In spite of its relative maturity as a group of
analytical methods, the field of liquid chromatography
continues to evolve with regard
to the development of new stationary
phases that may provide certain advantages
over existing materials. Stationary phases in
the form of inorganic or polymeric fibers
have been a curiosity of separation scientists
for many decades in the hopes of providing
highly directional mobile-phase flow with
minimal resistance.1–5
In principle, fibers can be extruded from
silica-based melts so that subsequent
derivatization chemistries can be performed
on the surface. Likewise, polymer
blends having specific functionality can
be employed as well. While the chemical
aspects of using micron-diameter fibers
have been realized in terms of performing
reversed-phase, ion-exchange, and capillary electrochromatography separations,
the performance-to-cost ratios have never
been such that there has been serious
commercial interest in developing fibrous
stationary phases. In particular, poor
repeatability and control of the packing
process, unexpectedly high flow resistance,
and relatively low specific surface
areas are the shortcomings typically cited.
A novel polymer fiber format is being
developed in the authors’ laboratory in an
effort to take advantage of the expected
benefits of fibrous stationary phases.6,7
Capillary-channeled polymer (C-CP)
fibers possess a very unique geometry (see
Figure 1a) that includes eight channels
that extend the entire fiber length (which
can be miles on a spool). The fibers are
nominally an oblong shape with diameters
ranging from 35 to 50 μm, with the individual
channels ranging in size from 5 to
20 μm. This class of materials was developed
for commercial applications wherein
spontaneous fluid movement (wicking) is
desired (e.g., disposable diapers, sanitary
napkins and tampons, and sportswear).8
For example, 1 g of polyester C-CP fiber
can wick 100 mL of water in 1 hr.
Figure 1 - Micrograph of C-CP fibers. a) Scanning electron micrograph of a polypropylene fiber.6
(Reprinted with permission from Elsevier.) b) Optical micrograph of a polyester fiber bundle.
These fibers have another very important
attribute. Depending on drawing conditions,
C-CP fibers have surface areas that
are 2.4–3.0 times those of conventional
circular cross-section fibers. The combination
of enhanced fluid mobility
and increased surface area is critical
to the potential use of C-CP fibers
as chromatographic stationary
phases. At this point in their development
as chromatographic stationary
phases, a few definitive
qualities have been identified and
are listed in Table 1. This paper presents
some basic properties and
examples of the use of the C-CP
fiber columns for chromatographic
separations as well as projected
areas of practical application.
Experimental
To date, three base polymer types
have been employed for chromatographic
separations: polypropylene
(PP), polyethylene terephthalate
(polyester, PET), and nylon. As seen in the structures below,
solute–surfaces interactions can take place
based on the polymer functionality. As PP
presents a totally aliphatic surface, PET
provides aromatic and carbonyl functionality,
and nylon includes amine and carbonyl
groups on an alkyl backbone. Atomic force microscopy (AFM) measurements
performed with native and carboxylated
polystyrene beads attached to the
AFM tips revealed the strongly hydrophobic
nature of the PP and PET fiber surfaces
(nylon was not available at the time).
The types of polymers that can be formed
into the C-CP structures are quite diverse.
The primary qualification here is that the
polymer solution must be amenable to a
melt-spinning process. In order for a polymer
to be melt-spinnable, the melt must
be sufficiently stable over a broad temperature
range. In a semicrystalline polymer
such as PET, the processing range extends
from the melting transition (Tm) up to the
degradation temperature. For glassy polymers
such as polysulfone that cannot crystallize,
the processing range will extend
from above the glass transition temperature
(Tg) to the thermal stability limit.
Examples of common classes of polymers
that fit into this general category include
polyamides, polyesters, polyesteramides,
polyolefins, polyetherketones, polyacrylates,
polysulfones, polycarbonates, and
polyurethanes. Finally, many copolymer
combinations are available that can be
melt-spun into C-CP fibers.
In addition to the surface chemistry affected
by the base polymer, there is a wealth of
fiber postprocessing that can be employed
to generate more specific surface characteristics.
For example, PET fibers were treated
in caustic solution to create a surface that is
anionic in character by oxidizing the carbonyl
groups to carboxylate functionalities.
In chromatographic terms, a reversed-phase
surface has been converted to a
cation-exchange surface. It must be stressed
here that the chemical change is not
achieved by coordinating a functional
group to the surface as in the case of derivatized
silica, for example. As such, a more
robust surface is created. The polymer base
materials also add a great level of chemical
resilience to pH extremes not seen in silica-based
supports where silica dissolution (typically
at pH >7) and other forms of surface
degradation/hydrolysis (pH <2) are limitations.
9 pH stability is the major driving
force in the use of polymer beads as supports
for biomolecule separations.10 In addition
to the choice of polymer base to achieve
specific separation chemistries, there is
some flexibility in the physical aspects of
the C-CP fibers. For example, the nominal
diameter of the fibers, and thus the channel
diameters, may be altered through the use
of different fiber drawing conditions. There
are also some alternative methods that may
be employed to add porosity (i.e., surface
area) to the fibers while still maintaining
the basic capillary structure.
The general process of column packing is
depicted in Figure 2. Columns were prepared
using stainless steel tubing with standard
commercial HPLC dimensions
(inner diameter and length). Additionally,
capillary formats were created in polymeric
tubing and glass capillaries. Columns are
prepared by winding the fibers onto a spool
mounted on an axle assembly, fitted with a
handle and an analog counter. When the
desired amount of fiber (i.e., the number of
fibers in the column) is obtained, the fibers
are removed from the spool as a loop. A
monofilament is then passed through the
fiber loop and fed through the column and
the fiber bundle pulled through, leaving a
short portion of the fiber sticking out of
each end of the column. Depending on the
polymer material, there is a potential for
fiber shrinkage in different organic mobile
phases. Interestingly, this shrinkage only
occurs along the main fiber axis, and not in
the radial direction. Therefore, columns
are soaked in acetonitrile for 30 min to
remove any organic contaminants and to
preshrink the fibers. The fibers are then cut
flush with the column ends and standard
HPLC endfittings are employed to mount
the columns, just as for commercial packed
columns. The fiber packing density is such
that frits are not required to retain the stationary
phase. In fact, standard frits can be
the major source of backpressure when
used on C-CP fiber columns.
Figure 2 - Diagrammatic representation of the C-CP fiber column packing procedure.
Figure 1b is an optical micrograph of a fiber
bundle showing the packing arrangement
likely present within the chromatographic
columns. Clearly seen is the interdigitating
of the fiber “fingers” as opposed to the perhaps
expected crushing of the capillary
walls. The primary limitation on the number
of fibers that can be pulled through a
column is the friction between the outermost
fibers and the inner wall of the tubing.
At the extreme, either the mechanical
force is too great to pull the fibers through,
or the monofilament (typically 50-lb test
line) used to pull the fibers breaks. In a
comparison of flow characteristics with
commercial 5-μm porous bead-packed
columns of the same dimensions (4.6 mm
i.d. × 250 mm long), C-CP fiber columns
operated over a flow range of 1–9 mL/min
with an upper backpressure of <2000 psi,
while the particle-packed columns reached
the maximum system pressure of 4000 psi
at 4 mL/min.11,12 The overall interstitial
fraction values are somewhat better than
expected for porous silica columns, with
the specific permeability values being quite
comparable. To be fair, these favorable
hydrodynamic characteristics are achieved for a stationary phase of fairly low specific
surface area (~3 m2/g) versus porous beads
(>100 m2/g). As such, the efficient flow
characteristics of the C-CP fiber columns
must be used to advantage in columns of
greater length to achieve high plate numbers
in isocratic separations.