I have an analyte which absorbs at 196 nm on PDA detector. So is it possible to shift its absorption between 200-300 nm by combination of mobile phase, buffer or modifier? how ?
Can you share some more information about the nature of compound?
its Gymenmagenin, reduced form of oleanane type triterpenoid saponins, gymnemic acids
Have you already tried to change the pH of the analyte solution?
Sometimes changing the pH solvent of the solution is sufficient to promote a different arrangement of the non-bonding electrons of the auxochromes and therefore change the absorption wavelength.
I tried at acidic PH using TFA but it didn't worked. Moreover, I tried methanol: water but UV cut off of methanol is 205 and for water is < 190, may be due to both it shows maxima at 196 nm
In my opinion, to change the UV absorbtion means to change the electronic arrangement of the molecule, to achieve such a big shift of absorbtion band in UV requests derivatization, the solvent itself cannot help.
Have you tried using indirect detection? Perhaps you could track the analyte by measuring the decrease in absorption of a UV active modifier in the mobile phase, as the analyte moves through the detection cell. Of course it would need to be optimised to have no interaction with the separation mechanism.
I tracked analyte at 210 nm by using phosphate buffer along with orthophosphoric acid but i observed large upward baseline drift !! Column is operating in column oven so temperature is not the cause of baseline drift. Moreover, ACN has low absorbance at lower wavelength despite large upward baseline drift is observed !!!!!
try this to see if you can derivatize R-OH group to increase UV absorbance or use indirect UV method.
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