"Ghost peaks" in reversed-phase liquid chromatography separation of an electron-rich aniline compound: Mechanism and solution for this phenomenon

Two "ghost peaks" appeared in the process of developing an impurities analytical method for an electron-rich aniline compound by RP-HPLC. Surprisingly, the "ghost peaks" disappeared when the mobile phase was re-prepared. The "ghost peak" slowly began to reappear and gradually increased after increasing the injection volume or changing different instruments and columns. In this paper, the structures of two "ghost peaks" were analyzed by LC-MS/MS, which were identified as the dimer of the main peak and the oxidation products of the dimer. The mechanism was confirmed by studying the "ghost peaks" appearance phenomenon in the mobile phase with different pH values, different types of buffer salts, and different brand chromatographic columns. The potential chemical behavior of this type of compound on the column was clearly revealed in the process of RP-HPLC separation. The coupling reaction of electron-rich aniline compounds can be induced by partial or complete dissociation of the silanol group to Si-O-in the stationary phase of a reversed-phase chromatographic column under neutral and basic conditions of the mobile phase. Adding inorganic salts to the mobile phase can inhibit the reaction, and the higher salt concentration, the stronger the effect.

Chromatographic studies of unusual on-column degradation of cefaclor observed in the impurity separation by HPLC

An unpredictable ghost peak was intermittently observed during the impurity separation of cefaclor and formulation by high performance liquid chromatography (HPLC) with a content from below the reported threshold to approximately 0.3% in different laboratories. Through a series of investigations, the ghost peak was identified as an unusual on-column degradant of cefaclor formed under elevated column temperature but was not an actual sample impurity. The chemical structure of the degradant was determined by spectroscopic methods, including high resolution mass spectrometry (HRMS) and ¹H-NMR. Consequently, the unknown peak was identified as a C-4 oxidative decarboxylation analog of cefaclor. The formation mechanism of the analog is proposed, and it is suggested that elevated column temperature during HPLC analysis has a profound effect on the degradation. Dissolved oxygen in the mobile phase may promote the formation of the ghost peak. The degradation can be suppressed by using a column temperature below 30 °C. Moreover, several other prevention measures are suggested based upon the results of the investigation.