Thermodynamic studies on the solvent effects in chromatography on molecularly imprinted polymers. 1. Nature of the organic modifier

Molecularly imprinted polymers (MIPs) are used as highly enantioselective stationary phases in liquid chromatography. To optimize the binding performance of MIPs, different types of polar modifiers are frequently used. Previous studies have shown that the hydrogen-bonding donor parameter (HBD) of the modifier has a large influence on the binding performance of MIPs in chiral separations. This possibility is addressed in a detailed thermodynamic study of a Fmoc-L-tryptophan (Fmoc-L-Trp) imprinted polymer, eluted with four different polar modifiers, i.e., THF, propan-2-ol, methanol, and acetic acid, which have different HBDs (0.00, 0.33, 0.43, and 0.61, respectively). Adsorption isotherm data for each enantiomer in each of these organic modifiers were acquired by frontal analysis over a 20 000 dynamic concentration range. Nonlinear regression of the isotherm data, along with independent calculation of the affinity energy distributions, identified four different types of binding sites coexisting for the enantiomers on the MIP. The exception was acetic acid, which has the highest HBD. In this case, three types of binding sites only coexist on the MIP. The isotherm parameters obtained from these data show the following: (1) The association energies of the two enantiomers with a given type of sites have a similar magnitude; however, the density of the sites is higher for the template than for its antipode. (2) The nature of the organic modifier has a larger influence on the density of high-energy sites than on the association constant of these sites. (3) The molecular size of the organic modifier has a larger influence on the site density (especially for Fmoc-D-Trp) than does HBD. (4) Using an organic modifier with a higher HBD reduces the enantioselectivity on each site. (5) High-energy sites are more enantioselective than low-energy ones. (6) Using an organic modifier with a high HBD causes a larger reduction in the density of high-energy sites approached by the template molecules.     

Comparison of the thermodynamic properties of particulate and monolithic columns of molecularly imprinted copolymers

A variety of polymerization techniques can be used to prepare molecularly imprinted copolymers (MIPs) for the purpose of the separation of enantiomers by HPLC. Unfortunately, the lack of thermodynamic and kinetic data characterizing the imprinted polymers prepared by these different techniques prevents the rational choice of the one most suited for a specific application. We investigated and compared the thermodynamic properties of copolymers imprinted for Fmoc-L-tryptophan and prepared by two different methods. The first was an in situ polymerization method that gives monolithic columns (monolithic MIPs), the second, a traditional method giving bulk MIPs. Examination of the thermodynamic properties on these two different MIPs showed that three types of binding sites coexist on their surface. The highest energy sites adsorb only the imprinted molecule or template. Most of the intermediate energy sites adsorb both the template and its antipode, although part of them may adsorb only the template. Finally, the lowest energy sites provide nonselective interactions of both the template and its antipode. On the nonimprinted copolymer, there are only two types of sites. The high-energy sites have a slightly lower energy that the intermediate sites of the MIPs, and the low-energy sites have properties close to those of the lowest energy sites on the MIPs. The monolithic MIPs have fewer nonselective sites than the bulk MIPs. However, the polar porogen that is needed to prepare the monolithic MIPs negatively affects the enantiomeric separation.     

Continuous-wave-laser versus pulsed-laser excitation for crossed-beam photothermal detection in small volume applications: comparative features

Crossed-beam thermal lens spectrometry can be implemented using continuous-wave- (cw) laser or pulsed-laser excitation. In both cases, the signal depends on the position of the sample with respect to the probe beam waist, the size of the excitation beam, the beamsize ratio into the sample, and the power or energy of the excitation beam. However, due to differences in the rate of formation and relaxation of the thermal lens, both methods exhibit distinct key features. Optimization of the experimental setup and understanding the thermal lens signal are more complicated under cw-laser excitation than with pulsed-laser excitation. Unlike that observed under pulsed excitation, the effect of the excitation beam waist, of the sample size, and of the flow rate are closely related to the effective size of the thermal element and depend on the chopping frequency. Although the intrinsic sensitivities are almost the same, the performance can significantly differ depending on the chopping frequency or pulse repetition rate, which should be high enough to allow fast data collection and efficient signal averaging.     

Separation of peptides from myoglobin enzymatic digests by RPLC. Influence of the mobile-phase composition and the pressure on the retention and separation

The influence of the mobile-phase composition and the pressure on the chromatographic separation of the peptides from the enzymatic digest of myoglobin was studied under linear conditions. The retention behavior of these tryptic peptides was measured under isocratic conditions with different mobile-phase compositions, ranging from 9 to 28% (v/v) acetonitrile in 0.1% (v/v) aqueous trifluoroacetic acid. The effect of the pressure was studied by analyzing the separation of the tryptic peptides under different average column pressures between 14 and 220 bar, at 13, 20, and 26% (v/v) acetonitrile. The differences between the partial molar volumes of these peptides in the stationary and mobile phases were derived from these results. All the measurements were performed on a 10-cm-long C18-bonded, end-capped monolithic column. The results obtained illustrate the highly complicated behavior of the complex peptide mixtures afforded by tryptic digestion. The capacity factors of the analyzed peptides do not depend linearly on the acetonitrile concentration but follow exactly a quadratic relationship. The adsorption changes of partial molar volumes are in good agreement with other literature data. The consequences of the influence of the average column pressure (hence of the flow rate) on the column phase ratio and on the retention factors of the peptides are discussed. The retention pattern of the complex mixture is affected by both the mobile-phase composition and the pressure, and the resolution of certain peptide pairs is so much affected by the pressure that inversions in the elution order of some pairs are observed.     

Variation thermique de la susceptibilite magnetique et de l'aimantation du ferrite bicalcique Ca2Fe2O5

An investigation of the magnetic susceptibility and the magnetization vs temperature has been carried out for Ca₂Fe₂O₅. The weak ferromagnetism behaviour of this compound was confirmed. The value of the ferromagnetic component was determined; an estimate of the various magnetic couplings was given and the importance of the thermal variation of the molecular field at high temperature pointed out.     

Wear behaviour of carbonitrided steel

A pin and disk study of carbonitrided steel revealed different types of wear. The best results were with carbonitrided material rubbing on carbonitrided material. The effect of load and the role of brittleness on wear has been studied. Differences in structure, hardness and work-hardening of the various surface layers as well as debris in the interface caused the difference in wear behaviour of the layers.     

Boundary lubrication with anti-wear additives: study of interface film formation by electrical contact resistance

It has been shown that a rapid measuring technique for electrical contact resistance (ecr) has potential applications in lubricant additives studies. In boundary lubrication of a metallic interface with zinc dialkyl dithiophosphate (DTPZn) or other anti-wear additives blended in the lubricant, anti-wear efficiency relies on interface film formation which acts also as an insulating barrier for electrical current. Investigations in the 1ω to 10 Mω range of resistance, with subsequent and appropriate ecr signal treatment and analysis, indicate that systematic use of ecr permits visualisation and study of interface film formation. This paper shows that two interface films can be detected, with two different levels of average ecr, 0–1 kω and 100k–10Mω. Discussion is based on an interface model and attempts to find what kind of further information on film formation may be obtained from ecr measurements     

Diffuse interstitial pneumopathy associated with limited Sj?gren's syndrome (author's transl)

Diffuse interstitial pneumopathy was found to be associated with limited Sj?gren's syndrome (i.e. without connective tissue involvement) in 5 women aged from 48 to 83 years. The respiratory symptoms (dyspnoea, unproductive cough and crepitations) appeared before the dry-eye-and-mouth syndrome was diagnosed in two patients and several years afterwards in three. Respiratory function studies showed a mixed restrictive and obstructive syndrome, perturbed Co transfer and increased total expiratory airways resistance. These changes reflected the underlying pathology, as revealed by bronchial and lung biopsies, which consisted of lymphocyte and plasmocyte infiltration and fibrosis of the interalveolar septa, peribronchial spaces and bronchial glands. Abnormal respiratory function tests in 5 other patients without any respiratory symptom suggest that subclinical and subradiological lesions of the lungs and bronchi are not uncommon in Sj?gren's syndrome.