Optimal heating rate in constant pressure and constant flow gas chromatography

Abstract

Optimal heating rate is the one resulting in the shortest analysis time for achieving a required separation performance of a column. The previously recommended default heating rate (RT,Def) was optimal for temperature‐programmed gas chromatography analyses in constant pressure mode. It has been shown herein that the same recommendation can be extended to constant flow mode with fixed heating rate (RT). The numerical value of RT,Def has been herein rescaled from previous 100.28em∘C/tnormalM (10°C per void time) where tM was measured at 50°C, to 120.28em∘C/tnormalM with tM measured at 150°C—a round number in the middle of the gas chromatography temperature range, chosen as a reference temperature for numerical values of all temperature‐dependent gas chromatography parameters. It has been experimentally found based on theory developed herein that RT,Def=120.28em∘C/tnormalM is optimal for columns with φ=0.001 (φ=dnormalf/d is dimensionless film thickness, d and df are the column internal diameter and film thickness, respectively) in constant pressure mode and constant flow mode with fixed RT. Theory shows that, for arbitrary φ, RT,Def=12(1000φ)0.090.28em∘C/tnormalM. The theory also shows that the fixed RT is optimal for constant pressure mode. In constant flow mode, however, the optimal RT should gradually increase with increasing temperature (T). The optimal theoretical curves RT(T), different for different flow rates, were found. However, only the optimization of the fixed RT was experimentally evaluated due to limited capability of existing gas chromatography instrumentation and resources. It has been shown that the separation‐time tradeoff in constant pressure mode is slightly better than that in constant flow mode. The experimental data are compiled in the Supporting information.