Determination of melamine and its conjugates in calcium oxalate stones by Fourier transform infrared spectroscopy

Objective To establish a method of two-times second derivative Fourier transform infrared spectroscopy （FTIR） for identifying melamine （MEL） and its conjugates with cyanuric acid （MEL-CYA） or with uric acid （MEL-UA） in human urinary calculi.

Methods MEL， MEL-CYA and MEL-UA were added to calcium oxalate stone samples， and then analyzed by two-times second derivative FTIR for identifying the characteristic response bands of MEL and its conjugates in stones as well as confirming the ratio of detection.

Results The second derivative FTIR could improve the detection of MEL in CaO_x stones by two orders of magnitude. When CaO_x being mixed with MEL （MEL∶ CaO_x）， the two-times second derivative FTIR showed the characteristic peak at 1 548 cm^-1 and the minimum mass ratio of detection at1∶100 （1%）. When CaO_x being mixed with MEL-CYA complex （MEL-CYA∶ CaO_x）， the two-times second derivative FTIR showed the characteristic peak on 1 740 cm^-1 and 1 538 cm^-1 and the minimum mass ratio of detection at 1∶500 （0.2%）. When CaO_x being mixed with MEL-UA complex （MEL-UA∶ CaO_x）， the two-times second derivative FTIR showed the characteristic peak at 1 117， 982 and 710 cm^-1 and the minimum mass ratio of detection at 1∶250 （0.4%）.

Conclusion Compare to the original spectra， the two-times second derivative FTIR can improve the detection ratio MEL in the CaO_x stones from 0.2% to 1.0%. The second derivative FTIR has the unique characteristic bands and sensitive detection limit for identifying MEL in kidney stones.