Current Chromatography - Volume 5, Issue 1, 2018

Background: The use of diamond as a chromatographic stationary phase has been studied for more than 45 years now. Detonation nanodiamonds (DND) have been regularly used to increase the low specific surface area of these non-porous stationary phases. Recently, core-shell particles with multiple layers of polyallylamine (PAAm) and DND have been reported for liquid chromatographic separations and solid phase extractions. However, retention mechanisms and selectivity of such core-shell particles are not well -understood due to their complex architecture. Objective: The objective of this work was to perform a detailed evaluation of the mixed mode behavior of three structural analogues of new nanodiamond (ND)/polyamine based core-shell stationary phases. Method: Particles were composed of a glassy carbon core coated with poly(allylamine) (PAAm) and ND, deposited using a layer-by-layer (LbL) approach, with the outer PAAm layer treated with a mixture of a hydrophobic cross-linker and modifier to introduce C8/C18 functional groups. Three types of the core-shell stationary phases having different shell thicknesses and purity of NDs were investigated. Type II was prepared with fewer PAAm/ND bilayers as compared to Type I, whereas Type III was prepared with a same number of bilayers but with an additionally purified form of ND as compared to Type II. Ion-exchange interactions of the phases were tested with a set of model anionic, cationic and ampholytic analytes at different eluent pHs. The relative degree of anionexchange and hydrophobic interactions was also evaluated using a set of dansylated amino acids (Dns-AAs) at varied mobile phase buffer concentrations and organic solvent contents. Results: Anion-exchange properties were noted for all phases at low pH, with the Type I phase exhibiting stronger retention of anionic analytes due to higher PAAm content in its shell. Typical reversed- phase behaviour was observed for the hydrophobic Dns-AAs (Phe, Ile, Leu, Trp, Val and Pro), while the retention selectivity for hydrophilic Dns-AAs (Gln, Asn, Thr, Ser) indicated mixed mode mechanism. Values of methylene selectivity obtained for homologues of carboxylic acids indicated relatively low hydrophobicity for all studied phases. Conclusion: All three columns demonstrated low to medium hydrophobicity, and the solute selectivity was dominated by their ion-exchange property. These nanodiamond columns result in lower performance as compared to conventional analytical columns, however, the inherently stable nanodiamond platform ascertains that nanodiamond columns offer better stability at extreme conditions.

Background: In everyday practice, it is necessary to carry out the most number of analyses in the shortest possible time. Therefore, chromatographic systems must usually work above the optimum linear velocity, in order to obtain efficient separations and fast analysis. Objective: This review illustrates the main advantages of the new generation of core-shell particles with respect to the modern totally porous sub 2-μm ones. Method: The benefits of core-shell particles can be summarized as follows: i) superior mass transfer kinetics that leads to better kinetic performance at high mobile phase flow-rate; ii) better column packing quality; iii) possibility to use sub 3-μm core-shell packing column to obtain approximately only half the operating backpressure of the sub 2-μm fully porous particles under their own optimal flow rate. Results: Comparisons between core-shell particles and sub 2-μm ones have been highlighted, making comparative examples of applications in different analytical fields, such as pharmaceuticals, food, environment, forensic and natural products. This multiplicity of applications is a demonstration of the wide use of core-shell particles both for the high efficient separation of small molecules and for that of macromolecules. Conclusion: However, porous silica sub 2-μm particles still have a good part of the market, in particular as concerns food and pharmaceutical analysis. In addition, it is not possible to utilize fully the real separation power of these very efficient core-shell columns because of the ineligible extracolumn effects of the commercially available UHPLC instrument.

Introduction: The development of new stationary phases is a continuing aspect in the improvement of separations in HPLC. Relatively few new separation materials get beyond the exploratory stage as reported in the research literature and become commercially viable products with wide-spread use. One such class of stationary phases, those based on silica hydride and chemically modified silica hydride, has become more universally accepted, particularly as some of their unique features with respect to columns utilizing ordinary silica are discovered. Objective: This review covers some of the basic fundamental properties of silica hydride-based separation materials highlighting where these are unique or enhanced with respect to conventional materials. Method: In particular, the aqueous normal phase mechanism for the separation and retention of polar compounds is discussed in some detail as well as the dual retention aspect of these phases since all hydride stationary phases also function in the reversed-phase mode. Results: This dual retention capability and other features based on the hydride surface are used to distinguish these materials from hydrophilic liquid interaction (HILIC) chromatography, a format also used for the analysis of polar compounds. Conclusion: Silica hydride phases offer unique advantages over other types of separation materials including versatility, stability, fast equilibration as well as both excellent intra-day and inter-day reproducibility.

Background: The retention mechanisms of hydrophilic interaction liquid chromatography (HILIC) have been reported as inconclusive and of mixed mode, and have not been explained quantitatively. Packing materials used in HILIC are bare silica gel, polar bonded-silica gels, and ion-exchangers. The chromatography is performed in aqueous solution. Under such condition, the stability of these packing materials is not guaranteed by manufacturers. The fundamental problem associated with such discussions is the lack of definition of experimental conditions, which led to inconclusive discussions. Objective: To define the experimental conditions, to clarify the differences of HILIC from other liquid chromatography techniques, and to quantitatively explain HILIC retention mechanisms. Method: Molecular interaction (MI) between an analyte and a simple model phase was quantitatively analyzed in silico in terms of MI energy value difference. MI between an analyte and a solvent molecule was also studied. An analyte was inserted between the model phase and the solvent molecule, and the movement of the analyte was analyzed in terms of the strength of MI energy values. Results: The analyte moved toward the favorite site based on van der Waals force in reversedphase liquid chromatography, hydrogen bonding in HILIC, and electrostatic interaction in ionexchange liquid chromatography. Conclusion: For explaining the retention mechanism in HILIC, the experimental condition should be defined, and not be mixed with other liquid chromatographic conditions based on the properties of analytes and packing materials. The HILIC technique described herein is aqueous HILIC, while normal-phase liquid chromatography should be non-aqueous HILIC. Ion-exchange should be clearly classified as an independent liquid chromatographic method.

Background: In the criminal expertise exams routine, documents with aged appearance are quite common. A frequent type of fraud is the use of tea or coffee to produce yellowish color on paper, making the paper look older. One way to identify this kind of fraud is caffeine detection on paper. Objectives: This paper introduces a case study from Brasilian Federal Police that involved office receipts with aged appearance, in which it was detected the presence of caffeine, using Gas Chromatography/ Mass Spectrometry in SIM mode. The criminal experts suspected, embased by other evidences, that those documents were not as old as they looked like. Based on those findings, a subsequent study tried to optimize the conditions for extraction and detection of caffeine in documents whose aging has been simulated with the use of caffeinated beverages. Methods: To achieve this, we used a factorial design 2k fractionated ½, with five factors at two levels, with one repetition and Gas Chromatography/Mass Spectrometry in SIM mode to detect de presence of caffeine. Results: The conditions that resulted in the best concentration of caffeine extracted from the paper were recycled paper, paper with high grammage, 15 minutes extraction in ultrasonic bath, using black tea and immersing the paper in the caffeinated solution. Conclusions: this method proved to be feasible to detect caffeine in artificially aged documents with the use of coffee or tea.

Background: Food comes into contact with many chemicals during cultivation and storage, such as alkylphenols and phthalates. These compounds can bring harm to human health and therefore its residual quantification in food is needed. Methods: In this study, a new method for determination of 4-octylphenol, 4-tert-octylphenol, 4-nonylphenol, 4-tert-butylphenol, dibutylphthalate and benzylbutylphthalate was developed using solid phase microextraction and comprehensive two-dimensional gas chromatography with flame ionization detection. Alkylphenols and phthalates were determined in commercially available frozen samples of carrots, potatoes, and corn. Results: The calibration curve was constructed in concentrations ranging from 0.5 to 6.0 μg L-1 for 4-tert-butylphenol, 4- nonylphenol, 4-octylphenol and benzylbutylphthalate and 0.2 to 6.0 μg L-1 for dibutylphthalate and 4-tert-octylphenol. The limits of detection ranged from 0.07 to 0.32 μg kg-1. The compounds 4-tert-octylphenol, dibutylphthalate and benzylbutylphthalate were found in carrot samples in concentrations that ranged from 0.4 to 2.5 μg kg-1. Dibutylphthalate was found in one corn sample in concentrations of 0.6 μg kg-1. None of the targeted compounds were found in potato samples. Conclusion: The developed method allows the quantification of alkylphenols and phthalates in low concentrations, and is therefore a good alternative to analysis of the studied compounds in vegetables samples.

Background: Usually, diphteria toxin concentration and purification are performed after detoxification due to its high danger. In this work, an alternative diphteria toxoid (DTx) concentration and purification method by affinity chromatography is proposed. Methods: Screening of 19 triazinic dyes for selecting the best ligand for DTx concentration and purification by affinity chromatography was performed. Results: Cibacron Blue 3GA and Reactive Green 10 showed the highest affinity for DTx. The adsorption isotherms of DTx on Cibacron Blue 3GA-Sepharose and Reactive Green 19-Sepharose showed a good fit of experimental data to a Langmuir-type isotherm and allowed the calculation of a maximum capacity (qm) of 1.21 ± 0.12 μmol DTx/mL matrix and 1.44 ± 0.14 μmol DTx/mL matrix, respectively and a dissociation constant (Kd) of 9.75 ± 2.90 μM and 1.58 ± 0.38 μM, respectively. Blue R-HE-Sepharose isotherm did not show a good fit of experimental data to a Langmuirtype isotherm due to its high Kd. Conclusion: Cibacron Blue 3GA and Reactive Green 10 are promising ligands for concentration and purification of DTx by affinity chromatography.