Volatile phenols analysis

Partial least squares regression analysis (PLS) has been used to predict intensity of sweet odour in volatile phenols. This is a relatively new multivariate technique, which has been of particular use in the study of quantitative structure-activity relationships. In recent pharmacological and toxicological studies, PLS has been used to predict activity of molecular structures from a set of physico-chemical molecular descriptors. These techniques will aid understanding of natural flavours and the development of synthetic ones. 

To test the potential of PLS to predict odour quality, it was used in a QSAR study of volatile phenols. A group of trained sensory panelists used descriptive analysis (28) to provide odour profiles for 17 phenols. The vocabulary consisted of 44 descriptive terms, and a scale fiom 0 (absent) to S (very strong) was used. The panel average sensory scores for the term sweet were extracted and used as the Y-block of data, to be predicted from physico-chemical data. 

Mejias, R.C., Matin, R.N., Moreno, M.V.G., Barroso, C.G. (2003). Optimisation of headspace solid-phase microextraction for the analysis of volatile phenols in wine. J. Chromatogr. A, 995, 11-20. 

Identification and quantification of low molecular weight and volatile phenols is usually performed by Gas Chromatography and Mass Spectrometry (GC-MS). For analysis and structural characterization of more polar compounds such as polyphenols, liquid-phase and Liquid Chromatography Mass Spectrometry (LC-MS) and Multiple Mass Spectrometry (MS/MS and MSn) techniques are used (Niessen and Tinke, 1995 de Hoffmann, 1996 Abian, 1999 Flamini et al., 2007). 

Figure 4.24 Analysis of the four volatile phenols (VPs) in a wine sample by HPLC-Coulometric array detector using 8 electrodes...

Preconcentration of analytes in aqueous solution may be performed by a miscible organic phase followed by salting out. Thus, microextraction of anionic solntes snch as phenol, cresols and xylenols in industrial effluents can be carried ont with a small amonnt of isopropyl alcohol, followed by demixing of the phases with ammoninm snlfate. End analysis of the extract by GC-MS in the selected ion monitoring (SIM) mode allowed a LOD of 1 ppb for 50 mL samples . The best conditions for eliminating petrolenm prodncts from the concentrate were found for the GC determination of volatile phenols in natnral waters. Losses of volatile phenols due to preconcentration were insignificant and cansed no increase in the relative error of determination by the internal-standard method. The concentration of phenol in the atmosphere can be determined by sorption on Chromosorb 102, desorption with benzene and 0.1 M NaOH and GC nsing a capillary colnmn. LOD was abont 1 p,gm, with accuracy within 15% . 

A preconcentration method that bears some resemblance to SDE consists of isolating the volatile phenols by steam distillation, followed by freeze-drying of the distillate. End analysis was by HPLC with ELD. The method was applied for determination of such phenolic components in foodstuffs and packing materials. Determination of phenolic antioxidants in polyolefins was carried out by dissolving the polymer sample in a hep-tane-isopropanol mixture (1000/5, v/v), at 160-170 °C, in an autoclave. The polymer precipitated on cooling the solution, and the dissolved antioxidant could be determined by LC with UVD. The advantage of the method is the relatively short time of analysis (about 2 h) and its reproducibility (RSD 3-5%) . 

TABLE 5.14. Multiple Headspace (MHS) SPME (n = 3) and GC/MS/MS Conditions for Analysis of Volatile Phenols in Wine2... 

TABLE 5.15. Multiple Headspace SPME-GC/MS/MS Method (n = 3) for Analysis of Volatile Phenols in Wines MS/MS Parameters and Performances"... 

Diez, J. Dominguez, C. Guillen, D.A. Veas, R. Barroso, C.G. (2004). Optimisation of stir bar sorptive extraction for the analysis of volatile phenols in wines. Journal of Chromatography A1025,263-267... 

Due to the labor-intensive nature of monitoring programs, alternatives have been sought. Enzyme-linked immunosorbent assay (ELISA) has been proposed (Kuniyuki et al., 1984). The advantage of ELISA is that viable yeasts need not be present. Unfortunately, the method is too sensitive for routine production applications and, at present, too costly. The volatile phenol, 4-ethylphenol, has been proposed as a marker for present/past growth of Brettanomyces and Dekkera in wine. For details regarding analysis, see Zoecklein et al. (1995). 

Gas Chromatographic Technique for the Analysis of Volatile Phenols in Serum... 

These compounds are of different types as summarized in Tables 37 and 38. Their characteristics are approximate but useful in understanding spent caustic treatment. Analysis must involve both entrainable and total phenols. In contrast with process condensates, many heavy nonvolatile phenols can predominate in spent caustic (Table 37). On the other hand phenol removal efficiency of acidification alone must be evaluated based on the measurement of total phenols. In this example, entrainable phenols represent 38 to 50% of the total phenols. Whereas volatile phenols represent 70 to 85% of total phenols in coking plants depending on coal type, in refineries the rate depends on intermediate cuts and their treatment before caustic scrubbing. Additionally, the ratio is not as well known as for coking plants. 

Gas chromatography (gc) has been used extensively to analyze phenoHc resins for unreacted phenol monomer as weU as certain two- and three-ring constituents in both novolak and resole resins (61). It is also used in monitoring the production processes of the monomers, eg, when phenol is alkylated with isobutylene to produce butylphenol. Usually, the phenoHc hydroxyl must be derivatized before analysis to provide a more volatile compound. The gc analysis of complex systems, such as resoles, provides distinct resolution of over 20 one- and two-ring compounds having various degrees of methylolation. In some cases, hemiformals may be detected if they have been properly capped (53). 

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. - ... 

Plasticiser/oil in rubber is usually determined by solvent extraction (ISO 1407) and FTIR identification [57] TGA can usually provide good quantifications of plasticiser contents. Antidegradants in rubber compounds may be determined by HS-GC-MS for volatile species (e.g. BHT, IPPD), but usually solvent extraction is required, followed by GC-MS, HPLC, UV or DP-MS analysis. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out. The determination of antioxidants in rubbers by means of HPLC and TLC has been reviewed [58], The TLC technique for antidegradants in rubbers is described in ASTM D 3156 and ISO 4645.2 (1984). Direct probe EIMS was also used to analyse antioxidants (hindered phenols and aromatic amines) in rubber extracts [59]. ISO 11089 (1997) deals with the determination of /V-phenyl-/9-naphthylamine and poly-2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) as well as other generic types of antiozonants such as IV-alkyl-AL-phenyl-p-phenylenediamines (e.g. IPPD and 6PPD) and A-aryl-AL-aryl-p-phenylenediamines (e.g. DPPD), by means of HPLC. 

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. 

Irradiation of at longer wavelengths (>280 nm) provided phenyl formate (6) as a major volatile product, together with minor amounts of phenol and phenoxyacetone (4), as well as other products. A possible pathway for formation of phenyl formate by oxidation and subsequent cleavage is provided in Scheme III. Phenoxyacetic acid (7) was also identified as a minor product by mass-gc analysis. Photolysis of phenoxyacetone ( ) and phenoxyacetic acid (7)12 yields phenol together with photo-Fries products (also shown in Scheme III). 

Until recently, most of the chemical research on the contents of these structures was directed at the identification of the constituents of castoreum. In the late 1940s Lederer [72, 73] identified 36 compounds and some other incompletely characterized constituents in castoreum of uncertain origin. Other constituents were subsequently identified in the material [74-77]. In a reinvestigation aimed specifically at the phenol content of the material, Tang et al [69] identified 10 previously unreported phenols in the castoreum from the North American beaver, Castor canadensis. Of the 15 phenols reported elsewhere, only five were confirmed in this analysis, in addition to 10 phenolic compounds that were not reported elsewhere. It was concluded that the 10 previously identified phenols that were not found in the study by Tang et al. were either absent or were not volatile enough to be detected by the methods employed. This was most probably because a relatively low maximum column temperature of only 210 °C was employed in the GC-MS analyses. The compounds identified by Lederer,... 

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