2-CEVE Issue on GC/MS

[Jaclyn]

I am trying to run an 8260 analyte list on an instrument that has not been used for several months. 2-chloroethyl vinyl ether is not detected, and several other analytes are showing inconsistent results/low recoveries (propionitrile, bromoform, trans-1,4-dichloro-2-butene, and 1,2-dibromo-3-chloropropane). Since 2-CEVE is detected on an identical instrument, the standard is not the issue. Replacing the trap and moisture trap did not help. The standards were not acidified, and 2-CEVE is not detected even when running a 50 ppb sample made directly from the stock (no other analytes present besides IS/surrogates).

For reference, here are the parameters I am using:

Agilent 6890 GC & 5973 MSD w/ helium
Vocarb K trap on Encon Evolution
Purge: 11 minutes @ 30C & 40 mL/min, Dry Purge: 1 min & 40 mL/min, Desorb preheat: 250 C, Desorb: 1 min @ 260C, Bake: 7 min @ 260C
Column: DB-624 - 0.18 mm ID, 20 m length, 1 um film
Temperature program: 45C initial for 2 min; rate 15 to 250C
Split ratio 50:1
Flow: 0.8 mL/min
Split flow: 39.6 mL/min
Inlet: 150C & 16.8 psi
MSD Quad: 150 C
MSD Source: 230 C

The data from last year indicate that 2-CEVE was detectable using the instrument in question with these parameters. Please let me know if you have ideas. I am completely new to this so please be detailed with suggestions.

[James_Ball]

I use an almost identical setup and we have seen the same problems before. There are several places you can get contamination that will reduce the 2CEVE response.

Have you cleaned the mount for the sparge tube? If you ever have samples that foam you can get residues up inside the mount that will cause problems, also using acid preserved samples can cause some corrosion in it too, especially now that no one used gold plating on them. You may even want to remove the mount and sonicate it to loosen up any deposits in it, but start by just cleaning it out with a swab and methanol.

The transfer line can also cause problems. I made an adapter to attach a disposable syringe to the purge and trap end of the transfer line. Just need a PEEK luer to 1/16" tubing adapter, usually used for HPLC. Remove the top from the GC inlet and place a paper towel below it and push methanol through from the purge and trap end to the inlet cap end, this will flush out contaminates, just heat it up and let it bake overnight before analyzing samples. I usually set it up with a really high split and low column flow for several hours until the methanol is gone.

Also change out the inlet liner, gold seal and scrub out the inlet.

It may take several standard injections after the cleaning to recover the lost analytes completely, and usually what I see is a slow increase over time when they come back.

Another place for loss that is more difficult to clean is the 8 port valve, sometimes you have to replace the rotor in it if it becomes too contaminated. The Evolution is less prone to these problems than the original Encon, but it can still happen. Also the transfer line from the autosampler to Evolution can cause problems as well as the stainless steel needle in the sparger. You can replace both of those with PEEK to help.

[sidz28]

James seems to have it covered. One other area where you can have acid hang out is the Moisture trap (MoRT). It might be worthwile to replace the MoRT or (if you just want to troubleshoot) to bypass it with a short piece of siltek tubing from port 4 to port 5 on the 8-port valve.

One thing about the Evolution is that the backpressure number can tell you a lot about the system. Normally, when generating 40mL/min of flow you will see a backpressure of 0.9-1.3psi. If your backpressure is much higher than that, it could signify that you have a restriction in your Vent valve or the restrictor tubing between the valve and the vent bulkhead. This could be causing issues during purge. If your backpressure is lower than that, it could mean that you may need to do a leak check and flow check and recalibrate your flow (always perform an automatic leak check before recalibrating flow - and make sure that it tells you that the leak check passed -- "leak check passed second attempt" is NOT a passing leak check. If you refer to the logbook as prompted, it will tell you there is a leak around your trap or MoRT).

If you have any questions specific to the Evolution, you can always contact EST tech support at 888-890-1404 or email me directly at msidler@estanalytical.com.

[Jaclyn]

Thanks for all of the suggestions. I was able to rinse the sample loop and sparge tube with methanol and I also cleaned the sparge tube mount and needle with methanol. The inlet liner was replaced recently. The pressure on the P&T device is 1.0 at 40 mL/min, and I replaced the MoRT just the other day. The leak check passes on the the first attempt. After doing these things I am still having the 2-CEVE issue so I am going to look into how to disassemble the transfer line between the P&T device and the GC so I can clean it with methanol as well.

[James_Ball]

Thanks for all of the suggestions. I was able to rinse the sample loop and sparge tube with methanol and I also cleaned the sparge tube mount and needle with methanol. The inlet liner was replaced recently. The pressure on the P&T device is 1.0 at 40 mL/min, and I replaced the MoRT just the other day. The leak check passes on the the first attempt. After doing these things I am still having the 2-CEVE issue so I am going to look into how to disassemble the transfer line between the P&T device and the GC so I can clean it with methanol as well.

It is fairly easy to disassemble the transfer line. First shutdown the instrument and allow about 15 minutes for the valve oven to cool. Remove the covers and the valve oven cover is right there in the front top behind the traps. If you follow the transfer line back from the GC you will see it run up to the column oven and attaches to one of the rear facing ports on the 8-port valve. Just use a 1/4" wrench to remove the connector.

From there you just need to fabricate something to connect a syringe to the line. The best I have found is a simple PEEK Luer to tubing adaptor(Upchurch has them) like what is used to prime LC pumps. Second best is some Silicone or Tygon tubing that will fit the Luer tip of a disposable syringe and is still small enough to seal if slipped over the ferrule on the end of the transfer line.

Sometimes you just have to end up replacing the silcosteel line inside the transfer line. It is 1/16" silcosteel tubing you can get from Restek, but I have also used some of their 0.53mm Mtx Hydrodeactivated transfer line or even 0.53mm Hydrodeactivated fused silica transfer line. They will give a little more restriction to the flow but can still handle normal desorb flows. For these you need to use the 0.8mm graphite/vespel ferrules to seal them in the port. The fused silica line is the most inert you can use, but also the most prone to breaking, but I have used them in emergencies before.