MULTIBAND SPECTROMETER FOR LIBS APPLICATIONS

Photon etc uses its unique technology based on volume Bragg gratings (non dispersive) to help scientists and process and control engineers improving the sensitivity, lower the cost, and reduce the overall size of conventional LIBS spectrometers. As show in Figure 1, Photon etc’s spectrometer is a multiband filter using ultrafast electronics and photodiode to detect simultaneously any desired spectral lines, within an extremely wide spectrum (typically from 380 to 2300 nm). Photon etc’s multiband spectrometer puts together all the needed optomechanics and ultrafast electronics into a single package. Optical properties of volume Bragg gratings allow this multiband spectrometer to have a larger equivalent entrance slit, which enhances the detectable plasma light. The flux density can thus be very high, without compromising the spectral bandwidth.

Photon etc is currently prototyping an instrument which will give the market a state-of-the-art LIBS multiband spectrometer, providing scientists and engineers significant combined advantages.

Advantages:
- High sensitivity;
- Integrated spectrometry technology, band detection, and optical gating into a single package;
- Reduced overall size compared to conventional spectrometers;
- Favorable cost breakdown compared to conventional spectrometers for similar sensitivity.

The measurement of each individual band compared to a conventional spectrometer measurement is shown in Figure 2. In this example, we see the three discrete bands used to perform the LIBS analysis using Photon etc’s unique technology. The three bands are measured simultaneously and give the user very high sensitivity together with a narrow bandwidth on each band. The efficiencies of simultaneous measurement of each band can be adjusted at the highest possible values (typical diffraction efficiencies are more than 85% per band).

Principle of Laser Induced Breakdown Spectroscopy (LIBS)

Laser Induced Breakdown Spectroscopy is used to characterize materials. It involves laser ablation of a material to create a plasma and spectroscopic technology to observe and analyze the plasma light spectrum. This allows the determination of the atomic constituents of any solid, liquid, or gas material.

Many current spectroscopic techniques use a grating to disperse the plasma light and a CCD or detector array to capture and analyze the plasma light spectrum. An example of such an arrangement is shown in Figure 3, in which a Q-switched pulsed laser is used to ablate the surface of a sample, thereby creating the plasma. Optical energy from the plasma is collected by optics and typically transferred to an optical fiber. At the opposite end of the optical fiber, the optical energy is output towards a spectrometer which, in this example, includes a diffraction grating and a detection element. The diffraction grating disperses the light from the fiber according to wavelength, and the spectrally dispersed light is detected by the detection element, a charge-coupled device (CCD). The output of the detection element is directed to a computer to perform the spectral analysis.

LIBS may be used to analyze a wide optical spectrum from the plasma of an ablated sample. However, if the constituents of a material to be analyzed are known, LIBS may be used to evaluate the relative abundance of each constituent element, or to monitor the presence of impurities. In such cases, only one or a few spectral lines might be of interest, along with an appropriate background continuum evaluation.

Because of the Bremsstrahlung and the ion-species recombination at the first nanosecond to microsecond following the spark, the optical background associated decreases more rapidly than the spectral line (Figure 4). LIBS system should then have an ultrafast and precise gating mechanism to properly trigger the integration of the signal. In this example, the integration process should start after a few tenth of µs (after the background attenuation), not before. In other applications, the gating process should involve only a few microseconds of delay with a resolution of 100 ns before integration of the signal.

As mentioned previously, in many LIBS qualitative or quantitative analysis, only a few spectral lines are requested. In such cases, the time required to acquire the whole spectrum with high sensitivity may be quite significant. In fact, in this high sensitivity regime, users typically have to couple an ICCD camera to the spectrometer in order to detect and properly gate the plasma light spectrum measurement.