Applications of SEPIL to the Solid State Defect Chemistry of Fluorites and Ultra-trace Inorganic Analysis [New analytical methods (16)]

Although it has been recognized for many years that the spectra of lanthanoid ions can provide useful information about short range phenomena in the neighborhood of these inos in a solid material, lanthanoid spectroscopy has only infrequently been used for studying complex materials because of the problem of line-sorting the complex spectra that are obtained. The advent of convenient tuneable lasers has eliminated this problem. By selectively exciting probe ion luminescence (SEPIL), it is possible to obtain fluorescence and excitation spectra from a single kind of crystallographic environment. Two applications of this method are dicussed in this paper. The first application is the study of the defect chemistry of fluorite materials (compounds with CaF₂ lattice). It is shown how this method can provide unique information about the solid state chemistry, thus clarifying many of the unexplained behaviors of this important class of material. The second application shows application shows how ultra-trace analysis can be carried out by causing an association between an analyte ion and a fluorecent probe ion. The unique crystal field levels of a probe ion associated with a particular analyte can be selectively excited so that traces of the ion to be analyzed can be detected with very high selectivity and with very low detection limits.     

Methods for extracting biochemical information from bacterial Raman spectra: an explorative study on Cupriavidus metallidurans

In this study, we explore Raman spectra of bacteria for their biochemical information. Therefore, a database of biomolecules was used and several approaches were applied such as the study of difference spectra, the calculation dot products, the usage of coefficients obtained from an EMSC procedure and the application of 2D correlation spectroscopy. These methods were applied on a dataset containing Raman spectra of Cupriavidus metallidurans LMG 1195 in five stages of its growth, aiming to extract information about the evolution of cell components during growth. EMSC coefficients seemed to be most promising for tracking metabolic products and the results were often confirmed by difference spectra or by 2D correlation spectroscopy.     

Estimation of trace vapor concentration-pathlength in plumes for remote sensing applications from hyperspectral images

A novel approach for quantification of chemical vapor effluents in stack plumes using infrared hyperspectral imaging are presented and examined. The algorithms use a novel application of the extended mixture model to provide estimates of background clutter in the on-plume pixel. These estimates are then used iteratively to improve the quantification. The final step in the algorithm employs either an extended least-squares (ELS) or generalized least-squares (GLS) procedure. It was found that the GLS weighting procedure generally performed better than ELS, but they performed similarly when the analyte spectra had relatively narrow features. The algorithms require estimates of the atmospheric radiance and transmission from the target plume to the imaging spectrometer and an estimate of the plume temperature. However, estimates of the background temperature and emissivity are not required which is a distinct advantage. The algorithm effectively provides a local estimate of the clutter, and an error analysis shows that it can provide superior quantification over approaches that model the background clutter in a more global sense. It was also found that the estimation error depended strongly on the net analyte signal for each analyte, and this quantity is scenario-specific.     

A life cycle approach to method management

This paper describes a life cycle model for method management, using existing published models for software life cycles as a basis. A direct analogy is drawn between the processes in method life cycles and those established for software development, and the constituent parts of the processes that occur in the method life cycle are explored in some detail. It is concluded that a life cycle model can usefully be applied to method management and the main principles are summarised. An example is included to illustrate how life cycle principles can be applied to a real situation. Received: 6 October 2000 Accepted: 7 March 2001     

Glow discharge excitation and matrix effects in the Zn–Al–Cu system in argon and neon

Matrix effects and other deviations from the standard model of glow discharge optical emission spectroscopy (GD-OES) have been investigated in the Zn–Al–Cu system in a Grimm-type discharge in argon and neon. In ionic spectra of the elements that can be ionized by asymmetric charge transfer with ions of the discharge gas, most observed deviations from the standard model can be explained by variations of the number density of ions of the discharge gas, caused by asymmetric charge transfer reactions with the matrix element. Similar mechanism, but involving metastables of the discharge gas, was observed for the Cu II spectrum in neon. Some matrix effects in atomic spectra of aluminium and possibly also copper suggest that three-body recombination of ions of the discharge gas, assisted by an analyte atom, is responsible for excitation of certain atomic levels of the analyzed elements. Excited atomic states of the analyzed elements have higher fractional populations in neon than argon, by factors that are similar for all three elements and the median of which is slightly less than 3. It is shown which lines are free of matrix effects and suitable for highly accurate analysis of Zn–Al–Cu alloys by GD-OES and how to optimize the calibration model. Neon can be a reasonable alternative to argon as the discharge gas for some applications.     

Methods for extracting biochemical information from bacterial Raman spectra: focus on a group of structurally similar biomolecules--fatty acids

In this paper we explore the possibilities of Raman spectroscopy in order to deduce information on the fatty acid composition of bacterial cells. Therefore, representative strains of two bacterial taxa were each cultured in different conditions and in parallel analyzed by Raman spectroscopy and gaschromatographic FAME analysis. Raman spectra of pure fatty acids were recorded and used as reference spectra. The culturing conditions for each strain could be easily distinguished by the fatty acid information retrieved from bacterial Raman spectra. Chemometric techniques such as EMSC and PCA allowed to extract information about groups of fatty acids, that was consistent with the results from FAME analysis. Although the information retrieved from Raman spectroscopy is not as refined as that from FAME analysis, the presented methods could be useful to obtain basic information on the fatty acid present in bacteria when performing Raman spectroscopic analysis for fast whole cell profiling, which provides information for different types of cell components (fatty acids, amino acids, primary metabolites, etc.).     

Density functional calculations of 9-diphenylaminoacridine fluorescent indicator and its interactions with analyte molecules: I. Structures of complexes in the ground electronic states and absorption spectra

The interaction of 9-diphenylaminoacridine dye (indicator) with several small analyte molecules (methanol, acetonitrile, acetone, tetrahydrofuran, benzene, ammonia, formaldehyde, and acetaldehyde) has been theoretically studied in relation to the problem of the development of optical chemosensors based on organic dyes. The structures of the resulting complexes and the absorption spectra of 9-diphenylaminoacridine and its complexes with analytes were calculated using density functional theory (DFT) with the PBE0 functional and the 6-31G(d,p) basis set. It was demonstrated that complexes of two types with different mutual arrangements of molecules corresponding to the lateral and stacking structures can be formed for each analyte. The calculated absorption spectrum only weakly changes upon complex formation, which is in agreement with experimental data on the absorption spectra of 2,7-dimethyl-9-ditolylaminoacridine in solutions of corresponding solvents. The method for the calculation of excited states that was used in this work can be applied to the calculation of the fluorescence spectra of 9-diphenylaminoacridine complexes.     

Chemotaxonomical identification of spores of macrofungi: possibilities of Raman spectroscopy

Confocal Raman spectroscopy is a non-destructive analytical method which is useful to obtain detailed information about the molecular composition of biological samples. Its high spatial resolution was used to collect spectra of single basidiospores of macrofungi of the genera Collybia, Gymnopus, Laccaria, Lactarius, Mycena and Russula. These spectra can be divided into three major taxon-related groups, with general compositional differences, such as the relative amount of lipids compared to proteins. In this study, collapsing of thin-walled spores during storage was often observed, a phenomenon which has been given little attention in the literature. The Raman spectra are treated with different chemometric preprocessing techniques, including Savitsky–Golay, standard normal variate (SNV) preprocessing and extended multiplicative scatter correction (EMSC). By using linear discriminant analysis, approximately 90% of the spectra can be assigned to the correct genus, but identification on the species level was not possible.     

Threshold detection using indicator-displacement assays: an application in the analysis of malate in Pinot Noir grapes

The mathematics for modeling indicator-displacement assay isotherms is presented and contrasted to the classical host-guest binding isotherm. It is shown that the signal response can be tuned to occur closer to 1 equiv of guest relative to a standard binding algorithm. This delay in response leads to a better triggering protocol for threshold detection schemes. The determination of malate in Pinot Noir must was calculated using this new mathematical model, which demonstrates how a color change can be tuned to occur near a desired concentration of analyte.     

Nanoelectrospray—More than just a minimized-flow electrospray ionization source

The comparison between electrospray ionization (ESI) mass spectra from NaCl solutions with and without analyte obtained under ionspray and nanospray conditions reveals different mass spectral behavior of the two ESI techniques. This can be attributed to the different initial droplet sizes which are in the microns range for ionspray, while in nanospray they are believed to be about one order of magnitude smaller. In the context of the widely accepted uneven-fission model, nanospray would then enter one fission generation later; in addition, a higher initial droplet surface charge density in nanospray results in early fissions without extensive evaporation and thus increase in sample and salt concentration. This rationalizes that ionspray spectra closely resemble nanospray spectra from solutions with about one order of magnitude higher salt concentrations, showing a higher tolerance of nanospray towards salt contamination. When the analyte is a peptide (in a solution containing a high molar surplus of salt), molecule ion formation effectively competes with salt cluster ion formation; when the analyte is a sugar, it is detectable beside a high salt concentration only with nanospray, indicating the supporting effect of surface activity on ion release in the case of peptides. A model is presented which explains the different mass spectral behaviour of ionspray and nanospray by suggesting different "predominant fission pathways" depending on the size of the initial droplets.     

Position and intensity of system (eigen) peaks in capillary zone electrophoresis

The intensity of system (or eigen) peaks encountered in capillary zone electrophoresis (CZE) can be predicted by considering mass balances for each of the analyte constituents and each of the constituents in the background electrolyte (BGE). As a result of coherence, in each zone the proportions in which the constituent concentrations vary are fixed; they are determined by the composition of the BGE and the nature of the analyte constituent (if present) and described as eigenvectors of a transport matrix. Considering the effect of an injection, the mass balances for all constituents can be satisfied only via the intensity of each zone. This leads to an n-equations, n-unknowns problem, with the intensities as the unknowns and the mass balances as equations.The latter can be easily solved to obtain the intensities of the zones, of analytes as well as of system peaks. In this work the approach has been applied to CZE systems with two co-ions in the BGE, and experimental results have been compared to the predictions obtained from the model. Agreement was seen to be reasonable, but the quantitative comparison often failed, probably due to experimental difficulties.     

Total internal reflection ellipsometry as a label-free assessment method for optimization of the reactive surface of bioassay devices based on a functionalized cycloolefin polymer

We report a label-free optical detection technique, called total internal reflection ellipsometry (TIRE), which can be applied to study the interactions between biomolecules and a functionalized polymer surface. Zeonor (ZR), a cycloolefin polymer with low autofluorescence, high optical transmittance and excellent chemical resistance, is a highly suitable material for optical biosensor platforms owing to the ease of fabrication. It can also be modified with a range of reactive chemical groups for surface functionalization. We demonstrate the applications of TIRE in monitoring DNA hybridization assays and human chorionic gonadotrophin sandwich immunoassays on the ZR surface functionalized with carboxyl groups. The Ψ and Δ spectra obtained after the binding of each layer of analyte have been fitted to a four-layer ellipsometric model to quantitatively determine the amount of analytes bound specifically to the functionalized ZR surface. Our proposed TIRE technique with its very low analyte consumption and its microfluidic array format could be a useful tool for evaluating several crucial parameters in immunoassays, DNA interactions, adsorption of biomolecules to solid surfaces, or assessment of the reactivity of a functionalized polymer surface towards a specific analyte.     

Quantitative ion mobility spectroscopy based on molecular collision rate theory

Atmospheric pressure chemical ionization and ion mobility spectrometry (IMS) have traditionally been viewed as a qualitative analytical technique for identifying specific chemicals in the atmosphere. This work employs a nonlinear model based on molecular collision rate theory for quantitative modeling of chemical analyte concentrations. The collision rate between any two molecules depends on the relative populations of each chemical species in the volume of air analyzed where most collisions between ions, or neutral molecules and ions, result in no charge transfer. The rate constants for formation of product ions and consumption of source ions are estimated using empirical data over a wide concentration range for several analytes and reagent gases. The rate constants are unique to the analyte and the reagent gas as well as the sensitivity of the particular IMS instrument and provide a quantitative model to relate the mobility peak amplitudes to the analyte concentration. The rate constants can also be normalized by the reaction ion consumption rate constant to remove the IMS instrument sensitivity and provide a qualitative metric for analyte identification independent of a particular IMS instrument. A quantitative example is given for an acetic acid plume measured by a hand-held IMS detector outdoors has the plume passes. The quantitative rate constants provide a reasonable basis for estimating analyte concentration from the ion mobility spectra over a wide range of analyte concentrations.     

Two-peak phenomena and formation origin in micellar electrokinetic capillary chromatography

Since Terabe et al.[1] developed micellar electrokinetic capillary chromatography (MECC) in 1984, a great number of important advances about separating neutral compounds have been achieved. In MECC mode, micellar of an ionic surfactant can form so-called pseudo stationary phase in the buffer solution when it is above the critical micelle concentration, and some portions of the solute may be distributed into the micellar phase when they are mobilized into the buffer solution from sample zone…     

Certified reference materials as a quality tool in food control: much used—often misused—sometimes abused

Certified reference materials (CRMs) are important tools in the quality control of food analyses. There are, however, many ways in which CRMs can be abused and misused. It can be due to ignorance, as well as to overuse of expensive materials. The major drawback of CRMs is probably that the analyst knows the level of the analyte. The statistical evaluation of CRM results in reports and publications is often limited to a comparison between the found and the certified levels, which yields little, and sometimes erroneous, information. Recoveries based on CRMs often give a picture that is far too bright, with little consideration of uncertainties. The way the use of CRMs is described in most scientific journals is often very crude and shows that CRMs are seldom used to their full capacity. The objective of this paper is to try to summarise the ways in which a CRM can be misused and thereby put into focus how to make better use of such materials. It also gives examples on how to evaluate CRMs, using a procedure that was recently introduced by the Nordic Committee on Food Analysis.     

Correction for relative interferences in a cholesterol determination

The detrimental influences of thiouracil, propylthiouracil, and bromide on the ferric perchlorate reaction for cholesterol have been previously reported (2, 3). On the basis of recovery studies it can be shown that their role in the reaction results in a relative rather than an absolute error of cholesterol measurement. Because a relative error causes a linear fanning effect on increasing concentrations of analyte which depends for its slope on both the concentration of the interference as well as on the concentration of constituent being measured, the method of standard additions can be used to correct a matrix effect on the reaction. A system in which the standard is used internally can eliminate the need to pretreat the sample in order to separate the interfering compound from the analyte. Therefore, both the interference and its obviation by this method of internal standardization are discussed here in a procedure which results in a means for simpler rescue of a contaminated sample than is afforded by the more laborious and time consuming pretreatment of previous workers.     

Improved partition equilibrium model for predicting analyte response in electrospray ionization mass spectrometry

A previously proposed partition equilibrium model for quantitative prediction of analyte response in electrospray ionization mass spectrometry is modified to yield an improved linear relationship. Analyte mass spectrometer response is modeled by a competition mechanism between analyte and background electrolytes that is based on partition equilibrium considerations. The correlation between analyte response and solution composition is described by the linear model over a wide concentration range and the improved model is shown to be valid for a wide range of experimental conditions. The behavior of an analyte in a salt solution, which could not be explained by the original model, is correctly predicted. The ion suppression effects of 16 : 0 lysophosphatidylcholine (LPC) on analyte signals are attributed to a combination of competition for excess charge and reduction of total charge due to surface tension effects. In contrast to the complicated mathematical forms that comprise the original model, the simplified model described here can more easily be employed to predict analyte mass spectrometer responses for solutions containing multiple components. Copyright © 2008 John Wiley & Sons, Ltd.     

Preliminary evaluation of laser-induced breakdown spectroscopy for tissue classification

Laser-induced breakdown spectroscopy (LIBS) is an on-line, real-time technology that can produce immediate information about the elemental contents of tissue samples. We have previously shown that LIBS may be used to distinguish cancerous from non-cancerous tissue. In this work, we study LIBS spectra produced from chicken brain, lung, spleen, liver, kidney and skeletal muscle. Different data processing techniques were used to study if the information contained in these LIBS spectra is able to differentiate between different types of tissue samples and then identify unknown tissues. We have demonstrated a clear distinguishing between each of the known tissue types with only 21 selected analyte lines from each observed LIBS spectrum. We found that in order to produce an analytical model to work well with new sample we need to have representative training data to cover a wide range of spectral variation due to experimental or environmental changes.     

Simultaneous determination of antazoline and naphazoline by the net analyte signal standard addition method and spectrophotometric technique

A novel net analyte signal standard addition method (NASSAM) was used for simultaneous determination of the drugs anthazoline and naphazoline. The NASSAM can be applied for determination of analytes in the presence of known interferents. The proposed method is used to eliminate the calibration and prediction steps of multivariate calibration methods; the determination is carried out in a single step for each analyte. The accuracy of the predictions against the H-point standard addition method is independent of the shape of the analyte and interferent spectra. The net analyte signal concept was also used to calculate multivariate analytical figures of merit, such as LOD, selectivity, and sensitivity. The method was successfully applied to the simultaneous determination of anthazoline and naphazoline in a commercial eye drop sample.     

Variable selection by modified IPW (iterative predictor weighting)-PLS (partial least squares) in continuous wavelet regression models

Variable selection is often used to produce more robust and parsimonious regression models. But when they are applied directly to the raw near-infrared spectra, it is not easy to select appropriate variables because background and noise will often overshadow or overlap the absorption bands of analyte. In this work, a new hybrid algorithm based on the selection of the most informative variables in the continuous wavelet transform (CWT) domain is described. The strategy is a combination of CWT and a procedure of modified iterative predictor weighting-partial least square (mIPW-PLS). After elimination of the background and noise in NIR spectra by CWT, the mIPW-PLS approach is used to select the most informative CWT coefficients. With the selected CWT coefficients, a PLS model is built finally for prediction. It is indicated that the extraction of most important variables in the CWT domain can effectively avoid the interference of background and noise, and result in a high quality of regression model with a very small number of variables and fewer PLS components.