Chromatography Forum

I am having a lot of problems analyzing VOCs using purge trap and it seems to me this problem stems from variation of internal standard. I am using 5975/ 7890 system coupled with OI 4660 and 4552. The trap I use now has a mixture of Tenex/silica gel/cms. I am splitting sample by 100 and I do not use dry purge. I was reading some of the posts related to this topic and it looks to me that my trap maybe the cause of this problem ( the trap is new and I have used different lots of this type of trap).

I appreciate any suggestions.

I haven't used the Tenax/silica gel/charcoal(or cms) traps in almost twenty years. For full list volatiles such as full EPA624 or EPA8260 I normally use a Supelco VOCarb3000. If only running BTEX supelco also makes a BTEX trap that works well but does not capture the gasses like Chloromethane or Freon very well if at all. If you are looking for compounds that are high molecular weight such as Naphthalene and the trichlorobenzenes but not the gasses try the VOCarb4000.

Currently I am using hand packed traps that contain Carboseive SIII, Carboxen 1000 and CarbopackB. Nearly the same thing as the VOCarb3000 except one packing for that one you can not purchase(or at least you couldn't several years ago when we purchased these).

Does your internal standard vary randomly or does it increase with increasing concentration of your standards? If it is with increasing concentration of standards and then returns to normal after the curve or even drops during the run, but returns to normal the next day as the run begins, and you have the normal K&M horn detector, consider switching to the film detector from ETP, that solved our problems. If it is varying randomly it is probably the trap. The water that is getting transferred from the silica gel portion of the trap is not handled so well by the newer MS units when running split injection. Running the VOCarb trap and using a 1 or 2 minute dry purge with do wonders for the early eluting gasses by eliminating much of the water and methanol that elutes at that portion of the run.

I'll second James' comments and add that if you can limit your desorb time to 1 minute (not if you are running 524.2) it will help a lot.
The only problems I have had with the Vocarb3000 is the occasional failure where only 1/2 of the trap heats properly.
I typically run 30 to 1 or 50 to 1 split into a .25mm 624 column.

I use the VOCARB 3000 trap for 524.2 and it works very well. Have tried other traps in the last 20 years and always came back to the vocarb 3000. If you are using a mass spec as the detector I would strongly suggest that you do not use any trap that contains silica gel.

These folks are correct - the VOCARB 3000 seems to be the best to use. OI equipment, however, seems to have some trouble with this trap. It is possible these were due to manufacturing problems at Supelco - this may no longer be the case. I had troulble with VOCARB on OI equipment years ago, but I do not recall what the issue was.

Our lab sticks with the OI #10 trap on our OI equipment - it works well enough; OI claims that thier water management is so superior that you do not need a VOCARB. I am not so sure; I think moving to the OI version of VOCARB would be worth a try (it is OI #11 trap). My lab won't make the move due to bureaucratic inertia, but I think it is worth a try.

The VOCarb 3000 for the Tekmar2000 had some problems about 15 years ago where it would not capture Trichlorofluoromethane. That is when we went to the hand packed traps that emulated them. That was fixed a few months later and I still use them but always keep some of the hand packed ones on hand when I need to swap one out and in the instrument that gets all the really nasty samples. Does OI still use the U shaped traps? Not sure how easy those would be to pack by hand.