Time-weighted average passive sampling with a solid-phase microextraction device

A modified Solid-Phase Microextraction (SPME) device has been used as a passive sampler to determine the time-weighted average (TWA) concentration of volatile organic compounds (VOCs) in air. Unlike conventional sampling with SPME, in which the fiber is extended outside its needle housing, during TWA passive sampling, the fiber is retracted a known distance into its needle housing. The SPME passive sampler collects the VOCs by the mechanism of molecular diffusion and sorption on to a coated fiber as collection medium. This process has been shown to be described by Fick's first law of diffusion, whereby determination of the amounts of analytes accumulated over time enable measurement of the TWA concentration to which the sampler was exposed. A series of fibers, 100-microm poly(dimethylsiloxane), 65-microm poly(dimethylsiloxane)/divinylbenzene, and 75-microm Carboxen/poly(dimethylsiloxane), were tested for their "zero sink", face velocity, and response time behavior. Of the fibers tested, that coated with 75-microm Carboxen/poly(dimethylsiloxane) was found to be an excellent passive sampler for VOCs. TWA passive sampling with a SPME device was shown to be almost independent of face velocity and to be more tolerant of high and low analyte concentrations and long and short sampling times, because of the ease with which the diffusion path length could be changed. It was found that environmental conditions, e.g., temperature, pressure, relative humidity, and ozone, have little or no effect on sampling. The 75-microm Carboxen/poly(dimethylsiloxane) fiber can retain VOCs for up to two weeks without significant loss. When the SPME device was tested in the field and the results were compared with those from National Institute of Occupational Health and Safety method 1501, good agreement was obtained.     

Time-weighted average sampling with solid-phase microextraction device: implications for enhanced personal exposure monitoring to airborne pollutants

The solid-phase microextraction (SPME) device is used as a time-weighted average (TWA) sampler for gas-phase analytes by retracting the coated fiber a known distance into its needle housing during the sampling period. Unlike in conventional spot sampling with SPME, the TWA sampling approach does not allow the analytes to reach equilibrium with the fiber coating, but rather they diffuse through the opening in the needle to the location of the sorbent. The amount of analytes accumulated over time gives the measurement of the average concentration to which the device was exposed to. Depending on the sorbent used as the sink, TWA sampling for various analytes is possible with times ranging from 15 min to at least 16 h. Both the poly(dimethylsiloxane) (PDMS) and poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB) fiber coating phases were tested, with the latter employing on-fiber derivatization for reactive carbonyl compounds, e.g., formaldehyde. Described herein are the theoretical and practical considerations for using the SPME device as a TWA sampler.     

Solid-phase microextraction field sampler

To facilitate the use of solid-phase microextraction (SPME) for field sampling, a new field sampler was designed and tested. The sampler was versatile and user-friendly. The SPME fiber can be positioned precisely inside the needle for time-weighted average sampling or exposed completely outside the needle for grab sampling. The needle is protected within a shield at all times, hereby eliminating the risk of operator injury and fiber damage. A replaceable Teflon cap is used to seal the needle to preserve sample integrity. Factors that affect the preservation of sample integrity (sorbent efficiency, temperature, sealing materials) were studied. The use of a highly efficient sorbent for the fiber is recommended as the first choice for the preservation of sample integrity. Teflon was a good material for sealing the fiber needle, had little memory effect, and could be used repeatedly. To address adsorption of high boiling point compounds on fiber needles, several kinds of deactivated needles were evaluated. RSC-2 fiber needles were the more effective. A preliminary field sampling investigation demonstrated the validity of the new SPME device for field applications.     

A diffusive badge sampler for volatile organic compounds in ambient air and determination using a thermal desorption-GC/MS system.

A sensitive personal badge sampler packed with Carbopack B for ambient levels of volatile organic compounds and an analytical system using a thermal desorption-preconcentration-GC/MS have been developed. The capacity of the new sampler was sufficient for an 8-h sampling period, and the analytical method was sensitive enough for the measurement of sub-ppb levels for a 2-h sampling period. The samplers were compared to diffusive samplers (OVM 3500) for typical environmental concentrations. There was a good correlation between the results obtained with the new samplers and the OVM samplers.     

Sampling and analysis of airborne particulate matter and aerosols using in-needle trap and SPME fiber devices

A needle trap device (NTD) and commercial poly(dimethylsiloxane) (PDMS) 7-microm film thickness solid-phase microextraction (SPME) fibers were used for the sampling and analysis of aerosols and airborne particulate matter (PM) from an inhaler-administered drug, spray insect repellant, and tailpipe diesel exhaust. The NTD consisted of a 0.53-mm o.d. stainless steel needle having 5 mm of quartz wool packing section near the needle tip. Samples were collected by drawing air across the NTD with a Luertip syringe or via direct exposure of the SPME fiber. The mass loading of PM was varied by adjusting the volume of air pulled through the NTD or by varying the sampling time for the SPME fiber. The air volumes ranged from 0.1 to 50 mL, and sampling times varied from 10 s to 16 min. Particulates were either trapped on the needle packing or sorbed onto the SPME fiber. The devices were introduced to a chromatograph/mass spectrometer (GC/MS) injector for 5 min desorption. In the case of the NTD, 10 microL of clean air was delivered by a gas-tight syringe to aid the introduction of desorbed analytes. The compounds sorbed onto particles extracted by the SPME fiber or trapped in the needle device were desorbed in the injector and no carry-over was observed. Both devices performed well in extracting airborne polycyclic aromatic hydrocarbons (PAHs) in diesel exhaust, triamcinolone acetonide in a dose of asthma drug and DEET in a dose of insect repellant spray. Results suggest that the NTDs and PDMS 7-microm fibers can be used for airborne particulate sampling and analysis, providing a simple, fast, reusable, and cost-effective screening tool. The advantage of the SPME fiber is the open-bed geometry allowing spectroscopic investigations of particulates; for example, with Raman microspectroscopy.     

Evaluation of solid-phase microextraction for time-weighted average sampling of volatile sulfur compounds at ppb concentrations

The potential of solid-phase microextraction (SPME) for time-weighted average (TWA) sampling of volatile sulfur compounds in air at ppb concentrations was investigated. The target compounds (hydrogen sulfide, methanethiol (MeSH), ethanethiol (EtSH), dimethyl sulfide (Me2S), and dimethyl disulfide (Me2S2)) were extracted using SPME with a Carboxen-poly(dimethylsiloxane) fiber coating, and diffusion was controlled by keeping the fiber retracted within the needle of the sampling device. The effects of several important experimental variables (air velocity, direction of air flow, analyte concentration, humidity, temperature, extraction time) were studied. The uptake by the fiber was not affected by the direction of the air flow or the air velocity. The effects of concentration, humidity, temperature, and extraction time were examined in experiments with a central composite face design. The results showed that all or most of the investigated parameters had a significant impact on the uptake rates of H2S, MeSH, EtSH, and Me2S, which invalidated time-weighted average sampling of these compounds by SPME under the tested conditions. Moreover, reverse diffusion of H2S, MeSH, and EtSH occurred at 40% relative humidity. For Me2S2, the uptake rate had a variation of only 8% within the whole experimental domain, and the experimental value derived for the uptake rate was consistent with the theoretical value. This result was confirmed by comparative analyses of industrial samples by the standard addition method. Therefore, SPME appears to be a suitable technique for TWA sampling of Me2S2 using the Carboxen-poly(dimethylsiloxane) fiber coating. Finally, in an investigation of potential losses during storage of the fiber, no significant losses of the target compounds were detected after 3 days at -80 degrees C.     

Air sampling with porous solid-phase microextraction fibers

A new, rapid air sampling/sample preparation methodology was investigated using adsorptive solid-phase microextraction (SPME) fiber coatings and nonequilibrium conditions for volatile organic compounds (VOCs). This method is the fastest extraction technique for air sampling at typical airborne VOC concentrations. A theoretical model for the extraction was formulated based on the diffusion through the interface between the sampled (bulk) air and the SPME coating. Parameters that affect the extraction process including sampling time, air velocity, air temperature, and relative humidity were investigated with the porous (solid) PDMS/DVB and Carboxen/PDMS coatings. Very short sampling times from 5 s to 1 min were used to minimize the effects of competitive adsorption and to calibrate the extraction process in the initial linear extraction region. The predicted amounts of extracted mass compared well with the measured amounts of target VOCs. Findings presented in this study extend the existing fundamental knowledge related to sampling/sample preparation with SPME, thereby enabling the development of new sampling devices for the rapid sampling of air, headspace, water, and soil.     

Theoretical and experimental studies on isothermal adsorption and desorption characteristics of a desiccant-coated heat exchanger

Dynamic characteristics of desiccant–coated heat exchangers (DCHEs) were experimentally measured by wind tunnel. The surface of the DCHE was coated with polymer sorbent desiccant. The isothermal adsorption and desorption experiments were conducted under the condition where the temperatures of the air and brine passing through the DCHE were identical.The experimental results were compared to that obtained from theoretical calculations. A diffusion model predicting the distribution of moisture concentration and temperature in the desiccant layer was introduced. The equivalent diffusion coefficient of the water inside the desiccant layer was determined from the experimental results.The adsorption and desorption speeds were at the maximum values at the beginning of the sorption processes, and then they gradually decreased. The equivalent diffusion coefficient was dependent on the temperature. Assuming the temperature dependence of the diffusion coefficient, the calculated sorption performance correlated well with that obtained from the experimental results.     

A sol-gel based solid phase microextraction fiber for analysis of aromatic hydrocarbons

A sol-gel based solid phase microextraction fiber for headspace sampling (HP-SPME) and GC determination of benzene, toluene, ethylbenzene and xylenes (BTEX) is introduced. The influences of fiber composition, microextraction conditions such as temperature and time on the fiber performance and desorption temperature and time were investigated. Under optimal conditions, the use of proposed fiber was thermally stable up to 250 degrees C and demonstrated high sensitive and fast sampling of BTEX from gaseous phase. Depending on the analysed substance, the linear range for a selected fiber and the applied GC-FID technique was from 4 to 80 ng mL(-1)with limit of detection (LOD) 0.2-0.7 ng mL(-1) and 100-1000 ng mL(-1) with LOD 8-20 ng mL(-1) for gaseous and soil samples, respectively. HP-SPME-GC analysis was highly reproducible-relative standard deviations (R.S.D.) were between 5.0 and 7.9%. The proposed fiber was successfully used for BTEX sampling from indoor air and headspace of soil samples.     

A dual-adsorbent preconcentrator for a portable indoor-VOC microsensor system

The development and testing of a miniature dual-adsorbent preconcentrator for a microsensor-based analytical system designed to determine complex volatile organic chemical (VOC) mixtures encountered in indoor working environments at low part-per-billion levels is described. Candidate adsorbents were screened for thermal-desorption bandwidth and breakthrough volume against 20 volatile organic vapors and subsets thereof as a function of several relevant variables. A glass capillary (1.1 mm i.d.) packed with 3.4 mg of Carbopack X and 1.2 mg of Carboxen 1000 provides sufficient capacity for a 1-L dry-air sample containing all 20 vapors at concentrations of 100 ppb as well as providing a composite half-height peak width of <3 s at a desorption temperature of 300 degrees C and a flow rate of 4 mL/min. Required adsorbent masses increase to 7.0 and 1.5 mg, respectively, for the same mixture at component concentrations of 1 ppm. Vapor breakthrough volumes for the Carbopack X are unaffected by humidity from 0 to 100%RH, but those for the Carboxen 1000 are significantly reduced, requiring an additional 0.9 mg of the latter to avoid premature breakthrough at the 100 ppb level. Good peak shapes, efficient chromatographic separation of preconcentrated sample mixture components, and detection limits in the low-parts-per-billion range using an integrated surface-acoustic-wave (SAW) sensor are achieved. Preconcentrator design and operating parameters are considered in terms of the vapor bed-residence times and breakthrough volumes in the context of the modified Wheeler equation.     

The effect of surfactants on the distribution of organic compounds in the soil solid/water system

The efficiency of soil remediation by surfactant washing was evaluated via the measured distribution coefficients of a number of nonpolar compounds in several soil-water mixtures. The studied compounds (contaminants) are BTEX (benzene, toluene, ethylbenzene, and p-xylene) and three chlorinated pesticides (lindane, alpha-BHC, and heptachlor epoxide), which span several orders of magnitude in water solubility (S(w)). A peat, and two natural soils were used that comprise a wide range in soil organic matter (SOM) content. The surfactants tested included cationic, anionic and nonionic types, with concentrations up to five to six times the critical micelle concentration (CMC). The K(d)(*)/K(d), values were used to evaluate the remediation efficiency under various operation conditions. For relatively water soluble BTEX compounds, the surfactant adsorption on the soil surface is the deciding factor on contaminant desorption from soil. For the less-soluble pesticides, surfactant micelles in solution influence the contaminant desorption more. The contaminants partitioning to SOM or adsorbed surfactants lowers the desorption efficiency. Anionic surfactants are found to be a better choice on soil remediation because they do not form admicelle on soil surface that enhances the SOM content. Cationic surfactant, which adsorb onto soil surfaces, leads to poor remediation efficiency. An improper selection of surfactant would result in inefficiency in soil remediation by surfactant washing.     

Gas chromatography on self-assembled, single-walled carbon nanotubes

High-performance stationary phases, which provide high resolutions and are stable at high temperatures, are of significant importance in gas chromatographic analysis. Carbon nanotubes are nanosized carbon-based sorbents that have a high surface area and a large aspect ratio and are known to be stable at high temperatures. It is, therefore, conceivable that gas chromatography can benefit from their unique properties. This paper reports gas chromatography separations in an open tubular format on self-assembled, single-walled carbon nanotubes (SWNTs) inside silica-lined steel capillary tubing. A SWNT film with an average thickness of 300 nm was self-assembled by a unique, single-step, catalytic chemical vapor deposition (CVD) process consisting of dissolved cobalt and molybdenum salts in ethanol. A variety of organic compounds with varying polarity were separated at high resolution, and the column efficiency demonstrated approximately 1000 theoretical plates/m. Comparison of capacity factors' and isosteric heats of adsorption with a packed column containing a commercial sorbent (Carbopack C) showed comparable results. This demonstrated high capacity and strong sorbate-sorbent interactions on the SWNT phase. Evaluation of the McReynolds constants suggested that the SWNT was a nonpolar phase.     

太阳能固体吸附式制冷吸附床的设计及数值模拟分析

设计了管内填充吸附剂,管外走传热介质的新型吸附床结构,建立了相应的数学模型.以太阳能为驱动热源,分析了吸附床内温度在某时刻的静态分布以及温度、压力和脱附速率随时间的动态变化.通过对吸附床传热效果的分析指出该吸附床结构有较好的传热效果.     

Calibration for on-site analysis of hydrocarbons in aqueous and gaseous samples using solid-phase microextraction

Rapid sampling and sample preparation methodology was investigated using adsorptive poly(dimethylsiloxane)/divinylbenzene and Carboxen/poly(dimethylsiloxane) solid-phase microextraction (SPME) fiber coatings and volatile aromatic hydrocarbons (BTEX: benzene, toluene, ethylbenzene, and o-xylene). A flow-through system was used to generate a standard aqueous solution of BTEX as model sample with known linear velocity. Parameters that affect the extraction process, including sampling time, concentration, water velocity, and temperature, were investigated. Very short sampling times from 10 s and sorbents with strong affinity and large capacity were used to ensure the effect of '"zero sink" and to calibrate the extraction process in the initial linear extraction region. Several different concentrations were investigated, and it was found that mass uptake changes with concentration linearly. The increase of water velocity increases mass uptake, though the increase is not linear. Temperature does not affect mass uptake significantly under typical field sampling conditions. To further accurately describe rapid SPME analysis of aqueous samples, a new model translated from heat transfer to a circular cylinder in cross-flow was used. An empirical correlation to this model was used to predict the mass-transfer coefficient. Findings indicate that predicted mass uptake compares well with experimental mass uptake. The new model was tested for rapid air sampling, and it was found that this new model also predicted rapid air sampling accurately. Findings presented in this study extend the existing fundamental knowledge related to rapid sampling/sample preparation with SPME.     

Theory and validation of solid-phase microextraction and needle trap devices for aerosol sample

Previous aerosol studies utilizing solid-phase microextraction (SPME) predominantly focused on volatile and semivolatile compounds in the gaseous phase. Difficulties were associated with quantitative analysis of these compounds when they were associated with atmospheric particles. The present study combines SPME technology with that of carboxen packed needles (needle trap, NT) for analysis of gaseous and particle-bound compounds in atmospheric samples. The NT device is constructed as a micro trap by placing some small sorbents in a needle. Aerosol samples are collected by drawing air through the NT device with a pump. The trapped components contain both gaseous chemical compounds as well as particulate matter present in the sample. The total concentration of analytes in an aerosol sample can be obtained on the basis of the exhaustive sampling mode of the NT device. Direct SPME is simultaneously used to determine gaseous compound in the aerosol sample. As a result, the SPME and NT devices, when used together, can provide a complete solution to highly efficient and accurate aerosol studies. The theoretical considerations of SPME and NT devices for aerosol sampling are validated by sampling seasalt aerosol, barbecue, and cigarette smoke. The concentrations of PAHs in the different phases of the samples are few ng/L. Result analysis shows that SPME and the NT device demonstrate several important advantages such as simplicity, convenience, and low costs under laboratory and on-site field sampling conditions.     

Adsorption of BTEX from aqueous solution by macroreticular resins

Theoretical and experimental investigations were conducted on the adsorption of benzene, toluene, ethylbenzene and xylene (BTEX) by macroreticular resins. A mass transfer model based on the squared-driving force principle is presented for describing the BTEX transfer between the aqueous and solid phases. Also proposed is a theoretical model for describing the BTEX breakthrough curves of the adsorption column. While the mass transfer model involves only an overall mass transfer coefficient, the column adsorption model has two model parameters. Those parameters are conveniently estimated using the observed mass transfer and breakthrough data. The predictions using the proposed models were found to compare well with the experimental data of batch and column BTEX adsorption tests.     

Purge-and-membrane mass spectrometry, a screening method for analysis of VOCs from soil samples

Purge-and-membrane mass spectrometry (PAM-MS) is a combination of dynamic headspace sampling and membrane extraction. A new and simple purge-and-membrane sampler is introduced and its basic testing results for the analysis of VOCs in soil samples are reported. Soil moisture had no effect on desorption times in the case of sand, but the desorption times increased when the content of organic matter in the soil sample (garden soil) increased. The longest desorption times were measured with dry garden soil samples. For both types of samples, minor differences in desorption peak areas were observed between 10 and 20% moisture. Detection limits of the VOCs varied in the range 2-150 microg/kg, depending on the soil type. Good linearity (correlation coefficient > 0.990) was observed in the range 0.5-50 mg/kg. Aging of the spiked soil samples had only a slight effect on desorption peak areas for samples stored at 5 degrees C up to two weeks, but after six months of storing, differences were observed between dry sand and moistened garden soil. In both cases, peak areas were diminished. On average, 46% of compounds could be desorbed from the aged sand and 86% from the aged garden soil. The modified vapor fortification method was used in preparing standard soil samples, which were analyzed by static headspace gas chromatography (HSGC) and PAM-MS. Some authentic soil samples were also analyzed using both of these techniques. Many of the vapor fortification samples and the authentic samples were also analyzed in another laboratory by HSGC. The agreement between the methods and the laboratories was generally good.     

An ion-imprinted functionalized silica gel sorbent prepared by a surface imprinting technique combined with a sol-gel process for selective solid-phase extraction of cadmium(II)

A new ion-imprinted thiol-functionalized silica gel sorbent was synthesized by a surface imprinting technique in combination with a sol-gel process for selective on-line, solid-phase extraction of Cd(II). The Cd(II)-imprinted thiol-functionalized silica sorbent was characterized by FT-IR, the static adsorption-desorption experiment, and the dynamic adsorption-desorption method. The maximum static adsorption capacity of the ion-imprinted functionalized sorbent was 284 micromol g(-1). The largest selectivity coefficient for Cd(II) in the presence of Pb(II) was over 220. The static uptake capacity and selectivity coefficient of the ion-imprinted functionalized sorbent are higher than those of the nonimprinted sorbent. The breakthrough capacity and dynamic capacity of the imprinted functionalized silica gel sorbent for 4 mg L(-1) of Cd(II) at 5.2 mL min(-1) of sample flow rate were 11.7 and 64.3 micromol g(-1), respectively. No remarkable effect of sample flow rate on the dynamic capacity was observed as the sample flow rate increased from 1.7 to 6.8 mL min(-1). The imprinted functionalized silica gel sorbent offered a fast kinetics for the adsorption and desorption of Cd(II). The prepared ion-imprinted functionalized sorbent was shown to be promising for on-line, solid-phase extraction coupled with flame atomic absorption spectrometry for the determination of trace cadmium in environmental and biological samples. All competitive ions studied did not interfere with the determination of Cd(II). With a sample loading flow rate of 8.8 mL min(-1) for 45-s preconcentration, an enhancement factor of 56, and a detection limit (3sigma) of 0.07 microg L(-1) were achieved at a sampling frequency of 55 h(-1). The precision (RSD) for 11 replicate on-line sorbent extractions of 8 mug L(-1) Cd(II) was 0.9%. The sorbent also offered good linearity (r = 0.9997) for on-line, solid-phase extraction of trace Cd(II).     

An Interfacial Solar‐Driven Atmospheric Water Generator Based on a Liquid Sorbent with Simultaneous Adsorption–Desorption

Water scarcity is one of the greatest challenges facing human society. Because of the abundant amount of water present in the atmosphere, there are significant efforts to harvest water from air. Particularly, solar‐driven atmospheric water generators based on sequential adsorption–desorption processes are attracting much attention. However, incomplete daytime desorption is the limiting factor for final water production, as the rate of water desorption typically decreases very quickly with decreased water content in the sorbents. Hereby combining tailored interfacial solar absorbers with an ionic‐liquid‐based sorbent, an atmospheric water generator with a simultaneous adsorption–desorption process is generated. With enhanced desorption capability and stabilized water content in the sorbent, this interfacial solar‐driven atmospheric water generator enables a high rate of water production (≈0.5 L m^?2 h^?1) and 2.8 L m^?2 d^?1 for the outdoor environment. It is expected that this interfacial solar‐driven atmospheric water generator, based on the liquid sorbent with a simultaneous adsorption–desorption process opens up a promising pathway to effectively harvest water from air.     

Adsorption properties of shungite in purification of water–alcohol solutions

Shungite adsorption properties to remove higher alcohols and other impurities from water–alcohol solutions is studied by the methods of nitrogen adsorption, thermogravimetric analysis, wetting, temperature-programmed desorption mass spectrometry, and FTIR spectroscopy. Shungite is shown to be able to effectively sorb impurities from water-alcohol solutions to form fairly strong adsorption systems. Heat treatment 180°C in vacuum results in almost complete decomposition of the adsorption systems, thus providing recovery and a slight increase of the specific surface and sorptive volume of shungite. The present findings demonstrate the theoretical possibility of recovery of adsorption centers on the shungite surface, which would extend its usable life as a sorbent.