Tunable Diode Laser Spectroscopy Technique Research Articles

Evaluating a Lagrangian inverse model for inferring isotope CO2 exchange in plant canopies

Multi-layer Lagrangian models could be useful techniques for studying stable isotope exchange within and just above plant canopies. The main objective of this study was to evaluate the use of an analytical Lagrangian analysis (localized near-field theory, LNF), to study 13CO2 and C18OO isotope exchange in different plant canopies by comparing the LNF estimates with those provided by the eddy covariance (EC) technique and the isotope flux ratio method (IFR). Mixing ratios of stable isotopes of CO2 were measured within and above a temperate deciduous forest, tallgrass prairie and corn field using a multi-port sampling system and the tunable diode laser spectroscopy technique. Wind velocity data and the net CO2 ecosystem exchange (NEE) were measured above the plant canopies using an EC system. The wind velocity data and CO2 stable isotope mixing ratios were combined with the LNF theory to infer NEE and source/sinks of isotopes inside canopies. The LNF NEE estimates were likely affected by the flux decoupling in the forest canopy, resulting in a low correlation (R2 ranging from 0.03 to 0.35) between LNF and EC NEE estimates. On the other hand, LNF NEE estimates for corn and grassland canopies showed better correlation with EC NEE estimates (R2 ranging from 0.58 to 0.85), suggesting better coupling between in and above canopy air flows. Although, both LNF and IFR estimates showed large variability, our results show that the LNF approach reduced the uncertainties of the isotope compositions of NEE when compared to the IFR approach. These results suggest that LNF is a useful tool to study CO2 isotope exchange within short canopies where flux measurements are more challenging than inside tall canopies.

Application of Tunable Diode Laser Spectroscopy for the Assessment of Food Quality.

Adequate food packaging is important to ensure food quality and prolong the shelf life of food. Frequently, the method of modified atmosphere packaging (MAP) is used. We used a tunable diode laser spectroscopy technique to detect the oxygen content in food packages, e.g., in packaged milk and bread. Different detection geometries were adopted to be able to collect scattering light that had passed through the gas volume in the packages, and to avoid the disturbance of ambient air. Furthermore, studies of the bread-baking process have been performed by measuring the water vapor signals in a fermenting dough. The results demonstrate that the technique has a great potential for nonintrusive assessment of food quality and the bread-baking process.

Pressure broadening of Ar and Kr (n+1)s[3/2]2→(n+1)p[5/2]3 transition in the parent gases and in He

Tunable diode laser spectroscopy technique was employed to determine pressure broadening coefficients for the (n+1)s[3/2]2→(n+1)p[5/2]3 transitions in Ar and Kr in RF discharge plasma. The widths of Gaussian and Lorentian components of the absorption lines were determined simultaneously by fitting absorption line profiles obtained in experiments to a Voigt profile. The technique was validated in measurements of pressure broadening coefficient for 811.5nm Ar absorption line in the pressure range of 15–75Torr. The obtained values for these coefficients reduced to 300K are ξAr–Ar=(2.8±0.1)×10−10s−1cm3 for broadening in the parent gas and ξAr-He=(3.2±0.2)×10−10s−1cm3 in helium.For krypton, isotope abundance was accounted for and appropriate fit function was constructed permitting to determine pressure broadening coefficients using the natural mixture of isotopes. For krypton 811.3nm line these coefficients are ξKr-Kr=(2.4±0.2)×10−10s−1cm3 and ξKr-He=(3.1±0.2)×10−10s−1cm3.

Use of the isotope flux ratio approach to investigate the C<sup>18</sup>O<sup>16</sup>O and <sup>13</sup>CO<sub>2</sub> exchange near the floor of a temperate deciduous forest

Abstract. Stable isotopologues of CO2, such as 13CO2 and C18OO, have been used to study the CO2 exchange between land and atmosphere. The advent of new measuring techniques has allowed near-continuous measurements of stable isotopes in the air. These measurements can be used with micrometeorological techniques, providing new tools to investigate the isotope exchange in ecosystems. The objectives of this study were to evaluate the use of the isotope flux ratio method (IFR) near the forest floor of a temperate deciduous forest and to study the temporal dynamics of δ18O of CO2 flux near the forest floor by comparing IFR estimates with estimates of δ18O of net soil CO2 flux provided by an analytical model. Mixing ratios of 12C16O2, 13CO2 and C16O18O were measured within and above a temperate deciduous forest, using the tunable diode laser spectroscopy technique. The half-hourly compositions of the CO2 flux near the forest floor (δ13CF and δ18OF) were calculated by IFR and compared with estimates provided by a modified Keeling plot technique (mKP) and by a Lagrangian dispersion analysis (WT analysis). The mKP and IFR δ18OF estimates showed good agreement (slope = 1.03 and correlation, R2 = 0.80). The δ13CF estimates from the two methods varied in a narrow range of −32.7 and −23‰; the mean (± SE) mKP and IFR δ13CF values were −27.5‰ (±0.2) and −27.3‰ (±0.1), respectively, and were statistically identical (p>0.05). WT analysis and IFR δ18OF estimates showed better correlation (R2 = 0.37) when only turbulent periods (u*>0.6 m s−1) were included in the analysis. The large amount of data captured (~95 % of half-hour periods evaluated) for the IFR in comparison with mKP (27 %) shows that the former provides new opportunities for studying δ18OF dynamics within forest canopies. Values of δ18OF showed large temporal variation, with values ranging from −31.4‰ (DOY 208) to −11.2‰ (DOY 221). Precipitation events caused substantial variation (~8‰) in δ18OF over a period of approximately 24 h. A diel trend of δ18OF was observed, with more depleted values present during the daytime. Model simulations indicate that the activity of the carbonic anhydrase enzyme was quite variable during the evaluated period. These simulations indicate that more frequent sampling of δ18O of soil water could improve the estimates of δ18O of net soil CO2 flux.

Reliable optical measurement of water vapor in highly scattering environment

Based on tunable diode laser spectroscopy with direct absorption measurement, we developed a reliable measurement scheme which can precisely detect water vapor concentration independent of the scattering (attenuation) caused by the non-water dust particles inside the observed humidity chamber. The relative water vapor concentration was measured at 2 ν 1 + ν 3 water vapor transition band by 938 nm distributed feed-back diode laser at different temperatures using the tunable diode laser spectroscopy technique. These relative water vapor concentrations are converted into the absolute water vapor concentrations and we confirmed that the experimental results at different temperatures are consistently independent of the attenuation. Our measurement system gives the consistent water vapor concentrations without regard to either high or low scattering caused by the non-water dust particles inside the observed humidity chamber.

On-line and in situ monitoring of oxygen concentration and gas temperature in a reheating furnace utilizing tunable diode-laser spectroscopy

Increased demands on energy savings and quality control in metallurgical processes have created incentives for new methods to monitor and control the process. In this paper we will present a field trial that shows the potential of tunable diode-laser spectroscopy (TDLS) for simultaneous contact free measuring and monitoring of the oxygen concentration as well as the gas temperature in a reheating furnace during production. The field trials were carried out at an oil-fueled reheating furnace during 7 weeks of production. The tunable diode-laser spectrometer was measuring in situ across the preheating zone and the soaking zone in the furnace. During the campaign the oxygen concentration and the gas temperature in the furnace environment were simultaneously monitored and instantaneous variations in these parameters could easily be recorded and subsequently correlated to actual changes in the process. Furthermore, the much shorter response-time of the TDLS technique compared with conventional measurement methods such as thermocouples and extractive gas analyzers was also demonstrated during the trials. The results show the potential for the TDLS technique to be used for energy savings as well as product quality improvements by controlling the burners in the reheating furnace. The results show that it would be possible to control and optimize the oxygen concentration with TDLS in the control loop of the reheating furnace.

Human breath trace gas content study by tunable diode laser spectroscopy technique

Measurements of carbon monoxide, ammonia, methane and carbon dioxide concentrations by registration of high resolution absorption spectra are described. A description of the automated tunable diode laser (TDL) system developed for simultaneous CO and CO 2 measure, and the TDL system developed for CH 4 and NH 3 measure is given. Calibration of concentration calculated on the base of a priori data on strength and broadening coefficients of detected lines is described. A clarified correct manner of value definition of spontaneous fraction of TDL' radiation allows an accuracy improvement of concentration measurement of several percent. The same level of CO surplus content of about 400–800 ppb in non-smoker's respiration was detected under different ambient conditions of CO content varied within one order of magnitude. A surplus content in human respiration of CH 4 equal to ∼3–8 ppm and NH 3 equal to 100–250 ppb were detected. The accumulation of CH 4 and NH 3 by the human organism seems to be rather negligible for a short time exposure, e.g. like smoking. Data obtained on non-invasive CO/CO 2 ratio detection in human breath shows that after 10–20 sec of breath holding time the organism's compensation reactions on acuity oxygen shortage for breath holding appear. Prospects for the development and application of diode laser spectroscopy to trace gas analysis in medicine are also discussed.