Amines, mobile aromatic

The dependence of chiral recognition on the formation of the diastereomeric complex imposes constraints on the proximity of the metal binding sites, usually either an hydroxy or an amine a to a carboxyHc acid, in the analyte. Principal advantages of this technique include the abiHty to assign configuration in the absence of standards, enantioresolve non aromatic analytes, use aqueous mobile phases, acquire a stationary phase with the opposite enantioselectivity, and predict the likelihood of successful chiral resolution for a given analyte based on a weU-understood chiral recognition mechanism. 

The chromatogram is freed from mobile phase, lightly but homogeneously sprayed with the spray solution or immersed in the dipping solution for 2 s. Triazines and aromatic amines must first be converted to chlorinated derivatives by exposing the chromatogram to chlorine gas (see Procedure Tested ). 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting), and CE as a detection technique for electroactive compounds. In this usage, the voltammetric cell has been miniaturised (to about 1 p.L) in order not to dilute the analytes after separation. This method of amperometric detection in the pulsed mode is very sensitive. However, this device makes it possible to detect few analytically important molecules besides phenols, aromatic amines and thiols. 

A similar study was carried out using a different set of synthetic dyes. The chemical structures of the dyes are listed in Fig. 3.62. The decomposition products were separated in an ODS column (250 X 4 mm i.d. particle size 7 /tm) using an isocratic mobile phase composed of methanol-water (45 55, v/v). The flow rate was 1.2 ml/min. The contents of aromatic amines determined by spectrophotometric and HPLC methods are compiled in Table 3.24. It was established that spectrophotometry can be used for the exact determination of the amines but it is inadequate for their separation. RP-HPLC proved to be a valuable method for the analysis of this class of decomposition products [131],... 

HPLC methods can be ntilized for the pre-concentration of aromatic amines from polluted waters on silica gel or octadecyl silica (ODS) colnmns [55], The determination is then performed by RP HPLC using ODS packings as the stationary phases and a mixture of methanol, isopropanol, and water as the mobile phase [55], RP HPLC with diode array detector (DAD) methods coupled on-line with a continnons seqnential anaerobic/aerobic reactor system have been employed in wastewaters treatments [56], A continnons monitoring of the possible presence of aromatic amines in azo dyes wastes is based on indncing in the waste, the reaction of a reduction of the dye, followed by HPLC/ UV or HPLC/MS analysis [57-59], The redncing agent solutions are sodium dithionite or tin(II) chloride in an aqneons acidic medinm at 70°C, followed by SPE [58,59], LLE [60,61], or SEE [60-62],... 

In amine-imine systems (75 76) the mobile proton can in principle be located at either of the two basic nitrogen sites in the anion (77). Since the canonical form with aromatic (benzenoid) structure is polar in the imine (76b) and non-polar in the amine (75a), the amine structure should be favoured, particularly in non-polar solvents. This seems generally to be the case, although solvent and medium effects do not appear to have been investigated the results available at the present time refer mainly to aqueous and other polar solvents. 

LCEC systems are used for a wide variety of applications, many of which have been published [1,32]. Space does not permit a thorough review however, it is possible to generalize by considering the classes of compounds that have most frequently been studied phenols, aromatic amines, thiols, quinones, and nitro compounds. By analogy with liquid chromatography with UV detection (LCUV), it is frequently desirable to obtain an electrochemical spectrum of a compound to assess Its suitability for LCEC. Cyclic voltammetry (CV) is the electrochemical equivalent of spectroscopy (Chap. 3). It is useful to carry out CV experiments in several possible mobile phases, since electrochemical reactions can be very dependent on the medium. 

The most recent paper on this topic has been published by Lu and Huang (213). The method consists of an online enrichment of the aromatic amines on a carboxymethyl-bonded silica precolumn and an HPLC-UV (at 254 nm) analysis. The mobile phase, ACN-acetate buffer (pH 4.66) (40 60, v/v), was used to desorb the analytes and for the subsequent separation. The method was applied to the determination of several compounds (4-aminoazobenzene (4-AAB), benzidine (Bz), 3,3 -methylbenzidine (DMBz), 4-aminobiphenyl (4-ABP), 3,3 -dichlorobenzi-dine (DCBz), and 2-naphthylamine (2-NA) together with some substituted naphthalens and phenols) in aqueous solution of four food dyes Direct Blue 6, Amaranth, Sunset Yellow FCF, and D C Orange No. 4. Detection limits ranged between 0.6 and 1.6 fig/g. Most part of the methods developed for this kind of determination are reported in Table 3. 

Plasticizers include the esters of a few aliphatic and aromatic mono and dicarboxylic acids, aliphatic and aromatic phosphorus acid esters, ethers, alcohols, ketones, amines, amides, and non-polar and chlorinated hydrocarbons. These additives are used in various mixtures. For their separation and qualitative detection, thin-layer chromatography (TLC) is preferred. Usually Kieselgur plates, 0.25 mm thick, activated at 110°C for 30 min, in the saturated vapor are used. Methylene chloride and mixtures of diisopropyl ether/petether at temperatures between 40 to 60°C have been successfully used as the mobile phase. Refer to Table 1. 

---