What Is Quantitative Mass Spectrometry?

Quantitative mass spectrometry is a method of determining both the molecular mass of a compound and from what it is made. Mass spectrometry works by exposing a sample to extreme conditions of heat and electricity, causing it to break into charged molecular fragments. The composition and abundance of these fragments is analyzed to reveal the mass and the composition.

There are many types of quantitative mass spectrometry, but each method employs the same set of processes. A sample is first heated to form a vapor, then ionized and accelerated using an electric field. The ions each have a single positive charge, and these charged particles are deflected by passing them through a magnetic field. Lighter ions are deflected more by a magnetic field than heavier ions, so varying the strength of the field channels ions of different mass into the detection apparatus.

The analysis of methane, the simplest hydrocarbon — or compound made up of hydrogen (H) and carbon (C) — reveals the presence of fragments that have atomic masses of 1, 12, 13, 14, 15 and 16 atomic mass units (amu). Methane has the formula CH₄, and sample analysis of methane reveals the presence, respectively, of H⁺, C⁺, CH⁺,CH₂⁺, CH₃⁺ and CH₄⁺. The abundance of these fragments is also measured, with the highest being at 16 amu, which corresponds with the mass of the unfragmented ion. This is because it takes an extreme amount of energy to strip away the hydrogens from the central carbon, meaning that the most abundant ion is going to be the most energetically favorable.

There is a very small abundance of a fragment weighing 17 amu on the mass spectrum for methane. This reading is because of the presence of an isotope of either carbon or hydrogen. Isotopes are elements that have the same chemical properties but different atomic weighs because they have different numbers of neutrons in their nuclei. The carbon-12 nucleus contains six neutrons and six protons, but the much rarer isotope carbon-13 contains seven protons. Similarly, a small amount of the hydrogen present will be hydrogen-2, also called deuterium, which has a nucleus of one proton and one neutron.

As well as analyzing the composition of organic compounds and the relative abundances of isotopes in a sample, quantitative mass spectrometry is also used to elucidate the composition of biological molecules such as proteins. Proteins are made up of a chain or sequence of amino acids, and mass spectrometry can be used to determine the sequence in which these amino acid residues occur. The molecular mass of a protein also can be found using quantitative mass spectrometry.