GB2079934A

GB2079934A - Integrating absorbance photometer

Info

Publication number: GB2079934A
Application number: GB8118525A
Authority: GB
Original assignee: Eflab Oy
Current assignee: Eflab Oy
Priority date: 1980-07-11
Filing date: 1981-06-16
Publication date: 1982-01-27
Also published as: FR2493521A1; IT8167927A0; GB2079934B; DE3124312A1; FR2493521B1; IT1144270B; JPS5749841A; FI802220A

Abstract

A photometer for the measurement of the absorbance of samples comprises a source of light, appropriate arrangements for selecting the colour of the light, and paths for respectively a reference and a measurement beam of light. The sample is placed in a cuvette in the measurement beam path. There is a detector of the beam from the sample and a detector for the reference beam, and an electronic circuitry for the processing of the values obtained from the measurement detector so that a desired reading is obtained concerning the properties of the substance placed in the cuvette. It is characterized by the reference detector signal being integrated from the beginning of the measurement period, and when the integrated signal reaches a threshold it terminates the measuring period. The signal of the measurement detector is integrated from the beginning to the end of the measurement period.

Description

SPECIFICATION Integrating photometer The present invention is concerned with a photometer by means of which, by measuring the absorbance of samples, measurement values can be obtained concerning the composition, concentration etc. circumstances of a sample that affect its transparency. The field of application of the device is in the first place photometers, in which samples are measured as extensive series while desiring to have the result from the sample at a certain reasonable precision.

The main parts of the photometer are the source of light, a cuvette containing the sample, and a detector, i.e. indicator of the quantity of radiation passing through the cuvette. Moreover, save certain exceptional cases, it is usually necessary to use an appropriate component for the selection of the colour of the light used, e.g. a prism or any other device, in itself well known in the art, to select a certain colour of the spectrum.

Since variations may occur in the quantity of light transmitted by the source of light, owing to ageing, temperature phenomena, etc. reasons, two paths of passage of the beam of light are usually used in a photometer, of which paths one is called the reference beam and the other one the measurement beam. The cuvette to be studied is placed on the path of the measurement beam, whereas the reference beam is passed along a path where the variable object to be measured cannot affect it, even though, on the path of this beam, there may be components corresponding the cuvette so that its conditions correspond the measurement beam quite closely.Both the measurement beam and the reference beam are then passed into an appropriate indicator, whereby there is most commonly an individual indicator for each of the beams, but in special cases it is also possible to employ one indicator only and to separate the reference beam and the measurement beam from each other, e.g., by means of time on the detector.

After the detectors, a processing circuitry for the quantities received from the detectors is likewise placed normally, in which circuitry the voltages, currents, or any other, corresponding quantities developed by these detectors are processed so that, as the final result, an output is obtained which illustrates the property of the sample to be measured. In view of the functioning of this "electronics", a breaker-type unit has been commonly used in some part of the beams of light to break the light used into pulsed light, because in this way it is possible to construct the amplifiers of the electronics as AC amplifiers and thereby as remarkably more stable and more reliable.

The object of the present invention is to provide a novel photometer, and the features characteristic of the invention are listed in detail in the attached claim 1.

In particular, the object of the invention is to provide a photometer in which the breaker of the beams of light has become unnecessary, because it is possible to use pulsed light, whereby the operation takes place within one pulse only.

Particularly favourabie embodiments of the invention can also be read out of the other attached patent claims, in which reference is made to claim 1.

Pulsed light has in itself already been used in photometers earlier. For example, the U.S. Patent No. 3,653,764 describes a photometer which has the said arrangement for controlling the beam of light of the photometer so that from the beam at least one pulse is obtained for each sample, the measurement interval of the pulse being adjusted favourably. When the light pulse is in this way adjusted, the said patent relies thereon that, when a certain stable portion of a pulse is selected, the output given by that portion from the sample can be passed directly, e.g., into an integrator and that the output reading produced by the pulse in the integrator can then be directly presented as a measurement value concerning the properties of the sample.In this solution one of course suffers from the drawback that, in spite of careful controlling of the voltage, no arrangement has been used in order to adjust the quantity of light developed, said quantity varying owing to ageing of the lamp and to other, corresponding phenomena, irrespective of the voltage; in other words, in the solution described in the said patent publication, there is no possibility to use a reference beam.

In the photometer in accordance with the present invention, it is possible to use an inexpensive and readily available lamp. Moreover, in the photometer in accordance with the present invention, a long service life of a lamp is obtained, for owing to the pulsed supply to the lamp, it can always be cooled between the operating pulses, which increases the service life. Since a reference beam is employed, no essential requirements have to be imposed on the stability of the intensity of light, and since no mechanical breaker is needed in the construction, e.g. a motor and a perforated disk to modulate the light, the whole construction can be made simple.

On the whole, the present invention can be understood better by examining it in the light of the exemplifying embodiments to be described beiow. In these examples, particular emphasis is given to possibilities of variation typical of this invention only, and the devices as a whole include many components that are also common in other photometers and that can therefore be employed by a person skilled in the art on the basis of his general knowledge.

The first component of such a general nature is the source of light used in the photometer and the subsequent selector of the colour of the measurement light, which selector may be, e.g., a filter. Likewise, the division into a reference beam and a measurement beam can be performed by means in themselves known by using, e.g., a partly reflecting mirror, whereby the beam passing through the mirror and the beam reflected by the mirror form the said two beams. The properties of transparency of the partly reflecting mirror do not have to be of the type 50%, but other ratios are also acceptable.

If it can be assumed that the colour distribution of the lamp in different situations remains sufficiently constant or if variations in the distribution can be compensated by means of special arrangements, it is also possible to use for the reference beam a portion of a different colour from the radiation coming from the source of light, which portion of different colour would otherwise be lost and remain unused.

It is an essential feature of the solution of the invention that two detectors are used. In theory, the detectors do not have to be alike, as compared with each other, but it is obvious that, when two similar detectors are used, it can be assumed that they age in a corresponding way both of them alike, whereas detectors ageing in different ways cause the formation of a new source of error.

The output of each detector is passed into an integrator. In electronics, several solutions are known for the formation of an integrating circuit, and the simplest one among them is a capacitor, which forms the output voltage as a time integral from the quantity of current developed by the radiation detected by the detector. A person familiar with electronics can easily provide different other integrators if the use of a capacitor alone is not appropriate as a solution in some particular cases.

The output value of the reference beam from the integrator is compared with a predetermined voltage, which may, according to one embodiment, be selected as several different voltages, in order that the photometer could operate in an even more versatile way. Thus, the output of the integrator is compared with this predetermined level and, when the output of the integrator reaches this level, a comparison unit gives an output signal, which is used in order to stop the integration process in the other integrator.

The simplest way to stop the integration is that the output of the comparison unit directly controls the source of light and puts it out when the determined integral value has been reached. Since the light given by the source of light is, however, thereby not cut-off instantaneously, but is reduced in accordance with a certain extinction curve, this means that in different situations different quantities of radiation may be received during the extinction time after the operation of the comparison unit. This of course means that this solution is not suitable for applications that require maximum precision.

A solution of substantially higher precision can be obtained when the current generated by both detectors is passed via electronic contactors to the integrators and the starting takes place when the source of light and the sample are ready for starting the measurement, whereas the disconnecting takes place on the basis of a comparison pulse given by the comparison unit. In this case the integration time depends on the quantity of light transmitted by the source of light, but the quantity received during the integration time is always constant in the way determined by the voltage set as the reference value of the reference beam. The integration sum formed in the integrator of the measurement beam is now the measurement value from the desired quantity in the sample to be measured.This voltage value can be controlled in a known way, e.g., into a numerical display by means of a pulse counter, or be guided for further processing into an appropriate processor, whereby any factors of unlinear nature in the sample can be taken into acount in the treatment. The result may also be written out on a specified strip, or it may be stored in an appropriate memory for future reference.

As was already stated, the service life of the source of light can be increased considerably when the operating power is supplied to the source of light only as short pulses for the periods of time when the light is really needed. Since the starting of a source of light is associated with particularly exceptional conditions, in this connection it is possible to proceed additionally so that the arrival of the cuvette at the measurement point gives a signal in the device about the readiness for starting the measurement. Then the voltage is connected to the source of light and a signal is given to a specified retarding unit, whereupon, after the delay determined by the said retarding unit, it can be considered that the source of light has reached its stable values. Then both of the detectors are switched onto their integrators and the measurement period proper starts.When the integrator of the reference beam then reaches its specified value, the comparison unit gives a pulse from its output, which pulse disconnects the integrator of the measurement beam from the detector. At the same time this comparison unit gives operating instruction to the display apparatus or equivalent, by means of which the voltage reached by the integrator of the measurement beam is read as a displayable measurement value from the absorbance of the measured sample. The circuit consists of an integrator, which is constructed so that its memory (capacitor) can be discharged by means of feedback so that the result is an exponent curve that illustrates the voltage as a function of time.

Thus, in this case, after the integrator of the reference beam of light has reached its final value, it is switched on for exponential feedback towards the initial value of the integrator for logarithmic modification.

In addition to this, the comparison unit, by means of its output, also produces disconnection of the source of light from the voltage, and thereby the source of light is extinguished for a period of time so that it has been possible to read the measurement result, to replace the sample by a new one, and to reset the electronics of the measurement device back to the zero situation for a new sample. This extends the service life of the lamp significantly.

Thus, a photometer has now been described by means of which essential advantages are obtained irrespective of the modes of implementation of certain details, which can be varied by a person skilled in the art. The present invention is thereby defined only by the scope of the attached patent

Claims

claims. CLAIMS

1. Photometer for the measurement of the absorbance of samples, which photometer comprises a source of light, appropriate arrangements for selecting the colour of the portion of the light that is used, two paths of passage of the beam of light, viz. reference beam of light and measurement beam of light, whereby the sample to be measured is placed in a cuvette onto the path of the measurement beam of light, after which sample there is a detector of the beam coming through the sample, in the same way as there is a detector for the reference beam of light on the path of the reference beam of light, after which there is an electronic circuitry for the processing of the values obtained from the detector so that a desired reading is obtained concerning the properties of the substance placed in the cuvette, characterized in that the quantity received from the detector of the reference beam of light and indicating the intensity of the light is integrated from the beginning of the measurement period and that the measurement circuit is connected so as to control the moment of end of the measurement period on the basis that the integration of the detector of the reference beam of light reaches a certain predetermined level.

2. A photometer as claimed in claim 1, characterized in that the light of the source of the measurement light is arranged as pulsed and that the integration of the detector of the reference beam of light is started right from the moment of beginning of the pulse.

3. A photometer as claimed in claim 1 or 2, characterized in that, when the integrator reaches a predetermined value, the value is made to extinguish the source of light.

4. A photometer as claimed in claim 1, characterized in that the output of the detector of the beam that passed through the sample is controlled into an integrator of its own beginning from the same moment when the integration of the reference sample is started and ending when the integration of the reference sample reaches the desired value.

5. A photometer as claimed in any of the preceding claims, characterized in that the preset desired final value of the integrator of the reference beam of light can, depending on each particular case of application, be set at several different levels.

6. A photometer as claimed in any of the preceding claims, characterized in that, after the integrator of the reference beam of light has reached its final value, it is switched onto exponential feedback towards the initial value of the integrator.