ケム 203有機分光学講義 04.質量分析 (Chem 203. Organic Spectroscopy. Lecture 04. Mass Spectrometry.)

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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
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
and then as I said we will
in what's called the magnetic sector instrument the oldest
sort of instrument have a magnet, an electromagnet.
The molecules will move in and depending on their mass
to charge ratio will go to a detector
and the detector basically measures electrical signal.
The molecules are charged and so you get a current
and you can amplify that current and therefore send that current
on to a recording device or a computer.
So this is called a magnetic sector instrument
and what you typically do will be vary the magnetic field
and in doing so as you increase the magnetic field those
molecules that are deflected less will then get deflected
more and if you plot
versus magnetic field the current then you basically get a
graph and that graph translates to mass to charge ratio
as a versus intensity.
So on the X axis you will see M to Z
and on the Y axis you'll see intensity of the current
and of course you'll see some patterns associated
with the molecule and with fragments and with isotopes
that we'll talk more about in a moment
and this will be called a mass spectrum
and in a wave techniques is a misnomer
because of course a mass spectrum is not a spectrum.
From the earliest points you're learning science
in school you learned
that spectrum is electromagnetic frequency and here
of course this only looks like a spectrum.
It looks like an NMR spectrum where you have frequency
on the X axis in hertz which translates to parts per million
or IR spectrum where you have frequency in wave numbers
which is just a frequency unit or UV spectrum
where you have frequency of wavelength.
>> So the detector produces a current.
How does it produce the current?
>> Well, very simple, if you have M plus, a charge going
to a detector that means you've got electricity going in
and so you send that to an amplifier just
like a microphone, like a microphone
in your cell phone generates a minuscule current
and then it goes to an amplifier and then gets broadcast.
It actually gets digitized in that case and gets broadcast,
goes to amplifier, to a computer,
actually in a modern system it would go to an analog
to digital converter and then it goes to basically a printer
but in the oldest systems of course it would go
to an amplifier and then a strip chart recorder
because you could literally just go ahead and have a needle go
or another electrical signal to write on a piece of paper.
Good question, other questions?
There are lots of variations and if you talk
to John Greaves he will wax,
John Greaves runs the mass spec facility.
We have one of the premier mass spectrometry facilities
in the country.
It's probably to best on the West Coast
because of John's innovation
in putting together a really great facility
with a whole bunch of instruments and great support
and open access and you can go there 24/7.
If you talk to him he will wax poetic about different sorts
of detectors and so forth,
so for example another detector that's used is called the
quadrupole detector and the idea on the quadrupole detector is
that you have four electrical rods, four metal rods.
The ions come into the rods, the detectors at the end.
You have alternating current of varying frequency on the rods
and ions of different mass charge to charge ratios to get
through at different times as you vary the frequency
and so a quadrupole detector is another way rather
than a magnet.
Another way that you'll see is time of flight or TOF
and the basic principle here is
that when you accelerate particles
across a certain voltage if they're heavier they're going
to be moving more slowly and so they'll take longer to fly
and that's used in particular with various laser techniques
like matrix assisted laser desorption.
So one of the problems with EI mass spec is that you put a lot
of energy into the molecules, often they fragment
so often you're not seeing the molecular ion.
You're not directly getting the molecular weight
but you're referring it from the fragments that are developed.
There are a whole bunch of other ionization techniques
and these are important because often you get less
fragmentation, so in addition
to EI mass spec electrical ionization there are a bunch
of techniques that are called soft ionization techniques
that are less prone to fragmentation.
The first developed is CI or chemical ionization and the big,
big, big difference between chemical ionization
and electrical ionization, the big difference between all
of the soft techniques and electrical ionization is instead
of knocking an electron
out of the molecule you're putting something charged
on to the molecule.
Often what you're doing is adding a proton to the molecule
which in way is much more intuitive to an organic chemist
because you're basically using a strong acid or using an acid
of various forms to protonate the molecule.
One of the problems of chemical ionization and one
of the problems of electrical ionization is
over here getting the molecule into the gas phase.
As organic chemists
and biomolecular chemists have become interested in bigger
and bigger molecules,
the targets of organic synthesis have gotten bigger.
People are interested in proteins
and nucleic acids and oligosaccharides.
As all of the molecules have gotten bigger the issue
of ionization has become more important.
Another technique that's been developed is called fast atom
bombardment and I'll show you more
about these in just a second.
There's a variant of this technique referred to as LSI.
In one case you're doing the business that I'll show you
with an atom and in the other case with an ion and maldi,
m-a-l-d-i is another technique for ionization
and getting molecules into the gas phase,
it's matrix assisted laser desorption ionization.
I'll talk more about all of these techniques in a second.
All right, so the gist behind chemical ionization is
that a reagent gas is going to be used
to protonate the molecule.
You're going to put a proton
on to the molecule sometimes it will be another ion
but you'll do so with a reagent gas.
What you're doing is first ionizing the reagent gas
but unlike the conditions where we're doing ionization
in mass spec which are very, very powerful vacuum,
very high vacuum we're doing that under a weak vacuum
at about .5 millimeters of mercury.
Now what's happening under those pressures is that your ions
that you're generating of the reagent gas of methane
or ammonia or isobutane are colliding with each other
and what they're doing is making acids for example,
as I said methane, isobutane or ammonia.
So let's look at the chemistry of the reagent gas so we saw
that if you take methane and you give it a good hard whack
with an electron you get a methane radical cation.
You get CH4 plus dot.
In the gas phase when CH4 plus dot collides
with another methane molecule what happens is you transfer a
hydrogen and so you get CH5 plus protonated methane,
in other words you basically glommed a proton
on to the methane structure.
As you might imagine this is not all the five hydrogens sort
of stuck happily around one carbon, one is sort of glommed
on to the side of the molecule.
As you might imagine this is a very strong acid
and if we're going to balance our equation we also get a
methyl radical.
So now when you have this very strong acid CH5 plus
and it collides with your molecule which has come off
of the heater coil, now it transfers a proton
to the molecule to give you MH plus, plus CH4
and that's very easy to conceptualize
if the molecule has a lone pair
of electrons you protonate the lone pair of electrons.
If you have an ether you protonate the ether.
If you have an alcohol you protonate the OH group
to give you a protonated alcohol.
If all you have in the molecule is an alkene you protonate the
alkene to give you a carbo cation.
So we call this species a quasi-molecular ion
and of course the big distinction is this is
at M plus 1, in other words it's one higher
than the molecular weight.
And sometimes you'll see other things glomming
on to the molecule including alkene fragments
in CI mass spec, so methane gives rise
to this CH5 plus as your reagent acid.
Isobutane gives rise to a tert-butyl carbo cation
and at first you might say, well wait a second that doesn't look
like an acid but of course, if you think
about it tert-butyl cation it can give up a proton off
of the adjacent carbon and give you isobutylene
so the tertbutyl cation is also an acid in a phase.
It's less of a strong acid than CH5 plus.
This is really unhappy.
This is only somewhat unhappy
so the ionization conditions generate a lower heat
of reaction.
That's important because that means when that proton,
remember this is in the gas phase
so when the reaction occurs
and the reaction is exothermic the molecule is hot.
It's vibrating very strongly
and it is still prone to fragmentation.
So the less energetic the ionization the less enthalpic
the ionization process, the less energy, the less strong to acid,
the less strong to molecule is to fragmentation
and the more likely you are
to actually see a quasi-molecular ion
and not some the fragments.
Ammonia, although we don't usually think
of the ammonium ion as being strongly acidic
in the gas phase the ammonium ion is a strong acid
because it gets its stability in water from being solvated
and here you have no solvation.
You don't have hydrogen bonding in the gas phase
so even the ammonium ion is a strong acid in the gas phase.
>> This is kind of like a useless question
but do you get polymerization in a mass spec of radicals?
>> Do you get polymerization of radicals in a mass spec?
Because mass spectrometry is conducted under conditions
where your molecules are not typically colliding you will not
see polymer.
In ESI mass spec which we'll talk about in a moment
because the molecules are actually starting
in solution phase you may ionize a pair of molecules
that are already stuck together, so you may for example,
see a molecular ion that's derived from two molecules
and let's say three charges, but yeah,
you do not typically see polymerization.
>> So pure methane with CH5 plus that is the N plus 1?
>> So you're not going to see the CH5 plus
but when you get your molecule,
so let's say your molecule is diethyl ether
so now what you'll see is not something diethyl ether is 29
plus 29 plus 16 but what you will see then is not something
at what's 29 plus 29 plus 16?
Not something at 64, if I'm doing the math correctly
in my head, no wait not something at 74
but rather something at 75 for the protonated ether.
>> So that's for methods that have
that when you see that M plus 1?
>> You see the M plus 1.
>> For isobutane it looks like it would be like one less.
>> So isobutane acts as an acid as well
and I'll draw a curved arrow mechanism
so I'll just draw this as base.
The base takes off the proton
and this is exactly the microscopic reverse
of the reaction that you get
with when you protonate an alkene so you end
up with isobutylene and BH plus and if you think about it
if you protonate isobutylene with a strong acid
like sulfuric acid for example
in a Friedel-Crafts reaction the first thing you do is you put a
proton here.
You get a tertbutyl cation,
so this is just microscopic reverse of that process.
Good question.
>> Why do you have the electron molecule specifically guarding
the methane and not your molecule?
>> Because you have an ionization chamber first.
So you basically have a chamber where,
remember I showed you the electron beam?
So you have a chamber with methane that's
at a higher pressure, that's at about 1.5 millimeters
and an electron beam going into that.
The methane is getting ionized.
It's colliding and then it's diffusing into a region
where you have the heater coil and your sample.
Now the problem with CI mass spec is you still have
to get your molecule into the gas phase and so for a very,
very big molecule this may not be feasible by heating it even
in a strong vacuum because the molecule may not vaporize.
It may just decompose, right?
If you go ahead and you heat
up sugar a lot you don't have the sugar boil you have the
sugar carbonize and similarly
for other organic molecules they may simply carbonize
and then you get those roasty, toasty caramel smells
but not the smell of actual sugar.
Soft ionization techniques
in which the ionization process gets the molecule
into the gas phase.
Solve this.
Fast atom bombardment was I think the first one developed
and in that case what happens is you take an atom that's moving
quickly and you actually do that by an electrical process
to ionize, accelerate and reprotonate that--
reneutralize that atom you have your sample on a target
and you have your sample in a matrix.
Matrix is just another way of saying a viscous solvent.
The matrix is like glycerol or nitro-benzyl alcohol
and what happens is when the atom fires into the molecule
in the matrix you dispute sputter off molecules that some
of which are protonated
so you basically have the molecule essentially get
protonated and you'll see MH plus or MH plus dot matrix.
In other words sometimes you'll see a molecule of glycerin
or a molecule of nitro-benzyl alcohol complex
with your molecule.
So this is good for highly polar compounds
and nonvolatile compounds
and higher molecular weight compounds.
It's also good for compounds that tend to fragment in CI
if you want to see the molecular ion.
As I said there's a variant of fab called LSI mass spec
in liquid secondary ionization mass spec rather
than firing an atom you're firing an ion
such as cesium plus but it's the same basic principle.
You put a good hard whack in there and you end
up ionizing the molecule and getting it into the gas phase.
I don't want to give hard numbers but let's say up to
about 20,000 molecular weight, so this really opened
up a whole new realm of mass spectrometry including
biomolecular mass spec.
>> Is that for fast atom bombardment?
>> For fast atom, well both of the techniques
but fast atom was the one that first was popularized.
John Greaves does the LSI technique
and again I am sure he will wax poetic on the differences
between the two techniques
but for your purposes they're pretty similar.
>> All right so does that change the type
of detector you can use [inaudible]?
>> It does.
You end up having to have, well you can go
with stronger magnetic fields or I think typically this is done
with a quadrupole and then for maldi which I'll tell you
about in a second often people do time of flight because time
of flight tolerates even bigger mass range.
ESI is another technique that's widely used now.
These are our open access instruments.
Mass spec has become very populaced, very cheap,
very easy to do and one of the reasons for this is
because a regular EI mass spectrometer is a relatively
fussy instrument although they're often made into parts
of gas chromatographs and so forth
but they often require a lot of care.
ESI now is a lot easier to care for.
It goes up to very high molecular weight.
I don't know, I'll say maybe 5 million
but basically just very, very large.
So the basic gist is you're spraying off
of an electrically charged nozzle.
You're spraying charged microdroplets,
sprayed into a vacuum and what happens is the droplets
in the vacuum, they're in solvent like methanol.
The solvent evaporates.
The charges which are put on electrically get closer
and closer together as the solvent evaporates
until they repel each other and the droplets shatter apart
and then you have more evaporation and more shattering
and eventually you get charged species free of solvent
so you often end up with multiply charged species
for big molecules.
I'll say big biomolecules so for example, you will end
up with MHN plus, so for example, you might end
up with three protons on your molecule.
Often you will pick up sodium so you will end up for example
with a certain number of protons and a certain number
of sodiums on your molecule.
The sodium cation will give charge to it as well.
So anyway I'm going to show you an example of this
in just a second, I'll show you an example
of an ESI mass spec. Let me just mention maldi, another technique
that John has in his facility.
So you're using a laser to blast the molecule in a matrix.
The matrix is a species with a chromophore
that absorbs the laser light
and again you get protonated molecules, so again you get
for example MH plus and again this is good
for very high molecular weight.
I don't know I'll say
up to approximately 300 thousand molecular weight
but again very, very large.
Mass spec has gotten coupled widely
with other technique including separation techniques
so you will see mass spec on a detector of a gas chromatograph.
You'll see mass spec on the back end of a detector
of a liquid chromatograph, for example HPLC
and you'll even see hyphenated techniques
where you have mass spec coupled to mass spec
where you fragment your ions in a controlled fashion
to learn about the structures.
So as I said in the soft ionization techniques what
you're doing is taking the molecule and putting a proton
on it or sodium ion so for example, to give you MH plus
for example, so I'll just give you two trivial examples you
wouldn't typically look at methanol
but it's a nice simple way to think about it.
If you put a proton on methanol as I indicated before
when I talked about diethyl ether you will end
up with protonated methanol.
You have sodium from glass everywhere and so
if you put a sodium on in your soft ionization for example,
an ESI mass spec you'll end up with a sodium on your molecule
and what I want to do now, sometimes you'll even see,
so this would be M plus one.
This would be M plus 23.
Sometimes you'll see potassium as M plus 39 and so what I want
to do is show you an actual ESI mass spectrum of a molecule.
I have a number of handouts here or a handout here.
And I think we may need to shoo a few handouts extras.
Try not to chop down too many trees here but I always
like to give a few extras all right, so this we're going
to be talking more about actual mass spectra in subsequent --
does everyone have a handout?
All right so this is a handout of a particular molecule.
This one happens to be an example of a peptide.
Now we're going to talk more next time but one
of the big concepts is you're separating molecule by molecule
which means you're looking at individual isotopomers.
Put simply, 99 percent of your carbons are carbon 12.
One percent of your carbons are carbon 13
so when you calculate a mass for mass spec you're going
to actually calculate the exact mass that's based
on the predominate isotopomers.
The exact mass of this molecule is 744.5 and so if you look
at the mass spectrum the first peak you see here is this peak
at 767.6.
Here you see a peak at 745.7.
So this peak is your M plus H plus the instrument is only good
to plus or minus a few tenths unless you're running
in high resolution mode so in other words we would expect
if we pick up a proton here we'd get to 745.5.
We're at 745.7.
That's within the limits of experimental error.
This peak over here corresponds to M plus Na plus.
Sometimes you will hear the biggest peak
in the spectrum referred to as the base peak in the spectrum.
This peak here corresponds to a C-13 isotopomer.
We'll talk more about that later.
That's a molecule with one C-13 in here.
You'll see the same over here
and you'll notice you'll even see molecules with two C-13s.
We typically don't get a lot of fragmentation in the mass spec
but you'll see for example here you have a fragment.
You're doing acid chemistry on the molecule.
What's happening in generating the fragment is you're
protonating on this nitrogen and then it's leaving
for that particular fragment to generate an acylium ion
and here's where the concept of charge comes in.
The fragment is uncharged
so this is basically I'll just write etcetera
for the rest of the molecule.
We're fragmenting right at this phthalein to cleave this bond.
The fragment, the charged peak, the charged species gives rise
to this peak here and this is another fragment over here.
All right, the last concept I want to bring
to mind is-- question?
The last concept I want to bring
to mind is something very simple.
It's what's often called the nitrogen rule
and I'll is just play with this for one second.
Nitrogen rule is that compounds with an odd number
of nitrogens give odd M plus in EI mass spec
and you can convince yourself of this.
In other words, if you look at trimethyl amine,
that contains one nitrogen.
Its molecular weight is 59.
If you look at isobutane
which contains no nitrogens it has molecular weight of 58
and you'd say, okay that's in the EI mass spec
and everything turns on its head in the soft ionization.
It's reversed so for example, M plus H plus,
for trimethyl amine now would be 60
and if you were somehow protonating this
which you might do in the CI mass spec it would be 59.
And so again on inspection of the mass spec if you look
at the mass spec you can go ahead
and say okay this compound has an odd number of nitrogens
or this compound has an even number of nitrogens.
The only caveat is with fragmentation
in EI everything can get messed up, so for example,
if you take tert-Butanol,
tert-Butanol has a molecular weight of 74
but you're often not going to see the tert-Butanol.
You'll often see a carbo cation
in the EI mass spec. You'll often see a tertbutyl carbo
cation and that's M minus 17, that's 57 and so
if you just look at the biggest peak in an EI mass spectrum
of tert-Butanol you'd say,
oh the highest molecular weight peak is 57 this has
seven nitrogen.
Reality, no it's a fragment.
Anyway that's something to keep in mind as a way
with small molecules of saying, okay what element are present?
We'll pick up next time talking about other elements present.
We're going to talk about chlorines, bromines.
We're going to review the concept
of exact mass a little bit more. ------------------------------f501d95fe3eb--