Core shell columns: What are they and how will they help with my HPLC analysis?
HPLC columns are the crux of a good separation and as we all learned, the smaller the particle size, the higher the efficiency of the column, and the better resolution you will get. This better resolution leads to more peaks being separated out or even close eluting peaks to be baseline resolved. With smaller particle size comes a major drawback of back pressure which can overload the pumps of the HPLC, or even stress the connection points causing a leak. So, what can an analyst do about balancing the particle size with backpressure?
This is where core shell columns are said to come into play. A standard column particle is fully porous, where a core shell column has a solid core. I’ve been told that columns with core shell particles (core shell columns) should create less back pressure that a column with the same dimensions with fully porous particles, so I was curious to put this to the test.
To put this to the test I tried several different columns all with the same length, ID, method, and test mix. I have outlined the columns in table 1 below.
| Column number | Type | Length | Inner diameter | particle size |
|---|---|---|---|---|
| 1 | Fully Porous | 150 mm | 4.6 mm | 5 μm |
| 2 | Core shell | 150 mm | 4.6 mm | 5 μm |
| 3 | Core shell | 150 mm | 4.6 mm | 2.7 μm |
Each of the columns were initially run using a gradient test mix from Sigma-Aldrich (p/n 48271) at 2.0 mL/min. The pressure was recorded 5 minutes into the re-equilibration phase of the run, which was 6 minutes long, so that there was no bias because of initial ramping of the method. The mobile phase at that point is 90% water and 10% methanol.
The pressure difference wasn’t what I expected. There was not a noticeable difference in back pressure from fully porous to core shell. But, as expected, the back pressure did increase with a reduction in particle size.
| Column number | Type | Length | Inner diameter | particle size | Pressure |
|---|---|---|---|---|---|
| 1 | Fully Porous | 150 mm | 4.6 mm | 5 μm | 4012 psi |
| 2 | Core shell | 150 mm | 4.6 mm | 5 μm | 3970 psi |
| 3 | Core shell | 150 mm | 4.6 mm | 2.7 μm | 8231 psi |
I was also curious about the comparable performance of these columns as well.
Chromatogram of the fully porous column
Chromatogram of the core shell equivalent of the fully porous column
Looking at peak shapes and elution time is what really blew me away. The core shell column had better peak shapes when compared to the fully porous column of the same dimensions, and the peaks came off a bit earlier on the core shell column than they did on the fully porous column.
To show how much better the core shell columns performed I calculated the efficiency of the first peak of each of the columns to give a concrete value on how each column should behave during the test. The efficiency equation used is as follows:
Efficiency = 5.54 x (tr/W1/2)2
| Column number | Type | Length | Inner diameter | particle size | Efficiency |
|---|---|---|---|---|---|
| 1 | Fully Porous | 150 mm | 4.6 mm | 5 μm | 17929 |
| 2 | Core shell | 150 mm | 4.6 mm | 5 μm | 24132 |
| 3 | Core shell | 150 mm | 4.6 mm | 2.7 μm | 54663 |
The efficiencies calculated show that in this study the core shell column was indeed more efficient than the fully porous column, and as expected, the column with smaller average particle size (2.7 μm particle size), was more efficient than the comparable column with larger particle size (5 μm particle size).
So although we didn’t see a reduction in back pressure when using the core shell column versus the fully porous column, we did see a nice increase in efficiency!
Overall, I am now a big fan of the core shell columns and I think that if you currently don’t use a core shell column, give it a try and you may be a convert too.