fronting or splitting peaks

[mattman]

I have a couple of early eluting peaks which have small shoulder peaks which elute just before the main part of the peak. I know the smaller peak is the same as the larger peak. almost like a peak on a peak. It only occurs with the early eluters as the late eluters are nice and gaussian. I pretty sure it is related to the injection.

Some things I've tried:
Using an air plug made no significant difference and modifing the solvent plug volume also did little. Slowing and speeding up the injection also did little. The injection is splitless for 0.15 minutes. I can't lower the GC temperature as the run starts at 35C. The extract solvent is MTBE. Iso-octane is used for the solvent plug. Any ideas or suggestions?

[chromatographer1]

Lower the injection temperature, you are splitting the injection plug by backflashing would be my guess.

best wishes,

Rod

[AICMM]

Mattman,

Just a hunch here but I would take out the isooctane. Just use the MTBE.

Best regards.

[Peter Apps]

Most likely this is some kind of solvent effect. Using MTBE as the solvent plug as AICMM suggests can only do good as a first step. For splitless injections you need a wide bore inlet liner.

What substances are giving split peaks, and which ones give symmetrical peaks ? - it could be that there is a solute - solvent mismatch.

What is the column stationary phase - the iso-ocatne might cause problems on a polar phase.

What are your sample and solvent plug volumes, and the i.d. of the column ? - the bigger the volume and the narrower the column the more likely you are to get solvent effects.

Paradoxically you might solve the problem by increasing the start temperature to prevent solvent recondensing on the column.

Peter

[AICMM]

Mattman,

One thing Peter brings to mind and of which I am often reminded by one of my good friends is "wettability". That is, is your solvent actually dissolving in the stationary phase to give you a focusing effect or is it simply beading up on the column. Put more simply, what stationary phase are you using as some are more "wettable" than others.

Best regards.