Elucidation of plume fluorescence mechanism for spectrochemical analysis

We developed an analytical technique capable of detecting 0.2 ppb of lead colloids in drinking water and atto-moles of trace elements in laser printed inks. This technique has sensitivity comparable to laser-excited atomic fluorescence (LEAF). Yet unlike LEAF when one wavelength only excited one transition, it induced numerous analytes to fluoresce with one single excitation wavelength. Multi-analyte fluorescence was intriguing. We believe the process was as follows. A nanosecond laser pulse vaporized the sample. The atoms in the dense plume interacted strongly. That resulted in the smearing of their energy levels. This was especially severe for highly excited states. Photoexcitation to these smeared levels was therefore non-selective. As the plume expanded, the atom-atom interaction diminished and the photoexcited species behaved like isolated atoms again. They decayed radiatively to emit sharp spectral lines.

So far, our experimental observations were consistent with the general ideas of our model. We now plan to elucidate the details by examining four key steps of the process. First, we will analyze the plume creation step and design ways to minimize optical interference while maximize volatilization. Second, we will study the absorption of the excitation laser beam by the dense plume. We will use various excitation wavelengths to map the smeared energy levels. Third, as the plume disperses, we will do time-resolved spectroscopy to track the metamorphosis of energized molasses to well-defined excited analytes. We will try to design and engineer the product states. Fourth, we will characterize the analyte spectral emissions, both their intensity and line shape, and tailor them for spectrochemical analysis. Ultimately, we aim at making ppb and atto- mole sensitivity an everyday performance.

Our analysis required little sample preparation, could be carried out rapidly in real- time, and featured a host of capabilities associated with laser beams, such as good spatial and temporal resolution for in-situ and non-contact sampling. We have applied it to matrices ranging from metals through ceramics to plastics. We have observed signature emissions from numerous elemental and molecular analytes. A technique with this combination of sensitivity and versatility is unheard of. The simultaneous capture of signals from multi analytes, including trace components, will present vast chemometric opportunities for the classification of complex samples. This can be real- time tissue identification during laser ablative surgery, or bio-agent screening in the field. The application horizon is beyond imagination.