Chemical differences that can assist selectivity

Now that the regular features of affinity for metals have been reviewed, some interesting irregular features can be discussed. Selectivity for particular ions is dependent on such irregularities. 

let us discuss the shapes of the chelated complexes. In most cases they are tetrahedral, but iron (both ferrous and ferric) forms octahedral complexes. However, the most remarkable contribution to selectivity is made by cupric ions which form planar complexes. This means that copper is uniquely sensitive to a type of steric effect imposed by bulky groups. Thus, in Table 11.2, the stability constants for the cupric complexes ofbipyridyl [11,19), phenanthroline [11.18), and folic acid lie below those of the corresponding nickel complexes, which is contrary to the natural order discussed in Section 11.2. 

The dimensions of the radii of anhydrous cations are constantly under revision (Ladd, 1968). All methods depend on inferences and assumptions. The data in Table 11.4 remain the most comprehensive and widely used. It will be 

Examination of the log K values for o-phenanthroline and 2,2 -bipyridyl (in Table 11.2) reveals the action of two principles which have been described above. The preference of Cu for a planar structure does not readily admit a third molecule of ligand. Hence takes an unusually low place in the table, 

See Section 14.2 for ionophores which, without any chelation, transport univalent ions (e.g. Na ) through natural membranes. 

Let us return to those ligands that are too bulky to approach one another closely and hence cannot form a strongly held 2 i-complex with a cation 

but this order is reversed by ligands that are too bulky for a pair of them to grip the metal. All dicarboxylic acids show this effect, e.g. succinic acid in (77.i6) (note oxalic acid in Fig. 11.5), but tartaric acid is outstanding in its preference for Ca over Mg + (Williams, 1952). Some relevant radii are given in Table 11.4. It should be noted that Fe, Co +, Ni +, and Zn have such similar diameters that they could not be separated through this steric effect. 

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Chapter 6, titled Selection of Ionization Methods of Analytes in the TLC-MS Techniques provides an overview of mass spectrometric techniques that can be coupled with TLC and act as specific detectors in this hyphenated approach. The mass spectrometric techniques discussed in this chapter are secondary mass spectrometry (SIMS), liquid secondary ion mass spectrometry (LSIMS), fast atom bombardment (FAB), matrix-assisted laser desorption/ionization (MALDI), atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), electrospray ionization (ESI), desorption electrospray ionization (DESI), electrospry-assisted laser desorption/ionization (ELDI), easy ambient sonic spray ionization (EASI), direct analysis in real time (DART), laser-induced acoustic desorption/electrospray ionization (LIAD/ESI), plasma-assisted multiwavelength laser desorption/ionization (PAMLDI), atmospheric-pressure chemical ionization (APCI), and dielectric barrier discharge ionization (DBDI). For the sake of illustration, the authors introduce practical examples of implementing TLC separations with detection carried out by means of individual mass spectrometric techniques for the systematically arranged compounds belonging to different chemical classes. 

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