unretained compound - analyte for which retention volume is close to the column dead volume (within the peak width).

See dead volume.

UV detector - the most popular detector for HPLC, a UV spectrophotometer (for variable-wavelength detection) or photometer (for single-wavelength detection) equipped with a low- volume flow through "curvette" , commonly referred to as a flow cell. Detects analytes which readily absorb light at the selected wavelength.

See UV detector.

van Deemter equation - theoretical relationship which describes sample band broadening as a function of three phenomena; eddy diffusion, molecular diffusion, and mass transfer.

See band broadening theory.

void - the formation of a space, usually at the head of the colurnn, caused by a settling or dissolution of the packing. A void in the column leads to decreased efficiency and loss of resolution. Even a small void can be disastrous for small microparticlate columns. The void can sometimes be removed by filing it with glass beads or porous packing.

void volume (Vo) - the total volume of mobile phase between the injector and detector flow cell. It includes all extracolumn volumes. Can be determined by injecting an unretained substance that measures dead volume plus extracolumn volume. Vi or Vm are sometimes used as a symbols.