Pressure increase after injecting!

[TOT]

Dear all,
I am using Agilent 1100 with autosampler, my column is Rp8 -150x4.6mm, 5mcm. The pressure increases from 81 bar to 95 bar when needle goes to needle seat; decreases back to 81 bar during run and increases again after each injection. My sample is aqueous with pH 9.5 when my mobile phase is ACN: 25mM acetate buffer pH 4 (80:20); I know that is big difference of pH but I don't have other choices (my sample is after derivatization). Is it the reason for increase of pH? What I have to do to overcome this difference?
Thanks inadvance!

[HW Mueller]

Sounds like a viscosity effect, water should be more viscous than your 80%ACN, apparently your injection amt is enough to show this. However, if the pressure is only high during the injection position it may be that you have a restriction in the injection path. If you know what the cause is the remedy is obvious.