Observation of intact (heme‐bound) myoglobin by electrospray ionization on a double‐focusing mass spectrometer

A noncovalently bound complex, the heme‐apomyoglobin protein system ( M r 17 568), was detected using electrospray ionization on a double‐focusing mass spectrometer with an array detector. The kilovolt energy conditions used for ion transmission and focusing did not cause significant amounts of fragmentation of the weakly‐bound complex. With a high‐performance array detector, femtomolar sensitivity was achieved for myoglobin in water.