Arsenate (As(V)) is a fatal poison and is widely distributed in the environment. The quick and accurate determination of As(V) is of great importance. Herein, we developed a novel competitive coordination strategy for the quantification of ultratrace As(V) using online internal extractive electrospray ionization mass spectrometry (iEESI-MS). Our strategy demonstrates great success for direct ultratrace As(V) detection in broad types of samples, from solids to liquids, such as food, water and biological samples.