Gas-diffusion Unit Research Articles

A Cost-effective Liquid Core Waveguide Based on a Concentrated Acid Medium for Colorimetric Determination of Sulfide.

A cost-effective long-path absorption liquid core waveguide (LCW) device was fabricated from a typical Teflon tube, and using a high refractive index (RI) medium of acid. It was incorporated in a flow injection-gas diffusion system for the sensitive determination of sulfide via methylene blue (MB) chemistry. The gas diffusion unit could prevent some interferences in samples. The limit of detection of 0.13 μmol L-1 was achieved without using a preconcentration procedure. The proposed device was successfully applied for sulfide determination in canal water samples with 88.7 - 103% recovery.

Reversible microfluidic measurement with zinc tetraphenylporphyrin aiming at slipping NH3detection

A microfluidic chip that can realise reversible and highly selective NH3 detection by separating NH3 from the complex environment is fabricated for slipping NH3 detection. The microfluidic chip consisting of the reaction unit, the gas-diffusion unit and detection unit applied the immobilised zinc tetraphenylporphyrin as the indicating material. The continuous spectrum change showed the light intensity decreased in the green range (500–550 nm). Based on the analysis of the spectrum change at 528 nm, a one-to-one relationship between the spectrum change and concentration was revealed. The reversibility of the detection process makes the detection advantageous. Also, the rapidity of the proposed method is proved by the time responses of the indicating and recovering processes. According to the 40 tests conducted for various concentrations, it was concluded that the results are stable and the entire system is reliable.

Simple flow system with in-line gas-diffusion unit for determination of ethanol employing hypsochromic shift of visible absorbance band of methyl orange

The effect of solvent composition on the uv–visible spectrum of methyl orange was investigated for application to the quantitative determination of ethanol. At fixed pH, there was a hypsochromic shift of the absorbance band of methyl orange with increasing ethanol concentration. Using acetate buffer at pH 3.40 the change of absorbance at 530 nm of a solution of methyl orange containing known concentrations of standard ethanol was measured to provide a calibration curve. In order to apply this method to the analysis of alcoholic samples, such as distilled spirits, blended spirits and liquid herbal medicines, a simple gas-diffusion unit coupled with flow system was employed to separate the ethanol from sample matrices. Using the gas diffusion-flow system and employing an evaporation time of 2 min, a linear calibration range of 5–45% (v/v) ethanol was achieved ((ΔA = (0.0078 ± 0.0002)x(ethanol, %(v/v)) + (0.040 ± 0.005)), r2 = 0.998). The limit of detection (3σ blank/slope) was 2.23% (v/v). The developed gas diffusion-flow system was applied to the analysis of colorless distilled spirits, yellow blended spirits and dark brown herbal medicines that are available in the local markets of Bangkok, Thailand. Validation of the method was carried out by comparing the results with analysis using gas chromatography. There was no statistically significant difference at the 95% confidence level for all alcoholic samples analyzed.

Environmentally friendly method for determination of ammonia nitrogen in fertilisers and wastewaters based on flow injection-spectrophotometric detection using natural reagent from orchid flower

ABSTRACTCrude aqueous extract from the orchid ‘Dendrobium Sonia earsakul’ was utilised as a natural product reagent in flow injection analysis (FIA) incorporating a gas diffusion unit (GD) for the determination of ammonia nitrogen. Sample solution was injected into a NaOH donor stream to generate ammonia gas (NH3). In the GD unit, NH3 diffused across a PTFE gas-permeable membrane into the acceptor stream of the orchid extract. As the result, the aqueous orchid reagent became more alkaline and its colour changed from purple to green. The change in the colour of orchid acceptor correlated with the concentration of ammonia nitrogen in the sample and its absorbance monitored by a spectrophotometer at 600 nm. Ammonia nitrogen in chemical fertiliser samples and wastewater samples from agricultural fields were determined and reported as %N (w/w) and mg N L−1, respectively. For chemical fertilisers which contained high content of ammonia nitrogen, a flow rate of 1.0 mL min−1 and injection volume of 100 µL were used with a linear range of 5–40 mmol L−1 and detection limit of 2.12 mmol L−1. However, a higher sensitivity was required for wastewater samples having low ammonia nitrogen content. The flow rate was reduced to 0.3 mL min−1 and the injection volume increased to 1000 µL. As a result, detection limit of 0.76 mmol L−1 was achieved with linear range of 1–5 mmol L−1. The results of our method agreed well with that using the OPA method employing fluorescence detection.

Green analytical method for simultaneous determination of salinity, carbonate and ammoniacal nitrogen in waters using flow injection coupled dual-channel C4D

A flow injection analysis system (FIA) for the simultaneous determination of salinity, carbonate and ammoniacal nitrogen has been developed and reported in this paper. FIA incorporating membrane units was used, not only for the separation of the gaseous carbon dioxide and ammonia, but also for on-line dilution in the salinity measurement. The sample was injected via a 10-port valve with two sample loops. One loop was used for salinity and carbonate measurements and the second loop for ammoniacal nitrogen determination. A dual-channel capacitively coupled contactless conductivity detector was assembled in a single shielding box. Input voltage from the same AC power supply was fed to the input electrodes of both C4D cells. One channel of the C4D was used to monitor the change in conductivity of an acceptor stream that carried a zone of the water sample that has passed through the on-line dilution unit. Conductivity of this zone relates directly to the salinity of the sample. The same sample zone was next acidified to generate carbon dioxide gas that diffused through a hydrophobic membrane of the first gas diffusion (GD) unit. The zone of dissolved carbon dioxide in acceptor stream of water flowed into the same C4D cell as for the salinity measurement, but arriving at a later time. Concurrently, the second channel of the C4D monitored the change in conductivity of the acceptor stream in the second GD unit due to the diffusion of ammonia gas generated by the reaction of base with the sample injected from the second sample loop. The change in conductivity at this second C4D cell correlates with the concentration of ammoniacal nitrogen present in the sample. The proposed method is low cost, simple, rapid and sensitive. The limit of quantitation for salinity, carbonate and ammoniacal nitrogen are 0.31mmolL−1, 1.85 µmol L−1, respectively. Throughput of 20 samples h−1 for simultaneous analysis can be achieved with RSD of less than 3.8%. The system had been applied to the determination of salinity, carbonate and ammoniacal nitrogen in 15 water samples, with results in agreement with those obtained using comparison methods.

A "Dual-acceptor Channel" Membraneless Gas-diffusion Unit for Simultaneous Determination of Ethanol and Acetaldehyde in Liquors Using Reverse Flow Injection.

A new design of membraneless gas-diffusion unit with dual acceptor channels for separation, collection and simultaneous determination of two volatile analytes in liquid sample is presented. The unit is comprised of three parallel channels in a closed module. A sample is aspirated into the central channel and two kinds of reagents are introduced into the other two channels. Two analytes are isolated from the sample matrix by diffusion into head-space and absorbed into the specific reagents. Non-absorbed vapor is released by opening the programmable controlled lid. The unit was applied to liquors for measurement of ethanol and acetaldehyde using reverse flow injection. Dichromate and nitroprusside were exploited as reagents for colorimetric detection of ethanol and acetaldehyde, respectively. Good linearity ranges (r2 >0.99) with high precision (RSD <2%) and high accuracy (recovery: 90 - 105%) were achieved. The results were compared to the results by GC-FID and no significant difference was observed by paired t-test (95% confidence).

Development of gas flow reactor with on-line monitoring system for nitrogen dioxide removal

In this work, a new flow system was developed for investigating the efficiency of synthesized TiO2 photocatalysts for NO2 degradation. The system consists of two parts: the gas flow reactor and an on-line NO2 monitoring system. The latter is based on the conversion of NO2(g) to NO2−(aq) with subsequent spectrophotometric detection of the nitrite anion by the Griess-Saltzman reaction. Simplex optimization was employed to obtain the required sensitivity for the NO2 monitoring. The developed flow system was employed to measure the catalytic efficiency of photodegradation of NO2 by various types of TiO2-based synthesized photocatalysts and a commercial TiO2 photocatalyst, ‘P25’. The TiO2 photocatalysts were synthesized using titanium isopropoxide (TTIP), tetrabutyl orthotitanate (TBOT) and titanium oxysulfate (TiOSO4) as precursors. Glass beads, coated with the TiO2 photocatalyst, were packed in a quartz tube reactor. Standard NO2 gas mixture (50ppmv) was continuously flowed through the reactor under UV radiation. Residual NO2 from the outlet of the reactor was measured using the on-line NO2 monitoring system which employs a gas diffusion unit and an acceptor stream of triethanolamine solution. The NO2(g) was converted into NO2−(aq) which was measured spectrophotometrically at 540nm using the Griess-Saltzman reaction. The photocatalytic efficiency of the synthesized photocatalysts was compared. The effects of oxygen and humidity were also studied in order to investigate the mechanism of photodegradation of NO2by the TiO2 catalyst. We observed that at low humidity (12% RH), the OH• radical generated from moisture plays an important role for photooxidation process of NO2.

Determination of trace levels of ammonia in marine waters using a simple environmentally-friendly ammonia (SEA) analyser

A simple environmentally-friendly ammonia (SEA) analyser based on programmable flow was developed for the determination of trace levels of ammonia nitrogen in marine waters. The proposed SEA analyser was based on a gas-diffusion spectrophotometric method. High sensitivity was achieved through the use of large sample volumes and a programmable-flow approach. The degree of pre-concentration of ammonia in the acceptor stream was determined by the controlled and programmable delivery of the sample to the gas-diffusion unit, and the sensitivity of the method was tailored to the concentration range of interest by simple modification of the sample volume used for analysis. Three different working concentration ranges were defined by varying the volume of the sample, thus making the SEA analyser versatile and applicable for use in estuarine or coastal seawaters: 0.028–5.6 μM (2.0 mL), 0.28–13.9 μM (1.0 mL), and 1.4–55.6 μM (0.25 mL). Typical limits of detection of 15, 88 and 440 nM, and repeatability of 0.71, 1.2 and 0.97% RSD (n = 10), were attained for each range, respectively. Good sample throughput was achieved with approximately 20 samples per hour for the lower concentration range, 30 samples per hour for the middle concentration range, and 40 samples per hour for the upper concentration range. Method validation involved the analysis of a certified reference material in artificial seawater, and spike-and-recovery analysis for seawater and estuarine samples collected from various locations around Port Phillip Bay, Victoria, Australia. Small volumes of waste (i.e. alkaline seawater feedstock and 1.5 mL indicator solution) were generated per sample. The reagents used were non-toxic, suitable for long-term storage, and cheap, costing as little as $2.40 AUD per 1000 determinations.

A flow-based platform for measuring the acidity parameters in wine

The present work describes a valuable tool for winemaking industry to measure the acidity parameters with rapid response, simple sample handling, with no or minimal pre-treatment. Thus, a sequential injection analysis (SIA) system with spectrophotometric detection was used as platform for the development of methodologies for the quantification of volatile and total acidity in wine samples. Both procedures make use of the same colour reagent, bromothymol blue (BTB) that changes its intrinsic colour in the presence of the acidity compounds. The volatile acidity value was attained with the separation of the volatile fraction of acids by means of a membrane separation technique, a gas-diffusion unit. For the total acidity value, the sample was merged with the colour reagent on the way towards detection. The fixed acidity is a result of the difference from the total and the volatile acidity.The presented tool displayed a low sample and reagent consumption, 346 and 102µL of sample and 37 and 32µg of BTB, for the volatile acidity (VA) and for the total acidity (TA), respectively. The observed limits of detection and quantification were 0.03 and 0.01gL−1 (VA) and 0.09 and 0.02gL−1 (TA) with high determination rates, 35 (VA) and 62 (TA) determinations per hour. The proposed system was successfully applied to the quantification of volatile, total and fixed acidity in white table wine samples. The obtained results were in good agreement with the ones obtained by the reference methods.

Flow injection with in-line reduction column and conductometric detection for determination of total inorganic nitrogen in soil

A cost effective flow injection (FI) conductometric system has been developed for determination of total inorganic nitrogen (TIN). The system is aimed for evaluation of nitrogen nutrient in soil for agricultural application. Inorganic nitrogen compounds were extracted from soil according to the standard method by using potassium chloride solution as an extractant, and the extracted solution was then injected into the FI system. Nitrate and nitrite are converted to ammonium ion by an in-line reduction column packed with a Devarda's alloy. A gas diffusion unit was incorporated into the FI system to separate ammonium ion from other ions in a donor stream by forming ammonia gas that can diffuse through a PTFE membrane to re-dissolve in an acceptor stream. Conductance of the acceptor stream was directly proportional to ammonium ion concentration. Various parameters affecting reduction efficiency of the column, e.g., column diameter, column packing procedure, and column length was investigated and optimized. A linear calibration graph in the range of 2.00–60.00mgL−1 N–NH4+ (y=0.123x+0.039, R2 =0.997) was obtained with a limit of detection of 0.47mgL−1. Sample throughput of 20samplesh−1 was achieved. The result of developed method was correlated with total Kjeldahl nitrogen (TKN) obtained from the Kjeldahl digestion method. The proposed method could be used as an alternative method to the Kjeldahl method for determination of TIN in soil.

A reagentless flow injection system for the quantification of ethanol in beverages based on the schlieren effect measurement

A flow injection system comprising a gas-diffusion unit and schlieren effect detection was arranged in order to develop a simple methodology for the quantification of ethanol in different alcoholic beverages. It was possible to establish a linear range up to 25% (v/v), with a LOD and a LOQ of 0.6% and 2.0% (v/v), respectively; with good repeatability, RSD<4.6%. Only 80μL of sample was needed for quantification with a determination rate of one determination per minute.To evaluate the accuracy of the proposed method, a total of 43 alcoholic beverages, red and white table wine, port wine, beers, liquors, spirits and brandies were analysed. The results obtained by the developed method were in good agreement with the ones obtained by the reference method.

Spectrophotometric determination of bromide in water using the multisyringe flow injection analysis technique coupled to a gas-diffusion unit

Spectrophotometric determination of bromide in water using a gas-diffusion unit.

Sequential injection methodology for carbon speciation in bathing waters

A sequential injection method (SIA) for carbon speciation in inland bathing waters was developed comprising, in a single manifold, the determination of dissolved inorganic carbon (DIC), free dissolved carbon dioxide (CO2), total carbon (TC), dissolved organic carbon and alkalinity. The determination of DIC, CO2 and TC was based on colour change of bromothymol blue (660nm) after CO2 diffusion through a hydrophobic membrane placed in a gas diffusion unit (GDU). For the DIC determination, an in-line acidification prior to the GDU was performed and, for the TC determination, an in-line UV photo-oxidation of the sample prior to GDU ensured the conversion of all carbon forms into CO2. Dissolved organic carbon (DOC) was determined by subtracting the obtained DIC value from the TC obtained value. The determination of alkalinity was based on the spectrophotometric measurement of bromocresol green colour change (611nm) after reaction with acetic acid. The developed SIA method enabled the determination of DIC (0.24–3.5mgCL−1), CO2 (1.0–10mgCL−1), TC (0.50–4.0mgCL−1) and alkalinity (1.2–4.7mgCL−1 and 4.7–19mgCL−1) with limits of detection of: 9.5μgCL−1, 20μgCL−1, 0.21mgCL−1, 0.32mgCL−1, respectively. The SIA system was effectively applied to inland bathing waters and the results showed good agreement with reference procedures.

A microflow analyzer with an integrated gas diffusion unit

This work describes the development of a microflow analyzer with an integrated gas-diffusion unit. Deep ultraviolet photolithography was employed to engrave the channels (width of 500 µm and depth of 440 µm) on two plates of urethane-acrylate (UA) photoresist, and a polytetrafluoroethylene (PTFE) membrane was adapted between the structures designed for manipulation of donor and acceptor solutions. To perform the conductivity measurements, three pairs of electrodes (six copper wires with 0.5 mm diameter) were coupled into guiding channels, orthogonal to detection channel. The proposed microfluidic device presented a total internal volume of 48 µL and supported flow rates up to 1.0 mL min-1 without leakages or membrane damages. Analytical signals for the determination of bicarbonate in mineral waters provided a linear response (R² = 0.999) for the concentration range studied (20 to 80 mg L-1), a limit of detection (LOD) of 2.3 mg L-1 and a relative standard deviation (RSD) of 2.5% (n = 5). A similar performance was observed for the determination of ammonium in medicinal tablets (R² = 0,998 (10-40 mg L-1), LOD = 2.9 mg L-1 and RSD = 3.0% (n = 5)). The sampling rates for HCO3- and NH4+ procedures were 15 and 25 injections h-1, respectively. For both applications, results agreed with those obtained by potentiometric titrations (HCO3-) and UV-Vis spectrophotometry (NH4+). In addition, by using a flow rate of only 40 µL min-1 for donor and acceptor solutions, a reduction of approximately 75% in residues generation was estimated by comparing the micro-analyzer with a conventional flow analysis system. These results demonstrate the viability of developing microflow analyzers with an integrated gas diffusion unit and conductometric detection.

Sequential injection system for the enzymatic determination of ethanol in alcoholic beverages with in-line dilution

A sequential injection system for the enzymatic determination of ethanol in wines with spectrophotometric detection was developed. The determination was based on the enzymatic reaction involving ethanol and NAD+ in the presence of alcohol dehydrogenase. A gas-diffusion unit was introduced in the system to perform an in-line dilution. It was possible to operate the system with a reagent consumption of 1.3 mg of NAD+, 0.45 U of ADH, and 32 μL of sample, per assay with a determination rate of 21 h−1.The results obtained from the analysis of wine samples were statistically equivalent to those obtained by the reference method, with good repeatability; the relative standard deviation (n = 10) was <1% with a LOQ of 0.3% (v/v).

Speciation of arsenic (III) and arsenic (V) based on quenching of CdS quantum dots fluorescence using hybrid sequential injection–stopped flow injection gas–diffusion system

A hybrid sequential injection–stopped flow injection system was developed for the speciation of arsenic based on the quenching of mercaptoacetic acid capped cadmium sulfide quantum dots (CdS–MAA QDs) fluorescence intensity. The analytical procedure involves the generation of arsine from As(III) by sodium borohydride in acetate buffer medium pH 6.0. The generated arsine (donor stream) diffuses across the PTFE membrane of the gas–diffusion unit into an acceptor stream and then interacts with CdS–MAA QDs. Total arsenic was determined after pre-reduction of As(V) to As(III) with 1% (m/v) mercaptoacetic acid. Concentration of As(V) in the sample solutions can be deduced from the difference of total arsenic and As(III). Optimization of the experimental conditions and instrumental parameters were investigated. Under optimal conditions, limits of detection were 20μgL−1 for As(III) and 40μgL−1 for As(V). Recoveries in the range 84–103% were obtained from sediment sample.

Determination of arsenic based on quenching of CdS quantum dots fluorescence using the gas-diffusion flow injection method

A sequential injection analysis system for determination of arsenic based on hydride generation and fluorescence quenching of mercaptoacetic acid capped cadmium sulfide quantum dots (CdS–MAA QDs) is described. The generated arsine diffused across the PTFE membrane in a gas-diffusion unit and subsequently interacted with CdS–MAA QDs. The parameters affecting the arsine generation and the fluorescence quenching of QDs were studied. Under the optimum conditions, it was observed that a increase in the concentration of As(III) corresponded well to a decrease in fluorescence intensity according to the Stern–Volmer relationship. The extent of quenching was dependent on the concentration of arsenic in the range of 0.08–3.20 mmol L −1, with the detection limit of 0.07 mg L −1. The precision (%RSD) from eight replicates of the determination of As(III) 1.0 mg L −1 was found to be 1.4%. The proposed method was applied to the determination of arsenic in ground water samples with satisfactory recoveries.

Flow injection colorimetric method using acidic ceric nitrate as reagent for determination of ethanol

Ceric ammonium nitrate has been used for qualitative analysis of ethanol. It forms an intensely colored unstable complex with alcohol. In this work, a simple flow injection (FI) colorimetric method was developed for the determination of ethanol, based on the reaction of ethanol with ceric ion in acidic medium to produce a red colored product having maximum absorption at 415 nm. Absorbance of this complex could be precisely measured in the FI system. A standard or sample solution was injected into a deionized water donor stream and flowed to a gas diffusion unit, where the ethanol diffused through a gas permeable membrane made of plumbing PTFE tape into an acceptor stream to react with ceric ammonium nitrate in nitric acid. Color intensity of the reddish product was monitored by a laboratory made LED based colorimeter and the signal was recorded on a computer as a peak. Peak height obtained was linearly proportional to the concentration of ethanol originally presented in the injected solution in the range of 0.1–10.0% (v/v) ( r 2 = 0.9993), with detection limit of 0.03% (v/v). With the use of gas diffusion membrane, most of the interferences could be eliminated. The proposed method was successfully applied for determination of ethanol in some alcoholic beverages, validating by gas chromatographic method.

A membraneless gas-diffusion unit – multisyringe flow injection spectrophotometric method for ammonium determination in untreated environmental samples

A new design of a membraneless gas-diffusion (MGD) unit coupled to a multisyringe flow injection system is proposed. The spectrophotometric determination of ammonium using an acid–base indicator was chosen to show the feasibility of this approach. Hence, in alkaline medium, ammonium ions are transformed into ammonia (donor channel) which diffuses through the headspace into the acceptor stream (bromothymol blue solution), causing a pH change and subsequently a colour change. The exploitation of the enhanced potentialities of this re-designed MGD device was the main purpose of the present work. Hence, several strategies concerning flow management were studied seeking to characterize and improve the analytical features of the methodology and moreover, untreated environmental samples were analysed without previous filtration. Consequently, stopped flow in acceptor channel with continuous flow in donor channel was chosen for the application to wastewater and spiked river water samples. A linear concentration range between 10.0 and 50.0mgL−1 of NH4+, a limit of detection of 2.20mgL−1 and a determination frequency of 11h−1 were obtained.

Fast and reliable analyses of sulphite in fruit juices using a supramolecular amperometric detector encompassing in flow gas diffusion unit

A new compact system encompassing in flow gas diffusion unit and a wall-jet amperometric FIA detector, coated with a supramolecular porphyrin film, was specially designed as an alternative to the time-consuming Monier-Williams method, allowing fast, reproducible and accurate analyses of free sulphite species in fruit juices. In fact, a linear response between 0.64 and 6.4ppm of sodium sulphite, LOD=0.043ppm, relative standard deviation of ±1.5% (n=10) and analytical frequency of 85 analyses/h were obtained utilising optimised conditions. That superior analytical performance allows the precise evaluation of the amount of free sulphite present in foods, providing an important comparison between the standard addition and the standard injection methods. Although the first one is most frequently used, it was strongly influenced by matrix effects because of the unexpected reactivity of sulphite ions with the juice matrixes, leading to its partial consumption soon after addition. In contrast, the last method was not susceptible to matrix effects yielding accurate results, being more reliable for analytical purposes.