by back titrating with 0.1165M hydrochloric acid. The pH was determined with a Digicord “Photovolt” model pH meter.
ACKNOWLEDGMENT
The author is indebted to the Wilson & Toomer Fertilizer Company; 1161 Tallyrand Avenue, Jacksonville, Fla. 32206, for the use of their laboratory for the ammonium sulfate analysis.
RESULTS AND DISCUSSION
The experiment found a 21.15% nitrogen content for ammonium sulfate. The calculated value is 21.20%, so the error was -0.24%. In addition to requiring only one standard solution, this method offers a single space-saving procedure both for original research and for industrial laboratory use.
RECEIVED for review December 15, 1970. Accepted February 9, 1971.
Inexpensive Method for Obtaining Deuterium Nuclear Magnetic Resonance Spectra on the XL-100 NMR Spectrometer Robert E . Santini Department of Chemistry, Purdue University, Lafayette, Ind. 47907
WE HAVE DEVELOPED a method for obtaining deuterium (2H) NMR spectra from the lock channel of a Varian XL-100-15 NMR spectrometer system. The circuit enables the accumulation of spectra in a Varian C-1024 time averaging computer (TAC) while the spectrometer is operated in an HR mode. With careful adjustment of the flux stabilizer for minimum drift, up to one hundred 25-second scans may be obtained in the computer memory. The scanning ramp generated by the TAC is interfaced to the field sweep coils in the magnet gap cia the existing calibrated sweep controls. In this way it is possible to obtain calibrated sweep widths over a four decade range. Sweep times may be selected via the TAC, or alternatively, by the X-Y recorder digital electronics in the XL-100 system. The circuit provides additional variable amplification of the time averaged analog output of the TAC in order that full scale recorder traces may be produced. This circuit may be constructed for about $100 parts cost, removing the need for a substantially greater investment in custom made high frequency components for deuterium NMR. The completed circuit, including the power supply, may be housed in a 6-inch X 6-inch X 6-inch enclosure. CIRCUIT DESCRIPTION
The circuit which interfaces the sweep signal generated by the TAC is shown in Figure 1. In operation, the full 25-Vp, ramp generated by the C-1024 (J-8) is applied to the input
networks consisting of R , , R1, and A l . The output of A1 is a 5-Vp, analog to the scanning ramp. This 5-V,, ramp is applied to ABwhere it is offset symmetrically about zero volts. The offset may be precisely adjusted by RI. This adjustment procedure is necessary to ensure that the center of the sweep ramp corresponds exactly to the center of the display on the monitor oscilloscope in the XL-100 system. The sweep ramp is inverted in A S . AQand A3 operate at unity gain, thus, SImay be used to choose the direction in which the magnetic field is swept. The output of As is applied, via R l cand R l l ,to the horizontal input of the monitor oscilloscope. The horizontal sweep of the monitor oscilloscope is therefore a linear function of the field sweep. The sweep waveform is also applied to the linear sweep module cia R12,R 1 3 and , A d . The exact ratio of RU to R13 is adjusted such that the sweep ramp is about 0.5 V,,. This value is adjusted to precisely match the sweep waveform produced by the normal circuitry in the linear sweep module. This measure maintains the calibration of that portion of the linear sweep module which is utilized for deuterium NMR experiments. The modification to the linear sweep module which is required for deuterium NMR is shown in Figure 2 (see also Varian schematic No. 87-126-840). A DPDT switch is installed such that in the lower position the sweep module operates normally. In the upper position the output of A4 is applied to the sweep width divider network and a 1.9-kQ load is applied to the existing ramp generator. If the adjustment of the ratio of Rll to R13is properly done, the calibration of the sweep width divider network is preserved.
Figure 1. Sweep ramp interface circuit and A 4 = Fairchild UA-741-C or equivalent RI = 20KQ,1% R P ,Rs, Ra = 5KQ, 1 % R3, R4,R8, Rs,and RPa= lOKQ, 1 % R7 = 5KO, 10 turn, cement type pot. Rio = 4KQ, 1% RII, Ria = IKQ, 1 % Riz = 9KQ, 1 % Power supply (for complete circuit) = Analog Devices No. 902 Al,A2,A3,
m
SCOPE
HORIZONTAL INPUT
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RAMP INPUT
system adjustments have been completed, the TAC may be placed under the control of the recorder digital electronics in the usual manner. The circuit associated with AT (Figure 3) is a variable gain, noninverting amplifier. This amplifier is connected, in parallel with the vertical input of the monitor oscilloscope, to the lock channel output. Its amplified signal is applied to the analog input of the C-1024 (J-1). The gain of this amplifier is adjusted to produce a l-Vppsignal into the C-1024. An amplifier identical to this circuit is placed between the analog output terminal of the C-1024 and the external input jack of the recorder. This second amplifier is used to produce proper scaling of the time-averaged signal when it is read out of the TAC and on to the recorder. The operation of the entire NMR spectrometer differs from the normal manner (HA mode) in that the observe channel is not used at all. The generation of the field sweep is under the control of the TAC with the field sweep width selected by the settings of the linear sweep module, and during time-averaging, the sweep times selected by the recorder in the main console. The TAC may be used independently to sweep the field for fast survey work with a visual display being presented on the monitor oscilloscope.
I
X-BREAK
Figure 2. Input circuit to the modified linear sweep module from A g c,
Rl4
1-'
RI?
R20
R21
RESULTS AND DISCUSSION
Re2
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OUTPUT
FROM NMR TO
R,?l
TO C-1024 INPUT
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SCOPE
The construction of the circuit is not critical, but a few points should be noted. It is suggested that a dual 15-V power supply with at least 0.1 regulation be used t o power the operational amplifiers. All resistors, except those in the 1-kHz oscillator should be of film type construction for low noise and thermal stability. The choice of operational amplifier is open. The Fairchild UA-741-C is suggested as being a good low-cost integrated circuit type but an equivalent modular type such as the Analog Devices No. 118 or Philbrick-Nexus 1009 can be used. A typical example of the results obtained with this circuit is shown in Figure 4. A sample of bicyclo [4.2.l]nona2,4,7-triene (D2) was run for J. Berson of Yale. The spectrum shown is the sum of 10-50 second scans on a 180-mg sample with CDCl3 and (CD& C = O as references. The circuit possesses an advantage for the normal operation of the N M R spectrometer. The TAC sweep ramp may be replaced by the output of a high quality function generator such as the Hewlett-Packard No. 3310-A, which is capable of producing a triangle function. The resultant display on the monitor oscilloscope of the *H lock signal being swept in both directions is especially useful in optimizing the curvature correction of the magnetic field. With minor changes in detail this circuit should be adaptable to NMR spectrometers, other than the XL-100 system, which are equipped with a deuterium lock capability and field sweep coils. It is not necessary that a C-1024 be used to provide the sweep ramp or to accumulate N M R data. The sweep ramp of a triggered oscilloscope may be used to drive
=-
Figure 3. Oscillator and buffer amplifier A6
and A7
=
Fairchild UA-741-C or equivalent
Ria = 240Kn, 10% RI:, = lOKn,lO% = 1MI2, 10% R I= ~ 10Ki2, 1 % R N = 30KI2, 1 % Rzi, RY?= 50Kn, 1 % R23 = 100Kn, 1 % CI = 0.1 pfd, 50 VDC CY = 1 Sfd, 50 VDC
Ri6
The circuit associated with A s (Figure 3) is a simple free running 1-kHz oscillator. This oscillator provides a trigger signal to J-2 of the C-1024 to initiate repetitive sweeps. On the XL-100 system the use of this circuit is optional as the C-1024 may be triggered by the recorder digital electronics. However, the use of the trigger oscillator enables the C-1024 to be swept independently of the rest of the system such that spurious signals generated during adjustments of the spectrometer cannot be read into the computer memory. After
n
0'
Figure 4. Deuterium NMR spectrum of bicyclo[4.2.l]nona-2,4,7 triene (D2) with deuterated chloroform and deuterated acetone references
CDCL,
7
6
5
4
6 802
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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the interface circuit. The spectrum would be presented on the vertical amplifier or accumulated on paper tape for later processing. Numerous other combinations are possible depending on the type of equipment which is available.
The circuit was constructed in the instrument laboratory of Purdue University’s Department of Chemistry. The instrument laboratory is directed by Jon W. Amy.
ACKNOWLEDGMENT
The author thanks John B. Grutzner of Purdue University for helpful comments during the development of this circuit.
RECEIVED for review December 11, 1970. Accepted February 10, 1971.
Galvanic-Coulometric Monitoring of Oxygen and Hydrogen in Glove Boxes Wanda Bahmet’ and Paul A. Hersch2 Gould, Inc., St. Paul, Minn. 5516.5
WHEN OPERATING under inert atmospheres one wishes to follow the progress of purging the enclosure, to keep an eye on the quality of the atmosphere, and to be warned of inadequate purification, leaks, or sudden accidental ingress of air. The literature offers but crude means such as turning a tungsten filament into a “puff of smoke” ( I , 2). Yet more precise implements are at hand. This paper describes how to adapt a known coulometric monitor for oxygen and hydrogen (3, 4 ) to glove box work. The sensor cathode is a fleece of graphite or porous silver exposed on one side to the sample gas stream while the other side is lined by a separator imbibed with caustic alkali. The separator in turn contacts the cadmium “negative” of a nickel-cadmium battery, acting as anode. The two electrodes are bridged by a microammeter. Any trace of oxygen in the gas stream creates electric current owing to the processes 1 / 2 0 2 H 2 0 2e- + 20H- and Cd 20H- -+ Cd(OH)? 2e-. From Faraday’s law, the maximum current attainable is 10.0 PA for each volume 0 2 per million in 37.4 ml/min inert gas (flow being measured at 20 O C and 1 atm). Hydrogen is indicated in terms of oxygen consumed when the gas stream is first provided with oxygen and then passed over a room temperature combustion catalyst. Workers in various fields have applied earlier versions of the galvanic sensing element to determine residual oxygen in inert gas streams, for example, in the study of the role of oxygen during the irradiation of tissues (5-7). However, there seems to be no reference to the application of the principle in work with enclosed atmospheres. The earlier cells had low and, more importantly, variable, coulombic yields. With the more recently described sensor, yields close to the theoretical are attainable, leaving little margin for variability with temperature or cell individuality.
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+
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Present address, 1725 James Street, St. Paul, Minn. 55105. Present address, 910 Franklin Terrace, Minneapolis, Minn. 55406.
(1) 1. D. Eubanks and F. J. Abbott, ANAL.CHEM.,41, 1708 (1969). (2) Specialty Gases and Equipment Catalog, Air Products and Chemicals, Inc.. Allentown, Pa., 1968, p 85.
(3) P. A. Hersch, “Galvanic Analysis” in “Advances in Analytical Chemistry and Instrumentation,” Vol. 3, Interscience Publishers, Inc. New York, N. Y . , 1964, pp 209-19. (4) P. A. Hersch, U. S . Patent 3223597 (1965). (5) E. J. Hall, J. S. Bedford, and R. Oliver, Brit. J . Rndiol., 39, 303, 896 (1966). (6) E. J. Hall and J. Cavanagh, ibid., 40, 128 (1967). (7) Zbid., 42,270 (1969).
A
1
Figure 1. Routine system for determining traces of oxygen in a glove box atmosphere Blower Wall of box; inside at left 3, 4. Needle valves 5. Blow-out 6. Sensor 7. Flowmeter 1. 2.
The adaptation of the coulometric cell to glove boxes requires only simple, generally available accessories, whether the glove box operates above, at, or below atmospheric pressure, and whether the sample gas stream can be allowed to run to waste or, carrying toxics, must be returned to the box. EXPERIMENTAL Simple Routine System. Only a few essentials are needed for oxygen readings when these are to be taken only intermittently, to f5 %, and the sampled gas can be allowed to go to waste. The essentials are, then: a blower, such as a fishbowl pump, inside the glove box; two needle valves; a “blow-out’’ column of water; the sensor, with a four-way dual channel stopcock permitting insertion and by-passing ; a multirange microammeter; a flowmeter (Figure 1). The needle valves are adjusted to allow a flow rate of F = 40 ml/min, plus 1-2 bubbles/sec rising through the blow-out ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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