Family retention plots

Laub and Tyagi investigate the analytical qualities of GC in general, and IGC specifically (52). They demonstrate the value of family retention plots from which the elution behavior of homologues or of related compounds in a series of volatile phase components may be estimated. In this way, the separability of such compounds via IGC methods is readily predicted. Also noted is the economy of sample sizes required for IGC, an invaluable consideration when only minute quantities of material are available. 

Table L Slopes, a, and Intercepts, k, of Family Retention Plots (Cf. Equation 16) of Indicated Solutes with OV Stationary Phases at 30-80°C ...

From the standpoint of physicochemical measurements, family and isothermal plots are useful for the determination of vapor pressures (and, in addition, recalling Equations 13 and 16, heats of vaporization as well) from the retention data obtained from just a few chromatographic runs (33). Furthermore, the GC technique is ideally suited to instances in which the solutes are available only in minute quantities or are substantially impure, where each of these constraints ordinarily precludes bulk vapor-pressure measurements by conventional static procedures. For example, Heath and Tumlinson (34) employed log(retention) plots to determine the vapor pressures of trace acetate ingredients used in pheromone formulations. An important aspect of their work was that family correlations were obtained with a chiral-nematic stationary phase, cholesteryl p-chlorocinnamate. 

The "family-plot retention data of 5 solutes with a series of poly(methylphenylsiloxane) stationary phases have been examined in terms of the saturation vapor pressure Pa of the solute, the methyl/phenyl ratio of the solvent, and the temperature. Plots of In Vg against In p for a given solute over a range of temperature were found to De linear, as were the "isothermaV, that is, the retention/vapor -pressure plots for an homologous series of solutes at a constant temperature. The family-plot slopes exhibited by the n-alkane probe-solutes were also found to be very sensitive to the aromatic content of the polymers. Thus, it appears that the "family" technique of GC data reduction can be a useful tool for characterizing the physicochemical properties of (polymer) stationary phases. 

However, it is not immediately clear what other advantages, if any, the mode of data analysis represented by Equation 16 might have over those founded upon xyA or wyA. We have therefore assessed in this work the family-plot behavior of several solutes with the Ohio Valley (OV) series of poly(methylphenylsiloxane) stationary phases, for which retention data of high accuracy are available for a variety of solutes (28-33). 

Plots of -a against phenyl content for n-hexane and benzene probe-solutes, provided in Figure 4, emphasize the dependency of the family-plot slopes of aliphatic and aromatic compounds (particularly those of the former) on the aromatic content of the OV solvents. The aromatic character of these stationary phases can therefore be gauged simply with the retentions of a few hydrocarbons. Presumably, with judicious choice of the probe-solute, other properties of polymer GC phases could be deduced from the slopes a of Equation 16 in a similar way. Family plots have also been shown recently to reflect discontinuities in retentions due to phase changes in mesomorphic polymers (16). 

However, the derivation of Equation 16 failed to take into account the free-volume ("structural") contribution to xy"(35), nor is the effect negligible with some polymer phases (36). In addition, there remains considerable doubt even as to what constitutes a "family" set of data e.g., with polymers that are liquid-crystalline. The characterization of polymer stationary phases via family-plot regressions of "inverse" gas-chromatographic retentions therefore invites further and comprehensive study. 

Plotting retention versus pressure for families of related compounds usually results in a series of parallel lines. Two lines almost never cross. Changing pressure is seldom effective in changing selectivity. 

---