Chromatography Forum

Can anyone tell me why do we get a void volume peak or a peak with negative absorbance at the start of each run in hplc?

Usually this is because the matrix you introduce the sample in has a lower response than the eluent being run. This slug of liquid does not interact enough with the mobile phase to concentrate to the same consistancy.

It comes from pressure impuls when you do the injection and you can see what ever compound is not retarded to the packing material. Pressure impuls when doing gradient elution can cause a slight change in the composition of your MP in the detector cell and can change the RI for a short moment. thant can give you a negative "Peak".

Also, difference in the refractive index between air and/or the sample solvent and the mobile phase you're using may contribute to a "void peak" that points in the opposite direction of your peaks of interest.

Thanks