Modern HPLC equipment have much lower extra-column band broadening contributions than conventional instruments of the late last century. Modern columns provide low extra-column band dispersion in the flow rate range between 0.1 and 5 mL/min but with that it is impossible to avoid a certain loss of the efficiency.  In practice, particularly under isocratic conditions, extra-column band broadening contributions and the heat friction effects are not negligible with 2.1 mm I.D. columns.

An easy solution to the heat problem is a further decrease of the column diameter, down less than so that the heat power generated per unit column length is sufficiently low at high linear velocities. The performance of the new UHPLC instruments not being consistent with the requirements of a generation of still narrower columns, 0.5 or 1 mm in diameter, Microflow UHPLC instruments are becoming popular among those interested in fast, high resolution separation methods for complex mixtures, especially available in small size samples.

There is little data available on the role of devices that are of primary importance in the measurements of the peak variances of sub-1 mm I.D. columns.  Besides the obvious reduced solvent usage, there are several inherent advantages performing chromatography in smaller ID columns. Less required mixing volume as a fraction of column volume allows for faster gradient separations. A lower flow resistance factor means the same separation can be run at lower backpressure.

In this paper we report on theoretical calculations and experimental results to demonstrate these advantages with columns of 0.5 mm ID. Particular attention will be given to the issue of frictional heating. A simplified model that provides a basis to evaluate the effects of frictional heating in UHPLC will be presented.

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