metal and the explosion proceeds easily. If there is a metallic substrate, it is the substrate that determines the course of the explosion. A portion of a microphotometer tracing showing Fe impurity lines from the explosion of a boron filament without a metallic substrate is shown in Figure 1. Almost all the lines not marked are due to iron. In other spectral regions molecular bands due to BO appear as well BS atomic lines. Considerable work will be required if
the conditions for explosion are to be properly understood and controlled. At present the technique can only be used for qualitative analyeis and estimates of relative concentrations. The advantages of this technique are: only a small section of the filament in.) and hence only a very small amount of materialisrequired. No sample preparation is required. Thus there is no contamination introdud by grinding, diasolution, Or from graphite electrodes.
The technique is extremely rapid and convenient. UTERATURE CITED
(1) ~ ~ bL., ~F i ~~ 1. , M.,~ Z. Physik bp, 547 (1930). (2) Labuda, A. A. MWt*ov, E. G., Nekrsshevich, I. &.,Zmeat.Akad.Nauk., SSSR, Ser. F i t . 22,720 (1958).
(x
This work was partially supported by the
Air Force Materials Laboratory under Contract AF 33(615)3212.
Simplified Conversion of a Direct-Current Polarograph to a low Frequency, Alternating-Current Instrument Herman Beckman and William 0. Gauer, Agricultural Toxicology and Residue Research Laboratory, University of California, Davis, Calif. HE ALTERNATINQ current polaroT g r a p h i c method described in this article is limited only to the use of low frequency (SO c.P.s.), constant a.c. voltage of a relatively small value. The recorded curve for each reducible constituent undergoing analysis ap-
A. C. POWERSUPPLY PLUG
signed around the Sargent Model XV recording d.c. polarograph. The Model XV records d.c. polarograms by automatically scanning through a preset applied voltage range, with a sensitivity of from 1.OOO to 0.003 pa./-. The a.c. circuitry is based upon the pre-
pears aa a peak and is similsr in shape to a derivative curve of the d.c. step polarogram. For analytical purposes, the amplitude of the peak has a linear relationship to the concentration of the reducible species. The instrument conversion is de-
-7
1
2
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I
3 456
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63 DEABH
I
1 SAA
CHOPPER PLUG
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1
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I
ACCESS. PLUG P4D
Figure 1.
1434
ANALYTICAL CHEMISTRY
Control panel modification of Sargent Model XV Polarograph
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,
I1
PLUG l l S V MI CYCLE
A.C. PLUG
Figure 2.
A S . power supply
vious work of Miller (6, 7) and the further modification of Caudill (3). In our laboratory modifications were made to include base line control, increased selectivity of applied voltage, control of input line voltage, and an a.c.4.c. switching network. The electrode assembly consista of a polarized and nonpolarked electrode. Although we tried no other combmations, excellent results were obtained hy using a dropping mercury electrode (DME) and a reference electrode consisting of a pool of mercury at the bottom of the electrode vessel. The DME capillary used was the Sargent S-29417, and the electrode vessel was the two-piece Sargent S29385. INSTRUMENT DESIGN
Figure 1 shows the modification of the Model XV polarograph control panel to include a.c. operations. For reasons of clarity and brevity, only those compon?nts involved directly in the modification will be identified in the subsequent text, with all other components being identified in the parts l i t of the operations manual of the Sargent Model XV Polarograph. The multi-pole tranafer switch is shown in the d.c. operating position, and is further shown to he a slide or toggle switch, for the sake of simplicity only. In the actual construction, a two-gang, rotary switch is used and is mounted in some readily accessible position on the front of the Model XV. One such means is to remove the cell input socket from the front panel, mount it on the side of the instrumtnt, and place the switch in the original socket position. Three capacitors are attached to the rotary switch as shown. Capacitors C17 and C18 are original components of the Model XV circuit. I t is important that the polarity of the W-pf. electrolytic capacitor be observed and not reversed. Three plug-in receptacles are incorporated into the circuit as follows. A four-contact receptacle is wired as required to accommodate the plug from the modified recorder chopper circuit, which will he described later. The shunt is removed from the circuit accessory plug P4D and the plug and receptacle are rewired as shown. On the back of the Model XV a S-contact aocket should be conveniently laced to accommodate the plug from t f e 8.0.
power supply circuit which is described in the paragraph following. The 8.c. power supply shown in Figure 2 has two primary functions. One is to superimpme a small constant 8.0. voltage upon the linear changing d.c. voltage that is being applied across the electrodes. The second is to apply a small constant a.c. voltage acrosa the slidewire of the recorder balance drive w m h l y . The voltage of the former is adjustable to 100 mv. and the latter is a small fixed m o u n t sufficient to oppose the a.c. input signal on the slidewire. I t is of critical importance that this opposing ax. voltage be 180‘ out of phase with the input signal. This is easily accomplished by ohserving the waveform of the signal of each transformer at the recorder slidewire with an oscilloscope and wiring accordingly. Since the recorder cannot be brought on scale without proper phasing, an alternate method would simply he one of trial and error. A metal cabinet with an 8 X 10 inch sloping control panel houses the entire power supply circuitry. The depth of the hox and
height of the back are 9 and 7 inches, respectively. The arrangement of the controls on the panel as shown in Figure 3 was found to be the most convenient. Filament transformen T2 and T3 (2.5 volt) are mounted on a 21/n x 5 x 5 inch metal chsssis base fastened to the inside of the cabinet. The primary leads are connected in parallel, through panel-mounted, 0-130 volt variable transformer TI, t.o 11!%volt, 60-cycle line voltage at P1. A low-load, 0-130 volt 8.c. voltmeter parallels the TI. secondary winding to visually aid in the correction of m y line voltage drift. Panel-mounted DPDT toggle switch SI serves a dual purpose. Fint, it is an o n 4 switch for the power supply, with neon bulb N1 as the indicat,or lamp. Second, it serves to switch the balance potential of the recorder slidewire to an a.c. or d.c. mode. Variable resistor R1 is a 10-turn, IO-ohm precision potentiometer with a 0-100 unit iudicator dial and lock. I t i5 panelmounted and used to adjust the applied 8.c. potential through a range of 0-100 mv. in 0.1-mv. increments. Variable resi4tor R4 is a panel-mounted, I-turn, lOO-ohm, standard wirewound potentiometer used to aid in base line control. R2 and R3 are 1-turn, 3M)-ohm and IO-Kohm wirewound potentiometers for the calibration of the output of T3 and T2,respectively. These two components are mounted at. the rear of the cabmet. The output lime from the power supply is a shielded, &conductor power supply cahle of sufficient length to connect the unit to the Model XV. It is important that the metal shield of the cable be grounded through the cabinets of both the power supply and the Model XV.
I. ”:‘,.”
Figure 3.
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AC. power supply and control VOL 38, NO. 10. SEPTEMBER 1966
1435
RESULTS AND DISCUSSION
1
0 TO 6.3V FILAMENT TRANS.
-l CHOPPER 8 PIN SOCKET CHOPPER PLUG
1
I
Figure 4.
3
Our interests have been directed primarily toward the use of polarographic techniques in the study of pesticides and their residues. Considerable data have been accumulated on the use of this technique in the pesticide field (I, 4-6). In our laboratory, great value and versatility was found in the interchangeable use of the d.c. and a.c. methods, an outstanding instance being the development of an analytical procedure involving the use of a polarograph for the determination of total bromide on crop materials (2).
Recorder 60-cycle chopper modification
LITERATURE CITED
Allen, P. T., Beckman, H., “Residue Reviews,” Vol. V, p. 91, SpringerVerlag, Berlin, 1964. (2) Beckman, H., Crosby, D. G., Allen, P. T., Mourer, C., unpublished data, University of California, Davis, Calif., (1)
Figure 4 shows the changes that are made at the Spin socket of the recorder 60-cycle chopper to enable it to accept either ax. or d.c. signal input. The chopper itself is a plug-in component of the circuit and requires no modification. The circuit is connected to the instrument transfer switch of Figure 1 by means of a k o n t a c t plug as shown. The d.c. calibration and operation of the modiied Model X V polsrograph is identical to that given in the instrument manual only when the control panel transfer switch is in the d.c.
position. With the switch in t,he ax. position the instrument voltage and current calibrating control and the displacement control are both inoperable. Therefore, the d.c. calibration is made only in the d.c. mode. The function of the d.c. displacement control is replaced with the previously described base line control on theax. power supply. The a.c. calibration is made by adjusting potentiometer R2 (Fi re 2) while a.c. power measuring the output of supply with a high impedance measuring device such as a precision oscilloscope.
tg
1966. (3) Caudill, P.
R., University of Kentucky, Lexington, Ky., pnvate communication, 1963. (4) Gajan, R. J., “Residue Reviews,” Vol. V, p. 7, Springer-Verlag, Berlin, 1om
-I-_.
( 5 ) Martens, P. H., Nangniot, P., Ibid., Vol. 11, p. 26, 1963. (6) Miller, D. M., Can. J . Chem. 34, 942 (1956).
(7)
ZM., 35,
1589 (1957).
Mass-Spectromelric Determination of Traces of Carbon Dioxide in the Presence of Oxygen P. J. Ross, Division of Soils, C.S.I.R.O., Cunningham Laboratory, St. Lucia, Brisbane, Australia
conditions, most Ucommercial mass spectrometers can NDER FAVORABLE
just detect 10-50 v.pm. (p.p.m. by volume) of a component in a 1-ml. sample of a gas mixture. Oxygen in mass spectrometers may produce a beam of C02+ ions (1,3, 4) which is probably formed by reaction of the oxygen with carbide in the filament (4). Brain and Evans ( I ) , using a Metropolitan-Vickers MS2 mass spectrometer, reported that this beam increased in size over a period of 50 minutes. Under such unfavorable conditions the 10-50 v.p.m. limit of detection for traces of COS in a sample cannot be reached by normal methods of analysis. One solution is to concentrate the CO, by freezing with liquid nitrogen; this also allows the detection of as little as 0.5 v.p.m. if sufticient sample is available (2). Another solution is to remove the oxygen chemically. However, both these methods are inconvenient and unnecessary for mess\Ilp ments on samples containing more than 100 v.p.m. COS,as will be shown. The method to be described allows the detection of 10-20 v.p.m. CO, in a 1-ml. 1436
ANALYTICAL C H E M I ~ Y
sample containing oxygen with a single analysis. Although a fast pumpout double-inlet mass spectrometer was used in these investigations, the principle of the method should be applicable to other instruinents. EXPERIMENTAL
Apparatus. The instrument used in the analyses was an MS3 doubleinlet maw spectrometer (Associated Electrical Industries Ltd.) modified in accordance with the manufacturer’s instructions to include a Mullard ME1403 electrometer pentode in the detection system. The inlet system of this instrument allowed the delivery of reproducible doses of approximately 1 ml. of sample to two expansion vessels, whence they were admitted to the ion source. The instrument is supplied with a tungsten filament; this was operated at a current of 4.5 amperes and an electron emission of 100 pa. The filament was not carburized and had been used only for the analysis of the permanent g-Procedure. The principle of the method was to estimate the effect of oxygen in the sample by admitting
pure oxygen at the same pressure as the sample oxygen. The mass spectrometer was conditioned for 15 minutes with a sample of air on days when analyses were to be carried out. This was found to be desirable to stabilize the peak at mass 44 induced by the presence of oxygen. This peak was larger than without conditioning, but its increase with time was smaller. To analyze a sample, the expansion vessels were first evacuated thoroughly (i a sample containing 1% C02 or more had just been analyzed, the expansion vessel concerned was first flushed with air). A dose of sample was expanded into one of the vessels and the displayed on the peak at mass 32 (02+) electrometer output meter. Sufficient pure oxygen to give the same peak height (to within 0.2y0) ww then admitted to the other expansion vessel. The instrument was focused on mass 44 and the amplifier output displayed on a chart recorder. The oxygen was admitted to the analyzer tube for 30 seconds; it was then shut off, and the sample admitted for 30 seconds. Oxygen was then admitted for 10 seconds, sample for 10 seconds, oxygen for 10 seconds, and 90 on to give seven peak heights (three for the sample, four for oxygen). Analysis for other components
