The eluent (its type, concentration, pH, and flow rate) constitutes another important factor affecting the separation efficiency and selectivity. It also depends on the stationary phase. Typical eluents applied in anion-exchange chromatography are: Na\({}_{2}\)CO\({}_{3}\), NaHCO\({}_{3}\), NaHCO\({}_{3}\) + Na\({}_{2}\)CO\({}_{3}\) NaOH/KOH, and Na\({}_{2}\)B\({}_{4}\)O\({}_{7}\). For cation-exchange chromatography, diluted water solutions of HNO\({}_{3}\), HNO\({}_{3}\), HCl, DPA (dipicolinic acid), tartaric acid, and oxalic acid are used. Carbonate and bicarbonate eluents are commonly applied in the anion-exchange ion chromatography. However, hydroxide seems the perfect eluent as it forms water that has virtually zero conductances after suppression. Unfortunately, the hydroxide eluents are difficult to use, because they absorb CO\({}_{2}\) from ambient air and form carbonates. The selectivity can be modified through the selection of the stationary phase and eluent composition, which has to be compatible with the detection system. The conductivity detector is still the most popular, but other types of detection modes (e.g., UV-VIS, amperometric, spectrometric ones) can also be applied for different analytes [22]. An overview of detection methods is given in Table 2. Figure 2 shows a division of detection methods used in ion chromatography. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Detection Mode** & **Principle** & **Example Applications** \\ \hline \multirow{2}{*}{Conductivity} & Electrical & \multirow{2}{*}{Anions and cations with pK\({}_{\text{a}}\) or pK\({}_{\text{b}}\) \textless{} 7} \\  & conductivity & \\ \hline Direct and indirect & UV-VIS light & UV-active anions and cations, heavy and transition \\ UV-VIS & absorption & metals after post column derivatisation reaction \\ \hline \multirow{2}{*}{Fluorescence} & Excitation and & \multirow{2}{*}{Ammonium, amino acids, and selected amines after post column derivatisation} \\  & emission & \\ \hline \multirow{2}{*}{Amperometry} & Oxidation or & \multirow{2}{*}{Anions and cations with pK\({}_{\text{a}}\) or pK\({}_{\text{b}}\) \textgreater{} 7} \\  & reduction & \\ \hline \multirow{2}{*}{MS} & Electrospray & \multirow{2}{*}{\
\begin{tabular}{} \end{tabular} } \\  & ionization & characterization of organic anions and cations \\ \hline \multirow{2}{*}{ICP-AES, ICP-MS} & Atomic emission & \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } \\  & & \\ \hline \multirow{2}{*}{Refractive index} & Change in & \multirow{2}{*}{High concentrated samples} \\  & refractive index & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detection methods applied in ion chromatography. Figure 2: Division of detection methods for ion chromatography. The most popular conductometric detector can be applied to determine all anions and cations of strong acids and bases (e.g., F\({}^{-}\), Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\), PO\({}_{4}\)\({}^{3-}\), SO\({}_{4}\)\({}^{2-}\), Na\({}^{+}\), K\({}^{+}\), NH\({}_{4}\)\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\)). The UV-VIS detection is a popular detection mode for high-performance liquid chromatography (HPLC), but its application in ion chromatography is limited because only a few inorganic ions have an appropriate chromophore [24]. The UV-VIS detection methods can be divided into direct and indirect modes. For the direct UV-VIS detection, the molar absorption of co-ions should be zero. The direct UV-VIS detection is applied for determinations of NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\), as well as Br\({}^{-}\) and I\({}^{-}\) in the presence of high Cl\({}^{-}\) concentrations. Moreover, sulfide, chromate, thiocyanate, thiosulfate, and selected metal chloro- and cyano-complexes can be determined in wastewater with this detection method [25]. The UV-VIS detection with post-column reactions is a versatile technique that combines enhanced sensitivity and selectivity for specific applications [26]. The chemiluminescence detection is generally performed in a post-column reaction mode [27].