JPH1010038A - Optical acoustic cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a measurement in a concentration range wider than in the past by about two digits by providing an optical diameter changing means for changing the optical diameter of a laser beam in a sample on the incident port side of the optical fiber of an optical acoustic cell for determining the sample concentration and optical absorption coefficient. SOLUTION: An optical acoustic cell is formed of an optical fiber 10, a casing 20, a quartz window 30, and a piezoelectric element 50, and the tip of the optical fiber 10 is spherically worked to form an optical diameter changing means. A sample is sent into the casing 20 through a sample injection port, and a laser beam 40 the optical diameter of which is changed in the sample is emitted to the sample through the incident port of the optical fiber 10. The sample in the emitted part is thermally expanded to generate a pressure wave propagated toward the circumference within the casing 20, and the pressure wave is detected by a pressure sensor. The resulting detection signal is taken out to determine the sample concentration and optical absorption coefficient.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoacoustic cell used for measuring water quality by laser photoacoustic spectroscopy.

[0002]

2. Description of the Related Art A conventional photoacoustic cell is disclosed in JP-A-6-28.
No. 8989 discloses this. This photoacoustic cell will be described with reference to FIG. 5. Reference numeral 1 denotes a casing; 1a, a sample injection port formed in the casing 1; 1b, a sample pouring hole formed in the casing 1; The incident port mounting flanges 5b and 5b are emission port mounting flanges mounted on the other end face of the casing 1.

[0003] 2a is at the exit port entrance port screwed via a screw 2a ₁ to the entrance port mounting flange 5a, which 2b is screwed through the screw 2b ₁ in the exit port mounting flange 5b, these entrance port 2a and the output port 2b face each other on the same axis, and the screw 2a
_The transparent body 6 is attached to each of the distal ends by a support ₁ and 2b ₁ so as to be movable and adjustable in the axial direction.

[0004] 4a is a sample inlet 1a of the casing 1.
Is a sample injection pipe connected to the sample injection port 1b of the casing 1, reference numeral 4b is a sample injection pipe connected to the sample injection pipe 4a, and reference numeral 7 is a sample liquid sending pump connected to the sample injection pipe 4a via a pipe. 3
Is a pressure sensor, and this pressure sensor 3 is
Is provided on the inner surface of the so as to be substantially orthogonal to the axis,
An amplifier 8, a digital oscilloscope 9, and a computer 10 are connected to the pressure sensor 3.

Reference numeral 11 denotes a portion between the casing 1 and the mounting flanges 5a and 5b, a portion between the mounting flanges 5a and 5b and the input / output ports 2a and 2b, and
And an O-ring interposed between the pressure sensor 3 and the pressure sensor 3.
Next, the operation of the photoacoustic cell shown in FIG. 5 will be described. The sample is transferred from the sample sending pump 7 to the sample injection pipe 4a to the sample injection port 1a.
To the casing 1 (photoacoustic cell). The sample is irradiated with a laser beam from the incident port 2a, and the sample at the irradiated portion is thermally expanded to generate a pressure wave.

The pressure wave, that is, the pressure wave propagating toward the surroundings in the casing 1 (photoacoustic cell) is transmitted to the pressure sensor 3.
, The detected signal obtained at that time is converted into an electric signal, sent to an amplifier 8, amplified here, and then sent to a digital oscilloscope 9, where it is measured. The measured data is processed by the computer 10 to determine the sample concentration and the light absorption coefficient.

At this time, if the concentration of the sample to be measured becomes higher, the input port 2a or the input / output port 2
By turning both a and 2b, the transparent bodies 6, 6 are brought close to the center of the pressure sensor 3 so that the pressure received by the pressure sensor 3 does not decrease even when the concentration becomes high. I / O port 2
For positioning of a, 2b, the input / output ports 2a, 2b are turned clockwise and fully tightened, and the respective transparent bodies 6, 2
6 at the center of the pressure sensor 3 and then input / output ports 2a, 2a by a pulse motor or a servomotor.
By turning b in the counterclockwise direction, the respective transparent bodies 6 are separated from the center of the pressure sensor 3. At this time, the input / output port 2
The value obtained by multiplying the number of rotations of a and 2b by the screw pitch is the distance L from the center of the pressure sensor 3.

When the sample concentration increases with the input / output ports 2a and 2b separated from the center of the pressure sensor 3, the pressure signal intensity decreases. Therefore, the input / output port 2
By bringing the transparent bodies 6 and 6 of a and 2b closer to the center of the pressure sensor 3, the pressure signal strength is increased. However, if the transparent members 6, 6 of the input / output ports 2a, 2b are set at the center of the pressure sensor 3 even though the concentration of the sample is low, the signal cannot be received over the entire pressure receiving surface of the pressure sensor 3, so that the pressure signal intensity Conversely decreases. Therefore, according to the change in sample concentration,
In each case, the input / output ports 2a, 2b are moved to set the input / output ports 2a, 2b at positions where the pressure signal intensity is strong and the signal intensity is highly linear with respect to the sample concentration.

[0009]

In the conventional photoacoustic cell shown in FIG. 5, the laser beam applied to the sample from the entrance port 2a through the transparent body 6 is subjected to energy absorption by the sample. It passes through the transparent body (quartz window) 6 and out of the cell. At this time, if the photoacoustic signal is P, the sample concentration is C, and the optical path length (the distance between the transparent body 6 at the entrance port 2a and the transparent body 6 at the exit port 2b) is L ', then P∝
Since e- ^{CL '} , when L' is constant, the signal is saturated for a high density, and the signal is too small for a low density. In any case, the density measurement is impossible. become.

As a countermeasure, the light path length L 'is made variable by moving the input port 2a and the light output port 2b. However, it is very difficult to reduce the light path length L' to 1 mm or less. Could not be sufficiently measured.
In addition, when the optical path length L ′ becomes longer, the intensity of the laser beam during that period is attenuated mainly by energy absorption in the sample (liquid), and it is not possible to sufficiently perform measurement up to a low concentration.

The present invention has been made in view of the above problems, and has as its object to provide a photoacoustic cell capable of measuring a concentration range about two orders of magnitude wider than that of a conventional photoacoustic cell. It is in the point.

[0012]

In order to achieve the above object, a photoacoustic cell of the present invention sends a sample from a sample injection inlet into a casing, and irradiates the sample with laser light from an input port of an optical fiber. Then, the sample at the irradiation site is thermally expanded to generate a pressure wave that propagates toward the surroundings in the casing, and the pressure wave is detected by a pressure sensor, and a detection signal obtained at that time is taken out to obtain a sample concentration and light. In the photoacoustic cell for obtaining the absorption coefficient, a light diameter changing means for changing the light diameter of the laser light in the sample is provided on the incident port side of the optical fiber.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a photoacoustic cell according to the present invention will be described with reference to FIGS.
Referring to the embodiment shown in FIG. 4, FIG.
FIG. 1 (b) is a vertical sectional side view showing another configuration example when applied to the measurement of the concentration of a high-concentration solution, and FIG. Is a longitudinal side view showing a configuration example when applied to the measurement of the concentration of a low-concentration solution, and FIGS. 3 and 4 are explanatory diagrams showing examples of effects of the photoacoustic cell shown in FIGS. 1 and 2.

First, FIG. 1 applied to the concentration measurement of a high concentration solution
The photoacoustic cell of (a) will be described. 10 is an optical fiber, 20 is a casing, 30 is a quartz window, 40 is a laser beam, and 50 is a piezoelectric element (or microphone) corresponding to a pressure sensor. The portion is spherically processed, and this spherically processed portion corresponds to a light diameter changing unit.

In the acoustic cell shown in FIG. 1A, the tip of the optical fiber 10 is spherically processed, and the laser light 40
Is spread in the solution, and the concentration of the highly concentrated solution is measured. Next, the photoacoustic cell of FIG. 1B applied to the measurement of the concentration of a highly concentrated solution will be described.
0 is a casing, 30 is a quartz window, 40 is a laser beam, 50
Denotes a piezoelectric element (or microphone) corresponding to a pressure sensor, 60 denotes a lens, and the lens 60 corresponds to a light diameter changing unit. This lens 60 may be installed inside the casing 20 or outside the casing 20.

In the photoacoustic cell shown in FIG. 1B, the light diameter of the laser light 40 is expanded in the solution by the lens 60, and the concentration of the high concentration solution is measured. FIG. 1 (a) above
In the photoacoustic cell shown in (b), the same effect can be achieved in any case as long as a predetermined spread angle can be obtained.
Next, the photoacoustic cell of FIG. 2 applied to the measurement of the concentration of a low-concentration solution will be described. 10 is an optical fiber, 20 is a casing, 21 and 21 are condenser lenses, 30 is a quartz window, and 40 ′.
Is a laser beam, 50 is a piezoelectric element (or microphone) corresponding to a pressure sensor, and each condenser lens 21 corresponds to a light diameter changing unit.

In the photoacoustic cell shown in FIG. 2, the diameter of the laser beam 40 is reduced by the condensing lenses 21 and 21,
Measure the concentration of the low concentration solution, for example, in parallel. The operation of each photoacoustic cell will be described more specifically. In the laser photoacoustic spectroscopy, the intensity of a pressure wave generated by light absorption by a sample to be measured is detected by a piezoelectric element or a microphone 50, and the sample is measured. FIG. 2 shows a method for determining the concentration.
When the laser light 40 ′ is parallel light as shown in FIG. 6, the following relationship theoretically exists between the acoustic signal P, the laser light intensity I, the sample concentration C, the absorption coefficient α of the sample, and the optical path length L ′. is there.

[0018]

(Equation 1)

However, as shown in FIG. 3, when the laser beam is spread in the sample, if the initial laser beam diameter is r, the distance from the laser incident position is x, and the spread angle of the laser beam is θ,
The formula is as follows:

[0020]

(Equation 2)

FIG. 4 is a trial calculation using the absorption coefficient of uranium. As is clear from FIG. 4, the equation shows that the saturation of the signal with respect to the concentration C is smaller than that of the equation. Concentration measurement can be performed without signal saturation.

[0022]

According to the present invention, as described above, the sample is sent from the sample injection inlet into the casing, and the sample is irradiated with laser light from the input port of the optical fiber, thereby thermally expanding the sample at the irradiated portion, thereby allowing the sample to enter the casing. A pressure wave that propagates toward the surroundings is generated, and the pressure wave is detected by a pressure sensor, a detection signal obtained at that time is taken out, and a photoacoustic cell for obtaining a sample concentration and a light absorption coefficient is used. A light diameter changing means for changing the light diameter of the laser light in the sample is provided on the incident port side, and it is possible to measure a concentration range that is about two orders of magnitude wider than that of a conventional photoacoustic cell. That is,
In the measurement of high-concentration samples, it is important how to reduce the region where the laser beam intensity is strong, and in the measurement of low-concentration samples, how high the total laser energy absorbed by the sample is important. The photoacoustic cell of the present invention also
Although the above point is aimed at, since the conventional photoacoustic cell moves the input / output port of the laser beam, there is a limit in adjustment, and it is difficult to set the distance between the ports to, for example, about 1 μm. However, with the photoacoustic cell of the present invention, the distance between ports can be set to, for example, about 1 μm, and a measurement in a concentration range that is about two orders of magnitude wider than that of a conventional photoacoustic cell becomes possible.

[Brief description of the drawings]

FIG. 1A is a longitudinal sectional side view showing an example of a configuration in which a photoacoustic cell of the present invention is applied to concentration measurement of a high concentration sample,
(B) is a longitudinal side view showing another configuration example when applied to the concentration measurement of a high concentration solution.

FIG. 2 is a vertical sectional side view showing a configuration example when the photoacoustic cell of the present invention is applied to concentration measurement of a low concentration sample.

FIG. 3 is an explanatory diagram showing an example of the effect of the photoacoustic cell shown in FIGS. 1 and 2;

FIG. 4 is an explanatory diagram showing an example of the effect of the photoacoustic cell shown in FIGS. 1 and 2;

FIG. 5 is a vertical sectional side view showing a conventional photoacoustic cell.

[Explanation of symbols]

Reference Signs List 10 optical fiber 20 casing 30 quartz window 40 laser beam 50 pressure sensor (piezoelectric element or microphone)

Claims (1)

[Claims] A sample is sent from a sample injection inlet into a casing, and the sample is irradiated with a laser beam from an input port of an optical fiber to thermally expand the sample at an irradiation site and propagate a pressure propagating in the casing toward the periphery. While generating a wave, this pressure wave is detected by a pressure sensor, and a detection signal obtained at that time is taken out, and in a photoacoustic cell for obtaining a sample concentration and a light absorption coefficient, a laser beam is applied to the incident port side of the optical fiber. A photoacoustic cell comprising a light diameter changing means for changing a light diameter in a sample.