Qualitative Analysis (1)
The identity of a substance may be found. Can be of elements, molecules, mixtures or objects.
May be done by comparison to tables of data, to standards of known composition, physical characteristics or by interpretation of analysis results.
Quantitative Analysis (1)
The concentration of a substance may be found. The effect often will relate to concentration.
This will use a range of units, separated into mass/mass, mass/volume and percentage.
Quantitative Analysis (2)
Concentration can have different uses in an investigation.
E.g. blood alcohol concentration above the legal limit can result in a conviction for drink driving.
Concentrations may help establish cause of death.
Stages in Chemical Analysis
Initial examination
Presumptive tests
Selective tests
Confirmation/Quantification
Reporting the Results
Initial Examination
This is looking at the characteristics, morphology, packaging and paraphernalia.
Does the evidence label description what you can see? Is the package in tact and appropriate?
Weight
The offence may depend on the weight of the drug found, and impacts whether destructive preliminary analysis can be conducted or not.
Trace samples - less than 0.1 gram.
Bulk samples - more than 0.1 gram.
Presumptive Testing (1)
Designed to quickly and easily screen samples, to indicate the possible presence of specific chemical compounds.
They rely on rapid chemical reactions to provide colour change.
Presumptive Testing (2)
These tend to be destructive, meaning that when only small amounts are present, other tests will take priority.
Unfortunately most results are for a class of material, not specifics. A systematic approach must be taken with presumptive testing.
Presumptive Test Advantages
Low cost
Allow rapid screening of large numbers of samples
Simple to perform, little training
Field use friendly
Presumptive Test Disadvantage
Colours may be subjective
Not specific, false positives occur
Interference from impurities
Not sensitive enough for trace samples
Destructive
Reagents are often harmful
Confirmation/Quantification
These are aided by results from the presumptive tests. They will typically use instrumental analysis to positively identify the substance.
Multi step process:
Separation of the individual compounds, then the identification of the separated compounds.
Forensic Science Involvement
FS is mainly involved in suspected drink-driving and alcohol-related death cases.
The courts need to know the level of alcohol in the body at the time of the incident, so knowledge of the following is important:
Absorption
Distribution
Metabolism
Elimination from the body
Legislation and technical defences
Alcohol - Central Nervous System
Alcohol depresses the central nervous system, affecting:
Higher brain functions (emotions),
Lower brain functions (breathing),
Motor functions (co-ordination).
Intoxication effect correlates to the blood-alcohol level.
Alcohol - Physiological Effects
20mg/100ml - some loss of judgement, begin to relax.
50mg/100ml - exaggerated behaviour, loss of small-muscle control, impaired judgement and lowered alertness.
Hip Flask Defence
You are thought to be in control of a vehicle while over the prescribed alcohol limit, however the alcohol was consumed after you stopped driving.
Absorption
Alcohol is absorbed by diffusion across the gut wall/gastrointestinal membrane.
20% happens in the stomach, the other 80% occurs in the small intestine.
Rate of absorption is affected by:
Presence/absence of food
Type of food
Still/carbonated beverage
Alcoholic strength of beverage
Distribution
For most people, blood circulates through the body in 90 seconds.
Absorbed alcohol is quickly and evenly distributed throughout the body.
Metabolism and Elimination
90-98% of absorbed alcohol is metabolised, mostly by oxidative decomposition.
The rate of metabolism varies from 10 to 25 mg/100mL per hour.
The remaining 2-10% is excreted unchanged via the lungs, kidneys and skin - hence why breath and urine tests are possible.
Road Traffic Act 1988
Section 4 - driving or being in charge when under influence of drink or drugs.
Section 5 - driving or being in charge of a motor vehicle with alcohol concentration above prescribed limit.
Section 5A - driving or being in charge of a motor vehicle with concentration of specified control drug above specified limit.
Section - the power to administer preliminary tests.
Preliminary Breath Tests
Acid Dichromate devices were based on chemical oxidation, where extent of colour change is related to alcohol level.
They are non-reusable and have a short shelf life, but are very cheap.
Fuel Cell Breathalysers
These are the majority of hand-held devices used by law enforcement officers.
Fuel Cell Breathalysers - Advantages
Sensor is highly specific and sensitive to alcohol
The measurements cannot be influenced by endogenous substances such as acetone, Carbon Monoxide or Toluene
Have long life
Fuel Cell Breathalysers - Disadvantages
Cannot detect if a breath sample was alveolar (deep lung air)
May produce a falsely high reading if a subject has recently drank and still has alcohol in their mouth
Infrared Optical Sensor Breathalysers - Advantages
Ensures that the breath sample is alveolar
Provides pinpoint accuracy
Can detect the presence of mouth alcohol
Does not have a limited life expectancy and will remain stable for many years
