![]() The main metal ions present in urine are sodium, potassium, and the divalent ions calcium and magnesium, however it is the divalent ions that are the main contributors (after pH) to peak chemical shift variability (Ackerman et al. However, differences in its other components (such as urea, salts and other ions) are common, and result in often large variations in metabolite peak chemical shifts (Lindon et al. Urine is a popular biofluid used for metabolomic investigations, as it consists of various metabolites that can provide insight into a number of metabolic processes and disease states (Lindon et al. However, metabolite chemical shifts are sensitive to the chemical environment and subtle matrix effects, such as differences in pH and ionic strength, generally lead to inter-sample peak position variation (Weljie et al. These data will be a valuable resource for 1H NMR metabolite profiling experiments and for the development of automated metabolite alignment and identification algorithms for 1H NMR spectra.ġH NMR is widely used for the metabolomic analysis of biofluids, as it provides quantitative, structural information on a wide range of metabolites, in a non-destructive and highly reproducible manner (Nicholson and Wilson 1989 Wishart 2008 Zhang et al. Furthermore, the peak variations induced by the main metal ions present in urine, Na +, K +, Ca 2+ and Mg 2+, were also measured. Peak pH titration curves for a further 65 unidentified peaks were also obtained for future reference. Nonlinear fitting was used to derive acid dissociation constants and acid and base chemical shift limits for peaks from 33 identified metabolites. Urine samples were either treated with either HCl or NaOH, or were supplemented with various concentrations of CaCl 2, MgCl 2, NaCl or KCl, and their 1H NMR spectra were acquired. Urine samples from five different individuals were collected and pooled, and pre-treated with Chelex-100 ion exchange resin. To investigate the acid, base and metal ion dependent 1H NMR chemical shift variations and limits of the main metabolites in a complex biological mixture. The problem is especially acute for urine, which can have a wide range of ionic concentrations between different samples. This frequency shifting results in a correspondence problem: it can be hard to register the same peak as belonging to the same molecule across multiple samples. ![]() These chemical shift changes are mainly due to differences in pH and divalent metal ion concentrations between the samples. Despite the use of buffering agents the 1H NMR spectra of biofluid samples in metabolic profiling investigations typically suffer from extensive peak frequency shifting between spectra.
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