Delayed responses of analyte emission in a pulse-modulated direct-current argon arc at atmospheric pressure

A pulse-modulated direct-current argon arc burning at atmospheric pressure has been investigated by analyzing temporally and spatially resolved analyte emission responses in a millisecond time domain. The arc current was a rectangular pulse modulated between 9 and 3.5 A with a modulation period of 250 ms and a low current interval of up to 50 ms. Both positive and negative step modulation in current strongly affected the analyte emission. Delayed responses of representative analytes with ionization energies ranging from 6.5 to 10 eV have been studied. Depending on the analyte ionization energy and the plasma zone observed, a sudden current change was in most cases followed by a line intensity increase. The magnitude of this increase is correlated with changes in the ionization-recombination balance, the extent of demixing and the gas dynamics processes invoked by a current modulation. For analytes with medium and low ionization energies a current drop is accompanied by a large increase in signal-to-background ratio, which opens up the possibility of the use of arc current modulation for sensitivity improvement.