Evaluation of Speciation Technology - OECD Nuclear Energy Agency

Photo-thermal spectroscopy
The thermal effect induced by non-radiative relaxation of a given excited light absorber can be
measured by different detection procedures [4], for which different names of spectroscopy are given.
There are two different detection procedures distinguished in principle, direct or indirect detection.
A schematic illustration of photo-thermal spectroscopic processes is shown in Figure 3.
Figure 3. Schematic illustration of photo-thermal spectroscopic process
Laser-induced Photothermal Spectroscopy
Modulated light source pulsed or chopped cw laser
Ion specific absorption at excitation wavelength
Generation of heat by nonradiative relaxation
Modulated
volume expansion
Propagation
of acoustic wave
Detection by sensitive
"microphone":
Modulated change
in refractive index
Generation of
photothermal lens
Detection by
probe laser beam:
Piezoelectric
transducer
Interferometer
Defocussing of
probe beam
Deflection of
probe beam
LPAS LIPDS TLS
PDS
Acoustic signals generated by thermal expansion can be detected either by a direct contact mode
known as laser-induced photo-acoustic spectroscopy (LPAS) [5,6,7] or by a remote contact mode via
interferometer known as laser-induced photo-thermal displacement spectroscopy (LIPDS) [8].
Difference in the two methods is simply whether the acoustic detector is contacted directly to a sample
cuvette or not. Another mode of measurement is deflection or de-focusing of a probe laser beam by the
change of the refractive index within the absorbing sample, which are known as thermal lensing
spectroscopy (TLS) [9] or photon deflection spectroscopy (PDS) [10], respectively. All four
spectroscopy with different names are the same for their function, i.e. they all function as conventional
absorption spectroscopy but with higher sensitivity. The detection of the relaxation process as
executed in the given spectroscopy is advantageous for the sensitivity over the measurement of light
transmission as done in conventional absorption spectroscopy.
A typical application of LPAS is shown in Figure 4 for the speciation of Np(IV). The concentration
of Np(IV) at 1.7 × 10 -7
mole/L can be well speciated. The speciation sensitivity is found to be
10 -8 mole/L. For this purpose, the aqueous background is depressed by using deuterium water.
LPAS provides the sensitivity of about 10 -6 cm -1 in absorbance for different oxidation states of
actinides [7]. The limiting factor is the water absorbance for the aqueous speciation. Other laser-induced
photo-thermal spectroscopic methods have the same limiting factor and comparable sensitivity.
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