Quantitative mass spectrometry determines molecular mass and composition by ionizing and analyzing charged molecular fragments. It can analyze organic compounds, isotopes, and biological molecules such as proteins.
Quantitative mass spectrometry is a method of determining both the molecular mass of a compound and what it is made from. 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 mass and 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, hence varying the strength of the field channel ions of different masses in the sensing apparatus.
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 mass units atomic (amu ). Methane has the formula CH4 and analysis of the methane sample reveals the presence of H+, C+, CH+,CH2+, CH3+ and CH4+, respectively. The abundance of these fragments is also measured, with the highest value at 16 amu, which corresponds to the mass of the unfragmented ion. This is because it takes an extreme amount of energy to strip hydrogens from the central carbon, meaning that the most abundant ion will be the most energetically favorable.
There is a very small abundance of a fragment weighing 17 amu in the mass spectrum of methane. This reading is due to the presence of an isotope of carbon or hydrogen. Isotopes are elements that have the same chemical properties but different atomic weights because they have different numbers of neutrons in their nuclei. The nucleus of carbon-12 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.
In addition to analyzing the composition of organic compounds and the relative abundance 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 are found. The molecular mass of a protein can also be found using quantitative mass spectrometry.
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