Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Detailed Description of the Invention
This invention relates to a detector for measuring the photoacoustic
output generated by a liquid sample which is irradiated by light. The liquid
sample may be flowing through a cell while being irradiated. The invention also
relates to a detector in which the liquid sample is allowed to flow through a
flow cell equipped with pressure sensing means.
In this apparatus, intense light such as a laser beam irradiates the
liquid sample in flowing state and the photoacoustic output generated by the
liqu~d sample is measured to indicate the solute components in the liquid sample,
without destroying the solute components and with high sensitivity. In general,
a material, which has absorbed light, emits fluorescent light or exhibits a
photoacoustic response, as shown in the following schema:
fluorescent
absorption light _~
light ~ material sample - _
photoacoustic
output
Among these three processes, the light absorption
process and the fluorescent light emission process are utilized practically for
detecting changes in a flowing liquid such as in a high-performance liquid
chromatographic detector. The light absorption process is used in both ultra-
violet or visible light absorption detectors and the fluorescent emission uses
a fluorescent light detector. In the absorption process the ratio of absorbed
light to amount of light not absorbed by a material sample is measured. However,
in the fluorescent light process or photoacoustic process the background light
is approximately zero when the material sample does not emit fluorescent light
or photoacoustic output, and thus it may be expected that highly sensitive
measurement can be obtained compared to a material sample capable of emitting
fluorescent light or photoacoustic output. In Eact, fluorescent light detectors
are available for measuring fluorescent materials with high accuracy, but there
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is a problem in that the number of fluorescen~ materials is limited.
It is therefore expected that photo-acoustic measurement can be applied
to many materials with high accuracy for example for in case liquid chromatography
etc., if the photoacoustlc measurement can be carried out while the materials
are flowing. Such photoacoustic detection may be used either singly or in
combination with fluorescence measurements. So far, the method of the photo-
acoustic de~ection has been applied to many gas samples and solid samples, using
condenser microphones as a sensor.
However, this method can not be applied to volatile materials, and
18 moreover water vapor is harmful to condenser microphones. This method has
heretofore been applied to only a small number of liquid samples. In addition,
no report has been made of using the photoacoustic detection for flowing liquid
samples as encountered in high-performance liquid chromatography, because of
pressure variations other than the photoacoustic effect of the samples.
As a result of our persevering research, the present inventors have
invented a flow type photoacoustic detector that has enabled highly accurate
detection of the photoacoustic output from a liquid sample which is not in a
stationary state or sealed state, but in a flowing state.
The flow type photoacoustic detector here described
comprises a sample cell; a light source for irradiating the sample in the sample
cell, and detecting means for detecting pressure variations in the sample,
characteri~ed in that the liquid inlet and outlet are spaced from each other
along the axis of irradiation of the sample cell, and light window plates are
arranged at each end of the cell.
More partlcularly in accordance with the invention there 1~ provided, a
flow type photoacoustic detector for liquids comprising:
a sample cell having a flow line formed therein by spaced upper and
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lower surfaces, side portions and end portions, at least one of said end por-
tions constituting an incldent light window plate;
a liquld inlet hole formed in said sample cell at one end of said
flow line and an outlet hole formed in said cell at the other end of said flow
line whereby liquid under pressure may flow therethrough;
a source of incident light arranged to irradia~e said flow llne through
said window plate; and
a piezoelectric detector positioned in said cell to be in acoustic
cor.tact with liquid passing through said cell to measure the optical sound
generated therein in the following sta~e.
Specific embodiments of the invention will now be described with
reference to the accompanying drawings in which:
Fig~ 1 is a schematic outline view of the optical system in the detector;
Fig. 2 A~ B, C, D and E are enlarged sectional views of the flow cell;
Fig. 3 is a diagram in which variations in the signal and noise
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magnitudes and the signal to noise ratio (S/N ratio) are plotted against
modulation frequencies of an photoacoustic filter type modulator;
Fig. 4 shows the comparative chromatograms showing the results of
concurrent measurements by the photoacoustic detector and an absorption detector
commonly used in high-performance liquid chromatography; and
Figo 5 shows the comparative diagrams obtained in case th~ background
noise of the absorption detector is approximated to that of the photoacoustic
detector.
Reference is now made to the accompanying drawings for illustrating
a preferred embodiment of the present invention.
Fig~ 1 shows the general outllne of the optical system used in the
new detector.
~ `he detector consists mainly of a light source 1 and a flow cell 4
containin~ sensor means. The incident light from the source l irradiates the
~iquid in the flow cell 4 under examination and the photoaGoustic output thus
produced is sensed by sensor means mounted in the flow cell 4.
Although laser light is most preferred as the light source employed,
emission lines from mercury lamps or light from xenon
lamps may also be used, and both visible and ultraviolet radiation may be used
as laser light.
As the sensors, pressure sensors SUCIl as Piezo~electric cera~ni~ and
other piezo-electric sansors may be employed advantageously.
When the present ~eaching is applied to a liquid chromatographic
detector, the volume of the flow cell within the detector should preferably be
less than 300 ~ ~.
Although the presentdisclosure is not ]imited concerning the flow rate
of the liquid in the cell, such flow rate should preferably be in the range of
0.1 m~/min to 10 m Q/min in case of liquid chromatographic detectors.
In case of monitoring devices, the flow cell capacity should preferably
be less than 10 m ~ for the above flow rate. Fig. 2 A, B, C, D and E show
different examples of the flow cell 4 in an enlarged sectional view respectively.
A flow line 21 in Fig. 2A is defined on upper and lower sides by a
diaphragm 13 and a metallic block 19 respectively. Both end portions of the line
21 are clamp`edly secured by sealing metallic fixtures 20 and quartz window plates
14, 14' that are also used as incident light window plates, while the side
portions thereof are clamped by suitable seals and metallic fixtures, not shown.
The line thus defined is placed on the path of the incident light.
The diaphragm 13 is bonded to the lower surface of a metallic block 18
having a through bore in which a piezoelectric ceramics element 12 îs accommodated
and there bonded to the diaphragm 13. The element 12 is connected by a copper
wire 10 to the foremost part of a connection terminal 8 connected in turn to the
upper portion of the block 18. The element 12 is also secured to the termlnal 8
through a Teflon*tube 9 and a rubber seal 11. The metallic block 19 has two
through holes opening on the side thereof facing the diaphrag,n 13, these holes
connecting to an inlet 16 and an outlet 17 for the liquid sample under examination.
Reference is now made to Figs. 1 and 2 for illustrating the detection
of photoacoustic output produced in the line from the liquid sample under
examination.
The laser light from the source 1 has its frequency modulated to a
desired frequency by a photoacoustic filter type modulator 2 fed by a generator
6 and condensed by lens 3. The light then passes through quartz window plate 14
as incident light so as to continuously irradiate the liquid flowing from the
inlet 16 towards the outlet 17. The photoacoustic output thus produced in
the cell is sensed by piezo-electric ceramic element 12 bonded to the
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diaphragm 13 and is suitably recorded in recorder 7 foll~wing
amplification by a lock-in amplifier 5. For using phase
sensing ~nplifiers such as lock-in amplifier 5, suitable modulators need to be
used for pulsed laser light or continuous incident light. Also, when the device
is used in conjunction with a flowing liquid as in the case of high~performance
liquid chromatography, it is desirable that any frequency may be chosen to cancel
external noises caused by pump pulsations. It is also desirable that the
liquid under examination by subjected to as small perturbational effects in the
cell as possible and hence the diaphragm should preferably be made of gold or
silver or other chemically stable material and have a smooth surface.
Detection of the photoacoustic output produced in the line from the
liquid can be made not only by the arrangement shown in Fig. 2, but by suitably
modified arrangements in which (b) the diaphragm 13 itself is designed as sensor
Fig. 2 B, ~c) sensor means are accommodated in the line 21, Fig. 2 C, or
(d) one or the other side of the liquid inlet 16 or the outlet 17 is deslgned as
sensor Fig. 2 D, (e) the window plate 14' itself is designed as sensor Fig. 2 E.
The new detector may be advantageously employed, by using any
one of the above configurations, for detecting the presence or concentrations of
the solute in the flowing liquid under examination and especially as a detector
for lLquid chromatography or other flow s~ate monitor.
Reference is made below to an embodiment as a `detector for liquid
chromatography.
Fig. 3 shows the results of a test in which the detector is connected
to a high performance liquid chromatographic system and the modulation frequencies
of the incident light are varied. Thus, Fig. 3 shows the modulation frequencies
on the abscissa and the changes in the magnitudes of signals and noises and the
signal to noise ratio on the ordinate. It may be seen fron Fig. 3 that the
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signal magnitude becomes ma~i~um at approximately 300 ~Iz, the noise level being
then rather high, and that the signal to noise ratio (S/N ratio) becomes
maximum in the neighborhood of S KHz. Thus it is ~pparent that external noises
of various frequeDcies such as those caused by liquid delivery pumps may be
produced in the course of flowing state measurement and that suitable means Eor
selecting desired incident ligh~ frequencies may be cmployed for improving
the detection sensitiv:ity.
Fig. 4 shows the results of a test in whlch the ~ ~LYe detector is
comlected in series with a visible light absorption detector that is commonly
employed as h:Lgh-performance liquid chromatographic detector and measurement was
made by the two detectors concurrently for comparison. The measurement conditions
were as listed below.
Liquid de1ivery pump, HLC-805 type pump for liquid chromatography,
manufactured by TOYO SODA MANUFACTURING CO., LTD.
Column, stainless column, 4 mm inside diameter and 30.0 cm length
packed with TSK-GEL LS 410 ODS SIL manufactured by TOYO SODA MANUFACTURING CO.,
LTD.
Sample A, 2-chlorodiethylaminoazobenzene
B, 3-chlorodiethylaminoazobenzene
C, 4-chlorodiethylaminoazobenzene
Amount of sample in~ection, 3 mg each
Visible light absorption detector
Measuring wavelength, 488 nm
Photoacoustic detector
Measuring wavelength, 488 rm
Modulation frequency, 4035 Hz
Flow Rate, 1.0 m Q /min
Eluent, methanol
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Fig. 4 shows that, with the photoacoustic detector, the signal to
noise ratio (S/N ratio) may bc improved by a factor of lO as compared to the
absorption cletector and thus the sensitivity may be improved by the same factor.
Fig. 5 shows the resu~t of a similar test in which the sample amount
is reduced to one thirtieth (100 pg each) urlder otherwise the same conditions.
It may be seen from the chromatograms of Fig. 5 that, with setting of
the detector sensitivity so that baseline noise levels of the two detectors are
approximately the same, and witll in~ection of trace amounts (100 pg each) of the
samples shown in Fig. 4, only small absorption peaks may be noticed with the
absorption detector, whereas quantitative determinatlon of the sample may be
possible with the photoacoustic detector.
Whereas the pieæo-electr$c element is arranged so as to contac~ the
outside of the foildiaphragm13 in the example of Fig. 2A above stated, the
element is embedded in seal 15 which forms the upper side of the line 21 in an
example of Fig. 2B. Likewise, said element can be set, projecting in the line
21 as in Fig. 2C, the element can be arranged in the liq~lid inlet 16 as in
Fig. 2D, or the element can be set in place of window plate ll!' as shown in
Fig. 2E.
