SPATIAL-DISTRIBUTION OF RADIANT INTENSITY FROM PRIMARY SOURCES FOR ATOMIC-ABSORPTION SPECTROMETRY .2. ELECTRODELESS DISCHARGE LAMPS

The spatial distribution of radiant intensity from electrodeless disch arge lamps (EDLs) used as radiation sources in atomic absorption spect rometry is investigated with a digital photodiode array imaging system . Intensity distribution over the radial and longitudinal sections of Pb and Hg lamps is measured for both atomic and ionic lines of the ana lyte and the filler gas. The plasma in the EDLs is highly structured, with metal and filler gas excited species being distributed nonuniform ly but in different ways, The clouds of emitting metal and Ar atoms ar e spatially separated in the volume of the Pb EDL. The excited Pb atom s detected from both the resonance and nonresonance lines have the for m of a thin layer concentric to the bulb walls located near the surfac e of the bulb (''optical skin effect''). In contrast, the emission dis tribution for Ar atomic lines is bell-shaped with a maximum at the cen ter of the plasma, The spatial distribution of emitting Ar ions is mor e complex-there is a bulk maximum coinciding with Ar atomic emission m aximum and another maximum concentric to the walls coinciding with the maximum of metal atom emission. In the Hg EDL the difference between the spatial intensity profiles of metal and filler gas (Ar) lines is l ess pronounced because of the use of an increased filler gas pressure in the lamp, Emitting species of both Ar and metal are primarily locat ed in the bulk of the plasma with, however, a small depletion in the v icinity of the lamp axis, Evolution of the spatial intensity profiles during warm-up of the lamps is investigated as well. In both lamps the radial and longitudinal intensity distributions of metal lines are es tablished during the first minutes after lamp ignition, after which th ere is a slow and monotonic increase of the established intensity prof iles, This result implies thermal vaporization as a mechanism of analy te supply to the plasma, The spatial intensity profiles for Ar lines a re established in the first seconds after lamp ignition, after which o nly the absolute values of the established distributions change. The a pproach to the steady-state intensity of Ar atomic and ionic lines is nonmonotonic; there is a clearly pronounced initial overshoot in inten sity of Ar atomic lines that coincides with a decline in the intensity of Ar ion lines, An interpretation for the observed spatial intensity profiles is given on the basis of radial cataphoresis theory.