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 (15NO) 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.
Highlights
Ratiometric analysis of nitrogen isotopologues (14N, 15N) provides unique insight into nitrogen cycling dynamics [1,2,3,4,5] and enables environmental diagnostics by differentiating various natural and anthropogenic chemical processes [6,7]
Stable-isotope mass-spectrometry is the state-of-art technique for precision isotopic ratiometry [5,6,7,11,12], but the substantial capital investment and maintenance requirements involved prevent nitrogen isotope analysis from being used as a mainstream diagnostic tool
We present a transportable analyzer based on dual-modulation Faraday rotation spectroscopy (DM-FRS) of nitric oxide (NO) [13,14,15] with near-fundamental quantum-noise limited sensitivity
Summary
Ratiometric analysis of nitrogen isotopologues (14N, 15N) provides unique insight into nitrogen cycling dynamics [1,2,3,4,5] and enables environmental diagnostics by differentiating various natural and anthropogenic chemical processes [6,7]. The lack of instrument transportability has eliminated the possibility of in situ measurements, requiring environmental samples to be shipped to the laboratory, which presents practical limits on sample sizes. The transportability of the system allows in situ measurements and practically eliminates the sample size limitation characteristic in the case of external laboratory analysis. Sub-per mille isotopic ratiometric precision for micro-mole nitrate samples has been achieved, demonstrating our spectrometer as an enabling technology for in situ environmental and medical diagnostics
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