Appendix: Simplified Instructions for Operating the HP 5890 GC/FID and Air Sampling Systems

Transcription

1 Appendix: Simplified Instructions for Operating the HP 5890 GC/FID and Air Sampling Systems The bucket brigade air sampler. For the square Tedlar bags with the push/pull valves: Put the valve of a tedlar bag through the hole in the lid of the bucket. The big part of the valve should fit snugly in the hole. Be careful to only handle the hard plastic part of the valve, since it s really easy to break the valve loose from the bag, ruining it. {They cost $50 each.} For the rectangular Tedlar bags with the twist valves: Put the tedlar bag in the bucket and attach the tubing to the valve stem and open the valve. Be careful to only handle the hard plastic part of the valve, since it s really easy to break the valve loose from the bag, ruining it. {They cost $50 each.} Put the lid on the bucket, being careful not to smash the tedlar bag in the lid. You will need to pound a bit on the lid to get it well seated all the way around. Connect the flexible tubing from the hand-held vacuum to the copper tube at the bottom of the bucket. When you re ready to grab a sample, open the valve on the tedlar bag (if it is the push/pull design) by pushing it in about a cm. {It should go in very easily, be careful not to push the valve back through the lid of the bucket. (You might want to try this before you put the lid on the bucket to see how it works.)} Turn on the hand-held vacuum. {The vacuum pulls air out of the bucket making the Tedlar bag inflate (similar to the way your lungs work) without letting the air come in contact with anything but the valve and the bag, both of which are made of clean, non-reactive Teflon.} Either wait for a suitable amount of time (after the first bag, you ll have a feeling for this) and then switch off the vacuum, or listen for the bag to stop crinkling which it does while it s filling. {Be careful about running the vacuum too long, we re not sure how long the batteries will last - but you need the bags to be full.}

2 Close the push/pull valve on the sampling bag by pulling out on it. Take the lid off the bucket, close the valve if you are using the twist type valve and remove the Tedlar bag from the bucket, again being careful to hold onto the hard part of the valve. If the bag didn t fill completely, you can put the lid back on and try some more vacuum. Otherwise, retain the bag for analysis and get the next one ready for sampling. Rinsing the bags After all samples have been tested Pull all of the air out of the bag using the house vacuum (yellow tap) and then fill the bag with the house air (orange tap) and re-empty the bag with the vacuum. Do the fill and empty cycle at least twice. {It is highly recommended that you do this before collecting any samples to ensure that the bags have been cleaned since the group before you might not have done this. Also, you can do more cycles if you are concerned that there may be carryover between air samples. Leave the bags empty with the valves closed for the next lab.} The water displacement method for measuring the volume of air in the bags (low tech, but reasonably accurate) Put the valve through the hole in the square piece of plexiglass with the valve facing up. Put the lid (with bag attached) on top of the volume sampling apparatus, with the bag pointed down (towards the water). Turn on the water and let run until the Plexiglas cylinder is full. (Make sure that the lid and bag are pushing firmly against the water in order to ensure that you are only measuring the bag s volume and not any extra air in the container. Remove the lid and bag from the volume sampler and measure the depth of the airspace above the water. Now calculate the volume of the bag. The GC/FID system including computer data system and printer

3 {Running this experiment is kind of involved, so if there s anything that isn t completely clear to you, please ask the TA. They ll light the FID for you and hang around the first couple of times you do an injection, just to make sure everything goes ok.} Lighting the FID (the TA may have done this already, if you aren t sure, ask) Make sure that the nitrogen gas is on and flowing (outlet pressure should be around 25 PSI and black outlet valve should be open.) Make sure that the computer and monitor are on if the screen is blank or shows a screensaver, just wiggle the mouse. If the computer is off, switch it and the monitor on; when it boots up, it will give an error message that you get around by pressing F1. It should start up the rest of the way by itself. The screen in the upper left hand of the display shows a set of lines, one black and the other green and a few controls: attenuation, time, and zero (button). Make sure that the attenuation is set at 0 (zero) and press the zero button. The black line should move on top of the green line. On the front panel of the GC, press the Detector A button the display should say Det A , meaning that the detector is set for 200 and that it s at 200 degrees C. Open the main tank valve and the black outlet valve on the hydrogen and compressed air cylinders. If you haven t worked with compressed gases before, have the TA help you, since this can be dangerous if done wrong. The outlet pressures for hydrogen should be 15 psig and for air should be 40 psig. Turn the black knob in the upper left hand panel of the GC (labeled air) all the way counter-clockwise (open) and then do the same with the hydrogen, which is right below the air, and finally open the open the aux gas (nitrogen) valve. (These are all on/off type valves, so you don t have to worry about setting them.) Let the gases run for a minute or so, and open the cover on the right side of the top of the GC. The little metal chimney is the FID. Press the light FID button - usually the gas will suck down in and you ll hear a gentle pop when the gas lights. You should also see a big jump in the signal (black line) trace on the computer monitor, about 10 seconds after you light the FID. What we re looking

4 for here is a big peak followed by a signal that is bigger than the zero level, which then usually drifts down slowly with time. If you don t see this, the signal will probably spike up and drop right back down to zero. Then you have to wait a few seconds and try again. Sometimes it works on the first try, but there have been times when I ve had to try up to twenty times. (If the FID ignition button isn t working, then you can also light the FID by using the fireplace lighter) Hopefully you won t need to fuss with this too much to get things going. Once it is on, we leave it on for the whole day. It needs to stabilize for around ten minutes before you run the first chromatogram of the day. If it blows out at some point during the experiment, you can just relight it again while the chromatogram is running (if you figure out that it has gone out.) Since the toluene comes out late in the chromatogram, you can usually get this done in time to avoid losing that run. At the end of the day, turn off the three valves on the GC (hydrogen then air then aux gas) and the tank and outlet valves on the hydrogen and compressed air bottles. DO NOT turn off the nitrogen cylinder as this will destroy the GC column {~500 dollars to replace. We usually leave the GC and computer running all the time during the week.} Running a Chromatogram (there are more detailed instructions in the experimental webnotes for this lab) Put the bag on the inlet tubing, open the valve on the bag, make sure the injection valve is in the LOAD position, and turn on (or leave on) the air pump. If you are doing a cryogenic preconcentration run, put the cup of cryogen around the metal sampling loop for a predetermined length of time (usually 5 minutes.) Without cryogenic preconcentration - When you are ready to inject a sample onto the GC, move the metal hook in lower left front panel of the GC from LOAD (horizontal) to INJECT (vertical - clockwise 90 degrees, from stop to stop - you ll

5 feel what I mean) and then quickly reach up and push the START button on the GC. With cryogenic preconcentration Remove the cup of cryogen, switch the valve from LOAD to INJECT, put the cup of water on the metal loop, and then push the START button on the GC. The GC sends the start signal to the computer which will initiate the temperature program. The display on the upper right of the computer should change color from Green to Blue and the status should change from Ready to Running. The running signal display in the upper left corner on the monitor should show a red vertical line, indicating the start of the chromatogram. You can watch peaks come out of the GC on this display, and you can adjust the attenuation control to keep things on screen (this doesn t change anything about the actual signal collection.) About 1.7 minutes into the chromatogram, you ll see a small peak which is the air that you injected coming out through the detector. After about 1 minute, switch the sampling valve on the GC back to LOAD (from INJECT.) {This isn t really time critical, but it s necessary to do it before you start the next sample. And if you leave the pump on all the time, the loop will be cleaned while it s in the LOAD position. Finally you sometimes see a little air peak from leakage that occurs when the valve is switched from one position to the other, and its helpful if that peak is always in the same place.} When the chromatogram is finished (18.75 minutes) the status display will turn to Red (and say Not Ready ) while the oven cools back down to 125 and stabilizes. {You will also see the chromatogram flash up on the screen and then disappear. To go back and look at the chromatogram, you can choose (on the computer) DataAnalysis MainScreen and to get back to the normal display Files Return to Top.}

6 After about 5 minutes the display will turn Green and say Ready again to indicate that you can initiate another run. {If you re doing a preconcentration run, the trapping step takes over 5 minutes, so you don t have to wait to start that part.} VERY IMPORTANT the thing that we can print out, that contains all the data you really need, is the Report Screen that is in a window on top of the normal display (and usually pushed way down to the bottom of the screen.) You have to print this BEFORE the next chromatogram finishes, or it will be overwritten by the next set of data. The report should print automatically, but if you need to print a report, select (on the Report menu) Tabulate Print. Usually you ll want to note (circle, underline, etc.) which of the peaks in the chromatogram is the toluene and xylene sometimes the chromatogram display (DataAnalysis MainScreen) can help you figure out which is which. It can be hard to figure out later. For TA s: Preparation of m-xylene standard Clean and half-fill a tedlar bag with house air. Add 2.5 μl neat m-xylene through the septum port. Fill the bag the rest of the way (be careful to not overfill and pop the bag) with house air and mix thoroughly. Measure the bag volume and provide it to the students so they can calculate the stock m-xylene concentration. Revised DBA