DE4310866A1 - Measuring arrangement having an optical measuring probe - Google Patents

Abstract

The invention relates to a measuring arrangement having an optical measuring probe, consisting of a radiation source which emits infrared light, an optical waveguide having a refractive index nL, which receives the infrared light emitted by the radiation source, a sensitive layer of silicon having a refractive index nS, nS being smaller than nL, which layer at least partly covers the optical waveguide and which is in contact with a measuring medium, and a photodetector which registers the intensity of the infrared light channelled in the optical waveguide. The object of the invention is, in the case of such a measuring arrangement, to make possible the use of quartz glass optical waveguides and to avoid the self-absorption of silicon in the measuring region. The object is achieved, according to the invention, in that the optical waveguide receives infrared light in the near infrared region and the sensitive layer consists of completely deuterised silicon.

Description

The invention relates to a measuring arrangement with an opti measuring probe consisting of

• a) an infrared light-emitting radiation source
• b) a light guide with a refractive index n _L , which receives the infrared light emitted by the radiation source,
• c) a sensitive layer made of a silicone with a refractive index n _S , where n _{S is} smaller than n _L , which at least partially covers the light guide and which is in contact with a measuring medium,
• d) a photodetector that measures the intensity of the light guide led infrared light registered.

Such a measuring arrangement is based on the principle of the evaneszen field. Ideally, light that is transported in a light guide with the refractive index n _L is totally totally reflected at the inner phase boundary. If the light guide is completely or partially covered with an optically transparent sheath with a smaller refractive index n _S , for example covered with a molecule-specific sensitive layer, the total reflection does not take place at the phase boundary between the light guide and the sensitive layer, but the light penetrates a short distance into it sensitive layer and is reflected here. The refractive index of the sensitive layer therefore influences the light conduction. Their refractive index changes when the sensitive layer is brought into contact with a measuring medium from which one or more substances in the measuring medium diffuse into the sensitive layer. The changed properties of the guided light due to changes in the refractive index or absorption by diffusing substances are a measure of the concentration of the constituents in the cladding and thus in the measuring medium.

The sensitive layer consists of a hydrophobic substance such as Silicon, only non-polar Be get from a measuring medium  components in the sensitive layer. Water or other polar Components of the measuring medium that would interfere with the measurement cannot penetrate the sensitive layer. The unpola Ren components of the measuring medium are in the sensitive Layer enriched and can by measuring the intensity Any change in the light guided in the light guide is detected the.

Such a measuring arrangement is known from EP 0 144 713 B1. An infrared light-emitting beam is used as the light source lenquelle, from which infrared light is inserted into a light guide is coupled. The light guide can be round or one have a rectangular cross section. It can be in the more common form Optical fibers or in the form of a plate. When A number of inorganic materials are used for the light guide salt, silicon or germanium.

The light guide is at least partially sensitive Layer covered. The sensitive layer is in contact with a measuring medium, such as an aqueous solution in which the Kon concentration of individual components is to be measured. Be If the light guide is made of an optical fiber, the sensitive layer around the entire circumference of the optical fiber give. If the light guide consists of a plate, it is enough off, if only one side of the record by the sensitive Layer is covered. As a material for the sensitive Layer is called silicone among other things. That the Lichtlei The light leaving is injected into a photodetector pelt.

Another, based on the principle of the evanescent field Measuring arrangement is known from DE 40 37 431 C2. At this measuring arrangement is a waveguide in on a substrate Strip shape made of an unspecified material intended. In the waveguide is over light-guiding fibers Light coupled in. The waveguide is in on the substrate  split two branches before the end of the exit again be brought together. The exit end is with a photode tector connected. Between the substrate and the branched There is a dielectric layer attached to the waveguide interrupted at a point in the area of one of the two branches and forms a window area here. At least in the fen is a branch of the waveguide with a sensiti ven layer covered with a measuring medium, for. B. with Air in contact.

The sensitive layer takes z. B. from the air Be individual components such as CO ₂ ; this results in a phase change in the guided light in this branch of the waveguide relative to the other, unaffected branch, which can be detected by measuring the interference after merging the two branches at the exit end of the waveguide. In the interference phenomena are a measure of the concentration of the components of the measuring medium, z. B. of CO ₂ . Compounds of the heteropolysiloxane class of substances are proposed as the material for the sensitive layer. An infrared light-emitting radiation source is not mentioned.

Another measuring arrangement, which is also based on the principle of evanescent field is known from US 4,399,099. The measuring arrangement described here corresponds essentially the measuring arrangement according to EP 0 144 713 B1 mentioned at the beginning; for them, however, is a UV or visible light emitter Light source provided. As an optical measuring probe Proposed a number of different embodiments. Of the Light guides can be made of quartz glass or other best materials hen. Silicon is used as the material for the sensitive layer called rubber and other fabrics.

In the measuring arrangement of the type mentioned at the outset, Ma material for the light guide inorganic salts, silicon or  Germanium proposed, but not quartz glass. Quartz glass has a significant ge for the classic infrared radiation lower transmission than for light with shorter wavelengths gen. Therefore, in the US patent in the under whose light guide made of quartz glass are proposed, UV or Visible light emitting radiation sources are provided. Quartz glass light guides are because of their chemical and me mechanical resistance and fiber optics for cost reasons preferred from the substances mentioned.

In the publications described at the beginning is used as material proposed silicone for the sensitive layer. Silicones have better enrichment properties for unpo lare substances on polymers known as other; you will be therefore for measuring arrangements of the type mentioned as sensi tive layer preferred.

However, it has been shown that the non-polar substances that selective through a sensitive layer of hydrophobic silicone be included and enriched in the UV or of visible light have only a few absorption bands. This is especially true in the event that the initially be Written measurement arrangement for detection and concentration measurement solution of chlorinated hydrocarbons or aromatics should be det.

A first object of the invention is therefore in egg ner measuring arrangement of the type mentioned a wavelength area for the light guided through the light guide find in which quartz glass has a higher transmission than in the classic infrared range, so that light guides made of quartz glass can be used, and in which at the same time non-polar, in a sensitive layer consisting of silicone Substances, especially hydrocarbons and chlorinated Hydrocarbons, a variety of absorption bands point.  

Such a wavelength range was with the near infrared range, preferably in the wave range between approx. 1600 and 1800 nm found.

If you look at a measuring arrangement of the type mentioned Radiation sources that emit in the near infrared range quartz glass light guides can be used; in In this area there are also a large number of absorption bands expected of non-polar substances.

However, it has been shown that a sensi in this area tive layer, which - like conventional silicone - from a Koh lenstoff and hydrogen in the form of protons containing Connection exists, causes interference. Through this disturbance the achievable detection limits of the Measuring arrangement. These disturbances are based on the one hand on the photo metric noise in the wavelength range of the C-H overtone Ab sorption bands mainly between 1600 and 1800 nm, the due to the strong light absorption of C-H bonds in the polyme ren silicone is caused. Both conventional silicone and also most of the non-polar substances found in silicone let enrich, contain C-H bonds, their absorption bands against each other are often only slightly shifted; the corresponding absorption bands therefore overlap. This results in a deteriorated signal / noise ver ratio and thus a reduced detection limit.

Another disruptive effect results from the fact that in the area the absorption bands of conventional silicone a very strong dependence of the baseline on refractive index and Density fluctuations, such as swelling, occur in the silicone. On the one hand, these fluctuations can be caused by temperature fluctuations and the changes in the refractive index caused thereby dex, on the other hand through the accumulation of po materials in the silicone newly setting mixed refractive index  dex are caused. As a result, the depth of penetration of the evanescent light field in silicone enlarged or ver shrinks; the light field can therefore do more or less C-H bonds interact in silicone. The baseline he seems structured by it. Tied up the absorption bands polarized substances are formed by a structured basis line strongly disturbed and distorted, so that the spec Microscopic evaluation result in errors. The influence of temperature can be eliminated by thermostatting; this he requires a lot of effort, especially for field measurements.

A second object of the invention is therefore a to propose such silicone as a sensitive layer in which such disturbances in the near infrared range (NIR range), ins especially in the wavelength range between 1600 and 1800 nm, in practice can be excluded.

The second object is achieved in that a sensitive layer made of a fully deuterated Si licon is provided.

It is known per se from DE 36 07 301 A1, Lichtlei ter fibers with an optically transparent sheath of low to surround the refractive index, the cladding consisting of a partially or fully deuterated, silicon-free poly always exists. The optical fibers coated in this way cause less attenuation during data transmission. Transparent polymeric, partially or fully deuterated Non-silicon materials and their use for low-loss light conduction are still from DE 36 39 117 A1 known. These coated optical fibers and Fa However, sern are not in a measuring arrangement of the beginning mentioned type used. It is not known whether such buttocks can absorb polymeric ingredients from measuring media.  

With the measuring arrangement according to the invention, the good anrei securing properties of silicone for non-polar substances uses and eliminates their disadvantages. Through the full Deuteration is the chemical properties of silicone not changed. The enrichment behavior for ingredients of a measuring medium is not changed. It can light the near Infrared range, especially in the wavelength range between 1600 and 1800 nm can be used with the light guide Quartz glass can be used. The detection limits for polar sub punches are lowered and the absorption measurements are not bothered. The unwanted self-absorption in conventional Chen silicone by C-H harmonics is at complete deuterated silicones by a factor of 1.36 after longer Wel length where they move the absorption bands of C-H Substances containing bonds do not interfere.

Because of the shift in the silicon self-absorption bands the baseline does not appear in longer wavelength ranges structured; Express temperature and refractive index changes just as small shifts in the baseline, wel che z. B. by a two-wavelength evaluation in simple Way can be corrected.

The invention can in principle with all known measuring arrangement can be realized. An embodiment is particularly preferred Form in which the measuring probe consists of an optical fiber There is quartz glass with the fully deuterated sili con is completely covered. Regarding the dimensioning of light guide and silicone cladding is referred to the US patent scripture referenced.

Laser diodes, for example, are suitable as the radiation source and tungsten halogen lamps. The radiation component in the near infra red range especially between 1600 and 1800 nm can exist if selected by bandpass filters. The light guide can e.g. B. consist of a 400 micron thick quartz glass rod, the  50 µm thick coated with silicone and for example U or is loop-shaped so that it can easily be measured with a measuring dium, e.g. B. an aqueous solution of chlorinated coal water in a cuvette that can be brought into contact. All commercial detectors can be used as photodetectors with which the resulting changes in intensity can be detected in the near infrared range, e.g. B. those with a PbS semiconductor crystal.

Particularly suitable as a sensitive layer are silicones of the general formula - [OSiR′R ′ ′) _n , where R ′ and R ′ ′ are straight-chain or branched, alkyl radicals partially substituted or unsubstituted by halogen or pseudohalogen groups or partially substituted by halogen or pseudohalogen groups or represent unsubstituted aryl residues. The substituents ten R 'and R''do not have to be identical; especially before preferred substituents are CD ₃ -, CF ₃ -CD ₂ -CD ₂ - and C ₆ D ₅ groups, with D being the hydrogen isotope deuterium.

According to German Pa tentschrift 895 650 are carried out, instead of usual starting materials preferably the corresponding, full continuously deuterated connections are to be used.

According to the method described herein, first the deuterated dihydroxypolysiloxanes, such as that Dihydroxypolydimethylsiloxane:

x (CD₃) ₂SiCl₂ + (x + 1) CD₃COOD + (x + 1) CD₃OD → DO - [CD₃) ₂SiO] _x - D + 2x HCl + (x + 1) CD₃COOCD₃

Cyclic siloxanes formed are removed by distillation distant. Then the curing takes place to solid polymers (only the terminal silicon groups are advertised):

The invention is explained in more detail below with reference to figures purifies.

Are shown in

Fig. 1 a first embodiment of the measuring arrangement;

Fig. 2 shows a second embodiment of the measuring arrangement;

Fig. 3 absorption spectra of CHCl = CCl ₂ , CH ₂ = CCl ₂ and CH ₃ -C ₆ H ₅ in the presence of proton-containing silicone as a sensitive layer;

Fig. 4 absorption spectra in pure water at 14, 17, 20 and 23 ° C in the presence of proton-containing silicone as a sensitive layer;

FIG. 5 shows absorption spectra of CHCl = CCl ₂ with a Single Point with reference measurement and sample measurement in water in trichlorethylene solution, and different temperatures in the presence of protonenhaltigem silicone as a sensitive layer;

Fig. 6 intensity spectra of protonated polydimethyl siloxane and fully deuterated polydimethylsiloxane for an optical layer thickness of 5 mm;

Fig. 7 absorption spectra of CHCl = CCl ₂ in protonated and deuterated polydimethylsiloxane.

In Fig. 1, an embodiment of the measuring arrangement is Darge provides. The light from a laser diode 1 is coupled into a gradient index light guide 2 made of quartz glass. The end of the optical fiber 2 is adapted to the surface waveguide 4 of the IO component 5 via a fiber / chip coupling piece. The loop-shaped surface waveguide 4 runs planar in the IO component 5 made of BGG 31 special glass. The measuring light is guided via a second fiber / chip coupling piece and a second Gra dient index light guide 2 to a PbS detector 3 and measured there. The surface waveguide loop 4 is covered on one side with a membrane made of deuterated polydimethylsiloxane 6 , which was applied and crosslinked by "spin coating". The membrane is in contact with a measuring medium (not shown).

In Fig. 2 shows another embodiment of the measurement arrangement is shown. The light from a 20 W tungsten halogen lamp 7 is focused using two lenses 10 . It is coupled via an adapter 9 into a 12 m long light guide 11 , the central part 12 of which is helically wound. The water area is supported by a Teflon plate 13 . The light guide, which has a core diameter of 400 µm, is covered with a 50 µm thick layer of a conventional sili cons. The end of the light guide opens into a second adapter 14 and then reaches a chopper 15 . It is spectrally broken down on a stepper motor-controlled halographic grating 16 with 300 lines / mm and focused in the direction of two detectors 17 , a germanium photodiode and a PbS detector.

With the measuring arrangement shown in FIG. 2, absorption spectra of trichlorethylene, 1,1- dichlorethylene and toluene were recorded (see FIG. 3). The hydrocarbons were dissolved in pure water. The middle part 12 of the light guide coated with conventional silicone was first immersed in pure water and a reference (intensity spectrum) was recorded and stored digitally. Subsequently, the middle part 12 of the light guide was immersed in an approximately 2-liter vessel which contained the aqueous solution of the hydrocarbons and carried out the test measurement. The concentrations of the hydrocarbons in water were (in the order mentioned) 5.9 ppm, 8.6 ppm and 7.7 ppm. The container was fitted with a gas-tight lid. From Fig. 3 it can be clearly seen that in conventional, protonated silicones, the absorption spectra of the hydrocarbons are strongly disturbed by a noise in the area of the silicon absorption lines.

Fig. 4 shows the temperature dependence of the spectral base line of protonated silicone. The sample light guide was brought into contact with pure water. The measuring arrangement speaks in principle of the arrangement shown in Fig. 2. Temperature differences between reference and sample measurements were set. For this purpose, the water was tempered to different values. The reference measurement was always carried out at 11 ° C, while the test measurement was carried out at 14, 17, 20 and 23 ° C. The spectral characteristic of the baseline is caused by the silicone absorption bands. Temperature changes cause the silicone to expand, reducing the refractive index and reducing the penetration depth of the measuring light in the evanescent field. Refractive index changes due to penetrating molecules lead to similar baseline characteristics.

In FIG. 5, the absorption band of trichlorethylene (see. Fig. 3) is shown. The measurements were carried out on the measuring arrangement shown in FIG. 2 with conventional silicone as a sensitive layer. The effect of temperature changes on the baseline is shown by distortions and asymmetries in the absorption band.

Fig. 6 shows the measurement of protonated (curve (b)) and deu tered (curve (a)) polydimethoxysiloxane in transmission cuvettes. The silicones were poured and cured in cuvette segments (open on both sides, obtained by cutting off normal cuvettes) in an optical layer thickness of 5 mm. The silicone samples thus obtained with a defined layer thickness were then clamped in a cuvette holder and connected to a NIR spectrometer (cf. FIG. 2) and measured via two transmission probes. The inherent absorption of protonated polysiloxane is clearly visible. Because of the absence of silicone self-absorption bands, the baseline is not structured for fully deuterated polydimethoxysiloxane. Changes in temperature and refractive index only manifest themselves as small baseline shifts, which can be corrected by a two-wavelength evaluation.

Fig. 7 shows NIR spectra of trichlorethylene, which was enriched in a silicone block with an optical thickness of 5 mm from conventional protonated silicone [curve (b)) and deuterated silicone (curve (b)) from aqueous solution, the Adjustment of the extraction equilibrium was followed by NIR spectroscopy. While with conventional silicone negative absorption lines and an increased spectral noise in the area of the silicone absorption lines clearly appear, the baseline with deuterated silicone is undisturbed and smooth. The spectral noise is greatly reduced by the higher measurement light intensity. Therefore, in contrast to the protonated silicone matrix, the smaller absorption line of trichlorethylene at 1700 nm can be seen undisturbed.

Claims (4)

1. Measuring arrangement with an optical measuring probe, consisting of

• a) an infrared light-emitting radiation source
• b) a light guide with a refractive index n _L , which receives the infrared light emitted by the radiation source,
• c) a sensitive layer made of a silicone with a refractive index n _S , where n _{S is} smaller than n _L , which at least partially covers the light guide and which is in contact with a measuring medium,
• d) a photodetector that registers the intensity of the infrared light guided in the light guide,
  characterized in that
• e) the light guide receives infrared light from the near infrared region and
• f) the sensitive layer consists of a fully deuterated silicone.

2. Measuring arrangement according to claim 1, characterized in that the infrared light of the near infrared range waves has a length between approx. 1600 and 1800 nm. 3. Measuring arrangement according to claim l or 2, characterized in that the fully deuterated silicone is a compound of the formula - [OSiR'R '') _n , where R 'and R''straight-chain or branched, partially with halogen or pseudohalogen groups represent substituted or unsubstituted alkyl radicals or aryl radicals which are partially substituted or unsubstituted by halogen or pseudohalogen groups. 4. Measuring arrangement according to claim 1, 2 or 3 characterized net that the light guide consists of quartz glass.