*3.3.* β*-Agonists*

β-agonists are widely used not only as bronchodilators, but also as muscle growth promoters to increase bovine, lamb, and pork meat production, mainly in Asian countries. They can be toxic for human health, particularly for their effect on the cardiovascular system, and, for this reason, the EU set a very low MRL for clenbuterol (0.05–0.5 μg/kg) in meat and milk samples [2].

NACE coupled with MS (NACE-MS) allowed trace amounts of β-agonists (clenbuterol, salbutamol and terbutaline, TER) to be detected. Anurukvorakun et al. developed a NACE-MS set-up, obtaining results comparable to those obtained by an HPLC-MS/MS method. In fact, the sensitivity is very high thanks to the combination of an SPE using mixed-mode reversed phase/cation exchange cartridges and hydrodynamic and electrokinetic injections in the CE-system [103].

For the quantification of ractopamine and dehydroxyractopamine in porcine meat, Wang et al. proposed an interesting MEKC-UV method with results totally in agreement with those obtained by MS techniques. The method was an on-line stacking method, including exhaustive CSEI and sweeping with a 900 fold higher sensitivity (ng/g level) in comparison to CZE-UV. The capillary was filled with a long plug of a higher conductivity buffer and an electrokinetic injection of the sample was then performed; at the end, the compounds were separated by a sweeping buffer with SDS, whose electrostatic forces contributed to the mobility and resolution of the two analytes [104]. Similarly, FASI-sweep-MEKC improved the MEKC sensitivity by 400–2000 times (ng/mL), allowing the analysis of eight β-agonists (R-albuterol hydrochloride, cimaterol, clenbuterolhydrochloride, colterol, TER, tulobuterol, ractopamine hydrochloride, and zilpaterol) in animal feed samples. A very efficient method was proposed by combining the sensitivity derived from the stacking effect of the FASI-sweeping procedure with the particular selectivity of a dialkyl-chain anionic surfactant in place of SDS [105].

The combination of FESI and CE-C4D represented another important alternative in the analysis of these drugs in animal feed. In 2014, Gao et al. proposed the first method able to detect TER, procaterol, formoterol, and bambuterol in pig feed. FESI as a pre-concentration step allowed an improvement in LOD values (0.02 mg/L) to be obtained in comparison to UV detection (Figure 3) [106].

**Figure 3.** CE-capacitively coupled contactless conductivity detection (CE-C4D) electropherograms of four β2-agonists in pig feed. (**a**) Samples (10 mg/L in background electrolyte (BGE)) without pre-concentration step; (**b**) samples (0.1 mg/L in methanol) with field-enhanced sample injection (FESI) preconcentration step; (**c**) samples (5 mg/L in methanol) with FESI preconcentration step [106].
