LC PHASES IMPROVE, BUT NOT ALL ASSAYS DO - METFORMIN BIOANALYSIS REVISITED

Two assay methods for the antidiabetic metformin, one developed and va lidated in 1990 and one developed and validated in 1996, are compared. The first method, using an octadecyl phase and an ion pairing agent i n the eluent, could not be reproduced some five years later, but anoth er method, using a phenyl phase and no ion pairing agent, could be suc cessfully applied. This paper shows that the retention mechanism of th e positively charged analyte is not due to ion-pair formation, as orig inally assumed, but to interaction with free silanol groups in the LC phase. It is suggested that the number of free silanol groups in octad ecyl phases was strongly reduced between 1990 and 1996, whereas for ph enyl phases this was not the case. For the second method, validation r esults on linearity, specificity, accuracy, precision, recovery and st ability as well as application of the method to samples from a clinica l trial are shown. The validated calibration range is from 20.0 to 200 0 ng.mL(-1), with accuracy (bias) and precision (coefficient of variat ion) being below 15% at all levels. Using automated solid-phase extrac tion for sample preparation, a sample throughput of typically 100 per day can be achieved.