Mass spectrometry 0.0 / 5 ? ChemistrySpectroscopyASOCR Created by: Kyra MullarneyCreated on: 15-04-19 14:38 Molecular ions and fragment ions When an organic compound is placed in the mass spectrometer, it loses an electron and forms a positive ion, the molecular ion. The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound. The molecular ion M+ is the positive ion formed when a molecule loses an electron. 1 of 5 Molecular mass from a mass spectrum To find the molecular mass, the molecular ion peak (M+) has to be located. The molecular ion peak is the peak on the far right with the highest m/z value. The small peak after the M+ peak is the M+1 peak. It exists because 1.1% of the carbon atoms present is the carbon-13 isotope. 2 of 5 Fragmentation Some molecular ions break down into smaller pieces known as fragments in a process fragmentation. The other peaks are caused by fragment ions, formed from the breakdown of the molecular ion. The simplest fgragmentation breaks a molecular ion down into two species - a positively charged fragment ion and a radical. Any positive ions formed will be detected by the mass spectrometer but the uncharged radicals are not detected. 3 of 5 Using fragmentation peaks to identify a molecule The mass spectrum of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures. Mass spectra can therefore be used to help identify molecules. Although two mat have the same molecular mass and the same molecular ion peak, the fragment ions may be different. 4 of 5 Common fragment ions CH3+ = 15 C2H5+ = 29 C3H7+ = 43 C4H9+ = 57 5 of 5
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