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  • >> Just start talking about mass spectrometry

  • and today we're going to talk a little bit

  • about how the technique works.

  • On our next lecture on Monday we're going to talk

  • about concepts and then

  • on Wednesday we'll spend one lecture on EI fragmentation

  • which is kind of special topics.

  • It used to be really, really central to mass spectrometry.

  • It's sort of part of pedagogy that's carried on

  • but EI mass spec is the historical first

  • in mass spectrometry but is a lot less important these days.

  • Mass spec is a super important technique.

  • Molecular weight and molar formula are some

  • of the most fundamental things that you can get

  • and mass spec is easily a technique

  • to give you molecular weight.

  • We'll talk about high resolution spectrometry.

  • From that you can get molecular formula.

  • We'll talk about that next time and the concepts

  • that are associated with that.

  • One thing that mass spec can easily, easily,

  • easily talk to you about is elements present

  • and this is really important

  • because you can easily see bromine and chlorine.

  • You can see sulphur and silicon if you know what you're looking

  • for and what's valuable about that is NMR is not going

  • to be a technique that talks to you about elements like that.

  • IR is not going to be a technique that talks to you

  • so this is why you should be reading these spectrometric

  • techniques and these the days mass spec can also be incredibly

  • valuable in getting structure.

  • It's in fact become central to biomolecular mass spectrometry,

  • to sequencing peptides and proteins but also

  • for more traditional organic structures

  • in natural products you can get structure

  • through fragmentation patterns which as I said we'll be talking

  • about a little bit on our third lecture and as I said

  • in biomolecular cases through slash techniques,

  • through techniques like MS/MS

  • where you're actually taking ions and deliberately bashing

  • into them and smashing them and see how they break up.

  • All right the basic principle of mass spectrometry is super,

  • super simple like beginning physics.

  • The basic principle--

  • [ Silence ]

  • -- and I love making these very simple-minded drawings

  • of scientific instruments because it's a good way to get

  • into our heads how the basic technique works.

  • So if you want to think

  • about the basic technique you can think of an ionized molecule

  • and that ionized molecule is moving along until you come

  • to some sort of magnetic field.

  • In the simplest and historical realm it is literally an

  • electromagnet and as the particle moves

  • into the magnetic field its path gets bent.

  • You have a force on it.

  • It's all that right-hand rule stuff from physics.

  • The degree of deflection depends on the mass to charge ratio.

  • [ Silence ]

  • In other words, any given particle whether it has 20 amu

  • and one charge or 40 amu and two charges is going

  • to get deflected the same amount so it's the mass to charge ratio

  • that you're seeing on the x axis, M to Z not mass.

  • This becomes particularly important

  • when you're doing EI mass spec which we do a lot

  • of here in the facility.

  • I'm sorry, ESI, electrospray ionization mass spec

  • and you do it on reasonably big molecules

  • where many times you get more than one charge on a molecule.

  • The degree of deflection depends on the mass

  • to charge ratio not surprisingly a heavier, h-e-v-i-e-r,

  • I can't spell today is deflected less.

  • A heavier particle is more massive so it's going

  • to be get bent less, more charged is going

  • to be deflected more and it's amazing how easy it is

  • for people to lose sight of these principles particularly

  • when you're starting to talk about fragmentation,

  • in that everything you see in the mass spectrum is going

  • to be charged, in other words a free radical or a dot

  • that has no charge on it is invisible.

  • Something has to have a charge.

  • Most of the mass spectrometry you're going to do will be

  • in the positive ion mode, in fact that's all we're going

  • to talk about today but one can also do it

  • in the negative ion mode

  • where you're looking for negative ions.

  • Most of the molecules that one works

  • with don't have a charge on them.

  • So the first question is how do you get a charge on a molecule?

  • Historically, the first technique developed is called

  • electron ionization.

  • You'll see that written as EI

  • or you'll see the whole technique written

  • as EI mass spec and the basic idea is a

  • little counter-intuitive.

  • You're going to use an electron

  • to ionize the molecule, so far so good.

  • You have a molecule.

  • You fire an electron at it.

  • You accelerate electrons and give it a good hard whack.

  • What's counter-intuitive

  • when you give a molecule a good hard whack

  • with an electron you knock an electron out of it.

  • So you get a cation.

  • Electrons weigh virtually nothing compared to molecules,

  • so for all intents and purposes the mass is the mass

  • of the molecule.

  • So for example if you take methane, CH4 and you hit it

  • with an electron you get CH4 plus.

  • You've taken an electron out of it

  • so you're getting a radical cation,

  • what mass spectrometrists call a molecular ion

  • and your two electrons.

  • As organic chemists we have trouble thinking

  • about odd electron species.

  • Most of the species we deal with have even numbers of electrons.

  • In fact I think by the time a student has taken sophomore

  • organic chemistry it gets more perturbing to see an structure

  • like this than when they're a freshman

  • because as a freshman you just learn, okay count up the number

  • of valence electrons from carbon.

  • You count up the number of valence electrons from hydrogen.

  • You take away electrons and so a freshman confronted

  • with the problem of writing a series of Lewis structures

  • and resonance structures for a molecule

  • like this will dutifully go ahead and say, well,

  • okay we've only got seven electrons so I guess we've got

  • to make do with our seven valence electrons

  • and I can write a resonance structure like this

  • and I can write a resonance structure like this

  • and I can write two more.

  • I'll just etcetera and we have a net positive charge

  • but by the time we get

  • to organic chemistry it gets perturbing to think about this.

  • If you like to think in orbitals you can think okay we're just

  • knocking an electron out of the highest occupied molecular

  • orbital and you can just think of this species and say,

  • okay instead of having a filled highest occupied molecular

  • orbital we have a half-filled highest occupied

  • molecular orbital.

  • Conceptually it gets easier when you have obvious orbitals

  • when you have things you can see rather than molecular orbitals.

  • So in the case for example, of anything with a lone pair

  • such as an ether if you go ahead and you take away an electron,

  • oops that's minus E minus.

  • If you take away an electron from this you can say okay,

  • it doesn't look very good but there's my molecular ion.

  • There's my radical cation.

  • If you have an alkene you can say,

  • well the pi orbital is the highest occupied molecular

  • orbital so we're going to take an electron away from it.

  • I can write a resonance structure like so

  • and a second resonance structure maybe perhaps a more minor

  • contributor where I just swap the charge and the odd electron.

  • Thoughts or questions?

  • >> [Inaudible] not being able to see a radical on-- ?

  • >> Exactly, so later on when we start to--

  • so the question was about not being able to see a radical.

  • So when we start to talk

  • about fragmentation you'll see a little bit of this

  • because at the end of today's class I'll even show you an ESI

  • mass spectrum where a molecule does break apart.

  • When one of these radical cations breaks apart

  • into two halves one half will end up with an even number

  • of electrons and a positive charge, the other half will end

  • up with an odd number of electrons and no charge

  • and the radical because it doesn't have a mass,

  • it doesn't have any charge won't be deflected and won't show up

  • and won't be detected because the detection depends upon

  • detecting an electrical current.

  • So for example, later on we're going to see

  • that if you have an ether like, I'll make it simple