Isotope ratio measurements

§) defined as the relative deviation from a standard reference material. E.g., a 30 ‰ variation in ¹³C corresponds to an abundance variation between 1.085 and 1.115 %.

Quantitative information on the natural variation of isotope abundance ratios is extremely useful, in fact often indispensable, information for a wide variety of research fields. For example, the ¹³C/¹²C ratio in CH₄ puts important constraints on the relative contributions to the global methane budget of the various CH₄ sources and their isotopic behavior [1–3]. Similarly, the data recorded on a global scale of the ¹³C/¹²C ratio in atmospheric CO₂ represent crucial independent information, which help to elucidate the role the oceans and the terrestrial biosphere have in the global carbon cycle, by virtue of the different rates of uptake of the two isotopic species [4–7]. The same ¹³C/¹²C isotope ratio in the CO₂ fraction of exhaled air for breath analysis is a valuable medical diagnostic tool, e.g., for the detection of Heliobacter pylori infections by means of a ¹³C labeled urea test, or the quantification of the amino acid metabolism by means of a ¹³C labeled leucine test [8].

But arguably it is the study of the isotopic composition of water that finds application in the widest variety of disciplines, from paleo-climatology, to hydrology, atmospheric chemistry, and bio-medicine.

IRMS

Isotope ratio mass spectrometry (IRMS) is the conventional method for measuring isotope ratios and has benefited from over 40 years of research and development. Nowadays IRMS instrumentation is commercially available that reaches a high throughput (several minutes analysis time per sample) and impressively high levels of precision, typically better than 0.1‰ for ¹⁸O/¹⁶O and ¹³C/¹²C, and 0.5‰ for ²H/¹H (see, e.g., [9-11]), where the precisions are expressed in the so-called delta-notation. Unfortunately, IRMS is incompatible with a condensable gas or a sticky molecule such as water. Therefore, in general, chemical preparation of the sample is required to transfer the water isotope ratio of interest to a molecule that is more easily analyzed. Several techniques, most still commonly used, exist, each with its specific properties, advantages, and shortcomings (see, e.g., ref. 12). It may be clear that for the larger part the required chemical conversion steps are time-consuming, and often compromise the overall accuracy and throughput [9-14]. Apart from these drawbacks specific to water, IRMS instrumentation is expensive, voluminous and heavy, confined to a laboratory setting, and usually requires a skilled operator. All or most of these issues may be addressed by optical measurement techniques.

SIRIS

Over the past 15 years we have developed an alternative method to IRMS, based on the, at least conceptually, very simple principle of direct absorption, infrared spectroscopy. Because of the quantitative nature, we call this Stable Isotope Ratio Infrared Spectrometry, or SIRIS.

The near to mid infrared is often referred to as the "finger print" region of the optical spectrum: most small molecules exhibit highly characteristic rotational-vibrational bands in this region, associated with rotational and vibrational bending and stretching motions of the nuclei. The changes in the rotational motions of the molecule that accompany the absorption or emission of an infrared photon give rise to the fine structure observed at sufficiently low pressure and high instrumental resolution (resolving power) (see Figure 1).

The resulting spectra are highly sensitive to isotopic substitution of the molecule. This can be exploited to analyse the isotopic composition of a gas phase sample, by measuring the intensity decrease of a laser beam that has traversed several meters (and in recent, more sensitive instruments more than 40 km ...) through a the gas (or vapour, in the case of water) as a function of the laser wavelength (‘color’). At wavelengths for which the laser radiation is in resonance with a molecular rotation-vibration transition, an absorption feature will be registered that can be uniquely assigned to one of the molecules’ isotopologues. The spectrum of Figure 1 shows absorption ‘lines’ of H¹⁶OH, H¹⁷OH, H¹⁸OH en H¹⁶OD. The Beer-Lambert law relates the laser intensity loss due to the molecular absorption to the molecular number density. In this way, by recording two spectra, one belonging to the sample, one to a known reference material, the isotope ratios of the sample may be determined.

Our early work has been carried out using an isotope ratio spectrometer with a color center laser (FCL, form the German Farbe: color) as light source. This laser is very broadly tunable in the 3-micron region, where most small molecules exhibit strongly absorbing fundamental vibrational bands. It is a great research tool, but difficult to handle and not adapted to field operation or a small instrument footprint. At present, we use mostly III-V type, DFB diode lasers in the near-infrared region of the spectrum. The lower absorption strength of the overtone vibrational bands that are excited in this case is easily compensated for by a more sensitive detection technique and better infrared detector performance. The recent spectrometers we developed all use a derivative of cavity ringdown spectroscopy (CRDS), which is in essence a clever way to increase the effective optical interaction length (up to tens of kilometres) and thus the sensitivity. For some molecular systems, such as CO₂ or N₂O, we use quantum cascade lasers (QCL), in order to exploit the much larger absorption strength in the mid infrared of these species, enabling measurements to be made with very high precision and/or on very small sample sizes.

The precision of our laser measurements of the ²H/¹H isotope ratio is currently about 0.3‰ and about a factor of 10 better still for the oxygen isotopes in water, depending on the humidity level [15]. This means we can tell the difference between a ²H/¹H isotope ratio of 0.000 155 75 and 0.000 155 80. This is comparable to the case of IRMS. A major advantage of the SIRIS method is the direct measurement on water vapor without any (chemical) pretreatment of the sample. Moreover, all three isotope ratios of interest , ²H/¹H, ¹⁷O/¹⁶O and ¹⁸O/¹⁶O, are measured simultaneously on one and the same sample. The measurement of ¹⁷O/¹⁶O is impractical with IRMS (due to the mass-overlap with the more abundant ¹³C in CO₂). Also, the laser-based measurement is non-destructive: in principle the sample can be recovered. Another major advantage is the possibility to build compact, light-weight, and energy efficient instrumentation (especially when using near-infrared diode lasers), for in-situ and real-time monitoring applications (as in airborne atmospheric monitoring). The latter advantages hold also for other molecules than water.

References

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[2] Schupp, M.; Bergamaschi, P.; Harris, G.W.; Crutzen, P.J. Chemosphere 1993, 26, 13-22.

[3] Bergamaschi, P.; Schupp, M.; Harris, G.W. Applied Optics 1994, 33, 7704-7716.

[4] Mook, W.G.; Koopmans, M.; Carter, A.F.; Keeling, C.D. J. Geophys. Res. 1983, 88, 10915-10933;

[5] Keeling, C.D.; Whorf, T.P.; Wahlen, M.; Plicht, J. van der Nature 1995, 375, 666-670.

[6] Ciais, P.; Tans, P.P.; White, J.W.C.; Trolier, M.; Francey, R.J.; Berry, J.A.; Randall, D.R.; Sellers, P.J.; Collatz, J.G.; Schimel, D.S. J. Geophys. Res. 1995, 100, 5051-5070.

[7] Francey, R.J.; Tans, P.P.; Allison, C.E.; Enting, I.G.; White, J.W.C.; Trolier, M. Nature, 1995, 373, 326-330.

[8] Koletzko, S.; Haisch, M.; Seeboth, I.; Braden, B.; Hengels, K.; Koletzko, B.; Hering, P. The Lancet 1995, 345, 961-962; and references therein.

[9] E.R.Th. Kerstel, Stable isotope ratio infrared spectrometry. Handbook of Stable Isotope Analytical Techniques. Chapter 34. pp. 759-787, P.A. de Groot (Ed.) Elsevier (2004).

[10] C. Horita and J. Kendall, Stable isotope analysis of water and aqueous solutions by conventional dual-inlet mass spectrometry, Chapter 1, P.A. de Groot (Ed.) Elsevier (2004).

[11] W. Brand, Mass spectrometry hardware for analysing stable isotope ratios, Chapter 38, P.A. de Groot (Ed.) Elsevier (2004).

[12] E.R.Th. Kerstel and H.A.J. Meijer, Optical Isotope Ratio Measurements in Hydrology (Chapter 9), in: Isotopes in the Water Cycle: past, present and future of a developing science. pp. 109-124, P.K. Aggarwal, J. Gat, and K. Froehlich (Eds.), IAEA Hydrology Section, Kluwer, 2005.

[13] W.A. Brand, H. Avak, R. Seedorf, D. Hofmann, Th. Conradi, Isotopes Environ. Health Stud. 32, 263-273 (1996).

[14] H.A.J. Meijer, Isotope ratio analysis on water: a critical look at developments. New Approaches for Stable Isotope Ratio Measurements (Proc. Advisory Group Meeting, 1999) IAEA-TECDOC-1247, IAEA, Vienna, 105-112 (2001).

[15] J. Landsberg, D. Romanini, and E. Kerstel, Very high finesse optical-feedback cavity-enhanced absorption spectrometer for low concentration water vapor isotope analyses, Opt. Lett. 39(7), 1795-1798 (2014).