*2.4. Sulfonamides*

These molecules are widely used for the treatment of bacterial and protozoan diseases (i.e., malaria) and as growth-promoters in farm animals. The EU fixed MRLs to 100 μg/kg in different animal tissues and milk [2]. Sulfonamides could be dangerous for human health. For example, studies on sulfamethazine carcinogenesis are very controversial, but nowadays, it is not yet classified as carcinogenic by the U.S. National Toxicology Program 14th Report on Carcinogens (2016).

For the antimalarials sulfadoxine and quinine, the old literature proposed CZE-UV as the main CE-mode in meat and water analysis with sensitivity values of μg/L and mg/L for sulfadoxine and quinine, respectively [68]. More recently, Mikus et al. set-up a capillary isotachophoresis (CITP) coupled on-line with the CZE method to detect quinine in commercial beverages. This approach could be potentially used to detect drug residues in food, as CITP combined on-line with CZE-UV significantly increased the CZE sensitivity. In fact, in CITP, leading and terminating electrolytes are used to create separated zones in which the ions migrate at the same velocity, giving origin to an on-line stacking effect [69]. This combination allowed the direct analysis of samples with a lower LOD than CZE-UV methods and with sensitivity values comparable to those obtained with HPLC-UV methods [70].

Nine sulfonamides in meat samples were efficiently separated by CEC-ESI-MS. The use of a poly(divinylbenzene-octyl methacrylate) (poly-DVB-OMA) monolithic stationary phase and an on-line concentration (obtained increasing sample injection time) provided a sensitive method for detecting trace residues that needed only a simple sample pretreatment (i.e., SPE) (Figure 2) [71].

Wang et al. set-up a microfluidic CE system with LIF detection, which was able to separate four sulfonamides (sulfamethazine, sulfamethoxazole, sulfaquinoxaline, and sulphanilamide) in milk and chicken muscle extracts in 1 min with LOD values of a few μg/L. The short time required for the analysis and the fact that the plastic chips proposed were cheap made this method very useful for a rapid screening of sulfonamide residues in food samples [72].

Recently, Dai et al. proposed a CZE method with on-line ECL detection to quantify sulfadimidine, sulfadiazine, and sulfathiazole in pork and chicken meat samples. Chemiluminescence (CL) emissions were generated by the oxidation of luminol in the Ag(III)-luminol system, and sulfonamides exhibited an inhibitory effect on CL signals. The careful optimization of different parameters (buffer type and pH, voltage, and injection time) allowed a promising selective and sensitive method to be obtained for the analysis of veterinary drug residuals in animal-derived food [73].

**Figure 2.** Capillary electrochromatography (CEC)-electrospray ionization (ESI)-mass spectrometry (MS) electropherograms obtained by using different monolithic stationary phases in the analysis of standard sulfonamides (**a**–**e**). The use of poly(DVB-OMA) capillary (**b**) allowed the best compromise to be obtained between resolution, efficiency, and analysis time [71].
