The preparation and characterisation of new stationary phases remain one of the most challenging and intensively developing
research areas in chromatographic sciences. There are two main areas in adsorbent development: surface chemistry and
adsorbent geometry. Chemical properties of adsorbent surface are the main factors that define chromatographic selectivity and
adsorbent geometry (particle size and porosity type) which are also the key elements that define column efficiency. These combined
factors determine overall chromatographic performance. Additionally, for some particular modes of liquid chromatography,
the significant improvement of auxiliary physico-chemical properties of stationary phases is often required. This includes
an elevated mechanical stability in ultra-high pressure liquid chromatography, a high thermal stability in high temperature liquid
chromatography, a superior biocompatibility in biochromatography, a broader pH range of hydrolytic stability for the separation
of various inorganic species and many other properties and others. Unsurprisingly, the development of advanced stationary
phases for liquid chromatography was selected as a theme for the first Special Issue of the journal Current Chromatography.
There are three major methodologies for the development of new sorbents. The first approach includes application of new
materials and testing their chromatographic performance. The finding of the right adsorbents was a key element since the discovery
of chromatography in 1903 by Russian scientist Mikhail Tswett. He investigated adsorption properties of more than 125
various materials according to their ability to retain and separate chlorophylls on short columns in normal-phase liquid chromatography
[1]. It is well known that Mikhail Tswett proposed a wide range of adsorption materials like silica, alumina, calcium
oxide, magnesium oxide, calcium carbonate, calcium phosphate, calcium sulphate (gypsum) and polysaccharides (cellulose,
inulin, dextrin, starch), which are used as a common matrices for the preparation of efficient and selective chromatographic
stationary phases or as additives to stationary phases (e.g. binders in thin layer chromatography) in the following years [2]. It is
less known, that he tested in his experiments also quite exotic type of carbonaceous materials like bone carbon and blood carbon
as well as superhard abrasive material known in Russia as nazhdak (natural mineral emery – ed.), which is composed of
corundum (alpha aluminium oxide), magnetite and titania. The idea of combination of advantages of carbonaceous and superhard
materials in one adsorbent by using diamond micro particles as of stationary phase in liquid chromatography appeared
significantly later [3]. Diamond has many superior properties including mechanical, thermal, hydrolytic stabilities, excellent
thermal conductivity, biocompatibility, but very low specific surface area and irregular shape of nonporous particles which
limit its practical applications in chromatography. The solution of this problem was found in the preparation of core-shell particles
with solid central core composed of glassy carbon microsphere and shell constructed from nanodiamond and cationic
polymer using layer-by-layer technology followed by grafting octadecyl functional groups [4]. The prepared mixed-mode reversed-
phase/ion-exchange stationary phase demonstrated excellent separation efficiency and hydrolytic stability. The current
investigation of Gupta et al. presented in the Special Issue is devoted to the characterisation of adsorption properties of core
shell diamond based stationary phases and to the identification of ion-exchange contribution of cationic polymer in the retention
of polar analytes [5].
Another productive way for the preparation of new stationary phases includes modification of the morphology and porous
structure of known adsorption materials e.g. bare silica. The evolution of silica based stationary phases started from the use of
irregular particles of porous silica gel produced by hydrolysis of inorganic silicates in early works of Mikhail Tswett [1]. The
next big step in the design of silica based phases was associated with the development of the technology allowing the preparation
of porous micro spherical particles by hydrolysis of ultra-pure organosilicon compounds in the presence of specifically
selected emulgator or by agglutination of 5 nm silica microspheres [6, 7]. It is believed, but not carefully proved yet, that micro
spherical particles can form more homogeneous column packings and provide more efficient separation at lower operating
pressures. Further improvement of chromatographic mass transfer, especially for large biomolecules, was achieved with chromatographic
columns packed with nonporous micro spherical particles [8]. The real breakthrough in chromatographic performance
of silica based materials was achieved with the development of monolithic porous columns, which provided excellent
separation efficiency at very low backpressures [9]. These types of silica morphology opened new possibilities for ultra-fast
separations, using extra-long separation column obtained by connection of shorter ones in series, low-pressure separations and
many other related options [10]. The current trend in the development of silica based stationary phases is connected with the
application of core-shell technologies for the preparation of micro particles composed of central solid core and porous shell
layer constructed from silica nanoparticles. This advanced structure of core-shell particles of typical diameter 1.3-2.0 microns
provides a shorter diffusion pathway for the separated molecules and therefore, excellent separation efficiency for short chromatographic
columns [11]. The updated overview on the application of core-shell particles in ultra-high performance liquid
chromatography for the analysis of food, environmental, forensic, biopharmaceutical, and natural products is presented in the
Special Issue by Dr. Gosetti and Marengo[12].
Finally, the easiest and the most accepted way for the preparation of new stationary phases is connected with modification
of the surface of bare matrices with functional groups to achieve the required separation selectivity. The immobilisation of various silanes. For example, for the preparation of the most popular octadecylsilica adsorbent, the use of mono-, di- or trichloro-
or alkoxy silanes is the standard procedure. The reaction is relatively simple, but in the obtained adsorbents, only half or
total 8.0 moles of reactive silanol groups per m2 of the silica surface can be converted into item functionalities due to bulkiness
of anchoring groups of alkylsilanes. Most modern HPLC silicas have bonding density from 2.5 to 3.5 mol/m2, while
maximum bonding density of 4.0 – 4.2 mol/m2 can be achieved for wide pore silica and the presence of residual silanol groups
influences peak shapes of basic organic molecules and their recovery from chromatographic columns. In 1989, Dr. Pesek proposed
the use of silica hydride for the preparation of chromatographic stationary phases. Silica hydride phases have a higher
density (up to 5 mol/m2) of bonded groups with improved hydrolytic stability as compared to the phases prepared by silane
treatment of silica [13]. Dr. Pesek's and Dr. Matyska's article provide excellent overview of basic fundamental properties of
silica hydride-based separation materials highlighting where these are unique or enhanced with respect to conventional materials.
The review is also focused on recent applications of hydride silica based phases in liquid chromatography [14].
