the invention relates to methods for investigating mixtures of substances by capillary electrophoretic separation , in particular of biopolymers such as proteins , proteoglycanes or other protein conjugates , or their digestion peptides . the invention comprises the analysis of specific , well - separated substances of analytical interest found in complex mixtures by subjecting the mixture of substances together to a selective derivatization process using charge - carrying groups prior to the electrophoretic separation , so that the substances of interest can be detected , essentially alone , within a specified time window .