REPORT FOR ANALYTICAL CHEMISTS
the —95 °C "slush" acetone bathtrap combination previously used. The variable cold trap utilizes liquid nitrogen as a coolant and a resistance thermometer for temperature control. The trap was constructed of electrical conducting glass with painted silver contacts for resistive heating. The glass tubing is electrically insulated with tape of Mylar and then encased in a copper jacket with attached copper coils. The temperature sensor is located between the coils and the insulation. Cooling the trap is accomplished by forcing liquid nitrogen through the copper coils. Warming the trap and temperature control above ambient is accomplished via the resistance heaters on the glass. The actual trap temperature is monitored by apparatus consisting of a resistance thermometer, power supply circuit, and analog-to-digital converter connected to appropriate control channels of the computer. The computer converts the signal from the detector into a digital value and compares it to a value set in the program. This comparison is made every second and the liquid nitrogen flow is turned on or off as required. By changing one word via the teletype in the computer program, the trap temperature can be adjusted to any value between —180 °C to + 100 °C. The sublimation-condensation stages normally carried out by lowering the level of the slush bath relative to the trap are now accomplished by selectively heating the small separately controlled zones A and B. This is also performed under computer control. The conventional gas buret-mercury manometer combination has been replaced by a large ( ~ 700 cc) buret volume and an automatic digitized 10-mm (full scale) MKS Baratron pressure transducer having a resolution of > 104. The pressure transducer and gas buret are contained in an oven regulated to better than 0.01 °C. A glass frit is inserted between the Toepler pump and the buret to provide an accurately reproducible gas buret that can be set automatically. On the last gas transfer step (pumping), the mercury in the
Toepler pump is forced up against the glass frit, thus providing a very reproducible buret volume. The Toepler pump is also controlled by the computer. In summary, this system is completely automated. As we gather more information and further refine our computer programs, we expect to routinely obtain determinations having an accuracy of ±0.03 per cent. The vacuum fusion apparatus is equally well controlled. The small mass spectrometer that is attached to the system is under full computer control as are the valves, sample dropper, and MKS pressure transducer. We have attempted, insofar as is practical, to completely control all apparatuses interfaced to the PDP-7 computer. We anticipate eventually having eight to ten different analytical instruments simultaneously operating asynchronously in real-time. All programs including the monitor will reside in the 8K words of core memory. The disk will be used for bulk storage of mass spectrometer data, hardware debugging programs, assemblers, etc. For this task all boundaries can be defined. Therefore, fixed point arithmetic is used for all computations. The usual approach is to use floating-point routines ; however, these are very large programs that are slowly executed. Where limits can be defined, all arithmetic should be performed in fixed point and scaled to the specific application. It is readily apparent from the foregoing that there are many approaches to analytical automation. This is a new and exploding field of experimentation. In all likelihood several "best" methods will be developed to answer to most of the automation needs in the scientific laboratories. The number of control computers is rapidly on the increase. It is going to be mandatory that we as scientists make effective use of these tools.
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Conclusions
We have tried to illustrate several different approaches to automation in the analytical chemistry laboratory. In conclusion, two
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