## 3 Results and Discussion  
### ESI-MS Analysis  
To achieve chiral recognition, metal-analyte-reference interactions play important roles in achieving the selectivity of the enantiomers due to their different stereochemical effects. In the study, \(\beta\)-CD and metal ions act as the reference respectively. As we know, single charged and double charged cluster ions are both possible to investigate and it has been shown to be a feasible alternative that the trimetric cluster ion [M\({}^{\Pi}\)(ref)(A)\({}_{2}\)-H]\({}^{+}\) can take the place of [M\({}^{\Pi}\)(A)\({}_{2}\)(ref)-H]\({}^{+}\). However, the single charged metal-analyte-reference cluster [M\({}^{\Pi}\)(\(\beta\)-CD)(EZM)\({}_{2}\)-H]\({}^{+}\) as well as the [M\({}^{\Pi}\)(\(\beta\)-CD)\({}_{2}\)(EZM)-H]\({}^{2+}\) were not detected in MS\({}^{1}\)spectra. Therefore, [M\({}^{\Pi}\)(\(\beta\)-CD)(EZM)\({}_{2}\)-H]\({}^{2+}\) was selected as the parent ion.  
The Effect of Cu\({}^{2+}\), Zn\({}^{2+}\), Mg\({}^{2+}\), Pb\({}^{2+}\), Hg\({}^{2+}\), Co\({}^{2+}\) on Recognizing EZM Enantiomers  
The effect of Cu\({}^{2+}\), Zn\({}^{2+}\), Mg\({}^{2+}\), Pb\({}^{2+}\), Hg\({}^{2+}\), and Co\({}^{2+}\) on distinguishing EZM enantiomers is different based on their interaction with ligands. With the addition of Pb\({}^{2+}\) and Hg\({}^{2+}\), there is no difference in MS/MS spectrogram and no corresponding [Pb(EZM)\({}_{2}\)(\(\beta\)-CD)-H]\({}^{2+}\) and [Hg(EZM)\({}_{2}\)(\(\beta\)-CD)-H]\({}^{2+}\) ternary complexes found. This demonstrates that Pb\({}^{2+}\) and Hg\({}^{2+}\) have no benefits with \(\beta\)-CD for increasing the distinguishing effect on EZM enantiomers.  
In addition, Zn\({}^{2+}\), Mg\({}^{2+}\), and Co\({}^{2+}\) have shown similar distinguishing effect to Cu\({}^{2+}\). They all show one abundance difference of fragment ion in corresponding [M\({}^{\Pi}\)(EZM)(\(\beta\)-CD)-H]\({}^{2+}\) (M = Cu\({}^{2+}\), Zn\({}^{2+}\), Mg\({}^{2+}\), Co\({}^{2+}\)) ternary complexes (\(SRS\)- EZM and \(RSR\)- EZM) MS/MS spectrogram. By comparing the abundance of this special fragment ion, we can discriminate SRS- EZM and RSR- EZM. Among that, Cu\({}^{2+}\) corresponds to fragment ion peak [EZM-H\({}_{2}\)O+H]\({}^{+}\) (\(m\)/z = 392.14) and Zn\({}^{2+}\), Mg\({}^{2+}\), Pb\({}^{2+}\) correspond to fragment ion peak [M\({}^{\Pi}\) (EZM)\({}_{2}\)(\(\beta\)-CD)-H]\({}^{2+}\) (M = Cu, Co, Zn, Mg, A. [Cu(EZM)\({}_{2}\)(\(\beta\)-CD)]\({}^{2+}\): \(m\)/z = 1007.7987, B. [Co(EZM)\({}_{2}\)(\(\beta\)-CD)]\({}^{2+}\): \(m\)/z = 1005.8000, C. [Zn(EZM)\({}_{2}\)(\(\beta\)-CD)]\({}^{2+}\): \(m\)/z = 1008.2984, D. [Mg(EZM)\({}_{2}\)(\(\beta\)-CD)]\({}^{2+}\): \(m\)/z = 988.3264). By comparation of the difference of fragment ion ratio of abundance between corresponding \(SRS\)-EZM and \(RSR\)-EZM ternary complexes, Cu\({}^{2+}\) shows the best distinguishing effect (Figure 2).  
Figure 2: _Cont._  
Taken together, the effects of Mg\({}^{2+}\), Zn\({}^{2+}\), Pb\({}^{2+}\), Hg\({}^{2+}\), and Co\({}^{2+}\) combined with \(\beta\)-CD as ligands on differentiating \(SRS\)-EZM and \(RSR\)-EZM were studied. It turned out that different metal ions produced different fragments from the ternary complexes, which can be used to recognize \(SRS\)-EZM and \(RSR\)-EZM. Besides, the efficiency of recognition is also different. After analyzing the characteristics of metal ions, we finally present a possible conjecture that the loss of electron capability and the size of the radius are important factors that affect the discrimination function of metal ions as ligands.