WO1985000224A1

WO1985000224A1 - Quantitative analysis method by absorption spectroscopy and device for its implementation

Info

Publication number: WO1985000224A1
Application number: PCT/FR1984/000160
Authority: WO
Inventors: Jean-Pierre Moy; Alain Mole; Jean-Louis Savoyet
Original assignee: Societe Civile De Brevets J.L.S.
Priority date: 1983-06-24
Filing date: 1984-06-22
Publication date: 1985-01-17
Also published as: EP0148225A1; CA1215307A; IT8448450A0; FR2547922A1; IT1177832B; FR2547922B3

Abstract

Quantitative analysis method by absorption spectroscopy of the components of a mixture. The method comprises the sending (1) of a single light beam to the sample (3) to be analysed, the sequential selection (4) for each constituant to be analysed in the above beam, the beam having the wave-length of the characteristic line and thereafter the beam having the reference wave-length, the sequential detection (8) of the intensity of the different selected beams, the demultiplexing (10) of the signal thus obtained into signals corresponding to the intensity of the different wave-lengths of the characteristic lines and of the references, and the processing of said signals to obtain a signal function of the absorbance for each characteristic line and a signal proportional to the concentration of the various constituents. The invention is particularly applicable to milk analysis.

Description

METHOD FOR QUANTITATIVE ANALYSIS BY ABSORPTION SPECTROSCOPY AND DEVICE FOR IMPLEMENTING SAME

The present invention relates to absorption spectroscopy and, more particularly, to a quantitative analysis method using absorption spectroscopy which makes it possible to determine the concentration of the constituents of a mixture mainly of a liquid or gaseous mixture such as , for example, the lipid, carbohydrate and milk protein concentration.

In fact, absorption spectroscopy is used in the usual way for the qualitative and quantitative analysis of systems with several constituents and this technique is used, for example, to determine the concentration of the various constituents of milk despite the difficulties of implementation. resulting from the high water content and radiation diffusion properties from milk. Consequently, there are currently methods and apparatuses which make it possible to determine by absorption spectroscopy, most often by infrared absorption spectroscopy, the concentration of the constituents of a mixture, in particular. the level of lipids, carbohydrates and proteins in milk.

We know in particular an apparatus consisting of a double beam spectrometer comprising two cells, one containing the pure solvent serving as a reference medium and the other containing the sample to be analyzed. In this apparatus, for each characteristic wavelength, the difference between the energy absorbed by the sample and that absorbed by the solvent is measured so as to obtain a signal proportional to the concentration of the component to be measured. However, this device does not take into account interference between the various components. On the other hand, it requires the use of two tanks which must be identical and very thin when used for the analysis of milk, in particular which leads to complex manufacturing problems. To partially remedy these drawbacks, we have developed an apparatus using a double wavelength beam analysis system which consists in selecting for the analysis of each component, two beams of different wavelengths, one having a wavelength corresponding to the absorption peak of the component to be analyzed and the other having a similar wavelength but for which the absorption by the component to be measured is very low. In this device, the two beams pass alternately through the cell containing the sample to be analyzed and are sent to a detector to measure the difference in energy absorbed at each of the two wavelengths. This device has the disadvantage of requiring complicated optics to obtain two beams of different wavelength and identical path. On the other hand, with the two devices described above, the measurements on the various constituents are carried out separately over time without the possibility of taking into account the physical modifications liable to affect the sample.

The present invention aims to remedy the drawbacks of the systems of the prior art by providing a method of quantitative analysis of the constituents of a mixture using absorption spectroscopy as well as a device for its implementation which allow perform substantially simultaneous measurements on the various constituents, these measurements being carried out with a device comprising optics and simple mechanical means. The subject of the present invention is a method of quantitative analysis by absorption spectroscopy of the constituents of a mixture, characterized in that a single light beam is sent to the sample to be analyzed, sequentially selected for each constituent to be analyzed in the above beam, before or after, preferably before the sample, the beam at the wavelength of the characteristic line and the beam at the reference wavelength, the intensity of the different beams is detected sequentially selected, the signal thus obtained is demultiplexed into signals corresponding to the intensity at the different wavelengths characteristic lines and references and the said signals are processed to obtain a signal which is a function of the absorbance for each characteristic line and then a signal proportional to the concentration of the various constituents. With the above method, a single beam is sent to a single tank containing the sample to be analyzed, which in particular makes it possible to avoid the complex system of mirrors used to focus the beams in the dual analysis apparatus. beam in wavelength. On the other hand, the detection is carried out sequentially and not successively as in the systems of the prior art, which avoids disturbances due to thermal or other modifications affecting the constituents during the measurements.

The present invention also relates to a device for implementing the above method. This device comprises a light source intended to send through a suitable optical system a light beam on the sample to be analyzed, a means positioned on the trajectory of the beam emitted to select sequentially in this beam for each constituent of the sample to be analyzed, the beam at the wavelength of the characteristic line and the beam at the reference wavelength, a detector for sequentially detecting the optical intensity of the different beams selected, means for demultiplexing the signal from the detector into signals corresponding to the intensity at the different wavelengths of the characteristic lines and of the references and of the means for processing the different signals in order to obtain a signal which is a function of the absorbance for each characteristic line and then a signal proportional to the concentration of the various constituents . According to a preferred embodiment, the sequential selection means can be constituted by any means passing sequentially at a given frequency through the beam emitted interference filters corresponding to the wavelengths of the characteristic lines and references of the constituents of the exchange tillon to be analyzed. Thus, the selection means can be constituted by a disc or similar element carrying at its periphery the very interference wires, said disc being driven in rotation so that the filters pass sequentially at a given frequency the beam coming from the light source.

Other characteristics and advantages of the present invention will appear on reading the description of various embodiments of the present invention given by way of illustration and not limitation, this description being made with reference to the attached drawings in which:

- Figure 1 is a block diagram of a device for implementing the quantitative analysis method by absorption spectroscopy object of the present invention;

- Figure 2 is a front view of the sequential selection means used in the device of Figure 1;

- Figure 3 shows the amplifier output signal;

- Figure 4 is a block diagram of the demultiplexer and the signal processing means from the demultiplexer used in the device of Figure 1; FIG. 5 is a block diagram of a preferred embodiment of the electronic device of FIG. 4.

In the drawings, the same references designate the same elements.

FIG. 1 represents a device for implementing the analysis method according to the present invention used in particular in the case of determining the concentration of the constituents of milk. In this figure, reference 1 designates the light source. This source will be, depending on the component to be analyzed, either an infrared source or a visible light source. Reference 2 designates a lens schematizing the focusing system used to obtain the image of the light source on the detector 8. This system can be constituted by a simple spherical lens or it can be constituted in known manner by a set of mirrors. On the path F 'of the beam focused by the lens 2, is positioned a tank 3 made of a transparent material such as glass. This tank 3 contains the sample to be analyzed. According to the present invention, a means 4 for selecting in the light beam, the beam at the wavelength of the line characteristic of the constituent to be analyzed and the beam at the reference wavelength is positioned between the source 1 and the detector 8.

This means 4 is constituted, for example, by a disc rotatably mounted on a shaft 5 driven by a motor 6. As shown in more detail in Figure 2, this disc carries at its periphery a number of interference filters (six in the embodiment shown) F ₁ to F ₆ corresponding respectively to the reference wavelengths and the characteristic line of the various constituents, namely, in the present case lipids, proteins and carbohydrates of milk. More specifically, the filter F ₁ corresponds to the reference wavelength of the first component, the filter F ₂ to the wavelength of the line characteristic of the first component, the filter F ₃ to the wavelength the reference of the second constituent and so on depending on the number of constituents. In the embodiment shown in FIG. 1, the disc 4 carrying the interference filters is positioned between the tank 3 and the detector 8, however it is possible to position the disc 4 before the tank, each position having advantages and disadvantages clean. Thus, when the disc 4 is positioned between the tank 3 and the detector 8, the interference filters can be of relatively small dimensions since the light beam is thinner. However, the sample receives all of the flux and heats up. In the other case, the filters must have larger dimensions but the sample receives a lesser amount of heat. As detector 8, use is made of a known detector chosen as a function of the transmitted wavelengths and of the performance required, such as a pyroelectric detector, a lead selenide detector, etc. On the other hand, in FIG. 1, the reference 9 designates an amplifier, the reference 10 a demultiplexer, the reference 1 1 a known device for processing signals from the demultiplexer to obtain signals proportional to the concentration of the various constituents, the reference λ2 a display device. All of these devices will be described in detail below with reference to the figure.

In the device described above, the light beam is passed through a tank containing the sample to be analyzed and the transmitted beam is detected. It is also possible, without departing from the scope of the present invention, to use a device working in reflection using the technique, MIR (for "Multiple Internai

Reflexipn "). This technique is particularly advantageous in the case of the analysis of highly absorbent liquids.

Similarly, the disc carrying the interference filters can be replaced by a monochromator system switched at a given frequency, although, in this case, the mechanical system for driving the monochromator is of relatively complex construction.

The operation of the device in FIG. 1 will be explained as follows. The light source 1 emits a light beam F which first passes through the focusing system 2 which makes it possible to obtain on the detector 8 the image of the source 1. The focusing beam F 'passes through the tank 3 containing l sample to be analyzed then the transmitted beam is successively crossed by the various interference filters F ₁ to F ₆ so as to transmit sequentially to the detector only the beam at the reference wavelength or at the wavelength of the characteristic line of a constituent. The detector 8 sequentially detects the different beams selected and emits a signal as a function of the intensity of the different beams which gives the signal shown in FIG. 3 at the output of the amplifier 9.

The signal in FIG. 3 represents the optical intensity of the different beams as a function of time. More specifically, S ₁ corresponds to the signal detected during the passage of the filter F ₁ , S ₂ at the signal detected during the passage of the filter F ₂ , etc. The disc 4 being driven in rotation at constant speed, the signal at the output of the amplifier 9 is reproduced for the same sample with a period T. The signal of FIG. 3 is then processed in an electronic device such as that represented in the figure Ψ to obtain at the output signals corresponding to the concentration of the various constituents of the mixture to be analyzed.

Thus, as shown in FIG. 4, the signals at the output of the detector 8 as a function of the optical intensity are first amplified by the amplifier 9 and then sent to a demultiplexer 10 to obtain at output a signal corresponding to each line. In the embodiment shown, the demultiplexer 10 consists of six analog gates 100. to 100 ^ corresponding to the six filters F ₁ to F ₆ . The opening of the analog doors 100 ₁ to 100 ₆ is synchronized by the arrival of the corresponding filter and lasts the passage time of this filter so as to integrate into each integrator 1 10 ₁ to 110 ₆ only the signal detected during the passage of the corresponding filter, namely the signal S ₁ in the integrator 110 ₁ , the signal S- in the integrator 110 ₂ , the signal S ₃ in the integrator 110 ₃ etc. The synchronization signal is referenced I ₁ to I ₆ in the figure

Once the signal representing the intensity of the beam at the reference wavelength S ₁ , S ₃ , S ₅ and the signal representing the intensity of the beam at the wavelength of the characteristic line

S ₂ , S ₄ , S ₆ integrated, the difference between these two signals is carried out in a subtractor referenced respectively 11 1 ₁ , 11 1 ₂ , 1 1 1 ₃ . One thus obtains a signal function of the absorbance for each constituent to be analyzed. This signal is then sent to a logarithmic transformer

1 12 ₁ , 112 ₂ , 1 12 ₃ , so as to obtain an output signal proportional to the concentration of the various constituents. The signals obtained can then be sent to a display device 12 which can either be an analog display device of the type recorder or a digital display device which, after transformation, gives direct reading of the concentration.

A preferred embodiment of an electronic device similar to that briefly described in FIG. 4 will now be described with reference to FIG. 5.

The light signal at the output of the various interference filters is firstly detected by a detection diode 20 1 which transforms the photons detected into an electrical signal. This signal is sent through a preamplifier 202 to a synchronous detector 203 which also receives as input two reference signals

S ₁ and S _{2 are} narrow. These reference signals are obtained using a system of the optical barrier type 204 which outputs a signal which, after having been amplified by the amplifier 205, is sent to a transistor 206. The collector of the transistor 206 is connected through a resistor R 'to a positive voltage and its transmitter is connected through a resistor R' of the same value to ground so as to obtain at the collector and at the transmitter two signals S ₁ and S ₂ symmetrical.

The signal at the output of the synchronous detector 203, namely a signal depending on the intensity of the light beam after passing through the various interference filters, is sent through an amplifier 207 at the input of an analog-digital converter 208 with 12 binary positions in output which is controlled by a pulse S ₃ at the start of conversion coming from a line recognition system.

The line recognition system to be analyzed consists of six optical barriers 210 offset by 60 ° each. On the other hand, the disc supporting the six interference filters is split at the height of the first filter F ₁ . The light barriers 210 are excited by a square wave signal supplied by an RC oscillator 209 on a Schmitt trigger. The output signal is sent to the six light barriers 210 via six amplifiers 21 1. The six light barriers 210 are connected to the six inputs of a multiplexer 212 which is controlled by a three-position code binaries ABC. The binary code ABC which allows the addressing of the different filters can be as follows:

F, 1 0 0

F ₂ 0 1 1 F ₃ 1 1 0

F ₄ 0 0 l

F ₃ 1 0 1

F ₆ 0 1 l A BC The code can be supplied by a microprocessor or by sequential logic.

Thus the multiplexer 212 makes it possible to select the filter by means of the selection of one of the optical barriers 210. The detected square wave signal is sent from the multiplexer 212 to a first monostable 213 which remains in position during the entire time when the slit is located in the center of the chosen light barrier.

A second monostable 214 is triggered simultaneously by the output Q of the first monostable 213 and provides the pulse S ₃ for the start of conversion. Once the conversion has been carried out, an end of conversion pulse S ₄ is sent from the analog-digital converter 208 to a monostable 215 which gives an output which is sent on the input of two AND gates 216 and 217. The another input of the AND gate 216 receives the least significant bit position A. which has the value 1 when one of the beams is analyzed at the wavelength, reference wave and O when one of the beams is analyzed at the wavelength of the characteristic line according to the code used. The signal at the output of the AND gate 216 is sent as a write signal E in a first memory 219 whose inputs are connected to the outputs of the analog-digital converter 208. Therefore, the memory 219 stores the digital information corresponding to one of the beams at the reference wavelength. On the other hand, the other input of the AND gate 217 receives the binary position A via an inverter 218. The signal at the output of the AND gate 217 is sent as a write signal E in a second memory 220 whose inputs are connected to the outputs of the analog-digital converter 208. Therefore, the memory 220 stores the digital information at the output of the converter which corresponds to one of the beams at the wavelength of the characteristic line. In addition, the signal at the output of the AND gate 217 is sent through two monostables 222 and 223 which act as a delay circuit, as a write signal from a memory 224 whose role will be explained below. The outputs of the two memories 219 and 220 are connected to the inputs A and B of a processing unit 221 mounted as a subtractor. At the end of a time determined by the monostable 222 triggered by the output of the AND gate 217, an impulse supplied by the monostable 223 makes it possible to store in the memory 224 the result of the subtraction.

This result can be directed, for example, either to a digital-analog converter 225, or to a digital display device via a binary BCD (decimal binary coded) converter 226 or be stored in a memory 227 for processing. ulterior.

On the other hand, the three binary positions ABC can be sent to a decoder circuit 228 some of whose outputs are connected via amplifiers 229 to three LEDs 230 representing the type of constituents which is analyzed. The device described above operates in the following manner.

When the filter F ₁ is in the path of the light beam, the diode 201 detects a light signal corresponding to the reference line of the lipids which is transformed into an electric signal sent through the amplifier at the input of the analog-digital converter. Simultaneously the optical barrier corresponding to the filter F ₁ is selected and triggers the operation of the converter 208 which transforms the received analog signal into a digital signal. In this case, the ABC code is worth 100. From this fact the memory 219 is selected when the conversion is finished, namely when the filter F ₁ is no longer on the beam path and the digital signal at the output of the converter is sent to the memory 219. At this time the filter F ₂ is on the path of the beam. As a result, a light signal corresponding to the characteristic line of the lipids is detected. This signal after transformation is sent to the input of the converter 208 and simultaneously the optical barrier corresponding to the filter F ₂ is selected and again triggers the operation of the converter 208. In this case, the code ABC is equal to 010. Therefore, at the end of the conversion, the memory 220 is selected and stores the digital information corresponding to the characteristic line of the lipids. The digital information corresponding to the characteristic line and to the reference line of the lipids is then subtracted in the unit 221 and the result is stored in the memory 224.

During the subtraction and storage, the light signal corresponding to the filter F ₃ is received as input and the operation described is repeated with reference to the filters F ₁ and F ₂ .

The devices of Figures 4 and 5 are given by way of example, other electronic circuits can be used to make them. The method of the present invention has been described with reference to infrared absorption spectroscopy. However, the same principle can be used in photoacoustic spectroscopy. In this case, the detection is carried out using a microphone which receives the vibration of the air on the surface of the sample.

Claims

1. A method of quantitative analysis by pectroscopy by absorption of the constituents of a liquid or gaseous mixture characterized in that a single light beam is sent to the sample to be analyzed, sequentially selected for each constituent to be analyzed in the above beam, before or after the sample, the beam at the wavelength of the characteristic line and the beam at the reference wavelength, the intensity of the different beams selected is sequentially detected, demultiplexes the signal thus obtained into signals corresponding to the intensity at the different wavelengths of the characteristic lines and of the references and said signals are processed to obtain a signal which is a function of the absorbance for each characteristic line and then a signal proportional to the concentration of the various constituents.

2. A device for implementing the method according to claim 1, characterized in that it comprises a light source (i) intended to send a light beam on the sample to be analyzed through an appropriate optical system, a means (4) positioned on the path of the emitted beam to select sequentially from this beam for each constituent of the sample to be analyzed, the beam at the wavelength of the characteristic line and the beam at the reference wavelength, a detector (9) for sequentially detecting the optical intensity of the different beams selected, means (10) for demuliplexing the signal from the detector into signals corresponding to the intensity at the different wavelengths of the characteristic lines and of the references and means (1 1) for processing the various signals in order to obtain a signal which is a function of the absorbance for each characteristic line then a si general proportional to the concentration of the various constituents.

3. A device according to claim 2, characterized in that the sequential selection means (4) consists of a disc carrying at its periphery interference filters (7), said disc being rotated so that the filters (7) pass sequentially at a given frequency the beam from the light source.

4. A device according to claim 2 characterized in that the means for demultiplexing the signal coming from the detector into signals corresponding to the intensity at the different wavelengths of the characteristic lines and of the references consists of an analog-digital converter ( 208) whose operation is controlled by the output of a multiplexing system (210, 212) designating the filter (F ₁ to F ₆ ) in operation.

5. A device according to claim 4 characterized in that the multiplexing system designating the filter in operation (210, 212) comprises optical barriers (210) offset by 60 ° connected at the input of a multiplexer (212) controlled by the 'through a binary code (ABC) corresponding to the characteristic lines and references.