MP56: High throughput Drop-to-drop liquid-liquid microextraction method coupled with real time concentration monitoring

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 P. Wijethunga1, Y. S. Nanayakkara2, P. Kunchala1, D. W. Armstrong2 and H. Moon1*
1Department of Mechanical and Aerospace Engineering, and 2Department of Chemistry and Biochemistry, The University of Texas at Arlington, USA

Liquid-Liquid Extraction (LLE) is among the most important pretreatment techniques in basic sciences. Although, various miniaturized LLE methods were emerged to overcome drawbacks in macro scale LLE methods, multiplexing LLE processes has never been considered. Therefore, we introduced a new drop-to-drop liquid microextraction (DTD-LLME) method that is capable of achieving high throughput LLE. The DTD-LLME device was built as an electrowetting on a dielectric (EWOD) based digital microfluidic (DMF) device, which can easily be automated. Due to its multiplexing capability, this DTD-LLME technique is an attractive solution for the need for high throughput micro total analysis (µTAS) applications, for instance preparing and screening samples from complicated biological fluids. In this paper, we completely demonstrate the proposed DTD-LLME technique experimentally, along with two further studies related to the device: (i) developing an image based real time concentration measurement technique to study extraction kinetics in the device and (ii) Investigating the potential use of AC frequency to enhance the extraction kinetics of the proposed DTD-LLME technique.
For the demonstration of DTD-LLME, we selected a solute, Acid Green 25; a dye soluble in both DI water and a chosen ionic liquid (IL), [bmim][PF6]. DI water and [bmim][PF6] were served as solvent and extractant, respectively. Four major steps in DTD-LLME (eg: dispensing droplets, moving and merging, mixing to effect extraction, and separating extractant phase from donor phase) are demonstrated experimentally. Specially, a successful phase separation of two immiscible droplets is presented for the first time on a digital microfluidic device.
To study the extraction performance, we developed an image based technique to read real time donor concentration while the extraction is in progress. Images of the droplets captured from the digital microscope system (HIROX KH 1300) were processed and color information was extracted (using MATLAB R2009a). The relationship between the color parameter and solute concentration of droplet was used as a calibration plot. An evaluation of the use of color parameters from three color models (RGB, HSV and CIELab) was carried out. Further, the technique was compared with a typical UV absorption based concentration measurement technique. Finally, we investigated the effect of AC frequency on the extraction kinetics.
The DTD-LLME technique was successfully implemented. The image based technique showed to be a precise method for real time concentration measurement. Results on the study of AC frequency effect showed that the application of high frequency (> 150 kHz) is an effective factor to accelerate the extraction. Overall, the DTD-LLME technique can be extended toward high throughput µTAS for various processes that involve liquid-liquid extraction, such as screening for drug development and biomarker preparation for point of care devices.

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