What is the difference between C18 and C18E? An approach to teach you to distinguish!

C18 and C18E SPE columns are the most commonly used SPE columns for various analytical tests. Not only are the names similar between the two, but also the functional groups and retention mechanisms are almost the same, which often makes people indistinguishable. So what exactly is the difference between the two?

Before that, let’s take a look at what is C18 adsorbent filter. C18 is a general term for a reversed-phase stationary phase linked to an octadecyl carbon chain. In addition to the common silica gel as the matrix, the reversed-phase stationary phase formed by bonding C18 chains with polymer beads, alumina, zirconia, etc. as the matrix can be called “C18”. There are many types of C18, common Silica-based C18 is usually called ODS, and C18 that can run in pure water is called C18-AQ. C18 packings are also divided into capped and uncapped.

For example, Welchrom® C18 is an unsealed C18 adsorbent filler. More residual silyl hydroxyl groups on the surface provide additional polar interactions, which makes the hydrophobic bonded silica gel contact more closely with the more polar extract, enhancing the retention ability of alkaline compounds and polar substances.

Welchrom®C18E is the sealed-end C18 adsorbent. It is the most hydrophobic silica matrix adsorbent with excellent retention properties for non-polar compounds and most organic compounds. It is also the most widely used SPE adsorbent.

In summary, the biggest difference between C18 and C18E lies in unsealed and sealed ends. Compared to Welchrom® C18E, Welchrom® C18 is a general-purpose stationary phase for extraction of polar and non-polar compounds. However, C18 retains the majority of organic matter in water-soluble matrices, resulting in low selectivity and is often used to treat analytes with multiple or significantly different structures. However, C18E can avoid irreversible adsorption, improve the recovery rate, and can significantly improve the chromatographic separation of basic compounds. For example, C18E column can usually be used to replace the ion exchange column for the desalination of some small molecules and some medium-sized molecules.

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