New membraneless vaporization unit coupled with flow systems for analysis of ethanol

Home-Made Membraneless Vaporization Gas-Liquid Separator for Colorimetric Determination of Ethanol in Alcoholic Beverages

This work utilized the simplicity of a so-called membraneless vaporization (MBL-VP) unit as a gas separator for the colorimetric determination of ethanol in alcoholic beverages. A beverage sample with a volume of 1 mL was directly injected into a small container which was hung from a lid inside a closed 40 mL reused glass bottle without pretreatment such as distillation. An acidified potassium dichromate (Cr₂O₇ ^2-) acceptor solution, preadded to the glass bottle, was reduced to Chromium (III) ion by the diffusion of vaporized ethanol from the sample. After 5 min, the absorbing solution was collected for colorimetric detection at 590 nm. The unit manually quantifies ethanol in the range 1.0-90% (v/v) with satisfactory interday precision but without matrix effect (recovery 89-109%). The method was validated with the conventional distillation/pycnometer method which showed no significant difference of ethanol contents between those two methods and the declared values of 12 alcoholic beverages, indicating sufficient accuracy. Analyses of alcoholic beverages using this method were successful with benefits of simplicity, cheapness, and less energy consumption.

A novel miniaturized electroanalytical device integrated with gas extraction for the voltammetric determination of sulfite in beverages

Voltammetry and amperometry are inexpensive and high-performance analytical techniques. However, their lack of selectivity limits their use in complex matrices such as biological, environmental, and food samples. Therefore, voltammetric and amperometric analyses of these samples usually require time-consuming and laborious sample pretreatments. In this study, we present a simple and cost-effective approach to fabricate a miniaturized electrochemical cell that can be easily coupled to a head space-like gas extraction procedure in such a way the sample pretreatment and voltammetric detection are performed in a single step. As a proof of concept, we have used the proposed system to quantify sulfite in beverage samples after its conversion to SO_2(g). Despite the simplicity and low cost of the proposed system, it provided good analytical performance and a limit of detection of 4.0 μmol L^-1 was achieved after only 10 min of extraction. The proposed system is quite versatile since it can be applied to quantify any volatile electroactive species. Also, the proposed system provides a unique way to assess real-time extraction curves, which are essential to study and optimize new gas extraction procedures. Therefore, the approach described in this study could contribute to both applied and fundamental Analytical Chemistry.

Rapid Fabrication of Superhydrophobic Virtual Walls for Microfluidic Gas Extraction and Sensing

Based on the virtual walls concept, where fluids are guided by wettability, we demonstrate the application of a gas phase extraction microfluidic chip. Unlike in previous work, the chip is prepared using a simple, rapid, and low-cost fabrication method. Channels were cut into double-sided adhesive tape (280 µm thick) and bonded to hydrophilic glass slides. The tape was selectively made superhydrophobic by ‘dusting’ with hydrophobic silica gel to enhance the wettability contrast at the virtual walls. Finally, the two glass slides were bonded using tape, which acts as a spacer for gas transport from/to the guided liquids. In our example, the virtual walls create a stable liquid–vapor–liquid flow configuration for the extraction of a volatile analyte (ammonia), from one liquid stream to the other through the intermediate vapor phase. The collector stream contained a pH indicator to visualize the mass transport. Quantitative analysis of ammonium hydroxide in the sample stream (<1 mM) was possible using a characteristic onset time, where the first pH change in the collector stream was detected. The effect of gap length, flow rates, and pH of the collector stream on the onset time is demonstrated. Finally, we demonstrate the analysis of ammonium hydroxide in artificial human saliva to show that the virtual walls chip is suitable for extracting volatile analytes from biofluids.

Development of a microfluidic membraneless vaporization flow system for trace analysis of arsenic

A new design of a membraneless vaporization (MBL-VP) unit coupled with a specific flow system is presented for the determination of arsenic at trace levels using a hydride generation process. The MBL-VP unit contains two concentric conical reservoirs, with the outer cone selected as the donor reservoir. The volume of the outer donor reservoir is thereby greater than the acceptor volume, necessary for holding sufficient sample and reagents for the generation of arsine gas by reaction between As(iii) and sodium borohydride under acidic conditions. The arsine gas diffuses into the narrow headspace and is absorbed by an aliquot of 150 μL of mercuric chloride acceptor solution. The resulting reaction produces hydronium ions which is monitored by the absorbance change at 530 nm of the methyl orange indicator added in the acceptor solution. To decrease the detection limit, the aspiration and removal of the donor plug, comprising the sample, borohydride and acid, into and out of the donor cone are repeated several times, while the acceptor solution is kept unchanged. As a result, analysis of arsenic was achieved in the range of 10 to 100 μg L-1 with a detection limit of 8 μg L-1. Application to surface water was investigated. Percent recoveries of spiked surface water samples were in the range of 94-110%. For comparison of total arsenic (As(iii) and As(v)), the results obtained from the developed method are not statistically different from the ICP-OES method.

Foldable paper-based analytical device for membraneless gas-separation and determination of iodate based on fluorescence quenching of gold nanoclusters

A new design of a paper-based analytical device (PAD) for membraneless gas-separation with subsequent determination of iodate is presented. The rectangular PAD was invented as the folded pattern, where two circular reservoirs: the donor reservoir and the acceptor reservoir were situated in "a single paper" for convenient use. The hydrophobic barrier of each reservoir was easily fabricated by painting with a permanent marker. The PAD was demonstrated for the quantitative analysis of iodate, based on the fluorescence quenching of the bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs). The BSA-AuNCs were fast prepared by a microwave-assisted approach. The nanoclusters solution was applied into the acceptor reservoir, while the sample, iodide and sulfuric acid were sequentially aliquoted into the donor reservoir. After folding the PAD, the donor and the acceptor were mounted together via a two-sided mounting tape. The headspace between the two reservoirs allows membraneless gas-separation of free iodine from the donor to diffuse into the acceptor. Etching of gold core of the nanoclusters in the acceptor resulted in quenching of the red emission, was monitored by two methods, i.e. "fluorometric detection" (λ_ex: 490 nm, λ_em: 630 nm) and "image capture" of the acceptor under the UV irradiation by a smart phone's camera. Two calibrations were plotted accordingly to their detections and good linearities (r² ˃ 0.98) were observed from 0.005 to 0.1 mmol L^-1 iodate. High accuracy (mean recovery: 95.1 (±4.6) %) and high precision (RSD < 3%) were obtained. The lower limits of detection were 0.005 mmol L^-1 (with fluorometric detection) and 0.01 mmol L^-1 (with image capture). The method was effectively applied for the measurement of iodate in iodized salts and fish sauces without prior sample pre-treatment.

Simple Flow-Based System with an In-Line Membrane Gas-liquid Separation Unit and a Contactless Conductivity Detector for the Direct Determination of Sulfite in Clear and Turbid Food Samples

This study presents a simple flow-based system for the determination of the preservative agent sulfite in food and beverages. The standard method of conversion of sulfite ions into SO2 gas by acidification is employed to separate the sulfite from sample matrices. The sample is aspirated into a donor stream of sulfuric acid. A membrane gas-liquid separation unit, also called a 'gas-diffusion (GD)' unit, incorporating a polytetrafluoroethylene (PTFE) hydrophobic membrane allows the generated gas to diffuse into a stream of deionized water in the acceptor line. The dissolution of the SO2 gas leads to a change in the conductivity of water which is monitored by an in-line capacitively coupled contactless conductivity detector (C4D). The conductivity change is proportional to the concentration of sulfite in the sample. In this work, both clear (wine) and turbid (fruit juice and extracts of dried fruit) were selected to demonstrate the versatility of the developed method. The method can tolerate turbidity up to 60 Nephelometric Turbidity Units (NTUs). The linear range is 5-25 mg L-1 SO32- with precision < 2% RSD. The flow system employs a peristaltic pump for propelling all liquid lines. Quantitative results of sulfite were statistically comparable to those obtained from iodimetric titration for the wine samples.

Novel Approach to Sample Preconcentration by Solvent Evaporation in Flow Analysis

A preconcentration module operated in flow mode and integrated with a sequential injection system with spectrophotometric detection was developed. Using the system, preconcentration was performed in continuous mode and was based on a membraneless evaporation process under diminished pressure. The parameters of the proposed system were optimized and the system was tested on the example of the spectrophotometric determination of Cr(III). The preconcentration effectiveness was determined using the signal enhancement factor. In the optimized conditions for Cr(III), it was possible to obtain the signal enhancement factors of around 10 (SD: 0.9, n = 4) and determine Cr(III) with precision and intermediate precision of 8.4 and 5.1% (CV), respectively. Depending on the initial sample volume, signal enhancement factor values of about 20 were achieved. Applicability of the developed preconcentration system was verified in combination with the capillary electrophoresis method with spectrophotometric detection on the example of determination of Zn in certified reference materials of drinking water and wastewater. Taking into account the enhancement factor of 10, a detection limit of 0.025 mg L⁻¹ was obtained for Zn determination. Zn was determined with precision less than 6% (CV) and the results were consistent with the certified values.

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.

T-shirt ink for one-step screen-printing of hydrophobic barriers for 2D- and 3D-microfluidic paper-based analytical devices

This work presents the use of polyvinyl chloride (PVC) fabric ink, commonly employed for screening t-shirts, as new and versatile material for printing hydrophobic barrier on paper substrate for microfluidic paper-based analytical devices (μPADs). Low-cost, screen-printing apparatus (e.g., screen mesh, squeegee, and printing table) and materials (e.g. PVC ink and solvent) were employed to print the PVC ink solution onto Whatman filter paper No. 4. This provides a one-step strategy to print flow barriers without the need of further processing except evaporation for 3-5 min in a fume hood to remove the solvent. The production of the single layer μPADs is reasonably high with up to 77 devices per screening with 100% success rate. This method produces very narrow fluidic channel 486 ± 14 μm in width and hydrophobic barrier of 642 ± 25 μm thickness. Reproducibility of the production of fluidic channels and zones is satisfactory with RSDs of 2.9% (for 486-μm channel, n = 10), 3.7% (for 2-mm channel, n = 50) and 1.5% (for 6-mm diameter circular zone, n = 80). A design of a 2D-μPAD produced by this method was employed for the colorimetric dual-measurements of thiocyanate and nitrite in saliva. A 3D-μPADs with multiple layers of ink-screened paper was designed and constructed to demonstrate the method's versatility. These 3D-μPADs were designed for gas-liquid separation with in-situ colorimetric detection of ethanol vapor on the μPADs. The 3D-μPADs were applied for direct quantification of ethanol in beverages and highly colored pharmaceutical products. The printed barrier was resistant up to 8% (v/v) ethanol without liquid creeping out of the barrier.

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).

Membraneless Gas-Separation Microfluidic Paper-Based Analytical Devices for Direct Quantitation of Volatile and Nonvolatile Compounds

This work presents new chemical sensing devices called "membraneless gas-separation microfluidic paper-based analytical devices" (MBL-GS μPADs). MBL-GS μPADs were designed to make fabrication of the devices simple and user-friendly. MBL-GS μPADs offer direct quantitative analysis of volatile and nonvolatile compounds. Porous hydrophobic membrane is not needed for gas-separation, which makes fabrication of the device simple, rapid and low-cost. A MBL-GS μPAD consists of three layers: "donor layer", "spacer layer", and "acceptor layer". The donor and acceptor layers are made of filter paper with a printed pattern. The donor and acceptor layers are mounted together with a spacer layer in between. This spacer is a two-sided mounting tape, 0.8 mm thick, with a small disc cut out for the gas from the donor zone to diffuse to the acceptor zone. Photographic image of the color that is formed by the reagent in the acceptor layer is analyzed using the ImageJ program for quantitation. Proof of concept of the MBL-GS μPADs was demonstrated by analyzing standard solutions of ethanol, sulfide, and ammonium. Optimization of the MBL-GS μPADs was carried out for direct determination of ammonium in wastewaters and fertilizers to demonstrate the applicability of the system to real samples.

A novel approach to Lab-In-Syringe Head-Space Single-Drop Microextraction and on-drop sensing of ammonia

A novel approach to the automation technique Lab-In-Syringe, also known as In-Syringe Analysis, is proposed which utilizes a secondary inlet into the syringe void, used as a size-adaptable reaction chamber, via a channel passing through the syringe piston. This innovative approach allows straightforward automation of head-space single-drop microextraction, involving accurately controlled drop formation and handling, and the possibility of on-drop analyte quantification. The syringe was used in upside-down orientation and in-syringe magnetic stirring was carried out, which allowed homogenous mixing of solutions, promotion of head-space analyte enrichment, and efficient syringe cleaning. The superior performance of the newly developed system was illustrated with the development of a sensitive method for total ammonia determination in surface waters. It is based on head-space extraction of ammonia into a single drop of bromothymol blue indicator created inside the syringe at the orifice of the syringe piston channel and on-drop sensing of the color change via fiber optics. The slope of the linear relationship between absorbance and time was used as the analytical signal. Drop formation and performance of on-drop monitoring was further studied with rhodamine B solution to give a better understanding of the system's performance. A repeatability of 6% RSD at 10 μmol L(-1) NH3, a linear range of up to 25 μmol L(-1) NH3, and a limit of detection of 1.8 μmol L(-1) NH3 were achieved. Study of interferences proved the high robustness of the method towards humic acids, high sample salinity, and the presence of detergents, thus demonstrating the method superiority compared to the state-of-the-art gas-diffusion methods. A mean analyte recovery of 101.8% was found in analyzing spiked environmental water samples.

Automated in-syringe single-drop head-space micro-extraction applied to the determination of ethanol in wine samples

A novel approach of head-space single-drop micro-extraction applied to the determination of ethanol in wine is presented. For the first time, the syringe of an automated syringe pump was used as an extraction chamber of adaptable size for a volatile analyte. This approach enabled to apply negative pressure during the enrichment step, which favored the evaporation of the analyte. Placing a slowly spinning magnetic stirring bar inside the syringe, effective syringe cleaning as well as mixing of the sample with buffer solution to suppress the interference of acetic acid was achieved. Ethanol determination was based on the reduction of a single drop of 3mmol L(-1) potassium dichromate dissolved in 8mol L(-1) sulfuric acid. The drop was positioned in the syringe inlet in the head-space above the sample with posterior spectrophotometric quantification. The entire procedure was carried out automatically using a simple sequential injection analyzer system. One analysis required less than 5min including the washing step. A limit of detection of 0.025% (v/v) of ethanol and an average repeatability of less than 5.0% RSD were achieved. The consumption of dichromate reagent, buffer, and sample per analysis were only 20μL, 200μL, and 1mL, respectively. The results of real samples analysis did not differ significantly from those obtained with the references gas chromatography method.