The most common geometry for capillary is circular, or round. Non-circular geometries, such as square cross-section capillary, have been produced in response to customer requests in CE and Flow Cytometry. Rectangular designs are popular as ferrules in telecom devices.  Other geometries, such as triangular, oval, and elliptical designs are being explored for cutting edge applications.  Custom configuration cross-sections can be produced, including multi-element tubing, and in some cases tubing with a polygon inside and/or outside.

Of the many geometric variations possible, square cross-section tubing has seen specific interest, and a line of standard products, referred to as WWP, is available. Polymicro’s standard “Square” tubing has square internal channels of 50mm, 75mm or 100mm. This product was developed as a solution to a customer problem, and also solved some optics issues associated with using circular silica capillary tubing in Capillary Electrophoresis.

Reported Advantages of Square Capillary:

• Creates 27% more volume and 2X the transverse optical interaction path length (using a collimated beam) when compared to a round capillary configuration.
• Eliminates the need to correct for the optical effect created by curved capillary surfaces.
• Offers larger effective internal surface area.

The optical advantage of reduced scattering and increased interaction volume is illustrated in the diagram shown in Figure 3-2.  Note that, when using square tubing with a collimated beam the refractive index of the internal fluid does not effect instrument focus.  Further, the collimated beam is not focused by a lens effect of the circular capillary.