Recently introduced compact mid-infrared (IR) lasers and optical components allowed for probing orders of magnitude stronger mid-IR transitions of atmospheric molecules, thus significantly increasing instrumental sensitivity. For example, optical radiocarbon detection is approaching sensitivity of the current state of the art technique: accelerator mass spectrometer . With only fraction of the size and cost, optical radiocarbon detector could in near future be applied for on-site applications.
VTT has years of experience of developing optical gas analysers for mid-IR spectral region. Recognising the potential, VTT has selected isotope metrology as a growth area.
The main isotopologue of carbon dioxide comprises 98.4% of all atmospheric CO2, and is followed by two stable isotopologues 13CO2 (1.1%) and 18O-CO2 (0.4%). The isotopic abundance varies with CO2 sources and sinks, shifting isotopic ratios locally and temporally. For example, diurnal cycle of carbon uptake and release by vegetation, forest fires, and annual growth cycle shift isotopic ratios of the atmospheric carbon dioxide.
The composition of isotopes is dictated by the source material isotopic composition and the physical process of the molecule formation; biogenic processes prefer lighter isotopes due to slight energy advantage. Photosynthetic organisms preferentially take up 12C over 13C due to faster diffusion and enzymatic preference. Anthropogenic processes tend to enrich heavier isotopes, especially in high-temperature combustion. Such isotopic fractionation effects, (see Figure 1) provide a reliable and widely used fingerprint for the origin of the gas species. VTT has been involved in various studies of stable isotopes over the years [2, 3] and activities are expected to increase with availability of the newly developed instruments.
Modern CO2 also only contains 1.2 parts per trillion (ppt) of the naturally occurring radioactive isotope of carbon, radiocarbon or 14C, produced from nitrogen by thermal neutrons, either naturally in upper atmosphere or in anthropogenic nuclear reactions, e.g. nuclear power plants or past atmospheric nuclear weapon tests. It is part of the carbon cycle, and found in every living organism at the same levels as in the atmosphere (see Figure 2). Due to its radioactive decay having half-life of 5700 years, radiocarbon in old organic material is scarcer and is practically absent in fossil oil and natural gas, making radiocarbon analysis a straightforward way to determine the age of the sample. Together with naturally occurring isotopic fractionation effects, also isotopic labelling can be employed for tracing substances and medical diagnostics.
Figure 1. Source identification map of CO2.
Isotopic analyses since the 1930s
Isotopic ratio analysis was discovered in 1930s, when naturally occurring variations in isotopic abundances were discovered. Since 1940s isotope ratio mass spectrometry (IRMS) has been the basic tool for distinguishing the isotopic ratios and has gradually improved. However, latest technological breakthroughs in the field of mid-IR optical spectroscopy closed the sensitivity gap between the laboratory-based IRMS analysis and field-deployable optical techniques. Advantages of optical isotope ratio measurements (OIRS) are capability of performing continuous measurements with sensitivities comparable to the state-of-the-art laboratory-based IRMS, but on-site in real-time with datarates exceeding 10Hz. Such performance is essential for reliable remote monitoring and flux analysis.
Commercial OIRS instruments are based on multipass tuneable diode laser absorption spectroscopy (TDLAS), cavity ring-down spectroscopy (CRDS) and integrated cavity output spectroscopy.
TDLAS and CRDS are the techniques employed by VTT for high-speed small sample volume and high sensitivity applications, respectively. TDLAS technique relies on fast scanning of the laser wavelength over absorption peaks, corresponding to different isotopologues of the probed species. The transmitted signal is measured with a photodetector and absorption is directly inferred from the detected signal. Sensitivity of the technique is enhanced via optical multipass beam path through the sample volume, thus increasing the interaction length by a cell length multiple. In CRDS technique interaction length between the light and the sample is further enhanced by introducing the optical cavity, formed by two highly reflective mirrors (see Figure 3). The laser light, once coupled into the cavity, is reflected back and forth by the highly reflective cavity mirrors, reaching on average several kilometres optical pathlengths before leaking out of the cavity. The absorption signal in CRDS technique is inferred from the cavity leak rate, i.e. ring-down times making the method immune to laser intensity variations.
Figure 3. Principle of the cavity ring-down spectroscopy technique. Optical cavity is pumped with a laser. The optical leak rate (ring-down) of empty cavity is decreased by absorbing molecules.
More reliable atmospheric monitoring
Atmospheric research employs isotopes as source identifiers, mostly for greenhouse gases (GHGs). The strength of the source or sink can be evaluated through flux measurements . The source strength information is essential for reliable modelling of atmospheric chemistry, playing major role in estimation of global warming.
EU member states are required to report their annual GHG emissions; however, current observation networks cannot verify reported emission reductions. Isotope specific flux measurements of GHGs cover source identification studies on a local scale and can be used for regulatory purposes for verification of the reported values. Back-trajectory calculations combining frequent remote measurements and monitored weather conditions can cover continental-scale source mapping .
The remotely measured air mass containing emission information can be back-propagated to form probability map of geographical locations indicating the longest residence time at lowest altitudes. Such maps, when combined with long-term monitoring, get more accurate with time.
Atmospheric dilution effects are accounted for via Keeling plot method. Keeling plot calculation assumes dilution of the background isotopic composition with the isotopic fingerprint of the emission source and relies on high number of samples. Therefore, optical isotopic ratio measurements providing data on second or minute timescale are ideal for such purposes. VTT develops portable instruments for on-site measurements of stable isotopes of atmospheric carbon dioxide and methane. Such instruments can be used for reliable atmospheric monitoring and various source identification studies.
Industrial applications for isotopic ratio analysis include stack emission monitoring for fuel source identification, soil nutrition monitoring for agriculture, waste water treatment monitoring, biofraction determination in biogas and numerous other possibilities. VTT is in the frontier of OIRS instrument development, having years of experience in mid-IR spectroscopic technology. The instrument development includes several case studies such as optical radiocarbon detection, analysis of isotopic ratios of atmospheric GHGs, determination of SI-traceable absorption line parameters and lately the development of compact isotopic analysers for medical breath air studies and biofraction determination in energy gases.
New research horizons
Novel spectroscopic techniques allow for approaching the existing methodology from a new perspective. For example, knowing the sources of biogas and natural gas in the energy gas mixture, stable isotopologues of methane can provide the information on their proportions. Knowledge of the amount of fossil fuel in a mixture is an effective way of carbon emission charge reduction. Currently the biofraction is determined by analysing samples in AMS facilities. Optical spectroscopy offers a real-time and cost efficient alternative to on-line measurements of biogas. Together with European partners VTT investigates the feasibility of on-line biofraction determination based on optical spectroscopy of stable isotopes.
Current spectroscopic techniques have reached sensitivity levels where uncertainties in preparation of standard gases became the limiting factor for reliable atmospheric GHG monitoring. Typically gravimetrically prepared standards rely on assumption of natural isotopic abundance of isotopes in the source materials.
The ratios of the most abundant isotopes can vary significantly with collection location and source of the species, thus affecting the number density of molecules in a given mass. The isotopic composition correction for gravimetric standard gas production can be readily provided by monitoring isotopic compositions of source materials.
However, high level of precision and SI-traceability are the prerequisite for such activities. VTT develops isotopic ratio analysers for CO2 and CH4 to support gravimetric standard gas preparation.
Detection of radiocarbon also plays an important role in many scientific and industrial applications, with its most known application is carbon dating. Several other applications are possible as it is the ideal tracer for discriminating between emissions of fossil origin or biogenic origin. Available methods to detect radiocarbon, such as accelerator mass spectrometry or liquid scintillation counting, are large, expensive and not suitable for in-situ measurement. In addition they usually require long and complex sample preparation, making real-time monitoring impossible.
Using some of the most advanced spectroscopic techniques VTT develops an instrument to detect radiocarbon with on-line on-site measurement capabilities. The advent of a portable instrument for the detection of radiocarbon might revolutionize many applications by providing users an alternative to conventional techniques. One might also envisioned new application in this field as this instrument can provide real-time measurement capabilities, which is not possible with current techniques. High sensitivity is achieved by using CRDS, and thanks to the use of a quantum cascade laser as light source and a simple design, the instrument is compact, ideal for field measurement.
Increasingly accurate radiocarbon content detection
The low natural abundance of radiocarbon makes its detection extremely challenging and part of our research has therefore focussed on applications related to nuclear facilities, where elevated levels of radiocarbon can be found, and potential applications are less demanding. Radiocarbon is present in all parts of nuclear power plants and most of it has potential for gas-phase release, mostly in the form of carbon dioxide .
For instance, gas emissions containing elevated levels of radiocarbon are produced in waste repositories, due to biodegradation of radioactive waste , (see Figure 4).
Sensitive measurement of the radiocarbon content could detect the smallest leaks of contaminated gases. In the future, the amount of waste to be treated and monitored will increase, and so will potential emissions. Operational gaseous emissions also take place inside nuclear facilities, and nuclear power plants are required to monitor their radiocarbon emissions through their stacks. For the moment this is done by collecting samples over large period of time. The instrument developed at VTT could provide on-line automatic monitoring of those emissions.
Recent results have shown that our instrument using advanced spectroscopic techniques is already able to detect low levels of radiocarbon, suitable for monitoring of emissions in nuclear facilities . In the future, further development of the instrument towards field capabilities should ensure a great impact by providing a new way of measuring the main source of radioactive emissions in nuclear facilities. With the shut down and decommissioning of the VTT research reactor and the opening of the VTT Centre for Nuclear Safety, this activity will also provide interesting link with other research areas of VTT.
Once the instrument can detect radiocarbon at level below 1ppt, i.e. below its natural abundance, several new exciting applications can be foreseen. The instrument is therefore currently upgraded towards greater sensitivity, using some of the most advanced spectroscopic techniques. Monitoring of anthropogenic emissions of fossil origin can be done by measuring the amount of the radiocarbon isotope in carbon dioxide, as fossil fuel does not contain any radiocarbon due to its old age.
Thanks to the on-site on-line measurement capabilities of the instrument developed at VTT, it will be possible to observe temporal and local variations of the radiocarbon content of the atmosphere, and help better understand climate change. It will provide information that is normally not available to climate scientists with traditional emissions monitoring techniques.
Radiocarbon analysis provides a straightforward and reliable way to determine biofraction of mixed fuels, which is crucial for the European Union Emission Trading System. Large producers of CO2 are obliged to monitor and report their emissions. However, the ratio of fossil and modern carbon in incinerated waste or mixed fuel is highly uncertain, thus affecting the reliability of the trading system [9, 10]. Monitoring is usually done by accounting, which is highly uncertain.
A precise determination of the 14C/12C isotope ratio with this instrument will provide information about the bio/fossil ratio on-site and in real-time, thus fundamentally changing this issue.
Portable tools for biomedicine
Finally, we also anticipate applications both for radiocarbon and for stable isotope analysis in the field of biomedical research. Microdosing studies using radiocarbon are a good way to reduce the amount of drug intake by the patient, allowing observing the effect of drugs with almost no risk of side effects .
However, current method uses accelerator mass spectrometry to analyse their samples, thus requiring transferring the samples to different facilities. The advent of a portable instrument could revolutionise this field by providing much more users with a way to analyse their samples. Stable isotope breath tests provide painless and non-invasive alternative for diagnosis and monitoring of numerous diseases and conditions. For instance 13C/12C ratio in exhaled CO2 can be used to diagnose and monitor bacterial infection, gastric emptying, liver function, bacterial overgrowth, absorption and numerous other conditions .
Typical way is to digest specific 13C-labelled compounds, which are converted and transferred into breath air. The air is sampled into bags before and after the digestion, and the samples are analysed in the lab, for example using IRMS. Delay time between the digestion of the labelled product and transfer into exhaled CO2 depends on the condition being monitored and is typically in the range of 10–100 minutes.
VTT develops a compact instrument capable of continuous measurements of stable isotopes in exhaled air. The instrument will be capable of measuring isotopic ratios directly from the breath air, removing the risk of inaccurate or contaminated bag sampling. The technique could be used for monitoring different isotopes of exhaled compounds, thus opening research pathways for development of new isotopic tracers.
Figure 4. Production of elevated levels of radiocarbon.
Isotopes provide invaluable information on origin, pathway and removal of gaseous species. Detection of both the stable isotopes and the radiocarbon is widely applied in various fields. Latest innovations in mid-IR technology offer spectacular improvements to spectroscopic detection of the most relevant light molecules, and allows for real-time in-situ monitoring of isotopes.
VTT cutting edge research in the field aims towards commercialisation of the technology to provide tools for industry, medical diagnostics and environmental monitoring. As an example, mobile optical radiocarbon analyser will be taken to on-site monitoring of 14CO2 at nuclear facilities. Also compact breath air analyser is planned to be applied to isotope-specific medical studies.
Dr Albert Manninen (PhD) became an employee of VTT when the Centre for Metrology and Accreditation and VTT were merged. He is a Senior Scientist in VTT’s environmental metrology team, and studies optical spectroscopy and hyperspectral remote sensing. Manninen is head of VTT’s isotope metrology growth area.
Dr Guillaume Genoud (PhD) is a Research Scientist in VTT’s environmental metrology team. He began working at the Centre for Metrology and Accreditation in 2011, after graduating from Swiss Federal Institute of Technology in Zurich and completing his doctoral dissertation at Lund University. He became an employee of VTT when the Centre for Metrology and Accreditation and VTT were merged. Genoud is responsible for the research on radiocarbon detection using laser spectroscopy.
Dr Mikko Merimaa (PhD)is a Research Manager and a Principal Scientist at VTT’s national reference laboratory, MIKES Metrology. Merimaa received his doctorate from the Helsinki University of Technology at the turn of the millennium, and his dissertation discussed diode laser-based reference frequencies. He is responsible for maintaining the official time in Finland. His most recent research projects have involved high-precision spectroscopy, optical clocks, and time links.
 Galli I, Bartalini S, Borri S, Cancio P, Mazzotti D, De Natale P, and Giusfredi G. “Molecular gas sensing below parts per trillion: Radiocarbon-dioxide optical detection,” Physical Review Letters. 107, 270802 (2011).
 Niemelä PS, Castillo S, Sysi-Aho M, and Orešic M. “Bioinformatics and computational methods for lipidomics,” Journal of Chromatography B 877, 2855 (2009).
 Kietäväinen R, Ahonen L, Kukkonen IT, Hendriksson N, Nyyssönen M, and Itävaara M. “Characterisation and isotopic evolution of saline waters of the Outokumpu Deep Drill Hole, Finland – Implications for water origin and deep terrestrial biosphere,” Applied Geochemistry 32, 37 (2013).
 Tuzson B, Hiller RV, Zeyer K, Eugster W, Neftel A, Ammann C, and Emmenegger L. “Field intercomparison of two optical analysers for CH4 eddy covariance flux measurements,” Atmospheric Measurement Techniques 3, 1519 (2010).
 Sturm P, Tuzson B, Henne S, and Emmenegger L. “Tracking isotopic signatures of CO2 at the high altitude site Jungfraujoch with laser spectroscopy: analytical improvements and representative results,” Atmospheric Measurement Techniques 6, 1659–1671 (2013).
 Yim M-S and Caron F. “Life cycle of carbon-14 from nuclear power generation,” “Life cycle and management of carbon-14 from nuclear power generation,” Progress in Nuclear Energy, 48, 2 (2006).
 Heikola T. “Leaching of 14C in repository conditions. Transport and speciation, Espoo, 35 pages,” VTT Technology 157 (2014).
 Genoud G, Vainio M, Phillips H, Dean J, and Merimaa M. “Radiocarbon dioxide detection based on cavity ring-down spectroscopy and a quantum cascade laser,” Optics Letters. Doc ID 230853 (25 February 2015, at the printers).
 Hämäläinen K, Jungner H, Antson O, Räsänen J, Tormonen K, and Roine J. “Measurement of biocarbon in flue gases using 14C,” Radiocarbon 49, 325 (2007).
 Mohn J, Szidat S, Fellner J, Rechberger H, Quartier R, Buchmann B, and Emmenegger L. “Determination of biogenic and fossil CO2 emitted by waste incineration based on 14CO2 and mass balances,” Bioresource Technology 99, 6471 (2008).
 Lappin G and Garner R. “Current perspectives of 14C-isotope measurement in biomedical accelerator mass spectrometry,” Analytical and Bioanalytical Chemistry 378, 356 (2004).
 Braden B, Lembcke B, Kuker W, and Caspary WF. “13C breath tests: current state of the art and future directions,” Digestive and Liver Disease 39, 795 (2007).