Ra. Multari et al., EFFECT OF SAMPLING GEOMETRY ON ELEMENTAL EMISSIONS IN LASER-INDUCED BREAKDOWN SPECTROSCOPY, Applied spectroscopy, 50(12), 1996, pp. 1483-1499
In laser-induced breakdown spectroscopy (LIBS), a focused laser pulse
is used to ablate material from a surface and form a laser plasma that
excites the vaporized material, Geometric factors, such as the distan
ce between the sample and the focusing lens and the method of collecti
ng the plasma light, can greatly influence the analytical results, To
obtain the best quantitative results, one must consider this geometry.
Here we report the results of an investigation of the effect of sampl
ing geometry on LIBS measurements. Diagnostics include time-resolved s
pectroscopy and temporally and spectrally resolved imaging using an ac
ousto-optic tunable filter (AOTF). Parameters investigated include the
type of lens (cylindrical or spherical) used to focus the Laser pulse
onto the sample, the focal length of the lens (75 or 150 mm), the len
s-to-sample distance (LTSD), the angle-of-incidence of the laser pulse
onto the sample, and the method used to collect the plasma light (len
s or fiber-optic bundle), From these studies, it was found that atomic
emission intensities, plasma temperature, and mass of ablated materia
l depend strongly on the LTSD for both types of Lenses, For laser puls
e energies above the breakdown threshold for air, these quantities exh
ibit symmetric behavior about an LTSD approximately equal to the back
focal length for cylindrical lenses and asymmetric behavior for spheri
cal lenses, For pulse energies below the air breakdown threshold, resu
lts obtained for both lenses display symmetric behavior, Detection lim
its and measurement precision for the elements Be, Cr, Cu, Mn, Pb, and
Sr, determined with the use of 14 certified reference soils and strea
m sediments, were found to be independent of the lens used. Time-resol
ved images of the laser plasma show that at times >5 mu s after plasma
formation a cloud of emitting atoms extends significantly beyond the
centrally located, visibly white, intense plasma core present at early
times (<0.3 mu s). It was determined that, by collecting light from t
he edges of the emitting cloud, one can record spectra using an ungate
d detector (no time resolution) that resemble closely the spectra obta
ined from a gated detector providing time-resolved detection. This res
ult has implications in the development of less expensive LIBS detecti
on systems.