Chromatography Forum

I have been trying to understand the basic principle of quadrupole mass analyzers. Although I could understand the basis of quadrupole analyzers from some books and websites, there are some doubts about the detection of the ions at the end of the analyzer, which are annoying me. Can someone please help?

In quadrupole analyzers what kind of detector is used at the end of the analyzer and how does it differentiate between ions having different m/z ratios?
Are all the ions having different m/z detected at the same time or are they detected at different time after separated by the quadrupole analyzer?
If later is the case, i.e. if ions are first separated based on their masses isn't same the principle of TOF?
In other words, if quadrupole is capable of differentiating between the ions of various m/z, why is there a need of ToF which separates the ion depending on the time which they require (and hence depending on their masses) to travel a partucular distance?
In other words, what is the difference between the final ion detectors (not the analyzers) of TOF and quandrupole? I just read somewhere that in TOF, all the masses are detected at the same time; which is not the case with quadrupole. But I am not convinced!
Thanks a lot
Ram

That is a complex topic for a short post but:

1. the detector basically collect and amplify the ions that are separated by the analyzer (quad or TOF). There are different detectors that can be used (get a book if you are interested in that) but are not necessarily dependant on the analyzer employed.

2. The quadrupole separate the ions according to differential tensions applied to its rods. This means that only one ion of a given m/z ratio can get through at a time. If you wish to perform scan mode with a quad, it will sequentially modify tensions to allow all ions to get through one after the other. With a TOF analyzer, all ions are pushed away by a high voltage and travel along the flight tube at different speeds according to their m/z ratios. They will arrive at different times at the detector which collects all of them. In other words, IN SCAN MODE, the duty cycle of a TOF is much higher than that of a quad, which loses about 99.9% of the ions when scanning from 100-1000 Da. Still, the duty cycle of a TOF is not 100%, especially for orthogonal-acceleration-TOF because the continous flow of ions that gets to the analyser has to be sent by packets through the flight tube and this induces loss.

Hope you are convinced



.

Thanks for your reply Gaetan Glauser!
Ya, some points are clear now; but some doubts are still there.
If the detector (for example, at the end of the quadrupole) is capable for detecting ions of different m/z, why should the ions by made to fly in the flight tube to get separated based on their masses? Can't they all be detected at the same time, just like in quadrupole (is that true? or "oh no, in quadrupole, all ions are not detected at the same time!") ? Or is it that the separation of ions after varying the voltage in quad is not as good as that during the ions are flying in TOF?
Thanks
Ram

The detector is not able to distinguish ions of different masses, it just detects ions and transforms the ionic current into a measurable signal. The analyser separates the ions and the detector detects them, and this is valid for a quad, a TOF or any other analyzer.

Just think of a variable wavelength UV detector: the photocell collects photons but is not able to distinguish between photons of different wavelenghts. You need a diffraction grating for that. To draw sort of a parallel, the quad or the TOF are the diffraction grating, and the detector (electron multiplier, MCP etc.) is the photocell.

thanks for the useful information Gaetan Glauser!