Chromatography Forum

Lately i've been having trouble with 3 compounds. Bromodichloromethane (BDCM), 1,2-Dichloropropane, & Dibromomethane. Using a 20m Restek column with a run time of 10min, all 3 are coming out around 5.02s & 5.28s. Any suggestions why only these 3 compounds, which seem to come out at roughly similar times, aren't recovering as well as all other compounds? Has anyone seen or heard of similar problems with these? I suspect it may be my Vendor (Restek) for my working standards so i'm going to try to run a calibration curve using an alternative vendor to see if I'm still recovering them low. Thanks.

More information. What are your P&T conditions and what trap? Vocarb3000? How old is the trap?

Trap is #9 and I'm using a Stratum. Trap was just changed last week.
Conditions:
Purge
at 11min @ 40mL/min
Dry purge 1min@100mL/min
Desorb
4min@250C@300mL/min
Bake
6min @ 280C@500mL/min

Too much dry purge flow. I use the same system but use a Vocarb 3000 trap.
I dry purge for 6 min @ 40cc/min. I never go above 250 cc/min with my traps per what Supelco has told me. I also desorb for 3 min as 524 says about 4 min and my auditor has accepted that 3 min is about 4!

Also traps can be bad out of the box. Try re installing the old trap .

On that does your vespel trap ferrule stick in the mount? In the 18 months I've had a Stratum, my ferrule has never come out with the trap.

I guess i can try that... just seems odd that it's only affecting 3 compounds. I'll let you know how it works. Thanks

I would say my ferrule gets stuck about 1/2 of the time when taking it out. I have to use a screw to get it out... a pain sometimes.

Bigbear, I wish I had your auditor. 3 minute desorb? dry purge? i think i'm beginning to feel a little jealous

Where are you Idla? I'm in region 1.

region 6

I'm handcuffed by my state and several others... would love to play around with the specs if i could...

Update:

Over the weekend i ran a batch through my GC/MS after i changed the inlet liner, a new #9 trap, and new source. I also lowered my dry purge flow from 100mL/min to 40mL/min as per bigbear's advice.

My THMs actually recovered worst. I was recovering all around 90-100% with only BDCM recovering at around 75%. After lowering the dry purge flow BDCM was still recovering at around 75% but now Bromoform is recovering at around 50-60% and even DBCM is low too. I'm gonna raise up the flow again and this time give Tekmar a call to see what they say.

I'm still getting low recoverings for the 3 compounds @ a 4 minute desorb but interestingly enough i'm getting full recoveries of them with a 2 min desorb

Any thoughts on ways to replicate these results but still stay at 4 min desorbs? Possibly raising my split from 40:1 to 80:1??

I hate this method!

I'd suggest playing with the desorb rate of flow next.

Purge for 11 min at 80cc/min.
Desorb for 4min@250C@200cc/min.

Have you ever gotten good recovery on these and the problems have started lately?

it started about 3 months ago... before that i've never had a problem

While developing the new 524.3 for EPA the contractors and manufacturers looked at the parameters that could be changed and not affect the method performance adversely. One of those was the desorb time. The new method allows a 1 minute desorb. It was determined that the longer desorb times only added to the water being transfered to the GC. Not an improvement.
OI Analytical has data on this. Sorry, I don't have the link handy.

Thanks Steve... I actually got the problem under control recently. We changed the heater sleeves on our Stratum traps and put new condensate traps in too. But the biggest step we did was get permission from our state auditor to lower our desorb time from 4 to 2 minutes and instantly we have gotten the results we are looking for. I guess we will getting to 524.3 faster than i thought. Thanks everyone for your help.