Retention of phenolic compounds

T. Hanai, C. Mizutani and H. Homma, Computational chemical simulation of chromatographic retention of phenolic compounds, /. Liq. Chromatogr. Relat Technol, 2003, 26, 2031-2039. 

The antioxidant capacity retained in carrots washed with ASC could be attributed to retention of phenol and flavonoid compounds as well as carotene content (Ruiz-Cruz and others 2007). These results suggest that the use of sanitizers such as ASC is helpful in preserving the antioxidant capacity of carrots. Similar ORAC values were observed between different sanitizers and both water conditions, without significant changes. This is a clear example of a postharvest treatment maintaining the nutritional value of produce. 

It may be concluded that PEF, HHP, and IR are adequate techniques for the retention of bioactive compounds in fruit and vegetable products and may even enhance bioactivity of juices, purees, and fresh-cut produce. A greater degradation of ascorbic acid in comparison with phenolics and carotenoids is usually observed. 

The retention times, limit of detection and repeatabilities of the analytes are compiled in Table 2.59. It has been concluded from the results that this easy-to-carry-out method can be used even in routine laboratories for the quantitative analysis (UV detection) and for the identification (ESI-MS) of this class of phenolic compounds in various complicated matrices [164]. 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

While identification of the peaks in a LC-UV chromatogram is possible by comparing retention times and UV spectra with authentic samples or a databank, this might not be possible for compounds with closely related structures, and wrong conclusions might be drawn. It has been established that in order to complete the characterization of phenolic compounds, reagents inducing a shift of the UV absorption maxima can be used. ... 

A dual-electrode liquid chromatography-electrochemistry (LCEC) system used in the detection and identification of flavanols and procyanidins in wines and grape seeds is a valuable tool (30). Voltammetric behavior of phenolic compounds by LCEC could provide information that cannot be obtained using HPLC with UV detection, for which the identification is usually based on a comparison of the retention time with that of standard compounds, especially for the identification of catechins and procyanidins with a small amount of sample available (30). Figure 10 shows the procyanidins commonly found in wines. 

The separation selectivity of a mixture of acidic, basic, and neutral compounds can be altered with the addition of chaotropic mobile-phase additives (Figure 4-62). The retention of the basic compounds can be increased by addition of chaotropic counterions in the mobile phase, while the retention of neutral and acidic compounds is generally unaffected. This is particularly useful during the development of impurity profile methods in the pharmaceutical industry where the retention of a polar protonated basic impurity may be adjusted such that adequate separation selectivity is obtained when union-izable, acidic, or basic (in neutral form) impurities in the drug substance are present. In Figure 4-62 the retention of protonated basic compounds, metoprolol and labetalol, increase while the retention of phenol (in its neutral state) remains constant. 

In order to reduce the LOD of the most polar EPA priority phenolic compounds to the required O.lUg/L level, PocuruU et al. [224, 225] suggested, first, to use tetrabutylammonium bromide as an ion-pair reagent enhancing the retention of phenols on PLRP-S and Envi-Chrom P extracting resins. Second, they coupled in series an ultraviolet (LJV) and electrochemical detector to significandy enhance the detection sensitivity toward phenolic water contaminants, including those in tap and river water. 

The retention time of phenolic compounds in reversed-phase liquid chromatography was predicted via molecular interaction energy values calculated using the MM2 program. The precision of the capacity ratios predicted by this new method was equivalent to a former method in which the retention time was predicted by log P calculated using the MOPAC program. Furthermore, the prediction of capacity ratios of phenolic compounds in reversed-phase... 

T. Hanai, Quantitative structure-retention relationships of phenolic compounds without Hammett s equations,/ Chromatogr., A, 2003, 985, 343-349. 

When a UV detector works at a fixed wavelength, the obtained information is still very limited. In this case, to identify a compound using different experimental conditions, cochromatography is necessary with authentic reference compounds. DADs are much more useful, because they yield the full record of the UV-Vis spectrum of each compound. Because each class of phenolic compounds has a characteristic spectrum, identification is facilitated, although a safe characterization still requires a standard to compare retention time and UV spectra (see Fig. 2 for an example). 

It is well known that in reversed phase the retention time of phenolic compounds is higher for substances that are less polar (myricetin, quercetin, kaempferol), while polar molecules as gallic acid, protocatechuic acid, epigallocatechin are eluted faster [43], Alternative high-strength silica (HSS) T3 columns have also been investigated. 

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