Preparation and characterisation of diamond-based stationary phases for high performance liquid chromatography

The idea of using diamond and diamond containing materials in separation sciences has attracted a strong interest in the past decade. The combination of a unique range of properties, such as chemical inertness, mechanical, thermal and hydrolytic stability, excellent thermal conductivity with minimal thermal expansion and intriguing adsorption properties makes diamond a promising material for use in various modes of chromatography. This Thesis is dedicated to the preparation and investigation of diamond based stationary phases for ultra high-performance liquid chromatography. The Thesis consists of 6 chapters: introduction and literature review, experimental Chapter, and four research Chapters. The information about the recent research on the preparation of diamond based stationary phases, their properties and chromatographic performance is summarised in Chapter 1. Special attention is devoted to the dominant retention mechanisms evident for particular diamond related materials, and their subsequent applicability to various modes of liquid chromatography, including liquid chromatography carried out under the conditions of high temperature and pressure. Advantages and drawbacks of diamond-based stationary phases described in literature are analysed. According to the literature review, it is decided to investigate chromatographic performance of non-modified non-porous high pressure high temperature (HPHT) diamond with 1.6 μm particles and a surface area 5.1 m²·g⁻¹. Chapter 2 contains the general information on materials and instrumentation used in this work, as well as the description of methods and calculations. A purification procedure for HPHT diamond is developed and its surface properties are characterised using various physical chemical methods in Chapter 3. It is shown that this material possesses a hydrophilic surface which displays various oxygen-containing groups, including carboxyl, carbonyl, hydroxyl, and others. Next, optimisation of particle shape and size distribution is performed by means of oxidative digestion and sedimentation. After that, the column packing procedure is developed. It is shown that better column packings can be achieved by using a water-based slurry with an addition of 10 mM NaOH, while use of organic solvents does not provide stable packed beds. Finally, it is established that packing pressure has to be kept as high as possible, and column after-packing conditioning with HNO₃ helps to improve peak shape. The resultant columns exhibit a maximum efficiency of 128,200 theoretical plates per meter in normal phase (NP) liquid chromatography, as shown in Chapter 4. The retention behaviour of several classes of compounds, including alkyl benzenes, polyaromatic hydrocarbons, alkylphenylketones, phenols, and aromatic acids and bases are studied using nhexane – IPA mixtures as a mobile phase. The results are compared with those observed for microdispersed sintered detonation nanodiamond (MSDN) and porous graphitic carbon. HPHT diamond revealed distinctive separation selectivity, which is orthogonal to that observed for porous graphitic carbon (Hypercarb); while selectivities of HPHT diamond and MSDN are similar. Owing to the non-porous nature of the particles, columns packed with HPHT diamond exhibit excellent mass transfer, and separations of 4, 6 and 9 model compounds are presented. Furthermore, HPHT diamond is tested as a stationary phase in aqueous normal phase (HILIC/ANP) chromatography using acetonitrile/water and methanol/water mobile phases in Chapter 5. The retention of several classes of compounds is investigated, including nucleobases, quaternary ammonium salts, carboxylic acids, phenols and their derivatives. The column performance is studied within a large pH range (2.2-12.7) using several buffering mixtures (trifluoroacetic, ammonium formate, ammonia and several hydroxides). The main retention mechanisms for HPHT diamond include cation exchange and hydrogen bonding. Both buffer and organic modifier (acetonitrile/methanol) have a crucial influence on the HPHT diamond column selectivity. A complete selectivity reversal is observed for different groups of solutes as a result of changes in mobile phase composition. During the research on chromatographic properties of HPHT diamond, an assumption is made that the adsorption of cations and anions on the surface of diamond has a strong effect on its chromatographic performance. For example, it is shown that column conditioning with HNO₃, H₂SO₄ or H₃PO₄ leads to different column selectivities in NPHPLC, and the use of NaOH and KOH solutions of equal concentration in ANP-HPLC results in different retention of nucleobases and phenolic compounds. Therefore, the adsorption of cations and anions on the diamond surface is studied in Chapter 6. MSDN is used for the investigation of the adsorption of inorganic cations and anions due to the higher surface area of MSDN as compared to HPHT diamond. The selectivity series Fe³⁺ > Al³⁺ > Cu²⁺ > Mn²⁺ > Zn²⁺ > Cd²⁺ > Co²⁺ > Ni²⁺ is found for MSDN, and the adsorption capacity for these metals is between 2 and 5 μmol·g⁻¹. Counter ions can contribute to the adsorption mechanism for transition metals, and the buffer influence on the adsorption of metals is revealed. Accordingly, the adsorption of inorganic anions (CH₃COO⁻, Cl⁻, B₄O₇²⁻, ClO₄⁻, I⁻, SO₄²⁻, C₂O₄²⁻, PO₄³⁻) on MSDN is investigated. The adsorption capacity for inorganic anions is at levels of 50-150 μmol·g-1, depending on the anion. For the first time, an anion exchange capacity is detected for detonation nanodiamond, exceeding its cation-exchange capacity. Electrostatic interactions, formation of complexes with hydroxyls and interactions with metal impurities contribute to the anion adsorption mechanism, so anion adsorption selectivity over MSDN is different from common anion exchangers. It is shown that the adsorption on MSDN obeys Langmuir law. The pH affects the adsorption of SO₄²⁻, PO₄³⁻ and B₄O₇²⁻ differently due to different adsorption mechanisms