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 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. The first (Q₁) and third (Q₃) quadrupoles serve as mass filters. Precursor ions selected in Q1 are dissociated in the collision cell in the presence of an inert gas such as Ar, He, or N₂ collision-induced dissociation. Resulting fragments are passed through to Q₃ where they may be filtered or scanned. This configuration is often abbreviated QqQ, here Q₁q₂Q₃.

Structural mass spectrometry

This alignment of two mass filters with a collision quadrupole between them allows to elucidate the structure of the ionised sample molecules. Four main modes can be performed as follows.

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 reaction monitoring (SRM) / Multiple reaction monitoring (MRM)

• In this method 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.

History

The arrangement of three quadrupoles was first developed by Morrison of LaTrobe University, Australia for the purpose of studying the photodissociation of gas-phase ions. The first triple-quadrupole mass spectrometer was developed at Michigan State University by Enke and Yost in the late 1970s.

See also

• Hybrid mass spectrometer