5.3.5 Tandem and Hybrid Techniques

Some mass spectrometers have more than one mass analyzer, and two is most common. If the two analyzers are of the same type, such as two magnetic sector analyzers, it is called a tandem instrument. On the other hand, if the analyzers are different types, the instrument is called a tandem hybrid. Generally, an rf only quadrupole (no offset, so no mass filtering) is placed between the analyzers and doubles as a collision cell. The technique of using mutiple analyzers is called MS/MS (for two analyzers), MS/MS/MS (for three), etc. Collision gas may be added between the cells to induce ion-atom or ion-molecule chemical decompositions.

Instrument geometries are abbreviated to communicate the analyzers used in a given experiment. If it does not matter in a particular context what type a given analyzer is, one may see MS1 (for the first analyzer), MS2 (for the second), etc. If, on the other hand, one needs to know the type of analyzer to draw the proper conclusions, the following shorthand can be used::

• E - an electrostatic sector (not a mass analyzer, but often specified to indicate hi-res is possible)
• B - a magnetic sector
• Q - a quadrupole mass filter
• q - an rf only quadrupole (that often doubles as a collision cell)

The instrument in this laboratory is a Q (meaning it is a quadrupole instrument. Staff in this laboratory also have experience with instruments of the following geometries: QqQ (a 'triple quad' instrument), EB (forward geometry double focusing sector instrument), EBqQ (tandem hybrid with forward geometry double focusing sector first followed by quadrupole; note this instrument combines a high resolution front-end with a nominal mass resolution analyzer as the secondary mass spectrometer), TOF and TOF-hE (time-of-flight with a hemispherical, as opposed to sector, electrostatic analyzer).

It is also worth noting that the quadrupole ion trap provides MS/MS/MS/MS/MS, etc capability in one small instrument with formally only one analyzer. This is accomplished by adding, removing, adding, removing (etc) the offset voltage that effectively alternates the trap between a mass analyzer and an rf only (no mass discrimination) trap. Collision gas can optionally be inserted during an rf only phase and pumped out for the next mass analysis phase to induce decomposition of ions selected in the previous mass analysis step.