**1. Introduction**

#### **1.1. Nanomaterials as stationary phases in separation science**

Nanomaterials are nanostructures with sizes in the range of approximately 1–100 nm [1]. These nanomaterials frequently have chemical and physical characteristics that are distinct from those of their macroscopic counterparts [1]. They can be exploited in many fields of science and technology including separation science [2]. In separation science, this term often refers to nano-materials-based stationary phases used to separate chemical compounds [2]. Widespread applications and outstanding performance of nanomaterials have not only accelerated the development of separation science, but also offer many opportunities in other related disciplines, and have a significant impact on many fields of science [2, 3]. Separation science is based on the application of broad chromatographic techniques to achieve separations of compounds. Such separation is achieved by regulating the magnitude of the distribution

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

coefficient between two distinct phases namely the stationary and the mobile phases [1–3]. The components separate as they migrate with different rates depending on their unique distribution coefficients, they separate [4]. Different chromatographic techniques are available depending on the type of the phases [4]. The chromatography is known to be liquid chromatography (LC) when it employs a liquid mobile phase [5]. The most sophisticated form of LC is High-performance liquid chromatography (HPLC) where the mobile phase passes through the stationary via a pump at high pressures [4, 5].

have resistance to mass transfer contribution from the stagnant mobile phase in the pores [8]. Decreasing the particle size and increasing the diffusion coefficient can improve the mass transfer of solutes in the mobile phase [3, 5]. Non-porous particles can provide lower mass transfer resistance and higher efficiency than porous particles [8]. However, porous particles have higher surface areas (**Figure 1**) and can provide much higher sample loading

Sub-2 μm Silica Particles in Chiral Separation http://dx.doi.org/10.5772/intechopen.79063 57

On the other hand, in porous particles, solutes transfer from the mobile phase exterior to the particles into the mobile phase within the pores to interact with the chiral stationary phase (CSP) [9]. Following this interaction, the solute molecule must diffuse out of the particle and continue its journey down the column ahead of the solute [5, 9]. This slow rate of mass transfer into and out of the porous particle is a source of HPLC band broadening [9]. **Figure 2** illustrates a reduction in particle size shortens the path length of this diffusion process, improves

**3. Effect of particle size on the separation efficiency, speed and**

**Figure 2.** Smaller particle sizes increase efficiency and result in a wider range of flow rates [9].

size is lowered, efficiency increases, and more resolution is achieved [10].

Particle size is known to be the mean diameter of the spherical support employed in column packing [6]. This physical dimension significantly impacts HPLC column performance [9]. A decrease in particle size increases peak efficiencies (**Figure 2**). This is based on the resolution equation (Eq. (1)) which comprise of three terms: selectivity, retention capacity, and efficiency [10]. The components of an analytical method alter each of these terms. In particular, the column's particle size affects the efficiency factor from the equation [10]. Efficiency is a qualitative term used to measure the number of theoretical plates in a column. Put simply, as particle

RS = 2(tR,2 − tR,1)/(wb,<sup>1</sup> + wb,2) (1)

**3.1. Relationship between particle size, and column efficiency**

capacity [8].

mass transfer, and provides better efficiency [9].

**resolution of chiral separations**
