Review of Chemometrics Applied to Spectroscopy: 1985-95, Part 2

INTRODUCTION

Before beginning Part 2 of this review, a caveat noted by Deming and Palasota is brought to the reader's attention: [1] “Press et al. [2] have emphasized that data ‘consist of numbers, of course. But these numbers are fed into the computer, not produced by it. These are numbers to be treated with considerable respect, never to be tampered with, nor subjected to a numerical process whose character you do not completely understand. You are well advised to acquire reverence for data that is rather diferent fiom the “sporty” attitude which is sometimes allowable, or even commendable, in other numerical tasks.’ Yet by and large within chemometrics, preprocessing often seems to be carried out with little understanding of its fundamental efect on the structure of the data.”     

Theoretical study of the structure of lead zirconate–titanate PbZr<Subscript>0.6</Subscript>Ti<Subscript>0.4</Subscript>O<Subscript>3</Subscript>

A model of the structure of the piezoelectric ceramic lead zirconate–titanate PbZr_1–x Ti _x O₃ (PZT) is proposed. The model is based on ab initio calculations for possible local structures using the density functional theory (DFT) approach. A comparison of the calculated neutron diffraction data for local structures and the measured diffraction data obtained for actual powder samples shows there is a partially established long-range crystalline order in the material, in the sublattice of Zr and Ti cations.     

Highly luminescent europium(III) complexes with naphtoiltrifluoroacetone and dimethyl sulphoxide

The synthesis, luminescence properties, experimental determination and theoretical calculation of the emission quantum yield of Eu(NTA)₃.2L complexes, where NTA is naphtoiltri-fluroacetone and L denotes H₂O or DMSO (dimethyl sulphoxide), were reported. The compounds were characterized by elemental analysis (carbon, hydrogen and europium), thermal analysis, UV-visible absorption and photoluminescence spectroscopies. The experimental quantum yields were determined based on a method previously proposed by Bril and collaborators. The Eu(NTA)₃.2DMSO compound shows a high value for the Ω₂ intensity parameter (35.8 × 10^?20 cm²), reflecting the hypersensitive nature of the ⁵D₀ → ⁷F₂ transition and indicating that the lanthanide ion is in a highly polarizable chemical environment. The experimental quantum yield measured for that compound, 0.75, is one of the highest so far reported for solid-state europium complexes. The theoretical calculations of the quantum yield were carried out by solving an appropriate set of rate equations and by using empirical spectroscopic parameters and energy transfer rates. The theoretical results agree well with the experimental data for both complexes. The photostability of Eu(NTA)₃.2DMSO at 358K was evaluated in order to verify whether this complex can be applied as a phosphor for blue light emitting devices.     

The flat-maximum effect and generic linear scoring models: a test

Received on 1 July 1991. Predicting human behaviour patterns with linear correlationmodels has absorbed researchers for the past five decades. Althoughmost observers generally concede that humans are inferior tosuch models in combining information, linear scoring modelsare unfortunately, plagued by the flat-maximum effect or the‘curse of insensitivity’. As Lovie & Lovie(1986)observe: ‘The predictive ability of linear models is insensitiveto large variations in the size of regression weights and tothe number of predictors.’ In essence, seemingly differentscoringmodels tend to produce indistinguishable predictive outcomes. Since its demonstration by Dawes & Corrigan (1974), observershave cast the flat maximum in a decidedly negative light. Incontrast, Lovie & Lovie (1986) present a provocatively contrarianview of the flat maximum‘s positive potential. In thissame vein, we examine the predictive power of a generic credit-scoringmodel versus individual empirically derived systems. If, asWainer (1976) noted in regard to the flat maximum, ’itdon‘t make no nevermind’, generic credit-scoringmodels could provide cheaper alternatives to individual empiricallyderived models. During the period 1984–8, a series of linear credit-scoringmodels were developed for ten Southeastern U.S. credit unions.For each credit union, stepwise multiple regression was employedto select a subset of explanatory variables to be used in adiscriminant analysis. A generic credit-scoring equation wasdeveloped from the resulting discriminant analyses using weightedaverage coefficients from five systems. The predictive powerof the generic model was compared to the predictive power ofholdout sample of the five remaining credit-scoring models. In all cases, the generic model's performance was very closeto that of the empirically derived models. Thus, our findingssupport Lovie & Lovie's (1986) challenge to the conventionalwisdom that the flat maximum casts a pall on the successfulmodelling of judgement processes. Indeed, the flat maximum impliesa positive role for simpler, and hence cheaper, generic models.Although further research is needed, it should be possible todevelop hybrid models with generic cores that perform as wellas empirically derived linear models.     

Hadamard Transform Time‐of‐Flight Mass Spectrometry: More Signal,More of the Time

Hadamard transform time‐of‐flight mass spectrometry (HT‐TOF MS) is a type of mass analysis that was developed to couple continuous ion sources to the inherently pulsed nature of time‐of‐flight measurements. Unlike conventional TOF MS, the Hadamard transform method offers a duty cycle of 50 %, with the possibility of extending it to 100 %. Because it is a multiplexing technique, the attainable signal‐to‐noise ratio (SNR) is also significantly higher than that of conventional TOF MS. This review covers the basic principles behind HT‐TOF MS. We illustrate, through examples, the source of the high‐duty cycle and the increase in SNR. These features translate to a mass spectral storage rate that is the fastest among similar instruments, which enables its use as a detector for high‐speed separations.     

Quantitative Determination of Reactive Species in Alkali Halide Disks

Abstract

As part of an infrared investigation of the kinetics of the reactions of KMnO₄ in a KI disk and of KIO₄ in a KI disk, the stoichiometry of the two reactions was needed. A previous study¹ has shown that MnO₄ ^? reacts in a KI matrix to produce IO₄ ^? (which then reacts further to form IO₃ ^?) and MnO₂. Iodate ion is the only observable product in the reaction of IO₄ ^? in a KI matrix¹. The infrared absorption at 740cm^?1, which is due to the presence of IO₃ ^?, was found to obey the Beer-Lambert Law. However, calibration curves for KMnO₄ in KI and for KIO₄ in KI could not be obtained because both species reacted with the dispersing medium during the preparation of the disk¹. It was also observed that no immediate reaction occurred when KBr or KC1 disks containing KMnO₄, or KIO₄ were prepared. Thus, an extrapolation method was sought which would permit the estimation of calibration curves for the reactive species.     

Tetraoxygen on reduced TiO2(110): oxygen adsorption and reactions with bridging oxygen vacancies

Oxygen adsorption on reduced TiO2(110) is investigated using temperature programmed desorption and electron-stimulated desorption. At low temperatures, 2 O(2) molecules can be chemisorbed in each oxygen vacancy. These molecules do not desorb upon annealing to 700 K. Instead, for 200 K相似文献