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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
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
2004 (English)In: Spectrochimica Acta Part B - Atomic Spectroscopy, ISSN 0584-8547, E-ISSN 1873-3565, Vol. 59, no 1, p. 67-92Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Oxford: Pergamon P. , 2004. Vol. 59, no 1, p. 67-92
Keywords [en]
double modulation, population modulation, wavelength modulation, diode laser, absorption spectrometry, dmas, wmas, wms, graphite furnace, background signal, etalon effects, trace amounts, rb
Identifiers
URN: urn:nbn:se:umu:diva-2702DOI: 10.1016/j.sab.2003.10.003OAI: oai:DiVA.org:umu-2702DiVA, id: diva2:140949
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2018-06-09
In thesis
1. Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
Open this publication in new window or tab >>Laser-based absorption spectrometry: development of NICE-OHMS towards ultra-sensitive trace species detection
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Laser-based absorption spectroscopy (AS) is a powerful technique for qualitative and quantitative studies of atoms and molecules. An important field of use of AS is the detection of species in trace concentrations, which has applications not only in physics and chemistry but also in biology and medicine, encompassing environmental monitoring, regulation of industrial processes and breath analysis. Although a large number of molecular species can successfully be detected with established AS techniques, there are some applications that require higher sensitivity, selectivity and accuracy, yet robust and compact instrumentation.

Various approaches have been made during the years to improve on the performance of AS, usually based on modulation spectrometry or external cavities. The most sensitive absorption technique of today is, however, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS). This technique elegantly combines several approaches: external cavities (for optical path length enhancement), modulation techniques (for noise reduction) and saturation spectroscopy (for enhanced selectivity). However, due to its complexity, the technique has so far not been applied to practical trace species detection.

This thesis provides the background for an understanding of NICE-OHMS and describes the construction of a first compact NICE-OHMS spectrometer based on a narrowband fiber laser. Moreover, it gives theoretical expressions for NICE-OHMS signal lineshapes, measured in various modes of detection, which can be fitted to the experimental data and thereby facilitate the assessment of species concentration. The sensitivity of the instrumentation is demonstrated by detection of acetylene (C2H2) and carbon dioxide (CO2) in the 1.5 μm region. A fractional absorption sensitivity of 3*10-9 (integrated absorption of 5*10-11 cm-1), could be achieved using a cavity with a finesse of 4800 and an acquisition time of 0.7 s. This results in a detection limit for C2H2 of 4.5 ppt (4.5*10-12 atm).

In addition, the thesis revives the idea of using an accurate (frequency) measurement of the free-spectral-range (FSR) of an external cavity for sensitive and calibration-free concentration assessment. A theoretical description of the expected signal lineshapes is given, and in a first experimental demonstration the FSR could be measured with a resolution of 5 Hz, resulting in a fractional absorption sensitivity of 1*10-7, and subsequently in a detection limit for C2H2 of 180 ppt (12.5 s acquisition time).

The thesis, finally, also contributes to the continuously ongoing development of conventional AS and wavelength modulated AS by addressing concepts related to when the light optically saturates the transition.

Place, publisher, year, edition, pages
Umeå: Fysik, 2007. p. 91
Keywords
absorption spectrometry, trace species detection, fiber laser, modulation, cavity-enhanced spectroscopy, optical saturation, laser frequency stabilization, free-spectral-range
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-1414 (URN)978-91-7264-403-8 (ISBN)
Public defence
2007-11-23, N430, Naturvetarhuset, Umeå Universitet Campus, Umeå, 13:15
Opponent
Supervisors
Available from: 2007-11-01 Created: 2007-11-01 Last updated: 2018-06-09Bibliographically approved
2. Development of wavelength modulation diode laser absorption spectrometry in transversely heated graphite atomisers for sensitive trace element analysis
Open this publication in new window or tab >>Development of wavelength modulation diode laser absorption spectrometry in transversely heated graphite atomisers for sensitive trace element analysis
2002 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Wavelength Modulation Diode Laser Absorption Spectrometry technique (WM-DLAS) is a laser-based spectroscopic technique for sensitive detection of atoms and molecules that so far mostly has been used for environmental monitor-ing. This thesis is concerned with the development of this technique for sensitive trace element analysis. The WM-DLAS technique is in general able to measure a variety of species at three orders of magnitude lower concentrations than conven-tional atomic absorption techniques since it shifts the detection to high frequen-cies where the 1/f-noise is significantly lower.

The WM-DLAS technique has in this work been combined with a Transversely Heated Graphite Atomiser (THGA) in order to make possible both ultra-sensitive trace element analysis and microanalysis. The new technique has shown a limit of detection of 10 fg ( g) for Rb in aqueous solutions (the pilot element under investigation), corresponding to a concentration of 0.2 ppt (0.2 parts-per-trillion or pg/mL).

The work has been pursued along several lines. The most important one has been to obtain a thorough understanding of the physical processes that might influence the performance of the technique. This part of the work has, in turn, been pur-sued in two different directions; to identify and understand the processes that give rise to the analytical signal (in order to find means to maximize it), and to identify and understand the processes that give rise to background signals and their drifts (in order to find means to minimize them).

A thorough understanding of the signal strengths and shapes of the analytical WM-DLAS signal has been obtained by the development and use of a program that simulates WM-DLAS signals for a variety of experimental situations. Quanti-ties of special importance have been the influence of laser centre frequency, fre-quency modulation amplitude, and the order of the harmonic detected on the signal strengths and shapes. The influence of hyperfine structure and isotope shifts on the elemental detection has also been investigated. It was found that the existence of hyperfine structure and isotope shift, as well as the pertinent broaden-ing mechanisms, has a pronounced effect on the detection. It was, in fact, found the WM-DLAS signal from a low pressure cell has little in common with that from an atmospheric pressure atomiser, such as the THGA, which puts sever restrictions on its use as a wavelength reference source.

It has also been found that WM-DLAS is, in most situations, not limited by the shot noise, but rather noise and drifts from the background signals. A significant amount of work has therefore been devoted to identify these background signals so as to find means to minimize them. The investigation has revealed that the most important background signals originate from multiple reflections in optical components, so-called etalon effects. Various techniques for reduction of such background signals have been proposed and examined.

Other research directions pursued have been to develop a new methodology for dealing with optically thick samples. This led to the development of a new tech-nique for extending the dynamic range of the WM-DLAS technique. The meth-odology does not require any prior knowledge of the analytical content of the sample, nor does it sacrifice the high sensitivity of the technique in order to obtain the extended dynamic range. The dynamic range was, by applying the methodol-ogy, increased to more than six orders of magnitude.

Place, publisher, year, edition, pages
Umeå: Fysik, 2002. p. 78
Keywords
Physics, Fysik
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-42 (URN)91-7305-358-9 (ISBN)
Public defence
2002-12-19, Umeå, 10:15
Available from: 2002-12-19 Created: 2002-12-19 Last updated: 2018-03-15Bibliographically approved

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Schmidt, Florian M.Axner, Ove

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