1,1,1-Trichloroethane contaminant/co-elution

[MMJ88]

Hello all,

I'm analyzing environmental samples for Chloroform, 1,1,1-Trichloroethane, and Carbon Tetrachloride. For each sample 3L of air is pulled through a Carbotrap 300 thermal desorption tube. We have a Gerstel thermal desorption autosampler which goes into a Gerstel CIS inlet into the Agilent 7890 GC, with an Agilent uECD detector.

I have an issue with an unknown peak coming out on top of 1,1,1-TCE. It's something from the sampled air, as it does not show up in any blanks or lab spikes. I am looking for help as to what this could possibly be. According to the Restek application on this column (http://www.restek.com/chromatogram/view/GC_EV1169), it could be dibromofluoromethane? My colleagues tell me it's unlikely that we'd see that in environmental samples. I was hoping for any other suggestions as to what it could possibly be, or really any insight as to what I can do. Here is what the relevant portion of the chromatogram looks like:



Background info:

CIS: Tenax TA packed liner
Trap at 5C, then 12C/sec to 275C, hold 3 min

Oven: 40C for 2 min, 5C/min to 100C, 120C/min to 240C, hold 4 min

PTV inlet:
Mode - Solvent Vent
Pressure - 11.905 psi
Gas saver - off
Purge flow to split vent - 20mL/min at 0.01min
Vent flow - 50 mL/min

Column: Restek Rxi-624sil MS
30m x 250um x 1.4 um

uECD:
240C
Makeup: Ar/CH4 40mL/min

[Bigbear]

The compounds identity is not important. what is important is to resolve the two peaks. If I'm not mistaken you can cold trap with a PTV inlet. This may help, or if you can run your oven sub ambient. With our Gerstel set up we have a cold trap in the oven.
The peaks look a little broad to me. You could also could try increasing your column flow.
What is the concentration of CCL4? You also may want to think of reducing your sample size, or split at the inlet ( if you are, increase the split) .

[MMJ88]

Hi Bigbear, thanks for the reply.

You are right, I suppose. The compound identity is not the most important thing. If I can get it separated I don't have to worry about it.

Regarding your suggestions:

Cold-trapping - I am essentially cold-trapping. I cool the CIS down to 5C. The CIS inlet liner is packed with Tenax TA, which adsorbs the analytes at the lowered temperature (I based this off the tables available on the SIS website when I was thinking this up. It seems to work well in practice too.) I desorb onto the column by ramping the CIS up to 275 quickly. The reason I am doing it like this is when we were using the inlet liner packed with glass beads and cooled down to -150C, we had a lot of problems with the inlet freezing up. My current method gives us the same separation (qualitatively - If I overlay samples done on each method, the peaks are the same distance apart).

Column flow - This could possibly be more optimized. I have it at just a standard 1mL/min.

CCl4 - The peak in the shown chromatogram corresponds to about 1.8 ng.

Split - Unfortunately I can't really split more at the inlet, as low concentration/LOD is an issue.

Thanks for your time.

[Bigbear]

1 ml/min is a little slow for that column. When I was doing volatiles on that size column I was using close to 2 ml/min.
I still think it would work better if you could cold trap the compounds on the column. For grins you could attach the LN2 to the oven cryo attachment and see if it helps.

[AICMM]

Bigbear,

The CCl4 is such a big peak because it is a fantastic responder on the ECD, one of the best.

MMJ88,

You might consider holding your isothermal longer. That might give you a bit better separation. If you have EPC you might consider a little less flow for a bit then program it up but I don't know exactly where you are on the curve so that is an experiment not an answer.

As far as identity, all I can suggest is that you not limit yourself to chlorinated compounds. ECD's will respond to things like like dienes or a sulfur compound or oxygenates so these are all in contention, especially at 3 L collected.

I am curious, what do you estimate the amount on column for the chloroform and 111TCA in that chromatogram?

Best regards,

AICMM

[MMJ88]

Hi AICMM,

My estimate for this sample:
Chloroform - 0.76 ng
1,1,1-TCE - 0.10 ng
Carbon Tet - 1.58 ng

When I have some time, I will try holding isothermal longer. I do have EPC, so that might be worth a shot too.