Chemistry....the real world...Gas Chromatography

Uploaded by mrphysh on 27.01.2011

So, what goes on in the world of chromatographic analysis
let's start with this diagram
we have a battery.... the battery goes up to these two plates
and that's it just sits on these two plates
what's going to happen?
well, the battery will not run down
current will not flow.. there will be an electrostatic charge
between these two plates
as long as there's nothing happening, nothing current will flow
if any ions appear between these two plates the ions will go flying to their respective places
that is, the negative ions will go to the positive plate the positive ions will go to the negative plate
and a current will flow and we can actually measure that current with an ammeter
the current is the signal were looking for
applying a voltage and looking for a current is a fairly common theme in detection systems
the response is sometimes this way, maybe often this way
we run a small hydrogen flame.... we run the hydrogen right up into this space
and light it..... if you want a clean flame you can't do better than hydrogen
hydrogen combines with oxygen somewhat explosively but flammably is probably more appropriate
it burns very nicely with oxygen producing water and no ions
so you can put this flame in this space and it will happily burn there
and nothing will happen unless something comes along that burns
so what does all this have to do with our situation
we run our column through this chamber
anything that comes along that burns which is a lot of things
maybe even most things, will burn producing ions
the ions will produce a current and the current can be measured
one of the, almost, miracles of our modern computer world
is that the computer people can take the current and turn it into numbers
I don't know exactly how they do that
but somehow they take the current and like they divided it and add it
and.... .. I don't know how it works but they turn it into numbers
the numbers of which are proportional to the amount of current
so as the current goes up, the numbers go up
as the current goes down the numbers go down
we make up a standard of benzene and toluene we make it up at fifty parts per million
and we subject it to our chromatographic analysis
we sit back and, according to our programming we wait about 19 min.
and we get a chromatogram that looks like this
when it's done it gives us a report the report tells us that the benzene, or whatever
something came off at 11.5 and gave us an area of 143 thousand that's the area under the curve
it gave us another peak at 18.56 and gives us a quantitation of 179 thousand
which is the area under the curve, okay, So we do another one
this one we know to be the 100ppm benzene and toluene
it gives us an area of ...let me find the right one......
it gives us an area of 284 thousand and the same retention time
and it gives us an area of 358 thousand, same retention time
this is what the instrument sees
we run it third time..... the peaks are getting bigger but they're staying in the same place
this time we get the same retention time of 11.51
this time we get an area of 432 thousand and the toluene comes in at 537 thousand
so, in real life how does this actually work
in real life we go into the computer and say "we need help with this"
" this is what we want to do...."
open up a method.... let's call the method {Benzene_Toluene}
then we say "okay... peak one.. we want you to look for it at 11.5 with a window of '.2'"
and we want you to call it benzene
we want you to look for another peak.... at 18.5 with the window of .2
and we want you to call it toluene
and then ... going to a different section of the method
and we say "okay we want you to have three levels for each of these equally
benzene and toluene at 50
benzene and toluene at 100
benzene and toluene at 200
close that out and go into the next section of the method and say "okay, we want to update this method using these three
quantitation reports"
we have three files with quantitation reports
we want you to take these three files, plug them in as a 50, a 100, and a 200
and set this method up for analysis
In about half a microsecond, the computer will generate two calibration curves
one of them for benzene and one of them for toluene
there are errors in the analysis
the computer, doing what computers do, will look at the errors
and generate a report which you may or may not want to print out
and may or may not be interested in
but the computer will do that
regression analysis is kind of a standard way to do it
but they don't always do it that way
computers are a little bit arbitrary
the method also has pages on ....several pages on details of how the integration parameters will work
there is at least a page on how the gas chromatograph will work
its.. it's.. it will take you a few minutes to put the method together
when it comes time to do the analysis however, all you do is enter, into a different screen,
the sample name ..the autosampler vial location
sample name ...vial location
sample name ....vial location
the autosampler typically has 100 slots
so you could 20 would be a pretty big run
the computer consists of the software to process the data
the software to run the autosampler
and the software to run the GC itself
all this stuff is communicated through one software package
as a functioning chemist in the modern laboratory
computer operation is a more important part of your job than chemistry.
computer operation is.... Your job is to make the computer do its job properly
so you don't have to work very hard
if the computers doing its job, instrumental analysis is pretty easy
And if you are skillful with the computer, you can make a pretty easy job
out of being a chemist.
And it is not boring and it is not particularly hard work.