Monolith

Theoretical Consideration

Flow Resistance Parameter (Φ):
(The smaller the Φ, the more permeable the column is.)
Φ = ∆P t0 dp2 / η L2

Permeability (K):
K = u η L / ∆P; u = L / t0

Column performance can be achieved by increasing column permeability
Monolithic
columns provides higher permeability that is lacking in particle-packed columns

The compromise of pressure drop is adjusting column length in order to achieve required efficiency
Theoretical Consideration

H = 1/ [(1/Cedp)+(DM /Cmdp2u)]+CdDM /u+Csmdp2u/DM = Au1/3 + B/u + Cu
(a) Scanning range: 1um
(b) Scanning range: 10 um
Why use monolithic columns?
Drawbacks of Particle-packed Columns

Higher efficiency in separation time achieved in HPLC by reducing particle size of packing materials Particle size reduction higher column efficiency
/david_ryan/84.314/MonolithicHPLC.pdf

What is a monolith? Why use monolith (Theoretical Consideration)? Preparation (Monolithic Silica Columns): Brief Info Structural properties (of Monolithic Silica Columns) Permeability (of Monolithic Silica Columns) Chromatographic performance (in Application section) Advantages and Disadvantages Application of the technology: Present an Example Conclusion References






What is a monolithic column?

Single piece of solid made of either porous crosslinked polymer or porous silica Interconnected skeletons and interconnected flow paths called “through-pores” (Macropores) that go through these skeletons; Mesopores: small pores on the surface area of the monolithic skeleton Three different formats of monoliths: porous rods; thin capillaries or thin membranes/disks


Structural Features of Monolithic columns
Scanning Electro Micrograph Photographs of monolithic silica prepared (a) in a mold and (b) in a 50µm capillary The bars denote 10 µm in (a) and 20 µm in (b)

Particle size reduction
higher column efficiency
- Smaller A term is from smaller eddy diffusion and mobile phase mass transfer - Smaller C term is from shorter diffusion path length
- High permeability (from reduced flow resistance) (Previous slide shows info on flow resistance parameter Φ) - High column efficiency (from reduced dispersion from Multi-path, from Longitudinal Diffusion and from Resistance to Mass Transfer)
Monolithic Columns
Mandy Amin and Lillian Fua Jaquinod Chem230 ember 30, 2010
“A single-piece permeable mass
can separate better than a cluster of packed particles.”
A silica-gel skeleton that contains macropores (throughpores) with diameters of approximately 2 μm and mesopores with diameters of approximately 13 nm.

Feature high (through-pore size)/(skeleton-size) ratio

Controllable sizes of silica skeletons and throughpores in monolithic columns provides better overall performance than that of particle-packed columns
Why use monolithic columns?

Higher number of theoretical plates (N) and a shorter separation time is achieved using UHPLC and CEC by overcoming high pressure drop, which is an issue of columns having tightly-packed bed

Reduction of flow resistance and diffusion path length not possible in a particle-packed column with a (through-pore (interstitial voids)-size)/particle-size) ratio
H = column dispersivity u = linear velocity of a mobile phase Cx= coefficient for the contribution of each term (A, B, C) A = eddy diffusion and mobile phase mass transfer B = longitudinal molecular diffusion C = diffusion path length

- Smaller eddy diffusion and “M” phase mass transfer term - Shorter diffusion path length

Drawbacks of small particle-sized packed columns:


Low permeability due to small-sized interstitial voids in particle-packed column with a (through-pore (interstitial void) size)/(particle size) ratio of 0.25–0.4 Limitation of high-speed operation (available pressure with current instrumentation is 350-400kg/cm2 (1 kg/cm2 = 98066.5 Pa)
/content/j990ba10mvt619ad/fulltext.html
Electron micrographs of (a) macroporous and mesoporous (b) structures in a monolithic silica rod
Preparation of Monolithic Silica Columns

Minakuchi et al. reported on the preparation of continuous porous silica columns by sol-gel method from alkoxysilanes in the presence of water-soluble organic polymers