P&T issues

[Bigbear]

We have just transitioned from an Agilent 5890/5973 to a 6890/5973 system to run EPA 524. We have a Tekmar Velocity/Aquatek 70. This P&T system was used with the old GC/MS. We are now having issues with the method that I feel are due to water. I have just changed to a 20M X 0.18 X 1.0 DB-624 column @ 0.6 cc/min. I had been using a 60 X 0.25 X 1.4 @ 1.7 cc/min.
I am getting quite a bit of caryover that Tekmar says is due either to the sample mount or the FFC. I changed the sample mount with no change. I'm now waiting for a new FFC.
This Tekmar system seeme to be a maintenance "hog". What systems do you all use? Are they stable?
I was running P&T on our old system for close to 20 years with a lot less problems that I am now having. I'm feeling kinda lost!

Thanks in advance
John

[JI2002]

what's split ratio with 5890 and what's the split ratio with 6890?

[Bigbear]

I used a jet separator with the 5890 and on the 6890 50:1 with the long column and 70:1 with the short.

[JI2002]

Assume there were no change on P&T parameters and the only thing changed was the column: With the long column, the desorb flow rate was about 90 ml/min (1.7 ml/min + 50*1.7 ml/min + 2 to 3 ml/min septum flow). With the short column, the desorb flow rate is about 45 ml/min (0.6 ml/min + 70*0.6ml/min + 2 to 3 ml/min septum flow). If the desorb time is the same and desorb flow is half what it was, that may explain why you have carryover problem.

Water is always blamed when problems arise. If you have adequate dry purge time, it's not the water.

[AICMM]

Bigbear,

Given the calculations that JI2002 did, both flows should be plenty adequate for clearing the trap (although 90 mL's a minute seems kind of high.) Then, the trap gets baked right after the desorb. So, I would not assume the carryover has anything to do with the P&T but rather with the inlet at that point.

Best regards.

[Bigbear]

Most of my caryover was due to the trap ( Supelco type K). I still get a little caryover with a new trap! I am now trying a Tekmar #9 trap.

[gpronger]

Here is a couple of tricks you can use with the 6890 to help get rid of water and improve sensitivity. To desorb your trap you need about 10mLs/min. Assuming your column flow is about 1.5ml/min the a spilt ratio of 1:7 will get you enough flow to desorb. But what about all the water? At about 2 minutes set your gas saver flow to 50 mls/min. This will effectively blow the water out the split vent. Effectively what we are doing is making an "injection" of our analytes with a low-split injection, and then going into a high-split using the gas saver to blow the water out the split vent.

You will need to tweak the times and ratios, but you should be able to get pretty decent sensitivity and lose your water at the same time.

Regarding the purge & trap units, we're not likely to be much help. We had originally Tekmar 3000's and 3100's and due to service difficulty and general reliability we swapped them for the older 2000 units. The benefit simply being that it is very straight-forward to back flush a 2000 where the newer units are generally more problematic. Regarding the autosampler we use Archons exclusively.