Development of the infrared hollow waveguide sampler for the detection of chlorophenols in aqueous solutions

A method based on the infrared hollow waveguide sampler was developed for sensing chlorophenols in aqueous solutions. This sampler was constructed by coating a suitable hydrophobic film onto the inner surface of an infrared hollow waveguide. By passing the aqueous solution through the hollow waveguide sampler, analytes can be absorbed into the hydrophobic layer. The adsorbed analytes can be sensed later by using Fourier transform infrared spectrometry. Six hydrophobic polymers were investigated for their performance in conjunction with the infrared hollow waveguide sampler for the detection of chlorophenols. Results indicated that poly(acrylonitrile-co-butadiene) was a most suitable hydrophobic material for absorption of chlorophenols in aqueous solutions. To further increase the detection sensitivity, factors such as sampling flow rate, sampling time, and thickness of the hydrophobic film were also investigated. Results indicated that the infrared signals were similar in the examined flow rates (2-30 mL/min), but that a higher flow rate tended to produce a higher analytical signal. Fast detection speed was an advantage of this method for the detection of chlorophenols, and the sampling/detection time can be <10 min. In addition, analytical signals were nearly proportional to the thickness of the hydrophobic film coating the inside of the hollow waveguide. With the optimal conditions found in this work, detection limits based on 3 times the peak-to-peak noise level were around 300 ppb for the chlorophenols examined. A high degree of linearity in the standard curves was also observed for this method in the concentration range of 10-100 ppm. The typical regression coefficients were >0.994 for the chlorophenols examined.     

Infrared reflection-absorption method for the detection of aromatic compounds in aqueous solutions with limited sample volumes.

An infrared reflection-absorption (IR/RA) method was developed to detect aromatic organic compounds in aqueous solutions where the required sample volume can be as low as 50 microL. Two aluminum plates were used to form the sampling cell for the detection of small amount of aqueous samples. One plate was used as an IR reflection substrate and a second plate, in which several holes were drilled, was placed tightly on the top of the reflection plate to form cavities for sampling. The cavities were further coated with hydrophobic film. After the hydrophobic film dried, a certain amount of aqueous sample was injected to the cavity. Analytes in the aqueous solution were attracted into the hydrophobic film through the solid phase micro-extraction principle. After residual water was removed from the cavity, organic compounds absorbed by the hydrophobic film could be sensed using IR radiation based on the reflection-absorption mode. To investigate the applicability of this type of sensing method for small-volume detection, factors such as the volume of the aqueous solution, the sample concentration, size of the cavity and the sensitivity of this method were investigated. An examination of the linear relationship between the signals and the analyte concentrations showed regression coefficients that were generally in the range of 0.992 to 0.999 for the examined analytes in the concentration range of 10 to 100 ppm. Under the condition that the sample volume was 100 microL and based on three-times the spectra noise level, the calculated detection limits for this method were found at around 1 ppm for the examined analytes.     

Cooled internal reflection element for infrared chemical sensing of volatile to semi-volatile organic compounds in the headspace of aqueous solutions

In this study, the cooling effect was applied to an evanescent wave type infrared (IR) chemical sensing method to effectively trap volatile organic compounds (VOCs), which have been absorbed in the hydrophobic film coated around the internal reflection element (IRE). The detection of VOCs in aqueous solutions was taken in the headspace of the aqueous solution. This method eliminates the long-term instability of hydrophobic film soaked in an aqueous solution and the potential spectral interference caused by the matrix of the aqueous solution. Thermal energy has been applied to the aqueous solution to assist in the evaporation of VOCs out of the aqueous matrix. By applying a cooling system to the IRE, the excess thermal energy can be removed leading to more stable IR signals. After examination of organic compounds with vapour pressure (P_v) ranging from 0.017 to 150 Torr, significant differences were found between IR signals from cooled and un-cooled systems. Because the thermal conductivity of the IRE used in IR detection is typically low; the efficiency in removing the thermal energy is limited. By heating the aqueous solutions to different temperatures, the IR signals showed that the sample temperature was limited to around 80 °C. The IR signal determination results for five different volatility organic compounds indicated that the optimal heating temperature was not necessary to match with the volatilities of organic compounds in cooling system. The linear regression coefficient (R²) of the standard curve for sample concentrations in the range 5-200 μg ml⁻¹ was generally higher than 0.991 and the detection limit was around a few hundred ng ml⁻¹, which was two to three times lower than that of un-cooled system.     

An infrared evanescent wave sensing system coupled with a hollow fiber membrane for detection of volatile organic compounds in aqueous solutions.

We have developed an on-line sensing method for the detection of volatile organic compounds (VOCs) in contaminated aqueous solutions by combining a microporous hollow fiber membrane with an infrared (IR) sensing system. Polypropylene microporous hollow fibers were used to separate the VOCs from the aqueous solution into the hollow fibers, which were purged countercurrently for detection by the IR sensing systems. An evanescent-wave-type IR sensing system was used to detect the VOCs that were purged from the hollow fibers. The sensing element was coated with polyisobutylene (PIB) to concentrate the VOCs for their detection. To study the performance of this system, we examined a number of factors, such as the purging flow rate, the sample flow rate, and the volatilities of the VOCs. The results indicate that an increase in the purging flow rate reduces the analytical signal significantly, especially for purging flow rates >2 mL/min. The pumping flow rate for the aqueous sample also influenced the analytical signals, but far less sensitively. The volatilities of the examined compounds also affected the analytical signals: the higher the volatility of the compound, the lower the intensity of the analytical signals and the shorter the time required to reach the equilibrium signal. From an examination of the dynamic range of this proposed method, a regression coefficient >0.994 was obtained for concentrations below 250 mg/L, even under non-equilibrium conditions. The response time of the system was studied in an effort to examine the suitability of using this sensing method for automatic detection. The results indicate that new equilibrium conditions were established within 3 min for highly volatile compounds, which suggests that on-line monitoring of the levels of VOCs can be performed in the field.     

Membrane-introduced infrared spectroscopic chemical sensing method for the detection of volatile organic compounds in aqueous solutions

A novel membrane-introduced infrared (IR) chemical sensing method has been developed for the detection of volatile organic compounds (VOCs) in aqueous solutions. In this method, a porous Teflon membrane was used to eliminate the problems associated with conventional IR spectroscopic sensing methods. The porous Teflon membrane was sealed below an IR spectroscopic sensing element pre-coated with a hydrophobic film and a two-channel flow cell configuration was established. In this configuration, the aqueous sample was allowed to pass through the lower channel and the VOCs that penetrated through the membrane to the upper channel were detected by the IR sensor. In this manner, the performance of the sampling at the headspace was improved while the problems caused by the presence of water were eliminated. Meanwhile, using a purging channel allowed the sensing element to be regenerated rapidly and enabled automation of the detection process. The parameters that influenced the analytical signals were studied, such as the sampling flow rate, the pH and ionic strength of the sample solutions, the effect of the volatilities of the VOCs, and the regeneration efficiency of the sensing element. The results indicated that the analytical signals were insensitive to the sampling flow rate and to the pH and ionic strength of the sample solutions. The results obtained from the detection of seven different volatile compounds indicated that this method is highly suitable for the detection of organic compounds that have vapor pressures >1 Torr and that it is potentially usable for organic compounds that have vapor pressures between 20 mTorr and 1 Torr. The regression analysis of the standard curves indicated that a regression coefficient (R(2)) > 0.99 was obtainable in the concentration range from 1 to 100 microg mL(-1). The detection limits for the tested compounds were around a few hundred ng mL(-1).     

Analysis of organochlorine pesticides in wine by solvent bar microextraction coupled with gas chromatography with tandem mass spectrometry detection

A solvent bar microextraction (SBME) technique combined with gas chromatography/tandem mass spectrometry (GC/MS/MS), for the determination of selected organochlorine pesticides (OCPs) in wine samples, is described. In this work the OCPs were extracted and dissolved in a 2-microL aliquot of organic extraction solvent (n-tetradecane) confined within a 1.7-cm length of hollow fiber. Both ends of the hollow fiber (solvent bar) were sealed, and it was placed in an aqueous sample solution for extraction. The effects of solvent selection, sample agitation, extraction time, extraction temperature, and salt concentration on the SBME performance were optimized. The influence of aqueous sample/organic solvent phase ratio was further investigated in detail. High enrichments (1900-7100-fold) could be obtained at an aqueous sample/organic solvent volume ratio of 20 mL/2 microL in this study. Good extraction reproducibility was obtained with relative standard deviation (RSD) values below 12.6%. Comparisons of sensitivity and precision between SBME and dynamic hollow-fiber liquid-phase microextraction were also investigated.     

Development of a solid-phase microextraction/reflection-absorption infrared spectroscopic method for the detection of chlorinated aromatic amines in aqueous solutions.

In this paper, the reflection-absorption infrared (IR) spectroscopic method combined with the principle of solid-phase micro-extraction (SPME) is proposed to detect chlorinated aromatic amines in aqueous solutions. This proposed method provides simplicity in both the optical system and equipment setup. Compared to the SPME/attenuated total reflection-IR method, this method reduces the cost for internal-reflection elements and optical systems. Meanwhile, it has no SPME/transmission IR method problems, which require high polymer film preparation techniques to obtain a standing film that has no physical/chemical property changes when immersed in an aqueous solution. The typical linear coefficients obtained using this method for chloroanilines in aqueous solutions are around 0.995 and the detection can be lower than 100 ppb. The thickness of the hydrophobic film is relatively important in the SPME/ATR-IR method, but the uncertainty caused by the film thickness can be partially eliminated in the proposed method. This is because the IR signals are proportional to the film thickness and can be corrected using hydrophobic film signals. The low detection limits have also indicated that this proposed method can compete with the currently existing IR methods, but allowing much simpler detection.     

Combination of a Mid-infrared Hollow Waveguide Gas Sensor with a Supported Capillary Membrane Sampler for the Detection of Organic Compounds in Water

A Fourier transform infrared (FT-IR) spectroscopy based gas sensor for continuous analysis of liquid phase samples has been developed, coupling a short hollow waveguide (HWG) gas cell with a supported capillary membrane sampler (SCMS) probe. Passing an inert carrier gas through the thin-walled tubular silicon membrane enables the permeation of volatile organic compounds (VOCs) present in aqueous solution and facilitates their continuous and quantitative detection in the infrared hollow fiber by multiple internal reflection spectroscopy. The sensitivity of the sensor system has been determined at the ppb (μg/L) concentration level and the response time ranges from few minutes to 30 min, depending on the analyte and the permeation properties of the sampling membrane.

The experimental set-up consists of Bruker Vector 22 FT-IR spectrometer with an externally aligned 50 cm long silica HWG coupled to the SCMS, which is immersed into a glass flask filled with analyte solution and kept under constant stirring.

Aqueous solutions of benzene, toluene, xylene isomers and chloroform were qualitatively and quantitatively analyzed confirming the feasibility of this sensor approach for environmental analysis.     

Determination of estrogens in wastewater using three-phase hollow fiber-mediated liquid-phase microextraction followed by HPLC

Three-phase hollow fiber-mediated liquid-phase microextraction followed by HPLC was used for the determination of three synthetic estrogens, namely diethylstilbestrol, dienestrol, and hexestrol, in wastewater. Extraction conditions including organic solvent, volume ratio between donor solution and acceptor phase, extraction time, stirring rate, donor phase and acceptor phase were optimized. The target compounds were extracted from a 10 mL aqueous sample at pH 1.5 (donor solution) through a 45 mm in length hollow polypropylene fiber that was immersed in 1-octanol in advance, and then the hollow fiber was filled with 10 microL 0.5 mol/L sodium hydroxide solution (acceptor phase). After a 40 min extraction, the acceptor phase was directly injected into an HPLC system for detection. Under the optimized extraction conditions, a large enrichment factor (more than 300-fold) was achieved for the three estrogens. The determination limit at an S/N of 3 ranged from 0.25 to 0.5 microg/L for the estrogens. The recovery ratio was more than 86% in the determination of these estrogens in wastewater.     

Development of the Headspace SPME/ATR‐IR Method for Detection of Chlorinated Aromatic Compounds in Soils

In this paper, a new method based on attenuated total reflection infrared (ATR‐IR) spectroscopy was developed to detect chlorinated aromatic compounds in soil. To eliminate the problems associated in inspection of soil samples by the ATR‐IR method, chlorinated compounds were evaporated from soil matrices and detected in the headspace. The sensing device was constructed by an internal reflection element (IRE) coated with a hydrophobic film to attract and concentrate chlorinated compounds evaporated to the headspace. Factors that influence the analytical signals were studied such as the moisture content, volatilities of analytes, and effect of heating temperature. Results indicated that the addition of thermal energy to the soil sample resulted in an increase of IR signal. However, the IRE was also warmed up and caused a slight decrease of the IR signals after a long detection time. The studies of the influence of moisture indicated that a small amount of water present in soils could tremendously increase the intensity of detected IR signals. The further increase of moisture contents resulted in a decrease of the analytical signals, and the optimal signal was found when soil samples contained 5% (v/w) water. Results in analyses of compounds with different volatilities indicated that even with vapor pressure lower than 0.017 Torrs, quality IR spectra could still be obtained. Using the optimal conditions found in this work, the results in determination of five compounds in soil samples indicated that the linear regression coefficients (R‐square) were higher than 0.992 with detection limits around a few hundreds of ppb.     

Development and application of microporous hollow fiber protected liquid-phase microextraction via gaseous diffusion to the determination of phenols in water

A new organic solvent-free microextraction technique termed liquid-gas-liquid microextraction (LGLME) was developed. In this technique, a small amount (6 microl) of aqueous acceptor solution (0.5M NaOH) is introduced into the channel of a 2.65 cm polypropylene hollow fiber. The hollow fiber is then immersed in an aqueous sample donor solution. The aqueous acceptor phase in the channel of the hollow fiber is separated from the sample solution by the hydrophobic microporous hollow fiber wall with air inside its pores. The analytes (phenols) passed through the microporous hollow fiber membrane by gas diffusion and were then trapped by the basic acceptor solution. After extraction, the acceptor solution was withdrawn into a microsyringe and injected into a capillary electrophoresis sample vial for subsequent analysis. Limits of detection of between 0.5 and 10 microg/l for eight phenols could be achieved. The relative standard deviations (n=6) of this technique between 2.7 and 7.6%. The technique also provides good enrichment factors for all the eight analytes.     

Three-phase system in solvent bar microextraction: An approach for the sample preparation of ionizable organic compounds prior to liquid chromatography

In this article, a simple new solvent microextraction technique is described for the extraction of ionizable organic compounds. This involves performing simultaneous forward- and back-extraction across an organic film immobilized in the pores of a porous polypropylene hollow fiber. Four chlorophenoxyacetic acid herbicides were chosen as model compounds. The target compounds are extracted from the stirred acidic aqueous sample (adjusted to 0.5 M HCl; donor phase) through a thin film of an organic solvent residing in the pores of a polypropylene hollow fiber; they are then finally extracted into another alkaline aqueous phase (1 M NaOH; acceptor phase). Both ends of the fiber are pressure-sealed. The acceptor phase was analyzed by liquid chromatography (LC). This method gave good enrichment (by a factor of 438-553) of the analytes in 40 min extraction time with reasonably good reproducibility. The analytical potential of the method was demonstrated by applying the method to spiked river water sample.     

Formation of a liquid organic ion associate in aqueous solution and its application to the GF-AAS determination of trace cadmium in environmental water as a complex with 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol

The formation of a liquid organic ion associate in an aqueous sample was applied to the concentration and determination of cadmium in environmental water samples. Cadmium was converted into a complex with 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (5-Br-PAPS) in a 40-mL sample solution, and was extracted into a liquid ion associate of phenolsulfonate and benzethonium during phase formation. More than 400-fold enrichment was easily attained by this technique, because the volume of the liquid organic phase formed was very small, ca. 2 microL. After dilution of the organic phase with a small volume of 2-methoxyethanol, the cadmium in the solution was determined by GF-AAS. The detection limit was 0.09 ng/L (3sigma(b)). This method was applied to the determination of cadmium in river water and seawater.     

Extraction of hydroxyaromatic compounds in river water by liquid-liquid-liquid microextraction with automated movement of the acceptor and the donor phase

Liquid-liquid-liquid microextraction with automated movement of the acceptor and the donor phase technique is described for the extraction of six hydroxyaromatic compounds in river water using a disposable and ready to use hollow fiber. Separation and quantitative analyses were performed using LC with UV detection at 254 nm. Analytes were extracted from the acidified sample solution (donor phase) into the organic solvent impregnated in the pores of the hollow fiber and then back extracted into the alkaline solution (acceptor phase) inside the lumen of the hollow fiber. The fiber was held by a conventional 10 microL LC syringe. The acceptor phase was sandwitched between the plunger and a small volume of the organic solvent (microcap). The acceptor solution was repeatedly moved in and out of the hollow fiber using a syringe pump. This movement provides a fresh acceptor phase to come in contact with the organic phase and thus enhancing extraction kinetics thereby leading to the improvement in enrichment of the analytes. The microcap separates the acceptor phase and the donor phase in addition to being partially responsible for mass transfer of the analytes from the donor solution to the acceptor solution. Under stirring, a fresh donor phase will enter through the open end of the fiber that will also contribute to the mass transfer. Various parameters affecting the extraction efficiency viz type of organic solvent, extraction time, stirring speed, effect of sodium chloride, and concentration of donor and acceptor phases were studied. RSD (3.9-5.6%), correlation coefficient (0.995-0.997), detection limit (2.0-51.2 ng/mL), enrichment factor (339-630), relative recovery (93.2-97.9%), and absolute recovery (33.9-63.0%) have also been investigated. The developed method was applied for the analysis of river water.     

“Reagentless” Flow Injection Determination of Ammonia and Urea Using Membrane Separation and Solid Phase Basification

Flow injection analysis instrumentation and methodology for the determination of ammonia and ammonium ions in an aqueous solution are described. Using in-line solid phase basification beds containing crystalline media. the speciation of ammoniacal nitrogen is shifted toward the un-ionized form. which diffuses in the gas phase across a hydrophobic microporous hollow fiber membrane into a pure-water-containing analytical stream. The two streams flow in a countercurrent configuration on opposite sides of the membrane. The neutral pH of the analytical stream promotes the formation of ammonium cations, which are detected using specific conductance. The methodology provides a lower limit of detection of 10 microgram/L and a dynamic concentration range spanning three orders of magnitude using a 315-microliters sample injection volume. Using immobilized urease to enzymatically promote the hydrolysis of urea to produce ammonia and carbon dioxide, the technique has been extended to the determination of urea.     

A gravity driven micro flow injection wetting film extraction system on a polycarbonate chip

A micro flow injection wetting film liquid-liquid extraction system has been developed for trace analyte concentration and on-chip detection. A hydrophobic channel fabricated on a polycarbonate chip was used to support the wetting film, and hydrostatic pressure generated by the difference in liquid levels was employed to drive the fluids. Sequential injection of segments of aqueous sample solution and organic solvent was conducted by switching the sample- or solvent-containing vials to an on-chip sampling probe, and detection was performed by a co-focused, laser induced fluorescence detector. Using butyl rhodamine B as a model analyte and butanol as the solvent for both film-coating and elution, various experimental conditions such as hydrostatic pressure, coating time, channel length, sampling volume, and sample acidity were investigated. Under optimized conditions, a 24-fold enrichment factor was obtained with the consumption of about 3 μL sample solution, and a detection limit (3σ) of 6.0 × 10⁻⁹ M butyl rhodamine B was achieved at the sampling rate of 19 h⁻¹. Eleven consecutive runs of a 1.0 × 10⁻⁵ M butyl rhodamine B solution produced a relative standard deviation of 1.5% for the detected fluorescence signals.     

Simultaneous quantification of polar and non-polar volatile organic compounds in water samples by direct aqueous injection-gas chromatography/mass spectrometry

A direct aqueous injection-gas chromatography/mass spectrometry (DAI-GC/MS) method for trace analysis of 24 volatile organic compounds (VOCs) in water samples is presented. The method allows for the simultaneous quantification of benzene, toluene, ethyl benzene, and xylenes (BTEX), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), as well as a variety of chlorinated methanes, ethanes, propane, enthenes and benzenes. Applying a liquid film polyethylene glycol or a porous layer open tubular (PLOT) divinylbenzene GC capillary column to separate the water from the VOCs, volumes of 1-10 microL aqueous sample are directly injected into the GC. No enrichment or pretreatment steps are required and sample volumes as low as 100 microL are sufficient for accurate quantification. Method detection limits determined in natural groundwater samples were between 0.07 and 2.8 microg/L and instrument detection limits of <5 pg were achieved for 21 out of the 24 evaluated VOCs. DAI-GC/MS offers both good accuracy and precision (relative standard deviations 相似文献   

Temperature-promoted large-volume solute enrichment in column-switching miniaturized liquid chromatography: determination of an antioxidant

A two-valve sub-ambient temperature-promoted reversed-phase packed-capillary liquid-chromatography column-switching system has been tailored for sensitive determination of hydrophobic compounds. Such compounds are not easily dissolved in solvent mixtures of non-eluting properties that traditionally are used for solute enrichment in reversed-phase liquid chromatography. Enrichment-column solute focusing of large sample volumes was promoted by use of sub-ambient temperatures only, allowing the use of sample solvents that were stronger or equal to the mobile phase solvent strength. Subsequent column switching and enrichment-column temperature increment provided efficient low-dispersion back-flushed enrichment-column solute desorption onto the analytical column, where the solute was subjected to temperature-programmed gradient action. The antioxidant, Irganox 1076 (octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) extracted from low density polyethylene with 100% acetonitrile served as a hydrophobic model compound. The mobile phase consisted of acetonitrile containing 10 mM triethylamine and formic acid, and the 0.25 mm id enrichment-column and analytical column in lengths of 27 and 250 mm, respectively, were packed with 3.5 microm Kromasil C18 particles. Sample volumes of up to 500 microL were successfully focused on the enrichment column at 5 degrees C using loading flow rates of up to 40 microL min(-1) prior to temperature programming to 90 degrees C. The concentration limit of detection of Irganox 1076 was 6 ng mL(-1) when using an injection volume of 500 microL. The within-assay precision was in the range 3.5-6.8% (n = 6) while the between-day precision was 7.5% (n = 3) relative standard deviation. The method was linear within the investigated mass range 3-100 ng (R2 = 0.9993).     

Characterization of cyclodextrin modified infrared chemical sensors. Part II. Selective and quantitative determination of aromatic acids

In this paper, the selectivity and sensitivity of cyclodextrin (CD) modified infrared (IR) chemical sensor in detection of aromatic acids in aqueous solutions were reported. To eliminate the interference from water, the technique of attenuated total reflection was employed. By surface treated with CD molecules on the internal reflection elements, the sensors were selective in sensing of aromatic acids compared to aromatic compounds with other functional groups. To facilitate the use of this method for the quantitative analyses of aromatic acids in aqueous solutions, analytical functions were also developed in this work and a linear relationship between analytical responses and concentrations of analytes can be obtained. To optimize the analytical conditions, the factors that influence the IR spectroscopic signals were examined. These factors included response time, CD loadings of the sensors, pH effect on response, regeneration efficiency and stability of sensors. Under the optimal conditions, the detection limits for aromatic acids at a detection time of 2 min can be <100 μg/L. Meanwhile, the dynamic linear range for detection was only ca. two orders of magnitude if direct IR signals were used. Using the analytical function developed in this work, the linearity can be extended up to a concentration of 100 mg/L.