Fig. 3From: Classification of gallstones using Fourier-transform infrared spectroscopy and photographyChemical structure of calcium bilirubinate and Fourier-transform infrared (FT-IR) spectrum of a calcium bilirubinate gallstone. The spectrum has a characteristic triplet absorption peak centered at around 1624 cm− 1 and a pyrrole N-H absorption peak at 3398 cm− 1Back to article page