Library search routines, computer

The mass spectra of drugs are as varied as the molecules from which they are formed (see Table 11.1). Two major sources that are available for identifying drugs are computer library search routines and Mass Spectral and GC Data of Drugs, Poisons and Their Metabolites. ... 

The data acquisition and processing operations on the MIKES instrument are under computer control and we are beginning to accumulate a library of reference spectra. A computer based library search routine has not yet been implemented on the MIKES instrument but existing gc/ms software can readily be adapted to the ms/ms data base. The construction of a ms/ms data base is underway in several laboratories. 

Library Search Routines To aid in the identification of hydrocarbons, computer programs are being developed for comparing unknown spectra to libraries of known compounds. As an example, a sample of 2-hexadecanol was compared to a library of over 100 compounds. The library search identified 2-hexadecanol as the... 

Once a mass spectrum from an eluting component has been acquired, the next step is to try to identify the component either through the skill of the mass spectroscopist or by resorting to a library search. Most modem GC/MS systems with an attached data station include a large library of spectra from known compounds (e.g., the NIST library). There may be as many as 50,000 to 60,000 stored spectra covering most of the known simple volatile compounds likely to be met in analytical work. Using special search routines under the control of the computer, one can examine... 

With regard to computer programming, the optimum seeking methods should be written separately and stored as a library program for repetitive use. The main program reads the input data, calls the search routine, performs any noniterative calculations, and handles the output. The calculation of iterative state variables and the calculation of the objective function should be performed within function subprograms. 

In the analysis of environmental samples, optoelectronic image devices allow for real time spectral acquisition and rapid identification by comparison tspectral libraries will be available for identifying HPLC eluates by computer search routines similar to those presently in use with MS and FTIR systems. 

During the 1960s further improvements made infrared spectroscopy a very useful tool used worldwide in the analytical routine laboratory as well as in many fields of science. Grating spectrometers replaced the prism instruments due to their larger optical conductance (which is explained in Sec. 3 of this book). The even larger optical conductance of interferometers could be employed after computers became available in the laboratory and algorithms which made Fourier transformation of interferograms into spectra a routine. The computers which became a necessary component of the spectrometers made new powerful methods of evaluation possible, such as spectral subtraction and library search. 

Computer library searching of spectral databases is routinely available. The database is usually a component part of the spectrometer although the search may be undertaken remotely. Several attempts have been made to develop artificial intelligence systems for direct spectral interpretation, but to date these have met with limited success. Advances in computer control have allowed multiexperiment analysis in which the spectrometer will follow a set of experiments sequentially while automatically adjusting operating parameters as directed by the results of the preceding experiment. Further advances in this area are anticipated. 

The number of occurrences of a certain substructure in the hitlist is compared with the corresponding number for the library and a probability is derived for the presence of that substructure in the unknown. This classification method is a variant of the well-known -nearest neighbour classification . Each mass spectrum is considered as a point in a multidimensional space the neighbours nearest to the spectrum of the unknown correspond to the most similar reference spectra in library search. If the majority of k neighbours (k is typically between 1 and 10) contain a certain substructure then this substructure is predicted to be present in the unknown. A drawback of this approach is the high computational effort necessary for classifying an unknown because a full library search is required. The performance has been described by Stein (1995, see Further reading section) as sufficient to recommend it for routine use as a first step in structure elucidation . 

Modem pulse height analysers essentially contain dedicated digital computers which store and process data, as well as control the display and operation of the instrument. The computer will usually provide spectrum smoothing, peak search, peak identification, and peak integration routines. Peak identification may be made by reference to a spectrum library and radionuclide listing. Figure 10.15 summarizes such a pulse height analysis system. 

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