This page explains how the M+1 peak in a mass spectrum can be used to estimate the number of carbon atoms in an organic compound. | |

Note: This is a small corner of mass spectrometry. It would be a good idea not to attack this page unless you have a reasonable idea about how a mass spectrum is produced and the sort of information you can get from it. If you haven't already done so, explore the mass spectrometry menu before you go on. | |

If you had a complete (rather than a simplified) mass spectrum, you will find a small line 1 m/z unit to the right of the main molecular ion peak. This small peak is called the M+1 peak. In questions at this level (UK A level or its equivalent), the M+1 peak is often left out to avoid confusion - particularly if you were being asked to find the relative formula mass of the compound from the molecular ion peak.
The M+1 peak is caused by the presence of the If you had a simple compound like methane, CH The mass spectrum will therefore have a line corresponding to the molecular ion [ The line at m/z = 17 will be much smaller than the line at m/z = 16 because the carbon-13 isotope is much less common. Statistically you will have a ratio of approximately 1 of the heavier ions to every 99 of the lighter ones. That's why the M+1 peak is much smaller than the M+ peak.
Imagine a compound containing 2 carbon atoms. Either of them has an approximately 1 in 100 chance of being There's therefore a 2 in 100 chance of the molecule as a whole containing one That means that the ratio of the height of the M+1 peak to the M+ peak will be approximately 2 : 98. That's pretty close to having an M+1 peak approximately 2% of the height of the M+ peak. | |

Note: You might wonder why both atoms can't be carbon-13, giving you an M+2 peak. They can - and do! But statistically the chance of both carbons being ^{13}C is approximately 1 in 10,000. The M+2 peak will be so small that you couldn't observe it. | |

If you measure the peak height of the M+1 peak as a percentage of the peak height of the M+ peak, that gives you the number of carbon atoms in the compound. We've just seen that a compound with 2 carbons will have an M+1 peak approximately 2% of the height of the M+ peak. Similarly, you could show that a compound with 3 carbons will have the M+1 peak at about 3% of the height of the M+ peak.
The approximations we are making won't hold with more than 2 or 3 carbons. The proportion of carbon atoms which are Consider a molecule with 5 carbons in it. You could work out that 5.55 (5 x 1.11) molecules will contain 1 Above 3 carbon atoms, then, you shouldn't really be making the approximation that the height of the M+1 peak as a percentage of the height of the M+ peak tells you the number of carbons - you will need to do some fiddly sums! | |

Important! Most syllabuses at this level (UK A level or its equivalent) don't ask for this. Before you get too bogged down in all this, check your syllabus and recent past papers to see whether you need to bother. This is a case where you really need to know what line your examiners are currently taking - for example, how far are they pushing the approximation.
If you are doing a UK-based exam, and haven't got a syllabus and past papers, follow this link to find out how to get them. | |

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© Jim Clark 2000 |