Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need Help Understanding Mass Spec

  1. Sep 13, 2004 #1
    I really need some help understanding mass spectrometry, this is what i know mass spec is used to find out the relative attomic mass and relative molecular mass of a compound or atom, there are 5 stages a sample goes through vaporisation,ionisation,acceleration,deflection and detection i understand what goes on, on the first 2 stages but i'm confused about the last 3, at the end of the process, the trace hits the detector where lines r shown. Could some 1 please help me understand mass spec i'll b extreemly greatful.
  2. jcsd
  3. Sep 14, 2004 #2
    Hi faisal,

    Mass spectroemtry has evolved with time, nowadays many different techniques are used. DIfferent ionisation techniques, detection techniques and even different techniques are used to control the particle (m/q)-depedent trayectory.

    The basic idea is what you said:

    -The sample is vaporised (heating or by vacuum free expansion).

    -Sample is ionisated normally by electron impact, but it can be also using laser or more recently with induced coupled plasma. This process normally fragments the molecules, so a mixture of different m/q ions is left.

    -Now you have a "cloud" of charged particles. You have to accelerate them, just by applying an uniform electric field E. This can be done by putting the sample between two plaques at different potential (the sencond one with a little hole just to let the particles pass). An ion beam is then generated. Bu this ion beam has different m/q particles which travel at different velocities.

    In order to know the (m/q) of the particles, the most simple (and ancient) method is to put a magnetic field B transverse to the beam. The Lorentz force makes the particles describe a circular trayectory whose radius depends on the m/q relationship
    The Lorentz force: F=q*(m X v)

    You just can put several detectors cutting different radius trayectories, so depending on which detector makes signal you know the m/q of the particle.

    The detectors are normally sensitive plaques that create an electric (or luminiscent) signal with the particle collides with them. They also can be spectroscopic detectors.

    Remember that this is the most ancient scheme of the MS. See this link (just to tello you one, there are plenty..):

  4. Sep 15, 2004 #3
    ''Sample is ionisated normally by electron impact, but it can be also using laser or more recently with induced coupled plasma. This process normally fragments the molecules, so a mixture of different m/q ions is left.''
    please take it a step at a time, i still dont understand :cry:
    your explanation is too complex for my understanding, please narrow it down
  5. Sep 16, 2004 #4
    Ok, don´t worry about it, let´s try..


    Forget the laser and the plasma, they are too complex methods. Let´s talk about electron impact method. An electron beam is used to ionisate the sample.

    What is an electron beam? It´s just a flow of free electrons traveling all of them in the same direction (in high vacuum of course, otherwise they would collide with air molecules loosing energy and changing their directions, all the MS process is carried out in high vacuum). If you wanna know how to create an electron beam, post it, it´s a simple scheme.

    Well, the sample has been properly vaporised. So we have only free molecules of the sample in the vaporisation chamber. Then we expose this molecules to the electron beam, electrons collide with them. A collision is just an energy transfer, so the electrons transfer energy to the molecules. As the molecules have increased their energy they reach a higher state of energy, they became unstable. So they fragmentate.

    Let´s talk about fragmentation. It consist on a covalent bond breaking. Imagine ethanol: CH3-CH2OH. Imagine that the fragmentation consist on the breaking of that central bond I´ve represented by "-". Normally a covalent bond involves two electrons (one from each part). So fragmentation can take place by two ways:

    -Each part takes it´s electron: CH3-CH2OH --> CH3· + ·CH2OH
    None of them have charge, they are neutral radicals.

    -One of them takes the two electrons while the other doesn´t take any:
    CH3-CH2OH --> CH3+ + CH2OH- Both are charged.

    We are talking about the second way, the molecules that follow this way can be accelerated in the next step of the mass spectrometry.

    Any problem?

    Tell me if you want to talk about next steps.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook