Solute identification techniques

Current trends in GC relate to miniaturisation, fast-GC, improved selectivity (mainly for short columns), stability of column stationary phases (reduction of bleeding) and increasing use of MS detection [117]. Finally, GC can be readily hyphenated with spectroscopic techniques without using involved interfaces and thus can easily provide unambiguous solute identification. 

Identification of unknown crystal structures and determination of phase fields by X-rays can be problematical if the characteristic patterns of the various phases are quite similar, for example in some b.c.c. A2-based ordered phases in noble-metal-based alloys. However, in many cases the characteristic patterns of the phases can be quite different and, even if the exact structure is not known, phase fields can still be well established. Exact determination of phase boundaries is possible using lattice-parameter determination and this is a well-established method for identifying solvus lines for terminal solid solutions. The technique simply requires that the lattice parameter of the phase is measured as a function of composition across the phase boimdary. The lattice parameter varies across the single-phase field but in the two-phase field becomes constant. Figure 4.12 shows such a phase-boundary determination for the HfC(i i) phase where results at various temperatures were used to define the phase boundary as a fimction of temperature (Rudy 1969). As can be seen, the position of is defined exactly and the method can be used to identify phase fields across the whole composition range. 

Due to the limited information given by most UV spectra and the inherent similarity between UV spectra of widely different types of compounds, this procedure is not very reliable for the identification of most solutes. The technique is useful, however, for determining the homogeneity of a peak by obtaining spectra from a sample on both sides of the peak. The technique is to normalize both spectra, then either subtract one from the other and show that the difference is close to zero, or take the ratio and show it is constant throughout the peak. 

In this review we have summarized the results obtained by different chromatographic techniques and a variety of sample preparation methods for the analysis of antioxidants in polymers and in solutions. Efficient techniques including liquid and gas chromatography, mass spectrometry, traditional low pressure extraction techniques and newer high pressure techniques have been developed. These have made possible the accurate quantification and identification of antioxidants. The newer techniques offer versatile tools for further developments in this area of polymer analysis. 

The mobile phase used for preparative separations must not react with the solutes, neither during chromatography, nor during removal of the solvent. The solvents used in the mobile phase must be very pure, redistilled or distilled in glass, in order not to leave any residue upon removal of the solvent. The mobile phase should preferably be totally volatile, or if residue is left it must not interfere with the proposed identification technique. 

The problem of damage detection is often solved by a system identification technique past research has aimed primarily at implementing efficient solutions for each relevant inverse problem, where the efficiency is usually related to the computational aspects. 

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