Chromatography Forum

This may be a silly question, but when using an isocratic method is it possible to get peaks caused by the mobile phase and/or sample solvent itself?

I'm using MeOH as the mobile phase and sample solvent right now and keep seeing a peak exactly at the UV cutoff of MeOH. I just want to know if it could be the mobile phase or does it have to be something eluting along with the solvent. Thanks.

Unless you're running very large injections of a realtively weak solvent, you should not be seeing peaks in an isocratic mode -at least not with any consistency.

What I really don't understand is that the peaks always elute at the same time (2.9 min) in my isocratic MeOH mobile phase. Even when I am only only injecting solvent blanks. Whatever it is, it is very consistent. And there's lots of it (area is >2,500,000).

Can anyone please offer some suggestions as to what may be causing these peaks.

In general, you have the possibility of seeing one "system peak" for every component in the mobile phase if you disturb the equilibrium of the system during the injection process. Most of these are invisible (transparent, unretained or very strongly retained), but if you're working down near your UV cutoff, you will be more susceptible to the problem.

The major (but not the only possible) cause of equilibrium disturbance is a difference in composition between the diluent and the mobile phase. Since you're dissolving in the mobile phase, that's obviously not the problem in this case. Other possibilities include the pressure pulse from the injection and dissolved air.

My first guess in this case would be dissolved air. Check out this reference:
LC-GC; 16(2) 112 (1998)

I think LC-GC magazine's on-line articles only go back to 2000, so I've stuck a copy here: http://www.lcresources.com/resources/TS ... 021998.pdf

I'm not sure I understand what you mean when you say you see a peak at the UV curoff of methanol. What wavelength are you using?

You should not see any peaks from the mobile phase using isocratic conditions but there is often a peak from the sample solvent that occurs at or near the column void volume.

Are you using 100% methanol for both the mobile phase and the injection solvent and are they from the same bottle?

What happens if you cycle the injector without loading the sample loop with your injection solvent?

Is it possible that your injector is dirty and that each time you inject the methanol, you are getting some unretained contamination/

Another question - what are your column's dimensions & flow rate? (maybe 2.9' corresponds to the system's void volume)