Chemistry: analytical and immunological testing – Oxygen containing – Inorganic carbon compounds
Reexamination Certificate
2000-03-23
2002-08-20
Snay, Jeffrey (Department: 1743)
Chemistry: analytical and immunological testing
Oxygen containing
Inorganic carbon compounds
C436S163000, C436S172000, C422S082080, C422S082090
Reexamination Certificate
active
06436717
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method and apparatus for optical chemical sensing and more particularly, to methods and systems for determining analyte contents of media by measuring spectral properties of a dye materials exposed to the analyte and processing the spectral properties in accordance with a family of ratiometric expressions.
BACKGROUND OF THE INVENTION
Various forms of analyte-measuring instruments have been designed and developed for use in biotechnology, industrial and environmental applications. Among these devices are those which rely on spectral properties of dye solutions responsive to a particular analyte. When a dye solution interacts with the analyte, the spectral properties of the dye solution change to a degree related to the concentration of the analyte in the surrounding medium. Thus, one may determine the analyte content in an analyte-containing medium by measuring the change in the spectral properties of the dye solution after the dye solution is exposed to the medium.
Two spectral properties of materials which have been used in optical chemical measurements are optical absorption and fluorescence. Absorption occurs when a material is illuminated and a portion of the illuminating light is neither transmitted through the material, scattered by it nor reflected from it. In the case of a layer of a solution, Beer's Law states that the amount of light energy thus absorbed by a solute in a solution will depend on the wavelength of the illuminating light, the concentration of the solute in the solution and the thickness of the solution layer:
A=&egr;bC,
where “A” is the “absorbance,” that is, the common logarithm of the ratio of the intensity of the illuminating light to the intensity of the absorbed light; “&egr;” is the “molar absorptivity” of the solute; “b” is the thickness of the layer of the solution; and “C” is the concentration of the solute in the solution. Typically, the molar absorptivity will vary with the wavelength of the illuminating light, reaching maximum values at a so-called “peak absorption wavelengths” of the solute.
Fluorescence occurs when light energy is absorbed by the material and subsequently emitted as light energy of a different wavelength. In the case of a thin layer of a dilute aqueous solution, the intensity “F” of the fluorescent emission can be expressed as follows:
F=
2.3
PK&egr;bC,
where “P” is the intensity of the illuminating light and K is the “quantum efficiency” of the solute. Like the molar absorptivity “&egr;,” the quantum efficiency will vary with the wavelength of the emitted light.
Certain prior art devices included sensor elements or probes in which aqueous dye solutions were trapped between analyte-permeable surfaces and distal ends of optical fibers. When the probes were exposed to analyte-containing media, analyte molecules diffused into the dye solutions. The dye solutions were illuminated at one or more selected wavelengths. The optical fibers gathered light returning from the dye solutions and conducted the light to one or more transducers. These transducers measured the intensities of the light to determine spectral properties of the dye solution. The spectral properties of the dye solutions were then used to determine the analyte content of the media.
Peterson et al. U.S. Pat. No. 4,200,110 proposed a fiber optic pH sensor using phenol red dye copolymerized with an acrylic base polymer to form microspheres. After the microspheres were exposed to a sample to be measured, they were illuminated sequentially by light at the peak excitation wavelength of the conjugate base (that is, a wavelength of illuminating light which maximizes the emitted intensity), given as 560 nm, and at an isosbestic wavelength (that is, a wavelength at which the optical absorbance of the dye microspheres was independent of the pH), given as 485 nm.
Peterson et al. taught that the pH of the sample could be determined from the intensity of the fluorescent emission of the microspheres measured after the microspheres were illuminated at the peak excitation wavelength of the conjugate base of the dye. In addition, they suggested normalizing this measured intensity by dividing it by the intensity of the fluorescent emission measured after the microspheres were illuminated at the isosbestic wavelength.
FIG. 3
of Peterson et al. suggested that the correlation of the pH to this normalized intensity was approximately linear over a range from about pH 6.5 to about pH 7.5.
Seitz et al. U.S. Pat. No. 4,548,907 proposed a fluorescence-based optical sensor including 8-hydroxy-1,3,6-pyrenetrisulfonic acid [hereinafter “HPTS”] dye immobilized on an ion exchange membrane. After the immobilized HPTS dye was exposed to a sample to be measured, it was illuminated sequentially by monochromatic light at peak excitation wavelengths of the undissociated HPTS dye, given as 405 nm, and of its conjugate base [hereinafter “PTS
−
”], given as 470 nm. The intensity of the fluorescent output of the dye when the dye was illuminated at each wavelength was measured using a photodetector having a narrow sensitivity band centered on 510 nm. A ratio was calculated by dividing the intensity of the fluorescent output of the dye after the dye was illuminated at the excitation wavelength of PTS
−
, that is, at 470 nm, by the intensity of the fluorescent output of the dye after the dye was illuminated at the excitation wavelength of HPTS, that is, at 405 nm.
Seitz et al. taught that the correlation between the pH of the sample and the foregoing ratio could be treated as approximately linear over a range from about pH 6 to about pH 8. They recommended using this ratio to determine pH instead of the absolute value of a single measured intensity because the ratio was insensitive to factors such as source intensity variations, fluorescence quenching and degradation of the dye material.
Seitz et al. also proposed the use of their fluorescence-based optical sensor for measuring dissolved carbon dioxide in accordance with the so-called “Severinghouse” method. More particularly, they proposed measuring dissolved carbon dioxide in a sample by exposing the HPTS to a solution of sodium bicarbonate of known concentration which, in turn, was exposed to the sample. Carbonic acid ions from the sample tended to lower the pH of the sodium bicarbonate solution. Seitz et al. taught that, by using their optical sensor to measure the pH of the sodium bicarbonate solution, they could determine the concentration of dissolved carbon dioxide in the sample.
Peterson et al. U.S. Pat. No. 4,476,870 proposed a fiber optic probe for determining the partial pressure of oxygen in blood by measuring the fluorescence quenching of a suitable dye exposed to a sample to be measured. More specifically, the dye was exposed to the sample and illuminated with blue light. The intensity of the fluorescent output from the dye, measured in the green range, was correlated to the partial pressure of oxygen by the following formula:
P
O
2
=
P
′
[
(
I
blue
I
green
)
m
-
1
]
n
,
where “P
O
2
” was the partial pressure of oxygen; “I
blue
”was the intensity of the blue illuminating light; “I
green
” was the intensity of the green fluorescent emission; “P′” was a constant having the dimensions of pressure; and “m” and “n” were non-dimensional constants.
One particularly demanding application for a chemical sensor is the measurement of the partial pressure of dissolved carbon dioxide in sea water. An oceanographic sensor may be called on to resolve differences in CO
2
partial pressure of as little as 1-2 ppm. The sensor must be compatible chemically with sea water and capable of providing accurate measurements despite relatively wide temperature variations.
Dr. David R. Walts of Tufts University proposed an oceanographic sensor in which dissolved CO
2
is detected in sea water by means of fluorescent emission intensity measurements performed on an aqueous solution of carboxy-seminapthofluorescein [hereinafter “c-SNAFL
Snay Jeffrey
Thompson Hine LLP
Yellow Springs Optical Sensor Co. PLL
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