June
2002
Light Guiding Fused Silica Capillary Tubing
By Joe Macomber & Gary Nelson
As printed in LCGC, The APPLICATION NOTEBOOK -- June 2002, Page 48
On the Web:
www.chromatographyonline.com/lcgc
Synthetic fused silica capillary tubing is widely used in the
separation sciences, including the popular fields of Gas
Chromatography, Capillary Liquid Chromatography, and Capillary
Electrophoresis. From its advent in the late 1970's, steady advances
in product quality have emerged. Light guiding fused silica capillary
tubing is the latest step in this continuing development.
Introduction
Improved purity of the synthetic fused silica, more durable external
coatings, and tightening of specification tolerances are cornerstones
in capillary tubing advancements. Researchers have successfully used
capillary tubing filled with a high refractive index liquid to form an
optical wave-guide. In one specific application, a low refractive
index, external fluoropolymer coated capillary was employed in a Raman
spectroscopy device1. Although a viable product for light guiding
applications, fluoropolymer coated capillary has never achieved the
low cost or durability desired. A non-liquid filled light guiding
capillary with a durable polyimide coating has been developed to fill
this void.
 Experimental Design
The tubing preform consists of a standard high-OH silica tube
over-clad with a layer of fluorine doped synthetic fused silica. The
tube interior is not modified from the pure fused silica surface as on
standard capillaries. This preform was then drawn using standard
operating procedures into capillary tubing. A polyimide coating was
applied, yielding products with dimensional specifications essentially
identical to the corresponding standard capillary products.
Results
It is clear from casual inspection that this capillary tubing embodies
the light guiding properties of a fiber, yet retains its basic tubing
design, internal surface chemistries and high strength. Figure 1 is an
image of the end of a LTSP250350 light guiding capillary tube, being
illuminated on its opposite end by a white light source.
Although additional testing continues, initial results indicate that
attenuation (with an air filled interior) is similar to a standard
high OH multimode step index optical fiber. Initial attenuation
results indicate values of ~20dB/km at 500nm and values < 15dB/km at
800nm. Several sizes have been successfully produced, including ID/OD
combinations (in µm) of: 50/150, 50/375, 75/200, 150/375, & 250/350.
No glass surface chemistry differences have been seen and none are
expected. Depending on the substance(s) introduced to the capillary
interior the light guiding properties can be modified significantly.
Conclusions
Light guiding capillary offers a new horizon in capillary tubing
applications, including:
Unique sample detection & analysis.
Increased path-length detection cells.
Evanescent wave optics.
UV transmission inside of capillary allows for coaxial light-activated
chemistries, curing of UV adhesives, & detection of light producing
reactions i.e. fluorescence.
Chromatographic columns with new frit technologies for improved
durability.
This latest advance in tubing technologies should prove to bring
exciting opportunities to both old new capillary techniques.
References
1 D. Che and S Liu, Long Capillary Waveguide Raman Cell, U.S. Pat.
5,604,587, 1997
About Polymicro
Technologies, LLC
Explore the
Capabilities™ Located in Phoenix, Arizona, Polymicro Technologies, LLC
is the world's leading manufacturer and supplier of silica capillary
tubing and specialty optical fibers, optical fiber and capillary
assemblies, discrete micro components and quartz optical fiber
ferrules. Since 1984, Polymicro Technologies has served the
analytical, medical, aerospace, military, manufacturing,
telecommunication and communication industries. Polymicro Technologies offers a total manufacturing solution,
providing initial product design, product & process development,
prototyping & beta trials and volume production.
Website:
www.polymicro.com
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