Carryover

[aes1311]

Need help troubleshooting a carryover issue, I am running EPA8260 (Archon Autosampler/PT OI4660/GCMS 6850/5975) and having trouble with carryover for the late eluting compounds Naphathalene, 124 Trichlorobenzene, 123 TCB and Hexachlorobutadiene. Parameters for P&T


Sample Temp

Sparge mount 40°C
Sample 40°C

Purge Time and Temp

Purge Time 11.00 min
Trap Temp 30°C

Water Mgmt Temp

Purge 100°C
Desorb 30°C
Bake 240°C

Bake Time 6.00min
Trap Temp 210°C
Desorb Time 2.00min
Trap Temp 190°C

Desorb Pre Heat

Trap Temp 180°C


Problem on mutiple instruments, tried replacing trap, sparge tube, increased temp on transfer line, so far no affect. Any ideas?

[Yama001]

I usually rinse/replace the purge flow path when this happens. The mount can get fouled as well. The water management fitting is often suspect.

Before spending a lot of money on parts, try the rinse first - sonicate the sparge filter as well.

[01310040231]

You have a few options:

1. Use VOCARB trap instead of #10
2. Clean the sample path weekly if you are analyzing sample with high concentration of hydrocarbons.
3. Increase the Bake time.
4. Increase the rinse.
5. If you have a foam sensor on your system, use the fritless sample adapter.

[Bigbear]

I agree, if you change to the Vocarb 3000 trap you can desorb @ 250 and bake @260.

[James_Ball]

I agree, if you change to the Vocarb 3000 trap you can desorb @ 250 and bake @260.

It helps but I have had the same carryover problems for years while using the Vocarb 3000 with the higher temperatures.

How bad is the carryover? Is it 1ppb after a 200ppb standard, or is it 5ppb after a 50ppb standard?

The only systems I have that I have gotten rid of this problem on are my newer ones with the Centurion autosampler where I can wash the sparge tube 3 times with 90C water then bake the sparge tube at 90C for the remainder of the bake time while also rinsing the sampling needle and lines of the autosampler with 90C water for 1 minute and flushing with He for another minute. Anything less and I will get carryover for these compounds at 1ppb after a 25ppb standard even on instruments that only run drinking water samples.

Another place that seems to contribute to the carryover problem is the moisture reduction systems. I found that if you bake at a lower temperature you get more time before needing to clean or replace the moisture trap tube. It seems the high temperature bake pyrolizes trace amounts of organics to form what I think is a char inside the tubes that will work as a trap later on(just a theory I haven't been able to prove it yet). If I bake at about 180C I get longer life from the moisture trap tubes before the carryover happens. If the tubes are clean, you shouldn't need to go any higher with the bake to remove the traces of the target analytes.

As mentioned above, heating the sparge tube between samples seems to give the lowest carryover of anything I have tried so far.