The method is for preparing a sample in a microfluidic device. A microfluidic device is provided that has a first reservoir in fluid communication with a second reservoir in fluid communication with and adjacent to a draining unit that has a first absorbing member disposed therein. The first reservoir contains a first liquid that is held in the first reservoir by a capillary stop valve connecting the first and second reservoirs. The second reservoir has a sample support disposed therein. A second liquid, containing substances, is added to the second reservoir. The second liquid contacts the first liquid and the first absorbing member. The first absorbing member absorbs the second liquid and the first liquid. The substances adhere to the sample support.