Inverted library search

The spray paint can was inverted and a small amount of product was dispensed into a 20 mL glass headspace vial. The vial was immediately sealed and was incubated at 80°C for approximately 30 min. After this isothermal hold, a 0.5-mL portion of the headspace was injected into the GC/MS system. The GC-MS total ion chromatogram of the paint solvent mixture headspace is shown in Figure 15. Numerous solvent peaks were detected and identified via mass spectral library searching. The retention times, approximate percentages, and tentative identifications are shown in Table 8 for the solvent peaks. These peak identifications are considered tentative, as they are based solely on the library search. The mass spectral library search is often unable to differentiate with a high degree of confidence between positional isomers of branched aliphatic hydrocarbons or cycloaliphatic hydrocarbons. Therefore, the peak identifications in Table 8 may not be correct in all cases as to the exact isomer present (e.g., 1,2,3-cyclohexane versus 1,2,4-cyclohexane). However, the class of compound (cyclic versus branched versus linear aliphatic) and the total number of carbon atoms in the molecule should be correct for the majority of peaks. 

In addition to the necessity of defining similarity, one other major choice must be made in developing library searching approaches. This involves the use of sequential search, the use of inverted (sorted) files, employing hashing methods, or the use of hierarchical trees. Each of these methods has its advantages and is suitable in some circumstances. 

Characterization of a LINE-like element can be quite difficult. First, the repetitive region of the isolated clone is sequenced. Then a computer analysis is performed to determine the characteristics of the sequence, such as the presence of inverted or direct terminal repeats. If terminal repeats are not found it is still possible that the element is one of the terminal repeat class which has a deletion at one end. Further clones are then obtained from the library by using the repetitive portion of the clone as a probe. By way of cross-hybridization, regions of homology between the two isolates can be delineated. The sequence of portions of other isolates should then be determined to search for the ends of the element. 

The second stage is to conduct a computer search of an appropriate database or library, for example of drugs, to test for an exact match or to compile an inverted list (Topic H4). 

Searching spectral libraries may involve the use of inverted lists. These consist of each characteristic absorption band or emission line along with a list of corresponding numbered library spectra that include that particular band or line. A list for the spectrum of an unknown analyte can then be rapidly checked against the library lists. An example of part of an inverted list for an infrared spectral library is shown in Figure 1. It includes spectrum No. 66 among those listed with an absorbance band at 1220 cm and spectrum No. 105 among those listed with an absorbance band at 2730 cm . ... 

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