ASimpleModelofGCxGCSeparations.pptVIP

A Simple Model of GC x GC Separations John V. Seeley Oakland University 3/6/07 Model Goals Generation of a “Simplified Chromatogram” from: 1-D retention times Linear free energy relationship parameters Retention indices Utility of the “Simplified Chromatogram” Demonstrates the underlying mechanisms of a GC x GC separation Approximate representation of relative peak position Quick screening new column sets Demonstrates the influence of stationary phase order on chromatogram structure Demonstrates the concept of “orthogonality” in GC x GC Model Goals The model does not attempt to: Predict absolute retention times (just relative retention position) Predict peak widths Find optimal flow, temperature, modulation conditions, and/or column dimensions Generate accuracy at the cost of convenience A 3-Step Solvation Model Conclusions Based on the 3-Step Solvation Model The “Logic” Behind a 2-D Chromatogram GC x GC Chromatograms generate separations in two dimensions one dimension is primarily a “size” separation one dimension is primarily a “polarity” separation Mixtures of monofunctional homologous organic compounds of the type Z – (CH2)n – H are the simplest samples to demonstrate the nature of GC x GC separations. Size determined by n and Z Polarity determined by Z Observations of Chromatogram Structure Each homolgous group (i.e., each Z) has a different starting primary retention time. Changing the value of n leads to a shift in primary retention time that is independent of Z. This suggests the use of a retention index, r, that is linearly related to n and has a Z-dependent offset, rz. r = n + rz rZ is a unique constant for each functional class and each column 1tR = f (r) f = monotonically increasing function Compounds of the same functional class generate peaks in a horizontal band. This means secondary retention time is independent of n and most likely determined by the rz factors on the primary and secondary column. Determinatio