Chromatography Forum

Jumpshooter,

The prep you describe above should indeed give you a big peak but it is also different than what you first described (10% versus 1% and no 100:1 dilution.) Having said all that, I would suggest you try the same mix with a different solvent (like hexane or DCM) since water is a nasty solvent to have to work in. It would be interesting to see if this helps the flash of the compounds you are interested in. Also, wool or no wool?

Best regards.

It may be worth while to run a sample of comercially prepared Grob mix through the column. The loss of phenols could be the result of activity on the column - or losses to glassware before that point. The aniline and nitrophenol in the mixture are intended to detect activity effects that would degrade results for acidic commpounds, like chlorophenols. While the concentration of your analytes may be high for this effect, it would be nice to be able to say that the problem is in the GC or not in the GC with a test of the GC with a mixture that gives results that have been shown elsewhere.

Hi Jumpshooter

We now have three different versions of your preparation of the solutions, and a range of amounts on the column spanning three orders of magnitude. Can you clarify for us whether this is three different preparations, or are you just re-doing the calculations. ?

5 ng of a chlorophenol on a 5% phenyl column should give a decent sized peak. I would expect a bit of tailing of the trailing edge of thepeak on anything except a very well deactivated system.

50 ng will give a front-tailed peak due to concentration overloading (by front tailing I mean that the peak goes up slowly and comes down quickly).

5 ug will give a grossly overloaded peak that I would expect to be a few minutes wide at baseline.

The detector has no effect on whether the column is overloaded (although the size of the peak is affected by the detector its shape is not) so if you are not seeing hugely distorted peaks with your latest version of the dilution you are not putting anywhere near as much on the column as you think you are. There are a number of possible reasons for this.

Peter

Notwithstanding a review of the calculations, a re-consideration of using Grob's Test Mix, and column phase effects, I am reviewing my entire experimental approach.
I found all the posted comments very helpful and will report back when I have made decent progress in the GC Lab. It's always amusing to experience the large Gulf between "theory" and "emprircal results".