Reproducibility chemometrics

Since reproducibility of the flow system is critical to obtaining reproducibility, one approach has been to substitute lower-performance columns (50-to 100-p packings) operated at higher temperatures.1 Often, improvements in detection and data reduction can substitute for resolution. Chemometric principles are a way to sacrifice chromatographic efficiency but still obtain the desired chemical information. An example of how meaningful information can be derived indirectly from chromatographic separation is the use of system or vacancy peaks to monitor chemical reactions such as the titration of aniline and the hydrolysis of aspirin to salicylic acid.18... 

Figure 3 Chemometric calibration of the elastic modulus (left) and the yield stress (right) of a series of polyethylenes. Reproduced from Gabriel et al. [23], Copyright 2003, with permission from Wiley-VCH Verlag GmbH.

One of the earlier tests [58] contains as hydrophobic samples toluene and ethyl benzene, as a weakly acidic component phenol, and weakly basic analytes like aniline and the isomeric toluid-ines. Chemometric analysis showed the proper selection of the analytes for characterization, with the surprising result that /V,/V-dimethyl aniline is not a signihcant analyte in characterization for silanophilic interactions [59]. As the mobile phase, a mixture of 49 Vol.% methanol with 51 Vol.% water has been used. In the beginning, an unbuffered mobile phase has been used because silanophilic interactions can be blocked by buffer constituents. For better reproducibility and transfer-ability, a 10 mM phosphate buffer of pH 7 is recommended. The comparison of RP columns for hydrophobic interaction by this test procedure is shown in Figure 2.8. The k value of toluene... 

Fig. 4. Chemometrics, the interface Iwtween the analysis and mathematics. From B. G. M. Vande-ginste. Anal. Chim. Acta 150, 203 (1983). Reproduced by permission of Elsevier Science Publishers, Amsterdam.

Fig. 8.3. A Acquired high SNR data and simulated noisy spectra (peak-to-peak noise = 0.001, 0.01, 0.1, and 0.4 a.u.), showing the degradation in data quality. Spectra are offset for clarity. B Spectra after noise reduction demonstrate the dramatic gains possible by chemometric methods. C Noise reduction was implemented to classify breast tissue and application of noise rejection allowed the same quality of classification (accuracy) to be recovered at higher noise levels. D In another example, image fidelity (here the nitrile stretching vibrational mode at 2227 cm-1) is much enhanced as a result of spectral noise rejection A and C are reproduced from Reddy and Bhargava, Submitted [165], D is reproduced from [43]...

In this situation, it would be ideal to produce a calibration on only one of the analyzers, and simply transfer it to all of the other analyzers. There are certainly cases where this can be done effectively, especially if response variability between different analyzers is low and the calibration model is not very complex. However, the numerous examples illustrated above show that multivariate (chemometric) calibrations could be particularly sensitive to very small changes in the analyzer responses. Furthermore, it is known that, despite the great progress in manufacturing reproducibility that process analyzer vendors have made in the past decade, small response variabilities between analyzers of the same make and... 

Chemometrics uses a very large variety of software. In this book we recommend two main environments, Excel and Matlab the examples have been tried using both environments, and you should be able to get the same answers in both cases. Users of this book will vary from people who simply want to plug the data into existing packages to those that are curious and want to reproduce the methods in their own favourite language such as Matlab, VBA or even C. In some cases instructors may use the information available with this book to tailor examples for problem classes. Extra software supplements are available via the publisher s www.SpectroscopyNOW.com Website, together with all the datasets and solutions associated with this book. 

Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form widiout the express written permission of Marcel Dekker, Inc. 2 Chemometrics in Chromatography... 

The application of chemometric or multivariate analysis techniques to nuclear magnetic resonance (NMR) spectroscopic data is reviewed. Descriptions of the different processing and data manipulation procedures being utilized to produce reproducible input data sets for chemometric analysis are discussed. A brief review of... 

The analyzed DNA was obtained through samples of adult populations of horn flies from five different Brazilian localities Boa Vista (RR), Mossoro (RN), Seropedica (RJ), Campo Grande (MS), and Rosario do Sul (RS), each one in a different geographical region of the country. These samples were amplified with 60 RAPD primers and their amplified products were assessed for the number and quality of polymorphic loci. Only 16 primers that amplified reproducible polymorphic bands were selected for chemometric analysis. 

Chemometric techniques can be valuable tools for the exploration of CE data as well as for the classihcation of samples based on electrophoretic data. The techniques maximally exploit the multivariate character of the data. In several applications, it was demonstrated that chemometric approaches can extract more information from electropherograms than only a visual inspection can. It is very important, especially when using entire electropherograms, that the CE data are preprocessed (e.g., aligned) in an appropriate way prior to other chemometric calculations, because CE analyses generally exhibit a rather poor reproducibility. 

Data processing is reproducible using a computer. Adequate software for chemometrics presents the signal obtained from analytical instruments as reproducible analytical information. The importance of obtaining a reproducible analytical process is its potential for automation and miniaturization. Miniaturization of the analytical process for electrometric analysis has been achieved for in vivo measurements. The use of electrometric analysis provides good reproducibility of analytical information. 

Precision of a chemometric method refers to the reproducibility of the method. For quantitative chemometric methods, it is important to test both the instrument and method precision. Instrument precision is done by repeating measurements on the same sample method precision is the closeness of replicate sample measurements while intermediate precision can be evaluated by running the same samples with different analysts on different days. 

Readers with experience in chemometrics will have noticed that, like principal components analysis (PCA), MDS is a dimensionality reduction method. For each molecule, a large number of attributes (similarity to each other molecule) is reduced to a much smaller number of coordinates in an abstract property space, which reproduce the original data within an established error. The pertinent difference is that PCA uses the matrix of correlations between a set of (redundant) properties, which are usually obtained from a table of those properties for an initial set of molecules. In contrast, MDS uses a matrix of similarities between each pair of molecules (or substituents). 

Figure 3 Response surface for the modeling of the hydrolysis of ATP as a function of two rate constants showing two minima A and B. T Is the location of the true minimum. (Reproduced with permission from Hibbert DB and Sandall JPB (1989) Mechanism of the hydrolysis of adenosine 5 -triphosphate A statistical evaluation of kinetic data. Journal of Chemometrics 3 569-577 John Wiley Sons Ltd.)...

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