Programmable Monochromator for Accurate High Speed Wavelength Isolation Emil Cordos’ and H. V. Malmstadtz School of Chemical Sciences, University of Illinois, Urbana, Ill. 61801 RECENTLY THE AUTHORS described the principles of a new automated atomic-fluorescence (AF) spectrometer for multielement analysis ( I ) . The development of this instrument required a programmable monochromator that could rapidly, accurately, and automatically slew from one preset wavelength to another and continuously recycle on command through a complete sequence of up to 8 (or 12) preset wavelengths. To obtain the required high speed programmable monochromator, a commercial scanning monochromator wdS modified and the logic circuitry developed. It has worked daily for over one year in the automated A F multielement spectrometer. The mpdified monochromator is generally applicable in other types of instruments such as absorption, emission, and fluorescence spectrometers where automated sequential selection of wavelengths is important. The programmable module described here is assembled entirely from readily available components. It can slew with a speed of about 200 Ajsec and stop at a preselected wavelength within an accuracy of 0.2 It is also designed with a self-calibration device which checks the accuracy after every measurement cycle. Any desired wavelengths can be selected by dialing switches on a switch panel. Basic Design. The basic design of the programmable monochromator is illustrated in Figure 1. The wavelength encoded monochromator is provided with a dc motor which rotates the monochromator grating and an encoder which generates a number of pulses proportional to the number of angstroms by which the wavelength is to be changed. The pulses are counted by a preset down-counter which, by means of a logic circuit, stops the motor when the number of pulses matches the number at which the counter was preset. The logic circuit then, through a multiplexer, presets the counter to a new wavelength, and the operation of starting and stopping the motor is repeated. In principle, the monochromator can stop at as many different wavelengths as desired. The only requirement is the availability of a sufficient number of switches on the wavelength selector switch panel and a suitable number of inputs to the multiplexer. Block Diagram. In Figure 2, a block diagram of the instrument is given. The components of the wavelength encoded monochromator are included within the dotted lines. The programmable monochromator is used in conjunction with additional modules to form a spectrophotometer. This instrument must have a logic circuit, which activates the monochromator logic when the spectrophotometric measurement is completed, and which in turn is activated and proceeds with the measurement when the monochromator has stopped at a new wavelength. When a spectrophotometric determination is completed, the spectrophotometer logic unit resets the timing circuit which controls the sequence of events in the remainder of the
MONOCHROMATOR
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Present address, University “Babes-Bolyai”, Cluj, Romania. To whom correspondence should be addressed. (1) H. V. Malmstadt and Emil Cordos, Presented at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 19 March, 1972; also, Amer. Lab., August 1972.
Figure 1. Simplified block diagram illustrating the basic design of the programmable monochromator
logic circuit. The first pulse from the timing circuit shifts the ring counter by one bit. The ring counter output, which is “l”, activates the corresponding group of switches from the wavelength selector switch panel. The number which has been dialed on this group of switches is fed through a multiplexer to the preset inputs of the down-counter. The second and third pulses from the timing circuits preset the down-counter outputs at the number dialed on the switch panel. Since a relative encoder has been used, the numbers on the switch panel correspond to the difference between the previous wavelength and the wavelength at which the monochromator must stop. The last pulse starts the dc motor and locks the timing circuit. It can be reactivated only by the spectrophotomettic unit logic circuit or by the down-counter if the preset number was zero. In the latter case, the motor does not start, and the timing circuit shifts the ring counter so the next number is fed to the preset inputs of the downcounter. The direction in which the motor rotates can be selected using a binary switch panel and a multiplexer circuit. The motor direction multiplexer output controls the encoder gating circuit and the polarity of the voltage applied from the power supply to the motor driver. The motor direction switch permits progression through the selected wavelengths in such a manner that the monochromator returns to the same initial wavelength upon completion of the working cycle. After the motor starts, the pulses generated by the encoder are counted by the down-counter, and when its output becomes “0”, a signal is fed to the start-stop circuit which stops the motor. Since the motor cannot be stopped suddenly, it is slowed down as the desired wavelength is approached. The motor speed control circuit allows the motor to be operated at three different speeds. The motor is maintained at full speed until 10-20 angstroms remain to be traversed. At this point the motor is operated at medium speed and when within 1-2 angstroms of the desired wavelength, at a low speed.
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