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Trace Evidence
Objectives
After reading this chapter, you will understand:
- How to apply deductive reasoning to
analytical data.
- How to follow qualitative analytical
schemes.
- How to gather and use information to solve
problems.
You will be able to:
**- Identify traces of white powder.
- Identify metals.
- Classify lip prints.
- Compare paint chips from hit-and-run**
cases.
**- Use chromatography to compare lipsticks.
- Design and conduct scientific**
investigations.
- Identify questions and concepts that guide
scientific investigations.
- Communicate and defend a scientific
argument.
Chapter
“Breadth of view is one of the essentials
of our profession. The interplay of ideas
and the oblique use of knowledge are
often of extraordinary interest.”
—Arthur Conan Doyle’s Sherlock Holmes, in The Valley of Fear
Metals: Product Liability?
Often investigators must identify small bits of metal that have been found
in the soil, swept up from the carpet or floor of a crime scene, or found
contaminating a consumer product.
For example, a lawyer calls you with a problem. He has a client who baked
a loaf of whole-wheat bread. As the family was eating it for supper, they
complained of hard bits and pieces in it; the client even broke her tooth
biting down on one of the pieces.
Examining the bread and flour, they found a number of what appeared to
be metallic particles. Family members had already swallowed some of these
particles, and now they were worried more about poisoning than about the
broken tooth.
The client called the lawyer, intending to sue Happy Miller, the company
that made and sold what seemed to be contaminated flour. The lawyer wants
you to analyze these particles. If indeed they are metallic, could they have
come from the machinery used to produce the flour? If so, shouldn’t Happy
Miller recall all its whole-wheat
flour packages? Or did the client
purposely add some contaminant
to the flour to extort money from
Happy Miller?
A few quick phone calls reveal
that the grinding, separating,
mixing, and fi lling equipment
at the flour mill is made of high-
chrome stainless steel and brass.
The family seems ordinary
enough. The client has had
problems with her teeth—lots of The bread in question
Powders: We will use a qualitative chemical scheme to identify a set of
white powders.
Lipstick: A comparison of lip prints leads to a robbery suspect, and
thin-layer chromatography of lipstick components solves a murder.
Paint: Microscopic examination can sometimes solve a hit and run.
Teacher Note,continued
- A red seed. Appears rather exotic; take it to a nursery or botanist.
- Red pigment. Perhaps used for making pottery or by an artist.
- Wood shaving, from a plane. Look for a carpenter.
- Short-grain rice. A recent wedding?
- A pearlescent button. From a shirt, blouse? Too bad no thread was attached.
Trace Evidence 233
fi llings, with not all costs covered by dental insurance. Her husband is a
welder at a local construction plant and is partially blind in his left eye.
Your task is to analyze the contaminating particles, determine what they
are, and give an opinion as to where they came from and how they got into
the flour. Construct a table in your notebook for your observations or use
the handout from your teacher.
Materials
Laboratory^ Identification of Metals Activity 9.
Caution: The chemical reagents that
you will be using are strong acids
and bases that can burn your skin
and ruin your clothing, so wear your
safety glasses, apron, and gloves;
follow instructions; don’t make a
mess; and wash up afterward. In
addition, some of the reagents smell
bad or can form noxious vapors and
gases. Be careful!
For each group:
copper, magnesium, aluminum,
lead, zinc, iron,nickel,
chromium
- test tubes
- hand magnifier or stereoscope
- 10-ml beaker with 5-ml
bromoform (keep under hood)
- magnet
- forceps
- 6 M sodium hydroxide (NaOH)
- 6 M nitric acid (HNO 3 )
- 6 M hydrochloric acid (HCl)
- 6 M ammonium hydroxide
(NH 4 OH)
- 6 M acetic acid
- 0.1 M sodium sulfide (Na 2 S)
(K 2 CrO 4 )
- 0.1 M potassium thiocyanate
(KSCN)
- 3 percent hydrogen peroxide
- S & O reagent
- aluminon reagent
- dimethyl glyoxime solution
- unknown sample(s) of
contaminating particles
- test tube rack
- safety goggles
- lab apron
- disposable gloves
- small beaker of alcohol
SAFETY ALERT! CHEMICALS USED
Always wear goggles and an apron when working in the labaratory
SAFETY NOTE Also wear disposable lab gloves. Avoid inhalation, !ingestion, and skin contact with chemicals.
Advance Preparation
Refer to Blackline Masters 9.1 and 9.2 for use with this activity. Check with your Kendall/Hunt representative for supplies. Bromoform is also available from chemical supply houses such as Aldrich (www. sigmaaldrich.com/catalog/ search/AdvancedSearchPage) or Flinn Scientific. It is also used in Chapter 10, “Soil and Glass Analysis.” Note: Bromoform is toxic. Use it in a well-ventilated place. Minimize students’ handling of it. Dispose of it by evaporation. Bromoform darkens with age, even stored in a brown bottle, making it useless for density measurements. It can be cleaned up, however, by mixing it with a fine activated charcoal and then filtering.
6 M sodium hydroxide (NaOH): 24 g/100 ml water 6 M nitric acid (HNO 3 ): dilute concentrated 1:2 (1 part acid to 2 parts water). Always add acid to water and wear your safety goggles.
234 Chapter 9
Laboratory Activity 9.1, continued
11. Repeat the tests you performed on the standards on your unknowns.
You may be pretty sure of what your samples are by now, but it is important,
especially in forensic science, to be as sure as you can possibly be. After
all, someone’s life may depend upon it. The confirmatory tests listed in
steps 12 through 19 are used to verify, or disprove, results. Do you have to
run all these confirmatory tests on your unknown flour contaminants? After
you have narrowed the possibilities, pick the appropriate tests to identify
the contaminating particles. Run a known standard if you are unsure of any
outcome.
12. If you think you have aluminum, confirm it by adding two drops of aluminon
reagent to the test tube containing your unknown sample in the NaOH. Mix
and let it set for a while. A pink, gel-like layer at the bottom of the test tube
confirms the presence of aluminum.
13. If you think you have copper, confirm it by adding NH 4 OH to the HNO 3
solution until you get a deep blue solution.
14. If you think you have magnesium, confirm it by adding one to two drops of
S & O reagent to the sample in the HNO 3 ; then slowly add NaOH until you see
a fluffy, blue precipitate or gel.
15. If you think you have zinc, confirm it by adding a few drops of sodium
sulfide solution (Na 2 S) to the metal in the HNO 3 and look for a yellowish or
white precipitate.
16. If you think you have iron, confirm it by diluting your acid solution 10 or
more times with water until it is almost clear. Add a drop of potassium
thiocyanate solution (KSCN) and note a brown-red or red color.
17. If you think you have lead, confirm it by adding an equal amount of acetic
acid (HAc) to the HNO 3 solution. Do it in the hood, because HAc smells pretty
bad—like very strong vinegar. Mix, and add a few drops of bright yellow
potassium chromate solution (K 2 CrO 4 ) to get an orange precipitate.
18. If you think you have nickel, confirm it by adding twice the amount of NH 4 OH
to the acid solution, mix, and add a few drops of dimethyl glyoxime reagent
to form a red-violet precipitate.
19. If you think you have chromium, confirm it by noting what happened when
you added the HCl to the chromium. Look for the same reaction and add
several drops of H 2 O 2. A momentary yellow color should appear.
Conclusions
Based on the results of your analyses, what are the contaminating particles in
the flour? Write an opinion paper as to where they came from and explain why
you think so. It is not enough in a court of law to say, “It looks like copper to
me”; you must back up your opinions with physical and chemical testing.
3 percent hydrogen peroxide S & O reagent: Dissolve 1 g NaOH in 100 ml water, then add 0.5 g p-nitrophenyl- azoresorcinol. Dilute 5 to 10 times. This produces a sky-blue precipitate with Mg. Co and Ni interfere. The solution is available from Flinn, as is solid p-nitrophenyl-azoresorcinol (Flinn Scientific; www.flinnsci. com). Aluminon reagent: Dissolve 1 g aluminon (ammonium salt of aurin tricarboxylic acid) in 1 L water. For best results, add ammonium acetate to make a pH of 4.5 to 5.5, then add ammonium hydroxide and ammonium carbonate to adjust the pH of the solution to 7. to 7.3. Under these conditions, only Fe interferes. Aluminon is available from Flinn also. Dimethyl glyoxime: Make a 1 percent solution in ethanol: 1 g dimethyl glyoxime in 100 ml ethanol. This is available from Flinn (don’t use their 0. percent solution; it’s too weak). Metals (Al, Cr, Cu, Fe, Mg, Ni, Pb, Zn): You can purchase these from supply houses, but most are easy to obtain. It is good to have two different forms of them, with one to use strictly as an unknown just to throw off any student preconceptions.
You can opt for contamination from the factory by using chromium and iron and nickel (stainless steel) or copper and zinc (brass). Or you can use aluminum and magnesium, which could come from a welding shop. Clean the Al and Cu with steel wool prior to cutting it up.
Flush all solutions and precipitates down the drain with lots of water. Save the metal pieces, dry, and discard in the wastebasket or use again.
Advance Preparation, continued
236 Chapter 9
Some analytical schemes are described in the flow chart shown in
Figure 9.1.
Figure 9.1 Metals analytical scheme for Laboratory Activity 9.
Al, Mg, Cr, Cu, Zn, Fe, Pb, Ni
magnetic
Fe, Ni Al, Mg, Cr, Cu, Zn, Pb
Density (bromoform)
yes (^) no
Cu, Cr, Zn, Pb (^) Al, Mg
sinks floats
HNO (^3) NaOH blue
rxn, br gas n.r.
rxn n.r.
Cu Zn Pb, Cr Al Mg
HCl
green (^) n.r (^).
Cr Pb
HNO 3
brown
Fe
green
Ni
Confirmatory Metal HNO 3 NaOH Tests Magnetic Density Al n.r bubbles pink gel no floats Mg violent rxn, br gas n.r. blue ppt no floats Cr n.r. n.r. gr in HCI yel in H 2 O 2 no sinks Cu blue n.r. deep bl no sinks Zn voilent rxn, br gas n.r. yel/wh ppt no sinks Fe bubbles, turns br n.r. red yes sinks Pb n.r. n.r. or ppt no sinks Ni slow bubbles n.r. violet ppt yes sinks
Table T9.1: Table of Observations
Teacher Note Figure 9.1 is also included as Blackline Master 9.2 on the TRCD for use with Laboratory Activity 9.1.
Trace Evidence 237
Industrial effluent into the Marggraf River
Laboratory Activity 9.
Testing for Environmental Contamination
In the previous problem, solid metal samples were individually
analyzed using their physical and chemical properties. In this
analysis, you will use paper chromatography to separate the dissolved
components, visualization techniques to observe them, and controls to
identify them.
Paper chromatography is based on the partitioning or separation of
substances between a solid absorbent medium and a liquid solvent. The
affinity of the unknown substance for each phase determines the degree
of separation; this affinity might be related to the sample’s solubility,
polarity, or even its size. In this activity, the absorbent is filter paper, also
referred to as the stationary phase, and the solvent is acetone/HCl,
also called the mobile phase.
stationary
phase: the fixed
adsorbing medium
of analytes in
chromatographic
separations
mobile phase:
the carrier of analytes
in chromatographic
separations; also called
the developing solvent
an operation producing specialty Alnico magnets for the
recycling industry
Mr. Calamine has sent samples from the water’s edge of
his farm to your environmental forensic lab, asking for a
qualitative analysis of the following dissolved metals: Al, Co,
Cu, Ni, Zn.
GO TO www.scilinks.org TOPIC environmental forensic science CODE forensics2E
Trace Evidence 239
Laboratory Activity 9.2, continued
Procedure
Record your observations in your notebook and use the data table provided by
your teacher.
1. Draw a line, in pencil, 1 cm from the bottom of each piece of
chromatography paper and put a tick mark in the center of the line.
2. Label the top of each paper with the name of the material to be analyzed.
3. Using an open-ended capillary for each of the metal ion standards, gently
touch the tick mark to transfer a drop onto the paper. Try to keep the spot
smaller than 3 mm. Let it dry. Repeat several times, drying in between.
4. Find a way to hang the chromatography strips in the developing tank so
that they do not touch each other and are 1 or 2 mm off the bottom (see
Figure 9.2).
5. Add sufficient mobile phase so that the bottom of each strip is immersed,
but keep the level well below the spot. This is important!
Figure 9.2 Preparation for paper chromatography
pencil line 1 cm from bottom eluting solvent beaker
strips
cover
For each investigative group:
with cover
- open capillary tubes
- safety goggles
- ruler
- strips of filter or chromatog-
raphy paper
- developing solvent (mobile
phase): HCl, acetone
- metal ion standards: Al^3 ,
Co^2 , Cu^2 , Ni^2 , Zn^2
- oxine (8-hydroxyquinoline)
- Beral pipettes
- ammonium hydroxide
- sodium sulfide solution
- UV light
- river water sample
Materials
SAFETY ALERT! CHEMICALS USED
Always wear goggles and an apron when working in the labaratory
SAFETY NOTE Also wear disposable lab gloves. Avoid inhalation, !ingestion, and skin contact with chemicals.
Advance Preparation
Hand out copies of Blackline Master 9.3, found on the Teacher Resource CD, to be used with this lab.
Metal ions: Prepare a set of standards by dissolving each of the following in 100 ml of distilled water:
Al^3 : 0.9 g AlCl 3 6H 2 O Co^2 : 0.4 g CoCl 2 6H 2 O Cu^2 : 0.3 g CuCl 2 2H 2 O Ni^2 : 0.4 g NiCl 2 6H 2 O Zn^2 : 0.2 g ZnCl (^2)
The weighing need not be precise, because you need only approximate 1 percent solutions of the metal ion. Dilute a portion of each solution about tenfold to make a 0.1 percent standard solution, bottle it up for each student’s or group’s experiment, and label it. Mix the 1 percent copper and zinc solutions about equally, dilute 1 to 4 with muddy water, bottle the mixture, and label it. This will be the unknown.
Use Whatman No. 1 filter paper or chromatography paper cut into strips about 12 cm long and 2 cm wide. Separate strips are better than having several different spots on a larger paper because of cross-contamination of eluted spots at the final solvent front.
For the developing solvent (mobile phase), add 10 parts 6 M HCl to 35 parts acetone (hardware purity is fine).Caution: The mobile phase is strongly acidic; wear safety goggles. A chromatography developing tank can be an 800–ml to 1000-ml beaker. Cover only the bottom 2 to 3 mm with the solvent.
The students should have found copper and zinc. You may have gotten a few “eeeww” reactions when they saw the muddy
240 Chapter 9
When investigators find substances at the scene of a crime and send
them to the laboratory for identification, the forensic chemist uses
several techniques or lab tests to identify them. In
the following laboratory activity, you will examine
some basic physical and chemical properties, such
as solubility, reaction with an acid, and reaction
with a base. You will be able to easily identify and
distinguish between substances that seem to be
alike in a preliminary examination.
Qualitative Analysis of Powders
Laboratory Activity 9.2, continued
16. What materials did you find in Mr. Calamine’s water sample?
17. Now the question arises: Where did these pollutants come from? Is there
a source upriver that can be identified? Here is where the detective work
comes in; use your knowledge and research, using the Web to support
any conclusions. Write a report defining the problem and describing your
lab procedure and results, analytical conclusions, method of determining
the polluting source, and any other pertinent observations and data to
reconstruct what happened. Be aware that you may be called as an expert
witness in a civil suit.
Some products that consist of white powders
from Fisher Scientific (https:// www1.fishersci.com). Dissolve 0.25 g in a mixture of 20 ml ethanol and 40 ml distilled water. The oxine solution should be stored in a dark, plastic bottle until pronounced discoloration occurs. Sodium sulfide: Prepare 100 ml of fresh 0.1 M solution by dissolving 2.4 g Na 2 S 9H 2 O in 100 ml of distilled water. Use the solution from the previous lab activity.
In the quantities used, all reagents and solutions can be flushed down the drain.
Advance Preparation,continued
Solution UV NH 3 Oxine-UV Na 2 S Rf* Al neg neg fluoresces light gray 0. Co neg red neg black 0. Cu neg blue neg brown 0. Ni neg light blue neg black 0. Zn neg neg fluoresces neg 1. Unknown neg blue fluoresces brown 0.75, 1.
*May differ slightly from these values depending upon HCl/acetone ratio, type of paper, and the like.
Teacher Note Table T9.2: Chromatography Visualization
Table T9.2 goes with Laboratory Activity 9.2.
242 Chapter 9
In 1912 Emile Gourbin, a bank clerk in Lyon, France, was suspected of strangling his girlfriend, Marie Latelle. Gourbin was arrested but had what appeared to be an air-tight alibi. Edmond Locard went to Gourbin’s cell and removed scrapings from under his fingernails. The scrapings contained tissue that was possibly from Marie’s neck, but this could not be proved. Locard noticed that the tissue was coated with a pink dust, which he identified as rice starch. On the particles he found bismuth, magnesium stearate, zinc oxide, and a reddish iron oxide pigment called Venetian red. Examination revealed that a face powder prepared for Marie by a Lyon druggist was similar in composition. In these days of mass-produced cosmetics, this would have far less significance; but in 1912, because of the special preparation, this evidence led to Gourbin’s confession.
Laboratory Activity 9.
You have ten numbered samples of unidentified white powders, listed on
the next page. You will observe their chemical and physical properties to
identify them. Record your observations in your notebook. Your goal is to
identify each of the numbered samples by its chemical name.
Analysis of White Powders
For each investigative group:
numbered 1 through 10
- test tube rack
- test tubes
- stereomicroscope
- iodine solution
- distilled water
- phenolphthalein
- spatula
- spot plate or weighing tray
- isopropyl alcohol
- acetic acid
- 0.3 M NaOH
- glass stirring rod
Materials
continued
White powders
Advance Preparation Refer to Blackline Master 9.4 for use with Laboratory Activity 9.3.
Trace Evidence 243
Laboratory Activity 9.3, continued
two more of the ten powders;
label them and set them aside.
5. The remaining solids are water-
soluble. Two of them, NaOH
and Na 2 CO 3 , produce strongly
alkaline solutions (basic, the
opposite of acidic) that will turn
bright pink with the addition of
the phenolphthalein indicator.
Choose the two that have the
most vibrant pink color. One is
NaOH, and the other is Na 2 CO 3.
6. Add 1 cm acetic acid to each
sample that turned bright pink.
The vigorous evolution of CO 2
indicates the presence of Na 2 CO 3.
If there is no gas, or only a small
amount, the sample must be
NaOH. Identify and label these
samples.
7. Add 1 ml of 0.3 M NaOH to
each of the remaining five
unknowns. They will turn violet
if phenolphthalein has already
been added. Identify the sample
that becomes cloudy; this means that a precipitate is being formed. The
sample is MgSO 4.
8. Prepare new samples of the four remaining solids. Adding acetic acid will
produce CO 2 gas that will identify the NaHCO 3.
9. Of the three solids left, only H 3 BO 3 is soluble in alcohol. Add half a test
tube of isopropyl alcohol to fresh samples of the three remaining powders.
Identify the H 3 BO 3.
10. Sucrose and NaCl are the only remaining solids. Sucrose is more soluble in
water than NaCl. Add a small amount of each solid to a clean test tube. Fill
halfway with water. Stir or hold the test tubes under hot tap water or in a
water bath. Identify the more soluble sample.
Check your data to make sure it is complete. In the next exercise, you will
analyze an unknown sample of a powder or powders using a flow chart of the
analysis scheme (Figure 9.4).
Photomicrographs of two different white powders
Trace Evidence 245
Figure 9.4 Qualitative analysis scheme for Laboratory Activity 9.
Modified from Solomon et. al., Qualitative Analysis of Eleven Household Chemicals
NaHCO 3 , CaSO 4 , Sucrose, CaCO 3 , NaOH NaCl, Cornstarch, MgSO 4 , H 3 BO3, Na 2 CO (^3)
Water
Iodine
Vinegar Vinegar
Vinegar
Insoluble Soluble
Deep blue Brownish
Alcohol Insoluble
Hot Water
Very soluble Less soluble
Soluble
Phenolphthallein Pink
Bubbles
Bubbles
Bubbles
Sodium Hydroxide
Precipitate
NaHCO 3 , Sucrose, NaCl, H 3 BO 3 , MgSO (^4)
Sucrose, H 3 BO NaHCO 3 , NaCl
CaSO 4 , CaCO 3 , Cornstarch
Cornstarch CaSO 4 , CaCO (^3)
CaCO 3 CaSO (^4)
Na 2 CO 3 NaOH
MgSO 4
NaCl, H 3 BO3 , Sucrose NaHCO (^3)
H 3 BO 3
Sucrose NaCl
Sucrose, NaCl
NaOH, Na 2 CO (^3)
Na 2 CO 3 , NaHCO 3 , Sucrose, NaCl, MgSO 4 , H 3 BO3, NaOH
Teacher Note
Figure 9.4 is also included as Blackline Master 9.4 on the TRCD for use in Laboratory Activity 9.3.
246 Chapter 9
Procedure
Follow the flow chart in Figure 9.4 and your identification procedure from the
earlier laboratory activity to identify the unknown powder and the thief.
Case Study 1.
Amanda Davies Case
From the files of coauthor John Funkhouser
Several years ago I was asked by a forensic science agency to
investigate a case involving white powder. A woman had complained
that she was being poisoned by powder being introduced into her high-rise
condo, coating her clothing, furniture, kitchenware, and just about everything.
She insisted it was arsenic.
I requested samples of various items that were coated with the material. In talking
to her, I asked about previous health issues and her present symptoms, which
were a rash and respiratory problems. These symptoms did not match those
associated with either ingestion or breathing of arsenic oxide. She also could not
opine as to how or why this was happening or who might be responsible.
Almost as an aside, she mentioned that the apartment directly above hers was
being renovated. Ding! Most construction in the United States uses sheetrock
(also called drywall or wallboard), which is gypsum (a natural mineral of
calcium sulfate) sandwiched between two paper layers. Pieces of sheetrock
are cut to the desired dimensions and nailed to studs, joists, and rafters.
Seams are taped, and spackle, which also may contain gypsum, is applied
9.1: White Powder
Laboratory Activity 9.4, continued
The following case study shows how a broad background of knowledge
and experience can aid in an investigation; however, preconceived ideas
should never interfere with an open mind and objective analysis. Forensic
science is largely based on comparative analysis, which can either eliminate
a source or link or provide a possible, maybe even a probable, relationship
between unknown and known. In this particular case, examination
eliminated arsenic as the problem and provided a possible source of the
powder. It also demonstrated that preconceived notions are hard to dispel,
even in the face of scientific and logical evidence. Or was there a motive in
this woman’s insistence that she was being poisoned?
Teacher Note
Balmer is likely to have been in contact with calcium sulfate (gypsum, a component of drywall) and magnesium sulfate (Epsom salts) for soaking his sprained wrist. Dalrymple is a cook, so he would be in contact with any white powder that might be found in the kitchen. Use whichever unknowns you choose.
248 Chapter 9
and sanded smooth when dry. Additionally, holes are cut into the sheetrock for
receptacles, light fixtures, etc. Consequently, lots of white powdered gypsum is
released. I had a possible lead as to what this contaminant might logically be.
Once samples of clothing, a purse, a belt, a shoe, and a soap dish arrived, I
was surprised to find so little powder adhering to the articles or in the plastic
bags in which they were packed, contrary to the woman’s description.
I used arsenic trioxide, arsenic pentoxide, and crumbled sheetrock as
controls (knowns in this instance) to compare to the unknown white powder.
Microscopic examination of a number of individual particles (all 1 mm or
smaller) in reflected, transmitted, and polarized light (see Chapter 10, “Soil
and Glass Analysis”) revealed a match of the unknowns with the gypsum
particles only. Comparative solubility studies of individual particles of the
unknown substance again ruled out the arsenic oxides, but showed the
substance to be consistent with the sheetrock control.
Rather than spend more time and money on confirmatory and sophisticated
analyses such as X-ray microprobe and SEM, I surmised that the white powder
was not arsenic but gypsum, and originated from the renovation site upstairs,
probably dispersed in the woman’s apartment through a faulty ventilation system.
When I reported my opinion and my reasoning, she still wanted to believe that
she was being poisoned. Oh, well.
Typical particles at 60, low-resolution magnification
Unknown particle:
Reflected light Transmitted light Polarized light
Gypsum control:
Reflected light Transmitted light Polarized light
Trace Evidence 249
