MassFinder 3: GC/MS Interpretation and Terpenoids Library
Retention Indices
Gas chromatography (without MS) often solely relies on reproducible retention times in order to identify compounds. However, in GC/MS automatic library searches rarely consider retention times, thus wasting precious information. Since the chromatographic retention time and the mass spectrum are two independent experiments, the combination of both drastically improves the accuracy and hit quality of library searches.
MassFinder's unique two-dimensional search algorithm finds matching library entries not only by their mass spectral similarity but also by comparing their retention indices.
Retention indices (RI) are normalised retention times relative to n-alkanes. By using RI instead of absolute retention times, you gain independence from instrumental conditions like carrier gas pressure or column length. RI are comparable world-wide between scientists using different GC/MS instruments and can be reproduced easily.
MassFinder supports the usage of retention indices in the most convenient way. All you have to do is to measure the "alkane pattern", i.e. the retention times of all n-alkanes relevant for your desired time range. This only requires one single injection, since ready-to-use alkane mixtures are commercially available.
Similar Mass Spectra with Distinguishable Retention Indices
The mass spectra below show to isomeric sesquiterpene hydrocarbons with pretty similar mass spectral pattern, but easily distinguishable retention indices on DB-1 stationary phase (RI 1507 vs. 1474).
Further information to download:
Retention Index Guide (pdf, 53 KB)
Short definition
By definition, n-alkanes with x carbon atoms have an RI of 100x (e.g. n-decane RI 1000, n-undecane RI 1100), and any compound eluting exactly in the middle between decane and undecane has RI 1050. Given a stationary phase you may think of RI as a natural property of a certain compound, determined by the chromatographic gas/liquid equilibrium constants. Instrumental parameters like column length, film thickness, carrier gas pressure or flow, injector and detector temperatures have negligible influence. Drastic changes in temperature programs may have slight to medium effects on the RI as do all kind of overloading or reactive processes that are known to influence relative retention times.
A more precise definition, an in-depth explanation, and examples are given in the retention index guide mentioned above.