Chromatography Forum

What would be happening if you have one peak shifting in RT with the same m.p. and column on two different systems?

All the other three peaks' RT remain the same.

1. The column is not equilibrated.
2. The column is changing.

More information needed: what is the column, what is the mobile phase, what are your analytes....

Are you running a gradient or isocratic method?

Which peak in the elution order is moving?

Do the systems have roughly equivalent dead volumes?

If isocratic, is your MP premixed or being mixed with a gradient controller?

I can forsee this happening pretty easily with systems that have broadly differing dead volumes, especially if you're running a gradient with a delay and the three earliest peaks elute within that delay (in the isocratic portion of the chromatogram).

If you're using gradient controllers, are you sure that they're equivalent?

Any change in peak shape in the early peaks?

The system has capabilities of both isocratic and gradient.

The method is isocratic.

Flow Rate: 1.0 mL/minute
Run Time: 35 minutes
Column Temperature: Ambient
Wavelength: 254 nm
Mobile Phase: 50:30:20 Buffer:Acetonitrile:Methanol (v/v/v)
Needle Wash: 50% acetonitrile / 50% purified water (v/v)

The mobile phase is premixed.

Only peak shifting is the last RT peak.

Phenothiazine is the analyte.

The RT change has revert back to the RT when first ran. Therefore, it does not appear that it is not the column changing. We are leaning to equipment.

I would also look at your buffer and buffer capacity.

1. Make sure the buffer you are using is within +/- 1 of the pKa of the buffer.
2. You may also need to increase buffer concentration

Hi srdales

If the change was to a longer Rt, it might have been a pressure drop, you were dealing with.
If your data system has the capability, try tracking the pressure during the run.
Also, did you experience the problem just ones, or did you re-inject with the same result?

Best Regards

It appears to repeat on the same system

Can you share what could cause a pressure drop in the mid of injection?

Hi srdales

I had this idea that the pressure dropped sometime before the last peak and caused a longer Rt.
An air bubble in the pump-head could’ve easily caused that, but it wouldn’t have been repeatable. As I understand you re-injected and saw the same retention offset, so it is obvious that the notion wasn’t applicable.

Best Regards

Is the shift gradual or abrupt?

For a brief discussion of why I'm asking look here:
http://www.lcresources.com/resources/TSWiz/hs390.htm