Anions adjusted retention times

Table 3,4, Adjusted retention times as a function of spacer arm length of anion exchange resins.

Experiments with several different hydrophobic quaternary ammonium salts as coating reagents showed that the adjusted retention times of test anions relative to t for chloride varied somewhat with the chemical structure of the coating [20]. However, it was found that the relative adjusted retention times also varied considerably... 

Table 3.9. Adjusted retention times for anions relative to chloride. Capacity of trimethylamine XAD-1 was 0.027 mequiv/g theoretical capacity of cetylpyridinium coated resins was 0.050 mcquiv/g. Eluent was 0.2 raM tetrabutylammonium phthalate, pH 6.5. Conductivity detection.

Table 6.9 lists the relative retention time of the chloride sample anion with the acid eluents chosen for this study. The adjusted retention time of chloride decreases as the retention time for the eluent anion increases and it also decreases as the amount of ionization of the eluent increases. The most satisfactory separations were obtained with either nicotinic acid or succinic acid as the eluent acid. 

Table 6JS. Relative adjusted retention times t lf Cl) of various anions with different acid eluents (Cl = 1()0)I . (From Ref. [14]).

Separations of metal cations are best carried out with a carboxyhc acid stationary phase. The mobile phase contains a chelating reagent, such as citric, ojaUc, or 2,6-pyridinedicarboxylic acid (PDA). The concentration and pH of the mobile phase are adjusted so that metal ions in the sample will be in an equilibrium between the cationic form, M, and a neutral or anionic metal ligand complex form. The rate at which a metal analyte moves down the column wiU depend on the fraction that remains in the free cationic form. A greater degree of complexa-tion results in a shorter retention time. 

Co-administration of ofloxacin and chitosan in eyedrops increased the bioavailabUity of the antibiotic [290]. Trimethyl chitosan was more effective because of its solubility (plain chitosan precipitates at the pH of the tear fluid). On the other hand, N-carboxymethyl chitosan did not enhance the corneal permeability nevertheless it mediated zero-order ofloxacin absorption, leading to a time-constant effective antibiotic concentration [291]. Also W,0-carboxymethyl chitosan is suitable as an excipient in ophthalmic formulations to improve the retention and the bioavailability of drugs such as pilocarpine, timolol maleate, neomycin sulfate, and ephedrine. Most of the drugs are sensitive to pH, and the composition should have an acidic pH, to enhance stability of the drug. The delivery should be made through an anion exchange resin that adjusts the pH at around 7 [292]. Chitosan solutions do not lend themselves to thermal sterilization. A chitosan suspension, however. 

---