Method And Detector For Detecting An Analyte (Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek TNO)

Method And Detector For Detecting An Analyte (Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek TNO)
United States Patent Application 20160116414
DAY; James Peter Robert ;   et al.   April 28, 2016
Applicant: Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek TNO
Abstract
The present disclosure concerns a method and detector (10) for detecting an analyte (1) in a sample volume (2), such as nitrosamine in an amine solvent. The method comprises measuring a resonance Raman spectrum (I1) with a first light beam (P1) matching an electronic transition of the analyte (1). The detection of the analyte is enhanced by measuring an off-resonance Raman spectrum (I2) using a second light beam (P2) that is shifted in wavelength at least 10 nm away from the electronic resonance. The resonance Raman signal (S1) of the analyte (1) is isolated from the background (Q1, Q2) by a difference analysis between the resonance and off-resonance Raman spectra (I1, I2). The method and detector (10) can be employed for detecting nitrosamine in a carbon capture process or plant (20) that employs an amine solvent.
FIELD AND BACKGROUND
[0001] The present disclosure concerns a method for detecting an analyte, a carbon capture process comprising the method, a nitrosamine detector for detecting presence of a nitrosamine analyte in an amine solvent, and a carbon capture plant comprising the detector.
[0002] Contribution of the greenhouse gas carbon dioxide (CO.sub.2) to global warming is well-documented. One promising step to limit the release of CO.sub.2 to the environment e.g. from power plants, is through post-combustion carbon capture. In one example, CO.sub.2 from flue gases is dissolved by an amine solvent in a carbon capture process or plant. The amine solvent comprises an aqueous solution of an amine such as monoethanolamine ("MEA"), to form carbonate and carbamate ions. Unfortunately, a by-product of this approach can be the formation in the amine solvent of nitrosamines such as N-nitrosodiethanolamine ("NDELA"), e.g. from a reaction of amines with nitrogen oxide compounds (NOx) that may be present in the flue gas. Such nitrosamines are considered to be carcinogenic even at very low concentrations, e.g. a few parts per million (ppm). Therefore, for carbon capture processes using amine solvents to become more acceptable, there is a need to deal with the possible nitrosamine contamination.
[0003] For example, WO2013/023919 describes a method and device for purification of a nitrosamine-contaminated product from a process plant wherein the contaminated product is treated with UV radiation from a UV light source such that nitrosamines are destroyed. There remains a desire for an improved method and/or detector for detecting the presence and/or concentration of nitrosamines in the amine solvent, e.g. to determine whether a treatment is efficient or even necessary at all. More in general there is a need for a detection method having improved specificity and sensitivity to a selected analyte which method can be implemented inline for continuous monitoring of a process flow such as carbon capture.
SUMMARY
[0004] A first aspect of the present disclosure provides a method for detecting an analyte in a sample volume, the analyte having an electronic transition, the method comprising directing a first light beam having a first wavelength into the sample volume, wherein the first wavelength matches the electronic transition of the analyte for generating a resonance Raman signal of the analyte; measuring a first Raman spectrum from the sample volume, the first Raman spectrum comprising the resonance Raman signal of the analyte and a first Raman background generated by the first light beam interacting in the sample volume; directing a second light beam having a second wavelength into the sample volume, wherein the second wavelength is shifted with respect to the first wavelength away from the electronic transition for generating an off-resonance Raman signal of the analyte, wherein the off-resonance Raman signal is lower than the resonance Raman signal; measuring a second Raman spectrum from the sample volume, the second Raman spectrum comprising the off-resonance Raman signal and a second Raman background generated by the second light beam interacting in the sample volume; and calculating the resonance Raman signal of the analyte from a difference analysis between the first and second Raman spectra.
[0005] By measuring a resonant Raman spectrum using a first pump wavelength that is specifically tuned to an electronic transition or resonance of the analyte to be detected, the Raman signal of the analyte is enhanced compared to any off-resonance background Raman signals. This can provide a detection method having improved specificity for the selected analyte. Furthermore, by using a second pump wavelength that is tuned away from the electronic resonance of the analyte, a reference or off-resonance Raman spectrum is obtained wherein the Raman signal of the analyte is not enhanced or less enhanced, while the non-resonant background Raman spectrum is affected less or not at all by the change in pump wavelength. By a difference analysis between the resonant and reference Raman spectra, the resonant Raman signal of the analyte, measured predominantly in the resonant Raman spectrum, can be enhanced or isolated while a contribution of the non-resonant background of the solvent and other solutes in the Raman spectrum can be removed or compensated. By removing or compensating the non-resonant background, a higher signal over background ratio is obtained providing an improved detection sensitivity of the resonant signal, i.e. the selected analyte. Thus, by using multiple light beams having different wavelengths for pumping resonant and non-resonant transitions of the analyte, and comparing the resulting different Raman spectra, a detection method having improved specificity and sensitivity is provided.
[0006] Furthermore, because the method measures a reference spectrum of the sample volume to subtract the background, a changing condition of the solvent can be compensated automatically. This makes the method suitable to be implemented inline for continuous monitoring of a process flow, in particular a process flow experiencing rapidly changing conditions, e.g. differing concentrations of solutes. By the dynamic subtraction of background signals, the method can be more robust in handling unexpected or unknown variations of the process flow. Furthermore, the method does not require any scanning or moving parts and can thus rapidly perform a detection of multiple spectra before changes in the process flow occur which changes may skew the reference spectra. By directing the first and second light beams to generate the resonance Raman signal and the off-resonance Raman signal at different locations in the sample volume, the said different locations can be imaged separately on the detector, e.g. by suitable projection optics. In this way interference between the signals can be minimized and the signals can be measured in parallel. By directing the light beams with distinct wavelengths simultaneously into the sample volume, the resonance and off-resonance Raman spectra can be recorded simultaneously and a relatively fast detection is provided. By passing a sample flow through the sample volume while repeatedly calculating the resonance Raman signal of the analyte, a process flow can be monitored continuously e.g. for changes in concentrations.
[0007] It is noted that US 2012/0162641 describes shifted-excitation Raman difference spectroscopy (SERDS) to eliminate a fluorescence background. While it is stated in this prior art that SERDS may be used in combination with other Raman techniques (e.g., SERS and resonance Raman spectroscopy), the specific combination of resonance and off-resonance spectra is not disclosed. Moreover, it is not disclosed how the prior art algorithm which relies on differences between off-resonance Raman signals, can be adapted to handle a combination of resonance and off-resonance spectra. Furthermore, according to the prior art method, the medium to be analysed is exposed in a time-shifted manner to first and second excitation radiation. Accordingly, the simultaneous excitation using two beams is not suggested. Also excitation at different locations in the sample is not suggested.
[0008] It is noted that US 2004/0127778 describes non-invasive spectrographic analysis of certain regions of the eye, such as the aqueous humor in the anterior chamber of the eye. In certain embodiments, for certain substances having molecules that generate resonant Raman spectra, the signal strength can be greatly enhanced for certain excitation wavelengths that, in turn, may reduce the amount of the analyte that is detectable over non-resonant Raman spectra at that wavelength. In certain other embodiments, the fluorescence spectrum for the aqueous humor can be subtracted from the Raman spectrum by stimulating the aqueous humor with a second excitation laser light pulse at a wavelength slightly different from that of the first pulse (e.g., up to two nanometers from the first pulse). However a difference analysis between on-resonance and off-resonance spectra is not disclosed. It is noted that a wavelength shift up to two nanometers is typically insufficient to move off-resonance. Furthermore, excitation at different locations in the anterior chamber of the eye is not suggested.
[0009] A second aspect of the present disclosure provides a carbon capture process comprising providing a flue gas comprising carbon dioxide; providing an amine solvent; passing the flue gas via the amine solvent for dissolving carbon dioxide from the flue gas into the amine solvent; providing the amine solvent in a sample volume; and using a Raman spectroscopy method, e.g. according to the first aspect, for detecting presence of nitrosamine as analyte in the sample volume. It is presently recognized by the inventors that a method according to the first aspect is especially suitable for detecting minute concentrations of nitrosamines that typically occur in the amine solvent of the carbon capture process, in particular due to the aforementioned tuneable specificity, high sensitivity, and inline applicability of the method as well as the specific Raman spectra of the amine solvent, dissolved carbon dioxide, and nitrosamine analyte.
[0010] A third aspect of the present disclosure provides a nitrosamine detector for detecting presence of a nitrosamine analyte in an amine solvent, the detector comprising a sample chamber, arranged for providing the amine solvent in a sample volume; a first light source, arranged for directing a first light beam having a first wavelength into the sample volume, wherein the first wavelength matches an electronic transition of the analyte for generating a resonance Raman signal of the analyte; a first photo detector, arranged for measuring a first Raman spectrum from the sample volume, the first Raman spectrum comprising the resonance Raman signal of the analyte and a first background generated by the first light beam interacting in the sample volume; a processor, arranged for receiving the first Raman spectrum from the first photo detector, calculating the resonance Raman signal in the first Raman spectrum, and determining a presence and/or concentration of the analyte from the calculated Raman signal.
[0011] The detector according to the third aspect provides an advantageous implementation of a resonance Raman process that make it particularly suitable for the said specific implementation. The inventors found that the specific band structures of the nitrosamine analyte and the amine solvent (including dissolved flue gases) allow a resonant electronic transition of nitrosamine to be specifically targeted by a first wavelength of the sensor and the resulting resonant Raman signal isolated from the amine solvent background. Furthermore by using a second pump beam at a non-resonant wavelength, e.g. according to the method of the first aspect, the resonant Raman signal can be further enhanced from a difference analysis, thus allowing lower concentrations to be measured. This makes the sensor especially suitable for detecting minute (ppm) concentrations of nitrosamines in an amine solvent.
[0012] A fourth aspect of the present disclosure provides a carbon capture plant for the capture of carbon dioxide from a flue gas by means of an amine solvent, the plant comprising a detector according to the third aspect, arranged for detecting presence of nitrosamine in the amine solvent.
[0013] By providing a sensor that is sensitive to minute traces of nitrosamine in the amine solvent of a carbon capture plant, the formation of nitrosamine can be monitored. By said monitoring, it can be ensured that the nitrosamine concentration stays below limits for preventing the carcinogenic substance from causing harm. By providing continuous and inline monitoring of the nitrosamine concentration, the process flow of the carbon capture plant does not have to be interrupted. Upon detection of nitrosamine concentration above a pre-set limit, appropriate measures can be taken for preventing contact with the nitrosamine, preventing escape of the nitrosamine, preventing the further creation of nitrosamine, and/or lowering the nitrosamine concentration. In one example, process conditions such as the acidity of the amine solvent is adjusted for lowering a reaction rate leading to the creation of nitrosamine. In another or further example, UV light is used for breaking down nitrosamine, e.g. as described in WO2013/023919.
[0014] It is noted that US 2004/0234958 describes methods for detecting analytes such as explosives and drugs. It is described that sensitivity of Raman scattering may be improved by surface enhanced Raman scattering (SERS). Combining SERS and resonance Raman scattering to give surface enhanced resonance Raman scattering (SERRS), provides more sensitivity. SERRS detection may be conducted between about 300 nm-1100 nm. While the selection and tuning of an appropriate light source, is within the capabilities of one of ordinary skill in the art, the prior art does not disclose or suggest to provide a light source with a resonance wavelength that matches the electronic transition of a nitrosamine analyte in an amine solvent. Instead the prior art teaches to adapt the analyte by derivatisation with a chromophore having a resonance matching the laser chosen for the SERRS analysis. However, introducing pollutants by mixing the sample with a reagent is not desirable in a carbon capture plant.
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