Ala Alak

Separation of mycotoxins, polycyclic aromatic hydrocarbons, quinones, and heterocyclic compounds on cyclodextrin bonded phases: An alternative lc packing

Abstract β-Cyclodextrin and γ-cyclodextrin chiral bonded phases were previously shown to be useful in the separation of enantiomers, diastereomers and structural isomers. In this work it is demonstrated that these stationary phases are also useful in more routine separations. As such, they provide an alternative to the popular reverse phase packings. Because the selectivity of cyclodextrin packings is often unique they can be used to compliment conventional columns, particularly when separating complex mixtures where peak overlap is a problem. The separation of several important classes of compounds is used to demonstrate the general utility of this packing.

Facile Separation of Enantiomers, Geometrical Isomers, and Routine Compounds on Stable Cyclodextrin LC Bonded Phases

The effectiveness of employing stationary phases composed of chemically bonded cyclodextrin molecules in the high performance liquid chromatographic separation of a variety of different types of compounds is summarized. Over one hundred compounds, including optical, geometrical, and structural isomers, diastereomers, and epimers were successfully separated from each other via use of beta- or gamma-cyclodextrin bonded phases and aqueous methanolic mobile phases. The mechanism of separation is based upon inclusion complex formation between the compounds being separated and the cyclodextrin molecules bonded to the stationary phase. The effects of temperature, mobile phase composition and flow rate upon the chromatographic selectivity and resolution are described. The results indicate that the cyclodextrin columns may be more versatile, flexible, and effective compared to the conventional normal or reversed phase columns.

Recent advances in surface-enhanced Raman spectrometry for chemical analysis

Abstract Surface-Enhanced Raman Scattering (SERS) is a relatively new approach in chemical analysis whereby the Raman scattering efficiency can be enhanced by factors of 10 3 to 10 7 when a compound is adsorbed on or near roughened metal surfaces, submicron structures, or particles. This paper presents an overview of recent developments of this new spectrochemical method for analytical applications. Various techniques to produce SERS-active solid substrates are presented. Some of the SERS-active surfaces evaluated include microsphere-coated surfaces or quartz post substrates. This study also reports the development of new substrates using materials, such as titanium oxide or fumed silica, which can be used as coating substances in the development of SERS-active substrates. The analytical figures of merit and limitations of each technique for analytical applications are discussed. Several examples of SERS measurements of individual compounds and mixtures of environmental and biological importance are presented to illustrate the usefulness of this potentially powerful new analytical tool.

Surface-active substrates for Raman and luminescence analysis.

This paper describes the development of active materials for optically enhanced Raman and fluorescence spectroscopy. The substrates for surface-enhanced Raman scattering investigated in this study involved silver-coated microspheres on glass plates. The effect of various experimental parameters, such as angle of incidence and excitation wavelength, were investigated. The substrate used for surface luminescence analysis consisted of a cellulose membrane coated with fumed silica microparticles, to enhance the sensitivity of analysis. Examples of analysis of benzo[a]pyrene and its derivatives are used to illustrate the efficacy of the analytical techniques.

Surface-enhanced Raman spectrometry of chlorinated pesticides

Abstract The utility of surface-enhanced Raman scattering (SERS) spectrometry for trace determination of various chlorinated pesticides including chlordan, carbophenothion, bromophos, methyl chloropyrifos, dichloran, linuron, and 1-hydroxychlordene is described. Silver-coated substrates consisting of submicron spheres on solid surfaces were used as SERS-active media. The detection limits for these pesticides are at nanogram levels. The sensitivity and spectral selectivity of this technique for toxic chemical agents are discussed. The results indicate that SERS shows promise for the determination of an important class of chlorinated pesticides.

Effects of Different Stationary Phases and Surfactant or Cyclodextrin Spray Reagents on the Fluorescence Densitometry of Polycyclic Aromatic Hydrocarbons and Dansylated Amino Acids

Abstract The detectable luminescence of twelve dansy1 amino acids and four polycyclic aromatic hydrocarbons (PAHs) spotted on five common TLC stationary phases was evaluated. The detectable luminescence varied appreciable for compounds associated with different stationary phases. The use of surfactant and cyclodextrin spray reagents caused luminescence enhancements on some stationary phases but not others. The reagents did not affect all compounds to the same degree indicating that qualitative information could be obtained in some cases. The largest luminescence increase for a compound spotted on silica gel was for pyrene (i.e., 47-fold) sprayed with sodium cholate. The degree to which the plates were dried also affected the luminescence intensity. Possible reasons for the observed effects are discussed.

Enhanced Room-Temperature Phosphorescence of Anthracene on Cyclodextrin-Treated Filter Paper

This study investigates the enhancement of room-temperature phosphorescence (RTP) for anthracene when the compound is adsorbed on cyclodextrin (CD)-treated filter paper. The results show that β-CD treatment induces a significant increase in anthracene RTP emission, which is normally extremely weak. The α-CD does not produce any strong RTP enhancement, while γ-CD produces relatively lower enhancement than does β-CD. The CD treatment procedure is very simple and improves the analytical usefulness of the RTP method. The binding constants of anthracene to cyclodextrin were measured with the use of thin-layer chromatographic retention values. Anthracene has a higher binding constant with β-CD than with γ-CD, in agreement with the RTP enhancement results.

Studies of cyclodextrin-enhanced room-temperature phosphorescence

Abstract The enhancement of room-temperature phosphorescence by α-, β- and γ-cyclodextrins in the presence of heavy atoms is described for p -aminobenzoic acid, anthracene and six of its derivatives, dibenzofuran and some other compounds. The sensitivity can be improved by treating the filter paper substrate with the cyclodextrin (preferably β-cyclodextrin) or by mixing the analyte with β-cyclodextrin prior to sample spotting on the substrate.

Resonance Raman analysis of fluorescent compounds using micellar solutions and ultraviolet laser excitation

Micelles composed of specifically functionalized surfactants can be used to great advantage in resonance Raman spectroscopy. In addition to circumventing the problem of luminescence background, micelles allow the use of aqueous solvents which stabilize labile species and improve the signal-to-noise ratio. The first examples of micelle-mediated resonance Raman analysis of fluorescent compounds using ultraviolet excitation are given.