GB2089041A

GB2089041A - A Flow Type Photoacoustic Detector

Info

Publication number: GB2089041A
Application number: GB8130315A
Authority: GB
Original assignee: Toyo Soda Manufacturing Co Ltd
Current assignee: Tosoh Corp
Priority date: 1980-10-07
Filing date: 1981-10-07
Publication date: 1982-06-16
Also published as: DE3139917A1; JPS5764145A; FR2491623A1; CA1186402A; JPH0219894B2; FR2491623B1; DE3139917C2; GB2089041B

Abstract

The invention relates to a detector for measuring the sound generated from a liquid sample by irradiation with light of the sample in a flowing state. The light from a source (1) irradiates the liquid in a flow cell (4) under examination and the sound produced is sensed by sensor means mounted in the flow cell (4). The light from the source (1), preferably laser light, is modulated to a desired frequency by a photoacoustic filter type modulator (2) and is condensed by a lens (3). The sound produced in the cell is sensed and recorded following amplification by a lock in amplifier (5). <IMAGE>

Description

SPECIFICATION A Flow Type Photoacoustic Detector This invention relates to a detector for measuring the optical sound generated from a liquid sample by irradiation with light of the liquid sample in flowing state. More particularly, the present invention relates to a detector in which the liquid sample is allowed to flow through a flow cell equipped with pressure sensor means, intense light, such as a laser beam, is used to irradiate the liquid sample in the flowing state and the optical sound generated from the liquid sample is subjected to measurement in order to measure solute components in the liquid sample without destroying the solute components and with high sensitivity.In general, a material which has absorbed the light emits fluorescent light or optical sound, as shown in the following schema:

fluorescent absorption light light 9 material sample optical sound Among the three processes of the above schema, the light absorption process and the fluorescent light emission process are practically utilized for detecting the liquid in the flowing state such as in a high-performance liquid chromatographic detector, wherein the former is made practical in the form of an ultraviolet or visible light absorption detector and the latter is made practical in the form of a fluorescent light detector. In the absorption process, the ratio of abosrbed light to the amount of light not being 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 optical sound, and thus it may be expected that a highly sensitive measurement can be obtained, with respect to a material sample capable of emitting fluorescent light or optical sound. In fact, fluorescent light detectors are available which measure fluorescent materials with high accuracy, but there is a defect in that the number of fluorescent materials is limited. It is therefore expected that the photoacoustic measurement can be applied to many materials with high accuracy, in the case of liquid chromatography etc, if the photoacoustic measurement can be carried out with materials of flowing state singly or in combination with fluorescent measurement.

So far, the method of the photoacoustic detection has been applied to a lot of gas samples and solid samples, using condenser microphones as the sensor.

However, as this method cannot be applied to volatile materials, and moreover water vapour is hostile to the condenser microphones, this method is applicable to only a small number of liquid samples, In addition, it has been hitherto unknown to apply the photoacoustic detection to liquid samples in flowing state, as encountered in high-performance liquid chromatography, because of pressure variations other than the photoacoustic effect of the samples which results in more difficulties.

As a result of perservering research by the present inventors, the flow type photoacoustic detector of the present invention ailows a highly accurate detection of the optical sound from a liquid sample which is not in a stationary state or sealed state, but in a flowing state.

According to the invention there is provided a flow type photoacoustic detector comprising a sample cell, a light source for radiating light to a sample in the sample cell, and detecting means for detecting pressure variations caused in the sample, wherein the sample cell comprises a line allowing the sample to flow therethrough and the said line is arranged to be in the path of incident light.

Embodiments of the invention are described below, by example only, with reference to the accompanying drawings, wherein: Fig. 1 is schematic outline view of the optical system in the detector; Figs. 2A 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 magnitudes and the signal to noise ratio (S/N ratio) are plotted against modulation frequencies of a photoacoustic filter type modulator; Fig. 4 shows the comparative chromatograms showing the results of concurrent measurements by the photoacoustic detector and an absorption detector which were connected in series the absorption detector being of the type which is commonly used in connection with highperformance liquid chromatography; and Fig. 5 shows the comparative diagrams obtained in a case where the baseline noise of the absorption detector is approximated to that of the photoacoustic detector.

The detector comprises a light source 1 and a flow cell 4 containing sensor means. The incident light from the source 1 irradiates the liquid in the flow cell 4 under examination and the optical sound thus produced is sensed by the sensor means mounted in the flow cell 4.

Although the laser light is most preferred as the light source employed in the present invention, emission line from mercury lamps or light from xenon lamps may also be used, and both visible and ultraviolet lights may be used as laser light.

Pressure sensors such as piezo-electric ceramics and other piezo-electric sensors may be employed advantageously as sensors in the present invention.

When the present invention is applied to a liquid chromatographic detector, the capacity of the flow cell within the detector should preferably be less than 300 yl.

Although the present invention is not limited to any specifice flow rate of the liquid in the flowing state, the flow rate should preferably be in the range of 0.1 ml/min to 10 ml/min in the case of the liquid chromatographic detectors.

In the case of monitoring devices, the flow cell capacity should preferably be less than 10 ml 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.

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 fixedly secured by sealing metallic fixtures 20 and quartz window plates 14, 14' which 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 21 thus defined is positioned in 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 peizo-electric ceramics element 12 is accomodated and 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 terminal 8 through a Teflon (a registered Trade Mark) tube 9 and a rubber seal 1 The metallic block 19 has two through holes opening into the side thereof opposite the diaphragm 13, which holes communicate with an inlet 1 6 and an outlet 1 7 for the liquid sample under examination.

Reference is now made to Figs. 1 and 2 A to E in order to illustrate the detection of optical sounds 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 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 toward the outlet 17. The light thus produced in the cell is sensed by piezoelectric ceramic element 12 bonded to the diaphragm 13 and is suitably recorded following amplification by a lock-in amplifier 5. To use phase sensing amplifiers such as the 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 the flowing liquid as in the case of highperformance liquid chromatography, it is desirable that any frequency may be selected to cancel external noises caused by pump pulsations. It is also desirable that the flowing liquid under examination be subjected to as small turbulent effects in the cell as possible and hence the diaphragm should preferably be made of gold or silver, or another chemically stable material, and have a smooth surface.

Detection of the sounds produced in the line from the liquid can be made not only by the arrangement shown in Fig. 2A, but by suitably modified arrangements in which the diaphragm 1 3 itself is designed as sensor (Fig. 2B), sensor means are accomodated in the line 21 (Fig. 2C), one or the other side of the liquid inlet 1 6 or the outlet 1 7 is designed as sensor (Fig. 2D), or the window plate 14' itself is designed as sensor (Fig.

2E).

The detector according to the invention may be advantageously employed, by using any one of the above configurations, for detecting the presence of concentrations of the solute in the flowing liquid under examination and especially as a detector for liquid chromatography or other flow state monitor.

Reference is made below to an example in which the invention is applied 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 from Fig. 3 that the signal magnitude becomes maximum at approximately 300 Hz, the noise level being then rather high, and that the signal to noise ratio (S/N ratio) becomes maximum in the neighborhood of 5 KHz. Thus it is apparent that external noises of various frequencies such as those caused by liquid delivery pumps may be produced in the course of flowing state measurement and that suitable means for selecting desired incident light frequencies may be employed for improving the detection sensitivity.

Fig. 4 shows the results of a test in which the detector according to the invention is connected in series with a visible light absorption detector which is commonly employed as highperformance liquid chromatographic detector and measurement was made by the two detectors concurrently for comparison. The measurement conditions were as listed below.

Liquid delivery 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 injection: 3 ng each Visible light absorption detector Measuring wavelength: 488 nm Photoacoustic detector Measuring wavelength: 488 nm Modulation frequency: 4035 Hz Flow Rate: 1.0 ml/min Eluent: methanol Fig. 4 shows that, with the photoacoustic detector, the signal to noise ratio (S/N ratio) may be improved by a factor of 10 as compared to the absorption detector and thus the sensitivity may be improved by the same factor.

Fig. 5 shows the result of a similar test in which the sample amount is reduced to one thirtieth (100 pg each) under otherwise the same conditions.

It may be seen from the chromatograms of Fig.

5 that, by setting the detector sensitivity so that baseline noise levels of the two detectors may be approximately the same, and with the injection of trace amounts (100 pg each) of the samples shown in Fig. 4, only small absorption peaks may be observed with the absorption detector, whereas quantitative determination of sample may be possible with the photoacoustic detector.

Whereas the piezo-electric element is arranged so as to contact the outside of the foil diaphragm 1 3 in the example of Fig. 2A described above, the element may be embedded in seal 1 5 which forms the upper side of the line 21 in the example of Fig. 2B. Furthermore, the element can be arranged to project into the line 21 as in Fig. 2C, the element can be arranged in the liquid inlet16 as in Fig. 2D, or the element can be set in the position of window plate 14' (Fig. 2A) as in Fig.

Claims

1. A flow type photoacoustic detector comprising a sample cell, a light source for radiating light to a sample in the sample cell, and detecting means for detecting pressure variations caused in the sample, wherein the sample cell comprise a line allowing the sample to flow therethrough and the said line is arranged to be in the path of incident light.

2. A flow type photoacoustic detector according to claim 1 wherein the line of the sample cell is equipped with a liquid inlet and a liquid outlet.

3. A flow type photoacoustic detector according to claim 1 or 2, wherein the channel of the sample cell is defined by a pair of two opposite faces on both ends thereof, at least one face being a window plate for the incident light.

4. A flow type photoacoustic detector according to any preceding claim, wherein an end of the detecting means is arranged in the line or in the vicinity of the line.

5. A flow type photoacoustic detector according to claim 4, wherein the remainder of one pair of two opposite faces or at least one face of the other pair of two opposite faces confining the line is the end of the detecting means.

6. A flow type photoacoustic detector according to claim 4, wherein the end of the detecting means is arranged in the liquid inlet or in the liquid outlet.

7. A flow type photoacoustic detector according to claim 4, wherein the end of the detecting means is a sensor with or without a diaphragm fitted thereto.

8. A flow type photoacoustic detector according to claim 7, wherein the sensor is a piezo-electric element.

9. A flow type photoacoustic detector substantially as herein described with reference to the accompanying drawings.