A review of the optical properties of capillary involves close examination of not only the glass substrate itself, but also the protective coatings often employed. These will be discussed further below. Optical properties such as transmission, fluorescence, refractive index, and in some cases numerical aperture are all important to consider. It should be noted that capillary is used as a substrate for liquid light guides and hollow silica waveguides. Further, if appropriate coatings or claddings are added, capillary can embody both fluidic and fiber optic properties simultaneously such as in Polymicro’s light guiding capillary.

When examining transmission of light through Polymicro's standard capillary products, it is important to consider both the direction of light propagation and the silica type. Since non-axial, or orthogonal, transmission (i.e. on-column detection schemes) is normally of key interest, the extremely thin walls of capillary make the actual transmission losses in the silica negligible. However, this is dependent upon the type of silica used to make the capillary. Polymicro has selected an appropriate -OH content glass for applications in the deep UV and Visible spectral regions.

Fused silica glass, especially the lower -OH types, can exhibit fluorescence when illuminated with 254nm light passed through a Schott UG-5 color glass filter. From Heraeus data, this apparently does not occur in the high -OH materials. High -OH fused silica glass is essentially free from visible fluorescence at excitation wavelengths greater than 290nm. Slight blue-violet fluorescence appears in materials such as natural fused quartz products, and significant fluorescence is observed in most borosilicate glasses. It can be concluded that fused silica is indeed the material of choice for most capillary products.

The refractive index of the fused silica used to make capillary is often of interest to researchers. The actual value is wavelength dependent. A table of fused silica refractive index versus wavelength can be found in the Appendix. On occasion, the numerical aperture of capillary is sought. It is of course dependent upon the gas, fluid, or coating that has been applied to the capillary surface. A discussion of numerical aperture and related topics is found in Fiber Optics & Optical Fiber.

As most capillary is coated with a thin layer of polyimide, this material will receive consideration here. Polyimide transmission varies significantly from that of the silica. Standard polyimide is relatively translucent down to about 550nm, with select custom polyimides exhibiting as much as 90% transmission at 425nm. All polyimides evaluated have shown less than 2% transmission below 350nm. This fact, when coupled with polyimide’s inherent fluorescence, has lead to the wide spread use of windowed capillary for on-column spectral analysis applications. Polyimide exhibits significant fluorescence across a broad range of excitation wavelengths. This has been studied and is summarized in a Polymicro application note.⁵ The polyimide coating on capillary has fluorescence essentially equivalent to that of a 1mM solution of Rhodamine B in a 50mm ID capillary.

As mentioned earlier, Polymicro does employ other protective coatings, and their optical properties vary significantly from polyimide. Acrylate coated capillary (TSA products) are more translucent in general and offer lower fluorescence. Teflon® AF coated capillary (TSU products) are produced specifically for their unique optical properties. This fluoro-polymer coating is UV transparent, with transmission at 214nm typically greater than 90%. The refractive index yields an NA of 0.66 and this, coupled with the low absorbance properties of the coating, make it an excellent optical cladding material. A number of applications take advantage of this by using TSU products as a light-guiding capillary. Others capitalize on the transmission properties by using TSU for on-column detection applications, as removal of the coating is not required. Unfortunately, these unique optical properties are offset by the lack of abrasion resistance of the fluoro-polymer. TSU products must be handled with care to avoid breakage during use.

In addition to fluid handling capabilities for analytical instruments, capillary tubing can be used as a waveguide. In hollow waveguides, the internal surface must be coated with an appropriate dielectric coating for the wavelength of interest. In this hollow core configuration, both metal and dielectric coatings have been applied to create a waveguide for wavelengths further into the infrared than would be possible with conventional optical fiber.

It is also possible to use silica capillary tubing as the cladding for a fluidic waveguide, or liquid light-guide. As long as the fluid has a refractive index higher than the capillary, a very effective light guide can be produced.

⁵ Macomber, J. et.al., LCGC North America, June Suppl. (2004) 72

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