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Laser-induced fluorescence spectroscopy: What's behind it?

The laser-induced fluorescence spectroscopy uses the energetic behaviour of individual molecules in the measured medium.

Principle of UV fluorescence

In contrast to infrared spectroscopy, in which the absorption behaviour of the infrared light is measured in a medium, laser-induced fluorescence spectroscopy uses the energetic behaviour of individual molecules in the medium to be measured.

Molecules - in this case the organic or inorganic compounds of the oil or lubricant - have different states which are referred to as energy levels.

Fluorescence spectroscopy is primarily concerned with electronic and vibrational states. Generally, the species (molecule of the substance) being examined has a ground electronic state (a low energy state) of interest, and an excited electronic state of higher energy. Within each of these electronic states there are various vibrational states. In fluorescence, the species is first excited, by absorbing a photon, from its ground electronic level to one of the various vibration states in the excited electronic level. Collisions with other molecules cause the excited molecule to lose vibration energy until it reaches the lowest vibrational state of the excited electronic level.  The molecule then drops down very fast (10-9 s range) to one of the various vibrational levels of the ground electronic state again, emitting a photon in that process.  As molecules may drop down into any of several vibrational levels in the ground state, the emitted photons will have different energies, and thus frequencies.

In the case of the EMG SOLID® LIF solution, a UV laser with a wavelength of either 255 nm or 355 nm is used. The wavelength selection depends on the respective usage Scenario.

This might lead to a fluorescence excitation of the molecules that should be detected (wanted signal), as well as a possible background signal caused by organic matter on the surface below the analytical target.Due to the broad fluorescence spectrum, registering only the spectral intensity distribution of the fluorescence does not necessarily lead to a significant separation of the substance spectra. Therefore, a time-integrating approach is included in the procedure to observe the decay times of fluorescence signals in a suitable wavelength range. In the case of EMG SOLID® LIF the spectrum is analysed in 3 time windows up to 1 µs after the initial laser excitation.

The applied method is therefore called: time-integrated laser-induced fluorescence spectroscopy, or short LIF(t).

The process of UV fluorescence is one of the most efficient interactions between light and matter, and it occurs as described above in dependency of the substances involved and substrate media, with measurably different speeds. A statistical registration of the single photons here offers a detection of very low quantities of material with particularly high sensitivity.

In comparison to infrared spectroscopy the effect is not very sensitive to the surface structures of the substrate. That means changing roughness values on a selected coating type (likegalvanised sheets) or different suppliers of the same material class do not disturb the measurement results. The same is true for lubricants applied in drop form. On the other hand, due to the underlying physical principle the measurement result is heavily depending on the sort of the oil and is affected by impurities of the oil or oil mixtures stemming from earlier batches. Therefore the calibration process needs to be targeted to the specific oil sorts and combinations. That means the calibration and the system usage needs to take into account the specific processing schedule of the production line and possible influencing pre-processes.

Optical excitation is performed by a specially designed, UV microchip laser with a repetition frequency in the range of 10 kHz. Single laser pulses are transferred via a quartz fibre bundle through the sensor head directly onto the material surface. With a second quartz fibre bundle (also part of the sensor head), the transmission of fluorescence signals to the detector is arranged. Therefore, the corresponding sensor head (see principle picture) contains no active components, and is suitable to be used in harsh and demanding environmental conditions.

Curious? Would you like to know more about EMG SOLID® IR or EMG SOLID® LIF? Please feel free to contact our responsible sales manager Karl-Heinz Fröhning (karl-heinz.froehning@emg-automation.com), our international sales organisation or our product manager Timo Gemmer (timo.gemmer@emg-automation.com).