Thermal Desorption Technique Research Articles

Quantification and speciation of mercury in soils from the Tripuí Ecological Station, Minas Gerais, Brazil

Contents of total mercury, organic carbon, total sulfur, iron, aluminum and grain size and clay mineralogy were used along with Pearson's correlation and Hg thermal desorption technique to investigate the presence, distribution and binding behavior of Hg in soils from three depths from the Tripuí Ecological Station, located near Ouro Preto, Minas Gerais State, Brazil. The soils studied had predominantly medium and fine sand texture (0.59–0.062 mm), acid character and Hg contents ranging from 0.09 to 1.23 μg/g. The granulometric distribution revealed that Hg is associated with coarse sand (2–0.59 mm) and silt and clay (< 0.062 mm) and presents similar Hg concentrations in both fractions. Mercury distribution in soil profiles showed that Hg was homogeneously distributed throughout the depths at most sites. Hg thermal desorption curves show that mercury occurs not only as Hg 2+ predominantly bound to organic components in most of the samples, but also in the form of cinnabar in some. Pearson's correlation confirmed that mercury is associated with organic matter and sulfur and possibly with sulfur-bearing organic matter in most samples.

Photooxidation of Acetone on TiO2(110): Conversion to Acetate via Methyl Radical Ejection

It is generally held that radicals form and participate in heterogeneous photocatalytic processes on oxide surfaces, although understanding the mechanistic origins and fates of such species is difficult. In this study, photodesorption and thermal desorption techniques show that acetone is converted into acetate on the surface of TiO2(110) in a two-step process that involves, first, a thermal reaction between acetone and coadsorbed oxygen to make a surface acetone-oxygen complex, followed second by a photocatalytic reaction that ejects a methyl radical from the surface and converts the acetone-oxygen complex into acetate. Designation of the photodesorption species to methyl radicals was confirmed using isotopically labeled acetone. The yield of photodesorbed methyl radicals correlates well with the amount of acetone depleted and with the yield of acetate left on the surface, both gauged using postirradiation temperature programmed desorption (TPD). The thermal reaction between adsorbed acetone and oxygen to form the acetone-oxygen complex exhibits an approximate activation barrier of about 10 kJ/mol. A prerequisite to this reaction is the presence of surface Ti3+ sites that enable O2 adsorption. Creation of these sites by vacuum reduction of the surface prior to acetone and oxygen coadsorption results in an initial spike in the acetone photooxidation rate, but replenishment of these sites by photolytic means (i.e., by trapping excited electrons at the surface) appears to be a slow step in a sustained reaction. Evidence in this study for the ejection of organic radicals from the surface during photooxidation catalysis on TiO2 provides support for mechanistic pathways that involve both adsorbed and nonadsorbed species.

The use of thermal desorption GC/MS to study weight loss in thermogravimetric analysis of di-acid salts

Thermal desorption gas chromatograph mass spectrometry (TD GC/MS) was used to study weight loss in thermogravimetric analysis (TGA). The technique of thermal desorption utilizes the same temperature heating rate as the TGA to thermally desorb volatiles from solid sample matrices. Volatiles were cryo-trapped at −60 °C. After thermal desorption is complete, the trapped volatiles are separated by a GC capillary column and identified by mass spectrometry. In this study, the TD GC/MS was applied in pharmaceutical development to understand the chemical reactions attributed to the weight loss in the thermal decomposition of two dicarboxylic acid salts of a drug substance. These two salts exhibited different thermal stabilities. The thermally induced chemical reactions obtained from these two salts included dehydration and decarboxylation. Thermal degradation compounds were identified and reaction pathways for decomposition were proposed. The stability of the salts is dependent on the identity of the dicarboxylic acids from which they were generated. The information obtained from TD GC/MS helps better understand the weight loss process in thermogravimetric analysis.

Preparation and application of poly(dimethylsiloxane)/β-cyclodextrin solid-phase microextraction membrane

A novel poly(dimethylsiloxane)/β-cyclodextrin (PDMS/β-CD) coating was prepared for solid-phase microextraction (SPME) in the form of membrane instead of fiber. The surface of the membrane was uniform and has not gel cracking after dipped into methanol, acetonitrile and acetone. Non-polar polycyclic aromatic hydrocarbons (PAHs) and polar phenolic compounds were used to evaluate the character of the PDMS/β-CD solid-phase microextraction membrane by direct extraction and solvent desorption, followed by GC–MS analysis. Parameters that affect the membrane extraction process were investigated, which included the matrix of sample, extraction time, desorption solvent and time. For PAHs, the linearity was from 0.10 to 1000 μg 1 −1. The limit of detection (LOD) was in the range of 0.01–0.2 μg 1 −1. The repeatability was in the range of 5.4–9.3%. For phenolic compounds, the linearity was from 0.10 to 5000 μg 1 −1 and LOD varied from 0.02 to 1.5 μg 1 −1. Compared with the SPME fiber, the SPME membrane has a large extraction capacity, low cost of preparation. The sensitivity can be further increased by developing large-volume-injection or thermal desorption technique.

The chemical composition of inorganic and carbonaceous materials in PM2.5 in Nanjing, China

PM2.5 samples were collected at an urban and a suburban site in Nanjing, China in 2001. They were analyzed for inorganic ions, elemental carbon, organic carbon (OC), water-soluble organic carbon (WSOC), and individual WSOC and nonpolar organic species. Sulfate and organic matter were the two most abundant constituents in these samples. Sulfate accounted for an average of 23% (urban site) and 30% (suburban site) of the identified aerosol mass. Organic matter accounted for an average of 37% (urban) and 28% (suburban) of the identified aerosol mass. WSOC was a significant portion of OC, accounting for about one-third of OC at the urban site and 45% of OC at the suburban site. The suburban–urban gradient in the WSOC/OC ratio also reflected that the aerosol OC was more aged at the suburban location. The correlations of WSOC with sulfate and nitrate suggest that the WSOC fraction was dominated by secondary organics. More than 30 individual WSOC species in the compound classes of organic anions, amino acids, aliphatic amines, and carbohydrates were quantified, accounting for approximately 8% of the WSOC on a carbon mass basis. In addition, 46 individual nonpolar organic compounds in the compound classes of n-alkanes, hopanes, and polycyclic aromatic hydrocarbons were quantified using an in-injection port thermal desorption technique. These nonpolar organic species accounted for less than 7% of the OC on a carbon mass basis. The quantification of individual compounds allowed the identification of major aerosol sources through principal component analysis. Coal combustion, vehicular emissions, secondary inorganic and organic aerosols, and road/sea salt were the major contributing sources to the identified PM2.5 aerosol mass.

Analysis of volatile components from Ziziphora taurica subsp. taurica by steam distillation, superheated-water extraction, and direct thermal desorption with GC×GC–TOFMS

Volatile components from the leaves of Ziziphora taurica subsp. taurica have been isolated by steam distillation, superheated-water extraction, and direct thermal desorption techniques. The volatile components were characterized by comprehensive two dimensional gas chromatography-time of flight mass spectrometry. The extraction yields from Z. taurica leaves were found to be 1.56% and 1.32% for steam distillation and superheated-water extraction respectively. The major compounds found in the volatile fractions of the leaves of Z. taurica were pulegone, terpinen-4-ol, cis-carveol, trans-carveol, and verbenone. The number of volatile components identified were 28, 30, and 41 for steam distillation, superheated-water extraction, and direct thermal desorption, respectively.

A Study on Hydrogen Desorption Properties of Multiwall Carbon Nanotubes

Hydrogen desorption properties in multiwall carbon nanotubes(MWNTs) were investigated from the view points of not only physical hydrogen molecules adsorption in nano-hole but also chemical hydrogen adsorption on graphite surface. MWNTs with intrinsic closed structure having the blocked hole and MWNTs with open structure were studied through hydrogen thermal desorption technique equipped with gas chromatograph. In order to grow MWNTs with open structure, novel method using oxygen added plasma(CH4/H2/O2) has been introduced. The structure of MWNTs was gradually changed with increasing oxygen addition amount, showing aligned MWNTs bundles and open tube cap at 10% oxygen addition. The precise analysis on thermal desorption spectra on MWNTs with closed structure showed that hydrogen gas was released at three major temperature ranges such as 100-230K, 290-350K, and 600-625K, where the evolved hydrogen amounts were about 1.65wt%, 0.64wt%, and 0.03wt%, respectively. However, in case of aligned and open MWNTs, the evolution peak around 290-350K was highly developed (about 1.9wt%) and the desorption peak at 600-625K was disappeared. The hydrogen desorption activation energy was also calculated. The obtained hydrogen desorption activation energy of MWNTs with closed and open structure at ambient temperature was -18.5kJ/mol H2 and 16.5kJ/molH2, respectively. The hydrogen desorbed between 290 and 330K in MWNTs with open structure was the hydrogen physi-sorbed in nano-hole. And Ni-doping (dispersion) effect on hydrogen storage properties was investigated comparatively. Ni nano-catalysts were expected to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding on carbon surface. The metal nanoparticles were homogeneously dispersed using incipient wetness impregnation method. Hydrogen desorption spectra of MWNTs with Ni nanoparticles showed that about 2.8wt% of hydrogen was released in the range of 340-520K.

Optimization of aqueous acetylation for determination of hydroxy polycyclic aromatic hydrocarbons in water by stir bar sorptive extraction and thermal desorption–gas chromatography–mass spectrometry

Stir bar sorptive extraction (SBSE) using a 10 mm-long stir bar coated with 24 μl of polydimethylsiloxane (PDMS) in combination with the thermal desorption technique and gas chromatography–mass spectrometry was used for determining hydroxy polycyclic aromatic hydrocarbons (OH-PAHs), particularly mono and diOH-PAHs having two- to four-ring structures, in water samples. To improve the recovery for OH-PAHs, in situ derivatization with acetic anhydride was applied prior to SBSE. The optimal conditions for a 10 ml water sample were attained with the addition of 100 mg of sodium hydrogen carbonate, followed by the addition of 20 μl of acetic anhydride and extraction for 360 min. In the selected ion monitoring (SIM) mode, the limits of detection were found to range from 0.27 ng l −1 for 2-hydroxynaphthalene (as 2-hydroxynaphthalene acetate) to 25 ng l −1 for 1,4-dihydroxynaphthalene (as 1,4-naphthoquinone) (S/N = 3). The low total recovery (<50%) of the entire procedure was mainly due to the low extraction efficiency of acetyl derivatives from water to PDMS, while the in situ derivatization step achieved more than 88% efficiency. Nine OH-PAHs, such as 1,4-naphthoquinone, 2-hydroxynaphthalene, 2,3-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,3-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, and 9-hydroxyphenanthrene, were detected in environmental samples under the optimal conditions. The detection of OH-PAHs in environmental water samples has not been reported so far.

Development and application of pyrolysis gas chromatography/mass spectrometry for the analysis of bound trinitrotoluene residues in soil.

TNT (trinitrotoluene) is a contaminant of global environmental significance, yet determining its environmental fate has posed longstanding challenges. To date, only differential extraction-based approaches have been able to determine the presence of covalently bound, reduced forms of TNT in field soils. Here, we employed thermal elution, pyrolysis, and gas chromatography/mass spectrometry (GC/MS) to distinguish between covalently bound and noncovalently bound reduced forms of TNT in soil. Model soil organic matter-based matrixes were used to develop an assay in which noncovalently bound (monomeric) aminodinitrotoluene (ADNT) and diaminonitrotoluene (DANT) were desorbed from the matrix and analyzed at a lower temperature than covalently bound forms of these same compounds. A thermal desorption technique, evolved gas analysis, was initially employed to differentiate between covalently bound and added 15N-labeled monomeric compounds. A refined thermal elution procedure, termed "double-shot analysis" (DSA), allowed a sample to be sequentially analyzed in two phases. In phase 1, all of an added 15N-labeled monomeric contaminant was eluted from the sample at relatively low temperature. In phase 2 during high-temperature pyrolysis, the remaining covalently bound contaminants were detected. DSA analysis of soil from the Louisiana Army Ammunition Plant (LAAP; approximately 5000 ppm TNT) revealed the presence of DANT, ADNT, and TNT. After scrutinizing the DSA data and comparing them to results from solvent-extracted and base/ acid-hydrolyzed LAAP soil, we concluded that the TNT was a noncovalently bound "carryover" from phase 1. Thus, the pyrolysis-GC/MS technique successfully defined covalently bound pools of ADNT and DANT in the field soil sample.

Modification of Electronic Properties of Mo2C Catalyst by Potassium Doping: Impact on the Reactivity in Hydrodenitrogenation Reaction of Indole

The effect of potassium doping on electronic surface properties of molybdenum carbide (Mo2C) was investigated by species-resolved thermal alkali desorption techniques, work-function measurements, a...

Structural and catalytic properties of skeletal Ni catalyst prepared from the rapidly quenched Ni 50Al 50 alloy

The textural and structural properties of a skeletal Ni catalyst prepared by alkali leaching of aluminum from the rapidly quenched Ni 50Al 50 alloy have been investigated by elemental analysis (ICP-AES), nitrogen physisorption, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and H 2 thermal desorption techniques. As compared to Raney Ni, such a skeletal catalyst has a residual Ni 2Al 3 phase, lower surface area, higher average pore diameter and porosity, larger mean crystallite size, and unit-cell parameter. These differences are attributable to the metastable character of the pristine rapidly quenched alloy and their relationship with the catalytic behavior is discussed and correlated.

How the iron oxide catalyst for EBDH is stabilized via Mn addition

The stability effect of Mn dopand on the potassium loss from β-ferrite, the active phase of a styrene catalyst, has been quantified for the first time in succinct measurements by means of a species-resolved (K and K +) thermal alkali desorption technique. Upon introduction of 2 wt% of Mn the desorption energies increased by 1.06 eV for ions, whereas the atomic barrier, which is crucial for the potassium volatilization, was enhanced by 1.52 eV. As a result, both K desorption fluxes are significantly extinguished.

ZnCl2-Zn2+-Montmorillonite composite: efficient solid acid catalyst for benzylation of benzene

Abstract A newer type of metal salt–metal ion exchanged Montmorillonite (Mont), MCl 2 -M n + -Mont (M=Ni, Cu, Zn; n =2) have been prepared and evaluated as solid acid catalysts for Friedel–Crafts reactions particularly for benzylation of benzene. The activity of such composites depends upon the amount of loading of metal salts on the metal ion exchanged-Mont support and 1.5 mmol loading per mole of the support shows the highest activity. Activation temperature of the composites also plays an important role and 150±10 °C is the optimum region. ZnCl 2 -Zn 2+ -Mont (1.5 mmol, 0.926 g) and Zn 2+ -Mont (1.5 mmol, 3.33 g) composites show about 97% conversion during 24 h of reaction time and therefore, the former composite shows an advantage over the latter in respect of total amount of catalyst weight to be used. The composites show both Bronsted and Lewis acidities which were determined by thermal desorption and differential scanning calorimetric (DSC) techniques. The composites were also characterized by thermal analysis and X-ray diffraction (XRD) measurements.

Recombination of deuterium atoms on the surface of molten Li–LiD

Measurements of the rate constant k r describing the surface recombination of deuterium atoms into molecules, were made for various molten mixtures of Li–LiD using a thermal desorption technique. The results are found to follow an Arrhenius relationship in the temperature range 573–873 K and are compared to the prediction of a kinetic model for the release of hydrogen from metals. The data are lower than would be predicted by the model for an atomically clean surface, but lower than predicted recombination rates are ubiquitous with this type of measurement and well known to result from surface contamination, which reduces surface-site availability and gives rise to variations in the surface chemisorption potential.

Low-flow active and passive sampling of VOCs using thermal desorption tubes: theory and application at an offset printing facility.

While air sampling techniques using adsorbent-based collection, thermal desorption and chromatographic analysis have found a niche in ambient air sampling, occupational applications have been more limited. This paper evaluates the use of thermal desorption techniques for low flow active and passive sampling configurations which allow conveniently long duration sampling in occupational settings and other high concentration environments. The use of an orifice enables flows as low as 0.5 ml min(-1) and sampling periods up to several days without significant biases. A model is used to predict sampling rates of a passive sampler encompassing an orifice, a void space, glass wool, and the adsorbent. Laboratory and field tests conducted at a commercial offset printing facility, which contained a variety of volatile organic compounds (primarily aromatic but also a few chlorinated and terpene compounds at levels from 1 to 67,000 microg m(-3)), are used to evaluate the approach. Tenax GR and Carbosieve SIII, both singly and together, were employed as adsorbents. Side-by-side tests comparing high flow, low flow and passive samplers show excellent agreement and high linearity (r = 0.95) for concentrations spanning nearly five orders of magnitude. Active samplers were tested at flows as low as 0.5 ml min(-1), compared to typical flows up to 40 ml min(-1). Passive samplers demonstrated a linear range and agreement with predictions for adsorbate loadings from approximately 1 ng to nearly 10 microg. Using a chemical mass balance receptor model, concentrations in the facility were apportioned to solvents, inks and other indoor and outdoor sources. Overall, the use of low flow active and passive sampling approaches employing thermal desorption techniques provides good performance and tremendous flexibility that facilitates use in many applications, including workplace settings.

Surface decomposition mechanism of Ti(OC 3H 7) 4 on a platinum surface

The decomposition behavior of titanium tetraisopropoxide [TTIP; Ti(OC 3H 7) 4] on a platinum surface has been studied using the thermal desorption technique. TTIP is dissociated on platinum below 230 K so that the platinum surface is covered with isopropoxy species and hydrogen originating from TTIP. The isopropoxy species adsorbed on the platinum surface are decomposed to acetone at 230 K. Two forms of surface hydrogen exist: molecular and atomic. Molecular hydrogen is desorbed at 230 K and atomic hydrogen is desorbed after recombination at 440 K. At higher dosages, TTIP is not completely dissociated, but some of isopropoxy ligands are still bound to titanium, even at 300 K. These intact ligands of TTIP produce acetone, isopropanol and propylene at temperatures between 300 and 500 K by disproportionation among themselves or by simple decomposition of the TiO bond in TTIP.

Hydrogen desorption properties of multiwall carbon nanotubes with closed and open structures

The multiwall carbon nanotubes (MWNTs) synthesized by microwave plasma enhanced chemical vapor deposition in 0.1% CH4 (H2 dilution) reaction gas were shaped in curly structures with the blocked nanoholes and closed caps. On the contrary, the carbon nanotubes with different structures could be obtained in oxygen loaded reaction gas, showing aligned structures with connected holes and open caps. The hydrogen desorption properties of the carbon nanotubes (CNTs) with these closed and open structures were compared by thermal desorption technique. The MWNTs with closed structure desorbed hydrogen at two different temperature ranges such as 290–330 and ∼420 K, where the evolved hydrogen amount were ∼0.64 and ∼0.03 wt %, respectively. In case of aligned and open MWNTs, hydrogen as high as ∼1.97 wt % was released at ambient temperature. The hydrogen desorption of MWNTs with open and closed structure at ambient temperature showed the hydrogen desorption activation energies of −16.5 kJ/mol H2 and −18.5 kJ/mol H2, respectively. The high temperature hydrogen desorption observed only in CNTs with closed and highly defective structure was high desorption activation energy of −124.4 kJ/mol.

Thermogravimetry (TG) applied to the study of the reaction of mercury with platinum–rhodium alloy

Thermogravimetry (TG) energy dispersive X-ray microanalysis (EDX), scanning electron microscopy (SEM), mapping surface, X-ray diffraction (XRD), inductively coupled plasma emission spectroscopy and atomic spectroscopy with cold vapor generation have been used to study the reaction of mercury with platinum–rhodium (Pt–Rh) alloy. The results suggest that, the electrodeposited Hg film reacts with Pt–Rh to form intermetallic compounds of different stability, when heated indicated by at least four weight loss steps. Intermetallic compounds as PtHg4 and PtHg2 was characterized by XRD. These intermetallic compound are the main product presents on the surface of the samples after remotion of the bulk mercury via thermal desorption techniques.

Workplace monitoring for volatile organic compounds using thermal desorption-gas chromatography-mass spectrometry.

The interest in the identification of volatile organic compounds in the workplace has been a major focus of many National Institute for Occupational Safety and Health (NIOSH) field studies. A primary technique for sampling and analysis of these compounds is summarized by NIOSH Manual of Analytical Methods (NMAM) 2549. This is a screening method that uses a multi-bed sorbent to trap a wide variety of compounds and compound classes. Thermal desorption techniques are used as a first attempt to characterize potential contaminants in a workplace and to determine what future sampling and analyses must be performed. Field examples are provided to show the versatility of thermal desorption methods and techniques. Due to their sensitivity, thermal desorption tube methods are sometimes required in order to measure the workplace concentrations of unusual compounds. In other situations, the exposures are too high or varied to make thermal desorption tubes practical. In these cases, the identification of contaminants with thermal desorption tubes leads to new method developments for the quantification of specific compounds using more conventional solid sorbent-solvent desorption based methods.

Analysis of low-molecular weight radiolysis products in extracts of gamma-irradiated polymers by gas chromatography and high-performance liquid chromatography

Estimating exposure to radiolysis products of polymers is an important part of the regulatory evaluation of packaging materials for use in food irradiation. However, as Koni Grob recently put it, the comprehensive analysis of migrants is a challenge. This paper discusses some of the analytical difficulties and presents results obtained with extracts of irradiated polystyrene and polyamide-6. The results indicate that headspace or thermal desorption techniques may, in some instances, lead to an overestimation of radiolysis product concentrations. It is concluded that validated analytical methods and a better understanding of the underlying radiation chemistry would greatly facilitate the safety assessment of irradiated packaging materials.