Chromatography Forum

Hi all. I am doing HPLC/UV using a Thermo Accella autosampler, Acella Pump, and Thermo PDA. I get a peak/blob thing at the beginning of every trace about a minute or so away from the injection depending on flow rate for any sample. I want to know if this is normal or if I am doing something wrong, and if so, how do I fix it. Thanks!

Search this forum for column volume calculations and look up your system's void volume. If you can't find it, search again... I'm betting that this blob you speak of is merely a normal void volume baseline disturbance. It is the natural result of having made an injection of something that is not 100% a match for your mobile phase. The result is, if nothing else, a momentary change in the refractive index as the "slug" of solution that was in your sample loop makes it to the detector cell. This is nothing to be alarmed about. If you want to be really thorough in your explanation of it, you can also note that the length of the disturbance is a function of your injection volume and linear velocity of mobile phase ± a diffusion term.