Dependence of thermal mismatch broadening on column diameter in high-speedliquid chromatography at elevated temperatures

In this paper, we compare a narrow-bore column (2.1-mm i.d.) to a conventio nal-bore column (4.6 mm i.d.) at elevated temperatures under conditions whe re thermal mismatch broadening is serious and show that narrow-bore columns offer significant advantages in terms of efficiency and peak shape at high er linear velocities. We conclude that the so-called thermal mismatch broad ening effect is largely due to a radial retention factor gradient and not a radial viscosity gradient. The lower volumetric now rates inherent with th e use of narrower columns lead to lower linear velocity in the heater tubin g and longer eluent residence times in the heater. Thus, with the same heat er tubing at the same column linear velocity, narrow-bore columns give bett er thermal equilibration between the eluent and the column compared to; wid er bore columns. This means that high-temperature, ultrafast liquid chromat ography no longer requires excessively long preheater tubing to thermally e quilibrate the eluent to the column temperature. Consequently, the use of n arrow-bore columns at high-temperature improves analysis speed and efficien cy over wider bore columns. We also discuss the advantages of using liquid heat-transfer media as compared to air as the heat-transfer media. We show that an air bath ought not be used to heat the mobile phase because at high temperature (> 80 degreesC) and high column linear velocity (>1.5 cm/s) th e length of tubing needed to heat the mobile phase to column temperature is prohibitively long. Using accurate, empirical heat-transfer correlations, we estimated the length of tubing needed to heat the eluent as a function o f the column linear velocity for both air and liquid heat-transfer media.