Mass Spectrometry

Professor Dave explains the basic concept of mass spectrometry: a sample is vaporized, ionized, and fragmented into pieces. These fragments are sent through a tube and curved section where their charge enables measurement of mass-to-charge ratio (m/z).

Using pentane as an example, the instructor shows how ionization and fragmentation produce various molecular pieces with different carbon chain lengths. Multiple fragment possibilities are illustrated from a single parent molecule.

The spectrum shows the molecular ion peak at m/z 72 representing the intact pentane molecule (radical cation), and explains why no data appears beyond this mass. Major peaks like m/z 57 (butyl cation), 43 (propyl cation), 29 (ethyl cation), and 15 (methyl cation) are identified.

Minor peaks in the spectrum arise from molecular fragments containing different isotopes such as carbon-13, carbon-14, deuterium, and tritium. These isotope variants produce slightly different m/z values but appear as smaller peaks since heavier isotopes are less abundant.

Mass spectrometry can be used alongside infrared spectroscopy to identify unknown compounds by comparing observed fragmentation patterns against predicted fragments from a proposed structure.

The mass spectrum helps differentiate between structural isomers of pentane that share the same molecular formula. Different isomers fragment in characteristic ways, allowing the actual structure to be confirmed by matching the observed spectrum to expected fragmentation.