Chemiluminescence emission intensity is modulated by modulating the po
tential of a working electrode which is used to generate a key species
in the electrogenerated chemiluminescence (ECL) reaction. The emissio
n is monitored synchronously using a lock-in amplifier. The reactions
used in the characterization are luminol with hydrogen peroxide and tr
is(2,2'-bipyridyl)ruthenium (II) (or Ru(bpy)3(2+)) with oxalate. Modul
ation widths of +/- 50 mV yield maximum signals for luminol when cente
red at 0.45 V (vs Ag/AgCl) and for Ru(bpy)3(2+) when centered at 1.05
V. The resulting signal decreases with increasing modulation frequency
and shows that luminol/H2O2 is a faster ECL system than Ru(bpy)3(2+)/
oxalate. Working curves for luminol and for oxalate have essentially t
he same linear range and slope with the modulated potential approach a
s with a DC electrode potential. This approach provides capability for
differentiating the analytical signal from constant background emissi
on or stray light.