Characteristic thermal constant and dimensionless heating rate. The links to optimum heating rate in GC

An initial step in the quest of deriving a generalized approach to optimiza tion of a temperature program in gas chromatography is presented. Central t o this is the introduction of a dimensionless heating rate, r. As a first s tep to defining r, a characteristic thermal constant, theta(char), defined as theta(char) = -dT/dk at k = 1, where T and k are, respectively, column t emperature and solute retention factor, is introduced and evaluated for our own experimental data and for thermodynamic data from the literature. It w as determined that, for silicone stationary phases with a phase ratio of 25 0, theta(char) ranged from about 23 degrees C for low molecular weight hydr ocarbons such as dimethylpropane to about 45 degrees C for high molecular w eight pesticides such as mirex, It was also found that, for a particular so lute and a stationary phase type, a 2 orders of magnitude increase in the f ilm thickness caused only about a 2-fold increase in the characteristic the rmal constant, Using theta(char) as a fundamental temperature unit in GC an d void time as a fundamental time unit, a dimensionless heating rate is int roduced and its potential utility for the evaluation of the separation-spee d tradeoffs in a temperature-programmed GC is demonstrated.