Evaluation of stationary phases based on silica hydride for the analysis of drugs of abuse

Silica Hydride: A Separation Material Every Analyst Should Know About

This review describes the development, special features and applications of silica hydride-based stationary phases for HPLC. The unique surface of this material is in contrast to ordinary, standard silica, which is the material most frequently used in modern HPLC stationary phases. The standard silica surface contains mainly silanol (Si-OH) groups, while the silica hydride surface is instead composed of silicon-hydrogen groups, which is much more stable, less reactive and delivers different chromatographic and chemical characteristics. Other aspects of this material are described for each of the different bonded moieties available commercially. Some applications for each of these column types are also presented as well as a generic model for method development on silica hydride-based stationary phases.

The utility of silica hydride-based stationary phases for dual-mode ultra high performance liquid chromatography separation of synthetic cathinone positional isomers

Emerging drugs usually mimic the effects of traditional drugs, but are not always controlled due to the chemically altered structures. Positional isomers of emerging drugs are difficult to analyze because they challenge the separation and detection techniques commonly employed by forensic laboratories. The utility of silica hydride stationary phases for the ultra-high performance liquid chromatography separation of synthetic cathinone positional isomers was studied in this manuscript. SiH phases are operable under both reversed phase and aqueous normal phase chromatographic conditions without the need to change solvent reservoirs. The separation of eight positional isomers of the synthetic cathinone, pentedrone, was investigated using five silica hydride phases, and compared to a classical dual column reversed phase, hydrophilic interaction chromatography system, and to a bimodal pentaflurophenyl column. Significant selectivity differences were observed using either a combination of a classical reversed phase C18 and NP Silica columns or the various bimodal columns. The silica hydride silica-C column, which contains no derivatized ligands attached to the silica hydride backbone, not only gave the most orthogonal separation of the bimodal columns, but provided a unique separation of all eight positional isomers (resolution ≥ 1) using the combination of reverse phase and aqueous normal phase chromatographic conditions.

Evaluating novel silica hydride-based stationary phases for the analysis of phytocannabinoids and other psychoactive drugs

Three silica hydride based novel chromatographic phases chemically-bonded with allyloxy-DL-alpha-tocopherol, allylpentafluorophenyl, and 1-eicosene moieties were evaluated as separation media for selected phytocannabinoids and other substances of abuse. In order to assess column selectivity, a series of reference standards was analyzed and detected by using liquid chromatography with mass spectrometry. Further, quantitative detections of cannabidiol and tetrahydrocannabinol were attempted for the extracts of cannabis plants and cannabidiol gummy formulation. For potential bioanalytical applications, the columns were evaluated for substance screening in a human urine matrix. In summary, the newly developed columns are functional and effective for the analysis of phytocannabinoids and various psychoactive drugs with or without the presence of biological matrices.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Ammonium fluoride as a mobile phase additive in aqueous normal phase chromatography

The use of ammonium fluoride as a mobile phase additive in aqueous normal phase chromatography with silica hydride-based stationary phases and mass spectrometry detection is evaluated. Retention times, peak shape, efficiency and peak intensity are compared to the more standard additives formic acid and ammonium formate. The test solutes were NAD, 3-hydroxyglutaric acid, α-ketoglutaric acid, p-aminohippuric acid, AMP, ATP, aconitic acid, threonine, N-acetyl carnitine, and 3-methyladipic acid. The column parameters are assessed in both the positive and negative ion detection modes. Ammonium fluoride is potentially an aggressive mobile phase additive that could have detrimental effects on column lifetime. Column reproducibility is measured and the effects of switching between different additives are also tested.

Phenyl-bonded stationary phases--the influence of polar functional groups on retention and selectivity in reversed-phase liquid chromatography

The chromatographic properties of four phenyl-bonded phases with different structures were studied. The columns used were packed with a stationary phase containing a phenyl ring attached to the silica surface using different types of linkage molecules. As a basic characteristic of the bonded phases, the hydrophobicity and silanol activity (polarity) were investigated. The presence of the polar amino and amide groups in the structure of the bonded ligand strongly influences the polarity of the bonded phase. Columns were compared according to methylene selectivity using a series of benzene homologues and according to their shape and size selectivity using polycyclic aromatic hydrocarbons. The measurements were done using methanol/water and acetonitrile/water mobile phases. The presented results show that the presence of polar functional groups in the ligand structure strongly influences the chromatographic properties of the bonded phase.

Silica, hybrid silica, hydride silica and non-silica stationary phases for liquid chromatography

Free silanols on the surface of silica are the "villains", which are responsible for detrimental interactions of those compounds and the stationary phase (i.e., bad peak shape, low efficiency) as well as low thermal and chemical stability. For these reasons, we began this review describing new silica and hybrid silica stationary phases, which have reduced and/or shielded silanols. At present, in liquid chromatography for the majority of analyses, reversed-phase liquid chromatography is the separation mode of choice. However, the needs for increased selectivity and increased retention of hydrophilic bases have substantially increased the interest in hydrophilic interaction chromatography (HILIC). Therefore, stationary phases and this mode of separation are discussed. Then, non-silica stationary phases (i.e., zirconium oxide, titanium oxide, alumina and porous graphitized carbon), which afford increased thermal and chemical stability and also selectivity different from those obtained with silica and hybrid silica, are discussed. In addition, the use of these materials in HILIC is also reviewed.

Sub-2 μm fully porous and partially porous (core–shell) stationary phases for reversed phase liquid chromatography

The need for increased throughput and superior performance has increased the demand for stationary phases with improved kinetic performance.