Broadening of peaks eluted before the solvent in capillary GC Part II: The role of phase soaking

Summary Phase soaking is a solvent effect which tends to reconcentrate peaks eluted after and to broaden peaks eluted before the solvent. The principles of the phase soaking effect on peaks eluted before the solvent are discussed. The broadening effect is distinguished from the broadening effect occurring in the flooded column inlet by partial solvent trapping. It was found that in most cases broadening by partial solvent trapping strongly predominated over broadening by phase soaking. Phase soaking was noticeable only in the neighbourhood of the solvent peak.Phase soaking does not broaden peaks eluted before the solvent as much as it re-concentrates those eluted after it. The two phase soaking effects on the front and the rear of the solvent band (that is, of the soaked zone) differ strongly from each other.Peak broadening by phase soaking is negligible for non-trapped components, because such solutes start their chromatography before a significant quantity of solvent enters the column. Phase soaking only broadens solute bands which were retained by the solvent in the column inlet, that is, bands already broadened by partial solvent trapping.     

Achieving high peak capacity production for gas chromatography and comprehensive two-dimensional gas chromatography by minimizing off-column peak broadening

By taking into consideration band broadening theory and using those results to select experimental conditions, and also by reducing the injection pulse width, peak capacity production (i.e., peak capacity per separation time) is substantially improved for one dimensional (1D-GC) and comprehensive two dimensional (GC×GC) gas chromatography. A theoretical framework for determining the optimal linear gas velocity (the linear gas velocity producing the minimum H), from experimental parameters provides an in-depth understanding of the potential for GC separations in the absence of extra-column band broadening. The extra-column band broadening is referred to herein as off-column band broadening since it is additional band broadening not due to the on-column separation processes. The theory provides the basis to experimentally evaluate and improve temperature programmed 1D-GC separations, but in order to do so with a commercial 1D-GC instrument platform, off-column band broadening from injection and detection needed to be significantly reduced. Specifically for injection, a resistively heated transfer line is coupled to a high-speed diaphragm valve to provide a suitable injection pulse width (referred to herein as modified injection). Additionally, flame ionization detection (FID) was modified to provide a data collection rate of 5kHz. The use of long, relatively narrow open tubular capillary columns and a 40°C/min programming rate were explored for 1D-GC, specifically a 40m, 180μm i.d. capillary column operated at or above the optimal average linear gas velocity. Injection using standard auto-injection with a 1:400 split resulted in an average peak width of ～1.5s, hence a peak capacity production of 40peaks/min. In contrast, use of modified injection produced ～500ms peak widths for 1D-GC, i.e., a peak capacity production of 120peaks/min (a 3-fold improvement over standard auto-injection). Implementation of modified injection resulted in retention time, peak width, peak height, and peak area average RSD%'s of 0.006, 0.8, 3.4, and 4.0%, respectively. Modified injection onto the first column of a GC×GC coupled with another high-speed valve injection onto the second column produced an instrument with high peak capacity production (500-800peaks/min), ～5-fold to 8-fold higher than typically reported for GC×GC.     

Influence of the errors made in the measurement of the extra-column volume on the accuracies of estimates of the column efficiency and the mass transfer kinetics parameters

The influences of the errors made in the measurement of the extra-column volume of an instrument on the accuracies of the estimates made of the column efficiency and of the parameters of the mass transfer kinetics were investigated from an experimental point of view. A standard HP1090 apparatus (extra-column volume, approximately 50 micro L) was used to measure the efficiency of a Sunfire-C(18) RPLC column (column hold-up volume, approximately 1.50 mL). The first and second moments of the peaks of phenol (a retained compound) and of thiourea (a practically non-retained compound) were measured at six different temperatures between 22 and 78 degrees C, for flow rates between 0.10 and 4.70 mL/min (i.e., for linear velocities between 0.025 and 1.179 cm/s). Each series of measurements was successively made with the instrument being fitted with and without the column. The experimental HETP data must be corrected for the solute dispersion in the connected tubes in order properly to assess the true column efficiency. Even with a modern, high performance instrument, the dispersion of a non-retained compound is essentially due to the band broadening phenomena that take place in the extra-column volumes, the sum of all these extra-column band broadening contributions accounting for more than 80% of the total band broadening measured. The contribution of the sampling device is particularly deleterious since, for a 2 mu L injection, the maximum solute concentration in the peak that enters into the column is nearly ten-fold lower than that of the sample. Nevertheless, the impact of the extra-column volumes on the estimates of the kinetic parameters (e.g., molecular diffusion coefficient D(m) and effective particle diffusivity D(e)) remains negligible. Obviously, the relative error made on the column efficiency of a retained compound depends much on its retention factor. It decreases from 8 to 1% when the retention factor increases from 5 to 17.     

High-speed gas chromatography: the importance of instrumentation optimization and the elimination of extra-column band broadening

This review provides a summary of chromatographic theory as it applies to high-speed gas chromatography. A novel method for determining the optimal linear flow velocity, u (opt), from specific experimental parameters, is discussed. An in-depth theoretical understanding of u (opt) and its relation to experimental parameters is presented, in the absence of extra-column band broadening, as a means of method evaluation and optimization. Recent developments in high-speed GC are discussed, in the context of the theory presented within this review, to ascertain the influence of extra-column band broadening. The theory presented herein can be used as a means of evaluating the various areas of GC instrumentation (injection, separation, detection, etc.) that need further development to further minimize the effects of extra-column band broadening. The theoretical framework provided in this review, can be, and is, readily used to evaluate high-speed GC results presented in the literature, and thus, the general practitioner may more readily select a specific capillary length and/or internal diameter for a given application. For example, it is theoretically shown, and prior work cited, that demonstrates a peak width of approximately 1 ms is readily achievable in GC, when extra-column band broadening is eliminated.     

Extra-column effects in multi-dimensional switching systems (MDSS) — GC

In order to permit valveless MDSS operation and easy column change, the hardware designed contains several connections. These can give rise to extra-column band broadening effects. In order to quantitate such effects, the packed-capillary column configuration (without trap) was used. The band broadening caused by the connections was found to be small: 120 ms². A more practical approach was used to check the performance of the trap (re-injection) in the capillary-trap-capillary column configuration. Here the results obtained with re-injection from the trap were compared with results obtained in transfer and heart-cutting modes while the trap was heated. Re-injection was tested with n-alkanes ranging from n-C₆ to n-C₂₈. A starting peak width of 1.6 s was found.     

Hydrogen bond network formation

The signs of the dynamic dichroism of the NH stretch and the amide I bands in nylons were found to be counterintuitive. Experiments show that polymer chains tend to align in the direction of an applied tensile strain. The CH stretching bands in nylons exhibit the expected negative dynamic dichroism indicating chain alignment in the strain direction. The ΔA′ peaks for the NH and amide I bands are positive. The ΔA′ peak for the NH band is also unusual in that it has a derivative shape. This can be explained by band shifts brought about by anisotropic changes in the intermolecular spacing in the glassy polymer. Above T_g the derivative shape disappears but the ΔA′ peak for both the NH and amide I absorption remain positive. We postulate that the positive ΔA′ peaks of the NH and amide I bands result from a hydrogen bonding network where stress is transmitted through a network consisting of covalent chains connected by hydrogen bonds.     

Interpretation of the real plate number concept: A fundamental analysis

The real plate number concept is interpreted in terms of the fundamental properties of a chromatograph. It is shown that the linear relationship between the total peak width at half height, (b_0.5)T, t, and k is only an approximation of the more general linear relationship between (b_0.5)²T, t and (1 + k)². In the case where the relationship between (b_0.5)T, t and k approaches linearity, it is concluded, firstly, that b₀ = a, i. e., b₀ is a column property and independent of extra-column phenomena and, secondly, that n_real=n_theor. When the relationship becomes non-linear, b₀ is determined partly by extra-column phenomena, but not in terms of a useful relationship. It is concluded that it is unnecessary to introduce the parameters a, b₀ and n_real, since they all have counterparts in the established theory of chromatography. These counterparts prove to be equally useful in the practical analysis of chromatographic systems.     

Flow-through radioactivity detection of 3H- and 14C-labelled compounds in reversed-phase liquid chromatography by a liquid scintillation counting technique based on post-column extraction

A method for flow-through liquid scintillation counting in reversed-phase liquid chromatography (RP-LC) based on the continuous extraction of aqueous column eluates with a water-immiscible liquid scintillator is evaluated in terms of sensitivity, reproducibility and extra-column band broadening. ³H- and ¹⁴C-labelled phenylthiohydantoinamino acid derivatives of widely different polarity serve as model compounds. For extractable derivatives, counting efficiencies of over 30 and 80% can be obtained for ³H and ¹⁴C, respectively. The reproducibility and extra-column band broadening depend on the mixing ratio of scintillator and LC eluent; relative standard deviations (peak areas) of less than 3% can be obtained. The sensitivity of flow-through counting can be increased at least 150-fold by storing the segmented scintillator/eluate stream in a capillary storage loop. After the separation is complete, the stream is re-introduced into the radioactivity detector at reduced flow-rates to increase the mean residence time, i.e., the counting time, in the detector.     

Vibrational spectra and vibrational states of simple gas-phase hydrogen-bonded dimers

The article aims to outline the growth of evidence and ideas about infrared band broadening for simple, gaseous, moderately strong, hydrogen bonded dimers B…HA, to draw attention to areas in need of further development and to collect together experimental information available at the present time about vibrational states associated with such dimers. The band associated with the modified HA stretching mode, ν_s, is observed for several dimers not only to be broad, but to have sub-band structure, which is satisfactorily interpreted as arising from combination bands of ν_s with the low frequency stretch-mode ν_σ, giving a progression ν_s ± nν_σ as a result of strong anharmonic coupling. For weaker dimers with lower values of ν_σ the sub-band structure is less evident, and may appear only as shoulders, while for still weaker dimers, the sub-bands may be merged into a featureless broad band. A major factor contributing to the breadth of the individual sub-bands in the band structure is the presence of hot-bands, especially a long series based on successive excited states of the low-frequency bending mode ν_β. A link is indicated with the interpretation of band-broadening for moderately strong hydrogen-bonded complexes in the liquid state. The anharmonic coupling of ν_s and ν_σ is again a central feature but there is a new factor, namely the coupling of ν_σ through a fluctuating potential with the surroundings, which has the result that the ν_s mode rapidly loses phase coherence resulting in a broad structureless band.     

Associative organotin polymers. I. Symmetric trialkyltin fluorides: Synthesis and properties

Four novel symmetric tri-n-alkyltin fluorides were prepared. In this homologous series, melting point decreases with increase in the carbon chain length while an x-ray diffraction study shows an increase in amorphousness. ¹H-NMR spectra in cyclohexane, a nonpolar solvent in which these compounds form long polymeric chain, show a peak broadening effect due to high solution viscosity at very low solute concentration. Contrary to this, the spectra in CDCl₃, a polar solvent in which these compounds are monomeric, show well resolved and narrow peaks. IR frequencies for Sn? C vibrations have been assigned.     

Influence of the eluent composition on column efficiency in reversed-phase high-performance liquid chromatography

Summary The dependence of column efficiency on the eluent (MeOH/H₂O) composition in a reversed-phase liquid chromatography system within a wide concentration range has been systematically examined. It is shown that when the intracolumn effect of mass transfer and diffusion is the main factor controlling band broadening the column efficiency decreases with the increase of the viscosity of the MeOH/H₂O mixture; on the other hand, when the extra-column effect is the main factor an increase in the viscosity of the eluents will help in improving column efficiency. The column efficiency is also related to the properties of the sample.     

Molecular complexes of iodine with metal acetylacetonates

The ability of metal acetylacetonates to act as electron donors and form molecular complexes with I₂ was studied by examining the electronic, vibrational, and NMR spectra of the complexes. The specific compounds used in the study were Al(acac)₃ Sc(acac)₃ Zr(acac)₄, and Th(acac)₄. The electronic spectra of mixtures of the metal acetylacetonates with I₂ in CHCl₃ had, in addition to the absorption peaks characteristic of the free components, two peaks that were due to the charge transfer complexes. For each complex, the highest wavelength peak (near 360 nm) was assigned to the blue shifted I₂ band, while the lower peak (between 270 nm and 305 nm) was attributed to the intermolecular charge transfer. In the i.r. spectra of each complex, the major effect of complexation was to cause the I₂ stretching frequency to appear between 145 cm⁻¹ and 160 cm⁻¹. The positions of the absorption peaks in both the electronic and vibrational spectra led to the conclusion that in these complexes, I₂ had received a large amount of charge from the donors. Complex formation had little effect on the NMR spectra of the donors. Association constants of 1:1 complexes were determined from the concentration dependence of the absorbance of the blue shifted I₂ bands. Values of ΔHdg and ΔS°₂₉₈ for the complex formation were obtained from the temperature variation of the association constants. The data indicate that the complexes are extremely stable species. Both the stability of the complexes and the high degree of charge transfer were rationalized by considering a model for the intermolecular interactions that involved two M(acac) rings simultaneously transferring charge from one donor to an I₂ molecule.     

Structural analysis of boron compounds using 11B-3QMAS solid state NMR

The structural analysis of three boron compounds, boron carbide (B₄C), silicon tetraboride (SiB₄) and hexagonal-boron nitride (h-BN), were performed using 2D ¹¹B-triple quantum MAS (3QMAS) solid state NMR capable of averaging the second-order quadrupolar interaction of ¹¹B that cause line broadening, splitting and low frequency shift of the central transition (−1/2, 1/2). The coordination number around the boron atom and structural symmetry of each boron compound is discussed by means of the isotropic chemical shift Δσ and the quadrupolar coupling constant C_Q calculated from 3QMAS spectra. Δσ of SiB₄ is quite larger than that of B₄C, which is thought to be caused by its structural distortion and distribution. Δσ of h-BN was found to be higher frequency shift obviously than that of B₄C and SiB₄ because of the difference of the boron coordination number, three-coordinated in h-BN and six-coordinated in B₄C and SiB₄. h-BN has very large C_Q compared to other two boron compounds since the h-BN forming a two-dimensional network has less structural symmetry than B₄C and SiB₄.     

Approaches to characterise chromatographic column performance based on global parameters accounting for peak broadening and skewness

Peak broadening and skewness are fundamental parameters in chromatography, since they affect the resolution capability of a chromatographic column. A common practice to characterise chromatographic columns is to estimate the efficiency and asymmetry factor for the peaks of one or more solutes eluted at selected experimental conditions. This has the drawback that the extra-column contributions to the peak variance and skewness make the peak shape parameters depend on the retention time. We propose and discuss here the use of several approaches that allow the estimation of global parameters (non-dependent on the retention time) to describe the column performance. The global parameters arise from different linear relationships that can be established between the peak variance, standard deviation, or half-widths with the retention time. Some of them describe exclusively the column contribution to the peak broadening, whereas others consider the extra-column effects also. The estimation of peak skewness was also possible for the approaches based on the half-widths. The proposed approaches were applied to the characterisation of different columns (Spherisorb, Zorbax SB, Zorbax Eclipse, Kromasil, Chromolith, X-Terra and Inertsil), using the chromatographic data obtained for several diuretics and basic drugs (β-blockers).     

Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part II. Micromechanism of CO chemisorption and a new concept of σ-π coordination

A micromechanism of CO adsorption and a new concept of σ-π coordination on transition metal are proposed in this article. Based on experimental facts, we assume CO 5σ- and/ or CO 1 π interacts with the representative M.O.s of the metal valence band, ψ(Mi, Vs) and ψ(Mi, Vd), to form the bonding M.O. group and antibonding M.O. group. The bonding group is located below the Fermi level (E_f), in which some M.O.s are much more characteristic of metal orbitais (denoted as M-CO σ-bondings) while some M.O.s exhibit slight metal orbital characteristics, which belong to the excited valence M.O.s of adsorbed CO, conventionally assigned as adsorbed CO 5σ, CO 1 π and CO 4σ. The calculated data indicate that the peak positions of adsorbed CO 5σ, CO 1 π and CO 4σ are significantly higher than their corresponding M.O.s in the gaseous CO molecule, i.e. adsorbed CO is in an excited (or activated) state. The total energy generated (ΔE) from adsorbed CO 5σ, CO 1 π and CO 4σ can be used as a qualitative parameter for characterizing the ability for CO dissociation. On the other hand, the antibonding empty M.O. group of M-CO is located above the E_f, which exhibits some characteristics of metal d orbitais. The hybridization of CO 2π with dπ- orbitais in the Vs, Vd bands and dπ orbitais of the antibonding M.O. group of M-CO bondings results in the formation of unoccupied M.O.s with CO 2π-M dπ character. These M.O.s plus those unoccupied M.O.s without CO 2π-M dπ character contribute the adsorbate-derived resonances, located 3-5 eV above E_F and observed by Inverse Photo-Emission (IPE) difference spectra. We have used orbital overlap integrals of S(CO 5σ, dσ, Vd) and S(CO 2π, dπ, Vd) to characterize the relative competitive abilities for hybridization of CO 5σ and CO 2π with d orbitais. The calculated results show that CO 5σ possesses a stronger ability to hybridize d orbitals in the Vd band than does CO 2π-, thus the peaks of adsorbate-induced empty levels are shifted farther from the d band when the competitive hybridizing factor [CHF=S(CO 5σ, dσ, Vd)/S(CO 2π, dπ, Vd)] is increased. The calculated data demonstrate that the peak positions of CO adsorbate-derived resonances of Cu, Ni, Pd and Pt metals, observed by IPE difference spectra, are in good parallel with their CHF values. Moreover, the values of CHE also demonstrate that CO σ-bonding stimulates d electrons to transfer upward from the d band to the Vs band, where much more CO 2π-M dπ character exists. We propose here a new concept of d back-donation, i.e. d electrons transfer from the occupied d band to the unoccupied M.O.s exhibiting CO 2π-M dπ character in the Vs and Vd bands, which weakens the π bond of C-O and simultaneously strengthens the M-C bond; these phenomena have been confirmed by IR spectroscopy and EELS. The d back-donation is represented by the B bonding function. The calculations of A, B and AB bonding functions indicate that the AB bonding function of CO adsorption on Cu is significantly smaller than that on Ni, Pd and Pt, so that CO adsorbtion is weak on Cu and is strong on Ni, Pd and Pt. Our micromechanism and our new concept of σ-π coordination provide a unified interpretation of various CO adsorption electronic spectra from below to above the E_F, i.e. from occupied orbitals to empty orbitals; and a unified interpretation of the adsorbate vibration spectra measured by EELS and IR spectroscopy. The advantages of our new concept have been discussed and compared with the conventional concepts of Blyholder and CO 2π-derived resonances.     

Magnetic circular dichroism studies 23. 1. Magnetic circular dichroism spectra of indole alkaloids

The magnetic circular dichroism spectra of a number of indole alkaloids show two B-terms of opposite sign in the 250–330 nm wavelength region associated with the ¹L_b and ¹L_a electronic transitions, the long wavelength, ¹L^b, band being of positive sign, whereas both bands strongly overlap in absorption. Various substituents in different positions of the indole ring cause a red shift of both bands and a broadening of the long wavelength B-term. The sign pattern, howver, remains unchanged in all examples thus far investigated. Dihydroindole and oxindole, on the other hand, exhibit MCD. bands with the opposite sign sequence as compared to the indole chromophore. This observation allows identification of the indole chromophore in alkaloids from the sign pattern of the MCD. bands.     

Vibronic coupling of pyrene in the first ππ* excited state

The two-photon fluorescence excitation spectrum of pyrene in n-hexane and n-heptane matrices has been measured at 10 K in the region of the first electronic transition (26800–30200 cm^?1). The spectrum consists of a rich number of sharp bands, being in general better resolved in n-hexane than in n-heptane matrix. Shpol'skii multiplets have been observed for the most intense bands. A strong two-photon band dominates the spectrum = 1495 cm^?1 from the 0—0 line and was assigned to B_1u × b_1u = A_g symmetry. Other weaker vibronic origins occur in the spectrum which were correlated to vibrational modes of b_1u, b_2u, b_3u and a_u symmetry. Intense vibronic bands are observed close to the origin of the second electronic transition and were interpreted as combination bands of B_1u × b_1u × b_3g symmetry. A two-photon vibronic theory to account for their intensity is proposed where the electronic moment is linearly expanded in powers of the nuclear displacements.     

An Experimentally Fast and Straightforward Method for the Direct Determination of Axial Dispersion σAd as Occurring in Size Exclusion Chromatography

The method for the direct determination of σ_ad (the extent of axial dispersion) under normal working conditions makes use of the peak variance of Poisson distributions and narrow peaks, which can be determined from the points of inflection. For such distributions the experimentally measured peak variance is the sum of the pristine or true peak variance and that stemming from the broadening processes during SEC measurements. The advantage of this method is that commercially available narrow polymer standard can be used which is demonstrated for four series of polymer standards. Knowledge of σ_ad finally enables the application of different correction procedures.     

Thermal [1,5] sigmatropic rearrangements in (σ-7-cycloheptatrienyl)triphenyltin and (σ-5-cyclohepta-1,3-dienyl)triphenyltin: A 1H and 13C NMR reinvestigation

It has been confirmed by ¹H and ¹³C NMR spectroscopies that Sn(σ-C₇H₇)Ph₃ undergoes either 1,4- or 1,5-shifts of the SnPh₃ moiety around the cycloheptatrienyl ring with ΔH³ = 13.8 ± 0.4 kcal mol^?1, ΔS3 = ?5.6 ± 1.2 cal mol^?1 deg^?1, and ΔG³₃₀₀ = 15.44 ± 0.14 kcal mol^?1. Similarly, (σ-5-cyclohepta-1,3-dienyl)triphenyltin undergoes 1,5-shifts with ΔH³ = 12.4 ± 0.6 kcal mol^?1, ΔS³ = ?11.2 ± 1.8 cal mol^?1 deg^?1, and ΔG³₃₀₀ = 15.76 ± 0.13 kcal mol^?1. It is therefore probable that Sn(σ-5-C₅H₅)R₃ and Sn(σ-3-indenyl)R₃ do not undergo 1,2-shifts as previously suggested but really undergo 1,5-shifts.     

Die rotationsisomerie des bicyclopropyls.: II. Die gauche/trans-isomerisierungsenthalpie und-entropie von bicyclopropyl aus ir-intensitäts-messungen. Ein experimenteller test

The determination of the enthalpy ΔH and entropy ΔS of the isomerization bicyclopropy l(trans)? bicyclopropyl(gauche) in the liquid phase by the IR intensity method is described. It is assumed that the ratio of the integral absorption coefficients of the two reference bands at 1351 cm^?1 (gauche) and 1291 cm^?1 (trans), which both belong to the same type of vibration, is temperature independent. The two values ΔH = ?160 ± 40 cal Mol^?1 and ΔS = ?0.4 ± 0.5 cal (Mol · Grad)^?1 respectively. ΔS_U = ?1.8 ± 0.5 cal (Mol · Grad)^?1 thus determined agree well with the corresponding results obtained from NMR and electron diffraction measurements. However, from the pair of reference bands at 695 cm^?1 (gauche) and 1291 cm^?1 (trans), which do not belong to the same type of vibration, strongly differing values for ΔH and ΔSresult under the same assumption as above, which apparently is not applicable in this case.It is shown through these data that the “Fateley-Test” does not provide a suitable tool to decide whether the absorption coefficients of the reference bands are temperature independent or not. The reason for this insignificance is the relatively poor accuracy and reproducibility of measured IR band intensities obtainable up to now.The relative density of bicyclopropyl between ?60°C and + 50°C was determined.