High sensitivity pollution detection employing tunable diode lasers

Abstract

A laser absorption spectrometer is described which employs a wavelength-tunable Pb_1−xSn_xSe diode in conjunction with a multipass White cell and which is capable of measuring SO₂ concentrations in the low ppb range. We describe in some detail the modulation techniques used in signal detection which enable us to measure absorption coefficients as low as 10⁻⁷ m⁻¹. In addition, calibration of the instrumentation using small sample cells is described, and the question of interference from unwanted molecular species is discussed. The instrumentation allows the measurement, basically at the same time, of a large number of other atmospheric gases which are of significance in pollution studies. For example, the present diode operates oyer 1050–1150 cm⁻¹ and can measure SO₂, O₃, N₂O, CO₂, H₂O, NH₃, and PAN. The addition of a second diode to the system will allow most gases of any atmospheric importance to be monitored. In general, these gases have much stronger ir absorption bands than SO₂ and hence can be detected at concentrations much less than 1 ppb.