Fluid handling – Processes – With control of flow by a condition or characteristic of a...
Reexamination Certificate
2000-11-17
2002-05-21
Hepperle, Stephen M. (Department: 3753)
Fluid handling
Processes
With control of flow by a condition or characteristic of a...
C137S606000, C137S607000, C250S288000
Reexamination Certificate
active
06390115
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and a device for producing a directed gas jet as they are known from Chun Hang Sin et al. “Supercritical Fluid/Supersonic Jet Spectroscopy with a Sheath-Flow Nozzle”, Analytical Chemistry, Vol 64, No. 2, Jan. 15, 1992 (1992-01-15), pages 233-238, XP000248258 ISSN: 0003-2700.
A fast on-line analysis for gaseuos samples is desirable in many areas of research but also in the industry. It could be used for research, for the surveillance of exhaust gases, waste combustion plants, roasting gases during roasting of coffee, headspace analysis of mineral oils and soil samples. The information received therefrom can be used as a parameter for the process control. Of particular interest are often the compounds with aromatic base structures such as polycyclical aromatic hydrocarbons (PAH) in exhaust gases of industrial combustion plants. Since different isomers of the various PAH have different environmental relevance or, respectively, toxicity, it is reasonable to detect them selectively.
For a rapid on-line analysis of gaseous samples, molecule-spectroscopic procedures using the supersonic molecule beam technique are particularly suitable. In this procedure, the sample gas beam is adiabatically expanded into a vacuum, which results in a reduction of the internal energy of the sample molecules. This reduction of the internal energy results in a reduction of the temperature, that is, the sample molecules are cooled by the adiabatic expansion. As a result, the energy bands become narrower and do not overlap—in contrast to samples, which have not been cooled. Since the energy required for the excitation of the molecules is different for different compounds and also for different isomers of a compound, the energy needed for the excitation of the molecules can be used for an isomer-selective identification. For example, by the excitation and subsequent photo-ionization (REMPI) by means of a narrow band laser, a very high optical selectively can be achieved in this way up to an isomer selectivity.
Generally, the supersonic molecule beam or jet is generated by the expansion of a continuous or a pulsed gas beam through a small nozzle into a vacuum. This method has been used so for mainly for spectroscopic examinations where the detection sensitivity does not play any role. Since, during expansion, the sample beam becomes rapidly wider, which results in a large reduction of the sample density, the achievable detection sensitivity is noticeably worse than with alternative inlet techniques such as an effusive gas inlet wherein the sample molecules are not cooled. The utilization of the selective supersonic molecule beam technique in the on-line analysis is therefore aimed at an improvement in the detection sensitivity.
A proposal herefor is offered in the article by S. W. Stiller and M. V. Johnston: “Supersonic Jet Spectroscopy with a Capillary Gas Chromatographic Inlet”, Anal. Chem. 1987, 59, 567-572. Stiller and Johnston developed a coupling of gas chromatography (GC) and laser-induced fluorescence spectroscopy (LIF) with supersonic molecular beam techniques (JET). To this end, they use an arrangement, wherein a GC capillary extends into the center of a concentric guide tube for an auxiliary gas beam. The sample gas supplied by way of the capillary is added into the core (center axis) of the auxiliary gas beam. The auxiliary gas beam and the sample gas beam centered in the auxiliary gas beam focussed along the center axis thereof are continuously expanded into a vacuum through a nozzle with a narrowed tip, whereby a continuous supersonic molecule beam is formed.
The adiabatic expansion of the gas beam into the vacuum results in a cooling of the auxiliary gas and the sample gas molecules. As a result of the adiabatic cooling more sharply defined bands for the excited states of the molecules are generated. With a sharply defined energy (laser wavelength), then only certain sample molecules can be excited, which provides for a high optical selectivity. With the high optical selectivity, the sample molecules can partially be detected even in an isomer-selective manner.
The concentric narrowing down of the gas beam guide tube toward the tip opening into the vacuum chamber causes an additional focussing of the sample gas beam onto the center axis of the auxiliary gas beam so that the expansion into the vacuum results in a delayed spatial expansion of the sample gas beam. With the subsequent ionization or fluorescence excitation, a larger part of the sample molecules can be irradiated (higher sensitivity) without the need for a reduction of the effective cross-section for the excitation or, respectively, ionization by a spatial expansion of the laser beam (lower power density).
Although the sample gas density in the excitation or, respectively, ionization volume can be increased with the arrangement as described by Stiller and Johnston, the high-vacuum conditions are detrimentally affected by the continuous gas beam to such a degree that collisions between the sample molecules and, respectively, the auxiliary gas molecules make sensible measurements impossible. Furthermore, a large part of the sample gas, which passes between the laser pulses and which is not ionized and can therefore not be detected, is wasted.
Another apparatus for improving the detection sensitivity by employing the supersonic molecular beam technique is described by B. V. Pepich, J. B. Callis, J. D. Sheldon Danielson and M. Gouterman in the article: “Pulsed free jet expansion system for high-resolution fluorescence spectroscopy of capillary gas chromatographic effluents”, Rev. Sci. Instrum. 57(5), 1986, 878-887. Pepich et al. represent therein a GC-supersonic molecular beam coupling for the laser-induced fluorescence spectroscopy. As a result of the pulsed inlet, among others, a first increase of the sample volume employed for the analysis in comparison with the effusive inlet system is achieved. In order not to interrupt the GC flow by the pulsed inlet, Pepich proposes to introduce the sample into a pre-chamber in an effusive manner. Into this pre-chamber, a pulsed carrier gas is injected, which also provides for the gas flow needed for the expansion cooling. This carrier gas compresses the sample gas in the pre-chamber and pushes it, like a piston, through a small opening downwardly into an optical chamber where the fluorescence excitation takes place. As a result of the pulsed compression and injection of the sample gases into the optical chamber, a larger number of sample molecules can be reached by the subsequent laser excitation (increase of the detection sensitivity). The opening of the valves and the laser pulses must be synchronized in order for the compressed sample gas pulse to be excited by the laser pulse.
With the supersonic molecular beam technique an adiabatic cooling of the sample is achieved, whereby the selectivity of the method is substantially increased.
The setup selected by Pepich et al. facilitates also a repetitive, timely limited compressions of the sample in gas flow direction and improves the detection sensitivity also for this reason. However, it does not prevent the rapid spatial expansion of the sample gases which is typical for the supersonic molecular beam technique and as a result of which a large part of the sample gas is outside the ionization volume when excitation or ionization takes place. A widening of the laser beam is again not feasible because of the deterioration of the effective ionization cross-section.
It is the object of the present invention to provide a method and apparatus of the type described above wherein however a maximum particle density can be generated.
SUMMARY OF THE INVENTION
In a method for producing a directed gas jet wherein a guided sample gas beam is generated and an auxiliary gas beam is generated and directed and guided in the same direction as, but separated from, the sample gas beam, a pulsed carrier gas stream is generated and combined with the sample gas beam such that the sample gas beam is separated
Boesl Ulrich
Dorfner Ralph
Heger Hans Jörg
Kettrup Antonius
Rohwer Egmont
Bach Klaus J.
GSF-Forschungszentrum für Umwelt und Gesundheit
Hepperle Stephen M.
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