edited by
the computer bulletin baord
RUSSELLH. BATT Kenyon College Gambier. OH 43022
Interfacing the PE 337 IR: An Inexpensive and Easy Mechanical "Interface" Michael J. Collins Viterbo College La Crosse. WI 54601 The old Perkin Elmer infrared spectrophotometers, such as the PE 337, are noteworthy for their optics, stability, and reliability. The recently developed Update Kit' testifies to their worth. This update, which consists of solid state electronics and an RS 232 cable, allows the data to be acquired, stored,audmsnipulated throughacompatihle computer. Additionally, a recent report ( I ) details the interfacing of a 337 with an IBM PC. The output voltage requires the use of the external platter accessory that was optional on the 337 and is no longer available for retro-fitting. For those who inherited an IR without this accessory, the interfacing becomes less straightforward, heeause the output ofthe pen on the drum recorder is based upon a null-potential servo motor that is used to move both the pen and s beam block so as to maintain zero output volts, giving the prospective interfacer no output proportioned to %T. This note reports a mechanical "interface" consisting of a l-MO pot-type variable resistor glued to the servo arm pivot. The installation cost less than $10 and took one afternoon. I have used the interface with the Interactive Microware Inc. ADALAB hoard', an Apple I k computer, and VlDlCHART-II' wftware to ohtain an expanded x s l e of the H U rotntion-vibratim spectrum with very goad accuracy and precision. The output is a voltage proportional to %T and is compatible with any AiJl converter. Equipment required: 1-MQ linear pat, 6V battery (screw terminals type easiest), 2X 4-in. right angle bracket, super glue, two self-tapping machine screws, Apple 11+ or IIe with ADALAB interface hoard. The figure, a view of the P E 337 with the housing removed, highlights the modification. The letters below are keyed to the letters in the figure. The potentiometer (A) is a 1-MO linear resistance pot. The ahaft was cut to about % in. length and filed smooth. Colored wires, 30 in. long, were soldered to the three pins.
' Ko~lowScientificCo.. Edge Water. NJ 07020. Interactive Microwave. Inc. P.O. Box. 139. State College. Pa 16804.
A230
Journal of Chemical Education
View of mounting bracket and potentiometer mounted to pivot arm.
The servo pivot arm (B) and the pot were hoth thoroughly cleaned with acetone. The pot was turned so as to have the variable resistance set at the maximum of about 1 MO and was bonded to the servo nivot arm with super glue while the pivot arm w a s w t to ita extreme position 1>1000bTl. Iused a piece of rubber tubing as insurance-see figure-but this was probably unnecessary. A right angle bracket was cut to 2 x 3%in. An extra hole was drilled in the short arm and the bracket was placed in position with the short arm on the optical compartment cover (C), and the long arm behind the pot. The drill hales on the optical compartment cover were marked, the cover removed, and the holes drilled. The cover was replaced, the bracket attached with self-tapping machine screws, and the pot glued to the bracke t with a drop of super glue. The wirea were threaded through to the side of the instrument and out through a mounting bracket notch. A voltage divider circuit was patched together usinga 6-V lantern battery and the 1-MOpot (A) with the variable resistance leads connected to the A/ D(+) and AD(-) leads of the ADALAB board and the third lead connected to the battery (-) terminal. TheAiJl(+) is jumped to the battery (+) lead. ~~
~~~~
~
~
.~
To test the new interface, the rotationvibration spectrum of HCl(g) was obtained. Our P E 337 was run in slow scan mode, nominal 2-m-' s scan rate, actually 1.97 cm-' s above 1300 cm-'. Data acquisition was begun as the mark on the drum crossed 3100 cm-I. The drum was calibrated using the polystyrene band at 2850.7 em-'. We were unable to resolve the H-CP7 a d HCIS hands using our gas cell, but our results were very favorable nonetheless (2)with r(u = 1) = 1.31 A;r(u = 0) = 1.29 A; extrapolated to ro = 1.27 A. The fits to second-degree polynomials (3) in the P and R branches gave correlation coefficients greater than 0.999 for each. In summary, although the simple modification described herein does not do all that a more sophisticated interface would do, it does allow for data acquisition and manipulation. By combining this modification to the PE 337 with the modification to VIDICHART-I1 (4) that allows one to convert output linear in %T into absorbance, one could also use the IR for base line corrected absorption spectra, which can he used for Beer's law plots, to analyze components of a mixture by spectral subtraction techniques, and to build a spectral library.
