Ammonia monitor based on intermodulated CO2 laser photoacoustic Stark spectroscopy

One of the causes of the acidification of the environment are high ammonia (NH ₃ ) emission rates. In environmental research it is an impetus to measure the ammonia concentration sufficiently accurate and fast. This thesis describes the development, construction and testing of a <sup>12</SUP>C <sup>16</SUP>O ₂ laser photoacoustic monitor for trace detection of ambient ammonia. It is for the first time that hyphenated photoacoustics has been applied to the direct detection of ambient ammonia.The difficulties met in the practice with NH ₃ monitors based on chemical or physicalchemical detection has stimulated the research towards the development of a monitor based on physical detection principles.Gas-phase photoacoustic spectroscopy has proven a sensitive and feasible technique for detection of pollutants. Owing to the availability of powerful, collimated, monochromatic and tunable light sources (such as the CO ₂ (waveguide) laser) the detection of trace gases at ppbv and even at pptv levels appears within reach.Although a sensitive technique, classical photoacoustic spectroscopy is limited by the complex interpretation and additive character of signal resulting from spectral interferences (when dealing with mixtures), occurring in the wavelength region of the emitting light source. Especially in the infrared (2-20 μm), molecular fingerprints of important trace gas molecules frequently overlap or are masked by water vapor and carbon dioxide absorption. Spectral interference from the previously mentioned and largely abundant ambient species impedes the unique determination of ambient ammonia in the <sup>12</SUP>C <sup>16</SUP>O ₂ laser emission frequency region.Using the Stark effect induced in ammonia and subsequent detection of the generated photoacoustic signal at the sum and difference sidebands of the modulated laser and Stark electric fields, detection of ammonia at trace levels employing the 10R(8) and 10R(6) CO ₂ laser lines has been successfully achieved. The sensitivity of the method was tested in a simulated atmosphere (present limit 2 ppbv NH ₃ ). Although materials exhibiting a low NH ₃ adsorption affinity have been used to construct the photoacoustic Stark cell, signal response time of the system is at present limited to 40 sec.In addition to a brief historical review Chapter 1 contains an outline of the methods used in detecting ammonia. Intrinsic difficulties associated with the different chemical and photoacoustic methods are described, as well as an experimental and theoretical description of the method employed in this thesis (InterModulated PhotoAcoustic Stark Spectroscopy or IMPASS). The theory of infrared rotational-vibrational transitions as well as the Stark effect applied to the case of symmetric rotators exhibiting inversion doubling are presented in Chapter 2. Details of the photoacoustic Stark cell, the three frequency phase-locked loop, progress and improvements made on the <sup>12</SUP>C <sup>I6</SUP>O ₂ waveguide laser as well as a new method for calibrating photoacoustic cells are given in Chapter 3. Experiments performed on detection of ammonia in a closely simulated atmosphere and in realistic air samples are discussed in Chapter 4.Finally, Chapter 5 presents closing remarks concerning suggestions for improving the performance of the present instrument.