Method to measure particle mobility in solution with scattered and unscattered light

A method to measure the electrophoretic mobility of particles in a sample solution comprising the steps of:

• A) filling a sample cell with an aliquot of said sample solution, said sample cell comprising:
• a. a chamber to hold said sample aliquot,
• b. electrodes between which a voltage is applied across said sample aliquot contained therein, and
• c. optical windows in said chamber through which a light beam may passes;
• B) providing means to generate a beam of coherent monochromatic light;
• C) providing means to divide said beam of coherent monochromatic light into
• a. a sample beam that will pass through said sample cell unscattered, and a fraction of the sample beam light scattered from said particles in said sample aliquot leaves said sample cell, and
• b. a reference beam;
• D) providing means to modulate the optical phase of said reference beam relative to the optical phase of said sample beam;
• E) providing means to collect and collimate said sample beam leaving said sample cell unscattered and said fraction of said sample beam light scattered from said sample aliquot in said sample cell;
• F) providing means to combine said reference beam with said collimated fraction of light scattered from said sample aliquot, forming thereby a third beam;
• G) providing means to measure the light intensity values of said third beam at each of a plurality of photo detector elements placed in the path of said third beam;
• H) providing means to convert said light intensity values measured at each photodetector element into digital representations;
• J) processing said digital representations to obtain therefrom said electrophoretic mobility of said particles in said solution.