AN AUTOMATED SOLID PHASE EXTRACTION POLYETHER-ETHER KETONE MICROFLUIDIC DEVICE: INFLUENCE OF SORBENT PACKING S. Heub1,2, N. Tscharner1, P.S. Dittrich2, S. Follonier1 and L. Barbe1* 1 CSEM, SWITZERLAND and 2 ETH Zurich, SWITZERLAND ABSTRACT The work presented here aims at developing a microfluidic device for automated sample enrichment of water samples. The device is optimized for the analysis of small organic pollutants by biosensing and environmental monitoring applications. Miniaturization and automation of solid phase extraction with our device enabled us to reduce the volume of sample and of eluent significantly, resulting in a fast sample processing. In this paper, we discuss the influence of the sorbent packing method on the performances of the solid phase extraction procedure. KEYWORDS: Microfluidics, sample preparation, solid phase extraction, immunoassay INTRODUCTION Miniaturization of sample preparation methods has been driven by the reduction of sample volume and need for portability. However, despite recent progress in the field [1-3], the chip-to-world interface is still an issue when dealing with real samples [4]. In our previous work [5], we developed a platform to perform automated solid phase extraction (SPE). We demonstrated the method through the analysis of the natural hormone 17β-estradiol (E2) at ng/l levels in seawater samples, followed by detection with an immunoassay. This instrument consisted of off-the-shelf components such as tubing, connectors, and valves and enabled to reduce the sample volume from a few liters to 100 ml. Further integration of the device was investigated, and a portable system was developed to allow the control of smaller volumes of solution and to increase the functionality [6]. Our device is made of polyether ether ketone (PEEK) which is totally compatible with the analysis of hydrophobic molecules. Our design bypasses the difficulties associated with traditional chip bonding methods due to its chemical resistance. The channels are cut in a fluoroelastomer gasket, pressed between two PEEK parts holding valves and connectors necessary for the fluid control. The materials are compatible with the study of organic molecules. The SPE column is prepared by the user and can be exchanged on the top of the main device. In this paper, we discuss the influence of the sorbent packing method on the reproducibility of the SPE procedure. Di-water samples spiked with E2 are studied and analyzed by a commercial enzymelinked immunosorbent assay (ELISA). EXPERIMENTAL The microfluidic device is depicted in Figure 1. The fluidic channels are laser cut in an FKM (fluoroelastomer) gasket (APSOparts AG, CH) and pressed between the two main PEEK parts with embedded 350 µm high spacers. The device is assembled using metallic screws. One PEEK part holds bistable surface mounted valves (Takasago, JPN) while the other is holding the exchangeable SPE chip. All solutions are pressure driven by using an air diaphragm pump (KNF, CH) and custom-made PEEK reservoirs. The user controls the pressure delivered to the solutions by means of a pressure regulator. Two SPE chip designs associated with two different sorbent packing methods were investigated and are shown in Figure 2. In Chip A, an 11 µm pore size Nylon membrane (Millipore AG, CH) is set at the bottom of the cylindrical hole. 6 mg of dry octadecyl-silica sorbent particles (Nucleodur® C18-ec, Macherey-Nagel, CH) is then added on top of it. In Chip B, an additional PEEK interlayer is added to the
978-0-9798064-8-3/µTAS 2015/$20©15CBMS-0001
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19th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 25-29, 2015, Gyeongju, KOREA
construction to enable the immobilization of the membrane using an O-ring. With this device, the sorbent suspension is sucked in using a syringe with a custom-made adapter on the bottom of the chip. The two methods are compared by measuring the flow rate of di-water at different pressures. For SPE experiments, 30 ml of de-ionized water samples spiked with E2 100 ng/l are injected through the sorbent for extraction. Elution is then performed using 90 µl of methanol 60 vol%. A sixtimes dilution is then done to reduce the methanol content, and E2 concentrations are measured with a commercial ELISA kit (Enzo Life Science, CH).
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Figure 1: Microfluidic device for automated SPE, with a) a schematic view of the assembly, b) a picture of the assembled device, and c) of the open chip with gasket, Dimensions of the chip are 7 cm x 6.5 cm x 2.5 cm, including the SPE microchip. From ref [6].
Figure 2: Schematics showing the two SPE micro-chip assemblies, and pictures. The red arrows show the direction of the flow. RESULTS AND DISCUSSION The results of a SPE procedure are influenced by various parameters including the sample volume to eluent volume ratio and the flow rate during the extraction step [7]. The latter is greatly influenced by the density of the sorbent packing. With our device, the SPE column is prepared by the user by inserting the sorbent between membranes in the SPE micro-chip. Figure 3 shows the results of flow rate measurements for the two SPE chip designs. With chip A, there is no clear relation between the pressure set by the user and the measured flow rate, demonstrating the non-reproducibility of the sorbent packing, despite good reproducibility of the sorbent mass (6.3 ± 0.1 mg, n=10) with this preparation method. Contrariwise, there is a linear relationship between the two parameters when using chip B. The performance of the SPE procedure can be evaluated by calculating the recovery of the target compound, comparing its amount in the sample before and after the pretreatment. The performances of SPE with spiked di-water samples are significantly different with the two chips. The E2 recovery obtained with chip A is not reproducible (56 ± 54 %), joining the previous observation. On the contrary, the recovery achieved with chip B is reproducible with 57 ± 10%.
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The sorbent packing by injection of the particle suspension provides dense and reproducible packing, providing a good control of the flow rate by the user. The stability of the system influences directly the performances of the SPE procedure.
Figure 3: Flow rate dependency on the pressure chosen by the user for both SPE chip designs, using di-water (1 data point = 1 fresh column).
Figure 4: E2 recoveries of the SPE procedure for 30 ml di-water samples spiked with 100 ng/l E2. Elution with 90 µl MeOH 50 vol% (n=3).
CONCLUSION We have developed a PEEK microfluidic device for automated and rapid SPE of water samples. Two designs of the SPE microchips were studied, and one method provided a good control of the flow rate linked to a reproducible sorbent packing density. Simulations of the flow velocity profile through the two chip designs would help to understand better the influence of the chip configuration on the SPE performances. ACKNOWLEDGEMENTS The research leading to these results received funding from the European Union Seventh Framework Program FP7/2007-2013 under grant agreement n° 265721. REFERENCES [1] A.J. de Mello and N. Beard, "Dealing with ‘real’ samples: sample pre-treatment in microfluidic systems”, Lab Chip (2003). [2] A. Ríos and M. Zougagh, “Sample preparation for micro total analytical systems (µ-TASs),” Anal. Chem. (2003). [3] A.J. de Mello and N. Beard, "Dealing with ‘real’ samples: sample pre-treatment in microfluidic systems”, Lab Chip (2003). [4] M. Miró and E. H. Hansen, " Miniaturization of environmental chemical assays in flowing systems: the lab-on-a-valve approach vis-à-vis lab-on-a-chip microfluidic devices”, Anal. Chim. Acta (2007) [5] S. Heub, N. Tscharner, V. Monnier, F. Kehl, P.S. Dittrich, S. Follonier and L. Barbe, " Automated and portable solid phase extraction platform for immuno-detection of 17β-estradiol in water”, J. Chromatogr. A (2015) [6] S. Heub, L. Barbe, S. Follonier and P.S. Dittrich, " Fully automated and portable platform for integrated extraction and pre-concentration of toxins and pollutants from liquid samples”, proceedings of the International Conference on Miniaturized Systems for Chemistry and Life Sciences (MicroTAS) (2014) CONTACT * L. Barbe, CSEM SA, Bahnhofstrasse 1, CH-7302 Landquart, Switzerland;
[email protected] 2013
