Identification of low-level impurities in drug prototypes of carbocisteine by means of liquid chromatography-high-resolution mass spectrometry and general unknown comparative screening

High-resolution tandem quadrupole time-of-flight mass analysers enable new automated workflows for untar-geted data evaluation of complex samples like drug products. An example of such procedure is the so-called general unknown comparative screening (GUCS), which is used for software-assisted, automated identification of components that are only present in a sample and not in a reference. The GUCS approach has been employed for the first time to detect both degradation products and reaction products in drug products. Two different carbocisteine containing syrup prototypes - one with sucrose and the other with artificial sweeteners - were selected as examples after nine months of storage at 40 degrees C and 75% relative humidity. The samples were analysed chromatographically using a Coresep SB mixed-mode column and high-resolution MS and MS/MS data were recorded in information dependant acquisition mode on a Sciex X500R quadrupole time-of-flight mass spec-trometer. Data analysis was considerably facilitated using the corresponding placebo formulation as reference samples. With the GUCS approach two hitherto unknown degradation products of carbocisteine, i.e. the car-bocisteine lactam of the sulfoxides and the disulfide between L-cysteine and thioglycolic acid, were detected at low concentrations in both of the syrup formulations. The presumed structures were confirmed by in silico analysis of the fragment spectra and high-resolution LC-MS experiments with reference substances. Two addi-tional impurities were found in the sucrose-containing sample and identified as the N-glycosides of carbocisteine and its lactam, respectively, using binary mixtures with a 13C-labelled monosaccharide.