mass spec data
let's you start out by taking a look at the flame ionization detector almost as a quick review
the idea of the FID is that there is a charge between these two plates
and there's a little hydrogen flame here... I have the hydrogen flame shown is a red dot
the hydrogen flame actually is invisible the hydrogen burns so cleanly that it is completely invisible
if you see a gas chromatographs and you see a little mirror next to it
or like a stainless steel spoon you know it's an FID
because the operator determines whether the flame this is lit or not
by holding a little mirror over it and looking for the vapors
kind of one of the little inside things for the FID
anyway .... so as the analytes comes through this space
they are ignited by the hydrogen flame they turn into ions
and the ions go flying off into their respective corners the positive ions go to the negative plate
the negative ions go to the positive plate
and current is formed and the current is our signal
in the old days the analog signal was run into with an analogue physical device
i.e. a strip chart recorder...The strip chart recorder made a peak on a piece of paper
and that was the signal
in today's modern world of computers we want to make the signal into a "computer useful" signal
in order to do that we need a time interval
lets pick a tenth of a second
so after a tenth of a second, the signal will be turned into a number
the next tenth of a second, the signal will be turned into a number
the next tenth of the second the signal will be turned into a number etc. etc. and you will end up with a
series of numbers ten numbers per second 10 seconds would be 100 numbers etc.
I take a tenth of the second as an arbitrary value
I think a tenth of a second would probably be good (small) enough
you could do a thousandth probably or maybe even 10,000th of a second
but a tenth of a second should be good enough for our purposes here
so then the idea of the computer is to take these numbers and group them in some fashion
and look the slopes so "what is the slope doing?"
well. the slope is going up ....the slope is going up but kind of slowing down
the slope is going from a positive slope to zero and then to a negative slope etc. etc.
it's not too hard to figure out that this is possible
take the signal ...make it into numbers
the computer people write software to help us sort through all this
let's look at the quadrupole
the quadrupole is designed by computer engineers
they say.... okay let's filter for mass thirty-five
okay now let's filter for mass thirty-six
okay now let's filter for mass thirty-seven
how about mass thirty-eight.... how about mass thirty-nine all the way up to mass 500
– the first second now the second second ......same as the first second
let's start with mass thirty-five and thirty-six and thirty-seven and go all the way to 500
now run the third second ....... well I think the point is clear
is that the engineers have it designed so the mass spec will filter selectively
and it's programmable by the operator
but from about thirty-five to 500 is the 8270
and these numbers are dropped into boxes
the engineers have designed the filter to match up the boxes in a very reproducible fashion
so that in a very short amount of time the mass spec
produces an absolutely incredibly large amount of data
so how does the calibration work?
well ...the calibration is pretty much like other calibrations
we give it a standard and we say
"okay we want this to be the standard" and then the boxes are lined up according to the standard
I think and it works a little bit like calibrating your bathroom scale
you get home from the gym and you know, with your cloths on
that you weigh a hundred and ninety
you get on your bathroom scale and it says hundred and ninety-six
so you say "well ...that's not right"
then you turn a little dial so this is 190
you get off the scale you get back on ...it says hundred ninety
you know you do, in fact, weigh a hundred and ninety
so you assume that all the other weights line up appropriately
I think that the mass spec works the same way
we give the mass spec PFTBA and then we say "match the boxes that we have... with this known"
and all the boxes line up
I've been operating a mass spec for many years and
the calibration of the masses is not much of a problem
I'm not going to say I've never seen a problem but it's not much of a problem
the routine the mass spec operator is to tune the mass spec every morning it just takes a few minutes
but the truth is ...it's not much of a problem
the equipment is very robust
you need to watch it but it's not much of a problem
this representation is about one 800th of the size of the true mass spec 8270 analysis
the analysis.... the GC/MS produces about the page numbers every second
and there is about 3000 seconds in 50 min. which is the 8270 run.
So that the 8270 produces a stack of papers maybe about this tall.... something like that
it is like.... just a staggering amount of data
let's turn our attention to this graphic
on the bottom we have time
and along the side we have mass
so that the scans go along this direction
and then a given ion is on this direction
so typical ion.... like for example...this might be the seventy ion
this might be the 194 ion ...something like that
the data it is very complicated, of course
computers are good at things like this (however)
we can cut the date up, kind of anyway we want, using the computer
let's say you we slice it up this way
we're basically looking at specific chromatograms of specific ions
so let's take a look at the at the 2-fluorophenol
2-fluorophenol is in black in this drawing 2-fluorophenol has a 112, a 92 and a 64 ... these ions match up for
their chromatograms
it is like the chromatogram of the 112 is the same as the chromatogram for 92
moving forward, in pink, we have regular phenol
phenol has an ion at 94 and an ion at 66
let's look at the 2-methyl phenol in the 4-methyl phenol in blue and red 2-methyl phenol and 4-methyl phenol,
of course, are similar molecules
they don't have the same retention time, but close
they don't have the same spectra, but it's close
that as you can see from looking and the blue one and red one
they're really quite different from the mass spec
the mass spec doesn't have any problem at all separating these two the quantitation is done with an ion that has
been picked by the operator
so if you pick the 107 for the blue and the 108 for the red
you can quantitative and separate them without the slightest problem
and then the last one.... in this example.... one of the internal standards we use
is dichlorobenzene D4 and it's in green here
It's our first internal standard and it has an ion at 78 and an ion at 115 and an ion at 150
the total ion chromatograph is... sort of ...the overall gas chromatography
....is the total ion chromatograph
It would be analogous to the FID for regular GC
It is kind of like the regular GC application
you can see from this model ...from this side
you can see it here we've got two peaks that are close but not the same
and this one is by itself
and the 2-methylphenol and the 4-methyl phenol are close but there but they're not the same
the 2-methyl and the 4-methyl do not collude but they're close
and then the 1-4 dichlorobenzene D4 makes a nice big peak but you can see the chromatogram from this side
let's look at a real-world application of mass spectrometry for the 8270
We have here... this is the total ion chromatograph(TIC), it's called, along the bottom
which is ....it's like the trace if it was an FID
everything kind of added up is on the bottom
we have these three peaks that are sort of semi-colluded and we're thinking "what are these three peaks?" and
"are they adequately separated ... do we have a problem with our chromatography"
we think the first one is phenol. we know the phenol has an ion at 66 and 94.
We have 94 and we have 66
so 94 has a nice big peak... 66 ...you see it as a shoulder here
I would call that shoulder a pretty big peak actually
so this looks like phenol based upon the ions
okay....the middle one, we think this is aniline
aniline ...we would be looking at 66 and 93
so 66 ... perfect... 93... it notice 93 is a double peak
but it's got a nice distinct peak matching up with this peak here
and then the last one is bis-2-chloroethylether
we will be looking for 63 and 93
so let's see we got here
63.... nice big peak 93..... nice big peak
so that pretty well tells us that these three peaks are phenol, aniline and bis-2-chloroethylether
notice the ..a.. 94..... we have a nice big peak
notice the 93 ....we got these two very distinct peaks
the 94 doesn't show up on the 93 at all
and 93 doesn't show up on the 94 at all
the quadrupole mass spec has no problem whatsoever
separating the 93 from the 94
it's pretty amazing
So now, instead of looking at it by time
we're looking look at it by specific.....
instead of by time... it's at a time
so at a certain time, we say " what is the spectrum at this location"
"what is the spectrum at this location"
and you get things like this
this is the first internal standard which is 1-4 dichlorobenzene D4
this is the first surrogate which is 2-chloro phenol
the mass spectrum are usually pretty messy and the peaks coelute
and, of course, if you can pick one spectra with a coeluting peak
you are going to get..... the spectrums are going to be mixed together
so the standard procedure is pick the spectra we want ...
pick a representation of the garbage and then subtract the spectra
and then... this is all a right-click function typically
and then when you get the one you want, do a library search again as a right-click function
and it will tell you what is depending upon how the software is set up
It'll typically give you like up to twenty possibilities
with a probability match according to each one
You can print them out or just look at them and see what it is if you have an unknown that ..... you're trying
to figure out what it is
the mass spec interpretation is not... is not as straightforward as you might think.
You might say what you say "Well you have the mass spectrum ....you know what it is"
sometimes you and sometimes you don't
the mass spec interpretation can be complicated in its own right and of course ,that's out of the topic of this video
so thank you