Valve Ports and Sample Loops...

[july]

We are looking to build a sample loop to deliver a vapor sample to the GC/MS. We would like to use liquid nitrogen to "concentrate" the sample and heat to drive the sample out of the tubing and into the GC/MS. I am looking to compile prices for an estimate of the final cost of putting the sample loop together, but I am not sure of all the pieces that are required:

Deactivated stainless steel - 2 lines
Deactivated stainless steel sample loop
six port vave
Heater for heating the line quickly - what is normally used?
Flow controller - I don't even know where to begin with this...I am not sure what is required to attached/use or anything.
I am still unsure how the sample gets to the sample loop from the sample container?
Internal standard - how is this mixed with the sample to be analyzed?
Some sort of something to allow the stainless steel tubing to be immersed in liquid nitrogen - I am not sure of how to do this reproducibly

I'm sure that I am missing some things...I would GREATLY appreciate suggestions on setting up, equipment, ANYTHING. I am in over my head with this.

Thanks for your time.

[pi3832]

What exactly are you trying to trap and concentrate?

Have you looked at O-I-Analytical's purge and trap equipment?

Way, way back in college we cryo-focused samples by putting the first loop of the capillary column in a dewar of liquid nitrogen. Pull the loop out and it would flash fast enough to give good resolution.

[july]

acetone
toluene
ethyl acetate
ethanol
hexane
butaldehyde
cis-2-butene

We have looked at several purge and trap instruments, but the least expensive we have found thus far is $20k...much more than our budget entails...so we plan to attempt construction of our own. We do have a Tekmar 3100 purge and trap concentrator, but I think it requires the use of a sorbent. We would like to avoid sorbents if at all possible.

I'm sorry - I'm not really sure I answered your questions.

Thanks.

[AICMM]

Having followed your earlier posts, I still think you have a very good chance of converting your Tekmar 3100 into the appropriate sampler that you want for your project. Granted you will be putting your materials onto the adsorbent but from the gas phase this list does not look too difficult. You would probably need to cut into the GC inlet nut to make the 3100 interface with the GC. You would also need to run the GC in split mode but this is a common way to interface purge and trap systems to GC's nowadays.
You could load samples into the sparger on the front of the 3100 and allow the automated operation to go from there. The use of the 3100 would allow you to operate without having to support liquid nitrogen for cooling the loop.

Best regards.

[july]

Is there not a way to use the Tekmar 3100 without an adsorbent? Everyone here is dead set against adsorbents. We don't like to do things that actually work...that would be too entertaining. jk

[AICMM]

July,

You can operate the 3100 as a simple valve injector. I have done this for a customer for multiple sample streams. However, it is very bad case of overkill (this customer just happened to have the equipment, idle, on hand.) To use the 3100 as a simple valve injector, replace the adsorbent trap with a sample loop of the desired volume. The loop will stay warm inside the 3100 valve oven and the system will perform injections as part of the "desorb" step. You would then connect the 3100 transfer line to the injection port of the GC/MS. If I am not mistaken, the minimum injection time would be 1 minute since that is the minimum desorb time on the 3100.

At that point, it is far more efficient and cost effective to simply buy a sample valve and mount it to the GC/MS system in question. By the way, you have not said what detection limits you are looking for which is certainly pertinent to the question of loop injector or concentrator.

[july]

I think ideally we would like to see down to about 10ppb. That is what I was told originally. The other day though, they came back and said they may have been mistaken...that they would only need to see down in the low ppm.

What we are hoping to do is prepare a gas sample containing different chemical classes of VOCs of known concentrations in air. The air will be filtered and we will determine the VOC concentrations after the air is filtered to determine what percentage of each compound is being removed from the air by the filter.

[AICMM]

July,

At about 1 ppm and 1 mL sample volume, you will only be putting about 5 ng of material on column. This will be challenging for your MS to see unless you go to selected ion mode. Second problem you are facing if you go to cryogenics and loop with air sample is that you are going to be freezing out water in the loop as well which will affect volatilization on warm up and move as a slug to the MS. Especially in humid Texas.

To see ppb levels, you either have to move to a more sensitive detector than the MS or you have to go to a concentration step.

[july]

Well you're no fun! jk

OK...so let's talk detectors then. What would be most appropriate FID or the helium pulse discharge detector? I'm not sure where to even begin.

[AICMM]

July,

FID is typically viewed as about as sensitive as MS in scan mode. Probably not the solution. Helium discharge means you will have to get the analytes well resolved from the air peak. However, the PDD with krypton in helium mode (Valco), my detector in argon mode (Advanced Industrial Chemistry Corp.) or a standard PID (OI Corp. or Hnu) should be able to do, substantially, what you want. Probably the least sensitive will be ethanol since it has the highest ionization potential.

[july]

Oh my goodness...You're THE guy at AICC. I looked at your website before and didn't catch that.

I was confused as to why you were calling it "your detector" ...I got it now...silly me for not catching that sooner.

You said your detector in argon mode...looking at your website I'm guessing that it is the DBD AID?

[AICMM]

July,

Sorry for the delayed response. DBD AID would do all of the compounds you are interested in and should do them into the ppb range with a sample injection valve. I cannot say how low in the ppb range for this particular list but doable. Also, do not have to worry very much about the air peak since, in argon mode, air peak will not significantly respond (too high an ionization potential). Feel free to contact me if you want more information.