Biomedical subjects

Daniel W Armstrong

Publications and source records attributed to Daniel W Armstrong.

At least 37 records · Page 2Linked to original sources

PubMed2005

Immobilized ionic liquids as high-selectivity/high-temperature/high-stability gas chromatography stationary phases.

Ionic liquids (ILs) are a class of nonmolecular solvents in which the cation/anion combination can be easily tuned to provide desired chemical and physical properties. When used as stationary phases in gas-liquid chromatography, ionic liquids exhibit dual nature retention selectivity. That is, they are able to separate polar molecules such as a polar stationary phase and nonpolar molecules such as a nonpolar stationary phase. However, issues such as optimization of the wetting ability of the ionic liquid on fused-silica capillaries, the maximum operating temperatures of the stationary phases, and nonuniform film thickness on the wall of the capillary at high temperatures have limited their use in gas chromatography. As described in this paper, these limitations are overcome by cross-linking a new class of ionic liquid monomers by free radical reactions to provide a more durable and robust stationary phase. By lightly cross-linking the ionic liquid stationary phase using a small amount of free radical initiator, high-efficiency capillary columns were produced that are able to endure high temperatures with little column bleed. Two types of cross-linked IL stationary phases are developed. A partially cross-linked stationary phase allows for high-efficiency separations up to temperatures of approximately 280 degrees C. However, by creating a more highly cross-linked stationary phase of geminal dicationic ILs, exclusively, an increase in efficiency is observed at high temperatures allowing for its use over 350 degrees C. In addition, through the use of solvation thermodynamics and interaction parameters, it was shown that the cross-linking/immobilization of the ionic liquid does not affect the selectivity of the stationary phase thereby preserving its dual nature retention behavior.

Journal Article↗

PubMed2005

Comparative study of three teicoplanin-based chiral stationary phases using the linear free energy relationship model.

Teicoplanin (T) is a macrocyclic glycopeptide that is highly effective as a chiral selector for enantiomeric separations. In this study, we used three teicoplanin-based chiral stationary phases (CSPs) - native teicoplanin, teicoplanin aglycon (TAG) and recently synthesized methylated teicoplanin aglycon (MTAG). In order to examine the importance of various interaction types in the chiral recognition mechanism the three related CSPs were evaluated and compared using a linear free energy relationship (LFER). The capacity factors of 19 widely different solutes, with known solvation parameters, were determined on each of the columns under the same mobile phase conditions used for the chiral separations. The regression coefficients obtained revealed the magnitude of the contribution of individual interaction types to the retention on the compared columns under those specific experimental conditions. Statistically derived standardized regression coefficients were used to evaluate the contribution of individual molecular interactions within one stationary phase. It has been concluded that intermolecular interactions of the hydrophobic type significantly contribute to retention on all the CSPs studied here. Other retention increasing factors are n- and pi-electron interactions and dipole-dipole or dipole-induced dipole ones, while hydrogen donating or accepting interactions are more predominant with the mobile phase than with the stationary phases. However, these types of interactions are not equally significant for all the CSPs studied.

Chromatography, High Pressure Liquid↗

PubMed2005

Using geminal dicationic ionic liquids as solvents for high-temperature organic reactions.

[reaction: see text] Several organic reactions conducted at high temperatures, including the isomerization reaction, the Claisen rearrangement, and the Diels-Alder reaction, were investigated in three geminal dicationic ionic liquids with high thermal stability. High to moderate yields of the products for most entries were obtained. Advantages of these approaches are discussed. These ionic liquids were shown to be recyclable. The utility of these ionic liquid solvents for high-temperature organic reactions was demonstrated.

Anthracenes↗

PubMed2005

High-performance liquid chromatographic enantioseparation of bicalutamide and its related compounds.

Direct high-performance liquid chromatographic methods were developed for the enantioseparation of (R,S)-bicalutamide (1) and its analogs (+/-)-3-chloro-N-(4-cyano-3-(trifluoromethyl)phenyl)-2-hydroxy-2-methylpropanamide (2), (+/-)-N-(4-cyano-3-(trifluoro-methyl)phenyl)-2-methyloxirane-2-carboxamide (3), (+/-)-4-fluorophenylsulfonyl-2-hydroxy-2-methylpropionic acid (4) and (+/-)-3-hydroxy-N-(4-cyano-3-(trifluoromethyl)phenyl)-2-hydroxy-2-methylpropanamide (5). The methods involved the use of a cellulose-based Chiralcel OD-H, macrocyclic glycopeptide-based Chirobiotic T, TAG and R, beta-cyclodextrin-based Cyclobond I 2000SN and t-butyl carbamate-derivatized quinine-based columns. The conditions affording the best resolution were found by selection and variation of the mobile-phase compositions, and the differences in separation capability of the methods were noted. The sequence of elution of the enantiomers was determined in all cases.

Anilides↗

PubMed2005

Gas-phase ion association provides increased selectivity and sensitivity for measuring perchlorate by mass spectrometry.

Perchlorate (ClO4-) competitively inhibits the uptake of iodide by the thyroid gland. Trace quantities of perchlorate are being increasingly detected in food and environmental samples. There is great concern that perchlorate contamination may be far more widespread than believed until now. Increasingly sensitive and unambiguous methods are needed for measuring perchlorate. We report here an ion chromatography-ion association-electrospray ionization-mass spectrometry (IC/IA-ESI-MS) method of substantially greater selectivity and sensitivity than other available single-stage MS approaches. A long chain dipositive cationic agent (D2+) is added postcolumn in low concentration. This ion associates with perchlorate, even in the gas phase. Perchlorate is, thus, detected as DClO4+ in the positive ion mode at an m/z value between 300 and 400 (depending on the choice of D2+). This results in much better S/N and selectivity, as compared to detecting 35ClO4- at m/z 99, where H34SO4- also responds. We show results for various dicationic agents which vary in their selectivity and affinity for ClO4-, typically being at least 1 order of magnitude more selective for ClO4- over HSO4-. For a 100-microL injected standard, limits of detection (LOD, S/N = 3) are as good as 25 ng/L on a single quadrupole mass spectrometer. Calibration for concentrations up to 100 microg/L displays an r2 value of > or =0.9993. We show applicability to various real samples. A number of the studied reagents are suitable for such applications.

Journal Article↗

PubMed2005

Separation and characterization of underivatized oligosaccharides using liquid chromatography and liquid chromatography-electrospray ionization mass spectrometry.

Native cyclodextrin-based columns are particularly useful for the analysis of oligosaccharides because the retention of these carbohydrates is based mainly on the hydrogen bonding interactions of oligosaccharide hydroxyl groups with the stationary phase. Thus, the retention time predictably increases with the number of analyte hydroxyl groups, which corresponds to the elongation of the oligosaccharide chain. High-performance liquid chromatography (HPLC) coupled to electrospray ionization (ESI) mass spectrometry (MS) was used for the separation and characterization of underivatized oligosaccharide mixtures. With the limits of detection as low as 50 pg, all individual components of oligosaccharide mixtures (up to 11 glucose units long) were baseline resolved on a Cyclobond I 2000 column and detected using ESI-MS. Low flow rates and narrow I.D. columns increase the ESI-MS sensitivity significantly. The method showed potential usefulness for the sensitive and quick analysis of hydrolysis products of polysaccharides, and for trace levels of individual oligosaccharide or oligosaccharide isomers from biological systems.

Acetonitriles↗

PubMed2005

Evaluation of ethoxynonafluorobutane as a safe and environmentally friendly solvent for chiral normal-phase LC-atmospheric pressure chemical ionization/electrospray ionization-mass spectrometry.

Coupling normal-phase LC separation methods to atmospheric pressure ionization (API)-mass spectrometry (MS) for detection can be problematic because of the possible detonation hazard and because nonpolar solvents do not support ionization of the analyte. Unlike achiral separations, enantiomeric separations can be very sensitive to small changes in the separation environment. Thus, completely substituting the main mobile phase component of a normal-phase LC solvent for an environmentally friendly, nonflammable fluorocarbon-ether as a safe and effective solvent must be thoroughly evaluated before it can be recommended for enantioselective separations with API-MS detection. Ethoxynonafluorobutane (ENFB) was used as a normal-phase solvent for the enantioselective separation of 15 compounds on two macrocyclic glycopeptide chiral stationary phases (CSPs) and a new polymeric chiral stationary phase. The chromatographic figures of merit were compared between results obtained with the ENFB mobile phases and traditional heptane-based mobile phases. In addition, the limits of detection (LOD) using the API-MS compatible ENFB were examined, as well as flow rate sensitivities and compatibilities with common polar organic modifier. ENFB is a safe and effective solvent for enantioselective normal-phase/API-MS analyses.

Atmospheric Pressure↗

PubMed2005

Chiral ionic liquids: synthesis and applications.

Over the last ten years, interest and publications involving ionic liquids have expanded exponentially. Thus far, they have predominantly been used in organic synthesis and separations. However, their use is rapidly expanding into other areas of science and technology. Research involving chiral ionic liquids has been much more limited and only recently has come to the forefront. In this work, we review the synthesis of chiral ionic liquids and their use. Today, this is an area of research that is poised for rapid development and expansion.

Alkanes↗

PubMed2005

Direct high-performance liquid chromatographic enantioseparation of beta-lactam stereoisomers.

High-performance liquid chromatographic methods were developed for the separation of the enantiomers of 12 beta-lactams. Direct separations were performed on chiral stationary phases (CSPs) containing cellulose-tris-3,5-dimethylphenyl carbamate (Chiralcel OD-RH and OD-H columns), the macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T column), or teicoplanin aglycone (Chirobiotic TAG column) as the chiral selector. It was clearly established that, with teicoplanin-based columns, the teicoplanin aglycone was most often responsible for the enantioseparation of the beta-lactams. The difference in enantioselective free energy between the aglycone CSP and the teicoplanin CSP was in the range between 0.02 and 0.97 kJ mol(-1) for these beta-lactam stereoisomer separations. The separations were carried out with high selectivity and resolution, and the method was therefore suitable for monitoring of the enantiomeric excess after chiral synthesis. The Chirobiotic and Chiralcel columns appear to be highly complementary to one another. The best separation of this class of beta-lactam compound could be obtained using the Chirobiotic TAG in the polar-organic mode plus the Chiralcel OD-H in the normal-phase mode. The elution sequence was also determined.

Carbohydrates↗

PubMed2005

Absolute configuration and predominant conformations of 1,1-dimethyl-2-phenylethyl phenyl sulfoxide.

The absolute configuration of the (+)-1,1-dimethyl-2-phenylethyl phenyl sulfoxide is determined to be (R), using three different chiroptical spectroscopic methods, namely vibrational circular dichroism (VCD), electronic circular dichroism (ECD) and specific rotation. Four solution conformations are identified for 1,1-dimethyl-2-phenylethyl phenyl sulfoxide. In each of the methods used, experimental data for the enantiomers of 1,1-dimethyl-2-phenylethyl phenyl sulfoxide were measured in the solution phase and concomitant quantum mechanical calculations of corresponding properties were carried out using density functional theory with B3LYP functional and 6-31G* and 6-31+G basis sets. Additional VCD and ECD calculations were also undertaken with 6-311G(2d,2p) basis set. A comparison of theoretically predicted data with the corresponding experimental data has allowed us to elucidate the absolute configuration and predominant conformations of (+)-1,1-dimethyl-2-phenylethyl phenyl sulfoxide.

Circular Dichroism↗

PubMed2005

On the capillary gas chromatographic separation of enantiomers of N-trifluoroacetyl-O-alkyl esters of selected amino acids on 2,3-di-O-pentyl-6-O-acyl cyclodextrins.

In this work, the separation of enantiomers of N-TFA-O-alkyl amino acids on the 2,3-di-O-pentyl-6-O-acyl alpha-, beta- and gamma-cyclodextrin stationary phases has been studied. The influence of structure differences in the alkyl substituents bonded to the stereogenic carbon atom (R1), as well as in the ester group (R2) of the selected amino acid derivatives, and the selectivity of modified alpha-, beta- and gamma-cyclodextrin phases in gas chromatographic separation of derivatized amino acid enantiomers was studied in detail. A model set of N-TFA-alkyl esters of four amino acids was separated on five columns. The separation of enantiomers was evaluated in terms of the interactions of the alkyl substituents bonded to the stereogenic carbon (R1) and/or the ester group (R2) of the N-TFA-O-alkyl amino acid derivatives as well as the nature of the 3-O-acyl group in the 2,6-di-O-pentyl-3-O-acyl alpha-, beta- and gamma-cyclodextrins. It was shown that the variation in the enantiomeric separation with temperature and the retention order of enantiomers on a given cyclodextrin capillary column depends both on the nature of the bonded R1 and R2 alkyl groups. It was found that the temperature dependencies of selectivity factors, ln alpha on 1/T, were mostly non-linear. The thermodynamic data [delta(deltaS) and [delta(deltaH)] which characterize the chiral recognition were used to gain more insight into the mechanistic aspects of enantio separation of the N-TFA-O-alkyl amino acid derivatives on 2,6-di-O-pentyl-3-O-acyl-alpha-, beta- and gamma-cyclodextrins.

Amino Acids↗

PubMed2005

Behavior of bacteria in the inductively coupled plasma: atomization and production of atomic ions for mass spectrometry.

The combination of perfusion chromatography (PC) and inductively coupled plasma mass spectrometry (ICPMS) is a fast, general way to monitor metal incorporation into bacteria. U+ signals from U incorporated intrinsically in Bacillus subtilis (B. subtilis) are measured with 4 ms time resolution to investigate the behavior of individual cells in the ICP. When intact B. subtilis cells are introduced directly into the ICP, occasional U+ spikes are observed. The positive U+ spikes suggest that bacteria behave more like solid particles than wet droplets in the ICP, compared to previous studies of such transient effects in the ICP. Drying the bacterial aerosol does not eliminate the spikes. Lysing the bacteria by sonication increases the U+ response by 30% compared to that from the untreated sample. PC results from a 10 ppb U standard, partially lysed and fully lysed bacteria samples show that the intracellular U-bound species are released by sonication and are small in size. The atomization-ionization efficiencies for different elements (U, Ca, and Mg) from cells differ somewhat. Reducing the aerosol gas flow rate by 0.1 L min-1 improves the relative U+ response for unlysed bacteria to 85% of that for lysed cells, although the absolute U+ signal is attenuated greatly.

Aerosols↗

PubMed2005

Chromatographic evaluation of poly (trans-1,2-cyclohexanediyl-bis acrylamide) as a chiral stationary phase for HPLC.

Chiral stationary phases (CSPs) based on polymeric (R,R)- or (S,S)-1,2-diaminocyclohexane (DACH) derivatives are synthesized. When bonded to 5 microm porous spherical silica gel, the poly (trans-1,2-cyclohexanediyl-bis acrylamide) based poly-cyclic amine polymer (P-CAP) stationary phases is proved to be effective chiral stationary phases that could be used in the normal-phase mode, polar organic mode and with halogenated solvents mobile phases, if desired. Since these are entirely synthetic CSPs, the elution order of all enantiomers can be reversed between the (R,R) P-CAP and (S,S) P-CAP columns. Because of the high loading of chiral selectors, the columns exhibit very high sample capacities. Thus, P-CAP columns are useful for preparative and semi-preparative enantiomeric separations. The application of these CSPs and optimization of their separations are discussed.

Acrylamides↗

PubMed2005

Separation of chiral furan derivatives by liquid chromatography using cyclodextrin-based chiral stationary phases.

The enantiomeric separation of a set of 30 new chiral furan derivatives has been achieved on native and derivatized beta-cyclodextrin stationary phases using high performance liquid chromatography (HPLC). The hydroxypropyl-beta-cyclodextrin (Cyclobond RSP), the 2,3-dimethyl-beta-cyclodextrin (Cyclobond DM), and the acetyl-beta-cyclodextrin (Cyclobond AC) stationary phases are the most effective chiral stationary phases (CSPs) for the separation of these racemates in the reverse phase mode. No enantioseparations have been observed on the native beta-cyclodextrin chiral stationary phase (Cyclobond I 2000) and only a few separations have been attained on the S-naphthylethyl carbamate beta-cyclodextrin (Cyclobond SN) and 3,5-dimethylphenyl carbamate beta-cyclodextrin (Cyclobond DMP) chiral stationary phases in the reverse phase mode. The polar organic and the normal phase mode on these CSPs are not effective for separation of these compounds. The characteristics of the analytes, including steric bulk, hydrogen bonding ability, and geometry, play an important role in the chiral recognition process. The pH affects the enantioseparation of compounds with ionizable groups and the addition of 0.5% methyl tert-butyl ether to the mobile phase significantly enhances the separation efficiency for some highly retained compounds.

Chromatography, Liquid↗

PubMed2005

Use of chiral ionic liquids as solvents for the enantioselective photoisomerization of dibenzobicyclo[2.2.2]octatrienes.

[Reaction: see text] Six chiral ionic liquids were prepared and evaluated as "chiral induction solvents" in which two different dibenzobicyclo[2.2.2]octatrienes were photoisomerized to chiral products. Enantiomeric excesses from 3 to 12% were obtained from the photochemical di-pi-methane rearrangement. Results indicate that the chiral induction derives from an ion pairing interaction of the deprotonated diacids with the ionic liquid cation. This is the first report on chiral induction via a chiral IL for an irreversible, unimolecular photochemical isomerization.

Alkenes↗

PubMed2005

Structure and properties of high stability geminal dicationic ionic liquids.

Thirty-nine geminal dicationic ILs were synthesized and characterized in terms of their surface tensions, densities, melting points, refractive indices, viscosities, and miscibilities with a polar and nonpolar solvent. Two imidazolium or pyrrolidinium cations were joined via different length hydrocarbon linkage chains (from 3 to 12 carbons long). The various geminal dications were paired with up to four different anions. The effect of the dication type, linkage chain, alkyl substituents, and anion type on the physicochemical properties of these compounds was examined. Among the more interesting findings for this class of compounds was that their liquid and thermal stability ranges generally exceeded those of the more conventional, better known ILs. Indeed, this range was from -4 to >400 degrees C for one of the pyrrolidinium-based geminal dicationic liquids. X-ray crystallography of the smaller solid ionic compounds indicated that there may be a correlation between the configurational degrees of freedom of the ILs and their melting points/glass transition temperatures. In one case, the crystal structure showed that a dicationic moiety had three distinct conformations in an asymmetric unit cell. The solvation properties of the geminal dicationic ILs tend to be similar to those of their monocationic analogues.

Journal Article↗

PubMed2005

Pharmacokinetics of theanine enantiomers in rats.

Theanine, first discovered in tea, is a chiral nonproteinic amino acid that has been reported to have cardiovascular, neurological, and oncological effects. It is being considered as a therapeutic/medicinal agent and additive in consumer products. The present study evaluated the pharmacokinetics of D-theanine, L-theanine, and D,L-theanine in plasma and urine using LC-ESI/MS in rats after oral (p.o.) and intraperitoneal (i.p.) administration. Oral administration data indicated that gut absorption of d-theanine was far less than that of L-theanine. However, after i.p. administration, plasma theanine concentrations of L- and D-theanine were similar. This indicated that D- and L-theanine may exhibit a competitive effect with respect to intestinal absorption. Regardless of the route of administration, p.o. or i.p., the presence of the other enantiomer always decreased theanine plasma concentrations, indicating D,L-theanine competition with respect to urinary reabsorption. Data on urinary concentrations of D-theanine suggested that the D-isomer may be eliminated with minimal metabolism. L-Theanine appeared to be preferentially reabsorbed and metabolized by the kidney while D-theanine was preferentially excreted. Clearly, the bioequivalencies of D,L-theanine and its enantiomers were found to be quite different from one another. Consequently, the efficacy of commercial theanine products containing D-theanine, L-theanine, or D,L-theanine may be quite different.

Animals↗

PubMed2005

Enantiomeric separation of furan derivatives and fused polycycles by cyclodextrin-modified micellar capillary electrophoresis.

The enantiomeric separations of highly hydrophobic furan derivatives and polycycles were performed and optimized using CD-modified micellar CE. The most effective chiral selector for the enantiomeric separation of these analytes was hydroxypropyl-gamma-CD. The effects of CD and SDS concentration and organic modifier were examined in order to optimize the separation conditions. The ratio of CD to surfactant concentration affected the enantiomeric separation significantly, with increases in the derivatized CD concentration generally enhancing resolution. Addition of an organic solvent modifier to the run buffer served to increase the analytes' solubility and enhance the separation efficiency. A highly acidic pH was necessary to effectively suppress the EOF when operating in the reverse polarity mode.

Chromatography, Micellar Electrokinetic Capillary↗