*2.1. Nitroimidazoles*

5-Nitroimidazoles (5-NDZs) are mainly active against Gram-negative and Gram-positive anaerobic bacteria. Some benzimidazoles (BZs) are also used as additives in stored fruit and vegetables, because of their fungicidal properties [35]. Regarding the veterinary use, these substances are prohibited [2,36], as they could be a potential risk for human health because of their genotoxicity and mutagenicity [37]. Therefore, no MRLs were established.

A rapid CEC-UV method for the analysis of 5-NDZ residues in bovine milk samples was set-up by Hernández-Mesa et al. The characteristic speed and high efficiency of CEC, which combines mobility and partition principles, are well-known to be affected by the complexity in fritting fabrication, which gives origin to pressure drops and bubble formation with loss of efficiency and reproducibility. In this work, the authors overcame this problem by proposing a simple capillary packing procedure and optimizing a sintering process (parameters: Time and temperature), thus obtaining a reproducible frit fabrication. A proper set-up of buffer composition and concentration, capillary length, and voltage allowed a reproducible analysis of eight 5-NDZs in milk samples in 15 min. In addition, a sample pretreatment using LLE and SPE methods highly increased the method sensitivity (LOQ range for all the 5-NDZs analyzed was 19–96 μg/L) [38]. This method, combining the advantages of CE and HPLC, was more selective and rapid than the simple HPLC [39] or CZE [40] methods.

The same technique (CEC-UV), but coupled with DLLME as a pre-concentration step, was used to analyze BZs in environmental and farm water [41]. DLLME, which belongs to liquid-phase microextraction (LPME) techniques, is based on the formation of a dispersion, created by adding an organic solvent mixture to an aqueous sample. It represented an ideal procedure to efficiently extract environmental pollutants and an interesting greener approach in their analysis [42].

This procedure was also used by Hernández-Mesa et al. to concentrate different 5-NDZ compounds in river water samples before the analysis with a cation-selective exhaustive injection and sweep (CSEI-sweep)-MEKC-UV method. DLLME and CSEI-sweep approaches combine an electrokinetic injection of charged cations and a sweeping in which the formation of micelles is promoted to focus the analyte. Thus, the MEKC method was more rapid, allowing the separation of six BZs in about 10 min [43]. The DLLME procedure coupled with MS detection could also improve the sensitivity of CZE. An example was the study of Tejada-Casado et al., who set-up a CZE-MS/MS method able to rapidly (about 30 min) detect and quantify twelve BZs in meat samples, as a valid alternative to HPLC methods [44].

Finally, SPE combined with CSEI-sweep-MEKC-UV allowed six nitroimidazole residues in egg samples to be quantified with LOQ values in the range of a few ng/g [45].
