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- 2019 (2) (entfernen)
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- Spatial Heterodyne Spectroscopy (1)
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Organisationseinheit der BAM
Spatial heterodyne spectroscopy (SHS) is used for quantitative analysis and classification of liquid samples. SHS is a version of a Michelson interferometer with no moving parts and with diffraction gratings in place of mirrors. The instrument converts frequency-resolved information into spatially resolved one and records it in the form of interferograms. The back-extraction of spectral information is done by the Fast Fourier transform. A SHS instrument is constructed with the resolving power 5000 and spectral range 522 - 593 nm. Two original technical solutions are used as compared to previous SHS instruments: the use of a high frequency diode pumped solid state (DPSS) laser for excitation of Raman spectra and a microscope-based collection system. Raman spectra are excited at 532 nm at the repetition rate 80 kHz. Raman shifts between 330 cm-1 and 1600 cm-1 are measured. A new application of SHS is demonstrated: for the first time it is used for quantitative Raman analysis to determine concentrations of cyclohexane in isopropanol and glycerol in water. Two calibration strategies are employed: univariate based on the construction of a calibration plot and multivariate based on partial least square regression (PLSR). The detection limits for both cyclohexane in isopropanol and glycerol in water are at a 0.5 mass% level. In addition to the Raman-SHS chemical analysis, classification of industrial oils (biodiesel, poly(1-decene), gasoline, heavy oil IFO380, polybutenes, and lubricant) is performed using their Raman-fluorescence spectra and principal component analysis (PCA). The oils are easily discriminated as they show distinct non-overlapping patterns in the space of principal components.
The spatial heterodyne spectrometer (SHS) concept, which is based on an interferometricoptical setup, boasts both the Fellgett and Jacquinot advantages. Theoretically it can provideboosted sensitivity and spectral resolution with respect to dispersion spectrometers in acompact, reasonably cheap arrangement without any moving components – this set ofcharacteristics can be attractive to a number of industrial, space and other field applications. The potential of SHS has already been demonstrated in IR and Raman spectroscopies(e.g.), and more recently also in LIBS. The scientific goal of our present project is toapply the SHS concept to the development of an optimized, but practical dual-grating,tunable SH-LIBS setup, which would possess appealing spectroscopic characteristics.During this development, we extensively rely on the computer-based simulation of theoptical setup, which is an efficient approach that we found to have been missing from earlierSHS efforts published. It can provide application-specific optimization of the SHS systemand predict the performance of the final system. In particular, we use optical simulation tostudy the effect of various important parameters on the relevant spectroscopic figures ofmerits of the system. We used the non-sequential ray-tracing mode of the Zemax/OpticStudio software for optical modelling of the SH-LIBS setup (Optical distortions were studiedin sequential mode). Characteristics of the setup and interferograms were calculated with atleast one million rays. All calculations were carried out for the visible spectral range(400-700 nm), using stepwise extension of monochromatic simulations with 5 nm steps.Wherever applicable, characteristic discrete visible wavelengths from the emissionspectrum of Hg discharge lamps (404.7 nm, 435.8 nm, 546.1 nm, 579.0 nm) were used for thecalculation of spectroscopy figures of merit and for the validation of simulation.