A method of detecting redispersion of particles into a solution using for example FTIR. The method including providing a sensor surface with dry particles; illuminating the sensor surface with light along a first optical path and detecting the light reflected by the sensor surface; providing a liquid to a volume in contact with the sensor surface; and detecting the reflected light while the dry particles redisperse into the liquid. The angle between the first optical path and the sensor surface fulfils the condition of total internal reflection. Further, an FTIR cartridge may be provided for use in said method. The cartridge including a sensor surface accessible for FTIR detection including at least one binding area wherein label particles are situated.