Polyimide coated fused silica
capillary tubing is widely used in the separation sciences. This
application note presents perspectives on the analysis of defects
found on capillary end-faces and the application of those findings
toward improved life time during usage.
Introduction
Fused silica capillary tubing used in
Gas Chromatography (GC) is produced in long lengths and then
provided to column manufacturers who employ a series of proprietary
techniques to produce finished GC columns used in laboratories
around the world. Whether one is manufacturing the column or using
it in the laboratory for sample analysis, breakage of the capillary
is never a welcomed event. Analysis of the end-face of a break can
lead to valuable information in an effort to minimize the frequency
of such occurrences. Elemental spectral analysis of any debris
found near the surface flaw can provide added detail.
The primary objective of the end-face
analysis is to determine the root cause of the material failure that
lead to the breakage of the capillary tubing. Breakage is
predominately traceable to a surface flaw on either the glass i.d.
or o.d. Knowing which surface was damaged can be instrumental in
the effort to find the root cause and take the appropriate
corrective action.
Experimental
Images of end-face samples were
collected by SEMTEC, Inc. (Phoenix, AZ) using a Jeol 6100 Scanning
Electron Microscope (Jeol Ltd., Tokyo, Japan). The secondary
electron mode of operation was employed. Although a number of
samples have been examined over the years, those shown in Figure 1
were actual samples provided by Polymicro customers. Samples were
readied for analysis by standard SEM preparation techniques,
including gold sputtering of the capillary end-faces.
Results
Surface flaws on both the glass i.d.
and o.d. have been found on many occasions. Figure 1a presents the
end-face of a break caused by a typical i.d. flaw. Figure 1b shows
a corresponding image of an o.d. flaw. Of particular interest is
the pattern of the flaw, which usually comprises two components
referred to as the mirror and hackle. The mirror portion penetrates
the surface to a depth of 5-10 µm and is semicircular in shape. It
is smooth in appearance and is formed when the surface is initially
damaged. When the capillary is subsequently stressed, typically by
bending, this mirror-smooth flaw propagates outward in a series of
jagged fractures referred to as the hackle. This fracturing
ultimately results in breakage of the tubing. This mirror-hackle
pattern can be seen in both figures.
The most common cause of i.d. flaws
is glass debris from cleaving; the importance of which has been
discussed previously (1). Introduction of debris from fittings,
connection lines, or unfiltered solvents is less common, but has
been verified on occasion. Debris is also the most common cause of
o.d. flaws. The break shown in Figure 1b was caused when debris
penetrated through the protective polyimide coating and contacted
the glass surface. Elemental spectral analysis confirmed that the
embedded debris was a piece of fused silica, most likely from a
dirty work surface. Cleanliness of the work area and any equipment
that contacts the capillary is critical (2). Determination of where
the failure began greatly aids in taking the appropriate action to
minimize breakage.
Conclusion
End-face analysis of damaged capillary is an important tool in
determining the root cause of capillary breakage. Glass debris from
poor cleaves or dirty work surfaces have been found to be the most
common sources of breakage.
References
-
“Cleaving
Procedure”, The Book on the Technologies of Polymicro,
Polymicro Technologies LLC Publication, p.A-2, (2005)
-
J. Macomber
and L. Begay, LCGC Application Notebook, 72 (Sept 2003).
