysis of the ink or toner identify the class characteristic the suspect office equipment. There may be defects on the typewriter, trash marks on the copier, or gripper marks on the printer that help identify the individual office equipment, the creator of the document.

Authenticity - The examiner will determine if a document is genuine by comparing the suspect or questioned document with a genuine or known document. Suspect documents can be currency, driver license, social security card or any other commercial or government document. Analysis of the print process, chemical composition as well as security features, both visible and latent, are used to determine authenticity.

Sometimes age determination of the paper or ink is needed to verify a genuine document such as a will or a painting. Analysis can be in the form of destructive or non-destructive testing. Non-destructive testing can be in the form of alternate light sources (ALS) which will temporarily react with the paper or the paint or the ink or whatever is the document. Destructive testing requires the permanent removal of a sample of the document. This can be in the form of microdots of ink for chemical analysis. Typically, when it will devalue the questioned document a written permission from the owner or legal representative is needed.

Latent or Hidden Images - This could be in the form of indented writing on a pad of paper. Using the Indention Materializer or the ElectroStatic Detection Apparatus (ESDA) or even an oblique or side lighting, the indention from the top sheet of a pad can be detected several sheets down. Once the indentions have been enhanced to a readable image, the content and writer can be identified.

Hidden images can also be in the form of security features in commercial or government documents. ALS has been mentioned as a tool to reveal some of the latent images. Heat can also be used to reveal heat sensitive inks such as lemon juice. The lack of these features will be used to automatically reject a questioned document.

Burned documents are a unique situation. Special handling at the scene, transportation to the lab and in the lab are paramount to the retrieving the content. Tightly pressing the charred document between two unexposed photographic plates can create a photographic image of the charred document.
Recommend reading:

Questioned Documents by Albert S. Osborn (the Father of Handwriting Examination)

Scientific Examination of Questioned Documents by Ordway Hilton

Introduction to Handwriting Examination and Identification by Russell R. Bradford & Ralph B. Bradford

Suspect Documents by Wilson R. Harrison

Controlled Substances (Drugs)

Controlled substances analysis (or drug analysis) deals with the identification of unknown plant materials, powders, crystals, pills, liquids, and paraphernalia items. Briefly the forensic scientist is tasked with the positive identification of an unknown substance, determining the quantity present, and in some cases whether the item is usable or not.

To determine quantity the forensic chemist will use a variety of techniques depending on the form of the substance found. For powders, crystals, and plant material the forensic chemist can weight the material. For liquids a graduated cylinder can be used to determine volume. For pills simply counting the number of discreet units will suffice.

Usability (in Arizona) is based on the form and condition of the substance. That is to say there is no minimum amount of a substance needed for a substance to be "usable". What matters in stead is the form and condition of the material. Can the material be manipulated, or can it readily be made usable, by the known practices of drug users and abusers. While 2.2 milligrams is a "small" amount, depending on its form and condition it can still be usable. Ten pounds of wet, moldy marijuana might not be. Collected vacuuming of an apartment carpet might result in a measurable amount of a controlled substance, but in the form and condition it was collected in it would generally not be in a condition known to be usable to by drug users, and therefore not "usable".

To tackle the problem of identifying the unknown substance, the forensic chemist has numerous analytical techniques at his disposal. The substance can be subjected to a variety of chemical color tests. Color tests are used to find a particular family of chemical compounds. Certain chemical compounds will turn specific colors when a given reagent is added to it.

After chemical color tests the forensic scientist can utilize micro-crystal tests. In these tests a chemical will grow unique micro-crystals when a specific reagent is added.

Many other techniques are available to the forensic chemist. Including Thin Layer Chromatography and Infra-red spectroscopy. One of the most popular tests used to conclusively identify an unknown substance is gas chromatography/mass spectronomy. Chromatography is a technique used in science to separate mixtures. The "gas" in "gas chromatography" denotes that the "mobile phase" is the vapor or gas state of matter. A gas chromatograph is basically a computer controlled oven that has a user defined temperature program. The unknown mixture is added to one end of a column (a small silica based tube with bonded substrate material on the inside designed to react in different way to different chemicals it comes in contact with). As the temperature increases, different chemicals in the mixture will volatilize (convert from a solid to a gas phase) and begin to travel down the column, pushed along by the carrier gas (typically Helium in the US, sometimes Hydrogen in other countries). The chemical will alternate forms between gaseous and condensed on the inside of the column at different rates than the other chemicals in the mixture. The end result is that the chemicals in the original mixture will take different amounts of time to exit the other end of the column.

At the end of the column is the detector. In this case a mass selective detector is used. In simple terms this detector bombards the chemicals exiting the end of the column with a stream of electrons, causing the chemical to become charged, and possibly breaking the chemical into smaller pieces. Once the chemical, and break-down pieces, are charged the begin to be pushed/pulled towards a detector while under the effect of a magnetic field set up in the detector. The detector rapidly changes the magnetic field to screen out only one weight of chemical at a time. The result is that the detector is able to determine what weights the original chemical was, as well as the break-down products the chemical produces. This results in something similar to a "fingerprint" (mass spectra) for the chemical. Using the retention time and mass spectra produced by known standards (either run in the laboratory, or one purchased from a standard library like NIST), the forensic chemist can compare retention time and mass spectra.

The information obtained by chemical color testing, micro-crystal tests, and any other instrument/chemical techniques can all be combined to determine the identity of the unknown substance.

Firearms Identification

Firearms identification, commonly incorrectly referred to as "ballistics", is the discipline in forensic science that pertains to all manners of firearms and firearms related evidence. Some of the types of analysis firearms identification deals with are:

Function testing - The firearms examiner will inspect the firearm for any signs of tampering with its action or safeties. Depending on the case, trace evidence will be collected, such as hairs, fibers, blood or other biological material. Generally the trigger-pull will be determined, that is the amount of force needed for cause the firearm to discharge. Sometimes there can be two trigger-pull forces, one for single-action another for double-action. A patch will be run through the barrel to collect any material left there since the last time the firearm was discharged, and to check for any obstructions in the bore. Finally the firearm will be test fired using ammunition obtained from inside the lab.

Predict-gun analysis - Sometimes only a bullet or casing will be recovered at a shooting scene, or from a post-mortem examination (autopsy). Depending on the marks impressed on the bullet or casing, the firearms examiner can measure these marks and check with a "general rifling characteristics" database maintained by the FBI, and produce a list of possible firearms that could have fired the bullet or casing. This can aid investigators with ongoing investigations, or bolster eye-witness testimony relating to what kind of weapon the suspect was supposed to have used, or if the suspect is known to have possessed that type of weapon in the past.

Bullet and casing comparison - The "bread and butter" analysis for a firearms examiner. In this type of analysis the examiner compares the microscopic markings on the evidence bullets and casings to other bullets and casings or test fires produced in the laboratory. During the manufacturing process of the firearm, there are unique toolmarks left on the firearm. These marks have to potential to be impressed on fired bullets and casings. If a sufficient quality and quantity of these marks are impressed on the bullets and casings, a qualified examiner can determine whether the bullet or casing was identified, excluded, or inconclusive as having been fired in the suspected firearm.

Distance determination - When a firearm is discharged many things are sent flying out the muzzle of the firearm. Besides the bullet, unburned smokeless gunpowder particles, lead and/or copper particles, soot, and primer residue particles. Different combinations of these particles, along with the general composition of the area in and around the bullet impact defect in the targeted object, can speak volumes about the distance the muzzle of the firearm was from the target. Using a suitable test medium, the same firearm, and same ammunition, an examiner can closely approximate the distance the muzzle was from the target. On some special cases, a rough distance can be determined without any testing, especially with very close muzzle to target distances.

For more information about firearms identification, the "bible" textbook is:
Hatcher, Jury, and Weller - Firearms Investigation, Identification, and Evidence. Recently back in print!

Other good firearms identification textbooks include:

Lucien C. Haag - Shooting Incident Reconstruction

Edward E. Hueske - Practical Analysis and Reconstruction of Shooting Incidents

Dean H. Darrison, Jr. - Practical Shooting Scene Investigation: The Investigation & Reconstruction of Crime Scenes Involving Gunfire

M.D., Vincent J.M. DiMaio - Gunshot Wounds

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. Continually updated textbook.

Saferstein, Richard. Forensic Science Handbook.

For more information on firearms identification, please visit:

Association of Firearms and Toolmark Examiners


Latent Prints

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Trace Evidence

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*This information is for educational purposes and should not be relied upon as professional advice. Information is current as of publication date.