Nuclear magnetic resonance using electronic referencing: method validation and evaluation of the measurement uncertainties for the quantification of benzoic acid in orange juice.

Quantitative nuclear magnetic resonance measurements have become more popular over the last decade. The introduction of new methods and experimental parameters has been of fundamental importance in the development of new applications. Amongst these new developments is the introduction of electronic referencing for quantifications. The use of electronic referencing eliminates errors in the analyses as a result of weighting of internal standards as well as undesired problems as a result of the solubility of the standards in the analyte solution and chemical interactions between the analyte and the internal standard. In this work, we have studied the quantification of a very important analyte in a food matrix, benzoic acid in orange juice, as a model to the validation and measurement uncertainty estimation of electronic referencing using (1)H NMR in food analyses. The referencing method applied was the pulse length-based concentration measurement. Method was validated and showed good results for the precision and accuracy parameters evaluated. A certified reference material and a reference material candidate were analyzed, and extremely good results were obtained. Reported relative expanded uncertainties are in the 1.07-1.39% range that can be considered an extremely good performance for the analysis of a food complex matrix. Measurement uncertainty was evaluated by two different approaches, and the pulse calibrations for the samples and for the reference have been shown to account for approximately 80% of the total uncertainty of the measurement.

Aqua-(4,4'-bipyridine-κN)bis-(1,4-dioxo-1,4-dihydronaphthalen-2-olato-κO,O)zinc 4,4'-bipyridine mono-solvate dihydrate.

The reaction of 2-hy-droxy-1,4-naphtho-quinone and 4,4'-bipyridine with zinc acetate produced the title compound, [Zn(C(10)H(5)O(3))(2)(C(10)H(8)N(2))(H(2)O)]·C(10)H(8)N(2)·2H(2)O. The bond lengths and angles around the metal atom indicate a deviation from octa-hedral geometry. The two naphtho-quinone ligands coordinate in a cis fashion, with the 4,4'-bipyridine ligand and the water mol-ecules completing the coordination sphere of the metal atom. The asymmetric unit contains also one free 4,4'-bipyridine mol-ecule and two uncoordinated water mol-ecules. These mol-ecules make contacts with the complex through O-H⋯N and O-H⋯O hydrogen bonds, creating a layer two-dimensional network parallel to (121).