Unknown compounds, identification

The scan mode determines the extent and quality of mass spectrometric data and has thus to be chosen with respect to the analytical requirements. Detection of unknown compounds, identification of unknown structures and confirmation of known molecules as well as quantification of distinct target analytes require different scan modes for reliable optimum analytical acuity. 

Tandem mass spectrometry (MS/MS) has attained an enviable status as an analytical tool to identify and quantify compounds in complex mixtures. MS/MS refers to the coupling of two stages of mass analysis, either in time or space. Of all the ionization techniques, only electron ionization (El) provides abundant sttuctural information. To obtain additional structure-specific information by other ionization techniques, it has become essential to perform MS/MS experiments [1,2]. MS/MS was first used in the late 1960s [3]. Since that time, its applications and popularity have continued to grow. Its major contributions are in the fields of structure elucidation of unknown compounds, identification of compounds in complex mixtures, elucidation of fragmentation pathways, and quantification of compounds in real-world samples. In recent times, several new generations of instruments have become available for tandem mass spectrometry applications. Basic concepts of tandem mass spectrometry and an account of these new developments are presented in this chapter. Additional reading material is listed at the end of the chapter. 

In this way the student s knowledge of the organic reactions is consolidated as he proceeds through the sections, his experience of the general method of identification steadily increases, and the investigation of the unknown compounds forms a welcome break from the systematic pursuit of the sectional work. 

Identification of unknown compounds in solutions, liquids, and crystalline materials characterization of structural order, and phase transitions... 

The Use of PLC for Isolation and Identification of Unknown Compounds from the Frankincense Resin (Olibanum) Strategies for Finding Marker Substances... 

To assure consistency and speed in multidisciplinary structure analysis of low-MW compounds involving various techniques (IR, NMR, MS, etc.) most industrial laboratories use a Standard Operating Procedure (SOP). In such schemes IR analysis is frequently used as a cheap filter for a quick starting control and as a means for verification. As IR detects only structural units identification of an unknown compound on the basis of IR is difficult. Mass spectrometry is used as the prime identification tool and is especially important in the determination of the exact mass and gross formulae. While structural prognostication on the basis of MS is difficult for the non-expert, a posteriori interpretation is quite feasible. H NMR is both easy and cheap, however requires greater sample quantities than either... 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

Positive identification of unknown compounds (library searchable EI/CI spectra)... 

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. 

The most commonly used method for the identification of carotenoids is high-pressure liquid chromatography (HPFC) combined with the UV-Vis absorption detection. The introduction of diode array detection enabled parallel collection of pigment spectra, which greatly aids the quantification and localization of unknown compounds. Coupling HPFC with the mass-spectrometer significantly... 

It is likely that when you have unknown compounds, such as those that can be extracted from an archaeological material, knowledge of their weight is important for their identification. MS can measure it. Hence, when you introduce a given compound into a mass spectrometer you can determine its molecular weight. Accordingly, you can restrict the number of possible compounds. 

Identification of flavonoids Quantification of individual flavonoids depends heavily on the availability of standard references. Only a limited number of common flavonoids are commercially available as standards. Standard references for flavonoid glycosides are particularly difficult to find thus direct quantification of the native glycosides is nearly impossible. Analysis of the aglycones after acid or enzymatic hydrolysis is therefore common practice. When standard flavonoids are not available, or when unknown compounds are encountered in a particular fruit or vegetable, use of a DAD... 

Visible and UV spectrometry are of secondary importance to other spectral methods for the identification and structural analysis of unknown compounds. This is a direct consequence of the broad bands and rather simple spectra which make differentiation between structurally related compounds difficult. As an adjunct to infrared, magnetic resonance and mass spectrometry, however, they can play a useful role. They can be particularly helpful in confirming the presence of acidic or basic groups in a molecule from the changes in band position and intensity associated with changes in pH (p. 369). 

The value of infrared spectrometry as a means of identification of unknown compounds and to investigate structural features is immense. Spectra are used in an empirical manner by comparison of samples with known materials and by reference to charts of group frequencies. A simplified correlation chart is shown in Table 9.8. The interpretation of infrared spectra is best considered by discussing the prominent features of a representative series of compounds. 

PMR spectrometry is an extremely useful technique for the identification and structural analysis of organic compounds in solution, especially when used in conjunction with infrared, ultraviolet, visible and mass spectrometry. Interpretation of PMR spectra is accomplished by comparison with reference spectra and reference to chemical shift tables. In contrast to infrared spectra, it is usually possible to identify all the peaks in a PMR spectrum, although the complete identification of an unknown compound is often not possible without other data. Some examples of PMR spectra are discussed below. 

The ideal imaging method should enable simultaneous detection and identification of known and unknown compounds present in the studied sample. During analysis, analyte atoms and molecules are removed from the sample surface, in most cases only... 

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