- Email: [email protected]
A microfluidic device with ion-exchange preconcentration column and photometric detection with Schlieren effect correction
Microchemical Journal 132 (2017) 161–166
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
A microfluidic device with ion-exchange preconcentration column and photometric detection with Schlieren effect correction Laiz de Oliveira Magalhães, Alexandre Fonseca ⁎ Instituto de Química, Universidade de Brasília, Cx Postal 4478, CEP 70910-900 Brasília, DF, Brazil
a r t i c l e
i n f o
Article history: Received 11 October 2016 Received in revised form 30 January 2017 Accepted 30 January 2017 Available online 01 February 2017 Keywords: Microfluidics Flow analysis Schlieren effect Preconcentration
a b s t r a c t This work describes the construction and evaluation of a microflow analyzer containing a Dowex® 1X8 packed column and LED-based photometric detection with Schlieren effect correction. Flow structures, including a reservoir for the preconcentration column with dimensions 25.0 × 2.0 × 1.0 mm (l × w × h), were manufactured in a urethane–acrylate resin, and the ion-exchange particles (50–100 mesh) were manually packed before sealing the device. Photometric measurements were performed using an integrated flow cell with a 5.0 mm optical path by using optical fibers to guide the radiation from a 3.0 W RGB LED to the microfluidic device and from the microdevice to a photodiode (OPT-101). It was demonstrated that the Schlieren effect is appropriately corrected by subtracting the signals for two diodes of the RGB LED and, by setting the intensity of light sources before measurements, the discordant response to the effect at different wavelengths is compensated. The preconcentration column was easily and successfully integrated into the microfluidic device without any clogging or leaking of the working solutions, and swelling of the packing particles caused no damage to the sealed structures because of the elastomeric nature of the microfluidic substrate. The application of the proposed device to the determination of nitrite in fresh water provides a linear response (R2 = 0.99) from 0.05 to 0.20 mg NO− 2 L−1 and an approximate increase in concentration by four fold was estimated when 3.80 mL of the sample/ standard solutions is percolated through the column, providing recoveries from 96% to 101% for tested samples. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Solid-phase extraction and preconcentration are efficient procedures for sample preparation that eliminates or minimizes matrix interferences and could adjust the concentration of analytes at detectable levels before instrumental analysis [1]. In order to enhance the performance of this strategy in terms of precision, reagent consumption, portability, and analytical throughput, miniaturized/automated devices with integrated monolithic or packed columns have been proposed and successfully applied to quantitative determinations [2–5]. However, in many cases, this integration is not easily performed, particularly when the particles of the commercial extractor need to be packed into the devices, which may lead to leaking or clogging in the microfluidic structures or adversely affect the sealing procedures. In fact, for usual flow analysis systems some alternatives have been proposed for minimizing these drawbacks as the use of fluidized beds [6] and combination of solid materials to prevent aggregation of particles [7]. But, the miniaturization of such strategies could further restrict the amount of usable solid particles in microsystems, leading to potential limitations of solid phase based procedures. ⁎ Corresponding author. E-mail address: [email protected] (A. Fonseca).
http://dx.doi.org/10.1016/j.microc.2017.01.020 0026-265X/© 2017 Elsevier B.V. All rights reserved.
Recently,the use of urethane–acrylate resin (UA) and photolithographic procedures were described to manufacture analytical microfluidic devices containing packed cadmium particles [8]. Authors demonstrated that devices were efficiently and rapidly constructed, but the packing of smaller particles and application to extraction/ preconcentration of analytes were not investigated, despite the great potential of the latter. It is also important to note that following the concept of Micro Total Analytical Systems (μTAS) [9], the combination of different analytical procedures in a single chip is necessary for the complete automation/ miniaturization of the method. Therefore, the integration of packed columns for preconcentration into microdevices should be accompanied by the introduction of reactors and detectors that will facilitate onchip sample preparation and detection. For UA microsystems, LEDbased photometric detection is easily performed with suitable results for many applications [10]. However, because of the highly laminar behavior of fluids in microchannels, analytical signals present the influence of the Schlieren effect that is caused by the deflection of the light beam because of the concentration gradients in nonhomogeneous fluids, thereby hindering the interpretation of analytical signals in continuous flow analysis [11]. In fact, considering the online solidphase extraction/preconcentration of inorganic analytes, the use of highly concentrated solutions that are required to elute the analyte
162
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
2. Experimental
For chromium spectrophotometry, a 0.5-g L−1 diphenylcarbazide (DFC) solution was prepared by dissolving 0.05 g of the reagent (Synth) in 3.0 mL of acetone followed by dilution with water to 100 mL. Solutions of H2SO4 (0.3, 0.1, and 0.05 mol L−1) were prepared by the dilution of a 95% m/V stock solution (Vetec). A 500-mg L−1 Cr (VI) stock solution was prepared by the dissolution of 0.2329 g of K2CrO7 (Synth) in 100 mL of H2SO4 0.1 mol L−1. Cr (VI) standard solutions (0.8 to 2.4 mg L−1) were prepared by diluting the stock solution in 0.30 mol L−1 H2SO4. A stock solution of nitrite (500 mg L−1) was prepared by the dissolution of pre-dried NaNO2 (Vetec) in water. Standard solutions of nitrite (0.05 to 0.20 mg L−1) were prepared by diluting the proper aliquots of stock solution in water. A chromogenic reagent solution containing 1.0 g L−1 of N-(1-Naphthyl)ethylenediamine dihydrochloride (NED) and 7.0 g L−1 sulphanilamide (SAM) was prepared by dissolving 0.7 g of SAM (Synth) in 20 mL of water with 1.0 mL of 37% HCl (Vetec) followed by the addition of 0.1 g of NED (Vetec) and completing the volume to 100 mL with water in a volumetric flask. Surface freshwater samples were collected from three different regions of Paranoá Lake (Brasília-DF, Brazil) and filtered using 0.45-μm pore cellulose nitrate membrane filter (512.047, Unifil). For recovery studies, samples were spiked with the nitrite stock solution to yield 0.12-mg L−1 nitrite added concentration.
2.1. Apparatus
2.3. Construction of the micro-analyzers
A lab-made photoexposer machine equipped with two UV lamps (Philips Actinic-BL, TL-D 15 W, 380 nm) was used for photolithography of the UA resist. Photomasks were designed using Auto-CAD software (Autodesk) and printed on an overhead transparency (Abezeta, PLT A4) at a resolution of 1200 dpi using a laser printer (HP LaserJet P2055dn). Ultrasonic bath (Unique, UltraCleaner 1400) was used to develop the structures in the elastomer. A peristaltic pump (Ismatec-Reglo Analog) equipped with Tygon® tubes (internal diameters of 0.38 mm and 1.30 mm) was used to propel or aspirate solutions, and three-port solenoid valves (Cole Parmer, 01540-11) controlled the flow directions. The detection system consisted of Ultra Brilliant RGB LED (LZ400MA10, LED Engin) emitting in 625 nm, 523 nm and 460 nm and a photodiode (OPT-101, Texas Instruments). An electronic circuit based on LM317 CI (Texas Instruments) was developed to actuate the individual diodes of the LED array and to perform the adjustment of brightness. During measurements, two diodes were actuated and switched off in sequence at intervals of 100 ms, permitting readings at two wavelengths. Another circuit based on OP07 CI (Texas Instruments) was used to amplify the analytical signal from the photodiode. One wavelength was selected to respond to both the analyte and Schlieren effect, and another wavelength was selected to respond to Schlieren effect only. By subtraction of the signals with two different wavelengths, the analytical signal was obtained. A software program developed using Microsoft VisualBasic 6.0 and a personal computer were used to control the fluidic and detection systems via an electronic interface (USB 6009, National Instruments). A UV/Vis spectrophotometer (USB 2000, Ocean Optics) and a flow cell with 10-mm optical path (178.010-QS, Hellma Analytics) were used for flow analysis studies.
The lab-made microfluidic device shown in Fig. 1 A is manufactured by using a previously described deep ultraviolet photolithographic method for UA resist [10]. U-shaped channels with approximately 360-μm width and 400-μm depth were engraved on the 2.0-mm-thick
often enhances the Schlieren effect, rendering photometric signal interpretation difficult. In order to correct the Schlieren effect, dual-wavelength spectrophotometry is commonly used [12] so that the suitable analytical signal is obtained by subtracting the absorbance of a wavelength that responds to both the effect and analyte from the absorbance of a wavelength that responds only to Schlieren effect. Although efficient correction is accomplished using this strategy for usual flow analysis systems, it was inefficient in the case of microfluidics because of the less effective mixing of solutions in the microchannels and the different responses to Schlieren effect at different wavelengths when medium is not homogenized [12]. Considering the abovementioned limitations, the present study describes an efficient alternative to perform solid-phase-based preconcentration integrated with photometric detection in UA microfluidic devices. The particles of a commercial solid-phase extractor (Dowex® 1X8) were manually and easily packed into a miniaturized column and the Schlieren effect was appropriately corrected using a pair of light-emitting diodes (LED) with controlled light intensity in order to compensate for the different responses to the effect at different wavelengths.
2.2. Reagents and solutions Urethane-Acrylate resist G50 (Gold News, Brazil) was purchased at a local market and used for the photolithographic construction of microanalyzers. All solutions used in the analytical procedures were prepared in reverse osmosis-purified water. A NaCl stock solution (0.5 mol L−1) was prepared by dissolving 2.9222 g of the salt (Vetec) in 100 mL of water. Less concentrated NaCl solutions (0.10 and 0.25 mol L−1) were prepared by appropriate dilution of the stock solution.
Fig. 1. Photograph of the proposed microfluidic device with Dowex® 1X8 packed column (A) and flow diagram used for nitrite determinations (B). Channels for commutation of solutions (CC), Dowex® 1X8 packed column (PC), polyurethane foam (PF), narrower channel to prevent particles displacement (NC), channel for eventual purpose (CP), channel for chromogenic reagent access (CR), reaction coil (RC), optical fibers guides (OF), flow cell (FC), chromogenic reagent solution (NED/SAM), purified water (PW), standard or sample solutions (SS), peristaltic pump with indications of flow rates in μL min−1 (PP), solenoid valves turned off (V1V4), waste (W), reservoir for reuse of solutions (RR), and photodiode (PD).
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
UA resist along with a 25 mm × 2 mm × 1 mm (l × w × h) reservoir used for column filling. Before permanent sealing with another UA plate containing no channels, 0.35 g of Dowex® 1X8 microspheres were manually packed into the reservoir using a small spatula. To avoid microsphere displacement to adjacent channels during flow procedures, a small portion of polyurethane foam (PF) was coupled at the reservoir output. To provide photometric detection, the microfluidic device was equipped with two plastic optical fibers (250-μm diameter, TorayEurope) in order to guide the electromagnetic radiation to and from a 5.0-mm-long channel used as the flow cell (FC). Hypodermic Needles with 0.45-mm external diameters (305111-BD) were inserted at the input and output channels to allow fluid access to the chip. 2.4. Procedures Preliminary studies for Schlieren correction were performed using a microfluidic device containing no preconcentration column that was substituted by a typical microchannel. Using hydrodynamic injection [10], approximately 3.0 μL of 0.1 to 0.5 mol L−1 NaCl standard solutions were inserted into a water carrier stream at 400 μL min−1 in a singleline flow configuration. A similar procedure was used for the injection of standard solutions of Cr(VI) with concentrations ranging from 0.8 to 2.4 mg L−1 in a converged carrier stream of 0.5-g L− 1 DFC and 0.1 mol L−1 H2SO4 at 100 μL min−1. In both the studies, detection was performed using diodes with emission wavelengths at 523 nm and 625 nm, respectively. The flow diagram shown in Fig. 1B is used for nitrite determination with online preconcentration, derivatization, and photometric detection. Prior to the calibration and analysis, a conditioning step was performed. For this purpose, purified water was allowed to percolate the solid phase during 100 s at 350 μL min−1 by turning V2 on and remaining valves off. Subsequently, 1.0 mol L−1 HCl solution was pumped through the column under the same conditions by turning V3 on and remaining valves off to complete the conditioning step. During the analysis, by turning V1 on and leaving the other valves off during 200 s, the extraction/preconcentration of the standard/sample solutions was performed so that a volume of approximately 3.8 mL of the solution was percolated through the column at a flow rate of 1.15-mL min−1. After this step, the unadsorbed species were removed by propelling water through the column during 100 s at 350 μL min− 1 by setting V2 on and remaining valves off. Elution, derivatization with NED/SAM, and photometric detection were performed by turning V3 and V4 on and V2 and V1 off so that the eluate that was flowing at a rate of 350 μL min−1 merged with the NED/SAM solution (350 μL min−1) before homogenization in the reaction coil (RC, 21.5 cm) and detection in the flow cell (FC). Table 1 summarizes the steps for the proposed procedures.
163
in the FC) by setting the individual current/brightness of the emitters. As shown in Fig. 2(A), when this dual-LED strategy was used with the proposed microfluidic device, the injection of 0.30 mol L−1 NaCl solution in a single-line water flow produces an elevated Schlieren effect with different responses at both the wavelengths so that the subtraction of individual signals do not provide efficient correction of the effect. As described earlier [12], Schlieren effect could occur with different intensities at different wavelengths, rendering dual-wavelength strategy unsuitable for correction purpose if no additional mathematical operations are used. Moreover, it is noteworthy that Schlieren effect could be more intense for microfluidic devices than for usual flowinjection analysis systems because of less effective mixing of solutions inside the microchannels. In fact, a similar experiment using a singleline flow-injection analysis system with 0.8-mm PTFE tubes and spectrophotometric detection yields significant differences for the Schlieren effect at 525-nm and 623-nm wavelengths (similar to LED emission) only if NaCl solutions with increased concentrations (2.0 mol L−1 at least) are injected into the water stream (Fig. 3(A)). During these studies, the visible spectrums of flowing solutions were acquired and are shown in Fig. 3(B), showing differences in absorbance for different wavelengths, particularly for point #3, wherein the undesirable effect was most pronounced.
3. Results and discussion 3.1. Schlieren effect correction In the preliminary studies for the Schlieren effect correction, two diodes of RGB LED with emission wavelengths of 523 nm and 625 nm were adjusted to yield the same response for the baseline (water stream
Table 1 Steps for the determination of nitrite in freshwaters. Valve state
Conditioning Calibration and analysis
Washing (water) Washing (HCl) Preconcentration Washing (water) Elution and detection
Interval/s
V1
V2
V3
V4
Off Off On Off Off
On Off Off On Off
Off On Off Off On
Off Off Off Off On
100 100 200 100 200
Fig. 2. Fiagrams acquired for injections of 0.3 mol L−1 NaCl in water stream using proposed microfluidic device and detection with 523-nm and 625-nm emitting diodes set to yield the same signal for baseline (A). Fiagrams for injections of 0.10, 0.25, and 0.50 mol L−1 NaCl solutions in water stream obtained via detection by diodes with brightness adjustment to correct Schlieren effect (B). Gray bars are used to hide the peaks acquired for changing the solutions. Difference = 523-nm LED signal – 625-nm LED signal. *Difference + 4.5 V for improved signal visualization in the graphic representation.
164
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
Fig. 4. Fiagrams for the photometric detection of Cr (VI) (0.8–2.4 mg L−1) with DFC. Gray bars are used to hide the peaks acquired for change of the solutions. *523 nm LED signal + 2.0 V. **(523 nm LED signal – 625 nm LED signal) + 5.5 V.
Fig. 3. Signal record for injection of 50-μL 2.0 mol L−1 NaCl solution in water stream using usual single-line flow-injection analysis system and spectrophotometric detection at 523 nm and 625 nm (A) and spectrums obtained for different points of signal record (B).
Considering these results, the current/brightness levels of the diodes were empirically adjusted until the Schlieren effect was corrected by the subtraction of signals for 525-nm and 623-nm emitting diodes. As shown in Fig. 2(B), this offset provides the appropriate correction of the effect even when solutions of 0.5 mol L− 1 NaCl were injected in the water stream, simulating a considerable difference in the concentrations of the flowing solutions that enhances the Schlieren effect. It can be noted that by adjusting the response signal for the 525-nm diode approximately 1.0 V over the signal for the 623-nm diode, the effect was efficiently corrected for all the studied NaCl concentrations; however, this compensation must be empirically determined for other experiments. It is also important to consider that the proposed correction could not be performed using usual commercial spectrophotometers, wherein the sensitivity or intensity at different wavelengths is not tunable. The performance for the detection of an analyte using solutions that causes the Schlieren effect was evaluated by the hydrodynamic injection of standard solutions of Cr(VI) containing 0.3 mol L−1 H2SO4 in a carrier stream containing less concentrated acid (0.1 mol L−1) and DFC. As shown in Fig. 4, the corrected signals (obtained using the difference for the signals acquired by the 523-nm and 623-nm diodes) for blank and standard solutions do not indicate the Schlieren effect that was observed for pure signals acquired by 625-nm and 523-nm
emitting diodes, confirming efficient correction. In this study, the signal for the 523-nm diode was displaced approximately 0.3 V over the 625nm-diode signal to permit efficient correction. For improved graphical visualization in Fig. 4, correction factors of 2.0 V and 5.5 V were added to the signals referring to the 523-nm LED and for the difference, respectively. Additional studies were conducted to evaluate the performance of the detection system by the photometric determination of Cu (II) using colorimetric reaction with cuprizone [13,14] and Fe (II) based on the reaction with 1,10-phenatroline [15,16]. In both the photometric determinations, the products absorb blue light so that the 460-nm emitting diode was used for the acquisition of the analytical signal and 625-nm emitting diode was used for reference correction, providing a similar performance to Cr (VI) determination, despite the use of another diode. Although the proposed strategy have indicated the appropriate Schlieren effect correction for some applications, it is important to report that all studies were performed using colorimetric complexes that do not absorb radiation at the reference wavelength; this aspect could render the efficient use of this dual-wavelength correction difficult. 3.2. Characterization of microfluidic device A photograph of the UA microfluidic device integrated with the solid-phase packed column (PC) is shown in Fig. 1(A). All the structures were arranged along a 4 cm × 7 cm UA plate containing channels to perform the commutation of solutions (CC), a channel for chromogenic reagent access (CR), and an extra channel for eventual purposes (CP). In addition, the flow analyzer comprises a reaction coil (RC) and a photometric flow cell (FC) with dimensions and volumes listed in Table 2. Originally, the microspheres of Dowex® 1X8 were directly packed into the column without the use of polyurethane foam at the output extremity of reservoir. In this case, to retain microspheres on column,
Table 2 Dimensions for different regions of microfluidic device. Region
Length/mm
Diametera/μm
Volume/μL
PC RC FC
25 215 5
2000 360 360
40b 11 0.3
a b
For semi-circular channel. For empty reservoir.
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
an adjacent channel (NC, detail on Fig. 2(A)) with diameter smaller than that of the microspheres and the other channels of chip were photolithographed, thus providing an efficient and simple method for packing the column. In fact, by propelling water through the column at flow rates up to 2500 μL min−1, no displacement of particles to the adjacent parts of the chip was observed. However, if little knocks were applied to the chip to eliminate air bubbles before analytical procedures, particle displacement to adjacent channels occurred because of the momentary deformation of elastomer. Therefore, polyurethane foam improved the retention of the particles making the system more robust. By using this procedure, no clogging or leaking was observed in the microfluidic device, indicating the efficient sealing of the structures. It is also important to mention that synthetic ion-exchange resins tend to swell in contact with the working solutions [17]; this could affect the microfluidic device performance owing to the increase in particle size. During the studies, this behavior did not seem to affect the integrity of chip or its application to ordinary determinations. 3.3. Determination of nitrite To calculate the concentration factor obtained using the online fluidic procedures, 2.0 mL of a 0.10-mg L−1 NO− 2 standard solution was mixed with 600 μL of NED/SAM solution followed by a batch absorbance measurement. An identical procedure was performed using 2.0 mL of collected eluate, and the ratios of absorbance for concentrated and dilute solutions showed an estimated four-fold increase in concentration. Considering the use of only 3.8 mL of the sample/standard solution in the proposed studies, this value can be considered adequate for nitrite determination because higher volumes could propitiate the enhancement of analyte concentration prior the photometric analysis. Fig. 5 shows the fiagram for the injections of standard solutions of nitrite by performing on-chip preconcentration, derivatization, and photometric detection with the Schlieren effect correction. Despite the use of highly concentrated solutions of HCl and NED/SAM, the proposed method provided an appropriate discrimination of peaks, demonstrating that LED-based dual-wavelength correction of signal was successfully applied. For this application, the signal for 523-nm LED was displaced approximately 1.0 V over the signal for 625-nm LED for the Schlieren effect correction. As shown in Fig. 5, a factor of 5.0 V was added to the corrected signal for improved visualization. A linear response (R2 = 0.99) was found for the studied concentration range (signal = −0.162 + 5.52[NO− 2 ]) and the limit of detection of −1 (c.a. 0.01 mg N L−1) was estimated, which is about 0.04 mg NO− 2 L
165
Table 3 Results for recovery studies. Sample
−1 Added NO− 2 /mg L
−1 Found NO− 2 /mg L
Recovery/%
1 2 3
0.12 0.12 0.12
0.115 ± 0.002 0.118 ± 0.002 0.121 ± 0.002
96 98 101
one hundred times lower than the maximum concentration level of nitrite allowed by Brazilian regulations for freshwaters (1.0 mg N L−1) [18]. The precision of 5.4% was determined by sequential injections (n = 5) of 0.10 mg L−1 nitrite standard solution, confirming the appropriate repeatability of on-chip procedures. Results for the studies performed using freshwater samples are listed in Table 3. Recoveries from 96% to 101% were determined using the proposed microfluidic device, indicating a significant accuracy for the method and the possibility of its application to nitrite determination in natural samples. As the samples solutions were introduced into the fluidic system immediately after conducting the procedures on standard solutions with higher concentrations, it can be affirmed that column cleaning was efficient, thereby avoiding contamination between injections. In addition, it was investigated that the proposed microdevice can be used for at least five similar determinations without the loss of analytical performance because of the efficient regeneration of the column after procedures [19,20]. Moreover, its economical use as a disposable device could be considered because the minimum cost of fabrication of one device is approximately US$ 5.00. By comparing the performance of the proposed system with microfluidic devices containing no SPE column for photometric nitrite determination in water samples, it was noted a reduction of the limit of detection (up to four fold) and the use of less concentrated Griess reagent, which reduces the costs of the analysis and the amount of residues [8,21]. In addition, the accuracy results for all studied systems are quite similar, indicating that preconcentration step causes no significant variation for this figure of merit. 4. Conclusion Combination of LED-based dual-wavelength photometric detection and ion-exchange packed column in a urethane–acrylate microfluidic device provided an easy, efficient, and low-cost alternative to perform on-chip extraction/preconcentration, derivatization, and detection of nitrite. Applications for additional analytes need to be evaluated considering the availability of emitting diodes for target methods and particular characteristics of the solid phase. Acknowledgments The authors are grateful to Instituto Nacional de Ciências e Tecnologias Analíticas Avançadas (INCTAA/CNPq – Grant 573894/2008-6), for financial support. L. O. Magalhães acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for a fellowship. References
−1 Fig. 5. Fiagrams for determining NO− ) by performing on-chip pre2 (0.05–0.20 mg L concentration, derivatization with NED/SAM, and photometric detection with Schlieren correction. Gray bars are used to hide the peaks acquired for changing the solutions. *(523 nm LED signal – 625 nm LED signal) + 5.0 V.
[1] A. Andrade-Eiroa, M. Canle, V. Leroy-Cancellieri, Trends Anal. Chem. 80 (2016) 641–654. [2] P.N. Nge, J.V. Pagaduan, M. Yu, A.T. Woolley, J. Chromatogr. A 1261 (2012) 129–135. [3] H. Zhai, J. Li, Z. Chen, Z. Su, Z. Liu, X. Yu, Microchem. J. 114 (2014) 223–228. [4] L.A. Legendre, J.M. Bienvenue, M.G. Roper, J.P. Ferrance, J.P. Landers, Anal. Chem. 78 (2006) 1444–1451. [5] J.D. Ramsey, G.E. Collins, Anal. Chem. 77 (2005) 6664–6670. [6] M.F.T. Ribeiro, A.C.B. Dias, J.L.M. Santos, J.L.F.C. Lima, E.A.G. Zagatto, Anal. Bioanal. Chem. 384 (2006) 1019–1024. [7] B. Parodi, A. Londonio, G. Polla, M. Savio, P. Smichowski, J. Anal. At. Spectrom. 29 (2014) 880–885. [8] L.N.N. Nóbrega, L.O. Magalhães, A. Fonseca, Microchem. J. 110 (2013) 553–557. [9] A. Manz, N. Graber, H.M. Widmer, Sensors Actuators B 1 (1990) 244–248.
166
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
[10] A. Fonseca, I.M. Raimundo Jr., J.J.R. Rohwedder, L.O.S. Ferreira, Anal. Chim. Acta 603 (2007) 159–166. [11] B.F. Reis, C.E.S. Miranda, N. Baccan, Quim Nova 19 (6) (1996) 623–635. [12] A.C.B. Dias, E.P. Borges, E.A.G. Zagatto, P.J. Worsfold, Talanta 68 (2006) 1076–1082. [13] P. Rumori, V. Cerdà, Anal. Chim. Acta 486 (2003) 227–235. [14] J.J. Pinto, C. Moreno, M. García-Vargas, Talanta 64 (2004) 562–565. [15] J. Kozak, J. Paluch, A. Wegrzecka, M. Kozak, M. Wieczorek, J. Kochana, P. Koscielniak, Talanta 148 (2016) 626–632. [16] C. Vakh, E. Freze, A. Pochivalov, E. Evdokimova, M. Kamencev, L. Moskvin, A. Bulatov, J. Pharmacol. Toxicol. Methods 73 (2015) 56–62.
[17] K. Dorfner, Ion Exchangers, De Gruyter, New York, 1991. [18] CONAMA, National Environmental Council, Resolução n.° 357/2005, Environmental Guidelines for Water Resources and Standards for the Release of Effluents (in Portuguese) 2005. [19] M.G. Martino, G.T. Macarovscha, S. Cadore, Anal. Methods 2 (2010) 1258–1262. [20] R.R.A. Peixoto, G.T. Macarovscha, S. Cadore, Food Anal. Methods 5 (2012) 814–820. [21] M.M. Baeza, N. Ibanez-Garcia, J. Baucells, J. Bartrolí, J. Alonso, Analyst 131 (2006) 1109–1115.
Contents lists available at ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
A microfluidic device with ion-exchange preconcentration column and photometric detection with Schlieren effect correction Laiz de Oliveira Magalhães, Alexandre Fonseca ⁎ Instituto de Química, Universidade de Brasília, Cx Postal 4478, CEP 70910-900 Brasília, DF, Brazil
a r t i c l e
i n f o
Article history: Received 11 October 2016 Received in revised form 30 January 2017 Accepted 30 January 2017 Available online 01 February 2017 Keywords: Microfluidics Flow analysis Schlieren effect Preconcentration
a b s t r a c t This work describes the construction and evaluation of a microflow analyzer containing a Dowex® 1X8 packed column and LED-based photometric detection with Schlieren effect correction. Flow structures, including a reservoir for the preconcentration column with dimensions 25.0 × 2.0 × 1.0 mm (l × w × h), were manufactured in a urethane–acrylate resin, and the ion-exchange particles (50–100 mesh) were manually packed before sealing the device. Photometric measurements were performed using an integrated flow cell with a 5.0 mm optical path by using optical fibers to guide the radiation from a 3.0 W RGB LED to the microfluidic device and from the microdevice to a photodiode (OPT-101). It was demonstrated that the Schlieren effect is appropriately corrected by subtracting the signals for two diodes of the RGB LED and, by setting the intensity of light sources before measurements, the discordant response to the effect at different wavelengths is compensated. The preconcentration column was easily and successfully integrated into the microfluidic device without any clogging or leaking of the working solutions, and swelling of the packing particles caused no damage to the sealed structures because of the elastomeric nature of the microfluidic substrate. The application of the proposed device to the determination of nitrite in fresh water provides a linear response (R2 = 0.99) from 0.05 to 0.20 mg NO− 2 L−1 and an approximate increase in concentration by four fold was estimated when 3.80 mL of the sample/ standard solutions is percolated through the column, providing recoveries from 96% to 101% for tested samples. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Solid-phase extraction and preconcentration are efficient procedures for sample preparation that eliminates or minimizes matrix interferences and could adjust the concentration of analytes at detectable levels before instrumental analysis [1]. In order to enhance the performance of this strategy in terms of precision, reagent consumption, portability, and analytical throughput, miniaturized/automated devices with integrated monolithic or packed columns have been proposed and successfully applied to quantitative determinations [2–5]. However, in many cases, this integration is not easily performed, particularly when the particles of the commercial extractor need to be packed into the devices, which may lead to leaking or clogging in the microfluidic structures or adversely affect the sealing procedures. In fact, for usual flow analysis systems some alternatives have been proposed for minimizing these drawbacks as the use of fluidized beds [6] and combination of solid materials to prevent aggregation of particles [7]. But, the miniaturization of such strategies could further restrict the amount of usable solid particles in microsystems, leading to potential limitations of solid phase based procedures. ⁎ Corresponding author. E-mail address: [email protected] (A. Fonseca).
http://dx.doi.org/10.1016/j.microc.2017.01.020 0026-265X/© 2017 Elsevier B.V. All rights reserved.
Recently,the use of urethane–acrylate resin (UA) and photolithographic procedures were described to manufacture analytical microfluidic devices containing packed cadmium particles [8]. Authors demonstrated that devices were efficiently and rapidly constructed, but the packing of smaller particles and application to extraction/ preconcentration of analytes were not investigated, despite the great potential of the latter. It is also important to note that following the concept of Micro Total Analytical Systems (μTAS) [9], the combination of different analytical procedures in a single chip is necessary for the complete automation/ miniaturization of the method. Therefore, the integration of packed columns for preconcentration into microdevices should be accompanied by the introduction of reactors and detectors that will facilitate onchip sample preparation and detection. For UA microsystems, LEDbased photometric detection is easily performed with suitable results for many applications [10]. However, because of the highly laminar behavior of fluids in microchannels, analytical signals present the influence of the Schlieren effect that is caused by the deflection of the light beam because of the concentration gradients in nonhomogeneous fluids, thereby hindering the interpretation of analytical signals in continuous flow analysis [11]. In fact, considering the online solidphase extraction/preconcentration of inorganic analytes, the use of highly concentrated solutions that are required to elute the analyte
162
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
2. Experimental
For chromium spectrophotometry, a 0.5-g L−1 diphenylcarbazide (DFC) solution was prepared by dissolving 0.05 g of the reagent (Synth) in 3.0 mL of acetone followed by dilution with water to 100 mL. Solutions of H2SO4 (0.3, 0.1, and 0.05 mol L−1) were prepared by the dilution of a 95% m/V stock solution (Vetec). A 500-mg L−1 Cr (VI) stock solution was prepared by the dissolution of 0.2329 g of K2CrO7 (Synth) in 100 mL of H2SO4 0.1 mol L−1. Cr (VI) standard solutions (0.8 to 2.4 mg L−1) were prepared by diluting the stock solution in 0.30 mol L−1 H2SO4. A stock solution of nitrite (500 mg L−1) was prepared by the dissolution of pre-dried NaNO2 (Vetec) in water. Standard solutions of nitrite (0.05 to 0.20 mg L−1) were prepared by diluting the proper aliquots of stock solution in water. A chromogenic reagent solution containing 1.0 g L−1 of N-(1-Naphthyl)ethylenediamine dihydrochloride (NED) and 7.0 g L−1 sulphanilamide (SAM) was prepared by dissolving 0.7 g of SAM (Synth) in 20 mL of water with 1.0 mL of 37% HCl (Vetec) followed by the addition of 0.1 g of NED (Vetec) and completing the volume to 100 mL with water in a volumetric flask. Surface freshwater samples were collected from three different regions of Paranoá Lake (Brasília-DF, Brazil) and filtered using 0.45-μm pore cellulose nitrate membrane filter (512.047, Unifil). For recovery studies, samples were spiked with the nitrite stock solution to yield 0.12-mg L−1 nitrite added concentration.
2.1. Apparatus
2.3. Construction of the micro-analyzers
A lab-made photoexposer machine equipped with two UV lamps (Philips Actinic-BL, TL-D 15 W, 380 nm) was used for photolithography of the UA resist. Photomasks were designed using Auto-CAD software (Autodesk) and printed on an overhead transparency (Abezeta, PLT A4) at a resolution of 1200 dpi using a laser printer (HP LaserJet P2055dn). Ultrasonic bath (Unique, UltraCleaner 1400) was used to develop the structures in the elastomer. A peristaltic pump (Ismatec-Reglo Analog) equipped with Tygon® tubes (internal diameters of 0.38 mm and 1.30 mm) was used to propel or aspirate solutions, and three-port solenoid valves (Cole Parmer, 01540-11) controlled the flow directions. The detection system consisted of Ultra Brilliant RGB LED (LZ400MA10, LED Engin) emitting in 625 nm, 523 nm and 460 nm and a photodiode (OPT-101, Texas Instruments). An electronic circuit based on LM317 CI (Texas Instruments) was developed to actuate the individual diodes of the LED array and to perform the adjustment of brightness. During measurements, two diodes were actuated and switched off in sequence at intervals of 100 ms, permitting readings at two wavelengths. Another circuit based on OP07 CI (Texas Instruments) was used to amplify the analytical signal from the photodiode. One wavelength was selected to respond to both the analyte and Schlieren effect, and another wavelength was selected to respond to Schlieren effect only. By subtraction of the signals with two different wavelengths, the analytical signal was obtained. A software program developed using Microsoft VisualBasic 6.0 and a personal computer were used to control the fluidic and detection systems via an electronic interface (USB 6009, National Instruments). A UV/Vis spectrophotometer (USB 2000, Ocean Optics) and a flow cell with 10-mm optical path (178.010-QS, Hellma Analytics) were used for flow analysis studies.
The lab-made microfluidic device shown in Fig. 1 A is manufactured by using a previously described deep ultraviolet photolithographic method for UA resist [10]. U-shaped channels with approximately 360-μm width and 400-μm depth were engraved on the 2.0-mm-thick
often enhances the Schlieren effect, rendering photometric signal interpretation difficult. In order to correct the Schlieren effect, dual-wavelength spectrophotometry is commonly used [12] so that the suitable analytical signal is obtained by subtracting the absorbance of a wavelength that responds to both the effect and analyte from the absorbance of a wavelength that responds only to Schlieren effect. Although efficient correction is accomplished using this strategy for usual flow analysis systems, it was inefficient in the case of microfluidics because of the less effective mixing of solutions in the microchannels and the different responses to Schlieren effect at different wavelengths when medium is not homogenized [12]. Considering the abovementioned limitations, the present study describes an efficient alternative to perform solid-phase-based preconcentration integrated with photometric detection in UA microfluidic devices. The particles of a commercial solid-phase extractor (Dowex® 1X8) were manually and easily packed into a miniaturized column and the Schlieren effect was appropriately corrected using a pair of light-emitting diodes (LED) with controlled light intensity in order to compensate for the different responses to the effect at different wavelengths.
2.2. Reagents and solutions Urethane-Acrylate resist G50 (Gold News, Brazil) was purchased at a local market and used for the photolithographic construction of microanalyzers. All solutions used in the analytical procedures were prepared in reverse osmosis-purified water. A NaCl stock solution (0.5 mol L−1) was prepared by dissolving 2.9222 g of the salt (Vetec) in 100 mL of water. Less concentrated NaCl solutions (0.10 and 0.25 mol L−1) were prepared by appropriate dilution of the stock solution.
Fig. 1. Photograph of the proposed microfluidic device with Dowex® 1X8 packed column (A) and flow diagram used for nitrite determinations (B). Channels for commutation of solutions (CC), Dowex® 1X8 packed column (PC), polyurethane foam (PF), narrower channel to prevent particles displacement (NC), channel for eventual purpose (CP), channel for chromogenic reagent access (CR), reaction coil (RC), optical fibers guides (OF), flow cell (FC), chromogenic reagent solution (NED/SAM), purified water (PW), standard or sample solutions (SS), peristaltic pump with indications of flow rates in μL min−1 (PP), solenoid valves turned off (V1V4), waste (W), reservoir for reuse of solutions (RR), and photodiode (PD).
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
UA resist along with a 25 mm × 2 mm × 1 mm (l × w × h) reservoir used for column filling. Before permanent sealing with another UA plate containing no channels, 0.35 g of Dowex® 1X8 microspheres were manually packed into the reservoir using a small spatula. To avoid microsphere displacement to adjacent channels during flow procedures, a small portion of polyurethane foam (PF) was coupled at the reservoir output. To provide photometric detection, the microfluidic device was equipped with two plastic optical fibers (250-μm diameter, TorayEurope) in order to guide the electromagnetic radiation to and from a 5.0-mm-long channel used as the flow cell (FC). Hypodermic Needles with 0.45-mm external diameters (305111-BD) were inserted at the input and output channels to allow fluid access to the chip. 2.4. Procedures Preliminary studies for Schlieren correction were performed using a microfluidic device containing no preconcentration column that was substituted by a typical microchannel. Using hydrodynamic injection [10], approximately 3.0 μL of 0.1 to 0.5 mol L−1 NaCl standard solutions were inserted into a water carrier stream at 400 μL min−1 in a singleline flow configuration. A similar procedure was used for the injection of standard solutions of Cr(VI) with concentrations ranging from 0.8 to 2.4 mg L−1 in a converged carrier stream of 0.5-g L− 1 DFC and 0.1 mol L−1 H2SO4 at 100 μL min−1. In both the studies, detection was performed using diodes with emission wavelengths at 523 nm and 625 nm, respectively. The flow diagram shown in Fig. 1B is used for nitrite determination with online preconcentration, derivatization, and photometric detection. Prior to the calibration and analysis, a conditioning step was performed. For this purpose, purified water was allowed to percolate the solid phase during 100 s at 350 μL min−1 by turning V2 on and remaining valves off. Subsequently, 1.0 mol L−1 HCl solution was pumped through the column under the same conditions by turning V3 on and remaining valves off to complete the conditioning step. During the analysis, by turning V1 on and leaving the other valves off during 200 s, the extraction/preconcentration of the standard/sample solutions was performed so that a volume of approximately 3.8 mL of the solution was percolated through the column at a flow rate of 1.15-mL min−1. After this step, the unadsorbed species were removed by propelling water through the column during 100 s at 350 μL min− 1 by setting V2 on and remaining valves off. Elution, derivatization with NED/SAM, and photometric detection were performed by turning V3 and V4 on and V2 and V1 off so that the eluate that was flowing at a rate of 350 μL min−1 merged with the NED/SAM solution (350 μL min−1) before homogenization in the reaction coil (RC, 21.5 cm) and detection in the flow cell (FC). Table 1 summarizes the steps for the proposed procedures.
163
in the FC) by setting the individual current/brightness of the emitters. As shown in Fig. 2(A), when this dual-LED strategy was used with the proposed microfluidic device, the injection of 0.30 mol L−1 NaCl solution in a single-line water flow produces an elevated Schlieren effect with different responses at both the wavelengths so that the subtraction of individual signals do not provide efficient correction of the effect. As described earlier [12], Schlieren effect could occur with different intensities at different wavelengths, rendering dual-wavelength strategy unsuitable for correction purpose if no additional mathematical operations are used. Moreover, it is noteworthy that Schlieren effect could be more intense for microfluidic devices than for usual flowinjection analysis systems because of less effective mixing of solutions inside the microchannels. In fact, a similar experiment using a singleline flow-injection analysis system with 0.8-mm PTFE tubes and spectrophotometric detection yields significant differences for the Schlieren effect at 525-nm and 623-nm wavelengths (similar to LED emission) only if NaCl solutions with increased concentrations (2.0 mol L−1 at least) are injected into the water stream (Fig. 3(A)). During these studies, the visible spectrums of flowing solutions were acquired and are shown in Fig. 3(B), showing differences in absorbance for different wavelengths, particularly for point #3, wherein the undesirable effect was most pronounced.
3. Results and discussion 3.1. Schlieren effect correction In the preliminary studies for the Schlieren effect correction, two diodes of RGB LED with emission wavelengths of 523 nm and 625 nm were adjusted to yield the same response for the baseline (water stream
Table 1 Steps for the determination of nitrite in freshwaters. Valve state
Conditioning Calibration and analysis
Washing (water) Washing (HCl) Preconcentration Washing (water) Elution and detection
Interval/s
V1
V2
V3
V4
Off Off On Off Off
On Off Off On Off
Off On Off Off On
Off Off Off Off On
100 100 200 100 200
Fig. 2. Fiagrams acquired for injections of 0.3 mol L−1 NaCl in water stream using proposed microfluidic device and detection with 523-nm and 625-nm emitting diodes set to yield the same signal for baseline (A). Fiagrams for injections of 0.10, 0.25, and 0.50 mol L−1 NaCl solutions in water stream obtained via detection by diodes with brightness adjustment to correct Schlieren effect (B). Gray bars are used to hide the peaks acquired for changing the solutions. Difference = 523-nm LED signal – 625-nm LED signal. *Difference + 4.5 V for improved signal visualization in the graphic representation.
164
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
Fig. 4. Fiagrams for the photometric detection of Cr (VI) (0.8–2.4 mg L−1) with DFC. Gray bars are used to hide the peaks acquired for change of the solutions. *523 nm LED signal + 2.0 V. **(523 nm LED signal – 625 nm LED signal) + 5.5 V.
Fig. 3. Signal record for injection of 50-μL 2.0 mol L−1 NaCl solution in water stream using usual single-line flow-injection analysis system and spectrophotometric detection at 523 nm and 625 nm (A) and spectrums obtained for different points of signal record (B).
Considering these results, the current/brightness levels of the diodes were empirically adjusted until the Schlieren effect was corrected by the subtraction of signals for 525-nm and 623-nm emitting diodes. As shown in Fig. 2(B), this offset provides the appropriate correction of the effect even when solutions of 0.5 mol L− 1 NaCl were injected in the water stream, simulating a considerable difference in the concentrations of the flowing solutions that enhances the Schlieren effect. It can be noted that by adjusting the response signal for the 525-nm diode approximately 1.0 V over the signal for the 623-nm diode, the effect was efficiently corrected for all the studied NaCl concentrations; however, this compensation must be empirically determined for other experiments. It is also important to consider that the proposed correction could not be performed using usual commercial spectrophotometers, wherein the sensitivity or intensity at different wavelengths is not tunable. The performance for the detection of an analyte using solutions that causes the Schlieren effect was evaluated by the hydrodynamic injection of standard solutions of Cr(VI) containing 0.3 mol L−1 H2SO4 in a carrier stream containing less concentrated acid (0.1 mol L−1) and DFC. As shown in Fig. 4, the corrected signals (obtained using the difference for the signals acquired by the 523-nm and 623-nm diodes) for blank and standard solutions do not indicate the Schlieren effect that was observed for pure signals acquired by 625-nm and 523-nm
emitting diodes, confirming efficient correction. In this study, the signal for the 523-nm diode was displaced approximately 0.3 V over the 625nm-diode signal to permit efficient correction. For improved graphical visualization in Fig. 4, correction factors of 2.0 V and 5.5 V were added to the signals referring to the 523-nm LED and for the difference, respectively. Additional studies were conducted to evaluate the performance of the detection system by the photometric determination of Cu (II) using colorimetric reaction with cuprizone [13,14] and Fe (II) based on the reaction with 1,10-phenatroline [15,16]. In both the photometric determinations, the products absorb blue light so that the 460-nm emitting diode was used for the acquisition of the analytical signal and 625-nm emitting diode was used for reference correction, providing a similar performance to Cr (VI) determination, despite the use of another diode. Although the proposed strategy have indicated the appropriate Schlieren effect correction for some applications, it is important to report that all studies were performed using colorimetric complexes that do not absorb radiation at the reference wavelength; this aspect could render the efficient use of this dual-wavelength correction difficult. 3.2. Characterization of microfluidic device A photograph of the UA microfluidic device integrated with the solid-phase packed column (PC) is shown in Fig. 1(A). All the structures were arranged along a 4 cm × 7 cm UA plate containing channels to perform the commutation of solutions (CC), a channel for chromogenic reagent access (CR), and an extra channel for eventual purposes (CP). In addition, the flow analyzer comprises a reaction coil (RC) and a photometric flow cell (FC) with dimensions and volumes listed in Table 2. Originally, the microspheres of Dowex® 1X8 were directly packed into the column without the use of polyurethane foam at the output extremity of reservoir. In this case, to retain microspheres on column,
Table 2 Dimensions for different regions of microfluidic device. Region
Length/mm
Diametera/μm
Volume/μL
PC RC FC
25 215 5
2000 360 360
40b 11 0.3
a b
For semi-circular channel. For empty reservoir.
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
an adjacent channel (NC, detail on Fig. 2(A)) with diameter smaller than that of the microspheres and the other channels of chip were photolithographed, thus providing an efficient and simple method for packing the column. In fact, by propelling water through the column at flow rates up to 2500 μL min−1, no displacement of particles to the adjacent parts of the chip was observed. However, if little knocks were applied to the chip to eliminate air bubbles before analytical procedures, particle displacement to adjacent channels occurred because of the momentary deformation of elastomer. Therefore, polyurethane foam improved the retention of the particles making the system more robust. By using this procedure, no clogging or leaking was observed in the microfluidic device, indicating the efficient sealing of the structures. It is also important to mention that synthetic ion-exchange resins tend to swell in contact with the working solutions [17]; this could affect the microfluidic device performance owing to the increase in particle size. During the studies, this behavior did not seem to affect the integrity of chip or its application to ordinary determinations. 3.3. Determination of nitrite To calculate the concentration factor obtained using the online fluidic procedures, 2.0 mL of a 0.10-mg L−1 NO− 2 standard solution was mixed with 600 μL of NED/SAM solution followed by a batch absorbance measurement. An identical procedure was performed using 2.0 mL of collected eluate, and the ratios of absorbance for concentrated and dilute solutions showed an estimated four-fold increase in concentration. Considering the use of only 3.8 mL of the sample/standard solution in the proposed studies, this value can be considered adequate for nitrite determination because higher volumes could propitiate the enhancement of analyte concentration prior the photometric analysis. Fig. 5 shows the fiagram for the injections of standard solutions of nitrite by performing on-chip preconcentration, derivatization, and photometric detection with the Schlieren effect correction. Despite the use of highly concentrated solutions of HCl and NED/SAM, the proposed method provided an appropriate discrimination of peaks, demonstrating that LED-based dual-wavelength correction of signal was successfully applied. For this application, the signal for 523-nm LED was displaced approximately 1.0 V over the signal for 625-nm LED for the Schlieren effect correction. As shown in Fig. 5, a factor of 5.0 V was added to the corrected signal for improved visualization. A linear response (R2 = 0.99) was found for the studied concentration range (signal = −0.162 + 5.52[NO− 2 ]) and the limit of detection of −1 (c.a. 0.01 mg N L−1) was estimated, which is about 0.04 mg NO− 2 L
165
Table 3 Results for recovery studies. Sample
−1 Added NO− 2 /mg L
−1 Found NO− 2 /mg L
Recovery/%
1 2 3
0.12 0.12 0.12
0.115 ± 0.002 0.118 ± 0.002 0.121 ± 0.002
96 98 101
one hundred times lower than the maximum concentration level of nitrite allowed by Brazilian regulations for freshwaters (1.0 mg N L−1) [18]. The precision of 5.4% was determined by sequential injections (n = 5) of 0.10 mg L−1 nitrite standard solution, confirming the appropriate repeatability of on-chip procedures. Results for the studies performed using freshwater samples are listed in Table 3. Recoveries from 96% to 101% were determined using the proposed microfluidic device, indicating a significant accuracy for the method and the possibility of its application to nitrite determination in natural samples. As the samples solutions were introduced into the fluidic system immediately after conducting the procedures on standard solutions with higher concentrations, it can be affirmed that column cleaning was efficient, thereby avoiding contamination between injections. In addition, it was investigated that the proposed microdevice can be used for at least five similar determinations without the loss of analytical performance because of the efficient regeneration of the column after procedures [19,20]. Moreover, its economical use as a disposable device could be considered because the minimum cost of fabrication of one device is approximately US$ 5.00. By comparing the performance of the proposed system with microfluidic devices containing no SPE column for photometric nitrite determination in water samples, it was noted a reduction of the limit of detection (up to four fold) and the use of less concentrated Griess reagent, which reduces the costs of the analysis and the amount of residues [8,21]. In addition, the accuracy results for all studied systems are quite similar, indicating that preconcentration step causes no significant variation for this figure of merit. 4. Conclusion Combination of LED-based dual-wavelength photometric detection and ion-exchange packed column in a urethane–acrylate microfluidic device provided an easy, efficient, and low-cost alternative to perform on-chip extraction/preconcentration, derivatization, and detection of nitrite. Applications for additional analytes need to be evaluated considering the availability of emitting diodes for target methods and particular characteristics of the solid phase. Acknowledgments The authors are grateful to Instituto Nacional de Ciências e Tecnologias Analíticas Avançadas (INCTAA/CNPq – Grant 573894/2008-6), for financial support. L. O. Magalhães acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for a fellowship. References
−1 Fig. 5. Fiagrams for determining NO− ) by performing on-chip pre2 (0.05–0.20 mg L concentration, derivatization with NED/SAM, and photometric detection with Schlieren correction. Gray bars are used to hide the peaks acquired for changing the solutions. *(523 nm LED signal – 625 nm LED signal) + 5.0 V.
[1] A. Andrade-Eiroa, M. Canle, V. Leroy-Cancellieri, Trends Anal. Chem. 80 (2016) 641–654. [2] P.N. Nge, J.V. Pagaduan, M. Yu, A.T. Woolley, J. Chromatogr. A 1261 (2012) 129–135. [3] H. Zhai, J. Li, Z. Chen, Z. Su, Z. Liu, X. Yu, Microchem. J. 114 (2014) 223–228. [4] L.A. Legendre, J.M. Bienvenue, M.G. Roper, J.P. Ferrance, J.P. Landers, Anal. Chem. 78 (2006) 1444–1451. [5] J.D. Ramsey, G.E. Collins, Anal. Chem. 77 (2005) 6664–6670. [6] M.F.T. Ribeiro, A.C.B. Dias, J.L.M. Santos, J.L.F.C. Lima, E.A.G. Zagatto, Anal. Bioanal. Chem. 384 (2006) 1019–1024. [7] B. Parodi, A. Londonio, G. Polla, M. Savio, P. Smichowski, J. Anal. At. Spectrom. 29 (2014) 880–885. [8] L.N.N. Nóbrega, L.O. Magalhães, A. Fonseca, Microchem. J. 110 (2013) 553–557. [9] A. Manz, N. Graber, H.M. Widmer, Sensors Actuators B 1 (1990) 244–248.
166
L. de Oliveira Magalhães, A. FonsecaMicrochemical Journal 132 (2017) 161–166
[10] A. Fonseca, I.M. Raimundo Jr., J.J.R. Rohwedder, L.O.S. Ferreira, Anal. Chim. Acta 603 (2007) 159–166. [11] B.F. Reis, C.E.S. Miranda, N. Baccan, Quim Nova 19 (6) (1996) 623–635. [12] A.C.B. Dias, E.P. Borges, E.A.G. Zagatto, P.J. Worsfold, Talanta 68 (2006) 1076–1082. [13] P. Rumori, V. Cerdà, Anal. Chim. Acta 486 (2003) 227–235. [14] J.J. Pinto, C. Moreno, M. García-Vargas, Talanta 64 (2004) 562–565. [15] J. Kozak, J. Paluch, A. Wegrzecka, M. Kozak, M. Wieczorek, J. Kochana, P. Koscielniak, Talanta 148 (2016) 626–632. [16] C. Vakh, E. Freze, A. Pochivalov, E. Evdokimova, M. Kamencev, L. Moskvin, A. Bulatov, J. Pharmacol. Toxicol. Methods 73 (2015) 56–62.
[17] K. Dorfner, Ion Exchangers, De Gruyter, New York, 1991. [18] CONAMA, National Environmental Council, Resolução n.° 357/2005, Environmental Guidelines for Water Resources and Standards for the Release of Effluents (in Portuguese) 2005. [19] M.G. Martino, G.T. Macarovscha, S. Cadore, Anal. Methods 2 (2010) 1258–1262. [20] R.R.A. Peixoto, G.T. Macarovscha, S. Cadore, Food Anal. Methods 5 (2012) 814–820. [21] M.M. Baeza, N. Ibanez-Garcia, J. Baucells, J. Bartrolí, J. Alonso, Analyst 131 (2006) 1109–1115.