Phenolic compounds structural identification

Various analyzers have been used to analyze phenolic compounds. The choice of the MS analyzer is influenced by the main objective of the study. The triple quadrupole (QqQ) has been used to quantify, applying multiple reaction monitoring experiments, whereas the ion trap has been used for both identification and structure elucidation of phenolic compounds. Moreover, time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) are mainly recommended for studies focused on obtaining accurate mass measurements with errors below 5 ppm and sub-ppm errors, respectively (Werner and others 2008). Nowadays, hybrid equipment also exists, including different ionization sources with different analyzers, for instance electrospray or atmospheric pressure chemical ionization with triple quadrupole and time-of-flight (Waridel and others 2001). 

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,... 

Phenolic compounds in Sicilian wines were directly detected by La Torre et al. [373] using an HPLC with a DAD coupled on-line with a MS system equipped with ESI source operating in the negative-ion mode and a quadruple mass analyzer. The structure was elucidated by recording MS spectra at different voltages, in addition to the molecular mass information. The method allowed both the identification and determination of 24 phenolic compounds in 22 different commercial Sicilian red wines by direct injection without any prior purification of the sample. Figure 19.10 reproduced an HPLC trace obtained in this work. 

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. ... 

Since 1951, the development of modern methods of separation and identification has made it possible to reexamine some structures that were looked upon as established. Here are a few significant examples. Bisabolangelone (34),91 a sesquiterpenoid hydrobenzofuran alcohol, was initially described as having a furocoumarin structure.92 A phenolic compound, once thought to have the structure of an auronol (35),93,94... 

Numerous papers have relied on only UV-visible spectra for their identification of phenolics, but for positive identification purposes, HPLC-mass spectrometry (MS) is another detection mode that can provide detection of all phenolic compounds in foods. This technique involves a hyphenated instrument that uses a mass spectrometer as a detector for HPLC or uses HPLC as cleanup step for mass spectrometry. After preparative HPLC, the MS technique has frequently been employed for structural identification of phenolics in many foods and essential oils because of its sensitivity and selectivity and its ability to provide structural information. 

The focus of this book is centered on structure, nomenclature and occurrence of phenolic compounds (Chapter 1), and their chemical properties (Chapter 2). Chapter 3 describes the biosynthetic pathways leading to the major classes of phenolics. This chapter presents an up-to-date overview of the genetic approaches that have been used to elucidate these pathways. Chapter 4 presents an overview of methods for the isolation and identification of plant phenolic compounds. Given that much of the recent... 

Fly ash as obtained from the incineration of municipal waste was subjected to clean up, and extractions designed to isolate the phenolic compounds from more acidic and from neutral or basic components. More than sixty phenolic compounds belonging to various structural systems (55-61) were identified in the concentrated extracts by GC-MS, most of them containing chloro and bromo substiments, at three levels of confidence based on the MS of each peak in the chromatogram Positive identification, presumed and tentatively presumed. Compounds of strucmres 59-61 belong to the latter two classes. ... 

When using MS/MS and more selective multiple reaction monitoring detection, it is recommended that the formation of sodimn adducts is suppresses, as these fragment poorly. ESI-MS/MS permits unambiguous identification and structure elucidation vmder negative ionization conditions of acidic alkyl-phenolic compounds (i.e., APECs) and fully de-ethoxylated alkylphenols. An example of MS/MS spectra of NP and NPjEC is shown in Figure 3. 

MS has a very important role for research and its analytical power is relevant for structural elucidation of phenolic compounds. The MS principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measure their mass-to-charge ratios. MS detector is critical for the identification of phenolic compounds, but selectivity and sensitivity can be increased using tandem MS (MS/MS) or MS". MS/MS and MS" produce more fragmentations of the precursor and product ions and, therefore, they provide additional structural information for the identification of these compounds. 

Attractive Compounds. Though the first report on the identification of a pheromone from a scarabaeid beetle dates back more than 30 years - phenol as an attractant for males of the gras grub beetle Costelytra zealandica [135] which turned out to be produced by beetle associated bacteria [136] - most of the pheromone structures known today have been elucidated during the last decade [3,137,138]. 

This limitation is well Illustrated by the failure of those methods to uncover the presence of another important group, the flavanoid, coumarin and cinnamic acid phenols (85 - 117). On the other hand, some 32 of these compounds were identified when isolation procedures specific for these types of compounds were employed. Using isolation and identification techniques which are particularly useful for alkaloids, it would be possible to determine whether any representatives of this class are present and, if so, to conduct subsequent studies for structure determination. 

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