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Thread: Apparatus Integrity: O2 Sensitive System

  1. #1 Apparatus Integrity: O2 Sensitive System 
    Lex
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    Hello

    My research requires my system to have very low O2 levels (preferably several ppb) to prevent oxidation of my material. I'm using UHP nitrogen (~6 ppm O2, according to detector), and it's going through an oxygen scrubber that should theoretically reduce that to less than 2 ppb. Nevertheless, the detector at the end of the gas route tells me I have over 20 (the actual value fluctuates, and ocasionally goes to below that level but usually only during a high-volume purge) ppm O2, and when I pump down my apparatus (pretty small total volume, about 4 L) with a vacuum pump that I know is good, I can't get below about 5-6 torr. Pretty clearly a leak, but I've tried everything I could think of (soap water, a thermal conductivity-based leak detector, even pouring liquid N2 all over the place to try to displace oxygen around the apparatus to see if I could see a change in O2 levels) and I can't find it. We don't have a spectrometer leak detector, unfortunately.

    The apparatus also uses plexiglass and copper tubing, so it's not an ideal system (oxygen is absorbed into pores in these materials), but I don't know if degassing could explain such a large presence of oxygen.

    Does anyone have experience with getting a system to very low oxygen levels? I'd appreciate any ideas about what could be causing this, or any hints for locating the leak.

    Thank you


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  3. #2  
    Forum Freshman Spin-1/2-nuclei's Avatar
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    Hello,

    I'd suggest the following:

    1. just like with a Schlenk line, take apart everything that you can - that is glass/metal, etc - clean, rinse with acetone, and place that all in the oven.

    2. How new is your oxygen scrubber? What are you using? You can try drierite-Gas Purifier if you aren't using it already - https://secure.drierite.com/catalog3/page6.cfm? (this stuff needs to be changed often and you place it in the same location you have your current scrubber in)

    3. Did you check your regulator? Some of them are crap.
    I'd suggest using - if you can/and you aren't already (this might be up to the powers that be in which case you have my condolences) - dual-stage, high-purity regulator - (preferably stainless steel)...
    These high-purity regulators contain less contaminates and are better at preventing atmospheric gasses from diffusing into your system.

    4. Are all your swaged fittings, etc from the same supplier? If they aren't they might not be compatible and this can most definitely lead to leaks/diffusion of atmospheric gas into the system..

    5. purge the system with argon - since it is better than nitrogen...
    The argon is more dense than air and if you do a purge it will push the oxygen out of the system if it is lingering from some unknown place - literally. You will want to do a purge glove box/Schlenk line style (like 4 or 5 times)...

    I prefer argon in the lab over nitrogen - if I get the option - because even if a hose blows off the reaction flask etc, so long as the schlenk line is set up properly, I will typically be able to save water/air sensitive reactions (if caught in time), due to the fact that the argon is more dense than air...

    Hope this helps..
    Cheers


    Last edited by Spin-1/2-nuclei; August 17th, 2011 at 12:16 AM.
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  4. #3  
    Lex
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    Hi, and thanks for replying.

    1. The substances I'm using are pretty dangerous (e.g. carcinogens), so it's a major pain to take them out of their dedicated hood. Plus, taking apart the system, mostly consisting of copper tubing connected by swagelok fittings, will probably invite disaster in the form of new leaks. I often clean the plexiglass jar and aluminum base with methanol (acetone eats plexiglass) though.

    2. It's a supelco scrubber from Sigma-Aldrich, and claims to be able to remove air from at least 3 cylinders (I'm on the 3rd right now, and I do have to admit that I didn't have a working O2 sensor before starting this tank) to <2 ppb, provided an incoming O2 level of <10 ppm.

    3. The regulator doesn't look super impressive. I agree that it could potentially cause a problem, since if enough O2 gets in the scrubber will probably not be able to cope. There is also the fact that connecting the tank directly to the sensor gives me a reading of ~6 ppm after steadily decreasing for about an hour, although it should be < 1 ppm for UHP nitrogen. But the tubing I used for the connection isn't that great, so I can't say for sure. In any case, if the regulator accumulates oxygen, the scrubber I have might have been titrated a while ago, and switching it would probably show an improvement. They're not too cheap though, so I'm hesitant to scrap one based on speculation. As for getting a new regulator, that might indeed be up to the powers...

    4. All are swagelok fittings, properly installed. I try not to mess with them, and only a few have ever been disconnected and reconnected.

    5. Since we're using UHP, argon is at least twice more expensive... That pretty much sums it up. I did have to connect a nitrogen (not UHP) tank to the system for a thermal conductivity leak detection test, but I don't think we'll be ordering UHP argon. I haven't done the pump/purge cycle that many times though: 2 at most. It seems fishy to me that the oxygen depletion, which should probably be exponential in a closed system, simply stabilizes around 20 or 10 ppm and then very slowly goes down over time, so I didn't think multiple purges would help.

    Do you have any idea if copper and plexiglass could be creating a degassing effect that large (several ppm O2, and not letting the pump get anywhere near its base pressure of a millitor)?

    Thanks again,

    Lex
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  5. #4  
    Forum Isotope Bunbury's Avatar
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    Quote Originally Posted by Lex View Post
    We don't have a spectrometer leak detector, unfortunately.
    You can hire a firm to come in and do a helium mass spectrometer leak test.
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