A method and apparatus for determining the amount of water in an organic solvent solution, such as a DMSO solution. The apparatus comprises an infrared LED for emitting near infrared light; a laser diode collimator for forming a collimated light beam from the light emitted by the LED; a sample container holder for accepting a sample container containing a sample solution; a photodiode for generating an output signal that is related to the intensity of the collimated light beam after the collimated light beam has passed through the sample solution; and a control means for controlling the stability of the near infrared light emitted by the infrared LED by controlling an amount of current flowing through the infrared LED.