Hi, My question: Why does a chemical under opservation need to be converted into ion form in mass spectroscopy? I understand that its easier to filter them using magnets but why cant MS detector pick neutral molecules up? Or can it? Cheers, Naat
Your message makes more sense if the word "don`t" has been erased, is that the case? So MS could detect neutral molecules as well as long as they are in gas form? I tried to look in my library and on internet for the exact detector mechanism but I have failed so far. Is it not that the detector "translates electric charges to peaks in the computer program"? Thus it could only "translate" charged molecules? So far I have thought that the "electric to peak" is done by the program, not the detector.
Naat: A MS for neutrals can be made, but it does not use any magnetic fields as the more conventional MS does. It uses time of flight of a brief "puff" of gas, which contains the various components of interest. When they are in thermal equilibrium the more massive have slower average velocities. If you have a long well evacuated tube with walls chilled to trap any molecules that hit the walls on the walls, then hydrogen will reach the detector first, then helium, etc. The detector can be just a very fine hot wire, probably tungsten, which has high thermal coefficient for resistivity change. (These detectors are readily available and widely used as vacuum gage sensors.) As the various gases pass by the fine wire, it is cooled and the resistance decreases - not a big effect, but easily measured.
I understand your concept of "neutral" time of flight, but I have never heard of an instrument that uses said "neutral" time of flight, all of them us ions, thus one must suppose there is little to no practical value out of achieving neutral time of flight measurements.
I think this would be a very specialized form of gas analysis. I have not seen such an instrument myself, either, though. The main reason to ionize the sample however, is that the separation technique relies on electric or magnetic fields, which of course only effect ions. The detector itself is not really the problem (though using electron multipliers or similar have far higher detection abilities). In TOFs for instance electric fields are used to accelerate the ions uniformly across the evacuated tube (and are also used to focus the ion beam according to mass with the help of the reflectron, though that is beside the point), whereas quads use oscillating electric fields to gate the ions.
In some sense the separation of U235 from U238 by "gasous diffusion" is time of flight MS. There are zillions of very short "tubes" in cascade and both isotopes are at the same temperature so the 235 is flying faster and what come out the end of the zillions of tubes is richer in 235 than 238.
I could imagine such a thing working in theory, but I have never heard of this sort of setup actually being used for anything.
I already gave an example in post 9, but there is no single long tube. I suspect that some organic structures would be destroyed if they were to be ionized. If gaschromographs did not exist, then probably some non-ionizing MS would. Fact that none of the long tube type exist (if true) is no comment on the feasibility of non-ionic time of flight MS.
Obviously there are uses for gas diffusion devices. But I have never heard of a mass spectrometer that works the way you describe. I suspect it would be very difficult to get any clear results out of it, because you would still have some of the lighter gasses hitting your detector at the same time as the heavier gases. There are plenty of "gentle" ionization techniques that mass spec labs use to ionize fragile molecules without breaking them up.
In the strictest sense this would not be considered mass spectrometry as it is defined as an instrument to measure mass to charge ratios.
