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Doc Brown's KS4 Science GCSE/IGCSE Industrial Chemistry Revision Notes

6. Instrumental Methods of Chemical Analysis

Fast, sensitive and accurate instrumental methods of chemical analysis are required to meet the demands of the modern chemical industry. These methods include mass spectroscopy, atomic emission spectroscopy, gas-liquid chromatography, nuclear magnetic resonance spectroscopy, infra-red spectroscopy, ultra-violet spectroscopy. In most cases only a small sample is required and modern analytical techniques have the advantages of greater sensitivity, more accurate data analysed by computer, automation of analysis, multi-samples efficiently analysed, a greater range and versatility of analytical techniques with greater reliability and consistency of analysis data.

Index of sections: 1. Limestone, lime - uses, thermal decomposition of carbonates, hydroxides and nitrates  *  2. Enzymes and Biotechnology  *  3. Contact Process, the importance of sulphuric acid  *  4. How can metals be made more useful? (alloys of Al, Fe, steel etc.) * 5. The importance of titanium  *  6. Instrumental Methods of Chemical Analysis * 7. Chemical & Pharmaceutical Industry Economics & Sustainability * 8. Products of the Chemical & Pharmaceutical Industries & impact on us * 9. The Principles & Practice of Chemical Production - Synthesising Molecules  and other web pages of industrial chemistry notes: Ammonia synthesis/uses/fertilisers * Oil Products * Extraction of MetalsHalogens - sodium chloride Electrolysis * Transition Metals * Extra Electrochemistry

Top of page - sub-index and links

6. Why are instrumental methods of detection so useful?

Typical chemical tests are on a separate web page  and a page on Mass spectrometry

Instead of testing for chemicals using standard laboratory equipment such as test tubes etc. Special instruments have been developed to carry out such testing. These are quick, accurate and can be used on very small samples.

  • Elements and compounds can also be detected and identified using a variety of instrumental methods. Some instrumental methods are suited to identify elements while other instrumental methods are suited to the identification of compounds.

  • Instrumental methods have several advantages over traditional analytical methods e.g.

    • (i) very accurate - as analysis technology has improved

    • (ii) sensitive - only a small amount of a sample is needed

    • (iii) rapid - again. through improved technology

    • (iv) methods can be fully automated - so large numbers of samples can be dealt with efficiently

  • Mass spectroscopy can be used to identify elements and their relative ratio of isotopes and for molecules it can help to determine a molecular structure (its expensive, and NMR is much better for molecular structure analysis - especially organic molecules, see below).

    • The atoms or molecules are vapourised and converted to positive ions (based on a single atom or molecular fragment)  by bombardment with high energy electrons an instrument called a mass spectrometer.

    • The gaseous ions (e.g. Na+ or CH3+ etc.) are analysed according to their mass in a powerful magnetic field.

    • The highest mass ion, known as the molecular ion peak, corresponds to the molecular mass of the molecule.

      • It can be measured to four decimal places and can even distinguish between molecules with a similar molecular mass e.g. nitrogen N2 and carbon monoxide CO, both 28, but not the same to four decimal places!

  • Atomic emission spectroscopy can be used to identify elements and analyse element mixtures

    • Basically atomic spectroscopy is about 'exciting atoms' with heat or electrical energy until they emit the absorbed energy as visible light. You see this effect when fireworks go off, most of the colour comes from the 'excited' metal atoms in the salts added to the explosive powder mixture.

    • In a simple way flame colour tests in the school laboratory are used to identify elements e.g. sodium is yellow, barium green etc. BUT these colours are formed from many specific frequencies of visible light added together, so how do you sort out e.g. two shades of greens from copper or barium?

    • The answer is that detailed analysis of the different emitted frequencies of visible light (e.g. using a (c) doc b prism) gives a 'finger print pattern' by which to identify elements.

    • AND the greater the relative intensity of light frequency the more there is of that element.

    • So atomic spectroscopy is used to identify elements and analyse a mixture of elements or detect traces of elements in a solid or solution.

    • This analytical method has many applications e.g.

      • Its used in the steel industry to monitor the composition of steel as the molten mixtures are being made

      • Astrophysicists can identify elements in distant stars from the light emitted.

      • Tiny traces of metal ions can be detected in water e.g. for pollution monitoring.

    • Advanced level notes on spectroscopy

A non-chemical test method for identifying elements - atomic emission line spectroscopy
An instrumental method for IDENTIFYING ELEMENTS from LINE SPECTRA

If the atoms of an element are heated to a very high temperature they emit light of a specific set of frequencies (or wavelengths). These are all due to electronic changes in the atoms, the electrons are excited and then lose energy by emitting energy as photons of light. These emitted frequencies can be recorded on a photographic plate, or these days a digital camera.

Each emission line spectra is unique for each element and so offers a different pattern of lines i.e. a 'spectral fingerprint' by which to identify any element in the periodic table .e.g. the diagram on the left shows some of the visible emission line spectra for the elements hydrogen, helium, neon, sodium and mercury.

Note the double yellow line for sodium, hence the dominance of yellow in its flame colour. In fact the simple flame test colour observations for certain metal ions relies entirely on the observed amalgamation of these spectral lines.

This is an example of an instrumental chemical analysis and is performed using an instrument called an optical spectrometer (simple ones are called spectroscopes). This method, called spectroscopy, is a fast and reliable method of chemical analysis. This type of optical spectroscopy has enabled scientists to discover new elements in the past and today identify elements in distant stars and galaxies. The alkali metals caesium (cesium) and rubidium were discovered by observation of their line spectrum and helium identified from spectral observation of our Sun.

  • Nuclear magnetic resonance spectroscopy (nmr) is one of the most powerful analytical tools for determining the molecular structure of an organic compound.

    • Its very expensive for routine analysis but is invaluable in designing and analysing new molecules or finding the structure of natural molecules that the drug industry might find useful in developing new pharmaceutical products.

  • Infra-red spectroscopy can help to determine molecular structure and identify an organic compound.

    • Each molecule has a 'fingerprint' pattern of absorption of different infrared frequencies. Can be used to determine alcohol concentrations in breath!

  • Ultra-violet spectroscopy can be used to the determine purity or concentration of solution of a substance that absorbs uv light.

  • Top of page - sub-index and linksGas-liquid chromatography (gc/glc) can be used to analyse liquid mixtures which can be vapourised (e.g. petrol, blood for alcohol content).

    • The instrument for doing gas chromatography is called a gas chromatograph.

    • a picture of 'glc': diagram a gas chromatogram and the resulting chromatograph

    • A sample of the substance under investigation is injected and vapourised into a tube containing a carrier gas (called the mobile phase, it moves). The gas carries the vaporised substance through a long 'separating' tube or column wound around inside a thermostated oven.

    • The substances in the mixture are partially and temporarily absorbed by an absorbent material held in the column.

      • The material in the column consists of fine particles of solid or a layer of very high boiling liquid, and is called the immobile phase or stationary phase - which doesn't move.

        • The column is prepared by filling it with a porous solid so gas can get through but passes over a large surface area OR it is coated in a very high boiling organic liquid which can also provide a large absorbing surface but still allows gas flow.

      • Depending on the strength of interaction between the different substances in the mixture and the column material, they are held back, or 'retained', for different times so that the mixture separates out in the carrier gas stream.

      • There is a dynamic equilibrium between the stationary and mobile phases and the separation of the components of a mixture by chromatography depends on the distribution of the components in the sample between the mobile and stationary phases.

    • The gases emerge from the oven into a detector system which electronically records the different signal as each substance comes through. A printout or computer display of the results from the gas chromatograph, called the gas chromatogram, shows a series of peaks in the graph line imposed on a steady baseline when only the carrier gas is passing through the detector.

    • The time it takes for a substance to come through is called the retention time and is unique for each substance for a particular set of conditions (flow rate, length of separating column, nature of separating column material, temperature etc.). Generally speaking, the greater the molecular mass of the mixture molecule, the longer the retention time. This is because the component molecule - immobile phase intermolecular force of attraction increases with the size of the component molecule, so it is absorbed/retained temporarily a bit more strongly (see right of diagram).

    • The height of the peak, or more strictly speaking, the area under the peak, is proportional to the amount of that particular substance in the mixture.

    • Therefore it is possible to identify components in a mixture and calculate their relative proportions in the mixture.

    • The chromatogram shown above (right of diagram) illustrates the separation of some alkane hydrocarbons in petrol (in reality it is far more complicated with dozens of hydrocarbon molecule peaks on the chromatogram). The different peak heights give the relative proportions i.e. hexane >pentane > heptane.

    • The retention time order follows the trend of increasing molecular mass gives increasing retention time i.e. in time heptane C7H16 > C6H14 > C5H12

    • The gas chromatographic instrument can be calibrated with known amounts of known substances.

      • So, the timing position of the peak identifies the component X in the gas and the height of the peak tells you much of X is in the mixture.

    • Don't confuse with 'non-instrumental' paper/thin layer chromatography.

    • You can also have more sophisticated analysis by attaching a mass spectrometer to the gas chromatograph and analyse each separated molecule as they exit the separating column. You can get the molecular mass of each component from the molecular ion peak (see mass spectroscopy further up the page)

  • Industry requires rapid and accurate methods for the analysis of its products. There have also been increasing demands from society for safe and reliable monitoring of our health and environment. The development of modem instrumental methods has been aided by the rapid progress in technologies such as electronics and computing.

  • Various factors have influenced the development of instrumental methods. With modern methods you get ...

    • greater sensitivity i.e. smaller amounts of material can be used OR much smaller amounts of a trace element or compound can be detected in a bulk mixture (drug testing of athletes)

    • more accurate data (perhaps analysed by computer)

    • automation of analysis, multi-samples efficiently analysed

    • a greater range of analytical techniques, today's laboratory is far more versatile these days

    • greater reliability and consistency once the instrument is set up and procedures in place and checked.

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