The relative formula mass of the compound is therefore 72. It has been simplified by omitting all the minor lines with peak heights of 2% or less of the base peak (the tallest peak).īecause the largest m/z value is 72, that represents the largest ion going through the mass spectrometer - and you can reasonably assume that this is the molecular ion. Note: This mass spectrum has 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. That isn't a problem you are likely to meet at A'level.įor example, in the mass spectrum of pentane, the heaviest ion has an m/z value of 72. A few compounds have mass spectra which don't contain a molecular ion peak, because all the molecular ions break into fragments. In the mass spectrum, the heaviest ion (the one with the greatest m/z value) is likely to be the molecular ion. Using the molecular ion to find the relative formula mass Note: If you are interested in a detailed look at fragmentation patterns you could follow this link. Fragmentation is irrelevant to what we are talking about on this page - all we're interested in is the molecular ion. These fragments produce the familiar stick diagram. The molecular ions tend to be unstable and some of them break into smaller fragments. 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. 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. Using a mass spectrum to find relative formula mass
It also shows how high resolution mass spectra can be used to find the molecular formula for a compound.
#EXACT MASS CALCULATOR HOW TO#
This page explains how to find the relative formula mass (relative molecular mass) of an organic compound from its mass spectrum.
MASS SPECTRA - THE MOLECULAR ION (M +) PEAK Mass spectra - the molecular ion (M+) peak
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