The recording spectrophotometer floods a specimen with the constituent colors of the spectrum sequentially, measures with a photocell the amount of light at each hue that the specimen transmits or reflects and with an automatic pen recorder plots a graph of the measured intensity of the light with respect to its wavelength. The graph fully de scribes the constituent hues that in sum constitute the color. The price of commercial instruments ranges from several hundred to several thousand dollars depending on their resolution, that is, the discreteness with which they plot differences in hue. An instrument that is adequate for many experiments by amateurs and that fully demonstrates the principles of the recording spectrophotometer has been improvised at a cost of about $50 by a Canadian high school student, Sean Johnston (4447 Venables Street, Burnaby, British Columbia, V5C 3A5). He describes his instrument as follows:

"Having built several spectroscopes, I decided last year to make a spectrophotometer that resembled the one described in 'The Amateur Scientist' several years ago. In it rays of light from an incandescent lamp that diverge from a slit are made parallel by a lens, dispersed into their constituent hues by a prism or a diffraction grating and finally made to converge through a second slit one color at a time, depending on the angle of the prism or the grating with respect to the axis of the optical train. Such a device is known as a monochromator. It can separate any color of light from a mixture of colors. If the dispersing element is rotated through an angle of sufficient magnitude, all hues can be made to appear sequentially at the output slit. A photocell on which this output is incident can be wired to generate a voltage that varies in proportion to the intensity of each hue.

"The addition of the photocell light meter converts the monochromator into a spectrophotometer [see "The Amateur Scientist, SCIENTIFIC AMERICAN, May, 1968]. To reduce the cost of materials I substituted a diffraction grating of the reflection type for the prism that served in the instrument described above. I also replaced the vacuum tubes of the photometer with solid-state devices. The reflection grating (No. 50,201) was purchased from the Edmund Scientific Co. (300 Edscorp Building, Barrington, N.J. 08007). The cost of the project was subsequently increased significantly when I added an automatic pen recorder, thus converting the instrument into a recording spectrophotometer. The motor that drives the recording pen was also obtained from the Edmund Scientific Co. (No. 71,702).

"The available literature indicated that the best spectrophotometers split the light from a single source into two parts, a principal beam and a reference beam. Both beams proceed through the monochromator portion of the instrument, in which the light is dispersed into its constituent colors. The color of each beam can be independently altered, however, before the light is dispersed. For example, one might arrange for the principal beam to be intercepted by a chemical solution and the reference beam to be intercepted by the solvent. A photometer can be designed that in effect subtracts the colors of the reference beam from those of the principal beam. The output of a photometer so designed would represent the true colors of the solute.

"Two general schemes have been devised for making the subtraction. One is known as the 'double beam in time' and the other as the 'double beam in space.' In the first scheme the principal beam and the reference beam as they emerge from the monochromator are directed sequentially onto a photocell by an oscillating mirror or an equivalent device. The photocell accordingly generates an alternating current that is amplified to run an electric motor, which operates the recording pen.

"The direction in which the motor runs is determined by the phase of the alternating current; therefore it is determined indirectly by the relative amplitude of the beams. Simultaneously the motor moves an 'optical comb,' or mask, to intercept the reference beam to a certain degree. The interception adjusts the reference beam to an intensity that matches the intensity of the principal beam. The scheme, which is also known as an 'optical-null a.c. servomechanism,' simultaneously and automatically moves the recording pen to the corresponding null position of the graph. The system is highly stable, primarily because alternating-current amplifiers are inherently more stable than direct-current ones.

"Notwithstanding this desirable feature, I resorted to the second scheme, involving the double beam in space, because the instrument is far simpler and cheaper to build. In this arrangement the dispersed color of each beam is incident on a companion photocell. I substituted inexpensive cadmium sulfide photocells for the photomultiplier tubes found in commercial instruments.

"To subtract the output of the reference beam from that of the principal beam I connected the photocells in series to function as adjacent arms in the circuit of a Wheatstone bridge, which has four arms. The other two adjacent arms are two variable resistors [see illustration at left]. The resistance of the photocells varies inversely with the incident illumination. When the incident illumination is equal on both cells, the resistances of the cells are equal. If the variable resistors are then adjusted to be equal, the bridge is balanced and no potential difference exists between the junction of the resistors and the junction of the photocells even though the circuit contains a three-volt battery.

"If the intensity of either light beam varies, however, a proportionate potential appears across the now unbalanced bridge and hence across the input terminals of the amplifier. The amplified potential operates the direct-current motor. The shaft of the motor assembly rotates the movable contact of one of the variable resistors in the Wheatstone bridge in the direction required to balance the circuit, thus reducing the input potential of the amplifier to zero. Simultaneously the motor moves a recording pen through a distance that corresponds to the algebraic sum of the light intensity of the two beams.

"The recorder consists of a felt-tipped pen that moves in a straight line parallel to the axis of a rotating drum around which the graph paper is wrapped. A graph is made by rotating the drum through 350 angular degrees. A system Of pulleys and belts that is coupled to the drum simultaneously and synchronously rotates the diffraction grating through an angle sufficient to scan the visible spectrum from violet (400 nanometers) to dark red (750 nanometers).

"A second system of pulleys and belts similarly rotates an opaque cam called an occulter. The cam partly intercepts the reference beam as required to compensate for deficiencies in the system, with the result that a graph indicating 100 percent transmission of all colors, as with a water-white specimen, approximates a straight line. The contour of the cam is being determined experimentally and filed by hand. I am still working to improve it.

"The drum consists of two soup cans fastened end to end with epoxy cement and adhesive tape. The assembly is covered by a length of rubber inner tube that is secured at the ends with broad rubber bands. The drum is rotated through frictional contact with a pair of typewriter erasers of the disk type that are fastened with epoxy cement to the shaft of a synchronous motor, which turns at one revolution per minute. Motors of this kind are common in electric clocks.

"The motor is fastened to a supporting bracket by a single screw on which it can pivot in the vertical plane. Firm contact between the rotating typewriter erasers and the rubber band at one end of the drum is maintained both by gravity and by the tension of a pair of rubber bands stretched between the motor and the base. The pulleys of the drive system came mostly from a Meccano set. The belts are of catgut.

"The specimen is scanned by the full spectrum during approximately one revolution of the drum. Thus the apparatus generates a graph that depicts the spectral response of specimens consisting of an unchanging mixture of colors, such as colored glass or gelatin filters. By disconnecting the belt that couples the drum to the diffraction grating, graphs can be made by plotting changes in the intensity of a selected color against time. This effect is frequently associated with chemical reactions. The angle of the diffraction grating can be set by hand to expose such specimens to any desired hue. When changes in the intensity of the transmitted hue are plotted against the rate at which the drum rotates, they measure the speed of the chemical reaction.

"The most difficult but most interesting part of the apparatus to develop was the pen mechanism. As I have mentioned, the electric power for operating the motor that moves the pen is the amplified potential that appears across the Wheatstone bridge. The signal appears whenever the bridge is out of balance in response to variations in the intensity of the light that falls on the photocells. The amplitude of the signal varies in proportion to the net intensity of the light beams.

"There is a minimum level below which the motor that moves the pen will not respond. Moreover, when balance is restored to the bridge by the operation of the balancing resistor that is coupled mechanically to the motor, the pen does not stop instantly at the point of balance. Inertia causes the mechanism to overshoot.

"For these reasons the graph of a smoothly increasing signal can appear as a series of stairsteps. When the signal current increases to the minimum required to start the motor, the pen moves upward abruptly. Because of the momentum it continues to move upward briefly after balance is restored. The movement of the pen then stops but the transverse motion of the paper, which is carried by the rotating drum, continues. The result is a graph in the form of a step instead of a smooth line. The solution is to minimize the mass of the moving parts, to minimize friction and to amplify the signal appropriately. By these stratagems the size of the steps can be reduced to the point at which they merge into a continuous line.

"Almost any small, reversible direct-current motor can be installed to operate the pen. The Edmund No. 71,702 motor that I selected includes a built-in set of reduction gears that turn the output shaft 60 revolutions per minute when the motor is connected to a source of .011 ampere at a potential of three volts. With this power input the motor develops a torque of eight inch-ounces. I increase the force at the pen severalfold by additional speed reduction.

"The shaft of the motor carries a rubber friction roller that I made by pushing a twist drill through a rubber eraser. Turning the assembly at high speed with an electric hand drill, I held a piece of sandpaper the eraser until the rubber was eroded to a smooth cylinder. The rubber was cemented to the shaft. It presses against the rim of a four-inch plywood disk that is locked on the shaft of a conventional potentiometer. A four-inch pulley is also locked on the shaft. The potentiometer, the motor, the rails and two secondary pulleys are mounted on an independent framework of quarter-inch plywood [see illustration upper right ]. The rails guide a sliding carriage made of coat-hanger wire. The carriage supports a helical coil of smaller wire that is free to swing in one vertical plane. The helix makes a snug fit with the felt tipped pen. Its freedom in the vertical plane enables the pen to follow irregularities in the surface of the paper.

"The secondary pulleys were bought at a local hardware store. Half of the block in which each pulley was mounted was cut off with a hacksaw and discarded. The cut face of each block was cemented to the plywood frame with epoxy. One end of the catgut cord that transmits power was tied to the carriage. The other end was then threaded over the pulley at the outer end of the carriage rails, around the motor-driven pulley, around the pulley at the inner end of the rails and then returned to the carriage, to which it was tied. The rails are three-eighth-inch iron rods of the kind sold for hanging window drapes.

"The two power supplies for the amplifiers represent the costliest part of the construction, particularly the step-down transformers that reduce 120 volts to 25 volts. I happened to have materials on hand for making the entire instrument except the electronic components. They cost approximately $40. Doubtless the cost could have been cut in half if I had taken the time to search the surplus market.

"The usual precautions should be observed when the electronic components are assembled. For example, in bending the leads of a solid-state component always grasp them close to the devices to avoid cracking the seals. In making solder joints connect an alligator clip to the leads or grasp them with long-nose pliers to obtain a heat sink. The 2N3567 and 2N5448 transistors or equivalent devices should be provided with heat sinks, which can be of the snap-on type. Incidentally, any transistors can be substituted in this application provided that they are rated at a beta of 40 to 100, a power dissipation of at least .3 watt and a collector-to-base potential of at least 40 volts.

"The optical system consists of a 12 volt incandescent lamp of the kind used in spotlights, a condensing lens for collimating the rays that diverge from the lamp, a pair of fixed entrance slits, companion pair of specimen holders, focusing lens, an aperture, a rotatable diffraction grating of the reflecting type, an exit aperture, an adjustable pair of exit slits and a cadmium sulfide photocell for each of the two beams. Specimens are inserted into the light beams at an arbitrary point between the entrance slit and the focusing lens.

"The supporting fixture consists of a plywood frame into which cuvettes are slid. (A cuvette is a clear, rectangular container of glass or plastic for holding solutions.) In my arrangement the specimen solution is placed in the upper cuvette for interception by the principal beam; the reference solvent is placed in the lower cuvette. Cuvettes can be made by cementing or waxing together appropriately sized rectangles of clear, flat glass of the optical grade used for projector slides. This scheme applies only if the experimenter excludes test solutions that would react with the cement or the glass. I also made a set of small frames that fit the supporting fixture to hold glass or plastic light filters.

"Cadmium sulfide photocells are designed for maximum sensitivity to a specific color. For example, the Clairex Type CL-702 is most responsive to the blue end of the spectrum and is suitable for use with fluorescent light. The sensitivity of the Clairex Type CL-705 is maximum at 550 nanometers and closely matches the spectral response of the human eye. This type is commonly employed for light measurements and is suitable for use with incandescent lamps. The response of the Type CL-703 peaks at 735 nanometers in the deep red portion of the spectrum. It can be used with either incandescent or neon lamps. Types that peak at intermediate wavelengths are also available from mail-order distributors that specialize in electronic supplies, such as Allied Electronics Corp. (2400 West Washington Boulevard, Chicago, Ill. 60612).

"The interception of the reference beam by the occulting cam tends to in crease the output of the amplifier just as though the intensity of the principal beam had been increased. In effect the occulting cam acts as a 'phantom specimen' of negative transmittance. Normally the adjustable resistor of the Wheatstone bridge is operated to shift the recording pen to a position on the graph that the experimenter arbitrarily selects as the point of 100 percent transmission. If the instrument were perfect, and if the specimen were completely clear, the resulting graph would be a straight line at the 100-percent-transmission level. The instrument is far from perfect. For this reason the uncorrected graph of 100 percent transmission is an irregularly undulating curve that extends above and below the 100-percent-transmission line. By altering the contour of the occulting cam experimentally, however, I have succeeded in limiting the excursion to less than 3 percent. By continuing to alter the shape of the cam I hope to reduce the error to less than .5 percent.

"The less than perfect response of the instrument can be traced to a number of obvious sources. For example, the monochromator could be improved by adding a telescope lens between the diffraction grating and the exit slit. The spectral response of the cadmium sulfide photocells is far from linear. Of greatest importance, however, is the relatively low quality of the diffraction grating, particularly with respect to resolution. The stock from which the grating is made was originally developed to split light into its constituent hues for a system of color photography that was later abandoned. I hope to replace the material eventually with a diffraction grating of instrument quality, such as the Edmund No. 41,028.

"Most supports of the optical elements were made from plywood either a quarter or an eighth of an inch thick. The grating support is a block of wood that measures 2 x 1-1/2 x 1-1/2 inches. The grating is fragile. To protect it I included the inverted lid of a plastic pill bottle as part of the mounting assembly. When the grating is not in use, I snap the pill bottle over it.

"For convenience in testing the apparatus during construction and in subsequent maintenance I made each functional element as a removable subassembly. Parts made in this way include the recording drum, the servomotor, the optical train and the electronics system. A panel on the front of the instrument includes on-off switches for the lamp, the drum motor and the electronic system and knobs for adjusting one variable resistor of the Wheatstone bridge and the gain of the amplifier. This latter control is rarely used because I operate the system at maximum gain.

"To begin operating the instrument I attach one edge of a sheet of blank paper to the drum with adhesive tape, rotate the drum by hand to wrap the paper snugly and fasten the remaining edge with another strip of tape. I then switch on the electronic system, including the lamp. Before inserting specimens in the cuvettes I set the diffraction grating by hand so that blue light (450 nanometers) falls on the photocells.

"The adjustable resistor of the Wheatstone bridge is then operated to the point where the pen carriage moves close to the left edge of the graph paper. I insert the pen in the carriage. The drum motor is switched on. The pen traces the 100-percent-transmission line at the top of the graph.

"After one full revolution the drum is stopped, the grating is returned to its initial position and specimens are inserted. The drum is again operated to record the graph. The first of the accompanying graphs depicts the characteristic opaqueness of a didymium filter to yellow light. The second graph was made by resetting the diffraction grating three times and recording three traces of the didymium filter on a single sheet of paper to demonstrate the repeatability, or precision, of the instrument. The remaining set of graphs was made by sequentially recording the transmission characteristics of a red, an infrared and a green filter on a single sheet of paper."

Bibliography

ANALYTICAL ABSORPTION SPECTROSCOPY: ANASORPTIMETRY AND COLORIMETRY. Edited by M. G. Mellon. John Wiley & Sons, Inc., 1950.

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