The new spectral-recording gas chromatographic (GC) detectors making computerised analyses of their records are the "state-of-the-art" in GC instrumentation. However, they are quite expensive, and many laboratories have to make do with cheaper, more traditional detectors. Even with these new luxury detectors it is valuable to have another method to confirm the spectral identification, especially if the computer conclusion does not have a high probability.
The flame ionisation detector is a universal, quickly set up GC detector for organic solutes, but unless molar response factors are considered, it is unable to distinguish between different chemical classes of solutes it detects.
Even before the invention of this detector attempts were made to use two stationary phases for the identification of substances. In 19.56, different straight line plots for alkanes, alkanols and alkanones were obtained1 by subtracting logarithmic expressions of retention times on a triethyleneglycol column from those on an ester (both polar!). Many papers appeared relating solute structure and retention values using two phases, for example that of Lewis et al. 2. Merritt and Walsh found3 that the only "very good" pairing was of two polyethyleneglycols for distinguishing homologous series. However, they then recorded4 that the "only pair (of columns) which showed a change in order of elution of (five mixed) components" was polyethyleneglycol with a silicone. Smith et a15, and later Ladon6, studied hydrocarbons only, on polar and non-polar (squalane) columns, and the lattermost could identify unknown peaks mathematically.
In 1970, Breckler and Betts' investigated the use of relative retention times (tR._,) to linalol on packed columns, as a guide to such distinction for the most volatile constituents of volatile oils. Betts' reviewed this work, where substances of low polarity like limonene, cineole and menthyl acetate showed higher tR,_l values on the low-polarity methyl polysiloxane column than on polar columns. More polar solutes such as anethole, citral and geraniol gave the opposite effect, with tR,& values on the low polarity column being well below the results for polar columns. It was also possible to detect an alcohol, as unlike other polar solutes it would give higher tR,& values on polyethylene glycol stationary phase (with its matching hydroxy groups) than on polyester. Five groups of volatile oil constitutents with tR.rel values usually in the range 0.24-4.00 could be distinguished by this method. Bettsg applied this concept to capillary columns in 1984.