Unknowns identification

The results of the two dtafts are rich in information about protein famihes and classes. One example is shown in Table 54—3, in which the major classes of proteins encoded by human genes are listed. As can be seen, the latgest class is unknown. Identification of these unknown ptoteins will be a major focus of activity for many laboratories. 

MCLAFFERTY, F.W., STAUFFER, D.A., LOH, S.Y., WESDEMIOTIS, C., Unknown identification using reference mass spectra. Quality evaluation of databases, J. Am. Soc. Mass Spectrom., 1999,10, 1229-1240. 

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. 

The mass spectrometer when used as a detector for GC is the only universal detector capable of providing structural data for unknown identification. By using a mass spectrometer to monitor a single ion or few characteristic ions of an analyte, the limits of detection are improved. The term mass selective detection can refer to a mass spectrophotometer performing selected ion monitoring (SIM) as opposed to operation in the normal scanning mode. Typical limits of detection for most compounds are less than 10 "l2 g of analyte. 

Analysis of Nontarget Compounds. "Complete Unknowns. This is a somewhat similar process in that the retention time and the type of LC column giving the best results also yields dues as to chemical classification, e.g., good retention and separation upon an anion exchange column suggests that the analytes are anionic. Confirmation information required for unknown identification is also obtained from other instrumentation induding UV spectrophotometry. 

Figure 3.12. Transmission IR spectra plotted in absorbance of a two-polymer laminate (A), one component of which is able to be measured (B) whereas the other is unknown. Identification is possible by spectral subtraction to give spectmm (C), which may be compared with a library spectmm (D). Spectrum courtesy of Dr L. Rintoul.

Ion trap-linear ion trap Full-scan MS/MS, or MS" Price, versatility, full-scan product ion spectra Targeted analysis [66,67], unknown identification, IDA MSMS [61,74]... 

Keywords product safety pharmaceutical product packaging quality control AMDIS deconvolution unknown identification Mass Frontier spectrum interpretation... 

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. 

Analysis. Excellent reviews of phosphate analysis are available (28). SoHds characterization methods such as x-ray powder diffraction (xrd) and thermal gravimetric analysis (tga) are used for the identification of individual crystalline phosphates, either alone or in mixtures. These techniques, along with elemental analysis and phosphate species deterrnination, are used to identify unknown phosphates and their mixtures. Particle size analysis, surface area, microscopy, and other standard soHds characterizations are useful in relating soHds properties to performance. SoHd-state nmr is used with increasing frequency. 

When the gas chromatograph is attached to a mass spectrometer, a very powerful analytical tool (gas chromatography-mass spectrometry, GC-MS) is produced. Vapour gas chromatography allows the analyses of mixtures but does not allow the definitive identification of unknown substances whereas mass spectrometry is good for the identification of a single compound but is less than ideal for the identification of mixtures of... 

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

The major STEM analysis modes are the imaging, diffraction, and microanalysis modes described above. Indeed, this instrument may be considered a miniature analytical chemistry laboratory inside an electron microscope. Specimens of unknown crystal structure and composition usually require a combination of two or more analysis modes for complete identification. 

The most useful application of ISS is in the detection and identification of sur-fece contamination, which is one of the major causes of product failures and problems in product development. The surface composition of a solid material is almost always different than its bulk. Therefore, surface chemistry is usually the study of unknown surfaces of solid materials. To better understand the concept of surface analysis, which is used very loosely among many scientists, we must first establish a definition for that term. This is particularly Important when considering ISS... 

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