Info

" From Tuazon et al. (1980). h From Schiff et al. (1994b). ' Resolution 0.5 cm-1. '' 150 m, integration time 3-5 min.

'' From Griffith and Schuster (1987); for a 15-L air sample. ' Based on laboratory spectra only. From E. Tuazon, personal communication, 1998.

" From Tuazon et al. (1980). h From Schiff et al. (1994b). ' Resolution 0.5 cm-1. '' 150 m, integration time 3-5 min.

'' From Griffith and Schuster (1987); for a 15-L air sample. ' Based on laboratory spectra only. From E. Tuazon, personal communication, 1998.

The advantages of TDLS over FTIR are increased resolution and sensitivity. The widths of the laser lines are less than ICT4 cm-1. This can be compared to typical pressure-broadened half-widths of infrared absorption bands of species of atmospheric interest, which are of the order of 0.05 cm-1 at atmospheric pressure; at low pressures (e.g., < 1 Torr), where the linewidth is limited by Doppler broadening, typical half-widths are 0.0005-0.005 cm-'. Thus the TDL output is usually sufficiently narrow to scan rotational absorption lines even at low pressures where Doppler broadening is the limiting factor on lineshape. This narrow laser linewidth allows one to measure weak absorptions between the ambient H20 and COz lines. Thus one can measure accurately small absorbances due to specific rotational lines in a vibration-rotation spectrum with high selectivity. However, for many molecules of interest, the presence of such rotational fine structure requires lowering the total pressure of the sample to ~ 10-30 Torr to minimize pressure broadening of the absorption lines. (For larger molecules, the absorption spectrum appears as a continuum even at these lowered pressures.)

A disadvantage of TDLS is that scanning the entire IR spectrum quickly is not possible since each diode normally covers a limited wavelength range and even the use of several diodes in one instrument does not provide the wide range of FTIR. Thus TDLS is more useful for following specific pollutants known to be present than for searching for previously unidentified species. In addition, the high-resolution capability is not of use for very large molecules with many overlapping bands. While reducing the pressure of the sample helps in reducing the absorbing linewidth, it also results in a loss of sensitivity through reductions in concentration and the possibility of interactions with the walls of the cell.

Commonly used tunable diode lasers are made of lead salt compounds such as PbS,_xSex, Pb,_xSnxTe, Pb,_xGexTe, Pb,_xSnxSe, and Pb!_xCdxS. Diodes made from Group III (Ga, Al, and In) and Group V (P, As, and Sb) elements are not in widespread use for atmospheric applications because they emit at wavelengths beyond 2 /¿m (5000 cm"1) where the molecular absorptions are much weaker overtone and combinaton bands, limiting the detection sensitivity (Schiff et al., f994a, f994b; Brassington, 1995). A p-n junction is formed in the crystal and the diode is mounted onto a support such as copper that serves as a temperature controller during operation. When an electrical current is applied, the diode emits light spontaneously at a wavelength corresponding to the energy band gap in the semiconductor. Laser action results from reflections from the end faces of the crystal. This gap de pends on the chemical composition of the laser and hence different wavelengths from 3 to 30 /¿m (3300-330 cm"1) can be produced by altering the diode composition. The actual structure of these devices is more complex than a simple p-n junction, typically involving double heterostructures (e.g., see Brassington, 1995).

Tuning of the emitted wavelength can be accomplished, in principle, through variation of one of three possible parameters: applied magnetic field strength, diode temperature, and hydrostatic pressure. In practice, temperature, which can be controlled by changing the current through the diode, is used. Typical variations of output with temperature are about 3 cm"1 per K (Brassington, 1995). Figure 11.5, for example, shows the output of laser frequency as a function of temperature from a lead salt diode laser (Werle et al., 1992). The output at a given current is a series of longitudinal modes whose separation, typically about 2 cm~', is determined by (2t/L)-1, where 17 is the index of refraction of the salt (usually 4.5-7) and L is the length of the laser cavity, i.e., separation of the end faces of the crystal (typically 300-400 /¿m). Tuning of such semiconductor lasers over ~ 100-200 cm~' can typically be carried out, which is sometimes sufficient to measure more than one pollutant with a single laser. Alternatively, several different diode lasers are included in the same apparatus.

A number of different modulation techniques can be used to increase the signal-to-noise ratio (e.g., see Schiff et al., 1994a, 1994b; and Brassington, 1995). For example, the laser beam can be mechanically chopped and detected using phase-sensitive detection with a lock-in amplifier. A more commonly used method for accurately measuring small absorbances is to modulate

FIGURE 11.5 Variation of laser frequency and signal with current for a typical lead salt diode laser (adapted from Werle et al, 1992).

0 0

Post a comment