[macattack]

And what is the difference between tandem in space and tandem in time

[hellboyz69]

MS tandem in space is separation elements are separated physically and distinct .although there is a physical connection between the elements to maintain high vacuum. These elements can be sectors, transmission quadrupole, or time-of-flight.
MS tandem in time is the separation is accomplished with ions trapped in the same place. So, the separated is not use physically

[fledarmus]

There is a pretty good description of the difference here:

http://pubs.acs.org/doi/abs/10.1021/ac00219a003

In response to your subject question, in general terms, MS-MS techniques are used to study reactions of charged particles (usually fragmentations), or to identify materials based on those reactions.

The technique uses the first mass spec to generate and separate ions, then allows those ions to react and uses a second mass spec to analyze the products. Between the generation of ions in the first mass spec and the detection of products in the second mass spec, the ion stream may be subjected to additional influences such as further ionization or bombardment with neutral particles.