Construction and validation of automated purge-and-trap-gas chromatography for the determination of volatile organic compounds

An automated purge-and-trap chromatographic system for the determination of dissolved volatile organic compounds in aqueous samples was built in the laboratory with minimum cost both in the construction and routine operation. This system was built upon a commercial gas chromatograph with full automation capability using self-developed hardware and software. The use of a multi-sorbent bed quantitatively trapped a wide range of volatile organic compounds at ambient temperature, including the extremely volatile ones such as dichlorofluoromethane (CFC-12). Flash heating for rapid desorption and adequate plumbing for minimizing dead volume resulted in excellent chromatographic separation at above-ambient temperatures, which eliminated the need for cryogen for cooling at the head of the column, a second refocusing stage, or entire GC oven for refocusing. This cryogen-free system was tested with standard solutions and environmental samples for determining hydrocarbons with flame ionization detection, and halogenated compounds with electron-capture detection. An innovative method was also developed for validating the system's linearity for extremely volatile compounds. By introducing ambient air, which usually contains constant levels of anthropogenic halocarbons, e.g., CFC-12 and CFC-11 (CCl3F), the need to prepare aqueous standards containing extremely volatile compounds is avoided, hence providing a convenient method for evaluating a purge-and-trap system.     

Thermal cycling of glasses. II. Experimental evaluation of the structure (or nonlinearity) parameter x

The structure parameter x (also known as the nonlinearity parameter), which in the KAHR model determines the relative contributions of temperature and structure to the retardation times of polymers in the glass transition region, is evaluated experimentally for a narrow molecular weight fraction of atactic polystyrene by the peak-shift method. This method is based on the dependence on heating rate and annealing time of the endothermal peak temperatures T_p, obtained by differential scanning calorimetry during the heating stage of well-defined three-step thermal cycles. The results obtained from the shifts of T_p, with both heating rate and the amount of isothermal recovery during the annealing stage before reheating, are consistent with the theoretical predictions of the KAHR model. The appropriate analysis of the data for a polystyrene fraction of molecular weight 30,100 leads to a value of x = 0.46 ± 0.02. Furthermore, this value of x is constant for a wide range of annealing times and heating rates and for two different annealing temperatures, in full agreement with the theoretical model. The advantages of the peak-shift method over that used by most other workers, the curve-fitting method, are critically discussed.     

Thermal Decomposition of Ni(II) and Fe(III) Nitrates and their Mixture

The thermal decompositions of nickel(II) nitrate hexahydrate and iron(III) nitrate nonahydrate were followed. It was found that the final decomposition products were NiO at 623 K and Fe2O3 at 523 K, respectively. The two salts exhibited only endothermic peaks and a loss in mass until constant mass was attained. The decomposition reactions and the compounds corresponding to each reaction were established. A heating rate of 1 K min-1 revealed several intermediates; higher heating rates shifted the peaks to higher temperatures. The use of an air flow during decomposition shifted the reactions to lower temperatures. The DTA for the mixed salts was found to be an overlap and the TG a summation of the results for the two individual salts. At 773 K, the decomposition products were composed of three phases: NiO, Fe2O3 and NiFe2O4. When these products were heated to 1773 K, only NiFe2O4 was identified by X-ray diffraction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of reductive stripping of Pu(IV) in the tributylphosphate-kerosene/nitric acid-water system using dihydroxyurea

The kinetics of the reductive stripping of plutonium(IV) by dihydroxyurea (DHU) in 30% TBP/kerosene-HNO₃ system was studied with a constant interfacial area cell. The stripping rate of plutonium(IV) increases with the increase of the stirring speed of two phases and the interfacial area. The activation energy of this process is 28.4 kJ/mol. Under the given experimental conditions, the mass transfer of Pu is not controlled by redox reaction, but controlled by molecular diffusion from the organic phase to organic film layer and from the aqueous film layer to aqueous phase. The rate equation of reductive stripping (process is controlled by diffusion) was obtained as: r ₀ = k′[Pu(IV)]₀[DHU]_a ^0.16[HNO₃]_a ^−0.34. The rate constant k′ is (5.0±0.4)·10⁻² (mol/L)^0.18·min⁻¹ at 18.0°C.     

Thermal desorption gas chromatography with mass spectrometry study of outgassing from polymethacrylimide foam (Rohacell®)

Polymethacrylimide foams are used as light structural materials in outer‐space devices; however, the foam closed cells contain volatile compounds that are outgassed even at low temperatures. These compounds ignite as plasmas under outer‐space radiation and the intense radio‐frequency fields used in communications. Since plasmas may cause spacecraft fatal events, the conditions in which they are ignited should be investigated. Therefore, qualitative and quantitative knowledge about polymethacrylimide foam outgassing should be established. Using thermogravimetric analysis, weight losses reached 3% at ca. 200°C. Thermal desorption gas chromatography with mass spectrometry detection was used to study the offgassed compounds. Using successive 4 min heating cycles at 125°C, each one corresponding to an injection, significant amounts of nitrogen (25.3%), water (2.6%), isobutylene (11.3%), tert‐butanol (2.9%), 1‐propanol (11.9%), hexane (25.3%), propyl methacrylate (1.4%), higher hydrocarbons (11.3%), fatty acids (2.2%) and their esters (1.3%), and other compounds were outgassed. Other compounds were observed during the main stage of thermal destruction (220–280°C). A similar study at 175°C revealed the extreme difficulty in fully outgassing polar compounds from polymethacrylimide foams by baking and showed the different compositions of the offgassed atmosphere that can be expected in the long term.     

A solvent recirculation trapping device for supercritical fluid extraction of polyaromatic hydrocarbons

A new device has been developed for the trapping of volatile pollutants in trapping solvents. The device allows solvent recirculation and cryogenic trapping of evaporated volatiles to minimize the stripping effect and any losses of volatile analytes. Due to solvent recirculation, the trapping solvent column height remains constant during the extraction without any need for replenishment. Also mass transfer conditions are favorable due to the flattened shape of bubbles of CO2 and the longer extraction time. The bubbles have higher interfacial area and they have to pass a three times longer distance in the solvent column. The device produces more concentrated extracts, reduces solvent consumption, and reduces or eliminates its evaporation to the environment. The cryotrapping part reduces losses of volatile analytes and the stripping effect. It also enables single-phase extraction into much smaller solvent volumes. Due to constant and favorable extraction conditions, the precision of the method was also greatly improved (RSDs decreased from 2.2 to 0.8%). As proved by a set of rapid spiked-sample extractions of highly volatile compounds at very high flow rates, the relative standard deviation of the experiments performed in the new device is 3.5 times lower.     

Dynamic headspace analysis of solid waste materials

A dynamic headspace stripping technique for the extraction of volatile organic compounds has been applied to a variety of solid and semisolid waste materials. A simple glass apparatus accommodates a wide range of sample sizes and allows for the volatiles to be stripped at elevated temperatures. Concentration on Tenax, followed by thermal desorption and analysis by fused silica capillary gas chromatography provides detailed information on the volatile content of waste samples of widely differing matrix composition.     

Synthesis and Thermal Properties of 4,4'-bipyridine Adducts of Mn(II), Co(II), Ni(II) and Cu(II) Monochloroacetate

The complexes with the empirical formula M(4-bipy)(ClCH₂COO)₂ ×nH₂O (where: 4-bipy=4,4'-bipyridine, L=ClCH₂ COO^–, M (II)=Mn, Co, Ni, Cu) were prepared and characterized via the IR and electronic (VIS) spectra and conductivity measurements. Thermal decomposition of these compounds was studied. During heating in air dehydration processes occur. The anhydrous compounds decompose at high temperature to oxides. The principal volatile mass fragments correspond to: H₂O, CO₂, CH₃Cl, HCl, Cl₂ and other. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of the separation characteristics of application-specific (volatile organic compounds) open-tubular columns for gas chromatography

The solvation parameter model is used to characterize the separation characteristics of two application-specific open-tubular columns (Rtx-Volatiles and Rtx-VGC) and a general purpose column for the separation of volatile organic compounds (DB-WAXetr) at five equally spaced temperatures over the range 60-140 degrees C. System constant differences and retention factor correlation plots are then used to determine selectivity differences between the above columns and their closest neighbors in a large database of system constants and retention factors for forty-four open-tubular columns. The Rtx-Volatiles column is shown to have separation characteristics predicted for a poly(dimethyldiphenylsiloxane) stationary phase containing about 16% diphenylsiloxane monomer. The Rtx-VGC column has separation properties similar to the poly(cyanopropylphenyldimethylsiloxane) stationary phase containing 14% cyanopropylphenylsiloxane monomer DB-1701 for non-polar and dipolar/polarizable compounds but significantly different characteristics for the separation of hydrogen-bond acids. For all practical purposes the DB-WAXetr column is shown to be selectivity equivalent to poly(ethylene glycol) columns prepared using different chemistries for bonding and immobilizing the stationary phase. Principal component analysis and cluster analysis are then used to classify the system constants for the above columns and a sub-database of eleven open-tubular columns (DB-1, HP-5, DB-VRX, Rtx-20, DB-35, Rtx-50, Rtx-65, DB-1301, DB-1701, DB-200, and DB-624) commonly used for the separation of volatile organic compounds. A rationale basis for column selection based on differences in intermolecular interactions is presented as an aid to method development for the separation of volatile organic compounds.     

Über eine einfache methode zur formierung von Rhenium-Kathoden

By treatment of a rhenium cathode with volatile organometallic thorium compounds such as Th(C₅H₅)₄ the emission characteristics are notably improved. One can work at reduced cathode heating currents and therefore at lower ion source temperatures. This effect is maintained for several weeks or months.     

Die ermittlung des schmelzpunktes von kristallinen polymeren mittels wärmeflusskalorimetrie (DSC)

Small and imperfect crystals in polymers reorganize during slow heating. This leads to an increase of their melting point T_m. In order to measure the melting point of the original crystals, high heating rates are needed. This is possible with the modern heat-flow-calorimeters, which work with very small samples. The thermal lag of a DSC cell causes a shift of the melting peak by ΔT to higher temperature. From the theory of a heat-flow-calorimeter, it follows that the error ΔT is proportional to the square root of heating rate. heat of fusion and sample mass. Measurements with sharp melting low molecular weight compounds confirm that this square root relation is quantitatively followed. In order to measure the true melting point of the crystals present in a polymer sample, one has to use different high heating rates and constant sample mass. By plotting the melting peak temperatures as a function of the square root of heating rate and linear extrapolation to zero heating rate, the true melting point is found. This method is applied to HDPE, LDPE and some polyamides.     

A microfluidic device for open loop stripping of volatile organic compounds

The detection of volatile organic compounds is of great importance for assessing the quality of water. In this contribution, we describe a miniaturized stripping device that allows fast online detection of organic solvents in water. The core component is a glass microfluidic chip that facilitates the creation of an annular-flowing stream of water and nitrogen gas. Volatile compounds are transferred efficiently from the water into the gas phase along the microfluidic pathway at room temperature within less than 5 s. Before exiting the microchip, the liquid phase is separated from the enriched gas phase by incorporating side capillaries through which the hydrophilic water phase is withdrawn. The gas phase is conveniently collected at the outlet reservoir by tubing. Finally, a semiconductor gas sensor analyzes the concentration of (volatile) organic compounds in the nitrogen gas. The operation and use of the stripping device is demonstrated for the organic solvents THF, 1-propanol, toluene, ethylbenzene, benzaldehyde, and methanol. The mobile, inexpensive, and continuously operating system with liquid flow rates in the low range of microliters per minute can be connected to other detectors or implemented in chemical production line for process control.

The pyrolysis and nonflaming oxidative degradation of poly(vinylfluoride)

The thermal degradation of poly(vinylfluoride) (PVF) was studied under slow heating conditions to 1000°C with and without the presence of air. The degradation products, classified as low-boiling volatiles, high-boiling volatiles, and nonvolatile residues, were analyzed quantitatively by gas chromatography—mass spectrometry and several spectroscopic methods. Initial stages of degradation begin at 420°C with the evolution of HF and benzene and rapidly reach a maximum in sample weight loss by 450°C. One-third of this weight loss was in the form of hydrofluoric acid (HF) and at least 70 low-boiling volatile compounds that consisted of substituted aromatics, unsaturated hydrocarbons, and multiple-ring compounds, many of which contained a fluorine atom. The high-boiling volatile fraction contained compounds with more aliphatic but less aromatic character than the low-boiling. The nonvolatile residue retained 4% of the original fluorine content and exhibited strong unsaturated character. In the presence of oxygen HF, CO, and H₂O were the major constituents of the low-boiling volatiles; the organic fraction was essentially unchanged in composition but reduced in overall concentration. The overall weight-loss process was bimodal in air and produced a thermally resistant residue that degraded by 650°C. A comparison of degradation products from poly(vinylchloride) with this work demonstrates that PVF forms more lower-molecular-weight, halogen-containing compounds, whereas the former produced more HCl and nonvolatile residue containing a lower halogen content.     

Influence of olive (cv Grignano) fruit ripening and oil extraction under different nitrogen regimes on volatile organic compound emissions studied by PTR-MS technique

Volatile organic compounds of extra virgin olive oils obtained from the local Italian cultivar Grignano were measured by proton transfer reaction–mass spectrometry (PTR-MS). Oils were extracted by olives harvested at different ripening stages across veraison, performing each extraction step and the whole extraction process in nitrogen atmosphere to observe the changes in the volatile profiles of the oils. Principal component analysis carried out on the full spectral signature of the PTR-MS measurements showed that the stage of ripening has a stronger effect on the global definition of volatile profiles than the use of nitrogen during oil extraction. The fingerprint-like chemical information provided by the spectra were used to construct a heat map, which allowed the dynamical representation of the multivariate nature of mass evolution during the ripening process. This provided the first evidence that some groups of volatile organic compounds displayed a time course of regulation with coordinated increasing or decreasing trends in association with specific stages of fruit ripening.     

Preliminary study of the retention behavior for different compounds using cryogenic chromatography at different initial temperatures

The Kováts indices (I) of 24 volatile organic compounds were measured at the different initial temperatures by a laboratory-made cryogenic chromatography. The results show that the correlations of the Kováts indices and the temperature are nonlinear at low initial temperature. Seven QSRR models were built using the heuristic method at different initial temperatures by calculating the quantitative chemical parameters of 24 organic compounds to study the relationship between the retention behavior and the molecular parameters. By studying the relationship between carbon and retention behavior, the capacity factor, and resolution of selected compounds, the contribution of gas–liquid interfacial adsorption process, which influences the retention behavior of solutes, is becoming more and more important with the decrease of the initial temperature.     

How to Identify Roast Defects in Coffee Beans Based on the Volatile Compound Profile

The aim of this study was to detect and identify the volatile compounds in coffee that was obtained in defect roast processes versus standard roasting and to determine the type and strength of the correlations between the roast defects and the volatile compound profile in roasted coffee beans. In order to achieve this goal, the process of coffee bean roasting was set to produce an underdeveloped coffee defect, an overdeveloped coffee defect, and defectless coffee. The “Typica” variety of Arabica coffee beans was used in this study. The study material originated from a plantation that is located at an altitude of 1400–2000 m a.s.l. in Huehuetenango Department, Guatemala. The analyses were carried out with the use of gas chromatography/mass spectrometry (GC–MS) and an electronic nose. This study revealed a correlation between the identified groups of volatile compounds and the following coffee roasting parameters: the time to the first crack, the drying time, and the mean temperatures of the coffee beans and the heating air. The electronic nose helped to identify the roast defects.     

Removal of VOCs from air by membrane-based absorption and stripping

Atmospheric emission of volatile organic compounds (VOCs) such as toluene, xylene, acetone etc. from industrial facilities causes serious environmental problems and financial losses. Existing technologies for VOC emission abatement have many strengths as well as considerable limitations. A regenerative absorption-based process for removal of VOCs from N₂ in an inert, nonvolatile, organic liquid flowing in compact hollow fiber devices has been studied here. These devices eliminate flooding, loading and entrainment encountered in conventional absorption units. Detailed experimental results and theoretical analyses for absorption studies were communicated elsewhere. The overall performance of the combined absorption-stripping process is described here; it appears to be controlled by stripping due to the low temperature and the lower membrane surface area in the stripper. The difference between only absorption and combined absorption-stripping results was more pronounced for VOC-absorbent systems having higher Henry's law constant and diffusivity. A theoretical model has been developed from first principles to simulate the behavior of the membrane stripper; this has been combined with the model for the membrane absorber to determine the overall process performance. Simulated results obtained from the mathematical models agree well with the experimental results for combined absorption-stripping. Simulation results suggest that higher stripping temperature and larger stripper area enhance the performance considerably.     

Effects of variation in modulator temperature during cryogenic modulation in comprehensive two‐dimensional gas chromatography

Many modulation systems in comprehensive 2D GC (GC×GC) are based on cryogenic methods. High trapping temperatures in these systems can result in ineffective trapping of the more volatile compounds, whilst temperatures that are too low can prevent efficient remobilisation of some compounds. To better understand the trapping and release of compounds over a wide range of volatilities, we have investigated a number of different constant temperature modulator settings, and have also examined a constant temperature differential between the cryo‐trap and the chromatographic oven. These investigations have led us to modify the temperature regulation capabilities of the longitudinally modulated cryogenic system (LMCS). In contrast to the current system, where the user sets a constant temperature for the cooling chamber, the user now sets the temperature difference between the cryo‐trap and the chromatographic oven. In this configuration, the cooling chamber temperature increases during the chromatographic run, tracking the oven temperature ramp. This produces more efficient, volatility‐dependent modulation, and increases the range of volatile compounds that can be analysed under optimal trap‐and‐release conditions within a single analytical run. This system also reduces cryogenic fluid consumption.     

Developmental variation in floral volatiles composition of a fragrant orchid Zygopetalum maculatum (Kunth) Garay

Emitted scent volatile profile of an orchid species Zygopetalum maculatum was studied using dynamic headspace sampling technique with four different adsorbent matrices, namely Porapak Type Q polymer (mesh size: 80/100), Tenax (mesh size: 60/80), activated charcoal and graphite. In addition, developmental variations in scent emissions and endogenous volatile levels were also investigated. Gas chromatography-mass spectrometry analysis revealed the presence of 21 volatile compounds in the headspace, which was predominantly enriched with benzenoid compounds. Among these benzenoids, o-diethylbenzene and p-diethylbenzene were the major compounds followed by benzyl acetate and methyl salicylate. Among the phenylpropanoid compounds, 2-phenylethyl acetate was the major volatile. However, as compared to benzenoids, the quantity was much lesser, indicating the inclination of phenylalanine flux towards benzenoid pathway. The outcome of this study has the implications in enhancing fragrance and vase life of orchids of the Sikkim Himalaya region and thus may further help to meet the growing market demand.     

2-(4-Alkylphenyl)-5-(alkenyloxy)pyrimidines: Synthesis,liquid crystal transition temperatures and some physical properties

Abstract

We have recently reported the introduction of a carbon-carbon double bond into a wide variety of 5-n-alkyl-2-(4-n-alkoxyphenyl)pyrimidines to produce the corresponding alkenyloxy derivatives. The position and nature (E/Z) of the double bond were varied systematically and the effect on the liquid crystal transition temperatures studied. The position and nature (E/Z) of the double bond changed the conformation of the alkenyloxy chain substantially. This resulted in higher smectic C and nematic transition temperatures for compounds with a trans-double bond (E) at an even number of carbon atoms from the molecular core. Significantly lower transition temperatures (including the melting point) were observed for materials with a cis-double bond (Z) at an odd number of carbon atoms from the molecular core. We have now performed the same operation on the related 2-(4-n-alkylphenyl)-5-n-alkoxypyrimidines to produce the corresponding alkenyloxy derivatives. An interesting feature of the new results is the high melting points of the trans-substituted materials and the low melting points of the terminally substituted compounds. The smectic C transition temperatures of both series are high. No nematic phases could be observed. However, in admixture with other smectic C components, the new compounds lead to surprisingly fast switching times, high smectic C transition temperatures and low melting points/crystallization temperatures in experimental mixtures designed for electro-optic display devices based on ferroelectric effects.