I do have a quite surprising result using an ELSD.
I was under linear gradient conditions, 1ml/min C18, 4.6x25cm; from 75%Methanol(MeOH) to 100, at2.5%/min; then, isocratic at 100% MeOH for 10 minutes, followed by a change of organic solvent (from 100% MeOH to 100% THF, through a 10 min linear gradient). Plateauing with 100% THF for another 10 min, and back to inital conditions (75:25 (v/v) MeOH:Water) in 5 minutes (column equilibrating for another 20 minutes).
I should add that the THF used is inhibitor free (therefore compatible with an ELSD), and came from a brand new bottle.
Here is my problem: When passing from 100% MeOH to 100% THF and back, I noticed two peaks (hardly gaussian) right around the switching time. I am positive those peaks are not part of my sample.
When plateauing for a longer period of time with THF (e.g., 20 min instead of 10), and then switching back to initial conditions, those peaks are moving along toward the end of the new THF plateau.

An ELSD is by principle mobile phase-related-peak free, when the mobile phase is non-volatile free. Would anybody have experienced the same occurrence? Thank you for your inputs.

What are your ELSD conditions and is it possible that you could post a chromatogram?

The ELSD conditions were as follows: Temp. 40C; Gain 6; N2 pressure 2.2 bar.
The temperature set up should be fine (bp. methanol 64.7 C; THF 66.0C). The gain gives a good S/N ratio. The pressure is 'regular'.
I do not have the technological background to post a chromatogram. However, the 'ghost peaks' look like a regular gaussian peak that would have been split in half and spread apart by 1-2 minutes.
Hopefully you see what I mean.

David,
If your solvents are clean,you should be able to use a sensitivity of 8 or 10.Do you see a baseline shift on running the gradient?Are sure of the column condition?What pumping system are you using?

An update:
- Solvents were clean (new bottles).
- Pumping system is Waters 600.
- Yes, we do see a slight baseline shift (when going from methanol to THF, and back) of about 5-10 mV max.
- 'ghost' peaks max. magnitude is about 50 mV.
- Increasing gain from 6 to 9 did not change signal output (we still have those same THF-related 'ghost' peaks, with an intensity of about 10-40 mV)
Any new theories, anybody?

HI David: Any peek tubing in your system?

Hi Bill: Yes we do (and I know why you ask, i.e. air going through PEEK tubing); we have PEEK tubings that run from the solvent reservoirs to the pump, and then a short length just before the ELSD (due to detector config.). We are also using some PEEK fittings here and there.
However, we run our system for about 15 columns at initial conditions for column conditioning, monitoring ELSD and PDA signals(BTW, we are running a PDA before it goes to the ELSD). No air bubble peaks on the PDA, though.

Hi, David!
Excuse, if I occupy your time (from me is not present ELSD, but there is an assumption). Probably, the problem is connected to formation of azeotropic mixture? It can be checked up by establishing at the end of a gradient not 100 % MeOH, but the mix MeOH:THF of such composition, which meets to time output of the ghost peak.

Excuse, if I vainly take away time.

V. Tkach.

Hi David
Check if THF is compatible in all conc.with peek.

Dear David,

There are several types of plunger seals available for the Waters 600 that you are using. The general purpose plunger seals are compatable with THF, the Buffer Seals are not. Is it possible that you have buffer seals installed? If so, that might be the cause of your mystery peaks.

It has been my experience that PEEK is NOT compatible with THF. As I recall, the PEEK is softened or even dissolved.

As it happens I checked with the Upchurch Co. recently about it. They considered PEEK to be resistant to THF, but I got the impression that they meant THF in aqueous solution.

I can't check on this experimentally because we don't keep THF around any more.

Hi Dave,
Did you have a bypass or column? In the former case, what was the column you used?

Hi,
i am trying to analize 1, 10 decanediol by HPLC and C18 column. Does anyone have expirence in this matter?
Thanks,
Eva