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

Katherine V Oliver,Peter R Rich,Amandine Maréchal

doi:10.1177/0003702816641263

Katherine V Oliver, Peter R Rich + Show 1 more

Open Access

https://doi.org/10.1177/0003702816641263

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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.

Highlights

There is a rapidly increasing literature on applications of vibrational infrared spectroscopy to analyses of complex biological tissues and fluids in order to identify unique signatures of cell types, diseased tissues and diagnostic markers of specific diseases.[1,2,3,4,5,6,7,8,9,10] The mid-infrared (MIR) spectra of such materials are inevitably complex combinations of many overlapping infrared (IR)-active components
When applying analytical protocols to IR data obtained from dried versus hydrated materials, it is important to take into account the fact that the IR spectra of many materials are different in aqueous solution and the fully dried states
A similar protocol was used to decompose spectra between 1520 and 1470 cm–1 of rehydrated creatinine during drying using combinations of Lorentzian[25,26] and pseudo-Voigt functions. Combinations of these same four urea components, together with an additional component from creatinine at 1492 cm–1, were fitted to the 1510–1410 cm–1 region of spectra of urine at different stages of dehydration, again with baseline offset and slope parameters being allowed to vary during the fitting and with all other parameters fixed according to Tables 2 and 4

Summary

Introduction

There is a rapidly increasing literature on applications of vibrational infrared spectroscopy to analyses of complex biological tissues and fluids in order to identify unique signatures of cell types, diseased tissues and diagnostic markers of specific diseases.[1,2,3,4,5,6,7,8,9,10] The mid-infrared (MIR) spectra of such materials are inevitably complex combinations of many overlapping infrared (IR)-active components. Combinations of these same four urea components, together with an additional component from creatinine at 1492 cm–1, were fitted to the 1510–1410 cm–1 region of spectra of urine at different stages of dehydration, again with baseline offset and slope parameters being allowed to vary during the fitting and with all other parameters fixed according to Tables 2 and 4.

Results

Conclusion