Lecture 18 - Glass Flashcards
1
Q
History of glass
A
- First human-made glass originates from 3500+ years ago!! Melted sand!!
- One of the earliest reports of glass usage as trace evidence in 1933 - linking splinters of glass in an brief case to that of a broken shop window by comparing RIs with 70 other samples
- Glass is fragile, likely to break and transfer in a controlled manner and persists long enough to be useful and recoverable
- ~10% of case work in the UK is glass evidence (greater in other countries)
2
Q
Glass Manufacturing
A
- Crystal Palace in 1851 became one of the first buildings to use glass as the main material for construction
- Glass produced by blowing cylinders, slicing them lengthwise and then flattening in an oven!!
- ~50 years later Pilkington developed the first semi-mechanical process to make flat glass by drawing between rollers from a molten source
- Inconsistent thickness, but could be strengthened by introducing wire etc. into the process
- Float glass developed in 1950s by Pilkington - Used today for the manufacture of the vast majority of flat glass
- The molten glass is delivered onto a bed of liquid tin where the glass “floats” over the metal
- Produces smooth, flat surface at large scales which can then be processed to customer needs including surface coatings.
3
Q
Glass coating
A
- Coating is a vital process in modern glass making for bespoke applications
- e.g. self-cleaning, photo-reactive, toughened etc.
- This gives it identifiable features through surface analysis
4
Q
Float glass
A
- Float glass is also generally identifiable due to side in contact with tin showing luminescence at 254nm and also a anisotropic gradient in RI in some cases.
- A unique thing for float glass is the side of the glass that touches the tin has changed ans tranferred the surface layer and it will flurese a little.
- Some glass with have post-modifications to remove/reduce this fluesecence but cheaper glasses will still most likely have this property.
5
Q
Molten tin in glass making
A
- Taking some kind of silica sand which is heated up to a very high temp to melt and mix it together and then in the tank there is a massive bath of molten tin metal.
- Molten tin is more dense than the glass material, so when the glass is poured it floats on top of it so you then end up with perfectly flat level glass
- As you flow the molten silica you get this nice flat level glass.
- It is then cooled down slowly to minimise defects like cracking. It is then divided into the shape desired.
6
Q
Glass composition
A
- Elemental composition of glass can vary based on manufacturing site and even within a single plant - complex variation!!
- Other components can be added to give specific properties other than colour:
- Boron oxide (B2O3) is added to improve heat durability in cookware, glassware and automobile headlamps
- Silver (Ag) added in sunglasses and Strontium in TV screens to absorb radiation
- Glass screens on phones are even more complex as they have had stuff added to them to make them stronger.
- Phone screens are deisnged to have defects and then it gets filled in.
- Don’t forget coatings!
- Chemistry of glass is inconsistent which makes things difficult. The orders of materials as they appear from the glass arent always consistent.
- A lot of glass evidence comes from hit and runs.
7
Q
Why is boron added to glass?
A
In order to toughen and strengthen it.
8
Q
Glass breakage and transfer
A
- Key elements in examining breakage are flexibility vs. strength of the glass and the nature of the impacting object
- An object cannot start a percussion flaw if it is too soft to do so!!
- Determination of side impact by comparison of hackle marks or rib marks
- Percussive cone more likely for projectiles impacting glass
- Transfer from a crime scene most likely in hit-and-run and ram-raids
- These produce large shards but it is usually only the small fragments that transfer on clothing.
9
Q
Recovery of glass evidence
A
- You should use oblique lighting particular on soles of shoes
- Look at transfer from river onto a seat
- Look at car for small bits of glass
- You can recover glass from taping or scraping
- Taping is good for car seats or when there’s small amounts but there will be a lot of noise.
10
Q
Analytical workflow for glass
A
- Gross examination, recovery and collection
- Preliminary evaluation of physical characteristics
- Physical fit assessment - incredibly unlikely as you’ll have lots of shards and missing pieces but sometimes it’s possible when you have larger pieces
- Microscopic Analysis - Refractive Index
- Density Measurements - In a lot of modern processes density isn’t included. It is offten done hand in hand with RI
- Elemental Analysis - SEM & XRF
- Elemetnal analysis will always be in the workflow for glass!
- We have to do these in order to get anything useful and be able to differentiate.
- Elemental Analysis - Mass Spectrometry
11
Q
Physical and microscopic examination
Glass
A
- Thickness, colour, edge comparison, fracture features
12
Q
Elemental analysis
Techniques for glass
A
- SEM-EDS
- uXRF
- ICP-MS
- LA-ICP-MS
13
Q
Physical examination of glass
Large fragments
A
- Size of the recovered fragments defines the analytical scheme employed
- Large Fragments:
- Comparison of thickness - Careful as need to take standard deviation into account
- Float glass still has variation in the thickness of glass so you need to acknowledge this.
- Comparison of colour - Remember this can be subjective!!
- Could put it into a MSP but this is unlikely as there are other methods that are more useful. Colour is good for an initial assessment.
- Matching edges - potential for physical fit assessment
- Density Comparison
- RI Measurement
14
Q
Physical examination of glass
Small fragments
A
- Confirmation it is glass!!
- Quartz & Minerals - Birefringent
- Plastic - Compresses under pressure
- Could be a small polymeric matieral
- Microscopic examination of surface fragments for distinguishing features and fluorescence etc. to identify float glass
- See if theres an outer layer.
- If we have fluorescence we will have an outer layer which will also have coatings on it.
- RI Measurement
15
Q
Density measurement
Glass
A
- Measure using a Density Gradient Column (graduated cylinder) where you put two different density materials and you add the fragment and keep adding liquid of different density until the fragment it sits in the middle and doesn’t float or sink to the top or bottom.
- Variation of the density of the liquid until the glass fragment ‘floats’
- Measuring both Density & RI gives more discrimination but many forensic labs moved to only measuring RI as standard
- In most cases there is a correlation between RI and density. However, there are some outliers so using both methods is better.
16
Q
Becke line measurement
Glass
A
- Take a focal point where everything is in focus and you move up to a secondard focal point but keep the focus as if its in focus point one. Illuminate the material from below.
- If the Ri of the particle is higher than the mounting medium you end up with focussin of light and you get a layer of bright light inside the fragment. If the RI of the particle is lower you end up with the halo effect because the light is being refracted out due to the difference in RI between mourning media and the particle.
- You then repeat this with different mounting media so you can find the point where theres no change in RI.
- This method is rarely used on glass cause we want something more precise.
17
Q
Low RI partilce
A
Lower RI particle in higher RI medium will direct light in opposite direction. Moving the line outside the particle.
18
Q
High RI
A
A particle with a higher RI mounted in a medium of lower RI, will focus axial illuminating rays toward a point above the particle.
19
Q
Becke line immersion measurements
A
- Becke Line immersion measurements can be made by mounting the substance in media of varying RI’s until little change is observed
- This has limitations as will only be true for one wavelength of light at a time (so averaged for white light) need a more precise method
20
Q
Variation method
A
Single Variation method is the more precise method we are after
1. Mount in a special High RI medium above that of sample
2. Fix light at a single wavelength (typically 589nm - Sodium Line)
3. Slowly heat the sample on a hot stage
4. The medium RI changes on heating much faster than the sample
5. The temperature of lowest contrast isbetween sample and medium noted (usually computationally)
This is when you can hardly see your media as it looks almost the same.
This is usually automated now.
6. Compare to table of RI value corresponding to temperature (if needed)
Double variation method is even more precise - Vary both the temperature and the wavelength in controlled manner.
21
Q
Double variation method
A
- Temperature is fixed and wavelength varied until a match is found
- Do this until there’s minimal contrast
- Temperature is then changed (by 5C+) and process repeated to find new matching wavelength
- Do this until there is minimal contrast.
- Data plotted on Hartmann net - wavelength at the match temperature are plotted on the net and RI read off
- Equation of straight line established and converted to RI based on calibration data of the immersion liquid
- To be ale to discriminate between two glass samples is very low due to the complexities.
22
Q
RI of float glass
A
- RI of float glass surface will be different to bulk - usually lower (due to enrichment with tin oxide) unless it’s been manipulated.
- This offers an additional aspect of discrimination!
23
Q
SEM-EDX for glass analysis
Drawbacks
A
- SEM‐EDX suffers from poor precision because variation in fragment orientation, shape, and thickness affecting the measurements and makes quantitative analysis very challenging.
- The same fragment in a different orientation will nlook different.
- Detection limit ~0.1% (1000ppm) so limited in sensitivity to major elements in glass - most discriminating trace is left undetected in glass
If we want a disrciminating value we need to go much lower than 1000ppm - The method is okay but generally uXRF is preferred for glass
24
Q
SEM-EDX glass analysis
Advantages
A
Advantages of SEM‐EDX are that it is minimally destructive of the sample, can analyse tiny fragments (<100 μm) and sample preparation is relatively easy.
25
SEM-EDX for glass analysis
* This method excites the sample using a beam of electrons, firing electrons at the surface.
* Limited by the penetration bc we’re firing the electrons at it.
26
uXRF for glass analysis
* uXRF uses same detection but excites using an X-Ray source rather than beam of electrons, so penetrates much deeper into glass
* Become a bulk analysis technique, less affected by fragment shape with a detection limit improved to 10-50ppm (depending on element)
* Can measure small fragments (100-300um)
* Firing an xray tube at the material.
* Allows us to actually penetrate much deeper within the sample.
* XRF picks out additional peaks that SEM doesn’t.
27
Plasma emission spectroscopy
* Enhanced sensitivity as much higher temperature (>5500K)
* More homogenous temperature and less interference
* Sample is pumped into the plasma torch instead of into the fuel mix
* Known as inductively coupled plasma (ICP) as energy supplied by electromagnetic induction from a RF coil
* Spark ionises Argon gas and the resultant ions/electrons gain energy from the RF induced magnetic field
* Sample is aspirated in by flow of gas
* Very high temperatures achieved (up to 10000 K)
* CAn have precis control of temperature depending on where in the flame you’re using
28
ICP Spectroscopy advantages
* More complete atomisation of sample
* Background emission is low in observed region
* Ionisation can be high (But ion lines can be used in preference to the atom lines)
* No oxide formation
* Minimal chemical interference
* Low self-absorption due to the high proportion of excited atoms
* Good detection limits (again related to high temperature)
* Multi-element determination
* Reproducible
* Combining with Mass Spec. is 10-100x more sensitive that AAS/AES
* For glass it needs to be in a liquid form which is a disadvantage. This is difficult to achieve. A solution for this is LDESI or MALDI.
29
Inductively coupled plasma Mass spectrometry
| Glass
* One disadvantage of ICP-MS for glass is that sample preparation in order to get the glass sample in to solution can take a long time
* Solution is to take note from LDESI and MALDI Mass Spectrometry
* Laser Induced Breakdown (LIBS) or Laser Ablation (LA) remove need for sample preparation so analysis takes ~ 2mins/sample
* Put our sample on a surface and fire a laser at it. The laser will hit it and ionise it and then take it up into the atompshere very quickie.
30
Inductively coupled plasma Mass spectrometry
| Advantages
* 10x faster than uXRF or ICP-MS Alone
* Laser to break down the surface is useful.
* Just 0.4-2ug sample needed!!
* Powerful technique enabling elemental analysis at speed for glass trace element samples
* Detection limit 10-50 ppb!! (really good) An order of magnitude smaller than the other methods.
31
Comparing discrimination capability
| Glass evidence
* Discrimination by RI is 3-10% (not much better than random match when taken alone) - Must be combined with Elemental Analysis
* Combining RI with uXRF gives discrimination capabilities >97.5%
* ICP-MS has been shown to have discrimination capabilities >98.8% for a variety of different glass fragment types
* Combining with modern chemometrics approaches are therefore vital for glass trace evidence
* Outside of DNA, glass is the highest amount of trace evidence.
