Estimation of the weddellite to whewellite ratio by infrared spectroscopy

Powder samples of 56 calcium oxalate stones the contents of weddellite, whewellite and apatite of which had been determined by thermogravimetry (TG) were studied by infrared spectroscopy (IR). When the ratio of the weddellite content to the total of weddellite and whewellite (weddellite ratio) calculated using Oka's method on the infrared spectra was compared with that determined previously by TG, the correlation coefficient between these ratios was 0.734. For the 24 calcium oxalate stones the apatite content of which was less than 10%, the correlation coefficient between these ratios was 0.976, although the weddellite ratio calculated using Oka's method was significantly higher than that determined by TG (P less than 0.01). In an attempt to estimate the weddellite to whewellite ratio by IR regardless of the content of apatite, the ratio of the depth of the absorption band at 780 cm-1 (H780) to the depth of the absorption band at 1,320 cm-1 (H1320) was compared with the ratio of the whewellite content to the total of whewellite and weddellite contents determined by TG. The correlation coefficient between these ratios was 0.953. We conclude that the ratio of H780 to H1320 determined from the infrared spectra obtained from a 0.3 to 1 mg powder sample of calcium oxalate stone is useful in the estimation of the weddellite to whewellite ratio.