CORE SHELL-TECHNOLOGY in LIQUID CHROMATOGRAPHIC COLUMNS:
The science behind the core shell columns giving a highly efficient performance in LC-Chromatography.
Always all chromatographic scientists willing to have fast separation of compounds, results in very high operating pressures, which places a huge burden on chromatographic instruments.
Core–shell silica microspheres: with a solid core and a porous shell, also known as fused-core or superficially porous microspheres, have been widely investigated and used for highly efficient and fast separation with reasonably low pressure for separation of small molecules, large molecules and complex samples.
Core-shell particles’ high separation efficiency is largely due to more rapid analyte mass transfer – from the mobile phase through to the stationary phase and back again. This is because diffusion only occurs via the porous, outer layer of the particle rather than the entire particle. In order to increase efficiency, it’s important to minimize sources of band broadening, such as diffusion. In terms of size and shape, core-shell particles are remarkably constant, which also helps to enhance separation efficiency by limiting variable analyte
movement between the particles.
Core-shell particle is not fully porous. Using sol-gel processing techniques that incorporate nano structuring technology, a durable, homogeneous porous shell is grown on a solid silica core. This highly optimized process combined with uniform particle size distribution produces a column that generates extremely high plate counts.
Why core–shell particles can perform better with low back pressure & improves chromatographic efficiency, in terms of "Van Deemter equation"?
Core shell technology results in Less Band Broadening, in order to maximize efficiency, sources of band broadening need to be minimized. With core shell particles, all three sources (A, B & C) are reduced compared to fully porous particles as seen in the graphs below. This reduction in band broadening results in chromatographic separations with better resolution, higher sensitivity, and improved peak capacities.
HETP:-Height Equivalent to Theoretical plate
The major factors in Van Deemter equation that affect chromatographic efficiency are “eddy dispersion, longitudinal diffusion, and resistance to mass transfer”- the A, B, C terms respectively from the equation.
A-Term: “Multipath effect” (Eddy dispersion):
A’ Term completely depend on column & only important in liquid chromatography. This multiple path effect tends to make the band of analytes broader as it moves through the column due to in-homogeneities in column packing and small variations in the particle size of the packing material. Eddy diffusion itself relates to the fact that an analyte molecule, within a ‘band’ of analytes, can take one of many ‘paths’ through the column. In fact, the Eddy Diffusion term in the Van Deemter equation (the A term), is often called the ‘packing’ term as it reflects the quality of column packing.
The unique core-shell morphology has very significant advantages in chromatographic performance over fully porous particles, specifically increasing column efficiency and reducing retention time.
Figure below, it shows how an analyte moves through a column with fully porous particles. Unwanted widening of peaks is caused by the wide particle size distribution and slow movement of the analytes into and out of fully porous particle leading to eddy dispersion.
If you look at below figure, you’ll see how an analyte moves through a column with core-shell particles. On a column packed with core-shell particles, the sample bands travelling through the column exhibit significantly less band broadening during the run from the reduced diffusion path. As a result, the peaks are eluted as much narrower bands, resulting in increased peak height and resolution.
⦁ Increased resolution and peak capacity: The core-shell particle decreased band broadening, making the peaks narrower. Therefore, an increase in resolution and peak capacity is seen.
⦁ Higher sensitivity: Again, the reduction in band broadening leads to a higher sensitivity, allowing chromatographers to see lower detection limits.
B Term- Longitudinal Diffusion:
One of the biggest advantages to the use of solid core materials is the reduction in the dead volume of the column. A fully porous material packed into a column will only occupy about one-third of the column volume whereas the amount of space occupied by the solid core material is dramatically increased by about 20–30%. This reduction in the accessible volume of the column results in less longitudinal diffusion occurring within the column.
A band of analyte molecules contained in the injection solvent will tend to disperse in every direction due to the concentration gradient at the outer edges of the band. This broadening factor is called ‘Longitudinal diffusion’ because inside tubes, the greatest scope for broadening is along the axis of flow. The band will broaden in all system tubing, but the worst effects will be encountered in the column itself.
Longitudinal diffusion occurs whenever the HPLC system contains internal volumes that are larger than necessary and some instances of this are:
- Tubing length too long
- Tubing that is too wide (internal diameter)
- Tubing joined by unions
- Incorrectly connected Zero Dead Volume fittings
- Incorrectly connected Zero Dead Volume fittings
- Using the wrong column nuts and ferrules
- Using a detector flow cell that has a large internal volume
Longitudinal diffusion has a much larger effect at low mobile phase velocity (flow). Therefore, using high linear velocity (high mobile phase flow with narrow columns), will reduce the effects of this broadening factor.
C Term- Resistance to mass transfer:
The analyte takes a certain amount of time to equilibrate between the stationary phase and mobile phase. If the velocity of mobile phase is high, and the analyte has a strong affinity for the stationary phase will move a head of the analyte in the stationary phase. The band of analyte is broadened. The higher the velocity of mobile phase, the worse the broadening becomes.
This term in the Van Deemter equation arises largely due to the fact that the stationary phase material is porous and the mobile phase within the pores is ‘stagnant’ or stationary. The packing material is porous to allow a very large surface area for separation to occur. As the analyte molecules move through the stagnant mobile phase to reach the surface of the packing material, they do so by diffusion only. Analyte molecules entering the pore, those that don’t enter the pore and those that penetrate more deeply into the pore, will all be held up at that point to different extents –causing a broadening of the band. This is the ‘C’ term.
Understand more from different articles on the evaluation of core shell technology in the chromatography columns & it’s impact on high performance chromatography by so many advantages towards fast & efficient separation of compounds. Hope, the information articulated here will give an idea on Core-shell technology by relating with Van deemtr equation.
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