The isotopic ratios of VSMOW water are defined as follows:   Although each water molecule () contains two hydrogen atoms and one oxygen atom, VSMOW composition is given in terms of the atoms and is indifferent to the isotopic combinations in molecules. The particular molecular combinations do not affect the properties of VSMOW, and in fact the molecules in a water sample exchange atoms rapidly and continuously. The International Committee for Weights and Measures specified in 2005 that the definition of the Kelvin thermodynamic temperature scale would refer to water with a composition of the nominal specification of VSMOW. The decision was welcomed in 2007 by Resolution 10 of the 23rd CGPM. This effectively defined the triple point of VSMOW to be exactly 273.16 °K and 0.01 °C. As of the 2019 redefinition of the SI base units, the Kelvin is defined in terms of the Boltzmann constant, which makes the definition completely independent of the properties of water. The triple point now has a measured value, not a defined value. The defined value for the Boltzmann constant was selected so that the measured value of the triple point is identical to the prior defined value to within the limits of accuracy of contemporary metrology. 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.