Gas concentration programming – a new approach to sample controlled thermal analysis

Sample controlled thermal analysis (SCTA) is the generic name used to describe a family of techniques where the heating rate is not pre-determined as in conventional thermal analysis, but altered as some function of a property of the sample (i.e. mass loss, rate of gas evolution, etc.). We demonstrate here a new form of SCTA, where the reaction rate for gas–solid reactions can be controlled by programming the concentration of the reactive gas whilst keeping the temperature constant.     

Temperature Control Modes in Thermal Analysis

Now we can use several temperature control modes, i.e., the isothermal run including stepwise heating and cooling, constant rate heating (or cooling), temperature control for sample thermal history, sample controlled thermal analysis (SCTA or controlled-rate thermal analysis, CRTA), temperature jump, rate jump, temperature modulation and repeated temperature scanning. Their advantages and drawbacks are reviewed with some illustrative examples, especially for application to kinetic analysis. The combined use of these varieties of temperature control mode is recommended by showing examples. Temperature modulation and repeated temperature scanning are discussed in comparison with temperature modulated DSC, and common and analogous points are elucidated. In relation to this, the possibility that an imaginary part of overall reaction rate constant in complex reaction is postulated. Finally,these modes are classified and tabulated from two viewpoints and other possible modes are shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

Composite adsorbents based on inorganic salts for the rapid preconcentration of low-molecular polar organic compounds from humid air followed by gas chromatographic determination

The regularities of the retention of organic vapors on composite adsorbents based on inorganic salts are determined. It is found out that such adsorbents possess very high adsorption capacity relative to low-molecular polar organic compounds and can be used for their adsorption preconcentration from an air flow, followed by thermal desorption and gas-chromatographic determination. To reduce the detection limits of analytes in humid air, a two-column system of adsorption preconcentration is proposed, involving passage of an air sample through two columns connected in a series, one with a KF-based desiccant and another with a Mg(ClO₄)₂-based adsorbent.     

Controlled release of volatile (?)-menthol in nanoporous silica materials

In this work, a series of nanoporous silica materials have been prepared as adsorbents for volatile (?)-menthol, a molecule widely used in food, pharmacy, and cosmetics. The isothermal release properties of (?)-menthol have been investigated and correlated with the structural parameters of nanoporous absorbents. A rotary evaporation method is used to effectively load (?)-menthol into the nanopores of adsorbents and to prevent the whisker growth during the adsorption. It is demonstrated that the pore size, structure, wall thickness and surface functionality of nanoporous adsorbents are four important parameters to influence the isothermal release of (?)-menthol. By tuning these parameters of nanoporous silica adsorbents, controlled release of (?)-menthol can be achieved. A vesicular silica material with thick wall and hydrophobic functional groups is shown to possess the slowest release performance. Our contribution provides important knowledge for the future applications of nanoporous silica materials in pharmacy and cosmetics.     

Kinetic approach to partially overlapped thermal decomposition processes

Practical usefulness of the kinetic deconvolution for partially overlapped thermal decomposition processes of solids was examined by applying to the co-precipitated basic zinc carbonate and zinc carbonate. Comparing with the experimental deconvolutions by thermoanalytical techniques and mathematical deconvolutions using different statistical fitting functions, performance of the kinetic deconvolution based on an accumulative kinetic equation for the independent processes overlapped partially was evaluated in views of the peak deconvolution and kinetic evaluation. Two-independent kinetic processes of thermal decompositions of basic zinc carbonate and zinc carbonate were successfully deconvoluted by means of the thermoanalytical measurements in flowing CO₂ and by applying sample controlled thermal analysis (SCTA). The deconvolutions by the mathematical curve fittings using different fitting functions and subsequent formal kinetic analysis provide acceptable values of the mass-loss fractions and apparent activation energies of the respective reaction processes, but the estimated kinetic model function changes depending on the fitting functions employed for the peak deconvolution. The mass-loss fractions and apparent kinetic parameters of the respective reaction processes can be optimized simultaneously by the kinetic deconvolution based on the kinetic equation through nonlinear least square analysis, where all the parameters indicated acceptable correspondences to those estimated through the experimental and mathematical deconvolutions. As long as the reaction processes overlapped are independent kinetically, the simple and rapid procedure of kinetic deconvolution is useful as a tool for characterizing the partially overlapped kinetic processes of the thermal decomposition of solids.     

Properties of new fibrous adsorbents with amidoxyme and hydrazidine groups

Data on sorption properties of new fibrous adsorbents with respect to heavy metals are presented. The POLYORGS 34 and POLYORGS 35 filled fibrous adsorbents and POLYORGS 33 fibrous adsorbent contain amidoxyme and hydrazidine groups. Is has been shown that these adsorbents can be used for the adsorption preconcentration of heavy, noble, and rare metals and radionuclides from aqueous salt solutions. Examples of using adsorbents under study for the dynamic preconcentration of copper from a 0.5 M NaCl solution during filtration through a series of disks, column, or adsorption cartridges are presented.     

The effect of chemical activation method on properties of activated carbons obtained from pine cones

A method for obtaining carbonaceous adsorbents from pine cones by chemical activation with NaOH is described. Activated carbons were obtained by two methods of activation (physical mixing and impregnation) and two variants of thermal treatment. It has been shown that pine cones can be successfully used as cheap precursor of carbonaceous adsorbents of well-developed surface area, large pore volume and good sorption properties. All activated carbon samples obtained show strongly microporous structure and surface of acidic character. The best physicochemical properties and greatest sorption capacity towards iodine were found for the carbon samples obtained by physical mixing of the precursor with the activating agent and then subjected to thermal activation at 600°C.     

CO_2 adsorption performance of different amine-based siliceous MCM-41 materials

A series of amine-based adsorbents were synthesized using siliceous MCM-41 individually impregnated with four different amines(ethylenediamine(EDA),diethylenetriamine(DETA),tetraethylenepentamine(TEPA) and pentaethylenehexamine(PEHA)) to study the effect of amine chain length and loading weight on their CO2 adsorption performances in detail.The adsorbents were characterized by FT-IR,elemental analysis,and thermo-gravimetric analysis to confirm their structure properties.Thermo-gravimetric analysis was also used to evaluate the CO2 adsorption performance of adsorbents.Longer chain amine-based materials can achieve higher amine loadings and show better thermal stability.The CO2 adsorption capacities at different temperatures indicate that the CO2 adsorption is thermodynamically controlled over EDAMCM41 and DETA-MCM41,while the adsorption over TEPA-MCM41 and PEHA-MCM41 is under kinetic control at low temperature.The chain length of amines affects the CO2 adsorption performance and the adsorption mechanism significantly.The results also indicate that CO2 adsorption capacity can be enhanced despite of high operation temperatures,if appropriate amines(TEPA and PEHA) are applied.However,adsorbents with short chain amine exhibit higher adsorption and desorption rates due to the collaborative effect of rapid reaction mechanisms of primary amines and less diffusion resistance of shorter chain length amines.     

Silica adsorbent prepared from spent diatomaceous earth and its application to removal of dye from aqueous solution

The objective of this work is to study the activation regeneration of spent diatomaceous earth (SDE) for the preparation of silica adsorbents using thermal regeneration and acid/alkaline activation methods. Under the experimental conditions investigated, it was found that the alkaline activation method carried out by sodium hydroxide under controlled conditions is significantly superior to other heat and activation methods. The porosities of solids thus obtained are over 0.2, indicating that they are basically mesoporous. The optimal porous material thus prepared was used as a mineral adsorbent for methylene blue at 25 degrees C. The adsorption equilibrium revealed that the silica adsorbent can take up over 50 mg/g at relatively low concentrations in aqueous medium from the fittings of Langmuir and Freundlich isotherms with high correlations. On the other hand, the adsorption kinetic of methylene blue under various adsorbent dosages can be well described with a pseudo-second-order reaction model.     

Physical adsorption of organic molecules on the surface of layered silicate clay platelets: a thermogravimetric study

Physical adsorption of various adsorbents on the surface of premodified montmorillonite platelets was performed to fully organophilize the inorganic platelets for the purpose of their easy nanoscale dispersion in the polymer matrices during compounding. Different extents of adsorption could be achieved owing to the nature and the functionality of the adsorbents. High molecular weight adsorbents not only enhanced the organic coverage of the platelets but also were observed to contribute toward the thermal stability improvement of the organic modification, thus further fitting the use of such clays for high temperature compounding. The amount of adsorption could also be quantified with respect to the initial amount of adsorbent used in the process. The importance of a clean surface free from any excess surface modification or adsorbent molecules was emphasized. The adsorption process is an effective means to generate such high potential montmorillonites and is much simpler in technique than the common methods of grafting of polymer chains from the clay surface.     

New inorganic (an)ion exchangers based on Mg-Al hydrous oxides: (alkoxide-free) sol-gel synthesis and characterisation

New inorganic ion exchangers based on double Mg-Al hydrous oxides were generated via the new non-traditional sol-gel synthesis method which avoids using metal alkoxides as raw materials. Surface chemical and adsorptive properties of the final products were controlled by several ways of hydrogels and xerogels treatments which produced the materials of the layered structure, mixed hydrous oxides or amorphous adsorbents. The final adsorptive materials obtained via thermal treatment of xerogels were the layered mesoporous materials with carbonate in the interlayer space, surface abundance with hydroxylic groups and maximum adsorptive capacity to arsenate. Higher affinity of Mg-Al hydrous oxides towards H(2)AsO(4)(-) is confirmed by steep adsorption isotherms having plateau (removal capacity) at 220 mg[As]g(dw)(-1) for the best sample at pH=7, fast adsorption kinetics and little pH effect. Adsorption of arsenite, fluoride, bromate, bromide, selenate, borate by Mg-Al hydrous oxides was few times high either competitive (depending on the anion) as compare with the conventional inorganic ion exchange adsorbents.     

Carboxyl modified multi-walled carbon nanotubes as solid-phase extraction adsorbents combined with high-performance liquid chromatography for analysis of linear alkylbenzene sulfonates

Recently, multi-wall carbon nanotubes (MWCNTs) as adsorbents of solid-phase extraction are attractive because they can be used for enrichment of organic compounds and metal ions at trace levels. In this study, we use the carboxyl modified multi-wall carbon nanotubes (CMMWCNTs) as adsorbents of solid-phase extraction for extraction of linear alkylbenzene sulfonates (LAS), which are widely used anion surfactant with different homologues, and detected by HPLC-UV. The effect of eluent and its volume, sample pH and flow rate, sample volume and the ultrasonic time of sample, the content of the electrolyte (NaCl) were investigated and optimized. The detection limit for LAS homologues was 0.02-0.03 μg L⁻¹ with R.S.D. (n = 6) ranging from 2.04 to 10.03%. The recoveries of LAS homologues in the spiked environmental water samples ranged from 84.8 to 106.1%. The proposed method has been applied successfully to the analysis of LAS in aqueous environmental samples, which demonstrates that CMMWCNTs-based solid-phase extraction is a precision and convenient enrichment method and can be used for analysis of LAS homologues in water samples.     

New method for bonding an adsorbent film to the walls of capillary columns

A new method for immobilisation of an adsorbent on the wall of a capillary column ensures high thermal and mechanical stability of the resulting coating. The method can be applied to a variety of adsorbents of natural or synthetic origin. Its effectiveness is illustrated by results obtained for coating with modified silica or molecular sieve 13X.     

TG/DTG/DTA for the oxidation behavior characterization of vegetable and animal fats

TG/DTG/DTA curves can be used to estimate alimentary fats quality and antioxidants insertion efficiency. Sunflower oil obtained from Carnia hybrid and pork lard were used as matrices for the non-isothermal measurements. The first stage of non-isothermal decomposition is mostly important for the characterization of the fats thermal stability. The corresponding onset temperature is a good value for the comparison of different fats thermal stability or for the effectiveness evaluation in case of antioxidant insertion. In this study, it can be seen a considerable improvement of the fats thermal stability by adding small amounts from a natural antioxidant liquid mixture (obtained by alcoholic maceration of equal amounts of seven plants, namely: milfoil, rosemary, marjoram, thyme, lovage, oregano, and basil). Chlorophylls removal from the plant extract using two different adsorbents was accompanied by a four time decrease of the antiradical activity (measured by the DPPH method) with Sephadex LH20 and seventeen times decrease when activated carbon was used.     

11种吸附剂上二噁英的热吸附/脱附性能评价

筛选在低温下高效捕集并在一定的高温下可以快速完全脱附二噁英的吸附/脱附材料是二噁英在线热捕集的关键.该研究以1,2,3,4-四氯代二苯并-对-二噁英(1,2,3,4-TCDD)和1,2,3,8,9-五氯代二苯并呋喃(1,2,3,8,9-PCDF)为二噁英模型物,以电子捕获检测器(ECD)作为检测器,利用填充柱气相色谱系...     

A comparative adsorption study with different industrial wastes as adsorbents for the removal of cationic dyes from water

Four adsorbents have been prepared from industrial wastes obtained from the steel and fertilizer industries and investigated for their utility to remove cationic dyes. Studies have shown that the adsorbents prepared from blast furnace sludge, dust, and slag have poor porosity and low surface area, resulting in very low efficiency for the adsorption of dyes. On the other hand, carbonaceous adsorbent prepared from carbon slurry waste obtained from the fertilizer industry was found to show good porosity and appreciable surface area and consequently adsorbs dyes to an appreciable extent. The adsorption of two cationic dyes, viz., rhodamine B and Bismark Brown R on carbonaceous adsorbent conforms to Langmuir equation, is a first-order process and pore diffusion controlled. As the adsorption of dyes investigated was appreciable on carbonaceous adsorbent, its efficiency was evaluated by comparing the results with those obtained on a standard activated charcoal sample. It was found that prepared carbonaceous adsorbent exhibits dye removal efficiency that is about 80-90% of that observed with standard activated charcoal samples. Thus, it can be fruitfully used for the removal of dyes and is a suitable alternative to standard activated charcoal in view of its cheaper cost.     

Recent advances in application and progress of advanced materials as adsorbents in sample preparation for plant growth regulators

Plant growth regulators are a class of physiologically active substances that could modify or regulate basic physiological processes in the plant and defense against abiotic and biotic stresses, including natural plant growth regulators and synthetic ones. Different from natural plant growth regulators with low content and high cost of extraction in plants, synthetic ones can be produced in large-scale production and widely used in agriculture for increasing and securing yield and quality of the harvested produce. However, like pesticides, the abuse of plant growth regulators will have negative impacts on human beings. Therefore, it is important to monitor plant growth regulators residues. Due to the low concentration of plant growth regulators and complex matrices of food, it is necessary to isolate and extract plant growth regulators by appropriate adsorbents in sample preparation for obtaining satisfactory results. In the last decade, several advanced materials as adsorbents have shown superiority in sample preparation. This review briefly introduces the recent application and progress of advanced materials as adsorbents in sample preparation for extraction of plant growth regulators from the complex matrix. In the end, the challenge and outlook about the extraction of plant growth regulators of these advanced adsorbents in sample preparation are presented.     

Development of a universal constant rate thermal analysis system for being used with any thermoanalytical instrument

The SCTA method implies to control the temperature in such a way that the reaction rate changes with the time according to a function previously defined by the user. Constant Rate Thermal Analysis (CRTA) is one of the most commonly used SCTA methods and implies achieving a temperature profile at which the reaction rate remains constant all over the process at a value previously selected by the user. This method permits to minimize the influence of heat and mass transfer phenomena on the forward reaction. The scope of this work is to develop a universal CRTA temperature controller that could be adapted to any thermoanalytical device. The thermoanalytical signal is programmed to follow a preset linear trend by means of a conventional controller that at the time controls a second conventional temperature programmer that forces the temperature to change for achieving the trend programmed for the thermoanalytical signal. Examples of the performance of this control system with a Thermobalance and a Thermomechanical Analyser (TMA) are given.     

Development of a New Type of Light Modulated DSC

A new type of the light modulated DSC was constructed. Optical fibers are used to separate the light source from the furnace. The sample can be handled accurately because the lamp and the intensity modulator are not above the furnace. Quality of the measured signal is much improved by extracting the analog signal from the electronic circuit board in the commercial DSC. Light intensities of the sample and reference sides are controlled independently from each other. A method to compensate for the asymmetry of the thermal system utilizing the independent control of the light intensity is proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal desorption-capillary gas chromatography for the quantitative analysis of dimethyl sulphate, diethyl sulphate and ethylene oxide in the workplace

Sampling on solid adsorbents followed by thermal desorption and gas chromatography was evaluated as a simple method for the quantitative analysis of dimethyl sulphate, diethyl sulphate and ethylene oxide in the workplace environment. Tenax TA and Carbosieve S-III (for ethylene oxide) were found to be suitable adsorbents. The charged tubes can be stored at 22 degrees C for 4 days. The recovery is better than 98% (greater than 80% for ethylene oxide) in the mass range 1 ng-1 micrograms. The recovery is not dependent on air humidity. The limits of detection tested were at the lower parts per billion level.