Spin-echo proton NMR spectroscopy of urine samples. Water suppression via a urea-dependent T2 relaxation process

Spin-echo methods have been used to effect solvent suppression during the collection of 400 and 500 MHz 1H NMR spectra of urine samples containing urea in the concentration range 0.7-1.2 M. There was a marked trough of water Tobs2 values in the pH range 2.6 to 4.1, due to acid-catalyzed proton exchange between urea and the solvent water. Minimum values of Tobs2 (9 ms at 500 MHz) of water were obtained at about pH 3.2 in urine samples containing 1.2 M urea. However, good solvent suppression factors were obtained with endogenous urea concentrations of only 0.7 M. There was a strong field dependence on the water, T2, over the pH range 2 to 4. Attenuation of the water signal was estimated to be at least an order of magnitude greater at 500 MHz rather than at 400 MHz. Using the Carr-Purcell-Meiboom-Gill spin-echo pulse sequence, water signals broader than 13 Hz were attenuated by factors of greater than 10(3), and factors of greater than 10(4) were possible with water linewidths greater than 20 Hz, thus allowing the signals from metabolites in the sample to be efficiently digitized. This approach allowed metabolite signals at or near the water resonance to be observed without serious distortion of their intensities. The utility of this approach to solvent suppression in real systems is illustrated by observation of paracetamol and benzyl penicillin metabolites in human urine samples.