High-resolution mass spectrometry measures molecular ion peaks to four decimals to determine the exact molecular formula of a compound.
The molecular formula is the formula indicating the number of atoms of each type of element in a molecule. In high-resolution mass spectrometry (HRMS), the m/z of each ion on the mass spectrum measures to multiple decimal places, enabling measurement of exact molecular mass. The exact molecular mass is the actual mass of a molecule calculated to four decimal places and obtained by high-resolution mass spectrometry. Chemists differentiate between compounds that have the same nominal molecular mass and would show up at the same peak on a standard mass spectrometer. The nominal molecular mass is the sum of the mass numbers of a molecule's constituent atoms.
For example, C2H6O3, C3H7Cl, C5H4N, and C6H6 all have the same nominal mass of 78. Using high-resolution mass spectrometry, a more precise m/z is calculated, and the compounds are identified.
Using Exact Molecular Mass to Distinguish Between Compounds
Exact Mass for Molecular Ion Peak
Compounds with the same nominal molecular mass are distinguished using high-resolution mass spectrometry to determine their exact molecular masses.
High-resolution mass spectrometry is a mass spectrometry method that allows for precise measurement of a molecule's mass. There is a variety of different methods for high-resolution mass spectrometry, including liquid chromatography/mass spectrometry (LC/MS), gas chromatography/mass spectrometry (GC/MS), electrospray ionization (ESI), and matrix-assisted laser desorption/ionization (MALDI).
Recall that mass spectrometry is an analytical method in which a molecule is ionized and the resulting fragments and ions are analyzed. Chromatography is a technique or series of techniques for separating a mixture into its components. Liquid chromatography is an analytical method in which a compound dissolved in a solvent is separated into its components. Liquid chromatography/mass spectrometry (or LC/MS) combines liquid chromatography with mass spectrometry. First, liquid chromatography separates liquid mixtures into their individual components. Then mass spectrometry specifically identifies the molecular components. LC/MS is commonly used in biotechnology, environment monitoring, food processing, and the pharmaceutical, agrochemical, and cosmetic industries.
Gas chromatography is an analytical method in which a compound is vaporized and separated into its components. GC/MS combines gas chromatography (GC) and mass spectrometry to separate the components of a mixture. In gas chromatography, molecules separate by boiling point as they travel through a capillary column. Then the molecules are sent through a mass spectrometer. GC/MS is frequently used in drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples—including some material samples collected in the 1970s from Mars.
Electrospray ionization is the technique used in mass spectrometry to produce ions by applying a high voltage to a liquid to create an aerosol. Electrospray ionization, or ESI, submits liquids to a high voltage, creating an aerosol that is then analyzed by mass spectrometry. ESI is commonly used to produce ionic macromolecules because it causes very little fragmentation. Therefore, ESI is useful to identify the molecular ion but is not used to glean structural information about the molecule from the resulting mass spectrum.
Matrix-assisted laser desorption/ionization (MALDI) is a mass spectrometric method used in determining the amino acid sequence of larger biomolecules. Matrix-assisted laser desorption/ionization, or MALDI, uses lasers to create ions from large molecules, with minimal fragmentation. This is useful in analysis of biomolecules, including DNA and proteins, and large organic molecules, which readily fragment when subjected to other methods of mass spectrometry. MALDI involves a three-step process: (1) the sample is mixed with a matrix material and applied to a metal plate, (2) the sample is irradiated with a pulsed laser, and (3) the resulting molecules are ionized by protonation or deprotonation using hot gases and then sent through a mass spectrometer for analysis.