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Miniaturized chromate-free chloride assay in fish sauce based on linear height calibration of AgCl precipitate in inverted microtubes
(2026-06-01)
Chantiwas, Rattikan
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Wilairat, Prapin
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Putthasa, Papawarin
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Cheotchinda, Benjamapohn
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Teerasong, Saowapak
A miniaturized, chromate-free methodology for chloride determination in high-salt food matrices is presented in accordance with the principles of Green Analytical Chemistry. Classical argentometric precipitation is adapted to a microvolume, instrument-light assay by confining silver chloride formation within inverted microcentrifuge tubes and directly measuring the height of the compact precipitate layer using a simple ruler. Because the cylindrical tube region has a constant cross-sectional area, precipitate height is directly proportional to chloride concentration, enabling straightforward linear calibration without optical detection, signal transduction, or complex data processing. Controlled tube inversion and low-speed centrifugation ensure that precipitation occurs exclusively within this cylindrical zone, producing a sharp, reproducible solid–liquid interface. Under optimized conditions (150 µL sample volume and 180 µL of AgNO₃ in nitric acid; centrifugation at 600 rpm for 3 min followed by 5 min settling), the method provides a linear working range of 50–500 mM chloride (r² = 0.9988), a detection limit of 14 mM, and relative standard deviations of 3–4 % at 200–300 mM. Validation of 12 commercial Thai fish sauce samples showed no statistically significant differences compared to AOAC potentiometric titration at the 95% confidence level, with recoveries of 100–103 % and %RSD values of 1–4.2 %. The primary advantage of this method is its strictly linear height–concentration relationship, enabled by geometry-controlled confinement of the precipitate. Unlike other miniaturized or paper-based assays, it eliminates the need for image analysis or device fabrication. From a green chemistry perspective, the assay eliminates the use of potassium chromate indicators and disposable paper-based devices, operates at the microliter scale using reusable plastic microtubes, and requires only low-speed centrifugation. Analytical Greenness (AGREE), Analytical Eco-scale, Green Analytical Procedure Index (GAPI), and White Analytical Chemistry (WAC) evaluations (AGREE score: 0.76; Eco-scale: 83; WAC score: 88.9) classify the method as an excellent example of green analysis. The proposed approach offers a robust and sustainable screening tool for chloride determination in high-salt foods.
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Semi-quantitative analysis of formaldehyde in food using calibration chart based on number of colored wells of microwell plate titration
(2024-10-30)
Tongdee, Mintra
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Wilairat, Prapin
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Praditweangkum, Wiboon
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Chantiwas, Rattikan
Microplate titration quantifies sodium hydroxide generated from formaldehyde reacting with excess sulfite in a 96-microwell plate. Phenolphthalein indicators change from red to colorless when all hydroxide ions react. Methodology optimized reagent concentrations, and reaction time and created a Calibration Chart for semi-quantitative determination. The chart shows formaldehyde concentration ranges corresponding to red well counts from 0 to 200 mM in 20 mM increments. Inter-operator repeatability demonstrates precision (3 replicates), correlating red wells with standard formaldehyde concentrations. This instrument-free technique uses readily available commercial plates, eliminating the need for specialized equipment and calibration. The methodology offers simplicity with its reliance on readily available commercial plates and minimal specialized equipment, hence it is cost-effective and easily transportable 96-microwell plates enhancing the methodology's portability, and efficient semi-quantitative analysis of formaldehyde. The analysis of twelve solutions from food samples agrees with the quantitative values using titration.
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Measurement of sucrose concentration using Imbibition length on paper: A device for equipment-free and environmentally-friendly detection
(2024-04-01)
Sitanurak, Jirayu
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Kumpong, Anongnat
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Yaimai, Orawan
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Wilairat, Prapin
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Teerasong, Saowapak
The Lucas-Washburn equation is commonly used to predict the distance (L) that a liquid travels through paper. This equation establishes that L2 is linear with time and inversely proportional to the viscosity of the liquid. However, there is currently no theoretical equation connecting the viscosity of a solution to its concentration. In this study, the imbibition flow of a sucrose solution was measured along the length of a horizontal strip of filter paper, featuring a printed, thermometer-shaped hydrophobic boundary. A sample (38 μL) was dispensed onto the bulb area, and the solution's flow was visually tracked using a red dye added to the sample. The imbibition length (L) was measured by a vernier caliper at 10.0 min after the sample addition. An empirical equation, based on literature values of the viscosity (η) and concentration (C) of sucrose solutions, was proposed. By integrating this empirical equation with the Lucas-Washburn equation, the following equation was derived: L = a⋅exp{-(bC + cC2)}, where ‘a’, ‘b’ and ‘c’ are parameters. This equation was fitted to the dataset of L and C, covering C values from 0 to 60 % w/w standard sucrose solutions, resulting in a coefficient of determination of 0.9987. The plot of L against C was observed to closely follow a linear line, with a fitting providing a coefficient of determination of 0.9986. The sucrose contents in samples, such as soft drinks, syrups, and sugarcanes, determined using the imbibition length method and conventional refractometry, were in statistical agreement via the paired t-test at the 95 % confidence level. This method is simple, instrument-free, requiring only a small amount of safe red food dye, and can be conducted on-site.
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Transparent Cross-Flow Platform as Chemiluminescence Detection Cell in Cross Injection Analysis
(2023-02-01)
Somboonsuk, Thachkorn
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Saetear, Phoonthawee
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Mantim, Thitirat
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Ratanawimarnwong, Nuanlaor
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Wilairat, Prapin
This work presents the use of a transparent ‘Cross Injection Analysis’ (CIA) platform as a flow system for chemiluminescence (CL) measurements. The CL-CIA flow device incorporates introduction channels for samples and reagents, and the reaction and detection channels are in one acrylic unit. A photomultiplier tube placed above the reaction channel detects the emitted luminescence. The system was applied to the analysis of (i) Co(II) via the Co(II)-catalyzed H<inf>2</inf>O<inf>2</inf>-luminol reaction and (ii) paracetamol via its inhibitory effect on the catalytic activity of Fe(CN)<inf>6</inf>3− on the H<inf>2</inf>O<inf>2</inf>-luminol reaction. A linear calibration was obtained for Co(II) in the range of 0.002 to 0.025 mg L−1 Co(II) (r2 = 0.9977) for the determination of Co(II) in water samples. The linear calibration obtained for the paracetamol was 10 to 200 mg L−1 (r2 = 0.9906) for the determination of pharmaceutical products. The sample throughput was 60 samples h−1. The precision was ≤4.2% RSD. The consumption of the samples and reagents was ca. 170 µL per analysis cycle.
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Multi-well plate as headspaces for paper-based colorimetric detection of sulfur dioxide gas: An alternative method of sulfite titration for determination of formaldehyde
(2023-01-25)
Yodpach, Nattapon
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Chantiwas, Rattikan
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Wilairat, Prapin
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Choengchan, Nathawut
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Praditweangkum, Wiboon
This work describes the analysis of formaldehyde using a 96-well microplate as multiple headspaces for the separation of sulfur dioxide gas generated from the sulfite remaining after its reaction with the formaldehyde in the sample. The quantitation of the gas is by colorimetric detection of an indicator paper placed over the microplate. The samples are aqueous extracts of various foods that are possibly adulterated with formaldehyde. A known excess amount of sulfite is added to the extract solution aliquoted in the well. The remaining sulfite is acidified with hydrochloric acid to generate sulfur dioxide gas which diffuses through the headspace above the solution to be absorbed at the moist strip of the indicator paper placed over the mouth of the wells. Anthocyanins extracted from the butterfly pea flower is used as the pH indicator giving a color change from the increase of hydrogen ions by hydrolysis of the absorbed sulfur dioxide gas. The exposed paper strip is scanned, and the digital images of the colored region analyzed using ImageJ software. The optimized method has a linear range of 200–1000 mg L−1 formaldehyde with limit of detection ((2.57*SD of intercept)/(slope of calibration line)) of the aqueous extract of 40 mg L−1 and coefficient of determination (r2) > 0.9979. Samples of fresh produce, such as seafood, meat, and vegetables, and various processed food were analyzed for their possible formaldehyde content. The results obtained from the headspace paper-based colorimetric detection are not statistically different from the values obtained from the titration method by paired t-tests.
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A ninety-six well plate as headspaces with moist starch indicator paper as a cover for the determination of ascorbic acid by iodate oxidation and formation of volatile iodine
(2022-02-21)
Nawalohakul, Thichaphat
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Charoenjiraroj, Pannarat
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Chantiwas, Rattikan
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Wilairat, Prapin
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Praditweangkum, Wiboon
This work presents the use of a 96-well plate as headspaces for the determination of ascorbic acid in samples loaded in the 96-well plate. Ascorbic acid in the sample is oxidized to iodide by the addition of excess acidic iodate solution into the well. The iodide is further oxidized by the remaining iodate to molecular iodine. A single sheet of moist starch indicator paper is immediately placed over the 96-well plate after the addition of the iodate with the moisture forming a gas seal. The iodine gas in each well diffuses through the headspace to react with the starch paper producing circular areas of a colored starch-iodine complex. After 15 min the indicator paper is scanned, and the digital images of the complex are analyzed by using ImageJ software to obtain blue intensity values. The precision of the intensity values from 12 wells containing 20 μL of 2.84 mM standard ascorbic acid is <2% relative standard deviation. Optimal conditions for detection were investigated, including the starch concentration, the acidic iodate reagent, and the measurement time. The linear calibration range of ascorbic acid is 0.284-2.84 mM, based on the plot of concentration vs. -log(reflectance). The coefficient of determination (r2) is >0.998. Samples of fruit juice and dietary supplements were analyzed for their ascorbic acid contents. The results obtained from the headspace reflectance method are not statistically different from values obtained from the titration method using paired t-tests (α = 0.05). This journal is
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Simple and fast fabrication of microfluidic paper-based analytical device by contact stamping for multiple-point standard addition assay: Application to direct analysis of urinary creatinine
(2020-04-01)
Mathaweesansurn, Arjnarong
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Thongrod, Suthathip
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Khongkaew, Putthiporn
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Phechkrajang, Chutima Matayatsuk
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Wilairat, Prapin
In this work, a microfluidic paper-based analytical device (μPAD) for simultaneous multiple-point standard addition assay was fabricated by rubber stamping the hydrophobic barrier pattern onto a laboratory filter paper. The μPAD has a central zone from which an applied sample flows into eight surrounding narrow channels to which had been added the standard solutions. Each channel is connected to a circular area loaded with the reagent. The opposite end of this reagent zone is connected by second narrow channel to the final circular detection zone. The μPAD was applied to the measurement of creatinine in human urine. After addition of a urine sample, the orange-colored product arising from the Jaffé reaction is formed at the eight detection zones. A digital image of the μPAD is then recorded and the ratio of the red/green (R/G) intensity obtained using the ImageJ™ program is used in the quantitation of creatinine. The normal standard addition calibration line is constructed using the intensity ratio against the added creatinine concentrations (50–1000 mg L−1). Good linearity was achieved (r2 ˃ 0.99). There were no significant differences between the creatinine content using an HPLC method (paired t-test at 95% confidence, t<inf>stat</inf> = 1.78, t<inf>critical</inf> = 2.26, n = 10). The use of simultaneous multiple-point standard addition calibration allows rapid determination of creatinine in urine with elimination of matrix interference.
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Microfluidic paper-based analytical device for convenient use in measurement of iodate in table salt and irrigation water
(2020-01-01)
Duangdeewong, Chomphunud
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Sitanurak, Jirayu
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Wilairat, Prapin
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Nacapricha, Duangjai
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Teerasong, Saowapak
We report a convenient microfluidic paper-based analytical device (µPAD) for the determination of iodate. The µPAD was fabricated using an ink-stamp with a simple circle design. Colorimetric detection of iodate employs the oxidation of hydroxylamine by the iodate to generate nitrite which reacts with the Griess reagent producing an intense magenta color. The images were recorded by a digital camera, and converted to a green scale color intensity by ImageJ. Calibration of iodate was linear in the range of 50–400 mg L−1 with a detection limit of 38.1 mg L−1. With such a wide linear range, the method can be applied to various kinds of samples with the entire process completed in 50 s. Good precision of measurement was obtained (<2 %RSD). Iodate levels were determined using the µPADs in table salt and irrigation water samples. The measured concentrations of the samples correlated well with the values as found by the standard iodometric titration (no statistical difference at a 95% confidence level). The developed device is simple with easy fabrication and rapid operation. It is portable and cost-effective and therefore suitable for quality control of iodate levels in the manufacturing and agricultural industries.
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Microfluidic analysis with front-face fluorometric detection for the determination of total inorganic iodine in drinking water
(2018-01-01)
Inpota, Prawpan
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Strzelak, Kamil
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Koncki, Robert
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Sripumkhai, Wisaroot
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Jeamsaksiri, Wutthinan
A microfluidic method with front-face fluorometric detection was developed for the determination of total inorganic iodine in drinking water. A polydimethylsiloxane (PDMS) microfluidic device was employed in conjunction with the Sandell-Kolthoff reaction, in which iodide catalyzed the redox reaction between Ce(IV) and As(III). Direct alignment of an optical fiber attached to a spectrofluorometer was used as a convenient detector for remote front-face fluorometric detection. Trace inorganic iodine (IO <inf>3</inf> - and I - ) present naturally in drinking water was measured by on-line conversion of iodate to iodide for determination of total inorganic iodine. On-line conversion efficiency of iodate to iodide using the microfluidic device was investigated. Excellent conversion efficiency of 93 - 103% (%RSD = 1.6 - 11%) was obtained. Inorganic iodine concentrations in drinking water samples were measured, and the results obtained were in good agreement with those obtained by an ICP-MS method. Spiked sample recoveries were in the range of 86%(±5) - 128%(±8) (n = 12). Interference of various anions and cations were investigated with tolerance limit concentrations ranging from 10-6 to 2.5 M depending on the type of ions. The developed method is simple and convenient, and it is a green method for iodine analysis, as it greatly reduces the amount of toxic reagent consumed with reagent volumes in the microfluidic scale.
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Poly(vinyl alcohol) capped silver nanoparticles for antioxidant assay based on seed-mediated nanoparticle growth
(2017-08-01)
Teerasong, Saowapak
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Jinnarak, Amornrassamee
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Chaneam, Sumonmarn
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Wilairat, Prapin
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Nacapricha, Duangjai
A simple and rapid method for measurement of total antioxidant capacity (TAC) was developed. In this work, gallic acid was used as the antioxidant standard. Poly(vinyl alcohol) embedded silver nanoparticles (PVA-AgNPs) were employed as a colorimetric sensor. The detection principle was based on the seed-mediated nanoparticle growth technique. The PVA-AgNPs act as a catalyst in the reduction of Ag+ by gallic acid by providing nucleation seeds. Ag+ was reduced to Ag° and accumulated on the PVA-AgNP surface, leading to an increase in the size of particles. The absorbance of the colloidal solution was drastically enhanced with a small red shift. Under optimal conditions, a linear response was established between the change in absorbance and the TAC value expressed in terms of gallic acid equivalents. The linear range was from 25 to 200 μM with a detection limit of 22.1 μM. Satisfactory precision was obtained with % relative standard deviation (RSD) of 2.17. The developed sensor was successfully applied for TAC assessment of commercial ginger products. The PVA-AgNP sensor offers rapid analysis (within 5 min) compared to other nanoparticle-based antioxidant assays. Synthesis of the particles and assay involved less-toxic chemicals, and is therefore a “greener” method.

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