Erik Kerstel

Isotope Ratio Infrared Spectrometry

Publisher Summary The application of optical isotope ratio techniques based on laser spectrometry depends on the easy availability of relatively powerful, compact narrow band laser sources with good mode structure and tunability. This chapter summarizes the optical techniques to accurately measure isotope abundance ratios as alternatives to isotope ratio mass spectrometry (IRMS). However, number of fundamental problems and limitations with IRMS leads to new measurement techniques. The optical techniques involving excitation in the infrared region of the spectrum, associated with molecular rotational-vibrational motions are discussed. Different approaches of the optical detection technique employed, is presented with an example of experimental work because of their successful application in the field of isotope ratio measurements. Some of the techniques are infrared spectrometry, absorption spectroscopy, and indirect spectroscopic techniques. Techniques based on absorption spectroscopy have the additional advantage of being conceptually simple. This translates in the ability to carry out isotope ratio measurements that require a much smaller normalization and scale correction than is often necessary with IRMS measurements. However, using optical techniques, isotope ratio instrumentation can be build that is potentially cheaper, compacter, and easier to operate than IRMS instrumentation.

Water isotope ratio (δ2H and δ18O) measurements in atmospheric moisture using an optical feedback cavity enhanced absorption laser spectrometer

Water vapor isotopes represent an innovative and excellent tool for understanding complex mechanisms in the atmospheric water cycle over different time scales, and they can be used for a variety of applications in the fields of paleoclimatology, hydrology, oceanography, and ecology. We use an ultrasensitive near-infrared spectrometer, originally designed for use on airborne platforms in the upper troposphere and lower stratosphere, to measure the water deuterium and oxygen-18 isotope ratios in situ, in ground-level tropospheric moisture, with a high temporal resolution (from 300 s down to less than 1 s). We present some examples of continuous monitoring of near-surface atmospheric moisture, demonstrating that our infrared laser spectrometer could be used successfully to record high-concentration atmospheric water vapor mixing ratios in continuous time series, with a data coverage of similar to 90%, interrupted only for daily calibration to two isotope ratio mass spectrometry-calibrated local water standards. The atmospheric data show that the water vapor isotopic composition exhibits a high variability that can be related to weather conditions, especially to changes in relative humidity. Besides, the results suggest that observed spatial and temporal variations of the stable isotope content of atmospheric water vapor are strongly related to water vapor transport in the atmosphere.

Development and airborne operation of a compact water isotope ratio infrared spectrometer

A sensitive laser spectrometer, named IRIS (water isotope ratio infrared spectrometer), was developed for the in situ detection of the isotopic composition of water vapour in the upper troposphere and the lower stratosphere. Isotope ratio measurements can be used to quantify troposphere–stratosphere exchange, and to study the water chemistry in the stratosphere. IRIS is based on the technique of optical feedback cavity-enhanced absorption spectroscopy. It uses a room temperature near-infrared laser, and does not require cryogenic cooling of laser or detectors. The instrument weighs 51 kg including its support structure. Airborne operation was demonstrated during three flights aboard the European M55-Geophysica stratospheric research aircraft, as part of the AMMA/SCOUT-03 (African Monsoon Multidisciplinary Analysis/Stratospheric Climate links with emphasis on the Upper Troposphere and lower stratosphere) campaign in Burkina Faso in August 2006. One-second averaged, vertical profiles of δ2H, δ17O and δ18O in the upper troposphere are shown, as are the δ17O–δ18O and δ2H–δ18O relations. The data are discussed with reference to a Rayleigh distillation model. As expected, there is no indication of non-mass-dependent fractionation (also known as mass-independent fractionation) in the troposphere. Furthermore, improvements to the thermal management system and a move to a (cryogen-free) longer-wavelength laser source are discussed, which together should result in approximately two orders of magnitude improvement of the sensitivity.

Kalman filtering real-time measurements of H2O isotopologue ratios by laser absorption spectroscopy at 2.73 mu m

Kalman adaptive filtering was applied for the first time, to our knowledge, to the real-time simultaneous determination of water isotopic ratios using laser absorption spectroscopy at 2.73 microm. Measurements of the oxygen and hydrogen isotopologue ratios delta(18)O, delta(17)O, and delta(2)H in water showed a 1-sigma precision of 0.72 per thousand for delta(18)O, 0.48 per thousand for delta(17)O, and 0.84 per thousand for delta(2)H, while sampling the output of the tuned Kalman filter at 1 s time intervals. Using a standard running average technique, averaging over approximately 30 s is required to obtain the same level of precision.

A Microdrop Generator for the Calibration of a Water Vapor Isotope Ratio Spectrometer

Abstract A microdrop generator is described that produces water vapor with a known isotopic composition and volume mixing ratio for the calibration of a near-infrared diode laser water isotope ratio spectrometer. The spectrometer is designed to measure in situ the water vapor deuterium and oxygen (17O and 18O) isotope ratios from the low troposphere up to the lower stratosphere with a high temporal resolution, and is based on ultrasensitive optical-feedback cavity-enhanced absorption spectroscopy. To calibrate the spectrometer, a commercial microdrop generator (Microdrop GmbH) is used to inject water droplets of known size at a preset repetition frequency into a stream of dry nitrogen or synthetic air. Complete evaporation of the small droplets ensures that there is no isotopic fractionation between the liquid phase and the generated moist “air.” The water mixing ratio of the synthetic air is controlled by the repetition rate and gas flow. The current system spans a water mixing ratio range from about 10 ...

Assessment of the amount of body water in the Red Knot (Calidris canutus): An evaluation of the principle of isotope dilution with 2H, 17O, and 18O as easured with laser spectrometry and isotope ratio mass spectrometry

We have used the isotope dilution technique to study changes in the body composition of a migratory shorebird species (Red Knot, Calidris canutus) through an assessment of the amount of body water in it. Birds were quantitatively injected with a dose of water with elevated concentrations of 2H, 17O, and 18O. Thereafter, blood samples were taken and distilled. The resulting water samples were analysed using an isotope ratio mass spectrometry (for 2H and 18O only) and a stable isotope ratio infrared laser spectrometry (2H, 17O, and 18O) to yield estimates of the amount of body water in the birds, which in turn could be correlated to the amount of body fat. Here, we validate laser spectrometry against mass spectrometry and show that all three isotopes may be used for body water determinations. This opens the way to the extension of the doubly labelled water method, used for the determination of energy expenditure, to a triply labelled water method, incorporating an evaporative water loss correction on a subject-by-subject basis or, alternatively, the reduction of the analytical errors by statistically combining the 17O and 18O measurements.

Obituary for Dr Peter Werle

It is with great sadness and shock that we learned of having lost on Friday 6 September 2013 a great friend, father, husband, scientist, mentor, symposium organiser, and a revolutionary intellectual thinker, who had a remarkable ability of addressing and describing fundamental problems in applied laser physics in new and enlightening ways. Although it is impossible to adequately capture the full extent of Peter Werle’s impacts in such a short space, this tribute to Peter attempts to highlight his vast scientific contributions in numerous areas and their relevance for optical isotope ratio measurements. Peter Werle was born in Kaiserlautern in 1958. He studied physics at Johannes Gutenberg University in Mainz and received the Dr rer. nat. degree in physical chemistry from Ludwig Maximilians University in München, with summa cum laude, for his thesis work on quantum-limited laser spectroscopy. True to his self-reliant and resourceful nature, he paid his way through his studies by working on a free-lance basis for IBM; certainly at the time, this was not very common in Germany. He headed different research groups at the Institute for Meteorology and Climate Research (IMK-IFU) in GarmischPartenkirchen, focusing on laser spectroscopy and integrated systems for environmental research. Peter brought this diverse background, combining outstanding experimental and mathematical skills, into atmospheric studies in the 1980s employing liquid-nitrogen-cooled lead salt diode lasers. At the time, such devices were fraught with many problems, and Peter methodically and systematically began to address each issue. He was particularly adept at introducing advanced algorithms and signal processing strategies and new data quality assurance concepts to diode laser measurements. In 1993, Peter and colleagues published a seminal paper in Applied Physics B in which they introduced the concept of Allan variance applied to laser diode measurements of gas concentration time series. The principle of using Von

Shining infrared light on isotope ratio measurements in applications from earthbound to the atmospheric

We present accurate water isotope ratio measurements using DFB diode lasers in laboratory based studies in biomedicine, paleoclimatology (ice-cores), and ecology (plant leave water), to airborne, in-situ, stratospheric measurements using optical-feedback cavity enhanced absorption spectroscopy.