Locating agents

Water 5 J diethylamine must be removed from the paper before locating agents are used... 

System 2 is quite non-polar and useful for discriminating between highly lipophilic bases, which include many of the antihistamines and narcotics, and sympathomimetic bases, which are often quite polar and move very little in mobile phase 2. The use of selective solvent systems is often combined with use of location agents that are selective for nitrogenous drugs. 

The plate is put into a tank containing iodine crystals. This treatment will produce brown spots with many organic compounds the staining is reversible, so that if it is necessary to recover the compound once it has been located, the iodine may be allowed to evaporate by exposing the plate to air and then the marked spot containing the compound of interest may be scraped off the plate. If a permanent record of the plate is required it has to be covered to prevent the iodine evaporating or the iodine spots may be sprayed with starch solution in order to stain them permanently. Iodine is used as a location agent in pharmacopoeial TLC tests of fixed oils and of cetrimide. 

Locating agent A substance used to locate, on a chromatogram, the separated parts of a mixture in chromatography. 

How can you determine which amino acids are present in a particular protein This involves hydrolysis of the peptide (amide) bonds in the protein so that the individual amino acids are released. This can only be done by heating the protein with dilute hydrochloric acid. The mixture of amino acids is then separated by thin layer chromatography (TLC) (Figure 15.24) or electrophoresis. In both cases, a locating agent (Chapter 2, p. 23), such as ninhydrin, is used. This ensures that the spots of amino acid are visible. 

The major drawbacks in the case of this system have been connected in the past with the unspecific nature and lack of sensitivity of the detectors employed (e.g. refractive index detection, UV monitor, flow calorimeter). These may be overcome by the use of a locating agent after the column as in ion-exchange chromatography or by the analysis of specific derivatives followed by colorimetric or fluorimetric detection. The latter, termed precolumn derivatisation, appears to be the more suitable approach since commercially available HPLC systems may be adopted without modification and the risk of peak broadening with increase in system dead volume are minimised. 

The chromatographic analyses described by Mopper and Larsson (1978), with an improved version described by Mopper (1978b), rely on the separation of complex uronic acid mixtures on anion-exchange columns in the acetate form with acetate buffers as eluant and are capable of detecting uronic acids in the nanomole range with Cu-bicinchoninate as locating agent. 

These oxazolines have cationic surface-active properties and are emulsifying agents of the water-in-oil type. They ate acid acceptors and, in some cases, corrosion inhibitors (see Corrosion). Reaction to oxazoline also is useful as a tool for determination of double-bond location in fatty acids (2), or for use as a protective group in synthesis (3). The oxazolines from AEPD and TRIS AMINO contain hydroxyl groups that can be esterified easily, giving waxes (qv) with saturated acids and drying oils (qv) with unsaturated acids. 

Unfortunately, there is a significant disadvantage resulting from appHcation of insect-resist agents from dyebaths it is impossible to ensure 100% exhaustion (transfer of pesticide from dyebath to fiber) and as a result, there is inevitably some environmental contamination. The extent of concern with this release of insect-resist agent depends on the spectmm of activity of the agent. If it is a broad-spectmm insecticide, especially one with reasonable persistence and lipophilic character, it is Hable to be reasonably toxic to aquatic insects and invertebrates, especially in certain environmental locations where... 

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