Unknown compounds structural elucidation

NMR spectroscopy is probably the singly most powerful technique for the confirmation of structural identity and for stmcture elucidation of unknown compounds. Additionally, the relatively low measurement times and the facility for automation contribute to its usefulness and industrial interest. 

Hyphenation of chromatographic separation techniques (SFC, HPLC, SEC) with NMR spectroscopy as a universal detector is one of the most powerful and time-saving new methods for separation and structural elucidation of unknown compounds and molecular compositions of mixtures [171]. Most of the routinely used NMR flow-cells have detection volumes between 40... 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

HPLC-NMR analysis in a closed-circuit reveals the stereochemical information for elucidating the structures of unknown compounds (Albert 2002). In contrast to the technique of off-line separation, sample collection, and peak identification closed-circuit analysis guarantees the absence of isomerization and degradation. Very often only small amounts of sample are available after extraction. 

There are many software tools available to help with the acquisition, processing and interpretation of NMR data. Attempts have been made to automate the verification process and even perform full structural elucidations of unknown compounds. As you might guess from the complexity of the interpretation chapters, these software solutions are not foolproof It remains to be seen whether they ever will be good enough but there have certainly been some major steps forward in all of these areas. 

Of particular importance with the use of LC-NMR as an experimental technique is that it is suited for only a limited number of applications in reference to structure elucidation. As will be discussed in greater detail, the sensitivity issues that arise between the amount of compound one is able to load onto a particular chromatographic stationary phase, and hence elute into the flow-cell of an LC-NMR probe, limit what type of structural analysis that can be performed. It is this author s current opinion that most complete structure elucidations of unknown molecular entities are not amenable to LC-NMR. In these... 

Spectroscopy is another topic, a good knowledge of which is expected from any good chemist. The spectroscopy finds immense applications in the identification of several unknown compounds in the field of research and medicine. Here one has to be very careful in elucidating the correct structure of entities. Moreover in every examination (competitive or regular) one has to show his skill in the area of spectroscopy and therefore chapters on spectroscopy giving a clear and lucid account is also noteworthy. 

The detection system should assist in the structural elucidation of unknown organic compounds. Moreover, accurate quantitative determinations should be feasible. 

Hyphenated analytical techniques such as LC-MS, which combines liquid chromatography and mass spectrometry, are well-developed laboratory tools that are widely used in the pharmaceutical industry. Eor some compounds, mass spectrometry alone is insufficient for complete structural elucidation of unknown compounds nuclear magnetic resonance spectroscopy (NMR) can help elucidate the structure of these compounds (see Chapter 20). Traditionally, NMR experiments are performed on more or less pure samples, in which the signals of a single component dominate. Therefore, the structural analysis of individual components of complex mixtures is normally time-consuming and less cost-effective. The... 

Determination of elemental compositions is an important step towards the structural elucidation of drug impurities. Elemental composition of an unknown compound can only be derived from the measurement of its exact mass. In combination with the knowledge of the physiological or chemical process, elemental compositions alone may be enough to propose a correct structure. 

Traditionally, conventional electron impact techniques have been used to effect sample ionization. The often complex fragmentation pattern which results can provide invaluable information for elucidating the structure of unknown compounds. Vast computer libraries of electron impact spectra are available to assist the analyst with identifications. However, electron impact is an ionization technique that is somewhat limited in scope. By contrast, sample ionization by chemical ionization techniques offers the potential for selectivity, increased sensitivity, complimentary fragmentation, and confirmation or determination of molecular weight. (7). In this paper we will describe several examples in which the versatility of the chemical ionization technique has simplified or improved an analysis. 

When considering the use of spectra for the classification of an unknown compound with the aim of elucidating its structure, some careful judgement on the relative merits of each spectroscopic tool is required. 

As mentioned in the previous section, triple-quadrupole instruments are very good at finding low levels and structurally related compounds in the presence of biological matrices as well as being the gold standard technique for quantitation. Ion trap mass spectrometers, on the other hand, have the capabilities to obtain high-sensitivity full-scan MS and MS/MS spectra therefore, they are widely used for qualitative analysis, such as structural elucidation and unknown identification. For complete metabolite identification, it is important to have both the sensitivity and selectivity of triple-quadrupole instruments and the full-scan data quality of ion traps. 

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