Identifying the Parent Molecular Ion Peak Identification Quiz

The molecular ion peak occurs when the entire molecule loses a single electron but does not break apart. This radical cation has a mass to charge ratio equal to the relative molecular mass of the original compound.

The notation M plus represents the molecule after it has been ionized by an electron beam. In most cases a single electron is removed resulting in a positive charge. This peak is usually found at the highest mass to charge value in a standard spectrum.

Once the molecular ion is formed it often possesses excess energy that causes it to break into smaller fragments. These fragments appear as peaks at lower mass to charge values and provide clues about the structural arrangement of the molecule.

The tallest peak is called the base peak and represents the most stable fragment formed during ionization. For many compounds the molecular ion is unstable and breaks down rapidly making its peak much smaller than the base peak.

If a molecule is very unstable it may fragment completely before reaching the detector leaving no molecular ion peak. Low concentrations make the signal weak and heavy isotopes can create smaller peaks next to the main one that confuse the analysis.

Naturally occurring carbon contains about 1.1 percent of the Carbon 13 isotope. If a molecule contains even one Carbon 13 atom it will appear at a mass to charge value one unit higher than the main molecular ion peak.

The spectrometer separates ions based on their mass divided by their charge. Since most ions have a charge of plus 1 the value on the X axis is numerically equal to the mass of the ion in atomic mass units.

Excluding the small isotope peaks like M plus 1 the molecular ion is the heaviest ion because it consists of the entire molecule. Any other peaks in the spectrum are fragments resulting from the loss of atoms from this parent ion.

The relative molecular mass of butane C4H10 is calculated as 4 times 12 plus 10 times 1 which equals 58. Identifying the mass to charge value of the molecular ion allows chemists to narrow down the identity of an unknown substance.

The exact position of the M plus peak gives the molecular weight. Isotope patterns for M plus 2 can signal the presence of Chlorine or Bromine. High resolution mass spectrometry can measure the mass so accurately that the exact number of each atom can be calculated.

This rule is a helpful shortcut for identifying molecular structures. If the molecular ion peak appears at an odd mass number like 45 or 59 it typically indicates the presence of 1 or 3 nitrogen atoms in the molecule.

Electron Impact is considered a hard ionization technique because it uses high energy electrons that often cause extensive fragmentation. Soft techniques like Chemical Ionization are used when the molecular ion is too fragile to survive standard Electron Impact.

Calculating the mass 2 carbons at 12 each plus 6 hydrogens at 1 each plus 1 oxygen at 16 gives a total of 46. Therefore a chemist looking at a sample of pure ethanol would expect to see the molecular ion peak at 46.

Halogens like Chlorine and Bromine have two significant isotopes separated by two mass units. This results in characteristic M plus 2 and sometimes M plus 4 peaks that serve as a chemical fingerprint for these elements.

If standard methods destroy the molecule before it can be weighed chemists switch to softer methods like Electrospray or MALDI. These techniques apply less energy allowing the molecular ion to reach the detector intact.