REPORT (Continued from page 32 A) sible the high speed recording instruments now so widely used in analytical work. W i t h o u t it, all direct-reading d a t a would still be obtained point by point, and only cumbersome photographic processes could be employed to give the t y p e of continuous record needed for high resolution spectral, electrochemical, and kinetic investigations. The instrumentation principles described above are combined with each other, and with still other circuit designs not considered here, in a variety of ways in the manifold instruments now in general use. These instruments have characteristics and limitations t h a t are inherent in their designs, and they cannot be used to best advantage unless these features are clearly understood.
a new human engineering
concept...
Choosing the Instrument Proper utilization of instrumentation starts with the proper choice of the instrument for the measurement problem in hand. Each situation confronting the a n a l y s t has its own unique features, and the decision as to which instrument to use often must be m a d e by t a k i n g into consideration noninstrumental factors, such as the local availability of instruments and operating personnel, need for certain kinds of records for legal purposes, etc. A number of general considerations apply to all instruments, and should be in the forefront of the chemist's thinking when considering an instrument for a given job. Signal-to-Noise Ratio. Electrical noise arises in electronic circuits from the random motion of electrons in conductors, resistors, and vacuum tubes, generating spontaneous voltage fluctuations t h a t are superimposed on whatever directed voltages are present. The three types of noise commonly encountered in electronic circuits are thermal-resistor, or Johnson noise, noise from fluctuations in plate and grid currents, and shot effect noise. The random fluctuations in the directions and velocities of the free electrons in metals lead to differences in electron density from one end to the other of the component; Circle No. 46 on Readers' Service Card
>
in the first medium-priced
M A S S SPECTROMETER for analytical
laboratories
C E C ' s all-new 21-130 Mass Spectrometer offers the same accuracy, precision, sensitivity and scan speed you'll find only in the largest instruments of its kind. All this plus a totally new "human" engineered packaging concept that means: Greater accessibility (modular electronics on pull-down chassis) . . . Convenience in the grouping of operating controls by function, with each operable and adjustable from the f r o n t . . . Lighter weight because it's built on a welded extruded aluminum frame with formica-over honeycomb cabinet panels. Look at its features: A built-in direct writing oscillograph recording syst e m using five galvanometers... a stainless steel inlet s y s t e m . . . a built-in micromanometer. And performance? Mass range from m/e 2 to 230 continuous with unit resolution up to m/e 200. Forcomplete information, call your nearest CEC sales and service office or write today for Bulletin CEC 21130-X5.
Analytical & Control Division
CEC
C O N S O L I D A T E D E L E C T R O D Y N A M I C S / pasadena, California A SUBSIDIARY OF
Bell
&
Howell
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FINER PRODUCTS THROUGH IMAGINATION
