Inside the laser optical box: two infra-red lasers (blue), a pack of mirrors and an astigmatic cell near the bottom.
Page 2 – working toward corrections
Page 3 – describing the laser
We acquired the dual Quantum Cascade laser isotope machine from Aerodyne a while back. It offers the promise of high precision isotopic analysis of carbon dioxide and water vapour and is therefore the perfect companion to plant gas exchange experiments. MMHP and HSW starting working on it as part of a usable system in a smaller room in the Robertson Building. We have subsequently been moved out of that building while it was renovated and that worsened some of our issues such as temperature control.
The laser system is a successor to a GC-based mass spectrometer system developed by HSW, Nerea Ubierna and MMHP. This earlier system used a computer controlled set up where air with carbon dioxide at ambient concentrations was passed through a tube for a minute or so, then the flow was switched to use helium to force the air through a molecular sieve-filled GC column that separated the carbon dioxide from the air and fed it into a Micromass Isochrom driven by a Sercon source-control. This produced very high precision d13C and d18O analyses, but only at 5 minute or so intervals.
The laser itself comprises two main boxes and a solid-state chiller. The lasers and optics sit in the top box (hidden by light green polystyrene foam below) and can be readily accessed by lifting off the insulating box and then removing the top of the temperature-controlled cabinet. This box has coolant circulating from a solid-state chiller under the bench and through the outside of the box. The electronics have to be slid out to be
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- Initial bent-piping heat exchanger, greatly improved later.
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- The whole assemblage boxed up.
accessed and were originally cooled only by a fan circulating room air. We found that the electronics were actually extremely temperature sensitive and that our signal was visibly responding to cycling in the air conditioning system. We largely solved this problem by wrapping the box in polystyrene foam and circulating air inside it with a fan while dumping heat through a liquid cooled heat exchanger to another chiller. Improved insulation, and some new versions of the electrical boards have helped greatly but temperature change is still an issue.
A second issue has been how to treat the data and make corrections that produce precise and accurate isotopic values. This is a general problem for users of laser systems and partly springs from many labs moving to lasers from mass spectrometers and try to correct their data the same way. As the results can be no better than the data corrections, MMHP, HSW and GDF (with assistance from Aerodyne) have put considerable effort into solving this issue and have a working system of corrections.
Software development has been a major part of the system integration. The Aerodyne software did not offer the level of immediate interaction and data capture that we wanted, so a new program was written to interact with the QCL, display the data in real time, control the gas mixing box (Gus) and switch valves etc. The resulting software is called Elsie (LC, Laser Controller).
The end result of all this effort is an integrated system for performing leaf gas exchange experiments.