The full NIST Mass Spectral Library consists of the NIST\EPA\NIH Electron Ionization (EI) mass spectral library (mainlib and replib), the NIST 17 Tandem Library (nist_mms and nist_msms), the NIST Mass Spectral Search Program v.2.3, Mass Spect Interpreter, AMDIS, and Lib2NIST.
The Tandem Library is compose of product-ion mass spectra which are produced by the collisional activated dissociation of precursor-ions produced primarily by electrospray ionization. A handful of these precursor-ions were produced by atmospheric pressure chemical ionization (APCI). The reason product-ion spectra are measured from LC generated precursor-ions is because the precursor-ions are very stable. This means that there is almost no fragmentation to product-ions that can be used to determine the structure of an unknown. All LC/MS techniques are called “soft-ionization” methods. The ion representing the intact molecule is formed by the addition of a hydrogen ion (a protonated molecule), the abstraction of a hydrogen nucleus to produce a deportended molecule (a negative ion), or the extraction of a hydride to produce an [M – H]+ ion.
The only way to get structural information is to cause these precursor-ions to fragment. This is done by isolating ions of a single m/z value using MS1 in a tandem mass spectrometer. The isolated ions are then subjected to collisions with an inert substance in the gas phase such as Ar atoms or N2 molecules. The resulting product-ions are then analysed by the second mass spectrometer. The resulting mass spectrum is then evaluated in the same way that an EI mass spectrum is evaluated using interpretation rules and libraries of like mass spectra.
In EI mass spectrometry, which is what most GC/MS is, the ion representing the intact molecule is the molecular ion, an ion formed by the removal of a single electron from a gas-phase molecule. This process is called “hard-ionization”. This molecular ions will under go extensive fragmentation. If you wanted to get structural information using MS/MS, you would isolate the molecular ion. Have it under go fragmentation in the collision area of an instrument and then obtain a mass spectrum using the second mass spectrometer. This spectrum would have little difference from the mass spectrum obtained by the standard EI process and would be far less sensitive.
With GC/MS/MS you are typically doing selected reaction monitoring (SRM a.k.a. MRM). This is a process where you isolate a fragment ion from the decomposition of a molecular ion using the first mass spectrometer; cause this ion to fragment in the collision area; and, then used the second mass spectrometer to measure the ion current of a specific product-ion. These data are then used for quantitation, not structure determination.
Libraries like the NIST\EPA\NIH EI Library and the NIST 17 Tandem Library are used to determine the structure of unknown analytes,
There are no GC/MS/MS libraries, because they would not contain any additional information to that found in in a library of EI spectra of the same compounds.
For more information on the NIST Mass Spectral Libraries, see: