A novel instrument for real time analysis of individual biological cells or other microparticles is described. The instrument is based on inductively coupled plasma time-of-flight mass spectrometry and comprises a three-aperture plasma−vacuum interface, a dc quadrupole turning optics for decoupling ions from neutral components, an rf quadrupole ion guide discriminating against low-mass dominant plasma ions, a point-to-parallel focusing dc quadrupole doublet, an orthogonal acceleration reflectron analyzer, a discrete dynode fast ion detector, and an 8-bit 1 GHz digitizer. A high spectrum generation frequency of 76.8 kHz provides capability for collecting multiple spectra from each particle-induced transient ion cloud, typically of 200−300 μs duration. It is shown that the transients can be resolved and characterized individually at a peak frequency of 1100 particles per second. Design considerations and optimization data are presented. The figures of merit of the instrument are measured under standard inductively coupled plasma (ICP) operating conditions (<3% cerium oxide ratio). At mass resolution (full width at half-maximum) M/ΔM > 900 for m/z = 159, the sensitivity with a standard sample introduction system of >1.4 × 10⁸ ion counts per second per mg L⁻¹ of Tb and an abundance sensitivity of (6 × 10⁻⁴)−(1.4 × 10⁻³) (trailing and leading masses, respectively) are shown. The mass range (m/z = 125−215) and abundance sensitivity are sufficient for elemental immunoassay with up to 60 distinct available elemental tags. When <15 elemental tags are used, a higher sensitivity mode at lower resolution (M/ΔM > 500) can be used, which provides >2.4 × 10⁸ cps per mg L^-1 of Tb, at (1.5 × 10⁻³)−(5.0 × 10⁻³) abundance sensitivity. The real-time simultaneous detection of multiple isotopes from individual 1.8 μm polystyrene beads labeled with lanthanides is shown. A real time single cell 20 antigen expression assay of model cell lines and leukemia patient samples immuno-labeled with lanthanide-tagged antibodies is presented.