Vibrational spectra of α-glucose, β-glucose, and sucrose: anharmonic calculations and experiment

The anharmonic vibrational spectra of α-D-glucose, β-D-glucose, and sucrose are computed by the vibrational self-consistent field (VSCF) method, using potential energy surfaces from electronic structure theory, for the lowest energy conformers that correspond to the gas phase and to the crystalline phase, respectively. The results are compared with ultraviolet-infrared (UV-IR) spectra of phenyl β-D-glucopyranoside in a molecular beam, with literature results for sugars in matrices and with new experimental data for the crystalline state. Car-Parrinello dynamics simulations are also used to study temperature effects on the spectra of α-D-glucose and β-D-glucose and to predict their vibrational spectra at 50, 150, and 300 K. The effects of temperature on the spectral features are analyzed and compared with results of the VSCF calculations conducted at 0 K. The main results include: (i) new potential surfaces, constructed from Hartree-Fock, adjusted to fit harmonic frequencies from M?ller-Plesset (MP2) calculations, that give very good agreement with gas phase, matrix, and solid state spectra; (ii) computed infrared spectra of the crystalline solid of α-glucose, which are substantially improved by including mimic groups that represent the effect of the solid environment on the sugar; and (iii) identification of a small number of combination-mode transitions, which are predicted to be strong enough for experimental observation. The results are used to assess the role of anharmonic effects in the spectra of the sugars in isolation and in the solid state and to discuss the spectroscopic accuracy of potentials from different electronic structure methods.     

Comparative study of different methodologies for quantitative rock analysis by Laser-Induced Breakdown Spectroscopy in a simulated Martian atmosphere

Laser-Induced Breakdown Spectroscopy was selected by NASA as part of the ChemCam instrument package for the Mars Science Laboratory rover to be launched in 2009. ChemCam's Laser-Induced Breakdown Spectroscopy instrument will ablate surface coatings from materials and measure the elemental composition of underlying rocks and soils at distances from 1 up to 10 m. The purpose of our studies is to develop an analytical methodology enabling identification and quantitative analysis of these geological materials in the context of the ChemCam's Laser-Induced Breakdown Spectroscopy instrument performance. The study presented here focuses on several terrestrial rock samples which were analyzed by Laser-Induced Breakdown Spectroscopy at an intermediate stand-off distance (3 m) and in an atmosphere similar to the Martian one (9 mbar CO₂). The experimental results highlight the matrix effects and the measurement inaccuracies due to the noise accumulated when low signals are collected with a detector system such as an Echelle spectrometer equipped with an Intensified Charge-Coupled Device camera. Three different methods are evaluated to correct the matrix effects and to obtain quantitative results: by using an external reference sample and normalizing to the sum of all elemental concentrations, by using the internal standardization by oxygen, a major element common to all studied matrices, and by applying the Calibration Free Laser-Induced Breakdown Spectroscopy method. The three tested methods clearly demonstrate that the matrix effects can be corrected merely by taking into account the difference in the amount of vaporized atoms between the rocks, no significant variation in plasma excitation temperatures being observed. The encouraging results obtained by the three methods indicate the possibility of meeting ChemCam project objectives for stand-off quantitative analysis on Mars.     

WE-ASCA: The Weighted-Effect ASCA for Analyzing Unbalanced Multifactorial Designs—A Raman Spectra-Based Example

Analyses of multifactorial experimental designs are used as an explorative technique describing hypothesized multifactorial effects based on their variation. The procedure of analyzing multifactorial designs is well established for univariate data, and it is known as analysis of variance (ANOVA) tests, whereas only a few methods have been developed for multivariate data. In this work, we present the weighted-effect ASCA, named WE-ASCA, as an enhanced version of ANOVA-simultaneous component analysis (ASCA) to deal with multivariate data in unbalanced multifactorial designs. The core of our work is to use general linear models (GLMs) in decomposing the response matrix into a design matrix and a parameter matrix, while the main improvement in WE-ASCA is to implement the weighted-effect (WE) coding in the design matrix. This WE-coding introduces a unique solution to solve GLMs and satisfies a constrain in which the sum of all level effects of a categorical variable equal to zero. To assess the WE-ASCA performance, two applications were demonstrated using a biomedical Raman spectral data set consisting of mice colorectal tissue. The results revealed that WE-ASCA is ideally suitable for analyzing unbalanced designs. Furthermore, if WE-ASCA is applied as a preprocessing tool, the classification performance and its reproducibility can significantly improve.     

Internal standard selection in the presence of matrix interactions in an inductively coupled argon plasma optimised for simultaneous multielement analysis by atomic emission spectrometry

Errors in element concentration measurement due to the effect of various matrix components have been reported in inductively coupled argon plasma emission spectroscopy (ICP-AES) analyses, optimised for simultaneous multielement determination.Two methods of correcting for these errors were examined by the use of six elements selected as internal standards, viz. As, Cd, Mn, Mo, Ni and Se. Both the usual internal standard ratio method and the analysis of covariance method were efficacious in reducing the influence of matrix components (K, Ca, NaSio₃, polymaleic acid (PMA) and diethylenetriaminepentaacetic acid (DTPA)) on analyte determination.Any one of As, Cd, Mn, Mo or Se could be used to correct for the main effects of NaSiO₃ and DTPA; Cd or Mo for K; but Cd + Mo + As are necessary to correct for Ca over the range used. Except for Al and Mg, element measurements could be corrected for PMA effects by Cd + Mo + Se.     

Reduction of interferences in graphite furnace atomic absorption spectrometry by multiple linear regression modelling

The multivariate effects of Na, K, Mg and Ca as nitrates on the electrothermal atomisation of manganese, cadmium and iron were studied by multiple linear regression modelling. Since the models proved to efficiently predict the effects of the considered matrix elements in a wide range of concentrations, they were applied to correct the interferences occurring in the determination of trace elements in seawater after pre-concentration of the analytes. In order to obtain a statistically significant number of samples, a large volume of the certified seawater reference materials CASS-3 and NASS-3 was treated with Chelex-100 resin; then, the chelating resin was separated from the solution, divided into several sub-samples, each of them was eluted with nitric acid and analysed by electrothermal atomic absorption spectrometry (for trace element determinations) and inductively coupled plasma optical emission spectrometry (for matrix element determinations). To minimise any other systematic error besides that due to matrix effects, accuracy of the pre-concentration step and contamination levels of the procedure were checked by inductively coupled plasma mass spectrometric measurements. Analytical results obtained by applying the multiple linear regression models were compared with those obtained with other calibration methods, such as external calibration using acid-based standards, external calibration using matrix-matched standards and the analyte addition technique. Empirical models proved to efficiently reduce interferences occurring in the analysis of real samples, allowing an improvement of accuracy better than for other calibration methods.     

电感耦合等离子体质谱中的基质效应

电感耦合等离子体质谱(Inductively coupled plasma mass spectrometry,ICP-MS)是痕量元素分析中最常用的检测技术,尽管ICP-MS在元素分析中表现出诸多优势,但其在检测复杂基质样品时,仍会遇到许多问题。复杂基质所引起的基质效应通常会导致分析物信号的抑制或者增强。基质效应影响程度取决于基质成分的绝对浓度,还与基质的种类、分析物的物理和化学性质以及仪器条件有关。该文介绍了ICP-MS中几种常见的基质效应及其影响因素,包括元素基质、含碳基质、酸基质和仪器条件等,探究了基质效应产生的可能原因,对国内外去除基质效应的方法,如样品前处理方法、样品引入系统、优化仪器参数和校准法等进行了系统的归纳和总结,并对基质效应的研究进行了展望。     

Application of solid-phase microextraction to the recovery of explosives and ignitable liquid residues from forensic specimens

A current review of the application of solid-phase microextraction (SPME) to the analysis of ignitable liquids and explosive residues is presented along with experimental results demonstrating the relative effects of controllable variables. Variables discussed include fiber chemistry, adsorption and desorption temperatures, extraction and desorption times, fiber sampling placement (direct, headspace, and partial headspace) and matrix effects, including water content. SPME is shown to be an inexpensive, rapid and sensitive method for the analysis of ignitable liquids and high explosives residues from solid debris samples and from aqueous samples. Explosives are readily detected at parts per trillion concentrations and ignitable liquids are reproducibly detected at levels below those using conventional methods.     

Why Does a Ga2 Dimer React Spontaneously with H2, but a Ga Atom Does Not?—A Detailed Quantum Chemical Investigation of the Differences in Reactivity Between Ga Atoms and Ga2 Dimers,in Combination with Experimental Results

The spontaneous and photoactivated reactions between Ga(2) and H(2) in a matrix of solid Ar at 12 K have been followed by using IR spectroscopy and have been shown to give access to several isomers of the subvalent hydride Ga(2)H(2). We now present Raman spectra for this system, to complete its characterization on the basis of vibrational spectra. In addition, the differences between the reactivity of a Ga atom and a Ga(2) dimer toward H(2) are evaluated. The matrix isolation experiments have shown that Ga(2) reacts spontaneously with H(2,) at 12 K, to give the cyclic subvalent hydride Ga(micro-H)(2)Ga (D(2h) symmetry), which can be transformed into two other isomers of Ga(2)H(2) by selective photoactivation. Interestingly, the spontaneous reaction is subject to a marked isotopic effect. In total, the experimental results provide detailed information about the reaction mechanism. In contrast to Ga(2), Ga atoms do not react spontaneously with H(2); on photoactivation they instead yield the radical species GaH(2). The quantum chemical calculations presented herein start with an analysis of the structures and relative energies of the relevant species at the MP2 level, by using extended basis sets, and lead on to a discussion of the correlation diagrams for both reactions. Finally, CASSCF and MRCI methods, in combination with moderate-sized basis sets, were employed to analyze in detail the mechanisms of the two reactions. It will be shown that the computational results, in concert with the experimental findings, provide a satisfying explanation of the contrasting reactivities of Ga and Ga(2).     

电感耦合等离子体原子发射光谱法测定N36锆合金中微量钼和铅

建立了电感耦合等离子体原子发射光谱（ICP-AES）法测定N36锆合金中微量钼和铅的分析方法.讨论了样品溶解、基体效应干扰、谱线选择和观测方式等对测定结果的影响.采用均匀试验设计确定最适合的仪器测定参数,包括等离子气流量、辅助气流量、雾化气流量和等离子体发生器功率.结果表明,锆基体对测定结果有较大影响,在试验中采用基体匹配消除干扰,在试验设计优化的仪器测定参数下,使用N36锆合金样品对方法的精密度与准确度进行验证,相对标准偏差（RSD,n=11）低于5%,加标回收率为93%~104%.所建立的方法快捷、简便、准确,满足核用N36锆合金中微量钼和铅元素的分析要求.     

Resolution of conformer-specific all-trans-1,6-diphenyl-1,3,5-hexatriene UV absorption spectra

All-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) exists in solution as a mixture of s-trans,s-trans and s-cis,s-trans conformers. The latter is higher in energy, and its contribution increases with increasing temperature. ttt-DPH UV absorption spectra broaden with increasing temperature and undergo blue shifts with decreasing polarizability. We describe here the resolution of two spectrothermal matrices of ttt-DPH UV absorption spectra into two conformer-specific components. The first matrix consists of DPH spectra measured in n-dodecane in the 283 to 374 K T range and the second of ttt-DPH absorption spectra measured in the even numbered n-alkanes (n-C(8)-n-C(16)) at temperatures selected to achieve isopolarizability (284-372 K). Principal component analysis (PCA) treatments showed that reasonable two-component systems are attained by compensation for T-induced broadening and shifting in the pure conformer spectra. The self-modeling (SM) method used to resolve the n-C(12) matrix is successfully tested on a simulated matrix closely mimicking ttt-DPH experimental spectra in n-C(12). Compensation for nonlinear effects yields robust two-component matrices from the experimental spectra. Their resolution into pure component spectra is based on the application of the Lawton and Sylvestre (LS) non-negativity criterion at the spectral onset to define the spectrum of the low energy s-trans-conformer and the optimum linearity van't Hoff (vH) plot criterion to find the spectrum of the higher-energy s-cis-conformer. Resolved spectra are somewhat sensitive to the choice of the spectral region in which the LS criterion is applied. The surprising result is that both resolutions lead to the conclusion that the molar fraction of the s-cis-conformer equals, or even exceeds, the molar fraction of the s-trans-conformer as the highest T's employed in our study are approached.