Forensic Science - Analytical Process

What is a ‘sample’?

The material that is being used for analysis.

What is an ‘analyte’?

The chemical entity that is being investigated.

What is a ‘matrix’?

Everything in a sample except the analyte. The matrix is the carrier of the analyte.

What is an ‘interferent’?

Any part of the matrix that interferes with analysis.

What are the 9 steps in analysis?

1. Formulate a question.

2. Sampling.

3. Sample preparation.

4. Screening.

5. Identification.

6. Confirmation.

7. Quantification.

8. Reporting & Interpretation.

9. Draw conclusions.

What happens during sample preparation?

The sample is reduced to a smaller amount for analysis to take place. ‘Cone & Quartering’ can be used where 50% of the sample is removed each cycle. This is more used for large samples. You can also take 10% of the mixture and use that for analysis, this needs to be representative of the sample as a whole though. Extract the components from the matrix e.g. removing components of blood to analyse a poison.

How should a sample be handled?

• Preserve it in the condition it was collected.

• Process it immediately (samples are instable).

• Consider contamination issues (equipment, containers).

• Remove unwanted material from sample.

• Get analyte in correct form.

• Put sample in the correct solvent for analysis.

• Concentrate the analyte in the sample.

What is centrifugation?

This is a process that separates particles from a liquid or gas. The sample is spun at a very high speed. This causes suspended particles to fall to the bottom due to gravitational force. This is known as sedimentation.

What is sample extraction?

This is the process of removing the analyte from the matrix.

What is headspace extraction?

This is a method to collect the vapours that come off of a solid or liquid. These vapour molecules are collected and then analyzed, usually through GC-MS (gas chromatography - mass spectrometry).

What is liquid-liquid extraction? (LLE)

A method used to separate an analyte from a liquid (e.g. drugs from blood). The liquid (blood) with the analyte (drugs) inside is mixed with a solvent. These two liquids repel each other and sit on top of each other separately. The drugs absorb better into the solvent rather than the blood, so when mixed the drugs move into the solvent. This solvent can then be separated from the blood, and the drug type can be found.

What is solid phase extraction? (SPE)

This is a method used to separate chemicals from a liquid (e.g. drugs in blood). This chemical can then be analysed.

What is chromatography?

This method is used to separate a mixture into individual parts (analytes). This can be done in a gas or a liquid. It can be separated because each analyte moves at a different speed.

What is screening?

Screening is a quick, initial test to check if certain substances might be present in a sample. Screening is carried out before more detailed tests are. 

Give two screening examples.

1. Spot test.

2. Immunoassays.

What is a spot test?

A spot test involves putting a small drop of a sample onto a surface and adding a reagent. If the substance you’re looking for is there, a colour change will occur.

What is an immunoassay?

A test that uses antibodies to find a presence of a certain substance within a sample. The rumoured substance acts as an antigen and binds with the antibodies. 

What is gas chromatography?

• A technique used to separate and analyze chemical mixtures.

• The mixture (blood, oil) is put into the machine and is heated up, turning it into a gas.

• The gas is carried through a long, thin tube (column) by another gas (carrier gas, usually helium or nitrogen).

• Inside the column, different chemicals move at different speeds.

• Because they move at different speeds they come out one by one and get separated.

• A detector at the end measures when each chemical comes out and how much there is.

• The result is a graph (a chromatogram) with peaks, each peak represents a different chemical.

What is UV-Vis?

• Ultraviolet-Visible Spectroscopy.

• It measures how much light a substance absorbs at different wavelengths.

• A beam of light passes through a sample.

• Some wavelengths of light are absorbed by the molecules in the sample.

• The instrument measures how much light passes through.

• The result is a UV-Vis spectrum, showing absorbance vs wavelength.

What is liquid chromatography? (HPLC)

• A technique used to separate, identify and measure chemicals that are dissolved in a liquid.

• The sample is injected into the HPLC system.

• A liquid solvent carries sample through the column.

• Inside the column, different substances stick to the solid material differently.

• As the separated substances exit the column, a detector measures them (usually using UV-VIS).

• The output is a graph called a chromatogram, showing peaks that represent each compound in the sample.

What is SEM-EDX?

• Scanning electron microscopy combined with energy dispersive x-ray spectroscopy.

• SEM uses a focused beam of electrons to scan a sample’s surface.

• This allows us to see very fine details like texture or particle shape.

• The image looks 3d.

• When the electron beam hits the sample it makes the atoms give off x-rays.

• Each element gives off unique rays.

• The EDX detector measures these x-rays and tells you which elements are in the sample and how much.

What is mass spectrometry?

• A technique used to identify a substance and measure how much of it there is.

• The sample is turned into charged particles (ions) using a high-energy source.

• The ions are sent through a magnetic or electric field that separates them based on mass and charge.

• A detector measures how many ions of each type there are.

• The machine draws a graph showing peaks at different masses, each peak represents a specific molecule.

What is infrared spectroscopy? (IR)

• A technique used to identify chemical compounds.

• A sample is placed in the instrument.

• The machine shines infrared light through it.

• The molecules absorb certain wavelengths which makes their bonds vibrate.

• The detector measures the intensity of transmitted light at each wavelength.

• The result is an IR spectrum, which is a graph of absorption vs wavelength. Peaks in the spectrum correspond to specific bonds or functional groups.