Forensics and the Environment (BJK)

Question 1

Why is metals analysis a possibility and why is it useful?

Answer 1

• environmental levels and distributions are controlled by the local geology and anthropogenic intervention
• profiles of metals can provide unique fingerprints of environment or activity

Question 2

What are the applications of metals analysis?

Answer 2

1. assessment of contaminated lands (harmful metals)
2. idenfication of oil spills
3. provenance studies (animal and crop products) using trace materials
4. origins of worked materials e.g. glass, ceramics

Question 3

Describe how glass is formed.

Answer 3

• glass is formed when a melt cools too fast for nucleation to occur, hence it has an amorphous structure - a disordered network
• various metal cations are accommodated within the structure randomly
• metals affect the properties of the glass

Question 4

Describe the composition of glass.

Answer 4

Two main components:

• the former - commonly SiO₂
• the flux - plant ash / natron; the metals lower the melting point and increase viscosity of the glass

Additional components:

• pigments - transition metals impart various different colours
• opacifiers - alter translucence to produce opaque regions

Question 5

Describe the metal contents of glass.

Answer 5

1. Major and minor: from principal components of former and flux
   
   • five key oxides
   • can reveal origins of the raw materials and different generic types of historical glass
2. Minor: pigments
   
   • often transition metals
3. Trace: low level species present in all materials used in manufacture
   
   • ultimately reflect the local geology of the sources of the raw materials

Question 6

How are metals analysed?

Answer 6

• inductively coupled plasma mass spectrometry (ICP-MS) often employed for multi-element analysis
• analyses > 50 trace elements and their isotopes simultaneously
• wide dynamic range = magnitude of the concentrations you can analyse at the same time

Question 7

Describe laser ablation ICP-MS.

Answer 7

• good for precious samples (no need to sacrifice)
• high energy laser
• spatial resolution: 100 - 350 μm spot, point or line analysis
• part per billion (ppb) detection level

Question 8

How does laser ablation ICP-MS work?

Answer 8

1. initial ablation to remove the surface at the spot you’re looking at (remove contaminants from the environment)
2. take another recording at the same spot from slightly deeper

Question 9

Describe the accuracy and precision of ICP-MS and laser ablation ICP-MS.

Answer 9

ICP-MS:

• good accuracy and precision

Laser ablation ICP-MS:

• very good accuracy but less precision
• largely due to the heterogeneity of the material but also impacted by the way the analysis is performed
  
  • losses are possible when gas flows in to carry the material, also can get differences in flow rate

Question 10

Describe replication.

Answer 10

• repeated measurements
• gives a frequency distribution described by the sample mean and the sample standard deviation
• repeat measurements approximate to a normal (Gaussian) distribution
  
  • symmetric about the mean
  • s is the spread of the curve

Question 11

Describe normal distribution.

Answer 11

• equation involves population mean (μ) and standard deviation (σ)
  
  • to determine these, measure each member of the population
• whatever the values:
  
  • 68% of the population lies within ± 1 σ
  • 95% within ± 2 σ
  • 99.7% within ± 3 σ

Question 12

Describe replication in analysis.

Answer 12

• repeat observations
  
  • the more repeat observations (n = repeats) the closer they represent a normal (Gaussian) distribution
  • a Gaussian distribution is described well by 50 points
• we can estime the population from a sample of n repeats provided we have enough coverage
  
  • estimate μ and σ from s and x̄
• selection of n depends on context of observation
  
  • practical considerations (ease of sampling, speed/cost) often limit choice of n

Question 13

Describe outliers.

Answer 13

• outliers are results that appear to differ considerably from the population mean
• often there is a tendency to discard these points in order to obtain a ‘better fit’
• statistical test to determine if a value should be excluded or not = Q test

Question 14

Describe the Q test.

Answer 14

• the difference between the questionable result and its nearest neighbour is divided by the spread (w) of the entire data set (including the outlier)
• Q_exp is compared with rejection values (Q_critical) at the right confidence level
  
  • if Q_exp > Q_critical then reject

Question 15

When/how can the Q test be used?

Answer 15

• suitable for limited sample sized (< 30)
• caution when dealing with small samples (lower limit of n = 4)
• must view the statistical test only in the light of the chemistry involved
• rejection is based on awareness of an error in measuring a particular result

Question 16

Describe chemical fingerprinting.

Answer 16

• use of diagnotic compounds or distributions of compounds in a sample to:
  
  • distinguish between types of source
  • identify or rule out a specific source
  • correlate with pollution history
• extremely challenging
  
  • distributions change as a function of time
  • pollution distribution overprints natural distribution
  • potential for multiple pollution sources
  • transport vectors may be complex

Question 17

Describe crude oil and oil-derived products.

Answer 17

• complex mixtures of components of varying toxicities, dominated by straight chain alkanes, contain other comounds useful for profiling (usually always contain these)
• profiles are characteristic of different oils
  
  • complexity allows fingerprinting to identify sources of spills
  • monitor specific compound classes by GC-MS

Question 18

Describe organic residues.

Answer 18

• anthropogenic organic inputs can influence environments
  
  • coal by-products, crude oil, refined oil, etc.
  • heterogeneous systems: residues spilt in the environment partition between different phases (aq + org)
  • dyanmic equilibrium driven by solubility in each phase - measured by distribution coefficient, K)

Question 19

Describe partitioning behaviour in the environment.

Answer 19

Partitioning controls the levels of organic compounds in different phases in the environment, driven by differences in solubility between organic and aqueous phases.

Question 20

Describe how different compounds partition in the environment.

Answer 20

• hydrophilic compounds associate with the aqueous phase
  
  • polar + poly-functional compounds
  • widely distributed through the environment via the water course and ground water
• hydrophobic compounds associate with the organic phase
  
  • apolar compounds
  • associate with the organic matter in natural solids (soils, sediments, suspended particles)

Question 21

What is the octonal-water partition coefficient?

Answer 21

Equilibrium phase partitioning of apolar organic molecules between immiscible phase (octanol and water) models the natural paritioning process.

K_ow = C_O/C_W

Apolar organic compounds shown to approximate to ideal solution behaviour.

Concentration in vapour phase is controlled by volatility.

Question 22

Describe oil spills.

Answer 22

Oil forms slicks, emulsions and tar balls.

• differences in composition and behaviour
• inhomogeneity must be overcome by sampling strategy

Question 23

Describe weathering processes and give an example.

Answer 23

Weathering:

• any chemical released into the environment is subject to physical, chemical and biological processes
  e. g. weathering of crude oil alters composition:
• loss of straight chain alkanes
• alteration to aromatic hydrocarbons prodile
• branched isoprenoids remain unaltered
• increased prominence of unresolved complex mixture

Question 24

Describe hopanes and steranes.

Answer 24

• polycyclic alkane (steranes and hopanes) profiles from MS are unaltered by weathering or biodegradation
  
  • reflect oil fingerprint
• ratios useful for confirmation
  
  • T_S/T_M - varies with source
  • C₂₉/C₃₀ hopane - varies with source
  • S/S+R hopane > 0.55 for petroleum

Question 25

What are polycyclic aromatic hydrocarbons (PAHs)?

Answer 25

Persistent organic pollutants, some are carcinogenic.

• hydrophobic hence accumulate in soils, sediments and organisms
• 16 are priority pollutants
• some have long residence times in environment

But:

• variations in volatility and aqueous solubility lead to alterations in profiles over time

Question 26

What are petrogenic PAHs?

Answer 26

Components of fossil fuels (petrogenic), transformation products of natural product molecules.

• enter the environment through oil seeps, oil spills + in process water from oil production platforms
• fresh oil dominated by low molecular mass + alkylated components
• distributions altered with weathering (more volatile components stripped away)

Question 27

Describe pyrogenic PAHs.

Answer 27

Mainly formed by incomplete combustion.

• anthropogenic combustion (fossil fuel/biomass burning)
• natural processes (forest fires, volcanic activity)

Typically higher in urban than in rural areas.

• distributions differ depending on land utilisation and activity

Question 28

Describe PAH molecular ratios.

Answer 28

Ratios are used to identify different sources. Linear PAHs/5-membered ring-containing PAHs are the less thermodynamically stable isomers.

• petrogenic sources (low T formation) lower concentrations
• pyrogenic sources (high T formation) higher concentrations

Note: ratios most meaningful when local sources are measured.

• need to consider factors that could alter ratio values e.g. solubility, volatility, etc.

Question 29

Describe compositional analysis.

Answer 29

Triacylglycerol profiles (main components of oils + fats) differ among oils by variations in:

• fatty acid chain lengths
• presence/number and position of double bonds
• position of fatty acids on the glycerol

Can be used to identify an oil and combat food fraud.

Question 30

Describe how to verify geographical origins of oil.

Answer 30

• stable isotope signatures verify geographical origins
• combustion elemental analyser coupled to stable IR-MS
• reference standard Vienna standard mean ocean water (V-SMOW)

Question 31

Describe the geographical considerations for stable isotope measurements.

Answer 31

Fractionation during evaporation/condensation of water:

• rain depleted in ¹⁸O and ²H relative to ocean (V-SMOW)
• δ values depend on meteorological factors - T, elevation, distance from sea

Leads to geographical variations in isotope ratios:

• reflected in δ²H and δ¹⁸O values of organic matter
• δ¹⁵N also reflects geographical variability in nitrogen cycle