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. 2015 Oct 14;15(10):25992-6008.
doi: 10.3390/s151025992.

Nitric Oxide Isotopic Analyzer Based on a Compact Dual-Modulation Faraday Rotation Spectrometer

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Free PMC article

Nitric Oxide Isotopic Analyzer Based on a Compact Dual-Modulation Faraday Rotation Spectrometer

Eric Zhang et al. Sensors (Basel). .
Free PMC article

Abstract

We have developed a transportable spectroscopic nitrogen isotopic analyzer. The spectrometer is based on dual-modulation Faraday rotation spectroscopy of nitric oxide isotopologues with near shot-noise limited performance and baseline-free operation. Noise analysis indicates minor isotope ((15)NO) detection sensitivity of 0.36 ppbv·Hz(-1/2), corresponding to noise-equivalent Faraday rotation angle (NEA) of 1.31 × 10(-8) rad·Hz(-1/2) and noise-equivalent absorbance (αL)min of 6.27 × 10(-8) Hz(-1/2). White-noise limited performance at 2.8× the shot-noise limit is observed up to ~1000 s, allowing reliable calibration and sample measurement within the drift-free interval of the spectrometer. Integration with wet-chemistry based on acidic vanadium(III) enables conversion of aqueous nitrate/nitrite samples to gaseous NO for total nitrogen isotope analysis. Isotopic ratiometry is accomplished via time-multiplexed measurements of two NO isotope transitions. For 5 μmol potassium nitrate samples, the instrument consistently yields ratiometric precision below 0.3‰, thus demonstrating potential as an in situ diagnostic tool for environmental nitrogen cycle studies.

Keywords: Faraday effect; isotopic ratiometry; nitric oxide; nitrogen cycle; optical sensing and sensors; spectroscopy.

Figures

Figure 1
Figure 1
Spectral schematic of DM-FRS signal frequencies. Modulation of the laser occurs at fL = 50 kHz, resulting in wavelength modulated carrier frequencies at N·fL and a corresponding decrease in 1/f noise. Further modulation of the magnetic field (fM = 100 Hz) results in generation of sum and difference frequency sidebands N·fL ± fM (scales in schematic are exaggerated for clarity). In DM-FRS, the demodulation bandwidth Δf must be considered around each sideband, resulting in 2·Δf total noise bandwidth.
Figure 2
Figure 2
Integrated sensor schematic. (a) Dual-modulation Faraday rotation spectrometer for NO isotopic analysis. The system is optimized for minor isotope sensitivity (15NO Q(3/2) transition, 1842.763 cm−1) with a detection limit of 0.36 ppbv·Hz−1/2 using a triple-pass gas cell (45 cm optical pathlength). A reference gas cell (~1% 15NO, and ~1% 14NO in N2) is utilized as wavelength reference for line-locking; (b) Photograph of spectrometer housed in 12U 19” rack, integrated with breath sampler; (c) Chemical system for nitrate/nitrite conversion to NO via heated acidic vanadium(III).
Figure 3
Figure 3
(a) Spectra of 14NO P(19/2)e (blue) and 15NO Q(3/2) (red) transitions from a G-Cal permeation device. Quasi-simultaneous isotopic analysis via line switching can be performed by alternately locking to the 3fL WMS zero-crossing each isotope; (b) Zero-gas spectrum using cylinder nitrogen demonstrating near-zero baseline spectra; (c) Measurement of 15NO spectra using non-certified NO in N2 mixture. Spectral modeling gives 15NO concentration of 4.56 ppbv, corresponding to 1.24 ppmv 14NO at natural abundance.
Figure 4
Figure 4
(a) Measurement of pure nitrogen by line-locking to the zero-crossing of the 15NO 3fL wavelength modulated spectrum. Slight baseline deviation is visible over the ~1 h of measurement, indicating EMI influence; (b) Linear baseline correction every ~500 s is necessary for liquid samples analysis and similar correction is applied here to demonstrate the Allan deviation in a practical measurement scenario; (c) Allan deviation of the baseline-corrected measurement, demonstrating white-noise performance up to ~1000 s at 2.8× the shot-noise limit. The inset is a typical 3fL WMS spectra used for line-locking.
Figure 5
Figure 5
Quasi-simultaneous isotope measurement of 1.24 ppmv 14NO in N2 mixture diluted using pure nitrogen. Line-switching occurs every 50 s, and polynomial interpolation of 14NO is used to calculate the isotopic ratio (top graph). Gray regions denote 14NO measurements and white regions denote 15NO. Permil ratiometric values are calculated according to Equation (12), demonstrating fractionation-free system performance. A concentration-normalized precision of Δ(δ15N) = 120‰·ppmv·Hz−1/2 was determined from a typical measurement segment.
Figure 6
Figure 6
(a) Real-time measurement of 5 μmol injection of KNO3 (500 μL of 10 mM solution) via line-switching every 15 s at 50% duty cycle for each isotope. The blue, red and black segments correspond to [14Ñ]S, [15Ñ]S and interpolated [14Ñ]S respectively. The resulting peak has FWHM of ~100 s, obtained in a flow rate of 64 sccm, and the inset shows three more repeat injections (only [15Ñ]S is shown for clarity); (b) Ratiometric curves derived from the quotient of [15Ñ]S(t) and interpolated [14Ñ]S. A clear time dependent curvature is apparent, with a maxima occurring at the measurement peak; (c) Calculation of ratiometric precision vs. sample size, using data obtained from (a). Generally, samples >1 μmol NO3 are required to ensure sub-permil precisions over the span of the peak.
Figure 7
Figure 7
(a) Multiple injections of known isotopically labeled references spanning from 5‰ to 5536‰, where the 15NO signal is clearly enhanced for enriched samples. The smaller 400 nmol injection size reduces ratiometric precision (Δ(δ15N) ≈ 2‰), consistent with Figure 6c and Equation (18); Plotted in (b) are multiple labeled reference injections (4 points per calculated δ15N value, and the plot demonstrates excellent linearity (R2 = 0.999) with measured δ15N offset consistent with fractionation observed in Figure 6a. The inset shows the shape of real-time fractionation curve of the most enriched reference injection (δ15Ncalc = 5536‰), demonstrating results similar to those in Section 5.2.

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