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In our research on open-tubular liquid chromatography we frequently need to accurately measure the internal diameter and length of capillaries of 2 to 30 micrometer diameter. For our purposes, an optimum diameter measurement technique would: 1) be simple, fast, and inexpensive;2) be as accurate as practically possible (at least 1.0%);3) give a measured diameter which is an average value over the 4) be non-destructive.We first tried optical microscopy and scanning electron microscopy. Both of these techniques produce a "site-specific'' diameter, not an average diameter. In order to ensure getting a good value for the average diameter, one must either assume the diameter does not vary along the capillary length, or break the capillary at several places and make measurements of all these diameters. This second approach, of course, destroys the capillary. In addition, both scanning electron microscopy and optical microscopy produce measurement errors i f a) the sample cut axis is not perfectly perpendicular to the longitudinal axis of the capillary; b) if the cut face is not perfectly planar; orc) if the capillary longitudinal axis and the microscope optical axis are not perfectly aligned. Case "a" produces an ellipsoid image whose minor axis is the true diameter. Case "c" also produces an ellipsoid image, but the major axis is the true diameter in this case. The image observed by either microscopic technique is a composite of these two errors. When combined, neitheraxis measurement is accurate and the errorscan not be resolved. An 8 O misalignment in either case is sufficient to produce a 1% error in the measured diameter. In terms of resolution, optical microscopy is fairly poor (1 pm at best) and is unacceptable for our purposes. Scanning electron microscopy has sufficient resolution but is expensive, and sample preparation is time-consuming.Our lab has devised an alternative diameter measurement method which fulfills all of the desired criteria we have listed, and which provides, simultaneously, an accurate measurement of the capillary diameter and length. The method used in our laboratory requires that the capillary be filled with mercury. The electrical resistance of the mercury plug is measured and then the mercury in the capillary is expelled and weighed. The measured resistance, R. will be: (4)Both the density and resistivity of mercury can be obtained from the literature with betterthan0.001%accuracy(d2O0 = 13.5462gl mL, p200 = 95.783 microohm-cm) [l]. Using a microbalance and a digital ohmmeter, radius and length are easily measurable to better than 1% accuracy. By this technique, a near average value for the capillary diameter is obtained.In our work, the capillaries were filled using mercury in the reservoir of a bomb we have previously described [2]. We found that electrical contact between the Hg reservoir in the bomb, the Hg plug in the capillary, and an outlet reservoir of Hg could not be guaranteed unless pressure in the bomb was maintained (i.e. Hg flow through the capillary was maintained). We fo...

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