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Comparing NanoPak-C All Carbon to Alkyl Bonded Silica-Based Stationary Phase Materials: Key Differences

We often get asked the following two questions regarding NanoPak-C All Carbon microbeads.

  1. Is it more expensive than alkyl-bonded silica?

  2. What are the differences between our carbon-based and alkyl-bonded silica?

In this blog post, we'll present and differentiate the characteristics of Nanopak-C All-Carbon microbeads with an alkyl-bonded silica-based stationary phase.

Our response to first question.

Economical. The way we make the All Carbon microbeads allows its cost per kg to be comparable or lower than alkyl bonded silica-based media.

The second questions needs a bit more elaborate response.

The table below lists fourteen fundamental properties of stationary phase media and their benefits to end-users. The table also presents these properties for NanoPak-C All Carbon microbeads synthesized using natural micro-graphite as starting material and silica-based stationary media used for reverse-phase chromatography.

The salient points of this table can be summarized as follows:

Structural Differences

NanoPak-C All carbon beads: It is composed entirely of carbon-containing material. Here, sp2 carbon sources are used as starting materials and then chemically linked to each other with crosslinkers (see Figure 1). All our products currently use graphite as a starting carbon source. However, if needed, we can also use, fullerene, carbon nanotubes, and graphene. This whole assemble process happens within tiny microdroplets [1].

Alkyl-bonded Silica: The Silica microbeads are attached to an alkyl group by chemical process. A covalent bond is formed between the Silica and alkyl group (Figure 2). Various types of alkyl groups have been explored, and depending on the number of alkyl groups, the terms C4, C8, and C18 are used.

Figure.1 Structure of Nanopak-C All Carbon microbeads            Figure.2 Structure of alkyl bonded silica microbeads

Differences in Chemical and Physical Properties

1) Intrinsically Hydrophobic. NanoPak-C graphite microbeads are inherently hydrophobic and thus suitable for reverse-phase chromatography. No additional functionalization with the alkyl group is required. Si microbeads are inherently hydrophilic suitable normal phase chromatography. Their surface must be further functionalized with alkyl groups to impact hydrophobic characteristics.

2) Tunable pore size, porosity, surface area. The NanoPak-C graphite microbeads synthesis method allows better control over microbead diameter, pore size, and surface area, allowing unrestricted tunability of these properties. These properties cannot be tuned easily by alkyl bonded silica synthesis process.

3) Tailorable Surface Composition. NanoPak-C carbon microbeads use crosslinkers that allow facile covalent or non-covalent functionalization of the interior and exterior surfaces of the spherical microbeads with atoms, molecules, or macromolecules to change the hydrophobicity, surface charge, or affinity to a particular analyte. This capability is mainly restricted to chemistries that are possible with silanol.

4) pH stable. NanoPak-C carbon microbead is chemically stable across the entire pH range. Alkyl bonded silica has pH restrictions. At acidic pHs, the alkyl can dissociate from the silica substrate. At alkaline pHs, the underlying silica substrate starts to dissolve. A number of strategies to mitigate these effects. These include the use of alkyl bridging groups to synthesize hybrid silica. However, the problem even though reduced, is not eliminated.

5) Temperature Stable. Carbon microbeads are thermally stable to high temperatures (up to 200◦C).

6) Seamless Scalability. It opens avenues for seamless method transition from analytical to semi-preparative and preparative chromatography without pH or temperature restrictions. Alkyl bonded silica, cannot be used in applications that require alkaline washes to regenerate the columns.

Author: Smruti Ranjan Das


1.     Parente, M. J. and B. Sitharaman (2023). "Synthesis and Characterization of Carbon Microbeads." ACS omega 8(37): 34034-34043.

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