Abstract:
Gas having stable isotopes is monitored continuously by using a system that sends a modulated laser beam to the gas and collects and transmits the light not absorbed by the gas to a detector. Gas from geological storage, or from the atmosphere can be monitored continuously without collecting samples and transporting them to a lab.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/126,299 entitled “Detection of Gaseous Stable Isotopes,” filed May 2, 2008, hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERAL RIGHTS 
     This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus and method employing frequency-modulated spectroscopy for monitoring the stable isotopes in various gases. 
     BACKGROUND OF THE INVENTION 
     Some gaseous molecules (CO 2 , CH 4 , O 3 , N 2 O, chlorofluorocarbons) are known to cause climate change and increase pollution (NO x , O 3 , SO 2 , H 2 S) in the atmosphere. Measuring the stable isotopes of these gases can provide information about their origin, can differentiate these gases in various settings (e.g. CH 4  produced from a wetland or a pipeline leak), and can help track them through production/processing scenarios or through biological and/or chemical and/or physical pathways. 
     Determining ratios of stable isotopes of CO 2  ( 12 CO 2 ,  13 CO 2 ) and other greenhouse gases (methane (CH 4 ), nitrous oxide (N 2 O), for example) is used for determining the source(s) of these gases. A challenge with measuring stable isotope ratios is measuring the minor isotopic species because the minor species may be present in very small amounts. The natural abundance of  13 CO 2  in atmospheric CO 2 , for example, is only approximately 1.1%; the rest is  12 CO 2 .  FIG. 1  shows ranges in isotopic variation of CO 2  from sources including natural gas, plants and microbes, air, magmatic sources, petroleum, and groundwater. 
     Current measurements of δ 13 C (i.e. the carbon isotope ratio of  13 C/ 12 C within the CO 2  of an unknown sample relative to a standard which is a limestone from the PeeDee formation (PDB)) involves collecting samples at the site and delivering them to the laboratory where they are analyzed using by optical spectroscopy using a commercially available absorption instrument, or by mass spectrometry with a highly precise sector mass spectrometer. The optical approach suffers from the limitations of any approach associated with collecting samples, including the lack of in situ detection. The optical approach also requires high electrical power and a continuous source of cryogens, and is thus severely limited by its continuous user interface and maintenance requirements. 
     Detecting CO 2  seepage from a geologic storage system is difficult to do using CO 2  concentration alone. Typically, other trace gases like CH 4 , inert tracers like perfluorocarbons, or natural tracers like the isotopes of CO 2  ( 13 C/ 12 C and  14 C/ 12 C ratios) are used to identify CO 2  leaks at the ground surface. Current efforts focusing on stable isotope identification of CO 2  seepage has focused on point measurements using traps that limit both the temporal and spatial resolution of the leak. Efforts have been made in creating systems that can be used in the field to conduct real time isotope measurements at a higher temporal frequency. Systems that can resolve isotopes over 5 to 15 minute windows of time are usually bulky, expensive, and not portable. 
     Stable isotopes (e.g.  13 C) are important for monitoring carbon sequestration potential in geologic CO 2  storage systems. Point source measurements are all that can be made at this time, but remote-sensing tools that will provide spatial analysis of CO 2  sources is what is needed. Furthermore, there is a need for a technology that can also provide information about where the CO 2  came from. 
     An object of the invention is a portable system and method for continuously monitoring stable isotopes of gas. 
     Another object of the invention is a portable system and method for continuously monitoring CO 2 . 
     Another object of the invention is a system and method for making in-situ measurements of stable isotopes of various gases. 
     Another object of the invention is a system and method for making remote measurements of stable isotopes. 
     Another object of the invention is a system and method for making real time measurement of stable isotopes of atmospheric gases. 
     SUMMARY OF THE INVENTION 
     In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a system for monitoring stable isotopes of gas. The system includes a cell for receiving a sample of gas, a tunable diode laser for providing a laser beam having an original carrier frequency (ω c ), and a modulation source operating in the radiofrequency regime for modulating the original carrier frequency (ω c ) of the laser beam from the tunable diode laser, thereby producing a modulated laser beam having the original carrier frequency (ω c ) and sidebands on either side of and evenly spaced apart from the carrier frequency by a modulation frequency (ω m ). The sample of gas inside the multipass cell absorbs energy from the modulated laser beam. The system also includes a detector that provides a detection signal for a stable isotope from the gas sample inside the cell. The detector includes a reference channel and a signal channel. The system also includes an amplifier for amplifying the detection signal from the detector. An optical fiber transmits light from the cell to the signal channel of the detector. A focusing means such as a lens focuses the light from multipass cell into the optical fiber. A beamsplitter directs the modulated laser beam alternately between the cell and the reference channel of the detector. 
     The invention also includes a method for continuous in-situ monitoring of gas having stable isotopes. The method involves directing a frequency modulated laser beam alternately between a reference channel of a detector and a cell containing a first sample of a gas having stable isotopes. The gas interacts with the modulated laser beam and produces a light emission resulting from the interaction. The light emission is collected and transmitted through an optical fiber to a signal channel of the detector. Afterward, the first sample is replaced with a second sample of gas and the steps are repeated and the results compared. This allows continuous gas monitoring without having to collect samples and send them out to a laboratory for analysis. 
     The invention also includes a spectroscopic system for monitoring gas having stable isotopes. The system includes a tunable diode laser for providing a laser beam having an original carrier frequency (ω c ). The system also includes a modulation source, either direct laser modulation or through an electro optical phase modulator, operating in the radiofrequency regime for modulating the original carrier frequency (ω c ) of the laser beam from the tunable diode laser. The laser produces a modulated laser beam having the original carrier frequency (ω c ) and sidebands on either side of and evenly spaced apart from the carrier frequency by a modulation frequency (ω m ). Gas from the sample absorbs energy from the modulated laser beam. The system also includes a detector that provides a detection signal that can be used to differentiate stable isotopes from a gas sample. The detector includes a reference channel and a signal channel. The system also includes an amplifier for amplifying the detection signal from said detector. An optical fiber transmits light from the gas sample to the signal channel of said detector. A focusing means such as a collection lens collects light from the gaseous sample and focuses it into the optical fiber. A beamsplitter directs the modulated laser beam alternately between the reference channel of the detector and the gaseous sample. 
     The invention also includes a method for continuously monitoring CO 2 . The method involves (a) directing a frequency modulated laser beam from a tunable diode laser simultaneously between a reference channel of a detector and gas remote from the detector, the gas comprising CO 2  that interacts with the modulated laser beam, whereby the gas absorbs energy from the modulated laser beam, (b) collecting the not absorbed by the gas, (c) transmitting the collected light through an optical fiber to a signal channel of the detector, (d) analyzing the transmission of collected light to provide a first  13 C/ 12 C ratio, (d) repeating steps (a), (b), (c), and (d) at a later time, thereby providing a second  13 C/ 12 C ratio for the gas, and (e) comparing first  13 C/ 12 C ratio with the second  13 C/ 12 C ratio, thereby monitoring the CO 2  as time passes. This method can be used to analyze gas from a geologic storage system that is remote from the detector. The method can also be used to monitor gas in the atmosphere. For atmospheric gas, the light collection step may involve reflecting light from an airborne platform in the atmosphere to a collector remote from the atmosphere. A mirror on an airplane might be used for the light reflection; thus the monitoring while the gas is in the atmosphere. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a graphical depiction of ranges in isotopic variation of CO 2  from sources including natural gas, plants and microbes, air, magmatic sources, petroleum, and groundwater. 
         FIG. 2   a - b  shows plots that illustrate the effect of modulating the original carrier frequency of a tunable diode laser with an electro-optical phase modulator operating in the radiofrequency regime. The plot shown in  FIG. 2   a  is a generic plot showing the original carrier frequency (ω c ) of the beam from a tunable diode laser (“TDL”), along with sidebands (ω c ±ω m ) evenly spaced about the original carrier frequency (ω c ) by the modulation frequency (ω m ).  FIG. 2   b  shows an actual spectrum with sidebands that result from frequency modulation. 
         FIGS. 3   a  and  3   b  are schematic block diagrams of two embodiments of the apparatus of the present invention 
         FIG. 4   a - d  shows a variety of spectra obtained using an embodiment system of this invention.  FIG. 4   a  shows a  12 C 16 O 2  FM spectra collected with the remote instrument with a 20 m standoff.  FIG. 4   b  shows  13 C 16 O 2  FM spectra collected with the remote instrument with a 20 m standoff.  FIG. 4   c  shows  12 C 16 O 2  FM spectra collected with the in situ instrument.  FIG. 4   d  shows  13 C 16 O 2  FM spectra collected with the in situ instrument. 
     
    
    
     DETAILED DESCRIPTION 
     The invention includes a system that optically monitors gaseous stable isotopes. The system employs frequency-modulated spectroscopy (“FMS”), which is a type of spectroscopy with a sensitivity that is orders of magnitude more sensitive than standard absorption spectroscopy. 
     An embodiment system of the invention includes a tunable diode laser (“TDL”) and an electro-optical phase modulator operating in the radiofrequency regime that modulates the laser beam from the tunable diode laser. The modulated laser beam has a frequency equal to that of the original carrier frequency (ω c ) of the beam from the TDL, along with sidebands (ω c ±ω m ) evenly spaced about the original carrier frequency (ω c ) by the modulation frequency (ω m ), as depicted in  FIG. 2 . This modulation of the original carrier frequency is analogous to a modulation of a carrier frequency from a radio station. The carrier frequency is the radio station that one tunes to, and the audio is the modulated frequency. A species of interest is detected by tuning the TDL and the modulation frequency such that one of the sidebands interacts with a specific spectral feature. Turning to  FIG. 2   a , the Lorentzian shaped spectral feature Δδ is probed by the right sideband (ω c +ω m ). One records the derivative shaped dotted line (Δφ) as the carrier and modulated frequency is tuned over the spectral feature. By using a detection frequency in the radiofrequency regime, the laser noise is low, thus improving the signal to noise ratio. The greatly enhanced signal to noise ratio is important for improving detection limits of both major and minor isotopes. 
       FIG. 3   a  and  FIG. 3   b  show schematic block diagrams of two embodiments of the invention apparatus.  FIG. 3   a  shows an embodiment apparatus for making in situ (i.e. closed path) measurements.  FIG. 3   b  shows an embodiment for making remote (i.e. open path) measurements. 
     Turning first to  FIG. 3   a , a schematic block diagram of an embodiment apparatus  10  for making in situ (i.e. closed path) measurements of stable isotopes is shown. Apparatus  10  is driven by a computer  12 . Apparatus  10  includes picoscope  14 , which is a commercially-available oscilloscope used to record the data. Apparatus  10  includes diode laser  16  (NEW FOCUS velocity tunable diode laser TLB-6330H, for example) and function generator  18  (SRS DS 345 function generator, for example) that drives the modulation of a laser beam from laser  16  through a laser driver  20  (NEW FOCUS velocity laser driver TLB-6330-LN, for example). Function generator  18  generates a sine wave for laser driver  20  that generates the modulation frequency. Function generator  18  also triggers picoscope  14 . Beamsplitter  22  (e.g. a beamsplitter cube) splits the modulated laser beam into a reference beam and a signal beam. The reference beam is sent directly to a reference channel  24  of a detector  26  (NEW FOCUS model 2017 Auto-balanced Receiver, for example). The signal beam is deflected by a mirror  28  to a multipass cell  30  (a 10 meter “White Cell,” for example) containing a sample of gas. In an embodiment, a polarizer  32  permits adjustment of the fraction of modulated laser light directed to the gas sample in the multipass cell and reference channel via the beamsplitter. In an embodiment, a pump  34  provides the means for pumping a sample of gas into the multipass cell. The modulated laser beam interacts with gas inside multipass cell  30 , and changes in the modulation sidebands are detected. Light emerging from multipass cell  30  is collected by lens  36  and focused into an optical fiber  38 , which transmits the collected light to signal channel  40  of detector  26 . Collection lens  36  can also act as a mount for optical fiber  38 . A power supply  42  (NEW FOCUS 3211 15V, for example) provides power to detector  26 . Preamplifier  44  (SRS SR 560 Low Noise Voltage Preamplifier, for example) amplifies the signal received by detector  26 . 
     A Remote FMS embodiment apparatus  46  is shown in  FIG. 3   b . Remote (i.e. open path) apparatus  46  is similar to the in situ instrument  10  of  FIG. 3   a . Apparatus  46  also includes computer  12 , picoscope  14 , laser  16 , function generator  18 , laser driver  20 , beamsplitter  22 , detector  26  with reference channel  24  and signal channel  40 , collection lens  36 , optical fiber  38 , power supply  42 , and preamplifier  44 , all configured the same way as for in situ embodiment apparatus  10 . Apparatus  46 , however, does not include a multipass cell for the gas sample, Instead, the modulated signal beam created by beamsplitter  22  is directed to a beam expander  48  that controls the beam divergence as it probes samples remotely. As the modulated beam exits beam expander, it interacts with a remote sample of gas. A remote mirror  50  reflects the light back to the FMS instrumentation. The returned laser light is collected and focused through collection lens  36  to optical fiber  38 , which transmits the collected light to signal channel  40  of detector  26  just as described for the in situ embodiment apparatus  10 . Power supply  42  provides power to detector  26 , and a low noise voltage preamplifier  44  amplifies the signal. 
     Apparatus  46  has been used for remotely monitoring stable isotopes of CO 2  wherein the modulated laser beam interacts with CO 2  and changes in the modulation sidebands are detected. It could also be used for monitoring other atmospheric gases besides CO 2 . An embodiment apparatus was constructed in the form of a tower on the ground in a field. A laser on the tower was directed up from the base of the tower to a mirror that directed the frequency modulated laser pulse to a detector. The laser could also be directed to another tower, which allows monitoring a region between the two towers. 
     Another embodiment of the invention employs an airborne platform (i.e. this embodiment is on an airplane) and may be used for regional monitoring of atmospheric gases (CO 2 , for example). In this embodiment, the frequency-modulated laser beam is directed to a mirror (or other reflective surface) on a wing of the airplane. This embodiment apparatus can monitor δ 13 C as the airplane flies over a region of interest. 
     An aspect of the invention is involved with carbon sequestration, in particular with monitoring CO 2  leaks and leak rates from geologic carbon storage sites. In this aspect of the invention, continuous, column averaged, spatially integrated measurements of δ 13 CO 2  were made using the apparatus  10  shown in  FIG. 3   a  and the measurements are analyzed. Natural gas was pumped into an underground pipe to simulate a storage reservoir. Natural sources of CO 2  (e.g. plants and microbes, see  FIG. 1 ) were distinguished from CO 2  released from underground pipe by using the δ 13 C signature of the CO 2 . Alternatively, natural gas could also be pumped into an underground aquifer or oil field so the method of the invention could be used to detect leakage from such an aquifer or oil field. 
     In another aspect of the invention, an embodiment apparatus can be used for monitoring leaks in a wellbore by fiber-optically coupling the laser to a probe that is lowered into the well. 
     There are other atmospheric gases besides carbon dioxide that have an impact on the changing climate. Some of these include methane, carbon monoxide, nitrous oxide, hydrogen sulfide, nitrogen oxides, and sulfur oxides (SO x ). An embodiment apparatus of the invention can be used to measure stable isotope ratios of atoms (e.g.  32 S,  33 S,  34 S,  14 N,  15 N,  16 O,  17 O,  18 O,  1 H,  2 H) of these other gaseous molecules. An embodiment apparatus has resolution sufficient for rotationally resolved spectra and therefore can be used to probe small molecules like CO or N 2 O or small highly symmetric molecules such as CH 4  and SF 6 . Thus, an embodiment apparatus can provide information from these other atmospheric species about the impact they may have on climate change. The stable isotopes of carbon in CO 2  and CH 4  have been researched over the past 50 or so years, yet the isotope variation in nitrogen and sulfur in the climate change gases is a relatively new field. Any effort to extend FMS to these other climate changing species would require an investigation into better interpretation of the isotope ratios of nitrogen and sulfur. This would involve extracting samples from specific sources from which one can correlate the relationship between source of the species and the stable isotope ratio. 
     The FMS experiment involves tuning the laser such that one of the sidebands in  FIG. 2  is absorbed by the species of interest. Ideally, the carrier (ω c ) and sidebands (ω c ±ω m ) are not absorbed by any other absorption features. The detector is sensitive to the relative changes between the sidebands. For  12 C 16 O 2  and  13 C 16 O 2 , this instrument monitors many of the absorption features in the 1595-1614 nm spectral region where the stable isotopes can be resolved. The peak-to-peak intensity is proportional to the concentration of CO 2  in the sample.  FIG. 4   a - d  shows a variety of spectra obtained using an embodiment system of this invention.  FIG. 4   a  shows a  12 C 16 O 2  FM spectra collected with the remote instrument with a 20 meter standoff.  FIG. 4   b  shows  13 C 16 O 2  FM spectra collected with the remote instrument with a 20 meter standoff.  FIG. 4   c  shows  12 C 16 O 2  FM spectra collected with the in situ instrument.  FIG. 4   d  shows  13 C 16 O 2  FM spectra collected with the in situ instrument. To obtain each of these spectra, the laser was tuned to the wavelength shown in the legend and one of the sidebands interacted with the absorption feature as depicted in  FIG. 2 , generating the FM spectra shown in the  FIG. 4   a - d.    
     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.