Optics: measuring and testing – By dispersed light spectroscopy
Reexamination Certificate
1999-12-03
2003-03-18
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
C250S341800
Reexamination Certificate
active
06535283
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for spectroscopic analysis of a fluid medium by attenuated reflection, preferably by internal reflection spectroscopy and especially by attenuated total reflection (ATR).
The invention relates particularly to a novel apparatus for continuous reaction monitoring, for example for in-situ or on-line reaction monitoring in the chemical industry.
2. Discussion of the Background
Optical analytical methods such as transmission spectroscopy have hitherto only been used to a very limited extent for the continuous monitoring of reactions on an industrial scale. Owing to the high concentrations arising in manufacturing processes, coupled with large extinction coefficients in some cases, the pathlengths of the measuring cells would have to be on the order of 1 micrometer to obtain useful absorption spectra. It is therefore necessary to take samples and to prepare them, for example by dilution, for a measurement in the laboratory. But this process of sample preparation may alter the chemical equilibrium of the sample, so that lab results are not necessarily applicable to conditions in the reactor.
It is known that these transmission spectroscopy problems can be avoided by conducting measurements utilizing the well known optical phenomenon of the total reflection of light. When light traveling within a first medium having a refractive index n
1
impinges upon a boundary between that medium and a medium of lower refractive index n
2
, it is totally reflected, i.e., does not pass into the second medium, when the sine of the angle of incidence &thgr; is greater than the ratio of the refractive index of the second medium to the refractive index of the first medium (sin &thgr;>n
2
1
). Although the reflection is referred to as total, the light, owing to its wave nature, does penetrate a short distance into the second medium. The depth of penetration is usually on the order of the wavelength of the light. If the light does not interact with the second medium, then the coefficient of reflection, i.e., the ratio of the intensity of the reflected light to the intensity of the incident light, is 1 and the reflection is indeed “total”. If, however, a portion of the light which penetrates into the second medium (the so-called evanescent wave) is absorbed or scattered therein, this results in a reduced coefficient of reflection and the effect is known as “attenuated total reflection”. Computing the negative decadic logarithm of the degree of transmission, i.e., the reciprocal of the coefficient of reflection R, gives the quantity of decadic extinction customary in absorption spectroscopy, which is usually known as the absorbance A:
A
=
-
log
(
1
R
)
Methods and apparatuses utilizing this simple relation so as to carry out absorption measurements by attenuated total reflection in chemical analysis are known. European Patent Application EP-A-0 206 433, for example, describes an ATR probe for measuring the concentration of a light absorbing substance in a fluid medium. An optical fiber is used to couple light at a certain angle into an ATR prism where it is totally reflected one or more times at a boundary between the prism and the medium to be analyzed. The reflected light emerges from the prism via a second optical fiber which transmits the light to two detectors via a bandpass filter each. One of the filters has a transmission wavelength at which no absorption is expected in the medium, and is used as reference signal, while the other filter has a transmission wavelength at which absorption does take place in the medium. The concentration measurement is effected by comparing the measured intensity ratio with calibration measurements carried out on solutions of known concentrations.
European Patent Application EP-A-0 221 011 discloses a method for analyzing dye solutions by attenuated total reflection. This reference also describes a probelike apparatus for spectroscopic analysis of a fluid medium by attenuated total reflection. It comprises a prism which is mounted in a holder and which shares with the medium to be analyzed one or more boundaries at which incident light is totally reflected and then transmitted to a detection unit. The reference proposes various applications in the chemical industry, especially in dye manufacture.
However, existing processes and apparatuses have disadvantages. For example, absorption spectra obtainable according to the prior art depend not only on the absorption coefficient of the sample, but also on its refractive index, which may vary, for example owing to temperature changes.
It is also well known that absorption spectra obtained via ATR measurements exhibit a bathochromic shift, i.e., a shift to longer wavelengths, compared with transmission spectra. This shift is due to the fact that the refractive index n of the absorbing medium to be analyzed and hence the depth of penetration of the evanescent light is wavelength-dependent (Harrick, J. Opt. Soc. Am. 55, 851-857, 1965). Accordingly, a single determination of the coefficient of reflection of the totally reflected light is not sufficient for accurate sample analysis.
From German Patent 12 69 816 C2 a device for measuring attenuated total reflection is known comprising a goniometer which allows for changing the angle of incidence of a single light beam. The time consuming mechanical adjustment of the angle of incidence does not allow for continuously monitoring chemical reactions.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus for continuous spectroscopic analysis of fluid mediums which permits precise and economical in-line monitoring of industrial reaction processes. The apparatus of the invention shall be useful in particular in an aggressive environment at comparatively high temperatures.
We have found that this object is achieved by the apparatus of the accompanying main claim. The present invention accordingly provides an apparatus for spectroscopic analysis of a fluid medium by attenuated reflection, comprising first means for directing a first light beam onto a boundary or interface of the medium to be analyzed and means for measuring the intensity of the first light beam reflected at the boundary. The apparatus according to the invention further includes second means for directing a second light beam onto a boundary of the medium to be analyzed and means for measuring the intensity of the second reflected light beam, the first and second light beams differing in their respective polarization state and/or in their angle of incidence upon the boundary.Said first and second light beams are directed substantially simultaneously onto said boundary.
By “angle of incidence” is means herein, as is customary in optics, the angle between the incident light beam and the perpendicular to the boundary. By “light” is meant in the present context not just visible light. The apparatus of the invention is also useful in the IR region or in the UV region. The preferred wavelength region for using the method of the invention ranges from 200 nm to 20,000 nm.
The invention is predicated on the concept that the reflection of a light beam at the boundary between two dielectric media is describable by the classic Fresnel equations. It is found that the coefficient of reflection is dependent, inter alia, on the angle of incidence of the light and on its polarization state.
The apparatus of the invention makes it possible to carry out two different, mutually independent reflection measurements, so that it is possible to determine decoupled dispersion spectra n(&lgr;) and absorption spectra k(&lgr;) of the medium. These spectra, unlike those produced by conventional ATR spectroscopy, are not bathochromically shifted, since the influence of the different depths of penetration of the evanescent light on the measured reflection spectra can be corrected.
Unlike the apparatuses known from EP-A-0 206 433 and EP-A-0 221 011, the apparatus of the invention is not restricted to
Beuermann Thomas
Ettmüller Jürgen
Heffels Camiel
Rädle Matthias
Rennig Alfred
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