Chromatography Forum

Hey folks,

Above is a chromatogram of a set of glycols at 1000mg/L obtained using on-column injection (1ul). As you can see the chromatography for 1,3-propanediol and 1,3-butanediol is very poor compared to the others. Any ideas as to why ?

Conditions:-
Sample is 50:50 MeOH/water
Using on column with oven tracking enabled
Oven start temp is 80 Deg C, ramp to 200 deg C at 20 Deg C per min.
Constant pressue, 5psi
Column, CP Wax 57CB, 25m x 0.53mm x 0.5um, there is also a pre-column installed, 5m of 0.53mm polar deactivated.

Changing start temp, or inlet temp has no impact on peak shape neither does the presence or absence of methanol.

As always any advice is appreciated. This is actually a revisit of a previous project with a couple of compounds added.

Rich

Hi Rich

Puzzling !

As a starting guess - the 2-affected compounds are co-eluting with water (which you do not see with the FID).

What happens if you dissolve them in dry methanol ?

Peter

Hi Peter,

I see what you're getting at. I've just run a standard prepared in MeOH only with no improvement. I can post a chromatogram if you wish.

So, I guess I'm left with instrument, instrument conditions and the column itself as sources of the problem. There aren't a lot of parameters to change in on-column, the column itself is 2 weeks old and hasn't had many samples put through it. The columns are pricey so at this point I'd rather not buy a replacement, the conditions suit the glycols at the start and end of the run which on the surface implies that the instrument is in good working order.

Rich

I suspect an enol may be forming? in equilibrium. A change in pH might help. Try putting the diols in something like an alkane or dioxane, perhaps toluene and see if that changes the chemistry.

best wishes,

Rod

Any idea what the surface chemistry of the retention gap is ? The only things that the distorted peaks have in common is neighbouring retention and -OH groups separated by one carbon - could be that there is some specific interaction.

Peter

Rod,

I'll check on that first thing in the morning.

Peter,

I don't know the surface chemistry of the retention gap exactly beyond the fact that it's polar deactivated. However, I can say that the peak shape remains the same whether the retention gap is installed or not.

Rich

Try injecting a smaller amount on column, like 0.1-0.2µL.

Let us see what happens then.

Hi Rich

Another thought - you have a fast temperature programme for the column - can the inlet oven tracking keep up with it ? If not you would get a positive temperature ramp from inlet to column which would de-focus the peaks. If the inlet temperature lags temporarily only some of the peaks would be affected. I would expect symmetrical broadening more than tailing, but it might be worth a try with a slower oven programme, or with the inlet heated faster than the oven if your hardware can do that.

Peter

Hey folks.

Above is the best I've gotten so far. I'll go through the suggestions and results below.

I've switched the column from a 6890 on-column system to a 7890 on-column system to confirm that the issue traveled between instruments - it did. All suggestions were tried on the 7890.

Dissolving the glycols in toluene didn't alter peak shape.

Injecting 0.1ul of a more concentrated solution didn't alter peak shape.

The inlet seems to be keeping up with the oven ramp without any issues. However, I did decrease the oven ramp from 20 Deg C / min to 5 deg C / min and the chromatogram above is the result. The two problem peaks are still tailing however the improved resolution between them has made the tailing less of an issue. I'll run a complete calibration set and test samples to see how the tailing affects things at low level.

I've ordered a stabilwax column from Restek and I'll see how that goes next week.

Thanks for the suggestions.

Rich

Water elutes near C10. Toluene elutes near C11 on Wax phases.

Since the analytes seem to improve the tailing if a slower ramp is used, then how about a hold at the beginning of the oven programming and see what happens.

It appears to be a 'solvent effect' at the head of the column which does not allow the 1,3- analytes to focus their plug. (Peter A. made the suggestion which I thought viable too.)

Try putting them into Ether or pentane to minimize the 'solvent effect' ?

best wishes,

Rod

Next guess !

With an inlet pressure of 5 psi, but depending what carrier gas you are using, you will have a very high volume flow rate. This can cause all sorts of funny effects with on-column or solvent effect injections.

Peter

I had exactly the same problem a couple of weeks ago, 1,3 propanediol and 1,2 butanediol were producing sloppy peaks (200mg/l top standard and below) while the others were fine this was using DAI on my FFAP column, I tried everything to sort it. In the end changed the collumn to a new FFAP column and the peaks sharpened up. Closer insepction of the dodgy column by holding up to the light reveled small black spots half way around the column... . I think this is definatley a column problem.

Hey folks,

This was a column problem as beetle said. New column went in on Friday and peak shape immediately sharpened. Thanks for all your input.

Rich