Assessment by Monte Carlo simulation of thermodynamic correlation of retention times in dual-column temperature programmed comprehensive two-dimensional gas chromatography

摘要

The correlation of retention times in comprehensive two-dimensional gas chromatography caused by correlation of enthalpy and entropy changes between two stationary phases, methylsilicone and poly(ethylene glycol), was examined using commercial GC software and in-house Monte Carlo simulation. The enthalpy change, ΔH~0, and entropy change, ΔS~0, of 93 compounds were predicted from isothermal one-dimensional gas chromatograms predicted by the software. These values then were mimicked by Monte Carlo simulation, which removed the strong correlation of ΔH~0 and modest correlation of ΔS~0 between the two phases. Retention times in a comprehensive two-dimensional gas chromatogram (GC * GC) and in simulations of it were predicted for typical dual-capillary temperature-programmed conditions using the actual, correlated values of ΔH~0 and ΔS~0 and their uncorrelated Monte Carlo counterparts, respectively. The uncorrelated ΔH~0 and ΔS~0 values caused the retention-time range of the simulations second dimension to expand substantially beyond that in the GC * GC. Other simulations were developed using a restricted range of uncorrelated ΔH~0 and ΔS~0 values to mimic more closely the retention-time range of the GC * GC's second dimension. The intervals between nearest neighbor retention-time coordinates were calculated in both the latter simulations and the GC * GC. The intervals were larger in the simulations than in the GC * GC because the former contained uncorrelated coordinates and the latter contained correlated ones, which clustered along or near the diagonal. The retention times in the first dimension of the GC * GC were Poisson distributed, as assessed by statistical-overlap theory. In contrast, the two-dimensional reduced retention-time coordinates in the GC * GC were not Poisson distributed, because retention times were correlated. However, the reduced retention-time coordinates in the simulations were Poisson distributed.