Go into nearly any gas chromatography (GC) lab and you will see shelves or cabinets stocked with columns; new, used, favorite,
and even broken ones will be on display. Yet in the midst of an abundance of columns, many chromatographers take them for
granted. In the years since 1979, when fused silica became generally available to the chromatographic community as a column
tubing material, the art of making capillary columns has advanced tremendously. Today's fused-silica columns, while outwardly
indistinguishable from all but the earliest predecessors, deliver both general and application-specific separations with much
higher mechanical stability, much lower column bleed levels, greatly enhanced inertness, and a high degree of column-to-column
reproducibility.
In this installment of "GC Connections," we'll review some of the developments in fused-silica column technology and then
take a brief tour through the process of column manufacturing.
Flexible Capillary Columns
In June of 1979, I was about to finish graduate school when word came of the presentation by Dandenau and Zerenner (1) at
the Third Hindelang Symposium, held in April that year, in which they discussed their development of flexible capillary columns
made of fused silica. Dandenau and Zerenner recounted their experiences in an LCGC article in 1990 (2). As far back as 1960, the chemical advantages of silica over softer borosilicate or sodium glasses as
a column material had been recognized (3), but the difficulties in working at the much higher temperatures required to soften
silica for drawing out and then coiling rigid silica capillaries had inhibited much progress up to 1979 (4,5). Dandenau and
Zerenner's breakthrough was the successful application of optical fiber technology to the production of flexible fused-silica
capillary tubing. With access to Hewlett-Packard's optical fiber facilities, where the production of step-refractive-index,
silica-clad optical fibers utilized hollow fused-silica tubing preforms, they made the leap from flexible fibers to flexible
capillaries.
John V. Hinshaw
I had personally drawn and coated innumerable rigid glass capillary columns in the course of completing my research, and so
I was very interested in the properties of flexible fused silica; especially considering that I might have saved all those
midnight hours nursing a cantankerous glass capillary drawing machine. Early in the summer of that year, a 30-m length of
the new capillary tubing found its way into the lab, and as the senior GC student, I was elected to coat the tubing with a
stationary phase and give it a try. I did so, using one of the chiral organosiloxane polymers from my research. I carefully
cut the ends of the column with a razor blade (the "proper" cutting techniques for fused silica were completely unknown to
me) and installed it in the GC inlet and detector. I set up the carrier gas pressure and split flow, lit the flame ionization
detector, closed the oven door, and heated the column to about 100 °C. Upon injecting a test mixture, I was thrilled to see
the expected peaks emerge, and I repeated the test several times. Then I turned off the oven heater and opened the oven door,
which turned out to be a fatal mistake for the column: it broke into about 50 pieces! Everywhere that the column had been
in contact with its metal cage created a stress point that broke as the column cooled off.
Fortunately, I was already set to graduate and did not need any more experimental results. Less than a year later, now employed
at an instrument company, I was thrust into the midst of the rush on the part of column producers to adapt their processes
to the new material. I was tasked with determining the feasibility of making fused-silica columns in-house and with comparing
that route to the purchase of columns from an outside supplier. At the time, there were no fused-silica column suppliers with
any track record, and so the decision was not trivial, especially considering that nearly all suppliers of glass columns were
engaged in essentially the same pursuit.
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