Dear colleagues, I need informatios on how to misured Dm the diffusion coefficient of solute in the mobile phase. I need to quantificate the factor for an oligopeptide in a eluent mix ACN/H20/MTBE/H3PO4 85% 445/475/85/1. Do you know the open-tube dynamic flow technique???

thank's.
Alessio Frizzarin, Italy

In order to do the measurement, you need an HPLC pump, injector and detector and a preferentially straight piece of tubing. I would recommend a 1 mm i.d. tubing, with a length of around 10 m. Run the mobile phase and inject the analyte at flow rates ranging from 0.5 mL/min to maybe 5 mL/min. Plot the HETP vs the linear velocity. You should get a straight line. I don't remember the equation out of the top of my head, but it should be something like H = d^2*u/(96*Dm), where H is the HETP, d is the diameter of the tubing, u is the linear velocity and Dm is the diffusion coefficient. Please look the equation up in the literature, since I am not 100% sure about the 96 (I don't have a book here at home to look it up).
Best regards
Uwe Neue