Temperature-programmed retention indices for g.c. and g.c.-m.s. analysis of coal- and petroleum-derived liquid fuels

Retention indices are very useful in identifying components in liquid fuels by gas chromatography (g.c.), even when g.c. is coupled with mass spectrometry (g.c.-m.s.). In this work, temperature-programmed retention indices of over 150 compounds were determined on an intermediately polar capillary column coated with 50% phenyl-50% methyl polysiloxane (Rtx-50) and a slightly polar column coated with 5% phenyl-95% methyl polysiloxane (DB-5) at three heating rates (2, 4 and 6°C min^t-1 from 40 to 310°C). Aliphatic compounds give nearly constant retention indices at different heating rates. However, the retention indices of polycyclic aromatic compounds exhibit a relatively large temperature dependence. The use of a short isothermal hold (5 min) prior to the programmed heat-up did not cause any significant difference in the retention indices. The column polarity can affect the retention indices significantly, depending on the compound type. The differences between the retention indices on the two columns are relatively small with aliphatic compounds but become larger with polycyclic and polar compounds. In general, retention indices and their sensitivity to temperature programming decrease with decreasing column polarity. The usefulness of the temperature-programmed retention indices was also demonstrated in the analysis of liquid fuels. Combined use of retention indices and mass spectra allows the identification of many more compounds with higher confidence in petroleum- and coal-derived JP-8 jet fuels. The knowledge on the effects of temperature and column polarity can be applied for selecting appropriate column and temperature programmes for the separation and reliable identification of compounds in given samples. In addition, the present results can be used in combination with a mass spectral library to accomplish faster and more reliable compound identification.