Qualitative Methods of GC/MS Analysis:Library Search
An Easy Tool for Identification
Library search is an easy and useful tool for identifying a chromatographic peak.
Since a mass spectrum pattern relates to the chemical structure of the compound, a mass spectrum acts as a fingerprint in chemical analysis. A database of mass spectra is called "mass spectral library". We can identify an unknown component by comparing its measured spectrum to reference spectra registered in mass spectral libraries.
Searching Libraries
An unknown spectrum is compared to reference spectra registered in spectral libraries and the similarity is estimated for each reference. In practical analysis, the completely same spectra can't always be obtained even though the same compounds are measured. Many practical conditions: the used instrument, the parameters for measuring, sample condition and statistical fluctuation, bring some ambiguity into a measured spectrum. A library search routinely informs us of the matched compounds with a greater similarity than the criterion determined by a user. The user identifie the unknown spectrum by taking the other conditions into consideration as well.
Search Methods of Libraries
There are various search methods with spectral libraries. The method fitting the user's purpose is selected.
Combined Use of Retention Index
A conventional library search neglects retention parameters, and the process includes only comparison of spectra. Some chemical components such as isomers have similar spectra, which may confuse chemists, but show chromatographic peaks at deferent retention times. The combined use of retention indices when searching libraries makes identification easier and more reliable. To perform this search, spectral libraries with retention indices are required. Using the same column and same analysis method described in the library is recommended to get good results
Examples of Spectral Library
Many mass spectral libraries are commercially available. Some of them are for general use while others are for specific application fields. In general, the data are obtained using 70eV EI. Additionally, users' personal libraries can be made for some GCMS application software.
