Computerized Matching of Spectra with Spectral Libraries

8 COMPUTERIZED MATCHING OF SPECTRA WITH SPECTRAL LIBRARIES 

A modem mass spectrometer is generally equipped with an on-line computer that can be used to collect, process, and display the mass spectral data. In addition, the computer often has one or more spectral libraries included in its memory. In some cases the instrument is equipped with libraries 

The information in Table 8.7 confirms that the unknown liquid is most likely l-chloro-2-methylbenzene, since the probability of a correct match is placed at 94%. It is interesting to note 

Assign a structure that would be expected to give rise to each of the following mass spectra. Note Some of these problems may have more than one reasonable answer. In some cases, infrared spectral data have been included in order to make the solution to the problems more reasonable. We recommend that you review the index of hydrogen deficiency (Section 1.4) and the Rule of Thirteen (Section 1.5) and apply those methods to each of the following problems. To help you, we have provided an example problem with solution. 

An unknown compound has the mass spectrum shown. The infrared spectrum of the unknown shows significant peaks at 

10 Computerized Matching of Spectra with Spectral Libraries 497 

The information in Table 8.9 indicates that the unknown liquid is most likely l-chloro-2-methylbenzene since the probability of a correct match is placed at 94%. It is interesting to note that the meta and para isomers show probabilities of 70% and 60%, respectively. It is tempting to simply accept the results of the computer-based hbrary search as correct, but the method is not an absolute guarantee that the identity of a sample has been correctly determined. A visual inspection of the experimental and library spectra must be included as part of the process. A computer can compare a mass spectrum it has determined with the spectra in these databases. 

There is also a band from aliphatic C—H stretching from 2879 to 2979 cm 

Once a digitized mass spectrum is in hand, a basic PC can compare that data set to a library of tens of thousands of mass spectra within seconds and provide a list of potential matches. Each peak in a spectrum is categorized by the search program by uniqueness and relative abundance. Higher mass peaks are usually more characteristic of the compound in question than are commonly encountered low mass peaks, so the peaks with larger miz may be weighted more heavily in the search algorithm. The output of a library search is a table that lists the names of possible 

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Computerized Matching of Spectra With Spectral Libraries 189... 

As with PDA detection and the construction of UV libraries, computerized data systems are used for data collection and processing with mass spectral detection. Unlike UV spectra, however, mass spectra are much less dependent on experimental conditions. Thus, it is possible to purchase commercial libraries of mass spectra for spectral matching. Mass spectral data systems range from systems that record the spectra and produce conventional relative abundance bar charts for subsequent analysis to those... 

Because the IR spectrum of each compound is unique, matching the IR spectrum of an unknown peak for peak to a reference spectrum of a known material is a very good way to identify the unknown. This is often done with the aid of computerized spectral libraries and search routines, as we saw for the polymer laminate (Figure 4.28). A number of companies, instrument manufacturers, government agencies, and other sources publish collections of reference spectra in electronic format... 

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