The sensitive detection of NO by Faraday modulation spectroscopy with a quantum cascade laser

Improved measurement method for the Faraday rotation distribution using beam splitting

In this work, two Faraday channels and one shadow channel are constructed by two non-polarizing beam splitters and one reflector to measure the Faraday rotation distribution. The intensity of the Faraday and shadow images is related to the state of polarization (SOP) of the incident light, thus achieving two-dimensional accurate measurement. The measurement sensitivity is influenced by the settings of two polarization analyzers in the Faraday channels and the parameters of beam splitters, which are explored numerically and verified with experiments. The fluctuation of the probe light is eliminated by using three evaluation indexes. Also, the measurement range and error sources under different experimental settings are discussed.

Quantum Cascade Lasers-Based Detection of Nitric Oxide

Despite the established importance of nitric oxide (NO) in many physiological and molecular processes in plants, most methods for quantifying NO are open to criticism This reflects the differing methods either lacking specificity or sensitivity, or even from an undue dependence of results on experimental conditions (i.e., chemical concentrations, pH, etc.). In this chapter we describe a protocol to measure gaseous NO produced by a biological sample using quantum cascade laser (QCL)-based spectroscopy. This technique is based on absorption of the laser light by the NO molecules which have been passed from a biological sample into an optical s cell that is equipped with two mirrors placed at both ends. This design greatly increases the interaction path length with the NO molecules due to multiple reflections of the light coupled inside the cell. Thus, the method is able to provide online, in planta measurements of the dynamics of NO production, being highly selective and sensitive (down to ppbv levels;1 ppbv = part per billion by volume mixing ratio = 1:10^-9).

Frequency-locked cavity ring-down Faraday rotation spectroscopy

Cavity ring-down Faraday rotation spectroscopy (CRD-FRS) is a technique for trace gas measurements of paramagnetic species that retrieves the molecular concentration from the polarization rotation measured as the difference between simultaneously recorded ring-down times of two orthogonal polarization states. The differential measurement is inherently insensitive to nonabsorber related losses, which makes off-resonance measurements redundant. We exploit this unique property by actively line-locking to a molecular transition for calibration-free trace gas concentration retrieval. In addition, we enhance the effective duty-cycle of the system by implementing a Pound-Drever-Hall laser lock to the cavity resonance, which allows for ring-down rates of up to 9 kHz. The system performance is demonstrated by measurements of trace oxygen with a minimum detection limit at the ppmv/√Hz-level.

Diamagnetic Raman Optical Activity of Chlorine, Bromine, and Iodine Gases

Magnetic Raman optical activity of gases provides unique information about their electric and magnetic properties. Magnetic Raman optical activity has recently been observed in a paramagnetic gas (Angew. Chem. Int. Ed. 2012, 51, 11058; Angew. Chem. 2012, 124, 11220). In diamagnetic molecules, it has been considered too weak to be measurable. However, in chlorine, bromine and iodine vapors, we could detect a significant signal as well. Zeeman splitting of electronic ground-state energy levels cannot rationalize the observed circular intensity difference (CID) values of about 10(-4). These are explicable by participation of paramagnetic excited electronic states. Then a simple model including one electronic excited state provides reasonable spectral intensities. The results suggest that this kind of scattering by diamagnetic molecules is a general event observable under resonance conditions. The phenomenon sheds new light on the role of excited states in the Raman scattering, and may be used to probe molecular geometry and electronic structure.

Laser-Based Methods for Detection of Nitric Oxide in Plants

Nitric oxide (NO) plays an important role in plant signaling and in response to various stress conditions. Therefore, real-time measurements of NO production provide better insights into understanding plant processes and can help developing strategies to improve food production and postharvest quality. Using laser-based spectroscopic methods, sensitive, online, in planta measurements of plant-pathogen interactions are possible. This chapter introduces the basic principle of the optical detectors using different laser sources for accurate monitoring of fast dynamic changes of NO production. Several applications are also presented to demonstrate the suitability of these detectors for detection of NO in plants.

Shot-noise limited Faraday rotation spectroscopy for detection of nitric oxide isotopes in breath, urine, and blood

Measurement of NO and/or its metabolites in the various body compartments has transformed our understanding of biology. The inability of the current NO measurement methods to account for naturally occurring and experimental NO isotopes, however, has prevented the scientific community from fully understating NO metabolism in vivo. Here we present a mid-IR Faraday rotation spectrometer (FRS) for detection of NO isotopes. The instrument utilizes a novel dual modulation/demodulation (DM) FRS method which exhibits noise performance at only 2 times the fundamental quantum shot-noise level and provides the record sensitivity in its class. This is achieved with a system that is fully autonomous, robust, transportable, and does not require cryogenic cooling. The DM-FRS enables continuous monitoring of nitric oxide isotopes with the detection limits of 3.72 ppbv/Hz^1/2 to¹⁴NO and 0.53 ppbv/Hz^1/2 to¹⁵NO using only 45 cm active optical path. This DM-FRS measurement method can be used to improve the performance of conventional FRS sensors targeting other radical species. The feasibility of the instrument to perform measurements relevant to studies of NO metabolism in humans is demonstrated.

Dual modulation Faraday rotation spectroscopy of HO2 in a flow reactor

The technique of dual modulation Faraday rotation spectroscopy (DM-FRS) has been applied to achieve technical-noise-limited detection of HO2 at the exit of an atmospheric pressure flow reactor. This was implemented by combining direct current modulation at 51 kHz of an external cavity quantum cascade laser system with 610 Hz modulation of the magnetic field generated by a Helmholtz coil. The DM-FRS measurement had a 1.5 times better signal-to-noise ratio than a conventional FRS measurement acquired under identical flow reactor conditions. High harmonic detection of the FRS signal also eliminated the substantial offset associated with electromagnetic interference pickup from the Helmholtz coils that is observed in the conventional FRS spectrum. A noise equivalent angle of 8.9×10(-9)  rad Hz(-1/2) was measured for the DM-FRS measurement, corresponding to a 3σ detection limit for HO2 of 0.35  ppmv Hz(-1/2).

Methods of NO detection in exhaled breath

There is still an unexplored potential for exhaled nitric oxide (NO) in many clinical applications. This study presents an overview of the currently available methods for monitoring NO in exhaled breath and the use of the modelling of NO production and transport in the lung in clinical practice. Three technologies are described, namely chemiluminescence, electrochemical sensing and laser-based detection with their advantages and limitations. Comparisons are made in terms of sensitivity, time response, size, costs and suitability for clinical purposes. The importance of the flow rate for NO sampling is discussed from the perspective of the recent recommendations for standardized procedures for online and offline NO measurement. The measurement of NO at one flow rate, such as 50 ml s(-1), can neither determine the alveolar site/peripheral contribution nor quantify the difference in NO diffusion from the airways walls. The use of NO modelling (linear or non-linear approach) can solve this problem and provide useful information about the source of NO. This is of great value in diagnostic procedures of respiratory diseases and in treatment with anti-inflammatory drugs.

Cryogen-free heterodyne-enhanced mid-infrared Faraday rotation spectrometer

A new detection method for Faraday rotation spectra of paramagnetic molecular species is presented. Near shot-noise limited performance in the mid-infrared is demonstrated using a heterodyne enhanced Faraday rotation spectroscopy (H-FRS) system without any cryogenic cooling. Theoretical analysis is performed to estimate the ultimate sensitivity to polarization rotation for both heterodyne and conventional FRS. Sensing of nitric oxide (NO) has been performed with an H-FRS system based on thermoelectrically cooled 5.24 μm quantum cascade laser (QCL) and a mercury-cadmium-telluride photodetector. The QCL relative intensity noise that dominates at low frequencies is largely avoided by performing the heterodyne detection in radio frequency range. H-FRS exhibits a total noise level of only 3.7 times the fundamental shot noise. The achieved sensitivity to polarization rotation of 1.8 × 10(-8) rad/Hz(1/2) is only 5.6 times higher than the ultimate theoretical sensitivity limit estimated for this system. The path- and bandwidth-normalized NO detection limit of 3.1 ppbv-m/Hz(1/2) was achieved using the R(17/2) transition of NO at 1906.73 cm(-1).

High sensitive detection of nitric oxide using laser induced photoacoustic spectroscopy at 213 nm

Trace level detection of nitric oxide (NO) is of great interest for a wide range of applications such as environment and human health. For this purpose, a high sensitive sensor based photoacoustic spectroscopy (PAS) principle has been developed at our laboratory for detection of NO at very low concentration (ppbV). For optimization of the PAS signal and to achieve higher sensitivity, parametric dependence investigation was carried out where PAS signal dependence on NO gas pressure, cell geometry, buffer gas (Ar, N2, He), and laser pulse energy used three PAS cells developed locally. The best sensitivity achieved with three cells was 41, 11, 20 ppbv, respectively. It is worth reporting that the best PAS signal to noise ratio was achieved by using a cylindrical cell having three acoustic filters and argon as a buffer gas.

Rotationally resolved magnetic vibrational circular dichroism of the paramagnetic molecule NO

Magnetic vibrational circular dichroism (MVCD) enables the measurement of molecular magnetic moments with modest spectral resolution. Due to its paramagnetism, NO gives a much stronger spectral response, about 3 orders of magnitude more intense, than do typical diamagnetic molecules. The molecule thus provides a convenient test for the experiment and theory of paramagnetic rotors. We have measured and analyzed the MVCD, equivalent to the molecular Zeeman spectra, of NO in co-linear magnetic fields of 0.1, 0.2, 2, 4 and 8 Tesla. Similar MVCD intensities were observed for both the (2)Π(1/2) and (2)Π(3/2) components of NO, particularly for high J values, which demonstrates a considerable deviation from pure Hund's case (a) for NO. The g(J)-values for the (2)Π(1/2) components of NO, which can be determined from our experimental spectra by moment analysis, agree well with the predicted values from Radford's theory. For the (2)Π(3/2) components, we tested this theory by simulating the MVCD and absorption spectra, and comparing them with our experimental spectra by use of moment analysis to show that they match well in terms of magnetic properties. While 0.2 T experiments easily develop sufficient MVCD for analysis of NO spectra and these low field intensities have a linear field dependence, spectra in the strong fields accessible in our study showed non-linear response due to onset of saturation effects. We also observed a strong field dependence for the absorption intensities for some (2)Π(3/2) components that was not encompassed in the theoretical model. Finally, a full coupling scheme provided analytical MVCD and absorption intensities that were in good agreement with the experimental values.

Exhaled nitric oxide monitoring by quantum cascade laser: comparison with chemiluminescent and electrochemical sensors

Fractional exhaled nitric oxide (F(E)NO) is considered an indicator in the diagnostics and management of asthma. In this study we present a laser-based sensor for measuring F(E)NO. It consists of a quantum cascade laser (QCL) combined with a multi-pass cell and wavelength modulation spectroscopy for the detection of NO at the sub-part-per-billion by volume (ppbv, 110(-9)) level. The characteristics and diagnostic performance of the sensor were assessed. A detection limit of 0.5 ppbv was demonstrated with a relatively simple design. The QCL-based sensor was compared with two market sensors, a chemiluminescent analyzer (NOA 280, Sievers) and a portable hand-held electrochemical analyzer (MINO, Aerocrine AB, Sweden). F(E)NO from 20 children diagnosed with asthma and treated with inhaled corticosteroids were measured. Data were found to be clinically acceptable within 1.1 ppbv between the QCL-based sensor and chemiluminescent sensor and within 1.7 ppbv when compared to the electrochemical sensor. The QCL-based sensor was tested on healthy subjects at various expiratory flow rates for both online and offline sampling procedures. The extended NO parameters, i.e. the alveolar region, airway wall, diffusing capacity, and flux were calculated and showed a good agreement with the previously reported values.

Sensitive and selective detection of OH radicals using Faraday rotation spectroscopy at 2.8 µm

We report on the development of a Faraday rotation spectroscopy (FRS) instrument using a DFB diode laser operating at 2.8 µm for the hydroxyl (OH) free radical detection. The highest absorption line intensity and the largest gJ value make the Q (1.5) double lines of the 2Π3/2 state (υ = 1 ← 0) at 2.8 µm clearly the best choice for sensitive detection in the infrared region by FRS. The prototype instrument shows shot-noise dominated performance and, with an active optical pathlength of only 25 cm and a lock-in time constant of 100 ms, achieves a 1σ detection limit of 8.2 × 10(8) OH radicals/cm3.

Improved approach for ultra-sensitive detection of NO

An improved approach has been developed for ultra-sensitive detection of the concentration of NO using Faraday Modulation spectrometry (FAMOS) combined with the strong electronic transition. By changing the modulating magnetic field attributing to linear absorption and refraction of gas sample, the polarized laser was rotated and absorbed by the complex refraction index of NO. We confirm the relation between the magnitudes of absorption and the optimum modulation magnetic field. Also, the accuracy and the precision of the technique have been evaluated at different pressures. It is shown that the system is capable of detecting NO concentration down to 0.34 ppb·m.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: I. Theory

We give a simple two-transition model of Faraday modulation spectrometry (FAMOS) addressing the electronic X(2)Π(ν('') = 0) - A(2)Σ(+)(ν(') = 0) band in nitric oxide. The model is given in terms of the integrated line strength, S, and first Fourier coefficients for the magnetic-field-modulated dispersive line shape function. Although the two states addressed respond differently to the magnetic field (they adhere to the dissimilar Hund coupling cases), it is shown that the technique shares some properties with FAMOS when rotational-vibrational Q-transitions are targeted: the line shapes have a similar form and the signal strength has an analogous magnetic field and pressure dependence. The differences are that the maximum signal appears for larger magnetic field amplitudes and pressures, ∼1500 G and ∼200 Torr, respectively.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: II. Experiment

A first demonstration of Faraday modulation spectrometry (FAMOS) of nitric oxide (NO) addressing its strong electronic X(2)Π(ν″ = 0) - A(2)Σ(+)(ν(') = 0) band is presented. The instrumentation was constructed around a fully diode-laser-based laser system producing mW powers of ultraviolet light targeting the overlapping Q(22)(21/2) and R(12)Q(21/2) transitions at ∼226.6 nm. The work verifies a new two-transition model of FAMOS addressing the electronic transitions in NO given in an accompanying work. Although the experimental instrumentation could address neither the parameter space of the theory nor the optimum conditions, the line shapes and the pressure dependence could be verified under low-field conditions. NO could be detected down to a partial pressure of 13 µTorr, roughly corresponding to 10 ppb·m for an atmospheric pressure sample, which demonstrates the feasibility of FAMOS for sensitive detection of NO addressing its strong electronic band.

Ultrasensitive detection of nitric oxide at 5.33 microm by using external cavity quantum cascade laser-based Faraday rotation spectroscopy

A transportable prototype Faraday rotation spectroscopic system based on a tunable external cavity quantum cascade laser has been developed for ultrasensitive detection of nitric oxide (NO). A broadly tunable laser source allows targeting the optimum Q(3/2)(3/2) molecular transition at 1875.81 cm(-1) of the NO fundamental band. For an active optical path of 44 cm and 1-s lock-in time constant minimum NO detection limits (1sigma) of 4.3 parts per billion by volume (ppbv) and 0.38 ppbv are obtained by using a thermoelectrically cooled mercury-cadmium-telluride photodetector and liquid nitrogen-cooled indium-antimonide photodetector, respectively. Laboratory performance evaluation and results of continuous, unattended monitoring of atmospheric NO concentration levels are reported.

Performance of an exhaled nitric oxide and carbon dioxide sensor using quantum cascade laser-based integrated cavity output spectroscopy

Exhaled nitric oxide (NO) is an important biomarker in asthma and other respiratory disorders. The optical performance of a NOCO(2) sensor employing integrated cavity output spectroscopy (ICOS) with a quantum cascade laser operating at 5.22 microm capable of real-time NO and CO(2) measurements in a single breath cycle is reported. A NO noise-equivalent concentration of 0.4 ppb within a 1-sec integration time is achieved. The off-axis ICOS sensor performance is compared to a chemiluminescent NO analyzer and a nondispersive infrared (NDIR) CO(2) absorption capnograph. Differences between the gas analyzers are assessed by the Bland-Altman method to estimate the expected variability between the gas sensors. The off-axis ICOS sensor measurements are in good agreement with the data acquired with the two commercial gas analyzers. This work demonstrates the performance characteristics and merits of mid-infrared spectroscopy for exhaled breath analysis.

Sub-part-per-billion monitoring of nitric oxide by use of wavelength modulation spectroscopy in combination with a thermoelectrically cooled, continuous-wave quantum cascade laser

We used a thermoelectrically cooled, continuous-wave, quantum cascade laser operating between 1847 and 1854 cm(-1) in combination with wavelength modulation spectroscopy for the detection of nitric oxide (NO) at the sub-part-per-billion by volume (ppbv) level. The laser emission overlaps the P7.5 doublet of NO centered around 1850.18 cm(-1). Using an astigmatic multiple-pass absorption cell with an optical path length of 76 m, we achieved a detection limit of 0.2 ppbv at 10 kPa, with a total acquisition time of 30 s. The corresponding minimal detectable absorption is 8.8 x 10(-9) cm(-1) Hz(-1/2).

Infrared Evanescent Field Sensing with Quantum Cascade Lasers and Planar Silver Halide Waveguides

We demonstrate the first midinfrared evanescent field absorption measurements with an InGaAs/AlInAs/InP distributed feedback (DFB) quantum cascade laser (QCL) light source operated at room temperature coupled to a free-standing, thin-film, planar, silver halide waveguide. Two different analytes, each matched to the emission frequency of a QCL, were investigated to verify the potential of this technique. The emission of a 1650 cm(-1) QCL overlaps with the amide absorption band of urea, which was deposited from methanol solution, forming urea crystals at the waveguide surface after solvent evaporation. Solid urea was detected down to 80.7 microg of precipitate at the waveguide surface. The emission frequency of a 974 cm(-1) QCL overlaps with the CH3-C absorption feature of acetic anhydride. Solutions of acetic anhydride in acetonitrile have been detected down to a volume of 0.01 microL (10.8 microg) of acetic anhydride solution after deposition at the planar waveguide (PWG) surface. Free-standing, thin-film, planar, silver halide waveguides were produced by press-tapering heated, cylindrical, silver halide fiber segments to create waveguides with a thickness of 300-190 microm, a width of 3 mm, and a length of 35 mm. In addition, Fourier transform infrared (FT-IR) evanescent field absorption measurements with planar silver halide waveguides and transmission absorption QCL measurements verify the obtained results.

Real‐Time Detection of Nitric Oxide Isotopes in Lung Function Tests

In lung function tests, the determination of the pulmonary diffusing capacity (D) using the single-breath method is a commonly applied technique. The calculation of D is performed on the basis of accurate measurements of indicator gas concentrations. In this chapter, we demonstrate the appropriateness of the stable nitric oxide (NO) isotopes 14NO and 15NO in revealing reliable data of D. We performed studies on animals (14NO) by using respiratory mass spectrometry (M3) and on humans (15NO) by applying laser magnetic resonance spectroscopy (LMRS). The equipment was characterized by sufficient detection limits of 70 parts/billion at [14NO] = 0.001% (M3) and 40 parts/billion at [15NO] = 0.002 % (LMRS), respectively. Lastly, we were able to show that D-values for 14NO indeed reveal the entire diffusive properties of the alveolar-capillary membrane and that 15NO is a useful indicator gas for reflecting disturbances of pulmonary gas exchange.

Quantum cascade semiconductor infrared and far-infrared lasers: from trace gas sensing to non-linear optics

The Quantum cascade (QC) laser is an entirely new type of semiconductor device in which the laser wavelength depends on the band-gap engineering. It can be made to operate over a much larger range than lead salt lasers, covering significant parts of both the infrared and submillimetre regions, and with higher output power. In this tutorial review we survey some of the applications of these new lasers, which range from trace gas detection for atmospheric or medical purposes to sub-Doppler and time dependent non-linear spectroscopy.

Sensitive and Isotope Selective (14NO/15NO) Online Detection of Nitric Oxide by Faraday–Laser Magnetic Resonance Spectroscopy

The monitoring of trace amounts of nitric oxide (NO) is of great interest for biomedical applications. High-resolution infrared spectroscopy, like laser magnetic resonance spectroscopy (LMRS), is a versatile technique for the quantitative and isotope-selective analysis of low concentrations of NO. The ability to distinguish between different NO isotopomeres is of special interest for pharmaceutical and biomedical applications when tracer investigations are performed. With LMRS using Faraday modulation, a sensitivity of 1-2 ppbV for 15NO can be achieved. With further improvements and development of the sensor, we expect a sub-ppbV sensitivity.

Parts per trillion sensitivity for ethane in air with an optical parametric oscillator cavity leak-out spectrometer

Spectroscopic detection of ethane in the 3-microm wavelength region was performed by means of a cw optical parametric oscillator and cavity leak-out. We achieved a minimum detectable absorption coefficient of 1.6 x 10(-10) cm 1/square root of Hz, corresponding to an ethane detection limit of 6 parts per trillion/square root of Hz. For 3-min integration time the detection limit was 0.5 parts per trillion. The levels are to our knowledge the best demonstrated so far. These frequency-tuning capabilities facilitated multigas analysis with simultaneous monitoring of ethane, methane, and water vapor in human breath.

Mid-infrared quantum cascade laser based off-axis integrated cavity output spectroscopy for biogenic nitric oxide detection

Tunable-laser absorption spectroscopy in the mid-IR spectral region is a sensitive analytical technique for trace-gas quantification. The detection of nitric oxide (NO) in exhaled breath is of particular interest in the diagnosis of lower-airway inflammation associated with a number of lung diseases and illnesses. A gas analyzer based on a continuous-wave mid-IR quantum cascade laser operating at approximately 5.2 microm and on off-axis integrated cavity output spectroscopy (ICOS) has been developed to measure NO concentrations in human breath. A compact sample cell, 5.3 cm in length and with a volume of < 80 cm3, that is suitable for on-line and off-line measurements during a single breath cycle, has been designed and tested. A noise-equivalent (signal-to-noise ratio of 1) sensitivity of 10 parts in 10(9) by volume (ppbv) of NO was achieved. The combination of ICOS with wavelength modulation resulted in a 2-ppbv noise-equivalent sensitivity. The total data acquisition and averaging time was 15 s in both cases. The feasibility of detecting NO in expired human breath as a potential noninvasive medical diagnostic tool is discussed.