Using Thermodynamics to Evaluate Stationary Phase Polarity in Gas Chromatography- A New Look

Author

Hetal R. RanaFollow

Date of Award

Spring 5-21-2022

Degree Type

Thesis

Degree Name

MS Chemistry

Department

Chemistry and Biochemistry

Advisor

Nicholas H. Snow, Ph.D.

Advisor

Wyatt R. Murphy, Ph.D.

Committee Member

Yuri. V. Kazakevich, Ph.D.

Committee Member

Stephen P. Kelty, Ph.D.

Keywords

gas chromatography, columns, stationary phases, intermolecular interactions, Kovats Retention Index, McReynolds constants, Mondello Polarity scale, Thermodynamics retention indices, van't Hoff plot, Equilibrium constant, Gibbs Free energy

Abstract

Abstract

Gas chromatography is a separation method in which the components of a sample partition between a gaseous mobile phase (carrier gas) and a solid or liquid stationary phase. The stationary phase is usually a polymer coated or chemically bonded to a solid support on the column wall and its function is to separate different components, based on their relative vapor pressure and their intermolecular interactions with the stationary phase. If the analyte polarity is similar to the stationary phase polarity, then the retention time increases because the intermolecular interactions are stronger and if the polarities are different then the retention time decreases.

Generally, the polarity of a stationary phase is calculated by determining phase constants using five probes: benzene, 1-butanol, 2-pentanone, 1-nitropropane, and pyridine. These are termed as McReynolds constants. The Kovats Retention Index of each probe on the stationary phase of interest is determined and then compared to the retention indices on squalane to determine the McReynolds constant for that probe. The five McReynolds constants can then summed to obtain polarity values and to calculate an overall polarity number. This method has some limitations, including the use of squalane, which is not readily available, as the standard stationary phase. In the present work, van’t Hoff plots are used to determine the thermodynamic quantities: ∆G^o, ∆H^o, and ∆S^o for the McReynolds test probes and alkanes on polar and non-polar stationary phases. Considering 100% polydimethyl siloxane (PDMS) as a reference, other stationary phases are evaluated using these thermodynamic retention indices. These values provide a more general means for comparing stationary phase relativeness than McReynolds constants or polarity numbers. Thermodynamics should be more accurate to estimate whether a given stationary phase will be selective for a proposed separation, since the thermodynamic values determine how retentive a column is for a specific analyte or class of analytes.

Recommended Citation

Rana, Hetal R., "Using Thermodynamics to Evaluate Stationary Phase Polarity in Gas Chromatography- A New Look" (2022). Seton Hall University Dissertations and Theses (ETDs). 3000.
https://scholarship.shu.edu/dissertations/3000