U.S. patent application number 09/823560 was filed with the patent office on 2001-09-20 for photoacoustic spectroscopy apparatus and method.
Invention is credited to Amonette, James E., Autrey, Tom, Daschbach, John L., Foster-Mills, Nancy S..
Application Number | 20010022657 09/823560 |
Document ID | / |
Family ID | 22307739 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022657 |
Kind Code |
A1 |
Autrey, Tom ; et
al. |
September 20, 2001 |
Photoacoustic spectroscopy apparatus and method
Abstract
The invention encompasses photoacoustic apparatuses and
photoacoustic spectrometry methods. The invention also encompasses
sample cells for photoacoustic spectrometry, and sample
cell/transducer constructions. In one aspect, the invention
encompasses a photoacoustic spectroscopy apparatus, comprising: a)
a sample reservoir and an acoustically-stimulable transducer
acoustically coupled with the sample reservoir, the transducer
comprising a detector surface having a substantially planar
portion; and b) a beam of light configured to be directed through
the sample at an angle oblique relative to the substantially planar
portion of the detector surface to generate sound waves in the
sample. In another aspect, the invention encompasses a
photoacoustic spectroscopy sample cell, comprising: a) a first
block of material having opposing front and back surfaces, the
front surface comprising a substantially planar portion configured
to be against a sample and the back surface comprising a
substantially planar portion configured to be joined to a
transducer, the back surface being parallel to the front surface;
and b) a pair of opposing side surfaces joined to opposite ends of
the front and back surfaces, one of the opposing side surfaces
being configured for passage of light therethrough and extending at
a first oblique angle relative to a plane containing the
substantially planar portion of the front surface.
Inventors: |
Autrey, Tom; (West Richland,
WA) ; Daschbach, John L.; (Richland, WA) ;
Amonette, James E.; (Richland, WA) ; Foster-Mills,
Nancy S.; (Richland, WA) |
Correspondence
Address: |
Intellectual Property Services
Battelle Memorial Institute
Pacific Northwest Divistion
P.O Box 999
Richland
WA
99352
US
|
Family ID: |
22307739 |
Appl. No.: |
09/823560 |
Filed: |
March 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09823560 |
Mar 30, 2001 |
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09105781 |
Jun 26, 1998 |
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6236455 |
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Current U.S.
Class: |
356/432 |
Current CPC
Class: |
G01N 21/1702
20130101 |
Class at
Publication: |
356/432 |
International
Class: |
G01N 021/00 |
Claims
1. A spectroscopy apparatus comprising means for enabling direct
measurement of absorbance across an entirety of a range of from
about 0.0001 absorbance units per centimeter to about 10,000
absorbance units per centimeter.
2. A photoacoustic spectroscopy apparatus comprising: a transducer
having a planar detecting surface; and means for directing a beam
of light through a sample obliquely relative to the plane of the
detector surface to generate acoustic signals detectable by the
transducer.
3. A photoacoustic spectroscopy apparatus comprising: a transducer
having a planar detecting surface; a wall of material having a
surface against a sample; and a light source oriented to direct
light through the wall of material and into the sample at an
oblique angle relative to the planar detecting surface to generate
a plurality of acoustic waves within the sample that are detectable
by the transducer.
4. The apparatus of claim 3 wherein the oblique angle is adjustable
to a first orientation and a second orientation, the light directed
at the first orientation predominately reflecting from the surface
against the sample, and the light directed at the second
orientation predominately penetrating the surface against the
sample and refracting into the sample.
5. The apparatus of claim 3 wherein the oblique angle is greater
than 20.degree. and less than 70.degree..
6. A photoacoustic spectroscopy sample cell, comprising: a first
block of material having: opposing front and back surfaces, the
front surface comprising a substantially planar portion for
contacting a sample and the back surface comprising a substantially
planar portion for joining to a transducer, the back surface being
substantially parallel to the front surface; and a pair of opposing
side surfaces joined to opposite ends of the front and back
surfaces, the opposing side surfaces being a first opposing side
surface and a second opposing side surface, the first opposing side
surface for receiving light therethrough and comprising a first
curved region between the front and back surfaces.
7. The sample cell of claim 6 wherein the second opposing side
surface comprises a second curved region between the front and back
surfaces.
8. The sample cell of claim 7 wherein the first and second curved
regions are convex.
9. The sample cell of claim 7 wherein the first and second curved
regions are concave.
10. The sample cell of claim 7 wherein the first and second curved
regions are shaped as arcs of circles.
11. The sample cell of claim 7 wherein the first and second curved
regions are shaped as arcs of circles and extend along an entirety
of the respective lengths of the first and second opposing side
surfaces.
12. The sample cell of claim 7 wherein the first and second curved
regions are shaped as arcs of circles and are substantially mirror
images of one another.
13. A photoacoustic spectroscopy sample cell, comprising: a first
block of material having: opposing front and back surfaces, the
front surface comprising a substantially planar portion for
contacting a sample and the back surface comprising a substantially
planar portion for joining to a transducer, the back surface being
substantially parallel to the front surface; and a pair of opposing
side surfaces joined to opposite ends of the front and back
surfaces, the opposing side surfaces being a first opposing side
surface and a second opposing side surface, the first opposing side
surface for receiving light therethrough and extending at a first
oblique angle relative to a plane containing the substantially
planar portion of the front surface, the second opposing side
surface extending at a second oblique angle relative to the plane
containing the substantially planar portion of the front
surface.
14. The sample cell of claim 13 wherein the first and second
oblique angles are greater than 20.degree. and less than
70.degree..
15. The sample cell of claim 13 wherein the first oblique angle and
the second oblique angle are the same.
16. The sample cell of claim 13 further comprising a second block
of the material.
17. The sample cell of claim 16 wherein said second block
comprises: opposing front and back surfaces, the front surface
comprising a substantially planar portion for contacting the sample
and substantially parallel to the substantially planar portion of
the front surface of the first block; and a pair of opposing side
surfaces joined to opposite ends of the front and back surfaces of
the second block, the opposing surfaces being a first opposing side
surface of the second block and a second opposing side surface of
the second block, the first opposing side surface for receiving
light therethrough.
18. The sample cell of 17 wherein said first opposing side surface
extends at an third oblique angle relative to a plane containing
the substantially planar portion of the front surface of the second
block, wherein the third oblique angle is equal to the first
oblique angle
19. The sample cell of claim 17 wherein the second opposing side
surface is substantially parallel with the first opposing side
surface of the first block.
20. The sample cell of claim 13 wherein the transducer is mounted
in acoustic communication with the substantially planar portion of
the front surface.
21. The sample cell of claim 20 wherein the transducer is along the
back surface.
22. The sample cell of claim 20 wherein the material comprises a
critical angle relative to a wavelength of light and wherein the
first oblique angle is less than the critical angle.
23. The sample cell of claim 20 wherein the material comprises a
critical angle and wherein the first angle is greater than the
critical angle.
24. A photoacoustic spectroscopy method comprising: providing a
transducer having a planar detecting surface; providing a wall of
material having a surface against a sample; directing light through
the wall of material and into the sample, the light passing through
the sample at an oblique angle relative to the planar detecting
surface; and generating a plurality of acoustic waves within the
sample that are detectable by the transducer.
25. The method of claim 24 wherein the directing comprises one or
more of refracting a predominate portion of the light from the
surface and reflecting a predominate portion of the light from the
surface.
26. A photoacoustic spectroscopy method comprising: providing a
sample and an acoustically-stimulable transducer acoustically
coupled with the sample, the acoustically-stimulable transducer
comprising a detector surface having a substantially planar
portion; directing a first beam of light through the sample at an
angle oblique to the substantially planar portion of the detector
surface to generate sound waves in the sample; and detecting the
sound waves with the transducer.
27. The method of claim 26 wherein the angle is greater than
10.degree. and less than 80.degree..
28. The method of claim 26 wherein the sample is against a first
planar portion of a mass, and wherein the detector surface is along
a second planar portion of the mass.
29. The method of claim 28 wherein the first and second planar
portions are opposing outer surfaces of the mass.
30. The method of claim 26 wherein the sample is between two
blocks, the sample being against a first planar portion of one of
the blocks, and the detector surface being along a second planar
portion of said one of the blocks.
31. The method of claim 26 wherein the angle is greater than
20.degree. and less than 70.degree..
32. The method of claim 26 further comprising directing a second
beam of light through the sample at an angle oblique to the
substantially planar portion of the detector surface to generate
sound waves in the sample.
33. The method of claim 32 wherein the directing the second beam of
light occurs after the directing the first beam of light.
34. The method of claim 32 wherein directing the first beam of
light is in a first direction within the sample and the directing
the second beam of light is in a second direction within the
sample.
35. The method of claim 34 wherein the first and second directions
are substantially opposite to one another
36. The method of claim 34 wherein the directing the second beam of
light occurs after the directing the first beam of light.
37. A method of photoacoustic spectroscopy, comprising: providing a
sample; providing a block of material proximate the sample, the
block having a substantially planar surface adjacent the sample and
comprising a critical angle relative to the substantially planar
surface; providing a transducer acoustically coupled with the
sample; directing a beam of light into the block at an angle
oblique to the substantially planar surface, the angle being
greater than the critical angle to generate an internal reflection
of the light from the surface, the light generating sound waves in
the sample during the reflection; and detecting the sound waves
with the transducer.
38. The method of claim 37 wherein the beam of light comprises
multiple wavelengths, and wherein only some of said multiple
wavelengths are internally reflected from the surface.
39. The method of claim 37 wherein proximate is inserting the block
into the sample.
40. The method of claim 39 wherein the inserting into the sample is
prior to directing the beam.
41. The method of claim 37 wherein the sample comprises a fluid
contained within a vessel.
42. The method of claim 41 wherein said fluid is a liquid.
Description
TECHNICAL FIELD
[0001] The invention pertains to photoacoustic spectroscopy,
including methods of photoacoustic spectroscopy and photoacoustic
spectroscopy apparatuses.
BACKGROUND OF THE INVENTION
[0002] Photoacoustic spectroscopy is an analytical method that
involves stimulating a sample by light and subsequently detecting
sound waves emanating from the sample. Typically, only a narrow
range of wavelengths of light are introduced into a sample. Such
narrow range of wavelengths of light can be formed by, for example,
a laser. Utilization of only a narrow range of wavelengths can
enable pre-selected molecular transitions to be selectively
stimulated and studied.
[0003] A photoacoustic signal can occur as follows. First, light
stimulates a molecule within a sample. Such stimulation can
include, for example, absorption of the light by the molecule to
change an energy state of the molecule. Second, an excited state
structure of the stimulated molecule rearranges. During such
rearrangement, heat, light, volume changes and other forms of
energy can dissipate into an environment surrounding the molecule.
Such forms of energy cause expansion or contraction of materials
within the environment. As the materials expand, sound waves are
generated. Accordingly, an acoustic detector mounted in acoustic
communication with the environment can detect changes occurring as
a result of the light stimulation of the absorbing molecule.
[0004] An exemplary prior art apparatus 10 for photoacoustic
spectroscopy is shown in FIG. 1. Apparatus 10 comprises a light
source 12 configured to emit a beam of radiation into a sample
holder 14. Light source 12 can comprise, for example, a laser.
Filters (not shown) can be provided between light source 12 and
sample holder 14 for attenuating the light prior to its impacting
sample holder 14.
[0005] Sample holder 14 comprises a sample cell 18 containing a
sample 16. Sample cell 18 can comprise a number of materials known
to persons of ordinary skill in the art, and preferably comprises a
material substantially transparent to the wavelengths of light
emanating from light source 12. Preferred materials of sample cell
18 will accordingly vary depending on the wavelengths of light
utilized in the spectroscopic apparatus. If the wavelengths of
light are, for example, in the range of ultraviolet through
visible, sample cell 18 can preferably comprise quartz.
[0006] Sample 16 comprises a material that substantially fills
sample cell 18. Such material can be, for example, a fluid such as
a liquid or a gas. Sample 16 can, for example, comprise a liquid
solution wherein the molecular vibrations that are to be studied
are associated with molecules dissolved in the liquid.
[0007] Apparatus 10 further comprises an acoustic detector 20
mounted to sample cell 18 and in acoustic communication with sample
16. Acoustic detector 20 can comprise a transducer, such as, for
example, a microphone and can be mounted such that a fluid (for
example, a grease) is provided between a surface of detector 20 and
sample cell 18. Detector 20 is typically removably mounted to
sample cell 18 by, for example, a clamp. Acoustic detector 20 is in
electrical communication with an output device 22. Device 22 can be
configured to display information obtained from detector 20, and
can be further configured to process such information. Output
device 22 can comprise, for example, an oscilloscope or a
computer.
[0008] In operation, a beam of light is generated by source 12 and
passed through sample cell 18 to stimulate molecular excitation
within sample 16. Non-radioactive decay or molecular rearrangements
cause expansions and/or contractions of a material within sample 16
to generate acoustic waves passing from sample 16 through sample
cell 18 and to acoustic detector 20. Acoustic detector 20 then
detects the acoustic waves and passes signals corresponding to, for
example, amplitudes and frequencies of the acoustic waves to output
device 22. Output device 22 can be configured to convert
information obtained from detector 20 to, for example, a graphical
display.
[0009] A difficulty in utilizing apparatus 10 is that acoustic
waves emanating simultaneously within sample 16 do not reach
detector 20 at the same time. As shown in FIG. 2, light from source
12 typically has a general shape of a cylinder 24 as it passes
through sample cell 18. Individual acoustic waves emanating from
cylinder 24 (shown as dashed lines 26) also have cylindrical
shapes. All portions of an individual acoustic wave 26 are
generated simultaneously within sample 16, and should therefore
desirably simultaneously impact detector 20. However, as acoustic
detector 20 has a flat detection surface, an individual acoustic
wave 26 will impact acoustic detector 20 at a later time at an edge
of the detection surface relative to a center of the detection
surface. Thus, there is a spread of a time interval during which an
individual acoustic wave impacts detector 20, rather than the
desired simultaneous detection event. It is desirable to reduce the
time interval during which an individual acoustic wave is detected
to enhance sensitivity.
[0010] One approach that has been utilized for reducing such time
interval is to utilize a detector 20 having a curved detection
surface approximately complementary to the curved cylindrical
shapes of acoustic waves 26. However, as such detectors can be
difficult to make the approach has had limited success. Another
approach is to use a slit to provide a planar acoustic wave.
[0011] Another approach that has been utilized for reducing a time
interval during which an individual acoustic wave is detected is
exemplified by a photoacoustic apparatus 10b shown in FIG. 3. In
referring to the apparatus of FIG. 3, similar numbering to that
utilized above in describing apparatus 10 of FIG. 1 will be used,
with differences indicated by the suffix "b" or by different
numerals. The primary difference between apparatus 10b and
apparatus 10 of FIG. 1, is that in apparatus 10b transducer 20 is
mounted directly in front of the beam of light emanating from light
source 12. Accordingly, apparatus 10b comprises a sample cell 14b
slightly modified from the sample cell 14 of apparatus 10 (FIG. 1).
As long as transducer 20 has a detector face that is smaller in
cross-sectional area than an area of the light beam emanating from
source 12, individual waves generated by the light beam will reach
the face at approximately the same time across an entire surface of
such face. Accordingly, apparatus 10b can eliminate the
above-discussed problem of individual acoustic waves reaching an
acoustic detector face at a spread of time intervals across a
surface of the face. A difficulty associated with apparatus 10b is
that the light emanating from source 12 shines directly into a
detector face of transducer 20 and can adversely heat such face.
Accordingly, a shield 26 is typically provided along an internal
sidewall of sample cell 18b to block radiation emanating from light
source 12 from reaching a detector face of transducer 20. Shield 26
is typically a thin film, and such thin films are typically only
suitable for very narrow ranges of light (about 20 nanometers on
average). Accordingly, a band of light entering sample holder 18b
must typically be kept to a very narrow wavelength range to avoid
having light pass through film 26 and into transducer 20.
[0012] As the above discussion indicates, the apparatuses 10 and
10b of FIGS. 1 and 3, respectively, both have advantages and
disadvantages. Specifically, the apparatus 10 of FIG. 1 can enable
relatively large bands of light to be utilized for photoacoustic
spectroscopy experiments, but has slow response times and
significantly lower sensitivity due to large time intervals wherein
individual acoustic waves impact different regions of an acoustic
detector surface. In contrast, apparatus 10b can have rapid
response times to acoustic waves generated within sample 16, but is
generally only useful for relatively narrow ranges of light. It
would be desirable to develop alternative photoacoustic detector
systems which could accomplish the advantages of both apparatus 10
of FIG. 1 and apparatus 10b of FIG. 3.
[0013] In another aspect of the prior art, it is recognized that
light can be either refracted or reflected by a material, depending
on an angle with which the light impacts a surface of the material.
Such is illustrated with respect to a material 50 in FIG. 4.
Material 50 comprises an upper surface 52. Upper surface 52 is
substantially planar. An axis "X" extends normal (i.e.,
perpendicular) to planar surface 52. A critical angle .theta. is
defined as an angle relative to normal axis "X" wherein a beam of
light impacting surface 52 passes from predominantly reflecting
from surface 52 to predominantly refracting within surface 52. A
critical angle is determined by the relative refractive indices of
materials joining at a surface. Specifically, if light passes from
a first material having a larger refractive to a second material
with a lesser refractive index, a critical angle can be defined
relative to an axis normal to a surface where the two materials
meet. In the example of FIG. 4, such surface corresponds to surface
52. If light impacts surface 52 at an angle greater than angle
.theta., the light will predominantly reflect from surface 52.
Also, if light impacts surface 52 at an angle less than angle
.theta., the light will predominantly pass into material 50 and
refract within material 50. A critical angle .theta. for particular
materials can be calculated from application of Snell's law and the
relative amount of refraction and reflection can be determined. For
a quartz/air interface a critical angle .theta. is about
40.4.degree., and for a quartz/water interface a critical angle
.theta. is about 59.7.degree..
[0014] FIG. 4 also illustrates that a beam of light 55 can be
directed into material 50 at an appropriate angle such that the
light reflects from surfaces of material 50 to be contained
internally of material 50. Such reflections are referred to as
internal reflections. It is known that some of the light will
actually extend slightly outward of a surface of material 50 (such
as surface 52) as the light reflects internally from the surface.
Such is illustrated by curved lines 57 in FIG. 4. Although the
light extends slightly outward of the surfaces of material 50 as it
is reflected within material 50, the light continues along the
general path illustrated by beam 55. Accordingly, if material 50 is
provided adjacent a sample, a light beam 55 can be provided to be
internally reflective within material 50 and yet to stimulate
molecules within the sample. Such use of internal reflections for
stimulating molecules within a sample can be advantageous in
situations wherein a sample is generally not transparent to a light
source, such as, for example, when the sample is relatively turbid
or optically dense. The amount by which light waves penetrate into
a sample can be adjusted by changing a wavelength of the light, or
by changing an angle at which the light internally reflects from
surfaces of material 50.
[0015] In yet another aspect of the prior art, it is recognized
that a sample's absorbance of light is directly proportional to a
path length of light through the sample, and to a concentration of
an absorbing species within the sample. Such relationship can be
represented by the formula A=abc, wherein A is absorbance, a is a
proportionality constant called absorptivity, b is a pathlength of
light through the sample, and c is a concentration of absorbing
species within the sample. Such relationship is referred to as
Beer's Law. The Beer's Law relationship indicates that an amount of
light absorbed is proportional to a concentration of an absorbing
species. Another way of describing absorbance is as Log P.sub.0/P,
wherein P.sub.0 refers to the initial power of a light beam
impacting a sample and P refers to the power of the beam exiting
the sample. Most spectroscopic methods can detect and quantitate
absorbing species only within a very narrow range of absorbance,
such as, for example, a range of from about 0.05 to about 1.0.
Accordingly, samples must be either diluted or concentrated to
bring an absorbance of the sample within the appropriate range for
the spectroscopic measurements. For samples that are extremely
dilute, such as minor contaminants in sea water, it can be
difficult and time consuming to adequately concentrate the samples
for spectroscopic measurements. Accordingly, it would be desirable
to develop spectroscopic methods that could be utilized over a wide
range absorbance.
[0016] In contrast to spectroscopy methods which measure absorbance
as Log P.sub.0/P, photoacoustic spectroscopy measures only P. This
can provide enhanced sensitivity relative to other forms of
spectroscopy in that it does not involve measuring a small signal
"P" in the presence of a large background "P.sub.0". Also, an
amplitude of a photoacoustic signal is believed to depend inversely
on a volume of an excitation source (i.e., P/V.sub.0). In other
words, Photoacoustic Theory predicts that an amplitude of a
photoacoustic signal is proportional to an energy/volume ratio,
wherein the energy is the energy generated by a measured transition
and the volume is the volume of a sample. Photoacoustic
spectroscopy can thus be advantageous over other forms of
spectroscopy.
SUMMARY OF THE INVENTION
[0017] In one aspect, the invention encompasses a spectroscopy
apparatus configured to enable direct measurement of absorbance
across an entirety of the range of from about 0.0001 absorbance
units per centimeter to about 10,000 absorbance units per
centimeter.
[0018] In another aspect, the invention encompasses a photoacoustic
spectroscopy sample cell. The sample cell includes a first block of
material. The first block of material has opposing front and back
surfaces. The front surface comprises a substantially planar
portion configured to be against a sample. The back surface
comprises a substantially planar portion configured to be joined to
a transducer. The back surface is substantially parallel to the
front surface. The first block of material also has a pair of
opposing side surfaces joined to opposite ends of the front and
back surfaces. The opposing side surfaces are a first opposing side
surface and a second opposing side surface. The first opposing side
surface is configured for passage of light therethrough and extends
at a first oblique angle relative to a plane containing the
substantially planar portion of the front surface. The second
opposing side surface extends at a second oblique angle relative to
the plane containing the substantially planar portion of the front
surface.
[0019] In yet another aspect, the invention encompasses a method of
photoacoustic spectroscopy. A sample is provided and an
acoustically-stimulable transducer is provided acoustically coupled
with the sample. The transducer comprises a detector surface having
a substantially planar portion. A first beam of light is directed
through the sample at an oblique angle relative to the
substantially planar portion of the detector surface. The first
beam of light generates sound waves in the sample. The sound waves
are detected with the transducer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Preferred embodiments of the invention are described below
with reference to the following accompanying drawings.
[0021] FIG. 1 is a schematic, diagrammatic view of a first prior
art photoacoustic spectroscopy apparatus.
[0022] FIG. 2 is a view along the line 2-2 of FIG. 1.
[0023] FIG. 3 is a diagrammatic, schematic view of a second prior
art photoacoustic spectroscopy apparatus.
[0024] FIG. 4 is a cross-sectional sideview of a prior art material
illustrating various relationships between angles and light waves
impacting the material.
[0025] FIG. 5 is a diagrammatic, cross-sectional view of a
photoacoustic spectroscopy sample cell of the present
invention.
[0026] FIG. 6 is diagrammatic view along line 6-6 of FIG. 5.
[0027] FIG. 7 is a second diagrammatic, cross-sectional view of a
photoacoustic spectroscopy sample cell of the present
invention.
[0028] FIG. 8 is a diagrammatic top view of a photoacoustic
spectroscopy sample cell holder apparatus of the present.
[0029] FIG. 9 is a diagrammatic cross-sectional sideview of the
photoacoustic spectroscopy sample cell holder of FIG. 8.
[0030] FIG. 10 is a schematic diagram of a photoacoustic
spectroscopy apparatus of the present invention.
[0031] FIG. 11 is a diagrammatic, cross-sectional sideview of an
alternative embodiment photoacoustic spectroscopy sample cell of
the present invention.
[0032] FIG. 12 is a diagrammatic, cross-sectional sideview of
another alternative embodiment photoacoustic spectroscopy sample
cell of the present invention.
[0033] FIG. 13 is a diagrammatic, cross-sectional sideview of yet
another alternative embodiment photoacoustic spectroscopy sample
cell of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] This disclosure of the invention is submitted in furtherance
of the constitutional purposes of the U.S. Patent Laws "to promote
the progress of science and useful arts" (Article 1, Section
8).
[0035] FIGS. 5 and 6 illustrate a photoacoustic sample cell 100
encompassed by the present invention. Sample cell 100 comprises a
first block of material 102 and a second block of material 104.
Blocks 102 and 104 can comprise a same material, or can comprise
different materials from one another. An exemplary material for
blocks 102 and 104 is quartz. Blocks 102 and 104 are separated from
one another by a shim 106. Shim 106 can comprise, for example, at
least one of a flexible gasket material (such as, for example,
rubber or plastic), or a metallic material. In preferred
embodiments, shim 106 will comprise an annular shape. In the
embodiment shown, block 104 has a rectangular shape and shim 106 is
an oval ring. In other embodiments (which are not shown), material
104 can have other shapes, such as, for example, square, oval, or
circular, and shim 106 can have other annular shapes corresponding
to circular rings, square-shaped rings, or rectangular-shaped
rings, for example.
[0036] Block 102 comprises front and back surfaces 110 and 112,
respectively, and opposing side surfaces 114 and 116. Front and
back surfaces 110 and 112 are preferably substantially parallel to
one another. Opposing side surfaces 114 and 116 are joined to
opposite ends of front and back surfaces 110 and 112. Opposing side
surface 114 can be referred to as a first opposing side surface,
and opposing side surface 116 can be referred to as a second
opposing side surface.
[0037] Cell 100 further comprises a sample reservoir 120 defined by
shim 106, and blocks 102 and 104. Sample reservoir 120 is
configured to hold a material, such as, for example, a liquid or
gas that is to be photoacoustically analyzed. Blocks 102 and 104
define walls of reservoir 120.
[0038] Front surface 110 comprises a substantially planar portion
configured to be against a material contained within reservoir 120.
The term "substantially" in reference to the substantially planar
portion of surface 110 indicates that a so-called "planar" portion
of surface 110 can have structural features which cause it to vary
from perfect planarity, and yet still be sufficiently planar for
purposes of the present invention. Such structural features can be
introduced as, for example, minor manufacturing defects. In
preferred embodiments, the substantially planar portion of surface
110 extends entirely across sample reservoir 120.
[0039] Second block 104 comprises a front surface 130, a back
surface 132, a first opposing side surface 134 and a second
opposing side surface 136. Front and back surfaces 130 and 132 are
preferably substantially parallel to one another. Front surface 130
of second block 104 comprises a substantially planar portion
configured to be against a material contained within reservoir 120.
In the shown embodiment, block 104 is substantially identical to
block 102. The term "substantially" indicates that block 104 can
vary from block 102 by the presence of minor manufacturing defects,
and yet still be identical for purposes of the present invention.
Blocks 102 and 104 are preferably identical in shape when the
blocks comprise identical materials. In embodiments in which blocks
102 and 104 comprise different materials, it can be preferable for
blocks 102 and 104 to have different dimensions from one
another.
[0040] In operation, a light beam 150 is passed through first
surface 114 to sample reservoir 120. Light beam 150 preferably
enters surface 114 at an angle perpendicular (normal) to surface
114 to minimize reflection of beam 150 from surface 114. Surface
114 extends obliquely at an angle .alpha. relative to a plane
containing the substantially planar portion of front surface 110
that is against sample reservoir 120. In the shown embodiment, an
entirety of surface 110 is within such plane. Accordingly, oblique
angle .alpha. is shown at a corner between surface 114 and surface
110. For purposes of interpreting this disclosure and the claims
that follow, an oblique angle is defined as an angle that is
neither 0.degree. nor 90.degree..
[0041] An axis "Q" extends normal to surface 110. Light beam 150
strikes surface 110 at an angle .beta. relative to axis "Q". Angle
.beta. is determined by the angle .alpha.. Specifically, angle
.beta. equals angle .alpha.. Accordingly, angle .alpha. can be
configured to provide beam 150 at less than, greater than, or equal
to a critical angle of the material of block 102 relative to
surface 110. If angle .beta. is less than such critical angle, a
predominate portion of beam 150 will penetrate sample reservoir 120
along a path such as that illustrated by dashed line 152. If angle
.beta. is greater than a critical angle of material 102 at surface
110, a predominate portion of light beam 150 will reflect from
surface 110 along a path such as that illustrated by dashed line
154. Accordingly, block 102 can be constructed for either internal
reflection of light beam 150 within block 102, or refraction of
light beam 150 through reservoir 120. Of course, the
above-discussed equality of angles .alpha. and .beta. only holds
true in situations wherein a is from 0.degree. to 90.degree..
Preferably, angle .alpha. is greater than 0.degree. and less than
90.degree., and more preferably is greater than 20.degree. and less
than 70.degree..
[0042] Although it can be preferred to have angles .alpha. and
.beta. equal to one another when .alpha. is between 0.degree. and
90.degree., it can also be preferred that angles .alpha. and .beta.
not equal to one another. For instance, it can be preferred to
change an orientation of sample cell 100 relative to a beam of
light (either by moving sample cell 100 or by moving the beam) to
vary the angle .beta. at which the light impacts surface 110. Such
can be preferred, for example, in circumstances in which it is
desired to perform some measurements on a sample under conditions
in which light travels along a predominately refractive path (such
as path 154) and other measurements under conditions in which light
travels along a predominately reflective path (such as path 152).
As angle .alpha. is generally fixed, angle .beta. will not equal
angle .alpha. at both the refractive conditions and the reflective
conditions.
[0043] Second opposing side surface 116 forms an oblique angle
.gamma. relative to the substantially planar portion of surface 110
configured to be against a material within sample reservoir 120.
Also, surface 130 of second block 104 comprises a substantially
planar portion configured to be against a sample in reservoir 120.
First and second opposing side surfaces 134 and 136 of second block
104 form oblique angles .delta. and .epsilon., respectively,
relative to such planar portion of surface 130. Oblique angles
.alpha., .gamma., .delta. and .epsilon. are preferably
substantially identical in embodiments in which blocks 102 and 104
consist of identical materials. Specifically, in such embodiments
it can be desirable for light beam 150 to enter first block 102
substantially perpendicular to surface 114 and to exit second block
104 at an angle substantially perpendicular to surface 136. If
blocks 102 and 104 consist of identical materials, such can be
accomplished by having oblique angles .alpha. and .epsilon. be
substantially identical to one another. If blocks 102 and 104
consist of different materials, it can be desirable to vary oblique
angle .epsilon. relative to oblique angle .alpha. such that light
exits block 104 in a direction substantially perpendicular to
surface 136.
[0044] It can be advantageous to have oblique angles .delta. and
.gamma. identical to one another in experiments in which at least
two beams of light are to be passed through a sample. In such
experiments, a first beam of light can be passed along the path of
beam 150, and a second beam of light can be passed along a path
which enters at surface 134 and exits at surface 116. Accordingly,
the paths of the two beams of light will intersect substantially
perpendicularly to one another within sample reservoir 120. The
beams of light can be passed through reservoir 120 simultaneously
with one another. Alternatively, the beams of light can be passed
in rapid succession such that the first beam of light excites
molecules to an initial state, and the second beam of light either
further excites the molecules to another state, or provides the
excited molecules with a path of relaxation. The beams of light can
comprise either identical wavelengths, or different wavelengths
from one another. Also, oblique angles .alpha. and .delta. can be
configured such that one beam of light predominately refracts
through reservoir 120, and another beam of light predominately
internally reflects from one of surfaces 130 or 110. Further, the
direction of one of the beams of light can be reversed relative to
a direction of the other beam of light. Additionally, it is noted
that the beams of light can comprise multiple wavelengths, some of
which predominately refract through reservoir 120 and others of
which predominately reflect from one or both of surfaces 110 and
130.
[0045] An advantage of utilizing refraction and reflection in a
common photoacoustic spectroscopy device is that such can enable
the device to be utilized for detecting and quantitating
characteristics of samples over a wide range of absorbances.
Specifically, refraction-based photoacoustic methods can enable
detection and quantitation of low concentrations of detectable
components in samples (for example, detection can occur to at least
as low as about 0.0001 absorbance units per centimeter), and
internal-reflection-based photoacoustic methods can enable
detection and quantitation of high concentrations of detectable
components in samples (for example, detection can occur to at least
as high as about 10,000 absorbance units per centimeter). Thus,
embodiments of the present invention can enable detection and
quantitation of sample components having absorbances of from about
0.0001 to about 10,000. The present invention can thus provide an
expanded useful absorbance range relative to other forms of
spectroscopy. Such expanded range can enable methods of the present
invention to be utilized for directly analyzing samples that would
need to be significantly diluted or concentrated for other forms of
spectroscopy. Experiments have been conducted to detect and
quantitate Cr(VI) absorbance of 372 nanometer light at various
concentrations of Cr(VI). Such experiments confirm that an
apparatus of the present invention can be utilized to directly
detect and quantitate a concentration of a sample component having
an absorbance of from about 0.0001 absorbance units per centimeter
to about 10,000 absorbance units per centimeter. For purposes of
interpreting this disclosure and the claims that follow "direct"
detection and quantitation of an absorbing species in a sample is
defined to mean spectroscopic detection and quantitation that
occurs without modifying a concentration (absorbance) of the
absorbing species (by, for example, concentration or dilution)
prior to the detection and quantitation. In other words, "direct"
detection refers to in situ, real time analysis.
[0046] It is noted that measurements of the detection limits of a
sample cell of the present invention (such as cell 100 of FIG. 5)
in both a refraction mode and a reflection mode indicate that
operation of the cell cannot be explained entirely by either Beer's
Law or Photoacoustic Theory. Specifically, the refraction mode has
a detection limit about twenty-times larger, relative to the
reflection mode, than that which would be predicted by
Photoacoustic Theory alone, and yet the signal is several times
smaller than that which would be predicted by Beer's Law alone. It
is to be understood that the scope of this disclosure is to be
determined by the claims that follow, and is not to be limited to
any particular mechanism except to the extent that such is
expressly claimed.
[0047] A transducer 170 is coupled to back surface 112 of block
102. Transducer 170 is preferably an acoustic microphone
acoustically coupled with a sample in reservoir 120 through block
102. In the shown embodiment, only one transducer is provided.
However, the invention encompasses other embodiments (not shown)
wherein a second transducer can be provided at, for example,
surface 132 of second block 104. An electrical interconnect 172
extends from transducer 170 to electrically couple transducer 170
with circuitry (not shown) for either processing or displaying
signals generated by transducer 170.
[0048] A method of operation of sample cell 100 is described with
reference to FIG. 7. A sample 190 is provided within reservoir 120
and a beam of light 180 is passed through surface 134 of block 104,
refracted through sample 190, and then exits from sample cell 100
through surface 116 of block 102. Sample 190 can comprise, for
example, a fluid. Alternatively, sample 190 can comprise a solid,
such as, for example, a powder or a block having a smooth surface
to align with an interior surface of block 102. As another example,
sample 190 can comprise an interface of two phases, such as a
liquid/solid interface.
[0049] The light stimulates molecules within sample 190 to generate
acoustic waves 185 which pass through block 102 and are detected by
transducer 170. It is noted that since the speed of light is
several orders of magnitude greater than the speed of sound, light
beam 180 effectively fills an entire thickness of reservoir 120
instantaneously prior to emanation of acoustic waves from sample
190. Acoustic waves 185 are thus generated to align parallel with
surface 110 of block 102 (and travel in a direction perpendicular
to surface 110).
[0050] Transducer 170 comprises a detector face 174 against surface
112 of block 102. In preferred embodiments, detector face 174 is
substantially parallel with surface 110. Accordingly, detector face
174 is substantially parallel to the alignment of waves 185.
Detector face 174 preferably comprises a surface area less than a
surface area of acoustic waves 185. Specifically, detector face 174
preferably comprises a surface area less than an area of sample 190
stimulated by light beam 180. In such preferred embodiments, an
entirety of detector face 174 can be stimulated simultaneously by
individual acoustic waves 185.
[0051] An exemplary apparatus 200 for holding sample cell 100 is
shown in FIGS. 8 and 9. FIG. 8 shows a top view of such apparatus,
and FIG. 9 shows a sideview. Apparatus 200 comprises a support
structure 202 with a flat base 204. A post 206 extends into support
structure 202 and can be configured to move within structure 202
for height adjustment of sample cell 100.
[0052] Apparatus 200 further comprises a holding box 208 supported
on post 206. Holding box 208 comprises sidewalls 210 and 212 and a
base 214. Sidewalls 210 and 212, as well as base 214, can be formed
of, for example, stainless steel. A tension adjustment pin 216 is
threadedly engaged within sidewall 210 and is coupled to transducer
170 with a cushioned end 218. Cushioned end 218 can comprise, for
example, a rubber material joined to pin 216. Pin 216 can be
screwed into sidewall 210 to provide tension against sample cell
100 for retaining sample cell 100 within box 208. The sample
cell/transducer assembly shown in FIGS. 8 and 9 comprises a second
transducer 190 joined to second block 104 of sample cell 100.
Transducers 170 and 190 are electrically coupled to processing
and/or output circuitry through electrical interconnects 172 and
192, respectively. In the shown embodiment, block 104 comprises an
inlet hole 220 and an outlet hole 230 for continuously flowing a
sample into reservoir 120 (FIG. 5). Holes 220 and 230 are connected
to ports 240 and 250, respectively. It is preferred to have outlet
hole 230 above inlet hole 220 so that if air is introduced into
reservoir 120 (FIG. 5) it will be readily expelled from sample cell
100. The embodiment shown in FIGS. 8 and 9 can be advantageous for
continuously monitoring samples. Such continuous monitoring can be
desired, for example, in environmental applications wherein samples
are to be monitored for pollution or other contaminants, and in
applications wherein samples are to be monitored for time-dependent
changes.
[0053] FIG. 10 schematically illustrates a photoacoustic
spectroscopic instrument 300 configured for incorporating a sample
cell 100 of the present invention. Instrument 300 comprises a laser
310 configured to emit a beam of radiation. Such beam of radiation
is directed by a wedge 320 through a filter wheel 330, a beam
splitter 340, and an iris 350, and into sample cell 100. Wedge 320,
filter wheel 330 and iris 350 can be provided to attenuate the beam
of radiation. Radiation that penetrates wedge 320 is directed to a
beam stop 410 which blocks the radiation from entering an
environment proximate apparatus 300. Beam splitter 340 splits light
from laser 310 into a first beam which penetrates sample cell 100,
and a second beam which enters an energy meter 400. Energy meter
400 is coupled to a processor 380 and outputs a signal to processor
380 indicating that a laser pulse has occurred. Such signal can be
utilized to trigger data acquisition by processor 380.
[0054] The beam passing through sample cell 100 impacts a
photodiode 360 configured to detect an intensity of the beam.
Photodiode 360 is coupled to an output device 370 such as, for
example, a digital oscilloscope, and to processor 380. Processor
380 can be configured to, for example, store information obtained
from photodiode 360, or to graphically output such information in
the form of, for example, a graph of intensity relative to
time.
[0055] The beam from laser 310 generates an acoustic signal within
sample cell 100 that is detected by a transducer 170. A signal from
transducer 170 is passed to an amplifier 390. Amplifier 390 outputs
a signal to output device 370 and processor 380. Processor 380 can
then, for example, store the signal, or process the signal to, for
example, output a graph of acoustic signal relative to time.
[0056] The above-described embodiments are sample cells in which a
sample reservoir is contained between two blocks of material. It is
to be understood, however, that the invention also encompasses
embodiments in which a sample reservoir is against a surface of a
block, regardless of whether a second block is provided against a
sample reservoir. For instance, FIG. 11 illustrates an embodiment
of the invention in which a sample cell comprising a single block
of material 400 is in contact with a fluid sample 440. Fluid 440 is
contained within a vessel 412. Block 400 comprises a surface 418 in
physical contact with fluid 440. A transducer 414 is mounted to
block 400 on a surface 416 parallel to surface 418. A beam of light
420 is directed into sample cell 400 at an angle which reflects
from surface 418. During the reflection, the light stimulates fluid
440 to form acoustic waves 430 which travel toward transducer 414.
Transducer 414 can then detect acoustic waves 430 and output a
signal through an electrical interconnect 434 to other circuitry
(not shown). Although transducer 414 is shown against a surface
(416) that is outside of fluid 440, in other embodiments (not
shown) transducer 414 can be mounted against a surface within fluid
440 (such as, for example, surface 418).
[0057] Contact of block 400 with fluid sample 440 can be
accomplished by insertion of block 400 either entirely or partially
into fluid sample 440, and can comprise more than one surface in
physical contact with fluid sample 440. Fluid sample 440 can
comprise, for example, either a liquid or a gas.
[0058] FIG. 12 illustrates a sample cell 500 corresponding to an
alternative embodiment of the present invention. Sample cell 500
comprises convex curved sidewall surfaces 502 and 504, a
substantially planar front surface 506 configured to be proximate a
sample, and a substantially planar back surface 508 configured to
be proximate a transducer 510. Curved surfaces 502 and 504 are
preferably shaped as arcs of circles,. and are preferably
substantially mirror images of one another. FIG. 12 further
illustrates a light beam 512 entering sample cell 500 through
sidewall surface 502, reflecting from surface 506, and exiting
through sidewall surface 502. As shown, curved sidewall surface 502
focuses beam 512 so that beam 512 is narrowed upon passing through
sidewall surface 502. Curved sidewall surface 504 then defocuses
beam 512 as beam 512 exits sample cell 500. In the shown preferred
embodiment, sidewall surfaces 502 and 504 comprise curved regions
extending an entirety of a length of the sidewall surfaces. It is
to be understood, however, that the invention encompasses other
embodiments (not shown) wherein the curved regions of sidewall
surfaces 502 and 504 extend along less than an entirety of the
length of sidewall surfaces 502 and 504.
[0059] An advantage of sample cell 500 over the above-discussed
sample cell embodiments having planar sidewall surfaces, in
addition to its focusing of a light beam, is that sample cell 500
can generate minimal amounts of reflection with light beams
entering sidewall 502 from a number of angular directions relative
to planar surface 506. In contrast, cells having planar sidewall
surfaces, such as planar sidewall surface 114 of cell 100 (FIG. 5),
will generally reflect a substantial portion of a light beam unless
the beam enters the sidewall surface at an angle normal to the
plane of the sidewall surface. Thus, cells having planar sidewall
surfaces (such as cell 100 of FIG. 5) can generate minimal amounts
of reflection with light beams entering the sidewall surfaces (such
as surface 114 of FIG. 5) from only a very limited number of
angular directions relative to a front planar surface adjacent a
sample (such as surface 110 of FIG. 5).
[0060] FIG. 13 illustrates a sample cell 600 corresponding to an
yet another alternative embodiment of the present invention. Sample
cell 600 comprises concave curved sidewall surfaces 602 and 604, a
substantially planar front surface 606 configured to be proximate a
sample, and a substantially planar back surface 608 configured to
be proximate a transducer 610. Curved surfaces 602 and 604 are
preferably shaped as arcs of circles. FIG. 13 further illustrates a
light beam 612 entering sample cell 600 through sidewall surface
602, reflecting from surface 606, and exiting through sidewall
surface 602. As shown, curved sidewall surface 602 defocuses beam
612 so that beam 612 is broadened upon passing through sidewall
surface 602. Curved sidewall surface 604 then refocuses beam 612 as
beam 612 exits sample cell 600.
[0061] It is noted that in the photoacoustic sample cell
embodiments described above, transducers are mounted to sample cell
blocks through which a light beam is passed. It is to be
understood, however, that the invention encompasses other
embodiments wherein transducers are mounted in other configurations
such as, for example, to other surfaces in contact with a sample,
or in acoustic contact with a sample without an intervening
surface.
[0062] In compliance with the statute, the invention has been
described in language more or less specific as to structural and
methodical features. It is to be understood, however, that the
invention is not limited to the specific features shown and
described, since the means herein disclosed comprise preferred
forms of putting the invention into effect. The invention is,
therefore, claimed in any of its forms or modifications within the
proper scope of the appended claims appropriately interpreted in
accordance with the doctrine of equivalents.
* * * * *