• Question: How does Mass Spectrometry work ?

    Asked by webbo to Meeks, Pete, Stephen, Steve, Tom on 19 Jun 2010 in Categories: .
    • Photo: Steve Roser

      Steve Roser answered on 18 Jun 2010:

      with MS you are finding the mass of a molecule so you can tell what atoms are in it (but not in which structure). The way it works is to make an ion by knocking off an electron and then speeding all the ions in a series of electric fields. You let them all start at once and then see which gets to the detector the quickest – this only takes a few nanoseconds, so you need really good timers! The lightest get there quickest, and hence you can work out the mass by the time of arrival. No-one uses the old fashioned ones where you bend the ions in a magnetic field to get the mass to charge ratio anymore…

    • Photo: Marieke Navin

      Marieke Navin answered on 18 Jun 2010:

      I’ve done about this recently. There are a few steps to mass spec: ionisation (knock some electrons off so you are left with positive ions), acceleration (accerate all the ions so they have the same kinetic energy), deflection (use magnets to deflect ions, the lighter they are the more deflected – although Steve said the other day they do it a different way now so perhaps he’ll comment on that) and detection.

      The purpose is to identify different types of molecules – for example they can find illegal drugs in a sportplayer’s blood sample

    • Photo: Tom Hartley

      Tom Hartley answered on 18 Jun 2010:

      I’m going to leave this to the others as I believe we may have some experts here. Probably should have done that for some of the other questions!

    • Photo: Stephen Curry

      Stephen Curry answered on 19 Jun 2010:

      Mass spec, as it’s known in the trade, is a great way for making accurate measurements of the weights of molecules.

      I’m no expert in the technique but small samples placed in the machine are ionised (or charged) – usually by stripping electrons off them in some way – and then accelerated in an electric field. At some point, they whizz through a magnetic field, which bends the flight path by an amount that depends on the ratio of the mass and charge of the molecules (m/z). The molecule then strikes a detector – it’s position on the detector is a measure of m/z.

      Because it is relatively easy (though I don’t know how!) to figure out how many charges are on the molecule, you can work out the mass. The method is amazingly accurate – even for very large molecules like proteins which molecular masses of 50,000 Da or more.

      I have not used mass spec myself but we have collaborated with people who do it. On one project we worked with Prof Carol Robinson (now at the Dept of Chemistry in Oxford). She has developed mass spec methods that allow you to investigate how molecules stick together. This is a very useful technique and I was delighted to learn last week that she has been awarded the Humphrey Davy Medal from the Royal Society because of the success of her work. Go Carol!

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