Triple quadrupole mass spectrometer
A triple quadrupole mass spectrometer (TQMS), is a tandem mass spectrometer consisting of two quadrupole mass spectrometers 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. Unlike traditional MS techniques, MS/MS techniques allow for mass analysis to occur in a sequential manner in different regions of the instruments. The TQMS follows this arrangement, known as tandem-in-space, due to ionization, primary mass selection, collision induced dissociation (CID), mass analysis of fragments produced during CID, and detection occurring in separate segments of the instrument. TQMS is most useful for quantitative analysis of small molecules. A mass range of roughly 4000 Daltons is efficient due to the employment of ionization techniques like electrospray ionization which can quantify smaller amounts of sample.
The arrangement of three quadrupoles was first developed by J.D. Morrison of LaTrobe University, Australia for the purpose of studying the photodissociation of gas-phase ions. After coming into contact with Prof. Christie G. Enke and his then graduate student Richard Yost, Morrison's linear arrangement of the three quadrupoles probed the construction of the first triple-quadrupole mass spectrometer. In the years following, the first commercial triple-quadrupole mass spectrometer was developed at Michigan State University by Enke and Yost in the late 1970s. It was later found that the triple-quadrupole mass spectrometer could be utilized to study organic ions and molecules, thus expanding its capabilities as a tandem MS/MS technique. 
Structural mass spectrometry
In the TQMS, a several ionization methods can be employed. Some of these include electrospray ionization, chemical ionization, electron ionization, atmospheric pressure chemical ionization, and matrix-assisted laser desorption ionization, all of which produce a continuous supply of ions. The first quadrupole mass filter, Q1, is the primary m/z selector once the sample leaves the ionization source. The quadrupole collision cell, denoted as "q", is located between Q1 and Q2, is where fragmentation of the sample occurs in the presence of an inert gas such as Ar, He, or N2. Both Q1 and q are continuously being exposed to ions, which is not time dependent. The fragmented ions then travel onto the second quadrupole mass filter, Q2, where m/z selection can occur again and this step in the analysis is time dependent. Employing two mass filters improves the mass selectivity and accuracy, which is usually limited when utilizing a regular transmission quadrupole. The arrangement of the instrument allows for four different scan types to be performed: a precursor ion scan, neutral loss scan, product ion scan, and selected reaction monitoring.
Product ion scan
The first quadrupole Q1 is set to select an ion of a known mass, which is fragmented in q2. The third quadrupole Q3 is then set to scan the entire m/z range, giving information on the sizes of the fragments made. The structure of the original ion can be deduced from the ion fragmentation information. This method is commonly performed to identify transitions used for quantification by tandem MS.
Precursor ion scan
A certain product ion is selected in Q3, and the precursor masses are scanned in Q1. This method is selective for ions having a particular functional group (e.g., a phenyl group) released by the fragmentation in q2.
Neutral loss scan
In this method both Q1 and Q3 are scanned together, but with a constant mass offset. This allows the selective recognition of all ions which, by fragmentation in q2, lead to the loss of a given neutral fragment (e.g., H2O, NH3). Similar to the precursor ion scan, this method is useful in the selective identification of closely related compounds in a mixture.
Selected and multiple reaction monitoring
In selected reaction monitoring (SRM) or multiple reaction monitoring (MRM), both Q1 and Q3 are set to a selected mass, allowing only a distinct fragment ion from a certain precursor ion to be detected. This method results in increased sensitivity. If Q1 and/or Q3 is set to more than a single mass, this configuration is called multiple reaction monitoring.
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