Method for determining optode quality

Abstract

An improved luminescent optode is disclosed that is capable of conducting a self-test to determine the quality of the optode's measurement of the concentration of a quenching molecule in a fluid, along with a method for conducting the self-test.

Claims

1. A computer implemented method for determining the quality of a first optode's measurement of the concentration of a quenching material in a fluid, comprising: instructions stored in a non-transitory computer-readable storage medium that, when executed by a processor, perform the steps of: (a) calibrating the optode by performing at least one measurement to determine a first mathematical relation between: (i) a first intensity of light emitted from an excited luminescent material in the fluid containing the diffused quenching material; and (ii) a first concentration of the quenching material in the fluid, for at least one temperature T1 of the luminescent material; (b) calibrating the optode by performing at least one measurement to determine a second mathematical relation between: (iii) a first fluorescence lifetime of light emitted from the excited luminescent material; and (iv) the first concentration of the quenching material in the fluid for at least the temperature T1 of the excited luminescent material; (c) exposing the excited luminescent material to a second concentration of the quenching material in the fluid at a temperature T2 and measuring values of: (v) a second intensity of light emitted from the excited luminescent material; and (vi) a second fluorescence lifetime of light emitted from the excited luminescent material; (d) using the first mathematical relation determined in step (a) and the second mathematical relation determined in step (b) to calculate a value of at least one measurement parameter using temperature T2 and at least one of the measured second intensity of light and the second fluorescence lifetime; and comparing the calculated value with the measured value of at least one of the second intensity of light and the second fluorescence lifetime measured in step (c) to identify any differences between the calculated value and the measured values; and (e) determining whether any differences between the calculated value and measured values exceed a predetermined quality factor threshold, indicating that the optode's actual measurement quality has degraded and if any differences between the calculated value and the measured values exceed the predetermined quality factor threshold, alerting a user.

2. The method of claim 1, wherein steps (a) and (b) are performed in a controlled calibration vessel holding a first body of the fluid containing the diffused quenching material at the first concentration and step (c) is performed in a second body of the fluid that is being tested to determine the second concentration of the diffused quenching material in the second body of the fluid.

3. The method of claim 2, wherein the fluid is water and the quenching material is oxygen.

4. The method of claim 3, comprising the further step, following steps (a) and (b), of: combining the first optode with at least one additional optode to form a string of spaced-apart optodes and immersing the string of optodes into the fluid.

5. The method of claim 3, further comprising the steps of: connecting the first optode with additional optodes on a string such that they are spaced apart; performing the method of claim 1 on each of the plurality of optodes on the string; and wherein alerting a user further comprises: generating and transmitting an individual alarm signal for any of the plurality of optodes for which the differences between the calculated value and measured values exceed the predetermined quality factor threshold, wherein the alarm signal alerts a user of the string that the individual optode's measurement quality has degraded.

6. An apparatus for measuring a concentration of diffused quenching molecules in a body of fluid, comprising: a plurality of luminescent optodes connected in a string for immersion into the body of fluid, wherein each of the plurality of luminescent optodes is operable to communicate with a common base station and each of the plurality of luminescent optodes is adapted to perform the computer implemented method of claim 1.

7. The computer implemented method of claim 1, wherein alerting the user further comprises signaling the user using an audible alarm.

8. The computer implemented method of claim 1, wherein alerting the user further comprises transmitting an alarm to a wirelessly connected base station.

9. The computer implemented method of claim 1, wherein alerting the user further comprises transmitting an alarm to a base station via a wired connection.

10. The computer implemented method of claim 1, wherein alerting the user further comprises indicating a sensor malfunction.

11. The computer implemented method of claim 1, wherein alerting the user further comprises indicating the first optode requires re-calibration.

12. The computer implemented method of claim 1, wherein the exposing and measuring values in step (c) are performed simultaneously.

13. An improved luminescent optode sensor, comprising: (a) a sensor cap that is coated with a luminescent material within a polymer matrix; (b) a light source for illuminating the luminescent material, causing the luminescent material to become excited; (c) a sensor for detecting the excited luminescent material's light emission; and (d) a microprocessor control circuit for controlling the light source and the optode sensor, the control circuit being further adapted to: (1) store, in non-transitory computer readable memory, calibration information reflecting a first mathematical relationship between: (i) a measured first intensity of light emitted from the excited luminescent material in a fluid containing a diffused quenching material; and (ii) a measured first concentration of the quenching material in the fluid, for at least one measured temperature T1 of the excited luminescent material; (2) store, in non-transitory computer readable memory, calibration information reflecting a second mathematical relationship between: (iii) a measured first fluorescence lifetime of light emitted from the excited luminescent material; and (iv) the measured first concentration of the diffused quenching material in the fluid, for at least the measured temperature T1 of the excited luminescent material; (3) upon exposure of the luminescent material to a second concentration of the quenching material diffused in the fluid at a temperature T2, measure a second intensity of light emitted from the excited luminescent material and a second fluorescent lifetime of light emitted from the excited luminescent material; (4) use the first mathematical relationship described in (d)(1) and the second mathematical relationship in (d)(2) and the measured second intensity of light emitted from the excited luminescent material and the measured second fluorescent lifetime of light emitted from the excited luminescent material described in (d)(3) to calculate measurement parameter values; (5) compare the measurement parameter values to determine a quality factor reflecting the quality of the measured second intensity of light emitted from the excited luminescent material and the measured second fluorescent lifetime of the light emitted from the excited luminescent material made by the optode; and (6) determine whether the quality factor exceeds a predetermined threshold and if the quality factor exceeds the predetermined threshold, generating a user alert.

14. The improved luminescent optode sensor of claim 13, wherein the sensor for detecting the excited luminescent material's light emission further comprises: a photo detector.

15. The improved luminescent optode sensor of claim 13, further comprising a thermometer.

16. The improved luminescent optode sensor of claim 13, further comprising a wireless transceiver.

17. The improved luminescent optode sensor of claim 13, wherein the polymer matrix is a chemical transducer.

18. The improved luminescent optode sensor of claim 13, wherein the light source emits blue light.

19. The improved luminescent optode sensor of claim 13, further comprising: at least one optical filter to prevent light from the light source from reaching the sensor.

20. The improved luminescent optode sensor of claim 13, wherein the light source is an LED.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 of the drawing is a schematic diagram showing various components of a typical fluorescence-based optode.

(2) FIG. 2 of the drawing is a schematic diagram showing various components of a system for measuring, storing and transmitting data from a plurality of optodes or from a limited number of optodes operated at multiple different locations.

(3) FIG. 3 of the drawing is an exemplary graph of data points and calibration curves for intensity versus dissolved oxygen concentration.

(4) FIG. 4 of the drawing is an exemplary graph of data points and calibration curves for characteristic time (shown as phase shift in the figure) versus dissolved oxygen concentration.

(5) FIG. 5 of the drawing is illustrates the concepts of phase shift and illumination time in a luminescent optode.

DETAILED DESCRIPTION OF THE INVENTION

(6) As discussed above, the present invention relates generally to luminescence-based optodes for measuring the concentration of a quenching molecule in a fluid. The following detailed description focuses primarily on the measurement of oxygen in water, but the measurement of some other quenching molecule in a different fluid will be substantially the same.

(7) As shown in FIG. 1, a luminescent optode 10 typically comprises a light source 12 (shown as an LED in the figure) that provides blue excitation light to a luminescent material 14 within a polymer matrix 16. The optode 10 also includes a photo-detector 18 (shown as a photo diode in the figure) for detecting and measuring light emitted from the luminescent material 14. The optode 10 also includes a thermometer 20 (shown as a thermistor in the figure), and associated electrical conductors 22 (wires) for connecting the afore-mentioned components into a microprocessor control circuit 24. The control circuit 24 is adapted and programmed to control the components and to make the various measurements and to perform the various calculations described herein. An optical filter 26 is included between the luminescent material and the photo-detector, to control the light that is detected by the photo-detector, to block blue excitation light from being detected, while permitting red emission light to be detected.

(8) As shown in FIG. 2, one or more optodes 10 can be used to measure the concentration of dissolved oxygen (or another quenching molecule) at one or more locations or depths within a body of water. A plurality of optodes 10 can be combined together to form a string 28 of separate optodes 10. In one embodiment, each of the optode's in the string of optodes communicates with a single base station 30 that is located, for example, on a buoy or other vessel 32 from which the string of optodes is suspended into the water. The base station can collect and process data from the various optodes.

(9) In another embodiment, the data can be processed by the microprocessor control circuit onboard each optode. The data from the various measurements can also be stored onboard the optode, for example, by storing the data on a storage device such as an SD card, or the data can be transmitted from the optode, through wires or via a radio signal or other type of wireless transmission, to a collection point, such as the base station or a central computer.

(10) As shown in FIG. 3, the optode is calibrated at various concentrations [C] and at various temperatures, T.sub.1, T.sub.2, T.sub.3 and T.sub.4, giving measurements of intensity. An interpolation function f.sub.i is mathematically generated from these measurements.

(11) As shown in FIG. 4, the optode is also calibrated at various concentrations [C] and at various temperatures T.sub.1, T.sub.2, T.sub.3 and T.sub.4, giving measurements of characteristic time, t (shown as phase shift in the figure). An interpolation function f.sub.t is mathematically generated from these measurements.

(12) One step of the present invention involves determining a mathematical relation, [C]=f.sub.i(L.sub.em/L.sub.ex,T), between the intensity of light emitted from an excited luminescent material into which the quenching material has been diffused and the concentration of the quenching material, at a variety of different temperatures (i.e., calibrating intensity versus dissolved oxygen concentration ([C]) at a variety of different temperatures).

(13) Another step of the invention involves determining a mathematical relation, [C]=f.sub.t(t, T) between the characteristic time of the emitted light and the concentration of the quenching material and temperature of the luminescent material, at a variety of different temperatures (i.e., calibrating the characteristic time (or phase shift) versus dissolved oxygen concentration ([C]) at a variety of different temperatures). The phase shift or characteristic time is illustrated in FIG. 5.

(14) The functions f.sub.i and f.sub.t, in these steps can be conveniently determined from measurements collected during a single calibration procedure but can also be individually determined from separate calibrations. In either case the calibration method is the same. Calibration of an oxygen optode consists of the experiment of exposing the optode to a number of different oxygen concentrations at a number of different temperatures, and recording the oxygen concentration [C], the temperature T, the intensity ratio L.sub.em/L.sub.ex, the characteristic time t. From these data values, the functional relationships f.sub.i and f.sub.t can be determined using any of a variety of well known mathematical techniques.

(15) Calibration experiments can be performed in different ways. One common way is to place the optode into a temperature controlled bath of water and to create different concentrations of oxygen dissolved within the water by bubbling air, oxygen, or nitrogen thru the water. Oxygen concentration is determined by a reference, either another sensor of some sort or by chemical assay techniques. Temperature is determined by a reference thermometer or by the optode's thermometer. At specific times the reference measurements of oxygen, [C], temperature, T, and the optode measurements of intensity, L.sub.em/L.sub.ex, and characteristic time, t, are obtained.

(16) Optode measurements are obtained by direct communication with the optode at the time of the measurement, or the optode may internally record the measured values and those obtained at the end of the experiment. As time passes a group of measurement points are obtained: <[C.sub.1], T.sub.1, L.sub.em1/L.sub.ex, t.sub.1>, <[C.sub.2], T.sub.2, L.sub.em2/L.sub.ex, t.sub.2>, <[C.sub.3], T.sub.3, L.sub.em3/L.sub.ex, t.sub.3>, and so on.

(17) The activity of determining f.sub.i and f.sub.t consists of selecting an appropriate mathematical expression containing unknown constant values P.sub.X, such as the polynomial Y=P.sub.0+P.sub.1+P.sub.2X+P.sub.3X.sup.2 and then determining the constants P.sub.0, P.sub.1, P.sub.2 and P.sub.3, such that the mathematical expression represents the true f.sub.i and f.sub.t in some optimal way. This activity is often referred to as fitting measurements by the method of least squares.

(18) Another step of the inventive method involves exposing the luminescent material to a particular unknown concentration of the quenching material and measuring the temperature of the luminescent material, T, the intensity of the light emitted from the excited luminescent material, L.sub.em/L.sub.ex, and the characteristic time, t, of the light emitted from the excited luminescent material. This step occurs during actual use of the optode as a sensor.

(19) Another step of the inventive method involves using these measurements, T, L.sub.em/L.sub.ex, t, together with the mathematical relationships, [C]=f.sub.I(L.sub.em/L.sub.ex, T) and [C]=f.sub.t(t, T) to produce two estimates of [C]. These estimates should agree if the optode is providing quality measurements. Disagreement between these two estimates indicates that the measurement quality has become questionable, and the optode can be removed from service. A threshold quality factor can be determined based on the two factors, using any of a variety of different techniques. For example, the quality factor may be expressed as a ratio of the two estimates, or as a percentage difference between the two estimates. So, for example, the quality factor may be set to flag any optode for which the first estimate of [C] differs from the second estimate of [C] by more than 5%, or any other chosen value.

(20) In the preceding paragraph, measurement quality is determined by the comparison of two concentration estimates. However the quality factor can also be expressed in different ways, with other expressions giving additional insight into the operation of the optode. For example it was previously mentioned that the optode is expected to gradually lose intensity due to photo bleaching of the luminescent material or other factors. A useful quality factor could be determined by determining the concentration estimate from the measured characteristic time and temperature and then using this concentration together with a reciprocal f.sub.i to determine the intensity at calibration time that would have produced the measured concentration. The measured intensity can then be compared to this calibration-intensity to give an estimate of photo bleaching. Mathematically this is be expressed as:
Q=(L.sub.em/L.sub.ex)m/f.sub.i.sup.1(f.sub.t(t.sub.m,T.sub.m),T.sub.m)

(21) where Q is the quality, (L.sub.em/L.sub.ex).sub.m is the measured intensity of emission, T.sub.m is the temperature of the luminescent material, and t.sub.m is the measured characteristic time of emission.

(22) Q might also be expressed as a difference, percentage difference, or some other form of comparison of (L.sub.em/L.sub.ex).sub.m and f.sub.i.sup.1(f.sub.t(t.sub.m,T.sub.m),T.sub.m).

(23) In a similar way the characteristic lifetime at calibration could be compared to the measured characteristic lifetime. Mathematically this is expressed as:
Q=t.sub.m/f.sub.t.sup.1(f.sub.i((L.sub.em/L.sub.ex).sub.m,T.sub.m),T.sub.m)

(24) where Q is the quality, (L.sub.em/L.sub.ex).sub.m is the measured intensity of emission, T.sub.m is the temperature of the luminescent material, and t.sub.m is the measured characteristic time of emission.

(25) Q might also be expressed as a difference, percentage difference, or some other form of comparison of t.sub.m and f.sub.t.sup.1 (f.sub.i((L.sub.em/L.sub.ex).sub.m,T.sub.m),T.sub.m).

(26) The invention also includes a device that implements the method described above. The device is similar to a regular optode device designed to operate under only the first (intensity-based) mode or under only the second (lifetime-based) mode, in terms of the arrangement of its light source, photo detector, thermometer, and the sensor cap coated with a luminescent material within a polymer matrix. However, the on-board circuit that controls the optode and its measurements is very different.

(27) In particular, the on-board control circuit is adapted and designed to control the electronics and to gather and manipulate the data measured by the optode, in the manner discussed above. The data from the various measurements can be stored onboard the optode, for example, by storing the data on a storage device such as an SD card, or the data can be transmitted from the optode, via a radio signal or other type of wireless transmission, to a collection point, such as a base station or central computer. The controller circuit can also instruct the optode to signal an audible alarm or to transmit an alarm to a base station or central computer, if the quality factor is exceeded.

(28) While the principles of the disclosure have been illustrated and explained in relation to the exemplary embodiments shown and described herein, the principles of the disclosure are not limited thereto and may be changed and modified without departing from the scope and spirit of the invention as defined in the claims.