Magnetophoretic velocimetry of manganese(II) in a single emulsion droplet at the femtomole level.

We developed a new experimental technique named magnetophoretic velocimetry to determine a small amount of paramagnetic species in a single microdroplet. The magnetophoretic velocity of an aqueous droplet containing paramagnetic metal ion dispersed in an organic medium could response to a very small amount of the metal ion under an inhomogeneous magnetic field. The paramagnetic droplet (2 approximately 8 microm diam) used as a test sample in this study was the aqueous droplet of manganese(II) chloride dispersed in ethylbenzoate whose density was nearly equal to water. A pair of small Nd-Fe-B magnets placed with a gap of 400 microm generated an inhomogeneous magnetic field between the edges, at which the product of the magnetic flux density and the gradient, B(dB/dx), was as high as 410 T2 m(-1). When a silica capillary containing the emulsion was inserted into the gap between the magnets, the magnetophoretic migration of the droplets was observed with a video microscope. The magnetophoretic velocity divided by the squared radius of the droplet was proportional to the MnCl2 concentration in the droplet, as predicted by a theoretical calculation. The estimated detection limit in this simple method was lower than 10(-16) mol for manganese(II).