Evaluation and prediction of the shape of gas chromatographic peaks

The evaluation and prediction of the shape of asymmetric gas chromatographic peaks is important as the knowledge of the amount of tailing permits to foresee the resolution between closely eluting peaks and to select the best analytical conditions for an efficient and rapid separation. A model function was tested in order to approximate the true peak shape obtained on non-polar column by injecting different compounds. The trend of the parameters involved in the used equation has been investigated as a function of column temperature and inlet pressure. The reproduction of the symmetrical or asymmetrical shape of gas chromatographic peaks is satisfactory and the method also permits to predict the shape of peaks obtained in different conditions of temperature and pressure.     

New liquid crystals derived from thiophene connected to the 1,2,3-triazole heterocycle

In this work, six new compound derivatives from thiophene were synthesised. The three symmetrical compounds were synthesised containing two heterocyclic 1,2,3-triazole on each side of thiophene, 2,5-disubstituted and the three non-symmetrical compounds were synthesised containing alkyne groups as a spacer in replacing one of these heterocycles. The structural modifications were made by changing the number of alkoxy groups in order to understand the relation between structure and mesomorphic behaviour. Some of the compounds presented liquid crystalline properties, smectic and nematic mesophases. The non-symmetrical compounds allowed for a low emission in the blue region.     

New liquid crystals derived from thiophene connected to the 1,2,4-oxadiazole heterocycle

In this study, eight new compounds derived from thiophene were synthesised and characterised. The four symmetrical compounds contain two heterocyclic 1,2,4- oxadiazole on each side of the 2,5-disubstituted thiophene, and the four non-symmetrical compounds contain alkyne groups as a spacer replacing one of these heterocycles. Some of the compounds presented liquid crystalline properties with smectic and nematic mesophases. The non-symmetrical compounds presented low emission of blue light.     

Effects of Energetic Surface Heterogeneity on Ion Adsorption at the Electrolyte/Oxide Interfaces: Non-symmetrical Binding-to-Surface Energy Dispersions

The equations developed by us for the surface complexation models, taking into account energetic heterogeneity of surface oxygens, are applied here to study the effects of the shape of adsorption energy distribution on the ion adsorption at the oxide/electrolyte interface. The paper presents comparison of two models: one assuming that the energetical heterogeneity of oxide surface is described by a symmetrical and next a non-symmetrical adsorption energy dispersion for the formation of various surface complexes. The comparison of these two models was based on the obvious assumption that the variances of both the symmetrical and the non-symmetrical distributions are equal. The potentiometric titration data are not sensitive enough to choose the right adsorption model. So, in addition the individual isotherms of adsorption of cation and anion, of the inert electrolyte measured radiometrically, have been taken into consideration. The symmetrical adsorption energy distribution seems to represent the features of these adsorption systems better.     

Symmetrical and Non-symmetrical Pd (II) Pincer Complexes Bearing Mesoionic N-heterocyclic Thiones: Synthesis,Characterizations and Catalytic Properties

In contrast to well-established symmetrical pincer complexes, non-symmetrical metal pincers with the loss of C_2v symmetry are much less studied. In this work, mesoionic NHTs (NHTs = N-heterocyclic thiones), which can be viewed as the sulfur adducts of mesoionic carbenes, are incorporated into pincer complexes for the first time. Two symmetrical and non-symmetrical phenylene-bridged bis (NHT) compounds 3a / 3b were synthesized as proligands via a “cycloaddition-alkylation-thionation” reaction sequence. In a case to access bis (NHT) compound 3c , N-dealkylation reactions occurred. The carbene NMR signals of NHTs are only partially correlated to the π-accepting abilities of carbenes, which is different from Bertrand's carbene-phosphinidene system. The structural analysis of 3a / 3b indicates that they possess C-S partial double bonds. 3a and 3b served as the precursors to access two aryl anion-linked [SCS/S′] pincer complexes 6a / 6b . An external base proved to be essential for this cyclopalladation process. The catalytic properties of 6a / 6b in the cycloisomerizations of alkynoic acids have been examined. Finally, non-symmetrical complex 6a shows superior catalytic performance in such transformations contrasting to its symmetrical counterpart 6b .     

Peak half-width plots to study the effect of organic solvents on the peak performance of basic drugs in micellar liquid chromatography

The addition of the anionic surfactant sodium dodecyl sulphate (SDS) to hydro-organic mixtures of methanol, ethanol, propanol or acetonitrile with water yielded enhanced peak shape (i.e. increased efficiencies and symmetrical peaks) for a group of basic drugs (β-blockers) chromatographed with a Kromasil C18 column. The effect can be explained by the thin layer of surfactant associated to the hydrocarbon chain on the stationary phase in the presence of the organic solvents, which covers the free silanols on the siliceous support avoiding their interaction with the cationic basic drugs. These instead interact with the anionic head of the surfactant increasing their retention and allowing a more facile mass transfer. The peak shape behaviour with the four organic solvents (methanol, ethanol, propanol and acetonitrile) was checked in the presence and absence of SDS. The changes in peak broadening rate and symmetry inside the chromatographic column were assessed through the construction of peak half-width plots (linear relationships between the left and right half-widths at 10% peak height versus the retention time). The examination of the behaviour for a wide range of compositions indicated that the effect of acetonitrile in the presence of SDS is different from ethanol and propanol, which behave similarly. Acetonitrile seems to be superior to the alcohols in terms of peak shape, which can be interpreted by the larger reduction in the adsorbed surfactant layer on the C18 column. However, the decreased efficiencies observed at increasing surfactant concentration in the mobile phase should be explained by the reduction in retention times, more than by a change in the stationary phase nature.     

The role of the theoretical plate concept and gamma density functions in studies on mass transport and mass loadability of chromatographic columns

Summary The validity of the use of the plate theory in transport processes with non-Gaussian peak shapes is discussed. It is shown that the application of the plate theory implies the assumption of a peak with a gamma density shape which, however, converges rapidly to symmetrical Gaussian shape for large plate numbers. A proposal for an extension of the plate theory, based on a gamma density function is given. This approach results in an extra parameter which characterizes the asymmetry of a peak. Use of the suggested fitting function permits an estimate of the moments of the peak, even without the use of a computer. The proposed model can be used to determine a transfer function for the chromatographic system, allowing a system theoretical approach to determine the influence of, for example, amplifiers, filters and detectors on the peak shape. The exponentially modified Gauss (EMG) is one of the possible peak shapes included in the model.     

Chromatographic behavior of new deazapurine ribonucleosides in hydrophilic interaction liquid chromatography

The chromatographic behavior of new biogenic purine nucleosides in hydrophilic interaction liquid chromatography was examined on three different stationary phases, namely bare silica, and amide‐ and cyclofructan‐based stationary phases. The effects of buffer concentration, pH and acetonitrile‐to‐aqueous‐part ratio in the mobile phase on retention and peak shape were assessed. The retention coefficients and peak symmetry values substantially differed with respect to analytes´ structures, stationary phase properties and mobile phase composition. The bare silica column was unsuitable for these compounds under the chromatographic conditions tested due to very broad and asymmetrical peaks. Furthermore, the cyclofructan‐based stationary phase provided almost Gaussian peak shapes of all deazapurine nucleosides under most conditions tested. Therefore, the cyclofructan‐based stationary phase is the most suitable choice for the chromatographic analysis of nucleosides.     

Instrument-induced effects in the analysis of polycyclic aromatic compounds by capillary gas chromatography with atomic emission detection (GC-AED)

Peak splitting of high molecular weight polycyclic aromatic compounds originating from the microwave plasma of an atomic emission detector (AED) coupled to a GC has been described and evaluated. The influence of the solute structure, solute concentration, and physical conditions in the AED (such as detector temperature, make-up gas flow, concentration of reagent gases and distance of column end from the plasma) have been studied. An explanation is presented for peak splitting, which is based on an insufficient solute decomposition and solute mass flow in the discharge tube. Modification of the instrument by introduction of additional make-up gas applied through the transfer line has been shown to improve peak shape and solute response.     

Mobile phase additives for enhancing the chromatographic performance of astaxanthin on nonendcapped polymeric C30‐bonded stationary phases

Astaxanthin shows peak deformation and reduced peak area response when eluted with methanol and methyl tert‐butyl ether on nonendcapped polymeric C₃₀‐bonded HPLC phases. The present study tested different column manufacturers, column batches, and ten mobile phase additives including acids, bases, buffers, complexing and antioxidant agents for improvement of peak shape and peak area response. Concerning chromatographic benefits and feasibility, ammonium acetate was found to be the best additive followed by triethylamine for all columns tested. Variation of the mobile phase pH equivalent and the column temperature showed no synergistic effects on peak shape and peak area response. Results indicate that peak tailing and variation of peak area response are due to different on‐column effects. Possible mechanisms of the observed phenomenon will be discussed.     

Analysis of peak asymmetry in chromatography

The knowledge of the symmetry of chromatographic peaks is extremely important regarding the digital signal processing. The significant deviation of the peak shape from the symmetrical peak makes hardly possible the acquisition of chromatographic signal information, such as the retention time, the peak area, the peak width at half peak height, the peak overlapping, etc. In the literature one can find many methods for the determination of the asymmetry factor. For example it is suitable to calculate the skewness from the third central moment. However in case of noisy baseline the value of the skewness oscillates highly depending on the number of points used for the mathematical calculation. In this work a new method is presented for the determination peak shape asymmetry. We order mathematical function to the chromatographic peaks by fitting, and then symmetrical curve is generated with the same peak maximum position and height, the peak width is fitted. The difference of the two functions is constituted and areas of the data differences are calculated, which are really characteristics of the peak asymmetry. Correlation between the area of the difference signal and the asymmetry factor is established. The method was applied for different types of chromatographic peak shapes and the results were interpreted.     

Factors affecting peak shape in comprehensive two-dimensional gas chromatography with non-focusing modulation

In the case of a non-focusing modulator for comprehensive two-dimensional gas chromatography (GC × GC), the systematic distortions introduced when the modulator loads the second-dimension column give rise to a characteristic peak shape. Depending on the operating conditions this systematic distortion can be the dominant component of the second-dimension elution profiles in the GC × GC peak. The present investigation involved a systematic investigation of peak shape in pulsed-flow modulation (PFM)–GC × GC. It is shown that low flow ratio can lead to significant peak skewing and increasing the flow ratio reduces the magnitude of peak skewing. Validation of the peak shape model is made by comparison with experimental data. The residuals from the fitting process (normalised to the maximum detector response) vary between –1.5% and +2.6% for an isothermal model and between –1.0% and +3.0% for a temperature-programmed model.     

辅助气燃烧接口对有机化合物碳同位素测定的影响

对多种烷烃及芳香化合物采用不同类型的反应管进行了碳同位素分析, 旨在阐明燃烧接口对有机化合物色谱行为的影响. 结果表明, 在反应管中填充较少的催化剂有助于提高化合物的分离度, 这可能是由于反应管中载气具有较高的线速度, 从而减少了化合物的扩散加宽. 然而, 不同反应管对分离度的改善作用相对有限. 对于不同的反应管, 化合物的灵敏度变化主要与开口分流处分流比的变化有关, 而这种变化可能是由反应管内载气流速的变化所致.     

Investigation of peak compression effects in packed column supercritical fluid chromatography using chemometrics

Summary In this paper chemometrics have been used to study and characterize peak compression phenomena in packed column SFC. A carbon dioxide/2-methyl-1-propanol mobile phase was used in the experimental design (modifier concentration, temperature and pressure) and modelling part of the investigation. A cubic interaction term was needed in the model to obtain a reasonable fit, suggesting that all three parameters are of significance in terms of controlling peak compression. At the optimum conditions derived from the model a narrow peak was obtained as predicted.     

An investigation of peak-broadening effects arising when combining CE with MS

In this study, peak-broadening effects caused by nebulizing gas flow and lack of temperature control have been investigated for separation capillaries with three different inner diameters. The study was performed with serial UV/ESI-MS detection in an effort to distinguish between peak broadening arising in the separation and peak broadening arising in the ion source. The nebulizing gas was found to significantly affect both migration time and separation efficiency when using capillaries with 50 and 75 microm id. If the nebulizing gas is on during injection, the injection volume increases to such an extent that significant peak broadening is induced. Reducing the id to 25 microm minimizes the parabolic flow induced by the nebulizing gas. Results indicate that the nebulizing gas pressure can be optimized to minimize peak broadening in the ion source. A decrease in detection sensitivity, possibly related to the orthogonal design of the interface, was observed when the nebulizing gas pressure was increased. A tapered capillary tip was found to provide superior separation efficiency as well as sensitivity.     

Azo functionalised achiral bent-core liquid crystals: observation of photo-induced effects in B7 and B2 mesophases

Three structural variants of azo substituted achiral bent-core compounds are reported. Here, the effect of symmetrical and non-symmetrical arms at 1,3-positions of the central phenyl ring on the mesogenic properties of the resulting bent-core azo compounds is studied. The structures of all the compounds synthesised are confirmed by the organic spectroscopic methods. The liquid crystalline properties are investigated using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction studies. It was found that the non-symmetrical molecules are more conducive to mesomorphism than the symmetrical ones. We observed B₁ (Col_r), B₂ (SmC_AP_A) and B₇ mesophases in these compounds. The B₇ mesophase was found to have a modulated layer structure. Interestingly, a reversible field-induced transition from the B₇-like structure to the racemic SmC_AP_F was also observed. We also report the photo-induced studies in the B₇ mesophase and make a comparison of these results with those obtained in a B₂ mesophase. From our studies, we observed that these effects are more profound in the case of B₇ mesophase when compared to the B₂ mesophase in such systems.     

Study of elution behaviour with gradient voltage in CEC using methacrylate monolithic columns

A theoretical study on the retention behaviour and chromatographic performance of neutral solutes using a lauryl methacrylate‐based monolithic column under voltage gradient mode in CEC was carried out. Through a flexible mathematical function based on a modified Gaussian model, the peak shape of compounds was firstly fitted under constant and gradient voltage. Using the peak shape parameters and retention time, the estimation of global chromatographic performance, efficiency and peak capacity under several voltage conditions was performed. The influence of voltage gradient on the separation efficiency is discussed and simple equations are presented to calculate retention and peak widths under voltage gradient conditions. A comparison in terms of chromatographic performance of a test mixture of neutral solutes under constant and gradient voltage modes was also carried out. The experiments carried out under gradient voltage showed better efficiencies (172 000 plates/m) and lower peak widths than those obtained under constant voltage (52 000 plates/m).     

Analysis of amine compounds by capillary gas chromatography using ammonia–helium carrier gas

The use of 10%, 1%, and 0.1% ammonia in helium as carrier gas was investigated as a means of improving poor chromatographic peak shape often associated with low level determinations of amine compounds using thin film capillary columns. The 1% ammonia in helium was found suitable for improving the peak shape of sterically unhindered amine compounds, such as urethane and certain aliphatic primary amines, during gas chromatographic analysis on thin film columns. There was a negligible effect on the peak tailing arising on thick film columns. The 0.1% ammonia in helium was suitable, but not as efficient as the 1% ammonia in helium, in eliminating the peak tailing associated with low level analysis of amine compounds. The signal-to-noise (S/N) ratio improved from < 1 using helium carrier gas to 20–25 (for certain test amine compounds) using 1% ammonia in carrier gas. The 10% ammonia in helium carrier gas had an effect on the chromatographic performance similar to that of the 1% ammonia in helium, but the baseline level was very high and this mixture was not used in further studies.     

Low generation PEGylated phosphorus-containing dendrons with phosphonate anchors

We present a simple strategy to obtain non-symmetrical polyethylene glycols equipped with telechelic phosphorus-containing dendrimeric moieties having adhesive phosphonate surface functions. The dendronized PEG tails were characterized by means of multi-nucleus NMR. The grafting abilities of model symmetrical PEG compounds equipped with telechelic amino-bis(methylene-dimethylphosphonates) and amino-bis(methylenephosphonic acids) were rapidly screened on plasma treated silicon wafers and activated silica.     

Peak clustering in two-dimensional gas chromatography with mass spectrometric detection based on theoretical calculation of two-dimensional peak shapes: the 2DAid approach

A method is presented to facilitate the non-target analysis of data obtained in temperature-programmed comprehensive two-dimensional (2D) gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-ToF-MS). One main difficulty of GC×GC data analysis is that each peak is usually modulated several times and therefore appears as a series of peaks (or peaklets) in the one-dimensionally recorded data. The proposed method, 2DAid, uses basic chromatographic laws to calculate the theoretical shape of a 2D peak (a cluster of peaklets originating from the same analyte) in order to define the area in which the peaklets of each individual compound can be expected to show up. Based on analyte-identity information obtained by means of mass spectral library searching, the individual peaklets are then combined into a single 2D peak. The method is applied, amongst others, to a complex mixture containing 362 analytes. It is demonstrated that the 2D peak shapes can be accurately predicted and that clustering and further processing can reduce the final peak list to a manageable size.