The NIST Search Procedure

After a significance weighting (square root of the product of mass and intensity), data reduction by a noise filter and a redundancy filter, the extensive reference database is searched for suitable candidates for a pattern comparison in a rapid pre-search. The NIST search uses all masses of the unknown spectrum in this pre-search (former INCOS only the eight most intense signals). The intensity ratios are not yet considered at this point, only the occurrence of the mass signal. Reference spectra, which only contain a small number or no matching masses are excluded from the list of possible candidates and are not further processed. 

The main search is the critical step in the search algorithm, in which the candidates selected in the pre-search are compared with the unknown spectrum and arranged in a prioritized list of suggestions. Of critical importance for the 

Two values are determined for spectral comparison as a result of the main search. The reverse match value (NIST RSI , former INCOS FIT ) value gives a measure of how well the reference spectrum is represented with its masses in the unknown spectrum (reverse search procedure, ignoring all peaks that are in the sample spectrum but not the reference spectrum). The forward looking mode of searching, whereby the presence of the unknown spectrum in the reference spectrum is examined (forward search procedure, all peaks of the sample spectrum are compared), is expressed as the match value (NIST SI , 

High High High Identification or that of an isomer very probable 

Low High Low Identification possible, but homologues, co-elution, or noise present 

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