Radiocarbon dioxide detection based on cavity ring-down spectroscopy and a quantum cascade laser

Mid-infrared trace detection with parts-per-quadrillion quantitation accuracy: Expanding frontiers of radiocarbon sensing

Detection sensitivity is a critical characteristic to consider during selection of spectroscopic techniques. However, high sensitivity alone is insufficient for spectroscopic measurements in spectrally congested regions. Two-color cavity ringdown spectroscopy (2C-CRDS), based on intra-cavity pump-probe detection, simultaneously achieves high detection sensitivity and selectivity. This combination enables mid-infrared detection of radiocarbon dioxide ([Formula: see text]CO[Formula: see text]) molecules in room-temperature CO[Formula: see text] samples, with 1.4 parts-per-quadrillion (ppq, 10[Formula: see text]) sensitivity (average measurement precision) and 4.6-ppq quantitation accuracy (average calibrated measurement error for 21 samples from four separate trials) demonstrated on samples with [Formula: see text]C/C up to [Formula: see text]1.5[Formula: see text] natural abundance ([Formula: see text]1,800 ppq). These highly reproducible measurements, which are the most sensitive and quantitatively accurate in the mid-infrared, are accomplished despite the presence of orders-of-magnitude stronger, one-photon signals from other CO[Formula: see text] isotopologues. This is a major achievement in laser spectroscopy. A room-temperature-operated, compact, and low-cost 2C-CRDS sensor for [Formula: see text]CO[Formula: see text] benefits a wide range of scientific fields that utilize [Formula: see text]C for dating and isotope tracing, most notably atmospheric [Formula: see text]CO[Formula: see text] monitoring to track CO[Formula: see text] emissions from fossil fuels. The 2C-CRDS technique significantly enhances the general utility of high-resolution mid-infrared detection for analytical measurements and fundamental chemical dynamics studies.

Mid-infrared supermirrors with finesse exceeding 400 000

For trace gas sensing and precision spectroscopy, optical cavities incorporating low-loss mirrors are indispensable for path length and optical intensity enhancement. Optical interference coatings in the visible and near-infrared (NIR) spectral regions have achieved total optical losses below 2 parts per million (ppm), enabling a cavity finesse in excess of 1 million. However, such advancements have been lacking in the mid-infrared (MIR), despite substantial scientific interest. Here, we demonstrate a significant breakthrough in high-performance MIR mirrors, reporting substrate-transferred single-crystal interference coatings capable of cavity finesse values from 200 000 to 400 000 near 4.5 µm, with excess optical losses (scatter and absorption) below 5 ppm. In a first proof-of-concept demonstration, we achieve the lowest noise-equivalent absorption in a linear cavity ring-down spectrometer normalized by cavity length. This substantial improvement in performance will unlock a rich variety of MIR applications for atmospheric transport and environmental sciences, detection of fugitive emissions, process gas monitoring, breath-gas analysis, and verification of biogenic fuels and plastics.

Cantilever-enhanced photoacoustic measurement of HTO in water vapor

A photoacoustic detection of tritiated water (HTO) is presented. The method uses cantilever-enhanced photoacoustic spectroscopy (CEPAS) to reach sub-ppb sensitivity for HTO in the gas phase. A noise equivalent concentration of 0.88 ppb is reached with a sampling time of 1 min. The high performance and small sample volume of CEPAS allows sensitive detection of HTO from a sample with low total activity.

Room-Temperature Optical Detection of 14CO2 below the Natural Abundance with Two-Color Cavity Ring-Down Spectroscopy

Radiocarbon's natural production, radiative decay, and isotopic rarity make it a unique tool to probe carbonaceous systems in the life and earth sciences. However, the difficulty of current radiocarbon (14C) detection methods limits scientific adoption. Here, two-color cavity ring-down spectroscopy detects 14CO2 in room-temperature samples with an accuracy of one-tenth the natural abundance in 3 min. The intracavity pump-probe measurement uses two cavity-enhanced lasers to cancel out cavity ring-down rate fluctuations and strong one-photon absorption interference (>10 000 1/s) from hot-band transitions of CO2 isotopologues. Selective, room-temperature detection of small 14CO2 absorption signals (<1 1/s) reduces the technical and operational burdens for cavity-enhanced measurements of radiocarbon, which can benefit a wide range of applications like biomedical research and field-detection of combusted fossil fuels.

On-Line Monitoring of Radiocarbon Emissions in a Nuclear Facility with Cavity Ring-Down Spectroscopy

There are currently no suitable methods for sensitive automated in situ monitoring of gaseous radiocarbon, one of the main sources of radioactive gas emissions from nuclear power plants. Here, we present a transportable instrument for in situ airborne radiocarbon detection based on mid-infrared cavity ring-down spectroscopy and report its performance in a 1-week field measurement at the Loviisa nuclear power plant. Radiocarbon is detected by measuring an absorption line of the 14CO2 molecule. The time resolution of the measurements is 45 min, significantly less than the few days' resolution of the currently used technique, while maintaining a comparable sensitivity. The method can also assess the prevalence of radiocarbon in different molecular species in the airborne emissions. The optical in situ monitoring presented is a completely new method for monitoring emissions from nuclear facilities.

Radiocarbon dioxide detection using cantilever-enhanced photoacoustic spectroscopy

In this Letter, we report on the sub-parts-per-billion-level radiocarbon dioxide detection using cantilever-enhanced photoacoustic spectroscopy. The ¹⁴C/C ratio of samples is measured by targeting a ¹⁴CO₂ absorption line with minimal interference from other CO₂ isotopes. Using a quantum cascade laser as a light source allows for a compact experimental setup. In addition, measurements of sample gases with ¹⁴CO₂ concentrations as low as 100 parts-per-trillion (ppt) are presented. The Allan deviation demonstrates a noise equivalent concentration of 30 ppt at an averaging time of 9 min. The achieved sensitivity validates this method as a suitable alternative to more complex optical detection methods for radiocarbon dioxide detection used so far, and it can be envisioned for future in situ radiocarbon detection.

Online Biogenic Carbon Analysis Enables Refineries to Reduce Carbon Footprint during Coprocessing Biomass- and Petroleum-Derived Liquids

To mitigate green-house gas (GHG) emissions, governments around the world are enacting legislation to reduce carbon intensity in transportation fuels. Coprocessing biomass and petroleum-derived liquids in existing refineries is a near-term, cost-effective approach for introducing renewable carbon in fuels and enabling refineries to meet regulatory mandates. However, coprocessing biomass-derived liquids in refineries results in variable degrees of biogenic carbon incorporation, necessitating accurate quantification to verify compliance with mandates. Existing refinery control and instrumentation systems lack the means to measure renewable carbon accurately, reliably, and quickly. Thus, accurate measurement of biogenic carbon is key to ensuring refineries meet regulatory mandates. In this Perspective, we present existing methods for measuring biogenic carbon, point out their challenges, and discuss the need for new online analytical capabilities to measure biogenic carbon in fuel intermediates.

Mid-infrared interference coatings with excess optical loss below 10 ppm

We present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of 4.54 μm with record-low excess optical loss below 10 parts per million. These high-performance mirrors are realized via a novel microfabrication process that differs significantly from the production of amorphous multilayers generated via physical vapor deposition processes. This new process enables reduced scatter loss due to the low surface and interfacial roughness, while low background doping in epitaxial growth ensures strongly reduced absorption. We report on a suite of optical measurements, including cavity ring-down, transmittance spectroscopy, and direct absorption tests to reveal the optical losses for a set of prototype mirrors. In the course of these measurements, we observe a unique polarization-orientation-dependent loss mechanism which we attribute to elastic anisotropy of these strained epitaxial multilayers. A future increase in layer count and a corresponding reduction of transmittance will enable optical resonators with a finesse in excess of 100 000 in the mid-infrared spectral region, allowing for advances in high resolution spectroscopy, narrow-linewidth laser stabilization, and ultrasensitive measurements of various light-matter interactions.

Optical interruption of a quantum cascade laser for cavity ring-down spectroscopy

We demonstrate optical unlocking of a cavity resonance in a mid-infrared (MIR) quantum cascade laser (QCL) cavity ring-down spectrometer using a low-power multimode near-infrared (NIR) laser diode. A NIR laser with a center wavelength of 1310 nm is injected into a QCL whose amplitude and frequency are modulated as a result. The shift in frequency leads to a rapid interruption of the cavity resonance. The optical method is compared to cavity interruption with a current step by measuring ring-down times in a high-finesse optical cavity, coupled with a QCL with a center wavelength of 4.5 μm. The results indicate the comparable performance of the all-optical method with the more conventional current modulation, but with significantly reduced bandwidth requirements for the QCL driver, opening the way to other potential applications in MIR laser spectroscopy.

Laser Spectroscopy for Monitoring of Radiocarbon in Atmospheric Samples

In-situ monitoring of radiocarbon emissions is challenging due to the lack of a suitable method for sensitive online detection of this isotope. Here we report on a complete system for automatized continuous on-site monitoring of radiocarbon gaseous emissions from nuclear facilities. By combining radiocarbon detection using mid-infrared cavity ring-down spectroscopy and an advanced sampling system, an elevated amount of radiocarbon in an atmospheric-like gas matrix was detected. Radiocarbon was detected in the form of ¹⁴CO₂ after extraction of the carbon dioxide from the air sample. The system is also able to discriminate between radiocarbon in organic or inorganic molecular form by converting ¹⁴CH₄ into ¹⁴CO₂. This work lays the groundwork for further use of this technology in nuclear facilities for online on-site monitoring of radioactive gaseous emissions as well as future work on in-situ monitoring of atmospheric radiocarbon.

Mid-infrared supercontinuum-based upconversion detection for trace gas sensing

Recent advancements of mid-infrared (MIR) supercontinuum light sources have opened up new possibilities in laser-based trace gas sensing. While the supercontinuum sources inherently support wide spectral coverage, the detection of broadband absorption signals with high speed and low cost is traditionally limited by the MIR detector arrays. In this work, we demonstrate that this limitation can be circumvented by upconverting the MIR signal into the near-infrared (NIR) region, where cost-effective silicon-based detector arrays can be utilized to measure broadband absorption. We also show that, by combining a MIR supercontinuum source with a MIR-to-NIR upconverter and an astigmatic multipass cell, fast detection (~20 ms) of ethane with sub-ppmv sensitivity can be achieved at room temperature. For multi-species detection, a least-square global fitting method is presented, showing a promising potential for applications such as environmental monitoring and biomedical research.

Laser-based radiocarbon detection in the laboratory: How soon?

Research over the past 25 years and the use of accelerator mass spectrometry (AMS) have demonstrated benefits of single-atom counting of ¹⁴ C compared with scintillation monitoring of ¹⁴ C radioactive decay for a multitude of applications in drug development studies. These include pharmacokinetics and metabolism studies, microdosing studies, and quantification of DNA adducts. In the last decade, the possibility of single-atom counting using lasers has been demonstrated, providing the possibility of simplified laboratory-based systems, which can equal or excel AMS sensitivity and provide scintillation system convenience without high levels of radioactivity. To achieve the required sensitivity, optical storage cavities have been used to enhance the laser interaction of the low densities of radiocarbon present. Two types of laser technologies have been used-cavity ring-down spectroscopy (CRDS) and intracavity opto-galvanic spectroscopy (ICOGS). Problems to be overcome to achieve routine use have included separation of the ¹⁴ C signal from backgrounds, achievement of acceptable precision and accuracy, reduction of measurement times for small samples, and improvement in the ease of use for the operator. Both technologies have achieved impressive results to date using samples of order 1 mg with CRDS and 10 μg with ICOGS. Commercial development is the next step.

Radiocarbon Tracers in Toxicology and Medicine: Recent Advances in Technology and Science

This review summarizes recent developments in radiocarbon tracer technology and applications. Technologies covered include accelerator mass spectrometry (AMS), including conversion of samples to graphite, and rapid combustion to carbon dioxide to enable direct liquid sample analysis, coupling to HPLC for real-time AMS analysis, and combined molecular mass spectrometry and AMS for analyte identification and quantitation. Laser-based alternatives, such as cavity ring down spectrometry, are emerging to enable lower cost, higher throughput measurements of biological samples. Applications covered include radiocarbon dating, use of environmental atomic bomb pulse radiocarbon content for cell and protein age determination and turnover studies, and carbon source identification. Low dose toxicology applications reviewed include studies of naphthalene-DNA adduct formation, benzo[a]pyrene pharmacokinetics in humans, and triclocarban exposure and risk assessment. Cancer-related studies covered include the use of radiocarbon-labeled cells for better defining mechanisms of metastasis and the use of drug-DNA adducts as predictive biomarkers of response to chemotherapy.

Broadband photoacoustic spectroscopy of CH414 with a high-power mid-infrared optical frequency comb

We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of CH₄14 is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon methane.

Nanotracing and cavity-ring down spectroscopy: A new ultrasensitive approach in large molecule drug disposition studies

New therapeutic biological entities such as bispecific antibodies targeting tissue or specific cell populations form an increasingly important part of the drug development portfolio. However, these biopharmaceutical agents bear the risk of extensive target-mediated drug disposition or atypical pharmacokinetic properties as compared to canonical antibodies. Pharmacokinetics and bio-distribution studies become therefore more and more important during lead optimization. Biologics present, however, greater analytical challenges than small molecule drugs due to the mass and selectivity limitation of mass spectrometry and ligand-binding assay, respectively. Radiocarbon (¹⁴C) and its detection methods, such as the emerging ¹⁴C cavity ring down spectroscopy (CRDS), thus can play an important role in the large molecule quantitation where a ¹⁴C-tag is covalently bound through a stable linker. CRDS has the advantage of a simplified sample preparation and introduction system as compared to accelerator mass spectrometry (AMS) and can be accommodated within an ordinary research laboratory. In this study, we report on the labeling of an anti-IL17 IgG1 model antibody with ¹⁴C propionate tag and its detection by CRDS using it as nanotracer (2.1 nCi or 77.7 Bq blended with the therapeutic dose) in a pharmacokinetics study in a preclinical species. We compare these data to data generated by AMS in parallel processed samples. The derived concentration time profiles for anti-IL17 by CRDS were in concordance with the ones derived by AMS and γ-counting of an ¹²⁵I-labeled anti-IL17 radiotracer and were well described by a 2-compartment population pharmacokinetic model. In addition, antibody tissue distribution coefficients for anti-IL17 were determined by CRDS, which proved to be a direct and sensitive measurement of the extravascular tissue concentration of the antibody when tissue perfusion was applied. Thus, this proof-of-concept study demonstrates that trace ¹⁴C-radiolabels and CRDS are an ultrasensitive approach in (pre)clinical pharmacokinetics and bio-distribution studies of new therapeutic entities.

Determining Biogenic Content of Biogas by Measuring Stable Isotopologues 12CH₄, 13CH₄, and CH₃D with a Mid-Infrared Direct Absorption Laser Spectrometer

A tunable laser absorption spectrometer (TLAS) was developed for the simultaneous measurement of δ¹³C and δD values of methane (CH₄). A mid-infrared interband cascade laser (ICL) emitting around 3.27 µm was used to measure the absorption of the three most abundant isotopologues in CH₄ with a single, mode-hop free current sweep. The instrument was validated against methane samples of fossil and biogenic origin with known isotopic composition. Three blended mixtures with varied biogenic content were prepared volumetrically, and their δ¹³C and δD values were determined. Analysis demonstrated that, provided the isotopic composition of the source materials was known, the δ¹³C and δD values alone were sufficient to determine the biogenic content of the blended samples to within 1.5%.

Optical Measurement of Radiocarbon below Unity Fraction Modern by Linear Absorption Spectroscopy

High-precision measurements of radiocarbon (¹⁴C) near or below a fraction modern ¹⁴C of 1 (F¹⁴C ≤ 1) are challenging and costly. An accurate, ultrasensitive linear absorption approach to detecting ¹⁴C would provide a simple and robust benchtop alternative to off-site accelerator mass spectrometry facilities. Here we report the quantitative measurement of ¹⁴C in gas-phase samples of CO₂ with F¹⁴C < 1 using cavity ring-down spectroscopy in the linear absorption regime. Repeated analysis of CO₂ derived from the combustion of either biogenic or petrogenic sources revealed a robust ability to differentiate samples with F¹⁴C < 1. With a combined uncertainty of ¹⁴C/¹²C = 130 fmol/mol (F¹⁴C = 0.11), initial performance of the calibration-free instrument is sufficient to investigate a variety of applications in radiocarbon measurement science including the study of biofuels and bioplastics, illicitly traded specimens, bomb dating, and atmospheric transport.

Quantifying Carbon-14 for Biology Using Cavity Ring-Down Spectroscopy

A cavity ring-down spectroscopy (CRDS) instrument was developed using mature, robust hardware for the measurement of carbon-14 in biological studies. The system was characterized using carbon-14 elevated glucose samples and returned a linear response up to 387 times contemporary carbon-14 concentrations. Carbon-14 free and contemporary carbon-14 samples with varying carbon-13 concentrations were used to assess the method detection limit of approximately one-third contemporary carbon-14 levels. Sources of inaccuracies are presented and discussed, and the capability to measure carbon-14 in biological samples is demonstrated by comparing pharmacokinetics from carbon-14 dosed guinea pigs analyzed by both CRDS and accelerator mass spectrometry. The CRDS approach presented affords easy access to powerful carbon-14 tracer techniques that can characterize complex biochemical systems.

Advances in Mid-Infrared Spectroscopy for Chemical Analysis

Infrared spectroscopy in the 3-20 μm spectral window has evolved from a routine laboratory technique into a state-of-the-art spectroscopy and sensing tool by benefitting from recent progress in increasingly sophisticated spectra acquisition techniques and advanced materials for generating, guiding, and detecting mid-infrared (MIR) radiation. Today, MIR spectroscopy provides molecular information with trace to ultratrace sensitivity, fast data acquisition rates, and high spectral resolution catering to demanding applications in bioanalytics, for example, and to improved routine analysis. In addition to advances in miniaturized device technology without sacrificing analytical performance, selected innovative applications for MIR spectroscopy ranging from process analysis to biotechnology and medical diagnostics are highlighted in this review.

Ultra-sensitive cavity ring-down spectroscopy in the mid-infrared spectral region

We describe an ultra-sensitive cavity ring-down spectrometer which operates in the mid-infrared spectral region near 4.5 μm. With this instrument a noise-equivalent absorption coefficient of 2.6×10-11  cm-1 Hz-1/2 was demonstrated with less than 150 nW of optical power incident on the photodetector. Quantum noise was observed in the individual ring-down decay events, leading to quantum-noise-limited short-time performance. We believe that this spectrometer's combination of high sensitivity and robustness make it well suited for measurements of ultra-trace gas species as well as applications in optics and fundamental physics.

Narrowing of the linewidth of an optical parametric oscillator by an acousto-optic modulator for the realization of mid-IR noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 10⁻¹⁰ cm⁻¹ Hz⁻¹/²

The linewidth of a singly resonant optical parametric oscillator (OPO) has been narrowed with respect to an external cavity by the use of an acousto-optic modulator (AOM). This made possible an improvement of the sensitivity of a previously realized OPO-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrument for the 3.2 - 3.9 µm mid-infrared region by one order of magnitude. The resulting system shows a detection sensitivity for methane of 2.4 × 10(-10) cm(-1) Hz(-1∕2) and 1.3 × 10(-10) cm(-1) at 20 s, which allows for detection of both the environmentally important (13)CH(4) and CH(3)D isotopologues in atmospheric samples.

Intracavity OptoGalvanic Spectroscopy not suitable for ambient level radiocarbon detection

IntraCavity OptoGalvanic Spectroscopy as a radiocarbon detection technique was first reported by the Murnick group at Rutgers University, Newark, NJ, in 2008. This technique for radiocarbon detection was presented with tremendous potentials for applications in various fields of research. Significantly cheaper, this technique was portrayed as a possible complementary technique to the more expensive and complex accelerator mass spectrometry. Several groups around the world started developing this technique for various radiocarbon related applications. The IntraCavity OptoGalvanic Spectroscopy setup at the University of Groningen was constructed in 2012 in close collaboration with the Murnick group for exploring possible applications in the fields of radiocarbon dating and atmospheric monitoring. In this paper we describe a systematic evaluation of the IntraCavity OptoGalvanic Spectroscopy setup at Groningen for radiocarbon detection. Since the IntraCavity OptoGalvanic Spectroscopy setup was strictly planned for dating and atmospheric monitoring purposes, all the initial experiments were performed with CO2 samples containing contemporary levels and highly depleted levels of radiocarbon. Because of recurring failures in differentiating the two CO2 samples, with the radiocarbon concentration 3 orders of magnitude apart, CO2 samples containing elevated levels of radiocarbon were prepared in-house and experimented with. All results obtained thus far at Groningen are in sharp contrast to the results published by the Murnick group and rather support the results put forward by the Salehpour group at Uppsala University. From our extensive test work, we must conclude that the method is unsuited for ambient level radiocarbon measurements, and even highly enriched CO2 samples yield insignificant signal.