Instrumental Methods of Analysis (Edexcel GCSE Chemistry)

Advantages of Instrumental Methods

• Advancements in technology and computing have allowed for the development of instruments designed to analyse chemical substances

• Methods of analysis include X-ray, Infra-Red and Mass Spectroscopy, Gas Chromatography and Flame Photometry

• These analytical techniques require modern day instruments which are a vital part of chemistry laboratories

• The advantage of using these instruments over more traditional methods include:

  • They provide greater accuracy

  • They are faster and easier to use

  • They are automated and can perform multiple simultaneous sampling and testing

  • Modern instruments are very sensitive and can work with very small sample sizes

Flame Photometry

• This technique is used to analyse metal ions in solution

• When substances are heated they often emit energy in the form of light

• This is due to electrons falling back to their original energy levels after becoming excited which causes them to jump up one or more energy levels

• Flame emission spectroscopy works by exposing the sample to a very hot flame and then measuring the intensity and wavelength of the light emitted

• The output is an emission spectrum in which different elements produce lines in different parts of the spectrum

Diagram of an emission spectrum for mercury obtained from flame photometry

• The emission spectrum consists of brightly coloured thin lines on a dark background and each element ion produces a unique spectrum

• Flame emission spectroscopy also works for mixtures of ions

• This is a major advantage over flame testing which can only analyze one ion at a time

• The intensity of the light produced is proportional to the number of ions vaporised, so the technique can be used to determine the concentration of metal ions in a solution by reference to a standard solution of known concentration

A calibration curve for solutions containing calcium ions. Different standard solutions have their intensity measured and plotted on a graph against concentration. This linear relationship allows the intensity of an unknown solution to be measured and its concentration read off the graph.

Reference Data

• Ions in unknown samples can be identified by comparing the sample spectrum to reference spectra

• This is particularly useful if the sample contains a number of different ions

• The following flame spectrum for example was obtained for solution containing an unknown metal:

Spectrum for an unknown element

• When compared to the reference spectra below we can see that the solution must contain sodium ions:

Reference spectra for elements