Identification and Structure Elucidation

In the case of known alkaloids, a complete isolation is not necessary. In fact an alkaloid can be identified with great certainty by co-TLC, co-GLC, and/or co-HPLC using several different separation systems. Also a mass spectrum (e.g., obtained after GC-MS of a crude extract) may be sufficient to identify a known compound however, closely related alkaloids can be difficult to distinguish by a mass spectrum only. 

Known alkaloids can be detected in complex mixtures also by NMR. H-NMR has been used for this purpose (Schripsema and Verpoorte 1991). C-NMR is even better suited because of the large shift range, which enables the observation of all signals with a sufficient signal-to-noise ratio, also when impurities are present. By comparison with reported spectra, the presence of a known compound can be established with great certainty. 

However, the drawback of this technique is that relatively large amounts of compound are needed to obtain a good spectrum. For the analysis of alkaloids, this approach has not yet been used. For essential oils it has been extensively described by Formacek and Kubeczka (1982). 

For identification of known alkaloids in crude mixtures 2D-NMR seems very suitable, especially 2D-COSY, which combines a good sensitivity with a better resolution. 

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Liaaen-Jensen, S., Combined approach identification and structure elucidation of carotenoids, in Carotenoids Spectroscopy, IB, Britton, G., Liaaen-Jensen, S., and Pfander, H., Eds., Birkhauser, Basel, 1995, 343. 

Owing to rapid development in analytical techniques, metabolite identification and structure elucidation have become possible even with trace levels of metabolites generated with in vitro or in vivo mammalian systems. However, the microbial bioreactor is still a valuable system for metabolite structure determination, especially when the metabolite of interest presents at a low level in in vitro or in vivo mammalian systems and the isolation from these matrices is hindered by the interference of other metabolites, the parent drug or endogenous compounds, or the structure determination requires appreciable amounts of samples due to structure complexity. 

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). 

ESI-MS-MS permitted unambiguous identification and structure elucidation of compounds detected under NI conditions (halogenated NPECs and NPs), while for NPEOs, detected under positive ionisation conditions no fragmentation was obtained and these compounds were analysed using a single stage MS as [M + Na]+ in selected ion... 

Compared to previously reported instrument configuration, is possible also to couple two different detector systems after 2D-HPLC in order to improve the identification and structural elucidation of targeted compound, for example mass spectrometric and NMR detectors. 

Mass Spectrometric Instrumentation Used in Infusion or Direct Analytical Methods for Chemical Identification and Structure Elucidation. 152... 

This chapter has two aims. First, techniques that use mass spectrometry for the identification and structural elucidation of neuroactive chemicals are reviewed. Second, and with greater emphasis, this chapter attempts to describe the routine use and pitfalls encountered when using mass spectrometry— especially GC/MS and LC/MS—for rapid, specific, quantitative analyses of neuroactive compounds and metabolites in a variety of biological matrices. 

Typically, sample detection in electromigration techniques is performed by on-column detection, employing a small part of the capillary as the detection cell where a property of either the analyte, such as UV absorbance, or the solution, such as refractive index or conductivity, is monitored. This section briefly describes the major detection modalities employed in capillary electromigration techniques, which are accomplished using UV-visible absorbance, fluorescence spectroscopy, and electrochemical systems. The hyphenation of capillary electromigration techniques with spectroscopic techniques employed for identification and structural elucidation of the separated compounds is also described. 

In summary, model studies are very efficient for the identification and structure elucidation of important flavor components. Most of the compounds reported here have not been identified in meat and have not yet been reported as constituents of food volatiles. Nevertheless, there are good reasons to believe that minute traces of these sulfur-containing components are present in roasted and/or cooked meat volatiles because our model system was based solely on naturally occurring precursors. We believe that only minute trace amounts of these types of components need to be present in natural products to be of prime significance due to their extremely low odor threshold values. 

Nuclear magnetic resonance spectroscopy (NMR) is one of the most powerful analytical methods for identification and structure elucidation of organic compounds. Since NMR spectra are recorded in solution, no phase transfer like in MS is necessary when coupled with LC techniques. Additionally, NMR is a non-destructive detection technique, allowing the analyte to be transferred for characterization using additional methods. As of today, LC—NMR coupling was used in a wide range of applications [65,66,67,68,69,70,71],... 

Fig. 8.2 General scheme for the detection, identification, and structural elucidation of related organic impurities in drugs [19]...

Mass spectrometry (MS), employing most of the ionization techniques, has been used either independently, or in combination with HPLC, for the determination of the dissociation scheme of paclitaxel or for the identification of taxanes and paclitaxel metabolites. Tandem MS has enhanced identification and structure elucidation capabilities in the analysis of either biological or plant samples. 

Isolation and Identification and Structural Elucidation of Tertiary Alkaloids from the Root of ThaUctrum minus L. Race C, by MOA ElSheikh, 223 pp., The Ohio State University, 1985 [14]... 

Specific detection is an analytical determination based on specific responses related to the chemical characteristics of a molecule excited by a certain type of irradiation. In this detection method, measurement of the molecule of interest may usually be performed without separation from matrix materials or from other ingredients if appropriate instrumental adjustments are made. The need for identification and structure elucidation for newly discovered compounds drives the progress of specific detection techniques with NMR and X-ray diffraction and MS. The UV-diode array detector often aids the recognition of chromatographic peak purity, ensuring complete resolution of a separation procedure. 

Various approaches to metabolite identification and structure elucidation have been extensively described in the literature [78,90,101,107,109,125,127,146-151], Fully identifying unknown metabolites in a metabonomics study remains a great challenge even today. For many metabonomics studies the most frequent goal is to understand... 

Although the use of spectroscopic methods can be of great value in the identification and structural elucidation of these alkaloids, IR, UV, and NMR spectra are not discussed in this section as these data are similar to those of the benzylisoquinoline alkaloids (see Table I). Therefore only mass spectra are described here. 

Roboz J. (1974) Combined GC-HRMS for identification and structure elucidation of organic compounds in natural product research. 6th lnt. Colloq. Chem. Coffee (Bogota, 4-9.6.1973) (ASIC, 1974), 81-8. 

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