Victoria L. McGuffin

Luminescence-based methods for sensing and detection of explosives

The detection of explosives and related compounds is important in both forensic and environmental applications. Luminescence-based methods have been widely used for detecting explosives and their degradation products in complex matrices. Direct detection methods utilize the inherent fluorescence of explosive molecules or the luminescence generated from chemical reactions. Direct detection methods include high-energy excitation techniques such as gamma-ray and x-ray fluorescence, detection of decomposition products by fluorescence or chemiluminescence, and detection following reduction to amines or another reaction to produce fluorescent products from the explosive. Indirect detection methods utilize the interference caused by the presence of explosive compounds with traditional processes of fluorescence and fluorescence quenching. Indirect detection methods include quenching of solution-phase, immobilized, and solid-state fluorophores, displacement of fluorophores, fluorescence immunoassay, and reactions that produce fluorescent products other than the explosive. A comprehensive review of these methods is presented.

Investigation of common fluorophores for the detection of nitrated explosives by fluorescence quenching

Previous studies have indicated that nitrated explosives may be detected by fluorescence quenching of pyrene and related compounds. The use of pyrene, however, invokes numerous health and waste disposal hazards. In the present study, ten safer fluorophores are identified for quenching detection of target nitrated compounds. Initially, Stern-Volmer constants are measured for each fluorophore with nitrobenzene and 4-nitrotoluene to determine the sensitivity of the quenching interaction. For quenching constants greater than 50 M(-1), sensitivity and selectivity are investigated further using an extended set of target quenchers. Nitromethane, nitrobenzene, 4-nitrotoluene, and 2,6-dinitrotoluene are chosen to represent nitrated explosives and their degradation products; aniline, benzoic acid, and phenol are chosen to represent potential interfering compounds. Among the fluorophores investigated, purpurin, malachite green, and phenol red demonstrate the greatest sensitivity and selectivity for nitrated compounds. Correlation of the quenching rate constants for these fluorophores to Rehm-Weller theory suggests an electron-transfer quenching mechanism. As a result of the large quenching constants, purpurin, malachite green, and phenol red are the most promising for future detection of nitrated explosives via fluorescence quenching.

Separation and characterization of tetracycline antibiotics by capillary electrophoresis

Abstract The electrophoretic behavior of the antibiotics tetracycline, chlortetracycline, demeclocycline, oxytetracycline, doxycycline, rnethacycline and minocycline has been characterized by capillary electrophoresis in phosphate buffer solutions in the pH range 4–11. A complete set of acid-base equilibrium constants and electrophoretic mobilities was determined and, subsequently, was used in a computer-assisted optimization program to assess the separation of the tetracyclines. Under the predicted optimum conditions (pH 7.5, 18.2 mM ionic strength, 4.3 mM buffer concentration, and constant current of 20 μA), the separation was performed satisfactorily and all tetracyclines were readily identified. Moreover, the common impurities in tetracycline resulting from dehydration and epimerization reactions were discriminated under the same conditions. The determination of tetracycline, doxycycline and minocycline was performed in commercially available pharmaceutical formulations. During the dissolution of the contents of hard-filled capsules, a minimum recovery of 95% of the active ingredient was obtained. A calibration curve of peak height versus concentration gave a slope of 6.15 · 10−4 cm M−1, intercept of −1.18·10−5 cm, and coefficient of determination equal to 0.9989. The analysis using UV-absorbance detection at 260 nm provided a detection limit of 1 · 10−5M at a signal-to-noise ratio of approximately 3, with a linear range of two orders of magnitude.

Selective fluorescence quenching of polynuclear aromatic hydrocarbons in microcolumn liquid chromatography

Abstract A selective fluorescence quenching method using nitromethane as the quenching agent is systematically studied in the absence and presence of absorption effects. Nitromethane is found to quench the emission intensity of polynuclear aromatic hydrocarbons (PAHs) with six-membered rings, while the emission of PAHs with five-membered rings is essentially unaltered. However, fluorescence attenuation caused by primary and secondary absorption may result in errors in the determination of dynamic quenching constants. In this work, a modified Stern-Volmer relationship is developed to distinguish and to compensate mathematically for absorption effects. Utilizing this expression, the Stern-Volmer quenching constant ( K q ) is determined to be 125 and 0.15 M −1 for pyrene and fluoranthene, respectively, using nitromethane as the quenching agent in methanol at ambient temperature. Because of the large difference in quenching constants, this analytical methodology is applied for the class-selective identification of PAHs in coal-derived fluids by microcolumn liquid chromatography with laser fluorescence detection.

Molar enthalpy and molar volume of methylene and benzene homologues in reversed-phase liquid chromatography

In this study, thermodynamic properties are measured for methylene and benzene homologues in reversed-phase liquid chromatography using octadecylsilica stationary phases and methanol mobile phase. The change in molar enthalpy (delta H degree) is determined from graphs of the logarithm of the capacity factor versus the inverse temperature (15 to 60 degrees C), whereas the change in molar volume (delta V degrees) is determined from graphs of the logarithm of the capacity factor versus pressure (830 to 5000 p.s.i.). For octadecylsilica phases with low bonding density (2.7 mumol m-2), delta H degree and delta V degree are small and are relatively unaffected by temperature and pressure. These thermodynamic parameters are linearly related to the homologue number for the methylene homologues, but not for the benzene homologues. For the ethylene group, delta delta H degree and delta delta V degree are in the order of -0.41 kcal mol-1 and -1.0 cm3 mol-1, respectively, at 30 degrees C. As the bonding density increases (5.4 mumol m-2), the molar volume and molar enthalpy decrease in a significant and nonlinear manner with the homologue number. Moreover, these thermodynamic parameters are markedly affected by temperature and pressure. For the ethylene group, delta delta H degree and delta delta V degree are in the order of -3.65 kcal mol-1 and -14.1 cm3 mol-1, respectively, at 30 degrees C. The theoretical and practical implications of these measurements are discussed with respect to the retention mechanism in reversed-phase liquid chromatography.

Theoretical and experimental studies of the effect of pressure on solute retention in liquid chromatography

Pressure is often assumed to have a negligible influence on solute retention in liquid chromatography because of the small compressibility of the mobile and stationary phases. The range of pressures commonly encountered in reversed-phase separations is considerable, however, and may give rise to significant changes in solute capacity factor. In this study, the retention of model solutes is measured directly along the chromatographic column as a function of the local pressure. The model solutes, a homologous series of derivatized fatty acids, exhibit a significant increase in capacity factor ranging from +9.3% for n-C(10) to +24.4% for n-C(20) for inlet pressures from 1500 to 5000 psi. These experimental results are compared with a thermodynamic model derived from regular solution theory. This model suggests that state effects alone are not sufficient to describe the measured change in solute retention and that variations in interaction energy with density must also be considered. By using the simple relationship of van der Waals for the interaction energy (E ∝ 1/V), the change in capacity factor with density is slightly underestimated. However, by using an extended relationship that better describes polar fluids (E ∝ 1/V(2)), good agreement is observed. Finally, the correlation of experimental results with this thermodynamic model reveals that all components in the chromatographic system, including the solute, mobile phase, and stationary phase, must be considered compressible. The results of this study have clear implications for the determination of fundamental physicochemical parameters, as well as for the everyday practice of liquid chromatography.

Quantitative Evaluation of Selective Fluorescence Quenchers for Polynuclear Aromatic Hydrocarbons

In this investigation, nitromethane, nitrobenzene, and are evaluated for their class selectivity and efficiency as fluorescence quenchers of polynuclear aromatic hydrocarbons (PAHs). Conditional Stern-Volmer quenching constants are utilized for qualitative and quantitative evaluation at known quencher concentration and fluorescence excitation and emission wavelengths. From these measurements, nitromethane is determined to be the most selective quencher for alternant PAHs, 1,2,4-trimethoxybenzene is somewhat less selective for nonalternant PAHs, and nitrobenzene is an effective but nonselective quencher. The utility of this technique is demonstrated by application to a complex coal tar sample after separation of the individual PAH components by microcolumn liquid chromatography.

Solvent modulation in liquid chromatography : extension to serially coupled columns

Abstract The concept and theory of solvent modulation are extended to serially coupled columns in liquid chromatography. Because the mobile phases are maintained in separate zones and the stationary phases in separate columns, solute retention is a simple, time-weighted average of the individual environments to which the solute is exposed. Thus, optimization of complex multidimensional chromatographic separations is more accurate and predictable. This approach is demonstrated by application to the separation of isomeric polynuclear aromatic hydrocarbons using octadecylsilica and β-cyclodextrin silica stationary phases with aqueous methanol and acetonitrile mobile phases.

Association and discrimination of diesel fuels using chemometric procedures

Five neat diesel samples were analyzed by gas chromatography-mass spectrometry and total ion chromatograms as well as extracted ion profiles of the alkane and aromatic compound classes were generated. A retention time alignment algorithm was employed to align chromatograms prior to peak area normalization. Pearson product moment correlation coefficients and principal components analysis were then employed to investigate association and discrimination among the diesel samples. The same procedures were also used to investigate the association of a diesel residue to its neat counterpart. Current limitations in the retention time alignment algorithm and the subsequent effect on the association and discrimination of the diesel samples are discussed. An understanding of these issues is crucial to ensure the accuracy of data interpretation based on such chemometric procedures.

Additivity of statistical moments in the exponentially modified Gaussian model of chromatography

Abstract A homologous series of saturated fatty acids ranging from C10 to C22 was separated by reversed-phase capillary liquid chromatography. The resultant zone profiles were found to be fit best by an exponentially modified Gaussian (EMG) function. To compare the EMG function and statistical moments for the analysis of the experimental zone profiles, a series of simulated profiles was generated by using fixed values for retention time and different values for the symmetrical (σ) and asymmetrical (τ) contributions to the variance. The simulated profiles were modified with respect to the integration limits, the number of points, and the signal-to-noise ratio. After modification, each profile was analyzed by using statistical moments and an iteratively fit EMG equation. These data indicate that the statistical moment method is much more susceptible to error when the degree of asymmetry is large, when the integration limits are inappropriately chosen, when the number of points is small, and when the signal-to-noise ratio is small. The experimental zone profiles were then analyzed by using the statistical moment and EMG methods. Although care was taken to minimize the sources of error discussed above, significant differences were found between the two methods. The differences in the second moment suggest that the symmetrical and asymmetrical contributions to broadening in the experimental zone profiles are not independent. As a consequence, the second moment is not equal to the sum of σ2 and τ2, as is commonly assumed. This observation has important implications for the elucidation of thermodynamic and kinetic information from chromatographic zone profiles.