Magnesium cations amine complexes

A second way to improve resolution is the modification of mobility by complexation of the analyte. Many buffers for analysis of cations use HIBA or 18-crown-6 to improve the resolution between sodium, potassium, calcium, magnesium, etc. as well as some aliphatic amines. By diluting an existing validated buffer, one can change the concentration of the complexation agent and thus also the selectivity of the system. 

Numerous tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions. Ethylenediamine tetra-acetic acid (EDTA) can be used for determination of 40 elements by direct titration using metal-ion indicators for endpoint detection. Direct titration procedures are limited to metal ions that react rapidly with EDTA. Back titration procedures are useful for the analysis of cations that form very stable EDTA complexes and for which a satisfactory indicator is not available. EDTA is also used for determining water hardness the total concentration of calcium and magnesium expressed in terms of the calcium carbonate equivalent. 

In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K", Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

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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