Importance of Debye and Keesom interactions in separating m-xylene and p-xylene in GC-MS analysis utilizing PEG stationary phase

In electron impact gas chromatography (GC)-mass spectrometry analysis of a complex mixture, such as gasoline, two coeluting solutes can be distinguished if each has a unique major ion. The boiling-point difference between m-xylene and p-xylene, which also has identical major ions (mz-1: 91 and 106 Da), is 0.77 degrees C. These cannot be separated even on a crossed-linked polydimethylsiloxane capillary column, which has a minimum of 5000 plates/m. They are separated on a crossed-linked polar polyethylene glycol (PEG) capillary column. GC separation on a stationary phase depends on the relative strengths of solute-solute, stationary phase-stationary phase, and solute-stationary phase interactions. Although the calculated molar electronic polarization and refractivity factor of Lorenz-Lorentz equation for m-xylene and p-xylene are nearly equal because of its greater dipole moment difference (0.30 and 0.02), the calculated orientation polarization of m-xylene is 80 times greater than p-xylene. This implies the dipole reinforcement through inductive polarization by the hydroxyl of PEG stationary phase molecules is greater on m-xylene than p-xylene. In addition, as the permanent dipole moment of m-xylene is 15 times greater than p-xylene, m-xylene has a stronger Keesom interaction with PEG. In order for m-xylene and p-xylene to solvate in PEG, analytes must overcome the PEG-PEG Keesom/hydrogen bonding interaction forces. Physical and chemical parameters indicate that compared with p-xylene, m-xylene has a greater Debye-Keesom interaction tendency with PEG molecules. This is supported by the 0.12-min. retention-time difference between them.