Lecture 17- Glass Flashcards
what was the first type of glass made from
melted sand
why can glass be a good source of trace evidence
it is fragile so likely to break and transfer
it is also able to persist for a long time enough to be recoverable
how is the flatness of glass controlled in the manufacturing of it
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
how is float glass normally identified
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
how does the composition of glass vary
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
what is added to glass to improve heat durability and where is this often seen
boron oxide (B2O3)
seen in cookware, glasses and car headlamps
what is added glass to absorb radiation and where is this often seen
silver (Ag) in sunglasses
strontium in TV screens
what are the 3 properties of glass associated with the possibility of it breaking
flexibility
strength
nature of the impacting object (force and shape)
how can a side impact on glass be determined
by hackle or rib marks
what shape is often seen when a projectile impacts glass
a percussive cone
when is glass transfer often seen in crime scenes
during hit and runs and ram-raids
do the smaller or larger fragments of glass tend to be transferred to clothing and found in investigations
smaller ones
more likely to see if a big chunk of glass is on your clothing
what are the three ways normally used to recover glass shards
scraping
taping
forceps
what is the analytical workflow for glass evidence
- gross examination recovery and collection
- prelim evaluation of physical characteristics
- physical fit assessment
- microscopic analysis - refractive index
- density measurements (often not included now)
- elemental analysis with SEM, XRF (non destructive)
- mass spec elemental analysis (destructive)
why is physical fit assessment matches very unlikely with glass evidence
therefore what type of information is heavily relied on from glass evidence
harder with smaller pieces of glass
glass can chage shape slightly - upon heat
rely on elemental analysis
what defined the analytical scheme employed in the analysis of glass
the size of the recovered fragment
large vs small
what is analysed for larger glass fragments
thickness - take an average and standard deviation
colour - can be subjective though
matching edges - physical fit
density
RI measurement
what is analysed for smaller glass fragments
confirm it’s glass
examine surface for distinguishing features
how can the analysis of colour be made less subjective
by using MSP - microspectrophotometry
what can be looked at to see if the sample is glass or a type of quartz/mineral
the birefringence
what can be looked at to see if the sample is glass or a type of plastic
see if it compresses under pressure if breaks = glass, if compresses = plastic
how is the density of a piece of glass measured
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
when measuring the RI of glass what method is normally used and why
the Single or double variation method as it is more precise than the Becke Line test
how is the single variation method carried out
how does the double variation method differ from this
- mount sample in a medium with higher RI
- fix light at a certain wavelength e.g 589nm
- slowly heat sample on a hot stage
- the medium’s RI will change quicker than the samples when heating
- temp of lowest contrast between sample and media is recorded computationally
- compare to a table of known RI values
double variation method varies temp and wavelength of light = even more precise
is a sample from the outer edge of glass or from the main bulk likely to have a lower RI
sample in bulk likely to be lower
why is elemental analysis done on glass samples normally
very little discrimination is often found comparing the RI of them
give 3 advantages of using SEM-EDX for glass analysis
minimally destructive
can analysis very small fragments (<100 micrometre)
what level does secondary electron mode reach of samples in SEM-EDX
surface
what level does back scatter electron mode reach of samples in SEM-EDX
just below the surface
SEM-EDX is good for the elemental analysis of glass but what technique is preferred and why
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
how does micro XRF differ from SEM-EDX
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
what is plasma emission spectroscopy
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
why is plasma emission spectroscopy good for the trace analysis of glass
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
what is two disadvantage of ICP analysis of glass
how can this be overcome
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
What are two methods of sample preparation that can be used for glass analysis that speeds up the process
LIBS = laser induced breakdown
LA = laser ablation
laser hits and ionises sample very quickly - no need for solution
= LA-ICP-MS
what is LA-ICP-MS
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
order these methods in how the provide discriminative value in the analysis of glass evidence
ICP-MS, micro XRF with RI and RI alone
RI alone = very low value
micro XRF and RI = 97.5% value
ICP-MS = high value
