Separations Using Partial Complexation

Our ability to separate free metal cations by CE is limited because many of the metal ions have similar electrophoretic mobilities. An excellent way to enhance the separation of metal ions is to add a relatively weak complexing ligand (L ) such as tartrate, lactate or a-hydroxyisobutyric acid (HIBA) to the BGE. Now part of each metal ion will remain as the free ion (M, for example) and part will be converted to a complexed form (ML , ML2, ML3, for example). The total mobility (p) will be the sum of the mole fraction of each species (a) multiplied by its mobility. 

Pos is the electroosmotic mobility. The free metal ion will make the greatest contribution to total mobility because of its higher positive charge. Different elements will in general be complexed to different degrees so that their net mobilities will vary even though the mobilities of uncomplexed cations may be almost the same. 

Jones et al. obtained exeellent separation of 15 alkali, alkaline earth, and divalent transition metal ions with 6.5 mM HIBA at pH 4.4 to partially complex some of the cations [11]. A protonated amine cation containing a benzene ring (Waters UV Cat 1) was used for indirect UV detection. All of the 13 lanthanides have been separated using HIBA under similar conditions (Fig. 10.12). 

Lactate has the same a-hydroxycarboxylate complexing group as tartrate and HIBA, but it is a smaller molecule and forms somewhat weaker complexes than tartrate with most metal ions. Shi and Fritz found that a lactate system gave excellent separations for divalent metal ions and for trivalent lanthanides. A brief optimization was first carried out to establish the best concentrations of lactate and UV probe ion and the best pH. Excellent separations were obtained for all thirteen lanthanides, alkali metal ions, magnesium and the alkaline earths, and several divalent transition metal ions. All of these except copper(II) eluted before the lanthanides. An excellent separation of 27 metal ions was obtained in a single run that required only 6 min (Fig. 10.13). 

It was not possible to separate ammonium and potassium by lactate alone. However, a mixture of lactate and 18-crown-6 permitted the separation of NH4 and K as well as most of the other ions in Fig. 10.13. 

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Enzymatic preparation with predominant content of pectinesterase (obtained from Penicillium fellutanum culture liquid by isolation by acetone was purified. Primary enzymatic preparation was re — precipitated by three volumes of ethyl alcohol and centrifuged (6000 rev/min.) during 20 min. The obtained precipitate of partially purified pectinesterase preparation was dried in vacuum — desiccator. Sephadexes G — 50, G-75, G-lOO, G-200 "LKB" (Sweden) and Toyopearl HW-55 (Japan) were used for separation of enzymatic complex by gel — filtration. 

The first resolution of trioxalatocobaltates was accomplished in 1916 with strychnine. The method of spontaneous crystallization of antipodes from a racemic mixture of a complex was first demonstrated with K3[Co(C204)s] above 13.2°, the optical antipodes may be crystallized and mechanically separated. In practice, however, the standard technique of fractional crystallization of the strychnine diastereoisomers has been used. Partial resolution on optically active quartz has also been reported. Selective decomposition of the antipodes by circularly polarized light... 

An advantage of NMR-based biomarker targeting is that it does not require any separation methods and thus we can use more complex mixtures. But this advantage brings with it a challenge it can be difficult to identify the compounds in the complex NMR spectrum. A limitation is that NMR, especially 1D-NMR, alone usually yields partial structures even for a pure compound. 2D-NMR gives more information and resolution for structure analysis, but its lower sensitivity is a disadvantage. Complete molecular identification often requires a combination of approaches. 

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