Eluents divalent cations

Ohta, K. and Tanaka, K. (1999) Simultaneous Determination of Common Mono- and Divalent Cations in Natural Water Samples by Conductimetric Detention Ion Chromatography with an Unmodified Silica Gel Column and Oxalic Acid/18-crown-6 as Eluent, Anal. Chim. Acta. 381, 265-273. 

Fig. 3-148. Gradient elution of monovalent and divalent cations. - Separator column Fast-Sep Cation I and II eluent (A) water, (B) 0.04 mol/L HC1 + 0.02 mol/L 2,3-diaminopropionic acid gradient 5 min 7% B isocratically, then linearly to 100% B in 10 min flow rate 1 mL/min detection suppressed conductivity injection volume 50 pL solute concentrations 2 ppm lithium (1), 5 ppm sodium (2), 10 ppm ammonium (3), and potassium (4), 20 ppm tetrabutylammonium (5), 10 ppm magnesium (6) and calcium (7), 20 ppm ethylenediamine (8).

Fig. 3-149. Separation of divalent cations with direct conductivity detection. - Separator column surface-sulfonated cation exchanger (Benson Co., Reno, USA) eluent 0.0015 mol/L ethylenediamine + 0.002 mol/L tartaric acid, pH 4.0 flow rate 0.85 mL/min injection volume 100 pL solute concentrations 10.3 ppm Zn2+, 9.1 ppm Co2+, 16 ppm Mn2+, 16.1 ppm Cd2+, 17.1 ppm Ca2+, 16 ppm Pb2+, and 20.3 ppm Sr2+ (taken from [148]).

Alternatively, a strongly conducting eluent may be used. In this case, elution of the solute ions is associated with a negative conductivity change. This indirect detection method is applied to the separation of anions with potassium hydroxide as the eluent [7], A corresponding chromatogram is displayed in Fig. 6-2. This indirect detection method is also utilized in the analysis of mono- and divalent cations, which are eluted by dilute nitric acid or nitric acid/ethylenediamine-mixtures. 

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). 

The properties and performance of a commercial weak-acid resin column (Dionex CS12) have been described [41]. The substrate is a highly cross-linked, macroporous ethylvinylbenzene-divinylbenzene polymer with a bead diameter of 8 pm, a pore size of 6 nm, and a specific surface area of 3(X) m /g. In a second step, this substrate was grafted with another polymer containing carboxylate groups. The exchange capacity is listed as 2.8 mequiv/column for a 250 mm x 4 mm i.d. column. With this column, simple eluents such as hydrochloric or methanesulfonic acid can be used to separate mono- and divalent cations rapidly and efficiently under isocratic conditions. 

Several divalent cations were also eluted with eluent containing the ethylenediam-monium- or w-phenylenediammonium 2+ cation. Capacity factors for these and other eluent are summarized in Table 5.8. It can be seen that the 2-v diamine cations are much more efficient than Mg ", Na", or H" ". 

Excellent separations of all the alkali metal cations plus ammonium in 10 min or less with a strong acid (sulfonic acid) cation exchanger and a dilute solution of a strong acid as the eluent (incomplete sentence). However, divalent metal cations are more strongly retained by this column and require either an eluent containing a divalent cation or a more concentrated solution of the H eluent. 

For cation chromatography there are basically two eluent systems used. In the case of monovalent cations, the normal eluent is 0.005M HC1. However, the concentrations of hydrogen ion required for divalent cations are such that hydrogen ion is impractical eluent for divalent cations. Instead, the preferred cation for divalent cations like the alkaline earth cations is m-phenylenediamine dihydrochloride. The divalent nature of m-phenylenediamine makes it an efficient eluent for other divalent cations, while its weakly basic character results in very little conductivity when it is converted to the free base form in the suppressor. 

Figure 10.7 shows ion chromatographic separation of some divalent cations using m-phenylenediamine eluent. 

Divalent cations such as alkaline earth metals exhibit a much higher affinity with stationary phase of strong acid cation exchangers and thus cannot be eluted with diluted mineral acids. Therefore, application of another eluent is necessary. 

Ohta, K. Indirect ultraviolet spectrophotometric detection in the ion chromatography of common mono- and divalent cations on an aluminum adsorbing silica gel column with t5framine-containing crown ethers as eluent. J. Chromatogr. 2000, 884, 113-122. 

A mixture of Li, Na, NH4, K, Rb and Cs was separated in less than 10 min with a blend of 0.17 meq g and unfunctionalized cation-exchange resins with 1.25 X 10 nitric acid as the eluent (Figure 1.5). Although separation of divalent cations with nitric acid was not practical, a fast separation of magnesium and calcium in tap water was obtained with a 1 x 10 M ethylene diammonium nitrate eluent. 

Several improved versions of sulfonated latex columns were developed over the next few years [41], but this type of cation-exchange column was abandoned in favor of materials with a weak-acid function. Catex columns with sulfonic acid functionalities have a relatively low selectivity for hydronium ions. A divalent cation component must be added to the eluent to efficiently elute divalent cations such as magnesium and calcium. 

The modification of silica gel with various metals is a simple and effective way to prepare ion exchangers that often have unique selectivities for analyte ions. Ohta et al. [49] described the preparation of a cation exchanger in which silica gel was first immersed in zirconium butoxide, Zr (OC4H9)4. Then the material was calcined (heated) at temperatures up to 1000 °C to form a sihca-zirconia product An excellent separation of all the alkaU metal ions plus ammonium was obtained with 10 mM tartaric acid as the eluent. Divalent metal cations were strongly retained. 

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. 

Figure 4.9 shows the respective separation of all alkali and alkaline-earth metals under the chromatographic conditions of Figure 4.8. In comparison to lon-Pac CS12, divalent cations are eluted with higher peak efficiencies, which can even be improved by increasing column temperature. In general, a decrease in retention of monovalent cations and an increase in retention of divalent cations are observed with increasing column temperature. Thus, by optimizing eluent concentration and column temperature, the analysis time for a baseline-resolved separation of the six standard cations can be reduced to 5 min.

Cation exchangers based on PBDMA-coated silica are offered by a number of companies. Metrohm (Herisau, Switzerland), for example, offers two columns under the trade names Metrosep Cl and C4. The 5 pm Metrosep Cl is considered to be the high-performance column for the simultaneous analysis of mono-and divalent cations with an analysis time of less than 20 min. It can also be used for the separation of various amines, which are typically eluted with a nitric acid eluent. Figure 4.41 shows an example chromatogram obtained under standard chromatographic conditions with an eluent mixture of tartaric acid and pyridine-2,6-dicarboxyiic acid (PDCA) (dipicohnic acid). Under these chromatographic conditions, calcium elutes ahead of magnesium, followed by strontium and barium. This unusual retention behavior can be attributed to the complex-ing properties of pyridine-2,6-dicarboxylic acid. With a pure tartaric acid eluent. 

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