The Development of Cavity Ringdown Spectroscopy as a Sensitive Continuous Emission Monitor for Metals Page: 3 of 5
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the graphite tube. Introduction of test samples followed standard electrothermal atomization
atomic absorption spectroscopy (ETA-AAS) methods.
Mercury and Lead were chosen for the initial study partly because of the need for mercury
monitoring within DOE and partly due to the fact that their volatility would present a challenge
to the technique. Preliminary experiments used the Pb 283.3 nm absorption line to confirm the
viability of ETA-CRDS for trace analysis. This Pb absorption line was chosen, rather than the
stronger 217.0 nm absorption, for convenience with the laser system and cavity mirrors. The Pb
solution data for Figure 2 was obtained by injecting 7.5 [l of 2 ng/ml lead standard solution for
each heating cycle. As is clear from equation 1, 1/it is proportional to the absorbance als.
The first experimental data, using ETA-CRDS, indicates that this technique has the potential to
significantly enhance the sensivity of electrothermal atomization systems. Preliminary results
for mercury and lead standard solutions gave detection limits of approximately 2.8 and 1 pg
respectively were obtained for peak height absorption measurements. No chemical matrix
modifiers were used. These detection limits, obtained in a clearly non-optimum experimental
setup, compare favorably to commercial GF-AAS systems (Hg DL ~30 pg) that have undergone
continuously optimization over many years. Additional experiments to measure a detection limit
for Hg using a Hg-hydride cold vapor system as the Hg source are in progress.
Continuing improvement of the ETA-CRDS technique includes optimization of the laser
linewidth, cavity stability during the furnace atomization sequence, data acquisition rate and
timing, and the operation of the graphite furnace itself. For example, the atomization of lead
within the GF is complete in well under one second while the laser repetition rate was only 20
Hz. This implies that the results obtained in this study would greatly benefit from the use of
higher repetition rate lasers such as an excimer (100-1000 Hz) or Cu vapor (2-32 kHz) lasers.
Finally, in a glance to the future, it is important to note that CRDS has been carried out in other
"non-traditional" configurations. Diode lasers have been employed for CRDS of molecular
species at near-infrared wavelengths, and provide a small, compact light source for CRDS
measurements. In addition, frequency-doubled diode lasers have been employed at ultraviolet
wavelengths for atomic absorption spectrometry (but not CRDS) for a few selected elements. As
the wavelength coverage offered by diode lasers and solid state lasers in general continues to
grow, the possibilities for constructing small, ultra-sensitive CRDS spectrometers for analytical
atomic spectrometry continue to increase.
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Miller, George P. The Development of Cavity Ringdown Spectroscopy as a Sensitive Continuous Emission Monitor for Metals, report, June 1, 1999; United States. (digital.library.unt.edu/ark:/67531/metadc781918/m1/3/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.