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Mass Spectrometry, Review of the Basics: Electrospray, MALDI, and Commonly Used Mass Analyzers

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Abstract

Mass spectrometry (MS) has progressed to become a powerful analytical tool for both quantitative and qualitative applications. The first mass spectrometer was constructed in 1912 and since then it has developed from only analyzing small inorganic molecules to biological macromolecules, practically with no mass limitations. Proteomics research, in particular, increasingly depends on MS technologies. The ability of mass spectrometry analyzing proteins and other biological extracts is due to the advances gained through the development of soft ionization techniques such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) that can transform biomolecules into ions. ESI can efficiently be interfaced with separation techniques enhancing its role in the life and health sciences. MALDI, however, has the advantage of producing singly charges ions of peptides and proteins, minimizing spectral complexity. Regardless of the ionization source, the sensitivity of a mass spectrometer is related to the mass analyzer where ion separation occurs. Both quadrupole and time of flight (ToF) mass analyzers are commonly used and they can be configured together as QToF tandem mass spectrometric instruments. Tandem mass spectrometry (MS/MS), as the name indicates, is the result of performing two or more sequential separations of ions usually coupling two or more mass analyzers. Coupling a quadrupole and time of flight resulted in the production of high-resolution mass spectrometers (i.e., Q-ToF). This article will historically introduce mass spectrometry and summarizes the advantages and disadvantages of ESI and MALDI along with quadrupole and ToF mass analyzers, including the technical marriage between the two analyzers. This article is educational in nature and intended for graduate students and senior biochemistry students as well as chemists and biochemists who are not familiar with mass spectrometry and would like to learn the basics; it is not intended for mass spectrometry experts.
... The generated charged (probably multiple charged) ions are sorted based on their m/z values in the mass analyzer. This method visibly solved the issue of studying the large protein using the traditional mass spectrometry and its efficiency can be further boost by coupling it with liquid chromatography (i.e., LCMS) (El-Aneed et al. 2009). Nano electrospray ionization tandem MS technology has the advantage not only of reduced flow rates but also enhanced the ion formation mechanism (Yates et al. 1997). ...
... If fingerprinting cannot recognize the proteins because of an inadequate number of proteolytic peptides or lack of an appropriate DNA database to align them confidently, then ESI MS/MS can be used for the sequencing of amino acids (He and Chiu 2003). ESI MS/MS is able to replicate the data, but it is worth noting that the relative abundance of different ions in an ESI spectra is not a true representation of the sample concentration (El-Aneed et al. 2009). ...
... The generated charged (probably multiple charged) ions are sorted based on their m/z values in the mass analyzer. This method visibly solved the issue of studying the large protein using the traditional mass spectrometry and its efficiency can be further boost by coupling it with liquid chromatography (i.e., LCMS) (El-Aneed et al. 2009). Nano electrospray ionization tandem MS technology has the advantage not only of reduced flow rates but also enhanced the ion formation mechanism (Yates et al. 1997). ...
... If fingerprinting cannot recognize the proteins because of an inadequate number of proteolytic peptides or lack of an appropriate DNA database to align them confidently, then ESI MS/MS can be used for the sequencing of amino acids (He and Chiu 2003). ESI MS/MS is able to replicate the data, but it is worth noting that the relative abundance of different ions in an ESI spectra is not a true representation of the sample concentration (El-Aneed et al. 2009). ...
... In most cases, samples are extracted, separated by reverse-phase liquid chromatography, which is coupled to tandem mass spectrometry (LC-MS/MS). The prerequisite for this type of analysis is a solution of the analytes; thus, compounds that are insoluble in the typically used solvents (methanol, acetonitrile, water, and their mixtures) cannot be analyzed [62]. ...
... Typical combinations are quadrupole/quadrupole, quadrupole/time-of-flight (q/TOF), and quadrupole or linear ion-trap coupled to Orbitrap. Tandem mass spectrometers allow for a unique type of analysis known as collision-induced dissociation, in which intact ions of a defined mass-tocharge ratio are selected and then deliberately fragmented to analyze the fragments [62]. This method is used to achieve ultra-high sensitivity, and it provides structural information on the molecules of interest. ...
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... ESI is considered as a soft ionization technique, and is able to create multiple charged ions for biomolecules such as proteins and peptides, which enables these large molecules to be analyzed within the mass over charge ratio (m/z) ranges of the instrument. When coupled to LC and including standards for calibration, ESI is more reliable for quantitative analysis than MALDI (59,60). ...
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