Multicomponent samples, identification

Identification of Multicomponent Samples without Previous Separation... 

Techniques in clinical analysis have undergone many advances in the last few decades. The basic needs in clinical chemistry are unambiguous analyte-specific assays that provide both identification of sample components and their concentration levels. The importance of this is self-evident, since most substances analyzed are part of a multicomponent biological fluid. Advances in enzyme and immunochemical assay techniques provide ideal systems for component... 

The isolation and concentration of petroleum products can be performed in several ways. The most efficient method is passive adsorption. In this method, the sample along with a tube filled with Tenax TA adsorbent is placed in a thermostated (60-70 °C) tightly closed container, such as a glass jar, for over 10 h. Under these conditions, a balance between compounds present in the headspace of the sample and the sample adsorbed on the polymer adsorbent is established. Adsorbed compounds are subjected to thermodesorbtion then, the desorbed compounds together with the carrier gas are injected onto a GC column, where they are separated and then identified. This approach has enabled easy detection and identification of trace amounts of petroleum products. Headspace analysis with passive adsorption on Tenax TA is normally used for separation and concentration of analytes. Gas chromatography coupled with an autothermal desorber and a mass spectrometer (ATD-GC-MS) is the best technique for separation of multicomponent mixtures... 

As shown by Helfferich and Peterson [110], if a small pulse of one of the mixture components, isotopically labeled, is injected in a multicomponent solution, the labeled component moves at a velocity that is proportional to the slope of the corresponding chord of the isotherm. In the same time, as many system peaks as there are components in the mixtiure arise and move at velocities related to the slopes of the corresponding isotherms (see Figure 4.27a,c,e). This assumes that equilibrium has previously been reached throughout the column. In the tracer pulse technique, only the labeled component is detected. Successive injections of a sample of each labeled component (or the simultaneous injection of all of them if their separate identification is possible) permit the direct determination of the competitive isotherms of all the components. From a theoretical point of view, the presence of large concentrations of the other components does not complicate the measurement nor its evaluation, since the retention time of each isotopic pulse is a linear fimction of the slope of the corresponding chord Aq / AC, see Eq. 2.15). 

Multidimensional fluorescence analysis is well suited for routine qualitative separation and identification of complex mixtures of PAHs in environmental samples. The separation and extraction of the individual component spectra of PAH mixtures affords valuable information about these hazardous compounds. Such information may allow for a detailed study of the means to remove their influence from the environment. The differences in the intensity contributions from a mixture of PAHs in a multicomponent system could be related to the differences in concentrations of the individual components and their respective quantum efficiencies. The problem is exacerbated by the fact that most solutions containing mixtures of PAHs display broad fluorescence spectral bands at room temperature, resulting in significant overlap and limited specificity for multicomponent analysis. This makes the success of multidimensional fluorescence measurements for complete spectral resolution of a complex multicomponent mixture dependent upon the degree of spectral overlap among the individual components and their fluorescence... 

Traditionally, IR spectroscopy has been one of the most popular physical methods in the polymer-characterization laboratory since it is useful in the elucidation of structures and the identification of organic and inorganic systems ahke. The quantitative analysis of samples down to picogram quantifies is straightforward for systems for which the spectra of the pure compounds are available. Yet, the most attractive advantage of the method is the potential for a rapid multicomponent analysis to be carried out from a single measurement (spectrum), once the methodology has been calibrated. 

Expected new developments are new sample stream interfaces (e.g., ATR techniques), the improvement of IR optical fibres [84] and data handling (chemometrics). A full spectrum approach provides the possibilities of multiple analysis from one measurement and correlations of the IR spectmm with other physical properties associated with composition. Discriminant analysis using principle components of mid-IR spectral data is a powerful quality identification tool where rigorous multicomponent analysis is not only costly but in many cases unwarranted. In combination with discriminant analysis, mid-IR spectroscopy becomes more readily available for QC validation by non-spectroscopists allowing validation without quantitation [85]. 

Information about the monomeric composition and structure can be obtained with pyrolysis MS but sequence information is lost [46]. The method was used in several applications, such as structural identification of homopolymers, differentiation of isomeric structures, copolymer composition and sequential analysis, identification of oligomers formed in the polymerization reactions, and identification of volatile additives contained in polymer samples [47]. One of the main challenges of the technique is the identification of the products in the spectrum of the multicomponent mixture produced by thermal degradation. 

Ions produced by the ICP are measured by the use of a mass spectrometer. The mass spectrometer is essentially a mass filter designed to isolate a specific mass-to-charge ratio (m/z) ion from the multi-ion beam. After separation, the individual ion beams, which are characteristic of specific charged isotopic or molecular species, are sequentially or simultaneously (multicollector spectrometer) directed to a detector devised to measure their individual ion currents. The magnitude of these ion currents is proportional to the population of the analyte ion species in the multicomponent ion beam sampled from the ICP. Therefore, the measurement of the m/z of the ion allows qualitative identification of the isotope or molecule being measured, and the magnitude of the ion current is used to provide quantitation of the amount of the analyte in the original sample. 

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