Technique for Continuous Intensity Standardization in Quantitative Analysis by Nuclear Magnetic Absorption R. H. ELSKEN
and
T. M. SHAW
Western Utilization Research Branch, Agricultural Research Service,
U. S. Department of Agriculture,
Albany 6, Calif.
tion curve slope are reduced by a narrow band audio amplifier following the oscillator unit output. The calibrator input signal and resulting artificial absorption signal are also sinusoidal variations. With the calibrator voltage originating from the same source or synchronized with the static field modulation voltage, the artificial absorption signal can be adjusted for any fixed phase relative to the nuclear magnetic resonance signal. I t is a 0 " or 180" phase relationship when a lock-in detector is employed if the detector is to respond to both the true and artificial signal. For use of the calibrator reference signal without the nullbalance feature, a specimen run is made and recorded. Immediately preceding or following this a calibrator standard signal is recorded. I t is proposed in the null-balance system that the specimen signal and calibrator signal be presented at the same time, and that the phase of the calibrator signal be adjusted BO that it is 180" out of phase with the specimen signal. Since the calibrator signal is introduced at the same point in the oscillator unit as the specimen signal, addition of the two signals occurs at the coil containing the specimen. .4 calibration signal amplitude then can be chosen so that the vector addition of the two signals results in a null-balance condition at the oscillator, a zero change in total conductance, G, of the tuned circuit and zero signal output from the oscillator unit as long as AG of the tuned circuit due to the calibrator influence is equal to and 180" out of phase with AG of the tuned circuit due to the influence of the specimen energy absorption. Then spectrometer output or error voltage is proportional to AGt0tni.
A technique for nuclear magnetic absorption measurements provides for continuous comparison of the absorption signal with a reference standard. A WatEns-Pound calibrator circuit serves as a source of the reference signal. A servo-system is used to compare mntinuous1~-the magnitudes of the reference and absorption signals. The method facilitates acrurate determination of absorption line intensity.
W
I T H expanding interest in analytical applications of magnetic resonance spectroscopy, there has developed a need for precise measurements of line intensity. The present communication is concerned with a method developed for this purpose. Measurement of line intensity involves the stabilization of spectrometer circuits and components to the degree of accuracy required, or the use of a standard of comparison. In the initial consideration of the problem of using a spectrometer for intensity measurements, a method was sought for providing a reference standard of intensity. Standard sample8 have proved only partially satisfactory, principally because of short time fluctuations in spectrometer operating conditions and changes in sample environment when switching to the standard. I t is possible to use a double coil arrangement or to combine a standard with the material being examined and thus reduce these effects. This paper, however, is concerned with another method more satisfactory for present purposes, which utilizes an electronic standard of reference (4, 5 ) in a manner so as to provide continuous comparison of the absorption signal with the standard. The method has been found useful in an application of proton resonance absorption for measurement of moisture in hygroscopic solids ( 2 , 8). The technique should be of interest in other applications of magnetic resonance absorption.
SaAGtata~= [AGspeclmen sin w t ]
+ [AGcahb sin (ut f 180")l
where wt = w,,t
THEORY
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r$
U
Cse of the absorption signal with present equipment ( I ) involves detection of small changes in input conductance G of a tuned circuit in a two-terminal negative resistance oscillator. In the presence of an absorption, G is modulated at the static field angular modulation frequency, urn. The oscillator is particularly sensitive to changes in tuned circuit conductance, and a small increase in conductance causes a large decrease in oscillation level. The calibrator unit supplies an "artificial" absorption in the form of a modulated electronic resistance connected across the coil containing the sample, and produces a known change in conductance AG,l,b at the spectrometer radio-frequency oscillator tuned circuit. This is analogous to the change in tuned circuit conductance AG,,,, caused by the absorption of r-f energy from the spectrometer by the sample. The spectrometer unit is operated so that the specimen signal is detected as a sinusoidal variation. The static magnetic field is varied slightly about a fixed value by applying a sinusoidal voltage of frequency, fm, to a pair of coils wound on the permanent magnet pole pieces. With the resulting static field fluctuation less than the line width of the resonance curve, the spectrometer output in the region of resonance is a sinusoidal voltage whose amplitude depends on the slope of the absorption curve at a specific operating point. This operating point is moved across the region of absorption by changing the r-f oscillator frequency. Effects of wave-form diqtortion caused by nonlinearity in ahsorp-
OSCILLATOR
SHIFTER PEN MOTOR
1 \
AUDIO OSCILLATOR
Figure 1. Block diagram of nuclear resonance spectrometer
290
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5
Figure 2.
Recorder pen m o t o r eoupling to calibrator u n i t input attenuator A. B.
Pen rnofo~shaft Helical potentiometer
29 1
verter and amplifier is the chopper amplifier normally supplied with the recorder, modified by disconnecting the potentiometer circuit and feeding the input directly t o the chopper. I n this condition the pen motor will continue t o rotate as long as a voltage is present a t the input to the recorder, or until the stops a t the chart extremities are reached. In the photograph, Figure 2 it can be seen that the pen motor is also directly coupled t o the shaft of a helical potentiometer which is the amplitude control a t the input to the calibrator unit. This potentiometer is of the ten-turn type which corresponds closely t o the number of revolutions required of the Brown recorder penmotor tomove the pen mechanism full scale on the chart. Although not investigated by the authors, an equivalent coupling may be applied to other recorders. This attenuator coupling completes a feedback loop through the calibrator hack t o the sample coil connection. When the specimen absorption signal amplitude changes BS the spectrometer frequency is moved through the absorption region and makes S > 0, the error voltage is detected and amplified which causes the pen motor to rotate the helical potentiometer until the calibrator signal and specimen signal are again equal. The only function of the spectrometer amplifier is to provide sufficient error voltage to operate the pen motor. Amplifier amplitude stability should be of minor importance in this operation. The calibrator input impedance is approximately 100 times the helical potentiometer impedance so the potentiometrr input is linear with rotation. The pen trace for the recorder which is proportional to the calibrator voltage is then
phase reversal of the cdibrator output as the speiimen signal goes through the maximum absorption portion of the line where the slope of the curve is zero. An audio-transformer with grounded center tap in the secondary supplies the calibrator attentuator with an input balanced to ground. This provides center scale zero for the potentiometer and recorder pen and accomplishes the necessary phase reversal either side of cero. It is necessary to provide adequate controls and checks of calibrator unit performance. A method of accomplishing this is to meter the calibrator input voltage and calibrator plate current. A primary standard sample, for calibrator calibration, may also be necessary for checking operation from time to time.
8
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6
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Figure 3. Test results on o u t p u t stability of conven. t ional system a n d null-balanoe system Voltage atabilirstion removed from radio-frequency npeotromctu narrow band audio-emplifier
Lent is conventional with the exception oflhe recorder pen m o h coupling to the edihrrttor input voltage attenuator. The net error signal, S, is fed to a narrow band amplifier and to the lockin detector. The output of the lock-in detector, which is a direct current voltage, is fed directly t o a synchronous converter, amplified, and used t o drive a motor connected t o the pen mechanism of a strip chart recorder. The synchronous con-
The time constant of the complete loop must not greatly exoerd the response time of the pen and helical potentiometer drive motor or severe hunting will result. PERFoRMANCE
I n order to determine the performance of the null-balance system relative to that of the conventional system without the calibrator feedback loop, an instrument was set up, and a number
ANALYTICAL CHEMISTRY
292 of tests were performed. As a test material an aqueous 0.15M solution of ferric nitrate was used. Complete absorption line derivatives were obtained with the null-balance system and the conventional system. Plots of the rate of change of magnetic absorption with frequency versus T-f oscillator frequency were obtained. A detailed comparison by superposition showed the identical shape of the two lines. This test indicated that the “electronic” standard functions in accord with the simple theory and does not introduce errors in line shape. Since the primary function of the spectrometer amplifiers is to provide sufficient error voltage to operate the pen and helical potentiometer motor in the null-balance system, tests were conducted to determine the effect of gain variations on the system. Deliberate changes in spectrometer audio amplifier gain were introduced and the peak-to-peak amplitude of the recorded derivative was measured for the various gain conditions. A maximum variation of f o.43yO in the average derivative was rccorded for a gain variation of & 18% from normal. This is approximately the limit of precision that can be expected of the prcsent equipment. A more realistic approach for checking the effects of circuit instability on the result8 obtained with the null-balance system was obtained by disabling the voltage stabilization circuit of thc spectrometer narrow band audio amplifier. Figure 3 shows the variations in peak-to-peak amplitude of the recorded line for the null-balance system compared with results for the conventional system, both operating under the above conditions. Readings were taken on a single specimen that remained untouched beginning approximately 10 minutes after the equipment v a s turned on from a cold start and extending over a period of 30 hours. Equipment wae alternated between conventional operation and null-balance operation during this test period with a minimum of disturbance and under controllable conditions. The standard
deviation over a period of 30 hours was 0.3% for the nullbalance system as compared with & 4.4y0for the conventional system. A source of difficulty encountered with the spectrometer operated conventionally is the long time required for the equipment to come to temperature equilibrium. With the conventional operation several hours warm-up time is required before the instrument readings become stable, even with stabilized amplifiers. Twenty minutes after start with the null-balance system, readings were consistent within z!= 0.3%. For the conventional system after a 1-hour warm-up the average amplitude had yet to reach a stable level. In addition to quantitative moisture measurements, the nullbalance system at present is being used to facilitate relaxation time measurements, or situations where a change in spectrometer radio-frequency energy level in the specimen coil is desired while holding the system gain constant. On the basis of performance to date, the system appears to provide performance of the type essential for precision amplitude measurements. It permits amplitude measurements utilizing amplifiers that do not have to be stabilized to a high degree, provides a continuous check of system gain with the specimen in place and in its normal environment, and warm-up time is conPiderably reduced where time is an important element in operation. LITERATURE CITED
(1) Pound, R. V., and Knight, W. D., Rev. Sei. Instr., 21,219 (1950). (2) Shaw, T.M., and Elsken, R. H., J . C h a . Phys., 18,1113 (1950).
(3) Ibid., 21, 565 (1953). (4) Watkins, G. D., “An
7.-f. Spectrometer with Applications to Studies of Nuclear Magnetic Resonance Absorption in Solids,” thesis, Harvard University, 1952. ( 5 ) Watkins, G. D., and Pound, R. V., Phys. Rep., 82,343 (1951).
RECEIVED for review August 10, 1954.
Accepted October 2, 1964
Nonaqueous Titration Method for Determination of the Purity of Hexahydro-l,3,5=trinitro-~-triazine And Its Content in Wax and Polyisobutylene-Motor Oil Compositions SEYMOUR M. KAYE Pieetinny Arsenal, Dover,
N. J.
A rapid, reliable method was needed for the determination of the purity of hexahydro-1,3,5-trinitro-s-triazine (RDX) and its content in explosive compositions. Its purity and amount present in composition with wax and polyisobutylenemotor oil were determined by titration in a dirnethylformamide medium using a solution of sodium methoxide in benzene-methanol as titrant. The end point of the titration is obtained visually, using azo violet as indicator.
T
HE determination of the purit! of hexahydro-1,3,5-trinitros-triazine (RDX, hexogen) has long been troublesome to analysts of explosives. Several attempts have been made through the years to establish a satiefactoiy method. In 1921 Rathsburg (6) first used titnnous chloride to determine the “nitration product of hexamethylenetetramine>” but he neither balancrd the equation nor identified the reaction ploduct He suggested thr equation: CsHsNa(N0z)s 12 Tic13 + CsHeNs
+
Desvergnes ( 2 )attempted to determine it by breaking it down to nitrogen in a nitrometer, but conceded failure because less than five sixths of its total nitrogen was liberated in elemental form. A later attempt to reduce hexahydro-1,3,5-trinitro-s-triasine with titanous chloride solution (6) succeeded only to the extent of about 6070, even on prolonged boiling. The use of ferrous chloride effected a negligible reduction. When both reducing agents were added to the same sample, st,rangely enough, the reduction was over 90% complete. By use of a 300% excess of titanous chloride, 20 ml. of 0.7N ferrous chloride, and a 30minute boiling period, the reduction was made to proceed to 98 to 99% of the theoretical. This method, however, was completely empirical, with even the slightest deviation from the above conditions resulting in errat,ic results. A characteristic color reaction with sodium nitroferricyanide has been utilized for a spect,rophotometric method of anal)-& ( 1 0 ) . A maximum absorbance for the color system is obtained at a wave length of 625 mg. Alt,hough the system does not conform to Beer’s law, a linear relationship exists between transmittance and oonccntration in the range of 100 to 200 p.p.m. This method, too, is
