Polar compounds, analysis

An approach that has been developed in response to the desire to extend the utility of SCFs to polar compound analysis is coupling of chemical derivatisation reactions to SFE. Table 3.20 lists situations when derivatisation may be useful [353]. 

Carraud, P., Thiebaut, D., Caude, M. et al (1987) Supercritical fluid chromatography/light-scattering detector a promising coupling for polar compounds analysis with packed columns. J. Chromatogr. ScL, 25, 395-8. 

W. Golkiewicz, C. E. Werkhoven-Goewie, U. A. Th Brinkman, R. W. Erei, H. Colin and G. Guiochon, Use of pyrocarbon sorbents for rtace enrichment of polar compounds from aqueous samples with on-line HPEC analysis , /. Chromatogr. Sci. 21 27-33 (1981). 

Various highly crosslinked polymers, with slightly different properties, such as Envi-Chrom P, Lichrolut EN, Isolute ENV or HYSphere-1, have been applied in environmental analysis, mainly for polar compounds. For phenol, for instance, which is a polar compound, the recoveries (%) when 100 ml of sample was analysed were 5, 16 and 6 for PLRP-s, Envi-Chrom P and Lichrolut EN, respectively (70). 

ESI-MS has emerged as a powerful technique for the characterization of biomolecules, and is the most versatile ionization technique in existence today. This highly sensitive and soft ionization technique allows mass spectrometric analysis of thermolabile, non-volatile, and polar compounds and produces intact ions from large and complex species in solution. In addition, it has the ability to introduce liquid samples to a mass detector with minimum manipulation. Volatile acids (such as formic acid and acetic acid) are often added to the mobile phase as well to protonate anthocyanins. A chromatogram with only the base peak for every mass spectrum provides more readily interpretable data because of fewer interference peaks. Cleaner mass spectra are achieved if anthocyanins are isolated from other phenolics by the use of C18 solid phase purification. - ... 

Induction forces, the so-called Debye forces ind> occur in the interaction between a permanent dipole of a solute or a polar solvent and an induced dipole in another compound. They are weak and appear during the analysis of the nonpolar polarized compounds, such as those with multiple... 

HPLC solvents (PDMS-coated fibres are incompatible with hexane). PDMS fibres are more selective towards nonpolar compounds and polyacrylate fibres towards polar compounds such as acids, alcohols, phenols and aldehydes. Another feature of SPME fibre selectivity is discrimination towards high-MW volatiles. SPME has successfully been applied to the analysis of both polar and nonpolar analytes from solid, liquid or gas phases. Li and Weber [533] have addressed the issue of selectivity in SPME. 

Freitag and John [96] studied rapid separation of stabilisers from plastics. Fairly quantitative extraction (>90% of the expected content) of stabilisers from a powdered polymer was achieved by MAE within 3 to 6 min, as compared to 16 h of Soxhlet extraction for the same recovery. MAE and Soxhlet extraction have also been compared in the analysis of cyclic trimer in PET [113]. On the other hand, Ganzler et al. [128] compared the extraction yields for various types of compounds from nonpolymeric matrices for microwave irradiation with those obtained by the traditional Soxhlet or shake-flask extraction methods. Microwave extraction was more effective than the conventional methods, in particular in the case of polar compounds. As expected, the efficiency of the former is high especially when the extraction solvents contain water. With the high dipole moment of water, microwave heating is more... 

Applications Desorption chemical ionisation has proven potential in the analysis of thermally labile, nonvolatile and polar compounds [40,67,68], for the identification of unknown polymers and the study of the thermal degradation mechanisms of polymers. Considering the overall ease of DCI operation, the capability of analysing nonvolatile compounds, and the selectivity provided by choosing different reagent gases, DCI has found surprisingly few practitioners in the analysis of polymer additives. 

Applications Off-line SFE-HPLC appears to be applicable and quantitative for a variety of samples in many real -world matrices. The main challenge lies in the use of this technique for the more polar compounds. Quantitative off-line SFE-SFC-UV analysis of HDPE/Ethanox 330 was described after extensive method development (varying modifiers, modifier concentration, temperature) [129]. Soxhlet extraction and SFE-RPLC-UV of PE samples were compared [127]. A sample size (inhomogeneity) problem was pointed out when a SFE reproducibility study was performed on five 3-mg samples of PE. This points to limits... 

On-line SFE-pSFC-FTIR was used to identify extractable components (additives and monomers) from a variety of nylons [392]. SFE-SFC-FID with 100% C02 and methanol-modified scC02 were used to quantitate the amount of residual caprolactam in a PA6/PA6.6 copolymer. Similarly, the more permeable PS showed various additives (Irganox 1076, phosphite AO, stearic acid - ex Zn-stearate - and mineral oil as a melt flow controller) and low-MW linear and cyclic oligomers in relatively mild SCF extraction conditions [392]. Also, antioxidants in PE have been analysed by means of coupling of SFE-SFC with IR detection [121]. Yang [393] has described SFE-SFC-FTIR for the analysis of polar compounds deposited on polymeric matrices, whereas Ikushima et al. [394] monitored the extraction of higher fatty acid esters. Despite the expectations, SFE-SFC-FTIR hyphenation in on-line additive analysis of polymers has not found widespread industrial use. While applications of SFC-FTIR and SFC-MS to the analysis of additives in polymeric matrices are not abundant, these techniques find wide application in the analysis of food and natural product components [395]. 

LC-MS is now a nature technology and operation of an LC-MS system is no longer the realm of an MS specialist. The proper choice of the LC-MS mode to be used in a specific situation depends on analyte class, sample type and problem (detection, confirmation, identification). On-line LC-MS is used more for specialised applications than for general polymer or rubber compound analysis. This derives from the fact that LC-MS method development (column, solvent system, solvent programme, ionisation mode) is rather time consuming. LC-MS (in particular with API interface) enables analysis of a wide range of polar and nonvolatile compounds which cannot be analysed by GC (icf. Scheme 7.7). 

Coupled LC-LC can separate high-boiling petroleum residues into groups of saturates, olefins, aromatics and polar compounds. However, the lack of a suitable mass-sensitive, universal detector in LC makes quantitation difficult SFC-SFC is more suitable for this purpose. Applications of multidimensional HPLC in food analysis are dominated by off-line techniques. MDHPLC has been exploited in trace component analysis (e.g. vitamin assays), in which an adequate separation for quantitation cannot be achieved on a single column [972]. LC-LC-GC-FID was used for the selective isolation of some key components among the irradiation-induced olefinic degradation products in food, e.g. dienes and trienes [946],... 

Compositional analysis shows a decrease in the percentage of polar compounds in the oils with increasing residence time (see Table II). The decrease in polar content is substantiated by a lower sulphur content and results in a lower viscosity (see Table II). The oil becomes more aromatic, as shown by n.m.r. spectroscopy (see Table II), with increasing time at temperature, while the molecular weights showed little change. G.l.c. analysis of the saturate hydrocarbon fractions from elution chromatography indicated little change in the saturates with residence time. 

Currently, LC-MS is widely used for the analysis of polar compounds, such as medicinal metabolites and bioactive peptides, since the interface has been improved and several new ionization methods have been developed. The sensitivity and reproducibility are sufficient for a daily quantitative analysis. The usefulness of the LC-MS has been demonstrated for studies on Type II pheromones using a time-of-flight MS with electrospray ionization (ESI) [180]. Each epoxydiene derived from the (Z3,Z6,Z9)-triene shows three ion series of [M+NHJ+, [M+H]+, and [M-OH]+ with high resolution and good sensitivity, indicating its molecular formula. In addition to these, characteristic fragment... 

A GC analysis of amino acids requires a derivatisation step to increase the volatility of the amino acids. Generally, norleucine and/or norvaline are the internal standards added to the hydrolysate to check the derivatisation yield. According to the experimental method applied, the limits of detection (LOD) vary in the range 10 100 pg for each amino acid. Regarding the chromatographic columns, as most of the derivatives are esters barely polar compounds the most commonly used are fused-silica capillary columns with a low... 

In the last decade modifications to the pyrolysis process have been developed to improve analytical efficiency and increase detectability. In the same way as in conventional GC, derivatization reagents may be used to improve the chromatographic separation and response of polar compounds. In order to reduce the time required for the analysis, the risk of contamination and of losing part of the sample, on-line derivatization methods should be preferred and those based on quaternary ammonium hydroxides are certainly the most widely used. 

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