Roland W. Frei

The application of spectral reflectance to thin-layer chromatography

Abstract The components of dye mixtures resolved on thin-layers plates were identified by direct examination of the plates by spectral reflectance The amounts of adsorbed dye were determined at the same time with a precision of approximatel ± 5%. Reflectance measurements carried out on spots removed from the plates and packed in an appropriate cell afforded a degree of precision identical to that attained with transmittance.

Fluorogenic labeling of organophosphate pesticides with dansyl chloride. Application to residue analysis by high-pressure liquid chromatography and thin-layer chromatography.

The analysis of some organophosphorus pesticides by fluorogenic labeling with dansyl chloride (5-dimethylaminonaphthalene--l-sulfonyl chloride) was investigated. The pesticides were hydrolysed in sodium hydroxide to the corresponding phenols. The reaction of dansyl chloride with the phenols was accomplished in a two-phase system. The resulting fluorescent derivatives were separated and analysed quantitatively by in situ thin-layer chromatography (TLC) and high-pressure liquid chromatography (HPLC). As little as 10-25 ng/spot of pesticide was detected by both TLC and HPLC.

The application of ultraviolet reflectance spectroscopy to thin-layer chromatography.

Abstract A method which can be employed for both preparative and analytical purposes was devised for the resolution of salicylic acid-aspirin mixtures on chromatoplates. Ultraviolet reflectance spectroscopy was then used to determine the amounts of resolved material by carrying out measurements on spots removed from the dried plates and packed in a suitable cell.

An improved method for the determination of amino acids by spectral reflectance

Abstract A nonspray method is described whereby amino acids resolved on thin-layer plates can be estimated by spectral reflectance. In addition to facilitating the determination the elimination of the spraying operation with its attendant irregularities has resulted in a substantial increase in precision and accuracy. In this respect the method is competitive with paper chromatography, used in conjuction with such techniques as total scanning and the electronic integration of curves.

The factors affecting the reflectance spectra of some dyes adsorbed on alumina

Abstract Various factors affecting the diffuse reflectance spectra of eosin B, rhodamine B, fuchsin, and o-nitrophenol adsorbed on Chromatographic alumina under various conditions were studied to explore the potential analytical application of reflectance spectroscopy to dyestuffs separated by thin-layer chromatography. Regeneration temperature, pH and grade of adsorbent, and humidity were found to influence the spectra but the technique may be utilized for the photometric analysis of food dyes adsorbed on alumina ; straight lines passing through the origin were obtained when the Kubelka-Munk function was plotted against a suitable concentration range of dye. For reproducible results, it is essential to employ the same grade of adsorbent and to prepare samples under uniform conditions of humidity.

A simple semimicro cell for the measurement of spectral reflectance

Abstract The construction and method of employment of a simple cell suitable for the measurement of the spectral reflectance of semimicro samples, such as those removed from chromatoplates, are described. An attempt is also made to correlate the performance of the cell with current theories of reflectance spectroscopy using data obtained with a system consisting of Merck silica gel G and McCormick blue, a food dye.

Trace enrichment of pyrimidine nucleobases, 5-fluoro-uracil and bromacil on a silver-loaded thiol stationary phase with on-line reversed-phase high-performance liquid chromatography

Abstract A silver-loaded thiol stationary phase was used for the trace enrichment of pyrimidine nucleobases (uracil, thymine and cytosine) and some related compounds (5-fluoro-uracil, bromacil, uridine). The critical parameters for the sorption of these compounds on Ag(I)-thiol are the pH value of the sample solution and the flow-rate with which they are flushed through the pre-column. Complete recovery for all compounds except uridine was obtained at pH 11.0 at a flow-rate of 0.6 ml/min. The desorption mechanism was based on protonation of the analytes. Efficient on-line desorption to the C 18 analytical column was achieved by injecting a plug of 60 μl HNO 3 (pH 1.2) onto the Ag(I)-thiol pre-column. After separation the analytes were detected by UV detection at 269 nm. For 5-fluorouracil a detection limit of 3 · 10 −9 M was obtained for the preconcentration of 1.0 ml with a reproducibility of 2.3% ( n = 6, 5 · 10 −7 M ). The column efficiency measured in terms of plate number for 5-fluorouracil was 11 000 for direct injections and 9000–10 000 for 1.0-ml preconcentrations, indicating that the desorption step causes no appreciable band broadening. Applications of the described method for the determination of 5-fluorouracil in plasma and for the herbicide bromacil in surface water are presented.

Cation analysis by thin-layer chromatography and reflectance spectroscopy : Part I. The rapid identificaiion of cations resolved on thin-layer plates

Abstract A rapid method has been devised whereby 14 cations — Al3+, Bi3+, Cd2+, Cr3+, Co2+, Cu2+, Fe3+, Pb2+, Mn2+, Hg2+, Ni2+, Ag+, Sn2+ and Zn2+ —. can be separated on one-dimensional, cellulose thin-layer plates and then identified by means of their reflectance spectra The procedure requires micro-amounts of sample and only one spray reagent.

Liquid chromatographic determination of pentachlorophenol in serum, using pre-column phase-transfer catalysed dansylation and post-column photolysis with fluorescence detection

Dansylation of pentachlorophenol was carried out after solid-phase extraction of acidified human serum and desorption with dichloromethane, in a dichloromethane-water two-phase system using tetrabutyl-ammonium bromide as the phase-transfer catalyst. Derivatisation was complete within 2 min at room temperature. After evaporation of an aliquot of the organic phase, the residue was dissolved in methanol and injected into a reversed-phase chromatographic system, equipped with a post-column photochemical reactor. In the reactor, dansylated pentachlorophenol is converted into highly fluorescent products. The recovery of the analyte from serum was 85 +/- 4%. Calibration graphs for horse and human serum were linear over two decades, with correlation coefficients ranging from 0.996 to 0.999. The detection limit of pentachlorophenol in horse and human serum was 400 pg ml-1. The reproducibility of the total procedure for a human serum sample containing 4 ng ml-1 was 4.5%.

The behavior of o-nitrophenol on alkaline earth oxides

Abstract A study of the behavior of o -nitrophenol adsorbed on alkaline earth oxides has revealed displacements of the wavelength maxima in the visible diffuse reflectance spectra of the two component systems. The shifts, which are bathochromic in order of increasing size of cation, beryllium providing an exception, are interpreted in terms of polarization theory. Moisture contamination studies show enhancement of bathochromic shifts upon progressive elimination of adsorbed water molecules in competition with the phenol for active adsorbent sites.