Effects of the Hydration State on the Mid-Infrared Spectra of Urea and Creatinine in Relation to Urine Analyses

Abstract

When analyzing solutes by Fourier transform infrared (FT-IR) spectroscopy in attenuated total reflection (ATR) mode, drying of samples onto the ATR crystal surface can greatly increase solute band intensities and, therefore, aid detection of minor components. However, analysis of such spectra is complicated by the existence of alternative partial hydration states of some substances that can significantly alter their infrared signatures. This is illustrated here with urea, which is a dominant component of urine. The effects of hydration state on its infrared spectrum were investigated both by incubation in atmospheres of fixed relative humidities and by recording serial spectra during the drying process. Significant changes of absorption band positions and shapes were observed. Decomposition of the CN antisymmetric stretching (νas) band in all states was possible with four components whose relative intensities varied with hydration state. These correspond to the solution (1468 cm–1) and dry (1464 cm–1) states and two intermediate (1454 cm–1 and 1443 cm–1) forms that arise from specific urea–water and/or urea–urea interactions. Such intermediate forms of other compounds can also be formed, as demonstrated here with creatinine. Recognition of these states and their accommodation in analyses of materials such as dried urine allows more precise decomposition of spectra so that weaker bands of diagnostic interest can be more accurately defined.

Soft and Robust Identification of Body Fluid Using Fourier Transform Infrared Spectroscopy and Chemometric Strategies for Forensic Analysis

“…1 . The average spectrum of each BF type showed a characteristic pattern, corresponding to previous reports 8 , 9 , 27 – 30 . Dried blood is abundant in hemoglobin from erythrocytes and human serum albumin in plasma.…”

“…Meanwhile, the average spectra of urine and sweat indicate little contribution from proteins. Urea is a major component in urine, and distinctive peaks for urea at 1592 cm −1 (C=O stretching), 1457 cm −1 (C-N stretching) and 1154 cm −1 (NH 2 deformation) are found in the average spectrum of urine 27 , 35 . Small contributions from creatinine, another typical component of urine, were found as weak peaks at 1334 cm −1 and 1237 cm −1 36 .…”

Pharmaceutical Hydrates Analysis—Overview of Methods and Recent Advances

“…Moreover, especially in the FT-IR transmission technique, because of the longer preparation process, a risk is posed by the environmental humidity, which could alter the analysis [ 59 ]. This is why, for the highly absorbing probes, ATR-IR (attenuated total reflectance) is used as it requires no sample preparation and is also a non-destructive method [ 60 ].…”

Infrared vibrational spectroscopy: a rapid and novel diagnostic and monitoring tool for cystinuria

“…Creatinine can be determined from IR spectra of whole urine with various deconvolution methods 16 17 18 19 35 . In the present study urea and creatinine were quantitated using the 1510–1445 cm −1 region of ATR-FTIR absorbance spectra of ‘as-collected’, undried urine since this region is dominated by urea and creatinine contributions 35 . For comparison, urinary creatinine was also assayed with the Jaffe reaction 36 , a protocol used in standard clinical practice.…”