Purge and trap is a pretty common gas chromatography application which looks for volatiles in water as often happens in chemistry the labels are determined by the situation so what is a volatile? In the case of purge and trap a volatile is anything that will come out with bubbles …… so if the sample will vaporize from the water into the bubbles then it is considered a volatile so what is a volatile? what does this encompass well …it encompasses anything that’s related to gasoline anything that’s related to diesel fuel, kerosene, jet fuel any kind of industrial solvent …like methylene chloride acetone, acetonitrile, tetrahydrofuran, hexane cyclohexane, pentane, cyclopentane any of the Freons, benzene, toluene, carbon tetrachloride a large range of bad guys are detected with purge and trap I think that intuitive….. like if you try to purge gasoline when you were done it will all be gone
because gasoline all volatile purge and trap is used for detection limits
down to parts per billion it is calibrated from 5 to 200 parts per billion so the sample is typically somewhere between
[kind of clean water] to [extremely clean water] so that we’re pulling out really really
really small amounts of these organic compounds the heaviest one is,…. generally speaking, if a compound is a liquid at room temperature, it can be purged. the heaviest one that comes to my mind is naphthalene which has a boiling point up over 150 or something like that… it’s pretty high…. but it will purge so, according to this definition,
naphthalene is considered to be a volatile so it’s under the conditions of the tests If it will purge, it is considered a volatile There’s three categories …
the flow which is related to the instrument and then time and temperature gas chromatography is all about time and temperature but let’s start out by looking at the flow the sample is put in a test tube and the test tube is raised up under this thing
and locked into place with a nut and a ferrule and then the gas goes in and is forced out in the appropriate fashion let’s turn our attention to this thing we have the…the helium comes in…bubbles,bubbles,bubbles the air (helium)is forced through this apparatus It’s called a six port valve and I want to take a quick look at it there is two configurations for the way the gasses can flow this way…. and …. this way what you’re looking at here …. this is part of the machine. It has these tubes that go into the manifold. on top of the manifold is this little thing that turns so..this thing turns on top of it it has an “A” / “B” position …. The “A” position and the “B” position so it doesn’t turn …. it just goes click…..click…..click
from A to B and if you look at the back of it, you can see these little groves that line up with the ports and so it goes from configuration one to configuration two ….. so it doesn’t turn this is the only moving part this little guy is the only moving part the rest of it is just part of the machinery I point this out because these valves are sort of common
especially in HPLC systems the six port valve and also, I point them out because I think they’re cool Okay ..so back to what we were doing the injection ……the purge stage is…. okay so …. the helium bubbles bubbles bubbles….
it goes through this six port valve through the trap and then it goes through it (the valve) again and is vented so it goes..through the valve.. through the trap… through the valve and then it is vented to waste in the meantime carrier gas is going into the GC column the gases are all helium at this point the gases are all helium….. these two streams are not really related they may come from the same tank but they’re not really related so purge purge purge trap trap trap okay now
after a period of time it goes to an injection form the injection …. the sparge is now out of the picture the carrier gas ….instead of going right into the GC column the carrier gas goes through this valve through the trap
notice it is going backwards compared to what it was doing before it goes through the valve again and then goes into the GC column and that’s all there is to it so that on the one stage….let’s go through it real quickly…. STAGE ONE…you’ve got the helium going bubbling into the sample goes through the valve, through the trap, through the valve to vent and , in the meantime, the carrier gas goes into the column STAGE TWO,,.. this is all out of the picture now the carrier gas which used to go into the column….it still goes into the column but it goes through the trap first That’s it for the flow again it uses this valve. It has these grooves cut in the top of it this valve is about ..in the real world ….
is about the size of an old fashion fifty cent piece …. not bigger than that it’s not very big So let’s talk about time for a minute the sparge,…. the bubbling… is ordinary extraction type physical science let’s say in the first minute, half of the pollutants will be pulled out of it in the second minute, half of the remaining pollutants will be pulled out of it in the third minute half, of still remaining pollutants will be pulled out of it so it needs to go a certain length of time but there is a diminishing return with a longer and longer …. A typical sparge is like five or six minutes….. something like that and then time for the desorption is…..we’ll talk about the temperatures in a minute but the time for the desorption is typically a minute ….something like that and ….then, of course, the gas chromatography is …a whole different we’re not talking about that at this point….. that’s a whole different story Let’s talk about the temperatures this sparge is sometimes heated but often not heated there would be advantages in heating it
because compounds will go into vapor easier but then there is more problems with water the lines are heated up..these lines go through a jacket they’re kept at about like 120 degrees centigrade or something like that this whole thing is in an oven that is also kept at about 120 to 150 degrees C so it’s all kept nice and warm and toasty so that the volatile liquids will remember that they are volatile (gases) and stay volatile(gas) and flow through the tubes properly as volatiles (gases) during the absorption period, during the trap period the trap is at ambient temperature ambient temperature being 30° C ….30°C is like 80° Fahrenheit so it’s pretty cool and then the trap is in it’s own little house let me describe the trap to you it’s about …maybe..a foot-long
it’s curled up into a “u” it’s got two nuts and ferrules so you snug it down …put it up and twist the bolts on and snug them down good the trap includes a plug so you plug the trap in the trap is its own little heating unit so that when you want to heat the trap up, you don’t heat up the oven you zap 110 volts through the trap itself and the trap is its own heating unit the heating up of the trap is the injection so that when the valve switches… this valve switches…. goes clunk the valve switches the trap gets zapped up to about 185 degrees centigrade so it’s pretty much the hottest thing in there and the injection all goes out as one plug so it goes basically to 185 and everything on it is immediately desorbed goes back through the two valves (ports) and goes into the GC column as the injection and then the GC column gets its start signal and starts doing its thing (is that all there is to it?) Still looking at the temperature …… all the lines have to be kept warm some of the lines are pretty long
its probably.. like ..a 24 inch line between the trap and the GC they come in specialized insulated and heated lines complete with thermostats the lines are…. the temperatures
are all controlled, of course, with the computer everything is, of course, controlled by the computer the purge and trap itself is about … maybe 10 x 10 x 20 (inches) something like that… it is not very big it has a typical “computer looking” interface on the top of it there is an autosampler that goes with this which has sixteen (typically) of these sparges and it rotates through them automatically so that the instrument will do its
work while the operator is home in bed as a scientist, instrumentation is a little bit aggravating because it is completely arbitrary how it’s designed I’ve see systems where the GC mass spec is completely independent the GC mass spec does one thing ..two things… it gets a start signal and
takes a signal from the quadrupole that’s all it does I think if you went to a modern ‘off the shelf’ purge and trap GC you would find it would be all operated by the chem station but it’s a little bit arbitrary …..it’s not a big deal if you understand how the system works
it’s not hard to pick up a new system A sales pitch on purge and trap Purge and trap is like elegantly simple and totally cool purge and trap produces no waste there is no extraction.. there is no acetonitrile you have to either disposed of….. a typical extraction involves a blow down technique and the volatile (solvents),, which are green house gases
and toxic,, are blow off into the air there is none of that goes on here there’s no sample prep there is no waste produced the only hazardous waste comes from the standards but it’s calibrated in parts per billion so it’s not really significant there is no waste produced the instrument is calibrated in parts per billion…. parts per billion the calibration five parts per billion to 200 parts per billion the purge and trap is an absolutely incredible concentration step you are concentrating everything in 5 mls you’re not concentrating it down to a milliliter or down to half a milliliter you are essentially concentrating it down to a microliter you’re concentrating it down to one injection that’s where the incredible detection limits come from is that you have this incredible concentration step. Another cool thing about purge and trap is that…. The GC…. gas chromatography…. the operative word is gas it likes gas …you separate the analytes based
upon their behavior is a gas and you never have anything going in the column that’s not a gas and the result of this you don’t have problems with your GC your column doesn’t degrade ..it lasts a long time there is no injection port that will cause you trouble people that operate normal GCs with injections
are constantly fighting with the instrument…. you don’t have those kind of problems with the purge and trap I think of this as related to mass spec you have small molecules with small ions you’re not injecting a bunch of junk into the vacuum space you don’t have problems with your source and the quadrupoles don’t get messed up so that it’s just a cool… totally cool and simple … elegantly simple technique you go into a big environmental lab … so if you’ve got ten GC’s three of them are going to be purge and trap this is not an obscure branch of chemistry the recent mess down in the Caribbean with the Deep Water Horizon oil spill the backbone ….the laboratory backbone of all that work is purge and trap EPA analysis “8240” is purge and trap for volatiles. I have made almost 40 videos.
This one may be my favorite!