Harold M. McNair

Geometry of chemically modified silica

The effect of alkyl chain length on adsorbent pore volume and void volume of HPLC columns is described. The results provide evidence that alkyl chains attached on silica surface are densely packed. A correlation of a decrease of pore volume with an increase of the alkyl modifier chain length was found. Effective molecular volume of bonded chains was found to be similar to the molecular volume of corresponding liquid alkanes. An absence of noticeable penetration of acetonitrile, methanol, or tetrahydrofuran molecules between bonded chains at any water-organic eluent composition was found.

Analysis of polyamines as markers of (patho)physiological conditions

The aliphatic polyamines, putrescine, spermidine and spermine, are normal cell constituents that play important roles in cell proliferation and differentiation. The equilibrium between cellular uptake and release and the balanced activities of biosynthetic and catabolic enzymes of polyamines are essential for normal homeostasis in the proliferation and functions of cells and tissues. However, the intracellular polyamine content increases in hyperplastic or neoplastic growth. Although the involvement of polyamines in physiological and pathological cell proliferation and differentiation has been well established, the role they play is quite different in relation to cell systems and animal models and is dependent on inducer agents and stimuli. Also, the experimental procedures used to deplete polyamines have been shown to influence the cell responses. In this paper, the assay methods currently in use for polyamines are reviewed and compared with respect to sensitivity, reproducibility and applicability to routine analysis. The relevance of polyamine metabolism and the uptake/release process in many physiological and pathological processes is highlighted, and the cellular polyamine pathways are discussed in relation to the possible diagnostic and therapeutic significance of these mediators.

Effect of the eluent pH and acidic modifiers in high-performance liquid chromatography retention of basic analytes.

The retention of ionogenic bases in liquid chromatography is strongly dependent upon the pH of the mobile phase. Chromatographic behavior of a series of substituted aniline and pyridine basic compounds has been studied on C18 bonded silica using acetonitrile-water (10:90) as the eluent with different pHs and at various concentrations of the acidic modifier counter anions. The effect of different acidic modifiers on solute retention over a pH range from 1.3 to 8.6 was studied. Ionized basic compounds showed increased retention with a decrease of the mobile phase pH. This increase in retention was attributed to the interaction with counter anions of the acidic modifiers. The increase in retention is dependent on the nature of the counter anion and its concentration in the mobile phase. It was shown that altering the concentration of counter anion of the acidic modifier allows the optimization of the selectivity between basic compounds as well as for neutral and acidic compounds.

Micellar electrokinetic chromatography employing sodium alkyl sulfates and Brij 35

Abstract Sodium decyl and dodecyl sulfates were evaluated as micellar phases for the micellar electrokinetic chromatographic separation of ASTM test mixture LC-79-2. Selectivity was similar in each system but different from the selectivity obtained in reversed-phase high-performance liquid chromatography. Despite separation efficiencies of approximately 150 000 theoretical plates per 50 cm, benzene and benzaldehyde coeluted in all concentrations of both surfactants employed. Separation was, however, readily achieved by addition of Brij 35® [polyoxyethylene(23)dodecanol] to the micellar phase.

Hydrocarbon group-type analyses by on-line multi-dimensional chromatography : II. Liquid chromatography-gas chromatography

Abstract An on-line multi-dimensional chromatographic system, coupling high-performance liquid chromatography (HPLC) and capillary gas chromatography (GC) is described. The system is controlled by a microprocessor and can be automated. The application of this system to hydrocarbon group-type analysis is demonstrated by using HPLC on silica gel to fractionate petroleum samples into saturates, unsaturates, aromatics and polar compound, followed by the characterization of these groups by capillary GC with flame-ionization detection. Analyses of gasoline and diesel fuel are demonstrated.

Influence of buffer concentration, capillary internal diameter and forced convection on resolution in capillary zone electrophoresis

Abstract The relative velocity difference (ΔU/Uav) of two zones, separated by capillary zone electrophoresis, increased with increasing buffer concentration, but remained constant for a given concentration regardless of electric field strength. For diffusionlimited band broadening, the increase in ΔUav offset a decrease in migration velocity to provide slightly better resolution. In practice, however, additional dispersion occurred as a result of Joule heating, especially when concentrated buffers, high electric field strengths and/or capillaries with large internal diameters were employed. To improve efficiency, under such conditions, forced air convection was investigated as a means for dissipating some of the Joule heat generated. In 100-μm capillaries, forced convection increased efficiency from 190 000 ± 3.1% relative standard deviation (R.S.D.) to 226 000 ± 3.3% R.S.D. theoretical plates. For comparison, 264 000 theoretical plates were observed in 50-μm capillaries under similar operating conditions. In the latter case the improved efficiency is, however, obtained at the expense of sample capacity.

Microbore high-performance liquid chromatographic columns: speed, efficiency, sensitivity and temperature programming

Abstract Microbore columns (1 mm I.D.) were compared with conventional 4.6 mm I.D. columns with respect to speed, efficiency and sensitivity. If the column lengths, particle diameter, packing efficiency, mobile phase and linear velocity are the same, most chromatographic properties such as speed (analysis time), efficiency, pressure drop and sensitivity are also the same. The major differences are the column volumes, the void volumes and the difference in volumetric flow-rates for equal linear flow velocities. The low flow-rates of microbore columns (30-200 μl/min), however, do save on solvent consumption and do make for easier and more efficient direct interfacing to other instruments. Temperature programming of microbore columns produces faster analyses for large k′values, increased sensitivity (due primarily to sharper peak shape) and increases the range of compounds that can be handled in an isocratic mode.

Adduct formation in electrospray ionization. Part 1: Common acidic pharmaceuticals

This study focuses on pseudo-molecular ion formation in electrospray ionization mass spectrometry (ESI-MS) of six anti-inflammatory pharmaceuticals with similar functionality. The formation of particular pseudo-molecular ions depends on ion affinity and molecular structure of the analyte as well as the solvent/buffer conditions used. Six common anti-inflammatory agents are mixed 1:1 with six different acetonitrile/aqueous buffer solutions at varying concentrations. The analytes are ibuprofen, carprofen, naproxen, ketoprofen, flurbiprofen, and fenoprofen. The pK a and surface activity of the analytes and the pH, concentration, and type of the solvent system strongly affect the ions formed [1,2]. The additives are common liquid chromatography (LC) mobile phase modifiers. The spectral intensities of three major pseudo-molecular ions were measured by flow injection analysis ESI-MS. The ions studied correspond to the deprotonated molecular ion ([M - H] - ), a deprotonated dimer ion ([2M - H] - ), and a deprotonated dimer ion pair with sodium ([2M - 2H+Na] - ). These ions were chosen due to their high relative abundance in a majority of the spectra. The pKa of the analytes studied range from 4.1 to 4.4, due to their aromatic acetic acid moiety. The common carboxylic acid group facilitates ESI of the compounds in the negative ionization mode. The changes in molecular structure of these model compounds allows for a wide variety of solution interactions. Some analytes are effectively declustered under the set conditions creating an intense [M - H] - peak, whereas others prefer to form dimers or complexes with sodium.

Adduct formation in electrospray ionization mass spectrometry II. Benzoic acid derivatives

This work serves as a follow-up to Part I of experiments designed to determine the underlying principles in the formation of pseudomolecular, or adduct, ions during electrospray ionization. Aromatic acids were studied by flow injection analysis in the negative ionization mode of electrospray ionization mass spectrometry. Part I dealt with common acidic anti-inflammatory pharmaceuticals. such as ibuprofen and related analogues. Part II deals with functionally less complex molecules, namely benzoic acid (BA) and substituted benzoic acids. Halide-substituted molecules are investigated to deduce the effect of electron-withdrawing substituents (bromo-, chloro-, and fluoro-) and ring position (ortho-, meta- and para-) on the response of a traditional deprotonated molecular ion ([M-H]-) and a sodium-bridged dimer ion ([2M-2 H+Na]-). Amino-substituted benzoic acids are also analyzed in order to study the effect of an additional ionizable group on the molecule, and para-tert.-butyl-BA is analyzed to study the effect of increased hydrophobicity, as they relate to the formation of pseudomolecular ions. This study shows that solution character [octanol-water partition coefficient (or log P) and pKa] of the model compounds controls the relative efficiency of formation of [M-H]- and [2M-2H+Na]- ions. However the relative gas phase character (gas phase basicity and proton affinity) also has a significant effect on the formation of the sodium-bridged dimer ion. For the halide-substituted species, placement of the electron-withdrawing atom at the meta-position gives the greatest enhancement in sensitivity. Observations also show that as the structural complexity of the model compound increases, predictions relating analyte acidity to sodium-bridged dimer ion formation give way to a stronger dependence between log P values and ionization efficiency. Supporting this hypothesis is the nearly ten-fold enhancement in signal for tert.-butyl BA relative to BA. due to the greater hydrophobicity, and consequently, increased surface activity in an electrosprayed droplet of the analyte molecule.

Vacuum ultraviolet detector for gas chromatography

Analytical performance characteristics of a new vacuum ultraviolet (VUV) detector for gas chromatography (GC) are reported. GC-VUV was applied to hydrocarbons, fixed gases, polyaromatic hydrocarbons, fatty acids, pesticides, drugs, and estrogens. Applications were chosen to feature the sensitivity and universal detection capabilities of the VUV detector, especially for cases where mass spectrometry performance has been limited. Virtually all chemical species absorb and have unique gas phase absorption cross sections in the approximately 120-240 nm wavelength range monitored. Spectra are presented, along with the ability to use software for deconvolution of overlapping signals. Some comparisons with experimental synchrotron data and computed theoretical spectra show good agreement, although more work is needed on appropriate computational methods to match the simultaneous broadband electronic and vibronic excitation initiated by the deuterium lamp. Quantitative analysis is governed by Beer-Lambert Law relationships. Mass on-column detection limits reported for representatives of different classes of analytes ranged from 15 (benzene) to 246 pg (water). Linear range measured at peak absorption for benzene was 3-4 orders of magnitude. Importantly, where absorption cross sections are known for analytes, the VUV detector is capable of absolute determination (without calibration) of the number of molecules present in the flow cell in the absence of chemical interferences. This study sets the stage for application of GC-VUV technology across a wide breadth of research areas.