Analytische Chemie
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The spatial heterodyne detection principle has a great potential in spectroscopy. It has an optical setup similar to that of a Michelson interferometer, with the mirrors replaced by diffraction gratings positioned at fixed, equal distances from the beamsplitter and are slightly tilted. The resulting interference pattern is recorded by a digital camera and the spectrum is recovered by using Fourier transformation. Although SHS was initially developed for astronomical and satellite-based atmospheric measurements, but in recent years it has been started to be applied in other branches of spectroscopy too. Recently the area of laser-induced breakdown spectroscopy (LIBS) has also discovered the potential of SHS. The main appeal of SHS detection in LIBS includes the compactness and robustness of the setup (in view of field applications) and the flexibility to optimize the setup for either high sensitivity or for high resolution, which can be benficially exploited in applications like stand-off measurements, quantitative analysis with isotope resolution, etc.
In the present work, we have improved and further optimized our initial LIBS-SHS setup described in a previous conference. By using optical simulations, we have modelled the light transmission efficiency, instrumental function and imaging properties of the system. We significantly improved and automated the spectral and image data processing sequence. The optimizations carried out resulted in an improved spectral resolution and repeatability, a lower spectral background and the elimination of the central line artifact originating from the Fourier transformation procedure. A detailed characterization of the LIBS spectroscopy performance (e.g. resolution, spectral coverage, tuning range, linearity, etc.), including a comparison with that of a LIBS setup based on a conventional dispersion CCD spectrometer was also performed.
Spatial heterodyne spectroscopy (SHS) is an optical setup that combines both dispersive and interference based methods to obtain spectroscopic information. It has the high light throughput characteristic for interference based methods, but at the same time it has the high resolution typical of grated spectrometers. The basic SHS optical setup is similar to that of the Michelson interferometer, with the mirrors replaced by diffraction gratings positioned at fixed, equal distances from the beamsplitter and are slightly tilted. The resulting interference pattern is recorded by a digital camera and the spectrum is recovered by using Fourier Transformation. Although initially SHS was developed for astronomical and satellite-based atmospheric measurements, where spectroscopy of faint but large light sources are investigated, but in recent years the application of SHS spectroscopy is gaining popularity.
Our research group is active both in Raman-SHS and LIBS-SHS, due to the fact that there are many overlapping challenges for the two spectroscopies in terms of optical and optoelectronic optimization. In the present study, we investigated the possibility of using SH detection for the qualitative and quantitative Raman spectroscopy of liquid samples. We constructed our own compact spatial heterodyne spectrometer using 300 mm-1 gratings (Newport), a 50:50 cube beamsplitter (Thorlabs), dischroic mirrors, bandpass and notch filters (Semrock), a Tamron telelens and a Retiga R1 CCD camera. A DPSS laser (532 nm, 20 ns) with variable energy and repetition rate (up to 100 µJ and 80 kHz) was used for excitation, with its beam driven through a 10x microscope objective (Thorlabs) to focus the laser light inside the liquid samples. The evaluation of the recorded interference patterns was carried out by self-developed software written in Octave.
In the qualitative experiments, we investigated several oils and additives and employed principal component analysis (PCA) for their classification. It was found that the recorded spectra could be separated well in the subspace of just two principal components. The quantitative experiments were conducted with two sets of binary solvent mixtures (isopropanol-cyclohexane, glycerol-water). The simple univariate method based on the net intensity of one spectral peak did not give good results, but principal component regression (PCR) gave rise to fairly good and robust calibrations.
Our results therefore show that a relatively simple and robust SHS setup can be advantageously used for both quantitative and qualitative Raman spectroscopy.