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- Gustafsson, Jörgen, et al.
(författare)
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Double modulation diode laser absorption spectrometry by simultaneous wavelength modulation and optically induced population modulation : application to trace element detection in window-equipped graphite furnaces
- 2004
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Ingår i: Spectrochimica Acta Part B - Atomic Spectroscopy. - Oxford : Pergamon P.. - 0584-8547 .- 1873-3565. ; 59:1, s. 67-92
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Tidskriftsartikel (refereegranskat)abstract
- A new diode laser-based double modulation absorption spectrometry (DMAS) technique for detection of species in trace amounts/concentrations is presented. The new technique makes use of a simultaneous modulation of the wavelength and population in order to reduce the background signals from multiple reflections in optical components (so-called etalon effects) that normally appear in ordinary wavelength modulation absorption spectrometry (WMAS). The simultaneous wavelength and population modulation are achieved by splitting the light from a wavelength-modulated diode laser into two beams—one strong pump beam and one weak probe beam—that subsequently are overlapped in an interaction region inside a sample compartment. The objective of the pump beam is to periodically transfer population from the state with which the probe beam interacts. The modulation of the population is achieved by modulating the pump beam with a chopper. The transmission of the probe beam is detected and demodulated at a frequency that is a combination of various harmonics of the wavelength modulation and chopping frequencies. The purely optical modulation makes the new technique more generally applicable than other DMAS techniques. The new DMAS technique is thoroughly described by a Fourier series-based theoretical description that previously has shown to be powerful for description of WMAS. The theoretical description is general in the sense that it considers DMAS for a variety of modes of operation and for any sample compartment providing homogeneously broadened transitions. The experiments were carried out on the 780-nm transition in Rb in a window-equipped graphite furnace (GF) used as an atomizer for aqueous solutions of Rb in ppt concentrations. The limit of detection obtained for the DMAS technique applied to a window-equipped GF was more than an order of magnitude better than that for the ordinary WMAS technique applied to the same type of window-equipped GF, and similar to that from an ordinary WMAS instrumentation coupled to a window-less GF, i.e. approximately 10 fg. Since the analytical DMAS signal was found to be approximately one order of magnitude smaller than the corresponding WMAS signal, it could be concluded that the noise from the background signal from the DMAS technique applied to a window-equipped GF was likewise about one order of magnitude smaller than the noise from ordinary WMAS applied to an open GF system. This implies in turn that the results so far published from the ordinary GF–WMAS technique are not shot noise limited and should therefore in principle also be improvable.
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