GB2141536A

GB2141536A - Photodiode arrray spectrophotometric detector

Info

Publication number: GB2141536A
Application number: GB08334656A
Authority: GB
Inventors: Hideo Iwasaki; Hiroji Kuwata; Yasuhiro Mito
Original assignee: Shimadzu Corp
Current assignee: Shimadzu Corp
Priority date: 1983-06-14
Filing date: 1983-12-30
Publication date: 1984-12-19
Also published as: DE3347603C2; GB8334656D0; DE3347603A1; JPH052931B2; JPS59231425A; GB2141536B

Abstract

A photodiode array spectrophotometric detector comprising a light source 2,3; a specimen cell 11 and a reference cell 10; a polychromator 14; a photodiode array 15; light selecting means 7 for allowing the photodiode array to receive a light which is separated by the polychromator after emerging from the specimen cell or the reference cell, or prohibiting the photodiode array to receive any light; a sample holding circuit; an A/D converter; and digital operating means for computing a spectrum of the specimen by distinguishing between the specimen signal, the reference signal and the dark signal through synchronization with the light selecting means; which is useful as a detector for a liquid chromatograph. <IMAGE>

Description

SPECIFICATION Photodiode array spectrophotometric detector This invention relates to a photodiode array spectrophotometric detector and particularly to one which can be used as a detector for a liquid chromatograph.

A photodiode array spectrophotometric detector is usable to obtain a spectrum in the substantially simultaneous measuring mode by corresponding a different wavelength to each of a number of photodiodes constituting a photodiode array.

A known photodiode array spectrophotometer is equipped with a measuring cell and a photodiode array and, by storing the output signal S(to) of the photodiode array obtained by allowing a carrier liquid to flow through the measuring cell at a certain point of time to and the output signal, or the dark signal D(to), obtained by cutting light at substantially the same time, it computes absorbance A(t) at the different time t when the output signal S(t) of the photodiode array is obtained by causing a specimen to flow through the measuring cell according to the following equation: S(n, to)-D(n, to) A(n, t) = log . .. (i) S(n, t)-D(n, to) However, n is assumed the number of a photodiode as one of the elements constituting the photodiode array.Since the photodiode number n can be made to correspond to the light wavelength A; in other words, as A(n, t) means the absorbance with the wavelength A at the point of time t, it can be referred to as A(A, t). As A(A, t) is preferred in view of physics, n will be referred to as A hereafter.

Incidentally, the following equation is generally known: S(A,t) = G(A,t).l(A,t).e-Ea(A)c-I+)] + D(A,t) . (ii) wherein G(A,t): sensitivity of the photodiode used to measure the wavelength A at the point of time t; I(A, t): intensity of light incident on the measuring cell with the wavelength A at the point of time t; a(A): absorbance of the molecuie of the specimen with the wavelength A; C: concentration of the specimen; I: length of the light path of the measuring cell; and y(A): absorbance of the carrier liquid with the wavelength A.

When only the carrier liquid is allowed to flow through the measuring cell, since C = 0, S(A, to) = G(A, to)l(A to) e-z(A) + D(A, to) . . . (iii) If the equations (ii) (iii) are applied to that of (i),

Assuming that G(A, to) = G(A, t), I(A, to) = I(A, t) and D (A, t) = D(A, to), that is, the sensitivity of the photodiode, the intensity of light and the dark signal do not fluctuate, A(A, t) = a(A) C I . . . (v).

Since the absorbance A(A, t) is proportional to the concentration of the specimen, the latter can be made known by the spectrum.

However, if the luminance of the light source, the sensitivity of the photodiode arrary and the dark signal vary, the above equation (v) will not conform to that of (iv). As a result, the concentration of the specimen can not be made known correctly from the spectrum.

Another known photodiode array spectrophotometric detector is described in the Journal of Chromatographic Science, Vol. 14/April 1 976/pp 195-200. This photodiode array spectrophotometric detector is equipped with a specimen cell, a reference cell, a measuring photodiode array and a reference photodiode array corresponding to each of the above cells. In the photodiode array spectrophotometric detector, since the output of the measuring photodiode array is compared with that of the reference photodiode array, the detector is unaffected by the change of the luminance of the light source; however, the problem is that the difference in the sensitivity of both the photodiode arrays and the characteristics of the dark signal causes errors.

An object of the present invention is to provide a photodiode array spectrophotometric detector capable of obtaining a specimen signal, a reference signal and a dark signal in the time sharing mode and suppressing the occurrence of an error by allowing the specimen signal to be corrected by the reference and dark signals.

Thus, the present invention provides a photodiode array spectrophotometric detector comprising a light source; a specimen cell and a reference cell both being acceptable for the light from the light source; a polychromator; a photodiode array; light selecting means for allowing the photodiode array to receive light which is separated by the polychromator after emerging from the specimen cell or the reference cell, or preventing the photodiode array from receiving any light; a sample holding circuit connected to the output of the photodiode array; an A/D converter connected to the output of the sample holding circuit; and digital operating means being employed to compute a spectrum of the specimen by distinguishing between the specimen signal, the reference signal and the dark signal through synchronization with the light selecting means.

The above described light selecting means can be a rotary sector mirror provided between the light source and the specimen cell and between the light source and the reference cell; or a double-trip mirror provided between the specimen cell and the spectrometer and between the reference cell and the spectometer.

The synchronization with the light selecting means can be accomplished in such a manner that the above digital operating means obtains the synchronous signal from the light selecting means or the digital operating means puts out the synchronous signal to the light selecting means.

For spectral analysis, it is preferred to obtain a spectrum with a wavelength of 200 to 699 nm and a light source abie to irradiate the light having such a wavelength. Moreover, if there are 500 photodiodes. 1 nm can be allotted to each photodiode within the range of wavelength 200 to 699 nm. For this reason, a photodiode array formed with about 500 photodiodes is desirous.

The present invention will now be described in more detail, with reference to the accompanying drawings, in which Figure 1 is a diagram illustrating the construction of an optical system as an exemplary embodiment of a photodiode array spectrophotometric detector in accordance with the present invention; Figure 2 is a diagram illustrating the construction of the rotating position detecting system of the sector mirror of the embodiment shown in Fig. 1; Figure 3 is a diagram illustrating the construction of the signal processing system of the embodiment shown in Fig. 1; Figure 4 is a time chart of each signal of the signal processing system shown in Fig. 3; and Figure 5 is an example of a three-dimensional spectro-chromatogram obtained by the photodiode array spectrophotometric detector in accordance with the present invention.

Numeral 1 in Fig. 1 indicates an optical system in the embodiment of the photodiode array spectrophotometric detector in accordance with the present invention. The light of a heavy hydrogen lamp 2 or a tungsten lamp 3 selected by a light source switching mirror 4 is reflected by a sector mirror 7 and troidai mirror 8 through a light source mirror 5 and a slit 6 and then allowed to pass through a cell 10 for the reference and again reflected by a sector mirror 7.The light is separated by a concave grating 14 through a condensing mirror 1 2 and a flat mirror 1 3 before being received by a photodiode array 1 5. However, when a cut portion 7a of the sector mirror 7 corresponds to the light passage I as the sector mirror 7 rotates, the light through the slit 6 is not reflected by the sector mirror 7 but by the troidal mirror 9 and allowed to pass through a flow cell 11 for the specimen and received by the photodiode array 1 5 through condensing mirror 1 3 and the concave grating 14.When the shaded portion 7b of the sector mirror 7 corresponds to the light passage I as the sector mirror 7 further rotates, the light through the slit 6 is cut by the shaded portion 7b, so that the light is not received by the photodiode array 1 5.

The photodiode array 1 5 is formed with about 500 photodiodes, each corresponding to roughly 200 in 700 nm wavelengths.

Numeral 1 7 in Fig. 2 shows a slit disc for detecting the rotating position of the sector mirror 7; the slit disc 1 7 is fixed to the rotary shaft 1 6 of the sector mirror and revolved together with the sector mirror 7. When the shaded portion 7b of the sector mirror comes over the light passage I, a photomicrosensor 1 8 generates an output signal because of a reflection mark 1 7d attached to the slit disc 17, whereas a photomicrosensor 1 9 produces an output signal because of a slit 1 7a. When the mirror reaches the light passage I as the sector mirror 7 rotates, only the photomicrosensor 1 9 generates an output signal because of the slit 1 7b. Furthermore, when the cut portion 7a reaches the light passage I as the sector mirror 7 rotates, only the photomicrosensor 1 9 generates an output signal. Since the output signal can be obtained in every half turn of the sector mirror, the rotating position of the sector mirror 7 is detected.

Numeral 30 in Fig. 3 indicates a signal processing circuit of the photodiode array spectrophotometric detector in accordance with the present invention. The photodiode array 1 5 has been incorporated in an output circuit 20. The output circuit 20 is used to integrate each output signal of the photodiode e1, e2. . . and, when the start signal SRT is inputted, it outputs each integrated value from its output terminal 20a synchronously with a clock signal CK. Each of the integrated values is cleared once after it has been outputted. The clock signal CK is a pulse signal with a frequency of 100 sec and given by an oscillator 21. An output signal E of the output circuit 20 is applied to a microcomputer 25 through an amplifier 22, a sample holding circuit 23 and an A/D converter 24.The signals from the photomicrosensors 18, 1 9 are inputted to the microcomputer 25. The microcomputer further outputs a driving signal for driving a motor 26 used to turn the sector mirror 7 and the slit disc 17; and an output signal as the start signal SRT for the output circuit 20.

The operation will be described.

At a time to, the operator makes only the carrier liquid flow in the flow cell 11 for the specimen and then gives instructions on the background processing BG to the microcomputer 25.

The microcomputer 25 monitors the signals from the photomicrosensors 18, 1 9 and, when the signals from the photomicrosensors 18, 19 becomes ON simultaneously as shown in Fig. 4, it gives the start signal SRT immediately. As a result, a series of output signals can be obtained from the output circuit 20 of the photodiode array 1 5. However, because these output signals are ihtegrated before and after the shading portion 76 of the sector mirror 7 reaches the light pasage I, they are inferior as data and thus ignored. Since the frequency of the clock signal CK was 100 y sec and the number of photodiodes e1, e2. . was about 500, outputting a series of output signals is completed at about 50 msec.In order to provide allowance, the start signal SRT is again given at about 60 msec. This makes a series of output signals available again.

These are the integrated values for 60 msec in such a state that the light is being cut by the shaded portion 7b and therefore stored as data D(to) of the dark signal. During about 1 20 msec from the first start signal SRT given and to the completion of sampling the second series of output signals, it is assumed that the dimensions and rotating speed of the shaded portion 7b of the sector mirror have so defined as to let the shaded portion 7b keep cutting the light.

Subsequently, the microcomputer 25 monitors the output signal of the photomicroprocessor 1 9 and gives the start signal SRT immediately after the signal becomes ON and ignores a series of output signals obtained thereafter. 60 msec later, it gives the start signal SRT again and stores a series of output signal obtained then as data R(to) of the reference signal. The reason for ignoring the first series of output signals is that they contain the integrated values at the time when the shaded portion 7b is over the light passage I and thus inferior. Assuming that the dimensions and rotating speed of the mirror of the sector mirror 7 have been so defined as to let the mirror stay over the light passage I for about 120 msec.

Then the microcomputer 25 operates in the same way and obtains data S(to) of the sample signal to store them.

Based on the data D(to), R(to) and S(to), the following equation is used to compute the background light absorbance B(to), which is then stored. However, the computation is carried out between data corresponding to the wavelength A.

R(A, to)-D(A, to) B(A, to) = log ... (vi) S(A, to)-D(A, to) Subsequently, at any time t the operator makes the specimen flow in the flow cell (11) for the sample and then gives instructions on the measurement processing to the microcomputer 25.

The microcomputer 25 obtains the dark signal D(t), the reference signal R(t) and the sample signal S(t) in the same way as above described and computes the light absorbance A(t) according to the following equation. However, the computation is carried out between data corresponding to the wavelength A.

R(A, t)-D(A, t) A(A, t) = log - B(A, to)... (vii) S(A, t)-D(A, t) A(A, t) thus obtained is stored as a spectrum or outputted to the recorder (not shown) as a chromatogram.

Fig. 5 shows an example of the A (A, t) thus obtained and put on the printer (not shown) in a three-dimensional chart.

Description will be made of obtaining the accurate concentration of the specimen from the A(A, t) given by the above equation (vii).

The above described equation (ii) is the general expression of the S(A, t); namely, S(A,t) = G(A,t) l(A,t) e~ra(A)C'+7(A)3 + D(A,t) . . . (ii) Since the cell (10) for the reference is empty, C = o, and y(A) = O; R(A, t) = G(A, t)-l(A, t) + D(A, .... . (viii), R(A, to) = G(A, to)-l(A, to) + D(A, to). . . (ix).

Moreover, since S(A, to) is only the carrier liquid, C = 0; S(A, to) = G(A, to)-l(A, to).e-Y(A) + D(A, to) ... (x) If the equations (ix), (x) are applied to the equation (vi) B(A, to) = ....... (xi).

If the equations (ii), (viii), (xi) are applied to the equation (vii), A(A, t) = a(A) C I . . . (xii).

In other words, the light absorbance A(A, t) computed from the equation (vii) is, even if the changes of the luminance of the light sources 2, 3 and the sensitivity of the photodiode array 1 5 and the dark signal occur between the time to and t unaffected by the changes and it accurately represents the concentration C of the specimen.

As another embodiment of the present invention, the carrier liquid or specimen is so arranged as to selectively flow to the flow cell 11 for the sample through a switching valve, which is controlled by a microcomputer. When the controlling microcomputer selects the carrier liquid with the switching valve, it is operated to output instructions on the background processing and when the microcomputer selects the specimen with the switching valve, it is operated to output instructions as to the measurement processing so that operation can be automated completely.

Since one photodiode array makes it possible to obtain the signal for the specimen and the other for the reference through the double beam method in the photodiode array spectrophotometric detector according to the present invention, the error caused by the changes of the luminance and the sensitivity of the photodiode can be cancelled. Therefore, measurement accuracy can be improved and a stable chromatogram becomes available. As a result, even if the warming-up time is shortened, measurement will be made stably.

Since the above as well as other modifications and changes are intended to be within the scope of the present invention, the foregoing description should be construed as illustrative and not in the limiting sense, the scope of the invention being defined by the appended claims

Claims

1. A photodiode array spectrophotometric detector comprising a light source; a specimen cell and a reference cell both being acceptable for the light from the light source; a polychromator; a photodiode array; light selecting means for allowing the photodiode array to receive a light which is separated by the polychromator after emerging from the specimen cell or the reference cell, or preventing the photodiode array from receiving any light; a sample holding circuit connected to the output of the photodiode array; an A/D converter connected to the output of the sample holding circuit; and digital operating means connected to the output of the A/D converter, the digital operating means being employed to compute a spectrum of the specimen by distinguishing between the specimen signal, the reference signal and the dark signal through synchronization with the light selecting means.

2. A photodiode array spectrophotometric detector according to claim 1, wherein the light source is a heavy hydrogen lamp and/or a tungsten lamp.

3. A photodiode array spectrophotometric detector according to claim 1 or 2, wherein about 500 photodiodes are used to constitute the photodiode array.

4. A photodiode array spectrophotometric detector according to any of the preceding claims, wherein the detector is used as a detector for liquid chromatograph.

5. A photodiode array spectrophotometric detector according to any of the preceding claims, wherein the light selecting means is a sector mirror provided between the light source and the specimen cell and between the light souce and the reference cell.

6. A photodiode array spectrophotometric detector according to any of the preceding claims, substantially as hereinbefore described and exemplified and with reference to the accompanying drawings.