WO1999056109A1

WO1999056109A1 - Article and method for optical and spectroscopic measurement of a dissolved gas

Info

Publication number: WO1999056109A1
Application number: PCT/US1999/009168
Authority: WO
Inventors: Amos J. Gottlieb
Original assignee: Gottlieb Amos J
Priority date: 1998-04-27
Filing date: 1999-04-27
Publication date: 1999-11-04

Abstract

A solid phase sample chamber (303) is formed of a copolymer of perfluoro(2,2-dimethyl-1,3-dioxole) or 'PDD' and preferably a copolymer of tetrafluoroethylene (TFE) and PDD with at least 20 mole % PDD. The apparatus includes a light source (301) and an optical fiber (302) for transmitting light from the source (301) to the sensor (303).

Description

1

ARTICLE AND METHOD FOR OPTICAL AND SPECTROSCOPIC MEASUREMENT OF A DISSOLVED GAS

U.S. Government License Rights

This invention was made with Government support under DMI- 9631648 awarded by the National Science Foundation. The Government has certain rights in this invention.

Background of the Invention

This invention relates in general to apparatus and processes for measuring the presence or concentration of an analyte component in a fluid mixture containing the analyte. More specifically, it relates to apparatus and methods for the optical or spectroscopic measurement of the concentration of a gas dissolved in a liquid.

In many situations, it is desirable to be able to measure the concentration of specific molecules, typically a selected type of gas, that are dissolved in a fluid, typically a liquid. Common examples include the measurement of dissolved oxygen in fresh water, seawater, or blood, and the measurement of carbon dioxide in an aqueous medium such as water, a carbonated beverage, sparkling wine, fermented beverages such as beer, and blood.

In oceanography, measurement of dissolved oxygen in seawater is critical to understanding mixing processes and oceanic circulation as well as for the description of water masses. These and other applications often require long-term, fixed-site measurements. An apparatus that transforms the gas concentration into a signal should be stable over long periods of time. More specifically, it should be substantially invariant when subjected to changes in salinity of the water, temperature, and external light. In oceanographic applications, it should fit into standard CTD (conductivity, temperature and density) measurement units that are widely used for vertical and horizontal profiling. Long term use such as oceanographic monitoring also makes maintenance-free operation important. Other applications include dissolved oxygen monitoring for agriculture and industrial applications such as monitoring of boiler water, a waste water for environmental purposes, and the monitoring of carbon dioxide in the manufacture of a variety of beverages. In food and medical applications, it is also valuable to be able to steam sterilize components in contact with the liquid being interrogated.

For reliable, maintenance-free, long-term use, measurement apparatus and processes should also involve no moving parts such as shutters, choppers or mechanical relays. There should also be no need to replace or replenish reagents, electrolytes, or membranes, e.g., semi- permeable membranes used in certain commercial units that measure dissolved gas concentrations.

Apparatus for such measurements has fallen into two broad categories, electrochemical and optical. The electrochemical units use electrodes exposed to the liquid with the dissolved molecules of interest. The most common type is polargraphic, as exemplified by the Clarke electrode units. It holds an inter-electrode potentially constant while measuring the current. The other type is galvanic.

The optical approach uses devices that, as described in U.S. Patent Nos. 5,244,810 and 5,460,971, both to Gottlieb, fall into one of three general types. The first uses an indicator material that reacts with the molecules being detected, and is then measured by, or interacted with, a source of incident radiant energy. The indicator luminesces, or causes some other detectable change. The indicators are typically dyes. They suffer from bleaching induced by the incident radiation, or the otherwise change chemically over time, particularly in an adverse, changing work environment. For these and other reasons listed in these patents, such "secondary indicator" systems can be unreliable, particularly for long-term use. The second typical optical unit measures the absorbency of an incident light beam by the molecule being measured held in solution in a chamber, or in a polymer body. Sensitivity and selectivity are problems. The light can be absorbed by molecules other than the one of interest, or lost for other reasons. The selectivity of the material forming the chamber or body - - its ability to pass the molecules of interest but block other molecules that would interfere with the measurement - - is therefore very important, and a source of problems.

The third type of optical unit relies on measuring the luminescence of the target molecule itself, not a dye that has reacted with it, when interrogated with a light source. This is termed direct luminescence measurement (DLM). The luminescence of materials other than the one being investigated (contamination) have limited the usefulness of this approach.

The present inventor's aforementioned '810 and '971 patents describe an analytical apparatus and method of this optical type that improves over the then-known optical measurement units. The general approach described in these patents is to dissolve the compound to be analyzed into a solid, semi-solid or liquid body, irradiate it with light at a frequency specific to that compound, and then detect and measure the luminescence or other optical characteristic of the compound. The double selectivity of the dissolving into the body and the light-wavelength-specific interaction produces an improved sensitivity.

The performance of this method, and apparatus constructed to use it, however, is dependent on the characteristics of the material forming the body being interrogated, and in particular, on its ability to (1) exclude molecules from the light-interacting region that would interfere with the measurement while readily passing the molecules to be analyzed and (2) to provide a controlled environment. The materials should also be size invariant during use because a change in size changes the path length of the light over which it can energize the molecules of interest. For a solid sensor, it must transmit the energizing light. It should also meet the other objectives noted above, e.g., be invariant to salinity changes when operating in seawater, be steam-sterilizable for applications where sterility is required, and not require replacement, replenishment, or containment. The '810 and '971 patents describe a number of materials to form semi-permeable membranes or solid sensor bodies with the required selectivity. Of the various polymeric materials mentioned, polydimethylsiloxane is preferred. It is described in one form as made into an optical fiber coupled to a standard 120 μm core optical fiber that receives pulses of laser light. The polydimethylsiloxane fiber is described, for example, as immersed in blood and adapted to measure oxygen dissolved in the sensor body optical (fiber) from the surrounding blood. While in practice polydimethylsiloxane has worked well for measuring benzene, it has not operated well when used to measure dissolved oxygen.

U.S. Patent No. 4,800,886 to Nestor describes an optical fiber sensor permeable to carbon dioxide for use in measuring its concentration in blood. An interrogatory light beam passes along the sensor, and is then reflected back through it. The '886 patent also lists various polymeric materials, such as polystyrene, polyurethane, and polyethylene, as suitable sensor materials. But it identifies silicones as preferred for their extensive permeability to gasses. One silicone, polydimethylsiloxane, is noted favorably for its permeability to carbon dioxide.

A principal difficulty with silicone is that the measurement sensor is contaminated by molecules other than those being analyzed, e.g., C0₂. In particular, silicones are known to exhibit permeability that increases with the molecular or weight of the molecule. For example, the permeability of silicone rubber to ethane, C₂H₆ is over 2.5 times its permeability to methane, CH₄. Butane, while heavier than ethane, is almost 10 times more permeable than methane. High molecular weight contaminants are therefore a particular problem. Another problem is that silicones swell when wet, and they are elastomers. Both characteristics can change the travel path of the light interacting with the dissolved compound under analysis, which in turn introduces error in the measured concentration. Perfluoro (2,2-dimethyl- l ,3-dioxole) polymers (referred to herein as

"PDD"), also known as 2,2-bistrifluoro-methyl 4,5-difluoro- l,2-dioxole, and copolymers formed from PDD including copolymers of TFE (tetrafluoroethylene) and PDD are materials that have been suggested for use in telecommunications applications. Mitsubishi Rayon Co., Ltd. has described its efforts to use these materials to form plastic optical fibers in U.S. Patent No. 4,966,435; 5,048,924; 5, 121 ,461; and 5, 136,002. DuPont has also showed interest in PDD copolymers as optical fibers for telecommunications. It owns U.S. Patent Nos. 5,076,659 and 4,530,569 relating to the use of these materials for fiber optic cores and cladding. Optical communication fibers are typically intended surrounded by air, not to operate while immersed in a liquid. Moreover, in telecommunications, it is very important that the signal being carried be immune to the environment and its chemical constitution. While use as a "window" has been suggested, as with the use of copolymers of PDD for telecommunications, the point is to use the window as a window, that is, to transmit light while isolating it from the environment outside the window. It is therefore not desired that the PDD copolymer fiber or window be permeable to any materials, let alone selectively permeable to a molecule (gas) dissolved in a liquid. It should also be noted that, as a general rule, with copolymers of PDD, the higher the molecule weight of a constituent, the lower its permeability. Moreover, PDD copolymers are difficult to work with and are quite costly, e.g., about $ 10, 000/kg. While there is some limited commercial use of PDD optical fibers for telecommunications, PDD and its copolymers remain an expensive, relatively obscure material.

It is therefore a principal object of this invention to provide an optical method and apparatus for measuring dissolved molecules, particularly oxygen and carbon dioxide, that are accurate, reliable, and stable over long term use.

Another object is to provide a method and apparatus with the foregoing advantages that are substantially invariant with the salinity of water being investigated and the ambient light. A further object is to provide the foregoing objects while also being compact, rugged and producible in a variety of forms to accommodate different liquids and different dissolved materials. Still another object is to provide a method and apparatus for optical and spectrographic measurement of molecules dissolved in liquids that is substantially maintenance free.

A further object is to provide a method and apparatus that can operate in conjunction with, and greatiy improve the analyzer of the type described in U.S. Patent Nos, 5,244,810 and 5,460,971.

A still further object is to provide a solid testing chamber for gases dissolved in liquids.

Another object is to provide a method and apparatus with the foregoing advantages that are compatible with steam sterilization.

Summary of the Invention

This invention can be summarized in its most basic aspect as an article made from copolymers of perfluoro (2,2-dimethyl- l,3-dioxole) (PDD) that is used as a solid phase sample chamber for carrying out a chemical measurement using an optical method such as an absorbance, transmittance, fluorescence, phosphorescence or the like. This article could take may forms. It could for instance be an optical fiber, a waveguide, a detection cell, or even a lens. In a second aspect of this invention is an analytical apparatus that uses an article made from copolymers of PDD as a solid phase sample chamber. In a third aspect of this invention is a method for indicating or measuring the presence or concentration of an analyte component in a fluid mixture containing the analyte wherein the method uses an article made from copolymers of PDD as a solid phase sample chamber.

Brief description of the Drawings

Fig. 1 is a schematic diagram illustrating one embodiment of this invention that is particularly adapted to measuring dissolved carbon dioxide gas;

Fig. 2 is a schematic diagram of another embodiment of this invention; Fig. 3 is a schematic diagram of another embodiment of this invention that is particularly adapted to measuring dissolved oxygen gas;

Fig. 4 is a timing sequence used in the operation of the embodiments shown in Fig. 3; Fig. 5 is a schematic diagram of an apparatus for producing clad optical fibers according to the present invention;

Fig. 6 is a schematic diagram of a circuit suitable for use in the embodiments shown in Figs. 1-3;

Fig. 7 is a graph of response time as a function of the concentration of oxygen dissolved in salt;

Fig. 8 is a scatter plot comparing measured values of dissolved oxygen concentration in salt water using the present invention to actual oxygen partial pressures in the salt water;

Fig. 9 is a graph of the output of the sensor platted against response time during step function changes in salinity of the sale water surrounding the sample chamber of the present invention; and

Fig. 10 is a graph of the output of the Fig. 3 embodiment as a function of temperature.

Detailed Description of the Preferred Embodiments This invention relates to the optical measurement of a specific molecular component (analyte) that is dissolved in a fluid, typically a liquid.

Typically the component under investigation is oxygen (0₂) or carbon dioxide (C0₂), but can include a variety of other materials that can be dissolved in other fluids such as water, both fresh and seawater, carbonated aqueous beverages such as soft drinks and sparkling wine, and fermented beverages such as beer.

The apparatus and method is generally of the type shown and described in U.S. Patent Nos. 5,244,810 and 5,460,971, the disclosures of which are incorporated herein by reference. A fundamental insight underlying all aspects of the present invention is that copolymers (hereinafter "CPoPDD") of perfluoro (2,2-dimethyl- l ,3- 8 dioxole) and perhalogenated monomers capable of polymerizing with PDD can be used as s solid phase sample chamber that can be immersed in a fluid to measure optically the presence or concentration of a dissolved substance. More specifically, thus been found that among perfluoropolymers, CPoPDD copolymers exhibit excellent optical clarity and high gas permeability. This combination of properties makes it possible to use articles made of copolymers of PDD as "solid phase sample chambers". CPoPDD of TFE and PDD are presently preferred. U.S. Patent No. 4,966,435 also teaches copolymers of PDD with either chlorotrifluoroethylene (CTFE) or perfluoromethyvinyl ether (PMVE) . They may also be utilized in the applications of the present invention. When an article made of a CPoPDD is placed in contact with a fluid containing a dissolved gas or other low molecular weight volatile component, a portion of the dissolved gas or volatile material will partition out of the fluid and become dissolved in the article to equilibrium. Optical or spectroscopic examination of the CPoPDD article will then reveal the presence and /or concentration of the gas or volatile materials. From the presence or concentration of the gas or volatile material in the CPoPDD material, the presence or concentration of the gas or volatile material in fluid can be determined.

As used herein luminescence is used to include both fluorescence and phosphorescence.

As used herein the term "dissolving" includes not only its conventional meaning of a gas or other material dissolved in a liquid but also includes within its means the physical phenomenon wherein a gas, liquid or other material is absorbed into or permeates a solid or semi- solid material whereby it is free to diffuse through and migrate in solid or semi- solid material in response to concentration gradiants. That is, so long as the behavior is similar to the material in a liquid solution, it is included in the scope of this invention even though the "solution" or "dissolving" is present in a solid or semi-solid material. As used herein the terms "molecules" and "chemical species" are used interchangeably to include not only actual molecules, but the atomic form of the elements as well. The atomic forms are included in the scope of this invention to the extent that it can be excited to an elevated energy state by source radiation and emit luminescent radiation when it returns to a lower energy state, or otherwise interact with light, e.g., be absorbed. The same is true for compounds or complexes which may not fit the conventional definition of molecules, but as long as they perform as indicted in the present invention, they are considered to be "molecules" or "chemical species" within the meaning of the term as used herein.

Example 1 : Measurement of Dissolved C0₂ Fig. 1 shows an apparatus 10 according to the present invention particularly adapted to measure carbon dioxide concentration in a liquid. An infrared source 1 emits light in a wavelength range including light with a wavelength in the range of 4.2 to 4.3 microns. This light travels down an infrared waveguide 2 (for instance, a hollow brass, aluminum, gold, or stainless steel tube with a polished bore and capped at the ends with a piece of sapphire, or alternatively, solid infrared (IR) transmitting waveguide made out of a heavy metal fluoride glass or other IR transmitting material) . It then enters a solid detection cell 3 formed of a CPoPDD copolymer sold by I.E. duPont de Nemours 85 Co. under the registered trademark "Teflon AF 2400". After traveling through the detection cell, reflector 4 reflects it back through the window and waveguide until it passes through optical filter 5 and strikes an IR detector 6. Teflon AF 2400 is a CPoPDD with a tetrafluoroethylene monomer. A 1/2 inch diameter, 0.5 mm thick detection cell 3 of Teflon AF 2400 can be prepared by compression molding Teflon AF 2400. When this detection cell is incorporated into the apparatus shown in Fig. 1 , the concentration of C0₂ in a fluid surrounding the detection cell can be determined and continuously monitored. The embodiment of this invention shown in Fig. 1 is a measurement system for determining the concentration of carbon dioxide in an aqueous medium such as water, a carbonated beverage, such as the beverage sold by the Coca Cola Company 10 under the registered trademark Coca-Cola®, a sparkling wine, beer, or blood.

The amount of CO2 present in detection cell 3 will be directly related to the amount of C0₂ in the fluid surrounding the detection cell. As the IR light with a wavelength near the region of 4.2 to 4.3 microns passes through detection cell 3, a portion of it will be absorbed by the C0₂ present in detection cell 3. Thus the signal coming from IR detector 6 will be related to the amount of C0₂ in the fluid. The signal can be amplified by amplifier 7 and processed and displayed by signal processor 8 and display 9. The higher the level of CO2 in the fluid, the higher the level of CO2 in detection cell 3, and the lower the signal detected by detector 6. Most typically this type of sensor system will be calibrated by exposing the sensor to one or more samples having a fixed and known concentration of C0₂. Note that while the detection cell has light transparent qualities of the CPoPDD, it is a solid phase sample chamber which is a controlled environment for the interaction of the light with a target molecule.

Example 2: Measurement of Oxygen Concentration Fig. 3 shows an apparatus 10' according to the present invention specifically adapted to measure the concentration of oxygen dissolved in a liquid. A laser diode 301 (MRV Communications) emits light of a wavelength near 1270 nm into the first end of optical fiber 302 (standard telecom 62.5/ 125 fiber). Attached to the second end of fiber 302 is a length (e.g., 4 inches) of an optical fiber 303 (with a 200 μm diameter core and 300 μm outer diameter cladding) with a core of the product sold by I.E. duPont de Nemours & Co. under the registered trademark "Teflon AF1600", and a cladding of Teflon AF 2400 material. Oxygen dissolved in a fluid surrounding the Teflon AF fiber 303 permeates it. A portion of the oxygen dissolved in the core of the fiber 303 will absorb a portion of the 1270 nm source light which is passing through it and luminesces. A portion of the luminescence is coupled into optical fiber 304 and is be carried to the InGaAs photodetector 305. The concentration of oxygen can then be determined based on the intensity of the luminescence detected by 11 photodetector 305, as described in greater detail in U.S. Patent No. 5,244,810.

In the DLM sensing method the dissolved oxygen concentration will be proportional to the ratio of the oxygen luminescence and the laser intensity. These two signals can differ in magnitude by six orders magnitude and therefore the photodetector circuit can sequentially measure both signals. Several different circuit architectures can be used, including ones that (i) electronically switch gain in the time between the laser pulse and the acquisition of the luminescence signal, (ii) use a set of matched transistors in a current mirror configuration to electronically switch the output of the photodiode between a high gain and low gain measuring circuit, or (iii) use an active correction circuit to remove current from the node between the photodiode and the high gain transimpedance amplifier during the laser pulse. At present, the first architecture is preferred. The circuit 308 (Fig. 6) consists of a classical transimpedance photodetector circuit with components that allow the circuit to automatically switch to a low gain mode when the laser pulse is present. For test purposes in measuring dissolved oxygen, a 1270 laser diode that was coupled to a four inch length of optical fiber 303 by conventional silica optical fiber. The optical fiber 303 is terminated with ST style optical fiber ferrules. The output of the optical fiber 303 is launched into the InGaAs photodetector 305 that serves as a front end to the auto- switching dual gain amplifier 310. The low gain output of the dual gain amplifier 310 was directly fed to channel 2 of a Keithley-Metrabyte data acquisition board 312. The high gain output was fed into both inputs of a dual channel integrator 314. The output from each integrator channel was fed to a separate channel on a data acquisition board 312.

Fig. 4 shows a timing sequence illustrating the operation of this architecture as used to obtain the test results described below for dissolved oxygen. The top line of Fig. 4 shows the laser illumination of the fiber 303 with intermittent pulses followed by a period where there is a luminescence of the target molecules, here oxygen. During luminescence, there is a 12 measurement over two time intervals that both have signal. One is subtracted from the other to get a D.C. stable signal. In Fig. 4, the times t are as follows: to - diode laser turned on. ti - acquisition of laser power level by A/D channel 0.

X.1 - acquisition of integrator channel A signed by A/D channel 1. t3 - acquisition of integrator channel B signal by A/S channel 2. t₄ - both integrator channels are reset. t$ - laser diode turned off and integrator channel A tuned on. tβ - integrator channel B turned on. t₇ - both integrator channels held and laser diode turned on.

Beginning the next cycle. The optical fibers 303 can be made from CPoPDD copolymers in at least two different ways. One method is disclosed in U.S. Patent No. 4,966,435 wherein optical fibers are made from PDD and a number of copolymers including CTFE and PMVE. A second method, presently preferred, is to draw optical fiber from a preform with a core of Teflon AF 1600 (a higher index of refraction copolymer of PDD) and a cladding made of Teflon AF 2400 (a lower index of refraction copolymer of PDD). With reference to Fig. 5, this second method involves pulling the fiber from a preform 500 that contains both the core 303a and the cladding 303b material, a known approach for making conventional optical fibers. The relative thickness of the core and cladding are fixed by the ratio of their dimensions in the preform 500. Since the preform is relatively large, it is easy to control the core to cladding thickness ratio. Optical fibers 303 of a fixed overall diameter but different cladding thickness can be made by preparing separate preforms 500 having the requisite core to cladding ratio.

To draw a fiber 303a the end of the composite preform 500 of the CPoPDD is placed in a drawing oven 502 and heated until it softens. The tip 500a of the preform is then pulled down and attached to a take up reel 504. The fiber 303 is drawn as the take up reel 504 is turned. The clad 13 fibers 303 are formed into sensors by termination with ST style fiber optical connectors.

Fiber made in either of these ways can be used as a solid detection cell according to the present invention. The oxygen concentration of both aqueous and gaseous samples can be determined by using the apparatus shown in Fig. 3 of U.S. Patent No. 5,244,810 wherein a length of 300 μm optical fiber with a core of the Teflon AF 1600 copolymer and a cladding of the Teflon AF 2400 copolymer was substituted for the linear filament of polydimethylsiloxane, the Toshiba laser diode was replaced by a diode from MRV Communications, Inc., and the Germanium Power Devices photodetector was replaced by an InGaAs detector made by Epitaxx. In addition, the strength of the laser pulse was measured during an interval between to and ti (Fig. 4 of U.S. Patent No. 5,244,810), and two measurements, of different time lengths, were made during the time interval ti to t₂ to enhance the performance of the system. Thus the updated embodiment of this invention illustrated in Fig. 3 shows a sensor made of a CPoPDD and method for the measurement of the concentration of oxygen in a fluid, such as water.

Although this specific example used a step-index optical fiber made from CPoPDD 's, a graded index optical fiber containing CPoPDD 's would also be suitable.

This DLM sensor 303 was successfully demonstrated to measure dissolved oxygen in both fresh and salt water. The background-corrected, laser-intensity-ratioed signal was found to be linear with oxygen concentration, independent of salinity, and temperature dependent in a near linear manner that can be compensated in the processing circuitry software with an algorithm. The performance of the sensor was unaffected by the presence of large amounts of carbon dioxide even though TFE copolymers of PDD are highly permeable to C0₂. The response of the DLM sensor 303 was found to be linear with oxygen concentration. Typical data from one sensor is shown in Figures 7 14 and 8. Figure 7 shows the response of a sensor as it was exposed to six different levels of dissolved oxygen in salt water with a salinity of 40 PSU.

Figure 8 is a scatter plot of typical values at each level of dissolved oxygen shown in Fig. 7. The measured values are in good agreement with the actual values indicating good accuracy and linearity. It should be noted that four of the gas mixes tested contained nitrogen and oxygen and two contained nitrogen, oxygen, and carbon dioxide ( 10% CO2 and 20% CO2). The presence of carbon dioxide, even at these high levels, did not induce an inaccuracy or affect the response of the sensor 303. It should also be noted that such high levels of CO₂ will cause the pH of the solution to change, and that the sensor was unaffected by these changes in pH.

Fig. 9 shows the response time of a sensor 303 as the salinity of a surrounding salt water solution was changed from O PSU to 40 PSU in 10 PSU incriments, as shown. The output of the sensor was found to be unaffected by the addition of salt.

Fig. 10 shows response of the sensor 303 operated immersed in a water, over a temperature range of 2°C to 30°C. The sensor signal, as shown, increases with decreasing temperature. The relative change in output signal over this temperature range is roughly the same as seen in a Clarke electrode, although in the opposite direction. The effect is believed to be due to at lest two factors, (i) the decreased solubility of oxygen in copolymers of PDD with increasing temperature, and (ii) a temperature dependence of the phosphorescence quantum. This temperature sensitivity can be corrected to the desired degree using known signal processing techniques. In summary, the testing of the DLM fiber optic sensor 303 of the present invention has been found to be suitable for the measurement of dissolved oxygen under the temperature and salinity conditions typical of seawater.

The response time of the fiber optic sensor 303 is determined by the time required for the dissolved oxygen to reach a constant value within it. For a cylindrical geometry such as that of an optical fiber, the time required 15 to reach a constant concentration, t, is proportional to the square of the radius of the fiber, r, divided by the diffusivity D of the gas, that is, t oc r /D

The response time of sensors made from optical fibers of three different diameters were examined in gas flows where the oxygen content varied from 0 to 21%. The cladding was less than 1.0 μm thick. Agreement was found between the predicted and measured response times. Defining the response time to be the time required to reach 90% of the final value following a step change in oxygen concentration, a 125 μm diameter fiber 303 had a response time t of about 4 seconds, for 300 μm the time t was about 32 seconds, and for 400 μm it was about 72 seconds. Note that for oceanographic instrumentation, the response time for oxygen sensors is sometimes defined as the time required to reach 63% of the final value, assuming an exponential decay. Defined in this way, the response times for the fibers 303 in various diameters would, of course, be substantially shorter.

Example 3: Measurement of the Concentration of Flammable Gases in Transformer Oil

Another embodiment 10" of the present invention is shown in Fig. 2. Here IR or near NIR (near infrared) light from light source 1' is launched in optical fiber 1 1. Optical fiber 1 1 has a core of Teflon AF 1600 and a cladding of Teflon AF 2400 as described above. When optical fiber 11 is immersed in transformer oil, a portion of the low molecular weight gases in the oil (such as methane, ethane, ethylene, and acetylene) will permeate the fiber and absorb a portion of the light passing through the Teflon AF fiber. The wavelength related loss of light will be detected by photodetector 12, which may contain a diffraction grating or other means for separating the wavelengths of light, and the concentration of gases will be calculated by signal processor 13 and display on display 14. There has been described a method and apparatus for measuring the presence and concentration of dissolved substances in fluids with reliability and accuracy. The system meets the objects set forth above. 16

While the invention has been described with respect to its presently preferred embodiments, it will be understood that various modifications and alterations will occur to those skilled in the art. Such modifications and alterations are intended to fall wil±iin the scope of the appended claims.

Claims

17 WHAT IS CLAIMED IS:

1. A method for measuring the concentration and presence of a selected chemical species dissolved in a fluid mixture comprising, providing a transparent article made of a copolymer of perfluoro (2,2-dimethyl 1,3-dioxole) and at least one additional perhalogenated monomer capable of polymerizing therewith; contacting said article with the fluid mixture to allow the selected chemical species to at least partially dissolve in the transparent article; exposing said transparent article to light with a predetermined wavelength characteristic; directing a portion of any light that exits from the transparent article to a detection means for measuring the exiting light; measuring said portion of exiting light; indicating the presence of the selected chemical species.

2. A method according to claim 1 comprising indicating the concentration of the selected chemical species in the fluid mixture.

3. A method according to claim 2 wherein said copolymer is a copolymer of perfluoro (2,2-dimethyl 1,3-dioxole) and tetrafluoroethylene.

4. A method according to claim 2 wherein said copolymer is a copolymer of perfluoro (2,2-dimethyl 1,3-dioxole) and chlorotrifluoroethylene. 18

5. A method according to claim 2 wherein said copolymer is a copolymer of perfluoro (2,2-dimethyl 1,3-dioxole) and perfluoro (methylvinyl) ether.

6. A method according to claim 2 wherein said chemical species in oxygen.

7. A method according to claim 2 wherein said chemical species is carbon dioxide.

8. A method for measuring the presence and concentration of a selected type of molecules dissolved in a fluid comprising, providing a solid phase sample chamber formed of a copolymer of PDD, contacting said sample chamber with a fluid to allow the selected type of molecules to dissolve into the sensor, transmitting light through the sample chamber with the selected molecules dissolved therein, said light being of a wavelength that selectively interacts with the selected type of molecules to energize them, and detecting and measuring a characteristic of said energized, selected type of molecules to provide said presence and concentration measurement.

9. The optical measuring method of claim 8 wherein said copolymer is one of TFE and at least 20 mole % PDD.

10. The optical measuring method of claim 9 wherein said forming produces a detection cell of said TFE and PDD copolymers and said transmitting includes providing a light source and a light waveguide 19 that transmits said light from its first end at said light source to its said second end at said detection cell.

11. The optical measuring method of claim 10 wherein said transmitting includes reflecting said light transmitted along said waveguide and through said detection cell back through said detection cell, and said waveguide and wherein said detecting and measuring occurs at said first waveguide end.

12. The optical measuring method of claim 11 wherein said selected type of molecule is carbon dioxide and said forming of said detection cell is includes making it sufficiently thick along a path followed by said light in traversing said detection cell to provide sufficient interaction between said light and said carbon dioxide in said detection cell for said detecting and measuring in said second interval to be accurate and reliable.

13. The optical measuring method of claim 12 wherein said detection cell thickness is about 0.5 mm.

14. The optical measuring method of claim 9 wherein said forming produces an optical fiber of said TFE and PDD copolymer and said transmitting includes providing a light source, transmitting said light from said light source along a first conventional optical fiber, to and through said TFE and PDD copolymer optical fibers, and transmitting luminescent light from said TFE and PDD optical fiber along a second conventional optical fiber to apparatus that perform said detecting and measuring. 20

15. The optical measuring method of claim. 14 wherein said TFE and PDD copolymer is a core having a diameter of about 200 ╬╝m or less.

16. The optical measuring method of claim 15 wherein said forming further comprises cladding said core with a layer of a TFE and PDD copolymer that has a lower index of refraction than that of the TFE and PDD copolymer fo╧Çriing said core.

17. The optical measuring method of claim 16 wherein said forming includes making said core with a diameter of about 200 ╬╝m or less and said cladding with an outer diameter of about 300 ╬╝m or less, provided that the radial thickness of said cladding is at least twice the wavelength of said light.

18. This optical measuring method of claim 16 wherein said forming includes making said copolymer with about 65 mole % PDD for said core and about 90 mole % PDD for said cladding.

19. The optical measuring method of claim 14 wherein said selected type of molecule is oxygen and the fluid is a liquid.

20. In an apparatus for detecting and measuring the concentration of a selected type of molecule dissolved in a fluid, said apparatus including a light source and means for transmitting light from said light source to a sensor immersed in said fluid and selectively permeable to said selected type of molecule that interacts with, and is energized by, said light after said interaction and energizing, the improvement comprising forming said sensor from a copolymer of PDD. 21

21. The apparatus of claim 20 wherein said copolymer is one of TFE and PDD.

22. The apparatus of claim 21 wherein said sensor is a solid block detection cell.

23. The apparatus of claim 22 further comprising a reflector operatively coupled to said detection cell that causes said light entering said detection cell to be reflected back through it to approximately double the distance transversed by the light in the detection cell.

24. The apparatus of claim 21 wherein said sensor is an optical fiber with a generally circular cross-section that conducts the light through said optical fiber using total internal reflection.

25. The apparatus of claim 24 where said core is clad with a copolymer of PDD having a lower index of refraction than that of said core.

26. The apparatus of claim 25 wherein said optical fiber has a core that is about 65 mole % PDD and 35 mole % TFE and is selectively permeable to oxygen.

27. The apparatus of claim 25 wherein said optical fiber is clad with said TFE and PDD copolymer and said core copolymer is about 65 mole % PDD and said cladding copolymer is about 90 mole % PDD, and both said cladding and said core are selectively permeable to oxygen.

28. Analytical apparatus for measuring the concentration and presence of a selected chemical species dissolved in a fluid mixture comprising: (i) a light source (ii) a transparent article made of a copolymer of perfluoro (2,2- 22 dimethyl 1,3-dioxole) and at least one additional perhalogenated monomer; (iii) a means for transmitting light from said light source to said transparent article; (iv) detection means for measuring light exiting from said transparent article; (v) a means for transmitting light from said transparent article to said detection means.