The invention relates to a method for separation of elements from a fluid using affinity-bearing particles suspended in the fluid and using ultrasonic standing waves and micro-fluidics. The method includes the steps of: mixing said fluid mixture with particles (10) having affinity to at least one element (9) to be separated; allowing the element (9) to be separated to bind to said affinity-bearing particles (10); subjecting the fluid to an ultrasonic wave field resulting in forces on the affinity-bearing particles (10) but substantially no forces on elements not bound to affinity-bearing particles; and allowing said forces to move said affinity-bearing particles (10) to a portion of the fluid thus obtaining a locally higher concentration of affinity-bearing particles. The method may be performed in a process with continuous flow.
