ANALYTICAL CHEMISTRY
1614 lower concentration recommended. Less than the 10 nil. recommended earlier resulted in hydrogen evolution, the latter increasing rapidly as the dilution was increased. It appears, therefore, that solutions 0.3 -1.I to 0.4 M with the titanic chloride are best, for such solutions give good results nithout unnecessary wasting of material.
and 0.18% for experiment 111. Although the average deviations (see Table I ) are somewhat higher for each, these results indicate that the titanium couple is suitable as an intermediate in coulometric reductions. In addition. its reduction potential is favorable for the reduction of many substances not readily reduced by any of the coulometric intermediates hitherto reported.
DISCUSSION
iilthough the primary purpose of this research mas to test the titanium couple as an intermediate rather than to devise a new method of analyzing iron samples, the results of these iion analyses provide evidence for the probable usefulness of this couple in coulometric reductions. Samples of iron of exactly the same size as those used in the runs listed in Table I, when analyzed by the well-established ferric oxide gravimetric method, gave average results for iron as follom: for experiment I, 0.1100 i0 0001 gram; for experiments I1 and I11 0.05585 zt 0.00009 gram. Comparing the averages found coulometrically with those determined gravimetrically and assuming the latter value to be correct, the actual error would be 0 27% for experiment I, 0.32% for experiment 11,
ACKNON LEDGRlEhT
The authors yish to express their gratitude to the Dow Cheniical Co., Midland, lIich., for its kindness in supplying titanium tetrachloride to complete this research r h e n other sources \%-ere unable to make delivery of this reagent. LITERATURE CITED
(1) Cooke, TT. D., and Furmnn. S . H.. ANAL.CHEX, 22, 896 (1950J. (2) Meier, D. J., Meyer, R. J., and Swift, E. H., J. Am. Chem. Soc., 71, 2340 (1949). (3) Szebelledy, L., and Somogyi. Z., Z. anal. Chem., 112, 313 (193s). RECEIVED for review February 4 , 1952. .4ccepted June 16, 1952.
Recording System for Mass Spectrometers K. K. JENSEN, W. E. BELL, AND F. E. BLACET D e p a r t m e n t of C h e m i s t r y , I-niversity of California, Los Angeles, Calif. SING a vibrating reed electrometer, a system for recording mass spectra has been installed on a Westinghouse Type LV mass spectrometer. This system is relatively economical in cost, simple in operation, accurate and dependable, and produces rapidly a directly readable chart on a pen recorder. The installation consists of a control box containing a Helipot and a magnet current meter, a vibrating reed electrometer and amplifier, a recorder, and a calibrating potentiometer. OPERATION
The main magnet current is varied from 100 to 7 ma. by means of an electronic circuit, controlled by a 15-turn Helipot which is driven by a synchronous motor through a gear reduction assembly. Scanning speed can be varied by changing the gear ratio. A Record switch controls both the Helipot motor and the chart drive motor in the recorder. The scanning motor drive is equipped with a magnetic clutch, which permits manual operation of the Helipot without strain on the motor gear-train. An interlock switch on the lid of the control box cuts off this scanning motor without disturbing the chart drive; hence, the scan can be interrupted a t any moment by simply raising the lid. This permits the magnet current to he manually set to any desired value and on closing the lid the scan starts from t h a t setting. A switch selects the direction of scan. The vibrating reed electrometer (manufactured by Ap lied Physics Corp., Pasadena, Calif.) consists of an electrometer t e a d which is mounted adjacent t o the ion collector plate, and an amplifier which is located on the control table. A decade range switch on the amplifier selects full scale sensitivities of 1, 10, 100, or 1000 mv. for recording mass peaks. Scale changes ap ropriate for each peak as the spectrum is being scanned are m a l e by the operator. With unfamiliar samples, a com lete scan is made on the 100-mv. scale; a second scan then can [e made over the desired mass range, and peaks easily recorded a t the proper sensitivity. The recorder is a 1-second response, 27.5-mv, strip-chart Brown Electronik potentiometer (manufactured by blinneapolis-Honeywell Regulator Co., Philadelphia, Pa.) with an auxiliary marker pen externally actuated by means of a toggle switch. An 11-inch chart having a 0.5-inch margin a t each edge for pertinent notes identifying mass peaks, magnet current, etc., is used. In order to record the full peak intensities, it was found that 23 minutes are required for scanning the mass range from 100 through 12. A section of the n-butane mass spectrum is shown in Figure 1. The signal to noise ratio is such that a peak intensity of 0.1 mv., corresponding to an ion current of 5 X 10-16 ampere, can easily be
II -
I I
I
--
~
I .
I
5~
I I
I
1 1 I 1
I I
I
I1
ol
R
R I1
I 1 I I I I
I1
I
55 1
Figure 1.
I
I
I1
, -
I
I1
SCAN
151
I
; 1
Section of Normal Butane Spectrum
Recorded a t ionizing potential of 70 volts and with ion accelerating voltage set at 750
V O L U M E 2 4 , NO. 10, O C T O B E R 1 9 5 2
L
1615
----------------- J BOX Figure 2. Circuit Diagram of Magnet Current Control and 3Iagnet Control BOK
MAGNET C O N T R O L
Helipot. Type BZ, 10 turn, 50 IC. Motor. Hayden, Series 1600, 1 r.p.m., CCW.,magnetic shift with Yo. 1 friction, shaft 616-3186 M . Meter, 0 to 100 m a , d.c. Si. Interlock GE CR1070-Dl12 S , . Switch, tbggld, DPST SS. Switch, toggle, DPDT Ti. Transformer, Kenyon T377 Tz. Transformer, Thordarson T13R13 8HY. Choke, Thordaraon 20C52
distinguished. Over the range of magnet current which is normally used, the mass peaks are almost uniformly spaced on the chart, permitting ready identification of the peaks. However, the characteristics of the control circuit are such that the scanning rate decreases slightly in the low mass range. This compensates, in part, for the increased sharpness of the peaks as lower mass numbers are approached, and results in improved recorder reJponse. CIRCUIT DESCRIPTIOS
The magnet control circuit, as shown in Figure 2, is a modification of that used by Westinghouse. This revised circuit yields a range of magnet current from 100 ma. to a minimum of 7 ma., and coupled with the synchronous drive gives a current which approaches linear variation with time. A constant potential across the 50,000-ohm Helipot is maintained by the VR-105 tube. Grid voltage of the 6SF5 is varied by the Helipot and the accompanying change in plate voltage is coupled to the grid of the 3C2.1, a-hich serves as a cathode follower whose output load is the 2.5K and 12.5K resistors t o ground. The voltage changes appearing across the 12.5K resistor are coupled to the 811 grid through the VR-150. Rectifier 5W4 and transformer T ) supply the voltage to maintain the potential across the VR-150. The change in voltage from the 3C24 is amplified by the first 811 and coupled to the second 811, which serves as a cathode follower whose load is the magnet, and two 2K resistors. These
resistors are manganin wire-wound of 1%tolerance, and rated a t 10 watts each. The magnet current creates a voltage drop acros9 these resistors and this voltage is coupled into the 6SF5 through the cathode, completing a feedback loop from the outuut staae to the input stage. SIis an interlock switch which stous the motor when the cabinet cover is opened for manual operation of the Helipot. S 1 reverses the potential across the Helipot to enable scanning in the reverse direction. Sz is the Record switch which closes the motor power circuit and also the chart-drive motor circuit in the recorder, assuring simultaneous starting of the recording chart and Helipot motor. Because of hysteresis effects, a scan is always started from the same magnetic field conditions; otherwise, the mass peaks are displaced slightly with reference to the starting point. The magnet current is returned to 100 ma. after each run and then reduced to any desired starting point.
-
~
ACKNOWLEDGMEYT
The Signal Oil and Gas Co. of Los dngeles donated the mas? spectrometer to the University of California a t Los hngeles, and, in addition, furnished financial aid which helped tox-ard completion of this development. Initial investigations and exploratory work leading to a coordinated system were due in large measure to R. K. Brinton, R. A4.Crane, and Arnold Miller. RECEIVED for review March 27, 1952. Accepted July 7,
1952.
