The units and notation above are used when dealing with the physics of mass spectrometry; however, the m / z notation is used for the independent variable in a mass spectrum. This notation eases data interpretation since it is numerically more related to the dalton. For example, if an ion carries one charge the m / z is numerically equivalent to the molecular or atomic mass of the ion in daltons (Da), where the numerical value of m / Q is abstruse. The m refers to the molecular or atomic mass number (number of nucleons) and z to the charge number of the ion ; however, the quantity of m / z is dimensionless by definition. An ion with a mass of 100 Da (daltons) (m = 100) carrying two charges (z = 2) will be observed at m / z 50. However, the empirical observation m / z 50 is one equation with two unknowns and could have arisen from other ions, such as an ion of mass 50 Da carrying one charge. Thus, the m / z of an ion alone neither infers mass nor the number of charges. Additional information, such as the mass spacing between mass isotopomers or the relationship between multiple charge states, is required to assign the charge state and infer the mass of the ion from the m / z. This additional information is often but not always available. Thus, the m / z is primarily used to report an empirical observation in mass spectrometry. This observation may be used in conjunction with other lines of evidence to subsequently infer the physical attributes of the ion, such as mass and charge. On rare occasions, the thomson has been used as a unit of the x-axis of a mass spectrum.