What is the reason behind the negative peak in chromatograms

[mobunvar]

In my method, mobile phase consists of 70% Buffer and 30% MeOH. Drug standards were prepared in Water (deionised). Then Why I am getting negative peak in first 3 minutes though the major portion of mobile phase if Water

[tom jupille]

Without knowing what detector you are using (and if UV, what wavelength) and what buffer you are using, it could be any of several common things:

- With UV detection at short wavelength, deionized water may well have a lower absorbance than your buffer/methanol mixture.

- with UV detection at any wavelength, injection of a different diluent from your mobile phase will generate a refractive index anomaly (UV detectors do respond somewhat to RI).

- with any detector, the equilibrium upset from injection can make the baseline jump around ("t0 noise").

The high probability of t0 noise is one reason the US FDA suggests that k' values should be >2 for any peak that you quantitate.