forensicsciencetraceevidence法医学物证.ppt

1,FORENSIC SCIENCE Trace Evidence,2,Introduction,Trace Evidence--any small pieces of material, man-made or naturally occurring Most common examples: Hair Fiber,3,Test Questions for Trace Evidence,What is it? Is it man-made or natural? What is its source? How common is it? Can it be identified to a single source?,4,Hair,Human hair is one of the most frequently found pieces of evidence at the scene of a violent crime. It can provide a link between the criminal and the act. From hair one can determine: Human or animal Race Origin Manner in which hair was removed Treated hair Drugs ingested,5,DNA,The hair shaft contains abundant mitochondrial DNA--inherited only from our mothers. It can be typed by comparing relatives if no DNA from the body is available. Only the root contains nuclear DNA.,6,Hair Morphology The Study of Structure and Form,Parts of the hair Shaft--part of the hair that sticks out of the skin Root--lies below the epidermis Follicle--structure from which the hair grows,,,,7,Hair Growth,Terminology Anagen--hair that is growing Catagen--hair at rest Telogen--hair that is dying Length--00.5 mm per day or 1 centimeter per month; approximately one half inch per month,8,Hair Cuticle,Mosaic Pectinate Imbricate Petal Diamond petal Chevron,The cuticle is the outermost layer of hair which is covered with scales. Scales also always point toward the tip of the hair. These scales differ between species of animals and are names based on their appearance. Some of these scales are variations of the same and include:,9,Scale Types,Mosaic,Chevron,10,Scale Types (cont),Pectinate,Imbricate,11,Scale Types (cont),Petal,Diamond Petal,12,HUMAN SCALES,In order to visualize the scales paint fingernail polish on a slide place a hair on the polish lift off the hair and observe the scale imprints What pattern is seen in this slide?,13,Hair Cortex,The cortex gives the hair its shape. It has two major characteristics: Melanin--pigment granules that give hair its color Cortical fusi--air spaces, usually found near the root but may be found throughout the hair shaft,14,The medulla is the hair core that is not always present. The medulla comes in different types and patterns. Types: Continuous Intermittent or interrupted Fragmented Absent--not present,,,,Hair Medulla,15,Hair Medulla Patterns,Uniserial Multiserial Vacuolated Lattice Amorphous (without a distinct pattern),16,HUMAN MEDULLA,Human medulla may be continuous, fragmented or absent.,17,RABBIT MEDULLA,Rabbit medulla is different depending on the type (location on the rabbit) of hair. The one to the left is multiserial. The one to the right is a uniserial ladder and is found in guard hair.,18,Medullary Index,Determined by measuring the diameter of the medulla and dividing it by the diameter of the hair.,Medullary Index for human hair is generally less than 1/3. For animal hair, it is usually greater than 1/2.,19,Hair Comparison,Color Length Diameter Distribution, shape and color intensity of pigments granules Dyed hair has color in cuticle and cortex Bleaching removes pigment and gives yellow tint,Scale types Presence or absence of medulla Medullary type Medullary pattern Medullary index,20,Collection of Hair Evidence,Questioned hairs must be accompanied by an adequate number of control samples. from victim possible suspects others who may have deposited hair at the scene Control Sample 50 full-length hairs from all areas of scalp 24 full-length pubic hairs,21,Hair Toxicology,Advantages: Easier to collect and store Is externally available Can provide information on the individual’s history of drug use. Collections must be taken from different locations on the body to get an accurate timeline.,22,HAIR TESTING Procedure,Collect an adequate sample, cut as closely to the scalp as possible. Wash the hair to remove lipids, oils, cosmetics and any drugs adhering to it Cut it into one centimeter sections Place hair in a digesting solution Screening test--antibodies are added to the hair that bind with the drugs. If this shows that drugs are present: A confirmation test is done by gas chromatograph and then a mass spectrometer.,23,Fiber Evidence,NOTE: Fabric is the type of material and fibers are the “threads” that make up the fabric The use of fiber evidence in court cases is used many times to connect the suspect to the victim or to the crime scene. In the case of Wayne Williams, fibers were the entire case. Williams was convicted in 1982 based on carpet fibers that were found in his home, car and on several murder victims. Although this case is unusual, fibers are generally considered of greater value as evidence than that of rootless hairs since they may contain a greater number of variables, thus showing more individual characteristics.,24,Polymers,Synthetic fibers are made of polymers which are long string of repeating chemical units. The word polymer means many (poly) units (mer). The repeating units of a polymer 。