A method for measuring the concentration of carbon dioxide in a medium

Abstract: A sensor system for determining the pH or carbon dioxide concentration of a liquid medium comprising, in combination, a pH-insensitive fluorescent indicator and a pH-sensitive fluorescent indicator which act in concert or a single fluorescence indicator which emits fluorescent signals of different wavelengths in different carriers, which system(s) produce diverging signals, the ratio of which provides an accurate and stable determination of the parameter being measured. A method for determining pH and CO₂ concentration is also disclosed.

Description: This invention relates to a method for measuring the concentration of carbon dioxide in a medium. This application is divided out from European patent application 0283116 (88300948.2). The measurement of desired parameters in various media, particularly in biological systems, is frequently required. For example, the measurement in blood of pH levels and concentration of gases, particularly oxygen and carbon dioxide, is important during surgical procedures, post-operatively, and during hospitalisation under intensive care and many devices for the measurement of said physiological parameters have been suggested in the art. U.S. Patent No. 4,003,707, Lubbers et al, and its reissue patent Re 31879, disclose a method and an arrangement for measuring the concentration of gases and the pH value of a sample, e.g. blood, involving the use of a fluorescent indicator at the end of a light-conducting cable which is sealingly covered by or embedded in a selectively permeable diffusion membrane. The radiation transmitted to and emitted from the indicator must be passed through various filtering elements and light elements, including reflectors, beam splitters and amplifiers before any meaningful measurements can be made. U.S. Patent No. 4,041,932, Fostick, discloses a method whereby blood constituents are monitored by measuring the concentration of gases or fluids collected in an enclosed chamber sealingly attached to a skin "window" formed by removing the stratum corneum over a small area of the patient's skin. The measurements in the enclosed chamber are made, inter alia, by determining the difference in intensity of light emitted from a fluorescent indicator. U.S. Patents Nos. 4,200,110 and 4,476,870, Peterson et al, disclose the use of a pH sensitive indicator in conjunction with a fibre optic pH probe. In each of these patents the dye indicator is enclosed within a selectively permeable membrane envelope. U.S. Patent No. 4,548,907, Seitz et al, discloses a sensor for measurement of pH values using fluorescent indicators. The sensor comprises a membrane immobilised fluorophor secured to one end of a bifurcated fibre optic channel for exposure to the sample to be analysed. Many fluorescent indicators sensitive to pH, and thereby useful for pCO₂ measurements, are known in the art. Examples of useful fluorescent indicators are disclosed in the above patents and also in "Practical Fluorescence" by George E. Guilbault, (1973) pages 599-600. Sensor devices using fluorescent indicators may be used for in vitro or in vivo determinations of components in physiological media. For in vitro determinations the size of the device is normally of no consequence, but for in vivo use, the size of the sensor may be extremely critical and there is an increasing need in the art to miniaturise sensor devices, particularly catheter-type devices, for the in vivo determination of components in physiological media, e.g. blood. However, diminution in size of the components of such devices, particularly in the size of the sensor itself, decreases the strength of the signal emitted by the indicator and consequently presents problems in the detection and measurement of said signal. These problems are aggravated when the detector system requires a multiplicity of components, such as filters, beamsplitters and reflectors to isolate and measure the emitted energy. Each of said components reduces the emitted signal strength resulting in a sequential loss of measurable signal. Consequently, the more components present in the system, the weaker the final signal strength. One approach for obtaining a meaningful measurement is to use the ratio of two signals which provides a signal with greater resolution than that obtainable from prior art systems based upon a single signal. Zhang ZHUJUN et al in Analytica Chimica Acta 160 (1984) 47-55 and 305-309 disclose that the fluorescent compound 8-hydroxy-1,3,6-pyrenetrisulfonic acid, referred to herein as HPTA, fluoresces when excited by excitation radiation having wavelengths of 470 and 405 nm and the fluorescence emission is sensitive to changes in pH in the physiological range of 6 to 9. In contrast to the system disclosed by Zhujun et al, which uses two excitation radiations to produce fluorescence, surprisingly, it has now been found that highly accurate, stable determination of pH can be obtained from a single external source of excitation radiation which is used to excite a first fluorescent indicator which in turn emits fluorescent radiation to excite fluorescence emission in a second fluorescent indicator, e.g. HPTA; said first indicator being insensitive to pH. According to the present invention, there is provided a method of measuring the concentration of carbon dioxide in a medium by determining the water content in a pH-independent sensor system comprising an optical fibre having a proximal end and a distal end, said distal end having attached thereto a fluorescence indicator embedded in a carrier matrix with a controlled water content, said carrier matrix containing a miscible mixture of water and non-aqueous solvent in controlled proportions and being separated from said medium by a gas-permeable, water-impermeable diffusion membrane, said indicator, when excited by excitation radiation of a predetermined wavelength λ o , emitting fluorescent emission at a wavelength λ w in the presence of water and at a wavelength λ s in the presence of said non-aqueous solvent, the intensity of each emission being dependent upon the ratio of water to non-aqueous solvent present in the system such that the ratio of the intensities of emitted radiation of wavelengths λ w and λ s is therefore proportional to the amount of water present and diffusion of carbon dioxide through said gas-permeable membrane and subsequent reaction with water to deplete the water content of the system induces a change in the intensities of said emissions, comprising transmitting through said optical fibre, from a source adjacent to its proximal end, excitation radiation of said predetermined wavelength λ o , and measuring the ratio of the intensities of the emitted radiation of wavelengths λ w and λ s , thereby obtaining a determination of the water content and thus a measurement of the carbon dioxide concentration in the surrounding medium. The term "stable" as used herein is intended to mean the stability of the determination with respect to all factors which might influence the measurement other than the parameter being measured. Thus the determination is not affected by, for example, changes in the strength of the excitation radiation, fluctuations in light or temperature or minor equipment defects. Since the quantity being measured is a ratio between two given intensities and this ratio remains constant when the value being measured is constant, irrespective of the actual size of the individual intensities, the resultant determination is necessarily stable. By using the method of the invention, enhancement of signal resolution is obtained due to the divergence of the fluorescence emission intensities as a function of pH of the surrounding medium. This phenomenon provides a higher degree of measurement resolution thus providing an increase in measurement accuracy in determining solution pH. The method of the invention will be further described by reference to two other embodiments which were claimed in the above-referred to European patent application 0283116, but are not claimed herein. The first of these embodiments comprises a sensor system for determining the pH of a liquid medium, said system comprising, in combination, a source of excitation of radiation of wavelength λ o , a first fluorescent indicator whose fluorescence emission is insensitive to pH and a second fluorescent indicator whose fluorescence emission is highly sensitive to solution pH, which indicator combination is adapted to respond when said source of excitation radiation is applied to the system such that said first fluorescent indicator is selectively excited by said excitation radiation to emit pH-insensitive fluorescence emission at wavelength λ₁, which emission overlaps the excitation radiation spectrum of said second fluorescent indicator, and said second indicator being excited by said emission radiation of wavelength λ₁ and in turn emitting a pH-dependent fluorescence emission of wavelength λ₂, the ratio of intensities of the radiation of wavelengths λ₁/λ₂ providing a highly accurate, stable determination of the pH of said liquid medium. The second of these embodiments comprises a sensor system as above described, further including an optical fibre having a distal end and a proximal end and in which said combination of fluorescent indicators, bicarbonate solution and membrane is attached to said distal end and said proximal end is adapted to receive excitation radiation from said source and said first fluorescent indicator is 6, 7-dimethoxycoumarin or a pH-insensitive coumarin derivative which is bonded directly to said optical fibre and said second fluorescent indicator is 8-hydroxy-1, 3, 6-pyrenetrisulfonic acid which is suspended in a gel of carboxymethyl cellulose containing said bicarbonate solution and bounded by a silicone rubber membrane. In an embodiment of the above-referred to system, for the determination of the concentration of carbon dioxide in a liquid medium, said indicator combination is associated with a bicarbonate solution bounded by a carbon dioxide-permeable membrane, such that the ratio of intensities of the radiation of wavelengths λ₁/λ₂ provides an indication of the solution pH within the membrane and thereby a highly accurate, stable determination of the concentration of carbon dioxide in the liquid medium. In an embodiment of the method of the invention, for determining the concentration of carbon dioxide in a liquid medium, said indicators are associated with a bicarbonate solution bounded by a carbon dioxide- permeable membrane, whereby said measurement provides an indication of the solution pH within the membrane and thus a highly accurate, stable determination of the concentration of carbon dioxide in the liquid medium. The first fluorescent indicator, which is insensitive to pH, is preferably 6,7-dimethoxycoumarin or a pH-insensitive coumarin derivative. A typical coumarin derivative is beta-methylumbelliferone, particularly in the form where it is chemically bonded to an acrylic polymer. The pH sensitivity of the umbelliferone polymer may be retarded by reacting the polymer solution with an excess of cross-linking agent such as poly (acrylic acid). The particularly preferred indicator for the purpose of the present invention is 6,7-dimethoxycoumarin which, when excited by excitation radiation having a wavelength of 337 nm emits fluorescent radiation at a wavelength of 435 nm. The characteristic excitation and emission spectra of 6,7-dimethoxycoumarin are illustrated in the accompanying drawings as described hereinafter. It is to be understood that when reference is made herein to a particular wavelength, for example with respect to excitation or emission, it is intended to mean that wavelength which is most representative of the condition being described; most typically the peak of a curve illustrating the spectrum which fully represents said condition. Thus, as shown by the curve for the excitation spectrum, 6,7-dimethoxycoumarin in excited by radiation over a spectrum of wavelengths from 310 to 380 with an optimum excitation at the peak wavelength of 337 nm. For convenience, unless otherwise defined, the wavelengths quoted herein are the peak wavelengths for the phenomenon in question. The preferred second indicator used in the first embodiment is HPTA. In the first embodiment excitation radiation having a wavelength, i.e. a peak wavelength, λo, of 337 nm, for example from a nitrogen gas laser, is transmitted from the proximal end of an optical fibre through the distal end where it excites a first indicator, preferably 6,7-dimethoxycoumarin, which emits fluorescent radiation having a wavelength, λ₁, of 435 nm. This fluorescence emission, in turn, excites the second indicator, preferably HPTA, to emit fluorescent radiation having a wavelength, λ ₂, of 510 nm. The intensity of the fluorescence emission of wavelength λ ₂ (510 nm) is dependent upon the intensity of the excitation emission of wavelength λ ₁ and upon the pH of the surrounding liquid medium, so that measurement of the ratio of the intensities of the emitted radiation of wavelengths λ ₁/ λ ₂ gives a highly accurate, stable determination of the pH of said liquid medium. It is to be noted that although the intensity of the fluorescence emission of wavelength λ ₁, derives from the pH-insensitive first-indicator, is itself independent of the pH of the medium, the fact that this intensity is affected by energy absorbed by the second indicator, which is pH-sensitive, means that the ratio derived from the peak of the emission spectrum curve of the first indicator and the isobestic point between the two emission curves (as described in detail hereinafter with reference to the drawings) also may be used to give an accurate, stable determination of the pH of the liquid medium. The sensor system of the second embodiment includes an optical fibre having a distal end and a proximal end, in which said combination of fluorescent indicators, bicarbonate solution and membrane is attached to said distal end and said proximal end is adapted to receive excitation radiation from said source of excitation radiation. As in the first embodiment, the preferred first fluorescent indicator is 6,7-dimethoxycoumarin or a pH-insensitive coumarin derivative, with 6,7-dimethoxycoumarin being particularly preferred. Also the particularly preferred second fluorescent indicator is HPTA. In a particularly preferred form of the second embodiment the 6,7-dimethoxycoumarin is directly bonded to the distal end of an optical fibre and HPTA is suspended in a gel of carboxymethyl cellulose containing a bicarbonate solution, preferably aqueous sodium bicarbonate solution, which gel is bounded by a silicone rubber membrane. The method of the second embodiment is preferably carried out by transmitting excitation radiation having a wavelength λ o, of 337 nm from a nitrogen gas laser through the optical fibre from its proximal end to its distal end where it excites the 6,7-dimethoxycoumarin to emit fluorescent radiation having a wavelength λ ₁, of 435 nm. This fluorescence emission, in turn, excites the HPTA to emit fluorescent radiation at a wavelength, λ ₂, of 510 nm. When the sensor is immersed in a liquid medium containing carbon dioxide, the latter permeates through the silicone rubber membrane and reacts with the bicarbonate solution thereby altering the pH of the solution around the sensor. The intensity of the fluorescence emission of wavelength λ ₂ (512 nm) is dependent upon the intensity of the excitation emission of wavelength λ ₁ and upon the pH of said surrounding solution. Therefore, measurements of the ratio of the intensities of the emitted radiation of wavelengths λ ₁/ λ ₂ provides an indication of the solution pH within the membrane and thus a highly accurate, stable determination of the concentration of carbon dioxide (pCO₂) in the liquid medium. The accompanying drawings comprise graphs illustrating excitation and emission spectra of the indicators used in the invention. Figure 1 illustrates excitation spectra for HPTA at varying pH levels; Figure 2 illustrates pH-insensitive excitation and emission spectra of dimethoxycoumarin in a solution of bicarbonate and ethylene glycol; Figure 3 illustrates spectra for HPTA in ethylene glycol at different pH levels; Figure 4 illustrates spectra for HPTA in different solvent mixtures; Figure 5 is a graph showing HPTA fluorescence as a function of the water content of the system; Figure 6 and Figure 7 illustrate spectra indication pCO₂ by a sensor system according to the first embodiment; Figure 8 illustrates spectra for varying carbon dioxide concentrations using HPTA in a 50/50 ethylene glycol/water solution; and Figure 9 is a graph showing the relationship between the ratio of fluorescence intensity and carbon dioxide concentration. The excitation spectra for HPTA illustrated in Figure 1 of the drawings taken over a wavelength range of 300 to 485 nm show that the intensity of the excitation radiation, which is a function of the area under the curve and is proportional to the height of the curve in each case, varies according to the pH of the surrounding medium. In this case the pH was varied from 6.66 to 8.132. Isobestic points were observed at 337 nm and 415 nm. The peak wavelength of the emission from HPTA subjected to the said excitation radiation was 510 nm (not shown). Figure 2 of the drawings illustrates excitations and emission spectra for dimethoxycoumarin in a solution of sodium bicarbonate and ethylene glycol. The concentration of dimethoxycoumarin is about 10⁻²M. The excitation spectrum exhibits a peak at a wavelength of about 340 nm and the emission spectrum has a peak at a wavelength of about 427 nm. The emission fluorescence is pH insensitive. It will be noted that the wavelength of the fluorescence emission for dimethoxycoumarin overlaps the wavelength of the excitation radiation for HPTA as illustrated in Figure 1. Figure 3 illustrates spectra for HPTA in ethylene glycol at pH 8.0 and pH 4.0, respectively. The HPTA is dissolved in ethylene glycol, one drop of pH 8.0 buffer is added and the solution is irradiated from a nitrogen laser with radiation of wavelength 337 nm. Two fluorescent emissions at wavelengths 440 nm and 510 nm are observed. One drop of pH 4.0 buffer is then added and the intensity of the spectra changes as illustrated in Figure 3. A peak at 510 nm appeared with the addition of water to the system, regardless of the pH. Figure 4 illustrates spectra of HPTA in different mixtures of ethylene glycol and water. 10⁻³M HPTA was dissolved in solution mixtures comprising, respectively, 100% ethylene glycol, 80% glycol/20% water and 50% glycol/50% water. Drops of each solution in turn were put on the tips of optical fibres and the HPTA was excited to fluoresce at a wavelength of 510 nm. The results are shown graphically in Figure 4. Additional results were obtained in a similar manner for solutions comprising 20% glycol/80% water and 100% water. The results for all runs are given in the following Table I. Solvent Relative Intensity I(Blue) λ = 440 nm Relative Intensity I(Green) λ = 510 nm Ratio 1/Ratio 100 % ethylene glycol 85.63 11.66 7.34 0.1362 80/20 glycol/water 69.97 49.64 1.41 0.7092 50/50 glycol/water 16.33 92.30 0.177 5.65 20/80 glycol/water 5.33 83.30 0.064 15.63 100% water 5.33 111.96 0.048 20.83 The fluorescence of HPTA as a function of the water content of the solvent system is illustrated graphically in Figure 5. Using excitation radiation of wavelength λ = 337 nm the ratio of the fluorescence peaks I (GREEN)/(BLUE) at λ = 510 nm and λ = 440 nm, respectively, was graphed for HPTA in ethylene glycol/water solutions of varying concentrations. The resulting graph indicates that the ratio of intensities increases substantially linearly as the water content of the solution increases. Figures 6 and 7 illustrate results obtained by performing the invention as illustrated in the following Examples: Example 1 A mixture of 1:1 dimethoxycoumarin:HPTA both at a concentration of 10⁻³M was dissolved in carboxymethylcellulose (CMC) and 5mM of sodium bicarbonate with the addition of 0.25 ml ethylene glycol to dissolve the dimethoxycoumarin. A carbon dioxide sensor was formed by depositing the resulting gel on the tip of an optical fibre formed by fused silica having a diameter of 400 m, and enveloping the gel in a carbon dioxide permeable silicone rubber membrane. The sensor was irradiated with excitation radiation of wavelength 337 nm from a nitrogen laser firing at 2 pulse/second. The fluorescence emission was detected with a linear array photodiode and monitored with an oscilloscope set to 0.1 volt/div. at 2 ms/div. A number of runs were conducted at varying carbon dioxide concentrations and the results for 0% CO₂ and 100% CO₂ are illustrated graphically in Figure 6 and are set out numerically in the following Table II: - % CO₂ I (Blue) I(Green) Ratio 0 5.66 53.97 9.53 100 30.32 20.99 0.69 Example 2 A gel containing mixture of 1:1 dimethoxycoumarin:HPTA at a concentration of 10⁻³ in CMC and 5mM of sodium bicarbonate was made up in a similar manner to that described in Example 1 and this gel was used to form a carbon dioxide sensor also as described in Example 1. A number of runs were conducted at varying carbon dioxide concentrations and the results are illustrated graphically in Figure 7 and set out numerically in the following Table III:- % CO₂ Baseline (COUM) Baseline (HPTA) IM (COUM) IM (HPTA) Ratio (HPTA/COUM) 0% 9.5 10.5 20.99 150.93 12.22 7% 9.5 10.5 40.65 97.29 2.79 100% 8.5 10 78.63 57.64 0.68 The results given in the above Examples show the accuracy with which quantitative results can be obtained using the sensor system according to the invention. Example 3 This example illustrates a carbon dioxide sensor utilising the relationship between the water content of the system and the carbon dioxide concentration. HPTA was dissolved in a 50/50 mixture of ethylene glycol and water and the solution embedded in a carboxymethylcellulose gel. This gel was deposited on the tip of an optical fibre and enveloped in a carbon dioxide permeable silicone rubber membrane to form a carbon dioxide sensor. The sensor was irradiated with radiation of wavelength 337 nm from a nitrogen laser at varying concentrations of carbon dioxide. The results are shown in Figure 8. It will be seen that the ratio of intensities of the fluorescence emissions at peak wavelengths of 460 nm and 510 nm is dependent upon the carbon dioxide concentration. The spectra exhibit an isobestic point at 485 nm. The fluorescence ratio as a function of carbon dioxide concentration is illustrated in Figure 9. This example illustrates the way in which a carbon dioxide determination can be obtained as a function of the water content of the sensor.

Claims: A method of measuring the concentration of carbon dioxide in a medium by determining the water content in a pH-independent sensor system comprising an optical fibre having a proximal end and a distal end, said distal end having attached thereto a fluorescence indicator embedded in a carrier matrix with a controlled water content, said carrier matrix containing a miscible mixture of water and non-aqueous solvent in controlled proportions and being separated from said medium by a gas-permeable, water-impermeable diffusion membrane, said indicator, when excited by excitation radiation of a predetermined wavelength λ o , emitting fluorescent emission at a wavelength λ w in the presence of water and at a wavelength λ s in the presence of said non-aqueous solvent, the intensity of each emission being dependent upon the ratio of water to non-aqueous solvent present in the system such that the ratio of the intensities of emitted radiation of wavelengths λ w and λ s is therefore proportional to the amount of water present and diffusion of carbon dioxide through said gas-permeable membrane and subsequent reaction with water to deplete the water content of the system induces a change in the intensities of said emissions, comprising transmitting through said optical fibre, from a source adjacent to its proximal end, excitation radiation of said predetermined wavelength λ o , and measuring the ratio of the intensities of the emitted radiation of wavelengths λ w and λ s , thereby obtaining a determination of the water content and thus a measurement of the carbon dioxide concentration in the surrounding medium. A method according to claim 1, in which said gas-permeable membrane is a silicone rubber membrane. A method according to claim 1 characterised in that said fluorescence indicator is 8-hydroxy-1,3,6-ppyrenetrisulfonic acid (HPTA) which emits a fluorescent emission at a peak wavelength λ w of 510 nm in the presence of water and a fluorescent emission at a peak wavelength λ s of 440 nm in the presence of non-aqueous solvent when excited with laser radiation of a wavelength λ o of 337 nm, said non-aqueous solvent is ethylene glycol or ethanol, and said gas-permeable membrane is a silicone rubber membrane. A method according to claim 3, in which said non-aqueous solvent is ethylene glycol or ethanol.

Inventors: KAMIYA TADAO, LEADER MATTHEW JAMES, KAMIYA, TADAO, LEADER, MATTHEW JAMES
Applicants: BIOMEDICAL SENSORS LTD, BIOMEDICAL SENSORS LTD.
Application year: 1988
Main classification: G01N21
IPC codes: G01N21