Real-time NMR spectroscopy. Through measurement of total Na⁺ per unit volume of cell culture through nuclear magnetic resonance or NMR, one can monitor cell concentration. NMR is based on the measurement of the reaction Na⁺ nuclei when culture is placed in a magnetic field and exposed to electromagnetic radiation. Since intracellular Na⁺ concentration is low relative to extracellular Na⁺ concentration, with increase in cell concentration, total Na⁺ will decrease, and will give an estimate of cell concentration. NMR is designed to monitor continuously the high density of viable cells (10⁸ cells ml^-1) in hollow fiber bioreactors. At such high concentration, other methods fail. However, the other methods also give an estimate of biomass (living or dead).