There are few methods in the literature to measure the inner diameter of very small capillaries. Although silica capillaries are more commonly used, synthetic polymer capillaries are preferred in some applications. The technology for producing them is not as mature. Aside from the absolute value of the inner diameter, the circularity, concentricity (a quantitative index is defined here for the first time) and the bore uniformity of such capillaries are of interest. Beyond microscopy, we describe multiple methods that determine the capillary inner diameter, averaged over a given length. The measurements variously depended on the capillary internal volume, length and cross section, and the resistance to fluid flow. The different approaches produced mutually consistent results. We show that when the internal diameter is not uniform, the different dependence on diameter that two such methods may exhibit, can be exploited to determine the true mean diameter as well as its variance. Finally, for open tubular liquid chromatography, where performance acutely depends on the inner diameter, we surprisingly find that while the mean i.d. may be the dominant determinant of efficiency, bore variance has little to no effect on the performance.
Keywords: Bore uniformity; Capillary diameter; Circularity; Concentricity; Open tubular liquid chromatography.
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