Instrumentation
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GENERAL
ELECTRIC
shows the type of correlation spec trometer which has been developed ; this is a dispersive Ebert spectro photometer. The incoming radiation to be ana lyzed is passed via reflection from the Ebert mirror to a grating which is the dispersive element. The radiation emerging from the grating is now dispersed into its various spectral components to be reflected a second time by the Ebert mirror and to pass through the exit aper ture of the spectrometer to a photodetector. The angular position of the grating determines the region of the spectrum that falls on the exit aperture. If the incoming radia tion is now made to vibrate periodi cally through a limited wavelength excursion, and a mask replica of the sought spectrum is positioned in the exit aperture, the incoming spec trum will be moved periodically in and out of correlation against the mask. A beat signal will result and will be detected by the photodetector. Phase-sensitive synchronous detection techniques can be used for the signal detection using, as the reference, the drive signal that causes the spectrum to vibrate. Signal Origin
The basic function of the correla tion spectrometer is the Beer-Lam bert law of absorption which de scribes the attenuation of a beam of monochromatic light transmitted through a medium which absorbs but does not scatter. Then where
H = H0e-acL
H0 == incident irradiance at a fixed wavelength H = emerging irradiance at same wavelength a = absorption coefficient at the same wave length c = concentration of me dium L = pathlength through medium
If the absorption coefficients are dif ferent at two known wavelengths, while H0 is the same, we have: Hi = H0-e~aicL and H, = H«e,-aich where αϊ and o2 are the absorption coefficients at wavelengths 1 and 2, respectively.
Circle No. 52 on Readers' Service Card 102 A
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(1)
ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 6, MAY 1 9 7 0
Thus,
- (ai — ai)cL
H*
(2)
In general, the exit plane slits have a finite width and H0 is not con stant with wavelength; similarly αχ and a2 vary with wavelengths. Thus, a more general power expres sion is: λι+Δλ
Nx-d\ ρ *2
/»λζ+Δλ /»λ2
J λ!
Nx-d\ (3)
In this expression etj and a2 are the average cross sections per molecule in the bands (λι, λι -f- Δλι) and (λ2, λ2 + Δλ 2 ). Pi and P 2 have the dimension of watts, that is, power passing through the slits. The passing power can be ana lyzed by means of a mask or array of slits for several exit slits at two different positions rather than for one slit in two positions. One would expect that if the slits are properly chosen, the effect of the spectral distribution of the light source and influences due to other absorbing gases, could be mini mized. In general, not only the quotient of Pi and P 2 can be used to determine cL, but also any other suitable relations between them. In this instrument, the physical ef fect selected to measure the product cL is the difference in power re ceived by the phototube at two dif ferent wavelength positions on the mask. The power difference nat urally results in a phototube cur rent difference. The actual proce dure used in the remote sensing spectrometer is as follows: (1) At one position of the spec trum with respect to the mask, the incident power, Plt on the phototube derives a current, J, which is then held fixed. The voltage, V, across the phototube dynode chain is variable and adjusted by an AGC loop so that the response to Pi watts is always J amps. (2) For the second position of the spectrum with respect to the mask, the incident power will be P 2 and will consequently generate J' amps. The correlation spectrometer measures the difference D between J and J'—i.e.,
