Towards Rapid Open
Tubular Liquid Chromatography
 main
 HPLC'96 |
Presented at HPLC'97
- 21th International Symposium
on High Performance Liquid Phase Separations
Birmingham, June 1997
|
Contents
Top | End
Abstract
We have recently shown the feasibility of rapid chromatographic separations
in open tubular serpentine columns of 100-250mm
i.d. [1]. The use of comparatively large column diameters
is permissible thanks to enhancement of native diffusion mass transfer
with a secondary mechanism of radial mixing. It is at high linear velocities
of flow that radial mixing occurs and hence open tubular serpentine systems
are inherently rapid. Rigorous mathematical treatment of the flow patterns
in shaped tubes appears consistent with the hydrodynamic studies. Serpentine
and helical tubes are compared from the point of view of band broadening.
We introduce an entirely novel concept of forming RP type stationary
phases in situ by the means of spontaneous self-assembly of a suitable
substrate on a modified column wall. In particular, it appears that the
chemical properties of the underlying substrate stop manifesting themselves
once the self-assembled monolayer reaches the thickness of (arbitrarily)
a few carbon atoms. In practice this means that a high quality RP material
can be produced on any substrate which will self-assemble. Thus obtained
RP systems are probed with the techniques of atomic force microscopy (AFM)
and X-ray photoelectron spectroscopy (XPS).
The limitations and requirements of the technique are discussed. It
is hoped that a limited use of standard chromatographic equipment will
be possible.
Contents | Top
| End
Key issues for rapid OTLC
-
Highly efficient open tubular GC is successfully performed in 530mm
i.d. (and larger) columns.
-
Diffusion in liquids is some 5-7 orders of magnitude slower than in gases.
-
Thus, in LC, the parabolic flow profile of the liquid pumped through an
OT column manifests itself in severe band broadening.
-
If the inner diameter of the OTLC column is (arbitrarily) below 10mm, the
radial diffusion is fast enough to alleviate the deleterious influence
of the parabolic flow profile (because any given fluid element can statistically
be present at any radial location at any given time).
-
However, for highly efficient OTLC columns above 10mm i.d., a secondary
mixing mechanism (radial swirl) must be provided to aid radial diffusion.
-
Shaped (twisted, deformed, helical, serpentine and knitted) tubes provide
such radial mixing mechanisms at elevated linear velocities of the flow
(Figure 1).
-
Radial mixing improves with the linear velocity of the eluent and depends
on its kinematic viscosity, i.e. the onset of radial mixing in e.g.
hexane takes place at a considerably lower linear velocity than in water
(Figure 2, Figure 3 and Figure
4).
-
Radial mixing in helices, serpentines and knitted tubes improves with increasing
the aspect ratio - the ratio of tube inner diameter to the diameter of
the curvature (Figure 5 and Figure
6). This work is mainly concerned with serpentine tubes up to 250mm
i.d.
-
Since high linear velocities of eluent are required (10-100cms-1),
the shear stress at the walls is of considerable magnitude.
-
Consequently, the stationary phases need to be chemically bonded to the
column walls in order to avoid stripping.
-
Because of high linear velocities, the interphase interactions have to
be rapid. This, in effect, limits the thickness of the stationary phase.
-
It follows, that any localized active centres in the stationary phase will
lead to very pronounced tailing.
-
Secondly, the mass loading is limited due to modest phase ratio.
-
However, self-assembled monolayers (SAMs) fit the stationary phase requirements
well.
-
SAMs are ordered molecular assemblies that form spontaneously on immersion
of an appropriate substrate into the organic solution of an active reagent.
Two of the known self-assembling systems include alkanethiols on gold and
silver (Figure 7).
-
In order to characterize the self-assembling layers, three distinct interactions
need to be considered: chemisorption of the head group on the surface of
substrate, inter-chain van der Waals interactions and thermal disorder
of terminal functionalities (Figure 8).
-
Due to the relatively high energies of chemisorption (ca 40-45kcalmol-1),
assembling molecules occupy every available binding site forming an ordered
and close-packed organic layer. Such monolayers cannot be washed off with
solvents and terminal functionalities can be chemically modified without
any apparent damage to the system (Figure 9).
-
Thus tailor-made RP-type stationary phases can be produced on the inner
walls of formed steel tubes by coating the walls with gold (by pyrolysis
of gold organometallics) or silver (from Tollens reagent) followed by spontaneous
self-assembly of an appropriate thiol.
Contents | Top
| End
Curved tubes
Figure 1. Serpentine tubes and a helix formed in AISI 316 stainless steel
tubing 1/32'' o.d., 125mm i.d. (Valco).
Contents |
Top | End
Onset of radial flow
Figure 2. Onset of secondary flow. The illustration of pressure differential
driving the radial swirl. The secondary flow is a result of competing inertia,
centrifugal forces, tangential pressure drop and friction.
Contents
| Top | End
Radial flow at moderate linear velocities
Figure 3. Diagrammatic illustration of radial flow at moderate linear velocities.
The pressure differential overcomes the inertia and friction and the liquid
begins to swirl. The cross section of the tube is divided into two domains
with centers of swirl near the centres of gravity of the semicircular sections
of the cross section of the tube. The lines of constant velocity are spaced
evenly around the centres of swirl. The flow profile deviates from the
parabolic profile of pumped flow in a linear tube.
Contents
| Top | End
Radial flow at high linear velocities
Figure 4. Diagrammatic illustration of radial flow at high linear velocities.
The centres of swirl move first up and then down in an arc towards the
outer wall of the curvature of the tube. The lines of constant velocity
are packed much closer together at the outer wall of the curvature than
anywhere else. It is therefore enough for a molecule to diffuse a small
fraction of the tube radius in order to be transported into an area radially
distant from the point of origin. Any given fluid element can statistically
be present at any radial location at any given time (in an analogy to linear
capillaries below (arbitrarily) 10mm i.d.).
Contents
| Top | End
Helices vs serpentines
Figure 5. Pressure contours for three types of curved tubes: helix, ideal
serpentine (consisting of semicircular sections 0f 180o curvature)
and steel tube formed into a serpentine (consisting of sections closely
approximating these of a 135o section of a circle, vide
Figure 1.). The aspect ratio of the tubes is 1:10. The flow is that of
water at 50cms-1. The model is scalable. The contours were obtained by
iteratively solving the Stokes-Navier equations in a periodic mass flow
model (of a developed flow as opposed to transients). The pressure image
of the cross-section of the helix is constant. The pressure image of the
serpentine changes as the flow progresses along the curve of the bend.
Downstream by PR, it is a mirror image
of the one shown here. Because of the frequent changes in the direction
of swirl, the serpentines produce less band broadening per unit length
then helices of the same curvature, even though the magnitude of swirl
in helices is larger. The pressure images shown for serpentines are at
maximum deflection - the changes in pressure differential are shown in
Figure 6. Larger_image_(1300x720).
Figure 6. Graphs of relative static pressure differential (driving the
swirl) along the curves of the tube. The profile is constant for the helix,
the curves for serpentines demonstrate the collapse and re-establishment
of swirl at the changes of curvature directionality. The wall texture on
the tube (effectively, a deformation) produces pressure gradients which,
in turn, decrease the band broadening somewhat at high linear velocities.
The aspect ratio of the curved tubes is 1:10. The flow is that of water
at 50cms-1. The model is scalable. The contours were obtained
by iteratively solving the Stokes-Navier equations in a periodic mass flow
model (of a developed flow as opposed to transients). Larger_image_(1030x620).
Contents
| Top | End
Stationary phases
Figure 7. Atomic force micrograph of passive electrolytic gold coating
on steel surface. Larger_image_(730x480).
Figure 8. Illustration of self-assembled thiols on gold. The carbon chains
are canted to the surface and remain tightly packed in an all-trans conformation.
Contents |
Top | End
Phase stability
Figure 9. Demonstration of chemical stability of a SAM system. Glass beads
(mean radius 5mm, surface area ca 0.25m2g-1
were surface-coated with silver via deposition form Tollens reagent and
subjected to dilute (20mM) anhydrous solution of dodecylthiol in anhydrous
ethanol. The resulting material was packed into a column and subjected
to 72h of methanol/water (70/30) under chromatographic conditions. Samples
from before and after chromatography were analysed for organic content
by thermal gravimetry (TGA). The weight loss is the same for both samples
and consistent with a monolayer of thiol. Larger_image_(1030x620).
Contents
| Top | End
Acknowledgements
We would like to acknowledge the funding from EPSRC; Ray Scott and Peter
Myers for useful discussions, John Warrack and Ian Marshall for AFM, XPS
and Auger spectroscopy, and Marianne
Odlyha and ULIRS for
TGA.
Contents
| Top | End
References
-
Grochowicz, P.R.; Simpson, C.F. Towards Rapid Chromatographic
Separations in Open Tubular Serpentine Columns; presented at HPLC'96, San
Francisco, June 1996; transcript on http://www.bbk.ac.uk/Departmentd/Chemistry/staff/prg/HPLC96.html.
main