Identification of unknown compounds

There are a few general rules that can be stated to help you to use ah infrared spectrum for the detemination of a structure. However, the most effective way to learn is obviously through practice. 

L Look first at the high-frequency end of the spectrum ( 1500 cm ) and concentrate initially on the major bands. 

For each band, short-list the possibilities by using a correlation chart. 

Use the lower-frequency end of the spectrum for the confirmation or elaboration of possible structural elements. 

Do not expect to be able to assign every band in the spectrum. 

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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... 

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)... 

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. 

TSI. The liquid is converted into a vapour jet and small droplets are generated with the help of a heated vapouriser tube. A buffer dissolved in the eluent assists the ionisation process through the formation of adduct ions, which are produced via statistical charging of individual droplets. Due to the softness of the procedure, no structurally characteristic fragments, which could aid identification of unknown compounds, are formed. 

The fingerprint region lies between 1300-400 cm-1 which is considered to be the most valuable component of the spectra and mainly comprises of a specifically large number of unassigned vibrations. Therefore, IR-spectroscopy aids in the identification of unknown compound by comparing its spectrum with a standard spectra recorded under exactly similar experimental parameters. Thus, pharmaceutical substances that exhibit the same infrared spectra may be inferred as identical. 

Most of the detectors permit peak recognition but provide no structural information, which can be particularly important for identification of unknown compounds. From this point of view, the spectro-metric detectors, specifically mass spectrometer and photodiode array detectors, add a third dimension to the multidimensional system and give additional information useful in components identification. 

Catalog of Infrared Spectrograms. Philadelphia Sadtler Research Laboratories, PA. 19104. Spectra are indexed by name and by the Spec-Finder. The latter is an index that tabulates major bands by wavelength intervals. This allows quick identification of unknown compounds. 

Identification of unknown compounds by retention times or peak enhancement is not conclusive or absolute proof of identity. It is possible for two different substances to have identical retention times under the same experimental conditions. For positive identification, the sample must be collected at the exit port and characterized by mass spectrometry, infrared, nuclear magnetic resonance, or chemical analysis. 

With the proliferation of mass spectrometers in dmg metabolism came a vast amount of data to be processed and understood. In particular, techniques and tools for identification of unknown compounds such as metabolites, degradation products, or impurities have seen continual improvement in recent years, especially with respect to data obtained under high-resolution (and presumed high-mass-accuracy) conditions. A few possible approaches are summarized here. 

A common method for identification of organic compounds is mass spectrometry (MS) in combination with GC. After separation of the component by GC the mass spectrometer transform the analyte into gaseous ions in vacuum in the ion source. For electron impact ionization this results in different mass fragmentation patterns with different mass-to-charge ratios (m/z). From this fragmentation pattern it will be possible to identify the compound by comparison with commercial mass spectral libraries. Identification of unknown compounds can be facilitated by... 

LC-MS uses different types of soft chemical ionization that produces molecular ions and no fragmentation pattern. In MS/MS instruments the molecular ions can be fragmented by collision with a gas for example, He. This fragmentation can be used for identification of a compound. No mass spectral libraries exist for LC-MS hence identification of unknown compounds is more time-consuming than for GC-MS. For known compounds LC-MS is a very sensitive and specific method, using LC-MS/MS systems the analytical performance can be increased even more. LC-MS analysis is especially suitable for non-volatile POMs such as non-ionic surfactants in house dust samples (Clausen et al., 2003). 

The Eight-Peak Index of Mass Spectra published by the Mass Spectrometry Data Centre of the Royal Society of Chemistry is a popular printed index of mass spectral data that now contains some 81000 spectra of over 65000 different compounds [4], These spectra are published in the shape of lists of the eight main peaks. The complete data are sorted in three different ways to allow easy identification of unknown compounds by (i) molecular weight subindexed on molecular formula, (ii) molecular weight subindexed on m/z value and (iii) m/z value of the two most intense ions. 

K. Clauwaert, S. Vande Casteele, B. Sinnaeve, D. Deforce, and W. Lambert, Exact mass measurement of product ions for the strnctnral confirmation and identification of unknown compounds nsing a qnadrnpole time-of-flight spectrometer A simplified approach using combined tandem mass spectrometric fnnetions, Rapid Commun. Mass Spectrom. 17 (2003), 1443-1448. 

Mass spectrometry (MS) and tandem mass spectrometry (MS/MS) play an important role in the identification of unknown compounds. Different ionization techniques have been used in studying isothiazole derivatives, such as electronic ionization (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), field desorption (FD), and chemical ionization (Cl). Different MS/MS experiments have been described, such as investigation of metastable and collision-induced dissociations to study isothiazoles gas-phase ion chemistry. In addition, MS and MS/MS have been used to differentiate isothiazole isomers differing in the position of endocyclic groups or exocyclic substituents <1998THS(2)471, 1999THS(3)369, 2000THS(4)405>. 

Applications of IR spectroscopy to qualitative analysis are mainly for the identification of unknown compounds. For a pure substance, an exact match of the IR spectrum of the compound with that of the... 

In any case, both biosensors and biosensing devices have been coupled to microdialysis and are considered among the non-separation-based methods [83]. The drawback of biosensing approaches is that they are usually able to measure just one analyte at a time, in contrast with separation-based methods such as chromatography and electrophoresis, which allow the detection of several analytes. However, if the primary interest is not the identification of unknown compounds, but, for example, the monitoring of variations in a single metabolite or drug, the optimization of therapeutic responses, or the control of a bioprocess via a marker analyte, the use of a specific sensor, which can be employed in a continuous manner, can provide useful information, and can also help to avoid the analysis of hundreds of samples or to reduce the number of animals necessary for a study. 

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