Let's have another look at the mass spectrum for pentane: This is generally the simplest thing you can be asked to do. Working out which ion produces which line You will find a link at the bottom of the page. That is covered in three other pages which you can get at via the mass spectrometry menu. This section will ignore the information you can get from the molecular ion (or ions). The base peak is the tallest peak because it represents the commonest fragment ion to be formed - either because there are several ways in which it could be produced during fragmentation of the parent ion, or because it is a particularly stable ion. This is usually given an arbitrary height of 100, and the height of everything else is measured relative to this. The tallest line in the stick diagram (in this case at m/z = 43) is called the base peak. You are very unlikely to come across such a case at A'level. Note: You have to be a bit careful about this, because in some cases, the molecular ion is so unstable that every single one of them splits up, and none gets through the machine to register in the mass spectrum. In the stick diagram showing the mass spectrum of pentane, the line produced by the heaviest ion passing through the machine (at m/z = 72) is due to the molecular ion. Note: If you are interested in the mass spectra of elements, you could follow this link. With a compound, each line represents a different fragment produced when the molecular ion breaks up. With an element, each line represents a different isotope of that element. It's important to realise that the pattern of lines in the mass spectrum of an organic compound tells you something quite different from the pattern of lines in the mass spectrum of an element. They have been simplified by omitting all the minor lines with peak heights of 2% or less of the base peak (the tallest peak). Note: All the mass spectra on this page have been drawn using data from the Spectral Data Base System for Organic Compounds ( SDBS) at the National Institute of Materials and Chemical Research in Japan. For example, the mass spectrum of pentane looks like this: The ion, X +, will travel through the mass spectrometer just like any other positive ion - and will produce a line on the stick diagram.Īll sorts of fragmentations of the original molecular ion are possible - and that means that you will get a whole host of lines in the mass spectrum. These uncharged particles will simply get lost in the machine - eventually, they get removed by the vacuum pump. Only charged particles will be accelerated, deflected and detected by the mass spectrometer.
The uncharged free radical won't produce a line on the mass spectrum. More complicated break-ups are beyond the scope of A'level syllabuses. Note: A free radical is an atom or group of atoms which contains a single unpaired electron. The simplest case is that a molecular ion breaks into two parts - one of which is another positive ion, and the other is an uncharged free radical. The molecular ions are energetically unstable, and some of them will break up into smaller pieces. That's one half of what was originally a pair of electrons - the other half is the electron which was removed in the ionisation process. The molecular ion is often given the symbol M + or - the dot in this second version represents the fact that somewhere in the ion there will be a single unpaired electron. Note: If you aren't sure about how a mass spectrum is produced, it might be worth taking a quick look at the page describing how a mass spectrometer works. This ion is called the molecular ion - or sometimes the parent ion. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. This page looks at how fragmentation patterns are formed when organic molecules are fed into a mass spectrometer, and how you can get information from the mass spectrum. FRAGMENTATION PATTERNS IN THE MASS SPECTRA OF ORGANIC COMPOUNDS