ETL 1110-1-171
31 Jan 96
provided by the excitation source. Hence, the resulting
wavelength of fluorescence or emitted light is always longer than
the wavelength of the excitation source. Because a population of
molecules is being considered and each molecule within the
population has its own micro environment, a variety of decay
times and wavelength emissions can be observed. The intensity of
light produced at a specified wavelength and time is indicative
of the number of molecules within the population producing the
fluorescence. Fluorometry data can be plotted in two ways:
first, the intensity of light produced is graphed against the
wavelength of emission. Second, the intensity of light produced
is graphed against time after excitation has occurred.
(c) The SCAPS POL sensor uses laser-induced fluorescence
(LIF) technology within a cone penetrometer probe. A diagram of
the fiber optic LIF probe is shown in Appendix C. The LIF sensor
= 10-9 meter) and pulses at a rate of 10 times per second. The N2
laser has enough energy to excite polynuclear aromatic
hydrocarbon (PAH) compounds with 3 or more rings with a high
degree of efficiency. Single ring aromatic, double ring
aromatic, and aliphatic hydrocarbons will not fluoresce
efficiently when excited at 337 nm. A 400 micrometer (Fm; 1
transmits light from the exit port of the laser, down through the
push rods and to a 6.35 mm diameter sapphire window located in
the sensor probe. The window is located 0.6 meter above the cone
tip. After the laser light reacts with the soil matrix and
fluorescence is produced, the light returns through the window
and is collected and transmitted back up the probe by another 400
Fm fiber optic cable. The return fiber terminates at the on-
board optical analyzer.
The analyzer is comprised of a
diffraction grating, a photo diode array, a multichannel
analyzer, and a data processor. The photo diode array consists
of 1024 diodes. The system records the light intensity at each
diode over the range between 300 and 800 nm; thus the optical
analyzer provides approximately 0.5 nm resolved spectra.
Wavelength maximum (peak wavelength) and intensity are used to
characterize the nature and concentration of the fluorescent
material in the soil matrix.
(d) Higher excitation energies (or shorter wavelengths) are
required to produce fluorescence in light aromatic hydrocarbons
such as benzene. The tunable dye laser (normally operated at 290
nm) developed by the Air Force is capable of detecting light
aromatic hydrocarbons of less than 3 rings.
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