Score Function for the Optimization of the Performance of Forward Fill/Flush Differential Flow Modulation for Comprehensive Two-Dimensional Gas Chromatography.

Modulation is the key element of the comprehensive 2D gas chromatography separation. Forward fill/flush flow modulation is cost effective, robust and suitable for analysis of a wide range of samples. Even though this modulation system is well known, studies regarding its optimization are sparse. In this work, based on hundreds of experiments involving multiple column sets and modulation conditions, an approach was proposed that permits to facilitate the choice of the forward fill/flush flow modulation parameters. A score function was developed that allows to predict the forward fill/flush flow modulation process efficiency as judged by the modulated peak shape. The score function was based on the physical rules for optimized and quantitative forward fill/flush flow modulation proposed in our previous work, which state that the sum of the fill and flush modulation distances should be close to the modulation channel length and that the ratio of the flush and fill distances should be sufficiently high for efficient channel flushing. The score function was embedded in a freely available tool in the form of a forward fill/flush flow modulation calculator, which allows the user either to quickly check the relevancy of the modulation operating conditions or to obtain a suggestion for optimal modulation parameters.

Advanced data preprocessing for comprehensive two-dimensional gas chromatography with vacuum ultraviolet spectroscopy detection.

Comprehensive two-dimensional gas chromatography with vacuum ultraviolet detection results in sizable data for which noise and baseline drift ought to be corrected. As the data is acquired from multiple channels, preprocessing steps have to be applied to the data from all channels while being robust and rather fast with respect to the significant size of the data. In this study, we have described advanced data preprocessing techniques for such data which were not available in the existing commercial software solutions and which were dedicated primarily to noise and baseline correction. Noise reduction was performed on both the spectral and the time dimension. For the baseline correction, a morphological approach based on iterated convolutions and rectifier operations was proposed. On the spectral dimension, much less noisy and reliable spectra were obtained. From a quantitative point of view, mentioned preprocessing steps significantly improved the signal-to-noise ratio for the analyte detection (circa six times in this study). These preprocessing methods were integrated into the plugim! platform (https://www.plugim.fr/).

Determination of vacuum ultraviolet detector response factors by hyphenation with two-dimensional comprehensive gas chromatography with flame ionization detection.

Two-dimensional comprehensive gas chromatography is an established technique, employed for the characterization of complex samples. Broadband vacuum ultraviolet absorption spectroscopy detection has recently attracted a lot of attention as it is a universal detection technique characterized by good selectivity but also ease of use and amenability to coupling with two-dimensional comprehensive gas chromatography. Vacuum ultraviolet spectroscopy is particularly interesting due to the possibility of performing spectral decomposition for species that coelute in gas chromatography analysis. This detector has quantitative capabilities, however not all species absorb vacuum ultraviolet radiation the same. Unfortunately, vacuum ultraviolet relative response factors for compounds are not always available. Methods to rapidly measure vacuum ultraviolet relative response factors and generate a large database that would allow calibration free quantitative analysis of complex mixtures are therefore of great interest. In this work, a universal methodology that permits rapid measurement of vacuum ultraviolet relative response factors is reported. It involves flow modulated two-dimensional comprehensive gas chromatography with dual vacuum ultraviolet and flame ionization detection. In this set-up, flame ionization detection is employed as a quantitative reference allowing to scale vacuum ultraviolet responses of investigated compounds. This approach was validated by flow measurements and by comparing relative response factors obtained for model compounds with literature data.

Quantitative performance of forward fill/flush differential flow modulation for comprehensive two-dimensional gas chromatography.

GC × GC is an advanced separation technique allowing to achieve quantitative and qualitative characterization of complex samples. In order to perform two-dimensional separation, the system must provide suitable peak modulation which will direct short impulses of first column flow towards the second column. Forward fill/ flush differential flow modulation is a cost effective and no cryogen requiring approach which allows modulation over a wide range of analytes with very different boiling points. However, optimization of the flow modulation process can be difficult to understand and quantification performance might be compromised if the parameters of the modulation process are not properly set. Modulated peak shape can be a good indication of the efficiency of the modulation process, however it is not sufficient to guarantee good quantification. Different average velocities in the beginning and the end of the thermally programmed GC run may cause different efficiency of the modulation process in various parts of the chromatogram. The purpose of this work is to investigate quantitative performance of the forward/fill flush modulation and delineate parameters that determine the effectiveness of the modulation process and its ability to properly reflect the quantitative composition of the investigated sample.

Gas chromatography vacuum ultraviolet spectroscopy: A review.

Accelerated technological progress and increased complexity of interrogated matrices imposes a demand for fast, powerful, and resolutive analysis techniques. Gas chromatography has been for a long time a 'go-to' technique for the analysis of mixtures of volatile and semi-volatile compounds. Coupling of the several dimensions of gas chromatography separation has allowed to access a realm of improved separations in the terms of increased separation power and detection sensitivity. Especially comprehensive separations offer an insight into detailed sample composition for complex samples. Combining these advanced separation techniques with an informative detection system such as vacuum ultraviolet spectroscopy is therefore of great interest. Almost all molecules absorb the vacuum ultraviolet radiation and have distinct spectral features with compound classes exhibiting spectral signature similarities. Spectral information can be 'filtered' to extract the response in the most informative spectral ranges. Developed algorithms allow spectral mixture estimation of coeluting species. Vacuum ultraviolet detector follows Beer-Lambert law, with the possibility of calibrationless quantitation. The purpose of this article is to provide an overview of the features and specificities of gas chromatography-vacuum ultraviolet spectroscopy coupling which has gained interest since the recent introduction of a commercial vacuum ultraviolet detector. Potentials and limitations, relevant theoretical considerations, recent advances and applications are explored.

BARCHAN: Blob Alignment for Robust CHromatographic ANalysis.

Two-dimensional gas chromatography (GC×GC) plays a central role into the elucidation of complex samples. The automation of the identification of peak areas is of prime interest to obtain a fast and repeatable analysis of chromatograms. To determine the concentration of compounds or pseudo-compounds, templates of blobs are defined and superimposed on a reference chromatogram. The templates then need to be modified when different chromatograms are recorded. In this study, we present a chromatogram and template alignment method based on peak registration called BARCHAN. Peaks are identified using a robust mathematical morphology tool. The alignment is performed by a probabilistic estimation of a rigid transformation along the first dimension, and a non-rigid transformation in the second dimension, taking into account noise, outliers and missing peaks in a fully automated way. Resulting aligned chromatograms and masks are presented on two datasets. The proposed algorithm proves to be fast and reliable. It significantly reduces the time to results for GC×GC analysis.

Complete elution of vacuum gas oil resins by comprehensive high-temperature two-dimensional gas chromatography.

The development of efficient conversion processes requires extended knowledge on vacuum gas oils (VGOs). Among these processes, hydrocracking is certainly one of the best suited to meet the increasing demand on high quality diesel fuels. Most of refractory and inhibiting compounds towards hydrocracking and especially nitrogen containing compounds are contained in a fraction of the VGO called the resin fraction, which corresponds to the most polar fraction of a VGO obtained by liquid chromatography (LC) fractionation on a silica column. However, the lack of resolution observed through existing analytical methods does not allow a detailed characterization of these fractions. A recent study showed that comprehensive high temperature two-dimensional gas chromatography (HT-GC×GC) methods could be optimized in order to elute heavy compounds. This method was implemented for the analysis of VGO resin fractions and complete elution was reached. Firstly, the method was validated through repeatability, accuracy, linearity and response factors calculations. Four VGO resin fractions were analyzed and their HT-GC×GC simulated distillation curves were compared to their GC simulated distillation (GC-SimDist) curves. This comparison showed that the method allows complete elution of most of the analyzed VGO resin fractions. However, a detailed characterization of these fractions is not yet obtained due to the very large number of heteroatomic and aromatic species that a flame ionization detector can detect. Current work aims at increasing the selectivity of GC×GC by using heteroatom selective detectors in order to improve the characterization of such products.

Overcoming the high-temperature two-dimensional gas chromatography limits to elute heavy compounds.

Overcoming high-temperature (HT) two-dimensional gas chromatography (GC × GC) limits is critical for the analysis of heavy hydrocarbons, particularly those contained in heavy petroleum fractions. To reach this goal, HT-GC × GC analysis was adapted from HT-GC Simulated Distillation (HT-GC SimDist) operating conditions as this analysis embraces compounds whose boiling point ranges from 35 °C to 700 °C and from a previous study that enabled the elution of linear alkane nC(60) and tetraaromatic compounds by HT-GC × GC. This paper shows that HT-GC × GC analysis using CO(2) cryogenic modulator and short wide bore columns with a thin film of stationary phase allows the elution of linear alkanes up to nC(68) (641 °C) as well as of highly aromatic hydrocarbons like coronene. Furthermore, compared to previous studies, an on-column injector was used to reduce discrimination of high boiling point compounds.

Norbadione a: kinetics and thermodynamics of cesium uptake in aqueous and alcoholic media.

Norbadione A (NbA) is a mushroom pigment, which is assumed to be involved in (137)Cs accumulation all over Europe during the Chernobyl nuclear accident. NbA bears seven acid-base functional groups, among which are two enolic and two carboxylic acid moieties. This work deals with complex formation of Cs(+) and NbA in ethanol, ethanol/water (9:1) (M1), and water with, when required, the support of two Cs(+) ionophore probes, calix[4]arene-bis(crown-6-ether)dioxycoumarine (A1) and its tetrasuslfonated form (A2). In ethanol, two Cs(+) complexes are formed, with the affinity constants K(1EtOH) = (1.1 ± 0.25) × 10(5) and K(2EtOH) = (2.1 ± 0.4) × 10(3). In M1, a single Cs(+) complex occurs when only the enols are deprotonated, whereas a bicomplex is formed when both enols and carboxylic acids are deprotonated: K(1M1) = (1.5 ± 0.3) × 10(5) and K(2M1) = (4 ± 2) × 10(3). These data are confirmed by stopped-flow and T-jump kinetics. In ethanol, a fast Cs(+) exchange occurs between NbA and A1: direct rate constant, k(1) = (3.1 ± 0.1) × 10(7) M(-1) s(-1); reverse rate constant k(-1) = (2.8 ± 1) × 10(5) M(-1) s(-1); and Cs(+) exchange constant, K(1Exchange) = (9 ± 4) × 10(-3). In M1, the quenching of A2 fluorescence by NbA is used to determine the kinetics of complex formation with Cs(+): k(2) = (1.8 ± 0.4) × 10(9) M(-1) s(-1); k(-2) = (1.80 ± 0.15) × 10(4) s(-1); and K(1M1) = (1.5 ± 0.5) × 10(5). The affinity of NbA for Cs(+) is probably the result of the particular structure in which the two pulvinic acid arms adopt a conformation that forms two complexation sites composed of the two enolates and/or the two carboxylates. This renders the efficiency in Cs(+) uptake comparable to that of some calixarenes or crown ethers.

Photoinduced cation translocation in a calix[4]biscrown: towards a new type of light-driven molecular shuttle.

The photophysics of a ditopic receptor of potassium ion consisting of a 1,3-alternate calix[4]biscrown with a merocyanine dye (DCM) inserted into each crown is reported. Thanks to the large difference between the binding affinity for one and two potassium ions, one can find relative total concentrations of ligand and potassium ion at which the 1:1 complex is most predominant with respect to the free ligand and the 2:1 complex whose amounts are a few percents. Investigation of the 1:1 complex by femtosecond transient absorption spectroscopy provides evidence for the ultrafast movement of a potassium ion through the calix[4]arene tube upon excitation at 400 nm of the dye. Phototranslocation occurs in the picosecond timescale with a non-exponential kinetics without competition with photoejection towards the bulk. The translocation time includes two main short components: 0.83 ps and 10 ps. A smaller-weighted third component of 101 ps might include a competition between phototranslocation and excitation energy transfer as shown by using Förster's theory. These findings open the way to new strategies for light-driven molecular shuttles with the aim of information storage and binary logic computing at a nanometric scale.

Kinetics, thermodynamics, and modeling of complex formation between calix[4]biscrowns and cesium.

Complex formations between calix[4]arene-bis(crown-6-ether) calix-COU2 (A1) and the tetrasulfonated species calix-COUSULF (A2) with Cs(+) are investigated in water and ethanol, and in 9:1 (M1) and 1:9 (M2) H(2)O/EtOH v:v mixtures, by chemical relaxation and molecular modeling. In ethanol and M2, two Cs(+) are included in A1 in two kinetic steps, whereas complex formation in M1 becomes controlled by a slow first-order kinetic process, which is accompanied by very fast Cs(+) inclusions, second-order rate constant: k'(1) = (3.4 +/- 0.8) x 10(7) M(-1) s(-1). In water and M1, A2 forms 1:1 and 1:2 cesium complexes in a single kinetic step, whereas in M2, two Cs(+) are included in two kinetic steps. The rate and thermodynamic constants involved are reported. They show that the second-order rate constants increase with the ethanol-to-water ratio, e.g., A2, second-order rate constant for the first Cs(+) in water: k(1A2water) = (9.7 +/- 0.3) x 10(4) M(-1) s(-1) and in M2: k(1A2M2) = (6.3 +/- 0.4) x 10(9) M(-1) s(-1). The affinities of both A1 and A2 for Cs(+) also increase with the ethanol-to-water ratio, e.g., first inclusion of A1 in M1: K(1A1M1) = (5 +/- 1.3) x 10(3) and in ethanol: K(1A1EtOH) = (7 +/- 3) x 10(6). The deviation from the expected mechanism of complex formation with alkali is attributed to the comparatively more difficult access of Cs(+) to the inclusion cavity of the capped calixarene. An analysis of calix-COU2 and calix-COUSULF and their Cs(+) complexes with only one rim capped by the crown ether confirms the thermodynamic and kinetic results, by showing that the inclusion cavity of calix-COUSULF is more adapted to Cs(+) than that of calix-COU2. This added to the presence of the shielding effect of the negative sulfonates can explain that the affinity of calix-COUSULF for Cs(+) is higher than that of calix-COU2. These results can be of interest in the search of an efficient Cs(+) decontaminant.

Multichromophoric supramolecular systems. Recovery of the distributions of decay times from the fluorescence decays.

The fluorescence decays of a tetrachromophoric calix[4]arene and a heptachromophoric cyclodextrin with appended pyridin-2'-yl-1,2,3-triazole groups, and their complexes with the cadmium ion, are analyzed with a sum of compressed hyperbola. Such functions are shown to be well suited for the recovery of the underlying distributions of decay times. The heterogeneity of the micro-environments experienced by the chromophores, and/or their mutual interactions in the excited state can in this way be displayed graphically. A fit with a sum of four exponentials is also found to be satisfactory, but the respective parameters lack direct physical meaning.

Photophysics of cyclic multichromophoric systems based on beta-cyclodextrin and calix[4]arene with appended pyridin-2'-yl-1,2,3-triazole groups.

The photophysical properties of the new pyridin-2'-yl-1,2,3-triazole chromophore have been investigated. Spectroscopic experiments and molecular modelling have provided evidence for a photoinduced charge transfer occurring from the triazole group to the pyridine ring. Hepta- and tetrachromophoric systems have been synthesized by covalently linking seven or four chromophores of this kind, respectively, to a beta-cyclodextrin and a calix[4]arene. They exhibit different fluorescence spectra, decays and quantum yields. Special attention has been paid to the binding of cadmium and zinc ions and to the resulting photophysical effects which are various and very different for the grafted beta-cyclodextrin and calix[4]arene systems.

Rhod-5N as a fluorescent molecular sensor of cadmium(II) ion.

The photophysical and complexing properties of Rhod-5N (commercially available) in MOPS buffer are reported. This fluorescent molecular sensor consists of a BAPTA chelating moiety bound to a rhodamine fluorophore. Its fluorescence quantum yield is low and a drastic enhancement of fluorescence intensity upon cation binding was observed. Special attention was paid to the complexation with Cd2+, a well known toxic metal ion. Possible interference with other metal ions (Na+, K+, Mg2+, Ca2+, Zn2+, Pb2+) was examined. Rhod-5N was found to be highly selective of Cd2+ over those interfering cations except Pb2+. The limit of detection is 3.1 microg l(-1).

Selective detection of cesium by a water-soluble fluorescent molecular sensor based on a calix[4]arene-bis(crown-6-ether).

A tetrasulfonated calixarene with two appended crowns, each including a dioxycoumarin fluorophore, is highly soluble in water and shows excellent selectivity for cesium ions.

Synthesis of novel rod-shaped and star-shaped fluorescent phosphane oxides--nonlinear optical properties and photophysical properties.

The design of a new class of fluorophores is presented. Some push-pull chromophores (D-pi-A) containing polyphenylethynyl units and a phosphane oxide moiety were efficiently prepared from common intermediates. Straightforward syntheses gave novel one-armed, rod-shaped and three-armed, star-shaped fluorophores. The optical properties of the resulting star-shaped derivatives were evaluated, showed high fluorescence quantum yields, and their excitation induces very efficient charge redistribution. Moreover, thanks to their push-pull character, the molecules exhibited significant second-order NLO properties with good transparency, up to 67x10(-30) esu at 1907 nm, with an absorption lambdamax at 369 nm. The effect of the donor group and of the number of phenylethynyl arms have been studied in this work.