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Why Are There so Many C18 Columns?

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Description

From end-capped to aqueous, there are a multitude of C18 phases to choose from. But why are there so many? A C18 is a C18, right?

In today’s Restek Tip, we’re going to explore the world of C18 phases. Are there benefits to choosing one specific to your analysis? And what parameters should you keep in mind when selecting a phase?

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Why are there so many C18 phases to choose from? A C18 is a C18, right? Well… maybe. In today’s Restek Tip, we’re going to explore the world of C18 phases. Are there benefits to choosing one specific to your analysis? And what parameters should you keep in mind when selecting a phase?

Let’s start with the basics.

While every C18 column comes with a chain of 18 carbon atoms, there can be substantial differences in retention and selectivity depending on the exact column you use.

Initially, many tend to focus on carbon load—the amount of carbon bonded to the surface of the particle—to determine whether there will be enough analyte retention, but there are many other factors to consider.

A traditional, end-capped C18 phase, which has about 20% carbon load, is a great place to start if you’re not sure where to begin. End-capped C18 columns have the advantage of offering the highest hydrophobic retention among C18 phases, while limiting other chromatographic interactions, due to end-capping and high surface coverage.

But, what if your mobile phase contains a high percentage of water or your analytes are more polar? Under highly aqueous conditions, a traditional C18 column will undergo a phenomenon known as “phase dewetting”.

When this occurs, the hydrophobic C18 chains repel water out of the particle pores, which limits analyte access to the stationary phase. So, in this case, an aqueous C18 column, which has about 15% carbon load, is recommended.

These phases are compatible with 100% aqueous mobile phases and also provide more retention for polar molecules that need an extra boost. This extra retention comes from a polar modification of the phase that also makes it 100 % water compatible.

Another factor to consider is pH. While low pHs can improve peak shape and the selectivity of a separation, these highly-acidic conditions can irreversibly damage your stationary phase. C18 phases with sterically protected ligands excel in low pH environments.

These bulky side chains protect the phase from being removed from the surface under acidic conditions, but also allows some extra polar interactions of your analytes with the silica surface for unique selectivity. These assets make them perfect candidates for multiclass and pesticide analyses.

So, is a C18 a C18? As you can see, there are many differences between C18 columns, and each have their own advantages. So, rather than selecting one you may be more familiar with, it’s worth taking the time to consider the needs of your analysis before making a choice.

And if you have any questions, we’re here to help! Leave a comment below or visit us at restek.com. Thank you for joining us for this Restek Tip!

GNAV3670-UNV

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