E. Horváth

Detection of four different OH-groups in ground kaolinite with controlled-rate thermal analysis

The thermal behaviour of mechanochemically treated kaolinite has been investigated under dynamic and controlled rate thermal analysis (CRTA) conditions. Ten hours of grinding of kaolinite results in the loss of the d(001) spacing and the replacement of some 60% of the kaolinite hydroxyls with water. Kaolinite normally dehydroxylates in a single mass loss stage between 400 and 600°C. CRTA technology enables the dehydroxylation of the ground mineral to be observed in four overlapping stages at 385, 404, 420 and 433°C under quasi-isobaric condition in a self-generated atmosphere. It is proposed that mechanochemical treatment of the kaolinite causes the localization of the protons when the long range ordering is lost.

Deintercalation of dimethylsulphoxide intercalated kaolinites : a DTA/TGA and Raman spectroscopic study

Abstract The deintercalation of dimethylsulphoxide intercalated kaolinites was studied using a combination of thermal analysis techniques and Raman spectroscopy. Thermal analysis shows three endotherms at 77°, 117° and 173°C attributed to the loss of water and the loss of DMSO in two stages. The use of a thermal stage enabled Raman spectra of the deintercalation process to be obtained in situ at elevated temperatures. The Raman spectra of the DMSO intercalated kaolinites show two bands at 3620 and 3660xa0cm−1 for the DMSO intercalated low defect kaolinite and 3620 and 3664xa0cm−1 for the high defect intercalated kaolinite. The 3620xa0cm−1 band is attributed to the inner hydroxyl of the kaolinite and shows no change upon deintercalation. The 3660 and 3664xa0cm−1 bands are attributed to the inner surface hydroxyls of the kaolinite hydrogen bonded to the DMSO. The intensity of these bands decreases upon thermal treatment and at the same time, the bands at 3695 and 3684xa0cm−1 increase in intensity. These changes clearly shows that the 3660xa0cm−1 band is attributable to the inner surface hydroxyls hydrogen bonded to the DMSO. Deintercalation may also be followed by the decrease in intensity of the CH-stretching bands. The application of the DTA/TGA patterns to determine the appropriate temperatures for Raman spectroscopy of the dimethylsulphoxide proved most useful.

Thermal Behaviour of Kaolinite Intercalated with Formamide, Dimethyl Sulphoxide and Hydrazine

The thermal behaviour of kaolinites intercalated with formamide, dimethyl sulphoxide and hydrazine has been studied by simultaneous TG-DTG-DTA-EGA and TG-MS techniques. The complexes can be decomposed completely without dehydroxylating the mineral. It was found that the amount of intercalated guest molecules per inner surface OH-group is close to unity for the formamide and dimethyl sulphoxide intercalates. For the intercalation of hydrazine it was found that hydrazine is locked in the expanded mineral as hydrazine hydrate and its amount is somewhat higher than that obtained for the other two reagents. The thermal evolution patterns of the guest molecules revealed that all the three reagents are bonded at least in two different ways in the interlayer space.

Vibrational spectroscopy of formamide-intercalated kaolinites

The vibrational spectroscopy of low and high defect kaolinites fully and partially intercalated with formamide have been determined using a combination of X-ray diffraction, DRIFT and Raman spectroscopy. Expansion of the high defect kaolinite to 10.09 A resulted in a decrease in the peak width of the d(001) peak attributed to a decrease in defect structures upon intercalation. Changes in the defect structures of the low defect kaolinite were observed. Additional infrared bands were observed for the formamide intercalated kaolinites at 3629 and 3606 cm(-1). The 3629 cm(-1) band is attributed to the hydroxyl stretching frequency of the inner surface hydroxyl group hydrogen bonded to the carboxyl group of the formamide. The 3606 cm(-1) band is ascribed to water in the interlayer. Concomitant changes are observed in both the hydroxyl deformation modes and in the carboxyl bands.

Effect of the preparation conditions on the surface-enhanced Raman-spectrometric identification of thin-layer-chromatographic spots

Abstract In situ surface enhanced Raman spectroscopic (SERS) analyses of thin layer chromatographic (TLC) spots of p-dimethyl-amino-benzylidene-rhodanine have been carried out using Nd-YAG and He-Ne lasers. With both lasers the enhancement factor was influenced by the analyte to metal (silver or gold) ratio as well. Since the enhancement factor with the Nd-YAG laser showed a stochastic distribution below 5 μg/TLC spot concentration, the NIR–SERS method can be used for in situ qualitative analyses, only. With the He-Ne laser, however, SERS spectra could be recorded in the pmol region using a concentrated silver sol. In the 1–5 μg/TLC spot region a linear relationship has been found between the SERS signal and the concentration for at least six different Raman bands. Therefore, an in situ qualitative and quantitative determination can be carried out in a single step.

Thermoanalytical Investigations on Intercalated Kaolinites

The thermal behaviour of ordered kaolinites from Hungary and Australia intercalated with potassium acetate, cesium acetate and urea has been investigated by simultaneous TG-DTG-DTA, TG-MS, Raman microscopy and XRD. Remarkable changes in the thermal decomposition pattern of the intercalates were obtained as a function of the nature of the intercalating re-agents. Replacing the potassium cation to cesium leads to a change in the OH environments resulting in a more complicated dehydroxylation pattern. The urea intercalates can be decomposed completely without dehydroxylating the mineral, although further treatments are necessary to restore the original d-spacing.

Thermoanalytical Investigation of Formamide Intercalated Kaolinites Under Quasi-isothermal Conditions

The thermal behaviour of fully and partially expanded kaolinites intercalated with formamide has been investigated in nitrogen atmosphere under quasi-isothermal heating conditions at a constant, pre-set decomposition rate of 0.20 mg min−1 . With this technique it is possible to distinguish between loosely bonded (surface bonded) and strongly bonded (intercalated) formamide. Loosely bonded formamide is liberated in an equilibrium reaction under quasi-isothermal conditions at 118°C, while the strongly bonded (intercalated) portion is lost in an equilibrium, but non-isothermal process between 130 and 200°C. The presence of water in the intercalation solution can influence the amount of adsorbed formamide, but has no effect on the amount of the intercalated reagent. When the kaolinite is fully expanded, the amount of formamide hydrogen bonded to the inner surface of the mineral is 0.25 mol formamide/mol inner surface OH group. While the amount of surface bonded formamide is decreasing with time, no change can be observed in the amount of the intercalated reagent. With this technique the mass loss stages belonging to adsorbed and intercalated formamide can be resolved thereby providing a complex containing only one type of bonded (intercalated) formamide.

Monitoring of enzyme catalysed reactions by Fourier transform Raman spectrometry

Esterification of mandelic acid catalysed by Candida antarctica lipase B was studied by Fourier transform (FT) Raman spectrometry in non-aqueous medium. It was found that there is a strict correlation between the intensity of the Raman scattering and the activity of the enzyme. FT-Raman spectrometry seems to be a fast and reliable method for selecting the appropriate enzyme and for determining the optimal enzyme water content. In addition, valuable information can be obtained from the spectra regarding the mechanism of enzyme–substrate bonding.

Investigation of mandelic acid bonding on Pirkle type chromatographic stationary phases by Raman spectroscopy

The bonding of mandelic acid enantiomers has been studied on benzene-leucine, dinitrobenzene-leucine and dinitrobenzene-phenylalanine type chiral stationary phases connected to zeolite A supports. The pi-donor, pi-acceptor and H-bonding interactions responsible for diastereomer pair formations can be studied under quasi in situ chromatographic conditions by Fourier transform Raman and surface enhanced Raman spectroscopic techniques. Structural differences between diastereomer pairs result in observable spectral differences at a phase load of approx. 50%. It was shown that the decreasing pi-acceptor character of the phase is associated with its increasing capability of H-bond formation. Correlating spectral data to chromatographic results it can be concluded that, in addition to H-bonding as well as to pi-donor-pi-acceptor interactions, steric hindrances due to bulky moieties of either the stationary phase or the analyte molecules are of importance in successful separations.

SURFACE-ENHANCED RAMAN SPECTROSCOPY AS A TECHNIQUE FOR DRUG ANALYSIS

In situ surface-enhanced FT-Raman (SERS) detection of heroin, codeine and cocaine samples after separation on a Kieselgel 60 type chromatographic thin layer has been performed. With a silver coating applied onto the TLC spots by vacuum evaporation a detection limit of about 0.2 μg/mm2 could be attained. Due to the low matrix effect of the silicate-based stationary phase, SERS detection of drugs can be used as a quick and reliable identification method.