Chromatography Forum

I have some 1/16" OD stainless steel capillaries handy which are meant to be used as mobile phase pre-heat tubes in high temperature liquid chromatography. A trivial problem is that the internal diameters (i.d.) of the capillaries are unknown. Could anyone give me some advice how to measure them conveniently? Thanks in advance for any info.

Take two capillaries of known diameter and the same lenght, use them as loop in a manual injector valve. Inject a well absorbing substance (0.1 % acetone in water) and overfill the loops, then inject into a UV detector at 254 nm at low flow rate (0.2 mL/min or so, depending on the length of the capillary). Measure the duration of the acetone peak of the known diameters and compare with the unknown one.
Or, if you have a really good analytical balance, you can weigh the empty capillary, then fill it with water, weigh again and calculate mg = µL.

Most capillaries have standard IDs such as 0.12mm , 0.17mm , 0.25mm , 0.5mm and 1.0mm.

You may even compare some of them ( 0.25mm , 0.5mm and 1.0mm ) by naked eye .
Use a magnifying glass for smaller IDs.

The easiest is to put them under a stereo microscope with a measuring graticule.

Slightly more work; weigh them, fill them with water (or better mercury), weigh them again, calculate the mass of liquid, from that and the density calculate the internal volume, from that and the length calculate the cross sectional area and from that calculate the diameter.

Peter

(Very) small modification of bunnahabhain's advice: you don't have to use them as injection loop, only connect unknown capillary to empty capillary with known ID. Inject (acetone) and repeat it again with only capillary with known ID. You might find volume of uknown capillary from elution volumes differences and then calculate ID.

Why don't using a cheap (~20 EUR) USB-microscope to determine the inner diameter. Is the inner diameter really that inhomogeneous for steel capillaries?

Thank you so much for those good tips!