OCR Scientific Detection

These cards are to summarise scientific detection module.


People and Organisations

People and Organisations

Scientific detectives fall into three categories:

1. Law enforcement i.e. crime scene investigators and forensic scientisits.

2. Environmental Protection i.e. the environmental agency monitoring the air, land and water.

3. Consumer Health and Safety i.e. the foods Standards Agency monitoring healthy and safety in kitchens and what goes into foods.

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Good Lab Practice

Good Lab Practice

It is important for labs to all work in the same way following the same set of standards.

Important things are that everyone is:

  • following health and safety regulations
  • regular maintanence of equipment to check it is all working correctly.
  • up-to-date training for staff.

If these three points are followed within all labs nationally then they will all work correctly and following the same standards.

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The Science

Collecting Samples

Analysts work with samples. The samples they use must be representative of the test they are carrying out i.e. they need to make sure they have a big enough sample.

It is important the samples are preserved so there is no damage to them.

They need to avoid tampering or contamination in any of the samples.

For all this there needs to be a good detailed plan of the test they are going to carry out.

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Carrying out the Tests

Carrying out the tests

It is important that standard operating procedures are used - i.e. all working in the same way.

It is also necessary that standard reference materials are used i.e. known samples/weights.

When calculating results you have to be accurate and sometimes a graph is drawn to help calculate an answer.

If a graph is drawn it is important to look for outliers. It is important to take into account that there can be systematic error (error with equipment), random error, human error.

Lastly you need to work out how significant the findings were.

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Recording Visual Information

Recording Visual Information

Visual information can be recorded in differen ways:

  • Written
  • Drawn
  • Photographs and Videos

This information can then be compared and features identified.

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Microscopic Examination

Light Microscopy

You need to know how to label a light microscope and how to work out magnification. I.e. if the eyepiece lens has a magnification of x10 and the objective lens has a magnification of x10 you multiply these two numbers together to get the overall magnifying power.

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Electron Microscopes

Electron Microscopy

Electron microscopes give a more detailed picture that is also 3D. There are two types of electron microscope:

1. Transmission Electron Microscope (TEM) - these pass a beam of electrons through the sample. Need very thin samples.

2. Scanning Electron Microscopes (SEM) - a fine beam of electrons scans the surface and the image is made by a detector. Samples are covering in a thin layer of gold or carbon.

Some electron microscopes have a resolving power of x500 000 which is massive!

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Paper Chromatography

Chromatography is used to separate mixtures and chemicals. The technique depends on the movement of a liquid or gas through a fixed medium. The moving is called the mobile phase and the medium is called the stationary phase.

The unknown solution is dropped onto paper with a few known solutions to see if they match up. The paper is stood in water or a solvent. The sample then moves up the paper and makes spots in various places. The unknown samples are then compared to known samples.

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Rf Factor

A chemical may also be identified by its retardation factor (Rf)

Rf = distance moved by chemical

distance moved by solvent

I.e. distance sample moved divided by distance water moved

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Gas Chromatography

This is used to separate complex mixtures. A machine is used to do it.

1. A very small amount of sample is injected in one end of a heated column where it turns to vapour.

2. A detector at the other end times how long it takes for the gas to travel through the column.

3. It draws a chromatogram which is then analysed.

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This is a technique used to identify chemicals, DNA and samples.

An unknown sample is run at the same time as known samples which are used for comparison.

The samples are loaded into small wells in a specially prepared gel. The gel is then put into a tank where an electric current is passed through it. This causes the samples to run from the negative to the positive electrode and separates the samples into a series of bands. The gel is then stained and the bands analysed.

If it was a sample of DNA, DNA profiling is carried out to analyse the DNA.

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Colour Matching

There are two ways in which colour matching can be done - by eye or by using a colorimeter.

Colour matching by eye is an easy method of comparing the colour/concentration of samples by setting up solutions of different concentrations.

Colourimetry is a more accurate method which uses a machine to work out the exact concentration.

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Procedures and Techniques

The last part of the module!!!

Reading a linear scale - just like reading a ruler, work out what the graduations represent and measure your object.

There are two ways to weigh something - weight by difference and weighing it direct.

When you use a pipette make sure you remember that water miniscus is curved, make sure you are at eye-level with the mark.

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good revision ty

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