Sequential Collinear Photofragmentation and Atomic Absorption Spectroscopy for Online Laser Monitoring of Triatomic Metal Species.

Industrial chemical processes are struggling with adverse effects, such as corrosion and deposition, caused by gaseous alkali and heavy metal species. Mitigation of these problems requires novel monitoring concepts that provide information on gas-phase chemistry. However, selective optical online monitoring of the most problematic diatomic and triatomic species is challenging due to overlapping spectral features. In this work, a selective, all-optical, in situ gas-phase monitoring technique for triatomic molecules containing metallic atoms was developed and demonstrated with detection of PbCl2. Sequential collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) enables determination of the triatomic PbCl2 concentration through detection of released Pb atoms after two consecutive photofragmentation processes. Absorption cross-sections of PbCl2, PbCl, and Pb were determined experimentally in a laboratory-scale reactor to enable calibration-free quantitative determination of the precursor molecule concentration in an arbitrary environment. Limit of detection for PbCl2 in the laboratory reactor was determined to be 0.25 ppm. Furthermore, the method was introduced for in situ monitoring of PbCl2 concentration in a 120 MWth power plant using demolition wood as its main fuel. In addition to industrial applications, the method can provide information on chemical reaction kinetics of the intermediate species that can be utilized in reaction simulations.

Rate constant and thermochemistry for K + O2 + N2 = KO2 + N2.

The addition reaction of potassium atoms with oxygen has been studied using the collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) method. KCl vapor was photolyzed with 266 nm pulses and the absorbance by K atoms at 766.5 nm was measured at various delay times with a narrow line width diode laser. Experiments were carried out with O2/N2 mixtures at a total pressure of 1 bar, over 748-1323 K. At the lower temperatures single exponential decays of [K] yielded the third-order rate constant for addition, kR1, whereas at higher temperatures equilibration was observed in the form of double exponential decays of [K], which yielded both kR1 and the equilibrium constant for KO2 formation. kR1 can be summarized as 1.07 × 10(-30)(T/1000 K)(-0.733) cm(6) molecule(-2) s(-1). Combination with literature values leads to a recommended kR1 of 5.5 × 10(-26)T(-1.55) exp(-10/T) cm(6) molecule(-2) s(-1) over 250-1320 K, with an error limit of a factor of 1.5. A van't Hoff analysis constrained to fit the computed ΔS298 yields a K-O2 bond dissociation enthalpy of 184.2 ± 4.0 kJ mol(-1) at 298 K and ΔfH298(KO2) = -95.2 ± 4.1 kJ mol(-1). The corresponding D0 is 181.5 ± 4.0 kJ mol(-1). This value compares well with a CCSD(T) extrapolation to the complete basis set limit, with all electrons correlated, of 177.9 kJ mol(-1).

In situ measurement technique for simultaneous detection of K, KCl, and KOH vapors released during combustion of solid biomass fuel in a single particle reactor.

A quantitative and simultaneous measurement of K, KCl, and KOH vapors from a burning fuel sample combusted in a single particle reactor was performed using collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) with a time resolution of 0.2 s. The previously presented CPFAAS technique was extended in this work to cover two consecutive fragmentation pulses for the photofragmentation of KCl and KOH. The spectral overlapping of the fragmentation spectra of KCl and KOH is discussed, and a linear equation system for the correction of the spectral interference is introduced. The detection limits for KCl, KOH, and K with the presented measurement arrangement and with 1 cm sample length were 0.5, 0.1, and 0.001 parts per million, respectively. The experimental setup was applied to analyze K, KCl, and KOH release from 10 mg spruce bark samples combusted at the temperatures of 850, 950, and 1050 °C with 10% of O2. The combustion experiments provided data on the form of K vapors and their release during different combustion phases and at different temperatures. The measured release histories agreed with earlier studies of K release. The simultaneous direct measurement of atomic K, KCl, and KOH will help in the impact of both the form of K in the biomass and fuel variables, such as particle size, on the release of K from biomass fuels.

Optical detection of potassium chloride vapor using collinear photofragmentation and atomic absorption spectroscopy.

A sensitive and selective optical technique to detect potassium chloride (KCl) vapor is introduced. The technique is based on the photofragmentation of KCl molecules, using a pulsed UV laser, and optical probing of the temporarily increased amount of potassium atoms with a near-infrared laser. The two laser beams are aligned to go through the sample volume along the same optical path. The performance of the technique is demonstrated by detecting KCl concentrations from 25 ppb to 30 ppm in a temperature-controlled cell.

Phase-sensitive method for background-compensated photoacoustic detection of NO2 using high-power LEDs.

A photoacoustic (PA) sensor has been developed for the detection of nitrogen dioxide (NO(2)). Ten amplitude-modulated high-power light emitting diodes (LEDs), emitting a total optical power of 9 W at 453 nm, are used to excite the photoacoustic signal in NO(2). The LEDs are attached to the circumference of a cylindrical PA cell. The induced longitudinal acoustics waves are detected using two electromechanical film stacks, located at the ends of the cell. Background signal cancelation is achieved by using phase-sensitive detection of the difference signal of the two pressure transducers. The phase-sensitive approach allows for improved dynamic range and sensitivity. A detection limit of 10 parts per billion by volume was achieved for flowing NO(2) gas sample in an acquisition time of 2.1 s, corresponding to a minimum detectable absorption coefficient of 1.6 × 10(-7) cm(-1) Hz(-1/2). The developed sensor has potential for compact, light-weight, and low-cost measurement of NO(2).

Optical detection of the anesthetic agent propofol in the gas phase.

The anesthetic agent propofol (2,6-diisopropylphenol) is the most widely used intravenously administered drug in general anesthesia. However, a viable online capability to monitor metabolized levels of propofol in patients does not currently exist. Here we show for the first time that optical spectroscopy has good potential to detect metabolized propofol from patients' exhaled breath. We present quantitative absorption measurements of gas phase propofol both in the ultraviolet and middle-infrared spectral regions. We demonstrate that a detection limit in the subparts-per-billion concentration range can be reached with photoacoustic spectroscopy in the UV spectral region, paving the way for the development of future optical monitors.

Transversely excited multipass photoacoustic cell using electromechanical film as microphone.

A novel multipass photoacoustic cell with five stacked electromechanical films as a microphone has been constructed, tested and characterized. The photoacoustic cell is an open rectangular structure with two steel plates facing each other. The longitudinal acoustic resonances are excited transversely in an optical multipass configuration. A detection limit of 22 ppb (10(-9)) was achieved for flowing NO(2) in N(2) at normal pressure by using the maximum of 70 laser beams between the resonator plates. The corresponding minimum detectable absorption and the normalized noise-equivalent absorption coefficients were 2.2 × 10(-7) cm(-1) and 3.2 × 10(-9) cm(-1) WHz(-1/2), respectively.

Electromechanical film as a photoacoustic transducer.

An electromechanical film, EMFi, is utilized as a transducer in a photoacoustic (PA) gas sensor. The film is a sensitive acoustic transducer, it is easily formable, and it exhibits a wide frequency response regardless of its large surface area. As a demonstration of its capabilities, the EMFi-based PA detector is used to measure NO(2) with pulsed excitation at 436 and 473 nm. The minimum detectable absorption coefficient is extrapolated to be 5.10(-7) cm(-1). Improvements for EMFi-based PA detector are discussed.

Fluorescence cross sections of bioaerosols and suspended biological agents.

Laser-induced fluorescence is used to investigate fluorescence properties of unwashed Bacillus thuringiensis and Bacillus subtilis spores, ovalbumin, and washed bacteriophage MS2. A fluorescence detector is calibrated to obtain absolute fluorescence cross sections. Fluorescence maps of biological aerosols and suspensions are measured at a wide excitation range from 210 to 419 nm and a wide detection range from 315 to 650 nm. The dominant features of the measured spectra are the amino acid peaks, having excitation maxima at 220 and 280 nm. The peaks are similar for the bacterial spores, both for aerosols and suspensions, whereas the peaks are shifted toward the shorter emission wavelengths for the suspended ovalbumin and MS2. Moreover, the fluorescence emission, excited above 320 nm is more intensive for the aerosols than the suspensions.

Wavelength modulation waveforms in laser photoacoustic spectroscopy.

Different wavelength modulation waveforms were studied comprehensively in tunable diode laser photoacoustic spectroscopy. The generation of the photoacoustic signal was studied by way of simulations and experiments. A cantilever-enhanced photoacoustic detector and CO(2) sample gas were used in the experiments. The modulation waveforms compared in this study were sinusoidal, triangular, shaped, and quasi-square waves. All four waveforms allow background-free detection of trace gases. Compared to the conventionally used sinusoidal modulation, the triangular, shaped, and quasi-square waves enhanced the photoacoustic signal by factors of 1.12, 1.42, and 1.57, respectively.

Resonant photoacoustic cell for pulsed laser analysis of gases at high temperature.

A new approach to high temperature gas analysis by means of photoacoustic (PA) spectroscopy is presented. The transverse modes of the resonant PA cell were excited with a pulsed laser and detected with a microphone. Changes in the properties of the PA cell resulting from a varying temperature are discussed and considered when processing the PA signal. The feasibility of the proposed method was demonstrated by studying PA response from saturated vapor of potassium chloride (KCl) in the temperature range extending from 410 to 691 degrees C. The PA spectrum, the detection limit, and the signal saturation of KCl vapor are discussed. At 245 nm excitation wavelength and 300 microJ pulse energy, the achieved detection limit for KCl is 15 ppb.