압축형 액체그로마토그래피 장치에서 층의 효율과 충진 특성

고준호*, B. Scott Broyles□, Georges Guiochon□

한전 전력연구원*, 테네시대학교 화학과 및 오크리지 국립연구소□

Packing Characteristics and Column Efficiency

of a Dynamic Axial Compression Column

Joon-Ho Koh*, B. Scott Broyles□, Georges Guiochon□

Korea Electric Power Research Institute*

Department of Chemistry, University of Tennessee, Knoxville, TN 37996□

Analytical Chemistry Division, Oak Ridge Nat□l Lab, Oak Ridge, TN 37831□

Introduction

Packed columns in liquid chromatography are not homogeneous in both the radial and axial directions [1].  Inhomogeneous column properties cause a loss of efficiency [2].  Column packing process is one of the key variables to determine packing properties.  Slurry packing and dynamic mechanical compression in the axial or the radial directions are the most widely-used packing procedures.  The former is more frequently used for analytical columns, and the latter for preparative columns.

The present study is concerned with the behavior of a preparative-scale chromatographic column under dynamic axial compression.  Dynamic axial compression columns are huge cylinders where a piston is moved by hydraulic force and packing materials are consolidated by this mechanical pressure.  The packing process of dynamic compression columns is similar to powder compaction mechanism [3, 4].  The information regarding the axial distribution of packing properties can be explained by a combination of column entrance effect and wall effect [5].  The column length seems to be more important in dynamic axial compression columns than conventional columns, because the end of the column is constantly under mechanical pressure.  Our study aimed at finding changes of column properties and column performance at different column lengths.

Experimental

A dynamic axial compression column (LC-50, PROCHROM) and a dual piston pump were used.  The column I.D. is 5 cm; its length is adjustable, depending on the nature and amount of the packing material used, up to a maximum of 50 cm.  The particular feature and the mode of operation of this column skid were described in detail by Sarker and Guiochon [6, 7, 8].  Two different commercial chromatographic materials, both chemically bonded octadecyl silica, were used for this study: Baker I-130 and Nova Pak.  The physical properties of these two packing materials are list in Table 1.  Four columns were packed with each of the two materials, using increasingly large amounts of them, at a typical compression pressure of this system (around 60 bar).  Four additional columns were packed with Baker I-130 at a very low pressure, because these particles are irregular and porous.

The packing procedure was the same for all columns.  Iso-propanol was used as the slurry solvent.  The ratio of dry weight of material to total slurry volume was kept constant for all columns packed with the same material.  A mixed slurry solution was poured into the column with the piston at its lowest position.  The column was closed and compressed to a pressure of desired level, and left to equilibrate for a few hours.  The frits (fine mesh grids installed at both ends of the column) act as a solvent distributor at the inlet and prevent particles from coming out of the column at the outlet.  Packing process was analyzed by column compression kinetics, the change of column length under mechanical pressure.  Column properties such as density, porosity, and permeability were analyzed from the measurements of final bed volume, inverse size exclusion data, and column pressure drop.  Column efficiency was evaluated from the width of peaks obtained following the injection of a 0.5 mL of uracil sample solution (0.8 g/L) at various flow rates.  Uracil is known for not being retained on the chemically bonded C18 silica materials used in reversed phase liquid chromatography.  The mobile phase was a methanol-water mixture (40/60).  The sample concentration was detected by UV absorbance at a wavelength of 254 nm.

Results and Discussions

The Nova Pak columns showed a nearly linear dependence of the final column length on the weight of dry packing material.  For Baker I-130, the column length increased more rapaidly than the weight of packing.  The packing density decreased with increasing column length for both column materials (Fig. 1), indicating that the shorter columns are more denser than the longer ones.  This result was expected, considering the nature of consolidation in dynamic axial compression columns.  The piston applies the compression pressure to one end of the column while the other end of the bed is merely pushed against the corresponding frit and flange, without experiencing any direct stress.  Compression stress in packed beds do not convey as pressure in liquids.  The decrease in the compression stress from the piston face to the opposite end of the column would be explained by friction between particles and between particles and the column wall.  The frictional force causes a distribution of the compression stress in the axial direction as well as in the radial direction.  The range of the stress distribution is expected to increase with increasing column length, the stress at the end opposite to the piston decreasing with increasing column length, which explains the decrease in the apparent column packing density under the constant applied pressure.  Note that the packing density shown in Fig. 1 is average values measured over the entire column volume.  The packing of the short columns is globally more homogeneous than that of the long columns, because of lower gradient of stress in the axial direction.

Reduced plate height and number of theoretical plates of each column were calculated at various mobile phase velocities.  The plots were mostly typical van Deemter curves, except the shortest columns, exhibiting a minimum plate height at a reduced velocity of 5 or lower.  The shortest columns had significantly lower efficiencies than the other columns (Fig. 2).  It appears that the column entrance effect exists in this dynamic compression column.  The column entrance effect can be explained in several ways.  First, we consider a non-uniform flow distribution through the inlet of the column, specifically the piston frit.  During the column operation, the fine particles may be trapped inside the frit, leading to uneven flow paths at the entrance.  Second, the local homogeneity of the column near the piston may be poorer than in the bulk of the column.  This is possible because the stress distribution inside the column is not uniform.  The intense stress near the piston can cause a strong disturbance of local homogeneity at the column inlet.  The particles may be broken in this region even for spherical particles, while they are not broken in other regions unless the compression pressure is too high.  Particle size analysis data revealed this interpretation is true [9].  Finally, the low efficiency of very short columns may be related to the total contact area between particles and the column wall.  This is related to the stress distribution especially near the wall which includes the column side wall and the column ends (frits).  The total wall area per bed volume is largely reduced as column length increases..  The total wall area per bed volume changes little for columns longer than 10 cm when the column inner diameter is 5 cm.  The frictional resistance is more influential in the short columns than in the longer columns, and this may be related to the low column efficiency of the short columns.

Conclusion

The properties of a dynamic axial compression column depend on its length in an unexpected, nonlinear fashion.  This is because the packed bed column is not axially homogeneous.  The region close to the piston experiences a higher degree of compression stress.  Accordingly, the average column packing density decreases with increasing column length.  The amplitude of this change is much larger with large, irregular-shaped particles than with small, spherical particles.  The column inlet is less homogeneous than in the bulk of the column packing, because of particle breakage and wall friction.

Acknowledgments

This work was supported in part by Grant DE-FG05-88ER13859 of the US Department of Energy and by the cooperative agreement between the University of Tennessee and the Oak Ridge National Laboratory.

References

1. Guiochon, G., Farkas, T., Guan-Sajonz, H., Koh, J.-H., Sarker, M., Stanley, B. J. and Yun, T.: J. Chromatography A, 762, 83(1997).

2. Farkas, T., Sepaniak, M. J. and Guiochon, G.: J. Chromatography A, 740 169(1996).

3. Train, D.: J. Pharm. Pharmac., 8 745(1956).

4. Train, D.: Trans. Inst. Chem. Eng., 36 224(1958).

5. Koh, J.-H. and Guiochon, G.: J. Chromatography A, in press.

6. Sarker, M. and Guiochon, G.: J. Chromatography A, 702 27(1995).

7. Guiochon, G. and Sarker, M.: J. Chromatography A, 704 247(1995).

8. Sarker, M. and Guiochon, G.: J. Chromatography A, 709 227(1995).

9. Koh, J.-H., Broyles, B. S., Guan-Sajonz, H., Hu, M. Z.-C. and Guiochon, G.: J. Chromatography A, submitted.

Table 1. Physical properties of the packing materials used.

	Baker I-130 C18	Nova Pak C18
Average particle size	130 □m	60 □m
Particle shape	Irregular	Spherical
Specific surface area	312 □/g	120 □/g
Average pore size	141 A	60 A
Specific density	0.48 g/mL	2.2 g/mL
Specific pore volume	1.1 mL/g	0.3 mL/g