Raman and near-infrared spectroscopy for quantification of fat composition in a complex food model system.

Raman and near-infrared (NIR) spectroscopy have been evaluated for determining fatty acid composition and contents of main constituents in a complex food model system. A model system consisting of 70 different mixtures of protein, water, and oil blends was developed in order to create a rough chemical imitation of typical fish and meat samples, showing variation both in fatty acid composition and in contents of main constituents. The model samples as well as the pure oil mixtures were measured using Raman and NIR techniques. Partial least squares regression was utilized for prediction, and fatty acid features were expressed in terms of the iodine value and as contents of saturated, monounsaturated, and polyunsaturated fatty acids. Raman spectroscopy provided the best results for predicting iodine values of the model samples, giving validated estimation errors accounting for 2.8% of the total iodine value range. Both techniques provided good results for predicting the content of saturated, monounsaturated, and polyunsaturated fatty acids in the model samples, yielding validated estimation errors in the range of 2.4-6.1% of the total range of fatty acid content. Prediction results for determining fatty acid features of the pure oil mixtures were similar for the two techniques. NIR was clearly the best technique for modeling content of main constituents in the model samples.

Development of a positive pressure driven micro-fabricated liquid chromatographic analyzer through rapid-prototyping with poly(dimethylsiloxane) Optimizing chromatographic efficiency with sub-nanoliter injections.

A rapid and low-cost means of developing a working prototype for a positive-displacement driven open tubular liquid chromatography (OTLC) analyzer is demonstrated. A novel flow programming and injection strategy was developed and implemented using soft lithography, and evaluated in terms of chromatographic band broadening and efficiency. A separation of two food dyes served as the model sample system. Sample and mobile phase flowed continuously by positive displacement through the OTLC analyzer. Rectangular channels, of dimensions 10 mum deep by 100 mum wide, were micro-fabricated in poly-dimethylsiloxane (PDMS), with the separation portion 6.6 cm long. Using a novel flow programming method, in contrast to electroosmotic flow, sample injection volumes from 0.5 to 10 nl were made in real-time. Band broadening increased substantially for injection volumes over 1 nl. Although underivatized PDMS proved to be a sub-optimal stationary phase, plate heights, H, of 12 mum were experimentally achieved for an unretained analyte with the rectangular channel resulting in a reduced plate height, h, of 1.2. Chromatographic efficiency of the unretained analyte followed the model of an OTLC system limited by mass-transfer in the mobile phase. Flow rates from 6 nl min(-1) up to 200 nl min(-1) were tested, and van Deemter plots confirmed plate heights were optimum at 6 nl min(-1) over the tested flow rate range. Thus, the best separation efficiency, N of 5500 for the 6.6 cm length separation channel, was achieved at the minimum flow rate through the column of 6 nl min(-1), or 3 ml year(-1). This analyzer is a low-cost sampling and chemical analysis tool that is intended to complement micro-fabricated electrophoretic and related separation devices.

A Raman waveguide detector for liquid chromatography.

A novel real-time liquid core Raman waveguide detector designed for liquid chromatographic applications is described. The Raman waveguide detector provides enhanced selectivity over typical high-performance liquid chromatography (HPLC) detectors. The waveguide detector also greatly improves the sensitivity of a typical Raman measurement without resorting to surface enhancement or resonance approaches and is compatible with the typical peak width volumes eluted by microbore and minibore HPLC (packed 1-2-mm-i.d. columns). Detection limit enhancements of over 1000-fold have been achieved for the measurement of alcohols in the aqueous phase with the Raman cell utilizing liquid core waveguide technology. The liquid core waveguides demonstrated in this study were constructed using Teflon AF 2400 tubing with a refractive index of 1.29. The low refractive index of the polymer material allowed HPLC separations with Raman detection to be performed with an aqueous mobile phase. A calibration curve for aqueous solutions of 2-propanol was generated and a limit of detection (LOD) of 2 ppm was determined. The Raman waveguide detector is demonstrated for the HPLC analysis of alcohol test mixtures, with LODs in the low-ppm range at the detector. By coupling the temporal separation achieved by HPLC with the vibrational information gleaned from Raman detection, an information-rich multivariate data matrix is obtained that can be deconvoluted to provide chemical speciation even when the HPLC resolution is poor. In this paper, we will discuss the physical and optical design of the Raman waveguide detector and the demonstration of the detector for HPLC detection.