Evaluation of Accuracy for the Quantitative Analysis Using Nuclear Magnetic Resonance as a Detector of HPLC

LC−NMR has been extensively used for determination and elucidation of chemical structures [1]. In such applications, NMR often suffers poor sensitivity of signal detection. Therefore, stop− flow LC−NMR method, in which flow of LC is stopped when an analyte enters NMR detector, is widely used in order to obtain spectra with sufficient signal to noise ratio to analyze. NMR has been also known that the area integration of the peaks is proportional to the number of nuclei; thus it can be applied to the quantitative analysis. However, accuracy and precision of quantitation using NMR are often poor because quantitative NMR requires its special parameter settings [2], which are quite different from those for structural determinations. Application of LC−NMR to quantitative analysis was reported with continuous−flow method [3], which used NMR as a detector of HPLC much like the conventional detectors such as UV. One of the requirements in quantitative NMR is a long relaxation delay; at the mean time, relaxation delay in continuous−flow LC−NMR is limited by flow rate. This contradicted problem must be overcome for obtaining the accurate continuous−flow LC−NMR measurements. Previous reports showed accuracy of the result was about 10%, precision was indicated worse. Here we aimed to obtain better accuracy of o−xylene purity assessments using NMR as a detector of HPLC. Our approach was following; two consecutive continuous−flow LC−NMR runs were measured; the o−xylene sample was measured during the first run, and the standard sample was measured during the second run. The ratio of NMR area integrations obtained by these two runs leaded the purities. In such a condition, we could maintain identical detection conditions in NMR measurements between the sample and the standard. The NMR results were compared with results obtained with UV detector. Furthermore, the results obtained with LC− NMR, GC−FID, and DSC were also compared.