A cardioplegic solution protects the ischemic myocardium from cell death. It is applied together with hypothermia e.g. in open heart surgery. Here the simultaneous determination of aspartic acid, glutamic acid, tris(aminomethyl)aminomethane (TRIS), sodium and potassium in such a solution is given. The two amino acids can be determined as they are partially in the triple protonated ammonium form at the eluent pH. Determination is achieved by direct conductivity detection.