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Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.
An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.
A mass spectrum is a histogram plot of intensity vs. mass-to-charge ratio (m/z) in a chemical sample, usually acquired using an instrument called a mass spectrometer. Not all mass spectra of a given substance are the same; for example, some mass spectrometers break the analyte molecules into fragments; others observe the intact molecular masses with little fragmentation. A mass spectrum can represent many different types of information based on the type of mass spectrometer and the specific experiment applied. Common fragmentation processes for organic molecules are the McLafferty rearrangement and alpha cleavage. Straight chain alkanes and alkyl groups produce a typical series of peaks: 29 (CH3CH2+), 43 (CH3CH2CH2+), 57 (CH3CH2CH2CH2+), 71 (CH3CH2CH2CH2CH2+) etc.
Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more stages of analysis using one or more mass analyzer are performed with an additional reaction step in between these analyses to increase their abilities to analyse chemical samples. A common use of tandem MS is the analysis of biomolecules, such as proteins and peptides.
Gas chromatography–mass spectrometry (GC–MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC–MS include drug detection, fire investigation, environmental analysis, explosives investigation, food and flavor analysis, and identification of unknown samples, including that of material samples obtained from planet Mars during probe missions as early as the 1970s. GC–MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. Like liquid chromatography–mass spectrometry, it allows analysis and detection even of tiny amounts of a substance.
In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy-absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.
Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry which utilizes gas-phase ion-molecule reactions at atmospheric pressure (105 Pa), commonly coupled with high-performance liquid chromatography (HPLC). APCI is a soft ionization method similar to chemical ionization where primary ions are produced on a solvent spray. The main usage of APCI is for polar and relatively less polar thermally stable compounds with molecular weight less than 1500 Da. The application of APCI with HPLC has gained a large popularity in trace analysis detection such as steroids, pesticides and also in pharmacology for drug metabolites.
The thomson is a unit that has appeared infrequently in scientific literature relating to the field of mass spectrometry as a unit of mass-to-charge ratio. The unit was proposed by R. Graham Cooks and Alan L. Rockwood naming it in honour of J. J. Thomson who measured the mass-to-charge ratio of electrons and ions.
In mass spectrometry, Orbitrap is an ion trap mass analyzer consisting of an outer barrel-like electrode and a coaxial inner spindle-like electrode that traps ions in an orbital motion around the spindle. The image current from the trapped ions is detected and converted to a mass spectrum by first using the Fourier transform of time domain of the harmonic to create a frequency signal which is converted to mass.
A mass chromatogram is a representation of mass spectrometry data as a chromatogram, where the x-axis represents time and the y-axis represents signal intensity. The source data contains mass information; however, it is not graphically represented in a mass chromatogram in favor of visualizing signal intensity versus time. The most common use of this data representation is when mass spectrometry is used in conjunction with some form of chromatography, such as in liquid chromatography–mass spectrometry or gas chromatography–mass spectrometry. In this case, the x-axis represents retention time, analogous to any other chromatogram. The y-axis represents signal intensity or relative signal intensity. There are many different types of metrics that this intensity may represent, depending on what information is extracted from each mass spectrum.
Electron-transfer dissociation (ETD) is a method of fragmenting multiply-charged gaseous macromolecules in a mass spectrometer between the stages of tandem mass spectrometry (MS/MS). Similar to electron-capture dissociation, ETD induces fragmentation of large, multiply-charged cations by transferring electrons to them. ETD is used extensively with polymers and biological molecules such as proteins and peptides for sequence analysis. Transferring an electron causes peptide backbone cleavage into c- and z-ions while leaving labile post translational modifications (PTM) intact. The technique only works well for higher charge state peptide or polymer ions (z>2). However, relative to collision-induced dissociation (CID), ETD is advantageous for the fragmentation of longer peptides or even entire proteins. This makes the technique important for top-down proteomics. The method was developed by Hunt and coworkers at the University of Virginia.
A prolate trochoidal mass spectrometer is a chemical analysis instrument in which the ions of different mass-to-charge ratio are separated by means of mutually perpendicular electric and magnetic fields so that the ions follow a prolate trochoidal path. These devices are sometimes called cycloidal mass spectrometers, although the path is not a cycloid.
A triple quadrupole mass spectrometer (TQMS), is a tandem mass spectrometer consisting of two quadrupole mass analyzers in series, with a (non-mass-resolving) radio frequency (RF)–only quadrupole between them to act as a cell for collision-induced dissociation. This configuration is often abbreviated QqQ, here Q1q2Q3.
A hybrid mass spectrometer is a device for tandem mass spectrometry that consists of a combination of two or more m/z separation devices of different types.
Electron capture ionization is the ionization of a gas phase atom or molecule by attachment of an electron to create an ion of the form . The reaction is
Selected reaction monitoring (SRM), also called multiple reaction monitoring (MRM), is a method used in tandem mass spectrometry in which an ion of a particular mass is selected in the first stage of a tandem mass spectrometer and an ion product of a fragmentation reaction of the precursor ions is selected in the second mass spectrometer stage for detection.
The linear ion trap (LIT) is a type of ion trap mass spectrometer.
Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions are usually accelerated by applying an electrical potential to increase the ion kinetic energy and then allowed to collide with neutral molecules. In the collision, some of the kinetic energy is converted into internal energy which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. These fragment ions can then be analyzed by tandem mass spectrometry.
A miniature mass spectrometer (MMS) is a type of mass spectrometer (MS) which has small size and weight and can be understood as a portable or handheld device. Current lab-scale mass spectrometers however, usually weigh hundreds of pounds and can cost on the range from thousands to millions of dollars. One purpose of producing MMS is for in situ analysis. This in situ analysis can lead to much simpler mass spectrometer operation such that non-technical personnel like physicians at the bedside, firefighters in a burning factory, food safety inspectors in a warehouse, or airport security at airport checkpoints, etc. can analyze samples themselves saving the time, effort, and cost of having the sample run by a trained MS technician offsite. Although, reducing the size of MS can lead to a poorer performance of the instrument versus current analytical laboratory standards, MMS is designed to maintain sufficient resolutions, detection limits, accuracy, and especially the capability of automatic operation. These features are necessary for the specific in-situ applications of MMS mentioned above.
Laser diode thermal desorption (LDTD) is an ionization technique that is coupled to mass spectrometry to analyze samples with atmospheric pressure chemical ionization (APCI). It uses a laser to thermally desorb analytes that are deposited on a stainless steel sheet sample holder, called LazWell. The coupling of LDTD and APCI is considered to be a soft-ionization technique. With LDTD-APCI, it is possible to analyze samples in forensics, pharmaceuticals, environment, food and clinical studies. LDTD is suitable for small molecules between 0 and 1200 Da and some peptides such as cyclosporine.
References
- ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " selected ion monitoring ". doi : 10.1351/goldbook.S05547
- ↑ Murray, Kermit K.; Boyd, Robert K.; Eberlin, Marcos N.; Langley, G. John; Li, Liang; Naito, Yasuhide (2013). "Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)". Pure and Applied Chemistry. 85 (7): 1515–1609. doi: 10.1351/PAC-REC-06-04-06 . ISSN 0033-4545.
- ↑ Kaufmann, Anton (2018). "Analytical performance of the various acquisition modes in Orbitrap MS and MS/MS". Journal of Mass Spectrometry. 53 (8): 725–738. Bibcode:2018JMSp...53..725K. doi:10.1002/jms.4195. ISSN 1076-5174. PMID 29708288.
- ↑ Lu, Wenyun; McBride, Matthew J.; Lee, Won Dong; Xing, Xi; Xu, Xincheng; Li, Xi; Oschmann, Anna M.; Shen, Yihui; Bartman, Caroline; Rabinowitz, Joshua D. (2024). "Selected Ion Monitoring for Orbitrap-Based Metabolomics". Metabolites. 14 (4): 184. doi: 10.3390/metabo14040184 . ISSN 2218-1989.