Discriminant matrices

The outer membrane of the cell is an organelle, richly endowed with receptors which recognise and react to signalling molecules from the environment, endowed with enzymes for the degradation and synthesis of nutrients within the cell and external to it, and bearing transport systems which control the entrance and egress of specific metabolites. Subsequent chapters of this series will deal with all aspects of this dynamic commerce of the cell membrane as mediated by these membrane proteins. The present chapter confines itself, however, to those properties of the cell membrane which arise from the lipid baekbone or substructure into which these more dynamic proteins are embedded. For a nutrient or any foreign molecule which finds no specific membrane component with which to interact, the lipid bilayer of the membrane provides the barrier which determines whether the molecule in question can cross the membrane. How then does this cell membrane matrix discriminate between possible permeants This question is the theme of the present chapter. 

The same research group (Ayora-Canada and Lendl, 2000) presented a similar configuration, but without the separation of the analyte from the sample matrix. In this case, spectral matrix discrimination was used to reduce the sample matrix contribution to the analytical signal. 

Studies of octylsilane (OS) phases, deactivated by end-capping, have shown that such stationary phases lead to a discrimination between compounds according to their H-bond donor capacity, as the stationary phase presents strong accessible H-bond acceptor groups (-Si-O-Si-) [22, 23]. For OS phases with a uniform matrix of cross-Hnked polysiloxane alkyl groups, relatively low correlations between log few and log Poet were found. 

So fiir, we have described only situations with two classes. The method can also be applied to K classes. It is then sometimes called descriptive linear discriminant analysis. In this case the weight vectors can be shown to be the eigenvectors of the matrix ... 

When all are considered equal, this means that they can be replaced by S, the pooled variance-covariance matrix, which is the case for linear discriminant analysis. The discrimination boundaries then are linear and is given by... 

Many more common problems start because the new users do not really understand their analytical systems, a problem described as the Nintendo scientist syndrome by Jenks (1995). Inexperienced scientists are often not sufficiently discriminating in their selection and use of CRMs. But incorrect choice can also be due to the unavailability of suitable matched matrix CRMs, or surprisingly often because the laboratory believes it cannot aftbrd the ideal product. 

Gas chromatography/electrolytic conductivity detection (GC/ELCD) and gas chromatography/halogen-speciflc detection (GC/XSD) are specific for halogenated compounds and were effective for discriminating between sulfentrazone compounds and the matrix components. Operating conditions are listed below. 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

Because neutron and y-radiation are highly penetrating, the method is virtually matrix-independent. NAA is a bulk technique. NAA can also be a selective technique, because appropriate choice of experimental parameters (such as irradiation and decay times), and use of neutrons of varying energies, can discriminate against unwanted elements. 

Bernardo, K. Pakulat, N. Macht, M. Krut, O. Seifert, H. Fleer, S. Hunger, F. Kronke, M. Identification and discrimination of Staphylococcus aureus strains using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Proteomics 2002, 2, 747-753. 

The most discriminating technique for proving the identity and purity of analyte peak of a chromatogram, especially for analyzing biological samples and natural products, is by using online LC-UV/MS or GC-MS/FTIR methods [15]. Alternatively, one could use a combination of TLC and MS, where direct determination on the TLC plates is made by matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) [16]. 

In addition to the analyte, the matrix will contain many other compounds. The method chosen must discriminate between the analyte of interest and other compounds also present in the sample. The test portion may have to pass through many analytical stages before the analyte is obtained in a form suitable for final measurement. First, the analyte may need to be separated from the bulk of the sample matrix. Further treatment may then be required to obtain an aliquot that is sufficiently clean (i.e. free from potential interferences) for the end-measurement technique. A general scheme of analysis is presented in Table 4.4 to illustrate the different approaches used depending on the nature of the analyte and of the matrix. 

PCBs in biological samples are usually extracted by a Soxhlet column and with a nonpolar solvent such as hexane. The sample is first mixed with sodium sulfate to remove moisture. The extraction of PCBs from sediments was tested with sonication, with two sonications interspersed at a 24-h quiescent interval, with steam distillation, or with Soxhlet extraction (Dunnivant and Elzerman 1988). Comparison of the recoveries of various PCB mixtures from dry and wet sediments by the four techniques and the extraction efficiency of four solvents showed that the best overall recoveries were obtained by Soxhlet extraction and the two sonication procedures. In comparisons of solvent systems of acetone, acetonitrile, acetone-hexane (1+1), and water-acetone-isooctane (5+1.5+1), recoveries of lower chlorinated congeners (dichloro- to tetrachloro-) were usually higher with acetonitrile and recoveries of higher chlorinated congeners (tetrachloro- to heptachloro-) extracted with acetone were superior (Dunnivant and Elzerman 1988). The completeness of extraction from a sample matrix does not seem to discriminate against specific isomers however, discrimination in the cleanup and fractionation process may occur and must be tested (Duinker et al. 1988b). 

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