1531
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 Mechanical Slit Control for the Beckman IR-2 Infrared Spectrophotometer. Dean H. Couch and A. L. Sstterthwaite, U. S. Naval Ordnance Test Station, Chins. Lake, Calif.
on, so that a warm-up period is not needed for the next run. Thus, after starting the instrument no further attention is required to complete the recording.
slit control for the Beckman IR-2 infrared with it the infrared absorption spectra. of compounds can be recorded directly in per cent transmittance. Blanks are recorded st 100 =t 5% transmittance. Beckman IR-2 infrared spectrophotometers have been in use for several years, and several mechanical slit controls have been developed. Some of these devices consisted of a simple spool and belt arrangement ( 1 , 2). Another type of slit control, essentially the same BS that described here, has been used by the California Research Carp. for aeverd years (S). The design considerations and the procedure for collecting data have been discussed by Tolberg and Boyd (4). The conversion of the data to information necessary for the construction of the control differs from that of Tolherg and Boyd in that an approximation method was used.
RESULTS AND CONCLUSIONS
MECHANICAL
A spectrophotometer has been devised;
*
This slit control gives a blank recording a t 100 5 % transmittanoe between 2.5 and 15 microns. For recording the blank, the gain control w u set on 10 and the variable control was set a t 627' of maximum. The carbon dioxide and water vapor hands artre alwaytys recorded.
CONSTRUCTION OF SLIT CONTROL
... . ...
A plot of diameter us. revolutions for the spiral was made for
the machinist. The spiral was made of brass plates 0.25 inch thick, which were bolted together as they were milled to shape. The coue-shilped drive spool w m made from solid brass. The final working model of the slit control iS shown in Figure 1, in operating position on the Beckman IR-2 infrared spectrophotometer. A No. 3o piano wire is used to link the spool and spiral together. Other modifications which add to the convenience of operation of the spectrophotometer are presented in Figure 2. OPERATION O F SLIT CONTROL
The instrument is turned on and allowed to warm up as usual. The starting conditions for the slit control are cheoked by turning the wavelength drive knob to the 2.50-micron position. The Nave-length drive clutch is engaged to hold this position. A mark on the bottom edge of the drive spool should line u with a mark on the frame below it, A similar mark on the spira7should line up with a vire on the left side of the frame, After these points are checked, the slit is adjusted to 0.880 revolution, and the Jacobs chuck is tight,ened. The wave-length drive clutch is apened and the device is turned hack manually to 15 microns.
Figure 2. Wiring Diagram Operating tests have shown this slit control is satisfactory for genera1 studies of infrared absorption spectra. It produces per cent transmittance recordings that are easily compared in contrast to the difficulties encountered with the original sawtooth type recordings. The Slit control has been in use since January 1952. No appreciable change in the blank recording has resulted from changes in the glower, sensitivity of the thermocouple, 07 other variables. LITERATURE CITED
(1) Badger, R. M., California Institute of Technology, private com-
munication.
J. Y . , paper presented at Ohio State Symposium on Moleeulsr Structure and Spectroscopy, Columbus, Ohio, June 1951. IRI Rhreve. 0.D.. and Henther. M. R.. ANAL. C ~ E M .22, . 835-7 (2) Beach,
Paper Ionograms. H. L. Demorest and Richard Baskin, Radioisotope Unit, Veterans Administration Hospital, Minneapolis, Minn. NEED
has arisen in this labomtory for a sensitive detector
A to be used for paper chromatogram and paper ionogram
scanning in which the compounds are tagged with carbon-14,
a weak beta emitter. T o date, the position of the compound
Figure 1. Mechanical Slit Control for the Bediman IR-2 Infrared Suectrouhotorneter
The recorder is standardized and the zero position is checked. The recording is started by engaging the wave-length drive clutch. Tlihen the wave-length scale reaches 9 microns, a knot on the load cord tips a mercury switch and closes a solenoid (Figure2). This changes the rotating shutter from glass to metal. A t the end of the run the load weight is drawn up against a limit switch. This shuts off the wave-length drive motor, recorder motor, and solenoid. The rest of t.he electronic equipment is left
tagged with carbon-14 has been located on the filter paper by moving i t heneath a thin-window Geiger counter [Frierson, TT-. D., and Jones, J . TV., ANAL. CHEM.,23, 1447 (1951)l ionization chamber [Jones, A. R., AN&[.. CHEM., 24, 1055 (195211, or by cutting the filt.er pnper into small sections which are then counted in a proportional counter. I n order to he able to improve the sensitivity of detection over the thin-window Geiger counter method and to avoid the tedious procedure of cutting the filter . Dmer . into sections. an instrument has been developed in which the sample is placed within the counting chamber of a gas How counter. The paper is then scanned within the chamber for the presence of radioacti INSTRUMI
As shown in Figure 1, the ...l... _.I_____._ a chsmher housing, D,with a slotted plate, H,a main chamber I_
___I
ANALYTICAL CHEMISTRY
1532 in which the filter paper is placed on the strip holder, L , and a reversible constant speed motor which moves the paper strip holder. The instrument is made of brass with the exception of the steel chamber housing, D, and the copper gas inlet and outlet tubes. There are two gas outlets to allow uniform gas pressure throughout the entire chamber. The copper gas tubes are soldered and the chamber, D, is secured to the main chamber by means of six screws. iln 0 ring, ZI,prevents leaking of the counting gas and diffusion of air into the chamber. The paper strip holder has been designed to keep the paper strip tight and smooth by shaping the top of the holder with a slight curvature a t the edges and with clips and phosphor bronze springs as shown in the cross-section view. The end plate is securely sealed to the main chamber using four screws and a tongue and lip arrangement with a rubber gasket. The longitudinal drive enters through this end plate and is sealed with another 0 ring, 12. A 3-mil tungsten wire, G, is used for the positive anode. The beta particles enter the counting chamber through a slit 1.5 inches long by 0.25 inch wide. Above the slit a coarse mesh is soldered to the plate, H . This gives a more uniform electric field a t the slit. BeloiT the slit and soldered to the slotted plate is a fine wire mesh to provide electrical shielding of the paper. The slotted plate can be removed and replaced by one having any desired slit width. The size of the slit width chosen depends on the senisitivity and resolution desired. T o prepare the filter paper for scanning, it is cut into a strip 13.5 cm. in length and mounted on the paper holder. If the width of the filter paper is less than 8.0 cm., the clips and phosphor bronze springs cannot be utilized. Paper strips less than 8.0 cm. in width can be conveniently fastened to the holder with transparent tape running lengthwise along the paper. The paper holder is coupled to the longitudinal drive, M , and placed inside the chamber. The end plate, K , is then secured tightly and the longitudinal drive is fastened to the longitudinal screw, 0, by means of the union, N . Q gas (a butane-helium mixture manufactured by Nuclear Instrument and Chemical Corp., Chicago) is put into the unit under a slight pressure and the complete assembly tested for leaks with a soap and water solution. Under running conditions, Q gas is constantly flushed through the chamber a t a rate of two to four bubbles per second as seen by oil bubblers attached a t each gas outlet tube. After 15 minutes of flushing, the chamber high voltage is applied and the paper is scanned. .3ao
C R O S S S E C T I O N V I E W OF
flow
Microswitches are arranged a t the end of the scanning length allowing the motor to reverse or stop as desired. SENSITIVITY
The counting rate as a function of voltage has been determined using a carbon-14 sample placed just beneath the 1.5 X 0.25 inch slit on the holder, The results of this determination show that the plateau of the counter is flat to within 2y0 for 150 volts. On the basis of this plateau the counter was operated a t 1350 volts. The same sample when counted in the proportional counter (Model PC-2, manufactured by Nuclear Measurement. Inc.) gave a net count of 14,030 counts per minute. Placing this sample 2 mm. beneath a 1.9 mg. per square cm. end-window Geiger counter, covered with aluminum containing a slit 1.5 X 0.25 inches, yielded a net count of 2160 counts per minutr
L
Figure 2.
Relay Circuit for Scanner
ilssuniing 50% geometry for the proportional counter, then the scanner which has approximately half the counting rate for a given sample will have a geometry of 25%. The scanner in turn gives a factor of approximately 3.5 times that of the 1.9 mg. per square cm. end-window Geiger counter when the sample is placed 2 mm. from the window. The data and results of these comparisons are given in Table I.
P A P E R S T R I P HOLDER
90s flow
flor
Table I.
Comparison of Scanner Counter and Two Other Counters
Counter X
1.9 mg./sq.
Paper scanner Proportional counter
A. B. C. D. E. F. 0. H. XI. 11. J.
K. L. .M. N.
0.
Axial drive coupled to motor Copper tubing Gas flow valve Chamber housing Lucite insulator Hinh voltage electrode Tungsten c i r e loop Slotted plate 0 type sealing ring Scanner chassis Sealing end plate Paper holder Longitudinal drive for scanner Union Longitudinal screw
The pulses from the chamber are amplified by a factor of 10, fed into a scaler and then into a counting rate meter which drives a 1-ma. Esterline-Angus recorder. Knowing the rate of speed of the paper strip and the rate of speed of the Esterline-Angus, the exact position of the radioactive spot on the paper can be detwmined. The rate of scanning in the present instrument is 0.48 cm. per minute. This slox rate of speed is necessary to reduce the error due to counting statistics, although this rate may be changed by changing the longitudinal screw.
Voltage cm.
1350 1350 1750
Net Counts/ Min. 2,160
Background Counts/Min.
7,070
7G 39
14,030
18
h'et Counts,/ Min. Background Counts/XIin. 29
180 780
The sensitivity of the instrument, as measured, gives a counting rate of twice the background for 0.15 X 10-3 microcurie of carbon14. The sensitivity of the instrument using a sample of aminofluorene that has actually been run as a chromatogram is approximately 1.5 X 10-8 microcurie for twice the background. This decreased sensitivity in an actual chromatogram is due to the nature of the sample, n-hich determines the resolution of the chromatogram itself and the amount of absorption present. .4s a point of interest the activity of acetylaminofluorene placed on Whatman No. 1 paper and run as an ionogram was determined by scanning the side of the paper on which the radioactive compound was placed and then scanning on the reverse side. It was found that for this compound the activity on the side of application was approximately tnice that of the reverse side. ACKNOWLEDGIlEhT
The authors wish to thank John H. Peters and Helmut R. Gutmann for the paper chromatograms and paper ionograms used in the testing of this instrument.
