Ion-exchange chromatography

HPHT and CVD produced diamonds have no charged groups at the surface and do not exhibit any significant ion-exchange selectivity without oxidative treatment. Hence, retention of neither anions nor of cations has not been reported using HPHT diamond based phases. However, DND possesses pronounced cation exchange properties, due to the functionalisation of its surface with carboxyl and hydroxyl groups, following the oxidative treatment applied during isolation and purification of DND from detonation soot. According to the results of potentiometric titrations [108,109], Boehm titrations [110] and FT-IR spectroscopic data [4], the surface of this form of synthetic diamond contains three types of cation-exchange groups, including strong acidic carboxylic, weak acidic carboxylic and phenolic. The type and surface concentration of such ionogenic groups depends on the type of oxidative purification procedure applied. Chukhaeva and Cheburina evaluated cation- exchange capacity of DND purified with strong oxidising acids as being equal to 0.7 mmol·g−1 [111]. Chiganova [110] reported the total concentration of protogenic groups as 0.66 and 0.91 meq·g−1 for DND samples with surface areas of 270 m2·g-1, which had been purified by wet (nitric acid) or gas phase oxidation in the air, respectively. The corresponding values for total surface concentrations of acidic functional groups were 1.47 and 2.02 groups per nm2, while the concentrations of strongly acidic carboxylic groups in these samples were 0.47 and 1.33 groups·nm−2.These values are comparable with the maximum concentration of

36 silanols known for silica, namely 4.6 groups·nm−2 [42]. The ratio of strong and weakly acidic functional groups evaluated by the Boehm method is significantly higher for the gas phase oxidised sample (0.60 mmol·g−1 against 0.18 mmol·g−1 = 3.33), as compared with wet oxidised sample (0.21 mmol·g−1 against 0.39 mmol·g−1 = 0.54). Obviously, the latter sample has a less charged surface (ζ = −30.1 mV) than gas phase sample (ζ = −36.6 mV) [110]. pKa values of carboxylic groups at the surface of diamond are usually around 4–6 [108,109].

As expected, most of the publications in this area are devoted to the investigation of cation-exchange properties of DND, while only a few papers have reported data on adsorption of inorganic anions [112]. Cation exchange selectivity of the following order, Ca2+> Ba2+> Li+> Na+> К+ has been reported for both oxidised synthetic diamond [113] and DND [110]. This selectivity is in agreement with the ion-exchange selectivity of nonaromatic carboxylic type cation exchangers towards alkali metal (Li+> Na+> Cs+> K+) and alkaline- earth metal ions (Ca2+> Sr2+> Ba2+), as recorded in basic media [114]. This typical retention order for alkali and alkaline-earth metals were obtained in acidic eluent on a column packed with MSND, and is shown in Fig. 1.12b. Obviously, MSND exhibits strong ion-exchange selectivity, as the baseline separation of alkali metal cations was possible (Fig. 1.12a). However, all chromatographic peaks tailed due to the microporous structure of MSND and slow diffusion of metal cations within these pores.

The complexing properties of the same type of MSND were evaluated by Nesterenko et al. [26]. A chromatographic column of dimensions 100 × 4 mm I.D. was used for the separation of Cd2+, Ca2+, Zn2+, Co2+, Mn2+, Mg2+, Sr2+, Ba2+, Ni2+, Pb2+, Cu2+, using 1.5–5.0 mM HNO3–0.1 M KNO3 as the eluent. Alkaline-earth, transition and heavy metal cations were retained under these conditions in an order correlating to the stability of the complexes formed with carboxylic groups at the surface of MSND [115]. Under optimised conditions, a mixture of Cd2+, Co2+, Mn2+, Mg2+could be separated within 20 min. The retention order for these metals on DND was similar to that obtained on commercially available carboxylic type cation exchangers. The effect of the column temperature and ionic strength on the retention of cations was evaluated, and the results confirmed the dominance of surface complexation within the retention mechanism.

For DND, there are some indications on the significant effect of counter-ions on the adsorption of cations [100], whilst no effect of counter ions was recorded for adsorption of cations on natural diamond [116].

37 Fig. 1.12. Separation of alkali metal cations on MSDN column 150 × 4.0 mm I.D. (dp = 3–6 µm). Eluent: 0.05 mM HNO3, 0.35 mL·min

−1

, 23 °C. Conductivity detector. Unpublished results.

38 1.4.2.4. Electro modulated liquid chromatography

The elevated electrical conductivity of boron doped diamond powder facilitates its use as a stationary phase in electromodulated chromatography [33]. This type of chromatography exploits the possibility of retention and selectivity control through changing the electric potential applied to the surface of the conducting stationary phase. The electric resistivity measured for 8–12 µm HPHT diamond particles powder was greater than 40 MΩ·cm, however after the coating of these particles with a boron doped CVD diamond layer, this resistivity dropped to 2.4 Ω·cm [33]. Therefore, boron doped diamond powder was packed in specially constructed columns, dimensions of 78 × 3.0 mm I.D., and applied to the electrochemically modulated separation of a mixture containing benzenesulfonate, 4- toluenesulfonate, 1,3-benzenedisulfonateand 1,5-napthalenedisulfonate, with 0.1 M LiClO4 as the mobile phase, delivered at a flow rate of 0.4 mL·min−1 [33]. A rise in applied electric potential from −1.2 V to +1.2 V increased the retention times for all solutes, in the order of 0.5–10 min. A linear correlation between voltage and retention factors was demonstrated for all solutes and separation of three aromatic acids within 3 min was achieved. Due to its inert nature and the wide potential window of conductive diamond, there is a possibility to use this adsorbent at higher positive and negative potentials without undesirable effects, such as surface oxidation, microstructural changes or solvent electrolysis, which are more likely with other carbon-based materials commonly applied to this niche variant of liquid chromatography. However, the very low specific surface area of nonporous borondoped diamond powder (~1 m2·g−1) greatly limits column loading capacity and influences interaction strength between solutes and such stationary phases.