Determination of calcium oxalate (mono- and dihydrate) in mixtures with magnesium ammonium phosphate or uric acid: the use of simultaneous thermal analysis in urinary calculi

Abstract

Background: The human urinary calculi are mainly formed from calcium oxalate, magnesium ammonium phosphate hexahydrate (MAPH) and uric acid (UA) crystals. It is important for the physician and surgeon to distinguish the two types of hydrates of calcium oxalate, mainly mono- (COM, Whewellite) and dihydrate (COD, Weddelite). This is because COM stones are evacuated two times more frequently than COD, which sometimes needs to be extracted surgically. Methods: Thermal analysis techniques, such as Differential Thermal Analysis (DTA) and Thermogravimetry (TG), are considered the best techniques for the characterization of the two hydrates of calcium oxalate. A simultaneous thermal analysis method (TG–DTA) was applied to determine the percentage of each hydrate (COM and COD) when present together, in the presence of MAPH or in the presence of UA. Results: The results performed on 156 urinary calculi were as follows (according to the major component): 74.36%, 7.05% and 10.26% for calcium oxalate hydrate, UA and MAPH, respectively. The COD/COM ratios were 0.75 and 1.75, when they are present in mixtures with UA and MAPH, respectively. Conclusions: Fourier transform infrared (FTIR) and X-ray diffraction (XRD) have been used in urinary calculi analysis. Simultaneous thermal analysis TG–DTA would be an alternative method for quantitative determination of the two types of calcium oxalate in urinary stones.