DIODE-LASER ATOMIC-ABSORPTION SPECTROMETRY BY THE DOUBLE-BEAM-DOUBLE-MODULATION TECHNIQUE

The limitations of absorption measurements in atomic-absorption spectr ometry with tunable diode lasers are investigated. It is shown that th e double modulation technique (diode-laser wavelength modulation and s ample modulation) with detection at the sum or difference frequency su ppresses spurious etalon effects, background absorption, residual diod e-laser-amplitude modulation and the noise which accompanies these eff ects, and enables measurement of detection limits determined by the la ser excess noise. Detection limits in absorption, defined as absorptio n equal to the root-mean square value of noise, as low as 1 x 10(-6) A U (absorption units) were achieved for metastable Cl atoms in a modula ted low-pressure microwave-induced plasma with a time constant of 1 s. In order to eliminate laser excess noise and signal variations due to changes of optical transmittance, a double-beam arrangement with loga rithmic subtraction of sample and reference detector currents was deve loped. It enables suppression of variations of the laser radiation pow er outside the detection pass-band and the achievement of a detection limit of about 2 x 10(-7) AU determined by shot noise only. (C) 1997 E lsevier Science B.V.