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Forensic Chemistry > Firearms > Flashcards

Firearms Flashcards

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1
Q

Where does the forensic chemist aid with firearms evidence?

A
  • identification of GSR
  • corrosion restoration
  • serial number restoration
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2
Q

How in energy transferred in firearm energetics?

A

Mechanical energy (hammer strikes primer) -> chemical energy (stored in the bonds of primer and propellant) -> heat and work (expanding gasses of combustion) -> kinetic energy (movement of projectile)

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3
Q

Components of modern ammunition?

A

Primer, propellant, bullet

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4
Q

What are the different types of primer?

A

Centre fire: primer at the base of cartridge with vents to propellant

Rim fire: primer wrapped around the base of the propellant; ignited from mechanical crushing

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5
Q

What are the four components of modern primers?

A

Initiating explosive (40%)
Oxidizing agent (40%)
Fuel (16%)
Sensitizer (4%)

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6
Q

What chemicals are used in the primer initiating explosives (40%)?

A

Lead styphnate (primary explosive in modern primers)
Lead azide
Potassium nitrate

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7
Q

What chemicals are used in primer oxidizing agents (40%)?

A

Barium nitrate
Barium oxide
Lead nitrate
Lead (IV) dioxide

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8
Q

What chemicals are used in primers fuels (16%)?

A

Antimony sulphide
Calcium silicide
Lead thiocyanate
Powdered metals (Al, Zr, Mg, Ti)

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9
Q

What chemicals are used in primers sensitizers (4%)?

A

Tetracene
PETN
TNT
Tetryl

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10
Q

What is black powder composed of?

A

Charcoal (15%) - carbon source
Saltpetre (75%) - oxidizer
Sulphur (10%) - fuel source; lower ignition temperature

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11
Q

What products are given when using black powder?

Why is this bad?

A

56% solid products
43% gaseous products
1% water

Solid products = reduced gases, chemical inefficiency, and lots of smoke

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12
Q

What does single-base smokeless powder contain?

A

Nitrocellulose

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13
Q

What does double-base smokeless powder contain?

A

Nitrocellulose + nitroglycerin

Nitroglycerin - plasticizer, water proofing, heat sink

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14
Q

What does triple-base smokeless powder contain?

A

Nitrocellulose + nitroglycerin + nitroguanidine

Large caliber military weapons

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15
Q

How does burn rate of propellant affect the kinetic energy produced?

A

Too fast = pressure drops before bullet is released

Too slow = waste, projectile leaves before all propellant burned - wasted propellant and energy

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16
Q

Why are additive added to smokeless powders?

What are some examples?

A

Nitrocellulose is susceptible to acid-catalyze breakdown -> gaseous nitrogen oxides + water -> nitric and nitrous acids -> more breakdown

Methyl centralite (deterrent)
Ethyl centralite (deterrent, stabilizer, plasticizer)
Diphenylamine (stabilizer)
Acardite II
Resorcinol
2,4-dinitrotoluene (deterrent, plasticizer)
2,6-dinitrotoluene (deterrent, plasticizer)

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17
Q

What does the particle cloud of GSR include?

A
  • Metallic particles from primer, bullet, and barrel
  • Unburnt propellant grains (nitrates, nitrites)
  • Partially burnt propellant grains
  • Solid combustion products (soot)
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18
Q

What are the different colour tests used in GSR analysis?

A 
Walker Test
Griess Test
Sodium Rhodizonate Test
Dithiooxamide (DTO)
Dimethylglyoxime (DMG)
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19
Q

What does the Walker Test test for?

What is a positive test?

A

Nitrites

Positive = Red dye

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20
Q

How is the Walker Test performed?

A

Desensitized photo paper used
Warm iron used to press evidence into photo paper
Production of azo dye

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21
Q

What does the Griess Test test for?

What is a positive test?

A

Nitrites! Can be modified for nitrates.

Positive = Orange colour

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22
Q

How is the Griess Test performed?

A

Desensitized photo paper (exposure to hypo solution; removes silver halides)
Photo paper impregnated with sulphanilamide and N-(1-naphthyl)ethylene diamine
Exhibit steam ironed with dilute acetic acid
Sulphanilamide is acidified and reacts with NO2- to form diazo

23
Q

What does the Sodium Rhodizonate Test test for?

What is a positive test?

A

Detects lead and barium
Positive test = pink colour
Confirmation of Pb test positive = blue colour

24
Q

How is the Sodium Rhodizonate Test performed?

A

Exhibit is moistened with tartrate buffer at pH 2.8
Fresh Sodium Rhodizonate solution is sprayed on surface
Pink regions are further sprayed with 5% HCl solution - blue (strontium also goes pink)

25
Q

What does the DTO Test test for?

What is a positive test?

A

Copper and Nickel
Positive Cu = dark green-black
Positive Ni = blue-pink

Cu confirmation test: pink, back to dark green
False positives = green for Fe and orange for Zn

26
Q

How is the DTO Test performed?

A

Filter paper moistened with ammonium hydroxide solution
Moistened side pressed against exhibit
Residue transfer photographed
Lift sprayed with DTO
Confirmation against false positives with 2-NN

27
Q

What does the DMG Test test for?

What is a positive test?

A

Nickel plated bullets

Positive = bright red

28
Q

How is the DMG Test performed?

A

Filter paper is prepared with a DMG solution
Suspected bullet hole is treated with ammonium hydroxide (looking for path of bullet)
Pretreated DMG filter pressed on exhibit and removed
If nickel red DMG complex seen immediately

29
Q

Within what range can you expect to find some form of GSR?

A

Within 18” firing distance.

30
Q

What confirmatory tests can be used for inorganic GSR?

A 
SEM-EDS
AAS
GFAAS
ICP
XRF
31
Q

What confirmatory tests can be used for organic GSR?

A 
Raman
IR
GC-MS
LC-MS
CE
32
Q

How is SEM useful in GSR analysis?

A

Information of physical nature of sample surface

Higher resolution than optical microscopes because of smaller wavelengths - smaller diffracted effects

33
Q

SEM instrumentation.

A

Electron beam production
Focussing electron beam
Image production
Detection

34
Q

SEM - electron beam production

A

Tungsten filament (super hot solid ejects electrons)
Crystals
Electrons accelerated by positive anode, produces smaller, more cohesive beam
Smaller eV gives better topographical information
Larger eV gives deeper information

35
Q

SEM - focussing the electron beam

A

Electromagnetic lenses and apertures reduce beam diameter
Pathways bent by electromagnetic radiation
Focal length controlled by varying current in coils
Electrons accelerated through objective and condenser lenses, reduce diameter
Condenser converges electron beam to spot below, height impacts spot diameters (low = large, high = small)
Resolution cannot be better than diameter of incident beam

36
Q

SEM - image production

A

Raster scanning - uses two pairs of deflection (scanning) coils within objective lens
Beam moves across sample
Electrical signal to coil controls movement
Coils deflect beams in X and Y direction, respectively
Size of rastered area gives rise to magnification

37
Q

SEM - detection

A

Multiple detector types: secondary electron detector, Backspatter electron detector, x-ray detector
Everhart-thornley: positive bias to attract and accelerate electron to a scintillator
Electrons are converted to photons
Photons pass through photomultiplier tube - amplifies signal

38
Q

SEM - electron sample interactions

A

Penetration of sample surface - electron beam produces secondary electrons, Backspatter electrons, xrays, and Auger electrons
The depth the electron come to rest within the solid is the penetration depth
The volume of sample containing most of these electrons is the interaction volume
Shallow depth = escape depth

39
Q

SEM - Electron beam energies and their applications.

A

1-5 keV = delicate or uncharted samples
5-10 keV = coated biological samples
10-30 keV = physical science samples

Higher the beam the more BSEs in addition to SEs

40
Q

SEM - Sample preparation

A

Conductive samples = easiest to study - minimize buildup of charge - buildup causes distortion of signal and image

Non-conductive samples = grounded or coated with conductive materials to minimize surface charging: copper tape, carbon paint, THIN metallic coatings (Os, Au, Pt, Ag)

41
Q

How is EDS/EDX useful for GSR analysis?

A

Investigates the elemental or chemical characteristics of a sample
Z > 3 (carbon and beyond)
When an electron beam interacts with a sample x-rays are emitted from the whole interaction volume

42
Q

What are the two types of x-rays produced in EDS/EDX?

A

Characteristic x-rays

Bremsstrahlung (continuum) x-rays

43
Q

EDS - Bremsstrahlung x-rays

A

X-rays produced due to interaction of electron with electric field atomic nuclei
Electrons are slowed, they lose energy, energy is converted to x-rays which are emitted
Sharper turn = higher energy x-rays, and vice versa
Lose energy in single interaction or many interactions
Whale shape: many low energy x-rays are absorbed by sample, x-ray intensity decreased at low energy

44
Q

EDS - characteristic x-rays

A

High energy electrons fill lower energy vacancies
Incoming electrons knock electrons in K and L shells shells, higher energy electrons fall down
When electron falls to lower shell an x-ray is emitted at equal energy to delta E
L -> K = Ka
M -> K = Kb
M -> L = La
N -> L = Lb
a=1 shell difference, b=2 shell difference

45
Q

EDS - analyzing characteristic x-rays

A

X-rays fluorescence lines have been tabulated for K and L lines
Energies independent of chemical form due to electron structure of atom
Characteristic x-rays have discrete energies but signal is averaged so line becomes peak
Energies of characteristic x-rays used to identify elements present
Ensure that both K lines or both L lines are present in the right ratios (Ka/Kb 5:1, La/Lb 1:1)
Quantitative information only for flat, polished, bulk samples

46
Q

ASTM E1588 GSR

A

Analysis of the particulates in GSR be conducted with SEM/SEM-EDS
SEM - morphology determination : spherical agglomerates
EDS - elemental profiles of individual particles
Identification on unique presence of Pb-Sb-Ba (Characteristic vs Consistent)

47
Q

Corrosion restoration

A

On environmental exposure bullets and cartridges may acquire films or undergo corrosion processes that cast obscure striation patterns or impressions and markings
Lead alloys react with atmospheric oxygen, water, and carbon dioxide to produce PbO, PbCO3, Pb3(CO3)2(OH)2 and other salts

-> comparative analysis of bullets

48
Q

Corrosion restoration: Lead Bullets

A
  1. Ultrasonication in DI for 30s
  2. Transfer to perchloric acid until bubbling stops
  3. Ultrasonicate in acid 10s
  4. Rinse with DI
    Removes corrosion products while leaving striation marking on bullet
49
Q

Corrosion restoration: Brass and Jacketed Bullets

A
  1. Sonication in MilliPore water
  2. After drying sonicate in reagent grade trichloroethylene
  3. Sonicate in HPLC-grade acetone
  4. Sonicate 50 C in aqueous metasilicate and ethylene glycol monobutyl, pH 12.5
  5. Immersion in acid cleaning solution - H2SO4, thiourea, sodium lauryl sulphate, pH 1.5
  6. Sonicate in MilliPore water
50
Q

Corrosion Restoration - alloys

A

Cu-Zn alloys (brass) are susceptible to dezincification
Preferential Zn dissolution
Simultaneous dissolution, redisposition of Cu as spongey Cu
Zn preferentially dissolves as anode

51
Q

Serial number restoration

A

Used to provide investigators with the ability to trace the original purchaser of a firearm
Above trigger and removable barrels

Damaged areas (mechanical deformation) etch at a different rate which is what allows for identification of the serial number

52
Q

Serial number restoration methods (3)

A

Chemical etching
Electrochemical etching
Magnetic particle restoration

  • depends on damaged crystals
53
Q

Serial number restoration - etching reagents

A

Reagent 1: Fry’s reagent

Reagent 2: most common (HNO3 and H2O)

Forensic Chemistry (4 decks)

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