No abstract

No abstract

No abstract

The ever-increasing number of commercially available reversed phases with which the analyst is confronted can cause problems in column selection. These and the non-standard test procedures used by the column manufacturers and packing companies cause further confusion. In order to independently compare and contrast a range of well established C18 stationary phases, we have performed a modified column characterization approach, based on Tanaka's methodology, on thirty commonly used phases in our laboratory. These results have been evaluated and presented in various formats as Principal Component Analysis, Cluster Analysis, Tabular Format and Radar Plots in order to assist in observing similarities and differences between phases. The results indicate that, while no two phases are exactly the same (with the exception of "me-to" clones), it is possible to characterize phases into different classes based on their chromatographic performance against selected test probes. The paper illustrates the similarities and differences between these phases. The column characterization described in the paper can form the basis of a rational column selection protocol by either careful matching of the appropriate column characteristics to the analyte's physico-chemical properties or by a systematic evaluation of columns from our various categories.Original

Abstract:The operational parameters and factors which control the performance Ž . of capillary electrochromatography CEC using commercially available CEC instrumentation are evaluated and discussed. The CEC of neutral or ion-suppressed acidic analytes shows marked advantages such as increased column efficiency and reduced analysis times compared to conventional high-performance liquid chro-Ž . matography HPLC . Development of the CEC method using C-18 stationary phases follows similar guidelines to that of HPLC and, due to the high efficiencies obtained, the time required for method development can be substantially reduced. The technique of CEC is extremely attractive for the analysis of pharmaceuticals due to its excellent quantitative characteristics: good repeatability, reproducibility, wide UV detector linearity, and excellent detection limits. The analysis of acidic and neutral analytes by CEC using reverse-phase material is highly successful.However, in order to analyze highly basic analytes a strong cation exchange stationary phase is needed. This phase can produce staggering and as yet unexplained ''apparent'' efficiencies of over 40 = 10 6 plates per meter; however, to-date these results are highly non-reproducible. The application of CEC to the pharmaceutical industry has been demonstrated in the analysis of a wide range of structurally diverse pharmaceutical compounds using capillaries packed with reverse-phase materials.

No abstract

The separation of ionized bases by reversed-phase liquid chromatography with alkyl silica columns often leads to severely tailed bands that are highly detrimental. Band shape and its dependence on sample mass are notably different when mobile-phase pH is changed, and this behavior has not been previously explained. Ionized silanols present in the stationary phase have been credited with a role in determining peak shape. In the present study, separations on two different polymer columns were compared with those previously obtained on alkyl silica phases. Because silanols are absent from polymer columns, this comparison enabled us to assess the role of silanols in separations on alkyl silica phases and to offer an explanation of why band shape changes with sample size and mobile-phase pH for both polymer and silica-based phases.

Chromatographic classification and comparison of commercially available perfluorinated stationary phases for reversed-phase liquid chromatography using Principal Component Analysis A range of ten perfluorophenyl and perfluoroalkyl stationary phases has been evaluated using standard chromatographic tests and probes. Principal Component Analysis of the data has indicated that the phases can be divided into distinct groupings. Extending the dataset to include standard alkyl and phenyl phases provided further data interpretation to support the orthogonal selectivity claims made for perfluorinated phases. The analysis of a range of basic analytes showed an unusual extended retention of hydrophilic basic analytes with perfluorophases. Furthermore, a nonlinear relationship between the amount of organic modifier and the logarithm of the retention factor was observed, for the hydrophilic bases, which could not be modelled with LC prediction softwares. This was in sharp contrast to the alkyl and phenyl phases examined. Basic analyte retention on perfluoroalkyl phases could be modelled adequately for the lipophilic bases. Exploration of the retention mechanism of these perfluoro phases indicated that silanol interactions were important in retention and selectivity. Using a rapid, isocratic, high organic modifier methodology, it was possible to analyse a mixture containing a lipophilic steroid, hydrophilic base and an internal standard in a 4 minutes with a perfluorophenyl phase. This had previously only been achievable with an alkyl phase under gradient elution conditions.