Quantitative Resonance Raman Spectroscopy

Abstract

Determination of organic species at low concentration levels, particularly the simultaneous determination of a number of related compounds, challenges the ingenuity of the analyst. Measurements for nonvolatile and thermally labile compounds in the complex materials derived from the environment and living systems are especially difficult, and existing techniques aremostly ill-suited to this task. Resonance Raman spectroscopy provides an appropriate basis for such analyses.     

Cold crystallization and postmelting crystallization of PLA plasticized by compressed carbon dioxide

The cold crystallization at temperature T_cc (melting > T_cc > glass transition) and the postmelting crystallization of polylactic acid plasticized by compressed carbon dioxide (CO₂) were studied using a high-pressure differential scanning calorimeter. The kinetics of the two kinds of crystallization were evaluated by the Avrami equation as a function of pressure at certain temperatures. The effects of using talc as a nucleation agent on the two types of crystallization under pressure were also investigated. The results show that compressed CO₂ increased the mobility of the polymer chains in solid state, resulting in an increased rate of cold crystallization. The decreased rate of postmelting crystallization was mainly in the nucleation-controlled region, which indicates that the number of nuclei was decreased by the compressed CO₂. The growth rate of the two crystallization types followed the Avrami equation, but the kinetics of each depended upon temperature and pressure. The inclusion of talc accelerated postmelting crystallization but had little effect on cold crystallization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2630–2636, 2008     

Use of near-infrared reflectance spectroscopy for shelf-life discrimination of green asparagus stored in a cool room under controlled atmosphere

This study sought to evaluate the ability of near-infrared reflectance spectroscopy (NIRS) to classify intact green asparagus, in refrigerated storage under controlled atmosphere, by storage time and post-harvest treatments applied. A total of 468 green asparagus (Asparagus officinalis, L., cultivar UC-157) were sampled after 7, 14, 21 and 28 days of refrigerated storage (2 °C, 95% R.H.) under three controlled atmosphere (CA) treatments: air (21 kPa O₂ + 0.3 kPa CO₂), CA₁ (5 kPa O₂ + 5 kPa CO₂) and CA₂ (10 kPa O₂ + 10 kPa CO₂). Two commercially available spectrophotometers were evaluated for this purpose: a scanning monochromator (SM) of 400-2500 nm and a combination of diode array and scanning monochromator (DASM) of 350-2500 nm. Models developed using partial least squares 2-discriminant analysis (PLS2-DA) correctly classified between 81-100% of samples by post-harvest storage time, depending on the instrument used. Using similar models, the DASM instrument correctly classified 85% of samples by post-harvest treatment, compared with 72% using the SM. These results confirmed that NIR spectroscopy, coupled with the use of chemometric techniques, provides a reliable, accurate method of predicting the shelf-life of asparagus under different storage conditions and as a function of post-harvest treatment applied; the method can be readily applied at industrial level.     

Estimates for the principal spectrum point for certain time-dependent parabolic operators

Non-autonomous parabolic equations are discussed. The periodic case is considered first and an estimate for the principal periodic-parabolic eigenvalue is obtained by relating the original problem to the elliptic one obtained by time-averaging. It is then shown that an analogous bound may be obtained for the principal spectrum point in the almost periodic case. These results have applications to the stability of nonlinear systems and hence, for example, to permanence for biological systems.     

C30 Stationary phases for the analysis of food by liquid chromatography

The introduction of a polymeric C₃₀ liquid chromatographic column by Sander et al. [Anal. Chem., 66 (1994) 1667] designed for the separation of carotenoid isomers, has led to the development of improved analytical methods for these compounds. Subsequent commercial availability of polymerically bonded C₃₀ columns has facilitated these advances, and applications to a wide variety of separation problems with biological samples have been described. This report provides a comprehensive review of applications of polymeric C₃₀ columns, utilized in the determination of carotenoids, retinoids, and other nutrients and related compounds in complex, natural-matrix samples.     

Flexible metathesis-based approaches to highly functionalised furans and pyrroles

A range of differentially functionalised furans and pyrroles have been synthesised in short order by the judicious use of a ring-closing metathesis/aromatisation strategy. Two contrasting approaches are described exploiting a palladium-catalysed union of allylic alcohols and sulfonamides in one case, and a titanium mediated methylenation of homoallylic esters in another. A number of groups that are difficult to install via traditional methods were incorporated successfully.