Endrin breakdown, possibly unrelated problematic baseline

[bisnettrj2]

Chromatographers:

My colleague is having a couple issues with her Agilent 7890 dual ECD GC. She's been having some extreme Endrin breakdown issues, to the point where all troubleshooting has failed, and she has switched to running on the back inlet instead of the front as usual. On the back inlet her Endrin is Ok on one column (~8%), and questionable on the other (~13%). However, the interesting issue (to me at least) is that there is a baseline disturbance after the Endrin peak elutes that lasts for about 2 minutes on one column and about half a minute on the second column which has occurred on both inlets and on a previous set of columns (both columns were replaced about two weeks ago as part of the extensive troubleshooting practice). The Endrin breakdown standard she is shooting is 200 ug/L. This baseline disturbance does not show up on hexane blanks. It only shows up in injections of standards. It does not show up on any other peak, only following the Endrin peak (she's running 504/505s on this instrument, as well as 608/8080 samples).

Parameters are as follows:

2 columns installed into one inlet via two-hole vespel-graphite ferrule. Columns are 30m by 0.25mm x 0.5um, one VF-5ms, one VF17ms. Columns have been replaced without fixing baseline issue or Endrin breakdown.

Carrier gas: Helium
Inlet: 250C
Detectors: 335C
Makeup: 30mL/min N2
Column Flow: 1.2mL/min constant flow
Injection volume: 0.6uL
Injection parameters: 25psi pulsed splitless for 0.75min, purge at 0.75 min at 80mL/min
Septum purge: 10mL/min
Straight liner with glass wool (she has tried packing her own glass wool and using purchased liners with glass wool, with little to no effect on Endrin breakdown).
Siltek-treated Restek double-sided split seal - recently replaced, no effect on Endrin breakdown.

Oven parameters:

40C initial, hold 4 minutes.
45C/min to 150C, hold 0 minutes.
5C/min to 205C, hold 0 minutes.
30C/min to 320C, hold 4.7min.

Does anyone have an idea as to what is causing this baseline issue? What can I suggest to help her resolve it? What more information does anyone need to give a more thorough diagnosis of this issue? I'm not a GC guy, so I'm stabbing in the dark when it comes to making suggestions as to resolving this issue. Oh, and if anyone has any ideas on the Endrin breakdown issue, I'm all ears. I've already suggested the "gun brush" cleaning or replacing the inlet weldment altogether.

Attached are several chromatograms of the baseline disturbance we're seeing. Hopefully they come through alright.

[bisnettrj2]

So my coworker cranked up the flow rate to 2.8mL/min (1.4mL/min per column, 25psi in the inlet at 40C) and the blob after Endrin disappeared, but Endrin still appears to be excessively tailing. However, breakdown is down to 10% on one column, below 10% on the other, so it seems the 'blob' was actually Endrin tailing horrendously at lower flow rates. Here's a question - what would be the optimum flow rate for this setup - two 30m x 0.25mm ID x 0.5um ft columns installed in one inlet through a two-hole ferrule?

[Bigbear]

I know it sounds odd, but you should clean or replace the injection port shell. I also have had endrin issues and when liner change in ineffective, it's the shell.
There is an instruction sheet on cleaning on Agilent's web site.

[bisnettrj2]

Is this the 'gun brush' cleaning I've read about?

[Bigbear]

Yes.

[bisnettrj2]

Fancy. I'll pass it along. Interestingly (at least to me), I was able to queue up a slew of different injection techniques on this instrument, and the technique that gave the least breakdown was a 1:10 pulsed split injection with a 2uL injection, followed very closely in terms of breakdown percentage by a simple 1:10 split injection. Which I think, if one considers the inlet to be excessively active, makes sense because essentially only a fraction of the sample that was injected was actually subjected to the inlet and also directed toward the column. Maybe I'm wrong in this supposition, but none of the other injection types I tried made any substantial difference.

[echae]

Do you guys think the gun brush cleaning would work on an MMI, where the gold seal is not removable? I'm thinking that it would not be a good idea, for the same reason that you would want to push the brush all the way through a gun barrel before pulling it back out. The bristles would be pointing towards the direction that the brush is being pulled, causing more wear/scratches than if pushed through and then pulled back out. Would it cause "significant" damage? Not sure. Thoughts?

[bisnettrj2]

You may want to call Agilent to confirm that this is in fact the recommended solution for your inlet, but I did find this document on their site:

http://www.chem.agilent.com/Library/use ... -90820.PDF