WO2019228593A1

WO2019228593A1 - Method for suppressing the cross sensitivity when detecting a trace gas and/or for simultaneously measuring the concentration of a plurality of trace gases by selective optical saturation spectroscopy

Info

Publication number: WO2019228593A1
Application number: PCT/DE2019/100468
Authority: WO
Inventors: Gernot Friedrichs; Ibrahim Sadiek
Original assignee: Christian-Albrechts-Universität Zu Kiel
Priority date: 2018-05-28
Filing date: 2019-05-27
Publication date: 2019-12-05
Also published as: DE102018112717A1

Abstract

The invention relates to a method for suppressing the cross sensitivity when determining the concentration of a trace gas, the absorptions and/or absorption spectra of which are disturbed by at least one further component, and/or for simultaneously determining the concentration of trace gases in a sample by measuring absorptions or absorption spectra, in each case when irradiating the sample with light, characterized by measuring the absorptions and/or absorption spectra at different intensities of the light irradiation, with the light intensity being systematically varied over a set range, this leading to a selective optical saturation of at least one of the absorbing components.

Description

Method for suppressing cross-sensitivity in the detection of a trace gas and / or for simultaneous concentration measurement of multiple trace gases by selective optical saturation spectroscopy

The invention relates to a method

to suppress the cross sensitivity in determining the concentration of a

Trace gas, its absorptions and / or its absorption spectra of

at least one other component is disturbed,

and or

- For the simultaneous concentration determination of trace gases in a sample

Measurement of absorptions or absorption spectra

each at the light irradiation of the sample.

In the method of suppressing cross-sensitivity in detecting a

Trace gas and the simultaneous determination of concentration of trace gases in a sample according to the invention in particular by measuring absorptions or of

Absorption spectra worked on the irradiation of the sample with light of different intensity.

Absorption measurements for accurate concentration determination of trace gases find a wide variety of applications in environmental monitoring, process monitoring, process control, and so on

medical diagnostics, etc. Numerous commercial instruments are available on the market which are mostly optimized for detecting one or a few trace gases (e.g., carbon dioxide, methane, nitrogen oxides). The quantitative detection of a species succeeds by applying Lambert-Beer's law,

/ = 7 ₀ exp (- a _{0 ί} Z), which allows to determine from the attenuation (absorption) of a light beam passing through a measuring cell of length /, the concentration c, of the absorbent material /. For this purpose, the light intensity I ₀ before the sample and the light intensity / behind the sample is measured and from this the absorption coefficient a _0, = s, - c _; calculated. With a known absorption cross-section s "which is dependent on the wavelength of the detection light and substance-specific, the concentration of the species to be detected is obtained directly. A general problem of the detection of gases by means of absorption spectroscopy is that the absorption spectra

different gases can overlap more or less strongly. This results in the investigation of real samples, which usually contain several different gases, the so-called cross sensitivity, ie it is no longer possible to detect a particular gas free of interference from other gases. Modern spectrometers counter this problem by working with spectrally narrow-band light sources (eg diode lasers). This makes it possible to perform the measurements at precisely selected wavelengths, in which the disturbance by

Cross-sensitivity is as low as possible. State of the art are diode laser spectrometers in which individual absorption lines of the trace gases are measured by TDLAS (tunable diode laser absorption spectroscopy) .Sensitivity enhancement also includes noise-suppressing modulation methods (for example, wavelength modulation spectroscopy) and methods with optical resonators such as CEAS (cavity enhanced absorption spectroscopy) ) and CRDS (cavity ring-down spectroscopy), the optical resonators also acting as a measuring cell, so that very high effective absorption lengths are achieved by the sample.

An alternative to measuring individual absorption lines is to measure an extended wavelength range of the spectrum with the highest possible spectral resolution. The measurement of infrared spectra over a large wavelength range, albeit with comparatively lower spectral resolution, succeeds with the frequently used Fourier transform infrared spectroscopy (FTIR). In the case of spectrally high-resolution laser-based methods, on the other hand, one generally limits oneself to smaller spectral windows, since in particular diode lasers are limited in their tunability. By mathematical modeling of the measured absorption spectra, however, the concentrations of several trace gases can be determined simultaneously in both cases. Since the measured total absorption is the sum of the contributions of the individual species, a _{0, total} = S _Ϊ « _{O, Ϊ>} , this is also possible for partially overlapping absorption lines, and thus it is also possible in principle to influence

To consider cross sensitivities.

Even in the measurement of absorption spectra over larger wavelength ranges, however, cross sensitivities can only be reliably corrected if as far as possible all components of the gas mixture and their absorption spectra are known. Especially for highly sensitive and high-precision measurement of a particular trace gas, the cross sensitivity or the correction of the absorption contributions of other trace gases thus remains an ever recurring and difficult to quantify experimental problem that limits the broad and easy usability of absorption spectrometers for trace gas analysis.

The object of the present invention presented method is by targeted

Exploitation of optical saturation absorption spectrometer with significantly reduced

To realize cross sensitivity. The non-linear effect of optical saturation with respect to the light intensity describes the specificity of a substance at high levels

Light intensity observed decrease in absorption coefficient:

a _p is the effective absorption coefficient of the species i measured at the light intensity / with the linear absorption coefficient a _{0 i} (thus measured for / ^ 0). h _g m _{g, i} denotes the saturation intensity of the species, the exponent «for absorption lines of trace gases is typically in the range of n = 0.5 (low cell pressure) to n = 1 (high cell pressure). For a gas mixture, the measured absorption is also given here as the sum over all absorption contributions of the different trace gases, a _total = S; _·

So far, the effect of optical saturation in particular to achieve the highest spectral resolutions and the realization of high-precision frequency standards, for example. Lamb dip spectroscopy, according to the document US 5,136,261 B, as well as to simplify measured spectra and thus achieved easier spectral interpretation, eg.

Double resonance, broadband decoupling, core Overhauser effect and

Saturation transfer difference used in nuclear magnetic resonance spectroscopy (STD NMR). The latter method finds particular application in the study of protein-ligand interactions.

In the publication "Saturated-absorption cavity-ring-down spectroscopy" by G. Giusfredi et al., Appeared in Journal Physical Review Leiters, vol. No. 104, 2010, Article 110801. describes the method Saturated-CRDS for quantitative trace gas detection. The method is used to increase the sensitivity of conventional CRDS spectroscopy by exploiting optical saturation effects. In conventional CRDS embodiments, the decay time of the detection light in the resonator must be measured once without and once with absorbing species in the optical resonator in two separate measurements. In both cases, the intensity decay curve of the light in the resonator (the so-called ringdown) can be represented by a simple exponential function, the absorption of the trace gas resulting from the difference between the two decay times. Alternatively, an absorption spectrum is often recorded and the signal without absorbing species is measured approximately at an adjacent wavelength at which no species is allowed to absorb. A disadvantage of the conventional CRDS is that additional errors occur due to the difference between two measurements. The determination of both cooldowns in a single experiment is therefore advantageous for the Saturated CRDS. By measuring at very high light intensities in the resonator, in the case of the Saturated-CRDS, the absorbing species initially becomes so saturated that first the decay time of the effectively empty resonator without absorber is measured. On the other hand, at later times of the ringdown, the intensity falls to such a low value that the absorption of the trace gas also leads to the Decay behavior of the resonator contributes. The quantitative determination of the concentration of trace gas is not possible by mathematical modeling of the observed

exponential ring-down signal. In "Theory of saturated-absorption cavity ring-down:

radiocarbon dioxide detection, a case study ", Journal of the Optical of America B, G. Giusfredi, et al. , received 21 May 2015, accepted 14 July 2015 (Doc ID 241162); published 1 October 2015, the underlying theory has been described in detail and the example of the radiocarbon dating important evidence of carbon dioxide isotopologue ¹⁴ C ¹⁶ 0 ₂ the experimental implementation. Furthermore, WO 2017/055606 A1 "Method for measuring the concentration of trace gases by SCAR spectroscopy" discloses detailed details on the mathematical modeling of the ring-down signals. The cross-sensitivity considered as interference by adjacent absorption lines of other trace gases or

Carbon dioxide isotopologue is countered by a global mathematical adaptation of the ring-down curves measured over an extended wavelength range. The usual superposition of the absorption contributions is taken into account and the saturation intensities required for the modeling of the absorptions are determined in separate calibration measurements.

A method for selectively using optical saturation to suppress cross-sensitivity and to improve the quantitative determination of the composition of a gas mixture has not previously been described. Rather, conventional absorption spectrometers intentionally operate at low light intensities to avoid the saturation variation caused by optical saturation, thus ensuring the ease of application of Lambert-Beer's law. Also in the Giusfredi et al. Saturated CRDS method described, the selective saturation of one of the components present in the gas mixture is not targeted for the suppression of

Cross-sensitivity exploited. Furthermore, the method described herein allows a quantitative analysis of a gas mixture without the need to acquire absorption spectra without prior calibration of the saturation behavior of the present two or more species. The method in this preferred

Fixed frequency light source embodiment is referred to as 2S1 W (Two Species - One Wavelength Detection) detection. The targeted selective saturation of one or more components in the gas mixture with simultaneous measurement of absorption spectra, however, is referred to as 2D LASSS (2D laser absorption selective saturation spectroscopy). In addition to the spectral axis, the 2D-LASSS method also uses the

Intensity axis recorded and evaluated. With increasing intensity of the detection light, the absorption contributions of the more optically saturable components in the spectrum are successively switched off. Compared with conventional analysis methods, 2D LASSS thus an improved quantitative analysis of complex gas mixtures, since the spectrally overlapping signals can be better detected separately by the targeted saturation of individual mixture components.

The task (s) and existing problems or

Requirements with a method according to the main claim.

The inventive method for suppressing the cross sensitivity and

improved simultaneous concentration determination of trace gases in a sample by measuring absorptions or absorption spectra in the light irradiation of the sample comprises measuring the absorbance at a particular wavelength or measuring absorption spectra at different intensities of the light irradiation;

Light intensity is varied systematically over a specified range, resulting in a selective optical saturation of at least one of the absorbing components. The suppression of the cross sensitivity or the simultaneous succeeds

The determination of the concentration of trace gases is the better, the more different the degree of optical saturation of the absorbing components that is achieved (saturation contrast).

The special feature of the method according to the invention for measuring the absorption of trace gases by means of selective optical saturation lies in the fact that the absorption of the sample is specifically measured at different intensities of the light irradiation. The systematic use of different light intensities ensures that the different species in the sample become highly optically saturated. It plays a minor role, whether the light intensity is varied gradually or continuously, as long as a specific light intensity can be assigned a unique absorption signal. Highly optically saturated substances have no or almost no absorption (note from the inventors: of course it is in principle impossible to saturate the system to 100%, since this would be possible only at infinitely high light intensity - almost can also have the meaning of : highly optically saturated substances have only negligible absorption, they are optically transparent) more, they become optically transparent. Conscious saturation due to particularly intense intensity means that, in the case of cross-sensitivity, an interfering substance or, in the case of quantitative analysis of a gas mixture of one of the target substances, becomes transparent for the measurement.

By measuring absorption signals at least two intensities, the

lead to different levels of saturation of trace gas present in the mixture, it is possible with the 2S1 W method, separately detected two different species with only one wavelength, so without absorption of an absorption spectrum. Ideally, it is possible to lower the absorption of the more optically saturable component

Switch saturation intensity off or almost completely turn off while the Absorption of the more difficult to saturate component with high saturation intensity is maintained. By simple subtraction then the absorptions of both components can be determined. It is therefore crucial for the method according to the invention that a selective saturation of one of the two components succeeds, and thus a higher one

Saturation contrast is achieved. In the general case, in which the saturation intensities of the different components do not differ sufficiently from each other, the separate detection succeeds only to a limited extent. At low light intensity, however, the sum of the absorptions of all species is still measured, and preferably the absorption of the less readily saturable components is measured at high light intensity. The determination of the individual concentrations is possible in this case by modeling the total absorption measured for different light intensities.

In principle, the non-saturated and the saturated absorption spectrum can be recorded separately in a normal absorption cell. In principle, all absorption spectroscopy methods including the modulation methods are suitable for this purpose. The method according to the invention can thus be operated with all spectrometers; it requires only a correspondingly different use or application.

The method according to the invention will be described with reference to diagram 1 in a preferred

Embodiment for a gas mixture of two components, which is located in a simple absorption cell explained. As the light intensity or light output increases, the total absorption measured at the maximum of two superimposed absorption lines decreases, since the more easily optically saturable component becomes increasingly transparent. By modeling the measured absorption intensity curve, the contributions of the absorptions and thus concentrations of both species can be determined quantitatively. The absorption spectra of the pure substances and of the mixture likewise shown in diagram 1 serve only to illustrate the selected wavelength of the detection light. The actual measurement according to 2S1W method takes place at a fixed wavelength.

In comparison to the prior art of linear absorption spectroscopy, this 2S1W embodiment of the method according to the invention allows simultaneous detection of two species of the trace gases present in the sample, even when using a fixed wavelength light source, or a cross sensitivity due to a second substance suppress. Unlike methods based on line shape analysis of two partially overlapping substances, the 2S1 W measurement is successful even when two absorption lines completely overlap spectrally.

In addition to the pure variation in light intensity, to increase the saturation contrast of the gas mixture within substance-specific limits, another non-absorbing buffer gas may be added. This influences the energy transfer effects Relaxation times of the energetically excited analyte molecules by the radiation absorption and thus also the absolute values of the saturation intensities of the trace gases present in the gas mixture. If the saturation intensity is influenced in

In this way, the degree of saturation of the different species in the gas mixture can be optimized substance-specifically.

In a further preferred embodiment of the method according to the invention complete absorption spectra are measured. The result is a new type of two-dimensional absorption spectroscopy (2D-LASSS method, 2D laser absorption selective saturation spectroscopy) in which the measured absorption is represented as a function of wavelength and light intensity. The absorption spectra measured under systematically varied light intensity can be classified in terms of saturation or unsaturation and thus quantitatively assigned to different trace gases. It is important for the accuracy of the method that at certain wavelengths a selective saturation of one of the present components occurs. In this case, the information obtained by the additional measurement variable of the light intensity makes it possible to better identify and suppress cross-sensitivities as compared with the prior art

More detailed analysis of the composition of more complex mixtures of trace gases.

For the implementation of the method according to the invention provide optical resonators, as used in the known spectroscopic method CEAS and CRDS, the advantage that with them the light intensity of a spectrally narrow-band light source in

Resonator can be amplified and thus the high light intensities required can be realized even with low-power beam sources.

In a further preferred embodiment of a spectrometer based on the method according to the invention, the object of simultaneous species detection or suppression of cross sensitivity is achieved by applying the known saturated CRDS methodology. Such an optional experimental setup is, for example, in the publication by Ibrahim Sadiek and Gernot Friedrichs "Saturation Dynamics and Working Limits of

Saturated Absorption Cavity Ringdown Spectroscopy "in Journal Physical Chemistry Chemical Physics, Vol. 18, 2016, pp. 22978 to 22989 and in patents.

In the preferred embodiment of the Saturated CRDS, the rapid decay of light intensity in the optical resonator, typically observed on the ps time scale, becomes

Determination of the total absorption of the sample at different light intensities according to the 2S1W-variant. In this case, the initial light intensity is selected to be high enough to optically saturate at least one of the two (or more) trace gases contained in the gas mixture. During the decay of the light intensity in the resonator, the saturation state of the sample gradually decreases. This leads to an increased absorption and thus a accelerated decrease in light intensity. By modeling the total measured decay curve of the light intensity in the optical resonator (the so-called ring-down curve), the concentrations of different trace gases can therefore be sufficiently determined

different saturation intensity can be determined or previously unknown

Cross sensitivities are revealed. In contrast to all previously described saturated CRDS spectrometers, which aim at increased sensitivity for the quantitative detection of a single trace gas, the described preferred embodiment of the inventive method allows the quasi-simultaneous determination of the concentrations of two (or more) species with overlapping absorption lines.

Unlike in the previously cited publication "Theory of saturated-absorption cavity ring-down: radiocarbon dioxide detection, a case study", Journal of the Optical of America B, G. Giusfredi, et al. , received 21 May 2015, accepted 14 July 2015 (Doc ID 241162); published 1 October 2015 and document WO 2017/055606 A1 "Method for measuring the

The method according to the invention presented here is based on a targeted, selective optical saturation of one of the mixture components and can also be used for measurements with only one detection wavelength even without the global spectral fit procedure described therein. Thus, the method according to the invention not only takes into account the superposition of the signal contributions of different species known from the literature, but deliberately switches off the signal contributions of one of the components at specific times of the ring-down signal by selecting the initial intensity and the optional non-absorbing additional collision partner.

On the basis of two further diagrams 2 and 3, the invention in the preferred form of Saturated CRDS on the example of the simultaneous detection of two trace gases in one

Gas mixture explained.

Diagram 2 illustrates the simultaneous acquisition of the absorption spectra of two substances using the preferred embodiment of the Saturated CRDS. The task of separate substance detection is solved by the evaluation of the measured ring-down curves directly for each measurement point and in the sense of the 2S1W variant delivers the pure absorption signals of the two substances in separate measurement channels. Diagram 2 shows that measuring channel 1 (optically non-saturated measuring channel) reliably delivers the spectrum of the substance with high saturation intensity when tuning the wavelength of the detection light. Measuring channel 2 (optically saturated measuring channel), on the other hand, supplies the spectrum of the substance with low saturation intensity. Although spectra are shown to illustrate the methodology in the diagram, the detection of the two substances is also possible at a selected wavelength with a fixed-frequency light source. Diagram 3 illustrates the quantitative detection of a trace gas to be detected in the presence of cross sensitivity. In the example, the absorption is measured at a fixed wavelength, which is set at the maximum of the superimposed absorption spectrum, so that the absorption of the species to be detected is quite deliberately superimposed by the spurious absorption of the second trace gas. In contrast to a linear absorption method in which it is not possible to quantitatively determine the concentration of the trace gas to be detected by superimposing both absorptions, measuring channel 1, which here corresponds to the measurement at high intensity, provides despite the present cross-sensitivity with the more easily saturable component over a wide dynamic range the expected linear relationship between measuring signal and

Concentration of the trace gas to be detected. Thus, in the illustrated example, it is possible to quantitatively detect chloromethane although there is a pronounced cross-sensitivity with methane.

For measurements and the analysis of complex gas mixtures according to the 2D-LASSS method according to the invention, the embodiment of the Saturated-CRDS is less suitable because the decoupling important for the modeling of the measured ring-down signal

different saturation intensities, which must be achieved by selective saturation, can not be ensured. In this case, as set forth in document WO 2017/055606 A1, it is necessary to perform calibration measurements for the different species and to apply global fit spectral procedures. The preferred

The embodiment for 2D-LASSS measurements is much easier to use

Absorption cells, wherein the measurement is performed with systematically varied intensity and optional addition of another collision partner for varying the saturation intensities.

Claims

1. Procedure

to suppress the cross-sensitivity in determining the concentration of a trace gas whose absorptions and / or absorption spectra are disturbed by at least one further component,

and or

for the simultaneous determination of the concentration of trace gases in a sample by measuring absorptions or absorption spectra,

each at the light irradiation of the sample,

marked by

measuring the absorbances and / or absorption spectra at different intensities of light irradiation, wherein the light intensity is systematically varied over a predetermined range, resulting in selective optical saturation of at least one of the absorbing components.

2. The method according to claim 1,

characterized in that

the light intensity at fixed wavelength of the detection light is systematically varied over a fixed range and the concentration of trace gas or trace gases in the sample is determined by modeling the absorption-intensity curve (2S1W method).

3. The method according to claim 1,

characterized in that

the absorption spectra for the selective saturation of the different components with a systematically varied light intensity over a defined range

Detection light is measured and the composition of the gas mixture by modeling the measurement data for absorption, wavelength and light intensity is determined (2D LASSS method).

4. The method according to claim 3,

characterized in that

for recording the absorption light intensity curves, the embodiment of the

Saturated CRDS is applied.

5. The method according to any one of the preceding claims,

characterized in that

To improve the saturation contrast of the gas mixture, a further, non-absorbing gas is added, which leads to a substance-specific variation of the saturation intensities.