Cavity Ring-down Spectroscopy System Research Articles

Simultaneous high-precision, high-frequency measurements of methane and nitrous oxide in surface seawater by cavity ring-down spectroscopy

An automated system was developed using commercially available Cavity Ring-Down Spectroscopy (CRDS) technology (Picarro LTD., G2508) which was interfaced to a custom-made system which automated the equilibration and analysis of seawater dissolved nitrous oxide (N2O) and methane (CH4). The combined system was deployed during two research cruises in the Atlantic Ocean, which combined covered 16,500 kms, one on a west to east transect between the United States and Europe at approximately 24°N, the second was a north to south transect which covered approximately 70° of latitude between the Tropic of Cancer and the Southern Ocean. Semi-continuous measurements using the CRDS (Approx. 73,000) were compared to discretely collected samples (n=156) which were analysed using gas chromatography (GC) with flame ionisation detection for CH4 and electron capture detection for N2O. Excellent agreement between the two approaches, though with an increase in analytical precision offered by CRDS compared to GC gives great confidence in the applicability of the CRDS system, whilst the significant (2 to 3 orders of magnitude) increase in measurement frequency offer an opportunity to greatly increase the number of dissolved N2O and CH4 data that are currently available. Whilst identifying a number of small-scale features, deployment during this study showed that whilst the surface of large areas of the Atlantic Ocean were in-balance with the overlying atmosphere with respect to N2O, the most of this region was offering a source of atmospheric CH4.

Trace Ammonia Equilibrium Pressure of Zirconium Phosphate in Moisture.

Zirconium phosphate-absorbed ammonia gas and the ammonia concentration (pressure) decreased to 2 ppm (ca. 20 Pa). However, it has not been clarified what the equilibrium pressure of zirconium phosphate is during ammonia gas ab/desorption. In this study, the equilibrium pressure of zirconium phosphate during ammonia ab/desorption was measured using cavity ring-down spectroscopy (CRDS). For ammonia-absorbed zirconium phosphate, a two-step equilibrium plateau pressure was observed during the ammonia desorption in gas. The value of the higher equilibrium plateau pressure at the desorption process was about 25 mPa at room temperature. If the standard entropy change (ΔS0) of the desorption process is assumed to be equal to the standard molar entropy of ammonia gas (192.77 J/mol(NH3)/K), the standard enthalpy change (ΔH0) is about -95 kJ/mol(NH3). In addition, we observed hysteresis in zirconium phosphate at different equilibrium pressures during ammonia desorption and absorption. Finally, the CRDS system allows the ammonia equilibrium pressure of a material in the presence of water vapor equilibrium pressure, which cannot be measured by the Sievert-type method.

Study on neural network algorithm for detecting respirable dust in photoacoustic cavity

The traditional photoacoustic cavity has the advantages of simple structure, low cost, and easy integration with optical cavity technology, so it has significant advantages in the measurement of the optical characteristics of respirable dust. In order to meet the demand of high-precision respirable dust measurements in practical applications, it is necessary to improve the measurement accuracy of respirable dust by traditional photoacoustic spectroscopy technology. Therefore, the structure size of the photoacoustic cavity was determined by theoretical and simulation analysis. A system for measuring respirable dust by photoacoustic spectroscopy was designed, which was applied to the atmospheric respirable dust detection simultaneously with the cavity ring-down spectroscopy system. The results showed that the correlation between the two systems was poor. Therefore, the three-layer back propagation neural network algorithm was used to correct the photoacoustic response values, and the measured value of the cavity ring-down spectroscopy system was used as the reference truth value. The calibration results showed that the output value of the neural network model was in good agreement with the reference true value: the slope was above 0.96. The results showed that the neural network algorithm could effectively improve the measurement accuracy of the photoacoustic spectroscopy system to respirable dust, improve the linearity, and reduce the detection error.

An open-source, automated, gas sampling peripheral for laboratory incubation experiments using cavity ring-down spectroscopy

Spectroscopic instruments are becoming increasingly popular for measuring the isotopic composition and fluxes of a wide variety of gases in both field and laboratory experiments. The popularity of these instruments has created a need for automated multiplexers compatible with the equipment. While there are several such peripherals commercially available, they are currently limited to only a small number of samples (≤16), which is insufficient for some studies. To support researchers in constructing custom, larger-scale systems, we present our design for a scalable gas sampling peripheral that can be programmed to autonomously sample up to 56 vessels – the “multiplexer”. While originally designed to be used with a Picarro cavity ring-down spectroscopy (CRDS) system, the multiplexer design and data processing approach implemented can be easily adapted to serve as a gas sampling/delivery platform for a wide variety of instruments including other cavity ring-down systems and infra-red gas analyzers. We demonstrate the basic capabilities of the multiplexer by using it to autonomously sample head-space CO2 from 14 laboratory-incubated soils amended with 13C-enriched pyrogenic organic matter for analysis in a Picarro G2201-i cavity ring-down spectroscopy system.

First result of CRDS system with cesium operation for radiofrequency negative ion source test facility at ASIPP

From September 2017, an advance study project of Negative ion-based Neutral Beam Injection (NNBI) prototype for the China Fusion Engineering Test Reactor (CFETR) was supported by the ministry of science and technology of China. The entire test facility will be integrated at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). In order to meet requirement of the development of negative ion based neutral beam injection system for CFETR, a negative ion source test facility with radio frequency source is being built. At the same time, cavity ring-down spectroscopy (CRDS) is employed for measurement of negative ion density. CRDS is an ultrasensitive absorption diagnostic technique for density measurement. Based on photo-detachment process of hydrogen negative ions, CRDS can measure integrated line-of-sight density of hydrogen negative ions in high power ion source. In this paper, the principle of CRDS diagnostic system has been introduced. Based on previous tests, the system setup and improvement are mentioned. In this paper, typical ring-down signals and time evolution of density of hydrogen negative ions are reported. With the first time cesium seeding, density of hydrogen negative ions increased obviously. It increased from 6.3E15 m−3 to 1.8E16 m−3, about 2.8 times under the 40 kW RF power discharge. Several days condition up, the density of hydrogen negative ions is up to 3.4 E16 m−3 under the 50 kW RF power discharge.

Erosion sensor using time-resolved cavity ring-down spectroscopy for Hall thrusters.

A high-sensitivity sensor to measure titanium atom density based on time-resolved cavity ring-down spectroscopy (CRDS) was developed to monitor the wall erosion and predict the lifetime of Hall thrusters. The minimum detection limit for the sensor was dependent on the discharge current oscillation in the Hall thruster. A Volterra engine management system was employed for time-resolved measurements to develop the time-resolved CRDS system, which was synchronized to the discharge current oscillation. The results confirmed that the path-integrated number density of sputtered titanium atoms was synchronized with the discharge current oscillation. The minimum detection limit was decreased by ∼30% from 2 × 1012 to 6 × 1011 m-2.

Cavity ring-down spectroscopy based on a comb-locked optical parametric oscillator source.

Spectroscopy of molecules in the mid-infrared (MIR) region has important applications in various fields, such as astronomical observation, environmental detection, and fundamental physics. However, compared to that in the near-infrared, precision spectroscopy in the MIR is often limited by the light source and has not shown full potential in sensitivity. Here we report a cavity ring-down spectroscopy system using a tunable narrow-linewidth optical parametric oscillator, which fulfills the requirement of high sensitivity and high precision in the MIR region. The Lamb-dip spectrum of the N2O molecule at 2.7 μm was measured as a demonstration of spectroscopy in the MIR with kilohertz accuracy.

An exploratory study on online quantification of isoprene in human breath using cavity ringdown spectroscopy in the ultraviolet

Breath analysis offers a promising method of noninvasive analyses of volatile metabolites and xenobiotics present in human body. Isoprene is one of the highest abundant volatile organic compounds (VOCs) present in human exhaled breath. Breath isoprene (50–200 part per billion by volume (ppbv) or higher) can be analyzed by using mass spectroscopy-based methods, yet laser absorption spectral detection of breath isoprene has not been much reported, partially due to its ultraviolet (UV) absorption wavelength and the spectral overlap with other breath VOCs such as acetone in the same wavelength region. These facts make it challenging to develop a spectroscopy-based breath isoprene analyzer for a potential portable instrument. Here we report on the development of a cavity ringdown spectroscopy (CRDS) system for detection of breath isoprene in the UV region near 226 nm. First, we investigated spectral absorption interferences near 226 nm and selected an optimal detection wavelength at 226.56 nm with minimum to no spectral interference. We then measured absorption cross-sections of isoprene at 225.5–227.4 nm under controlled cavity pressures, and the measured absorption cross-section 1.93 × 10−17 cm2/molecule at 226.56 nm was used to quantify isoprene in different cases including human breath gas samples. Finally, we validated the CRDS system by measuring breath gas samples from 19 human subjects using proton transfer reaction mass spectrometry (PTR-MS). The CRDS system shows good linear response (R2 = 0.999), high stability (0.2%), and high accuracy (R2 = 0.906 with PTR-MS). The limit of detection of the system was 0.47 ppbv, with average over 100 ringdown events (equivalent to 5 s). This work represents the first exploratory study of the detection of breath isoprene using CRDS. The results demonstrate the potential of developing a CRDS-based breath analyzer for online, near-real time, sensitive analysis of breath isoprene for further research that would help to elucidate its physiological and clinical significance.

Cavity ringdown spectroscopy of nitric oxide in the ultraviolet region for human breath test

We report the spectrum of nitric oxide (NO) in the ultraviolet (UV) (225.4–227.0 nm) region based on cavity ringdown spectroscopy (CRDS). A cavity ringdown system, which consisted of a tunable UV laser source and a vacuum-pumped ringdown cavity, was constructed to measure NO at room temperature and atmospheric or reduced pressure. The measured spectra were validated using LIFBase simulations. The absorption cross-section of NO at the strongest absorption peak at 226.255 nm was measured to be 7.64 × 10−18 cm2 molecule−1. Using the measured mirror reflectivity of 99.55% at 226.255 nm, the detection limit of NO was determined to be 7.4 ppb (parts per billion) based on the standard 3-σ criteria. The stability and reproducibility of this CRDS system were also tested. Furthermore, exhaled gas samples from 203 human subjects (105 healthy people and 98 lung cancer patients) were measured using the system. Results demonstrated that the cavity ringdown spectroscopy in the deep-UV region has potential for breath NO test.

Cavity ring-down spectroscopy system for the evaluation of negative hydrogen ion density at the ELISE test facility.

The large RF negative hydrogen (deuterium) ion source at the ELISE test facility (half of the ITER-NBI source size) has been equipped with a Cavity Ring-Down Spectroscopy (CRDS) system, in order to measure the negative hydrogen (deuterium) ion density in the region in front of the plasma grid (first grid of the extraction system). The challenge of this diagnostic for ELISE relies on the large size of the source and therefore on the plasma length across which the measurements are performed as well as the long pulses at RF power, which can affect the cavity mirror reliability. A dedicated experiment on the mirror reliability was performed, ensuring the feasibility of measurements for long pulses (several hundred seconds) at high RF power. Two horizontal lines of sight were dedicated to CRDS: the measured density was in the range between 4 × 1016 and 1 × 1017 m-3, with a slightly higher density for the bottom lines of sight, for both the isotope hydrogen and deuterium. Different temporal evolution was observed for the two isotopes, showing a higher instability for the deuterium case: this is in correlation with the extracted negative ion current density and inversely correlated with the coextracted electron current density. The CRDS system allowed performing the first measurements of negative ion density for a long pulse (1000 s) in a large source: the temporal behavior and the effect of the beam extraction will also be discussed.

Preliminary result of cavity ring-down spectroscopy system for radio frequency negative ion source test facility.

Negative ion source is a core part of the neutral beam injection system for magnetic confinement fusion devices. The density of produced hydrogen negative ions is a critical parameter of the negative ion source. Cavity ring-down spectroscopy (CRDS) is an ultrasensitive absorption diagnostic technique for density measurement. Based on the photodetachment process, CRDS can measure the integrated line-of-sight hydrogen negative ion density in a high power ion source. The CRDS diagnostic system has been applied to Hefei utility negative ion test equipment with the radio frequency (RF) source, which is now one of the references for the China Fusion Engineering Test Reactor neutral beam injection system. Typical ring-down signals are obtained to calculate the density of hydrogen negative ions. The time evolution of hydrogen negative ion density is successfully measured. Preliminary experiments show the accurate relationship between RF power and measured hydrogen negative ion density.

Amplifier-assisted CRDS (cavity ring-down spectroscopy) toward compact breath sensing

A cavity-ring-down spectroscopy (CRDS) system utilizing waveguide for breath sensing realizes several meter optical path integration that has possibility for ppm-order gas sensing within a compact area. We have proposed an optical amplifier assisted gas sensing system scheme to compensate waveguide propagation loss that may prevent ppm-order gas detection. The amplifier in a CRDS system, however, may result in self-lasing at specific wavelength in case of high pumping condition. Once self-lasing happens, signal light loose its gain from amplifier. The amplifier spontaneous emission that generated by an erbium-doped fiber amplifier affects the background noise level of cavity ring-down waveforms. In this work, we propose the scheme of polarization direction control so as to suspend the self-lasing, and the scheme of additional loss as to suppress ASE power. As a result, we confirmed the suppression of light intensity at a self-lasing wavelength below −50 dBm and the gain improvement of 24 dB at signal.

Negligible influence of livestock contaminants and sampling system on ammonia measurements with cavity ring-down spectroscopy

Abstract. Emission of ammonia (NH3) is a ubiquitous problem due to the adverse effects of NH3 on the environment and human health. The agricultural sector accounts for nearly all NH3 emissions in Europe. Hence, technologies for the abatement of NH3 emissions from this sector have been in strong demand in recent years. In order to document emissions and evaluate abatement technologies, there is a strong need for reliable NH3 measurement methods. Photoacoustic spectroscopy (PAS) is often used to measure NH3 concentrations, but recent research shows interference from compounds typically present in livestock production and during agricultural activities. In this work, the performance of cavity ring-down spectroscopy (CRDS) from Picarro, as an alternative to PAS, has been tested with respect to method validation under laboratory and field conditions. Potential interferences of 10 volatile organic compounds (VOCs) on CRDS NH3 measurement were tested with simultaneous VOC analysis performed by proton-transfer-reaction mass spectrometry (PTR-MS). Both laboratory and field calibrations show excellent linearity over a large dynamic range of NH3 concentrations. The analyzer shows a small humidity effect of up to a few ppb in the extreme case of a nearly water-saturated air stream. Apart from the negligible humidity dependency, no interferences of the tested VOCs were observed. Overall, the CRDS system performs satisfactory and is well suited for measurements of NH3 emissions from livestock production.

Prediction of high-order line-shape parameters for air-broadened O2 lines using requantized classical molecular dynamics simulations and comparison with measurements

Line-shape models such as the Hartmann-Tran (HT) profile have adjustable high-order parameters that are usually determined by fits to experimental spectra. As an alternative approach, we demonstrate that fitting the HT profile to theoretical spectra provides high-order line-shape parameters for O2 transitions that are consistent with experimentally determined values. To this end, normalized absorption spectra of air-broadened O2 lines were computed without adjustable parameters using requantized classical molecular dynamics simulations (rCMDS). These theoretical calculations were made at a pressure of 203 kPa and for values of the Doppler width that cover near-Doppler-limited to collisional-broadened pressure conditions. Hartmann-Tran (HT) line profiles with adjustable line-shape parameters were then simultaneously fit to the set of rCMDS-calculated spectra in a global multispectrum analysis. The retrieved high-order line-shape parameters (i.e. the speed dependence of the line broadening and the Dicke narrowing coefficient) were subsequently used as fixed HT parameters in the analysis of seven air-broadened O2 lines of the a1Δg←X3Σg−(0,0) band. The spectra were measured over a fifteen-fold range of total gas pressure at high spectral resolution and signal-to-noise ratio with a frequency-stabilized cavity ring-down spectroscopy system. We show that these predicted parameters enable all the measured lines to be fit to within 1%, which is much better than best fits of the Voigt line profile to the measured spectra. This approach opens the route for predicting high-order line-shape parameters from first-principles calculations and for their inclusion in spectroscopic databases. Furthermore, the temperature dependences of the broadening coefficient and its speed dependent component for air-broadened O2 lines were also calculated using rCMDS.

Soil δ13C-CO2 and CO2 flux in the H2S-rich Rotorua Hydrothermal System utilizing Cavity Ring Down Spectroscopy

Diffuse soil CO2 flux and stable carbon isotope surveys facilitate interpretation of the nature and structure of hydrothermal systems. Although soil CO2 flux measurements are completed in the field, isotopic carbon in CO2 (i.e., δ13C-CO2) analyses are more commonly performed in a stable isotope mass spectrometry laboratory, thereby, increasing the effort, time and resources required to complete the survey. Cavity Ring Down Spectroscopy (CRDS) overcomes these challenges by enabling both CO2 isotopic and flux analyses in field settings. However, H2S interference has limited CRDS applications in H2S-rich hydrothermal settings. Here, we assess the application of a mobile CRDS system equipped with a simple Cu wire trap to complete δ13C-CO2 and CO2 flux surveying in the relatively high H2S Rotorua Hydrothermal System (RHS), North Island, New Zealand. Laboratory experiments demonstrate that the Cu trap was able to remove H2S (~250 ppmv) enabling accurate and precise δ13C-CO2 measurements. In the RHS, the Cu trap successfully removed H2S concentrations during in situ soil gas analyses. Carbon dioxide fluxes determined using the CRDS were found be 102% of those values produced by a traditional soil gas flux meter (i.e., Licor). Carbon dioxide flux determined by CRDS was similar to previous studies in the RHS; however, zones of high CO2 flux in central Kuirau Park have moved ~100–200 m in the past twelve years. Isoscapes of δ13C-CO2 for the RHS were constructed from our soil gas survey results and were used to assess the apportionment of magmatic-hydrothermal versus biogenic CO2 release. Our findings demonstrate that a CRDS enabled system allows for faster in field survey measurement, lower cost, and reactive sampling for both δ13C-CO2 and CO2 flux measurement in H2S-enriched hydrothermal systems.

An external-cavity quantum cascade laser operating near 5.2 µm combined with cavity ring-down spectroscopy for multi-component chemical sensing

We report on the performance of a widely tunable continuous wave mode-hop-free external-cavity quantum cascade laser operating at λ ~ 5.2 µm combined with cavity ring-down spectroscopy (CRDS) technique for high-resolution molecular spectroscopy. The CRDS system has been utilized for simultaneous and molecule-specific detection of several environmentally and bio-medically important trace molecular species such as nitric oxide, nitrous oxide, carbonyl sulphide and acetylene (C2H2) at ultra-low concentrations by probing numerous rotationally resolved ro-vibrational transitions in the mid-IR spectral region within a relatively small spectral range of ~0.035 cm−1. This continuous wave external-cavity quantum cascade laser-based multi-component CRDS sensor with high sensitivity and molecular specificity promises applications in environmental sensing as well as non-invasive medical diagnosis through human breath analysis.

A variable-temperature cavity ring-down spectrometer with application to line shape analysis of CO2 spectra in the 1600 nm region

We present a new cavity ring-down spectroscopy system which was developed for variable-temperature absorption measurements (220–290 K) of atmospheric gases. This laser spectrometer was developed in the framework of the NASA Orbiting Carbon Observatory-2 project to improve our understanding of line shape parameters for carbon dioxide and oxygen. The apparatus consists of a monolithic, fixed-mirror ring-down cavity within a temperature-regulated enclosure, which is interrogated by a tunable, single-frequency diode laser. We experimentally characterize and model the dependence of the spectrum detuning axis at each setpoint temperature, and show that absolute frequencies are stable to within 200 kHz over several hours, corresponding to temperature stabilities better than 1 mK. We measure the R16e (30013-0001) 12C16O2 transition and carry out multi-spectrum analyses using two line profiles incorporating speed-dependent (quadratic approximation) and Dicke narrowing (hard collision assumption) effects. The resulting broadening coefficient and temperature exponent are in excellent agreement (0.05% level) with previous high-resolution Fourier-transform spectroscopy measurements, and the speed dependent broadening parameter is within 3% of the theoretical value.

Spectral shapes of rovibrational lines of CO broadened by He, Ar, Kr and SF6: A test case of the Hartmann-Tran profile

High signal-to-noise ratio spectra of the (3-0) band P(1) and P(17) lines of CO broadened by He, Ar, Kr and SF6 were measured with a frequency-stabilized cavity ring-down spectroscopy system. For each collision-partner and both lines, multiple spectra were measured over pressures spanning nearly three decades up to 130kPa. These data were analyzed with a multispectrum fitting procedure. Line shapes were modeled using the Hartmann-Tran (HT) profile with first-order line mixing as well as several other simplified profiles. The results show that for all considered collision partners (with the exception of SF6), the HT profile captures the measured line shapes with maximum absolute residuals that are within 0.1% of the peak absorption. In the case of SF6, which is the heaviest perturber investigated here, the maximum residuals for the HT profile are twice as large as for the other collision partners.

Cavity Ring-Down Spectroscopy measurements of Acetone concentration

The Cavity Ring-Down Spectroscopy system for medical applications is created and first results for detection of acetone are shown. The acetone concentration was measured in the air. In the system a pulsed 1 kHz 266 nm DPSS laser with pulse length of 10 ns is coupled in a gas cell with high reflectivity mirror.

Note: Reliable, robust measurement system for trace moisture in gas at parts-per-trillion levels using cavity ring-down spectroscopy.

We report a simple, robust cavity ring-down spectroscopy system to reliably measure trace moisture in gases at parts-per-trillion (ppt) levels. The performance of the system was evaluated on the basis of experiments performed in a manner traceable to the International System of Units. The obtained result was in good agreement with the primary trace-moisture standard at 12 nmol/mol (12 ppb) in N2 in amount-of-substance fraction. Measurement capability of residual moisture in high-purity dry N2 at ∼130 pmol/mol (130 ppt) was demonstrated, and background noise of 5.3 × 10(-12) cm(-1) was attained, corresponding to a minimum detectable H2O of 5 pmol/mol (5 ppt).