Fabrication of a Microfluidic Flame Atomic Emission Spectrometer: a

Technical Note Cite This: Anal. Chem. 2018, 90, 5995−6000

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Fabrication of a Microfluidic Flame Atomic Emission Spectrometer: a Flame-on-a-Chip Arpad Kiss and Attila Gaspar* Department of Inorganic and Analytical Chemistry, University of Debrecen, Egyetem ter 1., Debrecen 4032, Hungary S Supporting Information *

ABSTRACT: This work demonstrates for the first time the fabrication of a microfluidic flame atomic emission spectrometer (FAES), which incorporates a microburner and flame (flame-ona-chip). An essential part of the device is a thermospray system applied for effective sample introduction, which is more easily miniaturizable and integrable than the conventional nebulization methods. The merits and limitations of the microfluidic flame atomic emission device were demonstrated and discussed. Using a commercial cigarette lighter including butane gas, the flame temperature made the analysis of the most easily excitable alkali metals possible. The calibration diagrams for Li, Na, and K showed proper linearity in the range of 5−100 mg/L. The analytical applicability of the microfluidic FAES device was tested by analyzing various real samples.

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of mercury species was carried out22 and only one work combined (off-line) the microfluidic SPE system with flame AAS.23 But in these works the atomizer was positioned not in/ on the chip but outside the chip. The hyphenation between microchips and the recent atomic spectrometers is intricate, because only the detectors having minimal diffusion length between the chip and the detection cell and the ability to require only a few microliters or submicroliters of sample can be applied. Although the importance of the highly sensitive atomic spectrometric methods such as ICP/MS or graphite furnace AAS is obvious, the much cheaper and simple flame techniques like FAAS or FAES remain reliable element-selective methods. The FAES can be applied mainly for the detection of alkali or alkali earth metals, however, in several cases the qualitative and quantitative information is needed only for these macroconstituents (e.g., analysis of Na+/K+ content of body fluids, agricultural soil or soft drinks). Our work aims at developing a low-cost, microchip-based flame atomic emission spectrometer, which incorporates a direct, high efficiency sample introduction technique (thermospray (TS)) and a simple microburner with flame. This is the first work in which the flame as an atomic atomizer is placed on the microchip (flame-on-a-chip). For the application of the thermospray, which is a key element of the developed microfluidic system, our earlier experiences with thermospray flame furnace techniques24 have been utilized.

n the last few decades one of the most important efforts in analytical chemistry has been to develop miniaturized systems, because those offer many beneficial features over classical analytical methods including the need of small sample and reagent volumes, portability or fast and cheap ways of analysis. The highest degree of miniaturization can be commonly accomplished by microfluidic devices, and often the miniaturization is carried out through the size of a few square centimeters (microchip). Many analytical techniques (e.g., optical and electroanalytical devices, sensors, chromatographic or electrophoretic separation systems1,2) and procedures (e.g., enzymatic reactors,3 polymerase chain reaction (PCR)4) have already been miniaturized into a microchip, or if the analytical method is not miniaturizable enough (e.g., mass spectrometry (MS) or nuclear magnetic resonance (NMR) spectroscopy), those were hyphenated with microfluidic chips, including sample pretreatment processes (e.g., protein digestion5 or derivatization6). The miniaturization efforts have only barely touched atomic spectrometry. Although a few papers appeared about miniaturized plasmas (inductively coupled plasma (ICP),7 capacitively coupled plasma (CCP),8 microwave-induced plasma (MIP),9 or dielectric barrier discharge (DBD)10−18) used for element-selective detection, those did not become widely used. In addition, the graphite furnace or flame techniques have not been transferred to microchip format and only a very few papers appeared about hyphenation of microchips with atomic spectrometers. The ICP-MS has been coupled to chip electrophoresis19,20 and to chip-based magnetic solid phase extraction21 for speciation analysis. Only one paper was found where (flameless) atomic fluorescence spectrometry was coupled to a microchip, in which electrophoretic separation © 2018 American Chemical Society

Received: February 15, 2018 Accepted: April 11, 2018 Published: April 11, 2018 5995

DOI: 10.1021/acs.analchem.8b00774 Anal. Chem. 2018, 90, 5995−6000

Technical Note

Analytical Chemistry

Figure 1. Schematic construction of the miniaturized FAES system (left upper) and TS nebulizer/microburner (left lower) (a). Photographs of the miniaturized FAES system including microfluidic device and a propane/butane gas lighter (b) and the microfluidic device with flame colored by Li solution introduced via thermospray capillary (c).

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PDMS-Glass Device Fabrication. Microfluidic chips consisted of three layers of polydimethylsiloxane (PDMS) and the bottom pieces were sealed to a glass slide. The top and the middlemost pieces of PDMS included straight channels of 0.79 mm diameter in the middle of the pieces. These channels were created by positioning a straight PEEK tubing (OD: 1/ 32″) in the PDMS base just before its reticulation. After the PDMS was solidified, the tubing could be gently pulled out, gaining empty channels. The top piece of PDMS was used to hold the optical fiber in the proper height (4−5 mm) to detect the light emission of the flame. The middlemost PDMS piece was for introducing and transmitting the fuel gas. The bottom PDMS piece used for transporting the liquid sample toward the microburner was prepared by using a mold created by soft photolithography mainly according to the procedure described by Whitesides.25 Briefly, a channel pattern (straight line) was printed as a high-resolution (4000 dpi) photomask. Negativetype photoresist (SU-8 2025, Microchem, Newton, MA) was spin-coated onto a 3″ silicon wafer with 500 rpm for 30 s in order to obtain a 100 μm high layer. The PDMS chip was fabricated by cast molding a 10:1 mixture of PDMS oligomer and cross-linking agent (Sylgard 184, Dow Corning, Midland, MI). Although in this work only a simple, straight channel was prepared, the procedure is useful for the fabrication of such microfluidic channel patterns that are applicable for more complex liquid handling or sample pretreatment. A hole (300 μm diameter) was pierced through the PDMS device at the inlet end of the channel for liquid connection. The PDMS chip was sealed onto a glass slide of 1.2 mm thickness after oxygen plasma treatment (PDC-32G, Harrick, Ithaca, NY). A schematic depiction of the PDMS chip and the picture of the obtained microfluidic FAES device are given in Figure 1. The optical fiber (OD: 370 μm) of the photometer was introduced into the channel of the top piece of PDMS and pushed toward the flame. The PB gas tubing was connected to the middlemost piece of the PDMS and the tubing of the peristaltic pump (ID: 0.0075″, Tygon, Cole-Palmer, IL, U.S.A.)

MATERIALS AND METHODS

Reagents, Samples, and Apparatus. Analytical grade reagents were used. The 1000 mg/L concentration stock solutions of Li, Na, K, Rb, and Cs were prepared from their chloride salts (Reanal LLC, Hungary), respectively. The standard solutions were acidified to 0.1 M HCl. The 0.1 mL of human tear (male) sample was collected in our lab and analyzed after a 20-fold dilution with 0.1 M HCl, the blood sample (female, 1 mL) was digested atmospherically with 5 mL cc. HNO3 and 0.5 mL of H2O2 and completed to 10 mL with 0.1 M HCl. The 0.1 g soil and 0.3 g fodder crop samples were digested with 5 mL cc. HNO3 and 1 mL of H2O2, respectively, using microwave (ETHOS UP, Milestone Srl., Italy), and then completed to 25 and 50 mL with 0.1 M HCl. The Liticarb pill (500 mg lithium−carbonate from Pharma Produkt Gyógyszergyártó Ltd., Hungary) was pulverized and extracted with 100 mL of 0.1 M HCl and sonicated. The mineral water (Theodora Kékkúti bottled by Szentkirályi-Kékkúti Á sványviź Ltd.) was obtained from a local supermarket and it was directly analyzed (without dilution). The fuel (a propane butane (PB) mixture) supplying the flame was obtained from a common lighter. By means of a simple lab-made transformation of the lighter’s operating valve, PB gas could be transported into the microchip at a constant speed. For photometric detection of the emitted light a miniaturized spectrometer (AvaSpec-ULS2048LTEC, Avantes, The Netherlands) was used. The spectrophotometric measurements were controlled and recorded by AvaSoft software (Avantes). Peristaltic pump (IPC, Ismatec, Cole-Palmer, IL, U.S.A.) was used for the transportation of 0.25−2 μL volume of liquid samples through the microfluidic device. The length of the pump tubing was reduced to 25 cm and no extension with connecting pieces was applied in order to minimize the dispersion of the liquid sample in the capillary up to the microchip. 5996

DOI: 10.1021/acs.analchem.8b00774 Anal. Chem. 2018, 90, 5995−6000

Technical Note

Analytical Chemistry

(Although the flame temperature around the tip of the TS capillary amounts to approximately 1100 °C, the tip can not be heated up to such a high temperature.) When the small volume (∼1 μL (∼4 cm length)) of liquid plug reaches the heated and glowing tip of the capillary, small liquid bubbles are formed in the liquid phase close to the capillary wall and at the front of the liquid plug. From these bubbles larger droplets are formed in the vapor phase, which are diminished and finally vaporized leading to a fine, gaseous form of the sample. The TS capillary is glowing if the OD of the capillary is smaller than 2 mm and no liquid (Figure 2a) or liquid with small volume/rate is

was connected to the inlet of the bottom piece of the PDMS with a short metal capillary (0.51 mm/0.26 mm (OD/ID) × 20 mm). Preparation of a Microburner/Thermospray Nebulizer on the Microchip. The microburner/TS nebulizer system consisted of two coaxial capillaries placed tightly into the PDMS device (Figure 1a). The inner capillary (metal or fused silica, OD: 0.31−0.91 mm and ID: 0.1−0.6 mm) served as a TS nebulizer capillary and it was pushed down to the surface of the glass slide. The bottom PDMS layer was punched to tightly receive and hold the lower few-mm length of this TS capillary and a few mm upper part of the capillary stood out from the PDMS device. The outer capillary (metal, OD: 2.4 mm, ID: 1.8 mm) served as a microburner capillary and it was pushed down up to the bottom surface of the medium PDMS layer. The medium PDMS layer was punched to tightly receive and hold this capillary, and only shorter than 1−2 mm part of this capillary stood out from the surface of the PDMS device. Analytical Procedure. The 0.25−2 μL volume of the sample was sucked into the inlet end of the empty peristaltic pump tubing (0.5 μL sample resulted in 18 mm length of liquid plug), thus the sample plug was surrounded by air. The liquid plug was transported by the pump through the sampling inlet and microfluidic channel of the microchip toward the TS capillary. Different samples or washing solutions can be sucked in quick succession but the volume of the carrier air between the liquid plugs should be at least 4 times larger than the volume of the liquid plugs. During the analysis the PB gas was continuously let to flow from the lighter into the fuel gas inlet of the microchip. The fuel was ignited at least 30 s before the analysis (the flame heated up the thin TS capillary tip to its glowing for 10 s).

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RESULTS AND DISCUSSION Thermospray Aerosol Generation. The aerosol generation with thermospray was originally developed by Vestal26 as an interface between liquid chromatography and mass spectrometry. In atomic spectrometry, TS has been applied to ICP/AES or ICP-MS, but there are also a few efforts to use it for flame AAS.27 The theoretical aspects and the applications of TS for atomic spectrometry were summarized by Koropchak.28 In these papers it turned out that almost exclusively sophisticated electrical heating systems were applied to maintain a constant temperature for the sample vaporization. In contrast to this, Gaspar and Berndt24 heated the TS capillary with the flame of an atomic absorption spectrometer, and the thermospray flame furnace (TSFF) AAS technique was introduced. In this technique the liquid sample to be analyzed was transported through a hot metal or ceramic capillary tip acting as a flame-heated thermospray capillary. Besides the flame, the TS capillary was heated by the wall of flame furnace (metal tube placed into the flame) as well; thus, a sharp temperature gradient (900 °C on about 1 cm heated length) was formed in the tip of the TS capillary. This simple approach for TS generation led to many subsequent works.29−31 In the proposed microfluidic device the TS generation is similar to what was described in several TSFF works;24,31 however, there are some remarkable differences. The arrangement of the heating is axial, since the TS capillary is placed in the center of the flame and the tip of the capillary is positioned to the bottom of the middle zone of partial combustion, which is moderately hot. This approximately 1100 °C flame zone32 is able to heat quickly the last few mm length of the TS capillary.

Figure 2. Photographs of the flame when no liquid sample (a), 1 μL (b), and 2 μL (c) Li solution is pumped through the TS capillary. Schematic representation of the principle of thermospray sample introduction (d).

transported through it (Figure 2b). The volume of the sample plug reaching the tip of the TS capillary is critical. When larger liquid volume (e.g., 2 μL for 2 s) was pumped through the heated tip of the capillary, it was considerably cooled (Figure 2c) and larger liquid volume would result in the cease of the vapor generation. The above-described processes as the principle actions of thermospray sample introduction in the microfluidic TS capillary were schematically represented in Figure 2d. The operation of the TS is optimal when the tip of the TS capillary is glowing while the sample liquid is being pumped through it. During this process a continuous simmering sound of the TS can be heard and the shape of the flame is axially lengthened (Figure 2b). The jet of the formed aerosol stream and the operation efficiency of the TS can be studied by monitoring the colored flame when an alkali element (e.g., Li) was introduced. When capillaries with inner diameters smaller than 100 μm were used, the intensity of light emission fluctuated, since the bubbles rapidly forming at the tip of the TS capillary pushed the liquid sample backward in the capillary. This pulsation effect could be considerably reduced using capillaries with a larger inner diameter and only small sample 5997

DOI: 10.1021/acs.analchem.8b00774 Anal. Chem. 2018, 90, 5995−6000

Technical Note

Analytical Chemistry volumes (0.5−2 μL). However, capillaries with an inner diameter larger than 300 μm can not transmit enough heat to the liquid plug (which fastly passes the heated tip), thus, no TS aerosol was formed. During the TS operation some salts from the sample can deposit in the tip of the TS capillary, which can lead to memory effects for the subsequent measurements, therefore thorough rinsing of the TS capillary with 1 M HCl between the sample plugs is required. After injecting samples with high salt matrix content, the capillary should be rinsed at least for 1−2 min with low flow rate (large volume of liquid with higher flow rate would cool the capillary tip down preventing the operation of the TS). Analytical Characteristics of the Microfluidic FAES System. Our experiments have been performed with a cheap, easily available commercial cigarette lighter including butane or propane/butane gas, which has a flame temperature several hundreds °C lower than the acetylene/air flame (∼2400 °C) commonly used in flame atomic spectrometry. The lower flame temperature makes the excitation of the most easily excitable atoms like alkali metals possible only. Figure 3 shows the

calculated in absolute amounts (not in concentration), then these values are considerably better than in commercial FAES. The calibration diagrams for Li, Na, and K showed proper linearity in the range of 5−100 mg/L (R2 values were 0.998, 0.997, and 0.981 for Li, Na, and K, respectively), the differences obtained for the three elements can be explained with the different excitation energies of these elements (Figure 4). The good detection power of the described microfluidic FAES is aided by the realization of the total sample introduction. As it is known, the sensitivity of the flame spectrometers can be significantly improved by increasing the efficiency of aerosol generation and transport to the atomization cell.24,33 Although most of the commercial atomic spectrometers operate with pneumatic nebulization with less than 10% efficiency, the sample introduction efficiency of the microfluidic FAES can be considered close to 100%. The precision of the measurements obtained with this first lab-made microfluidic device was worse (∼5−50 RSD%) than those that are usual with commercial instrumentations. The precision of the measurements could be considerably improved by automation of the sampling (generation of the small volume of sample plugs in the pump tubing and their reproducable transportation into the TS capillary) and the improvement in robustness of the microchips’ fabrication and use. The developed microfluidic FAES device can be applied for the analysis of alkali metals. Because Na and K are often macroconstituents in samples, these samples should often be largely diluted. The fact that the microfluidic FAES system requires only a few microliters of sample is especially advantageous for the analysis of samples available only in limited volumes (e.g., tear, expensive materials). The analytical applicability of the microfluidic FAES device was tested by the analysis of several types of samples like mineral water, soil and fodder crop extract, pharmaceutical, human blood, and tear (Table 1 and Figure S-2). The obtained results were in acceptable agreement (2−28%) with ICP/AES measurements.

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Figure 3. Atomic emission spectrum of a solution of alkali metal ions obtained with the miniaturized flame emission spectrometer (concentration of alkali metal ions: 500 mg/L, fuel: butane gas, sample volume: 1 μL).

CONCLUSIONS In this work, we demonstrated for the first time the fabrication of a microfluidic flame emission spectrometer, where a microburner and flame are created in/on the microchip (flame-on-a-chip). The essential part of the system is the thermospray sample introduction, which was simply combined with the microburner in the microchip. The main objective of this work was to demonstrate the chance and possible advantages of the application of a flame in microfluidics for analytical use. The advantages of the described microfluidic device are the cheapness (even disposability), extreme simplicity, and portability. The microfluidic FAES requires only minimum, not more than 1 μL sample volume for analysis. The shown lab-made microfluidic chip (as its first prototype) obviously can not compete with the analytical performances (limit of detection, precision, accuracy) of the modern atomic spectrometers improved in the last few decades, but it might be considered as an initial stage device for microfluidic atomic spectrometers. The applicability of the developed microfluidic FAES could be largely extended if the temperature of the flame atomizer was elevated. However, the way to do that is probably not by replacing the simple propane/butane flame with other flames, but by miniaturizing and integrating higher temperature plasmas into the chip. (Efforts to develop plasmas used as detectors for microfluidic chips were made by Broekart,9 Niemax,10 and Hieftje,11 but none of those became widespread

atomic emission spectrum of 5 alkali metal ions obtained with the miniaturized flame emission spectrometer. Since K is more easily excitable than Na or even Li, K can be more sensitively detected than Na or Li. This is why the highest intensity spectral lines were obtained for K. (In contrast to this, the higher temperature atomizers (e.g., ICP) provide higher emitted light intensity for Li than K.) The limit of detection values obtained by the lab-made microfluidic FAES system (2.8, 0.68, 0.30, 0.66, and 1.1 mg/L for Li, Na, K, Rb, and Cs, respectively (based on 3× signal-tonoise)) were worse than those that could be obtained with the much more sophisticated, commercial FAES instruments, mainly due to the technical difficulties of miniaturization (imperfectness of fitting the fiber optic toward the microflame, darkening around the flame, positioning the TS capillary and microburner capillary related to each other and attaching those to the soft PDMS, etc.) and the very small sample volume/ amount used. While the commercial FAES instruments typically use up several milliliters (but at least 0.1 mL when microinjection technique of discrete samples34 is used), the microfluidic FAES consumes only 0.5−2 μL, which is a several orders of magnitude smaller volume. So if the LOD values are 5998

DOI: 10.1021/acs.analchem.8b00774 Anal. Chem. 2018, 90, 5995−6000

Technical Note

Analytical Chemistry

Figure 4. Signals obtained for different concentration of Li solutions using the microfluidic FES system. (fuel: butane gas, sample volume: 2 μL, TS capillary: OD: 0.51 mm, ID: 0.26 mm).

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Table 1. Concentrations of Different Solutions Determined with the Miniaturized Flame Emission Spectrometera

Corresponding Author

*E-mail: [email protected]. Tel.: +36-30-2792889. Fax: +36-52-518660.

concentration (mg/L) sample

K

Na

Li

mineral water soil sample extract crop sample extract blood tear Liticarb (pill)

12.4 12.5 54.5 13.7 1282 nd

30.9 5.1 12.9 80.9 1186 74.9

nd nd nd nd nd 956

ORCID

Attila Gaspar: 0000-0002-0172-2393 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the EU and cofinanced by the European Regional Development Fund under the Projects GINOP-2.3.2-15-2016-00008 and GINOP-2.3.3-15-201600004. The authors also acknowledge the financial support provided to this project by the National Research, Development and Innovation Office, Hungary (K111932). The assistance of Dr. Edina Baranyai in ICP/AES measurements and Adam Kecskemeti in taking photographs and video is greatly appreciated.

a

Conditions were the same as in Figure 4. The sample solutions were obtained by extractions/digestions/dilutions given in the Materials and Methods. bnd: not detectable.

or commercialized due to the difficulties to gain a robust, durable and precise device.) The shown thermospray implemented in a microchip might be applied as an effective sample introduction, which seems to be more easily miniaturizable and integrable than the conventional nebulization methods.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.8b00774. Figures S-1−S-3 and Table S-1 (PDF). Video S-1 is a real-time video about the flame during the thermospray operated in the microfluidic FAES device (AVI). 5999

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NOTE ADDED AFTER ASAP PUBLICATION This paper was originally published ASAP on April 17, 2018. There was an error in the scale of Figure 4, and the corrected paper was reposted on April 23, 2018.

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DOI: 10.1021/acs.analchem.8b00774 Anal. Chem. 2018, 90, 5995−6000