Characterization of silica bonded stationary phases

Synthesis and characterization of silica-based aliphatic ion exchangers

The chemical literature describing preparation of aliphatic ion exchangers is limited, and although such phases are available commercially, the synthetic schemes are proprietary. The course of our research required the preparation of silica-based aliphatic cation and anion exchangers for which the desired base silica and/or ligand properties were not commercially available. We developed synthetic schemes to prepare silica-based aliphatic sulfonic acid, carboxylic acid and quaternary ammonium ion-exchange phases with active exchange capacities of 0.2-0.9 mumol/m2. Multiple techniques were used to characterize the intermediate and final phases produced in the syntheses, including Fourier transform diffuse reflectance infrared spectroscopy, spot tests, elemental analysis, acid-base titration and elution analysis.

Monitoring of veterinary drug residues by a combination of continuous flow techniques and column-switching high-performance liquid chromatography. I. Sulphonamides in egg, meat and milk using post-column derivatization with dimethylaminobenzaldehyde

By using a combination of a low-pressure continuous flow module and a column-switching high-performance liquid chromatographic system, procedures have been developed for the automated residue analysis of sulphonamides and selected other drugs in meat, egg and milk. Aqueous extracts are purified by on-line dialysis and subsequent trace enrichment on a short column containing silica-based or polymer material. After backflush to the analytical column, detection is performed either at 280 nm or, after post-column derivatization with p-dimethylaminobenzaldehyde, at 450 nm. With these procedures the fully automated determination of both polar and apolar sulphonamides and dapsone above 5-20 micrograms/kg is possible. Coefficients of variation are 4-10%, recoveries compared to standards are 85-90%. The methods developed were tested in routine veterinary drug control.

Influence of solute polarity in column-switching chromatography for the assay of drugs in plasma and urine

The polarity of a drug is one of the most important parameters for the elaboration of switching systems. If the polarity of the drug is low or medium, "reversed-phase" chromatography is well adapted. The plasma or urine sample is diluted with water, centrifuged and injected first into a column of medium polarity (C2, CN or diol bonded phases). The compounds of interest are stopped on the top of the column and rinsed with water, then eluted and chromatographed on a C8 or C18 analytical column. A third column of still lower polarity can be added to improve the specificity of the system. In each successive step, the polarities of the mobile phases and columns should be decreased to reconcentrate the sample and reduce the band broadening that occurred in the previous step. Compounds of high polarity show almost no retention on reversed-phase columns, and normal-phase chromatography should be used. Aqueous solutions cannot be injected into polar bonded-phase columns as they lead to excessive band broadening. This problem can be solved by diluting plasma or urine with a large volume of a water-miscible organic solvent and injecting the clear supernatant. The compounds to be assayed are first reconcentrated on a polar column (NH2 or N(CH3)2 bonded phase) and then eluted. The selected "heart cut" of the eluate is chromatographed on another, more polar column. The influence of the polarity of drugs on the choice of switching systems is exemplified by assay methods for drugs of low, medium and high polarity.