Purity assessment problem in quantitative NMR—impurity resonance overlaps with monitor signal multiplets from stereoisomers
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This paper describes the situation that can emerge when the signals to be evaluated in quantitative NMR measurements—so-called “monitor signals”—consist of several resonance lines from the stereoisomers of the analyte in addition to an impurity signal underneath. The monitor signal problem is demonstrated in the purity assessment of two samples of 2-(isopropylamino)-4-(ethylamino)-6-chloro-1,3,5-triazine (atrazine), a common herbizide which served as analyte in a CCQM intercomparison. It is shown that, in DMSO-d6 solution, a mixture of stereoisomers leads to several individual overlapping singlets, which are further split by spin–spin coupling. A measurement protocol was developed for finding and identifying an impurity that has a signal that is positioned precisely beneath the methyl signal chosen as the monitor signal in one of the samples. Quantitative NMR purity assessment is still possible in this special case, but with higher uncertainty.
KeywordsQuantitative NMR Monitor signal Impurities Atrazine Purity assessment
Financial support by the Federal Ministry of Economics and Labour (BMWA) (AiF project: 13843 N/1), the German Federation of Industrial Cooperative Research Associations “Otto von Guericke” (AiF) and the German Research Association of Medicine Manufacturers (FAH) is gratefully acknowledged. Furthermore, we thank the referees for helpful suggestions.
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