a) What advantages do quadrupole ion traps offer over single quadrupole and even triple quadrupole instruments?
Quadrupole ion traps allow for multi-stage tandem MS experiments, i.e., not only MS/MS (MS2) but also MS/MS/MS (MS3) and higher order experiments (MSn) can be performed in a single QIT.
b) What is the difference between tandem-in-space and tandem-in-time instruments?
Tandem-in-space represents the “classical way” of tandem mass spectrometry. Ions are selected by the first mass analyzer, then allowed to fragment in transit to the next mass analyzer where the fragment ions are subsequently separated according to m/z.
Tandem-in-time repeatedly uses a single mass analyzer which of course must be able to perform precursor ion selection, ion activation, storage for the period of activation, fragmentation, and subsequent mass analysis of the fragments. Provided proper hardware is available, all sorts of ion traps are capable of tandem-in-space.
c) Can you find advantages/disadvantages of either tandem MS concept?
Tandem-in-space instrumentation is much more compact, usually very cost effective, and allows for MSn. Tandem-in-space is more flexible in the level of ion activation (in CID mode), less prone to distorting effects from overload, and thus better suited when quantitation is a major issue.