Assessment of the purity of pyributicarb with signals partially overlapping with a structural isomer impurity in NMR spectrum.

Abstract

Quantitative nuclear magnetic resonance (qNMR) spectroscopy is widely used as a quantification technique, the popularity of which is attributed to its ability to quantify various analytes using a single reference material. Accurate signal areas of target analytes are essential for accurate quantification by qNMR, but are difficult to obtain when impurities possessing similar chemical structures are also present. Here, we focused on accurate separation of impurity signals partially overlapping with target signals in NMR spectra and estimation of their signal areas using deconvolution. For accurate estimation of impurity signal areas, we applied manual adjustment after automatic deconvolution for pyributicarb, a carbamate herbicide, containing a structural isomer at 0.02 kg/kg. The level of impurity obtained by this method was from 0.0175 to 0.0245 kg/kg for five examined signals, which was comparable to 0.02 kg/kg. Purity obtained using five overlapping pyributicarb signal areas after subtracting impurity-related signals was from 0.9715 to 0.9834 kg/kg with small variations (between 0.02% and 0.41% in terms of relative standard deviation, n = 3). This result was much better than the purity obtained by automatic deconvolution alone (0.9811-1.0120 kg/kg, 0.52%-5.04%, n = 3) and better than that by a further optimized technique (the edited sum integration method; 0.9638-0.9820 kg/kg, 0.10%-1.41%, n = 3). The purity of pyributicarb estimated by our method using five overlapping signals [(0.9773 ± 0.0096) kg/kg, k = 2] was consistent with the reference value obtained by the mass balance approach [(0.9761 ± 0.0022) kg/kg, k = 2].