rubber band. The holes in the Teflon stopcock and in the base of the shielded electrode assembly for the tube, T,were drilled in such a way as to give a tight fit. No further sealing at these joints was necessary, as no leaking into or around the tube, T , was observed even over a long period of time. The Teflon-shielded electrode assembly (see Figure 1) is essentially identical, except in size, to that designed by Johnson (4). This electrode assembly was used instead of a platinum disk sealed in soft glass (2) because it was convenient at times to be able to remove the Pt disk for cleaning and polishing (4, especially when studying film deposition and dissolution and, also, some organic electrode reactions. The cell was designed so that the shield cap, C, could be unscrewed without removing the whole assembly from T. Electrical contact with the Pt disk was maintained through a steel coil spring (stretched to 21/2 times its original length) of approximately the same diameter as the inside diameter of the tube, T. The compression of this spring on
the side of a standard H cell and the shielded electrode wm mounted, as shown in Figure 2, on a bent (90’) glass tube, T, which was inserted through a hole drilled lengthwise in the Teflon stopper. Mounted in this manner, the shielded electrode can be rotated through 180’ by turning the Teflon stopcock and needs only to be turned in an upward orientation to expel any air bubble. It is then ro1;ated into a downward orientation for measurements requiring such an orientation. One added advantage of this cell is that any desired orientation for a measurement is readily available without having to make a separate electrode for each position. The bulge design in the wall of the sample compa,rtment of the H cell, through which the electrode rotates freely, was employelj to reduce the total volume of sample required for measurement. A rubber band, R, was crossed over the stopcock arms and attached to two small glass hooks, h, on the side of the cell to maintain pressure on the stopcock. The stopcock could be rotated freely without removing the
insertion into T resulted in a slight pressure of the end of the spring on the back of the Pt electrode which ensured a low resistance contact. The bevel, B, of the inside of the Teflon electrode shield cap, C, ensures a watertight seal between the Pt electrode and the bottom edge of the shield when screwed down tightly. This was tested by dipping the electrode in Fe+3 solution, washing and transferring the electrode to a 1.0 HCIOl ’ solution, and observing the resulting chronopotentiogram ( 3 ) . N o Fe+3 reduction wave was observed, indicating that there were no small cracks for Fe+3 to diffuse into ( 3 ) . LITERATURE CITED
(1) Adams, R. N., Symposium on Electrode Reaction Mechanisms, Division of Analytical Chemistry, 145th hleeting, ACS, New York, September 1963. (2) Bard, A. J., ANAL. CHEM. 33, 11 (1961); 35, 340 (1963). (3) Christensen, C. R., Anson, F. C., Ibid., p. 340. (4)Johnson, J. D., P1i.D. dissertation, University of North Carolina, Chapel Hill, N. C., 1962.
Automatic Mass Scanner for a Time-of-Flight Mass Spectrometer M. V. McDowell, R. S. Olfky, and F. E. Saalfeld, U. S. Naval Research Laboratory, Washington, D. C. 20390 a mass R group of masses by a mass spectrometer has many applications, for EPETITIOUS SCANNING O f
matic peak selector, type 21-065, made for the Consolidated Electrodynamics Corp. 21-620 mass spectrometer, allows up to six preselected peaks to be monitored, but does not have the ability to scan the entire mass spectrum), but there are no commercial automatic mass scanners available for the Bendix time-of-flight (T-0-F) mass spectrometer.
Or
example, in the moni1,oring of products from an effusion cell or in following the progress of a chemical reaction. Peak monitors employing a scanning circuit similar to that shown in Figure 1 exist for magnetic deflection-type mass spectrometers (for exitmple, the auto-
The scanning system described herein can be programmed so the circuit on the analog output chassis of the mass spectrometer and the recorder (Leeds and Northrup Speed-0-Matic G) are turned on automatically and remain on for any desired length of scan. This enables one peak or a series of consecutive peaks to be observed. After the scan has been completed, the scanner and the recorder are turned off and remain so for a preset length of time, after which the scan is automatically
I -+t 69 MIDDLE DISK2 DEC.
VDC
i3-P DISK 3
LEEDS 8 NORTHRUP RECORDER CONTROL
-TO
MOTOR(I-RPM) -DRIVING CAMS
POTTER 8 BRUMFIELD KRP14A 3PDT RELAY
VDC
*
SCHEMATIC IDENTIFICATIONS ARE FROM INSTRUCTION MANUAL &NALOG OUTPUT SYSTEM ELECTRONIC CHASSIS MODEL 30 MONITOR MODEL 311 CONTROLLER MODEL 311 SCANNER ACCESSORY FOR BENDIX T-0-F MASS SPECTROMETER
Figure 1 .
Scanning circuit
kl
I -
7 DPST
Figure 2.
Timing circuit VOL. 36, NO. 4, APRIL 1964
’
0
959
repeated. This scanner has been used in the study of the permeability of elastomers where one peak is monitored repeatedly while the mass transfer across the membrane comes to a steadystate condition. The automatic ma88 scanner frees the operator from the routine task of turning a switch off and on a t predetermined times in these lengthy experiments. Furthermore, this arrangement allows the experiments to he continued after working hours when the mass spectrometer is unattended. Each wiper of the scan switch is wired in series with the closed contacts of a 3PDT relay (Potter and Brumfield
KRP14.k). The original analog output circuit is maintained through the closed contacts of the nonenergieed relay permitting normal operation of the mass spectrometer. Activating the relay starts the maSS scan by switching the contmts from the manual scan circuit to the increase scan circuit. The mass scan continues until the relay is de-energized and the system is returned to the manual scan position. The timing circuit, shown in Figure 2, activates the switching relay through a cam-operated microswitch (Industrial Timer Corp. kit). Connections between the 3PDT relay and the scan switch of
Improved Technique for Determination of 0 'and
Ha by
the analog output system are made with shielded multiple conductor cable with the shielding grounded to the chassis of both the automatic scanner and the analog output. Although this automatic scanner has been used with only a single analog outnut svstem. it could easilv he modified t o operate more than one system by connecting, in parallel, 3PDT relays to each of the analog circuits heing used. In this manner all the analog scan systems could be started simultaneously and each could he set to scan either the entire mass spectrum or various portions of it.
Flask Combustion
Howard P. Baden, Research Laboratories, Department of Dermatology, Harvard Medical School at the Massachusetfr General Hospital, Boston 14,,Mass.
methods are available for E assaying C" and Ha labeled compounds. However, attempting to XCELLENT
measure these isotopes in biological materials can he a problem because direct counting is not possible. A method well suited to these situations is combustion with conversion of C" to C'400aand H3 to HiO. This paper describes a new inexpensive combustion and collection system which simplifies the counting of evolved Cl4O0,and HqO. The essential feature of this apparatus is the combustion head. As shown in Figure 1 two electrical leads A and B
Figure 1.
Combustion head
A, 8. EIechiml leads C, D. Electrodes E. Opening with serum stopper F. Needle
960
ANALYTICAL CHEMISTRY'
inserted
are fused into the glass. Attached to these by removable couplings are the stainless steel electrodes C and D across which the spark is produced. A wire basket which serves as electrode D is used to hold the sample. Both electrodes may be replaced when worn. A serum stopper is inserted into opening E of the glass stopper. The needle used to pierce the rubber stopper is of sufficient length to bypass the ignition apparatus. Commercial units at present have only an ignition device incorporated in the stopper. This new head allows for injection of an organic base into the reaction flask following combustion. The ground glass joint is a %'/do and fits a series of standard b s k s varying in siae from 100 to Zoo0 ml. With the older apparatus special and more expensive flasks had to be used and there wm the additional cost of adding a side arm for injection (I?).
calibratd syringe and a 6'/&ch hypodermic needle. After equilibration for 30 minutes an aliquot is removed and counted a8 described by Jeffrey ( 1 ) . When HfO is to he meamred 1fln'X ethanol & injected into the- flask-&; swirled around to melt the ice. An aliquot is counted in an ethanol-toluene scintillation mixture.
Procedure. The material may he spotted on filter paper, placed in a cellophane bag, or weighed into a paper boat (8). The specimen must he dried before combustion. Paper with a low ash content should he used and Wbatman No. 1 filter paper has Been found satisfactory for this purpose. Combustion is done in oxygen as previously described (8); When ?lo, is being collected a cooling bath of ice and water should be used, while for H:O a dry icealcohol mixture is required. For C W 2 collection a known volume of an ethylene glycol monomethyl ether and ethanolamine mixture is injected through the serum stopper using a
The author thanks Arthur Vash for his help in designing the apparatus and the Macalaster Scientific COT. of Cambridge, Mass., for its.construction.
DISCUSSION
With this system 95 to 97% of the C2' may be recovered as C*402with a 40% counting efficiency. The results with tritium are equally aa good with the exception that the counting efficiency is only 10%. The procedure is suited to routine use, and by employing three or four heads a continuous operation may be set up. The heads need not he cleaned before reuse, but. a new serum stopper should be inserted.
LITERATURE CITED
(1) Jeffrey, E., Alvarey, J., ANAL.CEEX.
33,612 (1961).
(2) Juvet, R. S., Chiu, J., Ibid., 32, 130 (1960).
(3) Kelley, R. C., Peets, E. A,, Gordon, S., Buyake, D. A,, Anal. Bioehem. 2, 267 (1961). TEESinvestigetion was aupported by Public Health Service Grant AM-06838 01.
