By Anonymous on Tuesday, January 23, 2001 - 10:55 am:
I have noticed retention time shift in my method when using the mobile phase composition of 60:20:20, Water:Acetonitrile:Methanol and C18 column. The peaks elutes faster if I do not filter and/or degass the Mobile phase under vaccum. Why it shows significant change of retention time shift?
By Anonymous on Tuesday, January 23, 2001 - 04:27 pm:
You evacuate the mobile phase for too long, loosing some of the organic solvents. The end results is that you have a higher water content in your mobile phase and therefore higher retention.
By chromgod on Wednesday, January 24, 2001 - 08:03 am:
Also, check your pump flow rate and pressure. What type of C18? Retention most generally is affected by mobile phase composition and flow rate. Look for your problem there.
By Anonymous on Wednesday, January 24, 2001 - 08:23 am:
Since this is an isocratic experiment, I would premix the solvents and run out of one bottle. This would take the proportioning (done by the pump) out of the question. As previously stated, watch the pressure.
By Uwe Neue on Thursday, January 25, 2001 - 03:56 pm:
Premixing of mobile phase is nearly guaranteed to give you different results than what you get from the system. Veronika Meyer has a nice page on this in her book "Pitfalls and Errors of HPLC in Pictures". I took her material and used it in one of my troubleshooting columns in American Lab.
I agree with the first "Anonymous". I think solvent evaporation explains this easily.
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