Biomedical subjects

Lloyd R Snyder

Publications and source records attributed to Lloyd R Snyder.

3 recordsLinked to original sources

PubMed2006

Peak compression in reversed-phase gradient elution.

Previous reports suggest that peak widths in linear gradient elution are consistently larger than predicted by theory; however, if gradient compression is ignored, experiment and theory are in reasonable agreement. This suggests that gradient compression might represent an incorrect or poorly understood concept. In the present study, an experimental program was carried out to better understand the role of gradient compression and the reason for past differences between experiment and theory. It is concluded that the concept of gradient compression is correct.

Chromatography, High Pressure Liquid↗

PubMed2004

Column selectivity in reversed-phase liquid chromatography. V. Higher metal content (type-A) alkyl-silica columns.

Retention measurements involving 16 test solutes have been carried out for 38 type-A alkyl-silica columns and three bonded-zirconia columns. These measurements have been analyzed in terms of a model previously developed for type-B columns, so as to yield values of five column selectivity parameters (H, S*, A, B, C) for each type-A column. Overall differences in selectivity between type-A and -B columns can be related to the average values of H, S*, etc. for each column type. Compared to type-B columns, type-A columns provide generally stronger retention for carboxylic acids, while solutes that are more hydrophobic or less bulky are more retained on type-B columns. Hydrogen-bond acceptors (e.g. aliphatic amides) and cations (e.g. protonated bases) are strongly retained on type-A versus type-B columns. Compared to type-B columns, bonded-zirconia columns show much increased retention of cations and reduced retention of hydrogen-bond acceptors. Because of relatively large differences in the selectivity of bonded-zirconia, type-A, and type-B columns, it will prove difficult to find columns of different type (e.g. a type-A and a type-B column) which have equivalent selectivity. Type-A columns also tend to be more different from each other (in terms of selectivity) than is the case for type-B columns. As a result, the replacement of a given type-A column by an "equivalent" type-A column also appears unlikely, except for samples that do not contain ionized compounds.

Chromatography, Liquid↗

PubMed2003

Column selectivity in reversed-phase liquid chromatography. IV. Type-B alkyl-silica columns.

Columns for reversed-phase HPLC (RP-LC) can be characterized by five, retention-related parameters: H (hydrophobicity), S (steric selectivity), A (hydrogen-bond acidity), B (hydrogen-bond basicity), and C (cation-exchange behavior). In the present study, values of the latter parameters have been measured for 92 type-B (low metals content)alkyl-silica columns and compared to column properties such as ligand length,ligand concentration, pore diameter, and the presence or absence of end-capping. With the exception of five columns of unusual design, retention factors, k, for 16 representative test compounds were correlated with values of H, S, etc., within an average +/- 1.2% (1 standard deviation, SD), suggesting that all significant solute-column interactions are recognized by these five column parameters. A single-valued function F(s) is proposed to measure differences in selectivity for any two RP-LC columns whose values of H, S, etc., are known. This allows the easy selection of columns whose selectivity is desired to be either similar to or different from a starting column, for application in either routine analysis or method development.

Chromatography, High Pressure Liquid↗