Problems with PCBs standards using ECD-GC

Chromatography Forum: GC Archives: Problems with PCBs standards using ECD-GC
Top of pagePrevious messageNext messageBottom of pageLink to this message  By ecabre on Saturday, May 19, 2001 - 12:01 pm:

Subject: Problems with PCBs standards using ECD-GC

I am very grateful to all the chromatographers who answered my first question (PCB s Determination by GC, date march, 2001). You can¢t imagine how difficult it is to work alone, with little experience, on a very small budget. Discussion Groups can make a big difference to people like me.

Right now, I am facing serious problems. I prepared 20 individual calibration standard solutions of PCB congeners (0,010 mg/ml) using neat (“ACCUSTANDARD”) standards and isooctane (pesticide grade) as solvent. The stocks and dilute solutions were stored in glass amber vials with Teflon lined caps, in a freezer. These solutions were analyzed using two different columns, and obtained, big, well-defined “extra” peaks, in addition to the PCB peak, when using the second column. The conditions and the results of both analyses are the following:

Conditions of the first analysis:

Gas Chromatograph: HP-5890 series II with ECD detector.
Column: HP-608 (35% phenyl methyl polisiloxane) 30m x 0.53 mm x 0.88 mm.
Carrier gas (N2) 7 ml/ml
Makeup gas (N2) 40 ml/min
Injector temperature: 250°C
Detector (ECD) temperature: 300°C
Oven temperature program: 150 to 275°C @ 5°/min. Final temperature 275°C, hold 5 min.
Splitless injection (1ml). Purge valve “time on” ( 1min).

Observations of the first:

In general, we observed just one peak in the chromatograms; but in some cases we got some “extra” very small peaks. With this column, the solvent showed no major impurities.

In order to determine the retention times of the PCBs on a second column (we don¢t have a MSD detector for confirmation) we repeated the same procedure using the following conditions:

Conditions of the second analysis:

Gas Chromatograph: HP-5890 series II with ECD detector.
Column: HP-5 (5% phenyl methyl polisiloxane) 30m x 0.53 mm x 0.88 mm.
Carrier gas (N2) 7 ml/ml
Makeup gas (N2) 30 ml/min
Injector temperature: 250°C
Detector (ECD) temperature: 300°C
Oven temperature program: 140 to 240°C @ 10°/min; 240 to 265°C @ 5°C/min. Final temperature 265°C, hold 18 min.
Splitless injection (1ml). Purge valve “time on” ( 1min).

Observations of the second analysis:

This time we observed big, well resolved, extra peaks, in addition to the PCB peak. Some times, we got broadened peaks or humps in the baseline.

Questions:

· Are the PCBs degrading in the standard solutions? We thought these are very stable compounds.
· Are they decomposing in the hot injector port? We tried lowering the injector temperature to 205°C, and observed no difference. Shall we try cool on column injection or cleaning the liner and the port? (No decomposition products were observed for PCB 8 and 18).
· Is the thick liquid phase bleeding at these high temperatures? The detector signal is stable around 40. We don¢t see extra peaks in the solvent. Increasing the make up flow to 40 ml / min, made no difference.
· Does anyone have experienced similar problems with PCBs standards?


I would really appreciate any kind of suggestion to my problem.

Thank you very much,
Socorro de Cabré ( Venezuela, Universidad Centroccidental)


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Monday, May 21, 2001 - 07:40 am:

Are your PCB peaks the same size (area) on both columns, or are they significantly larger on the second?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By craig on Monday, May 21, 2001 - 08:49 am:

PCB congeners are very stable and it is unlikely they are breaking down in solution or in the injection port. I would definately try cleaning your injection port, new liner, new septum, etc. Also, the final temperature of your second run is low, the HP-5 should be stable to 300 C, try baking the column for an hour or two.
Were the extra peaks on the second column at the same RT? Did you re-inject the standard from the first analysis, or did you take new aliquots from the stock solutions?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Monday, May 21, 2001 - 09:43 am:

If you re-injected from the same vials as the first set of injections, the extra peaks could be siloxanes leached from a pierced septums. It is not uncommon to see peaks like this after multiple injections from the same vial. I agree with Craig, the HP-5 column is thermally more stable than the HP-608, so I would condition at a higher temperature and see if things improve.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Wednesday, May 23, 2001 - 02:20 pm:

Preparation of pcb congeners at such a high concentration (ug/mL) will contribute to extra peaks. The extra peaks may be real PCB impurities, try to dilute the stock down to approx 20ng/mL. To eliminate other artifacts, run the standard with out a vial septum.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By LABMANAGER on Friday, December 21, 2001 - 12:48 pm:

The "extra" peaks could be phthalates. If you are using any type of plastic transfer pipets, or if the isooctane is coming into contact with anything plastic during the dilution process, this could very well be your problem. Phthalates leach from plastics and rubber products when they come in contact with a solvent.


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