Adsorption of creatinine to Fuller's earth

In search for a secondary reference method for the determination of creatinine, the adsorption of creatinine to seven different commercial batches of Fuller's earth have been investigated. Mineralogical investigation of the batches broadly divided them into two subgroups with quite different cation exchange capacities (CEC): dominant smectitic with and without calcium carbonate and dominant palygorskitic with calcium carbonate. Adsorption experiments using 14C-creatinine show, for all Fuller's earth samples, an incomplete adsorption of creatinine. The amount of creatinine adsorbed depends on pH, temperature, concentration of cations as Ca2+ and Na+, and other conditions. Weakly acidic suspensions of Fuller's earth in the course of time change their ability to adsorb creatinine. Compared to a synthetic strong acidic cation exchanger, Fuller's earth is more selective to creatinine. Because of the sensitivity of the adsorption to the various experimental conditions a determination of creatinine in biological fluids using Fuller's earth is too variable. Therefore, we cannot recommend this procedure as a secondary reference method for the determination of creatinine.