Peak shape and resolution was not good when I used acetonitrile on C18 column in method development of my compound. But using methanol as mobile phase gave good peak shape. What would be the reason?
separation is based on differential partitioning between the mobile and stationary phases. If you change the polarity of the mobile phase --> change the elution strength and selectivity. MeOH will not always be better than ACN. Depends on analytes, column, gradient, etc.
I guess the better peak shapes are due to "better" more rapid elution of analyte(s) from the column leading to less tailing/fronting...
Possibly improved solubility and interaction with the solvent? Methanol is a little more polar. Possibly an interaction with the hydroxy group on the methanol?
Just guesses
I don't know the exact answer for your question, but look at the article below for better understanding about the mobile phase.
http://www.chromatographyonline.com/lcgc/Articles/Reversed-Phase-HPLC-Mobile-Phase-Chemistry-mdash-E/ArticleStandard/Article/detail/859585
Methanol minimizes interactions between compounds and the silica part of the column, even in C18 column. This is due to chemical properties of this organic solvent. Compound tailing is due to interactions between silanols free bonds of the column and the compound itself. Methanol probably masks these free silica bonds instead of the compound and preventing the tailing of the peak.
it is depends on many factor like,,,, column, analytes, chemical properties of analytes, partition coefficient of your solvents..... so it is depends.
Another possible contributor is the solvent composition that's being injected vs the initial composition of the HPLC gradient. In general, the best results are when the injection-solvent composition matches the initial gradient composition. So, if the gradient starts at -say - 10% acetonitrile and 90% water, then your sample should be in a solvent of that composition.
MeOH is not good for the C18 columns. A mixture of H2O/ACN is a good choice but you have to test first the solubility of your compound in this mixture and determine the good ratio for your separation. I also authomatically add a 0.1% of formic acid to the mobile phase because it gives peaks with good shapes and protect your Equipment form algae.
I think the question he wanted to know is why the methanol works better than acetonitrile for his particular compounds. Not which solvent to use and how. Without knowing more informaiton about the compound, nobody can predict why it works that way.
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