OCR Text |
Show the light source spectrum, Im(v), matches the absorption spectrum, a(v), of the absorber of interest, a(v) can be replaced with an effective cross-section, ora, which depends on Im(v), F(v) and possibly foreign gas densities, "nT . These approximations lead to the transfer function i - i = CRTP exp (-£ n o ) /" dv I (v) F(v) exp [-1 E n. a, (v)] m b r m J0 ra k k (2) where R, T and P are the detector responsivity, the transmissivity of the optics and the attenuation due to particles, respectively and the summation involves only those species with frequency-dependent absorption within F(v). In general, C, R, T and P are not readily measured and the "n, are not known. Determination of the parameters, C, R, T and P, and of the concentrations ~n7 , in Equation 2 can be avoided if a second source profile Ir(v) is available such that C, R, T and P do not vary significantly for the two source beams. Then the ratio of the photocurrents of the two beams will eliminate C, R, T and P. Furthermore, careful wavelength selection can eliminate the dependence on the foreign gas densities. For nitric oxide, this leads to the equation for the number density n - [A <0 - 0 )] NO r m -I in m Xr0 ib0 i - i. i A - i. n r b mO bO (3) where or and am, the cross-sections for the reference and measurement beams, must be determined. ar and am can be functions of temperature so that evaluation of the concentration of nitric oxide is dependent upon the gas stream temperature prof Lie. III. Experimental A schematic diagram of the apparatus is shown in Figure 1. The basic idea is to generate an emission spectrum characteristic of nitric oxide which is intermittently altered such that the overlap of the emitted spectrum with the absorption spectrum of the nitric oxide in the flow is also altered. A monochromator is used to restrict the response of the photomultiplier to the electronic transition of interest and to remove system sensitivity to other absorbing species. The output of the |