Scientific Detection

Examination Techniques Visual; drawings, photos, sketches, video Identification; shape, colourings, markings Comparisons; fingerprints, hair samples, footprints, fibre samples. Samples must be collected, stored and prepared for analysis BUT we must make sure that… –We collect representative samples; –prevent change or deterioration to samples; –avoid contamination of samples; –avoid tampering of samples by keeping them in a secure place; This leads to more detailed analysis which follows………

Measuring: Increasing precision… estimate the readings between the increments e.g mm and not 12.1 mm Using a vernier scale (see opposite)… the value is mm since the zero is nearest to 0.3 and the lines of the bottom scale exactly line up on 58. The uncertainty of any measurement is usually +/- half the smallest increment If we work out the area the uncertainty will be squared (i.e. bigger!) If we work out the volume the uncertainty will be cubed (even bigger).

Errors arising from the techniques Example: Random errors You measure the mass of a ring three times using the same balance and get slightly different values: g, g, g Take more data. Random errors can be evaluated through statistical analysis and can be reduced by averaging over a large number of observations. Systematic errors The cloth tape measure that you use to measure the length of an object had been stretched out from years of use. (As a result, all of your length measurements were too small.) The electronic scale you use reads 0.05 g too high for all your mass measurements (because it is improperly tared throughout your experiment). Systematic errors are difficult to detect and cannot be analyzed statistically, because all of the data is off in the same direction (either to high or too low). Spotting and correcting for systematic error takes a lot of care

Light Microscopy Can you label… (i) Eyepiece (ii) Objective lens (iii) Focus knobs (iv) Stage (v) Mirror/ lamp (iv) Arm…? Key Terms: –Magnifying power = the magnification of the eye piece X the magnification of the lens –Depth of field… refers to the thickness of the object that can be seen clearly… this decreases with increasing magnification –Resolving power… the smallest distance you can still tell apart two objects Compound light microscopes have a max magnifying power of 1500X and a resolution limit of 200nm (0.2 microns)

Making a Slide Sample very thin Usually add a drop of water or a stain which can show specimen features more clearly.

Electron Microscopy 2 types; TEM (transmission electron) and SEM (scanning electron). More expensive and time consuming than light. Electrical focussing unlike manual of light. Can only view dead specimens unlike light. All have high magnification ( X); and very high resolution since they use electrons not light!

Sample Preparation TEMElectron beams pass through thin slice of specimens. 2D image (like Light). SEMCoated in gold so electrons can bounce off specimen for 3D view. Black + white image. Have a greater depth of view.

Chromatography (Paper + TLC) Mobile phase (in this case the solvent but could be a gas in GLC) Stationary phase (in this case paper/ silica gel) Solvent front Substances move depending on attraction to mobile phase Which spot above opposite is most attracted to the solvent?

Retention factors Calculate the retention factors (Rf) for spots found on the chromatogram using the formula Rf = X/Y where X = distance moved by ‘spot’ (measured to the centre of the spot), and Y = distance moved by the solvent

Background Paper + TLC are very similar but only 2 nd one really used in forensics (analysis of fibre dyes + drugs mainly Often need to be developed in order to see the spots e.g. by using iodine vapour or ninhydrin. Some can be seen under ultra violet (uv) light

Other types Gas-liquid chromatography GLC; used to separate complex mixtures. Mobile phase is a gas. Heat controlled and separated compounds identified electronically. High-performance liquid chromatography HPLC; similar to above but mobile phase is a liquid moved at high pressure. These 2 both have much greater separating powers and better sensitivity

Electrophoresis Useful for biological molecules Used in DNA profiling; –Crimes or to confirm whether a man is a father or not. Uses electrical voltage instead of moving solvent to separate particles. The sample contains positive and negative ions… the former are attracted to the negative plate and the latter to the positive plate [opposites attract!] The smaller the size of the ion and the more charge it has the further it moves

Colour Matching Intensity of a chemicals colour can show how much of a compound is present in a mixture. Test kits which normally give a YES/NO answer; e.g. pregnancy (ClearBlue), blood (anaemia = lack of red cells), urine for diabetes (Clinistix) or kidney disease (albustix). Some change colour (such as litmus – pink/blue); others part of an exact colour scale (as with pH and universal indicator).

Definitions Qualitative Test – simple colour change e.g. pregnancy kit, litmus (acid or alkali) Semi-Quantitative – specific colour change e.g. pH scale (more specific acid or alkali strength) Quantitative – precise measurement giving an actual number reading e.g. clinical thermometer, colorimeter.

Colorimetry: Gives the concentration by measuring the intensity of a colour looking at how much light passes through the sample compared with a reference sample (pass light through a pure, colourless solvent and set the meter to zero) Compared to colour matching: –Much more accurate if used with a calibration chart –produces quantitative results (not qualitative/ semi quantitative) –Lower uncertainty –range of concentrations that can be determined –More sensitive

Laboratories Proficiency Tests – all labs receive a set of samples to analyse. All must fall within a close area of accuracy to ensure they are all doing the testing properly. Those that meet standards are ‘accredited’ by UKAS

Good Practice GLP (good lab practice) –Health and Safety –Equipment; regular checks to ensure everything works, kept clean and stored correctly. –Training This system mainly for labs involved with things that come into contact with the body e.g. medicines, food additives, cosmetics.

Data Collection Sampling; use ‘representative’ samples and do same tests on each sample (standard procedures) Homogenous sample; made from sample material throughout e.g chocolate bar. Heterogenous sample; water taken from different parts of a stream. Calibration; equipment must be checked for accuracy before any testing; e.g. weigh scales have precise weights added (known as standard reference materials)