The EPA 8270
this is kind of fun
GC mass spec ....gas chromatography/mass spectroscopy
in my opinion .... my opinion
the mass spec is the king of the pile in a laboratory
the mass spec is like..... incredibly powerful
there's things you can do with a mass spec that you can't do with anything else
the people who make them are getting better and better at it instruments are getting more robust ...easier to operate ...cheaper
the mass spec is like..... you absolutely fall in love with those things they are not particularly difficult to operate
GC mass spec is a partnership between gas chromatography and mass spectroscopy
of that partnership the dominant player really is the GC
the gas chromatography forms the separation
If the separation is done properly
then the mass spec doesn't have to do much
If the separation is not done properly there isn't anything the mass spec can do about it
if the mass spec is running well they can't fix problems that are caused by poor chromatography
the EPA 8270 is a very sophisticated analysis largely because of the quality control...
The quality control constraints in 8270
are mostly... the mass spec, a very powerful detector,... being used to evaluate the performance of the GC
so quality control is all about the GC
it's not really about the mass spec so much
gas chromatography ........the operative word is gas
the GC separates compounds based upon their behavior as a gas
but the semivolatile 8270...the sample is injected as a liquid
the typical injection is one microliter in methylene chloride
that one microliter is injected into a space about the size of a thimble
it's pretty small..... it's about 260°C
260°C is hot enough to explode methylene chloride into vapor
but it's also hot enough that anything you want to analyze for , will be a gas
it is under pressure
so it's injected into this space under pressure
everything goes to vapor and the vapor is pushed into the column
once it is in the column, it's all pretty straightforward
the Achilles' heel of the GC is in the injection phase
getting the sample cleanly from the liquid into the gas phase
and into the column is really the key to it
the typical routine for mass spec operator is
you come to work...... the first thing you do is you turn off the GC oven
and then you turn off the GC injector to let them cool down
you go about your business....it is it's a lot of administration
go about your administrative business maybe get your standards ready
let the injector cool down .....put on a pair of gloves.... cotton
and .....let's... take a look at this
the injection system takes a lot of different forms with different gas chromatographs
but I think this is a pretty good general representation
the port liner is nothing more than a glass tube
its about the diameter of a pencil, about two a half inches long
it is held freely from the top and bottom
in a sleeve .....the sleeve accepts a nut both on the top and on the bottom.. let's look at the top first
the top has an "o"ring..
The "o" ring is squeezed down by a nut
so the "o" ring forms the seal between the sleeve and the port liner
and then this nut also has a little place that accepts the septum
and then another nut goes on top and holds the septum in place
So the needle goes through the top nut through septum through the bottom nut
the whole system is pressurized from the top and sealed with the "o" ring
to take it apart ..... it's ....fairly simple
the top nut which is green (in this representation) is unscrewed
revealing the septum. The septum is then pried out
And this loosens up the lower nut which is black in this drawing
and then that unscrews and that nut is taken off the port liner is just sitting there and the "o" ring is just sitting there
just pull them both out. Put the new port liner in
slide the "o" ring over it ...slide the black nut over that
snug it down .....put the new septum in
put the green nut back on and snug it down
and that's pretty much all there is to it
the bottom ...now this is inside the oven... also has a nut
The nut is part of the seal and this seal is actually through gold.... a little gold plate and then a little gasket
but it is snugged up against the bottom the port liner just sits on this plate
but .... the nut holds the seal.... makes this seal
and then, into this nut is yet another receiver which takes the column
let's turn our attention to column itself
the column is very very very small (diameter)...thirty meters long
so that's ...about...longer than a football field
is very long and very thin. The column goes in to the injector
with a nut and a ferrule
as an example... this is like... applied industrial engineering
up I want to take a quick look at this
this is ....looks like it came out of a bathroom
in fact, it did ... notice our working parts
we have a washer.. I think of it as a ferrule
I don't think plumbers call it that but whatever
and there is a nut and then this would be the column
so... put the nut on, put the washer on
notice the washer is tapered
such that it fits right in here exactly
so put the nut on... put the ferrule on put the whole thing in here
and then this guy comes up and
as this guy turns around and is tightened up, it pushes this washer into the groove around it and makes the seal
the technology, the engineering, for the GC column is identical
let's take a look at the column itself
it's...umm.... the column is so small it's hard to believe that it is actually hollow
this is the standard 8270 column
this, I believe, is a megabore ..I found these in a file so
I'm not exactly sure but I think this is a megabore from the 8240 which is the volatile GC mass spec
but it's it small to the point that it's just incredibly small
it is 30 m long ...so I don't know... ... but anyway that's the way it is
So the routine is this:
As part of regular maintenance... this is in the oven now.... open the oven door
it should be cooled off reasonably well
loosen the nut with a little wrench.. This is a little tiny nut with a little tiny wrench
loosen it....pull the column out
it will just come right out
make a note of how much column sticks out of the ferrule while you are at it
cut off about 18 inches of the column
just cut it off and throw away
slide the nut over that the remaining part, slide the ferrule over the remaining part
take the column cutter and cut off a little bit more of the end
the argument is that the graphite ferrule is a reactive substance
and the graphite pieces will stick to the cut glass
and cause problems so you want to cut off the and so you have a fresh cut
the way to do this is to gently scratch the column with the column cutter and then gently break it off
the column cutter is like a little piece of quartz
it's a material that's harder than glass
I've seen them made out of carbide steel
It is usually just a little quartz crystal
look around in your lab and you can find one
and then put the same amount of column
coming out of the ferrule that you saw
stick it back up where it belongs
take the nut ....twist it back on.. snug it down with a little wrench
gas chromatography is all about time and temperature
the temperature of the oven goes up and down as part of its programming
the temperature of the injector should stay the same
you need to keep these temperature zones separate
and the way to do it is to be sure to put the little cover back on
there is a little cover that goes around the injector
be sure that cover is put back on
When I was a new lab tech faced with this situation... like this
I was completely terrified
chemistry is abstract anyway.... the instruments are completely magical
and it was just really scary
it's not that difficult
is certainly no one complicated than other mechanical aspects in our lives
study the manual...... ask the people around you
if they have anything to say and.... just do it