A calibration device and a method of calibrating a mass spectrometer are described where two or more immiscible mass spectrometry calibration compounds in close proximity to each other share a common headspace volume above their liquid surfaces. This arrangement allows each calibrant to evaporate at differing rates while allowing the headspace concentrations to remain relatively unchanged over time (forming a quasi-equilibrium calibrant mixture). The mixture is either delivered to an ion source of a mass spectrometer or to a vacuum pump via a flow restrictor from a calibration vial. The calibrant gas mixture in the headspace volume may be used to calibrate a mass spectrometer.