Once a usable spectrum is displayed, the database of library spectra
can be searched. The search algorithm assigns a weight to each mass
peak according to how common that mass occurs in thousands of mass
spectra. For example, m/z 43 is quite common, so the presence of this
signal in an unknown is less informative than say an m/z 100 (which
is considerably less common). This type of weighting is called Probability
Based Matching, or PBM. In addition, the search is a reverse search,
which means the library spectral peaks are compared to the unknown
rather than vice versa.
Library search results have questionable meaning due to:
Exercise: Select the apex spectrum of the peak at 6.4 minutes in the
data file 00376.D Do a library search (by double clicking the right
mouse button while the curson is anywhere within the spectrum window).
Print the results of this search. Next, select an average spectrum
for the same peak and repeat the search. Is there a difference in
the search results? Why?
Exercise: From the data file 00677.D, select an average spectrum for the peak at 5.7 minutes and search the library for this spectrum. Does the 'best hit' identify the unknown compound? Which one of the hits is the unknown? Why did the library search fail in this instance? What does this exercise demonstrate?