guard column - a small column placed between the injector and the analytical column. Protects the analytical column against contamination by sample particulates and, perhaps, by strongly retained species. The guard column is usually packed with the same material as the analytical column and is often of the same i.d. It is much shorter, costs less, and is usually discarded when it becomes contaminated. Guard column is only useful for preparative separations in any other cases it is just waste of money and decrease of the system sfficiency.
H - Same as HETP.
head pressure - the pressure above gravity at the head of the column. Expressed in psig, bar, atm, or MPa.
height equivalent to a theoretical plate (HETP) - value obtained by dividing the column length by the number of theoretical plates; taken as an indication of column quality. A carryover from distillation theory; a measure of a column's efficiency. For a typical HPLC column well-packed with 5-um particles, HETP (or H) values are usually between 0.01 and 0.03 mm. HETP = L/N, where L is column length, and N is the number of theoretical plates.
See band broadening.