Measurements of chlorofluorocarbons and their replacement compounds at Monte Cimone: results obtained after eighteen months of observations

Chlorofluorocarbons and their substitutes are anthropogenic compounds strongly involved in global change phenomena. Therefore, their atmospheric mixing-ratios are monitored on a worldwide scale. In order to evaluate source strength of these compounds in Southern Europe, in 1999 a research activity, monitoring these compounds in the atmosphere of the Monte Cimone (MO, Italy) was started. Air samples, collected on a weekly base, were analyzed using a GC-MS methodology recently devised by our group. The reported results, obtained after eighteen months of observation, are relative to four fully halogenated halocarbons and four hydrogenated halocarbons.     

Studies about the desorption of volatile halocarbons from activated carbon by application of static headspace gas chromatography

The analysis of volatile halocarbons (VHCs) in air or ground air is often performed after their adsorption and enrichment on activated carbon. The current procedure for their subsequent determination is based on their extraction from the activated carbon with a volatile organic solvent such as n-pentane, followed by gaschromatographic (GC) analysis. In order to avoid extraction steps, the static headspace method in combination with GC analysis using diphenylmethane as a desorption agent has been applied. Satisfactory desorption rates for the chloromethanes, for 1,1,1-trichloroethane, trichloroethene and for tetrachloroethene have been obtained after a sample equilibration of 45 min at 120° C in the presence of diphenylmethane. The results have shown a higher recovering rate especially of the unsaturated VHCs compared to the extraction with n-pentane, whereby a potential loss of analytes by the latter procedure has been avoided.     

Studies about the desorption of volatile halocarbons from activated carbon by application of static headspace gas chromatography

The analysis of volatile halocarbons (VHCs) in air or ground air is often performed after their adsorption and enrichment on activated carbon. The current procedure for their subsequent determination is based on their extraction from the activated carbon with a volatile organic solvent such as n-pentane, followed by gaschromatographic (GC) analysis. In order to avoid extraction steps, the static headspace method in combination with GC analysis using diphenylmethane as a desorption agent has been applied. Satisfactory desorption rates for the chloromethanes, for 1,1,1-trichloroethane, trichloroethene and for tetrachloroethene have been obtained after a sample equilibration of 45 min at 120 degrees C in the presence of diphenylmethane. The results have shown a higher recovering rate especially of the unsaturated VHCs compared to the extraction with n-pentane, whereby a potential loss of analytes by the latter procedure has been avoided.     

Problems connected with the analysis of halocarbons and hydrocarbons in the non-urban atmosphere

The problems connected with the measurement of hydrocarbons outside urban areas are considerable: The atmospheric mixing ratios of most of the hydrocarbons are very low--from a few ppb down to some ppt; the mixture of hydrocarbons is extremely complex, ranging from light n-alkanes to alkyl benzenes and terpenes; for measurements in remote areas the logistic conditions often restrict the instrumentation which can be used for sample collection or in situ measurements (such as lack of electric power supply, weight restrictions etc.). Nevertheless, sensitive and sufficiently reliable measurements of hydrocarbons in the non-urban atmosphere are important. Hydrocarbons are important factors in the tropospheric photochemistry (e.g. ozone formation) and can be used as valuable tracers for man-made atmospheric pollutants etc. Other useful tracers for anthropogenic emission are halocarbons such as dichlormethane, tri- and tetrachloroethen etc. The impact of man-made hydrocarbons on the chemistry of the troposphere can only be understood if the extent of natural (biogenic) contributions is known. From measurements of a large variety of hydrocarbons and halocarbons it is often possible to obtain information about the sources of the most important atmospheric hydrocarbon species, even for trace gases with both significant anthropogenic and biogenic sources. In this presentation some of the problems and their solutions connected with such measurements of atmospheric hydrocarbons and halocarbons are presented and discussed. Some of the results obtained by several series of measurements are described, indicating that man-made as well as biogenic hydrocarbons can be important factors for the chemistry of the atmosphere.     

Microwave induced plasma atomic emission spectrometry: a suitable detection system for the determination of volatile halocarbons

Microwave induced plasma atomic emission spectrometry (MIP-AES), a highly sensitive detection system for organometal compounds, was coupled to an automated purge and trap gas chromatographic system for the determination of volatile halogenated hydrocarbons in environmental water samples. Optimisation of the parameters affecting the injection and detection system led to relative detection limits from 1 to 14 ng · L^–1 for chlorine- and bromine-compounds and from 10 to 75 ng · L^–1 for iodine-compounds, on basis of a 10 mL sample volume. A comparison of the analytical characteristics between atomic emission detection (AED) and electron capture detection (ECD) showed a lower sensitivity of the atomic emission detector for halocarbons, but the detection thresholds are low enough to use the method for the determination of volatile halocarbons in trace level concentrations. The ability of the atomic emission detector provides increased selectivity for monitoring individual halogenated compounds under simplified and rapid chromatographic conditions, within a total analysis time of only 30 min. The method was applied with gas chromatographic separation for the analysis of sea water samples. Concentrations for the different elements between 0.05 and 15.28 μg · L^–1 were determined.     

Separation of mixed halocarbons of environmental interest on a new type of silica-based porous-layer open tubular capillary gas chromatographic column

A new type of capillary porous-layer open tubular (PLOT) column consisting of a hydrophobic silica layer on a fused-silica capillary has been tested for the separation of a mixture of environmentally sensitive halocarbons present in tropospheric air. The column shows high retention for a wide range of halocarbons, with elution orders following both boiling point order and hydrogen bonding capability. The resolution of the halocarbons is good and only one pair of halocarbons [CHFClCF₃ (HCFC 124) and CH₃CF₂Cl (HCFC 142b)] cannot be resolved on this column type at all column temperature profiles. Unlike alumina PLOT columns, the silica PLOT column does not dehydrohalogenate labile halocarbons. Excellent reproducibility of retention times and peak areas for halocarbons on the column are reported.     

A Ten Years Monitoring of Chlorofluorocarbons at the Antarctica: Results and Perspectives

Abstract

Chlorofluorocarbons are man-made long lasting atmospheric pollutants of great environmental concern, responsible for important global change phenomena. Recently, they were replaced by hydrogenated halocarbons that, even if less persistent, do not lack in environmental impact. Atmospheric concentrations of these compounds were measured in Antarctica by gas chromatography. The extremely low atmospheric mixing ratios of these compounds require a pre-concentration step of the air sample on suitable adsorbent in order to meet the sensitivity of the analytical method Results obtained analyzing air samples collected in Antarctica since 1988 for the determination of CFC-12 and CFC-11 are reported, together with data concerning the less abundant species.     

Application of Headspace Liquid-Phase Microextraction to the Analysis of Volatile Halocarbons in Water

The determination of four volatile halocarbons (CHCl₃, CCl₄, C₂HCl₃ and C₂Cl₄) in water by headspace liquid-phase microextraction (HS-LPME) with gas chromatography using a micro electron capture detector (GC-μECD) is described. The effects of the type and volume of the extraction solvent, headspace volume, stirring rate, extraction temperature and time and ionic strength on the extraction performance are investigated and optimized. The developed protocol yields a linear calibration curve in the concentration range from 0.05 to 50 µg L^?1 for the target analytes; the detection limits ranged from 0.003 to 0.146 µg L^?1 and the relative standard deviation (R.S.D.) values below 8.45%. The results demonstrate that HS-LPME followed with GC-μECD is a simple and reliable technique for the determination of volatile halocarbons in water samples.     

Trace analysis of volatile polar organics by direct aqueous injection gas chromatography

Summary Procedures for the quantitative analysis of industrial effluents which involve concentration by solvent extraction or the purge-and-trap method are time-consuming, labor-intensive, and prone to error. Direct aqueous injection gas chromatography using an electron-capture detector for the analysis of volatile halocarbons at the ppb level is in routine use in many laboratories. We now discuss the development of a similar protocol for the analysis of volatile polar organics such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and tretrahydrofuran using a flame-ionization detector.     

Gas chromatography of low molecular weight halocarbons on macroporous silica modified by a thin liquid phase film

Summary The use of the silica adsorbent Silochrom C-80 modified by a thin liquid stationary phase film is proposed for the gas chromatographic analysis of low molecular weight halocarbons. Such modified adsorbents are characterized by good retention of the volatile halocarbons and high selectivity with respect to halocarbons. The thermodynamic sorption characteristics of alkanes, chloroalkanes and chloromethanes on Silochrom modified by three liquid phases (Apiezon L, methylsilicone and polyethylene glycol 20M) and on Chromaton N with thick films of the same phases were studied. The heats of sorption (−ΔH) and free sorption energy changes (−ΔF) of these substances and the increments −Δ(ΔH) and −Δ(ΔF) per chlorine atom in the chloralkane molecule compared with n-alkanes with the same number of carbon atoms were determined. The “chlorine” retention indices of low molecular weight volatile halocarbons on three sorbents were determined to identify them in aqueous solutions using the electron-capture detector. The analysis of drinking water by this technique is demonstrated.     

Preparation of a Carbon-Coated SPME Fiber and Application to the Analysis of BTEX and Halocarbons in Water

A carbon-coated fiber for solid-phase microextraction (SPME) has been prepared from powdered activated carbon (PAC) and a fused-silica fiber. Scanning electron microscopy of the coating revealed the carbon particles were uniformly distributed on the surface of the fiber substrate. Efficient extraction of BTEX (benzene, toluene, ethylbenzene, p-xylene, and o-xylene) and halocarbons (chloroform, trichloroethylene, and carbon tetrachloride), with short extraction and desorption times, was achieved by use of the coated fiber. The maximum working temperature of the coated fiber was 300 °C and the lifetime was over 140 desorption operations at 260 °C. Limits of quantification (LOQ) of the SPME method for the eight analytes ranged from 0.01 to 0.94 μg L⁻¹, and relative standard deviations (RSD) were below 7.2% (n=6). Recoveries were 87.9–113.4% when the method was applied to the analysis of BTEX and the halocarbons in real aqueous samples. An erratum to this article is available at .     

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.     

Element selective detection of chlorine in capillary gas chromatography by wavelength modulation diode laser atomic absorption spectrometry in a microwave induced plasma

A new element selective detector for gas chromatography is presented. The detector, based on wavelength modulation diode laser atomic absorption spectrometry in a microwave induced helium plasma, is used for measurements of chlorinated hydrocarbons by absorption of excited, metastable chlorine atoms. Indications of complete dissociation of the halocarbons in the plasma have been found. Preliminary detection limits of the order of 1 μg ml⁻¹ or 80 pg s⁻¹ have been found for different halocarbons.     

Theoretical study of halophilic reactions of stable silylenes with chloro- and bromocarbons

A theoretical study of the mechanism of the reaction of stable silylenes with halocarbons has been carried out using the B3LYP density functional method. The main findings are as follows: (1) Lewis acid-base complexes formed between silylenes and halocarbons do not play a role in silylene insertion chemistry into halocarbons; therefore, the acid-base complex mechanism proposed by West et al. (J. Am. Chem. Soc. 2002, 124, 4186) is not appropriate to describe the disilane formation reaction. (2) The disilane formation reactions follow the energetically favorable general reaction pathway (X = halogen): (i) Y2Si: + HCX3 --> TS1 --> Y2XSi-CHX2. (ii) Y2Si: + Y2XSi-CHX2 --> TS2 --> Y2XSi-SiY2CHX2. (3) The observed preference of stable silylenes to undergo C-X bond insertion rather than C-H bond has been investigated. The theoretical findings suggest that this preference is a result of the thermodynamic factor. (4) Stable silylenes prefer to insert into a C-Br rather than a C-Cl bond because the energy barrier to insertion is lower, and the reaction is more exothermic.     

Thermodynamic calculations for molecules with asymmetric internal rotors. II. Application to the 1,2-dihaloethanes

The thermodynamic properties of three halocarbon molecules relevant in atmospheric and public health applications are presented from ab initio calculations. Our technique makes use of a reaction path-like Hamiltonian to couple all the vibrational modes to a large-amplitude torsion for 1,2-difluoroethane, 1,2-dichloroethane, and 1,2-dibromoethane, each of which possesses a heavy asymmetric rotor. Optimized ab initio energies and Hessians were calculated at the CCSD(T) and MP2 levels of theory, respectively. In addition, to investigate the contribution of electronically excited states to thermodynamic properties, several excited singlet and triplet states for each of the halocarbons were computed at the CASSCF/MRCI level. Using the resulting potentials and projected frequencies, the couplings of all the vibrational modes to the large-amplitude torsion are calculated using the new STAR-P 2.4.0 software platform that automatically parallelizes our codes with distributed memory via a familiar MATLAB interface. Utilizing the efficient parallelization scheme of STAR-P, we obtain thermodynamic properties for each of the halocarbons, with temperatures ranging from 298.15 to 1000 K. We propose that the free energies, entropies, and heat capacities obtained from our methods be used to supplement theoretical and experimental values found in current thermodynamic tables.     

Infrared band strengths and their temperature dependence of the hydrohalocarbons HFC-134a,HFC-152a,HCFC-22, HCFC-123 and HCFC-142b

The absolute strengths of the IR absorption bands of five halocarbons, proposed as substitutes of chlorofluorocarbons, have been measured at 293, 273 and 233 K in the wavelength region 600–1500 cm⁻¹. The band strengths were derived from measurements performed using an FT-IR spectrometer operating at 0.1 cm⁻¹ instrumental resolution. The values obtained are compared to those from previous studies. Only a slight temperature dependence was observed, indicating a small increase of the band strengths at decreasing temperatures.     

Trace analysis of halocarbons in water; Direct aqueous injection with electron capture detection

Interest in monitoring halogenated organics in drinking water and natural surface and ground water in the low ppb range continues to grow. There is a tendency to include still more volatile halocarbons, the trace determination of which is known to be rather demanding. This prompted us to re-examine the feasibility of large-volume direct aqueous injection onto capillary columns, coupled with ECD. A primary problem was to avoid simultaneous elution of water with halocarbons, since water suppresses the ground current of the ECD. The following measures contributed to the solution of this problem. Apolar, extremely inert, columns are required to elute water completely, and even before very light halocarbons. Their coatings have to be far thicker (≈? 5 mUm) than commonly employed thick films since they must permit isothermal analysis at a column temperature around 100°C in order to ensure rapid and complete elution of water. Finally, it is essential that sampling be carried out on-column for two reasons: diffusion of water vapor in the injector, resulting in delayed elution, is then eliminated, and peak distortion during splitless injection is avoided. Although we now know that persilylated columns with immobilized coatings withstand routine water injections, more longterm experience is needed to provide detailed recommendations for the handling of these columns.     

A "teardown" method to create large mesotunnels on the pore walls of ordered mesoporous silica

A "teardown" method to create large mesotunnels (approximately 9 nm) on the pore walls of ordered mesoporous silicas is demonstrated by digesting the organic constituents from polymer-silicate nanocomposites. The ordered mesostructured polymer-silicate composites were first obtained via the evaporation-induced triconstituent co-assembly method by using a low-molecular-weight phenolic resin (resols) as an organic precursor; prehydrolyzed TEOS as an inorganic precursor, and triblock copolymer F127 as a template. All of organic components including F127 and phenolic resins are removed by the microwave digestion (MWD) method from mesostructured polymer-silica composites. While the removal of triblock copolymer F127 generates main pore channels, the phenolic resins can also be torn down from the pore walls, yielding mesotunnels between the channels. The resulting silica products exhibit ordered 2-D hexagonal mesostructure, large pore volume (up to 1.92 cm(3)/g), and very large pore size (up to 22.9 nm), which is even larger than their mesostructural cell parameter (14.2 nm). TEM images confirm the existence of mesotunnels on the silica pore walls. FT-IR and (29)Si solid-state NMR results reveal that these silica products have a large number of silanol groups.