The Optical-Nose (O-Nose™) Technology

Pranalytica has developed instrumentation for highly sensitive laser spectroscopy for detection of trace gases in medical, environmental, industrial, and national security applications. Laser spectroscopy has been extensively used for detection of a large number of industrially produced pollutant gases such as NO, NO₂ , NH₃ , SO₂ , and CH₄ . Many of these gases are found in large concentrations at their sources, e.g. nitric oxide at the tailpipe of an automobile, and at very low concentrations in the ambient atmosphere and stratosphere. Optical absorptivity of most gases is characterized through the measurements of a sample of known concentration. With this in hand, the concentration of an unknown sample may be readily determined.

Through its Nephrolux™ and Nitrolux™ instruments, Pranalytica is applying optoacoustic spectroscopy to the detection of minor constituents of human breath of patients undergoing hemodialysis and for analyzing ambient air in a variety of industrial processing arenas.

Spectroscopic Measurements of Gases at ppb levels

Molecular species absorb and emit light in a manner determined by their constituent atoms and their bonding. Chemists have long used this as a means of identifying what molecules are present in a mixture (qualitative analysis) and in what concentration (quantitative analysis). Most molecules with two or more atoms show distinct absorption bands in the infrared region of the spectrum, generally defined as light with a wavelength between 1mm and 15mm (1mm = 10^-6m). These features can be extremely sharp for molecules that are in the gas phase enabling both the qualitative and quantitative assay with very high selectivity of species through their so-called "fingerprint" absorptions.

Pranalytica has developed an "optical nose" platform that uses a compact, high-efficiency tunable carbon dioxide laser [1] (2W output power) in conjunction with a highly sensitive acoustic detector to measure trace gas concentrations (see Figure 1). Laser tuning is accomplished using proprietary algorithms and method [2] that do not require the use of a spectrometer or other wavelength measurement device for selecting a particular carbon dioxide laser transition. The laser is tuned to a known gaseous absorption line and passed through a cell containing the gas sample. If the trace gas species is present, the gas sample will be slightly heated. This heating can be measured very accurately and linearly by microphones dispersed in the cell [3] and the amplitude of the electrical signal from the microphones immediately correlated with the trace gas concentration. If there is no trace gas present, there will be no signal from the microphone. This is, therefore, the highly desirable "zero-background measurement". Traditional spectroscopic methods measure the total power of the laser beam with and without a sample in the path. For very weak attenuations that result from trace gas concentrations, this process requires taking the difference of two large numbers, each with a finite uncertainty, to compute a small real number. The optical-nose (O-Nose™) technology avoids this complication.

Figure 1. Schematic of the "optical nose" technology platform (O-Nose™)

Since the O-Nose technology is based on a platform concept, the carbon dioxide laser can be replaced with a carbon monoxide laser [4], a spin-flip Raman laser [5], or a tunable semiconductor laser [6]with or without obtaining power boost from an optical amplifier such as a fiber amplifier [7]. This flexibility will permit, in the future, virtually limitless options for measurement of any trace gas species [8]. Specifically, a combination of the CO₂ and a CO laser based instrumentation was shown to detect ppb levels of ammonia, benzene, 1,3-butadiene, 1-butene, ethylene, methanol, nitric oxide, nitrogen dioxide, propylene, trichloroethylene, and HCN [9,10]. The O-Nose technology is the engine of the Nephrolux™ that measures ppb level ammonia in expired human breath in presence of large number of other interfering species including >4% carbon dioxide, >10% water vapor, and more that 200 volatile organic compounds. This instrument is user friendly and has been deployed in UCLA's kidney dialysis center and in the OB/GYN center in the Olive View Medical Clinic in Sylmar, CA where they are used exclusively by physicians and nurse practitioners to collect clinical data on kidney dialysis patients and potential pre-eclampsia patients. The O-Nose technology is also the engine in the Nitrolux™ that is designed to measure sub-ppb levels of ammonia in semiconductor industry clean rooms and ambient air monitoring. Because at ppb and sub-ppb levels, the absorption caused by the gaseous species being detected is very small, a number of optoacoustic cells can be inserted in the path of the optical beam. Such "optical multiplexing" allows monitoring of multiple streams of gases simultaneously in the industrial environment. [11]

REFERENCES

1. C. K. N. Patel, "Interpretation of CO₂ Optical Maser Experiments", Phys. Rev. Lett. 12, 518 (1964).

2. Brian Wiemeyer U.S. Patent 6,658,032 ( December 2, 2003).

3. L. B. Kreuzer, and C. K. N. Patel, "Nitric Oxide Air Pollution Detection by Optoacoustic Spectroscopy", Science 173, 45-47 (1971).

4. C. K. N. Patel, "CW Laser on Vibrational-Rotational Transitions of CO", Appl. Phys. Lett. 7, 246-247 (1965).

5. C. K. N. Patel and E. D. Shaw, "Tunable Stimulated Raman Scattering from Conduction Electrons in InSb", Phys. Rev. Lett. 24, 451-455 (1970).

6. Michael E. Webber, Douglas S. Baer and Ronald K. Hanson, "Ammonia Monitoring near 1.5 mm with diode-laser absorption sensors," Applied Optics 40, 2031-2042 (2001); Michael E. Webber, Ricardo Claps, Florian V. Englich, Frank K. Tittel, Jay B. Jeffries and Ronald K. Hanson, "Measurements of NH₃ and CO₂ with distributed-feedback diode lasers near 2.0 mm in bioreactor vent gases," Applied Optics 40, 4395-4403 (2001).

7.  M.E. Webber,  M.B. Pushkarsky,  and C.K. N. Patel,  "Fiber-amplified enhanced photoacoustic spectroscopy using near-infrared tunable diode lasers,"  Applied Optics 42 (12), April 2003

8. C. K. N. Patel, "Opto-Acoustic Spectroscopy Applied to the Detection of Gaseous Pollutants", in Monitoring Toxic Substances, ed. Dennis Schuetzle, (ACS Symposium Series No. 94), pp. 177-194 (1978)

9. N. D. Kenyon, L. B. Kreuzer, & C. K. N. Patel, "Air Pollution Detection: Ten Pollutants Detected Using CO and CO2 Lasers with Sensitivity of 1 PPB", Science 177, 347-349 (1972).

10. R. J. H. Voorhoeve, C. K. N. Patel, L. E. Trimble, and R. J. Kerl, "Hydrogen Cyanide Production During Reduction of Nitric Oxide Over Platinum Catalysts", Science 190, 149-151 (1975).

11. M.B. Pushkarsky,  M.E. Webber,  O. Baghdassarian,  L.R. Narasimhan,  and C.K. N. Patel,  "Laser-Based photoacoustic ammonia sensors for industrial applications,"  invited paper for Applied Physics B Special Issue:  Trends in Laser Sources, Spectroscopy Techniques and Their Applications to Trace Gas Detection, 75 (2-3), September 2002.