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Trace Evidence > Lecture 17- Glass > Flashcards

Lecture 17- Glass Flashcards

1
Q

what was the first type of glass made from

A

melted sand

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2
Q

why can glass be a good source of trace evidence

A

it is fragile so likely to break and transfer

it is also able to persist for a long time enough to be recoverable

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3
Q

how is the flatness of glass controlled in the manufacturing of it

A

production of float glass - molten glass floats on a molten tin bath to become flat

can be controlled thickness, flat, smooth surfaces

gives identifiable features through surface analysis

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4
Q

how is float glass normally identified

A

the side that was in contact with the tin shows a luminescence at 254nm and can have a anisotropic gradient in the refractive index

but in the manufacturing this luminescence can be removed

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5
Q

how does the composition of glass vary

A

depending on the manufacturer the mixture and ratio of components changes

this addition of things for colour and strengthening

the order and amount of things added is likely to be different

presence of coatings

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6
Q

what is added to glass to improve heat durability and where is this often seen

A

boron oxide (B2O3)

seen in cookware, glasses and car headlamps

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7
Q

what is added glass to absorb radiation and where is this often seen

A

silver (Ag) in sunglasses

strontium in TV screens

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8
Q

what are the 3 properties of glass associated with the possibility of it breaking

A

flexibility
strength
nature of the impacting object (force and shape)

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9
Q

how can a side impact on glass be determined

A

by hackle or rib marks

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10
Q

what shape is often seen when a projectile impacts glass

A

a percussive cone

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11
Q

when is glass transfer often seen in crime scenes

A

during hit and runs and ram-raids

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12
Q

do the smaller or larger fragments of glass tend to be transferred to clothing and found in investigations

A

smaller ones

more likely to see if a big chunk of glass is on your clothing

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13
Q

what are the three ways normally used to recover glass shards

A

scraping
taping
forceps

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14
Q

what is the analytical workflow for glass evidence

A
  1. gross examination recovery and collection
  2. prelim evaluation of physical characteristics
  3. physical fit assessment
  4. microscopic analysis - refractive index
  5. density measurements (often not included now)
  6. elemental analysis with SEM, XRF (non destructive)
  7. mass spec elemental analysis (destructive)
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15
Q

why is physical fit assessment matches very unlikely with glass evidence

therefore what type of information is heavily relied on from glass evidence

A

harder with smaller pieces of glass
glass can chage shape slightly - upon heat

rely on elemental analysis

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16
Q

what defined the analytical scheme employed in the analysis of glass

A

the size of the recovered fragment

large vs small

17
Q

what is analysed for larger glass fragments

A

thickness - take an average and standard deviation

colour - can be subjective though

matching edges - physical fit

density

RI measurement

18
Q

what is analysed for smaller glass fragments

A

confirm it’s glass

examine surface for distinguishing features

19
Q

how can the analysis of colour be made less subjective

A

by using MSP - microspectrophotometry

20
Q

what can be looked at to see if the sample is glass or a type of quartz/mineral

A

the birefringence

21
Q

what can be looked at to see if the sample is glass or a type of plastic

A

see if it compresses under pressure if breaks = glass, if compresses = plastic

22
Q

how is the density of a piece of glass measured

A

using a density gradient column - vary the density of the liquid and see when the glass fragment remains even - doesn’t sink or float

not most accurate but get a rough idea

combine with RI measurement to be more discriminative

23
Q

when measuring the RI of glass what method is normally used and why

A

the Single or double variation method as it is more precise than the Becke Line test

24
Q

how is the single variation method carried out

how does the double variation method differ from this

A
  1. mount sample in a medium with higher RI
  2. fix light at a certain wavelength e.g 589nm
  3. slowly heat sample on a hot stage
  4. the medium’s RI will change quicker than the samples when heating
  5. temp of lowest contrast between sample and media is recorded computationally
  6. compare to a table of known RI values

double variation method varies temp and wavelength of light = even more precise

25
Q

is a sample from the outer edge of glass or from the main bulk likely to have a lower RI

A

sample in bulk likely to be lower

26
Q

why is elemental analysis done on glass samples normally

A

very little discrimination is often found comparing the RI of them

27
Q

give 3 advantages of using SEM-EDX for glass analysis

A

minimally destructive
can analysis very small fragments (<100 micrometre)

28
Q

what level does secondary electron mode reach of samples in SEM-EDX

A

surface

29
Q

what level does back scatter electron mode reach of samples in SEM-EDX

A

just below the surface

30
Q

SEM-EDX is good for the elemental analysis of glass but what technique is preferred and why

A

micro XRF is preferred

SEM-EDX is limited by its sensitivity to some of the major components in glass

also it has poor precision due to the variation in fragments orientation, shape and thickness

31
Q

how does micro XRF differ from SEM-EDX

A

the detection method is the same but XRF uses X-Rays as the excitation source so is able to penetrate deeper into glass rather than the use of electrons in SEM (backscatter and secondary electron modes)

this can analyse the bulk of the glass and is less affected by the fragments shape

can measure smaller fragments and has a better detection limit - this is element dependent though

32
Q

what is plasma emission spectroscopy

A

sample pumped into a plasma torch

also known as ICP (inductively couple plasma)

spark ionises argon gas and resultant ions gain energy from an induced magnetic field

can achieve very high temps

sample aspirated by a flow of gas

33
Q

why is plasma emission spectroscopy good for the trace analysis of glass

A

higher temps reached so greater sensitivity - can be couple with mass spec

more sensitive than AAS and AES

background emission is low in the observed region

no oxide formations = minimal chemical interference

low risk of self absorption

good detection limits

multi element determination

reproducible

ionisation chance = high

34
Q

what is two disadvantage of ICP analysis of glass

how can this be overcome

A

sample prep can take a long time as sample is needed in a solution

destructive

overcome this issue is to do LDESI or MALDI mass spec instead as don’t need sample in solution

35
Q

What are two methods of sample preparation that can be used for glass analysis that speeds up the process

A

LIBS = laser induced breakdown

LA = laser ablation

laser hits and ionises sample very quickly - no need for solution

= LA-ICP-MS

36
Q

what is LA-ICP-MS

A

laser ablation - inductively coupled plasma - mass spectrometry

allows fast elemental analysis of glass samples

detection limits in ppb range

very small amount of sample can be used

37
Q

order these methods in how the provide discriminative value in the analysis of glass evidence
ICP-MS, micro XRF with RI and RI alone

A

RI alone = very low value

micro XRF and RI = 97.5% value

ICP-MS = high value

Trace Evidence (16 decks)

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