DE2927432A1 - Photo-acoustic spectrometer for solid and liq. samples - has connected gas-filled reference and sample chambers - Google Patents

Abstract

A photoacoustic spectrometer has a light source emitting a beam in a defined spectral range which is periodically interrupted. The samples are contained in solid or liquid form in a gas-filled chamber (11) with light transmitting windows (4). Periodic pressure variations in the chamber are converted into electrical signals. It is a relatively simply constructed and is operated by a spectrometer enabling examination of all types of fluid and solid samples, e.g. emulsions, granular material, dust, coatings or solid objects and living organisms. A reference chamber is connected to the sample chamber by a channel containing a flow transducer.

Description

Photoacoustic spectrometer

The invention relates to a photoacoustic spectrometer with Light sources radiating in certain spectral ranges, means for periodic Interrupting radiation, containing samples in solid or liquid state, sample chambers filled with gas with light entry windows for the radiation, means to convert the periodic pressure fluctuations in the sample chambers into electrical ones Signals.

From the absorption of radiation of certain wavelengths in or on certain properties of the surface of substances can be deduced. In the case of non-gaseous substances, however, such an analysis is relatively difficult and expensive, since measures have to be taken to avoid the spread of the To obtain radiation on the surface or in the material sample unaffected measurement signal.

There is accordingly the object of a relatively simply constructed and to set up the photoacoustic spectrometer of the type mentioned above to be operated in such a way that that solid and liquid samples of all kinds, such as emulsions, granulates, dusts, coatings on solids and living organisms, can be examined spectrometrically.

One solution to the problem is in a photoacoustic spectrometer seen, which is characterized by an adjacent to the sample chamber and with this reference chamber connected in a gas-conducting manner and with a flow sensor in the the duct connecting the chambers.

The two chambers forming a measuring cell are filled with a gas, that does not absorb in the spectral range used for the investigation, preferably air, nitrogen or helium is used.

The radiation entering the sample chamber through the entrance window is absorbed by the sample located there, depending on the test conditions mainly on their surface.

The absorbed energy is at least partially converted into heat, whereby the gas surrounding the sample is heated and expanded. This expansion causes a pressure increase and thus a pressure-equalizing gas flow into the Reference chamber. Since the radiation emanating from the light source periodically with a Frequency between 1 and 1000 Hz is interrupted, occurs in the connection line a pulsating flow generated by the flow sensor, for example after Kind of a hot wire anemometer can be built into an electrical alternating signal is transformed, the amplitude of which is proportional to the absorption of the sample.

With the spectrometer equipped according to the invention, it is thus possible to precise qualitative analyzes of solid and liquid substances in the form of emulsions, Powders or Granules or thin layers on the surface Carry out examinations of a solid body without any preparative effort on living organisms, for example the absorption of light by plants.

Further possibilities and advantages when using the invention Measuring device emerge from the subclaims in connection with the figures and their description.

Figure 1 shows a schematic representation of a photoacoustic Spectrometers to explain the measuring principle, in Figures 3 to 7 are different Versions of measuring cells shown.

The same reference symbols denote the same parts.

Figure 1: A light source radiating in a certain spectral range 1, for example a laser emitting monochromatic radiation or a xenon lamp in connection with a monochromator, emits radiation in the beam path S. Between the light source 1 and a measuring cell 2 are means 3 for periodic interruption the radiation or arranged to deflect it, for example a rotating one Aperture or mirror arrangement. The periodically interrupted radiation passes through an entry window 4 into the measuring cell 2, with a non-absorbent Gas is filled and in which the sample 5 is located, its absorption-related Heating leads to thermal expansion of the surrounding gas.

The associated increase in pressure of the measuring cell 2 is with the help of a suitable sensor 6 converted into an electrical signal, which in the with the Interrupter frequency controlled amplifier 7 amplified rectified and the Arithmetic unit 8 is supplied. There the FUh- learning signal with others Signals that correspond to, for example, wavelength and beam strength and from the Control device 9 of the monochromator are supplied, and optionally with Measurement signals from other sensors are mathematically linked. At the exit 10 stands then a calibrated or standardized output signal for further processing or for Display.

In FIG. 2, a cross section is schematically shown in accordance with the invention trained basic shape of the measuring cell 2 shown.

It contains a sample chamber 11 with an entry window 4 in the beam path S. The sample 5 to be examined is on an exchangeable sample carrier 12, which by means of a sealing element 13, for example an O-ring, against the Wall of the sample chamber 71 seals.

The sample carrier can accommodate liquid or powder samples 5 12 with a recess, not shown here, or a container inserted therein be provided.

The sample chamber 11 is located adjacent in the measuring cell 2 a Reference chamber 14 which is not in the beam path and which is connected via a line 15 is connected to the sample chamber 11 in a gas-conducting manner. In this line 15 is preferably a thermal flow sensor 16 is arranged at its mouth in the reference chamber 14, which is arranged in a known manner from two at a small axial distance from each other Grids can consist of resistance material, the branches of a resistance measuring bridge form.

Sample chamber 11, reference chamber 14 and line 15 are filled with a gas, for example air filled, which the coming from the light source 1 radiation not absorbed in a certain spectral range.

The amount converted into heat during the absorption of radiation in sample 5 Part of the radiant energy is transferred to the gas and causes it to expand and thus a nen pressure increase in the sample chamber 11, the one Equalizing flow in the non-thermally loaded gas volume of the reference chamber 14 via line 15 causes. This flow is measured with the help of the thermal flow sensor 16 measured. During the periodic interruption of the radiation, the cools down Gas in the sample chamber 11 again, so that an equalizing flow in reverse Direction through line 15 flows. That obtained with the help of the sensor 16 Alternating signal, the amplitude of which corresponds to the thermal energy conversion and thus the absorption on the sample is processed, as already described.

When recording an absorption curve over a larger spectral range or in examinations with light of a larger spectral bandwidth there is a risk of that the gas in the measuring cell absorbs itself at certain wavelengths and this leads to a falsification of the measurement result.

To avoid this, the measuring cell shown in FIG 2, the reference chamber 14 is arranged coaxially in the beam path S in front of the sample chamber 11 and provided in an entry window 4 'and an exit window, the same here the entry window 4 of the sample chamber 11. The two chambers 11, 14 are thus optically one behind the other and switched pneumatically against one another via the line 15. The reference chamber 14 has the same dimensions as the sample chamber 11, the flow sensor 16 is arranged in the middle of the line 15, so that a symmetrical Construction is achieved. The pressure increases resulting from the absorption of radiation in the gas in the two chambers act against each other and cancel each other out.

There is often the task of spectrometrically measuring the properties of two samples compare with each other, especially a sample with unknown properties with a known reference sample.

This can be done easily with the spectrometer according to the invention Carry out manner, at the same time and under the same metrological conditions.

In the measuring cell 2 shown in Figure 4, which is structurally according to 2, two samples 5 and 5 'are next to one another on the sample carrier 12 arranged in a plane perpendicular to the beam axis of the beam path S.

For alternating irradiation of the two samples 5 and 5 ', the Beam path S divided into two identical beam paths S1 and S2, for example by alternating beam deflection of the radiation coming from the light source 1 or by using two identical light sources.

This ensures that the comparison of the absorption properties of the two samples 5 and 5 'under the same measurement conditions. The changing the two samples 5 or 5 'associated with signals from the flow sensor 16 compared with one another in the arithmetic unit 8 by forming the difference or ratio and an output signal is formed which shows the spectrometrically detectable properties which images one sample in relation to the other sample.

FIG. 5 shows an embodiment of the measuring cell 2 in accordance with the measuring cell in FIG Figure 3 for the investigation of two samples 5 and 5 'shown. That in the description What was said about FIGS. 3 and 4 applies here accordingly.

Are the spectroscopically detectable properties of several samples to investigate simultaneously and under the same measuring conditions, so can in the Measuring cell 2 contain several pairs of sample and comparison chambers according to FIG as shown in the example of FIG. The structure of the measuring cell with sample chambers 11 and 11 arranged next to one another or in a star shape radial reference chambers 14 located behind it is very compact. The irradiation of the in the Samples 5 located in sample chambers 11 can be displaced with the aid of a rotating beam deflection take place cyclically, with the signals obtained from the sensors 16 one after the other fed to the arithmetic unit 8 and processed there. However, it is also possible to irradiate all sample chambers 11 in unison, but what an amplifier 7 for each flow sensor 16 and an arithmetic unit 8 with a corresponding number of inputs.

Another embodiment for the simultaneous examination of a Sample 5 and a reference sample 5 'are shown in FIG. The one next to the sample chamber 11 arranged reference chamber 14 is here provided with an entry window 4 ', by which radiation of the second beam path S2 on a located therein Reference sample 5 'falls. The chambers are about a flow sensor 16 containing Line 15 connected to one another. With periodically changing irradiation of the two Chambers, the non-irradiated chamber acts as a reference chamber for the irradiated one. When the jet is applied at the same time, a pneumatic difference is formed instead, which can be tapped with the aid of the flow sensor 16.

Claims (7)

Patent application Photoacoustic spectrometer with in certain spectral Areas of radiating light sources, means for periodically interrupting the radiation, Gas-filled sample chambers containing samples in the solid or liquid state with light entry windows for the radiation, means for implementing the periodic Pressure fluctuations in the sample chambers in electrical signals, marked with e by a) one of the sample chamber (11) adjacent and connected to it in a gas-conducting manner Reference chamber (14), b) a flow sensor (16) in which the chambers (71, 14) connect Line (15). 2. Photoacoustic spectrometer according to claim 1, d a -d u r c h g it is not shown that the dimensions of the sample chamber (11) which are identical Reference chamber provided with radiation-permeable entry and exit windows (4 ') (14) is arranged coaxially in the beam path (S) in front of the sample chamber (71). 3. Photoacoustic spectrometer according to claim 1, d a -d u r c h g e k e n n n n z e i c h n e t that the reference chamber (14) in the measuring cell (2) is axially parallel to the beam path (S) is arranged to the side of the sample chamber (71). 4. Photoacoustic spectrometer according to claim 1, 2 or 3, d a d u r c h e k e n n n n z e i c h n e t that in the sample chamber (11) two samples (5, 5 ') arranged side by side in a plane perpendicular to the beam axis and periodically alternately exposed to identical radiation. 5. Photoacoustic spectrometer according to claim 2 or 3, g e k e n n z e i h n e t d u r c h an arrangement of at least two pairs of sample and reference chambers (11, 14) in a measuring cell (2) with different samples (5) and cyclically at the same time or alternating loading of the sample chambers (77) with identical radiation. 6. Photoacoustic spectrometer according to claim 1 and 3, d a d u r c h g e k e n n n n e i c h n e t that a reference chamber (14) with an entry window (4 '), contains a reference sample (5') and is axially parallel to the beam axis is arranged next to the sample chamber (11) and that both chambers at the same time or alternately exposed to identical radiation. 7. Photoacoustic spectrometer according to one of claims 1 to 6, d a d u r c h e k e n n n z e i c h n e t that in an arithmetic unit (8) means for functional linkage of the electrical output signals of the flow sensors (16) are provided according to one of the basic calculation types.