High-Sensitivity Spontaneous Raman Spectrometer for Gaseous Media

Abstract

We have developed a spectrometer designed to record spontaneous Raman scattering (SRS) spectra in gaseous media. The spectrometer has improved sensitivity due to using a high-performance spectral instrument and also a device for compressing the analyte gaseous medium. We present the results of an experimental test of the spectrometer, and show that its limiting sensitivity for methane is <1 ppm.Introduction. Diagnostics of the composition of gaseous media at a new technical level are greatly needed in today's society. Environmental protection agencies and agencies of the Ministry for Civil Defense, Emergencies, and Disaster Response especially need high-sensitivity monitoring instruments solving problems for both environmental monitoring of atmospheric pollution and identifying emergency situations associated with emission of toxic gases into the atmosphere [1]. In connection with development of the corresponding instrumentation, refi ning analysis methods, and improving metrological characteristics of gas analyzers, the list of their fi elds of application is steadily expanding. Today they are acutely needed to solve such problems as diagnosis of diseases from the composition of exhaled air [2][3][4][5], detection of explosives [6-8] and narcotics [9], optimization of combustion processes in heating and power plants [10], high-precision monitoring of the composition of fuel gases [11][12][13], etc.Despite the broad range of physical principles for monitoring gaseous media and instruments based on them, optical diagnostics methods are especially important. This is because of their high sensitivity, selectivity, and speed at which results are obtained. The most attractive optical diagnostics method is spontaneous Raman scattering (SRS) spectroscopy. The SRS method for gas analysis makes it possible to simultaneously record any molecular components of the gaseous medium using a single laser with fi xed wavelength, where the SRS signal for each gaseous component is strictly individual, proportional to its concentration, practically instantaneous, and does not depend on the composition of the gaseous medium. Owing to such a unique set of properties, the SRS method is a general-purpose optical method for analysis of multicomponent molecular gaseous media, including media whose composition is not known ahead of time. The major disadvantage of this method is the low intensity of the SRS signals in a gaseous medium, due to the low density of molecules and the small light scattering cross section (~10 -30 cm 2 /sr). However, when using high-sensitivity multichannel photodetectors, and also with improved effi ciency of detection of the SRS signals, the possibility arose to design a high-sensitivity SRS spectrometer and an SRS gas analyzer.Design of the Spontaneous Raman Spectrometer. An SRS spectrometer for gaseous media was modifi ed in the Environmental Instrumentation Laboratory of the Institute for Monitoring Climatic and Ecological Systems (Siberian Branch, Russian Academy of Sciences). The block diagram for this inst...

Depolarization Ratios of Methane Raman Bands as a Function of Pressure

“…It is present in atmospheric air, often found in inclusions in minerals, and is also the dominant component of natural gas. Development of Raman gas analysis [1][2][3][4][5][6][7][8][9] brings up the need of information that allows taking into account changes in the methane Raman spectrum under various conditions (temperature, pressure, environment). At present, shifts and broadenings of its fundamental bands [10][11][12][13][14][15], as well as changes in the ratio of peak intensities [13,[15][16][17] are known.…”

“…Pressure dependence of integrated intensities in the range of 2890-2940 cm −1 for polarized (a) and depolarized (b) methane spectra which were normalized using Equation(2).Molecules 2020,25, 1951 6 of Pressure dependence of integrated intensities of ν3 band in the range of 2820-2870 cm −1 for depolarized methane spectra which were normalized using Equation 2.…”

Pressure dependence of the Raman signal intensity in high‐pressure gases

“…Among them are the application of multipass and intracavity systems of excitation of Raman scattering, hollow‐core fibers, and surface enhancement . However, the most efficient and easiest approach used to increase the Raman signals is the compression of the gas being analyzed . It is well known that broadening and shift of spectral lines are observed when the gas pressure changes.…”

“…[19][20][21][22][23] However, the most efficient and easiest approach used to increase the Raman signals is the compression of the gas being analyzed. [4,5,24,25] It is well known that broadening and shift of spectral lines are observed when the gas pressure changes. A large number of works have been devoted to this fact from the moment of discovery of the phenomenon of Raman scattering.…”

Optimization of parabolic cell for gas Raman analysis

“…Moreover, several sample cells with special optical structures can also be found, such as the near‐confocal cavity reported by Li et al and Yang et al, by which high‐sensitivity detection with tens of parts per million for multitrace gas was achieved . The similar work was also reported by Petrov et al, which is an improved multipass optical system that reaches LODs with several parts per million for gas media by 30‐s registration time under the exciting laser power of 5 W. Normally, the key element of multipass system for signal enhancement is the design and alignment of the reflectors, so high‐precision manufacture and excellent mechanical stability are all needed.…”