U.S. patent application number 09/801165 was filed with the patent office on 2001-08-02 for method and apparatus for characterizing materials by using a mechanical resonator.
Invention is credited to Bennett, James, Matsiev, Leonid, McFarland, Eric.
Application Number | 20010010174 09/801165 |
Document ID | / |
Family ID | 26831126 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010174 |
Kind Code |
A1 |
Matsiev, Leonid ; et
al. |
August 2, 2001 |
Method and apparatus for characterizing materials by using a
mechanical resonator
Abstract
A method and apparatus for measuring properties of a liquid
composition includes a mechanical resonator, such as a thickness
shear mode resonator or a tuning fork resonator, connected to a
measurement circuit. The measurement circuit provides a variable
frequency input signal to the tuning fork, causing the mechanical
resonator to oscillate. To test the properties of a liquid
composition, the mechanical resonator is placed inside a sample
well containing a small amount of the liquid. The input signal is
then sent to the mechanical resonator and swept over a selected
frequency range, preferably less than 1 MHz to prevent the liquid
being tested from exhibiting gel-like characteristics and causing
false readings. The mechanical resonator's response over the
frequency range depends on various characteristics of the liquid
being tested, such as the temperature, viscosity, and other
physical properties. Particular mechanical resonators, such as
tuning fork resonators, can also be used to measure a liquid
composition's electrical properties, such as the dielectric
constant and conductivity, because the tuning fork's structure
allows a high degree of electrical coupling between the tuning fork
and the surrounding liquid. The mechanical resonator can be covered
with a coating to impart additional special detection properties to
the resonator, and multiple resonators can be attached together as
a single sensor to obtain multiple frequency responses. The
invention is particularly suitable for combinatorial chemistry
applications, which require rapid analysis of chemical properties
for screening.
Inventors: |
Matsiev, Leonid; (Cupertino,
CA) ; Bennett, James; (Santa Clara, CA) ;
McFarland, Eric; (Santa Barbara, CA) |
Correspondence
Address: |
RADER, FISHMAN & GRAUER PLLC
39533 WOODWARD AVENUE
SUITE 140
BLOOMFIELD HILLS
MI
48304-0610
US
|
Family ID: |
26831126 |
Appl. No.: |
09/801165 |
Filed: |
March 7, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09801165 |
Mar 7, 2001 |
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09133171 |
Aug 12, 1998 |
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09133171 |
Aug 12, 1998 |
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08946921 |
Oct 8, 1997 |
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6182499 |
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Current U.S.
Class: |
73/592 ;
73/64.53 |
Current CPC
Class: |
G01N 9/002 20130101;
G01N 2291/014 20130101; G01N 29/42 20130101; G01N 29/348 20130101;
G01N 2291/0423 20130101; G01N 2009/006 20130101; G01N 5/04
20130101; G01N 11/16 20130101; G01N 29/036 20130101; G01N 2291/0427
20130101; G01N 2291/02863 20130101; G01N 2291/0255 20130101; G01N
2291/0422 20130101; G01N 2291/0426 20130101; G01N 29/022 20130101;
G01N 2291/02818 20130101; G01N 29/4418 20130101; G01H 13/00
20130101; B01J 2219/00704 20130101; G01N 2291/0224 20130101 |
Class at
Publication: |
73/592 ;
73/64.53 |
International
Class: |
G01N 029/00; G01H
003/00 |
Claims
What is claimed is:
1. A method for measuring a property of a fluid composition using a
tuning fork resonator, the method comprising: placing the tuning
fork resonator in the fluid composition such that at least a
portion of the tuning fork resonator is surrounded by the fluid
composition; applying a variable frequency input signal to a
measurement circuit coupled with the tuning fork resonator to
oscillate the tuning fork resonator; varying the frequency of the
variable frequency input signal over a predetermined frequency
range to obtain a frequency-dependent resonator response of the
tuning fork resonator; and determining the property of the fluid
composition based on the resonator response.
2. The method of claim 1, wherein the fluid composition is a liquid
composition, wherein a plurality of liquid compositions are
measured by a plurality of tuning fork resonators, and wherein said
method further comprises: providing an array of sample wells;
placing each of said plurality of liquid compositions in a separate
sample well; placing at least one of said plurality of tuning fork
resonators in at least one sample well; applying a variable
frequency input signal to a measurement circuit coupled with each
tuning fork resonator in said at least one sample wells to
oscillate each tuning fork resonator associated with each of said
at least one sample well; varying the frequency of the variable
frequency input signal over a predetermined frequency range to
obtain a frequency-dependent resonator response of each tuning fork
resonator associated with said at least one sample well; and
analyzing the resonator response of each tuning fork resonator
associated with said at least one sample well to measure a property
of each liquid composition in said at least one sample well.
3. The method of claim 2, wherein said method measures a physical
property of each liquid composition in the array of sample
wells.
4. The method of claim 2, wherein the physical property measured by
said method is selected from the group consisting of specific
weight, temperature and viscosity.
5. The method of claim 2, wherein said method measures an
electrical property of each liquid composition in the array of
sample wells.
6. The method of claim 5, wherein the electrical property measured
by said method is selected from the group consisting of dielectric
constant and conductivity.
7. The method of claim 2, wherein said method measures an
electrical property and a physical property of each liquid
composition in the array of sample wells simultaneously.
8. The method of claim 7, wherein the electrical properties and
physical properties measured by said method are selected from the
group consisting of specific weight, viscosity, temperature,
dielectric constant and conductivity.
9. The method of claim 8, wherein said method simultaneously
measures at least two properties selected from the group consisting
of specific weight, viscosity, temperature, dielectric constant and
conductivity of each liquid composition in the array of sample
wells.
10. The method of claim 1, further comprising the step of coating
the tuning fork resonator with a material that modifies the
characteristics of the tuning fork resonator.
11. The method of claim 10, wherein the material used in said
coating step is a functionality designed to change the resonator
response of the tuning fork resonator if a selected substance is
present in the fluid composition.
12. The method of claim 11, wherein said functionality used in the
coating step contains receptor molecules for attracting molecules
of the selected substance in the fluid composition to change the
resonator response.
13. The method of claim 1, wherein the placing step includes
placing a plurality of tuning fork resonators in the fluid
composition.
14. The method of claim 13, wherein the varying step varies the
frequency of the variable frequency input signal over a plurality
of predetermined frequency ranges to obtain a plurality of
frequency dependent resonator responses from the plurality of
tuning fork resonators.
15. The method of claim 2, further comprising the step of coating
at least one of said plurality of tuning fork resonators with a
material that modifies the characteristics of said at least one
tuning fork resonators.
16. The method of claim 15, wherein the material used in said
coating step is a functionality designed to change the resonator
response of said at least one tuning fork resonator of a selected
substance is present in the liquid composition associated with said
at least one tuning fork resonator.
17. The method of claim 16, wherein said functionality used in the
coating step contains receptor molecules for attracting molecules
of the selected substance in the liquid composition associated with
said at least one tuning fork resonator.
18. A method for monitoring a change in a property of a liquid
composition, the method comprising: placing the mechanical
resonator in the liquid composition such that at least a portion of
the mechanical resonator is submerged in the liquid composition;
applying a variable frequency input signal to a measurement circuit
coupled with the mechanical resonator to oscillate the mechanical
resonator; varying the frequency of the variable frequency input
signal over a predetermined frequency range to obtain a
frequency-dependent resonator response of the mechanical resonator;
determining the property of the liquid based on the mechanical
resonator response; repeating the applying, varying, and
determining steps over time; and monitoring over time a change in
the mechanical resonator response reflecting the change in property
of the liquid composition.
19. The method of claim 18, the change monitored in the monitoring
step is a physical change in the liquid composition.
20. The method of claim 19, wherein the physical change in the
liquid composition is a liquid-to-solid state transformation of the
liquid composition.
21. The method of claim 18, wherein the change monitored in the
monitoring step is a chemical transformation of the liquid
composition.
22. The method of claim 21, wherein the chemical transformation
monitored in the monitoring step is a polymerization reaction.
23. The method of claim 18, wherein said method measures a property
selected from the group consisting of specific weight, viscosity,
temperature, dielectric constant and conductivity of the liquid
composition and monitors said property over time.
24. The method of claim 23, wherein said method simultaneously
measures at least two properties selected from the group consisting
of specific weight, viscosity, temperature, dielectric constant and
conductivity of the liquid composition and monitors said properties
over time.
25. The method of claim 18, further comprising the step of coating
the mechanical resonator with a material that modifies the
characteristics of the mechanical resonators.
26. The method of claim 25, wherein the material used in said
coating step is a functionality designed to change the resonator
response of the mechanical resonator if a selected substance is
present in the liquid composition.
27. The method of claim 26, wherein said functionality used in the
coating step contains receptor molecules for attracting molecules
of the selected substance in the liquid composition to change the
resonator response.
28. The method of claim 18, wherein the placing step includes
placing a plurality of mechanical resonators in the liquid
composition.
29. The method of claim 28, wherein each of said plurality of
mechanical resonators has a different resonator response
characteristic, and wherein the varying step varies the frequency
of the variable input signal over a plurality of frequency
dependent resonator responses from the plurality of mechanical
resonators.
30. The method of claim 18, wherein the mechanical resonator placed
in the placing step is a multiple-mode resonator that can be
operated in more than one mechanical mode, and wherein the varying
step comprises varying the frequency of the variable frequency
input signal to obtain a plurality of frequency-dependent resonator
responses corresponding to said more than one mechanical mode.
31. An apparatus for measuring a property of a fluid composition,
comprising: a tuning fork resonator; means for containing the fluid
composition; a measurement circuit coupled with said tuning fork
resonator, said measurement circuit having a signal generator for
generating a variable frequency input signal to cause said tuning
fork to oscillate; and a receiver coupled to the measurement
circuit to output a frequency response of said tuning fork
resonator.
32. The apparatus of claim 31, wherein the fluid composition is a
liquid composition, the apparatus further comprising: an array of
tuning fork resonators; and an array of sample wells for holding a
plurality of liquid compositions, and wherein said measurement
circuit and said receiver are coupled with said array of tuning
fork resonators to obtain a frequency response associated with each
of said plurality of liquid compositions.
33. The apparatus of claim 31, wherein the tuning fork comprises at
least two tines, each tine including a quartz crystal center
portion having at least two faces and an electrode on at least one
of the two faces of said quartz crystal center portion.
34. The apparatus of claim 31, wherein said quartz crystal center
portion of each tine has four faces and wherein four electrodes are
connected to said quartz crystal center portion such that one
electrode is coupled to each face.
35. The apparatus of claim 31, further comprising a coating
material on said tuning fork resonator that modifies the
characteristics of the tuning fork resonator.
36. The apparatus of claim 35, wherein said coating material is a
functionality designed to change the resonator response of the
tuning fork resonator if a selected substance is present in the
fluid composition.
37. The apparatus of claim 36, wherein said functionality contains
receptor molecules for attracting molecules of the selected
substance in the fluid composition to change the resonator
response.
38. The apparatus of claim 31, further comprising a plurality of
tuning fork resonators, each tuning fork resonator having a
different resonator response characteristic.
39. The apparatus of claim 38, wherein each tuning fork has a
different functionality, each functionality designed to change the
resonator response of its associated tuning fork resonator if a
selected substance corresponding with the specific coating is
present in the fluid composition.
40. A method for measuring a property of a plurality of liquid
compositions using a plurality of mechanical resonators, the method
comprising: providing an array of sample wells; placing each of
said plurality of liquid compositions in a separate sample well;
placing at least one of said plurality of mechanical resonators
into at least one of said sample wells such that at least a portion
of the mechanical resonator is submerged in its associated liquid
composition; applying a variable frequency input signal to a
measurement circuit coupled with at least one of said plurality of
mechanical resonators to oscillate said at least one mechanical
resonator in its associated liquid composition; varying the
frequency of the variable frequency input signal over a
predetermined frequency range to obtain a frequency-dependent
resonator response of the at least one mechanical resonator; and
determining the property of the liquid based on the mechanical
resonator response to measure a property of each liquid
composition.
41. The method of claim 40, further comprising the step of
distinguishing between at least two of said plurality of liquid
compositions based on the mechanical resonator response.
42. The method of claim 40, wherein the mechanical resonator placed
in the placing step is a thickness shear mode resonator.
43. The method of claim 40, wherein the mechanical resonator placed
in the placing step is a tuning fork resonator.
44. The method of claim 40, further comprising placing a plurality
of mechanical resonators in the placing step.
45. The method of claim 44, wherein the plurality of mechanical
resonators placed in the placing step are selected from the group
consisting of tridents, cantilevers, torsion bars, length extension
resonators, bimorphs, unimorphs, membrane resonators, surface
acoustic wave devices, thickness share mode resonators, and tuning
fork resonators.
46. The method of claim 45, wherein each of the plurality of
mechanical resonators placed in the placing step is a different
type of mechanical resonator from the other mechanical resonators
in the plurality of mechanical resonators.
47. The method of claim 40, wherein the mechanical resonator placed
in the placing step is a multiple-mode resonator that can be
operated in more than one mechanical mode, and wherein the varying
step comprises varying the frequency of the variable frequency
input signal to obtain a plurality of frequency-dependent resonator
responses corresponding to said more than one mechanical mode.
48. The method of claim 40, wherein the mechanical resonator placed
in the placing step is one selected from the group consisting of
tridents, cantilevers, torsion bars, bimorphs, unimorphs, membrane
resonators, and surface acoustic wave devices.
49. The method of claim 40, wherein said method measures a physical
property of each liquid composition in the array of sample
wells.
50. The method of claim 49, wherein the physical property measured
by said method is selected from the group consisting of specific
weight, temperature and viscosity.
51. The method of claim 40, wherein said method measures an
electrical property of each liquid composition in the array of
sample wells.
52. The method of claim 51, wherein the electrical property
measured by said method is selected from the group consisting of
dielectric constant and conductivity.
53. The method of claim 40, wherein said method measures an
electrical property and a physical property of each liquid
composition in the array of sample wells simultaneously.
54. The method of claim 53, wherein the electrical properties and
physical properties measured by said method are selected from the
group consisting of specific weight, viscosity, dielectric constant
and conductivity.
55. The method of claim 54, wherein said method simultaneously
measures at least two properties selected from the group consisting
of specific weight, viscosity, temperature, dielectric constant and
conductivity of each liquid composition in the array of sample
wells.
56. The method of claim 40, further comprising the steps of:
calibrating each of said tuning fork resonators against a standard
liquid having known properties to obtain calibration data; and
determining the property of each liquid composition based on the
calibration data.
57. The method of claim 40, further comprising the step of coating
at least one of said mechanical resonators with a material that
modifies the characteristics of the mechanical resonator.
58. The method of claim 57, wherein the material used in the
coating step is a functionality designed to change the resonator
response of the mechanical resonator if a selected substance is
present in the liquid composition.
59. The method of claim 58, wherein the functionality used in the
coating step contains receptor molecules for attracting molecules
of the selected substance in the liquid composition to change the
resonator response.
60. A method for measuring a property of a liquid composition
flowing through a conduit, the method comprising: placing a
mechanical resonator in the conduit such that at least a portion of
the mechanical resonator will be surrounded by the fluid
composition as the fluid composition flows through the conduit;
applying a variable frequency input signal to a measurement circuit
coupled with the mechanical resonator to oscillate the tuning fork
resonator; varying the frequency of the variable frequency input
signal over a predetermined frequency range to obtain a
frequency-dependent resonator response of the mechanical resonator;
and determining the property of the fluid composition based on the
mechanical resonator's response.
61. The method of claim 60, further comprising the steps of:
calibrating the mechanical resonator against a standard fluid
having known properties to obtain calibration data; and determining
the property of the fluid composition based on the calibration
data.
62. The method of claim 60, wherein the mechanical resonator placed
in the placing step is a thickness shear mode resonator.
63. The method of claim 60, wherein the mechanical resonator placed
in the placing step is a tuning fork resonator.
64. The method of claim 60, further comprising placing a plurality
of mechanical resonators in the placing step.
65. The method of claim 64, wherein the plurality of mechanical
resonator s placed in the placing step are selected from the group
consisting of tridents, cantilevers, torsion bars, length extension
resonators, bimorphs, unimorphs, membrane resonators, surface
acoustic wave devices, thickness share mode resonators, and tuning
fork resonators.
66. The method of claim 65, wherein each of the plurality of
mechanical resonators placed in the placing step is a different
type of mechanical resonator from the other mechanical resonator in
the plurality of mechanical resonators.
67. The method of claim 60, wherein the mechanical resonator placed
in the placing step is a multiple-mode resonator that can be
operated in more than one mechanical mode, and wherein the varying
step comprises varying the frequency of the variable frequency
input signal to obtain a plurality of frequency-dependent resonator
responses corresponding to said more than one mechanical mode.
68. The method of claim 60, wherein the mechanical resonator placed
in the placing step is one selected from the group consisting of
tridents, cantilevers, torsion bars, length extension resonators,
bimorphs, unimorphs, membrane resonators, and surface acoustic wave
devices.
69. The method of claim 60, wherein said method measures a physical
property of the fluid composition flowing through the conduit.
70. The method of claim 69, wherein the physical property measured
by said method is selected from the group consisting of specific
weight, temperature and viscosity.
71. The method of claim 60, wherein said method measures an
electrical property of the fluid composition flowing through the
conduit.
72. The method of claim 71, wherein the electrical property
measured by said method is selected from the group consisting of
dielectric constant and conductivity.
73. The method of claim 60, wherein said method measures
simultaneously an electrical property and a physical property of
the fluid composition flowing through the conduit.
74. The method of claim 73, wherein the electrical properties and
physical properties measured by said method are selected from the
group consisting of specific weight, viscosity, temperature,
dielectric constant and conductivity.
75. The method of claim 74, wherein said method simultaneously
measures at least two properties selected from the group consisting
of specific weight, viscosity, temperature, dielectric constant and
conductivity of the fluid composition flowing through the
conduct.
76. The method of claim 60, further comprising the step of coating
the mechanical resonator with a material to modify the
characteristics of the mechanical resonators.
77. The method of claim 76, wherein the material used in said
coating step is a functionality designed to change the resonator
response of the mechanical resonator if a selected substance is
present in the fluid composition to change the resonator
response.
78. The method of claim 77, wherein said functionality used in the
coating step contains receptor molecules for attracting molecules
of the selected substance in the liquid composition.
Description
RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of
commonly assigned, copending U.S. application Ser. No. 08/946,921,
filed Oct. 8, 1997, the disclosure of which is incorporated by
reference.
TECHNICAL FIELD
[0002] The present invention is directed to using mechanical
oscillators for measuring various properties of fluids (including
both liquids and vapors), and more particularly to a method and
system using a mechanical oscillator (resonator) for measuring
physical, electrical and/or chemical properties of a fluid based on
the resonator's response in the fluid to a variable frequency input
signal.
BACKGROUND ART
[0003] Companies are turning to combinatorial chemistry techniques
for developing new compounds having novel physical and chemical
properties. Combinatorial chemistry involves creating a large
number of chemical compounds by reacting a known set of starting
chemicals in all possible combinations and then analyzing the
properties of each compound systematically to locate compounds
having specific desired properties. See, for example, U.S. patent
application Ser. No. 08/327,513 (published as WO 96/11878), filed
Oct. 18, 1994, entitled "The Combinatorial Synthesis of Novel
Materials", the disclosure of which is incorporated by
reference.
[0004] The virtually endless number of possible compounds that can
be created from the Periodic Table of Elements requires a
systematic approach to the synthesizing and screening processes.
Thus, any system that can analyze each compound's properties
quickly and accurately is highly desirable. Further, such a system
would be useful in any application requiring quick, accurate
measurement of a liquid's properties, such as in-line measurement
of additive concentrations in gasoline flowing through a conduit or
detection of environmentally-offending molecules, such as hydrogen
sulfide, flowing through a smokestack.
[0005] It is therefore an object of the invention to measure
simultaneously both the physical and the electrical properties of a
fluid composition using a mechanical resonator device.
[0006] It is also an object of the invention to detect differences
clearly between two or more compounds in a fluid composition by
using a mechanical resonator device to measure a composition's
physical and electrical properties.
[0007] It is a further object of the invention to use a mechanical
resonator device to monitor and measure a physical or chemical
transformation of a fluid composition.
[0008] It is also an object of the invention to use a mechanical
resonator device to detect the presence of a specific material in a
fluid.
SUMMARY OF THE INVENTION
[0009] The present invention includes a method for measuring a
property of a fluid composition using a tuning fork resonator, the
method comprising:
[0010] placing the tuning fork resonator in the fluid composition
such that at least a portion of the tuning fork resonator is
submerged in the fluid composition;
[0011] applying a variable frequency input signal to a measurement
circuit coupled with the tuning fork resonator to oscillate the
tuning fork resonator;
[0012] varying the frequency of the variable frequency input signal
over a predetermined frequency range to obtain a
frequency-dependent resonator response of the tuning fork
resonator; and
[0013] determining the property of the fluid composition based on
the resonator response.
[0014] The method can also measure a plurality of fluid
compositions, wherein the fluid compositions are liquid
compositions, using a plurality of tuning fork resonators, wherein
the method further comprises:
[0015] providing an array of sample wells;
[0016] placing each of said plurality of liquid compositions in a
separate sample well;
[0017] placing at least one of said plurality of tuning fork
resonators in at least one sample well;
[0018] applying a variable frequency input signal to a measurement
circuit coupled with each tuning fork resonator in said at least
one sample wells to oscillate each tuning fork resonator associated
with each of said at least one sample well;
[0019] varying the frequency of the variable frequency input signal
over a predetermined frequency range to obtain a
frequency-dependent resonator response of each tuning fork
resonator associated with said at least one sample well; and
[0020] analyzing the resonator response of each tuning fork
resonator associated with said at least one sample well to measure
a property of each liquid composition in said at least one sample
well.
[0021] Accordingly, the present invention is directed primarily to
a method using a mechanical piezoelectric quartz resonator
("mechanical resonator") for measuring physical and electrical
properties, such as the viscosity density product, the dielectric
constant, and the conductivity of sample liquid compositions in a
combinatorial chemistry process. The detailed description below
focuses primarily on thickness shear mode ("TSM") resonators and
tuning fork resonators, but other types of resonators can be used,
such as tridents, cantilevers, torsion bars, bimorphs, or membrane
resonators. Both the TSM resonator and the tuning fork resonator
can be used to measure a plurality of compounds in a liquid
composition, but the tuning fork resonator has desirable properties
that make it more versatile than the TSM resonator.
[0022] The mechanical resonator is connected to a measuring circuit
that sends a variable frequency input signal, such as a sinusoidal
wave, that sweeps over a predetermined frequency range, preferably
in the 25-30 kHz range for the tuning fork resonator and in a
higher range for the TSM resonator. The resonator response over the
frequency range is then monitored to determine selected physical
and electrical properties of the liquid being tested. Although both
the TSM resonator and the tuning fork resonator can be used to test
physical and electrical properties, the tuning fork resonator is an
improvement over the TSM resonator because of the tuning fork's
unique response characteristics and high sensitivity.
[0023] Both the TSM resonator and the tuning fork resonator can be
used in combinatorial chemistry applications according to the
present invention. The small size and quick response of the tuning
fork resonator in particular makes it especially suitable for use
in combinatorial chemistry applications, where the properties of a
vast number of chemicals must be analyzed and screened in a short
time period. In a preferred embodiment, a plurality of sample wells
containing a plurality of liquid compositions are disposed on an
array. A plurality of TSM or tuning fork resonators are dipped into
the liquid compositions, preferably one resonator per composition,
and then oscillated via the measuring circuit. Because the
resonating characteristics of both the TSM resonator and the tuning
fork resonator virtually eliminate the generation of acoustic
waves, the size of the sample wells can be kept small without the
concern of acoustic waves reflecting from the walls of the sample
wells. In practice, the tuning forks can be oscillated at a lower
frequency range than TSM resonators, making the tuning forks more
applicable to real-world applications and more suitable for testing
a wide variety of compositions, including high molecular weight
liquids.
[0024] In another embodiment of the invention, the mechanical
resonator is coated with a material to change the resonator's
characteristics. The material can be a general coating to protect
the resonator from corrosion or other problems affecting the
resonator's performance, or it can be a specialized
"functionalization" coating that changes the resonator's response
if a selected substance is present in the composition being tested
by the resonator.
[0025] To obtain a more complete range of characteristics for a
selected fluid composition, multiple resonators having different
resonator characteristics can be connected together as a single
sensor for measuring the fluid composition. The resonator responses
from all of the resonators in the sensor can then be correlated to
obtain additional information about the composition being tested.
By using resonators having different characteristics, the fluid
composition can be tested over a wider frequency range than a
single resonator. Alternatively, a single resonator that can be
operated in multiple mechanical modes (e.g. shear mode, torsion
mode, etc.) can be used instead of the multiple resonators. The
resonator responses corresponding to each mode would be correlated
to obtain the additional information about the composition.
[0026] The mechanical resonator system of the present invention,
particularly a system using the tuning fork resonator, can also be
used to monitor changes in a particular liquid by keeping the
resonator in the liquid composition as it undergoes a physical
and/or chemical change, such as a polymerization reaction. The
invention is not limited to measuring liquids, however; the quick
response of the tuning fork resonator makes it suitable for
measuring the composition of fluid compositions, both liquid and
vaporous, that are flowing through a conduit to monitor the
composition of the fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIGS. 1a and 1b are cross-sectional views of a TSM resonator
plate and a tuning fork resonator tine used in preferred
embodiments of the present invention, respectively;
[0028] FIG. 2 is a block diagram illustrating an example of a
composition testing system of the present invention;
[0029] FIG. 3 is a representative diagram illustrating oscillation
characteristics of the tuning fork resonator used in a preferred
embodiment of the present invention;
[0030] FIGS. 4a and 4b are simplified schematic diagrams
illustrating a tuning fork resonator connection with the
measurement circuit in a preferred embodiment of the present
invention;
[0031] FIG. 4c illustrates a sample response of the representative
circuit shown in FIG. 4b;
[0032] FIGS. 5a and 5b are examples of traces comparing the
frequency responses of the TSM resonator and the tuning fork
resonator of the present invention, respectively;
[0033] FIGS. 6a and 6b are examples of graphs illustrating the
relationship between the viscosity density product and the
equivalent serial resistance of the TSM resonator and the tuning
fork resonator of the present invention, respectively;
[0034] FIGS. 7a and 7b are examples of graphs illustrating the
relationship between the dielectric constant and the equivalent
parallel capacitance of the TSM resonator and the tuning fork
resonator of the present invention, respectively;
[0035] FIGS. 8a and 8b are examples of graphs illustrating the
relationship between the molecular weight of a sample composition
and the equivalent serial resistance of the TSM resonator and the
tuning fork resonator of the present invention, respectively, in a
polymerization reaction;
[0036] FIGS. 9a and 9b illustrate another embodiment of the
invention using a resonator that is treated with a coating for
targeting detection of specific chemicals; and
[0037] FIGS. 10a, 10b and 10c illustrate examples of different
multiple resonator sensors of yet another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] The method and apparatus of the present invention focuses on
using a mechanical resonator to generate and receive oscillations
in a fluid composition for testing its characteristics in a
combinatorial chemistry process or other process requiring analysis
of the fluid composition's physical and/or chemical properties.
Although the detailed description focuses on combinatorial
chemistry and the measurement of a liquid composition's
characteristics, the invention can be used in any application
requiring measurement of characteristics of a fluid composition,
whether the fluid is in liquid or vapor form. The fluid composition
itself can be any type of fluid, such as a solution, a liquid
containing suspended particulates, or, in some embodiments, even a
vapor containing a particular chemical or a mixture of chemicals.
It can also include a liquid composition undergoing a physical
and/or chemical change (e.g. an increase in viscosity).
[0039] Mechanical resonators, such as thickness shear mode (TSM)
quartz resonators 10, are used in the present invention for
measuring various physical properties of fluid compositions, such
as a liquid's viscosity, molecular weight, specific weight, etc.,
in a combinatorial chemistry setting or other liquid measurement
application. Referring to FIG. 1a, TSM resonators 10 usually have a
flat, plate-like structure where a quartz crystal 12 is sandwiched
in between two electrodes 14. In combinatorial chemistry
applications, the user first generates a "library", or large
collection, of compounds in a liquid composition. Normally, each
liquid composition is placed into its own sample well. A TSM
resonator 10 connected to an input signal source (not shown) is
placed into each liquid composition, and a variable frequency input
signal is sent to each TSM resonator 10, causing the TSM resonator
10 to oscillate. The input signal frequency is swept over a
predetermined range to generate a unique TSM resonator 10 response
for each particular liquid. Because every compound has a different
chemical structure and consequently different properties, the TSM
resonator 10 response will be also be different for each compound.
The TSM resonator response is then processed to generated a visual
trace of the liquid composition being tested. An example of traces
generated by the TSM resonator 10 for multiple liquid compositions
is shown in FIG. 5a. Screening and analysis of each compound's
properties can then be conducted by comparing the visual traces of
each compound with a reference and/or with other compounds. In this
type of application, the TSM resonator 10 serves both as the wave
source and the receiver.
[0040] Two types of waves can be excited in liquids: compression
waves (also called acoustic waves), which tend to radiate a large
distance, on the order of hundreds of wavelengths, from the
wave-generating source; and viscose shear waves, which decay almost
completely only one wavelength away from the wave-generating
source. In any liquid property testing, acoustic waves should be
kept to a minimum because they will create false readings when
received by the resonator due to their long decay characteristics.
For typical prior art ultrasonic transducers/resonators, the
resonator oscillation creates acoustic waves that radiate in all
directions from the resonator, bounce off the sides of the sample
well, and adversely affect the resonator response. As a result, the
resonator response will not only reflect the properties of the
liquid being measured, but also the effects of the acoustic waves
reflecting from the walls of the sample well holding the liquid,
thereby creating false readings. Using a sample well that is much
greater than the acoustic wavelength does minimize the negative
effects of acoustic waves somewhat, but supplying thousands of
sample wells having such large dimensions tends to be
impractical.
[0041] TSM resonators 10 primarily generate viscose shear waves and
are therefore a good choice for liquid property measurement in
combinatorial chemistry applications because they do not generate
acoustic waves that could reflect off the sides of the sample wells
and generate false readings. As a result, the sample wells used
with the TSM resonators 10 can be kept relatively small, making it
feasible to construct an array of sample wells for rapid,
simultaneous testing of many liquids. The high stiffness of TSM
resonators 10, however, require them to be operated at relatively
high frequencies, on the order of 8-10 MHz. This stiffness does not
adversely affect measurement accuracy for many applications,
though, making the TSM resonator an appropriate choice for
measuring numerous liquid compositions.
[0042] However, TSM resonators 10 can be somewhat insensitive to
the physical properties of certain liquids because the load
provided by the surrounding liquid is less than the elasticity of
the resonator. More particularly, the high operating frequencies of
TSM resonators 10 make them a less desirable choice for measuring
properties of certain liquid compositions, particularly
high-molecular weight materials such as polymers. When high
frequency waves are propagated through high molecular-weight
liquids, the liquids tend to behave like gels because the rates at
which such large molecules move correspond to frequencies that are
less than that of the TSM resonator's oscillations. This causes the
TSM resonator 10 to generate readings that sometimes do not reflect
the properties at which the liquids will actually be used (most
materials are used in applications where the low-frequency dynamic
response is most relevant). Although it would be more desirable to
operate the TSM resonator 10 at lower frequencies so that
laboratory conditions reflect real world conditions, the stiffness
of the TSM resonator 10 and its resulting high operating
frequencies can make operation at lower frequencies rather
difficult. Further, even when the TSM resonator 10 can accurately
measure a liquid's properties, the differences in the visual traces
associated with different compositions are relatively slight,
making it difficult to differentiate between compositions having
similar structures, as shown in FIG. 5a.
[0043] TSM resonators and other plate-type resonators, while
adequate, may not always be the best choice for measuring the
electrical characteristics, such as the dielectric constant, of the
liquid composition being measured. As shown in FIG. 1a, the
cross-section of a TSM resonator 10 has the same structure as a
flat capacitor, resulting in relatively little coupling between the
electric field of the resonator and the surrounding composition.
While there can be enough electrical coupling between the resonator
and the composition to measure the composition's electrical
properties, a greater amount of electrical coupling is more
desirable for increased measurement accuracy. Electrical coupling
will be explained in greater detail below when comparing the
electrical characteristics between the TSM resonator 10 and the
tuning fork resonator 20.
[0044] FIGS. 1a and 1b show a cross-section of a TSM resonator
plate 10 and a tuning fork tine 22, respectively. The tuning fork
resonator 20 is preferably made from a quartz crystal 24 and has
two tines 22, as represented in FIG. 2, each tine having the quartz
crystal center 24 and at least one electrode 26 connected to the
quartz crystal 24. The tuning fork tines 22 in the preferred
structure have a square or rectangular cross-section such that the
quartz crystal center 24 of each tine has four faces. The
electrodes 26 are then attached to each face of the quartz crystal
center 24, as shown in FIG. 1b. The method and system of the
present invention can use any type of tuning fork resonator, such
as a trident (three-prong) tuning fork or tuning forks of different
sizes, without departing from the spirit and scope of the
invention.
[0045] The cross-sectional views of the TSM resonator 10 and the
tuning fork resonator 20 shown in FIGS. 1a and 1b also illustrate
the relative differences between the electric coupling of each
resonator with the surrounding liquid. Referring to FIG. 1a, the
structure of the TSM resonator 10 is very flat, making it close to
a perfect capacitor when it is placed in the liquid to be measured.
As noted above, the quartz crystal 12 in the TSM resonator 10 is
sandwiched between two electrodes 14, causing most of an electric
field 16 to travel between the two electrodes through the quartz
crystal 12. Because most of the electric field 16 is concentrated
within the quartz crystal 12 rather than outside of it, there is
very little electric coupling between the TSM resonator 10 and the
surrounding liquid except at the edges of the resonator 10. While
there may be sufficient electrical coupling to measure the
electrical properties, such as the conductivity or dielectric
constant, of the liquid composition being tested, a greater degree
of coupling is desirable to ensure more accurate measurement.
[0046] By comparison, as shown in Figure lb, the structure of each
tuning fork tine 22 allows much greater electrical coupling between
the tine 22 and the surrounding liquid because the tuning fork
tine's cross-sectional structure has a much different structure
than a flat capacitor. Because the tuning fork tine 22 is submerged
within the liquid being tested, an electric field 27 associated
with each tine 22 does not concentrate in between the electrodes 24
or within the quartz crystal 24, but instead interacts outside the
tine 22 with the surrounding liquid. This increased electrical
coupling allows the tuning fork 20 to measure accurately the
electrical properties of the liquid as well as its physical
properties, and it can measure both types of properties
simultaneously if so desired.
[0047] One unexpected result of the tuning fork resonator 20 is its
ability to suppress the generation of acoustic waves in a liquid
being tested, ensuring that the resonator's 20 physical response
will be based only on the liquid's physical properties and not on
acoustic wave interference or the shape of the sample well holding
the liquid. As explained above, TSM resonators 10 minimize
excitation of acoustic waves because it generates shear
oscillations, which do not excite waves normal to the resonator's
surface. As also explained above, however, the TSM resonator 10
requires high frequency operation and is not suitable for many
measurement applications, particularly those involving
high-molecular weight liquids.
[0048] Without wishing to be bound by any particular theory, the
inventors believe that the tuning fork resonator 20 used in the
present invention virtually eliminates the effects of acoustic
waves without having to increase the size of the sample wells to
avoid wave reflection. Tuning fork resonators 20, because of their
shape and their orientation in the liquid being tested, contain
velocity components normal to the vibrating surface. Thus, it was
assumed in the art that tuning fork resonators were unsuitable for
measuring liquid properties because they would generate acoustic
waves causing false readings. In reality, however, tuning fork
resonators 20 are very effective in suppressing the generation of
acoustic waves for several reasons. First, the preferred size of
the tuning fork resonator 20 used in the invention is much smaller
than the wavelength of the acoustic waves that are normally
generated in a liquid, as much as one-tenth to one-hundredth the
size. Second, as shown in FIG. 3, the tines 22 of the tuning fork
resonator 20 oscillate in opposite directions, each tine 22 acting
as a separate potential acoustic wave generator. In other words,
the tines 22 either move toward each other or away from each other.
Because the tines 22 oscillate in opposite directions and opposite
phases, however, the waves that end up being generated locally by
each tine 22 tend to cancel each other out, resulting in virtually
no acoustic wave generation from the tuning fork resonator 22 as a
whole.
[0049] A simplified diagram of one example of the inventive
mechanical resonator 20 system is shown in FIG. 2. Although the
explanation of the system focuses on using the tuning fork
resonator 20, the TSM resonator 10 described above can also be used
for the same purpose. To measure the property of a given liquid,
the tuning fork resonator 20 is simply submerged in the liquid to
be tested. A variable frequency input signal is then sent to the
tuning fork resonator using any known means to oscillate the tuning
fork, and the input signal frequency is swept over a predetermined
range. The tuning fork resonator's response is monitored and
recorded. In the example shown in FIG. 2, the tuning fork resonator
20 is placed inside a well 26 containing a liquid to be tested.
This liquid can be one of many liquids for comparison and screening
or it can simply be one liquid whose properties are to be analyzed
independently. Further, if there are multiple liquids to be tested,
they can be placed in an array and measured simultaneously with a
plurality of tuning fork resonators to test many liquids in a given
amount of time. The liquid can also be a liquid that is undergoing
a polymerization reaction or a liquid flowing through a
conduit.
[0050] The tuning fork resonator 20 is preferably coupled with a
network analyzer 28, such as a Hewlett-Packard 8751A network
analyzer, which sends a variable frequency input signal to the
tuning fork resonator 20 to generate the resonator oscillations and
to receive the resonator response at different frequencies. The
resonator output then passes through a high impedance buffer 30
before being measured by a wide band receiver 32. The invention is
not limited to this specific type of network analyzer, however; any
other analyzer that generates and monitors the resonator's response
over a selected frequency range can be used without departing from
the scope of the invention. For example, a sweep generator and AC
voltmeter can be used in place of the network analyzer.
[0051] An equivalent circuit of the tuning fork resonator 20 and
its associated measurement circuit is represented in FIGS. 4a and
4b . FIG. 4a represents an illustrative tuning fork resonator
system that measures a liquid's viscosity and dielectric constant
simultaneously, while FIG. 4b represents a tuning fork resonator
system that can also measure a liquid's conductivity as well.
Referring to FIG. 4a, the measurement circuit includes a variable
frequency input signal source 42, and the resonator equivalent
circuit 43 contains series capacitor Cs, resistor Rs, inductor L,
and parallel capacitor Cp. The resonator equivalent circuit 43
explicitly illustrates the fact that the quartz crystal 24 in the
tuning fork resonator 20 acts like a capacitor Cp. The
representative circuit 40 also includes input capacitor Cin, input
resistor Rin and an output buffer 44.
[0052] The representative circuit shown in FIG. 4b adds a parallel
resistor Rp in parallel to capacitor Cp to illustrate a circuit
that measures conductivity as well as dielectric constant and
viscosity, preferably by comparing the equivalent resistance found
in a given liquid with a known resistance found via calibration.
These concepts will be explained in further detail below with
respect to FIGS. 5a-b, 6a-b, 7a-b, and 8a-b. Rp represents the
conductivity of the liquid being tested. The resistance can be
calibrated using a set of liquids having known conductivity and
then used to measure the conductivity of a given liquid. For
example, FIG. 4c shows a sample trace comparing the resonator
response in pure toluene and in KaBr toluene solution. A liquid
having greater conductivity tends to shift the resonator response
upward on the graph, similar to liquids having higher dielectric
constants. However, unlike liquids with higher dielectric
constants, a liquid having greater conductivity will also cause the
resonator response to level out somewhat in the frequency sweep, as
can be seen in the upper trace 45 between 30 and 31.5 kHz. In the
example shown in FIG. 4c, the difference between the upper trace 45
and the lower trace 46 indicates that the equivalent resistance Rp
caused by the additional KaBr in solution was about 8
mega-ohms.
EXPERIMENTAL EXAMPLES
[0053] FIGS. 5a-b, 6a-b, 7a-b and 8a-b are examples demonstrating
the effectiveness of the invention. These figures show some
differences between the frequency responses, for various liquid
compositions, of the plate-type TSM resonator 10 and the tuning
fork resonator 20. FIGS. 5a, 6a, 7a and 8a are examples using the
TSM resonator 10, and FIGS. 5b, 6b, 7b and 8b are examples using
the tuning fork resonator 20.
[0054] The experimental conditions for generating the example
tuning fork resonator traces in FIGS. 5b, 6b, 7b, and 8b are
described below. The experimental conditions for generating the
comparative TSM resonator traces in FIGS. 5a, 6a, 7a and 8a are
generally similar to, if not the same as, the conditions for the
tuning fork resonator except for, if needed, minor modifications to
accommodate the TSM resonator's particular geometry. Therefore, for
simplicity and clarity, the TSM resonator's particular experimental
conditions will not be described separately.
[0055] All of the solvents, polymers and other chemicals used in
the illustrated examples were purchased from Aldrich, and the
polymer solutions were made according to standard laboratory
techniques. Dry polymers and their corresponding solvents were
weighed using standard balances, and the polymer and solvent were
mixed until the polymer dissolved completely, creating a solution
having a known concentration. The solutions were delivered to and
removed from a 30 ul stainless steel cylindrical measurement well
that is long enough to allow a tuning fork resonator to be covered
by liquid. Liquid delivery and removal to and from the well was
conducted via a pipette or syringe.
[0056] Before any experiments were conducted with the solutions,
the tuning fork resonator response in air was measured as a
reference. The actual testing processes were conducted in a
temperature-controlled laboratory set at around 20 degrees
Centigrade. Once the liquid was delivered to the well, the tuning
fork was placed in the well and the system was left alone to allow
the temperature to stabilize. Alternatively, the tuning fork can be
built into a wall portion or a bottom portion of the well with
equally accurate results. The tuning fork was then oscillated using
the network analyzer. The resonator response was recorded during
each measurement and stored in a computer memory. The measured
response curve was fitted to a model curve using an equivalent
circuit, which provided specific values for the equivalent circuit
components described above with respect to FIGS. 4a and 4b and the
traces in FIGS. 6a through 8b.
[0057] After the measurement of a given solution was completed, the
resonator was kept in the well and pure solvent was poured inside
the well to dissolve any polymer residue or coating in the well and
on the tuning fork. The well and tuning fork were blown dry using
dry air, and the tuning fork response in air was measured again and
compared with the initial tuning fork measurement to ensure that
the tuning fork was completely clean; a clean tuning fork would
give the same response as the initial tuning fork response. Note
that the above-described experimental conditions are described only
for purposes of illustration and not limitation, and those of
ordinary skill in the art would understand that other experimental
conditions can be used without departing from the scope of the
invention.
[0058] Although both the TSM resonator 10 and the tuning fork
resonator 20 are considered to be part of the method and system of
the present invention, the tuning fork resonator 20 has wider
application than the TSM resonator 10 and is considered by the
inventors to be the preferred embodiment for most measurement
applications because of its sensitivity, availability and
relatively low cost. For example, note that in FIGS. 5a and 5b, the
frequency sweep for the TSM resonator 10 is in the 8 MHz range,
while the frequency sweep for the tuning fork resonator 20 of the
present invention is in the 25-30 kHz range, several orders of
magnitude less than the TSM resonator frequency sweep range. This
increases the versatility and applicability of the tuning fork
resonator 20 for measuring high molecular weight liquids because
the operating frequency of the tuning fork resonator 20 is not high
enough to make high molecular weight liquids act like gels.
Further, because most applications for the solutions are lower
frequency applications, the laboratory conditions in which the
liquid compositions are tested using the tuning fork resonator 20
more closely correspond with real-world conditions.
[0059] Also, the operating frequency of the tuning fork resonator
20 varies according to the resonator's geometry; more particularly,
the resonance frequency of the tuning fork 20 depends on the ratio
between the tine cross-sectional area and the tine's length.
Theoretically, it is possible to construct a tuning fork resonator
20 of any length for a given frequency by changing the tuning
fork's cross-sectional area to keep the ratio between the length
and the cross-section constant. In practice, however, tuning fork
resonators 20 are manufactured from quartz wafers having a few
selected standard thicknesses. Therefore, the cross-sectional area
of the tuning fork 20 tends to be limited based on the standard
quartz wafer thicknesses, forcing the manufacturer to change the
tuning fork's resonating frequency by changing the tine length.
These manufacturing limitations must be taken into account when
selecting a tuning fork resonator 20 that is small enough to fit in
minimal-volume sample wells (because the chemicals used are quite
expensive) and yet operates at a frequency low enough to prevent
the tested liquids from acting like gels. Of course, in other
applications, such as measurement of liquids in a conduit or in
other containers, the overall size of the tuning fork resonator 20
is not as crucial, allowing greater flexibility in selecting the
size and dimensions of the tuning fork resonator 20. Selecting the
actual tuning fork dimensions and designing a tuning fork resonator
in view of manufacturing limitations are tasks that can be
conducted by those of skill in the art after reviewing this
specification.
[0060] Referring to FIGS. 5a and 5b, the solutions used as examples
in FIGS. 5a and 5b have somewhat similar structures and weights. As
a result, the TSM resonator responses for each solution, shown in
FIG. 5a, create very similar traces in the same general range.
Because the traces associated with the TSM resonator 10 overlap
each other to such a great extent, it is difficult to isolate and
compare the differences between the responses associated with each
solution. By comparison, as shown in FIG. 5b, the increased
sensitivity of the tuning fork resonator 20 causes small
differences in the chemical structure to translate into significant
differences in the resonator response. Because the traces generated
by the tuning fork resonator 20 are so distinct and spaced apart,
they are much easier to analyze and compare.
[0061] Using a tuning fork resonator 20 to measure properties of
liquids also results in greater linearity in the relationship
between the square root of the product of the liquid's viscosity
density and the equivalent serial resistance Rs (FIGS. 6a and 6b)
as well as in the relationship between the dielectric constant and
the equivalent parallel capacitance Cp (FIGS. 7a and 7b) compared
to TSM resonators 10. For example, the relationship between the
liquid viscosity and serial resistance for a tuning fork resonator
20, as shown in FIG. 6b, is much more linear than that for the TSM
resonator, as shown in FIG. 6a.
[0062] Similarly, the relationship between the dielectric constant
and the equivalent parallel capacitance is more linear for a tuning
fork resonator 20, as shown in FIGS. 7a and 7b. This improved
linear relationship is primarily due to the relatively low
frequencies at which the tuning fork resonator 20 operates; because
many liquids exhibit different behavior at the operating
frequencies required by the TSM resonator 10, the TSM resonator 10
will tend not to generate testing results that agree with known
data about the liquids' characteristics.
[0063] FIGS. 8a and 8b illustrate sample results from real-time
monitoring of polymerization reactions by a TSM resonator and a
tuning fork resonator, respectively. The graphs plot the equivalent
resistance Rs of the resonators oscillating in 10 and 20 mg/ml
polystyrene-toluene solutions versus the average molecular weight
of polystyrene. As explained above, high molecular weight solutions
often exhibit different physical characteristics, such as
viscosity, at higher frequencies.
[0064] The size and shape of the TSM resonator 10 make the
resonator suitable, but not as accurate, for real-time monitoring
of polymerization reactions compared with the tuning fork resonator
20. This is because the TSM resonator's high operating frequency
reduces the accuracy of measurements taken when the molecular
weight of the polymerizing solution increases. As shown in FIG. 8a,
a high operating frequency TSM resonator is not very sensitive in
monitoring the molecular weight of the polystyrene solution used in
the illustrated example. A tuning fork resonator, by contrast, has
greater sensitivity to the molecular weight of the solution being
measured, as shown in FIG. 8b. This sensitivity and accuracy makes
it possible, for many reactions, to estimate the amount of
converted solution in the polymerization reaction and use the
conversion data to estimate the average molecular weight of the
polymer being produced.
[0065] Although the above-described examples describe using a TSM
or a tuning fork resonator without any modifications, the resonator
can also be treated with a "functionality" (a specialized coating)
so that it is more sensitive to certain chemicals. The resonator
may also be treated with a general coating to protect the resonator
from corrosion or other problems that could impede its performance.
A representative diagram of an embodiment having a functionalized
resonator is shown in FIGS. 9a and 9b. Although FIGS. 9a and 9b as
well as the following description focuses on coating or
functionalizing a tuning fork resonator, any other mechanical
resonator can also be used without departing from the scope of the
invention.
[0066] The tuning fork resonator 20 can be coated with a selected
material to change how the resonator 20 is affected by a fluid
composition (which, as explained earlier, includes both liquid and
vapor compositions). As mentioned above, one option is a general
coating for providing the tuning fork resonator 20 with additional
properties such as corrosion resistance, chemical resistance,
electrical resistance, and the like. Another option, as noted
above, is using a "functionality", which coats the tines with
materials that are designed for a specific application, such as
proteins to allow the tuning fork resonator 20 to be used as a pH
meter or receptors that attract specific substances in the fluid
composition to detect the presence of those substances. The coating
or functionality can be applied onto the tuning fork resonator 20
using any known method, such as spraying or dipping. Further, the
specific material selected for the coating or functionality will
depend on the specific application in which the tuning fork
resonator 20 is to be used. J. Hlavay and G. G. Guilbault described
various coating and functionalization methods and materials to
adapt piezoelectric crystal detectors for specific applications in
"Applications of the Piezoelectric Crystal Detector in Analytical
Chemistry," Analytical Chemistry, Vol. 49, No. 13, November 1977,
p. 1890, incorporated herein by reference. For example, applying
different inorganic functionalities to the tuning fork resonator 20
allows the resonator to detect organophosphorous compounds and
pesticides.
[0067] An example of a tuning fork resonator that has undergone a
functionalization treatment is illustrated in FIGS. 9a and 9b. FIG.
9a represents a tuning fork tine 22 that has been treated by
absorbing, coating, or otherwise surrounding the tine 22 with a
functionality designed to change the tuning fork's resonance
frequency after being exposed to a selected target chemical. In the
illustrated example, the tuning fork tine 22 is covered with
receptor molecules 90, represented in FIGS. 9a and 9b by Y-shaped
members, designed to bond with specific target molecules. Because
the resonance frequency and the damping of the tuning fork
resonator depends on the effective mass of the tine 22 and the
amount of "drag" of the tine 22 within the fluid, any change in the
tine's mass or the amount of drag will change the tuning fork's
resonance response. More specifically, the resonance frequency of
the tuning fork resonator is proportional to the square root of the
inverse of the tuning fork's mass. An increase in the tuning fork's
mass will therefore reduce the tuning fork's resonance
frequency.
[0068] This mass-frequency relationship is used to detect the
presence of a specific target chemical in a fluid composition in
this example. When the functionalized tuning fork tine 22 is placed
in a fluid composition containing the target chemical, the
receptors 90 on the tuning fork tine 22 will chemically bond with
molecules of the target chemical 92, as shown in FIG. 9b. The
resonance frequency of the tuning fork resonator will consequently
decrease because of the increased mass and the additional drag
created by the additional molecules 92 attached to the tuning fork
tines 22 via the receptor molecules 90. Thus, when screening a
plurality of fluid compositions to detect the presence of a target
chemical in any of them, only the fluid compositions containing the
target chemical will cause the tuning fork's resonance frequency to
change. Fluid compositions without the target chemical will not
contain molecules that will bond with the receptor molecules 90 on
the tuning fork tine 22, resulting in no resonance frequency change
for those fluids. Alternatively, the tuning fork tines 22 can be
functionalized with a material that physically changes when exposed
to molecules of a selected chemical such that the material changes
the mechanical drag on the tuning fork tine 22 when it is exposed
to the selected chemical. For example, adding a hydrophobic or
hydrophilic functionality to the tuning fork tine 22 allows the
tine 22 to attract or repel selected substances in the medium being
analyzed, changing the mass or effective mass of the tuning fork
and thereby changing its resonance frequency.
[0069] In yet another embodiment of the present invention, multiple
mechanical resonators can be attached together in a single sensor
to measure a wider range of responses for a given fluid
composition, as shown in FIGS. 10a, 10b and 10c. The multiple
resonator sensor can be fabricated from a single quartz piece such
that all of the resonators are attached together by a common base,
as shown in the figures. The multi-resonator sensor could also be
attached to multiple frequency generating circuits, such as
multiple network analyzers 28, to measure properties of the fluid
compositions over multiple frequency sweeps so that the generated
data can be correlated to obtain additional information about the
liquid compositions. Because different resonator structures are
best suited for measurement over different frequency ranges and for
materials having different characteristics, a sensor combining a
plurality of different resonators can provide a more complete
representation of the fluid composition's characteristics over a
wider frequency range than a single resonator. FIGS. 10a, 10b and
10c show specific examples of possible multi-resonator
configurations, but those of skill in the art would understand that
sensors having any combination of resonators can be constructed
without departing from the scope of the invention.
[0070] FIG. 10a illustrates one possible sensor 100 configuration
containing both a tuning fork resonator 102 and a TSM resonator
104. This type of sensor 100 can be used to, for example, measure
the mechanical and electrical properties of very thick liquids such
as polymer resins and epoxies. This sensor 100 can also be used to
monitor a material as it polymerizes and hardens. For example, the
sensor 100 can be placed in a liquid composition containing
urethane rubber in its diluted state so that the tuning fork 102 is
used initially to measure both the composition's density viscosity
product and its dielectric constant. As the rubber changes to a gel
and finally to a solid, the sensor 100 can switch to using the TSM
resonator 104 to measure the rubber's mechanical properties,
leaving the tuning fork resonator 102 to operate as a dielectric
sensor only.
[0071] A sensor 106 for observing a fluid composition over a wide
frequency range is shown in FIG. 10b. High polydispersity polymer
solutions are ideally measured over a wide frequency spectrum, but
most resonators have optimum performance within a relatively
limited frequency range. By combining different resonators having
different resonance frequencies and different response
characteristics, it is possible to obtain a more complete spectrum
of resonator responses for analyzing the fluid's characteristics
under many different conditions. For example, due to the wide
spectrum of polydisperse solution relaxation times, it is generally
predicted that high molecular weight compositions will react at
lower frequencies than lighter molecular weight compositions. By
changing the temperature, observing the frequency response of
different resonators, and correlating the different resonator
responses, it is possible to obtain a more accurate picture of a
composition's relaxation spectrum than from a single resonator.
[0072] A low frequency tuning fork resonator 108 and a high
frequency tuning fork resonator 110 in one sensor will probably
suffice for most wide-frequency range measurements. For certain
cases, however, the resonators in the multi-resonator sensor 106
can also include a trident tuning fork resonator 112, a length
extension resonator 114, a torsion resonator 116, and a TSM
resonator 1 18, membrane oscillators, bimorphs, unimorphs, and
various surface acoustic wave devices, as well as any combination
thereof, or even a single resonator structure than can operate in
multiple mechanical modes (e.g. compression mode, axial mode,
torsion mode). Of course, not all of these resonators are needed
for every application, but those of skill in the art can select
different combinations that are applicable to the specific
application in which the sensor 106 will be used.
[0073] Alternatively, multiple resonators having the same structure
but different coatings and/or functionalities can be incorporated
into one sensor 120, as shown in FIG. 10c. In this example, a
plurality of tuning fork resonators 122, 124, 126 have the same
structure but have different functionalities, each functionality
designed to, for example, bond with a different target molecule.
The high sensitivity of the tuning fork resonators 122, 124, 126
makes them particularly suitable for "artificial noses" that can
detect the presence of an environmentally-offending molecule, such
as hydrogen sulfide or nitrous oxide, in industrial emissions. When
the sensor 120 is used in such an application, one tuning fork
resonator 122 can, for example, be functionalized with a material
designed to bond with hydrogen sulfide while another resonator 124
can be functionalized with a material designed to bond with nitrous
oxide. The presence of either one of these molecules in the fluid
composition being tested will cause the corresponding tuning fork
resonator 122, 124 to change its resonance frequency, as explained
with respect to FIGS. 9a and 9b.
[0074] The tuning fork resonators 122, 124, 126 can also be
functionalized with a polymer layer or other selective absorbing
layer to detect the presence of specific molecules in a vapor.
Because the tuning fork resonators 122, 124, 126 are highly
sensitive to the dielectric constant of the surrounding fluid, the
tuning fork resonators 122, 124, 126 can easily detect changes in
the dielectric constant of the fluid and recognize a set of
solvents with different dielectric constants in the fluid. This
information, combined with other observable parameters, makes
tuning fork resonators particularly adaptable for use in artificial
noses.
[0075] The method and system of the present invention has been
described above in the combinatorial chemistry context, but it is
not limited to such an application. Because the resonators in the
method and system of the present invention have high sensitivities
and quick response times, it can be also be used for in-line
monitoring of fluid compositions flowing through conduits or
pipelines. For example, the invention can be used in a feedback
system to monitor properties of liquids flowing through a gas or
oil pipeline to monitor and control the concentration of additives
in the gas or oil, or to detect the presence of impurities in water
flowing through a water pipe. The additives or impurities will
change the physical and electrical characteristics of the liquid
flowing through the conduit. A functionalized tuning fork resonator
20 can further detect the presence of a specific chemical in a
fluid composition, whether it is a liquid or a vapor, and can be
used to monitor the presence of, for example, a known chemical
pollutant in a smokestack. The high sensitivity and quick response
time of the resonator, and the tuning fork resonator 20 in
particular, makes it uniquely suitable for such an application. The
circuitry and system used to generate the visual traces from the
resonator's response can be the same as described above or be any
other equivalent resonator analysis system.
[0076] Further, although the above description focuses primarily on
using TSM resonators and tuning fork resonators, any other
mechanical resonators exhibiting similar characteristics can be
used. Tridents, cantilevers, torsion bars, bimorphs, and/or
membrane resonators can be substituted for the TSM resonator or
tuning fork resonator without departing from the scope of the
claimed invention.
[0077] It should be understood that various alternatives to the
embodiments of the invention described herein may be employed in
practicing the invention. It is intended that the following claims
define the scope of the invention and that the methods and
apparatus within the scope of these claims and their equivalents be
covered thereby.
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