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Claims  |
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We claim:
1. A sensor system for determining the pH of a liquid medium which
comprises, in combination, 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 a source of
excitation radiation of wavelength .lambda..sub.o 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 .lambda..sub.1, which emission overlaps the excitation
radiation spectrum of said second fluorescent indicator, and said second
indicator being excited by said emission radiation of wavelength
.lambda..sub.1 and in turn emitting a pH-dependent fluorescence emission
of wavelength .lambda..sub.2, the ratio of intensities of the radiation of
wavelengths .lambda..sub.1/ .lambda..sub.2 providing a highly accurate,
stable determination of the pH of said liquid medium.
2. A sensor system according to claim 1, which includes an optical fiber
having a distal end and a proximal end and in which said combination of
fluorescent indicators is attached to said distal end and said proximal
end is adapted to receive excitation radiation from said source.
3. A sensor system according to claim 1, in which said first fluorescent
indicator is 6,7-dimethoxycoumarin or a pH- insensitive coumarin
derivative.
4. A sensor system according to claim 3, in which said second fluorescent
indicator is 8-hydroxy-1,3, 6-pyrenetrisulfonic acid (HPTA).
5. A method for determining the pH of a liquid medium which comprises
contacting said medium with a sensor system comprising, in combination, 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, subjecting said sensor system to
excitation radiation of a predetermined wavelength .lambda..sub.o, thereby
selectively exciting said first fluorescent indicator to emit a
pH-insensitive fluorescence emission at a wavelength of .lambda..sub.1,
which emission overlaps the excitation radiation spectrum of said second
fluorescent indicator and thus excites said second indicator to emit a
pH-dependent fluorescence emission of wavelength .lambda..sub.2, and
measuring either (i) the ratio of intensities of the emitted radiation of
wavelengths .lambda..sub.1/ .lambda..sub.2, or (ii) the ratio derived from
the intensity of the emitted radiation of wavelength .lambda..sub.1 and
the intensity at the isobestic point between the emission curves of said
first and second indications, thereby obtaining a highly accurate, stable
determination of the pH of said liquid medium.
6. A method according to claim 5, in which the sensor system includes an
optical fiber having a distal end and a proximal end, said combination of
said first and second fluorescent indicators is attached to the distal end
of said optical fiber and said proximal end is adapted to receive
excitation radiation from an appropriate source.
7. A method according to claim 6, in which said excitation radiation is
laser radiation of wavelength, .lambda..sub.o, 337 nm, said first
fluorescent indicator is 6, 7-dimethoxycoumarin which emits fluorescent
radiation of wavelength, .lambda..sub.1,435 nm and said second,
pH-sensitive, fluorescent indicator is 8-hydroxy-1, 3, 6-pyrenetrisulfonic
acid which emits fluorescent radiation of wavelength, .lambda..sub.2, 510
nm.
8. A sensor system for the determination of the concentration of carbon
dioxide in a liquid medium which comprises, in combination, a first
fluorescent indicator whose fluorescence emission is insensitive to pH and
a second fluorescent indicator whose fluorescent emission is highly
sensitive to solution pH, which indicator combination is associated with a
bicarbonate solution bounded by a carbon dioxide-permeable membrane, and
is adapted to respond when a source of excitation radiation of wavelength
.lambda..sub.o is applied to the system such that said first fluorescent
indicator is selectively excited by said excitation radiation to emit a
pH-insensitive fluorescence emission at wavelength .lambda..sub.1, which
emission overlaps the excitation radiation spectrum of said second
fluorescent indicator, said second indicator being excited by said
emission radiation of wavelength .lambda..sub.1, and in turn emitting a
pH-dependent fluorescence emission of wavelength .lambda..sub.2, the ratio
of intensities of the radiation of wavelengths .lambda..sub.1
/.lambda..sub.2 providing 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.
9. A sensor system according to claim 8, which includes an optical fiber
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.
10. A sensor system according to claim 9, in which said first fluorescent
indicator is 6, 7-dimethoxycoumarin or a pH-insensitive coumarin
derivative.
11. A sensor system according to claim 10, in which said second fluorescent
indicator is 8-hydroxy-1, 3, 6-pyrenetrisulfonic acid.
12. A sensor system according to claim 11, in which said first fluorescent
indicator is bonded directly to said optical fiber and said second
fluorescent indicator is suspended in a gel of carboxymethyl cellulose
containing said bicarbonate solution and bounded by a silicone rubber
membrane.
13. A method for determining the concentration of carbon dioxide in a
liquid medium which comprises contacting said medium with a sensor system
comprising, in combination, 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 indicators are associated with a bicarbonate solution bounded by a
carbon dioxide-permeable membrane, subjecting said sensor system to
excitation radiation of a predetermined wavelength .lambda..sub.o, thereby
selectively exciting said first fluorescent indicator to emit a
pH-insensitive fluorescence emission at a wavelength of .lambda..sub.1,
which emission overlaps the excitation radiation spectrum of said second
fluorescent indicator and thus excites said second indicator to emit a
pH-dependent fluorescent emission of wavelength .lambda..sub.2, and
measuring either (i) the ratio of intensities of the emitted radiation of
wavelengths .lambda..sub.1 /.lambda..sub.2, or (ii) the ratio derived from
the intensity of the emitted radiation of wavelength .lambda..sub.1 and
the intensity at the isobestic point between the emission curves of said
first and second indications, thereby obtaining 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.
14. A method according to claim 13, in which the sensor system includes an
optical fiber having a distal end and a proximal end, said combination of
said first and second indicators, bicarbonate solution and membrane is
attached to said distal end and said excitation radiation is transmitted,
from a source adjacent to said proximal end, through said optical fiber
from said proximal end to said distal end.
15. A method according to claim 14, in which said excitation radiation is
laser radiation of wavelength, .lambda..sub.o, 337 nm, said first
fluorescent indicator is 6,7-dimethoxycoumarin which emits fluorescent
radiation wavelength, .lambda..sub.1, 435 nm and said second,
pH-sensitive, fluorescent indicator is 8-hydroxy-1, 3, 6-pyrenetrisulfonic
acid which emits fluorescent radiation of wavelength, .lambda..sub.2, 510
nm. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to a sensor system, particularly to a system for
determining the pH of a liquid medium and a system for determining the
concentration of carbon dioxide in a liquid medium. The invention is also
concerned with a method for measuring the concentration of carbon dioxide
in a medium.
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 hospitalization under intensive care and many devices for the
measurement of said physiological parameters have been suggested in the
art.
U.S. Pat. No. 4,003,707, Lubbers et al, and its reissue patent Re 31879 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 Pat. 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 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. Pat. Nos. 4,200,110 and 4,476,870, Peterson et al, disclose the use of
a pH sensitive indicator in conjunction with a fiber optic pH probe. In
each of these patents the dye indicator is enclosed within a selectively
permeable membrane envelope.
U.S. Pat. No 4,548,907, Seitz et al, discloses a fluorescent-based optical
sensor comprising a membrane immobilized fluorophor secured to one end of
a bifurcated fiber optic channel for exposure to the sample to be
analyzed.
Many fluorescent indicators sensitive to pH, and thereby useful for
pCO.sub.2 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 miniaturize 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.
The problems associated with miniaturization of sensor devices are
substantially solved by a device involving a radiation-transmissible
junction of optical fibers encased in an opaque radiation reflective
jacket as described and claimed in commonly assigned patent application
Ser. No. 874,927 (U.S. Pat. No. 4,927,222).
With the aid of said device the emission signal from radiation-sensitive
indicators, particularly fluorescent indicators of the type disclosed in
the prior art references discussed above, may be received substantially
unattenuated in a suitable detector without the necessity of filters, beam
splitters, reflectors or other light elements used in the prior art.
Another 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, a new improved system is obtained by
the use of two fluorescent indicators acting in concert or by the use of a
single fluorescence indicator which emits fluorescent signals of different
wavelengths in different carriers which signals have intensities
proportional to the parameter under investigation. Under this approach the
parameter being measured is determined by the ratio of two diverging
signals which provides greater resolution and a highly accurate, stable
determination.
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.
When the excitation radiation used to actuate the system according this
invention is introduced through a device as claimed in U.S. Pat.
application Ser. No. 874,927 even greater signal strength and resolution
may be obtained.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a sensor system
for determining the pH of a liquid medium which comprises, in combination,
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 a source of excitation radiation of wavelength .lambda.o 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 .lambda..sub.1, which emission
overlaps the excitation radiation spectrum of said second fluorescent
indicator, and said second indicator being excited by said emission
radiation of wavelength .lambda..sub.1 and in turn emitting a pH-dependent
fluorescence emission of wavelength .lambda..sub.2, the ratio of
intensities of the radiation of wavelengths .lambda..sub.2 /.lambda..sub.2
providing a highly accurate, stable determination of the pH of said liquid
medium.
The invention also provides a method for determining the pH of a liquid
medium which comprises contacting said medium with a sensor system
comprising, in combination, a first fluorescent indicator whose
fluorescence emission is insensitive to pH and a second fluorescent
indicator whose fluorescence emmission is highly sensitive to solution pH,
subjecting said sensor system to excitation radiation of a predetermined
wavelength .lambda..sub.o, thereby selectively exciting said first
fluorescent indicator to emit a pH-insensitive fluorescence emission at a
wavelength of .lambda..sub.1, which emission overlaps the excitation
radiation spectrum of said second fluorescent indicator and thus excites
said second indicator to emit a pH-dependent fluorescence emission of
wavelength .lambda..sub.2, and measuring the ratio of intensities of the
emitted radiation of wavelengths .lambda..sub.1 /.lambda..sub.2 thereby
obtaining a highly accurate, stable determination of the pH of said liquid
medium.
The sensor system and method described above are referred to herein as the
first embodiment of the invention.
By using the system and 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 invention further provides a sensor system for the determination of the
concentration of carbon dioxide in a liquid medium which comprises, in
combination, 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 associated with a bicarbonate solution bounded by a carbon
dioxide-permeable membrane, and is adapted to respond when a source of
excitation radiation of wavelength .lambda..sub.o is applied to the system
such that said first fluorescent indicator is selectively excited by said
excitation radiation to emit a pH-insensitive fluorescence emission at
wavelength .lambda..sub.1, which emission overlaps the excitation
radiation spectrum of said second fluorescent indicator, said second
indicator being excited by said emission radiation of wavelength
.lambda..sub.1, and in turn emitting a pH-dependent fluorescence emission
of wavelength .lambda..sub.2, the ratio of intensities of the radiation of
wavelengths .lambda..sub.1 /.lambda..sub.2 providing 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.
The invention still further provides a method for determining the
concentration of carbon dioxide in a liquid medium which comprises
contacting said medium with a sensor system comprising, in combination, 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 indicators are associated with a
bicarbonate solution bounded by a carbon dioxide-permeable membrane,
subjecting said sensor system to excitation radiation of predetermined
wavelength .lambda..sub.o, thereby selectively exciting said first
fluorescent indicator to emit a pH-insensitive fluorescence emission at a
wavelength of .lambda..sub.1, which emission overlaps the excitation
radiation spectrum of said second fluorescent indicator and thus excites
said second indicator to emit a pH-dependent fluorescence emission of
wavelength .lambda..sub.2, and measuring the ratio of intensities of the
emitted radiation of wavelengths .lambda..sub.1 /.lambda..sub.2, thereby
obtaining 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 sensor system and method for pCO.sub.2 determination described above
are referred to herein as the second embodiment of the invention.
This second embodiment, as with the first embodiment, provides enhancement
of signal resolution due to divergence of fluorescence emission
intensities.
The invention yet further provides a method of measuring the concentration
of carbon dixoide in a medium by determining the water content in a
pH-independent sensor system comprising an optical fiber having a proximal
end and an 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 predetermined
wavelength .lambda..sub.o, emitting fluorescent emission at a wavelength
.lambda..sub.w in the presence of water and at a wavelength .lambda.s 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 .lambda..sub.w and .lambda..sub.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, which method comprises transmitting through said optical fiber,
from a source adjacent to its proximal end, excitation radiation of said
predetermined wavelength .lambda..sub.o, measuring the ratio of the
intensities of the emitted radiation of wavelengths .lambda..sub.w and
.lambda.s, thereby obtaining a determination of the water content and thus
a measurement of the carbon dioxide concentration in the surrounding
medium.
The above-described method of measuring pCO.sub.2 as a function of the
water content in a pH-independent sensor system and the system used in
such method is referred to herein as the third embodiment of the
invention.
Here again, enhancement of signal resolution is obtained from divergence of
fluorescence emission intensities.
DETAILED DESCRIPTION OF THE INVENTION
The sensor system of the first embodiment of the invention preferably
includes an optical fiber having a distal end and a proximal end, in which
said combination of fluorescent indicators is attached to said distal end
and said proximal end is adapted to receive excitation radiation from
s-aid source of excitation radiation.
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 chemically bonded to an acrylic polymer. The pH sensitivity of
the umbellilferone 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 is 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 of the
invention is HPTA.
In the preferred first embodiment of the invention excitation radiation
having a wavelength, i.e. a peak wavelength, .lambda..sub.o, of 337 nm,
for example from a nitrogen gas laser, is transmitted from the proximal
end of an optical fiber through the distal end where it excites a first
indicator, preferably 6,7-dimethoxycoumarin, which emits fluorescent
radiation having a wavelength, .lambda..sub.1, of 435 nm. This
fluorescence emission, in turn, excites the second indicator, preferably
HPTA, to emit fluorescent radiation having a wavelength, .lambda..sub.2,
of 510 nm.
The intensity of the fluorescence emission of wavelength .lambda..sub.2
(510 nm) is dependent upon the intensity of the excitation emission of
wavelength and upon the pH of the surrounding liquid medium, so that
measurement of the ratio of the intensities of the emitted radiation of
wavelengths .lambda..sub.1 /.lambda..sub.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 .lambda..sub.1, derived 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 of the invention preferably
includes an optical fiber 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
fiber 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 .lambda..sub.o, of
337 nm from a nitrogen gas laser through the optical fiber from its
proximal end to its distal end where it excites the 6,7-dimethoxycoumarin
to emit fluorescent radiation having a wavelength .lambda..sub.1, of 435
nm. This fluorescence emission, in turn, excites the HPTA to emit
fluorescent radiation at a wavelength, .lambda..sub.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
.lambda..sub.2 (512 nm) is dependent upon the intensity of the excitation
emission of wavelength .lambda..sub.1 and upon the pH of said surrounding
solution. Therefore, measurement of the ratio of the intensities of the
emitted radiation of wavelengths .lambda..sub.1 /.lambda..sub.2 provides
an indication of the solution pH within the membrane and thus a highly
accurate, stable determination of the concentration of carbon dioxide
(pCO.sub.2) in the liquid medium.
DESCRIPTION OF THE DRAWINGS
The accompanying drawings comprise graphs illustrating excitation and
emission spectra of the indicators used in the sensors of the invention.
FIG. 1 illustrates excitation spectra for HPTA at varying pH levels.
FIGS. 2 illustrates pH-insensitive excitation and emission spectra of
dimethoxycoumarin in a solution of bicarbonate and ethylene glycol;
FIG. 3 illustrates spectra for HPTA in ethylene glycol at different pH
levels.
FIG. 4 illustrates spectra for HPTA in different solvent mixtures.
FIG. 5 is a graph showing HPTA fluorescence as a function of the water
content of the system.
FIG. 6 and FIG. 7 illustrate spectra indicating pCO.sub.2 by a sensor
system according to the invention.
FIG. 8 illustrates spectra for varying carbon dioxide concentrations using
HPTA in a 50/50 ethylene glycol/water solution
FIG. 9 is a graph showing the relationship between the ratio of
fluorescence intensity and carbon dioxide concentration.
The excitation spectra for HPTA illustrated in FIG. 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).
FIG. 2 of the drawings illustrates excitations and emission spectra for
dimethoxy coumarin in a solution of sodium bicarbonate and ethylene
glycol. The concentration of dimethoxy coumarin is about 10.sup.-2 M. 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 FIG. 1.
FIG. 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 FIG.
3. A peak at 510 nm appeared with the addition of water to the system,
regardless of the pH.
FIG. 4 illustrates spectra of HPTA in different mixtures of ethylene glycol
and water. 10.sup.- M 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 fibers and the HPTA was excited to fluoresce at a wavelength of
510 nm. The results are shown graphically in FIG. 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.
TABLE I
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Relative Relative
Intensity Intensity
I (Blue) I (Green)
Solvent .lambda. = 440 nm
.lambda. = 510 nm
Ratio 1/Ratio
______________________________________
100% ethylene
85.63 11.66 7.34 0.1362
glycol
80/20 69.97 49.64 1.41 0.7092
glycol/water
50/50 16.33 92.30 0.177 5.65
glycol/water
20/80 5.33 83.30 0.064 15.63
glycol/water
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 FIG. 5. Using excitation radiation of
wavelength -337 nm the ratio of the fluorescence peaks I(GREEN)/I(BLUE) at
.lambda.=510 nm and .lambda.=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.
FIGS. 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.sup.-3 M was dissolved in carboxymethyl-cellulose (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 fiber formed by fused silica having a diameter of 400
.mu.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 nitrogen laser firing at 2 pulse/second. The fluroescence emission
was detected with a linear array photodiode and 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% CO.sub.2 and 100% CO.sub.2 are illustrated
graphically in FIG. 6 and are set out numerically in the following Table
II.
TABLE II
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% CO.sub.2
I (Blue) I (Green) Ratio
______________________________________
0 5.66 53.97 9.53
100 30.32 20.99 0.69
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EXAMPLE 2
A gel containing mixture of 1:1 dimethoxycoumarin:HPTA at a concentration
of 10.sup.-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 FIG. 7 and set out
numerically in the following Table III.
TABLE III
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Ratio
% Baseline Baseline IM IM (HPTA/
CO.sub.2
(COUM) (HPTA) (COUM) (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 utilizing 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 fiber 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 FIG. 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 FIG. 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.
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