Abstract: Electrospray ionization mass spectrometry systems and methods are disclosed in which multiple source materials may be introduced into a mass spectrometer at the same time without mixing the different source materials. Integrity of a sample material may be maintained before and during analyses that require the introduction of a reference material at the same time as the sample material. Independent control may be obtained over, e.g., the voltage level of the different source materials, flow rates of the different source materials through the different capillaries, and alignment of the different capillaries used to introduce each of the source materials.