Enantioseparation of pesticides by gas chromatography. Measurement of association constants enantiomer-chiral selector.

In this study, the association constants of sixteen pesticides with the chiral selector octakis(6-O-tert-butyldimethylsilyl-2,3-di-O-acetyl)-γ-cyclodextrin were determined. The procedure only involved a few experimental measurements; namely, gas hold-up time and retention time of pesticides in capillary columns, as well as column phase ratio at each temperature condition. Fundamental equations of gas-liquid chromatography were used to estimate association constants. Two sets of columns containing different concentrations of the mentioned chiral selector dissolved in (14 %-cyanopropyl-phenyl)-86 %-methyl-polysiloxane were used. One set included capillary columns without any chemical treatment and the other group included columns that were crosslinked. The systematic comparison between both groups indicated a deleterious effect of the crosslinking on enantioselectivity. Our main objective is to promote the use of gas chromatography for the analysis of volatile and semi-volatile chiral pesticides. Thus, we proposed a simple methodology, based only on chromatographic measurements, to obtain information about the enantiorecognition ability of a particular chiral selector constituting the stationary phase and the influence of the selected polymer on the selectivity experimentally obtained.

Enantioseparation of agrochemicals by gas chromatography. Exploring columns based on cyclodextrin derivatives dissolved into polysiloxanes.

The main goal of this work is to expand the availability of chiral columns for the analysis of agrochemicals by gas chromatography. A broader offer of chiral stationary phases would allow shifting toward enantioselective analytical techniques environmentally more friendly for those compounds. We prepared seven chiral capillary columns based on derivatives of either, ß-cyclodextrin or γ-cyclodextrins dissolved at high concentrations, in two typical polysiloxanes with different polarities, demonstrating not only the significance of the chiral selector but also of the polymer solvent for achieving adequate enantioseparation of some agrochemicals. The enantiorecognition ability of each column was evaluated with 20 volatile and semivolatile agrochemicals, possessing one or two chiral centers. Besides, to elute more polar agrochemicals, as well as to enhance enantioselectivity, three derivatization procedures targeting the carboxyl and/or amine group were evaluated. The results revealed that the prepared column consisting of octakis(2,3-di-O-acetyl-6-O-tertbutyldimethylsilyl)-γ-cyclodextrin dissolved in (14%-cyanopropyl-phenyl)-86%-methyl-polysiloxane provides the broadest enantiorecognition capacity. This column allowed the enantioseparation of seventeen chiral agrochemicals, including metalaxyl, furalaxyl, and four imidazolinones, which were not enantioseparated in the remaining columns. To the best of our knowledge, glufosinate, fluorochloridone, fenarimol, furalaxyl, and four imidazolinones were enantioseparated by gas chromatography for the first time.

Determination of gas-polydimethylsiloxane distribution constants of major Cannabis terpenes and terpenoids by capillary gas-liquid chromatography.

Terpenes and terpenoids are the principal responsible for the aroma of Cannabis, playing an important role in the interaction with the environment. Analytical determination of these compounds can be done by headspace coupled to solid phase micro-extraction (HS-SPME) and then injected in a gas chromatograph. In the present study, we determined distribution constants between gas and polydimetylsiloxane (PDMS), a conventional SPME liquid phase, at three temperatures between 303.15 and 343.15 K for major Cannabis terpenes and terpenoids employing a method based in gas chromatography using four capillary columns for monoterpenes and five columns for sesquiterpenes. In addition, van't Hoff regressions (logKfg vs T-1) were obtained in order to estimate logKfg at 298.15 K aiming to compare with bibliographic values (experimental or estimated ones). An excellent agreement was found between them. The method, based on chromatographic theory is robust and relatively simple. It is expected that the herein obtained data could be useful for selecting SPME fiber type and dimensions, estimating extraction efficiencies, as well as to develop prediction models and validate them.

Determination of variability of terpenes and terpenoids in Cannabis sativa by gas chromatography-flame ionization detection and gas chromatography-mass spectrometry.

Absolute content of terpenes in inflorescences of two strains of Cannabis sativa L., CAT 1 and CAT 3, has been determined. Twenty terpenes commonly present in these samples were quantified by solid phase microextraction combined with gas chromatography and flame ionization detection (SPME/GC-FID). High amounts of ß-myrcene, α-pinene, ß-pinene, limonene, (E)-ß-ocimene, ß-caryophyllene, α-humulene, (E)-nerolidol, and linalool, were found in both strains. Lower concentrations (< 20 µg·g-1) of other terpenes were also determined. Only (E)-ß-ocimene was detected at 50 µg·g-1 in CAT 3 whereas it was below the LOD in CAT 1. Concentrations of other compounds for which standards were not available, were estimated based on a response factor obtained from the calibration curves of compounds with similar chemical structures. Fingerprints of both CAT strains were obtained and the identities of most volatile compounds were assigned using gas chromatography coupled to mass spectrometer detector (GC-MS). Additionally, an assessment of variability of terpenes was achieved by analyzing ten plants of each strain grown under controlled conditions and harvested at the same time. This variability was about 20%, considering terpenes at concentration above 20 µg·g-1.

Solid-Phase Microextraction-Gas Chromatography Analytical Strategies for Pesticide Analysis.

Due to their extensive use and the globalized commerce of agricultural goods, pesticides have become a global concern. Despite the undoubtful advantages of their use in agricultural practices, their misuse is a threat to the environment and human health. Their analysis in environmental samples and in food products continues to gain interest in the analytical chemistry community as they are challenging matrices, and legal concentration limits are particularly low (in the order of ppb). In particular, the use of solid-phase microextraction (SPME) has gained special attention in this field thanks to its potential to minimize the matrix effect, while enriching its concentration, allowing very low limits of detection, and without the need of a large amount of solvents or lengthy procedures. Moreover, its combination with gas chromatography (GC) can be easily automated, making it a very interesting approach for routine analysis. In this review, advances and analytical strategies for the use of SPME coupled with GC are discussed and compared for the analysis of pesticides in food and environmental samples, hopefully encouraging its further development and routine application in this field.

Development of a background electrolyte for the determination of inorganic cations in high ionic strength samples by capillary electrophoresis with indirect UV-absorption detection.

In this study, a background electrolyte capable to separate and quantify inorganic cations in high ionic strength samples by UV-absorption indirect detection was designed. In this regard, the four most abundant monovalent and divalent cations in earth crust (K+, Na+, Ca+2, Mg+2) were selected as model compounds. A group of small carboxylic acids and, several toluidines and pyridines were evaluated as mild strength complexing agents and chromophoric probes, respectively. The optimized background electrolyte was composed of 200 mM 2,4,6-trimethylpyridine as the chromophoric probe, 250 mM lactic acid as the weak complexing agent and pH buffering reagent (adjusted to pH 4.5), and 5% v/v methanol as organic solvent modifier. Based on a minimum number of components, it provided outstanding separation performance in less than 4 min in a wide linear dynamic range (10 - 2500 µg·mL-1). Performances were contrasted against a reference method based on conductometric detection. Furthermore, studies of separation efficiency and peak shape were carried out at different analyte concentrations in high electric conductivity solutions. The herein developed method demonstrated exceptional features in terms of limits of detection (~10 µg·mL-1), resolution, speed of analysis, sensitivity and peak capacity in high electric conductivity samples. Moreover, the method was successfully applied to high ionic strength samples such as rock digest, sea water, soy sauce and isotonic drinks.

Study of enantioseparation of ß-blockers using amylose tris(3-chloro-5-methylphenylcarbamate) as chiral stationary phase under polar-organic, reversed-phase and hydrophilic interaction liquid chromatography conditions.

In this study, we describe the experimental variables influencing enantioseparation of twelve ß-blockers when analyzed under polar-organic, reversed-phase and hydrophilic interaction liquid chromatography conditions on a column with immobilized amylose tris(3-chloro-5-methylphenylcarbamate) as chiral stationary phase. Regarding polar-organic mode, two component mobile phases consisting of methanol, ethanol or acetonitrile with the addition of basic additives such as diethylamine, triethylamine, mono-ethanolamine, ethylendiamine or trifluoroacetic acid/diethylamine mixture were evaluated. Studies of retention at different temperatures were also performed. In reversed-phase mode, mixtures consisting of methanol or acetonitrile with either aqueous boric acid-borate buffer or sodium hydrogen carbonate-carbonate buffer solutions were compared aiming to study the influence of organic modifier as well as buffer type and pH on resolution. In addition, a systematic evaluation of the retention factors of ß-blockers enantiomers in hydro-organic eluents containing acetonitrile in presence of diethylamine as additive was carried out by increasing progressively the water content, in order to check for retention dependencies indicative of the interplay of both hydrophilic interaction liquid chromatography and reversed-phase modes.

Chiral separation of several pesticides on an immobilized amylose tris(3-chloro-5-methylphenylcarbamate) column under polar-organic conditions. Influence of mobile phase and temperature on enantioselectivity.

In this work, the use of different solvents and temperatures was explored, aiming to evaluate their influence on the enantioseparation of pesticides by HPLC in polar-organic conditions, employing a column containing immobilized amylose tris(3-chloro-5-methylphenyl-carbamate). The chiral separation of seventeen different pesticides widely used as herbicides, fungicides, insecticides and precursors were studied. The mobile phases included methanol, ethanol, iso-propanol, n-propanol and acetonitrile; either pure or containing additives such as diethylamine, trifluoroacetic acid, formic acid, acetic acid or mixtures thereof. We studied the influence of these eluents on chiral separation of those pesticides in terms of retention factor, enantioselectivity, enantioresolution and peak symmetry. Regarding temperature influence, evaluated within the range 5 - 40 °C, nearly all the compounds decreased their retention and selectivity factors with the increase in temperature, although the effect was dependent on the mobile phase solvent. Moreover, estimation of thermodynamic parameters was performed based on linear van´t Hoff plots.

Efficiency of capillary GC columns based on phosphonium ionic liquids.

Gas chromatographic columns based on ionic liquids (ILs) are very promising since the selectivity of these columns can be tuned by both the cation and the anion chemical nature. In this paper, efficiencies of capillary columns based on four phosphonium ionic liquids were studied. The performance of seven columns containing the cation trihexyl(tetradecyl)phosphonium and the anions bromide, chloride, and bis(trifluoromethylsulfonyl)imide was evaluated by measuring the solute band broadening as a function of gas velocities at three temperatures. Hence, classical height equivalent to a theoretical plate (H) against gas velocity (u) plots corresponding to those columns were generated and the data were fitted to the Golay-Guiochon equation with the aim of seeking the optimum conditions to be operated each of them. Band broadening at practical gas velocities is mainly due to poor mass transfer properties of solutes in the (viscous) liquid phases, which limits the achieved efficiencies. These H/u plots proved to be necessary to characterize the column quality at a given temperature, to interpret the band broadening phenomena and thus, to establish the lower temperature limits and the expected plate counts at that temperature.

Determination of thermodynamic binding constants by affinity capillary electrophoresis.

A strategy to study thermodynamic binding constants by affinity capillary electrophoresis (ACE) is presented. In order to simplify mathematical treatment, analogy with acid-base dissociation equilibrium is proposed: instead of ligand concentration [X], negative logarithm of ligand concentration (or activity), pX = -log[X], is used. On this base, and taking into account ionic activities, a general procedure for obtaining thermodynamic binding constants is proposed. In addition, the method provides electrophoretic mobilities of the free analyte and analyte-ligand complex, even when binding constants are low and thus, the complexed analyte fraction is also low. This is useful as a base to rationally analyze a diversity of situations, i.e., different mathematical dependencies are obtained when analytes and ligands with different charges are combined. Practical considerations are given for carrying out a full experimental design. Enantiomeric ACE separation based on the use of chiral selectors is addressed. 2-hydroxypropyl-ß-cyclodextrin was chosen as a model ligand, and both enantiomeric forms of four pharmaceutical drugs (propranolol, pindolol, oxprenolol and homatropine methylbromide) were considered as model analytes. Practical aspects are detailed and thermodynamic binding constants as well as free and complexed analytes mobilities are determined.

Enantiomeric separations by capillary electrophoresis: Theoretical method to determine optimum chiral selector concentration.

A method to optimize the ligand concentration [S] in the background electrolyte of capillary electrophoresis separations is presented. It is based on the use of a model which predicts apparent electrophoretic mobilities as a function of ligand concentration (expressed as p[S] = -log[S]). This model is employed to compose the expression of a recently proposed criterion to qualify separations in electrophoresis. Two strategies to find the optimum p[S], leading to the best separation of all compounds, are explained: 1.- a graphical method using a windows map depicting the single separation criteria between all possible combination of compounds by pairs, and 2.- an analytical method where an extended multicriterion optimization function is composed and optimum p[S] is found by mathematical maximization. The procedure is applied to a hard-to-separate model system: enantiomeric separations of racemic mixtures. 2-Hydroxypropyl-ß-cyclodextrin was chosen as a model ligand, and four pharmaceutical drugs as model analytes. In order to demonstrate the performance of the procedure, results of electrophoretic separations obtained at p[S] found as optimum are compared with separations obtained at p[S] values slightly higher and lower than the optimum.