Workshops Flashcards
Give reasons why the inclusion of trace evidence in a criminal investigation may be beneficial.
- Trace evidence doesn’t lie, have an agenda, forget things, succumb to emotion or misinterpret scenarios.
- Traces can provide evidence of association or contact, between people, places and/or objects.
- They can assist in crime reconstruction, i.e. activity level propositions.
- They provide robust and complementary evidence in casework.
- We can no longer rely exclusively on DNA evidence as the sensitivity of its analysis means contamination is a problem.
- It can provide valuable evidence where DNA and fingermarks presence would be unhelpful. (For example a domestic assault case in the victims home)
- Where no matches are revealed on either the DNA or fingerprint databases, traces can provide investigative leads.
What do we mean by the term chemometrics
- Computationally intensive, multivariate statistical analysis, applied to chemical systems or processes.
- Reduction of data dimensionality in complex datasets.
- Pattern recognition techniques that reveal sample groupings or underlying trends.
- An aid to objective and reproducible data interpretation or parameter optimisation.
- Techniques that reveal important variables within datasets and those that demonstrate covariance.
- An aid to data visualisation.
Explain how Raman microspectroscopy is advantageous for the analysis of trace evidence specifically.
- It is a non-destructive technique so the sample’s integrity is maintained.
- It is a rapid technique.
- Microspectroscopy has a high spatial resolution so it is good for trace samples.
- Organic and inorganic components within a sample can be profiled simultaneously.
- It is a sensitive technique and is therefore able to detect slight changes in analyte concentration.
- It is superior to IR for the analysis of inorganic components, i.e. metal-ligand bonds.
- It aids identification of a sample through functional group assignments and provides additional, complementary information.
- Minimal sample preparation means sample integrity is maintained; sample alteration or contamination is unlikely.
- Polymorphs may be determined.
- Mapping function allows for sample heterogeneity to be ascertained.
- Microscope imaging allows precise focussing of laser on specific sites of interest within a sample.
Name factors you would need to consider when interpreting the evidential value of fibres recovered from a deceased victim’s garment.
- Are they relevant in the context of the case?
- Do they “match” or share similar characteristics with anything relating to the suspect?
- Background levels – how common is the fibre?
- Must consider 2⁰, 3⁰ and even 4⁰ transfer.
- Non-contact transfer?
- The fibre’s material and shedding likelihood.
- The material of the garment.
- How many fibres were recovered.
- The location and surface area of fibre recovery.
- Any evidence of emergency services intervention?
- Any contamination from the scene?
- Have elimination samples been taken from emergency personnel and excluded as a source?
- Attempts at cleaning/laundering
- Environmental conditions at an indoor scene.
- Weather conditions at an outdoor scene.
- Wild animal interference? Or even household pet
- Time elapsed since crime to recovery and recovery to analysis.
Name differences between raman and infrared
Raman
- Light scattering
- Requires change in polarisability (C=C, C-C, C-H)
- Little or no sample preparation necessary
- Measure through transparent packaging
- Aqueous samples
Name differences between raman and infrared
IR
- IR absorption
- Requires change in dipole moment (O-H, N-H, C=O)
- Sample preparation usually necessary
- Short optical path length required
- Non-aqueous samples
Which part of the analytical workflow for both fibre and paint evidence, would provide a result with the highest probative value?
- Physical fit
In fibre analysis, name physical properties that we use microscopy to examine.
- Surface treatments
- Dye penetration
- Cross section
- Chemical composition
- Colorant/dye
- Texture
- Morphology
- Diameter
- Dichorism/pleochroism
- Isotopy/anisotropy
- Refractive index/birefringence
Give pieces of information that are available for a particular colourant (pigment or dye) in the Colour Index International list.
- It’s application
- It’s hue
- It’s properties
- Chemical composition
- Generic name
- Colurant properties
- CI number
- Companies that manufacture it
- Trade names
Explain why Principal Component Analysis (PCA) is a better unsupervised exploratory data analysis technique than Cluster Analysis (CA).
- CA groups samples into clusters based on their calculated distance (similarity), but the resultant dendrogram does not tell you why the samples are similar/different.
- PCA provides a loadings plot as part of the visual overview; this is a map of the variables, which in turn is related to the chemistry of the samples.
- The loadings plot can be used in conjunction with the scores plot - which is a map of the samples - to work out how the samples are similar/different based upon their chemical compositions.
Equation for F
F = -kx
Equation for stress
force / cross sectional area (A)
Equation for strain
Extension/length
Young modulus equation
- Plot stress on x axis
- Plot strain on y axis
- Gradient = young modulus
What is the most common form of paint evidence?
When considering this evidence type with a comparison microscope, what particular features should you note?
- Automotive paint
- Colour of layers, thickness of layers, pigment distribution, textural chanes, layer sequence, pigment size and distribution, defects eg weathering, dirt and solvent traps
What is the definition of the term “resolution”? What are the parameters that
limit the resolution of a light microscope?
- The limit of resolution in light microscopy is given by rayleighs criterion, half of the airy disk diameter (d) the airy disk diameter os given by d = 1.22 x wavelength x (focal length / lens diameter) acknoweldeg that wavelength, focal length and lens diameter matter for resolution.
- Resolution is the ability to determine between two points of specimen.
In a light microscope explain what is the purpose of the… (i) Lamp at the base of the stand. (ii) Eyepiece lens.
- Lamp in the base of the stand - Supplies light to the specimen. Without it the overall illumination and contrast would not be sufficient for imaging.
- Eyepiece lens - Further magnifies the image from the objective and puts it in a form and position suitable for viewing.
What is the role played by light in a light microscope?
In a reflected light microscope a light source (usually visible) is directed through a tube, reflects off the surface of the sample, and is then sent through a series of lenses to magnify the sample. The image is relayed to the eyepiece which puts it into a form suitable for viewing for the operator looking through the eyepiece.
What are the different types of signal detected in SEM and typically what depth does each signal originate from, in relation to the surface of the sample? In your answer you should consider how these signals arise, following the interaction of the incoming probe beam with the sample.
Background
- SEM images are generally formed by detecting either the secondary SE (low-energy) electrons emitted from the sample, or the backscattered (high-energy) electrons. While secondary electrons can be formed anywhere within the volume of interaction, only those generated within a few nanometers of the surface actually escape to be detected. Most of these are generated before the beam spreads out, and thus provide information about a very small volume of material at the e-probe’s point of impact on the surface.
- Different types of signal originate from different depths.
What is the role played by the beam of electrons in an Electron Microscope?
Electron microscopes use a beam of electrons rather than visible light to visualise the object. The beam interacts with the sample and produces several types of electron signals. Some of the electrons that are scattered are collected by a detector to produce an image, with magnifications of the order of 100,000x.
What is the principal difference between SEM and TEM?
In a Scanning electron microscope (SEM) the focussed electron beam is scanned across the specimen and is reflected from the surface, while in a Transmission Electron Microscope (TEM) the beam passes through the specimen.
What are the different types of signal detected in SEM and typically what depth does each signal originate from, in relation to the surface of the sample? In your answer you should consider how these signals arise, following the interaction of the incoming probe beam with the sample.
BSE
- The backscattered electrons able to escape from the sample are formed only in the first part of the electron trajectory, before the beam has lost too much energy, while they still stand a good probability of getting back to the surface, but still at greater depths than the secondary electrons.
- Hence the detected backscattered electrons (BSE) originate from a larger volume of the sample than do the secondary electrons, and thus form an image of lower resolution.
- BSEs that re-emerge from the sample surface can also generate secondary electrons, but in most cases these are few enough, and sufficiently diffuse, that they do not mask the high-resolution detail in the main secondary electron signal.
What are the different types of signal detected in SEM and typically what depth does each signal originate from, in relation to the surface of the sample? In your answer you should consider how these signals arise, following the interaction of the incoming probe beam with the sample.
X-rays
- Production of X-rays can take place at any depth where the electron beam has enough energy to ionise the atoms.
- The probability of X-ray production will vary with depth, in a way that will vary from element to element, but it is clear that the volume from which the X-rays originate is substantially the whole interaction volume of the electron beam.
- While the X-rays can be absorbed as they pass through the material of the sample, they have, in general, a much longer path than electrons, and so have a good probability of leaving the sample and reaching the detector.
- Thus the spatial resolution of the X-ray signal is variable, and is frequently of the order of a micron or more.
Introduction to pollen
- Pollen is the powder containing the male gametophytes of seed-producing plants - dispersed in order to reproduce by wind, water or animals.
- A single anther can produce 50-250,000 pollen grains depending on the plant species
