Wavelength modulation diode laser atomic absorption spectrometry in modulated low-pressure helium plasmas for element-selective detection in gas chromatography. Plenary lecture

Abstract

A new technique for element-selective detection in gas chromatography is reported. It is based on wavelength modulation diode laser atomic absorption spectrometry (WM-LAAS) in modulated low-pressure dc or microwave-induced plasmas (MIP). The double modulation laser atomic absorption spectrometry (DM-LAAS) eliminates not only flicker noise from the laser as well as from the plasma but also etalon effects which limit the detection in WM-LAAS. DM-LAAS allows the measurement of absorbances of about 10^–7. The analytical power of the technique is demonstrated by the analysis of haloform samples applying DM-LAAS of metastable chlorine and bromine atoms which are generated in the low-pressure plasmas by dissociation and excitation. Since the species are dissociated completely, the signals reflect the relative element concentration in the molecules, which allows calibration by internal standards. The detection limits are very low. For example, 3s detection limits were found to be below 3 ng ml^–1 for species such as CHCl₃ or CCl₄, applying splitless injection of 0.5 µl samples and DM-LAAS of chlorine in the MIP. The detection limits found in the dc plasma were slightly higher.