WO1991010895A1

WO1991010895A1 - Process and device for determining the concentration of isotopes

Info

Publication number: WO1991010895A1
Application number: PCT/EP1991/000031
Authority: WO
Inventors: Horst Preier; Hanno Wachernig
Original assignee: Mütek Gmbh
Priority date: 1990-01-10
Filing date: 1991-01-10
Publication date: 1991-07-25
Also published as: DE4000584A1

Abstract

To determine the concentration of isotopes in a sample by radiation absorption spectrometry, the damping of a radiation generated by a laser diode, whose wavelength can be varied by means of adjusting parameters, is determined as it passes through the sample. The concentration is derived from the ratio of the dampings of the absorption lines of the isotopes. The wavelength of the radiation is adjusted on a weak absorption line of the most concentrated isotope in the sample at which slight damping occurs. The intensity of the radiation at this wavelength after it passes through the sample is measured and stored. The radiation is then adjusted on a strong absorption line of the least concentrated isotope in the sample at which greater damping occurs. The intensity of the radiation at this wavelength after passing through the sample is measured and stored. The concentrations of the isotopes are derived from the ratios of the intensities.

Description

Method and device for determining the concentration of isotopes

description

The invention relates to a method for determining the concentration of isotopes according to the preamble of claim 1 and a device for carrying out the method according to the preamble of claim 13.

A method of the type mentioned is known from US 46 84 805. A laser is used here, the output radiation of which is divided into two beam paths by a beam splitter. One beam path runs through a measuring cuvette with the sample contained therein, the other runs through a second dining cuvette, which is also filled with the sample. After passing through the sample, the radiation intensities in the two beam paths are measured using detectors. The radiation we l in this case coordinated in such a way that first the absorption line of the more common isotope and then the absorption line of the less common isotope is "hit". A delay device connected downstream of a detector and a blanking circuit assigned to the other detector ensure that both signals corresponding to the absorptions can be fed to a differential amplifier at the same time. By mechanically lengthening or shortening the one measuring cell, it can be achieved that both detectors are essentially d

10 record or display the same radiation intensity. By measuring the change in length, concentration ratios or changes in the concentration ratios can thus be detected. In particular, this is about the analysis of breathing air. The known method thus exposes one

^ 5 digits mechanics ahead. In addition or as a result of this prerequisite, the measuring sensitivity or measuring accuracy ve deteriorated.

The invention is based on the object of further developing methods and devices of the type mentioned at the outset in such a way that a reliable and precise determination of the concentration is achieved in a simple manner.

This object is achieved procedurally by the features specified in the characterizing part 25 of claim 1 and in terms of the device by the features specified in the characterizing part of claim 13.

An essential point of the invention is therefore that on the one hand only a single sample (in a single measuring curve

30 te) is examined by means of a single beam path, but on the other hand the frequency is tuned in such a way that d absorption lines are of similar strength or the radiation absorption at the corresponding wavelengths is essentially the same. Preferably used as absorption lines

35 selected those whose absorptions are in a ratio between 0.5 and 1.5 to each other. This guarantees that the detector is operated in a cheap working area and that the output signals of the detector are in one and the other same circuit can be processed without switching a gain or the same.

The accuracy becomes particularly high when the entire width of the absorption lines is scanned by successively shifting the wavelength of the radiation. The resulting intensities are determined so that only their minimum values can then be used, which correspond to a maximum absorption. The intensities of the radiation next to the absorption lines are determined as a correction, stored and used for further calculation.

In order to minimize the data processing effort or the storage capacity device and to increase the working speed, it is advantageous if the wavelength is shifted in discrete steps and the step width chosen during the scanning of the absorption lines is smaller than in the other areas. As a result, the maximum absorption can be determined very precisely, without unnecessarily requiring many scanning steps for the determination of the background.

Since the wavelength of the radiation is not predetermined from the outset, but depends on various parameters (temperature, current), a particularly preferred embodiment of the invention stores a multiplicity of absorption lines in the area around at least one of the absorption lines of interest as a pattern. The wavelength of the radiation is now changed over this range and the pattern of the radiation intensity is determined after passage through the sample. A precise calibration can now be carried out by comparing the stored pattern with the determined pattern. It is important here that the "pattern" is a relative absorption curve, that is to say nothing initially needs to be known about the absolute values of the absorption.

By using the known methods for pattern recognition, for example a cross correlation between the stored and the determined pattern, it is therefore more precise and nevertheless calibration is possible in a relatively simple manner. In particular, the wavelength of the radiation can be adjusted in one detection step until the determined amplitude ratios of the absorption line correspond to one another with the amplitude ratios of the stored amplitude ratios, in order to determine the sorption lines to be measured or the corresponding setting parameters (temperature, Current) for the radiation source.

After passing through the sample, the radiation is preferably subjected to a band filtering (directly before hitting the detector), the transmission bandwidth of which encompasses the two absorption lines as closely as possible, but without changing their height. This reduces interference that leads to noise.

In a preferred embodiment of the invention, a calibration of the setting parameters (temperature, current) for the radiation source in accordance with the band filtering is carried out (if necessary additionally). In particular, this is a "coarse adjustment", since available band filters (monochromators, etalons) are very broadband measured based on the width of absorption lines.

When determining the isotope ratio from 12C02 to 13C02, e.g. for breathing gas analysis, it is advantageous if mmaann ddii <e choose absorption lines of 2295.796 cm and 2295.846 cm.

Further preferred embodiments of the invention result from the subclaims.

The invention is explained in more detail below with the aid of an exemplary embodiment, for the clarification of which the enclosed illustrations serve. Show here

Fig. 1 is a block diagram of an embodiment of the inven fertilizer, and

Fig. 2 shows the intensity curve of the detector signal as a function of the wave number after passing through a Pr

The embodiment of the invention shown in FIG

Determination of the concentration or concentration ratio of isotopes is intended for the investigation of gaseous p ben. Here, a sampling system 1 is provided to which sample bags 2 can be connected, which are filled with the gas to be tested. The gas in the sample 2 can be, for example, a patient's breathing gas. The sample bags 2 are pushed onto the nozzle, d are connected to a main line 5 via solenoid valves 3, 4. The main line 5 of the sampling system is connected on the one hand to an evacuation line 6, which (where appropriate with the interposition of a reservoir container) a pump solenoid valve 7 is connected to a vacuum pump 8. On the other hand, the main line 5 is connected via a measuring magnet valve 12 to a measuring line 11. The measuring line 11 opens into a measuring cell 14.

When the solenoid valves 3 and 4 are closed and the Pumpenma netventil 7 and the solenoid valve 12 are open, a vacuum can be generated with the help of the vacuum pump 8 in the measuring cell 14. When a desired final pressure is reached (v

-2 preferably less than 10 Torr), the pump solenoid valve

7 closed. By opening the solenoid valve 3 in the assigned sample bag 2 can be introduced into the measuring cell 14. The volume ratios and the vacuum generated with the vacuum pump 8 are selected so that a pressure of approximately 1 to 10 Torr is established in the interior of the measuring cell 14. After carrying out a measurement, the measuring cell 14 is pumped empty with the help of the vacuum pump 8, so that the contents of the second pro bag 2 can be introduced into the measuring cell 14 by opening the solenoid valve 4.

The sample application presents a problem insofar as the cuvette has a "memory effect", ie sample molecule stick to the inside of the cuvette. This memory effect is particularly problematic in that the memory effect is different for different isotopes. For example, lead H2018 is more attached to most materials than H2016. In a preferred embodiment of the invention, E directions are provided to heat and / or rinse the measuring cell 14. In a further embodiment of the invention, not shown separately in the figures, a magazine of cuvettes 14 is provided which can be connected to the measuring line 11 one after the other for the examination of samples.

The measuring cuvette 14 is located in a beam path 15 between a radiation source 16 and a broadband detector 17 which is sensitive in the infrared range. Immediately in front of the detector 17 is a narrowband radiation filter 18, the filter passband of which is shown by the front and rear edges shown in FIG. 2 9 and 10 is indicated.

The measuring cell 14, the radiation source 16 and the radiation filter 18 are preferably housed in a chamber, u to ensure that there are no interfering gases between the measuring cell 14, the radiation source 16 and the radiation filter 18. This chamber (not shown in the drawing) can be filled with a non-interfering gas or evacuated.

Fig. 2 shows the course of the intensity of the detector signal occurring at the output 19 d detector 17 when the Strahlu source 16 is tuned over a frequency range, which includes the filter pass band. Here, the Meßk vette 14 is filled with breathing gas at a pressure of about 1 to 10 torr. The intensity curve is plotted against the frequency or wave number of the radiation. This mode of application in turn corresponds to the time when the radiation source is tuned perically over a predetermined frequency range. As can be seen in Fig. 2, the filter pass band comprises a frequency range in which two absorption lines occur. The first absorption line 21 on the left in FIG. 2 is assigned to a wave number of 2295.769 cm. It comes from a weak absorption line of the more common isotope 12C02.

The second absorption line 22 shown on the right of this in FIG. 2 is assigned to a wave number of 2295.846 cm ^" and comes from a strong absorption line of the secondary isotope 13C02. The one opposite the first absorption line 21 in FIG. 2 somewhat lower decrease in intensity stems from the fact that the greater line thickness is compensated for by a smaller concentration or (depending on the concentration ratios) overcompensated.

It can be seen from the course of the intensity of the detector signal of the detector 17, which can be seen in FIG. 2, that a filter passband is provided for detecting the concentration ratio from 12C02 to 13C02, which extends beyond the range which is determined by the two wave numbers. It is important here that this filter pass band is at a point where the more common isotope 12C02 never has a weak line and the rarer isotope 13C02 never has a strong Li. No further lines of these gases or other gases occurring in the breathing air are present in the filter pass area. Instead of the specified absorption lines, other absorption lines can also be used, which have the properties just mentioned, that is to say strong absorption lines of the rarer and weak absorption lines of the more frequent isotope. It does not depend on the order of the absorption lines, but only on the fact that they are as close as possible. If there are no further absorption lines between them, the method according to the invention can be carried out particularly easily or the measurement can be carried out particularly quickly.

The filter passband shown in FIG. 2 of the narrow-band filter 18 also defines the frequency or waves length range in which the radiation source 16 should emit radiation. The filter 18 can optionally be designed as a monochromator or etalon together with an interference filter and, in addition to the selection of the two selected absorption lines 21 and 22, serves to suppress background radiation into the detector 17. This ensures a constant detector operating point. Laser modes, which are half of this pass band, do not reach the detector. The optics associated with the detector 17 and the radiation source 16 designed as a laser is not shown in FIG. 1 for reasons of simplification.

When selecting the absorption lines 21 and 22, it was taken into account that the line thickness of the first absorption line 21 never reversed to the line thickness of the second absorption line 22, like the natural isotope ratio between the isotopes 12C02 to 13C02. This ratio is approximately 100: 1. Accordingly, the ratio of the line widths is 1: In this way it is achieved that the first absorption line 21 and the second absorption line 22 are approximately the same in the intensity profile of the detector signal at the output 19 of the detector 17, so that Changes in relation to the natural isotope ratio can be recorded particularly easily and without oversteering problems.

The output 19 of the detector 17 is connected to the input of an amplifier 24, the output of which is connected to the input of a signal processing device 25. The signal processing device 25 detects the measured values for the calculation of the isotope ratio, prepares the detected data and stores them in a memory 23.

The signal processing device 25 continues to perform a fault function. For this purpose, it is connected to outputs via lines 27 with the vacuum pump 8 and the solenoid valves 3, 4, 5 and 7. In this way, automatic control of the sampling is possible. Furthermore, the signal processing device 25 is connected via an output 33 to an input 35 of a temperature controller 29, which outputs a cooling device 28, which is formed, for example, as a thermoelectric heat sink a. The temperature of the cooling device 28 is sensed by a temperature sensor 30, the output signal of which is led via a line 31 to the input 32 of the signal processing device 25. A regulation of the temperature of the cooling device 28 is thus possible via this arrangement. Of course, this can also be carried out in a different way.

The radiation source 16 is mounted on the cooling device 28 in a conductive connection. In this embodiment, the radiation source 16 is designed as a PbTe DH laser, the temperature of which, depending on the desired wavelength of the output radiation, is maintained between 20 K and 120 K during continuous operation of the laser and up to 300 K in pulsed operation. Of course, the regulation of the temperature can also be carried out via a separate control circuit, so that then only the setpoint value for the temperature is to be transmitted to the controller 29 from the signal processing device 25.

The radiation source 16 is connected to a via a line 41

Power control device 40 connected. The current control device 40 has two inputs which are connected to the signal processing device 25 via lines 38 and 39. A constant base current can be set via one input, and a superimposed periodic modulation current can be set via the other input, which are fed (together) to the radiation source 16. The strength of the basic current is between 300 mA and 2 A, the amplitude of the modulation current is around 20 mA. The ratio of modulation current to base current is chosen to be approximately 1/10. To specify the area in which the radiation source works, on the one hand the basic current and on the other hand the temperature can be set, which is to be transferred during the actual measurement during scanning. range is entered via the modulation current The modulation frequency is a few Hertz.

The concentration is determined by precisely measuring the absorption maxima and the associated base points (background), which e.g. is obtained by a polynomial approximation of the intensities of the non-absorbed radiation in the region of the absorption lines. The modulation current can also itself be modulated with a modulation frequency of a few kilohertz (in addition) in order to permit the use of derivative spectroscopy, in which the first derivative for determining the time or frequency and the second derivative for determining the concentration and then determining the concentration quotient is needed.

Preferably, the sampling of the output signals a occurs to the detector 17 in such a way that in the area of the absorption maxi mum is sampled at a higher sampling rate than in the area of the background.

When tuning the radiation frequency, a signal sequence results at the detector 17, which is assigned to the two absorption lines 21 and 22 in the manner shown in FIG. 2. The abscissa in this figure shows the wave number or the number after the start of the modulation minimum. Here, a monotonic relationship between time and frequency or wave number is assumed. An output signal is present at the output .19 of the detector 17 only when the radiation lies within the filter passband of the filter between its edges 9 and 10.

The detector lines assigned to the absorption lines 21 and 22 are further processed in the signal processing device 25 to determine the isotope ratio. From amount of the intensity drop of the radiation, the molecule concentration, ^'absorption coefficient can at bekan th from the Beer-Lambert'sehen Ge translated are calculated for both isotopes when the length of the measuring cell is known fourteenth Since d

1 device, a memory is provided in which the operations of the radiation source 16 can be stored. When the device is shuttered (after filling the measuring cell 14 with Pr gas) the radiation source 16 is initially at the temperature egg

5 represents, which led to satisfactory results when the device was last operated. After reaching this temperature, the power supply for the radiation source is also brought to the value stored during the last operation.

0 If no laser beam now appears on the detector after switching on, the temperature of the heat sink 18 of the radiation source is first increased by a fixed amount, eg. lowered by 10 K using the signal processing device 25. During cooling, the detector signal occurring at the output 19 of the detector 5 is observed.

As soon as a detector signal appears, the corresponding actual temperature is registered via the temperature sensor 30, the line 31, the temperature measurement input 32 in the signal processing device 25 and recorded as the target temperature for the temperature control process. If no output signal appears on the detector 17 during cooling, the radiation source 16 is heated starting from the last temperature mentioned at the beginning. Here too it is observed whether at the outlet 19 of the D

25 tectors 17 a signal appears.

Of course, this process of "temperature sensing" can be repeated several times until a temperature range has been investigated, within which experience has shown that a correct result must be achieved. Can be checked as a whole

If there is no radiation at the temperature range at the detector 17, there is a service case which is displayed on a corresponding display device.

35. If a radiation is registered at the output 19 of the detector 17 and is thus within the pass band of the filter, it is checked whether the two absorption lines 21 are also detected. For this the temperature of the street

according to the method described above instead of gaseous water molecules.

Related list:

34 - Temperature control line

38,

39 - Power control lines

40 - Power control device

41 - Power line

42 - Data output

43 - output unit

gear

Claims

1. A method for determining the concentration of isotopes in an egg by means of radiation absorption spectrometry, wherein the attenuation of radiation generated by a laser diode, the wavelength can be changed via setting parameters (temperature, current), as it passes through the sample, and the ratio of the attenuations in absorption lines of the Isot pen the concentration can be derived, characterized by the following steps:

- The wavelength of the radiation is set on a sc che absorption line to be measured of the more common isotope of the sample, in which there is a slight attenuation;

- The intensity of the radiation at this wavelength after D gang through the sample is measured and stored;

the radiation is adjusted to a strong absorption line of the rarer isotope of the sample to be measured, at which a greater attenuation takes place;

- The intensity of the radiation at this wavelength after passing through the sample is measured and stored;

- The concentrations of the isotopes are derived from the intensity ratios.

2. The method according to claim 1, characterized in that the absorption lines selected are those whose absorptions are in a ratio between 0.5 and 1.5 to one another.

3. The method according to any one of the preceding claims, characterized in that the entire width of the absorption lines is scanned by successive shifting (scanning) of the wavelength of the radiation, the resulting intensities are determined and only their minimum values (maximum absorption) are used further.

4. The method according to any one of the preceding claims, characterized in that the wavelength of the radiation is shifted successively and the intensities of the radiation are determined in addition to the absorption lines as correction values indicative of the background and are stored.

5. The method according to any one of claims 3 or 4, characterized in that the wavelength is shifted in discrete steps and the step width across the width of the absorption lines is set lower than in the other areas.

6. The method according to claim 5-, characterized in that the intensity of the radiation at an absorption line width of over a total width of essentially 4 ^ 11 is considered.

7. The method according to any one of the preceding claims, characterized in that storing a plurality of absorption lines in the area around mi least one of the (weak or strong) A sorption lines of interest (of the more frequent or less common isotope) as a pattern ("Atlas"), the wavelength of the radiation changes over this range, the pattern of the intensity of the radiation after passing through the sample is determined and the setting parameters (temperature, current) are calibrated on the basis of a comparison of the stored pattern with the determined pattern.

8. The method according to claim 7, characterized in that the comparison is carried out via a cross-correlation between the stored and determined pattern.

9. The method according to any one of claims 7 or 8, characterized in that the wavelength of the radiation is adjusted in a detection step until there is a match between the determined amplitude ratios of the absorption lines and the amplitude ratios of the stored absorption lines and then determined the absorption lines to be measured.

10. The method according to any one of the preceding claims, characterized in that the radiation after passing through the sample is subjected to a band filtering whose pass band closely embraces the two absorption lines.

11. The method according to claim 10, characterized in that a calibration of the setting parameters in Ubereinsti with the band filtering.

12. The method according to any one of the preceding claims, characterized in that when determining the isotope ratio of 12 to 13C02, in particular for respiratory gas analysis, the wavelength is set to absorption lines with 2295.796 cm and 2295.846 cm e.

13. Device for determining the concentration of isotopes i of a sample by means of radiation absorption spectrometry, with a laser diode (16) whose radiation wavelength is adjustable, with a controllable cooling device (28) for cooling the laser diode (16), with a controllable current control device (40) Supply of current to the laser diode (16), with e detector device (17) for converting radiation from the laser diode (16) after passing through the sample, with a signal processing device (25) connected to the current control device (40) and the detector device (17 ) is characterized in that the signal processing device (25) includes memory devices (23) and is designed such that the wavelength of the radiation can be set to a weak absorption line to be measured of the more common isotope in the sample, at which a lower one Attenuation occurs after the intensity of the radiation at this wavelength Throughout the sample is measurable and storable that the radiation can be adjusted to a strong absorption line of the rare isotope of the sample to be measured, in which a greater attenuation takes place that the intensity of the radiation at this wavelength after passage through the sample is measurable and safe, and that the K concentrations of the isotopes can be derived from the intensity ratios.

14. The apparatus according to claim 13, characterized by a narrow-band filter (18), in particular a monochrome gate or an etalon, which is substantially immediately before de Detector device (17) is arranged.

15. Device according to one of claims 13 or 14, characterized in that the laser diode (16) is a double heterostructure laser diode, in particular a lead telluride diode.

16. Use of a device according to one of claims 13 to 15 for breathing gas analysis.