Comparison of various detection limit estimates for volatile sulphur compounds by gas chromatography with pulsed flame photometric detection

This paper addresses the variations that presently exist regarding the definition, determination, and reporting of detection limits for volatile sulphur compounds by gas chromatography with pulsed flame photometric detection (GC-PFPD). Gas standards containing hydrogen sulphide (H(2)S), carbonyl sulphide (COS), sulphur dioxide (SO(2)), methyl mercaptan (CH(3)SH), dimethyl sulphide (DMS), carbon disulphide (CS(2)), and dimethyl disulphide (DMDS) in concentrations varying from 0.36ppb (v/v) up to 1.5ppm (v/v) in nitrogen were prepared with permeation tubes and introduced in the gas chromatograph using a 0.25-ml gas sampling loop. After measuring the PFPD response versus concentration, the method detection limit (MDL), the Hubaux-Vos detection limit (x(D)), the absolute instrument sensitivity (AIS), and the sulphur detectivity (D(s)) were determined for each sulphur compound. The results show that the MDL determined by the US Environmental Protection Agency procedure consistently underestimates the minimum concentrations of volatile sulphur compounds that can be practically distinguished from the background noise with the PFPD. The Hubaux-Vos detection limits and the AIS values are several times higher than the MDL, and provide more conservative estimates of the lowest concentrations that can be reliably detected. Sulphur detectivities are well correlated with AIS values but only poorly correlated with MDL values. The AIS is recommended as a reliable and cost-effective measure of detection limit for volatile sulphur compounds by GC-PFPD, since the AIS is easier and faster to determine than the MDL and the Hubaux-Vos detection limit. In addition, this study confirmed that the PFPD response is nearly quadratic with respect to concentration for all volatile sulphur compounds.     

Measurements Of Biogenic Hydrocarbons And Their Atmospheric Degradation In Forests

Abstract

An analytical procedure for the sampling and quantitative determination of biogenic hydrocarbons in the atmosphere is presented. The method is based on the adsorptive preconcentration of the hydrocarbons using Tenax TA/Carbopack B filled sampling tubes followed by thermodesorption of the trapped compounds and gas chromatographic analysis. To avoid losses of the biogenic alkenes as a consequence of their reaction with ozone on the adsorbents during the sampling step, an ozone scrubber is used in front of the adsorption tubes. Diurnal variations of the monoterpene- and ozone-concentrations are determined at different heights in and above a forest stand (coniferous and deciduous trees) in the Vosges (Col du Donon, 760 m a.s.l.) in order to elucidate the importance of terpene-oxidation reactions on the formation of peroxidic products in forest air.     

Air sampling of aromatic hydrocarbons in the presence of ozone by solid-phase microextraction

Effects of ozone on air sampling of standard gas mixtures of aromatic hydrocarbons were tested using solid-phase microextraction (SPME). Standard concentrations of ozone ranging from 10 ppb (v/v) to 6400 ppm (v/v) were generated using an in-house built ozone generator based on corona discharge. Effects of temperature, discharge voltage, and oxygen flow on the ozone generation were tested. The working dc voltage had the greatest effect on generated ozone concentration and was proportional to the ozone concentration. Generation temperature and oxygen flow rate were inversely proportional to ozone concentrations. Produced ozone was mixed with standard benzene, toluene, ethylbenzene, and xylenes (BTEX) gas at less than 100 ppb (v/v). Air samples were collected with poly(dimethylsiloxane) (PDMS) 100 microm SPME fibers and analyzed by gas chromatography (GC)-flame ionization detection (FID) and GC-MS. Significant reductions of BTEX concentrations were observed. In addition, some products of BTEX-ozone-oxygen reactions were identified. SPME worked well as a rapid sampler for BTEX and BTEX-ozone-oxygen reaction products. No significant deterioration of the PDMS coating and no significant reduction of absorption capacity were observed after repeated exposure to ozone.     

Continuous measurement of ozone in air by chemiluminescence using ndigo-5,5′-disulphonate

Gas-phase ozone was continuously measured by using the chemiluminescent reaction between indigo-5,5′-disulphonate and ozone. A detection limit of 0.4 ppb (v/v) was obtained using a reaction cell of 0.7 ml at a sampling rate of 15.0 ml min^?1. The method was applied to the monitoring of ozone in the atmosphere.     

Identification of oxidation products of solanesol produced during air sampling for tobacco smoke by electrospray mass spectrometry and HPLC

Solanesol, a 45-carbon, trisesquiterpenoid alcohol found in tobacco leaves and tobacco smoke, has been used as a quantitative marker for tobacco smoke for years. However, solanesol appears to be unreliable as a quantitative marker for tobacco smoke during environmental air sampling because it can be degraded substantially when present as a component of tobacco smoke and by as much as 100% when present as pure solanesol on fortified filters during air sampling. Since there is strong evidence that ozone is the agent responsible for the degradation, solanesol appears to be unreliable as a quantitative marker during indoor air sampling when indoor levels of ozone are greater than about 15 ppb. The degree of loss of pure solanesol is directly proportional to the concentration of ozone and the length of the sampling period and depends on the type of 37 mm membrane filter used for air sampling (PTFE or quartz fiber). While the degree of loss of solanesol is inversely proportional to the relative humidity of the air at a sampling rate of 1.7 L min(-1), the degree of loss is virtually independent of relative humidity at a lower sampling rate; i.e., 0.25 L min(-1). A curve of loss of solanesol on a filter versus concentration of ozone from an ozone generator is virtually identical to a curve segment based on atmospheric ozone under the same conditions of air sampling. Oxidation of solanesol by ozone to approximately 25 to 60% completion produces at least three series of products for a total of at least 26 compounds: (1) isoprenoid acetones, (2)omega-hydroxyisoprenoid acetaldehydes, and (3) isoprenoid oxoaldehydes. All products in each series were tentatively identified as their derivatives with 2-(p-aminophenyl)ethanol (APE) by electrospray mass spectrometry (ES-MS). Ten ozonation products were detected as their 2,4-dinitrophenylhydrazine derivatives by HPLC at 360 nm: 4-oxopentanal and nine isoprenoid acetones (acetone, 6-methyl-5-hepten-2-one, geranylacetone, farnesylacetone, tetraprenylacetone, geranylfarnesylacetone, farnesylfarnesylacetone, farnesylgeranylgeranylacetone and bombiprenone.     

Dynamic solid phase microextraction sampling for reactive terpenes in the presence of ozone

Dynamic gas sampling using solid phase microextraction (SPME) was evaluated for recovery of reactive terpenes and terpenoids in the presence of ozone. For limonene, α-terpineol and dihydromyrcenol in the 20-60 ppb range, this method achieves >80% recovery for ozone mixing ratios up to 100 ppb. Both the experimental results and a model analysis indicate that higher ozone concentrations and longer sampling times result in lower percent recovery. Typically greater than 90% recovery and ppb level method detection limits were achieved with a 5 min sample time. Increasing the flow rate from 100 to 400 sccm flow (5-20 cm s⁻¹) through the active sampler did not significantly affect sensitivity or recovery in most cases, probably due to negligible mass-transfer improvements. The recovery for each compound improves when sampling from a mixture of different species than that from a single compound sample. This may be due to competition for ozone amongst adsorbed species. Dynamic SPME sampling can improve detection and quantification of terpenes in reactive environments, especially for low vapor pressure (<5 mm Hg at 25 °C) compounds that can be adsorbed to ozone scrubbers used in other methods.     

Water-Nafion equilibria. absence of Schroeder's paradox

Water-Nafion phase equilibria and proton conductivities were measured in two ways. First, Nafion was in contact with saturated water vapor. Second, Nafion was in contact with liquid water at the same temperature. At 29 degrees C, for preboiled, vapor-equilibrated Nafion exposed to water with an activity = 1 and air pressures ranging from 0 to 0.96 bar, the water content was lambda = 23 +/- 1 mol H(2)O/mol SO3-. For the preboiled, liquid-equilibrated membrane, lambda = 24 +/- 2. At 100% relative humidity (RH), the water content of preboiled Nafion decreased as the temperature rose from 30 to 80 degrees C but did not recover its initial water content when the temperature returned to 30 degrees C. The water content of predried Nafion at 1 atm and 30 degrees C was lambda = 13.7 +/- 0.2 when vapor-equilibrated and lambda = 13.1 +/- 0.5 when liquid-equilibrated. A Nafion membrane originally boiled in water had much higher liquid- and 100% RH vapor-equilibrated proton conductivities than the same membrane originally dried at 110 degrees C with a RH less than 2%. The liquid-equilibrated and 100% RH vapor-equilibrated membrane conductivities were the same when the membrane had the same thermal history. The conductivity data was fit to a model, and the water content was determined at different temperatures. The predried membrane water content increased with temperature, and the preboiled membrane's water content changed slightly with temperature. Both water sorption and proton-conductivity data do not exhibit Schroeder's paradox. These studies and previous results suggest that Schroeder's paradox is resolved when attention is given to the thermal history of the absorbing polymer.     

Development of a quantification method for the analysis of malodorous sulphur compounds in gaseous industrial effluents by solid-phase microextraction and gas chromatography-pulsed flame photometric detection

A quantification method for malodorous sulphur compounds in gaseous industrial effluents using solid-phase microextraction sampling followed by gas chromatography-pulsed flame photometric detection has been developed. A comparative study showed that polydimethylsiloxane-Carboxen fibre led to sufficient sensitivity to achieve the microg m(-3) human perception levels of the five analytes studied (hydrogen sulphide, methanethiol, ethanethiol, dimethyl sulphide, dimethyl disulphide). However, this coating is known to suffer from competitive adsorption, which may lead to inaccurate quantification. Therefore, external calibration can only be used under a limited range of concentrations, which were determined from Fick's diffusion law. This approach was tested on a real gaseous sample and compared with the standard addition method. Good correlations were found for ethanethiol, dimethyl sulphide and dimethyl disulphide. However, for more volatile sulphur compounds (i.e., hydrogen sulphide and methanethiol), the easy-to-use external calibration could not be applied and standard additions had to be performed for accurate quantification.     

Measurement of reduced sulphur compounds contained in aqueous matrices by direct injection into a gas chromatograph with a flame photometric detector

An analytical method was developed to measure the concentration of hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide contained in aqueous matrices (distilled water, tap water, kraft mill condensates and membrane bioreactor mixed liquor) by direct injection of aqueous samples into a gas chromatograph with a flame photometric detector. The analytical method requires a small sample volume (2 ml), sample preparation and analysis can be completed within 20 min and no complex sampling apparatus is needed. Consistent results and good recoveries were observed in all matrices investigated over the range of concentrations examined. The relationship between the normalized peak area obtained from GC–flame photometric detection and the concentration of the reduced sulphur compounds (RSCs) examined did not follow the theoretical power law exponent of two. The power law exponent appeared to decrease with the organic fraction associated with each RSC. The observed power law exponents for hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide were 1.92, 1.90, 1.66 and 1.72, respectively.     

Atmospheric ozone measurement with an inexpensive and fully automated porous tube collector-colorimeter

The bleaching action of ozone on indigo and related compounds is well known. We describe sensitive automated instrumentation for measuring ambient ozone. Air is sampled around a porous polypropylene tube filled with a solution of indigotrisulfonate. Light transmission through the tube is measured. Light transmission increases as O(3) diffuses through the membrane and bleaches the indigo. Evaporation of the solution, a function of the RH and the air temperature, can, however cause major errors. We solve this problem by adding an O(3)-inert dye that absorbs at a different wavelength. Here we provide a new algorithm for this correction and show that this very inexpensive instrument package (controlled by a BASIC Stamp Microcontroller with an on-board data logger, total parts cost US$ 300) provides data highly comparable to commercial ozone monitors over an extended period. The instrument displays an LOD of 1.2ppbv and a linear span up to 300ppbv for a sampling time of 1min. For a sampling time of 5min, the respective values are 0.24ppbv and 100ppbv O(3).     

Optical fibre sensor for hydrogen sulphide monitoring in mouth air

A reversible optical fibre chemical sensor for hydrogen sulphide monitoring in mouth air based on reflectance measurements has been developed. The active sensing phase has been prepared by immobilising the colorimetric reagent 2,6-dichlorophenolindophenol (DCPI) in a silica gel support. The principle of the determination is based on the increase of reflectance of such solid sensing phase when hydrogen sulphide reduces the colorimetric reagent with the subsequent decolouration process. The addition of 1.26 μg of Cu(II) per gram of solid support improved the response time and reversibility of the sensing phase.The detection limit is 10 ppb (v/v) of hydrogen sulphide. The linear range using the Kubelka-Munk function extends at least up to 1000 ppb (v/v). The sensor exhibits a response time of less than 2 min for hydrogen sulphide concentrations in the linear range and the signal is reversible.The optical sensor has been successfully tested for human malodour monitoring and the results validated by comparison with those obtained for the same individuals using a commercially available electrochemical instrument.     

Aliphatic/aromatic polyimide ionomers as a proton conductive membrane for fuel cell applications

To produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide ionomers containing aliphatic groups both in the main and in the side chains have been synthesized. The title polyimide ionomers 1 with the ion exchange capacity of 1.78-2.33 mequiv/g were obtained by a typical polycondensation reaction as transparent, ductile, and flexible membranes. The proton conductivity of 1 was slightly lower than that of the perfluorinated ionomer (Nafion) below 100 degrees C, but comparable at higher temperature and 100% RH. The highest conductivity of 0.18 S cm(-)(1) was obtained for 1 at 140 degrees C. Ionomer 1 with high IEC and branched chemical structure exhibited improved proton conducting behavior without sacrificing membrane stability. Microscopic analyses revealed that smaller (<5 nm) and well-dispersed hydrophilic domains contribute to better proton conducting properties. Hydrogen and oxygen permeability of 1 was 1-2 orders of magnitude lower than that of Nafion under both dry and wet conditions. Fuel cell was fabricated with 1 membrane and operated at 80 degrees C and 0.2 A/cm(2) supplying H(2) and air both at 60% or 90% RH. Ionomer 1 membrane showed comparable performance to Nafion and was durable for 5000 h without distinct degradation.     

Röntgenfluorimetrische Bestimmung von Sulfidspuren im ppb-Bereich in wäßrigen Lösungen nach Austauschreaktion an dünnen Silberhalogenidschichten

Sulphide concentrations in the ppm and ppb range can be determined by X-ray fluorescence spectrometry in aqueous solutions with a volume up to 3 l. The solution to be analysed is passed through a thin layer of a silver halide precipitate whereby the sulphide ions are held back as silver sulphide. The membrane filter with the silver halide deposit is directly used for measuring the X-ray intensity from sulphur. The experimental conditions used include: 2?=23.20°, KAP, X-ray tube with a chromium anode, 50 kV, 40 mA, and a throughflow proportional counter. Interferences of numerous accompanying components were investigated as well as the stability of solutions with very low sulphide concentrations. These solutions can be stabilized sufficiently with ascorbic acid at pH 12, but in the case of natural water only in presence of zinc ions. The method is applicable to sulphide amounts greater than 0.1 μg. The variation coefficient for sulphide amounts of ≈ 10 μg was found to be 2.3%.     

Phosphotungstic acid functionalized silica nanocomposites with tunable bicontinuous mesoporous structure and superior proton conductivity and stability for fuel cells

A novel proton exchange membrane using phosphotungstic acid (HPW) as proton carrier and cubic bicontinuous Ia3d mesoporous silica (meso-silica) as framework material is successfully developed as proton exchange membranes for fuel cells. Meso-silica is functionalized by 80wt% HPW using a vacuum impregnation method. The HPW-functionalized meso-silica (HPW-meso-silica) nanocomposites are characterized by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), N(2) adsorption/desorption isotherms, thermogravimetric analysis (TGA), water uptake and four-probe conductivity. The results show that the mesoporous structure of silica hosts can be altered by the hydrothermal temperature. Conductivity measurements indicate that meso-silica host with pore diameter of 5.0 nm has the highest proton conductivity of 0.11 S cm(-1) at 80 °C and 100% relative humidity (RH) with an activation energy of ～14 kJ mol(-1) and better stability as compared to that with large mesopores. The proton conductivity and performance of HPW-meso-silica nanocomposites also increase with the RH, but it is far less sensitive to RH changes as compared to conventional perfluorosulfonic acid (PFSA) polymers such as Nafion. The maximum power density of the cell with HPW-meso-silcia nanocomposite membranes is 221 mW cm(-2) at 80 °C and 100% RH and decreases to 171 mW cm(-2) when RH is reduced to 20%, a 20% decrease in power output. In the case of a cell with Nafion 115 membranes, the decrease in power density is 95% under identical test conditions. The results demonstrate that the HPW-meso-silica nanocomposite has an exceptionally high water retention capability and is a promising proton exchange membrane material for fuel cells operating at reduced humidity and elevated temperatures.     

Kinetic and product yield study of the heterogeneous gas-surface reaction of anthracene and ozone

Little quantitative information exists regarding the products of the heterogeneous reaction of polycyclic aromatic hydrocarbons (PAHs) and ozone. We have, therefore, performed the first quantitative study investigating the kinetics and products of the heterogeneous gas-surface reaction of anthracene and ozone as a function of ozone concentration and relative humidity (RH). The reaction exhibited pseudo-first-order kinetics for anthracene loss under dry conditions (RH < 1%) and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on the gas-phase ozone concentration, which yielded the following fitting parameters: the equilibrium constant for ozone adsorption, K(O3) = (2.8 +/- 0.9) x 10(-15) cm3 and the maximum pseudo-first-order rate coefficient, k(I)max = (6.4 +/- 1.8) x 10(-3) s(-1). The kinetics were unchanged when experiments were performed at approximately 50% and 60% RH. In the product study, a nonlinear dependence, similar to a Langmuir adsorption plot, of the anthraquinone product yield as a function of ozone concentration was observed and resulted in the following fitting parameters: K(O3) = (3.4 +/- 1.5) x 10(-15) cm3 and the maximum anthraquinone yield, ANQmax % = 30 +/- 18%. Experiments performed under higher relative humidity conditions ( approximately 50% and 60% RH) revealed that the anthraquinone yield was unaffected by the presence of gas-phase water. It is noteworthy that both the anthracene loss kinetics and the anthraquinone yields have a similar dependence on the degree of ozone partitioning to the surface. This can be understood in terms of a mechanism whereby the rate-determining steps for anthracene loss and anthraquinone formation are both driven by the amounts of ozone adsorbed on the surface. Our results suggest that at atmospherically relevant ozone concentrations (100 ppb) the anthraquinone yield from the ozonolysis of anthracene under dry and high relative humidity conditions would be less than 1%.     

7,7,8,8-tetracyanoquinodimethane chemiluminescence sensitized by Rhodamine B on surfactant bilayer membrane assemblies for determination of sulphide by a flow-injection method

A new chemiluminescence system is described for selective determination of sulphide by a flow-injection method. The weak light emitted from the reaction between 7,7,8,8-tetracyanoquinodimethane and sulphide is efficiently sensitized by Rhodamine B in alkaline solution containing dioctadecyldimethylammonium chloride bilayer membrane aggregates and acetonitrile. The limit of determination is 0.05 ng (injection of 30 mul of 5 x 10(-8)M sulphide), the linear range is two orders of magnitude, the sampling rate is 240/hr, and the relative standard deviation is 3.0% for 0.3 ng of sulphide. Manganese(II), the strongest enhancing species after sulphide ion, provides a signal 1.1% of that for sulphide ion. Manganese(II), iron(II), and species forming precipitates with sulphide ion interfere, but other sulphur anions such as sulphite, sulphate, thiosulphate, and thiocyanate do not.     

A New Nitric Oxide Gas Sensor Based on Reticulated Vitreous Carbon/Nafion and Its Applications

Reticulated vitreous carbon (RVC), and Nafion membrane are used to fabricate a composition electrode to measure nitric oxide (NO) concentration amperometrically in the gas phase. Limit of detection was found to be 6 ppb at an applied voltage of 0.66 V (vs. mercury sulfate reference electrode) with average response time of less than 30 seconds. The response of the sensor was linearly dependent on the concentration over the whole tested range from 19 ppb‐50 ppm of NO. Simplicity in electrode fabrication and consistent performance between individual sensors make RVC and Nafion attractive materials for detecting very low levels of nitric oxide gas in routine analysis.     

Effect of complexones and tensides on selectivity of nitrogen dioxide determination in air with a chemiluminescence aerosol detector

Modification of the luminol solution by means of addition of various complexones and surfactants has been investigated to eliminate interferences from gaseous co-pollutants in the determination of ambient nitrogen dioxide using a chemiluminescence aerosol detector. The simultaneous presence of EDTA and triton X-100 or X-405 together with sulphite and iodide in the luminol solution suppressed interferences from ozone and peroxyacetyl nitrate to a negligible level and no scrubbers or corrections of the NO₂ measurements were needed.

In general, the best composition of the reagent solution included luminol, KOH, Na₂SO₃, KI, Na₂EDTA and triton X-100. From the point of view of selectivity of NO₂ determination, an optimum reagent solution consisted of luminol (0.002 M), KOH (0.5 M), Na₂SO₃ (0.2 M), KI (0.1 M), Na₂EDTA (0.05 M) and triton X-100 (0.5 vol.%). Interferences from ozone (170 ppb (v/v)) and peroxyacetyl nitrate (81 ppb (v/v)) were 0.2 and 1.2%, respectively, for nitrogen dioxide at a concentration of 50 ppb (v/v) and 0.25 and 1.7%, respectively, for 0.5 ppb (v/v) NO₂. The calibration graph was linear for NO₂ concentrations ranging from 3 to 665 ppb (v/v). Below 3 ppb (v/v) NO₂ the detector response to nitrogen dioxide can be fitted with a linear equation of the third order.     

A gravimetric procedure for the determination of wet precipitated sulphur, dissolved sulphur, soluble sulphides and hydrogen sulphide

Elemental sulphur (in wet precipitated form or dissolved in organic solvents) and hydrogen sulphide have been determined gravimetrically at room temperature by conversion into copper sulphide by elemental copper in presence of an organic solvent such as benzene or acetonitrile. Any solvent in which sulphur is soluble can be used. The black copper sulphide formed can be weighed or determined iodometrically. Analysis indicates the black compound to be Cu(1.8)S. This room temperature method is a versatile one-step procedure sensitive to microgram or macro amounts of sulphur. It has been used for determining the solubility of sulphur in tetrahydrofuran and dioxan. The apparent heat of solution indicates that sulphur dissolves in these solvents without any marked solute-solvent interactions.     

Stability of fluoroquinolone antibiotics in river water samples and in octadecyl silica solid-phase extraction cartridges

Airborne particulate matter has been sampled at a location close to a metallurgical plant in North-Rhine-Westphalia, Germany, and first results on the chromium(VI) content in the collected dust are presented. A special procedure using a scrubber as sampling device was used to preserve Cr(VI) during the sampling procedure. The scrubber solution which consisted of 0.1 mol L^–1 TRIS-buffer solution was adjusted to a slightly alkaline pH of 8.6 to reduce the oxidation potential of Cr(VI) and to avoid possible oxidation of Cr(III) to Cr(VI), for example by oxygen (or ozone at ambient concentrations). After sampling Cr(VI) was pre-concentrated on an anion-exchange material and eluted with aqueous 0.6 mol L^–1 sodium perchlorate solution. After elution, a species-selective complex of Cr(VI) with diphenylcarbazide (DPC) was prepared; this was extracted into n-hexanol and quantified by UV–visible spectrophotometry. A detection limit of 0.9 ng m^–3 for Cr(VI) in ambient aerosols can be achieved with this method.