Analytica Chimica Acta 912 (2016) 97e104
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Microfluidic distillation chip for methanol concentration detection Yao-Nan Wang a, Chan-Chiung Liu b, Ruey-Jen Yang c, Wei-Jhong Ju c, Lung-Ming Fu d, e, * a
Department of Vehicle Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Department of Food Science, National Pingtung University of Science and Technology, Pingtung 912, Taiwan c Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan d Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan e Department of Biomechatronics Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An integrated microfluidic distillation system was developed for separating a mixed ethanol-methanol-water solution. A chromogenic detection process was proposed to measure the methanol concentration with high accuracy. An average methanol distillation efficiency of 97% was achieved using the integrated microfluidic distillation system.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2015 Received in revised form 22 January 2016 Accepted 26 January 2016 Available online 1 February 2016
An integrated microfluidic distillation system is proposed for separating a mixed ethanol-methanolwater solution into its constituent components. The microfluidic chip is fabricated using a CO2 laser system and comprises a serpentine channel, a boiling zone, a heating zone, and a cooled collection chamber filled with de-ionized (DI) water. In the proposed device, the ethanol-methanol-water solution is injected into the microfluidic chip and driven through the serpentine channel and into the collection chamber by means of a nitrogen carrier gas. Following the distillation process, the ethanol-methanol vapor flows into the collection chamber and condenses into the DI water. The resulting solution is removed from the collection tank and reacted with a mixed indicator. Finally, the methanol concentration is inversely derived from the absorbance measurements obtained using a spectrophotometer. The experimental results show the proposed microfluidic system achieves an average methanol distillation efficiency of 97%. The practicality of the proposed device is demonstrated by detecting the methanol concentrations of two commercial fruit wines. It is shown that the measured concentration values deviate by no more than 3% from those obtained using a conventional bench top system. © 2016 Elsevier B.V. All rights reserved.
Keywords: Distillation Methanol Ethanol Microfluidic
1. Introduction Methanol is formed as a natural byproduct when fermenting beverages with a high pectin content. However, methanol is toxic to
* Corresponding author. Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan. E-mail address: [email protected] (L.-M. Fu). http://dx.doi.org/10.1016/j.aca.2016.01.047 0003-2670/© 2016 Elsevier B.V. All rights reserved.
the human body, and thus its concentration must be carefully controlled [1e3]. According to the Department of Alcoholic Beverage Control Standards in Taiwan, the methanol content in wine must not exceed 2000 ppm, while that in fruit wines or distilled fruit spirits should be no higher than 3000 ppm. The most commonly used methods for methanol detection include gas chromatography (GC), potassium permanganate oxidation and methanol oxidase (MOX) [4e11]. Among these methods, GC has an
98
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
extremely high detection accuracy. However, the detection apparatus and operating materials are both expensive, and hence GC testing is cost-prohibitive for the general population and small businesses. Moreover, most wines and distilled spirits contain not only methanol, but also ethanol. Ethanol interferes with the methanol concentration measurement process, and must therefore be removed via distillation before detection is carried out. However, the distillation process requires the use of bulky, specialist equipment. Thus, for reasons of both expense and equipment constraints, the methanol detection process must be performed in a wellequipped laboratory. In practice, however, it is often desirable to perform methanol detection in a more local, self-administered manner (e.g., by health and nutrition practitioners, wine producers, and so on). Accordingly, this study develops a miniaturized system for methanol distillation to facilitate a cheaper and more convenient methanol detection process. Integrated microfluidic device technology enables the fabrication of microfluidic systems providing hundreds or more of functions, such as mixing, metering, trapping, reaction, filtering, separation and detection, on a single chip [12e17]. In this regard, integrated microfluidic device technology is analogous to electronic large-scale integration in the semiconductor industry or microelectromechanical systems (MEMS) field. The objective of integrated microfluidic chip systems is to realize automated standard laboratory processes and chemical/biochemical analyses on a single microfluidic platform [18e21]. Such platforms provide the potential for highly-parallelized and automated fluidic processes, and have many practical advantages, including a high throughput, precise metering, the possibility for automation, a shorter processing time, a lower power consumption, a greater portability, and reduced fabrication and operating costs. The literature contains many integrated microfluidic devices for performing liquid mixture separation and/or purification and solvent exchange [22e38]. Timmer et al. [22] presented a micro evaporation concentrator in which a hydrophobic vapor-permeable membrane allowed the passage of the gas components of the electrolyte solution under a forced convection effect, but trapped the analyte within the aqueous solution, thereby producing a concentration effect. Wootton and deMello [23] presented a microfluidic distillation system for the continuous purification of fluid streams. The proposed device comprised three sections, namely heating, condensation and separation, for separating volatile mixtures in accordance with differences in their vapor pressures. Cypes and Engstrom [24] presented a microfabricated stripping column (MFSC) for the removal of trace amounts of toluene from water via a convective mass transfer process through a porous silicon substrate under the effects of a nitrogen flow. Hibara et al. [25] presented a surface-modified microchip for gaseliquid chemical operations based on a microstructure with a deep main channel and a shallow side channel. In the fabrication process, a capillarity restricted modification (CARM) method was used to selectively pattern the microchip surface such that the shallow channel surface became hydrophobic, while the deep channel surface retained its original hydrophilic properties. It was shown that the difference in the surface properties of the two channels enabled the removal of dissolved oxygen in pure water across the channel boundary by means of the forced convection effect induced by a flow of nitrogen gas in the shallow channel. Kralj et al. [26] developed a microfluidic device for continuous flow liquideliquid phase separation incorporating a hydrophobic polytetrafluoroethylene (PTFE) membrane sandwiched between two microchannels. The experimental results showed that the device was capable of completely separating organiceaqueous and fluorouseaqueous liquideliquid systems, even given high fractions of partially-miscible compounds in the sample stream. Boyd et al.
[27] presented a separation method referred to as bubble-assisted interphase mass-transfer (BAIM), in which a small amount of heat was added close to the liquidevapor interface of captive gas bubbles in a microchannel. It was shown that the localized heating effect prompted a controlled mass-transfer through the bubble and was suitable for low-throughput applications such as biomolecule preconcentration and bioproduct recovery from single living cells. Hibara et al. [28] presented a microfluidic distillation system consisting of a hydrophilic-hydrophobic microchannel structure for gaseliquid separation and nanopillar structures with a radius of 270 nm for vapor condensation. Zhang et al. [29,30] designed a polymer-based multilayer microchip for methanol/water separation comprising a cooling channel, a separated liquid phase channel, and a vapor phase channel separated from the liquid channel by a PTFE membrane. Hartman et al. [31,32] proposed a multistep chemical synthesis method based on microfluidic distillation technology for performing liquidegas phase separation and masstransfer on a single microfluidic platform. Lam et al. [33e36] presented a multi-stage microfluidic distillation chip based on the heat pipe principle of counter-current liquid and vapor flows in the presence of a temperature gradient. To prevent flooding between the two flows, micropillar structures were fabricated on either side of the channel. The performance of the proposed device was evaluated by separating acetone-water and methanol-toluene mixtures, respectively. The results showed that a highly efficient (>95%) separation performance was obtained for both samples given an appropriate setting of the heating and cooling temperatures. Ju et al. [38] presented a miniaturized distillation system comprising a power control module and a carrier gas pressure control module for separating sulfurous acid (H2SO3) into sulfur dioxide (SO2) and water (H2O). In the proposed device, the gaseous SO2 was driven into a collection chamber filled with DI water and the SO2 concentration was deduced from the absorbance measurements obtained using a spectrophotometer. The experimental results showed that the device achieved a distillation efficiency as high as 94.6%. This study presents a polymethylmethacrylate (PMMA)-based micro-distillation chip for methanol detection purposes. The microchip is fabricated using a CO2 laser system, and comprises an inlet reservoir, a multi-stage distillation serpentine channel, and a cooled collection tank containing de-ionized (DI) water. In the proposed device, a mixed ethanol-methanol-water solution is injected into the inlet reservoir and driven through the serpentine microchannel by a nitrogen carrier gas. As the solution travels through the microchannel it passes repeatedly through boiling and heating regions, resulting in a multi-stage evaporation and condensation process. Finally, the vapor passes into the collection chamber, where it condenses and dissolves into the DI water. Following a chromogenic reaction with a mixed indicator, the methanol concentration is deduced from the absorbance measurements obtained using a spectrophotometer. 2. Fabrication and experimental details The proposed micro-distillation chip was designed using commercial AutoCAD (2010) software. The reservoir and microchannels were then patterned on a PMMA (General grade, Mai-Yu Co., Taiwan) substrate using a CO2 laser ablation system [39,40]. As shown in Fig. 1(a), the chip comprised an inlet port, a reservoir, a serpentine evaporation/condensation microchannel, a collection chamber, and two microfluidic valves (one on the inlet side and one on the outlet side). The device had overall dimensions of 115 mm 65 mm 8 mm (Fig. 1(b)). As shown in Fig. 1(c) and (d), the serpentine microchannel contained both a boiling zone and a heating zone and the collection chamber was positioned over a
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
99
Fig. 1. (a), (b), (c) Schematic illustrations showing arrangement of proposed micro-distillation device, and (d) photograph of proposed device.
cooling module. Fig. 2 presents an exploded view of the micro-distillation chip, showing all of its major components. Structurally, the device comprises two PMMA substrates with a thickness of 6 mm, one PMMA substrate with a thickness of 1.6 mm, and two aluminum back cover layers (one lower and one upper) with a thickness of 200 mm. The two microfluidic valves have a channel depth and width of 300 mm. On the inlet side, the valve connects the sample inlet port (ø ¼ 4 mm) with the reservoir (ø ¼ 16 mm), while on the outlet side, the valve connects the serpentine evaporation/ condensation channel with the collection tank (20 mm 8 mm 6 mm). In the distillation process, the inlet valve
prevents liquid flow from the reservoir/serpentine channel to the inlet port, while the outlet valve prevents condensed liquid flow from the serpentine channel to the collection tank. The serpentine channel comprises five inverted V-shaped channels and a single Ishaped channel. The channels have a width of 4 mm in the upper (heating) region and 2.2 mm in the lower (boiling) region. The serpentine channel has a depth of 6 mm and a total length of 495 mm. Each section of the channel (V-shaped or I-shaped) induces distillation of the injected sample. Consequently the sample undergoes six separate distillation processes as it is transported through the channel. The boiling zone and heating zone within each segment of the channel are separated by a temperature buffer
Fig. 2. Exploded view of micro-distillation device.
100
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
zone with a depth of 300 mm and a width (i.e., buffer distance) of 18 mm. The temperature within the boiling and heating zones is controlled by means of two heating layers beneath the chip (one upper layer with dimensions of 64 mm 34 mm 1 mm and one lower layer with dimensions of 64 mm 10 mm 1 mm) and one heating layer above the chip (dimensions of 70 mm 65 mm 1 mm). As shown in Fig. 3(a)e3(c), each heating layer comprises a temperature sensor and an ultra-thin flexible heater layer (TS020073B, KLC Co., Taiwan) sandwiched between two pieces of aluminum foil. During the distillation process, the temperatures of the front heating layer, back upper heating layer (corresponding to the heating zone) and back lower heating layer (corresponding to the boiling zone) are maintained at constant temperatures of 80 C, 80 C and 95 C, respectively, by means of a
power supply module. Finally, the collection chamber of the microfluidic chip is maintained at a constant temperature of approximately 8 C by passing chilled water through a cooling channel located beneath the chamber (see Fig. 2). Fig. 3(d) presents a schematic illustration of the fully-assembled device. 3. Materials and methods The sample reagents included methanol (CH4O), ethanol (C2H6O), potassium permanganate (KMnO4), oxalic acid (H2C2O4), basic fuchsin (C20H20N3HCl), hydrochloric acid (HCl), and DI water. The following samples and reagents were prepared: (1) mixed ethanol-methanol-water solutions (prepared by mixing methanol with concentrations ranging from 100 to 800 ppm, ethanol with
Fig. 3. (a), (b), (c) Heating layer consisting of temperature sensor and ultra-thin flexible heater sandwiched between two pieces of aluminum foil. (d) Schematic illustration of fullyassembled microfluidic distillation chip.
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
concentrations ranging from 5% to 13%, and DI water); (2) potassium permanganate solution (prepared by mixing 15% phosphoric acid, 3% potassium permanganate and DI water); (3) oxalic acid (prepared by mixing 5% oxalic acid, 5% sulfuric acid and DI water); and (4) basic fuchsin solution (prepared by mixing basic fuchsin (in 1% Na2SO3 and 1% H2SO4 solution) with DI water and then adding 1 N HCl solution). The details of the experimental methanol distillation and detection procedure are briefly summarized as follows: (1) 500 ml of ethanol-methanol-water solution was injected into the inlet reservoir using a syringe pump (KDS200, USA). (2) 500 ml of DI water was injected into the collection tank. (3) the inlet port was connected to a micro-gas valve, and carrier gas (N2) was flowed into the reservoir, causing the ethanol-methanol-water solution to pass continuously through the serpentine channel. The carrier gas N2 was setup to 10 ml/min which used the commercial high-pressure vessel and connected with a flow-rate meter. (4) After 10 min of heating, the ethanol-methanol-water solution reached the boiling point of 100 C, resulting in an evaporation process. The vapor was transported into the heating zone and partially condensed into its constituent components as a result of differences in their vapor points. Most of the methanol remained in a gaseous CH4O form and passed through the microfluidic valve into the collection chamber, where it subsequently condensed due to the cooler temperature (8 C) and dissolved into the DI water. However, the DI water and a portion of the ethanol condensed and flowed back into the heating zone region of the serpentine channel. (5) Following heating for an additional 20 min to allow for further distillation in the microchannel, the gaseous CH4O and DI water were extracted from the collection tank and blended with the mixed indicator in an eppendorf in order to induce a chromogenic reaction. (6) A mixture of reacted ethanol-methanol-water solution (1000 ml) and potassium permanganate solution (20 ml) was injected into a test tube and mixed mechanically. The tube was then placed in an ultrasonic cleaner (40 kHz, 1500DTH, BRANSON, USA) containing water at a temperature of 45 C for 15 min (7) Oxalic acid (200 ml) was added to the test tube and mixed mechanically. The tube was then placed in a tank of water at a temperature of 45 C for 6 min without ultrasonic agitation. (8) Fuchsin (200 ml) was added to the test tube and mixed mechanically. The tube was then cleaned ultrasonically in water at a temperature of 45 C for 15 min (9) The tube was left to cool to room temperature and the contents of the tube were then transferred to a cuvette. (10) The cuvette was inserted into the detection trough of a commercial UV spectrophotometer (595 nm, Model U-2000, Tokyo, Japan) in order to measure the corresponding absorption spectrum. A spectrophotometer is employed to measure the amount of light absorbed (called absorbance) by the sample solution. The absorbance is proportional to the concentration of solute in sample solution, which is quantitatively described by Bill-Lambert Law. The concentration of solute in sample solution is therefore determined by measuring light absorbance at particular wavelength via spectrophotometer. Fig. 4 shows the main steps in the distillation and methanol concentration detection process. (11) For reference purposes, the methanol concentrations were also evaluated using the by the CAAPIC (Center for Agriculture and Aquaculture Product Inspection and Certification, ISO/IEC 17025 Accreditation by Taiwan Accreditation Foundation) at National Pingtung University of Science and Technology in Taiwan using a traditional distillation system and a gas chromatography (GC) method. 4. Results and discussion Fig. 5(a) presents the absorbance values obtained for mixed methanol-ethanol-water solutions containing 100 ppme800 ppm
101
(methanol) and 5%e13% (ethanol). Note that the absorbance values represent the average values obtained over five separate tests performed using different samples. Note also that the results correspond to non-distilled samples. In other words, each sample was mixed to produce a chromogenic reaction and was then transferred to a cuvette and inserted into the detection trough of the spectrophotometer. It is seen that for a given methanol concentration, the absorbance value decreases as the ethanol concentration increases. In addition, it is observed that the absorbance values gradually converge toward a constant value as the ethanol content increases. The concentration of ethanol in spirits is typically 40e50%. Thus, it is extremely difficult to obtain accurate measurements of the methanol concentration since the absorbance value is dominated by the ethanol content and tends to have a constant value irrespective of changes in the methanol concentration. Consequently, to improve the accuracy of the methanol detection results, the ethanol content must be significantly reduced. The efficiency of the proposed micro-distillation chip was quantified as follows:
Distillation efficiency¼ ¼
Distilled number of moles of component Reference number of moles of component
Distilled component concentrationðppm=%Þ Reference component concentrationðppm=%Þ (1)
A mixture of ethanol-water solution was injected into the micro-distillation chip, and part of the ethanol was flowed into the collection tank via a suitable control of the distillation parameters. The ethanol concentration in the collected sample was then measured using a refractometer. To calibrate the micro-distillation chip, a series of preliminary tests were performed to measure the absorbance values of standard methanol-ethanol-water mixed solutions with methanol concentrations ranging from 300 to 800 ppm and an ethanol concentration of 50%. In each case, the methanol-ethanol-water solution was injected into the micro-distillation chip and distillation performed under temperature and time conditions carefully controlled so as to achieve a 25% ethanol concentration in the collection tank (as evaluated by a refractor using 250 ml of solution extracted from the tank) following the distillation process. The remaining 250 ml of liquid in the collection tank was injected into a test tube and diluted with 250 ml of DI. Thus, the methanol concentration was halved and the ethanol concentration reduced to 12.5%. The methanol concentrations of the diluted methanol-ethanol solutions were evaluated using the basic fuchsin-methanol detection method. The initial volume of DI water in the collection tank prior to the distillation process was 500 ml, but increased slightly following the distillation process due to the addition of methanol and ethanol condensate. (Note that the flow of DI water from the serpentine channel into the collection tank was prevented by the microfluidic valve on the exit side of the channel.) Suppose that the final volume of fluid in the collection tank after the distillation process is V ml, of which 500 ml is the volume of the original DI water, X ml is the volume of distilled methanol and Y ml is the volume of distilled ethanol. In other words, the total volume is equal to V ¼ 500 þ X þ Y. Suppose also that the known methanol concentration of the sample is 500 ppm. In other words, assuming a methanol distillation efficiency of 100%, the collection tank contains 0.25 ml of distilled methanol. Thus, the total volume of fluid in the collection tank is given by V ¼ 500 þ 0.25 þ Y. As stated above, the ethanol concentration in the collected sample is equal to 25%. In other words, the ethanol concentration can be expressed as Y/
102
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
Fig. 4. Main steps in methanol distillation and concentration detection process.
Fig. 5. Variation of absorbance value with (a) methanol concentration as function of ethanol concentration, and (b) methanol concentration for control group (dashed line, 12.5% ethanol) and experimental group (solid line).
V ¼ Y / (500.25 þ Y) ¼ 0.25. Solving the two equations simultaneously, it is found that Y ¼ 166.67 ml. In other words, the ethanol volume in the collection tank is equal to 166.67 ml. The methanol concentration in the collection tank can therefore be determined as follows: methanol concentration ¼ methanol volume/total collected solution volume. In other words, the methanol concentration is equal to 374.9 ppm (the real concentration in the collection tank). Suppose that the absorbance value of a hypothetical solution consisting of 374.6/2 ppm of methanol, 12.5% of ethanol, and DI water is measured. The methanol concentration value of this hypothetical solution should be the same as that of the diluted collected solution. Given the correspondence, it is not necessary to actually prepare solutions with 374.6/2 ppm of methanol. Rather, mixed solutions consisting of 300e800 ppm of methanol, 25% of ethanol, and water can be used directly. In other words, the distillation efficiency is 96%. The initial
ethanol volume in the collecting tank is 250 ml (50% ethanol concentration). Table 1 summarize the related experimental results. The results presented in Table 1 show that for mixed ethanolmethanol-water samples with methanol concentrations in the range of 400e700 ppm and an ethanol concentration of 50%, the proposed micro-distillation chip achieves a maximum distillation efficiency of 99.8% (700 ppm) and an average distillation efficiency of 97.9%. In other words, the effectiveness of the proposed distillation chip is confirmed. Fig. 5(b) shows the absorbance measurement data used to calculate the control and experimental values of the methanol concentration in Table 1. The feasibility of the proposed micro-distillation chip for realworld methanol detection applications was investigated using two commercial fruit wine samples, namely Sample 1 (Star fruit wine, Risin Co., Taiwan) and Sample 2 (Star fruit ice wine, Risin Co., Taiwan). According to Taiwan FDA (Food and Drug Administration)
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
103
Table 1 Methanol distillation efficiency for mixed methanol-ethanol-water solution samples with methanol concentrations in the range of 300e800 ppm and ethanol concentration of 50%. Concentration of methanol (control group) (ppm)
Concentration of methanol (experiment group) (ppm)
Distillation efficiency (%)
800 700 600 500 400 300 Average distillation efficiency
787 699 586 492 384 293
98.3 99.8 97.6 98.3 96 97.5 97.9
Table 2 Methanol concentration results obtained for two commercial fruit wines using proposed micro-distillation system and conventional bench top (Gas chromatography) system, respectively. Note that for each sample, the absorbance data represent the average values obtained over three separate measurements. Samples
Relative concentration (ppm)
Actual concentration (ppm)
NPUST CAAPIC detection (ppm)
Wine #1 Wine #2
944 500
964.25 515.46
974 527.3
Relative con.: relative concentration, real con. ¼ relative concentration/distillation efficiency, NPUST: National Pingtung University of Science and Technology, CAAPIC: Center for Agriculture and Aquaculture Product Inspection and Certification.
regulations, the methanol content in wine must be lower than 2000 ppm, while that in fruit wines or distilled fruit spirits must be lower than 3000 ppm. The two samples were distilled using the proposed micro-distillation system and the absorbance values of the reaction solutions were measured using the spectrophotometer. The corresponding methanol concentrations were found to be 944 ppm and 500 ppm, respectively (see Table 2). (Note that the reported concentration values represent the average values obtained over three separate measurements.) Thus, given a distillation efficiency of 97.9% (see Table 1), the two samples were determined to have actual methanol concentrations of 964.25 ppm and 515.46 ppm, respectively. Notably, the concentration measurements deviate from the measurement results obtained using a commercial bench top (Gas chromatography) system by no more than 3%. Consequently, the feasibility of the proposed device is confirmed.
300e800 ppm, the proposed microfluidic system achieves an average distillation efficiency of 97.9%. Moreover, it has been shown that the methanol concentration measurements obtained by the proposed system for two commercial fruit wines deviate by no more than 3% from the measurements obtained using a commercial bench top (Gas chromatography) system. In other words, the proposed system provides a feasible solution for performing methanol detection in a low-cost and convenient manner.
5. Conclusion
References
This paper has presented a microfluidic distillation chip for methanol detection purposes. The proposed chip is patterned on PMMA substrates using a commercial CO2 laser and comprises a serpentine channel with boiling and heating zones and a cooled collection chamber filled with DI water. In the distillation process, a mixed ethanol-methanol-water solution is transported continuously through the serpentine channel by a carrier gas (N2). As the solution passes through the channel, it is repeatedly evaporated in the boiling region (95 C) and condensed in the heating region (80 C). Due to the difference in the vapor points of the mixture, the DI water and a portion of the ethanol condense and are returned to the boiling region of the channel. By contrast, the methanol remains in vapor form and (together with the remaining ethanol vapor) is flowed into the collection chamber under the effects of the carrier gas. On entering the collection chamber, the vaporous methanol and ethanol condense into the DI water. Following a sufficient heating/distillation time (30 min), the solution is removed from the collection tank and reacted with a mixed indicator. Finally, the methanol concentration is inversely derived from the absorbance value obtained using a spectrophotometer. (Note that in evaluating the methanol concentration, the ethanol content is accounted for by means of a prior calibration experiment.) The experimental results have shown that for ethanol-methanol-water solutions with methanol concentrations in the range of
Acknowledgment The authors would like to thank the Ministry of Science and Technology of Taiwan for the financial support of this study under Grant Nos. MOST 103-2320-B-020-001-MY3, MOST 103-2221-E020-025-MY3, and MOST 103-2622-B-020 -007-CC2.
[1] K.E. McMartin, J.J. Ambre, T.R. Tephly, Methanol poisoning in human subjects: role for formic acid accumulation in the metabolic acidosis, Am. J. Med. 68 (1980) 414e418. [2] P. Marquardt, H.W.J. Werringloer, Toxicity of wine, Food Cosmet. Toxicol. 3 (1965) 803e810. [3] F. Bindler, E. Voges, P. Laugel, The problem of methanol concentration admissible in distilled fruit spirits, Food Addit. Contam. 5 (1988) 343e351. [4] C. Kee, C. Lai, X. Ge, M. Low, L.A. Ciolino, Partial conversion of methanol to formaldehyde in the gas chromatography injection port and reactivity with amines, J. Chromatogr. A 1257 (2012) 158e170. [5] C.Y. Zhang, N.B. Lin, X.S. Chai, Z. Li, D.G. Barnes, A rapid method for simultaneously determining ethanol and methanol content in wines by full evaporation headspace gas chromatography, Food Chem. 183 (2015) 169e172. [6] C. Plaziac, P. Lachapelle, C. Casanova, Effects of methanol on the retinal function of juvenile rats, Neuro Toxicol. 24 (2003) 255e260. [7] M.C. Wu, C.M. Jiang, S. Shen, H. Chang, Convenient quantification of methanol in juices by methanol oxidase in combination with basic fuchsin, Food Chem. 100 (2007) 412e418. [8] Y.N. Wang, R.J. Yang, W.J. Ju, M.C. Wu, L.M. Fu, Convenient quantification of methanol concentration detection utilizing an integrated microfluidic chip, Biomicrofluidics 6 (2012) 034111. [9] S.J. Kim, K. Kurabayashi, Uniform-temperature, microscale thermal modulator with area-adjusted air-gap isolation for comprehensive two-dimensional gas chromatography, Sens. Actuators B Chem. 181 (2013) 518e522. [10] S.J. Kim, S. Paczesny, S. Takayama, K. Kurabayashi, Preprogrammed, parallel on-chip immunoassay using system-level capillarity control, Anal. Chem. 85 (2013) 6902e6907. [11] L.M. Fu, W.J. Ju, R.J. Yang, Y.N. Wang, Integrated microfluidic array chip and LED photometer system for sulfur dioxide and methanol concentration detection, Chem. Eng. J. 243 (2014) 421e427. [12] S.H. Huang, S.K. Wang, H.S. Khoo, F.G. Tseng, AC electroosmotic generated inplane microvortices for stationary or continuous fluid mixing, Sens. Actuators
104
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
B Chem. 125 (2007) 326e336. [13] C.H. Lin, Y.N. Wang, L.M. Fu, Integrated microfluidic chip for rapid DNA digestion and time-resolved capillary electrophoresis analysis, Biomicrofluidics 6 (2012) 012818. [14] L.M. Fu, W.C. Fang, H.H. Hou, Y.N. Wang, T.F. Hong, Rapid vortex microfluidic mixer utilizing double-heart chamber, Chem. Eng. J. 249 (2014) 246e251. [15] P.H. Huang, Y. Xie, D.l Ahmed, J. Rufo, N. Nama, Y. Chen, C.Y. Chan, T.J. Huang, An acoustofluidic micromixer based on oscillating sidewall sharp-edges, Lab Chip 13 (2013) 3847e3852. [16] W.H. Chang, S.Y. Yang, C.H. Wang, M.A. Tsai, P.C. Wang, T.Y. Chen, S.C. Chen, G.B. Lee, Rapid isolation and detection of aquaculture pathogens in an integrated microfluidic system using loop-mediated isothermal amplification, Sens. Actuators B Chem. 180 (2013) 96e106. [17] Y.H. Lin, P.Y. Peng, Semiconductor sensor embedded microfluidic chip for proteinbiomarker detection using a bead-based immunoassay combined with deoxyribonucleic acid strand labeling, Anal. Chim. Acta 869 (2015) 34e42. €gli, Y.C. Chang, A. Homsy, L. Hvozdara, H.P. Herzig, N.F. de Rooij, [18] P. Wa Microfluidic droplet-based liquideliquid extraction and on-chip IR spectroscopy detection of cocaine in human saliva, Anal. Chem. 85 (2013) 7558e7565. [19] L.M. Fu, Y.N. Wang, C.C. Liu, An integrated microfluidic chip for formaldehyde analysis in Chinese herbs, Chem. Eng. J. 244 (2014) 422e428. [20] R. Liu, C. Zhang, M. Liu, Open bipolar electrode-electrochemiluminescence imaging sensing using paper-based microfluidics, Sens. Actuators B Chem. 216 (2015) 255e262. [21] K.L. Dornelas, N. Dossi, E. Piccin, A simple method for patterning poly(dimethylsiloxane) barriers in paper using contact-printing with low-cost rubber stamps, Anal. Chim. Acta 858 (2015) 82e90. [22] B.H. Timmer, K.M.V. Delft, W. Olthuis, P. Bergveld, A.V.D. Berg, Micro-evaporation electrolyte concentrator, Sens. Actuators B 91 (2003) 342e346. [23] R.C.R. Wootton, A.J. deMello, Continuous laminar evaporation: micron-scale distillation, Chem. Commun. (2004) 266e267. [24] S.H. Cypes, J.R. Engstrom, Analysis of a toluene stripping process: a comparison between a microfabricated stripping column and a conventional packed tower, Chem. Eng. J. 101 (2004) 49e56. [25] A. Hibara, S. Iwayama, S. Matsuoka, M. Ueno, Y. Kikutani, M. Tokeshi, T. Kitamori, Surface modification method of microchannels for gas-liquid twophase flow in microchips, Anal. Chem. 77 (2005) 943e947.
[26] J.G. Kralj, H.R. Sahoo, K.F. Jensen, Integrated continuous microfluidic liquideliquid extraction, Lab Chip 7 (2007) 256e263. [27] D.A. Boyd, J.R. Adleman, D.G. Goodwin, D. Psaltis, Chemical separations by bubble-assisted interphase mass-transfer, Anal. Chem. 80 (2008) 2452e2456. [28] A. Hibara, K. Toshin, T. Tsukahara, K. Mawatari, T. Kitamori, Microfluidic distillation utilizing micro-nano combined structure, Chem. Lett. 37 (2008) 1064e1065. [29] Y.P. Zhang, S.J. Kato, T. Anazawa, Vacuum membrane distillation on a microfluidic chip, Chem. Commun. (2009) 2750e2752. [30] Y.P. Zhang, S.J. Kato, T. Anazawa, Vacuum membrane distillation by microchip with temperature gradient, Lab Chip 10 (2010) 899e908. [31] R.L. Hartman, J.R. Naber, S.L. Buchwald, K.F. Jensen, Multistep microchemical synthesis enabled by microfluidic distillation, Angew. Chem. Int. Ed. 49 (2010) 899e903. [32] R.L. Hartman, H.R. Sahoo, B.C. Yen, K.F. Jensen, Distillation in microchemical systems using capillary forces and segmented flow, Lab Chip 9 (2009) 1843e1849. [33] K.F. Lam, E. Cao, E. Sorensen, A. Gavriilidis, Development of multistage distillation in a microfluidic chip, Lab Chip 11 (2011) 1311e1317. [34] K.F. Lam, E. Sorensen, A. Gavriilidis, Towards an understanding of the effects of operating conditions on separation by microfluidic distillation, Chem. Eng. Sci. 66 (2011) 2098e2106. [35] K.F. Lam, E. Sorensen, A. Gavriilidis, Review on gaseliquid separations in microchannel devices, Chem. Eng. Res. Des. 91 (2013) 1941e1953. [36] M. Foerster, K.F. Lam, E. Sorensen, A. Gavriilidis, In situ monitoring of microfluidic distillation, Chem. Eng. J. 227 (2013) 13e21. [37] T. Mansfeldt, H. Biernath, Determination of total cyanide in soils by microdistillation, Anal. Chim. Acta 406 (2000) 283e288. [38] W.J. Ju, L.M. Fu, R.J. Yang, C.L. Lee, Distillation and detection of SO2 using microfluidic chip, Lab Chip 12 (2012) 622e626. [39] T.F. Hong, W.J. Ju, M.C. Wu, C.H. Tsai, L.M. Fu, Rapid prototyping of PMMA microfluidic chips utilizing a CO2 laser, Microfluid. Nanofluid. 9 (2010) 1125e1133. [40] L.M. Fu, W.J. Ju, Y.N. Wang, R.J. Yang, Prototyping of glass-based microfluidic chips utilizing two-pass defocused CO2 laser beam method, Microfluid. Nanofluid. 14 (2013) 479e487.
Contents lists available at ScienceDirect
Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Microfluidic distillation chip for methanol concentration detection Yao-Nan Wang a, Chan-Chiung Liu b, Ruey-Jen Yang c, Wei-Jhong Ju c, Lung-Ming Fu d, e, * a
Department of Vehicle Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Department of Food Science, National Pingtung University of Science and Technology, Pingtung 912, Taiwan c Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan d Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan e Department of Biomechatronics Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An integrated microfluidic distillation system was developed for separating a mixed ethanol-methanol-water solution. A chromogenic detection process was proposed to measure the methanol concentration with high accuracy. An average methanol distillation efficiency of 97% was achieved using the integrated microfluidic distillation system.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2015 Received in revised form 22 January 2016 Accepted 26 January 2016 Available online 1 February 2016
An integrated microfluidic distillation system is proposed for separating a mixed ethanol-methanolwater solution into its constituent components. The microfluidic chip is fabricated using a CO2 laser system and comprises a serpentine channel, a boiling zone, a heating zone, and a cooled collection chamber filled with de-ionized (DI) water. In the proposed device, the ethanol-methanol-water solution is injected into the microfluidic chip and driven through the serpentine channel and into the collection chamber by means of a nitrogen carrier gas. Following the distillation process, the ethanol-methanol vapor flows into the collection chamber and condenses into the DI water. The resulting solution is removed from the collection tank and reacted with a mixed indicator. Finally, the methanol concentration is inversely derived from the absorbance measurements obtained using a spectrophotometer. The experimental results show the proposed microfluidic system achieves an average methanol distillation efficiency of 97%. The practicality of the proposed device is demonstrated by detecting the methanol concentrations of two commercial fruit wines. It is shown that the measured concentration values deviate by no more than 3% from those obtained using a conventional bench top system. © 2016 Elsevier B.V. All rights reserved.
Keywords: Distillation Methanol Ethanol Microfluidic
1. Introduction Methanol is formed as a natural byproduct when fermenting beverages with a high pectin content. However, methanol is toxic to
* Corresponding author. Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan. E-mail address: [email protected] (L.-M. Fu). http://dx.doi.org/10.1016/j.aca.2016.01.047 0003-2670/© 2016 Elsevier B.V. All rights reserved.
the human body, and thus its concentration must be carefully controlled [1e3]. According to the Department of Alcoholic Beverage Control Standards in Taiwan, the methanol content in wine must not exceed 2000 ppm, while that in fruit wines or distilled fruit spirits should be no higher than 3000 ppm. The most commonly used methods for methanol detection include gas chromatography (GC), potassium permanganate oxidation and methanol oxidase (MOX) [4e11]. Among these methods, GC has an
98
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
extremely high detection accuracy. However, the detection apparatus and operating materials are both expensive, and hence GC testing is cost-prohibitive for the general population and small businesses. Moreover, most wines and distilled spirits contain not only methanol, but also ethanol. Ethanol interferes with the methanol concentration measurement process, and must therefore be removed via distillation before detection is carried out. However, the distillation process requires the use of bulky, specialist equipment. Thus, for reasons of both expense and equipment constraints, the methanol detection process must be performed in a wellequipped laboratory. In practice, however, it is often desirable to perform methanol detection in a more local, self-administered manner (e.g., by health and nutrition practitioners, wine producers, and so on). Accordingly, this study develops a miniaturized system for methanol distillation to facilitate a cheaper and more convenient methanol detection process. Integrated microfluidic device technology enables the fabrication of microfluidic systems providing hundreds or more of functions, such as mixing, metering, trapping, reaction, filtering, separation and detection, on a single chip [12e17]. In this regard, integrated microfluidic device technology is analogous to electronic large-scale integration in the semiconductor industry or microelectromechanical systems (MEMS) field. The objective of integrated microfluidic chip systems is to realize automated standard laboratory processes and chemical/biochemical analyses on a single microfluidic platform [18e21]. Such platforms provide the potential for highly-parallelized and automated fluidic processes, and have many practical advantages, including a high throughput, precise metering, the possibility for automation, a shorter processing time, a lower power consumption, a greater portability, and reduced fabrication and operating costs. The literature contains many integrated microfluidic devices for performing liquid mixture separation and/or purification and solvent exchange [22e38]. Timmer et al. [22] presented a micro evaporation concentrator in which a hydrophobic vapor-permeable membrane allowed the passage of the gas components of the electrolyte solution under a forced convection effect, but trapped the analyte within the aqueous solution, thereby producing a concentration effect. Wootton and deMello [23] presented a microfluidic distillation system for the continuous purification of fluid streams. The proposed device comprised three sections, namely heating, condensation and separation, for separating volatile mixtures in accordance with differences in their vapor pressures. Cypes and Engstrom [24] presented a microfabricated stripping column (MFSC) for the removal of trace amounts of toluene from water via a convective mass transfer process through a porous silicon substrate under the effects of a nitrogen flow. Hibara et al. [25] presented a surface-modified microchip for gaseliquid chemical operations based on a microstructure with a deep main channel and a shallow side channel. In the fabrication process, a capillarity restricted modification (CARM) method was used to selectively pattern the microchip surface such that the shallow channel surface became hydrophobic, while the deep channel surface retained its original hydrophilic properties. It was shown that the difference in the surface properties of the two channels enabled the removal of dissolved oxygen in pure water across the channel boundary by means of the forced convection effect induced by a flow of nitrogen gas in the shallow channel. Kralj et al. [26] developed a microfluidic device for continuous flow liquideliquid phase separation incorporating a hydrophobic polytetrafluoroethylene (PTFE) membrane sandwiched between two microchannels. The experimental results showed that the device was capable of completely separating organiceaqueous and fluorouseaqueous liquideliquid systems, even given high fractions of partially-miscible compounds in the sample stream. Boyd et al.
[27] presented a separation method referred to as bubble-assisted interphase mass-transfer (BAIM), in which a small amount of heat was added close to the liquidevapor interface of captive gas bubbles in a microchannel. It was shown that the localized heating effect prompted a controlled mass-transfer through the bubble and was suitable for low-throughput applications such as biomolecule preconcentration and bioproduct recovery from single living cells. Hibara et al. [28] presented a microfluidic distillation system consisting of a hydrophilic-hydrophobic microchannel structure for gaseliquid separation and nanopillar structures with a radius of 270 nm for vapor condensation. Zhang et al. [29,30] designed a polymer-based multilayer microchip for methanol/water separation comprising a cooling channel, a separated liquid phase channel, and a vapor phase channel separated from the liquid channel by a PTFE membrane. Hartman et al. [31,32] proposed a multistep chemical synthesis method based on microfluidic distillation technology for performing liquidegas phase separation and masstransfer on a single microfluidic platform. Lam et al. [33e36] presented a multi-stage microfluidic distillation chip based on the heat pipe principle of counter-current liquid and vapor flows in the presence of a temperature gradient. To prevent flooding between the two flows, micropillar structures were fabricated on either side of the channel. The performance of the proposed device was evaluated by separating acetone-water and methanol-toluene mixtures, respectively. The results showed that a highly efficient (>95%) separation performance was obtained for both samples given an appropriate setting of the heating and cooling temperatures. Ju et al. [38] presented a miniaturized distillation system comprising a power control module and a carrier gas pressure control module for separating sulfurous acid (H2SO3) into sulfur dioxide (SO2) and water (H2O). In the proposed device, the gaseous SO2 was driven into a collection chamber filled with DI water and the SO2 concentration was deduced from the absorbance measurements obtained using a spectrophotometer. The experimental results showed that the device achieved a distillation efficiency as high as 94.6%. This study presents a polymethylmethacrylate (PMMA)-based micro-distillation chip for methanol detection purposes. The microchip is fabricated using a CO2 laser system, and comprises an inlet reservoir, a multi-stage distillation serpentine channel, and a cooled collection tank containing de-ionized (DI) water. In the proposed device, a mixed ethanol-methanol-water solution is injected into the inlet reservoir and driven through the serpentine microchannel by a nitrogen carrier gas. As the solution travels through the microchannel it passes repeatedly through boiling and heating regions, resulting in a multi-stage evaporation and condensation process. Finally, the vapor passes into the collection chamber, where it condenses and dissolves into the DI water. Following a chromogenic reaction with a mixed indicator, the methanol concentration is deduced from the absorbance measurements obtained using a spectrophotometer. 2. Fabrication and experimental details The proposed micro-distillation chip was designed using commercial AutoCAD (2010) software. The reservoir and microchannels were then patterned on a PMMA (General grade, Mai-Yu Co., Taiwan) substrate using a CO2 laser ablation system [39,40]. As shown in Fig. 1(a), the chip comprised an inlet port, a reservoir, a serpentine evaporation/condensation microchannel, a collection chamber, and two microfluidic valves (one on the inlet side and one on the outlet side). The device had overall dimensions of 115 mm 65 mm 8 mm (Fig. 1(b)). As shown in Fig. 1(c) and (d), the serpentine microchannel contained both a boiling zone and a heating zone and the collection chamber was positioned over a
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
99
Fig. 1. (a), (b), (c) Schematic illustrations showing arrangement of proposed micro-distillation device, and (d) photograph of proposed device.
cooling module. Fig. 2 presents an exploded view of the micro-distillation chip, showing all of its major components. Structurally, the device comprises two PMMA substrates with a thickness of 6 mm, one PMMA substrate with a thickness of 1.6 mm, and two aluminum back cover layers (one lower and one upper) with a thickness of 200 mm. The two microfluidic valves have a channel depth and width of 300 mm. On the inlet side, the valve connects the sample inlet port (ø ¼ 4 mm) with the reservoir (ø ¼ 16 mm), while on the outlet side, the valve connects the serpentine evaporation/ condensation channel with the collection tank (20 mm 8 mm 6 mm). In the distillation process, the inlet valve
prevents liquid flow from the reservoir/serpentine channel to the inlet port, while the outlet valve prevents condensed liquid flow from the serpentine channel to the collection tank. The serpentine channel comprises five inverted V-shaped channels and a single Ishaped channel. The channels have a width of 4 mm in the upper (heating) region and 2.2 mm in the lower (boiling) region. The serpentine channel has a depth of 6 mm and a total length of 495 mm. Each section of the channel (V-shaped or I-shaped) induces distillation of the injected sample. Consequently the sample undergoes six separate distillation processes as it is transported through the channel. The boiling zone and heating zone within each segment of the channel are separated by a temperature buffer
Fig. 2. Exploded view of micro-distillation device.
100
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
zone with a depth of 300 mm and a width (i.e., buffer distance) of 18 mm. The temperature within the boiling and heating zones is controlled by means of two heating layers beneath the chip (one upper layer with dimensions of 64 mm 34 mm 1 mm and one lower layer with dimensions of 64 mm 10 mm 1 mm) and one heating layer above the chip (dimensions of 70 mm 65 mm 1 mm). As shown in Fig. 3(a)e3(c), each heating layer comprises a temperature sensor and an ultra-thin flexible heater layer (TS020073B, KLC Co., Taiwan) sandwiched between two pieces of aluminum foil. During the distillation process, the temperatures of the front heating layer, back upper heating layer (corresponding to the heating zone) and back lower heating layer (corresponding to the boiling zone) are maintained at constant temperatures of 80 C, 80 C and 95 C, respectively, by means of a
power supply module. Finally, the collection chamber of the microfluidic chip is maintained at a constant temperature of approximately 8 C by passing chilled water through a cooling channel located beneath the chamber (see Fig. 2). Fig. 3(d) presents a schematic illustration of the fully-assembled device. 3. Materials and methods The sample reagents included methanol (CH4O), ethanol (C2H6O), potassium permanganate (KMnO4), oxalic acid (H2C2O4), basic fuchsin (C20H20N3HCl), hydrochloric acid (HCl), and DI water. The following samples and reagents were prepared: (1) mixed ethanol-methanol-water solutions (prepared by mixing methanol with concentrations ranging from 100 to 800 ppm, ethanol with
Fig. 3. (a), (b), (c) Heating layer consisting of temperature sensor and ultra-thin flexible heater sandwiched between two pieces of aluminum foil. (d) Schematic illustration of fullyassembled microfluidic distillation chip.
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
concentrations ranging from 5% to 13%, and DI water); (2) potassium permanganate solution (prepared by mixing 15% phosphoric acid, 3% potassium permanganate and DI water); (3) oxalic acid (prepared by mixing 5% oxalic acid, 5% sulfuric acid and DI water); and (4) basic fuchsin solution (prepared by mixing basic fuchsin (in 1% Na2SO3 and 1% H2SO4 solution) with DI water and then adding 1 N HCl solution). The details of the experimental methanol distillation and detection procedure are briefly summarized as follows: (1) 500 ml of ethanol-methanol-water solution was injected into the inlet reservoir using a syringe pump (KDS200, USA). (2) 500 ml of DI water was injected into the collection tank. (3) the inlet port was connected to a micro-gas valve, and carrier gas (N2) was flowed into the reservoir, causing the ethanol-methanol-water solution to pass continuously through the serpentine channel. The carrier gas N2 was setup to 10 ml/min which used the commercial high-pressure vessel and connected with a flow-rate meter. (4) After 10 min of heating, the ethanol-methanol-water solution reached the boiling point of 100 C, resulting in an evaporation process. The vapor was transported into the heating zone and partially condensed into its constituent components as a result of differences in their vapor points. Most of the methanol remained in a gaseous CH4O form and passed through the microfluidic valve into the collection chamber, where it subsequently condensed due to the cooler temperature (8 C) and dissolved into the DI water. However, the DI water and a portion of the ethanol condensed and flowed back into the heating zone region of the serpentine channel. (5) Following heating for an additional 20 min to allow for further distillation in the microchannel, the gaseous CH4O and DI water were extracted from the collection tank and blended with the mixed indicator in an eppendorf in order to induce a chromogenic reaction. (6) A mixture of reacted ethanol-methanol-water solution (1000 ml) and potassium permanganate solution (20 ml) was injected into a test tube and mixed mechanically. The tube was then placed in an ultrasonic cleaner (40 kHz, 1500DTH, BRANSON, USA) containing water at a temperature of 45 C for 15 min (7) Oxalic acid (200 ml) was added to the test tube and mixed mechanically. The tube was then placed in a tank of water at a temperature of 45 C for 6 min without ultrasonic agitation. (8) Fuchsin (200 ml) was added to the test tube and mixed mechanically. The tube was then cleaned ultrasonically in water at a temperature of 45 C for 15 min (9) The tube was left to cool to room temperature and the contents of the tube were then transferred to a cuvette. (10) The cuvette was inserted into the detection trough of a commercial UV spectrophotometer (595 nm, Model U-2000, Tokyo, Japan) in order to measure the corresponding absorption spectrum. A spectrophotometer is employed to measure the amount of light absorbed (called absorbance) by the sample solution. The absorbance is proportional to the concentration of solute in sample solution, which is quantitatively described by Bill-Lambert Law. The concentration of solute in sample solution is therefore determined by measuring light absorbance at particular wavelength via spectrophotometer. Fig. 4 shows the main steps in the distillation and methanol concentration detection process. (11) For reference purposes, the methanol concentrations were also evaluated using the by the CAAPIC (Center for Agriculture and Aquaculture Product Inspection and Certification, ISO/IEC 17025 Accreditation by Taiwan Accreditation Foundation) at National Pingtung University of Science and Technology in Taiwan using a traditional distillation system and a gas chromatography (GC) method. 4. Results and discussion Fig. 5(a) presents the absorbance values obtained for mixed methanol-ethanol-water solutions containing 100 ppme800 ppm
101
(methanol) and 5%e13% (ethanol). Note that the absorbance values represent the average values obtained over five separate tests performed using different samples. Note also that the results correspond to non-distilled samples. In other words, each sample was mixed to produce a chromogenic reaction and was then transferred to a cuvette and inserted into the detection trough of the spectrophotometer. It is seen that for a given methanol concentration, the absorbance value decreases as the ethanol concentration increases. In addition, it is observed that the absorbance values gradually converge toward a constant value as the ethanol content increases. The concentration of ethanol in spirits is typically 40e50%. Thus, it is extremely difficult to obtain accurate measurements of the methanol concentration since the absorbance value is dominated by the ethanol content and tends to have a constant value irrespective of changes in the methanol concentration. Consequently, to improve the accuracy of the methanol detection results, the ethanol content must be significantly reduced. The efficiency of the proposed micro-distillation chip was quantified as follows:
Distillation efficiency¼ ¼
Distilled number of moles of component Reference number of moles of component
Distilled component concentrationðppm=%Þ Reference component concentrationðppm=%Þ (1)
A mixture of ethanol-water solution was injected into the micro-distillation chip, and part of the ethanol was flowed into the collection tank via a suitable control of the distillation parameters. The ethanol concentration in the collected sample was then measured using a refractometer. To calibrate the micro-distillation chip, a series of preliminary tests were performed to measure the absorbance values of standard methanol-ethanol-water mixed solutions with methanol concentrations ranging from 300 to 800 ppm and an ethanol concentration of 50%. In each case, the methanol-ethanol-water solution was injected into the micro-distillation chip and distillation performed under temperature and time conditions carefully controlled so as to achieve a 25% ethanol concentration in the collection tank (as evaluated by a refractor using 250 ml of solution extracted from the tank) following the distillation process. The remaining 250 ml of liquid in the collection tank was injected into a test tube and diluted with 250 ml of DI. Thus, the methanol concentration was halved and the ethanol concentration reduced to 12.5%. The methanol concentrations of the diluted methanol-ethanol solutions were evaluated using the basic fuchsin-methanol detection method. The initial volume of DI water in the collection tank prior to the distillation process was 500 ml, but increased slightly following the distillation process due to the addition of methanol and ethanol condensate. (Note that the flow of DI water from the serpentine channel into the collection tank was prevented by the microfluidic valve on the exit side of the channel.) Suppose that the final volume of fluid in the collection tank after the distillation process is V ml, of which 500 ml is the volume of the original DI water, X ml is the volume of distilled methanol and Y ml is the volume of distilled ethanol. In other words, the total volume is equal to V ¼ 500 þ X þ Y. Suppose also that the known methanol concentration of the sample is 500 ppm. In other words, assuming a methanol distillation efficiency of 100%, the collection tank contains 0.25 ml of distilled methanol. Thus, the total volume of fluid in the collection tank is given by V ¼ 500 þ 0.25 þ Y. As stated above, the ethanol concentration in the collected sample is equal to 25%. In other words, the ethanol concentration can be expressed as Y/
102
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
Fig. 4. Main steps in methanol distillation and concentration detection process.
Fig. 5. Variation of absorbance value with (a) methanol concentration as function of ethanol concentration, and (b) methanol concentration for control group (dashed line, 12.5% ethanol) and experimental group (solid line).
V ¼ Y / (500.25 þ Y) ¼ 0.25. Solving the two equations simultaneously, it is found that Y ¼ 166.67 ml. In other words, the ethanol volume in the collection tank is equal to 166.67 ml. The methanol concentration in the collection tank can therefore be determined as follows: methanol concentration ¼ methanol volume/total collected solution volume. In other words, the methanol concentration is equal to 374.9 ppm (the real concentration in the collection tank). Suppose that the absorbance value of a hypothetical solution consisting of 374.6/2 ppm of methanol, 12.5% of ethanol, and DI water is measured. The methanol concentration value of this hypothetical solution should be the same as that of the diluted collected solution. Given the correspondence, it is not necessary to actually prepare solutions with 374.6/2 ppm of methanol. Rather, mixed solutions consisting of 300e800 ppm of methanol, 25% of ethanol, and water can be used directly. In other words, the distillation efficiency is 96%. The initial
ethanol volume in the collecting tank is 250 ml (50% ethanol concentration). Table 1 summarize the related experimental results. The results presented in Table 1 show that for mixed ethanolmethanol-water samples with methanol concentrations in the range of 400e700 ppm and an ethanol concentration of 50%, the proposed micro-distillation chip achieves a maximum distillation efficiency of 99.8% (700 ppm) and an average distillation efficiency of 97.9%. In other words, the effectiveness of the proposed distillation chip is confirmed. Fig. 5(b) shows the absorbance measurement data used to calculate the control and experimental values of the methanol concentration in Table 1. The feasibility of the proposed micro-distillation chip for realworld methanol detection applications was investigated using two commercial fruit wine samples, namely Sample 1 (Star fruit wine, Risin Co., Taiwan) and Sample 2 (Star fruit ice wine, Risin Co., Taiwan). According to Taiwan FDA (Food and Drug Administration)
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
103
Table 1 Methanol distillation efficiency for mixed methanol-ethanol-water solution samples with methanol concentrations in the range of 300e800 ppm and ethanol concentration of 50%. Concentration of methanol (control group) (ppm)
Concentration of methanol (experiment group) (ppm)
Distillation efficiency (%)
800 700 600 500 400 300 Average distillation efficiency
787 699 586 492 384 293
98.3 99.8 97.6 98.3 96 97.5 97.9
Table 2 Methanol concentration results obtained for two commercial fruit wines using proposed micro-distillation system and conventional bench top (Gas chromatography) system, respectively. Note that for each sample, the absorbance data represent the average values obtained over three separate measurements. Samples
Relative concentration (ppm)
Actual concentration (ppm)
NPUST CAAPIC detection (ppm)
Wine #1 Wine #2
944 500
964.25 515.46
974 527.3
Relative con.: relative concentration, real con. ¼ relative concentration/distillation efficiency, NPUST: National Pingtung University of Science and Technology, CAAPIC: Center for Agriculture and Aquaculture Product Inspection and Certification.
regulations, the methanol content in wine must be lower than 2000 ppm, while that in fruit wines or distilled fruit spirits must be lower than 3000 ppm. The two samples were distilled using the proposed micro-distillation system and the absorbance values of the reaction solutions were measured using the spectrophotometer. The corresponding methanol concentrations were found to be 944 ppm and 500 ppm, respectively (see Table 2). (Note that the reported concentration values represent the average values obtained over three separate measurements.) Thus, given a distillation efficiency of 97.9% (see Table 1), the two samples were determined to have actual methanol concentrations of 964.25 ppm and 515.46 ppm, respectively. Notably, the concentration measurements deviate from the measurement results obtained using a commercial bench top (Gas chromatography) system by no more than 3%. Consequently, the feasibility of the proposed device is confirmed.
300e800 ppm, the proposed microfluidic system achieves an average distillation efficiency of 97.9%. Moreover, it has been shown that the methanol concentration measurements obtained by the proposed system for two commercial fruit wines deviate by no more than 3% from the measurements obtained using a commercial bench top (Gas chromatography) system. In other words, the proposed system provides a feasible solution for performing methanol detection in a low-cost and convenient manner.
5. Conclusion
References
This paper has presented a microfluidic distillation chip for methanol detection purposes. The proposed chip is patterned on PMMA substrates using a commercial CO2 laser and comprises a serpentine channel with boiling and heating zones and a cooled collection chamber filled with DI water. In the distillation process, a mixed ethanol-methanol-water solution is transported continuously through the serpentine channel by a carrier gas (N2). As the solution passes through the channel, it is repeatedly evaporated in the boiling region (95 C) and condensed in the heating region (80 C). Due to the difference in the vapor points of the mixture, the DI water and a portion of the ethanol condense and are returned to the boiling region of the channel. By contrast, the methanol remains in vapor form and (together with the remaining ethanol vapor) is flowed into the collection chamber under the effects of the carrier gas. On entering the collection chamber, the vaporous methanol and ethanol condense into the DI water. Following a sufficient heating/distillation time (30 min), the solution is removed from the collection tank and reacted with a mixed indicator. Finally, the methanol concentration is inversely derived from the absorbance value obtained using a spectrophotometer. (Note that in evaluating the methanol concentration, the ethanol content is accounted for by means of a prior calibration experiment.) The experimental results have shown that for ethanol-methanol-water solutions with methanol concentrations in the range of
Acknowledgment The authors would like to thank the Ministry of Science and Technology of Taiwan for the financial support of this study under Grant Nos. MOST 103-2320-B-020-001-MY3, MOST 103-2221-E020-025-MY3, and MOST 103-2622-B-020 -007-CC2.
[1] K.E. McMartin, J.J. Ambre, T.R. Tephly, Methanol poisoning in human subjects: role for formic acid accumulation in the metabolic acidosis, Am. J. Med. 68 (1980) 414e418. [2] P. Marquardt, H.W.J. Werringloer, Toxicity of wine, Food Cosmet. Toxicol. 3 (1965) 803e810. [3] F. Bindler, E. Voges, P. Laugel, The problem of methanol concentration admissible in distilled fruit spirits, Food Addit. Contam. 5 (1988) 343e351. [4] C. Kee, C. Lai, X. Ge, M. Low, L.A. Ciolino, Partial conversion of methanol to formaldehyde in the gas chromatography injection port and reactivity with amines, J. Chromatogr. A 1257 (2012) 158e170. [5] C.Y. Zhang, N.B. Lin, X.S. Chai, Z. Li, D.G. Barnes, A rapid method for simultaneously determining ethanol and methanol content in wines by full evaporation headspace gas chromatography, Food Chem. 183 (2015) 169e172. [6] C. Plaziac, P. Lachapelle, C. Casanova, Effects of methanol on the retinal function of juvenile rats, Neuro Toxicol. 24 (2003) 255e260. [7] M.C. Wu, C.M. Jiang, S. Shen, H. Chang, Convenient quantification of methanol in juices by methanol oxidase in combination with basic fuchsin, Food Chem. 100 (2007) 412e418. [8] Y.N. Wang, R.J. Yang, W.J. Ju, M.C. Wu, L.M. Fu, Convenient quantification of methanol concentration detection utilizing an integrated microfluidic chip, Biomicrofluidics 6 (2012) 034111. [9] S.J. Kim, K. Kurabayashi, Uniform-temperature, microscale thermal modulator with area-adjusted air-gap isolation for comprehensive two-dimensional gas chromatography, Sens. Actuators B Chem. 181 (2013) 518e522. [10] S.J. Kim, S. Paczesny, S. Takayama, K. Kurabayashi, Preprogrammed, parallel on-chip immunoassay using system-level capillarity control, Anal. Chem. 85 (2013) 6902e6907. [11] L.M. Fu, W.J. Ju, R.J. Yang, Y.N. Wang, Integrated microfluidic array chip and LED photometer system for sulfur dioxide and methanol concentration detection, Chem. Eng. J. 243 (2014) 421e427. [12] S.H. Huang, S.K. Wang, H.S. Khoo, F.G. Tseng, AC electroosmotic generated inplane microvortices for stationary or continuous fluid mixing, Sens. Actuators
104
Y.-N. Wang et al. / Analytica Chimica Acta 912 (2016) 97e104
B Chem. 125 (2007) 326e336. [13] C.H. Lin, Y.N. Wang, L.M. Fu, Integrated microfluidic chip for rapid DNA digestion and time-resolved capillary electrophoresis analysis, Biomicrofluidics 6 (2012) 012818. [14] L.M. Fu, W.C. Fang, H.H. Hou, Y.N. Wang, T.F. Hong, Rapid vortex microfluidic mixer utilizing double-heart chamber, Chem. Eng. J. 249 (2014) 246e251. [15] P.H. Huang, Y. Xie, D.l Ahmed, J. Rufo, N. Nama, Y. Chen, C.Y. Chan, T.J. Huang, An acoustofluidic micromixer based on oscillating sidewall sharp-edges, Lab Chip 13 (2013) 3847e3852. [16] W.H. Chang, S.Y. Yang, C.H. Wang, M.A. Tsai, P.C. Wang, T.Y. Chen, S.C. Chen, G.B. Lee, Rapid isolation and detection of aquaculture pathogens in an integrated microfluidic system using loop-mediated isothermal amplification, Sens. Actuators B Chem. 180 (2013) 96e106. [17] Y.H. Lin, P.Y. Peng, Semiconductor sensor embedded microfluidic chip for proteinbiomarker detection using a bead-based immunoassay combined with deoxyribonucleic acid strand labeling, Anal. Chim. Acta 869 (2015) 34e42. €gli, Y.C. Chang, A. Homsy, L. Hvozdara, H.P. Herzig, N.F. de Rooij, [18] P. Wa Microfluidic droplet-based liquideliquid extraction and on-chip IR spectroscopy detection of cocaine in human saliva, Anal. Chem. 85 (2013) 7558e7565. [19] L.M. Fu, Y.N. Wang, C.C. Liu, An integrated microfluidic chip for formaldehyde analysis in Chinese herbs, Chem. Eng. J. 244 (2014) 422e428. [20] R. Liu, C. Zhang, M. Liu, Open bipolar electrode-electrochemiluminescence imaging sensing using paper-based microfluidics, Sens. Actuators B Chem. 216 (2015) 255e262. [21] K.L. Dornelas, N. Dossi, E. Piccin, A simple method for patterning poly(dimethylsiloxane) barriers in paper using contact-printing with low-cost rubber stamps, Anal. Chim. Acta 858 (2015) 82e90. [22] B.H. Timmer, K.M.V. Delft, W. Olthuis, P. Bergveld, A.V.D. Berg, Micro-evaporation electrolyte concentrator, Sens. Actuators B 91 (2003) 342e346. [23] R.C.R. Wootton, A.J. deMello, Continuous laminar evaporation: micron-scale distillation, Chem. Commun. (2004) 266e267. [24] S.H. Cypes, J.R. Engstrom, Analysis of a toluene stripping process: a comparison between a microfabricated stripping column and a conventional packed tower, Chem. Eng. J. 101 (2004) 49e56. [25] A. Hibara, S. Iwayama, S. Matsuoka, M. Ueno, Y. Kikutani, M. Tokeshi, T. Kitamori, Surface modification method of microchannels for gas-liquid twophase flow in microchips, Anal. Chem. 77 (2005) 943e947.
[26] J.G. Kralj, H.R. Sahoo, K.F. Jensen, Integrated continuous microfluidic liquideliquid extraction, Lab Chip 7 (2007) 256e263. [27] D.A. Boyd, J.R. Adleman, D.G. Goodwin, D. Psaltis, Chemical separations by bubble-assisted interphase mass-transfer, Anal. Chem. 80 (2008) 2452e2456. [28] A. Hibara, K. Toshin, T. Tsukahara, K. Mawatari, T. Kitamori, Microfluidic distillation utilizing micro-nano combined structure, Chem. Lett. 37 (2008) 1064e1065. [29] Y.P. Zhang, S.J. Kato, T. Anazawa, Vacuum membrane distillation on a microfluidic chip, Chem. Commun. (2009) 2750e2752. [30] Y.P. Zhang, S.J. Kato, T. Anazawa, Vacuum membrane distillation by microchip with temperature gradient, Lab Chip 10 (2010) 899e908. [31] R.L. Hartman, J.R. Naber, S.L. Buchwald, K.F. Jensen, Multistep microchemical synthesis enabled by microfluidic distillation, Angew. Chem. Int. Ed. 49 (2010) 899e903. [32] R.L. Hartman, H.R. Sahoo, B.C. Yen, K.F. Jensen, Distillation in microchemical systems using capillary forces and segmented flow, Lab Chip 9 (2009) 1843e1849. [33] K.F. Lam, E. Cao, E. Sorensen, A. Gavriilidis, Development of multistage distillation in a microfluidic chip, Lab Chip 11 (2011) 1311e1317. [34] K.F. Lam, E. Sorensen, A. Gavriilidis, Towards an understanding of the effects of operating conditions on separation by microfluidic distillation, Chem. Eng. Sci. 66 (2011) 2098e2106. [35] K.F. Lam, E. Sorensen, A. Gavriilidis, Review on gaseliquid separations in microchannel devices, Chem. Eng. Res. Des. 91 (2013) 1941e1953. [36] M. Foerster, K.F. Lam, E. Sorensen, A. Gavriilidis, In situ monitoring of microfluidic distillation, Chem. Eng. J. 227 (2013) 13e21. [37] T. Mansfeldt, H. Biernath, Determination of total cyanide in soils by microdistillation, Anal. Chim. Acta 406 (2000) 283e288. [38] W.J. Ju, L.M. Fu, R.J. Yang, C.L. Lee, Distillation and detection of SO2 using microfluidic chip, Lab Chip 12 (2012) 622e626. [39] T.F. Hong, W.J. Ju, M.C. Wu, C.H. Tsai, L.M. Fu, Rapid prototyping of PMMA microfluidic chips utilizing a CO2 laser, Microfluid. Nanofluid. 9 (2010) 1125e1133. [40] L.M. Fu, W.J. Ju, Y.N. Wang, R.J. Yang, Prototyping of glass-based microfluidic chips utilizing two-pass defocused CO2 laser beam method, Microfluid. Nanofluid. 14 (2013) 479e487.
