DE19526943C2 - Multiple reflection device for generating and measuring conventional signals and saturation signals of fluorescence and scattering - Google Patents

Description

The invention relates to a multiple reflection device for generating and measuring conventional signals and saturation signals according to fluorescence and scattering the preamble of claim 1, as known from DE 43 37 227 A1.

Methods are described in DD 301863 A7 and DE 43 37 227 A1, using the Fluo Rescence and scattering spectroscopy not only fluorescent substances, but also fluorescence to detect incapable substances in liquids or the absorption coefficient with a significantly higher sensitivity than classic absorption spectrometry to determine. It is proposed the conventional and the saturation fluorescence to measure or scatter (90 ° measurement geometry). These two signals arise when the exciting radiation on the one hand short distances (conventional fluorescence and scatter signals CF and CS) and, on the other hand, due to multiple reflection, long distances (multipath Saturation fluorescence and scattering (MPSF and MPSS) in the sample to be examined travels. With the MPSF and MPSS, the exciting radiation is almost completely in the Sample absorbed. DE 43 37 227 A1 also describes multiple reflection devices with which a separate generation and measurement of the CF, CS and MPSF, MPSS signals in cuvettes for liquids should succeed.

Short paths of the radiation incident in the sample to generate the conventional fluorescence and scattering are realized in conventional fluorimeters or scatterometers by cuvettes with sufficiently small dimensions (e.g. 10 × 10 × 45 mm ³ ) (G. Schwedt, fluorimetric analysis , Verlag Chemie, Weinheim et al., 1981; Th. Förster, fluorescence of organic compounds, Vanderhoeck & Ruprecht, Göttingen, 1982).

Longer paths of the incident radiation are used in fluorescence techniques in order to Increase sensitivity. For example, in total internal reflection significant increases in sensitivity can be achieved (DE 30 36 656 A1, N.L. Vekshin, Multipass cuvettes for spectrofluorimetry, Anal. Chim. Acta, Vol. 227, pp. 291-295, 1989). The multiple reflection becomes different with the help of two opposing ones large prisms reached. The stimulating radiation leaves after a few reflections again the cuvette or sample room. The cuvettes are comparatively small Dimensions. The fluorescence can thus be relatively easy with simple optical Means are measured.

Long distances have to be traveled in absorption spectroscopy, especially with gaseous media be realized in order to obtain evaluable signals (G. Baumbach, air pollution control, Springer publishing house, Berlin-Heidelberg u. a., 1990; H. D. Kronfeldt, I. Berger, in SPIE, Vol. 1780, Lens and optical system design, 1992, pp. 650-657; M. Tacke, in Lasers in the Environment - Lasers in Remote Sensing, Laser 91, Munich, pp. 11-14). The injected radiation is after Running through a defined sample thickness to measure its weakening pelt. Laser beam sources are increasingly being used. A very simple arrangement to generate a multiple reflection of the excitation radiation consists of itself  standing plane mirrors. However, in order to be able to realize a high number of reflections, special mirror arrangements and shapes are used. Are common in Distance d opposite concave mirror; on the one hand as a pair of mirrors (HERRIOTT- Arrangement) and on the other hand as a device with 3 mirrors (WHITE arrangement).

DE 41 04 316 A1 presents an internally mirrored spherical cell in which the exciting radiation is reflected back and forth several times and then coupled out again. In DE 41 24 545 A1 describes a gas absorption cell from which the injected Radiation also emerges after multiple reflections. According to DE 31 22 896 A1 long beam paths generated by the radiation with the one to be examined Liquid-filled light guide passes through and is coupled out.

Furthermore, very long paths of excitation radiation in route measurements (e.g. Control of air pollutants near roads, control of landfill gases) reali siert. Here a laser beam is aimed at a distant mirror, which is the laser photons for measuring the transmission (absorption) on an optoelectronic receiver ger reflected back. For LIDAR measurements in water and in the atmosphere (R. Reuter, R.H. Gillot (ed.), Remote Sensing of Pollution of the Sea, Proceedings of the International Colloquium, University of Oldenburg, March 31 - April 3, 1987; H. J. Kölsch, Probing the Atmosphere: Air Pollution Studies by LIDAR, dissertation, FU Berlin, 1990) are getting high energetic laser photons irradiated into the medium to be examined and the return scattered interaction photons (fluorescence, Raman scattering, elastic scattering) measured with an optoelectronic receiver.

Fluorescence and scattering methods are compared to classic absorption spectrometry characterized by a very high sensitivity. Therefore, they are well suited for the Detection of trace concentrations, which in many cases with absorption spectrometry not succeed. A special fluorescence / scattering method is that in the prior art mentioned multipath saturation fluorescence / scatter MPSF / MPSS. To the Schwä determination or absorption coefficients at the excitation wavelength, it is required to measure in 90 ° geometry.

There is no multiple reflection device with which conventional and saturation signals measured in classic, self-contained measuring cells and in open routes can. The task consists on the one hand, the interaction photons of a larger one radiating or scattering volume (e.g. classic gas measuring cell) to a smaller one Bring recipient. On the other hand, the conventional signals and the saturation signals can also be registered if the medium to be examined is expanded (Road, landfill), has aggressive properties (exhaust gas fireplace) and its flow behavior must not be disturbed by protruding measuring heads (chemical processes in Reactors).

When analyzing optically thin media (gases, liquids), usually Cuvettes or Measuring cells used by longer lengths or by multiple reflection cells larger mirror distances are characterized.  

The problem is solved in a multiple reflection device according to the Oberbe Handle of claim 1 by the feature specified in its characterizing part le. Advantageous further developments are characterized in the subclaims.

The interaction volume spanned by two opposing mirrors is fully or partially encased according to the invention with optical fibers, the change effective volume is exclusively the measuring volume and from the other Measurement volume and a separate indicator volume. The measuring volume is too investigating medium. The indicator volume can e.g. B. a scintillating or scattering Be solid. The generated in the interaction volume by the excitation radiation Fluorescence or scattering photons hit the optical fibers and call there, depending on the type of the optical waveguide, an elastic scattering (scattering quartz glass fiber) and / or Szintila tion (scintillating fiber), which is then directed to optoelectronic receivers will. It should be noted that a roughly 90 ° measuring geometry is implemented, what can be achieved with simple mechanical aids (see execution examples).

A high reflection number of the excitation radiation in the interaction volume can a mirror pair consisting of a cylinder and concave mirror can be achieved. A sol The mirror pair generates reflection curves of the excitation radiation on the specular one Areas that move helically around the entrance opening under defined conditions, see above that the excitation radiation is not decoupled early.

This mirror combination delivers even with comparatively small cuvette lengths (e.g. 1 cm) good results. Here, however, an encasing with optical fibers should not be uncommon must be necessary since small volumes per simple optical components can be easily mapped.

Classic gas measuring cells have lengths of up to approximately 1 m. In such a comparison The entire cell body or the entire measuring volume can still be of small dimensions can be encased with optical fibers with little effort. The optical fibers can be arranged in multiple layers and / or are located in a scintillation block. That has the advantage that so many fluorescence and scattering photons through the optical fibers catch ".

The measuring volumes for route measurements are much longer (some hundreds Meter). A complete encapsulation of the measuring volume would be very expensive or not at all possible. The measurement of fluorescence and scattered radiation succeeds here with the help of a partial cladding with optical fibers. This will be explained in more detail below (see also embodiment (b) with mathematical consideration):

The intensity of the fluorescence and scattered radiation decays exponentially with the path of the excitation radiation (e.g. laser) ( Fig. 1a). In the case of complete encapsulation, the fiber optic sensor integrates over the entire path x and delivers a saturation function as a sum ( Fig. 1b). In the case of only partial wrapping, e.g. B. as Lichtwellenlei terring the width a at the site immediately after coupling the excitation radiation, the exponential function is only integrated in segments; namely always at the location of the light wave conductor ring. The sum also results in a saturation function, the value range of which is smaller by a constant factor ( Fig. 1b). This factor depends on the length a of the optical waveguide ring and the mirror spacing d. Based on this, MPSF and MPSS can also be measured in 90 ° geometry for route measurements.

When measuring in aggressive media or in media whose flow behavior is not the interaction volume is disrupted by protruding measuring heads two separate and separate volumes are formed. On the one hand that's the under measuring volume searching and on the other hand a fluorescent or / and scattering indicator volume whose optical properties are known and constant quantities. Both volumi na are between the mirrors and do not interact with each other. The indicator volume is covered with optical fibers, the measuring volume is not. Similar to the routes measurement there is also the case of a partial covering with optical fibers. The Interaction signals of the measuring volume to be examined are, however, here via the Interaction of the excitation radiation with the indicator volume determined.

Favorable training courses describe options for generating and measuring CF and CS (short paths of excitation radiation in the interaction volume) and MPSF and MPSS (long paths of excitation radiation).

For the generation of MPSF, MPSS are mirrors with very high reflectivity necessary. Such mirrors can be realized with dielectric layers. The disadvantage is however, that dielectric mirrors are very high only in a limited spectral range Guarantee reflectivity. So z. B. for the UV / VIS range at least 3-4 Mirror (cylinder / concave) with different coating materials required. Several pairs of mirrors can now be arranged on a rail or a rotating disc are arranged, rail / disc and excitation beam path movable to each other are. Such a mirror holder also contains positions that are not cylindrical and Concave mirrors are occupied. At these positions, the excitation radiation is only short Paths back in the interaction volume, which leads to the generation of the CF, CS.

For applications that only need one pair of mirrors (e.g. with limited waves length ranges with regard to the excitation or with optically thick media, for which for the Generation of the MPSF, MPSS only a few reflections are necessary), the following configuration should be tion: The pair of mirrors is immobile in relation to the excitation radiation. For Generation of the MPSF, MPSS is, as usual, the excitation radiation almost parallel in the Interaction volume coupled in and the fluorescence and scattering over the one enveloping optical fiber brought to the optoelectronic receiver. For generation the CF, CS, a lens is placed in the excitation beam path, which shows the parallelism in Divergence transformed. The result is that the now divergent excitation radiation only short distances in the interaction volume or in a small interaction volume focused. The focus can be viewed through a window free of optical fibers Mapped to an optoconverter using simple optics.

For the sake of clarity, a brief mathematical review should first be made the example of fluorescence (CF, MPSF). It will be the 90 ° Considered measurement geometry, d. H. the fluorescence always becomes perpendicular to the excitation radiation measured. It is assumed that there is no noticeable weakening of the 90 ° fluo Resence in the interaction volume takes place, which is due to short distances in detection direction can be realized.  

Fig. 1a shows the behavior of fluorescence along the path of the excitation radiation. The differential fluorescence dF observed at a location x is smaller, the further the location x is from the location x = 0:

The parameter H contains the intensity of the excitation source and the fluorescence quanta yield α _F is the absorption coefficient of the fluorescent substance at the excitation wavelength, α _T is the total attenuation coefficient (sum of fluorescent and non-fluorescent substances). The product α _T x is the absorption or the extinction E. The integration of equation (1) provides:

This is the total sum fluorescence measured along path x ( Fig. 1b, Graph 1). If the path x is sufficiently small, then equation (2) can be linearized:

F (0, a) = H. α _F. a (3)

Equation (3) describes the linear, conventional fluo generated in the interval x = 0 to x = a resence.

Now the path x is divided into n intervals of width a with an equidistant distance d ( Fig. 1a). We want to know how large is the total fluorescence generated at all intervals on path x. This fluorescence is called interval fluorescence IF. The differential fluorescence dIF is generated in each interval nd - (n - 1) d with n = 1, 2, 3, ... The differential equation applies:

with x = n. d. The integration over x in the limits x = 0 to x = n. d then gives the total fluorescence generated in the intervals a of width a located along path x with distance d ( Fig. 1b, graph 2):

If the interval distance d is reduced to a = d (synonymous with n = 1, since then it is there is only a single interval of length a) equation 5 goes because of the smallness of a (linearization) in equation 3 above.

The invention will now be described with reference to exemplary embodiments shown in the figures described. Show it:

Fig 1 a, b different fluorescence curves

Fig. 2 a flow cell completely enveloped with fiber optic cable

Fig. 3 a multiple reflection cell for route measurements

Fig. 4 a multiple reflection cell for chimney measurements

(a) Flow cell:

Fig. 2 shows the design of a flow cell completely encased with fiber optic cables.

The arrangement consists of two sub-modules. First, there is a cylindrical flow measuring cell with inlet and outlet openings ( 10 ) for the measuring volume ( 11 ) to be examined (liquid or gas). On the other hand, this is a device (base plate ( 13 ) with rigid and movable elements) for receiving the measuring cell. This holding device essentially consists of movable supports for holding the optical fibers of the optical fiber ring ( 8 ) (e.g. two foldable half-shells) which enclose the cell, two opposing mirrors, namely a cylindrical mirror ( 6 ) and a concave mirror ( 12 ), with brackets ( 9 ), which bring about multiple reflection of the excitation radiation and the coupling and receiving optics with laser ( 3 ), swiveling converging lens ( 4 ), window ( 7 ), optical fiber bundle ( 5 ), spectrally selective element ( 2 ) and receiver ( 1 ) . The optical fibers are arranged along the cylindrical measuring cell or the excitation radiation. The fluorescence and scattering photons generated in the measuring volume enter the optical fibers and are noticeably scattered when using optical fibers with a sufficiently large interaction cross section. The photons generated in the optical fiber are then z. T. transported in the optical fiber direction. Direct excitation radiation does not reach the optical fibers.

In order to be able to roughly approximate a 90 ° measuring geometry, regular Distances ring-shaped diaphragms or lamellae between measuring cell and optical fiber orderly. These prevent the stamens from being at a very different angle from 90 ° fluorescence or scattering photons from the measurement volume onto the optical fibers to meet. These slats can also be used for mechanical fastening of the light wave conductor can be used (adhesive connections could possibly cause annoying adhesive Cause fluorescence).

The base plate holds a cylinder mirror, which has an opening for coupling the stim radiation (e.g. UV laser) and an opposing concave mirror. Both Mirrors are movably mounted for adjustment purposes (translation, rotation, tilting). At  Using several pairs of mirrors, the mirrors are on one perpendicular to the excitation Radiation movable bracket (rail or disc) positioned.

The optical fibers are guided on one side over the mirror, compressed and with coupled to an optoelectronic receiver. The other end of the optical fiber can be mirrored. The optical fibers are optically shielded from the outside, so that none Extraneous light can penetrate. Both non-scintillating come as optical fibers also scintillating into consideration. Non-scintillating fibers (e.g. undoped quartz fibers) conduct both fluorescence and scatter photons of the measurement volume. Between the ver sealed end of the non-scintillating fibers and a receiver is a spectral positioned element. That can e.g. B. be a filter wheel with bandpass filters and Edge filtering is occupied. Bandpass filters and edge filters are alternating on the filter wheel arranged. The elastic spread of the measuring volume is measured by means of the bandpass filter integral fluorescence using the edge filter. Alternatively, a spectrometer can also be used be used. In contrast, scintillating fibers do not conduct the fluorescent or scatter photons of the measurement volume to be examined, but the one generated by them Fiber scintillation light. As a result, a spectrally resolved measurement of the fluo Resence / scatter of the measuring volume is not possible. Fiber scintillation serves as an indicator Tor for the interaction signals generated by the measurement volume.

The following options are available to increase the intensity generated in the fibers:

• - The optical fibers are arranged in multiple layers.
• - The optical fibers are covered with an outer mirror or on the measuring side facing away from the volume.
• - The measuring volume is encased in an externally mirrored, scintillating block. In this block is again scintillating fibers for guiding light onto the receiver embedded.

The generation and measurement of the saturation signals are always by means of cylinders / concaves mirror pair performed. To generate the conventional signals, the stimulating Laser radiation with the help of a lens that can be swiveled in front of the cylinder mirror Measuring volume focused. This leads to a divergent radiation, which is necessary for multiple reflections xion and therefore unsuitable for long paths of excitation radiation. The focus is over a window free of optical fibers by means of collecting optics on an optoelectroni brought recipient. Another way to generate the conventional signals gene, consists of moving the mirrors out of the excitation beam path into a mirror-free position (e.g. using a rail). The excitation radiation then leaves due to the lack of Multiple reflection the measurement volume after a short distance, which the generation of the con conventional signals.

This is a very different way of separating conventional and saturation signals separately measure is the time-resolved measurement. The conventional signals CF, CS arise at short because of the excitation radiation in the measuring volume, the saturation signals MPSF and MPSS for long distances. Short distances are characterized by correspondingly short times; long ways through long times. That is, in a short time period starting from zero Signals are measured that are characterized by short paths of excitation radiation are: CF and CS. In a long time window, however, signals are measured that pass through Long paths of excitation radiation are characterized: MPSF and MPSS. So you can by means of a receiver arrangement consisting of a short and a long time window, the optical measured variables CF, CS and MPSF, MPSS separately by purely electronic means be determined.  

Based on the above mathematical considerations, this flow cell (almost) completely enveloped with optical fibers detects an MPSF, which results from equation 2. For long paths x of the excitation radiation due to the multiple reflections between the cylinder and concave mirror, equation 2 changes to equation 6 (see DE 43 37 227 A1 and K.-H. Mittenzwey and G. Sinn, EARSeL Advances in Remote Sensing, Vol. 4, No. 1-IX, 1995, pp. 87-95):

This is exactly the fluorescence that is shown in Fig. 1b by the saturation value of curve 1 re.

(b) Open multiple reflection for route measurements:

In contrast to the above-mentioned apparatus, Fig. 3 shows an open route as the measuring volume ( 11 ), which is partially enveloped with an optical waveguide ring ( 8 ). The concave mirror ( 12 ) is located at a greater distance from the cylinder mirror ( 6 ) on a separate base plate ( 14 ). Both units can be mounted on movable supports ( 15 ) for a rough optical adjustment. The optical waveguide ring detects the fluorescence and scatter of the measurement volume located within the optical waveguide ring in a 90 ° measuring geometry.

This optical waveguide ring is reflected in Fig. 1a in the intervals of width a. The intervals have the distance d, which corresponds to twice the mirror distance. The interval MPSIF thus measured is the saturation value which results from equation 5 and is characterized in Fig. 1b by the plateau of curve 2 :

Equation 7 also results from another consideration. The MPSF is measured when a fictitious elongated sensor detects along the entire measurement volume or the entire route length (90 °). The fiber optic ring, on the other hand, "sees" only a section of the route with the intensity MPSIF. The two intensities behave like the corresponding lengths d (length of the entire measuring volume) and a (length of the optical waveguide ring):

Equation 6 used in equation 8 gives the one measured using an optical fiber ring Saturation fluorescence in Equation 7.  

With the exception of the constant and known variable a / d, MPSIF shows the same behavior as the MPSF measured over the entire distance. If a / d is known, the MPSF can with the help of an optical fiber ring, the length of which is smaller than the route length, be be true. Such an optical waveguide ring can of course at any point in the The route.

(c) Laser measuring device for non-contact measurements on aggressive media (e.g. in a Stack):

Fig. 4 shows a multiple reflection device for contactless control on an exhaust gas fireplace (e.g. a furnace), in which the optical parts are not in contact with the exhaust gas to be examined as the measuring volume ( 11 ). The chimney wall contains two opposing protective windows ( 16 ). The radiation source (eg laser ( 3 )) with cylindrical mirror ( 6 ) and optical fiber ring ( 8 ) is located behind one window; the concave mirror ( 12 ) is positioned behind the other window. The laser radiation is coupled through an opening in the cylindrical mirror into the interaction volume spanned by the two mirrors and reaches multiple reflection. The laser shines through the cylinder mirror, optical fiber ring, the one protective window, the measuring volume to be examined ( 11 ) in the chimney, the other protective window, hits the concave mirror and is reflected back, etc. that the optical waveguide ring envelops a separate indicator volume from the measuring volume. The indicator can be a solid (e.g. quartz glass), a liquid or a gas with constant and known optical properties. This indicator volume provides fluorescence and scattered radiation that the optical waveguide ring picks up. These signals are a measure of the conventional and saturation signals generated by the measuring volume. The mathematical representation is similar to that in point (b), except that the scattering and absorption coefficients of the indicator material must also be taken into account here. The window ( 7 ) is used here to determine the optical properties of the indicator and to check it regularly.

The exemplary embodiments (a) to (c) are summarized:
In the flow cell, the interaction volume spanned by the mirror consists exclusively of the measurement volume to be examined, which is completely coated with light waveguide. In the route measurement, the interaction volume spanned by the mirrors consists exclusively of the measurement volume, which is only partially enveloped with an optical waveguide ring. In the chimney measurement, the interaction volume spanned by the mirrors consists of the measurement volume and a separate indicator volume, only this indicator volume being encased with an optical waveguide ring.

Claims (3)

1. Multiple reflection device for generating and measuring conventional signals and saturation signals of fluorescence and scatter due to short and long paths of the excitation radiation in the measurement volume to be examined with excitation source, spectral selective elements, reference channel and opposing concave mirrors for multiple reflection of the excitation radiation and with one for the Measurement of conventional signals, optical window, which is arranged directly at the point of entry of the excitation radiation, characterized in that the interaction volume spanned by the concave mirror is completely or partially encased with optical fibers, to which optoelectronic receivers are arranged, and one concave mirror is a cylindrical mirror and the other a concave mirror is, the interaction volume on the one hand only the measurement volume to be examined or on the other hand the measurement volume plus a separate, fluorescent or scattered is the indicator volume. 2. Device according to claim 1, characterized by that on a bracket movable perpendicular to the excitation radiation from several pairs Cylinder mirrors and concave mirrors arranged with different reflection properties are and that other positions are provided on the movable bracket, which are not are covered with such mirrors. 3. Device according to claim 1, characterized by that a focusing and a collimating between the radiation source and the coupling mirror of the optical element are alternately arranged to measure conventional Signals and saturation signals are brought into the excitation beam path, so that the radiation coupled into the measuring volume is divergent on the one hand and almost on the other is parallel.