Remote Coupling Unit for Radiofrequency Inductively Coupled Plasma Discharges in Spectrochemical Analysis Robert G. Schleicher and Ramon M. Barnes‘ Department of Chemistry, University of Massachusetts. Amherst, Mass. 0 1002
Experimental demonstration of modifications of available radiofrequency equipment for use as an inductively coupled plasma discharge generator for spectrochemical analysis is presented. Specific design details are illustrated for the “conjugate matching” system approach, and calculations show close agreement with experimental measurements. Two independent plasma input power measurements indicate a coupling efflciency of 65% and an overall efficiency of 35%.
Recent demonstrations of the utility of the inductively coupled plasma (ICP) discharge as a source for emission spectrochemical analysis (1-9) prompted a survey of ICP equipment arrangements and requirements. An apparent lack of systematic guidelines exists in the spectrochemical literature with which to develop an optimized operational system. Since until recently no commercial ICP instruments designed specifically for spectrochemical analysis were available, one possible approach is to reproduce apparatus already tested and described in references (1-9). The objective of this article is to demonstrate how available rf equipment, such as radio transmitters and induction heaters, can be modified for use as an ICP generator. Specific design details are given for one type of system, so that those not familiar with rf design may construct an ICP source without difficulty. The important problem of determining plasma power input is also considered, since this critical parameter is often not clearly reported, which may lead to complication in comparing the results of different systems. For spectrochemical applications, the ICP source is conveniently mounted on a movable platform which can be accurately positioned with respect to the entrance optics of a spectrometer or some type of plasma diagnostic probe. Two basic approaches in the design of a movable ICP source have been described. In the case in which the dc power supply is separate from the rf oscillator (5, 6, I O ) , the entire oscillator section can be mounted on a movable platform and the plasma discharge tube assembly can be located directly in the oscillator tank coil. In this situation, the induction coil and the discharge are part of the oscillator tank circuit. T o provide reliable source operation, a special circuit which maintains constant plasma input power but permits variable generator frequency was developed by Boumans e t al. (6). Whenever the discharge is formed in the generator tank coil, the generator may require manual or feedback tuning of the oscillator circuit to correct for reflected reactance of the plasma load. Even with power-controlled circuits, the physical size of the rf oscillator section makes awkward the accurate positioning of the discharge for some types of measurements such as plasma diagnostics. An alternative approach is to separate the induction coil from the rf generator and maintain electrical connection by Author to whom correspondence should be sent. 724
ANALYTICAL CHEMISTRY, VOL. 47, N O . 4, APRIL 1975
means of a transmission line ( I , 8). This arrangement provides great flexibility in positioning the ICP discharge, because only the induction coil, discharge tube assembly, and a few electrical components need be moved. Since these components can be mounted on a small, lightweight assembly, the mechanical requiremqnts of the positioning mechanism are less severe than those for a bulky system. This class of system requires only a few components for optimum matching of the rf generator with the ICP discharge.
THEORY AND CALCULATIONS Matching Techniques. Basically two matching techniques can be applied in rf transmission systems (11). In the “conjugate matching” technique, maximum power transfer is achieved when the load impedance a t any point on the transmission line is equal to the complex conjugate of the source impedance a t the same point. In the ‘ ‘ 2 0 matching” technique, the load impedance is made equal to the characteristic impedance of the transmission line. This condition results in minimum reflections. In order to employ 20matching, standard rf transmitters provide a “load control” which consists of a LC circuit combination to transform the SO-ohm impedance of the transmission line into an impedance which correctly loads the transmitter. The 20matching technique is used in the equipment described by Scott et al. ( 8 ) , and Allemand (IZ),who has treated this approach in ICP generator design extensively. Radiofrequency oscillators and amplifiers built for communications applications are designed to deliver power to a load having constant impedance. Maximum power transfer is usually achieved by 20 matching. For industrial rf heating applications, obtaining maximum power transfer is more complicated because the impedance of the load may change during the processing cycle. Thus tubes and circuits employed in industrial rf heating applications are designed so that the generator output power remains relatively constant despite changes in the impedance of the load (13). For these rf generators, which are not specifically designed to deliver power into a standard 50-ohm transmission line, the use of the conjugate matching technique is most convenient in plasma applications for spectrochemical analysis. In inductively coupled plasma operation, the load impedance seen by the generator varies during the discharge initiation and when the plasma composition is changed. The generator must be able to deliver power in spite of these changes for ease of initiation and operating stability. Marynowski and Monroe ( 1 4 ) and Boumans et al. (6) discussed problems of plasma initiation for systems in which the plasma was formed in the tank circuit coil. In the application of the conjugate matching technique with a remote coupling unit and an industrial rf generator, an untuned link coil (Figure 1) is coupled to the oscillator tank coil, and the transmission line length is adjusted so that the line input impedance is capacitive and equal to the inductive reactance of the link coil. Adjusting the line termination is more practical than adjusting the length of the transmission line, since then the remote coupling unit can
be easily tuned for different loading conditions at the induction coil. A circuit representation of this system is given in Figure 1. The rf oscillator tank coil L1 is loosely coupled to the link or pick-up coil Lp. The signal transmitted through the line T is coupled to the work coil L3 through a series capacitor C1 and a variable parallel capacitor Cz. Conjugate Matching. In designing a conjugate matching system for remote ICP tuning in spectrochemical analysis, the values of the components must be correctly selected to obtain maximum power transfer. On any transmission line, a certain amount of power is required to supply the P R losses. The resistance and the length of the line must therefore be minimized. In the system represented in Figure 1, which is used in our laboratory and in a similar system employed by Fassel et al. ( 1 , 7 ) , multiple lengths of coaxial cable were connected in parallel to provide a line with the necessary current-carrying capability (15). All coils are water cooled to prevent heating and increased resistive losses. The line length must be considered when resonant transmission lines are used. The input impedance 2, of a dissipation-free transmission line terminated in an impedance 2, is given in Equation 1 (16).
2, ij R , tan ps (11 R , i-jZ, tan Bs R , is the characteristic impedance of the line, p = 2rIX is the phase constant in radians per unit length, and s is the electrical length of the line. The electrical length is equal to the physical length of the line multiplied by the velocity factor for the type of transmission line used. The velocity factor for RG-B/U cable is 0.66. From Equation 1, a wide range of input impedances can be calculated for a given termination impedaqce if the length of the line is varied. If s is made equal to X/2, then tan @s = 0 , and the input impedance of the line is equal to the termination impedance. On the other hand, if either random lengths of lines are used or if the frequency is varied, then the effect of s on 2, must be considered in choosing a termination impedance for suitable matching. In our system, a half-wavelength line is used, and the sample calculations made in the following sections assume a halfwavelength line. The values of the components Lz, L3, C1, and C2 in Figure 1 must also be correctly chosen. Coils commonly used in ICP systems for spectrochemical analysis consist of 2 to 4 turns of 0.25-inch o.d. copper tubing formed into solenoids of approximately 1 to 2 inches in diameter. If Lz is a solenoid 1.7 inches i.d. composed of 2 to 4 turns, its inductive reactance will be in the range of 40 to 90 ohms at 30 MHz. For the coupling system to be resonant, the transmission line input capacitive reactance must be 40 to 90 ohms, and therefore the net capacitive reactance must be 40 to 90 ohms for the X/2-line termination consisting of C1, C L , and L:i. The variable capacitor C2 permits compensation for stray reactance and changing load conditions. The choice of the series capacitor C1 is critical, because its reactance dominates the line termination in this design. Its reactance must be in the correct range for the magnitude of the link coil Lp used. If C1 is too small, the net capacitive reactance of the termination will be high and a link coil with a large number of turns will be necessary for resonance. If C1 is too large, a very small link coil will be required and the stray inductance in the connectors may be the same order of magnitude as the inductance of the coil winding and can result in ineffective coupling with the tank coil. For example, at 26.5 MHz, if the net capacitance of the termination is 12 pF, 21 link coil with an inductance of 3000 nH would be required. This inductor corresponds to a sole-
c1
r
3 Figure 1. Schematic diagram of the remote coupling unit. Rf generator tank coil L1, link or pick-up Lp, pick-up coil resistance Rp, transmission line T,series capacitor C,, work coI Ls, work coii resistance R3, varhble tunhg capacitor Cp
2, = R ,
Figure 2. Line termination reactance as pacitor c2
a function of variable ca-
Curve A, the reactive component of the combination C1, Cp, Ls, and R3; and Curve 8 , the net reactive component of resonant line indicating the system to be resonant when C2 equals 105 pF. Frequency, 26.5 MHz; L2, 350 nH; Rp. 0.1 ohm; CI, 50 pF; L3, 180 nH; R3,0.05 ohm
noid of approximately 10 turns of 0.25-inch 0.d. copper tubing 1.75 inches in diameter. If the net capacitance of the termination is 50 pF, an inductor of 720 nH (-5 turns) is necessary for resonance. Large link coils are impractical a t this frequency for efficient coupling to the tank coil, because of the added resistance of the windings and poor coupling geometry. If the net termination capacitive reactance is 300 pF, a coil inductance of only 120 nH is required. At 26.5 MHz, a value of 50 pF for C1 was chosen because the net termination reactance permitted resonance with a link coil inductance of approximately 350 nH (2lh turns). For this inductance, the coil was of a physical dimension that could be effectively coupled to the tank coil in the oscillator. When C1 is fixed a t 50 pF, the net reactance of the transmission line termination can be calculated as a function of the value of the variable capacitor Cn. Results of these sample calculations are plotted in Figure 2. The capacitive reactance of C1 must combine with the inductive reactance of Ls, R, and Cz to provide a net impedance which balances the inductive reactance of the link coil Lz. For a 350-nH pickup coil, the system is resonant when Cz equals 105 pF. The work coil resistance R3 was calculated after corrections for the space between turns (17). The presence of a load in the work coil changes the inductance of the coil and the tuning characteristics of the coupling unit, because the load acts effectively as a resistor in series with L3 (18).The effective resistance of the coil R3 is increased by AR3, and the inductance of the coil L3 is decreased by ALa. Although AL3 can be estimated by approximate calculations, it was measured experimentally to avoid inaccuracies which arise from the assumption of idealized geometries made for computational purposes (19ANALYTICAL CHEMISTRY, VOL. 47,
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I
O'
I
e'o
.n,-
li/
20
140
c2/ PF Figure 3. Tuning cwves of impedance as a function of C2 for Curve A non-loaded conditions and for Curves S,-& loaded conditions
Table I. Summary of Equipment Used R F generator Forrest Electronics Co. tuned platetuned grid oscillator modified to operate at 26.5 MHz. Maximum plate power 10 kW. Tank coil w a s 1.5 turns of 6-mm 0.d. copper tubing, 4-cm i.d., 2.5-cm long. Pickup coil 2.5 turns, 2.3-cm long, 3.8-cm i.d. G a s flows Argon was used f o r coolant flow at 10 liters/min. No sample o r plasma gas flow were used. Plasma tube An all quartz plasma tube identical in d i asseniblv mension to that used by Scott et a l . (8). The coupling w o r k coil w a s 1% turns of 6mni 0.d. copper tubing, 23-mm i.d. 2-cm long. Tuning c i r C: was a parallel combination of two 25cuit pf (10-kV) Jennings Radio (JC52-25) vacuum capacitors. C3 was a variable vacU U I i I capacitor, VACAP Corp (VVC-30042-15) covering the range 10-300 pf (15 kV). Transmission line was six parallel 15-foot lengths of JANtype 8A/U coaxial cable.
+
Frequency, 26.5 MHz; L2, 350 nH; L3, 180 nH;C,,50 pF; L3 1 L 3 = 162 nF loaded. For Curve B1, R3 = 3.0ohms; for B2. R3 = 1.5 ohms; and for B3,R3 = 0.5ohms
21). Based upon these experimental values, the network impedance can be calculated from Equation 2.
systems, the considerable impedance mismatch between loaded and unloaded conditions prevents the formation of a discharge. To obtain maximum power input, the discharge CS is set to the maximum of the loaded impedance curve, but under some mismatch circumstances, the C2 value for initiation must be reduced from the maximum loaded impedance condition toward the unloaded maximum until enough energy becomes available to form a pilot discharge which can grow into a fully stable discharge. Thus, the shape of the loaded and unloaded impedance curves determine the ease of plasma initiation, and a flatter curve provides less critical tuning requirements. Correct initiation conditions of the coupling unit are especially crucial for generators with limited power capability, because trying to force plasma initiation under mismatch conditions can result in power supply overload. EXPERIMENTAL
Tuning Characteristics. The tuning characteristics of the laboratory coupling circuit with and without a load in the working coil calculated from Equation 2 are illustrated in Figure 3. Curve A represents the impedance of the transmission system without a load in the coil. This also corresponds to the condition during the initiation of a discharge when no plasma is present. Curves B1 to B3 represent tuning curves for loaded conditions with different values of reflected equivalent resistance. The reflected equivalent resistance of the load A R 3 can be calculated from the dimensions of the coil and load, and the resistivity of the load for solid samples. For an actual plasma, A R s can be only approximated from the metallic cylinder model (22). The magnitude of h L 3 was estimated by measuring the inductance of the coil with a graphite cylinder load inserted. The curves B1 and B2 are in the range expected for an argon plasma load, and B3 is expected for a graphite cylinder load. A shift in the value of Cz corresponding to the maximum 2 results from the load in the coil. The plasma-loaded condition B1 illustrates the non-critical dependence of tuning for remote coupling, once a plasma is initiated. The plasma can be easily initiated if the impedance maxima for the loaded and unloaded conditions are not significantly different. With improperly designed transmission 726
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, A P R I L 1975
Apparatus. The general coupling arrangement is similar to that used by Dickinson and Fassel ( I ) , and the plasma tube assembly follows the design of Greenfield as modified by Scott et al. (8). Table I summarizes the equipment used. Power Measurement. A water-cooled graphite calorimeter, constructed with a graphite cylinder (1.9-cm long, 1.59-cm diameter) and mounted on a Plexiglas base, was based on the design by Reed and Roddy (23). The graphite (POCO Graphite, Decatur, Texas) has a resistivity of 1.9 X ohm-cm. Thermometers were mounted in water lines to measure the inlet and outlet cooling water temperature. A vacuum capacitive voltage divider (Jennings Model JP-325) was used to measure the coil voltage after calibration with a standard 26.5-MHz signal. RESULTS AND DISCUSSION Tuning Behavior. The tuning and coupling behavior of the remote coupling unit shown in Figure 1 was determined by recording the changes in work coil voltage drop under loaded and unloaded conditions and by measuring the power transferred to a graphite load. In Figure 4,the working coil voltage drop is presented as a function of the tuning capacitor C2 value for three conditions. Curve A represents the unloaded tuning curve, curve B shows the tuning curve for a water-cooled graphite cylinder load and curve C illustrates tuning characteristics for an argon plasma discharge. As C2 is increased, the coil voltage increases as resonance is approached for curves A and
60
,
1
0.6 1
05
5
I
-
04
J 85
90
95
100
105
'1G
C2 ,PF
Figure 4. Experimental tuning curves as a function of Cp at 26.5 MHz 95
(A) Unloaded coil voltage drop, (61graphite loaded coil voltage drop, and (c) argon plasma discharge loaded coil voltage drop. Coil voltages are peak-topeak values in kV
B. The shape of curve A depends upon the resistance of the wires and connectors in the transmission system and the power loss in heating the wires. In curve B, the added resistance resulting from the graphite load limits the currents in the coupling system; the curve is broader and the maximum coil voltage smaller than A as is predicted by curve B3 of Figure 3. The tuning curve for the argon plasma is flat and resembles Bl in Figure 3 as predicted. The tuning curve for argon is flatter than for the graphite load because the electrical conductivity of argon between 8000 and 10000 O K is approximately 10-20 ohm-' cm-', whereas, the electrical conductivity of the graphite used is approximately 500 ohm-'-crn-l. Thus, the reflected equivalent resistance of the graphite load is less than for the argon discharge. Comparison of curves in Figure 3 and 4 shows the differences between the calculated and experimental results. The maximum of the experimental curve A occurred 13 pF lower in C2 value than the calculated curve. The maxima of curves B and B3 differ by 18 pF. This difference in the unloaded case ( i e . , curves A) corresponds to stray reactances in the connectors and lines which were not included in the calculations for Figure 3. The difference in the loaded case ( i e . , curves B) results from the evaluation of Ls. The asymmetry of curves A and B in Figure 4 may result from heating the transmission line especially in the unloaded case as C2 is increased to reach resonance. The shapes of these curves also depend upon the direction from which resonance condition is approached. By decreasing C2, for example, a tuning curve with a less pronounced maximum is observed. P o w e r Measurements. The power input to a graphite load as a function of C2 is presented in Figure 5. Curve A shows the power input to a water-cooled graphite load measured by using the cylindrical load as a wattmeter (23), and curve B is the power input to the same load calculated from the measured coil voltage drop (curve B, Figure 4) and a reflected equivalent resistance of the load of 0.5 ohm. Graphite can be used as a dummy load for measuring the efficiency of the ICP system and thus for determining the power input to an argon plasma from the generator conditions (23),because the resistivities of graphite and argon plasma are both very much higher than the resistivity of copper used in the coils, connectors, and transmission line, and because the frequency is sufficiently high so as not to affect the electrical efficiency. In effect, the coupling efficiency measured with a graphite load will be the same as that with a plasma load. However, comparison of curves C and B in Figure 4 shows clearly a disagreement in tuning behavior for graph-
105
100
c2 ,PF
Flgure 5. Power input to a graphite load as a function of C1 (A) The power (kW) measured using a water-cooled graphite cylindrical load under conditions corresponding to curve E, Figure 4. (4The power input to the same load calculated from the measured coil voltage drop (curve 6,Figure 4) with a reflected equivalent resistance of 0.5 ohms
ite and argon plasma loads. Because of the magnitude of this disagreement, graphite is not a useful dummy load for studying experimental tuning properties for a plasma or more subtle aspects of the ICP. These subtle aspects may include the analysis of the spatial distribution of temperature and sample emission in a spectrochemical experiment, for example. This limitation results from a basic restriction of the metallic cylinder model when applied to the argon discharge because, in the model, the discharge is assumed to be a cylinder like the graphite load of fixed dimension and constant electrical conductivity (22). The discharge, however, changes volume and shape with Cz (Le., power input) and other operating conditions. Thus, graphite is a good dummy load material only for studying the efficiency of power transfer. From the maximum power input values measured in Figure 5 , the total overall efficiency of power transferred to the load averages to 35%. The efficiency of power transfer through the coupling system is approximately 65%.
CONCLUSION The selection of the matching technique to be used for spectrochemical analysis depends on the equipment that is employed as the ICP generator. Both conjugate matching and 20 matching can be very efficient for the short line lengths needed in ICP systems. If power is being transmitted over distances which are large compared to the wavelength, standing waves should be avoided. Loss is greater for the resonant line than on the same line operated as a non-resonant transmission line. For short lines with sufficient voltage breakdown and current capacity, the operation of a resonant line presents no major difficulty. When 20matching is used, both ends of the line must be tuned to match the impedance of the load to the line and to transform the input impedance of the line into a value that will correctly load the rf generator. If radio transmission equipment is being used, the equipment will have a loading control to match the transmitter to a 50-ohm line. Thus, the design of a coupling unit based on the 20 matching technique is most convenient. If induction heating equipment is to be used, a loading control generally will not be available, and conjugate matching is simpler and less expensive to use since only a pick-up coil is required for the input end of the transmission line. All tuning can be accomplished a t the load end of the line in the remote coupling unit. ANALYTICAL CHEMISTRY, VOL. 47,
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Using the approach outlined in this article, a good approximation of optimum remote tuning conditions can be obtained by straightforward calculations, and a systematic evaluation can be made of ICP equipment for spectrochemical analysis.
ACKNOWLEDGMENT We thank R. N. Kniseley for the detailed descriptions of the coupling arrangements employed in reference I , and C. Allemand for introducing us to concepts of loaded and unloaded impedance calculations.
LITERATURE CITED G. W. Dickinson and V. A. Fassel, Anal. Chem., 41, 1021 (1969). S.Greenfield and P. B. Smith, Anal. Chim. Acta, 59, 341 (1972). J. C. Souilliart and J. P. Robin, Analusis, 1, 427 (1972). G. F. Kirkbright, A. F. Ward, and T. S. West, Anal. Chim. Acta, 62, 241, (1972); 64, 353 (1973). (5) P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part B, 27, 391 (1972). (6) P. W. J. M. Boumans, F. J. de Boer, and J. W. de Ruiter, Philips Tech. Rev., 33, 50 (1973). (7) R. N. Kniseley, V. A. Fassel, and C. C. Butler, Clin. Chem., 19, 807 (1973). (8)R. H. Scott, V. A. Fassel, R. N. Kniseley, and D. E. Nixon. Anal. Chem., 46, 75 (1974). (1) (2) (3) (4)
D. E. Nixon, V. A. Fassel, and R. N. Kniseley, Anal. Chem., 46, 210 (1974). G. R. Kornblum and L. de Galan, Spectrochlm. Acta, Part B, 29, 249 (1974). R. A. Schrank, "Radio Frequency Power Measurements," Nat. Bur. Stand. (U.S.) Circ. 536 (1953). C. Allemand, Jarrell-Ash Division, Fisher Scientific Corporation, Waltham, Mass.. private communication, Nov. 1973. H. F. Dittrich, "Tubes for R. F. Heating," N. V. Philips Gloeilampenfabrieken, Eindhoven, The Netherlands, 197 1. C. W. Marynowski and A. G. Monroe, Intern. Symp. High Temp. Tech. Proc., 1963, 67 (1964). Lepel Rev., I@), p. 12; Lepel High Frequency Laboratories, Maspeth, N.Y. R. W. P. King, H. R. Mimno, and A. H. Wing, "Transmission Lines, Antennas and Waveguides," McGraw-Hili. New York, N.Y., 1945. C. A. Tudbury, "Basics of Induction Heating," Vol. 1, John F. Rider Publ., New York, N.Y., 1960. H. Brown. C. N. Hoyler, and R. A. Bierwirth, "Theory and Application of Radio Frequency Heating," Van Nostrand, New York. N.Y.. 1947. E. A. Bamberg and S.V. Dresvin, Sov. Phys.-Tech. Phys., 8, 43 (1963). S. V. Dresvin, A . V. Donskoi, and V. M. Gol'dfarb. Sov. Phys.-Tech. Phys., 10, 1270 (1966). R. V. Mitin and K. K. Pryadkin, Sov. Phys.-Tech. Phys., 11, 672 (1966). R. M. Barnes and R. G. Schleicher, Anal. Chem., 46, 1342 (1974). T. B. Reed and J. T. Roddy, Rev. Sci. Instrum., 36, 620 (1965).
RECEIVEDfor review July 3, 1974. Accepted November 18, 1974.
NOTES
Determination of Lithium-7, Molecular Tritium, and Helium-3 in Lithium Tritide by Pulsed Nuclear Magnetic Resonance Spectrometry Albert Attalla and Robert C. Bowman, Jr. Monsanto Research Corporation, Mound Laboratory, Miamisburg, OH
This paper describes a pulsed NMR technique using a Bruker B-KR 323s, 4-100 MHz, pulsed NMR spectrometer for the nondestructive determination of the radiolysis products (7Li metal, 3Hz, and 3He) formed by the natural decay of tritium (12.3-yr half-life) in LiT. Nuclear magnetic resonance (NMR) is a particularly valuable method for studying radiation damage in lithium tritide or externally irradiated lithium hydride ( I , 2) since the radiolysis products are easily detected by NMR because of the motional narrowing of their NMR spectra. Following a a/2 or 90' rf pulse ( 3 ) , the duration of the FID (free induction decay) signal is usually about 10-20 Msec for bonded nuclei in the intact solid while that of the radiolysis products is in the millisecond range. The FID signal is the Fourier transform ( 4 ) of the continuous wave, steady state, spectrum. Hence, the transform at time zero is the sum of all the Fourier components of the continuous wave spectrum and is directly proportional to the number of nuclear spins contributing to the resonance. The concentration of spins is determined by a linear comparison of the initial amplitude ( M o )of the FID signal with a corresponding set of data for standard samples. This determination is independent of the nuclear relaxation times ( 5 ) if the time between radio frequency (rf) pulses is many times (at least five times) the spin-lattice relaxation time (2'1) and if the rf pulse width (wsec) is much less than the spin-spin relaxation time ( 2 ' 2 ) . Besides the determination of nuclear spin concentrations, pulsed 728
A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
NMR allows quantitative measurement (6) of the relaxation times 2'1 and 2'2 of various nuclear species.
EXPERIMENTAL Samples are irradiated with rf pulses of 2- to lO-jmc duration and 500- to 1000-W power. The FID signals are observed between pulses by an oscilloscopic display. The pulses are spaced 1-300 sec apart depending on the time required for the particular nuclear species to recover from the previous exposure to rf power. All signals were phase-detected before recording; :Li was determined at 23.3 MHz and 3He, 3H, and 'H were determined at 45.7 MHz. The same probe was used for all experiments. A different rf insert was used a t each frequency. Six I-ml and six %-ml standard samples, each containing equal concentrations of 'Li and 'H (1-25 X lozo nuclear spins per sample), were used for the calibration of the NMR system. Water was selected as the calibration material for the 3Hz and 3He determinations because tritium is unstable because of its radioactive decay and impractical to handle, and because helium exists as a gas with a very long relaxation time (>50 min). The standards were prepared by dissolving :LiCI and H20 in 99.99 at. % D20. Anhydrous MnC12 (1.1 mg/lO ml D 2 0 ) was added to the DzO to shorten the relaxation times of the nuclear spin systems. ;LiC1 was synthesized from 99.99 wt % 7Li metal by reacting the metal with reagent grade hydrochloric acid and evaporating to dryness a t 110 "C for 24 hr. The solid 'LiC1 was crushed to a fine powder, dried repeatedly a t 110 "C until a constant weight was attained, and then stored in a desiccator. Distilled, deionized, boiled water was the source of protons in the standards. Instrument response was studied as a function of sample size. I t was determined that the NMR response is linear with sample size
