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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°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.

Importance of Debye and Keesom Interactions in Separating m -Xylene and p -Xylene in GC-MS Analysis Utilizing PEG Stationary Phase