Quantitative structure-wavelength relationship modeling of porphin: derivative photosensitizers

A linear quantitative structure-visible light absorption wavelength (λ) relationship model for one hundred and forty-two photosensitizers was proposed using heuristic method and multiple linear regression analysis. The statistical parameters of the model were R2 = 0.916; F = 372.16; and RMSE = 5.0873. A fivefold cross-validation algorithm was applied, and the results indicated that the model has a satisfactory statistical stability and validity. The proposed model was evaluated for predictive ability with an external validation set, and the statistical parameters obtained were and R2EXT = 0.908; Q2EXT = 0.897; F = 118.14; and RMSEEXT = 5.6338 for the external test set. The results obtained demonstrated that the simple linear quantitative structure-wavelength relationship model was robust and satisfactory. It could be a feasible and effective tool for predicting λ of photosensitizers, which is an important parameter for their effect on photodynamic therapy for cancer, and could be a potential way for instructing synthesis of this kind of new photosensitizers.     

Comparative study of several programs used in the potentiometric evaluation of equilibrium constants including an error sensitivity analysis

The programs MINIQUAD, MINIPOT, SUPERQUAD and PHCONST are applied to the evaluation of the protonation constants of several hypothetical polyprotic substances using simulated titrations, and the precision and accuracy of the results are discussed and compared. When statistical weights are used the results are very similar, the accuracy being better with PHCONST. Error sensitivity analysis was used as a means of establishing the influence of a systematic error of an experimental variable on the accuracy of the determination. An algorithm for the accurate calculation of error sensitivities is proposed and checked. Error sensitivities can be used to make a choice between the various minimization functions or to decide if a variable must be included in the optimization process.     

Assessment of the Accuracy of Estimating Adsorption Parameters of Organic Substances from Differential Capacitance Curves: Monte-Carlo Simulation of the Random-Error Effect in Capacitance Measurements

Monte-Carlo method is used for elucidating the effect of a random normally distributed error of capacitance measurements on estimated adsorption parameters of organic substances on an ideally polarizable electrode. Within the model of two parallel capacitors and a Frumkin isotherm with preset adsorption parameters, initial differential capacitance vs. potential dependences are calculated for ten organic-substance concentrations. Then, a random normally distributed error is introduced at every point in these dependences. For estimating the adsorption parameters, 100 series of dependences thus obtained are processed by a nonlinear regression analysis technique. Histograms of distributions of found parameters are plotted. The accuracy of the parameter calculation is assessed for 5 and 10% errors of capacitance measurements.     

Wang-Landau algorithm: a theoretical analysis of the saturation of the error

In this work we present a theoretical analysis of the convergence of the Wang-Landau algorithm [Phys. Rev. Lett. 86, 2050 (2001)] which was introduced years ago to calculate the density of states in statistical models. We study the dynamical behavior of the error in the calculation of the density of states. We conclude that the source of the saturation of the error is due to the decreasing variations of the refinement parameter. To overcome this limitation, we present an analytical treatment in which the refinement parameter is scaled down as a power law instead of exponentially. An extension of the analysis to the N-fold way variation of the method is also discussed.     

A statistical analysis of the precision of reweighting-based simulations

Various advanced simulation techniques, which are used to sample the statistical ensemble of systems with complex Hamiltonians, such as those displayed in condensed matters and biomolecular systems, rely heavily on successfully reweighting the sampled configurations. The sampled points of a system from an elevated thermal environment or on a modified Hamiltonian are reused with different statistical weights to evaluate its properties at the initial desired temperature or of the original Hamiltonian. Often, the decrease of accuracy induced by this procedure is ignored and the final results can be far from what is expected. We have addressed the reasons behind such a phenomenon and have provided a quantitative method to estimate the number of sampled points required in the crucial step of reweighting of these advanced simulation methods. We also provided examples from temperature histogram reweighting and accelerated molecular dynamics reweighting to illustrate this idea, which can be generalized to the dynamic reweighting as well. The study shows that this analysis may provide a priori guidance for the strategy of setting up the parameters of advanced simulations before a lengthy one is carried out. The method can therefore provide insights for optimizing the parameters for high accuracy simulations with finite amount of computational resources.     

X-ray absorption spectroscopy of hemes and hemeproteins in solution: multiple scattering analysis

A full quantitative analysis of Fe K-edge X-ray absorption spectra has been performed for hemes in two porphynato complexes, that is, iron(III) tetraphenylporphyrin chloride (Fe(III)TPPCl) and iron(III) tetraphenylporphyrin bis(imidazole) (Fe(III)TPP(Imid)2), in two protein complexes whose X-ray structure is known at atomic resolution (1.0 A), that is, ferrous deoxy-myoglobin (Fe(II)Mb) and ferric aquo-myoglobin (Fe(III)MbH2O), and in ferric cyano-myoglobin (Fe(III)MbCN), whose X-ray structure is known at lower resolution (1.4 A). The analysis has been performed via the multiple scattering approach, starting from a muffin tin approximation of the molecular potential. The Fe-heme structure has been obtained by analyzing independently the Extended X-ray Absorption Fine Structure (EXAFS) region and the X-ray Absorption Near Edge Structure (XANES) region. The EXAFS structural results are in full agreement with the crystallographic values of the models, with an accuracy of +/- 0.02 A for Fe-ligand distances, and +/-6 degrees for angular parameters. All the XANES features above the theoretical zero energy (in the lower rising edge) are well accounted for by single-channel calculations, for both Fe(II) and Fe(III) hemes, and the Fe-N p distance is determined with the same accuracy as EXAFS. XANES evaluations of Fe-5th and Fe-6th ligand distances are determined with 0.04-0.07 A accuracy; a small discrepancy with EXAFS (0.01 to 0.05 A beyond the statistical error), is found for protein compounds. Concerns from statistical correlation among parameters and multiple minima in the parameter space are discussed. As expected, the XANES accuracy is slightly lower than what was found for polarized XANES on Fe(III)MbCN single crystal (0.03-0.04 A), and states the actual state-of-the-art of XANES analysis when used to extract heme-normal parameters in a solution spectrum dominated by heme-plane scattering.     

对直接标准化算法的改进及其应用

由于各种仪器之间存在差异,主机上建立的定量模型用于从机会导致预测结果出现较大偏差。目前主要通过有标样方法和无标样方法来减小预测偏差。该文对现有标样方法中的直接标准化算法进行改进,在转移矩阵的建立过程中,对从仪器数据矩阵进行主成分分解,以预测均方差为判定标准,确定最终的转移矩阵。并以玉米和烟草数据为对象,测试了该法的有效性。玉米样品含有2种成分:水分和蛋白质;烟草样品含有4种成分:还原糖、总糖、总氮和总碱。结果表明,对于玉米样品中的2种成分,采用改进的方法可显著提高预测的准确度;对于烟草中的4种成分而言,采用改进的方法可获得稳健的预测结果。     

A numerical study of expected accuracy and precision in Calibration-Free Laser-Induced Breakdown Spectroscopy in the assumption of ideal analytical plasma

Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS) has been proposed several years ago as an approach for quantitative analysis of Laser-Induced Breakdown Spectroscopy spectra. Recently developed refinement of the spectral processing method is described in the present work. Accurate quantitative results have been demonstrated for several metallic alloys. However, the degree of accuracy that can be achieved with Calibration-Free Laser-Induced Breakdown Spectroscopy analysis of generic samples still needs to be thoroughly investigated. The authors have undertaken a systematic study of errors and biasing factors affecting the calculation in the Calibration-Free Laser-Induced Breakdown Spectroscopy spectra processing. These factors may be classified in three main groups: 1) experimental aberrations (intensity fluctuations and inaccuracy in the correction for spectral efficiency of a detection system), 2) inaccuracy in theoretical parameters used for calculations (Stark broadening coefficients and partition functions) and 3) plasma non-ideality (departure from thermal equilibrium, spatial and temporal inhomogeneities, optical thickness, etc.). In this study, the effects of experimental aberrations and accuracy of spectral data were investigated, assuming that the analytical plasma is ideal. Departure of the plasma conditions from ideality will be the object of future work. The current study was based on numerical simulation. Two kinds of metallic alloys, iron-based and aluminum-based, were studied. The relative weight of the error contributions was found to depend on the sample composition. For the here-investigated samples, the experimental aberrations contribute to the overall uncertainty on the quantitative results more than theoretical parameters. The described simulation method can be applied to the Calibration-Free Laser-Induced Breakdown Spectroscopy analysis of any other kind of sample.     

Independent assessment of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) sample preparation quality: A novel statistical approach for quality scoring

Preparation of samples according to an optimized method is crucial for accurate determination of polymer sample characteristics by Matrix-Assisted Laser Desorption Ionization (MALDI) analysis. Sample preparation conditions such as matrix choice, cationization agent, deposition technique or even the deposition volume should be chosen to suit the sample of interest. Many sample preparation protocols have been developed and employed, yet finding the optimal sample preparation protocol remains a challenge. Because an objective comparison between the results of diverse protocols is not possible, “gut-feeling” or “good enough” is often decisive in the search for an optimum. This implies that sub-optimal protocols are used, leading to a loss of mass spectral information quality. To address this problem a novel analytical strategy based on MALDI imaging and statistical data processing was developed in which eight parameters were formulated to objectively quantify the quality of sample deposition and optimal MALDI matrix composition and finally sum up to an overall quality score of the sample deposition. These parameters can be established in a fully automated way using commercially available mass spectrometry imaging instruments without any hardware adjustments. With the newly developed analytical strategy the highest quality MALDI spots were selected, resulting in more reproducible and more valuable spectra for PEG in a variety of matrices. Moreover, our method enables an objective comparison of sample preparation protocols for any analyte and opens up new fields of investigation by presenting MALDI performance data in a clear and concise way.     

The optimal P3M algorithm for computing electrostatic energies in periodic systems

We optimize Hockney and Eastwood's particle-particle particle-mesh algorithm to achieve maximal accuracy in the electrostatic energies (instead of forces) in three-dimensional periodic charged systems. To this end we construct an optimal influence function that minimizes the root-mean-square (rms) errors of the energies. As a by-product we derive a new real-space cutoff correction term, give a transparent derivation of the systematic errors in terms of Madelung energies, and provide an accurate analytical estimate for the rms error of the energies. This error estimate is a useful indicator of the accuracy of the computed energies and allows an easy and precise determination of the optimal values of the various parameters in the algorithm (Ewald splitting parameter, mesh size, and charge assignment order).     

Quantitative validation of a flow-injection determination of penicillin in pharmaceutical formulations by means of a validation program based on an expert system

The quantitative validation of the results of a flow-injection determination of penicillin in pharmaceutical formulations is described. The validation procedure is done by using VALID, which is a generally applicable validation program based on an expert system program. The automated penicillin assay is based on the enzymatic hydrolysis of the penicillin to the corresponding penicilloic acid, which reacts with iodine generated on-line; the iodine consumption is detected amperometrically. The method is evaluated for applicability in pharmaceutical quality control. The complete validation procedure is described. During the program run, the system evaluates the calibration procedure, the drift of the analytical systemm and the effect of the sample matrix. The reliability of the flow-injection method is estimated by evaluating the maximum total error (MTE), which includes both random error and systematic error. The latter was assessed by comparing of the results of the flow-injection method with the results obtained by titration with mercury(II) as the reference method. The user requirement for the assay was an MTE of 10%. The validation procedure showed that the analytical method complied with the requirements for the major part of the concentration range (0.066–0.25 mM).     

Retention time alignment of LC/MS data by a divide-and-conquer algorithm

Liquid chromatography-mass spectrometry (LC/MS) has become the method of choice for characterizing complex mixtures. These analyses often involve quantitative comparison of components in multiple samples. To achieve automated sample comparison, the components of interest must be detected and identified, and their retention times aligned and peak areas calculated. This article describes a simple pairwise iterative retention time alignment algorithm, based on the divide-and-conquer approach, for alignment of ion features detected in LC/MS experiments. In this iterative algorithm, ion features in the sample run are first aligned with features in the reference run by applying a single constant shift of retention time. The sample chromatogram is then divided into two shorter chromatograms, which are aligned to the reference chromatogram the same way. Each shorter chromatogram is further divided into even shorter chromatograms. This process continues until each chromatogram is sufficiently narrow so that ion features within it have a similar retention time shift. In six pairwise LC/MS alignment examples containing a total of 6507 confirmed true corresponding feature pairs with retention time shifts up to five peak widths, the algorithm successfully aligned these features with an error rate of 0.2%. The alignment algorithm is demonstrated to be fast, robust, fully automatic, and superior to other algorithms. After alignment and gap-filling of detected ion features, their abundances can be tabulated for direct comparison between samples.     

专家系统方法选择负载型催化剂的溶液浸渍制备的参数

负载型催化剂的浸渍制备涉及到大量的经验规律,这就使得其条件参数的选择成为一个比较复杂的不确定性问题,借助于专家系统方法可望解决此类问题。本文在单组分浸渍系统的基础上,形成了一个多组分浸渍制备的专家系统原型IACES—Ⅱ,该系统由Prolog和Fortran语言编码,具备处理数值和非数值信息的功能。在IACES-Ⅱ系统中,采用集总的方法将浸渍参数的优化选择问题分解为若干子问题,并采用“规则架+规则体”这种基本结构来形成知识库的整体结构。然后,通过基于目标的推理给出如下浸渍参数:(1)吸附形式,(2)离子形式,(3)浸渍体系稳定的pH值范围,(4)竞争吸附剂,(5)操作参数(初始浓度,载体用量和溶液体积之比,浸渍时间等)。     

An interlaboratory comparison of sample presentation methods for the analysis of aqueous solutions using fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is becoming much more widely used as an analytical technique. It appears to be particularly useful for quantitative analysis, especially of aqueous solutions. However, there is a considerable variation in the cost and quality of instrumentation as well as a large number of accessories for sample presentation employing attenuated total reflectance (ATR). The objectives of the work reported in this paper were to investigate the effect of the choice of instrumentation on the quality of quantitative analyses as well as the effect of variable acquisition parameters. A series of experiments was designed in which samples were distributed to a number of workers using a range of FTIR instruments from different manufacturers and of differing quality. The workers were allowed to use the accessory of their own choice and to carry out a quantitative analysis of glucose solutions using their normal operating parameters. In following experiments the effects of instrumental and purge were considered and quantitative analysis was performed using a set of prescribed acquisition conditions. The results showed that there were significant differences in the quality of the spectra produced on the different combinations. However, although each worker produced quite different linear calibrations (in terms of slope and intercept), they all predicted the concentration of an unknown with a similar accuracy. However, later experiments showed that the low-cost bench-top instruments were not always so successful when the peaks used in the analysis were affected by residual water vapour interferences. Instrumental drift was noticeably worse for the low-cost instruments but this was an effect that could easily be eliminated by suitable baseline correction. Received: 12 December 1994/Accepted: 13 February 1995 Correspondence to: R.H. Wilson     

Analytical service: systematic approach to the activity optimization according to accuracy criterion of analysis results

Summary It is taken as an initial conception that the general aim of quantitative analysis is measurement by specific, mainly analytical, methods and instruments. Another important initial position is the necessity to specify the required accuracy of analytical data, taking into account the contribution of analytical error within the error of the whole test result which includes contributions from other sources (sampling, sample storage, transporting, etc.).The approaches to validation of accuracy of field analysis are considered. Taking into account all factors mentioned above, the principles and methods of demonstration of accuracy are stated.The main sources of uncertainty of the really insured accuracy are pointed out. The typical scheme of activity optimization of analytical subdivisions and those systems which use the results on the chemical composition of substances is described. The peculiarities of obtaining basic data and their usage for quality control of field analytical results are discussed.

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Systemoptimierung durch Genauigkeitskriterien analytischer Ergebnisse

     

提高能谱无标样定量分析准确度的参数选择

针对如何提高X射线能谱仪(EDS)无标样定量分析的准确度,进行了一系列实验研究和理论分析.讨论了样品制样的基本要求,能谱分析过程中加速电压、收集时间和分析区域等方面对能谱分析结果的影响及其最佳的选择参数,如对于轻元素分析应采用较低电压、分析时间过长可能导致样品在电子束轰击下引起更大碳沉积等,通过合理的参数选择均能得到较为满意的分析结果,提高了定量测试的准确度.     

Die Bestimmung des Sauerstoffs mit hoher Präzision und Richtigkeit

Oxygen has been determined with high precision and accuracy by activation analysis via N-16, using a cyclic irradiation and counting technique. The sources of accidental errors have been revealed and eliminated by means of statistical tests and the systematic errors due to counting errors and matrix effects have been corrected. The standardization errors arising in stationary and birotary sandwich arrangement of sample and standard are compared and the procedure of statistical error analysis is discussed. The results of oxygen analysis in organic compounds, oxides, carbonates, sulfates and other compounds with a precision 0.05% and an accuracy <0.1% are reported.     

Forward flux sampling-type schemes for simulating rare events: efficiency analysis

We analyze the efficiency of several simulation methods which we have recently proposed for calculating rate constants for rare events in stochastic dynamical systems in or out of equilibrium. We derive analytical expressions for the computational cost of using these methods and for the statistical error in the final estimate of the rate constant for a given computational cost. These expressions can be used to determine which method to use for a given problem, to optimize the choice of parameters, and to evaluate the significance of the results obtained. We apply the expressions to the two-dimensional nonequilibrium rare event problem proposed by Maier and Stein [Phys. Rev. E 48, 931 (1993)]. For this problem, our analysis gives accurate quantitative predictions for the computational efficiency of the three methods.     

Sensitivity of model-based quantitative FTIR to instrumental and spectroscopic database error sources

We report a theoretical study of the error sources associated with the quantification of gas-phase FTIR spectra using synthetic calibrations. A forward model was constructed based on a Bruker IFS66 FTIR spectrometer, modelling the instrument line shape from theory and taking line parameters from a line-by-line spectral database. Default values were set in the forward model for an ‘ideal’ system where the spectrometer is perfectly aligned and all spectroscopic parameters are exactly known. Using a re-iterative non-linear least squares routine input values were perturbed allowing assessment of the model sensitivity to each parameter. Using the sensitivity information and knowledge of the possible absolute uncertainties (e.g. the accuracy to which a user might be able to axially align the field stop aperture) the possible quantitative contribution of each parameter was ranked in the following order (greatest first); field stop aperture axial alignment, line intensity, line centre, air-broadening line width, pressure, lateral field stop aperture/collimating optic alignment, pathlength, temperature. Whilst this ranking was specific to the type of measurement modelled it was discussed which elements would remain consistent for different measurement acquisition set-ups and which would alter. Consequently, it was proposed that the error analysis presented here could be used to determine which parameters in a forward model should be set as variables (i.e. those with the highest potential error contribution) and which should remain fixed in attempting to minimise the errors in using optimisation routines for the purpose of gas quantification.     

Excitation of X-ray fluorescence of massive samples by bremsstrahlung: Analytical possibilities of the monochromatic model

An algorithm was proposed for calculation of the equivalent analytical wavelength of the bremsstrahlung spectrum during X-ray fluorescence excitation in multicomponent objects of arbitrary thickness. A correlation is found between the analytically obtained approximate values of a given parameter and its exact values found numerically taking into account the design features of an X-ray tube. The approach makes it possible to minimize the error of the results of quantitative X-ray fluorescence analysis of samples of various thicknesses taking into account the matrix effects.