RAMAN APPLICATION IN EMULSION POLYMERIZATION SYSTEMS - Raman Spectroscopy for Soft Matter Applications - page 125

optic technology to transfer the signal to the reactor and back to the
detector. This means that the most sensitive part of the Raman equip-
ment (the laser source and the detector) can be installed in the control
room in a less demanding environment. In any case, there are a number
of aspects that must be considered in the implementation of advanced
hardware instruments in industrial environment.
5.5.1.1 Safety An important issue of implementing Raman spec-
troscopy in industrial reactors is the use of a laser source in a dispersion
plant. Two main risks are associated with the use of lasers in working
environments: (1) exposure to direct and indirect light from a laser can
cause irreversible damage to the eye and also severe burning; (2) the
laser, at suffi ciently high power, can set fi re in an explosive atmosphere
(it has been shown for styrene and butadiene that the power limit is of
about 200mW [89]). Fortunately, the laser power typically measured
out of a Raman probe is 100 mW (for an FT-Raman equipment with a
NIR laser [1064nm] and a power of 1.5W) and furthermore, beam
attenuation derivation shows that 8cm of water is enough to absorb
80% of the beam power and hence signifi cantly reduce the risk of the
laser light when implemented in industrial emulsion polymerization
reactors. Nevertheless, very stringent safety procedures must be fol-
lowed to implement Raman spectroscopy in a safe manner in an indus-
trial environment. Typically, procedures are necessary to avoid eye
contact with the laser when the reactor is empty and installing the
optical fi bers into nitrogen-purged steel tubes through the plant to
avoid light leakage to the environment.
5.5.1.2 Selection of the Excitation Wavelength of the Laser
Source A balance between emission intensity (sensitivity) and fl uo-
rescence in technical systems which are seldom very clean, and there-
fore fl uoresce, is necessary. As discussed in other chapters and in Section
5.2 , fl uorescence can be avoided by using excitation in the NIR region,
but this implies the use of FT technology and more expensive detectors
(e.g. liquid N 2 cooled Ge detectors) to enhance the sensitivity. Also,
self - absorption of Raman - emitted light may complicate quantifi cation
and requires a constant distance between the probe and the analyzed
material or constant focus depth of the light-collecting lenses in the
reaction mixture, especially in transparent mixtures.
If excitation wavelengths in the visible range are selected, one has
to cope with strong fl uorescence, which requires spectra pretreatment.
From an industrial point of view, the decision on which laser is chosen
is generally made based on availability and cost criteria (i.e. replace-