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

like diethyl ether and this molecule breaks apart

because when you give it a whack with an electron you put a lot

of vibrational energies.

You've done double damage to the molecule.

You've decreased the number of bonding electrons.

You've weakened the bonds in the molecule and you've put a lot

of kinetic energy into the molecule in the form

of the impact from the electron,

so the molecule now is vibrating.

It is hot and it has a tendency to fragment and so for example,

if diethyl ether fragments

and I can write a curved arrow mechanism for the fragmentation,

we'll talk more about it later, you get CH3 plus

and you get this charged species

and you will observe this species

but you will not observe the radical.

Does that make sense?

All right let me give a little more detail

on the instrumentation of an actual EI mass spectrometer,

so what I showed you before was sort of a simplified diagram

and I'll still give you a simplified diagram.

Now if first thing that you need to think

about is all this chemistry and this is true for all

of mass spectrometry occurs in the gas phase.

In fact for common techniques an organic chemist would use the

only experiment where you're doing it in the gas phase.

IR you could do gas phase IR but most

of the molecules organic chemists work with are going

to be in the liquid phase or in the solid phase

or the solution phase.

So the first problem is how do you get the molecule

into the gas phase?

So typically what you do is you have a heater, a coil of wire

like a filament and you put your sample on the filament

and you have this in a vacuum and it's going to need

to be a pretty good vacuum at least

by the time the molecule is flying along,

the ionization part can have some pressure to it

but by the time the molecule is actually moving you've got

to have movement in the vacuum

without it colliding into other molecules.

In fact one of the experiments

that people sometimes do is collisional experiments

where you're deliberately trying to stop the motion

of the molecule but short

of those experiments you need the molecule in a vacuum.

You then have to, so you have to get it into the gas phase.

Already this means that EI mass spec is going to be limited

to molecules that can be evaporated.

That means that by the time you get to very big molecules

like strychnine which are going to have very,

very low vapor pressures even

at high temperatures you're fighting getting it

into the gas phase because if you heat it a lot to get it

into the gas phase you're going to basically cook

and decompose the molecule.

Once you get the molecule into the gas phase you hit it

with an electron beam.

That electron beam typically ends

up being at 70 electron volts.

The molecule now in the gas phase is ionized

but it's not moving at any particular rate

so then what you do is you have a pair

of accelerating plates those impart velocity to the molecule