INSTRUMENTATION
SPECTROSCOPY
multiplied from 0.000 to 5.998 times and t h e rate reading could be multiplied from 0.000 to 11.996. As will be seen below, the digital readout is incorporated in a very simple and straightforward manner in the logarithm conversion operation of t h e circuit. A permanent record should be pro vided, also. This system incorporates a digital printout and can give a continuous analog o u t p u t for a pen recorder etc., by means of a repeater potentiometer on the servo. Instrument Design
Cryo-Τiρ® Refrigerators: inexpensive solutions to difficult cryogenic interfaces. • Temperatures down to 3.6° Κ • Temperature control to ± 0 . 1 ° Κ • Uses gaseous, not liquid, helium. • Wide variety of interfaces available. A single Cryo-Tip* Refrigerator serves many operations simply by changing the inexpensive vacuum shroud interface. These refrigerators operate by the Joule-Thomson expansion of economi cal, convenient cylinder gas, eliminat ing the need for liquid helium. Gives precise temperature control from 3.6° Κ to 3 0 0 ° Κ by simply varying gas pressure. Cryo-Tip refrigerators are now used for low-temperature experiments in UV, IR, visible and nuclear spectroscopy — w i t h i n t e r f a c e s f o r m a n y m a k e s of spectrometers. Other uses include x-ray diffraction, Hall effects, field-ion mi croscopy, semiconductor studies, ESR, EPR, NMR and cooling of lasers and low-noise receivers. A v a i l a b l e f o r o p e n - or c l o s e d - c y c l e " p l u g - i n " operation. For full technical i n f o r m a t i o n , write: Advanced Products Dept., Air Products & Chemicals, Inc., Box 538, Allentown, Pa. 18105.
Air Products and Chemicals INC.
General Considerations. A general block diagram of the rate instrument is given in Figure 1. T h e i m p o r t a n t feature is the digital logic sequencer module which is programmed by the operator to control sequentially all operations (sample handling, stirring, washing, in duction, measurement, etc.) of a par ticular analytical method. Once set for a particular analysis, the timing of the reaction and its measurement are auto matically controlled and, hence, known to a high degree of accuracy. This module is constructed with conventional digital logic circuitry. T h e design and operation of the control sequencer will be discussed in detail in a subsequent paper and, hence will not be discussed here. It should be noted, however, t h a t it is designed in a flexible manner with extra unused logic circuits so t h a t it can be programmed for any future analysis sequence t h a t is developed. Also, the details of sample handling, reaction chambers, and cuvette will be discussed elsewhere. Only the unique design fea tures of accurate measurement of the reaction course will be discussed here.
For operation as a spectrophotometer in transmittance or absorbance (a non linear element converts transmittance to absorbance) as function of time mode, the basic operational amplifier circuitry is conventional for a single-beam spectro photometer and the servo system acts only as a digital voltmeter. Extreme care must be taken to eliminate noise and drift in the light source and the photocell. This is especially important when operating in a rate mode, because the derivative of a signal containing noise and drift is impossible to handle. It was decided to use a single beam rather than dual beam, because excessive noise problems and both mechanical and electronic complications arise from the dual-beam system. Also, with care in design and the use of quality operational amplifiers, the long term drift of the single-beam system is less than 0.003 absorbance unit per hour. Thus, no significant improvement could be ob tained with a dual-beam system. Noise. T o start with, 60 cycle line noise and transient pulses from switches, etc., must be eliminated by careful at tention to shielding, line filtering, and ground paths. Light source, photocell, and amplifier noises are more difficult to deal with because they contain low frequency components, some of which have frequencies of the same magnitude as the period of the signals to be mea sured. This, of course, limits the use of filtering techniques, and these low fre quency noises must be eliminated a t their source. First, the light source must be stabi lized. Because typical line voltage fluc tuations have periods on the same order of magnitude of the rates to be measured, a system with poor line voltage regu lation might give the same light intensity a t the beginning and end of an hour's operation, b u t will have short term fluc tuations which seriously interfere with rate readings. For example, if a light
Sample Reaction Chambers
Constant Temperature Regulator
Cuvette
Separate Analog Output to Plotter, Recorder, or Outside Digital Computer
Analog Printing Module
Spectrophotometer
Sample Handling Digital Logic Control, Stir, Wash, Sequencer Induction, etc.
Wash Water
Spectrophotometer Light Regulator
Vacuum and Pressure Pumps
Circle No. 92 on Readers' Service Card
Figure 1. 104 A ·
ANALYTICAL CHEMISTRY
Block diagram of the Digecon system
