Mass spectrometry is an analytical method used to identify different compounds based on the atomic sample constitution of the molecules and their charge state, which enables “blind” analysis of an unknown sample without any prior knowledge of its composition.
This kind of analytical power made this technique an indispensable tool for both qualitative and quantitative applications. Over the past two decades, prodigious technological advancements allowed for its use in studying peptides, proteins, carbohydrates, nucleic acids, drugs, and a plethora of other biologically pertinent molecules.
Principles of Mass Spectrometry
The basic principle of mass spectrometry lies in ionizing chemical compounds in order to generate charged molecules (or fragments) and determine their mass-to-charge ratio. Hence even the name “mass spectrometry” is sort of a misnomer, as we are not measuring strictly mass, but already mentioned mass-to-charge ratio (or a property related to it).
The analysis of data generated by mass spectrometry is a complicated issue specific to the type of experiment used for its production. There are general subdivisions of data fundamental for proper understanding; in addition, it is highly important to know if the observed ions are negatively or positively charged.
Mass spectrometry can be used to measure molecular structure, molar mass, or sample purity. As each of these queries necessitates a different experimental approach, suitable definition of the experimental goal is a prerequisite for obtaining and analyzing the data.
The first mass spectrometer that analyzed only small inorganic molecules was developed in 1912, but today’s mass spectrometers can be used for analysis of biological macromolecules – practically with no mass limitations. Essentially, any information acquired from a mass spectrometer is a result of the analysis of gas-phase ions.
Mass spectrometer consists of three main modules: an ionization source that converts gas phase molecules into ions, a mass analyzer that sorts ions on the basis of their masses (using electromagnetic fields), and a detector that measures the value of an indicator and provides data for calculating the plenitude of present ions.
A key factor that decides the sensitivity of a mass spectrometer is the mass analyzer where ion separation takes place. Therefore, combining two or more analyzers in the same mass spectrometer can yield high performance and resolution.
Types of Mass Spectrometry
Tandem mass spectrometry found its use in peptide sequencing, as well as in structural characterization of small oligo-nucleotides, carbohydrates and lipids. It employs two separate stages of mass analysis for selectively examining the fragmentation of particular ions present in a mixture of ions.
One of the commonly used combinations is gas chromatography-mass spectrometry (often abbreviated as GC/MS). In this technique, different compounds are separated by a gas chromatograph and subsequently ionized by a metallic filament to which voltage is applied. The intact ions and fragments are then detected by a mass spectrometer’s analyzer.
Liquid chromatography-mass spectrometry (LC/MS) first separates compounds chromatographically before they are directed to the ion source and the mass spectrometer. The mobile phase is a liquid, and an electrospray ionization source is most commonly employed in this technique.
In ion mobility spectrometry, ions are initially separated by drift time via some neutral gas under electrical potential gradient before they are introduced into a mass spectrometer. A recently introduced technique for structure clarification in mass spectrometry is called precursor ion fingerprinting, which identifies individual structural pieces by searching the tandem spectra of the molecule against a library of the product-ion spectra of characterized precursor ions.