We often receive questions about the selectivity of our all-carbon graphitic stationary phase media. This topic is of significant interest as it directly impacts the quality of our chromatographic separations. They're curious if it's different from silica-C18 or other alkyl-bonded silica-based stationary phases. The answer, in short, is a definitive yes.
A key follow-up question is whether this is different from other graphitic carbon-based stationary phases. The short answer is both yes and no. This might seem confusing now, but it will become clear by the end.
In this blog, we examine these responses in detail to help you understand the differences in selectivity among these chromatography stationary phases.
Recapping Selectivity
In a previous blog, we discussed the term resolution, which describes a separation quality between two adjacent peaks in a chromatogram. This term depends on three key factors: efficiency, selectivity, and retention. The master resolution equation shows how these factors are related to each other.
Selectivity has the most significant impact on resolution. Selectivity increases resolution linearly; therefore, even a tiny increase in selectivity can significantly improve resolution.
Selectivity (α) defines the ratio of retention times of two sequentially eluting analytes and is a descriptor for the distance between them. We can change the mobile phase composition to make analytes stay on the stationary phase for a longer or shorter time. Still, the main factor that controls selectivity is how the stationary interacts differently with each analyte.
Selectivity Differences: Silica C18 vs. Graphitic Carbon vs. All Carbon
In silica-C18 or other alkyl-bonded silica-based stationary phases, the analyte molecules interact with the alkyl chain (e.g., C18) bonded to silica. The differences in the hydrophobic content in each analyte lead to differences in the hydrophobic effect of the alkyl chain, allowing the stationary phase to exert selective interactions.
Changes in the chemistry of the groups bonded to silica can further alter selectivity. For example, amide, cyano, or other polar groups are embedded into the alkyl-bonded silica stationary phases. These combinations of possible solute-stationary phase interactions influence overall selectivity.
The surface of all graphite-based stationary phases, including all carbon media, comprise of a network of carbon carbon double bonds delocalized electrons in the π orbital. These carbon atoms are known as aromatic sp2 hybridized carbon. π-π interactions occur between an aromatic or unsaturated analyte and graphitic stationary phases. The differences in these π-π interactions contribute to analyte selectivity.
These π-π interactions (charge) and the planar shape of the aromatic sp2 carbon network (elaborated in our blog on Carbon’s role in Reversed-Phase Liquid Chromatography Retention and Selectivity) are the reason for the differences in selectivity compared to silica-C18 or other alkyl-bonded silica-based stationary phases.
Furthermore, unlike other graphitic carbon-based stationary phases, the graphite particles in all carbon stationary phases are held together with a crosslinker. The chemistry of the crosslinker, a chemical that binds the graphite particles together, can be designed to introduce specific interactions with analytes, thereby further altering the selectivity of the stationary phase. For example, embedding polar groups in these crosslinkers can alter the selectivity.
This is what we meant by our yes or no answer to whether the selectivity of all carbon media differs from other graphitic carbon-based Stationary Phases. The yes is due to the graphitic phase, which gives rise to selectivity like other graphitic carbon phases. The no is due to the crosslinker, which alters the selectivity and could make it different than other graphitic carbon phases.
The bottom line
Complex mixtures comprise various chemical compounds with differing physicochemical properties. All Carbon stationary phases could be tailored to meet the need for newer chemistries with different selectivity for the ever-expanding chemical diversity of analytes, which includes a wide range of organic compounds, from small molecules to large biomolecules.
Additional References
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