Multicomponent samples, analysis

A thorough treatment of the multicomponent quantitative analysis of samples based on Beer s law, and the analysis of samples for which the pathlength is indeterminate is found in the following review article. 

Haaland, D.M., et.al. "Application of New Least-squares Methods for the Quantitative Infrared Analysis of Multicomponent Samples", Appl. Spec. 1982 (36) 665-673. 

Certain disadvantages of this method of analysis should be enumerated. The preparation of standards becomes a major task if a large variation in concentrations of multicomponent samples is expected. The cost of preparing standards for expensive elements is a major consideration however, recovery and purification are possible. 

The most common method of isolation and sample cleanup involves contacting a filtered aqueous solution with an appropriate immiscible organic solvent in a. aboratory separatory funnel of appropriate size. Some specific examples are discussed later. With multicomponent samples a single solvent or solvent mixture is unlikely to extract all components equally causing discrimination. Ihis discrimination may be useful if the solvent discriminates against the extraction of solutes that are not of interest in the analysis. 

The atomic absorption characteristics of technetium have been investigated with a technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 /ig/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. Only calcium, strontium, and barium cause severe technetium absorption suppression. Cationic interferences are eliminated by adding aluminum to the test solutions. The atomic absorption spectroscopy can be applied to the determination of technetium in uranium and its alloys and also successfully to the analysis of multicomponent samples. 

Has the potential to replace chromatography as a method for more rapid analysis of multicomponent samples. 

Haaland, D.M., et. al. "Multivariate Least-Squares Methods Applied to the Quantitative Spectral Analysis of Multicomponent Samples",/ / / /. Spec. 1985 (39) 73-84. 

Sojo, L.E. and de Haan, H. (1991) Multicomponent kinetic analysis of iron speciation in humic lake Tjeukemeer comparison of fulvic acid from the drainage basin and lake water samples. Environ. Sci. Technol., 25, 935-939. 

Molecular fluorescence spectrometry has long been regarded as a useful technique for the determination of polycyclic aromatic hydrocarbons (PAHs) and related materials, due to the very high sensitivities which can be achieved. However, molecular fluorescence spectra measured in liquid solution usually are broad and relatively featureless hence, spectral interferences are common in the liquid-solution fluorometric analysis of multicomponent samples. Moreover, the fluorescence of a particular component of a complex sample may be partially quenched by other sample constituents if quenching occurs to a significant extent, the fluorescence signal observed for a particular compound present at a particular concentration will also depend upon the identities and concentrations of other substances present in the sample. Under these conditions, it is virtually impossible to obtain accurate quantitative results. Therefore, it is generally observed that molecular fluorescence spectrometry in liquid solution media is useful for quantitative determination of individual components in complex samples only if the fluorescence measurement is preceded by extensive separation steps (ideally to produce individual pure compounds or, at worst, simple two- or three-component mixtures). 

MIP sensor elements are also suitable for the analysis of multicomponent samples. The cost-effective, miniaturised, non-covalent MIP sensor arrays, when combined with computational data evaluation, make weak artificial recognition phenomena highly applicable for smart sensors. In comparison to gas or liquid chromatography, the results with mass-sensitive MIP sensors are faster and cheaper to obtain [32]. For effective on-line monitoring, the ideal MIP sensor or actuator should allow reversible analyte enrichment without dependencies on intermediate washing procedures (with organic solvents, for example). 

The method-development laboratory is olfen set with a task of identifying unknown constituents in a complex sample. The identity or quantity of analytes is not well defined at this stage. Complicated sample-preparation steps and multiple HPLC analyses may be required to ascertain the nature of a sample. The diode array detector simplifies this process. In the case that more than 15 different peaks have been separated in a analysis of a multicomponent sample of known origin more than a full day s work might be required just to establish the retention times of the unknowns with the use of a standard absorbance detector. A number of long-run HPLC analyses would have to be made with pure standards so that retention time correlations could be determined. This necessity is accomplished by the DAD s advantage of acquiring data in both the time and spectral domain. 

Synchronous scanning techniques have also been applied to the quantitative analysis of fluorescent substances. Synchronous scanning involves scanning both the excitation and emission monochromators simultaneously, while maintaining a constant wavelength interval between them. The technique has been employed in the analysis of multicomponent preparations. The technique is reported to simplify the spectra of multicomponent samples and reduce the bandwidths of fluorescence spectra. The equation relating the measured fluorescence to concentration is given by... 

For complex mixtures where components to be separated include a wide range of polarities, the k values are too dissimilar, and the peaks tend to crowd together near the start, while later peaks are broadened too greatly to be measured with accuracy (Figure 7.5A). This common difficulty encountered in the analysis of multicomponent samples is referred to as the general elution problem. 

The main advantages of gradient over isocratic systems are provided in the analysis of unknown or multicomponent samples where a range of solvent strengths may be required both to ensure that aU the components loaded are eluted and also that the retention times of later eluting peaks are minimised. Gradient systems may be subdivided into two classes those where solvents are mixed prior to the pump (low pressure systems) and those where solvents are mixed after the pump (high pressure systems). 

Quantification requires knowledge of the beam path through the sample. Hence, the sample often needs to be modified to allow for a known geometry. Spectral subtraction, least square regression analysis, PLS and spectral deconvolution are some of the spectroscopic techniques widely used to quantify constituents in a multicomponent sample. For almost any type of spectroscopic analysis, this is usually the first step. It is especially important for polymers given the variations in spectra for the same polymer due to molecular weight, conformation, crystallinity, sample preparation, age and sampling method. 

It is apparent from the scenario described above that conventional fluorescence methods are of limited applicability for the analysis of multicomponent mixtures. Therefore, for routine laboratory analysis of a multicomponent mixture, the most important prerequisite is to develop a simple and easy-to-use qualitative and quantitative fluorescence-based method that is not limited to single-component samples. Additionally, the fluorescence technique adopted should not sacrifice the sensitivity and selectivity of conventional fluorescence measurements and should minimize the problem of overlap in emission spectra of complex multicomponent samples. 

The new computerized spectrofluorimeters in combination with the availability of selective multichannel detectors allow for a reliable and quick collection and analysis of spectral information that was difficult to accomplish using a single-wavelength scan. The new multichannel detectors used for making multiwavelength measurements also allow the ability to study a wide variety of multicomponent samples. 

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