4.2.5 Instrumental Analysis

Flame Photometry and Line Emission Spectra

• Flame tests can distinguish between distinctive colours but when colours are similar then the test is more difficult
• Flame tests also only work when only one metal ion is present
• Instrumental analysis using a flame photometer (also called a flame emission spectrometer) gives a more detailed analysis and can be used when multiple colours from different ions are present
• 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 image is viewed as a line emission spectra, and each element has a characteristic pattern of emission lines a bit like a fingerprint
  • If multiple ions are present then the overall emission spectra can be compared to their individual spectra to identify the individual ions present
• The colour that we see depends on the colours that are most strongly emitted in the visible 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
• 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
• The output is an emission spectrum in which different elements produce lines in different parts of the spectrum

Using Reference Spectra

• 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

Using a Calibration Curve

• 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 on a calibration curve
• Whilst it is called a curve, this is a general term and the concentration is directly proportional to the intensity so these graphs will be straight for analysing the intensity of light

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.

Mass Spectrometry

• Mass Spectrometry is a powerful analytical technique
• It is the most useful instrument for accurate determination of the relative atomic mass of an element, based on the abundance and mass of each of its isotopes
• It is also used to find the relative molecular mass of molecules
• As a sample passes through the mass spectrometer, a spectrum is produced of mass / charge ratio against abundance
  • Mass / charge ratio is often referred to as m/z or m/e on the axis of graphs
• The spectrum can be used to find the relative isotopic abundance, atomic and molecular mass and the structure of a compound
• The peak with the highest mass is the molecular ion peak, M⁺, which is the peak the furthest to the right
• There are several types of mass spectrometer, but all of them are based on an ionised sample being accelerated through the mass spectrum, and being separated based on the ratio of their charge to their mass

Using Mass Spectrometry to Identify Isotopes

• The heights of the peaks in mass spectroscopy show the proportion of each isotope present:

The peak heights show the relative abundance of the boron isotopes: boron-10 has a relative abundance of 19.9% and boron-11 has a relative abundance of 80.1%

Using Mass Spectrometry to Identify a Molecule

• Molecules, not just atoms, can be analysed using mass spectrometry
• The molecular ion peak can be used to identify the molecular mass of a compound, however, different compounds may have the same molecular mass
• Molecules can fragment as they get ionised and the fragments can pass through to give a range of different peaks on the mass spectrum
• Fragments may appear due to the formation of characteristic fragments or the loss of small molecules
  
  • For example, a peak at 29 is due to the characteristic fragment C₂H₅^+­­
  • Loss of small molecules, still ionised, give rise to peaks at 18 (H₂O⁺), 28 (CO⁺), and 44 (CO₂⁺)
• Each peak in the mass spectrum corresponds to a certain fragment with a particular mass/charge ratio (m/z)
• The most important peak to look for is the one with the highest m/z value, the molecular ion (M⁺) peak which gives information about the molecular mass of the compound and is the first thing used to identify a compound

Mass spectrum showing the fragmentation of C₁₀H₂₂

• The carbon compound that this mass spectrum is from is C₁₀H₂₂ as the molecular ion peak, the peak with the greatest mass to charge value, is at 142
• The mass of the C₁₀H₂₂⁺ ion is 142
• Some other common fragments are labelled too

• 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