Most chromatographers know whether they’re using a C18 or a C8 column. You most likely know their particle size.
But very few stop to consider two finer details that can make a big difference in your separations: HPLC carbon load and surface area.
These are the specifics that explain why one C18 column doesn’t behave like another.
What Is Carbon Load in HPLC Columns?
Carbon load is simply the percentage of carbon bonded to the silica surface of the stationary phase.
It’s measured by gravimetric analysis:
- Take one gram of packing material.
- Weigh it.
- Put it in a muffle furnace at 800 °C.
- Burn off the bonded ligands.
- Weigh it again.
The difference is the carbon load.
Typical Carbon Load Values
Not all columns are created equal. Real examples that I have used include:
- Many C8 columns: 15–18% carbon load
- Waters Nova-Pak C18: 7% carbon load
- Phenomenex Ultrarb C18: 30% carbon load
- Most modern C18 columns: 12–18% range
Why Carbon Load Matters for Chromatographers
A higher carbon load means the column is more non-polar. That translates to:
- Stronger retention — molecules stay on the column longer.
- Greater capacity — you can inject more material without overloading.
- Prep-scale advantage — useful when working with large sample amounts.
A lower carbon load, on the other hand, brings compounds off the column faster. That can be helpful when you’re analyzing very non-polar molecules that would otherwise stick.
Endcapped C18 columns often fall in the 16–18% range, since additional bonding covers exposed silanols and increases carbon content.
What Is Surface Area in HPLC Columns?
Along with carbon load, surface area is a key factor.
- Typical HPLC silica: about 100 m² per gram — equal to 1,000 square feet of surface area in one gram of material.
- High-surface silica: about 300 m² per gram — like fitting a 3,000 square foot home inside your column.
Why is this important? Separation happens on the surface. The more surface available, the more interactions your molecules can have.
How Surface Area Affects Column Performance
Higher surface area means:
- More retention sites for analytes
- Greater column capacity before overload
- Stable retention times even with large injections
That’s why prep-scale HPLC often relies on high-surface area materials: you can load more sample without shifting peaks.
Carbon Load vs. Surface Area: The Nuances
Carbon load and surface area aren’t identical. Bonding density and chemistry also matter.
Traditional C18 chemistry only bonds to about half of the available silanols due to steric hindrance. Newer chemistries (like t-butyl groups) can increase bonding and add more carbon.
So, while higher carbon load and higher surface area often go together, they don’t always scale in the same way. Looking at both values gives you a clearer picture of how a column will behave.
Practical Takeaways for HPLC Column Selection
While you could run many separations without knowing these details, understanding HPLC carbon load and surface area gives you more control. They help you:
- Fine-tune column selection
- Avoid column overload
- Understand why retention changes with different columns
What to Remember
- Carbon load: Higher = stronger retention, more capacity. Lower = faster elution.
- Surface area: Higher = more interactions, greater stability, less overload.
- Together: Both help you choose the right column for your sample and method.
Keep Learning with Axion Labs
Understanding column properties is just one piece of the puzzle. If you’d like to go deeper, check out our online chromatography courses. In HPLC Demystified Part 1, Dr. Polite explains the theory of separation with an unforgettable Kool-Aid demonstration. Together, these lessons give you both the fundamentals and the finer details you need to run better separations.
