Photoacoustic spectroscopy on trace gases with continuously tunable CO_2 laser

Abstract

A novel photoacoustic (PA) system that uses a continuously tunable high-pressure CO(2) laser as radiation source is presented. A minimum detectable absorption coefficient of 10(-6) cm(-1) that is limited mainly by the desorption of absorbing species from the cell walls and by residual electromagnetic perturbation of the microphone electronics has currently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, the dependence on energy exhibits a nonlinear behavior owing to saturation effects in excellent agreement with a theoretical model. The calibration of the laser wavelength is performed by PA measurements on low-pressure CO(2) gas, resulting in an absolute accuracy of ± 10(-2) cm(-1). PA spectra are presented for carbon dioxide (CO(2)), ammonia (NH(3)), ozone (O(3)), ethylene (C(2)H(4)), methanol (CH(3)OH), ethanol (C(2)H(5)OH), and toluene (C(7)H(8)) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies with line-tunable CO(2) lasers is demonstrated with the aid of multicomponent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations calculated on the basis of a fit with calibration spectra. Finally, the perspectives of the system concerning air analyses are discussed.

Optical detection of chemical warfare agents and toxic industrial chemicals: Simulation

“…L-PAS involves absorption of the modulated laser radiation followed by deactivation of the excited molecule via collisions, which convert the absorbed energy into periodic local heating at the modulation frequency, and generate acoustic waves that can be monitored using low-noise microphones. [9][10][11][12][13][14][15][16] The photoacoustic signal S in volts is…”

“…Figure 3, showing the absorption spectra of eight target nerve agents and surrogates at wavelengths accessible with 12 CO 2 and 13 CO 2 lasers, indicates that the absorption spectrum of DIMP ͑a relatively harmless surrogate for nerve agents͒ has excellent overlap with the available laser transitions from a 13 CO 2 laser. Consequently, the modeling was performed for a 13 CO 2 L-PAS sensor for DIMP detection. The libraries of spectra were converted into the spectra specific to a 13 CO 2 L-PAS by evaluating the absorption coefficients at the 87 discrete 13 CO 2 laser wavelengths in the 9.6-11.5-µm range.…”

“…Consequently, the modeling was performed for a 13 CO 2 L-PAS sensor for DIMP detection. The libraries of spectra were converted into the spectra specific to a 13 CO 2 L-PAS by evaluating the absorption coefficients at the 87 discrete 13 CO 2 laser wavelengths in the 9.6-11.5-µm range. The spectral signatures of the 45 interferents were then used to evaluate matrix elements involved in the least-squares fit ͑LSF͒ spectral decomposition to be described in Sec.…”

Detection of Greenhouse Gases Using the Photoacoustic Spectroscopy

“…These microphones convert the sound signal into an electric signal, which is filtered and detected by a lock-in amplifier. Photoacoustic spectroscopy is widely used for the detection of several gases in the concentration range of ppbv and sub-ppbv Berrou et al, 2010;Sthel et al,2011;Harren et al,2008;Thomas, 2006;Elia et al, 2009;Sorvajärvi et al, 2009;Sigrist et al, 2008;Angelmahr et al,2006;Filho et al, 2006;Schramm et al, 2003;Harren et al, 2000;Miklos et al, 2001;Gondal, 1997;Repond & Sigrist, 1996;Sigrist, 1994aSigrist, , 1994b. There are some types of trace gas detection systems based on continuous wave (CW) CO 2 laser, optical parametric oscillator (OPO) in combination with photoacoustic spectroscopy and quantum cascade laser (QCL).…”

Atmospheric methanol measurement using selective catalytic methanol to formaldehyde conversion

“…Techniques include photoacoustic spectroscopy (PAS) (Repond and Sigrist, 1996 ;Prasad and Thakur, 2003), Fourier transform infrared spectroscopy (FTIR) (Yokelson et al, 1997(Yokelson et al, , 2003, pre-concentration followed by gas or liquid chromatography (Snider and Dawson, 1985;Goldan et al, 1995a, b;Riemer et al, 1998;Lamanna and Goldstein, 1999;Nguyen et al, 2000;Schade and Goldstein, 2001;Kesselmeier et al, 2002;Millet et al, 2004;Singh et al, 1995Singh et al, , 2000Singh et al, , 2004, and chemical ionisation mass spectrometry (CIMS) (Lindinger et al, 1998;Holzinger et al, 2001;de Gouw et al, 2000de Gouw et al, , 2003Karl et al, 2001Karl et al, , 2003. In spite of this impressive array of tools, the experimental measurement of methanol at typical tropospheric abundances can still be quite challenging.…”