Eluents ethylenediammonium

Alkaline-earth metals, which elute in the order Mg24 < Ca2+ < Sr2+ < Ba2+, can also be determined using both conductivity detection modes. While an eluent mixture of hydrochloric acid and 2,3-diaminopropionic acid is used in the suppressed mode, ethylenediammonium ions are suitable for the direct conductivity detection mode. Fig. 3-143 shows a separation of alkaline-earth metals on Shimpack IC-C1 obtained with this eluent. Because of the high elution power of the mobile phase, all monovalent... 

For the separation of cations, a cation exchange column of low capacity is used in conjunction with either a conductivity detector or another type of detector. With a conductivity detector, a dilute solution of nitric acid is typically used for separation of monovalent cations, and a solution of an ethylenediammonium salt is used for separation of divalent cations. Because both of these eluents are more highly conducting than the sample cations, the sample peaks are negative relative to the background (decreasing conductivity). 

For effective elution of divalent metal ions a divalent eluent cation such as the ethy-lenediammonium cation is also needed. Thus, divalent metal cations are eluted nicely by an eluent of 2.0 x lO"" M ethylenediammonium tartrate, but not by an eluent containing sodium or ammonium tartrate. 

Sevenich and Fritz found an eluent containing 2.0 x 10 - M ethylenediammonium tartrate to be effective for separating several divalent metal cations [13]. Calculations from ionization constants showed ethylenediamine to be fully protonated (EnH2 ) at pH 5.0 or below. The adjusted retention times for several metal ions were obtained as a function of eluent pH (Table 7.6). The retention times increased in almost every case between pH 5 and 6. Furthermore, some minor extraneous peaks appeared in this pH region. A buffer pH of 4.5 was selected as giving generally the best results. 

Efficient and selective cation chromatographic separations are possible with an ethylenediammonium tartrate eluent at pH 4.5 and sulfosalicylic acid added to the sample before injection. At pH 4.5 zinc(Il) can be chromatographed without interference from a 20-fold excess of thorium(IV), vanadium(IV), or uranium(Vl), or from a 100-fold excess of iron(lll) when sulfosalicylic acid is added to the sample [14]. 

An early paper by Sevenich and Fritz [14] described a separation of several divalent metal cations with an eluent containing 2.0 mM ethylenediammonium tartrate at pH 4.5. A sulfonic acid column was used with direct conductivity detection. Eluents containing ammonium tartrate and no ethylene-diammonium 2+ salt were ineffective for elution of the metal ions studied. The elution mechanism was described as an ion-exchange pushing action of the 2+ cation and a weakly com-plexing pulling effect of the tartrate anion. 

Simple calculations showed that EDTA does not complex metal ions such as magnesium(II) and calcium(II) at pH 4 but it does complex many other metal cations. Therefore, experiments were performed in which ETDA was added to the metal ion sample and the column was eluted with ethylenediammonium tartrate as before [14]. The amount of EDTA used was more than enough to complex the metal ions present, but an unduly high concentration of EDTA was avoided. The results obtained show that conditions can easily be established whereby magnesium and the alkaline earth cation peaks are hardly affected but metal ions that form stable EDTA complexes at about pH 4 are rapidly eluted. Because EDTA is added only to the sample and not to the eluent, it moves rapidly through the column and appears as part of the pseudo peak . 

A much faster separation of divalent metal cations is obtained by replacing with a divalent cation such as ethylenediammonium (with a 2+ charge) as the predominant cation in the mobile phase [16]. Figure 7.19 shows excellent resolution of divalent cations with an acidic eluent containing ethylenediammonium PDA. 

---