Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of mobile phase composition.

Raman spectroscopy is used to examine the effect of mobile phase composition on the orientation of octadecyl-bonded silica-based reversed-phase liquid chromatographic (RPLC) stationary phase ligands. The effect of ligand bonding density is also investigated. The present experimental set-up utilizes a direct, noninvasive, on-column approach to examine the solvent dependent conformational behavior of the bonded ligands under flow-rate and back pressure conditions similar to those used during conventional RPLC measurements. Neat, single-component, mobile phase solvents including water, acetonitrile, methanol and chloroform are used to investigate the hypothesized collapsing and extension of stationary phase ligands with changes in mobile phase composition. No evidence of phase collapse was observed upon changing the mobile phase composition from an organic to an aqueous content. Also, Raman spectroscopic measurements allowed the differentiation between associated and free acetonitrile solvent.

Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of temperature.

This study represents the first time that both the mobile phase composition and the temperature are simultaneously controlled to examine silica-bonded octadecylsilyl (C18) ligands spectroscopically at typical liquid chromatographic (LC) mobile phase flow-rates and back-pressures. Raman spectroscopy is used to characterize the behavior of the C18 bonded ligands equilibrated at temperatures from 45 to 2 degrees C in neat, single-component, mobile phase solvents including: water, acetonitrile, methanol, and chloroform. In addition, the effect of stationary phase ligand bonding density is examined by using two different monomeric reversed-phase liquid chromatographic (RPLC) stationary phases, a 2.34 and a 3.52 micromol m(-2) Microporasil C18 stationary phase, under identical conditions. The direct, on-column, spectroscopic analysis used in this study allows direct evaluation of the temperature-dependent behavior of the bonded C18 ligands. The temperature-dependent ordering of the stationary phase ligands is examined to determine if the ligands undergo a phase transition from a less-ordered "liquid-like" state at higher temperatures to a more-ordered "solid-like" state at lower temperatures. A discrete phase transition was not observed, but rather a continual ordering as temperature was lowered.

Characterization of liquid chromatographic stationary phases by Raman spectroscopy. Effect of ligand type.

This study represents the first Raman spectroscopic characterization of conventional chemically-bonded liquid chromatographic (LC) stationary phases under typical flow-rate and pressure conditions. Raman spectra were obtained for amino propyl (NH2), cyano propyl (CN), phenyl (Ph), octadecyl (C18), octyl (C8), and methyl (C1) chemically-bonded silica-based stationary phases in 100% aqueous mobile phases. The present experimental set-up has allowed Raman spectra of various stationary phase ligands, present in sub-monolayer coverages on the siliceous supports, to be obtained. This study: (1) demonstrates that conventional Raman spectroscopic techniques can be used to study LC stationary phases; (2) presents the experimental set-up, conditions, and approaches utilized to obtain Raman spectra of conventional stationary phases; (3) examines the spectroscopic differences observed for a variety of different types of bonded ligands that are typically used in reversed-phase (RPLC) and normal-phase (NPLC) liquid chromatographic separations; and (4) considers other future studies that are possible with this experimental approach, including mobile phase composition and temperature studies.

Direct assay and shelf-life monitoring of aspirin tablets using Raman spectroscopy.

A comparison was made between Raman and high-performance liquid chromatography (HPLC) analysis of aspirin tablets. The basis was an assay of aspirin content and the determination of salicylic acid produced by decomposition. Raman observations were performed directly on both intact and powdered tablet material. The limit of detection of HPLC with an ultraviolet detector is lower than that of the Raman measurement, but both are adequate for this application. The reproducibility of the Raman measurement is somewhat better than that of the HPLC measurement. Both methods were used in a degradation study in which samples were stored in a humid atmosphere for a maximum period of 8 weeks. Aside from somewhat higher salicylic acid responses from the HPLC method, which were attributed to hydrolysis during chromatography, results from the two methods were comparable. Direct Raman measurements are faster and do not require the use of solvents.

Determination of phenylpropanolamine hydrochloride and acetaminophen in pharmaceutical preparations by Raman spectroscopy.

Phenylpropanolamine hydrochloride and acetaminophen in mixtures have been determined, without prior separation, by Raman spectroscopy. Results are reported for both solution and solid samples. Direct measurement of solid samples, requiring only a brief period of mixing and grinding with potassium nitrate added as an internal standard, is shown to be particularly convenient and effective. Various data treatments to compensate for spectral overlap have been compared, and a correlation method has been shown to be most effective. For a sample typical of a cold remedy, in which the concentration of acetaminophen is 10 times that of the phenylpropanolamine hydrochloride, accuracy and precision were shown to be better than 1% for the measurement of acetaminophen and better than 3% for the measurement of phenylpropanolamine hydrochloride.