What is the cause of negative peaks?
A negative peak is a peak that dips below an established flat baseline. Negative peaks arise when the refractive index or UV absorbance of the solute is less than that of the mobile phase. When using an RI detector, simply reversing the polarity of the recording device will make the peak show up as a positive signal. The same ‘trick’ is applicable when using a UV detector, although this is rarely needed since most mobile phases have much lower UV absorptivity at the wavelength where the solutes have significant absorption. Negative peaks are most often seen at the beginning of the chromatogram, where differences in the composition of the sample solvent and the mobile phase may give rise to an unretained solvent peak that first shows a positive deflection, but then goes negative before returning to the baseline. Since measurements are best performed when peaks elute after two or three column volumes have passed through the column, i.e., k=1 or 2, such ‘solvent’ peaks are usually ignored
