Metal cations elution orders

The Dionex column caused each of the metal ions to be completely retained. The authors reasoned that this behavior was due to the presence of unreacted surface-sulfonated PS-DVB macroparticles which were capable of adsorbing the metal cations. A mixed retention mechanism was theorized for the other three columns. In addition to anion-exchange of negatively charged metal-phthalate complexes, the authors suggest that adsorption of neutral metal-phthalate complexes might contribute to the retention of each metal ion. This theory was based on the fact that since the stationary phase of the Hamilton, Waters, and Vydac columns were comprised of different materials, these materials would have different hydrophobicity which would lead to differences in adsorption of the neutral metal-phthalate complexes on each column. This was corroborated experimentally, as the metal ions were retained more strongly on the more hydrophobic column (PS-DVB from Hamilton). The dominant mechanism of retention was similar for each column because there were no differences in metal ion elution orders between the three columns. 

In contrast to conventional cation exchangers, a reversed elution order is observed with crown ether phases, which is mainly determined by the size ratio between crown ether ring and alkali metal ion. Due to the high affinity of poly(benzo-15-crown-5) toward potassium and rubidium ions, these are more strongly retained than lithium, sodium, and cesium ions, respectively. However, the complexing properties of crown ethers also depend on the counter ion being employed. Thus, in potassium salts, for example, an increase in retention in the order KC1 < KBr < KI is observed with an increasing size of the counter ion. 

A reversal of elution order for the ions Pb2+, Co2+, Zn2+, and Ni2+ is obtained with the use of oxalic acid as the sole complexing agent. Such a separation is displayed in Fig. 3-155. Under these conditions, the metal separation is controlled by anion and cation exchange processes. The degree to which both mechanisms contribute to the separation process is different for each metal ion. The anion exchange mechanism predominates where stable anionic oxalate complexes are formed. In metal ions which do not form stable oxalate complexes, such as Cu2+, the cation exchange mechanism is... 

The method of anion exchange chromatography for heavy metal analysis may also be applied to ion-pair chromatography. With oxalic acid as the complexing agent and tetrabutylammonium hydroxide as the ion-pair reagent, an elution order opposite to cation analysis is obtained. The separation of anionic metal complexes on MPIC phases... 

Calcium(II), which shows no appreciable complexing, has a distribution coefficient of 147 in 0.5 M perchloric acid and 191 in 0.5 M hydrochloric acid. Strelow. Rethc-meyer, and Bothnia [10] also reported data for nitric and sulfuric acids that showed complexation in some cases. Mercury(II), bismuth(III), cadmium(II), zinc(II), and lead(II) form bromide complexes and are eluted in the order given in 0.1 to 0.6 M hydrobromic acid [11]. Most other metal cations remain on the column. Aluminu-m(III), molybdenum(VI), niobium(V), tin(IV), tantalum(V), uranium(VI), tung-sten(VI), and zirconium(IV) form anionic fluoride complexes and are quickly eluted from a hydrogen-form cation-exchange column with 0.1 to 0.2 M HF [12]. 

The selectivity of sulfonated ion-cxchange resins for metal cations is often expressed qualitatively in terms of elution orders. Numerical selectivity data for cations are limited [3-5]. Strelow and co-workers [6-8] published comprehensive lists of distribution coefficients for metal ions with perchloric acid and other mineral acid eluents. However, their data are for sulfonated gel resins of high exchange capacity. 

Elution order of alkali and alkaline earth metals and ammonia cations depends on many factors (e.g., column selectivity, eluent type) but is usually as follows Li+, Na, NH4+, K+, Mg +, Ca +, and Ba +. 

The elution order of metal cations should follow the order of decreasing com-plexation. This is indeed the case for the separation in Figure 7.17. Here, spectro-photometric detection at 530 nm was used after post-column addition of pyridyl-azoresorcinol (PAR) as a complexing reagent. At highly alkaline pH values the eluted metal ions form colored complexes with PAR that are more stable than the complexes with the mobile phase. However, the use of post-column detection requires somewhat more complicated equipment. 

Retention times of divalent metal cations can be manipulated by addition of the sodium salt of an anion with a strong pairing propensity. The retention times of the cations were all longer when iodide or thiocyanate was added to the sample (Figure 10.6). The elution order of the cations was entirely different from the order obtained in classical ion chromatography. 

The sequential analysis of alkaline-earth metals, which elute in the order Mg " < Ca " < Sr < Ba on strong acid cation exchangers, can also be performed using both conductivity detection modes. An eluent mixture of hydrochloric acid and 2,3-diaminopropionic acid is used for suppressed conductivity detection ethylenediammonium ions are suitable for nonsuppressed conductivity detection. Figure 4.56 shows a separation of alkahne-earth metals on Shimpack IC-Cl obtained with this eluent Because of the high elution power of the mobile phase, all monovalent cations present in the sample are eluted as one peak within the void volume of the column. As an alternative to strong eluents, shorter separator columns may be employed to reduce the retention of cations that have high affinities toward the stationary phase. 

The separation of cations is usually carried out on lonPac CS12A with sulfuric acid or methanesulfonic acid as an eluant. For anion analysis, carbonate-selective anion exchangers such as lonPac AS12A or AS14 have proved to be suitable. Alternatively, gradient elution on lonPac AS 11 can be used. As a matter of principle, 2-mm columns are utilized in order to reach the required sensitivity. With a gas flow of 1 L/min, sampling times are between 10 and 40 minutes the preconcentrated volume ranges from 1 mL to 10 mL. Metal cations are rarely detected in clean room air because they are usually bound to particles. Because... 

Karcher and KruII [30] used complexometric calculations to determine the mobile phase concentrations of HIBA and tartaric acid needed to fine tune their separation of eleven metal cations on C and Ci silica-based reverse-phase columns which had been dynamically modified with n-octanesulfonate. Isocratic elution was used to separate the metals into three distinct windows each window corresponding to one of the three dififerent valence states spanned by the eleven metal cations. The metals eluted in the order of increasing valence, with the sole exception of La(III) which did not elute within the trivalent ion window. Figure 6.15 illustrates the separation of ZrdV), Ga(III), Sc(III), Y(III), Al(III), In(III), Zn(II), La(III), Cd(II), Ca(II), and Mg(II) using a Cis column and an eluent comprised of 2.27 mM n-octanesulfonate, 8.18 mM tartaric acid, 52.9 mM HIBA, and 10.7% (v/v) methanol. 

The authors indicate that the elution orders for samples of metal ions can be accurately predicted using complexometric calculations. The algorithm used requires inputs of mobile phase ligand concentrations, mobile phase pH, and the formation constants for reactions between metal ions and the mobile phase ligands. In addition to their own experimental results, the authors have successfully calculated elution orders for many of the literature reported in situ complexation-based separations which have been achieved on either cation-exchange or d3mamically modified reverse-phase columns [31]. 

The technique separates components of a mixture in order of their molecular size, practionally their molecular weight. In a typical example [14], a sample of a crude oil was dissolved in toluene or tetrahydrofuran and pumped through a column of 60A Styragel. The effluent was monitored by a differential refractometer. In the case of cations, separations based on molecular size have been achieved with alkali metals eluting in order potassium, sodium, lithium, magnesium and calcium. 

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