Automatic Base Line Drift Corrector Device For Use In An Integrator For Chromatographical Analysis

Abstract

An automatic base line drift corrector device for use in an integrator for chromatographical analysis, essentially including means for generating an instantaneous correction signal by which a correcting operation with respect to the base line value of the signal can be instantaneously carried out and means for generating a non-instantaneous correction signal by which a correcting operation with respect to the base line value of the signal can be carried out with a time lag with respect to the detected signal, either of said first and second generating means being employed by a switching means for excluding an interval between the starting and terminating points of the peak appearing in the signal in connection with the wave form or time of the signal.

Description

The present invention relates to an automatic base line drift corrector device for use in an integrator for chromatographical analysis adapted to automatically detect the starting point and terminating point of the peak in a chromatograph to integrate the area under the peak so that correction of the base line drift can be effected without altering the wave shape or amplitude of the analytical signals detected. In the art of chemical analysis, chromatographs are used to analyze the chemical contents of samples and are designed so as to provide output analytical fluctuations indicative of the occurrence and concentration of the chemical constituents. Nevertheless a proper technique for this purpose includes the use of actual onset analysis which results in a higher or lower base line value. Obviously, it is sometimes desirable or even necessary to alter the base line value to accomplish various chromatographic analyses in which the analytical signals provided by various sensors, detectors, voltage sources or other transducers may normally integrate the areas under respective peaks with respect to the base line thereof. Because of the importance of obtaining analytical signals having a base line which is essentially drift-free, and for many reasons, it has been found desirable to provide a device for correcting base line drift.

The correction of the base line for the chromatograph constitutes, in principle, instantaneously correcting to zero a point on the curve which is above the zero point of the base line before detecting the peak. However, according to such a method, a part of the curve of the peak is corrected to zero by a drift of the base line, when the detection of the start of the positive slope of the peak has been delayed as shown in FIG. 1(a). The vertical axis of FIG. 1(a) and the horizontal axis thereof show the height of the peak and the time interval thereof, respectively. If the peak is wide with a gentle slope when the peak starts at the time point A, there will be a considerable amount of time elaspe until the slope reaches a fixed value or more. Accordingly, the detection of the peak is often delayed. If the peak is detected, for example, starting at time point B, the base line is instantaneously corrected so that the peak starts at zero at the time point B, whereby the portion of the peak prior to the time point B is corrected to zero by a drift of the base line. Therefore, only the lined area of the peak between B and C is calculated by means of an integrator. Since the peak actually ends at D a considerable error is caused with respect to the actual area under the peak because the area under the line B C and between A and D is not calculated.

Thus, a fixed time lag is generally used for the correction of the base line and the part of the curve of the peak before the start of the calculation of the area under the peak is not cancelled, and the area of the hatched portion defined by B, E, F and C in FIG. 1(b) is calculated.

However, in such a method of correcting the base line a considerable error is likely to be caused with respect to a peak which is difficult to separate. This is described hereinafter with reference to FIGS. 2 and 3.

The vertical axis of FIGS. 2 and 3 shows the peak and the base line, respectively, while the horizontal axis thereof, shows the time. The start of the positive slope of the first peak is detected at point A, the end thereof being detected at point B, the start of the next peak being detected at point C and the end thereof being detected at point D. At first, in FIG. 2(a), it is assumed that the base line has drifted from the height designated at point A to the height designated at point C when the end of the first peak has been detected at point B. In this case, as for the second peak, the area covered with oblique lines between C and D has to be calculated.

However, in the conventional system, the base line correcting device is actuated after the end of the first peak has been detected at point B. If there is a lag in the correcting operations of the correcting device, the base line can not be shifted completely between points B and C when the time interval from the point B to the start point C of the next peak is short. As shown in FIG. 2(b), the base line can drift as far as point F. Accordingly, the area under the second peak calculated by the integrating device becomes the portion covered with the oblique lines between C, F, G and D of FIG. 2(b), which is a considerably bigger size than the actual area, that is, the area of the portion covered with the oblique lines between C and D of FIG. 2(a).

In order to prevent the errors, as shown in FIG. 2(b), from being caused, the area of the portion covered with the oblique lines between C and D of FIG. 2(a) must be calculated as if the instantaneous correction were carried out without a time lag in the correcting action for the base line. However, in this case, a considerable error is caused with respect to the type of peak shown in FIG. 3.

FIGS. 3(a) and 3(b) show the case where the second peak is produced on the negative slope of the first big peak, in which condition, as shown in FIG. 3(a), the respective areas of the portions, defined by A, E and B and C, F and D have to be calculated. However, if the correction of the base line is carried out instantaneously after the slope of the first peak has become a fixed value or less at point B and the end of the first peak has been detected, in the second peak the area of the portion covered with the oblique lines between C and D of FIG. 3(b) is calculated, which area is considerably smaller than the actual area CFD in FIG. 3(a).

The conventional method of calculating the area of the portion covered with the oblique lines in FIG. 3(a) has been carried out by the interposition of a delay circuit wherein the base line correcting device is not operated at all during a predetermined time period which is longer than the time interval between B and C of FIG. 3(a), and is actuated after the lapse of the predetermined time.

However, the area can not be calculated correctly by any conventional method for the curves of both FIG. 2(a) and FIG. 3(a). Either the area of FIG. 2(a) or that of FIG. 3(a) can be calculated correctly, but not both.

However, the delay circuit means for correction of the base line for a gentle curve as shown in FIG. 3 can not keep up with variations of the base line when the area of the peak on the negative slope as shown in FIG. 4 is calculated, whereby the error becomes considerable. FIG. 4 shows changes of the peak and the base line with respect to the time elapsed, in which the waveforms vary in the direction of the arrow as time passes. When the first peak is finished at time point B, the point B of the figure becomes a base line. But the slope of the curve to the start time point C of the second peak is so relatively steep that the correction of the base line can not catch up with the slope. At the start time point C of the second peak, the base line is corrected to a value before the time point C, for example, a value in the time point E, whereby the area of the second peak becomes the area between E and F covered with the oblique lines, and is considerably smaller than the actual peak area between the time point C and the time point D where the second peak actually begins and ends. Moreover, in practical gas chromatography which employs gas as the mobile phase, the base line of the detected signals is normally shifted all of a sudden from the high value X of the carrier gas to the low value Y of the sample gas, as shown in FIG. 5. Therefore, when the correcting operation for the base line delayed by means of a delay control circuit is subjected to the detected signals of the carrier gas in gas chromatography, the first peak of the detected signals appearing adjacent to the turning point A at which the sample gas is mixed with the carrier gas may be integrated on a false base line, i.e., the line D-E in FIG. 5, which is much higher than the real base line, i.e., the line B-C in FIG. 5, thereby reducing the detected area and giving misleading information as to contents of the sample gas. Accordingly, it is necessary to carry out an instantaneous correcting operation for the base line to follow the detected signals at once when the output of the base line correcting device is detected to have negative polarity at the turning point.

An object of the present invention is to provide a base line correcting device wherein a exact correction of the base line can be carried out with respect to any peak which is insufficiently separated from adjacent peaks.

Another object of the present invention is to provide a base line correcting device for an integrator for chromatographical analysis, which is adapted to delay the start of the correcting operations for the base line by means of a delay control circuit, which can select the delay time after the end of the peak has been detected, to carry out a correcting operation instantaneously for a very short time, unless the start of the next peak is detected, after the delay time has elapsed, and to carry out the correcting operation for the base line with an ordinary time lag after the very short time has been elapsed.

A further object of the present invention is to provide a system wherein the slope is instantaneously corrected even when the slope of the negative slope of the peak is steep, whereby the peak area on the negative slope can be calculated more correctly. The instantaneous correction of the base line is carried out when the base line falls steeply, while the correction of the base line is carried out with time lag, as before, on the other occasions.

A still further object of the present invention is to provide a base line correcting system for an integrator for chromatographical analysis wherein the polarity of the output of the base line correcting device is discriminated, the correcting operation of the base line correcting device being carried out with a time lag when the polarity of the output is positive or zero while the correcting operations of the base line correcting device are instantaneously carried out when the polarity of the output is negative.

These and other objects and features of the present invention will become apparent from the following full description of the present invention taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings, in which:

FIGS. 1(a) and 1(b) are schematic diagrams each showing examples wave forms of the signal for chromatographical analysis, this diagram being shown only for the purpose of illustration,

FIGS. 2(a) and 2(b) are schematic diagrams each showing another example like that of FIG. 1,

FIGS. 3(a) and 3(b) are schematic diagrams each showing a further example like that of FIG. 1,

FIGS. 4(a) and 4(b) are schematic diagrams each showings a still further example like that of FIG. 1,

FIG. 5 is a schematic diagram showing a still further example like that of FIG. 1,

FIG. 6 is a circuit block diagram showing a preferred embodiment of the present invention,

FIG. 7 is a circuit block diagram showing the details of part of FIG. 6,

FIG. 8 is a circuit block diagram showing another embodiment of FIG. 7,

FIG. 9 is a schematic diagram of a wave form shown only for the purpose of illustration, and

FIG. 10 is a schematic diagram of another wave form shown only for purpose of illustration.

Considering the invention broadly, attention is directed to FIG. 6 which illustrates a Gas Chromatography detector 10 in schematic block form for providing an input signal to the device 11 of the present invention which is the base line corrector. The signal from the detector 10 is amplified by a DC amplifier 12 which provides an input for two branches of the circuit, i.e., to a peak detector 13 and the base line corrector 11. The peak detector 13 indicates the existence of a peak in the signal from the detector 10 and maintains the peak indicating signal for an interval of time beginning from the occurrence of the onset of the analytical voltage fluctuation and ending at the termination of the analytical fluctuation. The signal in the peak detector 13 serves as one means for informing the base line corrector 11 of the occurrence of the peak so that the base line corrector 11 can withhold correction during the analytical fluctuation. The base line corrector 11 is adapted to co-operate with the input signal to correct the base line drift as will be more throughly described hereinafter.

The output of the base line corrector 11 is supplied to digital integrator including a voltage-to-frequency converter 14, a gate 15 and a counter 16. The base line corrector 11 provides an input current to the converter 14 to effect a drift correction in accordance with the present invention. The V-F converter 14 provides to the gate 15, an output in the form of pulses having a repetition rate proportional to the amplitude of the input voltage. The gate 15 can be switched on to transfer the pulses from the converter 14 to the counter 16 when the peak detector 13 detects the existence of a peak. The output of the counter 16 for counting the number of the pulses is then fed to a printer 17 which indicates the measured size of the peak. The base line corrector 11 in the gas chromatograph is usually operated in the range of the low level signal the value of which is lower than that of a threshold level provided selectively in accordance with the sampling gas. The operation of the base line corrector 11 is illustrated more in detail in FIG. 7.

FIG. 7 is a block diagram showing one embodiment of the base line correcting device in accordance with the present invention. Referring to FIG. 7, 21 is a base line correcting device input to which signals from the chromatograph are provided, 22 being an amplifier, 23 being a base line correcting device output, 24 and 25 being normally closed and normally open contact points, respectively, for a relay which is described later, and 26 being an amplifier or an integrator which consists of, for example, a resistor 27 with the resistance value of R and a capacitor 28 with the capacity of C. The output signal of the amplifier 22 is supplied to the integrator 26 through the contact point 24, and the input of the integrator 26 is fed back to the input side of the output of the amplifier 26. The contact point 25 is disposed in parallel with the resistor 27. 29 is a detector for the peak start, 30 being a detector for the peak end, 31 being a flip-flop circuit which is set or reset by means of detectors 29 and 30, and 32 and 33 being relays each of which is operated by means of the flip-flop 31 for closing the contact points 24 and 25, respectively.

The amplifier 22 is provided with a signal from the chromatograph at the input 21 thereof. When the contact point 24 is closed and the contact point 25 is open, the output 23 of the amplifier 22 is supplied to the integrator 26 and the output of the integrator 26 is fed back to the input side of the amplifier 22, whereby the output 23 is reduced zero. This is the correcting operation for the base line, which, as long as contact 24 is closed is continuously carried out with a time lag determined by the time constant of the resistance R and capacitance C of the integrator 26. It is well known that the correcting operation for the base line is carried out with a time lag by negative feedback of the integrator output to the input side of the amplifier 22.

Next, the operations of the relay contacts 24 and 25 will be described. When the start of the peak is detected by means of the detector 29, the flip-flop circuit 31 is set by the output thereof. At the setting by the output of the flip-flop circuit 31, the relay 32 is operated to open the contact point 24. At this time, the relay 33 is not operated so that the contact point 25 is maintained open. When the end of the peak is detected by means of the detector 30, the flip-flop circuit 31 is reset whereby the output disappears. At this time, the relay 32 holds the contact point 24 for a fixed time which is a time lag, while the relay 33 closes the contact point 25 for a very short time with the same time lag as in the relay 32.

FIG. 9 is a wave form of a chromatograph for illustrating the operation of the device shown in FIG. 7. Before the positive slope of the peak is detected at point A, the contact point 24 in FIG. 7 is closed while the contact point 25 is open, and the correcting operation for the base line is carried out with a fixed time lag. When the start of the positive slope of the peak has been detected at the point A, an output of the detector 29 is provided to set the flip-flop circuit 31, and the relay 32 opens the contact point 24 by the setting output of the flip-flop circuit. Therefore, since the output of the amplifier 22 is not provided to the integrator 26, the correcting operation for the base line is not carried out, whereby the value at the point A is retained. When the end of the peak is detected at point B, the flip-flop circuit 31 is reset by the output of the detector 30. At this time, the relay 32 closes the contact point 24 after a fixed time lag, which is the time interval corresponding to the time between B and C of FIG. 9, and also, the relay 33 closes the contact point 25 for a very short time after the same delay time. The contact point 24 remains open for a fixed time after the end of the peak has been detected, whereby the correcting operation for the base line is not carried out during this period. At the time C, the contact point 24 is closed and the contact point 25 closes for a very short time. Accordingly, the contact point 24 remains closed during the very short time, and the resistor 27 is short-circuited by the contact point 25, whereby the correcting operation for the output 23 is carried out instantaneously. If the output 23 has diverged from the zero point, the output 23 is corrected to zero instantaneously. After the correcting operation is completed, the contact point 25 opens while the contact point 24 remains closed. Accordingly, the correcting operation for the base line is carried out again with the usual time lag until the point D at the start of the positive slope of the next peak.

When the start of the next peak has been detected by the detector 29 between B and C, of FIG. 9, or during the delay time when the correcting operation for the base line is not carried out after the end of one peak has been detected, the contact point 24 is opened immediately by the output of the detector 29, whereby the instantaneous correcting operation for the base line and the following ordinary correcting operation are not carried out. The area of the second peak is integrated with the same base line as that of the first peak.

The relay 33 comprising, for example, a delay circuit, a not circuit and a series circuit for a monostable multi-vibrator, can close the contact point 25 for a short time after a fixed delay time when the setting output from the flip-flop disappears.

The present invention is designed as described hereinbefore, so as to delay the start of the correcting operation for the base line during a fixed time upon detection of the peak end, to carry out the instantaneous correcting operation for the base line during a very short time, unless the start of the next peak is detected during a fixed delay time, to instantaneously correct the divergence of the base line during the detection of the first peak and thereafter, to carry out the correcting operation for the base line with the ordinary time lag. Therefore, if the time interval, i.e., delay time, or the time between B and C of FIG. 9 during which the correcting operation for the base line, after the end of the peak has been detected for peak as in FIG. 2(a), is not carried out, is selected to be shorter than the time interval between B and C of FIG. 2(a), the instantaneous correcting operation for the base line can be carried out at the point C of FIG. 2, whereby a correct calculation of peak area can be carried out, as in FIG. 2(a), with respect to the second peak. If the second peak is wide and gentle, and the detection of the start of the peak is delayed (if the peak of FIG. 1(b) is the second peak, and the first peak has ended prior thereto) as shown in FIG. 1(b), the instantaneous correcting operation for the base line is completed by point B, and the correcting operation for the base line is carried out with the ordinary time lag at the point B. Thus, the shaded area of FIG. 1(b) can be calculated correctly.

On the other hand, if the time between B and C of FIG. 9 is selected to be longer than the time between B and C of FIG. 3(a) for a peak as shown in FIG. 3(a), the correcting operation for the base line is not carried out at all between B and C of FIG. 3(a), whereby the area covered with oblique lines in FIG. 3(a) can be calculated correctly. The time between B and C of FIG. 9 can be changed at will by a change of the delay time of the relays 32 and 33. Therefore, according to the present invention, the correct calculation of peak area can be carried out with respect to any chromatograph.

The start of the correcting operation for the base line after detection of the end of the peak can be delayed by means of the flip-flop circuit 31, and the relays 32 and 33. Since the delay time is variable, the circuit comprising these elements 31, 32 and 33 has been called a delay control circuit in view of the fact that the relay 32 opens the contact point 24 without the delay time upon detection of the peak start, but operates with the delay time when the contact point 24 is closed.

FIG. 8 is a block diagram showing another example of the device in accordance with the present invention, wherein 41 is an input terminal to which the signal from the chromatograph is provided, 42 being an amplifier, 43 being a feedback circuit for correcting the base line, 44 being a polarity discriminating means, 45 being a relay having two contacts 46 and 47, 48 being a switch, and 49 being an output terminal.

The circuit including the amplifier 42 and the feedback circuit 43, for carrying out the correcting operation for the base line with the time lag, is known to those skilled in the art. In this case, the feedback circuit 43 is, for example, an integration circuit, the signal of output side P of the amplifier 42 being fed back to the amplifier 42 as a negative feedback signal by the feedback circuit 43, whereby the signal at point P is controlled to be zero. The signal supplied from the chromatograph through the terminal 41 appears at point P through the amplifier 42. It is well known that the signal of point P is controlled to zero with a fixed time lag to carry out the correcting operation for the base line.

Also, a circuit for carrying out the correcting operation for the base line by feeding the signal at P back to the amplifier 42 is also known.

Referring to FIG. 8, the signal at point P is fed through the feed-back circuit 43 to the input side of the amplifier 42 with a fixed time lag when the contact point 46 of the relay 45 is closed, whereby the signal at point P is controlled to zero with a fixed time lag. On the other hand, when the contact point 47 of the relay 45 is closed, the signal at point P is instantaneously controlled to zero. The switch 48 opens when a peak has been detected. The correcting operation for the base line is not carried out during a period in which the switch 48 is open, with the result that the feedback from the feedback circuit to the amplifier is maintained at a constant value.

The polarity discriminating device 44 detects the polarity of the signal at point P. The polarity of the signal which is applied to the input terminal 41 negative slope, is negative, and the polarity of the signal at point P when the signal at point P has been shifted from zero by such a signal is negative. The polarity discriminating device 44 closes the contact point 46 of the relay 45 when the signal at the point P is positive or zero, while the contact point 47 of the relay 45 closes when the signal at the point P is negative.

Referring to FIG. 10, the vertical axis shows the level of the signal at the input terminal 41 and the signal at point P, while the horizontal axis shows time. Assuming now that the signal (a) at the input terminal 41 has been changed to the negative direction after the time point t.sub.O, the signal (b) at the point P, which has been corrected with a fixed time lag, also changes to the negative direction, whereby the contact point 47 of the relay 45 closes, and the contact point 46 thereof is opened by the output of the polarity discriminating device 44. When the contact point of the relay 45 is switched to 47, the signal at the point P is instantaneously fed back to the amplifier 42 through the feed-back circuit 43 to instantaneously set the signal at the point P to zero. At the time point t.sub.1, the correction for the base line is instantaneously carried out to set the signal at the point P to zero, whereupon the relay 45 is switched to complete the circuit through the contact point 46 by the output of the discrimination circuit 44. At this time, the signal at point P is fed back to the amplifier 42 with a fixed time lag through the feed-back circuit 43. If the signal (a) at the input 41 still remains in the negative direction, the signal at point P is changed to negative again with the fixed time lag. At the time point t.sub.2, the relay 45 is again switched to complete the circuit through the contact point 47 by the discriminating circuit 44, whereby the signal at the point P is instantaneously fed back to the amplifier 42 to again set the signal at point P to zero. Such operation is repeated so long as the signal (a) at the input 41 is in the negative direction. Each time the signal at point P is instantaneously set to zero, the instantaneous correcting operation for the base line is carried out. The waveforms (b) of FIG. 10 show the signals at the point P which have been corrected as described hereinbefore.

The advantages obtainable by the present invention as described with reference to FIG. 8 are described hereinafter with reference to FIG. 4(b). FIG. 4(b) shows the changes of the base line for the peaks as in FIG. 4(a) with respect to the time elapsed. At the point B, the output of the amplifier 42 is zero, corresponding to t.sub.0 of FIG. 10. As the slope, corresponding to (a) of FIG. 10, of the negative slope from point B of FIG. 4(b) to point C thereof, is corrected such as shown by (b) in FIG. 10, the detection point C of the start of the second peak becomes the base line for the second peak, whereby the area of the portion defined by C, E, F and D covered with the oblique lines in FIG. 4(b) can be calculated. It will be found that the error is greatly reduced as compared with the shaded portion defined by E and F in FIG. 4(a).

As described hereinbefore, in the present invention, the output the signal, signal at point P of the amplifier 42 which receives the signal from the chromatograph, is fed back through a feed-back circuit 43. The output signal of the amplifier 42 is set to zero to carry out the correcting operation for the base line of the integrator for the chromatograph, and the amplifier 42 and feed-back circuit 43 carry out the correcting operation for the base line as described hereinbefore. Therefore, they are hereinafter refered to as a base line correcting device. However, the output signal of the base line correcting device produces an output signal of the amplifier 42 or the signal at the point P. In this case, a circuit 44, which detects the polarity of the output of the base line correcting device, decides whether the polarity of the output signal of the base line correcting device is negative or positive or zero. When the polarity of the output signal of the base line correcting device is detected as being positive or zero by the the polarity discriminating circuit, the correcting operation for the base line is carried out with a fixed time lag, while, when the polarity of the output signal of the base line correcting device is negative, the correcting operation for the base line is instantaneously carried out. Accordingly, when the base line is shifted in the positive direction, the correcting operation for the base line is carried out by a conventional fixed-time-lag-system as described with reference to FIG. 1(b). The correcting operation for the base line is instantaneously carried out on the negative slope or when the base line suddenly shifts in the negative direction due to a pressure change or the like. Thus, the area under the peak can be calculated more correctly, even on the negative slope or when the base line suddenly shifts in the negative direction. Although the present invention has been fully described by way of example, various changes and modifications will be apparent to those skilled in the art. For example, the correcting operations for the base line hereinbefore described in connection with FIG. 8 are applied to the initial state of a gas-chromatographical measurement in which the base line is normally shifted all of a sudden from the high value of the carrier gas to the low value of the sample gas beyond the small value of the usual shifts thereof. In addition, both means described as a first embodiment shown in FIG. 7 and a second embodiment shown in FIG. 8 can be employed together so as to use properly either of the above two means in accordance with the detected peak of the wave form, for example, in such a manner that, when the wave form of signal has a plurality of integrated small peaks on the negative slope thereof, the first means are employed with respect to the first small peak while the second means are employed with respect to the other small peak. Also, the detailed construction of means 44 and 47 described in the second embodiment shown in FIG. 8 is the same as the construction of means 25, 26, 27 and 28 described in the first embodiment shown in FIG. 7 as a unit.

Claims

What is claimed is:

1. An automatic base line drift corrector device for use in an integrator for chromatographical analysis, comprising:

a. means for receiving a signal from a measuring instrument;

b. peak detecting means coupled to said signal receiving means for detecting the start and end time points of a peak appearing in said signal;

c. correction signal generating means coupled to said signal receiving means and said peak detecting means for generating a correction signal by which the base line value of the signal can be carried out;

d. first switching means coupled to said correction signal generating means and said peak detecting means for switching said correction signal generating means off between the start and end time points of a peak in said signal;

e. instantaneous correction signal means coupled to said correction signal generating means for causing instantaneous generation of a correction signal;

f. time delay correction signal means coupled to said correction signal generating means for causing generation of a correction signal after a predetermined time delay;

g. further switching means coupled to said instantaneous correction on signal means and said time delay correction signal means for switching from one to the other for causing said correction signal generating means to generate a correction signal instantaneously or with a time delay at the end time point of a peak appearing in said signal; and

h. integrating means coupled to said correction signal generating means and said peak detecting means for integrating the area under a peak in the wave form of the signal.

2. An automatic base line drift corrector device as claimed in claim 1, in which said further switching means includes delay means for delaying the operation of said further switching means for a certain period and after said certain period, said further switching means switches said instantaneous correction signal means when the start time point has been detected by said peak detecting means and switches said time delay correction signal means when no start time point has been detected.

3. An automatic base line drift corrector device as claimed in claim 2 wherein said delaying means is operable to vary the certain period.

4. An automatic base line drift corrector device as claimed in claim 1 wherein said further switching means comprises discriminating means for detecting the polarity of the wave form of the signal and for switching said instantaneous correction signal means when the discriminating means detects a negative polarity and for switching said time delay correction signal means when the discriminating means detects a positive polarity.

5. An automatic base line drift corrector device as claimed in claim 4 wherein said discriminating means is operable when fluctuations in the signal are much larger than a predetermined value of drift.