CHAPTER Forensic Laboratory Techniques

CHAPTER

CHAPTER

1

Forensic Laboratory

Techniques

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WHERE IS LACI?

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REUTERS/LOU DEMATTEIS/FILE LD/GN

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Laci Peterson was

27 years old and in

her eighth month

of pregnancy when

she

disappeared

from her home in

Modesto, California,

on December 24,

2002. She was last

seen walking her

dog that morning at

10:00 a.m. Her husband Scott returned

home from a fishing

trip and discovered

that Laci was gone.

Figure 3-1. Laci Peterson.

Trying to locate her,

he contacted friends and family. He reported her

missing shortly before 6:00 p.m.

Over the next three days, police, firefighters,

and volunteers searched for Laci, but there was

no sign of her. A grid-pattern search along Dry

Creek revealed no evidence. Three days after

her disappearance, the FBI joined in the investigation, and the Peterson home was searched.

Peterson had been a suspect in his wife¡¯s disappearance almost from the beginning. When he

confessed to having a romantic relationship with

another woman, he became a very strong suspect.

Peterson maintained that he had told Laci about

his indiscretion and that it was not something that

would have ended their marriage. At first, Laci¡¯s

parents supported Peterson. When it was also

discovered that he had taken out a $250,000 life

insurance policy on Laci, they stopped.

As time passed, evidence against Peterson

mounted. A powdery substance found on

Peterson¡¯s boat was identified as concrete

dust. A long, dark hair caught on pliers was

found in the same fertilizer warehouse where

Scott, a fertilizer salesman, had stored his

boat. Forensic scientists used comparison

microscopes to compare this hair to hair

known to be Laci¡¯s. They examined the cuticle,

pigmentation, and medulla. Recall from previous coursework that the cuticle is the outer

layer of hair. The medulla is the center core.

The hair found at the warehouse was consistent with Laci¡¯s hair. Scott Peterson had purchased a four-day fishing license on December

20, but he told police he hadn¡¯t decided to go

fishing until the morning of Laci¡¯s disappearance (December 24). His blood was found on

the driver¡¯s door inside his truck.

On April 13, 2002, the body of a fetus with the

umbilical cord still attached washed ashore near

Point Isabel in Northern California. The next day, a

female¡¯s body was discovered in a park near Point

Isabel. DNA testing revealed them to be the bodies

of Laci and her baby, Conner.

Scott Peterson was convicted of the two murders and sentenced to death by lethal injection.

CHRIS HARDY/SAN FRANCISCO CHRONICLE/

CORBIS

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Figure 3-2. Laci¡¯s body being loaded into

the van.

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Objective

s

By the end of th

is chapter, you wi

ll be able to:

een physical an

d chemical prop

3.2 Describe

erties.

presumptive an

d confirmatory te

3.3 Compare

sts.

and contrast di

fferent types of

microscopes.

3.4 Explain

how qualitative

analysis differs

from quantitativ

analysis.

e

3.5 Different

iate between th

in-layer chromat

matography, an

ography, gas ch

d high-performan

roce liquid chrom

3.6 Calculat

at

og

ra

ph

e Rf (retention fa

y.

ctor).

3.7 List and

describe three ty

pes of spectrosc

3.8 Compare

opy.

and contrast te

chniques for visu

3.9 Describe

alizing fingerprin

the structure of

ts.

DNA.

3.1

Distinguish betw

Vocabula

ry

chemical p

roperty property

substanc

of a

e that d

escribes

physical pro

reacts i

how it

perty - p

n the pr

roperty

esence o

s

ubstance

substanc

of a

f other

es

that can

be obser

o

r

m

easured

chromatog

ved

without

raphy - an

the chem

changing

y of sev

processe

i

c

a

l identi

eral

s used t

ty of th

substanc

o separa

mixture

e

e

te a

into its

individu

p

re

componen

s

u

m

ptive test

al

ts based

- a test

on their

evidence

attracti

to scree

and narr

on to a

n

ow down

s

tationar

p

o

or solid

s

s

i

b

the

le type

y liquid

of a sub

stance

Rf value con?rmato

retentio

ry test n

t

i

f

e

n

actor;

st done

establis

paper an

to

h with c

d thin-l

ertainty

c

a

h

y

characte

r

er

o

m

a

t

o

the

graphy,

ristics

ratio of

of a sub

distance

the

frequency

stance

a substa

- the num

nce trav

the dist

ber of wa

e

that pass

led to

ance the

ves

a specifi

solvent

wavelength

c point w

traveled

a given t

- the di

ithin

ime; usua

stance b

crests,

lly expre

in cycles

etween

o

r

s

sed

peaks, o

per secon

f two

consecut

d or

hertz (Hz

i

v

e waves

)

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CHAPTER

Obj. 3.1

I N T ROD U C T I ON

Hairs and fibers found at a crime scene can give investigators a great deal

of information. Microscopy and other analytical techniques can be used to

determine whether the hairs came from a human or another animal. These

techniques also help investigators determine whether the fibers came from

carpet, clothing, or something else.

In the crime lab, forensic scientists compare samples of evidence from

the crime scene to known samples. The evidence samples are called questioned samples. The known samples are called controls. Ultimately, investigators hope to identify the evidence samples through specific physical and

chemical properties.

Physical properties are properties that can be measured without changing the identity of the evidence. For example, when forensic scientists calculate the density of glass, they divide the mass of the glass by its volume.

Measuring mass and volume does not affect the chemical makeup of the

glass. Therefore, density is a physical property. Other physical properties

include color, melting point, boiling point, odor, and viscosity. Changes

to substances that do not alter the chemical makeup of the substance¡ª

cutting, shredding, melting, or freezing¡ªare physical changes.

Chemical properties determine how a substance behaves in the presence of other substances. For example, iron will react with oxygen in the

presence of water to produce rust, or iron oxide. Changes to a substance

that alter its chemical identity are chemical changes. Rusting, burning, and

decomposing are chemical changes. When chemical testing is done on evidence, the original evidence sample is often destroyed.

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Obj. 3.2

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PRESUMPTIVE AND CONFIRMATORY

Y

TESTS

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? PABLO PAUL/ALAMY

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Figure 3-3. Presumptive test for blood.

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At a crime scene, field investigators must make immediate d

decii

sions regarding potential items of evidence. For example, if

an investigator finds a red stain at a homicide scene, he or she

must conduct initial tests to narrow down the possible identity

of the stain. Although it is easy to assume that the red stain is

human blood, it could also be paint, ketchup, or blood from

an animal. Presumptive tests allow a field investigator to screen

evidence to reduce the number of possibilities and to get a preliminary identification. If presumptive tests at the scene show

that the red stain is blood, investigators will collect additional

samples. The tests do not, however, tell crime-scene investigators whether the blood is from a human or another animal.

These samples are sent to the lab for confirmatory tests.

Confirmatory tests are used to make a more specific identification. A confirmatory test would determine whether the blood

belonged to a human or some other animal (see Figure 3-3).

Presumptive tests exist for saliva, semen, blood, urine,

and vaginal secretions. There are also presumptive tests for

many kinds of drugs. Presumptive tests screen for chemicals

in each fluid. At the crime scene, investigators might use a

Forensic Laboratory Techniques

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UV (ultraviolet) light, or another alternative light source, to determine

whether a stain is a body fluid. If so, a sample will be collected and sent

to the lab. At the lab, forensic scientists may perform presumptive tests

to determine what kind of body fluid. For example, semen contains the

enzyme acid phosphatase. Although this enzyme is found in other fluids,

including vaginal secretions, it is found in much higher concentrations

in semen. The presumptive test for semen is actually screening for acid

phosphatase. To definitively identify the evidence as semen, confirmatory

testing would be completed at the crime lab. For example, the fluid may

be viewed under a microscope. If sperm is present, the fluid is semen. In

this case, microscopy is a confirmatory test. To identify the person who

produced the semen, the sample must undergo more specialized confirmatory tests, such as DNA profiling. Presumptive testing reduces costs

and aids field investigators in collecting evidence essential to the case.

Confirmatory tests are necessary to identify, with certainty, a piece of evidence. Each test plays an integral role in forensic investigations.

MI C RO S C O P Y

Obj. 3.3

Prior to the mid-1600s, microscopes could magnify a specimen only about

b

six to ten times its original size. In 1665, Robert Hooke published a book

called Micrographia. In it, he described cork cells. Hooke was the first to

observe cells; he used the term cells because the dead cork cells looked like

small rooms. In 1676, Antoine van Leeuwenhoek was the first to observe single-celled microscopic organisms. Today, there are several different kinds of

microscopes. Some can magnify an object hundreds of thousands of times.

COMPOUND LIGHT MICROSCOPE

IMAGE COPYRIGHT BRIAN MAUDSLEY , 2010.

USED UNDER LICENSE FROM SHUTTERSTOCK.

COM

The compound light microscope is probably the most widely used microscope today. This microscope has a light source and multiple lenses to

obtain high magnification. The compound microscope usually has a magnification between 40¡Á (40 times) and 1,000¡Á (1,000 times). Compound

microscopes are powerful enough to view hair, fibers, and cells. Figure 3-4

shows a cross-section of an artery through a compound light microscope at

different magnifications.

Some of the earliest

microscopes were

simply magnifying

glasses. Looking at

small insects like

gnats and fleas

was very common.

For that reason,

microscopes were

sometimes referred

to as flea glasses.

Figure 3-4. The image on the left has been magnified 40¡Á.

The image on the right has been magnified 100¡Á.

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CHAPTER

STEREOMICROSCOPE

A compound microscope works by sending light

through the specimen. Sometimes, a specimen is

too thick or opaque to be seen through a compound

microscope. The light of a stereomicroscope, or

dissecting microscope, is reflected from the surface

of the specimen. Because the light is reflected, the

stereomicroscope produces a three-dimensional

image useful for dissecting. Surface details are also

more visible with the stereomicroscope. Forensic

investigators use a stereomicroscope to examine

insect larvae, paint chips, and other small items of

evidence.

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OMAR TORRES/AFP/GETTY IMAGES

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COMPARISON MICROSCOPE

The comparison microscope is another useful tool (see

Figure 3-5). The comparison microscope is actually

two microscopes connected to one eyepiece. When

the investigator looks through the eyepiece, he or

she sees a circular, split-view window. The image on

the right is of the specimen under the microscope

on the right and can be compared side-by-side to

the image on the left. The comparison microscope is

particularly useful when comparing bullet striations,

fibers, and hair samples. Investigators are able to

make comparisons while viewing two samples at the

same time (see Figure 3-6). Usually, the investigators

will compare a known sample to a questioned sample.

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Figure 3-5. Comparison microscope.

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PHILIPPE PSAILA

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Figure 3-6. Images of two bullet casings as seen

through a comparison microscope. One casing was

found at the crime scene. The other was taken from

the suspect¡¯s gun.

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