Biomedical subjects

J A Blackwell

Publications and source records attributed to J A Blackwell.

8 recordsLinked to original sources

PubMed2001

The role of modifiers in high selectivity chiral separations on Pirkle-type columns.

Atypically high selectivity (alpha > 2) chiral separations have been observed on Pirkle type supports. These separations were examined in terms of their response to various modifiers, thermodynamic behavior and LSER modeling. Atypical thermodynamic and LSER results were observed along with large changes in selectivity between modifiers. The use of bulky modifiers gave increased selectivity resulting from dramatic increases in the retention of the second eluting enantiomer. These findings lead to the conclusion that increased retention is due to an inability of bulky modifers to penetrate a tight complex between selector and this enantiomer.

Journal Article↗

PubMed2000

A comparative study of factors affecting chiral separations under reversed-phase conditions using cellulosic and Pirkle-type chiral selectors.

Limited studies have been conducted in reversed-phase chiral chromatography despite the relevance to water soluble chiral analytes. Systematic investigation and interpretation of factors affecting chiral separation in reversed-phase LC is crucial to efficient chiral method development. Two of the most useful chiral stationary phases (CSPs), a cellulosic (Chiralcel OJ-R) and a Pirkle-type (Chirex 3005), were chosen as the chiral selectors in this study to systematically investigate the factors affecting chiral separation of a series of Fmoc-amino acids. The effects of temperature, pH, buffer concentration, buffer type and organic modifiers on retention and chiral selectivity were compared between the cellulosic and the Pirkle-type selectors. Temperature affected retention and selectivity, as predicted by thermodynamics, using both types of chiral selectors. On the cellulosic CSP, buffer pH was the only factor which caused a significant change in chiral selectivity, while several factors, including buffer concentration and organic modifier type, affected chiral selectivity on the Pirkle-type CSP. The results also indicated that the cellulosic CSP had a retention mechanism analogous to conventional reversed-phase LC, however, the Pirkle-type selector appeared to show a mixed mode retention mechanism for these analytes.

Journal Article↗

PubMed1999

Empirical relationship between chiral selectivity and mobile phase modifier properties.

An empirical relationship was derived which relates properties of the mobile phase modifier to the chiral selectivity factor for a given analyte/chiral selector combination. Using carbon dioxide and heptane-based mobile phases, the effect of various mobile phase modifiers on Pirkle-type stationary phases may be accurately modeled using a two-parameter equation. Similar results are obtained using cellulosic stationary phases with carbon dioxide-based mobile phases. Modeling separations performed using heptane-based mobile phases with cellulosic stationary phases were not successful. The predictive ability of this modeling approach was demonstrated using novel modifiers and chiral analytes.

Chromatography, Liquid↗

PubMed1997

Quantitation and determination of molecular weight distribution of residual water soluble extractable polyamines in DMP 504, a poly-alkylamine bile acid sequestrant, by aqueous high performance size exclusion chromatography.

A high performance size exclusion chromatography (HPSEC) method with refractive index detection for quantitation and molecular weight determination of extractable water soluble polyamines (SPA) in a novel proprietary polymeric pharmaceutical compound (DMP 504) is described. The extracted polyamines are synthetic impurities as well as potential degradation products of the polymer. Data for calibration, precision, solution stability, and limits of detection and quantitation are presented to validate the suitability of this method for its intended purpose. Response linearity for the poly(2-vinylpyridine) standard is demonstrated between 10 and 1000 micrograms ml-1, equivalent to 0.005% and 0.5% (w/w) in DMP 504. Molecular weight distributions of SPA extracted from DMP 504 are presented. No increase in SPA is detected in DMP 504 over 6 months storage at room temperature.

Calibration↗

PubMed1992

The role of Lewis acid-base processes in ligand-exchange chromatography of benzoic acid derivatives on zirconium oxide.

Porous microparticulate zirconium oxide shows very different selectivities and pH dependencies for the separation of benzoic acid derivatives than do conventional bonded-phase anion-exchange supports. This results from a very significant ligand-exchange contribution to the retention of hard Lewis bases on the surface of transition-metal oxide supports. We have found that the capacity factors of a wide variety of derivatives of benzoic acid are closely correlated with their Bronsted acidities. The eluent pH is also a critical factor in determining the magnitude of the capacity factor, but it does not have much influence on chromatographic selectivity. The differential selectivity of this phase in comparison to conventional polymeric and bonded-phase anion exchangers can be attributed to complexation and steric effects which profoundly alter the elution patterns of certain solutes.

Benzoates↗

PubMed1992

Development of an eluotropic series for the chromatography of Lewis bases on zirconium oxide.

Ligand-exchange interactions, which dominate the retention characteristics of Lewis base solutes on zirconium oxide, can be strongly attenuated by the addition of a competing Lewis base to the eluent. The chromatographic effects of these competing bases vary significantly in their thermodynamics and kinetics depending on their structure and Lewis basicity. An eluotropic scale of mobile-phase strength for ligand-exchange/ion-exchange chromatography on zirconia has been developed which ranks the overall elution strength of a variety of Lewis bases in terms of their ability to elute a wide variety of benzoic acid derivatives. This series generally holds true for similar solutes; however, deviations are noted with some solutes and eluents where chelation and steric factors alter the kinetic and thermodynamic characteristics of the retention process.

Benzoates↗

PubMed1992

Ion- and ligand-exchange chromatography of proteins using porous zirconium oxide supports in organic and inorganic Lewis base eluents.

The applicability of an eluotropic scale pertaining to the desorption of low molecular weight Lewis base solutes from zirconium oxide is examined for its ability to rationalize the retention of proteins on this substrate. The strongest Lewis base eluents (phosphate and fluoride) are able to bring about elution of nearly all proteins provided that their initial mobile phase concentration almost saturates the eluent's adsorption isotherm. In contrast, weaker Lewis bases such as borate, sulfate and bromide are able to elute only those proteins which are retained primarily by ionic interactions. In weak eluents, proteins that contain a large number of accessible Lewis base sites are not eluted from the support. The effect of ionic strength and a variety of Lewis base eluents were also examined.

Borates↗

PubMed1986

Reproductive responses, progesterone profiles and ovarian characteristics of cycling, fine-wool ewes treated with an inhibitor of progesterone synthesis (epostane).

In Exp. 1, 45 fine-wool ewes received (sc) either 0, 50 or 100 mg epostane (3-beta-hydroxysteroid dehydrogenase inhibitor) to examine effects on return to estrus and conception rates. Treatments were imposed on d 10 of an estrous cycle (estrus = d 0) and jugular blood samples were collected once daily on d 8 and 9, twice daily on d 10 through 13 and once daily from d 14 until first post-treatment estrus or d 20 (15 ewes/treatment). Intensive samples were obtained hourly for 6 h after treatment (d 10; five ewes/treatment). Serum progesterone (P4) before treatment was similar among groups; but by 2 h after treatment, epostane-treated ewes had lower (P less than .10) values than controls. By 6 h post-treatment, serum P4 in ewes receiving 50 (1.0 ng/ml) and 100 (.9 ng/ml) mg epostane was well below control values (3.2 ng/ml). By 7 d after treatment, 93.3% of ewes treated with 50 mg epostane had recycled compared with 66.7 and 33.3% of those receiving 100 and 0 mg, respectively (P less than .10). Similarly, 93.3, 53.3 and 26.7% of ewes receiving 50, 100 or 0 mg epostane, respectively, lambed to breeding within 7 d of treatment (P less than .05). Overall conception rates during a 34-d breeding season were similar among groups. Preweaning performance of offspring did not differ among maternal treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Androstenols↗