Laser Pyrolysis

Both ohmic and inductive pyrolyses are initiated by heating the back of the sample (which can be a serious disadvantage: see Sampling, below). In contrast, laser heating involves surface initiation of the degradation process. Table 2 compares the pertinent elements of the two processes.

One of the recommended sources of energy is a pulsed neodynium-YAG laser which is mounted in parallel with a neon laser aligned on an identical light path to afford a sight line to aid selection of the required target.

Very rapid heating requires a high thermal flux; perhaps the idea of a pulsed laser is far too simplistic when only the thermal aspects of laser-induced pyro-lysis are considered, for the process involves a short, intensive photolysis which radically differs from our present understanding of the pyrolytic process.

A phase-coherent laser beam delivers packets of photons in a nanosecond pulse into the surface of the sample. It should be noted that if the sample is optically transparent, a pigment must be added to provide absorbing centres to promote the ionization of the molecules in the sample surface. Table 2 Comparison of two heating processes

Figure 4 Diagrammatic cross-sectional representation of the pre-column concentration unit (not to scale).

The ionization process can be explained either by electron tunnelling or by multi-photon absorption. After initial ionization, photon energy is selectively absorbed by electrons above the sample surface and the hot electron cloud or laser plume collapses into the sample surface whereupon molecular fragments are pumped into the hot plasma. On termination of the pulse the system rapidly returns to ambient.

The plasma, consisting of unbound electrons, free atoms and those few radicals of unusual stability, is in kinetic equilibrium but when the unbound electrons return to their accustomed atomic states the resultant species quench directly from the plasma and provide an informative series of products. However, other species can arise from thermal scissoring within the solid sample and a further series of products can then result from interaction of those species with certain plasma components.

Ohmic inductive heating

Pulsed laser heating

Substrate Sample

Heating geometry Heating rate Cooling rate Pyrolysis condition Time

Limitations

Metal platten or coil, or ferromagnetic probe Solution or sonic dispersion Sample back 103 Ks~1 - 103 Ks~1

Sample continuum (pigmented if necessary) Solid chip or compacted powder Sample front 107 Ks~1 - 106 Ks^1

100-2000 ms

Possible catalytic effect(s). 'Blow-off' due to gaseous products at substrate causing molten undegraded sample to balloon and burst

Both in an inert atmosphere 4-800 js

Heat input/take-up indeterminant. Possibility of plasma reactions and photolysis. Transparent samples need pigmenting

Figure 5 Cross-section of laser pyrolysis cell insert (not to scale).

A laser pyrolysis cell insert for the chromato-graphic injection port receptor (Figure 2) is shown in Figure 5.

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Solar Panel Basics

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