Triple quadrupole mass spectrometer

Waters Quattro II triple quadropole mass spectrometer (center). This photo was taken in the old mass spec facility in Whitmore Lab of Pennsylvania State University.

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.

History

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.

Principle of Operation

Paul Patent 2939952 Fig5

Essentially the triple quadrupole mass spectrometer operates under the same pretenses as the single quadrupole mass analyzer. Each of the two mass filters (Q1 and Q3) contains four parallel, cylindrical metal rods. Both Q1 and Q3 are controlled by direct current (dc) and radio-frequency (rf) potentials, while the collision cell, q, is only subjected to RF potential. The RF potential associated with the collision cell (q) allows all ions that were selected for two pass through it. In some instruments, the normal quadrupole collision cell has been replaced by hexapole or octopole collision cells which improve efficiency.

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 the tandem-in-space arrangement, 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. Sector magnetic field instruments tend to surpass the TQMS in mass resolution and and mass range. However, the triple quadrupole has the advanagtage of being cheaper, easy to operate, and they are highly efficient. Also, when operated in the selected reaction monitoring mode, the TQMS has superior detection sensitivity as well as quantification. The triple quadrupole allows the study of low-energy low-molecule reactions, which is useful when small molecules are being analyzed.

Instrumentation

Schematic of a triple quadrupole mass spectrometer

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. Both the first mass analyzer and the collision are continuously exposed to ions from the source, in a time independent manner. It is once the ions move into the third mass analyzer that time dependence becomes a factor. The first quadrupole mass filter, Q1, is the primary m/z selector once the sample leaves the ionization source. Any ions with different mass-to-charge ratios other than the one selected for will not be allowed to infiltrate Q1. The collision cell, denoted as "q", is located between Q1 and Q3, is where fragmentation of the sample occurs in the presence of an inert gas such as Ar, He, or N2. A characteristic ion is produced as a result of the collisions of the inert gas with the analyte. Upon exiting the collision cell, the fragmented ions then travel onto the second quadrupole mass filter, Q3, where m/z selection can occur again. The TQMS enables enhanced selectivity, accuracy, and reproducibility, which are all limited in ordinary single quadrupole mass analyzers.

Scan Modes

Settings associated with selecting m/z values in both mass filters of a triple quadrupole mass analyzer

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. In the product 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. When utilizing a precursor 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. In the neutral loss 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. When employing selected reaction monitoring (SRM) or multiple reaction monitoring (MRM) modes, 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.

See also

• Hybrid mass spectrometer