Chromatography Forum

Is using a mixture of 70/30 MeOH/Water from a singular bottle the same as using two separate mobile phase bottles? One containing water and one containing MeOH, set to 75% MeOH and 25% Water. HPLC-grade of course, with 4g of formic acid. I realize the amounts % are different. I am more asking about the method. I know this seems like a simple question, which is why I am asking. Sometimes the simple things are too simple and I am suspicious. I am a beginner HPLC analyst and have made much progress with help from members in this online community. Thanks!

I recently had a question about it being possible to switch out my current mobile phase set up with a similar set up, but only drawing from one bottle. This is sort of a part B to that question.

We are testing out a different method of using 70/30 MeOH/Water from a singular bottle instead of two separate bottles. We currently run at 75% and 25% but want to do some tests to see if we can maintain efficiency and lower costs.

The column I have on now is a Raptor ARC-18 and is about 3 months used but relatively clean, maintaining pressure and retention times. I have read that it is bad practice to test out a new method on an old/moderately-used column. I have a feeling that what we are trying to do is exactly what that warning is referring to.

We are testing a sample with the current set-up and then will be changing the mobile phase to one bottle of 70/30 MeOH/Water and testing the same sample. Then we will compare results.

Should we replace the column first or can I equilibrate the used column with the new mobile-phase and then just run it as is?

Thanks in advance!

I'd test it on the used column.

Make sure to write down exactly how to make the MeOH/water mixture. Taking X mL MeOH and adding water to volume mark Y will not give the same mixture as doing it the other way around.

W buy our 70/30 pre-mixed. Thank you for your advice. I ran the sample today with the 70/30 mobile phase and it did not go well. Meaning the chromatography was pretty ugly. No worries though as this was just a test! Thanks again!

the chromatography was pretty ugly

The description above is not valid/informative in this case.
Ugly in which way - fused peaks, or broad peaks, or unacceptable retention time............?