Very pure, carefully distilled VSMOW remains important in the manufacture of high-accuracy temperature measurement reference standards. Due to differences in isotopic composition, water samples from various sources may exhibit slight differences in physical properties, including triple point, density, boiling point, and vapor pressure. Snow, river water, and rainwater, all of which are recently evaporated ocean water, tend to be depleted in the heavier isotopes of hydrogen and oxygen, which affects the triple point. Temperature reference cells filled with water of improper isotopic composition can cause errors of several hundred microkelvin in the measured triple point.