negative peak at blank

[tubish]

The method that I am using for impurities of injectable product determination is UV HPLC system.We have analyzed and negative peak obtained from the injection of blank. Blank is water for injection. The mean peak comes at the end of negative peak. The blank can not be changed because it was manufactured injectable product wiht water. We have to injection of product directly. How can we separate these peaks? (Negative peak and mean peak)
Mobile phase: 0.2 M potassium dihydrogen phospate : 0.2 M 1-heptane sulfonic acid sodium salt (1:1) pH :2.0
Column: THERMO HYPERSIL BDS C-18 250mmx4,6mm 5µm


Thanks in advance

[tom jupille]

A bit more detail would be useful:
what is your flow rate?
what is the retention time of your peak

The reason for asking is that if your peak is weakly retained, the negative dip may be t0 noise and your problem is simply lack of retention. As a good rule of thumb, retention time should be at least 3x t0.

If your peak is well retained, then the negative dip is probably a system peak generated by the equilibrium disturbance from injection. You could try changing the potassium concentration in your mobile phase (with no organic solvent in the mobile phase, it looks like you are doing essentially cation exchange chromatography) to move your product away from the system peak. Another approach would be to add potassium phosphate and heptanesulfonate to your sample to match the mobile phase.

[LCbob]

Hi

To follow on what Tom has said, a negative peak in a water blank at t0 tends to tell us the mobile phase is not entirely transparent in the UV, therefore as the slug of water enters the detector, the signal drops.