Approaches to using retention indices with coupled column pressure tuning in gas chromatography

Pressure tuning (PT) of a coupled column ensemble in gas chromatography (GC) changes the retention time of analytes by altering the contribution of each column in the dual column system. This study investigates the effect of PT between the two columns on calculation of compound retention indices (RI), and proposes an approach to align RI data with values reported in a database library. The study first describes a model to predict the relative coupled column contribution to separation (φ) according to the PT process. The first column contribution could be tuned from 0.20 to 0.97 φ₁ by altering the mid-point pressure (p_j), which implies the property of the two-column ensemble can range from behaving similar to the first column, or to the second column, or some intermediate property depending simply on the junction pressure. The second part establishes a relationship between φ and oven temperature by using multivariate analysis based on isothermal experimental data, showing a significant change of φ in a T programmed GC analysis. The deviation for PT RI from single column RI values is related to two reasons, (1) variable retention on the coupled column ensemble, and (2) contribution of each column arising from PT. RI values can be tuned according to the mid-point pressure setting; for example, γ-terpinene, d-limonene and o-cymene had RI value changes of 3, 8 and 16 I unit, respectively simply by changing p_j by 1 psi. A correction factor may be calculated using chi-square fitting to predict single column RI data from experimental PT coupled column RI values at a particular φ, allowing the PT system to match the analyte RI with library database values.