Use of chemometrics

As in various areas of analytical chemistry, there is a growing trend in operational speciation studies to apply chemometric techniques in order to both (1) improve experimental design and (2) gain as much useful information as possible from experimental results. 

Analysis of variance (ANOVA and MANOVA) has been used to investigate the influence of location on forms of metals in roadside soil (Nowak, 1995). Multiple regression analysis has proved valuable in processing sequential extraction data to obtain information on plant availability of trace metals in soils (Qian et al, 1996  

Zhang et al., 1998) whilst, in the aquatic environment, chemometric methods have provided insight into sources of heavy metal contamination (Borovec, 1996 Tsai et al., 1998). 

Chemometrics can also be used to overcome some of the intrinsic deficiencies of sequential extraction, such as non-specificity. Barona and Romero (1996a) used principal components analysis (PCA) to establish relationships between the amounts of metals released at each stage of a sequential extraction procedure and bulk soil properties, and demonstrated that carbonates played a dominant role in governing metal partitioning in the soil studied. The same workers employed multiple regression analysis to study soil remediation (see Section 10.11.1.1). Zufiaurre et al. (1998) also used PCA to confirm their interpretation of phase association and hence potential bioavailability of heavy metals in sewage sludge. 

An interesting, and somewhat radical, departure from traditional extraction methodology was proposed by Cave and Wragg (1997). They demonstrated that, with an appropriate chemometric mixture resolution procedure, a simple, nonspecific extraction could provide information on metal binding in soil SRM 2710 similar to that obtained by a Tessier sequential extraction. The method used a central composite design, with extraction time, nitric acid concentration and sample extractant ratio as variables, together with PCA. 

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Terrado M, Kuster M, Raldua D, Lopez de Alda M, Barcelo D, Tauler R (2007) Use of chemometric and geostatistical methods to evaluate pesticide pollution in the irrigation and drainage channels of the Ebro river delta during the rice-growing season. Anal Bioanal Chem 387(4) 1479-1488... 

C.E. Miller, The use of chemometric techniques in process analytical method development and operation, Chemom. Intell. Lab. Syst, 30, 11-22 (1995). 

Despite the lack of inherent selectivity, it is still possible to obtain good quantitative data from online UV/vis monitoring by making use of chemometric techniques to resolve the overlapping spectra. The most common application is in dissolution testing [73, 74], where results that are at least as accurate as those of the reference (and much slower and more costly) HPLC method have been demonstrated. 

There are broadly two uses of chemometrics that interest the process chemist. The first of these is simply data display. It is a truism that the human eye is the best analytical tool, and by displaying multivariate data in a way that can be easily assimilated by eye a number of diagnostic assessments can be made of the state of health of a process, or of reasons for its failure [ 153], a process known as MSPC [154—156]. The key concept in MSPC is the acknowledgement that variability in process quality can arise not just by variation in single process parameters such as temperature, but by subtle combinations of process parameters. This source of product variability would be missed by simple control charts for the individual process parameters. This is also the concept behind the use of experimental design during process development in order to identify such variability in the minimum number of experiments. 

Anyway, whatever the instrument and the application, an aspect is unvarying the only way to obtain valuable information from these analytical systems is to process data by means of multivariate tools, that is to say, to make use of chemometrics. 

In recent years the term qualimetrics has been coined to refer to the use of chemometrics for the purposes of quality control (Massart et al. 1997). ft relates particularly to the use of multivariate analysis of process control measurements. Other texts on quality assurance in chemical laboratories include the latest edition of Garfield s book published by AOAC International (Garfield et al. 2000), material published through the Valid Analytical Measurement program by the LGC (Prichard 1995), and books from the Royal Society of Chemistry (Parkany 1993,1995 Sargent and MacKay 1995). Wenclawiak et al. (2004) have edited a series of Microsoft PowerPoint presentations on aspects of quality assurance. 

In order to overcome, or at least minimise, such drawbacks we can resort to the use of chemometric techniques (which will be presented in the following chapters of this book), such as multivariate experimental design and optimisation and multivariate regression methods, that offer great possibilities for simplifying the sometimes complex calibrations, enhancing the precision and accuracy of isotope ratio measurements and/or reducing problems due to spectral overlaps. 

A. M. Carro, I. Neira, R. Rodil and R. A. Lorenzo, Speciation of mercury compounds by gas chromatography with atomic emission detection. Simultaneous optimisation of a headspace solid-phase microextraction and derivatisation procedure by use of chemometric techniques, Chro-matographia, 56(11/12), 2002, 733-738. 

Miller, C.E., The Use of Chemometric Techniques in Process Analytical Method Development and Operation Chemometrics Intell. Lab Syst. 1995, 30, 11-22. 

R. Petkovska, C. Cornett, A. Dimitrovska, Development and validation of rapid resolution RP-HPLC method for simultaneous determination of atorvastatin and related compounds by use of chemometrics, Anal. Lett. 41 (2008) 992-1009. 

The use of chemometric methods will cut the costs of environmental monitoring in the future this can be carried out on the basis of an optimal number of indispensable parameters to be determined, without any loss of significant information on environmental pollution. 

G., Expert systems for multivariate calibration, trendsetters for the wide-spread use of chemometrics, Chemom. Intell. Lab. Syst., 15, 171-184, 1992. 

The use of multivariate spectral information is particularly advantageous where quantification of a particular metabolite in a complex biological background is being attempted and application of the technique necessitates the use of chemometric processing techniques for quantification of components. 

To undertake QSRR studies one needs two kinds of input data. One is a set of quantitatively comparable retention data (dependent variable) for a sufficiently large (for statistical reasons) set of analytes. The other is a set of quantities (independent variables) assumed to account for structural differences among the chromatographed analytes. Through the use of chemometric computational techniques, retention parameters are characterized in terms of various descriptors of analytes (or their combinations) or in terms of systematic knowledge extracted (learned) from these descriptors. 

Fig. 13.5 Schematic representation of the use of chemometrics to deconvolute the multiplex spectrum of six dye-labeled oligonucleotides, (a) Shows the individual spectra of six dye-labeled oligonucleotides and (b) shows the spectrum of the mixture of the six labeled oligonucleotides...

Use of chemometrics to devise procedures suitable for the most crucial stage of optimization, optimization of selectivity, is generally performed in three steps ... 

The price of being able to observe the process in near real-time with NIR or other monitors is dealing with the large amount of data and the interpretation. This requires the use of multivariate methods to reduce the dimensions of the data into something that can be handled. This is accomplished through the use of chemometrics. 

Chemometric or multivariate methods are routinely utilized in other forms of spectroscopy for the analysis of complex mixtures. The use of chemometrics in NMR is more limited, but has quickly become an important tool for the NMR spectroscopist. " For this review, we define chemometrics as the application of statistical and mathematical methods to extract chemically relevant information produced in NMR... 

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