MS is an analytical technique used to find the composition of a chemical or biological sample by generating a spectrum representing the mass-to-charge (m/z) ratios of sample components. The technique has many applications that include (a) identifying unknown compounds by the masses of the compound molecules or their fragments, (b) determining the isotopic compositions of elements in a compound, (c) determining the structure of a compound by observing its fragmentation, (d) quantifying the amount of a compound in a sample, (e) studying the fundamentals of gas phase ion chemistry, (f) determining other physical, chemical, or even biological properties of compounds.
Throughout its various applications, all MS is based on the measurement of the m/z ratios of charged particles in a vacuum. In order to measure the m/z ratio of samples, all mass spectrometers contain three main components: ionization source, mass analyzer, and ion detector.
The ionization source is the mechanical device that allows ionization to occur. Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) are now the most common ionization sources for biomolecular MS, because both of them are soft ionization techniques; that is, the sample ionization process generates few or no fragments even for large biomolecules, such as proteins and oligosaccharides, so that the intact molecular ions can be easily observed.
The mass analyzer is used to separate sample ions. Commonly used analyzers include time-of-flight (TOF), quadrupole (Q), quadrupole ion trap (QIT), and Fourier-transform ion cyclotron resonance (FT-ICR) (9-14). These analyzers provide a wide mass range, high accuracy, and resolution for biomolecular analysis.
In MS, resolution is the ability of a mass spectrometer to distinguish ions with slightly different m/z ratios. The resolution can be measured from a single peak based on the following equation:
where M corresponds to m/z, DM is the peak width at half maximum of peak height. The narrower the peak is, the higher the resolution. Resolution calculated in this way is called full width at half maximum (FWHM). For example, a peak with m/z 800 and full width at half maximum 0.5 Da has a resolution of 1600.
The detectors are used to generate a signal current from the incident ions to record the m/z ratios and abundances of the ions. A variety of methods are used to detect ions depending on the type of mass spectrometer, such as channel electron multipliers for quadrupoles and ion traps, and microchannel plate for TOF mass spectrometers, inductive detector for Fourier-transform mass spectrometry (FTMS).
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