bg Ralph H.MulCer of this magnitude greatly simplify the process of measuring and recording. By m a n s of appropriate changes in ion source and the reversal of potentials this spectrometer can be used for the study of negative ions. chemical systems, taken in all the permutations and combiThe instrument is now being adapted to the rapid scanning nations, there is sufficient subject material for doctoral disof mass spectra. By sweeping the ion accelerating voltage sertations for the next century. But the mischievous process from 50 to 250 volts while modulating the radio-frequency of instrument development continues unabated and in those potential with a 1OOo-Eyclesignal, it is possible to sweep twice casm in which the new development is unusual, entirely new a second through the mass range from 10 to 50 and display analytical possibilities are presented. the measured mass components directly on the screen of E Three-Stage Nonmagnetic Mass Spectrometer cathode ray oscilloscope. Studies are currently under way at the bureau to utilize the instrument in the field for the autoAn excellent example is the progress which has been made matic analysis of small percentages of hydrogen in helium, in the development of nonmagnetic mass spectrometers. We and for the continuous monitoring of air in enclosed spaces have referred to these investigations several times because for the presence of chlorine 01 hydrogeu, and another project they gave indications of what might be expected in the future. is under way to utilize the spectrometer as an extremely sensiThe three-stage nonmagnetic mass spectrometer, employing tive carbon monoxide detector. the principle of velocity selection, has been developed by WilAnalysis of the upper atmosphere now becomes feasihIe, lard B. Bennett of the National Bureau of Standards. This and in arrangements made with the Applied Physics Labors, work has been yeported briefly [Rev.Sn'. Instruments, 21, 578 tory of the Johas Hopkins University the bureau plans to send (1950)l. The principal advantages of the new instrument are one of the new spectrometers aloft in a rocket. Before the simple operation, small size and light weight, high sensitivity, flight, the spectrometer will he evacuated and sealed. At the and unusual possibilities for scientific and industrial applipeak altitude, an ann of the tube will be broken open to the cation. rarefied air and the relative densities of the atmospheric comIons produced in an ionization chamber travel in parallel ponents will he telemetered to the ground station. This may paths through the velocity selector tube, which is a glass tube seem very startling to analysts who ai- not particularly air8 inches long and 2 inches in diameter. Each of the three minded, but it may be somewhat reassuring to note that the stages consists of three tungsten-wire grids, the middle of successful researches on the proximity fuse demonstrated which is excited by a radio-frequency potential. The 6nal how electronic equipment may be made immune to high acstage is followed by an additional grid, to which a negative celerations. The matter of telemetering data is au old story, potential is applied in order to turn back any electrons which and for many years temperature, pressure, humidity, light, may have originated anywhere along the tube. The final elecand cosmic ray intensity have been transmitted from stratotrode is a collector plate, the potential of which is d c i e n t l y spheric heights to recorders on the ground. The transmission positive to repel all but the desired positive ions. of specific analytical information by similar means should add Iutergrid and interstage distances are chosen very accumeatlv to our knowledge rately, so that for any given ion mass there will be a single, - of atmospheric and meteorological phenomena. characteristic radio-frequency a t which the ions will be The impact and full simificance of the Bennett spectromspeeded through the three stages to the collector plate. The eter for a h y s i s will become apparent when we consider successive distances between stages must be chosen so that how many problems ean be solved. The conventional mas8 the radio-frequency potential will complete an exactly intespectrometer is highly developed and in many kinds of investigral number of cycks during the time of flight of the desired gations it is well nigh indispensable. For a compromise inspecies between stages. For a three-stage tube the best strument, of necessarily mole limited range and precision, but combination of integral numbers was found to be 7 and 5,and no more complicated than a television receiver, the analyst with a 7- and 5-cycle tube it has been possible to separate the may now turn to applications for which the conventional inchlorine isotopes. From this it is estimated that a four-stage strument has been prohibitive. Perhaps elementary organic tube with integral numbers of 13, 11, and 7 would permit microanalysis will yield to this treatment wherein carbon, the resolution of masses differing by 1%. hydrogen, and nitrogen may be determined more readily than One of the outstanding advantages of the Bennett mass by the absorption and weighing techniques. Physiologists spectrometer is the relative independence of resolution on have used simplified mass spectrometric methods in respire source slit width. A new positive ion source has been detion studies, and the new approach may well make this methodl veloped which will deliver a positive ion current of 100 microstill more attractive. amperes a t a pressure of 4 x 10-5 mm. of mercury, Currents 21 A T PRESENT the analytical chemist has
so many instruments
A a t his disposal that their routine application to analytiea1 problems can continue for a long t h e . Indeed, for n
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22 A
ANALYTICAL CHEMIST
INSTRUMENTATION Densitometer and Photometer
An unusual densitometer-photometer has been described recently by H. P. Kalmus and &I.Sanders [Electronics, 23, No. 7, 84 (1950)l. It requires no shielding from stray light, operates at low levels of light intensity, and possesses a stable zero reading. It was designed for reflectivity and density measurements and in one form has been used as a hemoglobinometer. The instrument makes use of a two-tube multivibratoramplifier circuit to excite a small incandescent lamp usetl as the light source. The 20-cycle square wave produced by the niultivibrator-amplifier causes 100% li4ht output modulation in a bulb of the G.E. 104 type. This is a conventional dial light, dissipating 6 watts at 125 volts. The average light flux is held constant by this Circuit and is independent of linevoltage fluctuations. The photometer portion of the instrument contains an a.c. amplifier with an over-all voltage gain of 150,000. A parallel-T network is used a3. a negative feedback element and another parallel-?' network, tuned to 120 c.P.s., is inserted between the first and second stages of the amplifier to eliminate the 120-cycle modulation due to artificial light. The over-all response of the photometer amplifier shows that a t the 20-cycle modulation of the light source, 160 microvolts of signal will produce full scale deflection of the 0- to 1ma. output meter. The rejection of 120-cycle component, arising from any stray light, is 98 db. down from this value. Full scale deflection of the meter corresponds to a light flux of 0.5 microlumen. (For rough purposes of comparison this is approximately 20,000,000 times the output which would be obtained from a simple harrier-layer photocell-meter combination.) For those applications in which it is desirable to house the phototube in a separate search unit, the authors encountered microphonic difficulties with the shielded cable connecting the search unit with the amplifier. These were due not to capacitance changes but presumably to piezo-electric effects. For such applications, the authors found a s m l l barrier-lager cell to be preferable to a phototube. This could be coupled to the amplifier through a transformer and the low impedame coupling was immune to hum pickup or microphonic effects. This instrument wouId seen1 to have many uses, especially where high stability and sensitivity are required. The paper discusses the circuits in great detail and treats the signal-tonoise relationship and time condnnts in II careful, explivit nlanner. Phototransistor
New instruments invariably follow the discovery of new components. To the Bell Telephone Laboratories' well known thermistors and transistors now comes the latest relative, the phototransistor. This device, still in the experimental stage, is similar to the transistor except that current flow is controlled by light rather than by the current of the emitter probe [Electronics, 23, No. 7, I22 (1950)l. This tiny photoelement consists of a fine wire probe in con- tact with a disk of germanium at n point where the germianium film is only 0.003 inch thick. A variation in the amount of light falling on the obverse side of the gernlaniuni mill cause variations in the collector current. Normally the collector wire is biased negative with respect d t h e d i s l t h w g h a load resistor. Outputs of the order of tenths of a milliwatt per niillilumen have been realized and light modulation up to 200 kc. has been followed with good fidelity. It is essentially a low-impedance device and it responds to light in the visible, although no spectral response curves have been released ye:.
