Analysis apparatus

Details Analysis apparatus Analysis apparatus

2001-09-26

2004-07-13

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S345000

Reexamination Certificate

active

06762410

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an analyzer for determining the concentration of one or more substances in a mixture by measuring the concentration-dependent molecule-specific extinction.
BACKGROUND OF THE INVENTION
Nondispersive photometers for determining the concentration of a substance in a mixture are widely known and used for a great variety of measuring tasks. Thus, commercial medical measuring instruments for determining CO
2
content in tidal air, so-called capnometers, are based on this principle. These devices evaluate the attenuation of introduced infrared radiation at the wavelength of 4.26 micrometers characteristic of CO
2
according to Lambert-Beer's law
I=I
0
exp[−
kCL]
where
I: Detected intensity
I
0
: Irradiated intensity
k: Specific extinction coefficient
C: Concentration
L: Optical path length
as a measure of the CO
2
concentration present in the sample.
In the simplest form, nondispersive photometers work according to a single-beam method (See EP 0 794 423 A1).
IR radiation having an intensity assumed to be constant is passed from a radiation source through the volume penetrated by the sample under testing and measured therebehind for its intensity. The measurement is performed using an optoelectric detector as a radiation receiver. The selectivity for the substance to be detected is ensured by restricting the IR spectrum to the characteristic wavelengths) by a narrow-band filter disposed either behind the radiator or in front of the receiver.
Alternatively, unfiltered light is passed behind the absorption chamber in a closed chamber filled with the substance to be determined, in which chamber the radiant energy of the radiation source attenuated at the characteristic wavelength(s) in accordance with the concentration present in the absorption chamber is converted into thermal energy by optical excitation at exactly the characteristic wavelength(s) and detected as pressure (optopneumatic detector).
In order to obtain the signal-to-noise ratio required for the necessary measuring accuracy, periodic modulation of the signal is imperative. This is classically effected by the use of rotating beam chopping disks, so-called choppers. Since these are mechanically moving parts, this solution has inherent disadvantages with respect to minimum attainable size, interference susceptibility due to the action of external force, and perturbing effects such as vibrations or sound caused by the rotation. In modern devices one therefore uses compact, thermal thin-film or thick-film radiation sources which are operated with clocked current and thus emit periodically modulated radiation themselves.
The single-beam method with one radiation source and one receiver is seldom employed. This is because temperature and intensity fluctuations and aging phenomena of the radiation source, optical elements and receiver lead to strong drift of the output signal. In order to compensate for these effects, one usually employs a double-beam method which uses a second radiation path not influenced by the substance to be measured as a reference. The signals of the first (measuring) and second (reference) radiation paths are ratioed. The ratio is used for determining the concentration.
Double-beam methods by which the radiation from the radiation source is split into measuring and reference radiation paths to two receivers can be realized with optical devices differing in the number of radiation sources, cells and/or receivers. A device with two radiation sources, one cell and two receivers is described in U.S. Pat. No. 3,734,631. Compensation of thermal and aging effects in the cell is intrinsic to the method. Moreover, considerable measuring and automatic-control effort is necessary for keeping the radiant power of the two radiation sources constant, for instance by using two additional receivers for measuring the radiant power emitted by the radiation sources before the cell is traversed. A simplification of this assembly in that both radiation paths are passed onto one receiver is described in U.S. Pat. No. 4,899,053. However, no stabilization of the intensities of the two radiation sources is provided.
Intensity fluctuations due to temperature fluctuations or signs of aging of the radiation source can be compensated intrinsically if both radiation paths, i.e. the reference and measuring radiation paths, are operated from the same radiation source. For this purpose one must perform beam splitting which is typically realized by prisms or semitransparent, partially dichroic mirrors in front of or behind the absorption chamber (See EP 0 834 732 A2).
However, such optical components reduce intensity, which reduces the signal-to-noise ratio and thus worsens the lower detection limit. Furthermore, the spectral properties of such elements can change in the course of time through deposits or attack by aggressive media, which can lead to a shift of the intensity ratio between measuring and reference paths.
Advantageous beam splitting via imaging mirrors within the sample chamber doing without intensity-attenuating elements is described in DE 44 37 188 C2. The central requirement for the reference path is that its intensity not be influenced, or influenced substantially less than the measuring path, by concentration changes in the substance to be measured. For this purpose the reference path is guided almost completely through a transparent block of calcium fluoride in the interior of which there is no attenuation of light by the substance to be measured. However, such a block can cloud in the course of time through attack by aggressive media. More-over, exact adjustment of the mirrors and thus an elaborate adjusting device is necessary. Also, the adjustable mirrors form gaps and similar cavities which delay the exchange of the substance to be measured and thus lead to memory effects.
According to EP 0 780 681 A2, the reference beam traverses a reference cell filled with reference gas, but this involves the above-described disadvantages of prismatic beam splitting. Furthermore, it does not permit changes in the optical properties of the measuring cell to be detected. The miniaturization of such assemblies is limited.
Alternatively, measuring and reference beams are both guided through the measuring cell but measured in different wave ranges. The reference beam is either measured in such broadband fashion that the intensity change through extinction at the characteristic wavelengths of the substance to be measured is irrelevant, or it is measured in narrow-fashion band like the measuring beam but at another wavelength. The disadvantage of the first method is that a change in spectral distribution of the radiation source due to temperature fluctuations or signs of aging will generally influence measuring and reference signals differently. The disadvantage of the second method is the uncertainty about the nonoccurrence of absorption at the reference wavelength due to unknown substances. This is dangerous specifically in the case of ambient air monitoring for toxic gases since absorption at the reference wavelength leads to a reduction of sensitivity in the measuring path.
According to U.S. Pat. No. 4,281,248 the radiation of an IR radiation source is supplied to optopneumatic detectors with a chopper alternatively via a reference radiation path and a measuring radiation path. The gas to be measured flows through a long cell in the measuring radiation path and then through a short cell in the reference radiation path.
According to U.S. Pat. No. 5,876,674 the radiation of a radiation source is split into two radiation paths and the gas to be measured guided through an absorption chamber having in each radiation path two optical elements formed as aligned glass rods each at different distance so that the optical path length in the absorption chamber is greater between one pair of optical elements than with the other pair.
SUMMARY OF THE INVENTION
The object of the invention is to provide an analyzer for determining concentration by transmission measurement

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