Optics: measuring and testing – Sample – specimen – or standard holder or support – Fluid containers
Reexamination Certificate
1998-06-26
2001-05-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Sample, specimen, or standard holder or support
Fluid containers
C356S432000
Reexamination Certificate
active
06236455
ABSTRACT:
TECHNICAL FIELD
The invention pertains to photoacoustic spectroscopy, including methods of photoacoustic spectroscopy and photoacoustic spectroscopy apparatuses.
BACKGROUND OF THE INVENTION
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.
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.
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
.
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.
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.
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.
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.
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.
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.
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
10
b
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
10
b
and apparatus
10
of
FIG. 1
, is that in apparatus
10
b
transducer
20
is mounted directly in front of the beam of light emanating from light source
12
. Accordingly, apparatus
10
b
comprises a sample cell
14
b
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
10
b
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
10
b
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
18
b
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
18
b
must typically be kept to a very narrow wavelength range to avoid having light pass through film
26
and into transducer
20
.
As the above discussion indicates, the apparatuses
10
and
10
b
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
10
b
can have rapid response times to acoustic waves generated within sample
16
, but is genera
Amonette James E.
Autrey S. Tom
Daschbach John L.
Foster-Mills Nancy S.
Battelle (Memorial Institute)
Font Frank G.
Lauchman Layla
Wells, St. John, Roberts Gregory & Matkin P.S.
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