We present results obtained with a novel mass spectrometer capable of determining the mass of multiply charged electrospray ions generated from samples of macromolecules in the megadalton (MDa) size range. The instrument utilizes a sensitive amplifier which can detect the charge on a single ion as it passes through a tube detector. A velocity measurement of an ion with known electrostatic energy provides the ion's mass-to-charge ratio. Simultaneous detection of the ion charge permits a mass assignment to be made for each ion. Electrospray ions of DNA and polymer molecules with masses greater than 1 x 10(6) Da and charge numbers (z) in excess of 425 e(-) are readily detected in this mass spectrometer. The weights of small particles were also measured. The on-axis single-ion detection configuration provides a duty cycle of nearly 100% and extends the practical application of electrospray mass spectrometry to the analysis of very large molecules with relatively inexpensive instrumentation.

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The simultaneous measurement of the time‐of‐flight and the number of charges on each electrosprayed ion provides a new way to determine the mass of a whole intact virus. The weights of an icosahedral virus (rice yellow mottle virus) consisting of a single‐stranded RNA surrounded by a homogeneous protein shell with a mass of 6.5×106 Da, and a rod‐shaped RNA virus (tobacco mosaic virus) with a mass of 40.5×106 Da were measured with this technique.

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