Characterization by the solvation parameter model of the retention properties of commercial ionic liquid columns for gas chromatography

Assessment of physicochemical properties of sorbent materials in passive and active sampling systems towards gaseous nitrogen-containing compounds

The adsorption and desorption behavior of volatile nitrogen-containing compounds in vapor phase by solid-phase microextraction Arrow (SPME-Arrow) and in-tube extraction (ITEX) sampling systems, were investigated experimentally using gas chromatography-mass spectrometry. Three different SPME-Arrow coating materials, DVB/PDMS, MCM-41, and MCM-41-TP and two ITEX adsorbents, TENAX-GR and MCM-41-TP were compared to clarify the selectivity of the sorbents towards nitrogen-containing compounds. In addition, saturated vapor pressures for these compounds were estimated, both experimentally and theoretically. In this study, the adsorption of nitrogen-containing compounds on various adsorbents followed the Elovich model well, while a pseudo-first-order kinetics model best described the desorption kinetics. Pore volume and pore sizes of the coating sorbents were essential parameters for the determination of the adsorption performance for the SPME-Arrow sampling system. MCM-41-TP coating with the smallest pore size gave the slowest adsorption rate compared to that of DVB/PDMS and MCM-41 in the SPME-Arrow sampling system. Both adsorbent and adsorbate properties, such as hydrophobicity and basicity, affected the adsorption and desorption kinetics in SPME-Arrow system. The adsorption and desorption rates of studied C₆H₁₅N isomers in the MCM-41 and MCM-41-TP sorbent materials of SPME-Arrow system were higher for dipropylamine and triethylamine (branched amines) than for hexylamine (linear chain amines). DVB/PDMS-SPME-Arrow gave fast adsorption rates for the aromatic-ringed pyridine and o-toluidine. All studied nitrogen-containing compounds demonstrated high desorption rates with DVB/PDMS-SPME-Arrow. Chemisorption and physisorption were the sorption mechanisms in MCM-41- and MCM-41-TP- SPME-Arrow, but additional experiments are needed to confirm this. An active sampling technique ITEX gave comparable adsorption and desorption rates on the selective MCM-41-TP and universal TENAX-GR sorbent materials for all the compounds studied. Vapor pressures of nitrogen-containing compounds were experimentally estimated by using retention index approach and these values were compared with the theoretical ones, calculated using the COnductor-like Screening MOdel for Real Solvent (COSMO-RS) model. Both values agreed well with those found in the literature proving that these methods can be successfully used in predicting VOC's vapor pressures, e.g. for the formation of secondary organic aerosols.

Regulation of hydrogen bond acidity and its effect on separation performances

Ionic liquid bonded polysiloxanes (PILs) are a class of polysiloxanes whose side chains contain ionic liquid (IL) moieties. They not only inherit the character of "dual nature" from ILs but also inherit the excellent film-forming ability and thermal stability from polysiloxanes. In this paper, the solvation parameter model is introduced to investigate the interaction characteristics of PILs. The experimental results show that the b values of PILs occur in a wider range than those previously reported for the stationary phases. The hydrogen bond acidity can be effectively adjusted by varying the ionic liquid content or substituents. Hindering the formation of the hydrogen-bonded networks and increasing the exposed hydrogens may be intrinsic to the strong hydrogen bond acidity of PILs. Subsequently, the separation performances of these PIL stationary phases were demonstrated by separating various mixed samples of aromatic isomers, dichloroanilines, substituted alkanes, alcohols, esters, etc. The results show that the PILs with strong hydrogen bond acidity have excellent selectivity performances for aromatic position isomers, alcohols, and substituted alkanes. This study is significant for understanding the hydrogen bond acidity and broadening the range of hydrogen bond acidity of ionic liquid stationary phases.

Separation of Fatty Acid Dimethyl Esters on an Ionic Liquid Gas Chromatographic Column

An ionic liquid (IL) 111 column was compared with other commonly employed stationary phases including polydimethyl siloxane and polyethylene glycol for the separation of fatty acid monomethyl and dimethyl esters. The fatty acid esters employed in this study were derived from metathesis reactions of vegetable oils both with and without olefins. The IL 111 column demonstrated enhanced performance compared with conventional columns for the separation of these esters. These advantages included significantly enhanced retention of dimethyl esters relative to monomethyl esters, excellent cis/trans isomer separation and the ability to analyze higher carbon number dimethyl esters. As a result, these columns are highly suited for the analysis of mixtures of mono- and dimethyl fatty acid esters found in lipid metathesis reaction products or to determine monofunctional impurities in samples of commercial dimethyl esters.

Polymeric Ionic Liquids Derived from L-Valine for the Preparation of Highly Selective Silica-Supported Stationary Phases in Gas Chromatography

A series of silica-supported polymeric ionic liquid (PIL)-based stationary phases derived from a vinylic L-valine ionic liquid monomer and divinylbenzene (DVB) as the crosslinking agent have been prepared and studied as gas chromatographic stationary phases. These coated gas chromatographic columns exhibited good thermal stabilities (230-300 °C) and high efficiencies (1700-2700 plates/m), and were characterized using a linear solvation parameter model in order to understand the effects of the amount of DVB on the features of the resulting composite systems. Their retention behavior and separation efficiencies were demonstrated using the Grob test. By tuning the crosslinking degree for the IL-derived stationary phase, the separation selectivity and resolution of different compounds were improved. The different retention behaviors observed for many analytes indicate that these stationary phases may be applicable as new types of GC stationary phases.

The Current Role of Graphene-Based Nanomaterials in the Sample Preparation Arena

Since its discovery in 2004 by Novoselov et al., graphene has attracted increasing attention in the scientific community due to its excellent physical and chemical properties, such as thermal/mechanical resistance, electronic stability, high Young's modulus, and fast mobility of charged atoms. In addition, other remarkable characteristics support its use in analytical chemistry, especially as sorbent. For these reasons, graphene-based materials (GBMs) have been used as a promising material in sample preparation. Graphene and graphene oxide, owing to their excellent physical and chemical properties as a large surface area, good mechanical strength, thermal stability, and delocalized π-electrons, are ideal sorbents, especially for molecules containing aromatic rings. They have been used in several sample preparation techniques such as solid-phase extraction (SPE), stir bar sorptive extraction (SBSE), magnetic solid-phase extraction (MSPE), as well as in miniaturized modes as solid-phase microextraction (SPME) in their different configurations. However, the reduced size and weight of graphene sheets can limit their use since they commonly aggregate to each other, causing clogging in high-pressure extractive devices. One way to overcome it and other drawbacks consists of covalently attaching the graphene sheets to support materials (e.g., silica, polymers, and magnetically modified supports). Also, graphene-based materials can be further chemically modified to favor some interactions with specific analytes, resulting in more efficient hybrid sorbents with higher selectivity for specific chemical classes. As a result of this wide variety of graphene-based sorbents, several studies have shown the current potential of applying GBMs in different fields such as food, biological, pharmaceutical, and environmental applications. Within such a context, this review will focus on the last five years of achievements in graphene-based materials for sample preparation techniques highlighting their synthesis, chemical structure, and potential application for the extraction of target analytes in different complex matrices.

Potential of topological descriptors to model the retention of polychlorinated biphenyls in different gas chromatography stationary phases, including ionic liquid-based columns

The aim of this study was to develop a statistical model based on a set of intuitive topological descriptors that will help to determine the influence of the polychlorinated biphenyls (PCBs) structural features on the chromatographic behavior of these analytes in a variety of gas chromatographic stationary phases, including the highly polar ionic liquid (IL)-based SLB-IL76 and SLB-IL60 columns. The model was developed using the stepwise multiple linear regression method, and constructed through several levels of increasing complexity to make evident the relative influence of the selected descriptors. The proposed model was easy to implement and provided similar satisfactory results irrespective of the dependent variables used (i.e., retention index or retention time) or the chromatographic conditions applied (i.e., pseudo-isotherm and programmed temperature) for IL-based phases. The model also allowed the correct prediction of the elution order of selected PCBs in these and other less polar phases evaluated (i.e., SW-10, DB-17, ZB-5 and HT-8). To our knowledge, this is the first models based on topological descriptors described in the literature that provided a satisfactory fitting of the PCB behavior in IL-based phases. Our results indicated that the mechanism governing the chromatographic separation of PCBs in these highly polar columns showed significant differences compared with those observed in other less polar stationary phases.

Room-temperature-ionic-liquid coated graphitized carbons for selective preconcentration of polar vapors

Most adsorbent materials used for preconcentrating and thermally desorbing volatile and semi-volatile organic compounds (S/VOCs) in portable or "micro" gas chromatographic (GC/µGC) instruments preferentially capture non-polar or moderately polar compounds relative to more polar compounds. Here, we explore the use of a known trigonal-tripyramidal room-temperature ionic liquid (RTIL) as a surface modifier for the graphitized carbons, Carbopack B (C-B) and Carbopack X (C-X), with the goal of enhancing their capacity and selectivity for polar S/VOCs. Breakthrough tests were performed by challenging tubes packed with ∼2.5 mg of C-B or RTIL-coated C-B (RTIL/C-B) with 13 individual S/VOCs, including several organophosphorus compounds and reference alkyl and aromatic hydrocarbons of comparable vapor pressures, at concentrations ranging from 14 to 130 mg/m³. The 10% breakthrough volume, V_b10, was used as the measure of capacity. For the RTIL/C-B, the V_b10 values of the five organophosphorus vapors tested were consistently ∼2.5 times larger than those for the untreated C-B, and V_b10 values of the four non-polar reference vapors were 11-26 times smaller for the RTIL/C-B than for the untreated C-B. For compounds of similar vapor pressure the capacity ratios for polar vs. non-polar compounds with the RTIL/C-B ranged from 1.8 to 34. Similar results were obtained with C-X and RTIL/C-X on a smaller set of compounds. Tests at 70% relative humidity or with a binary mixture of a polar and non-polar compound had no effect on the capacity of the RTIL/C-B, and there were no changes in V_b10 values after several months of testing that included cycling from 25 to 250 °C. Capacity was strongly correlated with vapor pressure. Attempts to reconcile the selectivity using models based on linear-solvation-energy relationships were only partially successful. Nonetheless, these results indicate that RTIL coating of carbon adsorbents affords a simple, reliable means of rendering them selective for polar S/VOCs.

Ionic liquids as gas chromatographic stationary phases: how can they change food and natural product analyses?

The volatile fraction of natural products often consists of complex mixtures of isomeric and/or homologous components with similar structural and physical characteristics (e.g. mono- and sesquiterpenoids) that are not easy to separate simultaneously with conventional GC stationary phases, even when used with multidimensional systems. The introduction of ionic liquids (ILs) as stationary phases has opened up new perspectives in this field as their unique solvation properties result in uncommon selectivity, which is completely different to that of classic polydimethylsiloxane (PDMS)- and polyethyleneglycol (PEG)-based columns. Because of their peculiar selectivity and high inertness, IL-based columns have already found several applications in the natural product field in mono- and multidimensional GC and preparative GC, leading to the exhaustive analysis of complex samples (including aqueous solutions), and the separation of challenging pair(s) of compounds. This article provides an overview of how IL-based columns can be exploited in the fields of food and natural products, explores the wide range of applications that have already been developed and highlights the main features of these promising stationary phases, which are expected to be further extended in the near future, in particular, for routine use. Graphical abstract.

Retention characteristics of silica materials in carbon dioxide/methanol mixtures studied by inverse supercritical fluid chromatography

In this work, inverse supercritical fluid chromatography was applied to characterize the surface of four silica materials (three commercial Kromasils and one silica aerogel) from chromatographic retention data. Retention factors at various pressures (150-300 bar), temperatures (25-60 °C) and modifier concentrations (5-20 vol.% methanol in CO₂) for a set of representative 17 solutes were correlated with the solute properties by the linear solvation energy relationships (LSER). Two types of the LSER models were identified based on different criteria. Firstly, a generally valid model with two descriptors concerning dipolarity/polarizability and solute hydrogen-bonding acceptor ability was constructed. Secondly, a group of specific models for each particular silica material was proposed. According to the statistical analysis of the modeling results, the acid-basic interactions were demonstrated to have a major contribution to the retention for all studied silicas. The intensity of these interactions decreases with increasing methanol concentration in the mobile phase, possibly due to the mixed mechanism of competitive adsorption of the modifier on silanol groups and modification of mobile phase property. Moreover, retention factors measured under constant conditions (p, T, methanol concentration) for a pair of the materials were found to be proportional in logarithmic scale implying the transferability of the adsorption free energies and the adsorption constants across four studied silica materials.

A detailed structural study of cytotoxicity effect of ionic liquids on the leukemia rat cell line IPC-81 by three dimensional quantitative structure toxicity relationship

In the present study, a very thorough and in-depth three-dimensional quantitative structure-toxicity relationship (3D-QSTR) analysis has been implemented to make a correlation between the structural information of the ionic liquids (ILs) and their cytotoxicity towards Leukemia rat cell line IPC-81, as one of the ILs' toxicological consequences. To do this, alignment free GRid-INdependent Descriptors (GRINDs), which were derived from molecular interaction fields (MIFs), were correlated to the cytotoxicity values by partial least squares (PLS) and support vector regression (SVR). Genetic algorithm (GA), as a powerful linear tool, was used to select the best and interpretative subset of variables for the predictive model building. The selected variables with the capability to screen the effective structural features, showed direct and inverse contribution to the cytotoxicity. In silico modeling can reduce the amount of cellular testing necessary by predicting the toxicological functions of the chemical structures. Acceptable predictions of both internal and external validation sets made it possible to develop the predictive models for a large set of 269 diverse ILs containing 9 cationic cores and 44 types of anions. The constructed 3D-QSTR models use simple and interpretable descriptors to provide an in-depth and mechanistic interpretation of structural characteristics. This helps provide a clear understanding of the cytotoxicity effects of the understudy ILs. The effects of the nature of the cations, anions, and substituents on the cytotoxicities were evaluated and discussed.

Recent advances on ionic liquid uses in separation techniques

The molten organic salts with melting point below 100°C, commonly called ionic liquids (ILs) have found numerous uses in separation sciences due to their exceptional properties as non molecular solvents, namely, a negligible vapor pressure, a high thermal stability, and unique solvating properties due to polarity and their ionic character of molten salts. Other properties, such as viscosity, boiling point, water solubility, and electrochemical window, are adjustable playing with which anion is associated with which cation. This review focuses on recent development of the uses of ILs in separation techniques actualizing our 2008 article (same authors, J. Chromatogr. A, 1184 (2008) 6-18) focusing on alkyl methylimidazolium salts. These developments include the use of ILs in nuclear waste reprocessing, highly thermally stable ILs that allowed for the introduction of polar gas chromatography capillary columns able to work at temperature never seen before (passing 300°C), the use of ILs in liquid chromatography and capillary electrophoresis, and the introduction of tailor-made ILs for mass spectrometry detection of trace anions at the few femtogram level. The recently introduced deep eutectic solvents are not exactly ILs, they are related enough so that their properties and uses in countercurrent chromatography are presented.

A QSPR approach to the ecotoxicity of ionic liquids (Vibrio fischeri) using VolSurf principal properties

Recently derived in silico structural descriptors for both IL cations and anions allowed the development of a QSPR model correlating ionic liquid structures to Vibrio fischeri toxicity using the partial least squares (PLS) approach. Interpretation of the PLS model confirmed the effect of IL cationic structural features such as the influence of cation side chain length, presence of heteroatoms, and non-aromaticity of the heterocyclic scaffold on toxicity. The PLS model also provided a quantitative evaluation of anion effects, previously not evidenced due to the structural similarity of the anions considered. A simple equation in which three descriptors (two for the cations and one for the anions) allow the prediction of Vibrio fischeri toxicity for over 8000 ILs is reported.

Modelling the aquatic toxicity of ionic liquids by means of VolSurf in silico descriptors

VolSurf+ in silico physicochemical descriptors for both the cationic and the anionic counterparts of ionic liquids (ILs) have been derived. These descriptors, suitable for molecular modelling of IL structures which, due to their amphiphilic nature, interact strongly with biological matrices, can be related to aquatic toxicity by means of a partial least squares statistical model. This model gives an insight into the relationships between structural physicochemical properties and aquatic toxicity as well as a satisfactory quantitative structure-property correlation, allowing prediction of aquatic toxicity scores of ILs.

Prediction of ionic liquid's heat capacity by means of their in silico principal properties

VolSurf+ in silico principal properties of ionic liquids were used to develop a QSPR model providing affordable heat capacity predictions which were experimentally validated.

A multivariate insight into ionic liquids toxicities

A multivariate insight into the toxicities of ionic liquids provides a comprehensive picture and guidelines for the evaluation of their eco- and bio-sustainability.