The 100 mm version of the Metrosep C 4 column is intended for rapid determinations of standard cations. Very short retention times are achieved; however the elution times of sodium and ammonium still differ by 25 s. When a special eluent is used, the six cations lithium, ammonium, sodium, calcium, magnesium and potassium can be determined in less than 5 minutes with the Metrosep C 4 - 100/4.0.