Different retention time of a standard solution

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Good morning,

Two months ago in UPLC system i found retention time of masclinic acid at
1,9 min and oleanolic acid at 2,9 min, which is in accordance with the
polarity of the standards. Two days before under the same conditions i
found retention time of masclinic acid at 3,1 min and oleanolic acid at 2,9
min..Is there any explanation about that?? Thank you in advance!
What is the pH of your MP and what is the pKa of your wandering analyte?
If they are too close, this can happen.
Thanks,
DR
Image
The pKa of maslinic acid is ~4.81.
Good morning,

The pH value of the mobile phase is 1,9...Is that possible?
I am wondering because the system is under the same conditions in both cases..
Thank you!
Hi Aggeliki,

Your mobile phase can certainly be that low, however, some columns will have shortened life-spans at low pH's and it may end-up damaging them in the future. Of course, I am not the expert on your columns and this may be completely fine!

That leads me to my next question, what column are you using and what are your mobile phase additives?

Mind you, if your mobile phase was actually at a pH of 1.9, your analytes would have been fully protonated and eluted correctly on both runs. Interestingly, your second analyte does not change retention which makes me believe that there was some sort of change in pH to allow for this compound to elute later. Is there any possibility that the standard analyte is sensitive to anything or was potentially contaminated? Some standard analytes can decompose under particular conditions like exposure to UV light or spending too much time in water.

Tyler
Hi Tyler and thank you for your help..

The column I am using is C18
and the mobile phase is
A: Water plus 0.2% phosphoric acid and
B: Methanol

The standard solution was prepared and stored in methanol.

Because I am using different methods and mobile phases I am not sure if the pH value is 1.9 or around 3 I will check..
Is the stationary phase a "simple" C18 or a C18 phase with some polar groups? What samples do you analyse? What is the concentration of MeOH in the MP? Do you clean the column with a strong eluent (up to 100 % MeOH or MeCN) after the analyses? Are the peaks symmetrical? Does the reduction of the injection volume influence the peak shapes and/or retention times?
It is a simple C18, we analyse samples from olives but the difference in the retention time is about masclinic acid as a standard solution. The concentration of MeOH is 92% and I clean the system with MeOH 100%
Aggeliki Kaloge wrote:
A: Water plus 0.2% phosphoric acid and
B: Methanol
...The concentration of MeOH is 92%

Try to use 0.2 % H3PO4 in MeOH instead of pure MeOH for B.
Thank you! I will try it
I would bet a chocolate bar on this being a problem of either identification or standards, and nothing to do with chromatography. Either one of these peaks in the past wasn't what you thought, or one of the standards isn't what you think now. These compound are so totally similar, differing only in a hydroxyl group, that any change to the column or solvents should affect both of them in a similar way. If they've swapped the order in which they elute, then either they weren't what you thought when you ran them before, or they aren't what you think when you're running them now... (sorry!)
Did injection volume change?

Sometimes when injecting high concentration methanol into a MP that is mostly water, increasing injection volume can cause an analyte to move through the column with the methanol solvent front and elute faster.
The past is there to guide us into the future, not to dwell in.
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