Excimer UV fluorescence detection

[0001] The present invention relates to a system and method for excimer UV fluorescence detection. More specifically, the present invention provides a system and method for enhancing the detection of sulfur dioxide using ultraviolet fluorescence. And even more specifically, the present invention provides a system and method for detecting sulfur dioxide using a KrCl dielectric barrier-discharge (DBD) excimer lamp.

[0002] Efforts to reduce the environmental impact resulting from combustion of hydrocarbon fuels continues to place increasingly stringent requirements on acceptable levels of total bound sulfur content in motor fuels. Recently issued federally mandated guidelines will eventually pose a significant challenge to present ASTM methods for laboratory and on-line analysis of these products with desired concentrations even lower than presently imposed. As a result, improved or alternative methods need to be developed in order to keep pace with strict requirements being placed on these fuels. The following describes a proposed improved method and the associated predictive results.

[0003] At present, the X-Ray Fluorescence (XRF) method has limited ability to provide repeatable analysis of sulfur content below present guidelines. Flame Photometric Detection (FPD) and Sulfur Chemiluminescence (SCD) methods possess much greater sensitivity, but suffer from a variety of problems. Both methods require the use of hydrogen, a hazardous gas, which increases the cost and complexity of these systems in order to meet safety requirements. The FPD method also has a characteristic non-linear response to sulfur requiring multi-point calibration and/or sulfur addition to approximate linear results. Chemiluminescence analysis additionally requires a stable vacuum source and ozone generator, increasing system complexity and making it notorious for high maintenance and long-term stability issues.

[0004] The simplest, most practical low-level sulfur analysis technique that has been widely used and proven over many years of utilization is the UV Fluorescence method. This method involves the optical excitation of sulfur dioxide and detecting a longer wavelength, secondary emission continuum. Total sulfur content in fuels is determined by complete combustion of all hydrocarbon and sulfur species, which are oxidized to CO₂, H₂O and SO₂.

[0005] Originally developed UV fluorescence detectors were continuous types, but sensitivity of these detectors is somewhat limited due to inherent background noise or baseline instability, which is partially due to lamp intensity variations from plasma instabilities within the UV excitation lamp. Both cadmium and zinc excitation lamps have been successfully applied utilizing the 228 nm and 214 nm emission lines respectively, but the present accepted ASTM method favors the zinc lamp due to minimal fluorescence quenching influence from variations in the moisture content of the sample.

[0006] In an attempt to further decrease lower detectable limits, the Pulsed UV Fluorescence (PUVF) method was developed. The operating principle of PUVF is very similar to continuous type UV Fluorescence detectors, except that it uses a xenon flashlamp as the excitation source. Since the light is electronically pulsed, signal-to-noise ratios are improved and a lower level of detectability can be routinely achieved. However, since flashlamp intensity varies substantially from pulse-to-pulse, flash intensity is electronically normalized utilizing a separate light detector and induced fluorescence is averaged over a period of time. The PUVF detector is ideally suited for atmospheric monitoring and similar type applications where typically encountered SO₂ concentrations generally change more slowly with time. Unfortunately, the pulsing excitation source also renders the PUVF a poor detector for higher speed detection requirements, such as total sulfur peak integration or sulfur speciation applications by chromatographic analysis.

[0007] In addition, the complexity of additional electronics required for flashlamp operation, signal synchronization and subsequent data averaging yields a detector that is considerably more complex and costs more than the continuous type UV detectors.

[0008] Although some additional improvement to accepted UV fluorescence detection methods may be realized by optimizing detector design, inherent limitations prevent the present methods from any quantum increase in sensitivity. Most efforts to further enhance sensitivity are primarily applied to reduction of background noise through improved detector geometry and careful selection of optical components to maximize excitation and fluorescence signals. Since several manufacturers of SO₂ fluorescence detectors have attempted to push the limits of detection for many years, it is this author's opinion that significant improvement to present detection levels is unlikely utilizing present methods.

[0009] There exists, therefore, a need for improved UV fluorescence detection of sulfur dioxide having enhanced sensitivity.

[0010] A new excitation source can yield a significant improvement to present SO₂ sensitivities achieved with ASTM accepted standards utilizing UV Fluorescence methods. This new excitation source, or Excimer lamp, creates a high intensity UV emission that possesses higher spectral purity than traditional zinc or cadmium lamps.

[0011] Excimer lamps are barrier-discharge devices based on the excited dimer principle and contain either an inert fill gas or a specific halogen and inert fill gas mixture. An alternating high voltage field is applied to electrodes separated by an insulating medium or dielectric and induces the generation of microscopic internal filament discharges, which excite fill gas atoms into briefly forming an excited molecular species. These short-lived transient molecules rapidly dissociate and release photons of specific wavelengths directly related to the quantum energy lost in the transition to ground state.

[0012] A feature of the present invention is to provide a UV fluorescence detection system and method having emission that is caused by excitation at a wavelength that correlates with absorption bands of the species under investigation.

[0013] A feature of the present invention is to provide a UV fluorescence detection system and method having an excitation wavelength that correlates with peak absorption bands of SO₂ which create maximum fluorescence intensity.

[0014] Another feature of the present invention is to provide a UV fluorescence detection system and method possessing maximum fluorescence intensity at or near an excitation wavelength of 222 nm so as to correlate with absorption bands that induce the greatest fluorescence emission.

[0015] Another feature of the present invention is to provide a UV fluorescence detection system and method utilizing a dielectric barrier-discharge excimer lamp.

[0016] Another feature of the present invention is to provide a UV fluorescence detection system and method utilizing a krypton-chloride excimer mixture.

[0017] Another feature of the present invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral emission purity to decrease the effects of background measurement limitations.

[0018] Another feature of the present invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral emission purity so as to increase signal to noise ratio.

[0019] Yet another feature of the invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral purity so as to possibly eliminate the need for an excitation source filter.

[0020] Still another feature of the present invention is to provide a UV fluorescence detection system and method having an excitation source with higher spectral purity so as to possibly eliminate the need for dielectric reflective mirrors as used with pulsed ultraviolet fluorescence systems.

[0021] Another feature of the present invention is to provide a UV fluorescence detection system and method that can be operated in either continuous or pulsed modes of operation.

[0022] Yet another feature of the present invention is to provide a pulsed UV fluorescence detection system and method for further lowering minimum detectable levels or concentrations.

[0023] Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.

[0024] To achieve the foregoing objects, features, and advantages and in accordance with the purpose of the invention as embodied and broadly described herein, a UV fluorescence detection system and method is provided.

[0025] In one embodiment of the present invention, a UV fluorescence detection system is provided comprising a dielectric barrier-discharge excimer lamp, an excitation source for the dielectric barrier-discharge excimer lamp; a fluorescence chamber containing a sample gas that may also contain sulfur dioxide, a light sensitive detector is in operative association with the sample chamber; and a signal conditioning and analyzing device is provided for determining the sulfur dioxide content of the sample in the fluorescence chamber. The dielectric barrier-discharge excimer lamp contains a krypton-chloride excimer gas mixture within the lamp envelope. An excitation filter and a fluorescence filter may be used in association with the fluorescence chamber for excitation and detection enhancement.

[0026] In another embodiment, a UV fluorescence detection system is provided comprising an excimer lamp having a krypton-chloride excimer mixture, an exciting source for the excimer lamp, a sample chamber for housing a sample of gas containing sulfur dioxide and having an excitation interference filter at one location and a fluorescence filter at another location, a light sensitive detector in operative association with the fluorescence filter of the sample chamber; and analyzing devices for determining the sulfur dioxide content of the sample in the sample chamber.

[0027] A preferred UV fluorescence detection method for detecting a specific species comprising the steps of exciting an excimer gas mixture for generating an emission spectrum having a wavelength that causes a fluorescence emission, and engaging the wavelength with the specific species for generating a fluorescence emission that is characteristic of the excited species having a characteristic spectral profile specific to the species under investigation.

[0028] Another UV fluorescence detection method for detecting a specific species comprises the steps of exciting an excimer gas mixture, generating an emission spectrum from the excited excimer gas mixture having an optimal wavelength that enhances the fluorescence emission of the specific species under investigation, engaging the optimal wavelength with the specific species, generating a fluorescence emission from the specific species engaged with the optimal wavelength that is characteristic of an excited form, inducing a fluorescence emission from the specific species having a characteristic spectral profile specific to that species under investigation, and analyzing the emission from the characteristic spectral profile for evaluating the qualitative or quantitative content of species under investigation.

[0029] Additional advantages and modification will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and the illustrative examples shown and described herein. Accordingly, the departures may be made from the details without departing from the spirit or scope of the disclosed general inventive concept.

[0030] The accompanying drawings which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and together with the general description of the invention given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.

[0031] FIG. 1 is a schematic illustration of a preferred embodiment of the UV fluorescence detection system of the present invention.

[0032] FIG. 2 is an exploded view of the excimer lamp illustrated in FIG. 1.

[0033] FIG. 3 is a cross section of the excimer lamp taken along the section 3-3 as illustrated in FIG. 2.

[0034] FIG. 4 is a flow chart illustrating a preferred UV fluorescence detection method of the present invention.

[0035] FIG. 5 is a flow chart illustrating another preferred UV fluorescence detection method of the present invention.

[0036] The above general description and the following detailed description are merely illustrative of the generic invention, and additional modes, advantages, and particulars of this invention will be readily suggested to those skilled in the art without departing from the spirit and scope of the invention.

[0037] Excimer Lamp Operation

[0038] Excimer lamps generate high intensity, narrow band UV radiation through what is known as a dielectric barrier discharge and have also been referred to as silent discharge devices. Excimer lamps can have numerous configurations, but always contain two electrodes separated by at least one dielectric medium and a rare-gas or rare-gas-halogen mixture or complex. The dielectric medium most often used is a high purity quartz or fused-silica.

[0039] The operation of an excimer lamp begins when an alternating current, high-voltage field is applied to the electrodes. Since the dielectric medium will initially resist electrical current flow due to the highly resistive nature of all dielectric materials, a charge will begin to build between the electrodes. As the voltage or strength of the applied field continues to increase, a critical point known as dielectric breakdown is reached where the dielectric medium can no longer resist the increasing pressure of the applied field.

[0040] When the voltage reaches this point, electrical current will be transported through the dielectric and be transferred by a multitude of randomly distributed microscopic discharges into the excimer gas. The plasma created by these micro discharges ionizes the surrounding inert gas into an excited state. The excited rare-gas species will then transfer the loosely bound electron to the halogen molecule to form an excited dimmer or excimer complex. The excimer complex, sometimes referred to as an exciplex, is a molecular species whose induced formation does not allow it to exist in a ground state under normal conditions.

[0041] Due to the highly unstable nature of these exciplex molecules, their atoms will rapidly separate and decompose, usually within a few nanoseconds. During the process of molecular dissociation back to the ground state, their excitation energy is given up in the form of a UV photon. The wavelength of the emitted photon will depend on both the rare-gas or rare-gas-halogen species of the excimer complex and is directly related to the transitional energy lost between the excited and ground states of these molecules.

[0042] The typical emission spectrum of the KrCl* exciplex, displays a primary emission wavelength of 222 nm, although other much lower intensity wavelengths associated with similarly relevant excimer transitions are unavoidably present. The spectral distribution and level of relevant emission wavelengths has been found to be related to purity of the excimer gases, fill pressure and operating temperature.

[0043] UV Fluorescence Detector Operation

[0044] Once the photons leave the emission aperture of the lamp, they are collected by a lens, or pair of lenses made from an appropriate material for high UV transmission of the excimer wavelength. The function of the first lens is to collect and collimate expanding light rays emitted from the lamp. The second lens subsequently focuses the collimated light rays from the first lens to a focal point within the center of the fluorescence chamber and preferably directly in front of or normal to the light detector or photomultiplier axis.

[0045] Before the light enters a fluorescence chamber, it passes through a narrow band-pass interference filter. The function of this filter is to pass the desired excitation wavelength, in this case 222 nm, and reject or eliminate unwanted wavelengths from entering the chamber. Any wavelength other than the excitation wavelength can create an undesirable background that interferes with optimal fluorescence detection and particularly any wavelengths that fall within the fluorescence band of the excited or measured species.

[0046] The sample gas intended for measurement is directed to flow through the fluorescence or sample chamber. This gas may be atmospheric air or exhaust from an internal combustion engine that is drawn by vacuum pump or pressure, as might be the case for environmental monitoring or vehicle emission applications. Alternately, the sample could be exhaust from an oxidizing combustion furnace typically found in total sulfur analyzers for petroleum fuels and related products, as well as any unrelated sample that may contain SO₂, or other sulfur compounds that are converted to SO₂, for quantitative and/or qualitative analysis.

[0047] The energy from excitation light photons is absorbed by the SO₂ molecules which is then subsequently lost or degraded by various mechanisms. It can be re-emitted as radiation of the same wavelength in what is known as resonant fluorescence. Energy may also be lost in the form of heat as the net energy level decays to the lowest rotational or vibrational level within the same electronic singlet level.

[0048] The form of lost energy this detector is primarily intended to detect is most commonly referred to as just fluorescence, a phenomenon that occurs when SO₂ decays from the lowest rotational and vibrational level within the singlet levels back to the ground state. Fluorescence radiation contains less photon energy than the excitation source and is usually a broadband continuum of longer wavelengths. Fluorescence decay times are extremely fast, and for most practical purposes are considered near instantaneous.

[0049] However, such detectors additionally detect SO₂ phosphorescence, yet another form of radiated energy that takes place when a nonradiative transition first occurs as an excited singlet state molecule shifts to a corresponding triplet state before eventually decaying back to the ground state. Unlike fluorescence which has no significant persistence, phosphorescence is metastable and possesses a much lower probability of reverting back to the ground state resulting in an emission that lasts for measurably longer periods of time.

[0050] Since emission photons radiate equally in all directions, a relatively small fraction is collected by a second filter, known as the fluorescence filter. It is typically a broadband transmission filter that allows photons from fluorescence and phosphorescence emission of SO₂ to pass and possesses an ideal wavelength transmission range of approximately 250-450 nm.

[0051] The photons that pass through the fluorescence filter then impinge upon a light detector that should have reasonably high sensitivity to the fluorescence emission wavelengths. A detector commonly used for this type of application is a photomultiplier tube that utilizes a series of resistively coupled electrically charged plates, also called a dynode chain, which converts photon energy to an electrical current. Once the SO₂ emission has been converted to an electrical representation, it can be processed by numerous analog and/or digital signal conditioning electronic configurations. These may include, but are not limited to signal filtration, peak detection, integration, data acquisition, signal averaging and statistical data analysis functions. Real-time and accumulated data may also be represented by a variety of measurement display configurations.

[0052] Operation

[0053] Reference will now be made in detail to the present preferred embodiments of the invention as described in the accompanying drawings.

[0054] FIG. 1 is a schematic illustration of a preferred embodiment of the UV fluorescence detection system 10 of the present invention. The primary components of the UV fluorescence detection system 10 are an excimer lamp 100, a light collection and lensing member 200, a fluorescence chamber 300 and an electronics/analysis member 400.

[0055] FIGS. 2 and 3 illustrated the details of the excimer lamp 100. It can be appreciated by the discussions above that the embodiment of the excimer lamp 100 can take many and numerous forms. The presently preferred embodiment of the excimer lamp 100 comprises an outer electrode 1 10, an inner electrode 120 and a quartz envelope 130. The quartz envelope 130 is bounded on an interior cylindrical surface by the inner electrode 120 and on an exterior cylindrical surface by the outer electrode 130.

[0056] The excimer lamp 100 is powered by the electronics 400. An alternating current, high-voltage field is applied to the electrodes 110, 120 of the excimer lamp 100 by the electronics 400. The dielectric medium will initially resist electrical current flow due to the highly resistive nature of the dielectric materials. A charge will begin to build between the electrodes 110, 120. As the voltage or strength of the applied field continues to increase, a critical point known as dielectric breakdown is reached.

[0057] When the voltage reaches the dielectric breakdown point, electrical current will conduct through the dielectric and be transferred by a number of randomly distributed microscopic discharges into the excimer gas within the quartz envelope 130. A plasma is created by the micro-discharges, which ionize the surrounding inert gas into an excited state. The excited rare-gas species will then transfer the loosely bound electron to the halogen molecule to form an excited dimmer or excimer complex. Due to the highly unstable nature of the exciplex molecules, their atoms will rapidly separate and decompose, usually within a few nanoseconds. During the process of molecular dissociation back to the ground state, their excitation energy is given up in the form of a UV photon. The wavelength of the emitted photon will depend on either the rare-gas or rare-gas-halogen species of the excimer complex within the quartz envelope 130 and is directly related to the transitional energy lost between the excited and ground states of the molecules.

[0058] The photons will leave the lamp 100 via the emission aperture 132. The emitted photons will pass through the lensing member 200 into the fluorescence chamber 300. In the lensing member 200, the photons will pass through an aperture 202A in an aperture member 202. Then, the photons are collected by a pair of lenses 204, 206 made from an appropriate material for high UV transmission of the excimer wavelength. The function of the first lens 204 is to collect and collimate expanding light rays emitted from the lamp 100. The second lens 206 subsequently focuses the collimated light rays from the first lens 204 to a focal point or foci 312 within the fluorescence chamber 300 and preferably directly in front of or normal to the light detector or photomultiplier 450.

[0059] Before the photons or light enters the fluorescence chamber 300, the photons pass through a narrow band-pass interference filter 306. The function of the narrow band-pass interference filter 306 is to pass the desired excitation wavelength, in this case 222 nm, and reject or eliminate unwanted wavelengths from entering the fluorescence chamber 300. Any wavelength other than the excitation wavelength can create an undesirable background that interferes with optimal fluorescence detection and particularly any wavelengths that fall within the fluorescence band of the excited or measured species, such as for example sulfur dioxide.

[0060] A sample gas intended for measurement is directed to flow through the fluorescence chamber 300 by entering via an inlet 302 traveling through the chamber 310 and exiting via an outlet 304. The sample gas may be, for example, atmospheric air or exhaust from an internal combustion engine that is drawn by vacuum pump or pressure, as might be the case for environmental monitoring or vehicle emission applications. Alternately, the sample could be exhaust from an oxidizing combustion furnace typically found in total sulfur analyzers for petroleum fuels and related products, as well as any unrelated sample that may contain SO₂, or other sulfur compounds that are converted to SO₂, for quantitative and/or qualitative analysis. The energy from excitation light photons is absorbed by the species under investigation, e.g. SO₂ molecules, which are then subsequently lost or degraded by various mechanisms. The energy can be re-emitted as radiation of the same wavelength in what is known as resonant fluorescence. Energy may also be lost in the form of heat as the net energy level decays to the lowest rotational or vibrational level within the same electronic singlet level. The detector 450 primarily detects fluorescence, a phenomenon that occurs when SO₂ decays from the lowest rotational and vibrational level within the singlet levels back to the ground state. However, it can be appreciated that other types of energy can be detected using the present invention. Such detectors additionally detect SO₂ phosphorescence, yet another form of radiated energy that takes place when a nonradiative transition occurs as an excited singlet state first shifts to a corresponding triplet state before eventually decaying back to the ground state. Unlike fluorescence which has no significant persistence, phosphorescence is metastable and possesses a much lower probability of reverting back to the ground state resulting in an emission that lasts for a measurably longer time period.

[0061] Emission photons radiate equally in all directions, and thus, a relatively small fraction are collected by a fluorescence filter 308 of the fluorescence chamber 300. The fluorescence filter 308 is typically a broadband transmission filter that allows photons from fluorescence and phosphorescence emission of the species under investigation to pass and possess an ideal wavelength transmission range, such as 250-450 nm for SO₂.

[0062] The photons that pass through the fluorescence filter 308 then impinge upon a light detector 450 that should have reasonably high sensitivity to the fluorescence emission wavelengths. A detector 450 commonly used for this type of application is a photomultiplier tube which converts photon energy to an electrical current. Once the SO₂ emission has been converted to an electrical representation, it can be processed by the electronics 400. The electronics 400 can be numerous analog and/or digital signal conditioning electronic configurations. These may include, but are not limited to signal filtration, peak detection, integration, data acquisition, signal averaging and statistical data analysis functions. Real-time and accumulated data may also be represented by a variety of measurement display configurations. The preferred configuration comprises a high-voltage supply 402 to power the detector 450, an electrometer/amplifier 404, data acquisition/signal processing 406, integration/ averaging electronics 408, recorder/display 410 and a high voltage RF/AC power supply 412 to power the excimer lamp 100.

[0063] FIG. 4 is a flow chart illustrating a preferred UV fluorescence detection method of the present invention. The UV fluorescence detection method for detecting a specific species comprises the steps of exciting an excimer gas mixture, generating an emission spectrum from the excited excimer gas mixture having an optimal wavelength that enhances the fluorescence emission of the specific species under investigation, engaging the optimal wavelength with the specific species, generating an excited form of the specific species, inducing a fluorescence emission from the specific species having a characteristic spectral profile specific to that species under investigation, and analyzing the emission from the characteristic spectral profile for evaluating the content of species under investigation.

[0064] FIG. 5 is a flow chart illustrating another preferred UV fluorescence detection method of the present invention. The other preferred UV fluorescence detection method for detecting a specific species comprises the steps of exciting an excimer gas mixture that generates an emission spectrum having a wavelength that induces an emission of the specific species, engaging the wavelength with the specific species for generating a fluorescence emission that is characteristic of the excited species, analyzing the emission from the excited species under investigation.

[0065] Additional advantages and modification will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and the illustrative examples shown and described herein. Accordingly, the departures may be made from the details without departing from the spirit or scope of the disclosed general inventive concept.