DMEM-HEPES medium incubated with 0.4 mM Na2CrO4 at 37°C under room air generates Cr(V) (left), whereas LHC-9 medium does not (right). The times of incubation with Na2CrO4 are indicated. Representative ESR spectra obtained at room temperature are shown. Instrument settings were: 5 G modulation amplitude, 50 mW microwave power, 6.32 × 104 receiver gain, 40.96 msec time constant, 9.76 GHz microwave frequency, sweep width = 200 G, field set = 3510 G, modulation frequency = 100 kHz, scan time = 42 sec; number of scans, 9.