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One of the common issues to be solved in microfluidic devices is high sensitive detection because the sample volume in micro chip is limited.Plasma optical emission spectrometry is one of the candidates for high sensitive elemental analysis.However,it requires a certain amount of plasma gases and high power sources to generate plasma,so that it is not easy to be miniaturized.We have been developed a novel elemental analysis method for liquid sample in micro chip,namely liquid electrode plasma atomic emission spectrometry (LEP-AES)[1].The principle is as follows (Fig.1).A micro channel whose center was made narrower was fabricated on a quarts glass chip.Solution samples were introduced into the channel.A high voltage (500 - 1500 V) was applied to the solution from the both ends of the channel.The sample solution was locally heated by confined electric current at the narrower part,to evaporate and to make discharge with element specific spectra.From the spectra,we can measure the elemental concentration in the liquid sample.This method requires neither plasma gas nor nebulizer,and is compact and suitable for on-site use.Figure 2 shows the experimental setup.The liquid sample solution was introduced from the tube through the chip to waste using a syringe pump.The sample flow remove bubbles remaining after each plasma pulse,resulting in highly reproducible plasma generation,to enable a low limit of ditection (LOD).Figure 3 shows the Cd emission spectra after background subtraction in a 0.1 mol/L HNO3 aqueous solution.The spectra for the con-centration range from 0.001 to 0.01mg/L are shown in the figure inset.The emission line for Cd at 228.8 nm can be clearly observed even in the spectra for the 0.001 mg/L Cd solution.Figure 4 is a calibration curve for Cd.The current limit of detection of this method for Cd and Pb were derived as 0.5 ppb and 5 ppb,respectively,in 0.1 M HNO3.Those values were low enough for the environmental analysis applications.We are trying to apply this LEP-AES as a high sensitive ditector in other microfluidic applications.For an example,a protein sensing was studied employing Ag nanoparticle labeled antibody[2].Human chorionic gonadotropin (hCG) was used as a model target protein.Figure 5 shows a schematic representation of this immunoassay system.hCG was sandwiched between two antibodies,of which one was immobilized onto the microwell and the other was conjugated to Ag nanoparticles.The Ag nanoparticles were dissociated oxidatively,and the silver ion concentration was measured by LEP-AES.A linear relationship between the logarithmic signal intensity and the logarithmic concentration of hCG was obtained in the range from 10 pg/mL to 1 ng/mL (Fig.6).The detection limit for hCG was estimated at 1.3 pg/mL (22.8 fM).The proposed detection method has a wide variety of promising applications in element-labeled biomolecule detection.