Chromatography Forum

Dear fellows,

I'm currently working on the validation of a GC-ECD method for the determination of THMs in drinking water. The preconcentration is through LLE and the seperation of the compounds takes place on a 60m long capillary column. The peaks have a very unusual appearance: each peak starts off normally and sharply, reaches its apex and then drops extremely slowly. Near the end of the peak there's a sharp drop to the baseline creating something that looks like a step. Do you think the slow return to the baseline is due to the length of the column? Has anyone else observed that final drop? Since the peak is extremely assymetric (almost impossible to decide where it stops) would you recommend using the peak height instead of its area as the response?

Thanks in advance for any valuable comment.

What you are describing is not normal. THMs should give nice sharp peaks, even on a 60-meter column. You will have to provide us with much more information about your analytical GC conditions before we can offer suggestions.

Check to be sure that your inlet liner is intact and has an o-ring. I assume that this is a splitless injection?

Please take a screenshot of the chromatogram and post it.

The problem seems to come from the inlet or the column installation I think.

Please post details on your splitless conditions. Perhaps the sudden drop is related to the time where the purge is turned on. Does the solvent peak also have the same shape? Are all the peaks the same shape, or are the early eluters better or worse?