Chromatography Forum

Hi everyone! I was just wondering if anyone has ever had problems with their Niacin peaks on HPLC coming out as hills instead of nice sharp peaks? Or if not, any advice on how to correct this? Here are my specs: mobile phase is 10% methanol/90% water/1.7g tetrabutylammonium dihydrogen phosphate(per 100mL). Flow rate is 1.2mL/min and we are using a c18 column from Thermo. The method we are using is JAOAC vol 70, #4, 1987. Thanks! ~M

In general, "hills" instead of sharp peaks is the result of lots of diffusion. This can take place between the column and detector, if a piece of tubing is not properly installed (resulting in a gap) or it can be the result of "channeling" in a column (decay of stationary phase allowing gaps or channels to form in it, diffusion results).
Changes in peak shape can also be the result of "dirt" on the column (dirt being defined as undissolved particulates small enough to pass through a syringe filter or salt if the aqueous phase of your mobile phase has too much dissolved solids for the highest organic concentration of your gradient to maintain in a dissolved state, or crud in the tert... salt that was not filtered out).
Doing a lengthy low to high water/organic flush with the column reversed (sometimes this helps, but should not be done w/ columns whose inlet frits are sized very differently from outlet frits) may help restore desired peak shape.

Thanks! Ill be sure to try it!