It would be of interest to extend the Dresent investigation in three directions: First, a system of practical interest, such as some biological fluid, could be investigated by the above method, and the results compared with those obtained by other analytical methods. Next a more sensitive (though perhaps less convenient) parameter than VScould be employed -e.g., the frequency of the absorbed light. Finally, the treatment described in the theoretical section could be applied to some other analytical methods, in which the instrument reading is a linear function of concentration-e.g., pdarograph y.
ACKNOWLEDGMENT
We are indebted to Prof. M. Anbar of the Isotope Dept. of the Weizmann Institute of Science, under whose direction this investigation was carried out, for his interest and advice. We are gratefulto Dr. s, szapiro of the Weizmann Institute of Science for his many helpful suggestions. RECEIVED for review January 15, 1968. Accepted July 31, 1968. Paper based on work performed under Grant No. 5x5121 of the National Institutes of Health, U.S.A.
urce Mass Spectrometry
Ti
C. A. Evans, &.,I and G . H. Morrison Department of Chemistry, Cornell Unicersity, Ithaca, N . Y . 14850 The application of time resoiution to rf spark source mass spectrometry has been studied and has shown e scale previously unpredicted. Ion yield variations were found over a 100-@ec pulse length and were dependent on the element and matrix under consideration. A comparison was made of a metallic iron sample and a biological ash-graphite matrix. The electronics necessary for pulse synchronikation, instrumental requirements, and a method of data reduction are described. In addition to providing information on the spark source excitation, the use of time resolution to improve the analytical method is illustrated.
THESPARK SOURCE MASS SPECTROGRAPH utilizes a pulsed oscillatory excitation causing the resultant ion beam to vary with time. Essentially each rise in the radio-frequency voltage causes a spark breakdown across the electrode gap followed by a period of low-voltage, high-current excitation ( I ) . This process, requiring about 0.1 psec, is repeated throughout the duration of the rf pulse. Thus the ion beam consists of ion bursts, and the ion intensity is time dependent. An analogous situation exists in emission spectrometry. In many of the spectrometric light sources, important parameters, such as current, voltage, and temperature, vary with time. As a consequence, it is often useful to time-resolve the radiation to better understand or explain the excitation as well as improve the analytical method. With time resolution, workers have investigated such parameters as the order of appearance of various spectral lines (2, 3), the temperature of the discharge (4), and line shifts (5). One of the most important benefits of time resolution is the improvement of detection limits in spectrochemical analysis (6-9). (1) R. E. Honig in “Mass Spectrometric Analysis of Solids,” A.J. Ahearn, Ed., Elsevier, New York, 1966, p 16. (2) A. Schuster and G. Hemsalech, Trans. Roy. Soc. (London), 193, 189 (1900). (3) S. L. Mandelstam, Specirochim. Acta, 11, 245 (1957). (4) C. M. Cundall and J. D. Craggs, ibid., 9, 68 (1957). (5) . , A. Bardocz. U. M. Vanvek, . . and T. J. Voros,. J. Opt. . SOC.Amer., 51, 283 (1961). ( 6 ) G. H. Dieke and H. M. Crosswhite. ibid., 36, 192 (1946), (7j H. M. Crosswhite, D. W. Steinhaus, and G. H. Dieke, ibid., 41, 299 (1951). (8) D. W. Steinhaus, H. M. Crosswhite, and G. H. Dieke, ibid., 43, 257 (1953). (9) D. W. Steinhaus, H. M. Crosswhite. and G. N. Dieke, Spectrochim. Acta, 5, 436 (1953). 2106
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The success of time resolution when applied to the emission spectrograph suggests that information on excitation and an improved analytical method may result from study of the time-dependence of the spark source ion beam. In spite of the attractiveness of this method, Franzen and Schuy ( I O , II) are the only workers to have time resolved the ion beam from a solid source mass spectrometer. The ions they studied were not produced by the radio-frequency spark but by the condensed vacuum discharge (also called a pulsed dc source). Because time resolution has shown a utility in emission spectrometric analysis and with the pulsed dc source, a method was developed which allows the study of the time variance of the ion beam emanating from an rf spark source. The electronics necessary for pulse synchronization, instrumental requirements, a method of data reduction, and preliminary results will be discussed. During the analysis of biological materials, the biological ash-graphite electrodes exhibited quite an unusual behavior (12). There were a large number of inorganic molecular ions produced, the sensitivity of the rare earths was depressed, and multiply-charged ion production was suppressed. Because of these unusual characteristics, the time behavior of these samples will be compared to that of a “normal” metallic matrix, iron, EXPERIMENTAL
Mass Spectrograph. The Nuclide Analysis Associates GRAF 2.1 mass spectrograph previously described (12) was used in this study. Figure 1 details the modifications to the instrument necessary to achieve time resolution. A trigger pulse of +IO V, 1 psec length generated by the E-H Model 131 pulse generator determines time zero. This pulse is shaped and amplified by a Stromberg-Carlson Model AR-410 amplifier and then used to trip the monostable multivibrator in the rf pulse generator of the mass spectrograph. The pulse duration is determined by the settings on the control panel of the rf pulse 1 Present address, Ledgemont Laboratory, Kennecott Copper Corp., 128 Spring Street, Lexington, Mass. 02173
(10) J. Franzen and K. D. Schuy, 2. Naturforsch., 20a, 176 (1965). (11) J. Franzen and K. D. Schuy, Z . Anal. Chem., 225-2,295 (1967). (12) C. A. Evans, Jr., and G. H. Morrison, ANAL.CHEM.,40, 869
(1968).
Ion Beam Deflector ( Faraday Cup 1
Figure 1. Electronics for time resolution
generator and used to drive the internal 1 MHz oscillator. This output of the multivibrator (called the driving pulse) is amplified to about 6 kV and stepped-up to 60 kV by the Tesla coil. The rf spark pulse from the Tesla coil then causes the electrode breakdown to occur. The repetition rate of the trigger pulse from the E-H pulse generator is continuously variable allowing selection of any desired spark pulse repetition frequency. To time resolve the ion beam a square pulse of variable width and delay after t = 0 is generated by the E-H pulse generator. This time resolving pulse is imposed on a retractable Faraday cup located just after the beam monitor (The Faraday cup has a length of 1.4 cm parallel to the ion path). The ion beam is deflected while this voltage pulse is on the Faraday cup and travels a slightly different path through the magnetic sector. Both ion beams (time selected and normal) are resolved according to their mass-to-charge ratio, striking the photoplate slightly separated. The time resolved ions can be made to fall at either higher or lower apparent mass and at a variable distance in relation to the undisturbed ions by chosing the appropriate square pulse polarity and amplitude, respectively. In this study a $48 V square pulse was used for time selection of the ions. Several time resolving pulse lengths were used for study of the two matrices. The resolving pulse was set at a given time after time-zero and 2 to 4 photoplate exposures were made at predetermined beam monitor values to allow the detection of elements at various concentration. The resolving pulse was then advanced to another time period and the same exposures were made. This process was repeated until the desired time period was covered. Although data was taken for some 15 different species, only a few examples will be given for any one type of behavior. Unless otherwise stated, a 10-psec square pulse was used for the time resolution of the 60-psec radio-frequency pulses. A constant electrode separation was maintained by the use of the oscilloscope as presented under the section on Results and Discussion. RG-58A/U, 50-ohm impedance shielded cable was used where necessary to maintain pulse rise times. A Narda
22625, 50-ohm terminator was used to match the impedance of the external circuit to the output impedance of the E-H pulse generator. Oscilloscope. A Tektronics Model 551 dual-beam oscilloscope with Type K Fast Rise Time and Type 1A2 Dual Trace Preamplifiers were used. A Beattie-Coleman Oscillotron MI1 oscilloscope camera and Polaroid Type 47 film were used to record any desired oscilloscopic traces. Direct electrical connections were used for observation of the square pulse and the output of the rf Pulse Generator. A five-turn coil of wire placed near the Tesla coil was used to pick up the spark signal, since the very high voltages involved precluded a direct connection. Samples. As an iron sample, National Bureau of Standards Standard Iron No. 464 was cut and machined to serve as self-electrodes. Ten different elements (Eu, Tm, Y , Cd, Cu, Co, Cr, Mn, S, and Fe) were doped into spectrographic grade graphite (National Carbon SP-1C) at about 5000 ppm (weight) by the addition of high purity oxides (Mn and S were added as MnS). For the studies of the 50:50, biological ash : graphite matrix, the necessary amounts of air-ashed sugar beet leaves, the 5000-ppm graphite mix, and spectrographic grade graphite were blended to yield a 1000-ppm doped mix. All blending was accomplished by using a tungsten carbide capsule and shaking in the Spex Industries No. 8000 mixer/mill. Data Reduction. The elemental ion yield for a given time period was calculated by ratioing the intensity of the photographic line produced during that period to the intensity of the ions produced during the entire discharge. The transmission readings were converted to intensity using the Seidel function and background corrected. The total ion intensity was calculated by adding the intensity of the timeresolved and the undeflected ions. The ratio of the timeresolved image to the total ion image intensity was then used as a measure of the relative ion yield. When possible, the two intensities were taken from the same photoplate exposure in order to minimize the effects of sample inhomogeneity. If the two intensities were taken from different exposures, the appropriate correction was made for exposure VOL. 40, NO. 14, DECEMBER 1968
a
2107
values but no correction could be made for any scatter caused by the inhomogeneity of the electrode. For presentation and ease of examination and discussion the calculated ratios have been multiplied by 103 and labeled “Relative Ion Yields.” RESULTS AND DISCUSSION
Oscilloscope. I n addition to allowing time and voltage measurements pertinent to the time resolution study, the oscilloscope provided other valuable information. Several workers in the field (13, 14) have mentioned the advantage of using an oscilloscope in conjunction with the spark source mass spectrograph to monitor the electrode gap and for tuning the rf circuitry (15). The peak to peak voltage of the rf signal during sparking is dependent on the electrode separation. Figure 2 shows the spark signal and its relation to the driving pulse for shorted electrodes (a) and sparking electrodes (b) and 2(c) compares the spark signal for a wide and a narrow electrode gap. Figure 2(d) is the oscilloscope pattern observed while sparking iron electrodes with a 340-psec rf pulse. The oscilloscopic traces were quite different when the biological ash-graphite electrodes were sparked. Figure 3(a) is the pattern seen for a single rf pulse. The normal rf signal from the Tesla coil is seen for the first 20 psec and then a trace is seen which is similar to the shorted electrode type signal [Figure 2(a)]. The relative amounts of rf and shorted electrode type signal in a given pulse varied from very little rf to complete rf type breakdown. Figure 3(b) presents the integrated oscilloscope trace for 10 rf pulses. Also shown is the square pulse used for time resolution. If enough pulses were observed, as in Figure 3(c), there would be rf signal throughout the full pulse duration, but the occurrence of the rf during a single packet was quite unpredictable. This is in direct contrast to the oscilloscope pattern of the iron electrodes with spark breakdown occurring during the entire 60-psec rf pulse. The rf discharge and/or the shorted electrode type signal observed with the ash-graphite electrodes lasted past the “cutoff)’time of the iron electrodes. As is seen in Figure 3(b),the third channel of the oscilloscope displays the time resolving pulse with its relation to time-zero and the rf pulse. As mentioned above, the delay from t = 0 and the width of the square pulse (100 and 10 psec, respectively, in Figure 3(b)) are both variable to allow selection of the desired time segment. Iron Electrodes. The iron sample was chosen to test the time resolution method and as a “normal” matrix for comparison to the ash-graphite electrode. The iron sparks similar to most metallic electrodes, generally yields sensitivity factors within a factor of three of unity (as noted in the authors’ laboratory) and presents a reasonably uncomplicated rf signal on the oscilloscope. Figures 4,5, and 6 present the relative ion yield GS. time for representative elements in the iron electrodes using resolving pulses of 2, 5 , and 10-psec duration, respectively. All three figures present the time behavior of the 60-psec rf pulse shown in Figure 2. The 2-psec and 5-psec resolution studies were conducted in an attempt to localize in time the initial ion yield maximum noted when using the 10-psec pulse. As is seen in Figures 4 and 6, a shift in the maximum from 6 psec to 10 psec is caused by the increased resolving pulse length. This expected shift and a slight decrease in the height of the maximum (13) W. E. Harrington and J. R. Woolston: private communication, David SarnoffResearch Center, Princeton, N.J. 08540. Novem-
ber, 1967. (14) J. Franzen, Z . Natuuforsch., 18a, 410 (1963). (15) W. L. Harrington, Cornell University Materials Science Center Report No. 496, Ithaca, New York, 1966, p 27. 2108
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ANALYTICAL CHEMISTRY
(a)
E l e c t r o d e s s h o r t e d . (20 p.sec/div)
(b)
Electrodes sparking.
(20 Wsec/div)
Tesla coil-wide e l e c t r o d e gap T e s l a coil-narrow e l e c t r o d e gap
(.c) , E l e c t r o d e s s p a r k i n g .
(d).
(20 Wsec/div)
E l e c t r o d e s s p a r k i n g , 340 psec p u l s e .
Figure 2. Oscilloscope traces for iron electrodes (a) electrodes shorted (20 psec/div), (b) electrodes sparking (20 psecldiv), (c) electrodes sparking (20 psec/div), ( d ) electrodes sparking (340 psec pulse)
are the only differences noted for resolving pulses of 2 and 10 psec. The initial maximum in the ion yield us. time distributions indicates that a greater number of ions are produced by the first spark discharge (or discharges) than by any others during the rf pulse. The maximum occurs at 6 to 8 psec after
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Single pulse.
20
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I 30 40 60 70 10 Time ( p s e c 1 20
Figure 4. Ion yield pulse)
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for iron matrix (2 psec resolving
2 .o (b)
(c).
Ten superimposed pulses.
(20 li.sec/div)
I .o
Multiple superimposed pulses. (10 ysec/div)
Figure 3. Oscilloscope traces for biological ash-graphite electrodes (a) single pulse (20 psec/div), (b) ten superimposed pulses (20 psec/ div), (c) multiple superimposed pulses (10 psecidiv)
time-zero which is approximately the time after time-zero that the first ions would be passing the deflector plates. (The oscilloscope shows the first discharge occurring at 3 to 4 psec and 2 to 3 ksec are required for time-of-flight to the Faraday cup.) The ion production is reasonably constant during the majority of remaining pulse and dies out as the driving pulse “ringsout” (see Figure 2). Except for a slightly more intense initial maximum for Fe+, no definite correlation between ion yield us. time and element or classes of elements was observed. In addition, the + 2 ion yields for the metallic elements (Fe, Cu, Mn, etc.) closely parallel that of the singly-charged species. Figures 4, 5 , and 6 show +1 to +2 ratios which are essentially constant. The minor variations seen are much less than the variations from unity found in other matrices. As discussed by Honig and Woolston (16), the radio-fre(16) J. R. Woolston and R. E. Honig, Twelfth Annual Conference on Mass Spectrometry and Allied Topics, Montreal, Canada, 1964, p 377.
20
IO
30
40
50
60
70
Time ( p s e c ) Figure 5. Ion yield pulse)
US.
time for iron matrix (5 psec resolving
quency spark source produces ions with a large range of initial energies. After acceleration and passage through the electrostatic sector, the ions are distributed into an energy spectrum formed at the beam monitor as shown in Figure 7. These data are obtained by varying the accelerating potential while the electrostatic field is maintained constant, thereby shifting the energy spectrum across the energy slit. This energy slit is set to admit approximately 600-V portions of the energy spectrum to the magnetic sector. Constant value exposures, as measured by the ion beam monitor, are taken over the range of accelerating potentials, the plate is developed, and the % transmission data are converted to intensity via the Seidel function. Because the iron comprises the vast majority of the VOL. 40, NO. 14, DECEMBER 1968
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2109
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Figure 6. Ion yield us. time for normal energy ions (iron matrix)
1.0
Low
Accelerating Voltage (kv.) Initial Ion Energy / charge
High
Figure 7. Ion intensity us. accelerating voltage for iron matrix ion beam, the use of constant ion beam exposures results in data which are normalized to iron. The flat distribution shown for Gr+ and @u+was characteristic for the normal metallic ions (Fe+, Mn+, V+, Co+, etc.). The singly-charged ions of S, P, As, and Sn gave the double peaked distribution characterized by the curves for Sf and P + in Figure 7. The distribution for K+ was very similar to that shown for Na+. The plateau at the high accelerating voltages (denoting low primary energy) continued to rise for Na+ and K+, as opposed to the break noted for the non-metals, S and P. The doubly-charged metal ions all showed a two-peaked ion yield us. energy distribution with the valley falling at the accelerating potential normally used in an analysis. The energy distributions for the non-metals, S and P, and the doublycharged species are quite similar to those found by Woolston and Honig for their MS-7 (16). As is shown on the abscissa of Figure 7, the ions can be thought of as having a low, medium, or high primary energy depending on whether a high, normal, or low accelerating potential is necessary to bring them into focus on the energy pass window. Those ions which exist at a final energy less than that of the accelerating potential are thought to be produced from neutrals which drifted into the accelerating region and were created by secondary electron collision (17). Because the ions were formed in the accelerating region, they did not experience the full effect of that voltage and had a final energy less than the accelerating potential. As the previous time resolution data were taken at the normal accelerating voltage, it was decided to examine the initial maximum in the ion yield us. time curves as a function of ion energy. The ion beam was time resolved at three accel(17) J. Franzen, K. D. Schuy, and H. Hintenberger,Twelfth Annual
Conference on Mass Spectrometry and Allied Topics, Montreal, Canada, 1964, p 251. 21 18
@
ANALYTICAL CHEMISTRY
erating potentials (12.5 kV, 11.7 kV, and 11.1 kV) giving time dependency data for ions with low, medium, and high primary energy, respectively. Figures 6, 8, and 9 present the variations in ion yield us. time distribution for the early part of the rf pulse for low, medium, and high primary energy, respectively. For all three cases, a 10-psec time resolving pulse was used so that the intensity ratio for the resolved and unresolved ions could be calculated for data from the same exposure. This prevented distortions caused by electrode inhomogeneities. Only the first 30 to 40 psec were covered in Figures 6 and 8 allowing the use of the entire photographic plate to better define this time region. Since there was little initial peaking for the high primary energy ions, Figure 9 presents the data for the entire rf pulse. As mentioned earlier, the instrumental conditions for the data presented in Figure 6 are the same as in Figures 4 and 5, except for the use of a 10-psec time resolving pulse. The ion yield us. time maximum shows a definite energy dependence. For species with a high primary energy, the ion production is fairly constant throughout the radiofrequency pulse, whereas at a low primary energy the yield us. time maximum is sharper than under normal experimental conditions. As with the ions of medium primary energy, there seems to be no variation of yield us. time with element or element class for the low and high primary energy ions. In general, each electrode discharge is composed of a spark break-down (high voltage, low current) and an arc discharge (low voltage, high current). The presence and length of the arc period is very dependent on the amount of' energy in the rf power circuit. This whole discharge cycle lasts only 0.1 psec and is repeated throughout the rf pulse as dictated by the circuit design. The design of the spark source does not dictate that the initial discharges be any different than the
following ones, yet the time resolution data indicate an initial burst of ions. Possibly there is more energy available from the inactive circuit for the first discharge(s) than from an active circuit for the later closely spaced sparks. Two possible explanations for the energy dependence of the ion yield 6s. time maximum depend on the premise that all or a major part of the low primary energy ions are neutrals ionized by collisions (with electrons, ions, or neutrals) in the accelerating region.
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It may be that there are more neutrals ejected by sputtering, etc. near time-zero, and consequently more neutrals available for collision excitation. Alternately, the production of neutrals is constant with time and there is an initial burst of particles, causing more collisions, and, consequently, more neutrals are ionized in the accelerating region. This energy dependence of the ion yield us. time could also be attributed to limitations of the accelerating potential power supply. After the first few microseconds, the ion current could be sufficiently high enough to load the high voltage power supply causing the accelerating potential to decrease with a consequent shift in the ion energy spectrum at the beam monitor energy slit. If the ion yield us. energy distribution were as in Figure 10 as discussed by Franzen and Hintenberger (18), the observed ion yield us. time distributions could be explained by a shifting of the energy distribution across the energy window. The apparent time differences in the rf discharge could be observed if at the lower accelerating voltages, the energy window were passing ions from the flat portion of the ion intensity cs. voltage distribution. Thus a change in accelerating potential would not cause a change in the ion intensity. Whereas, at higher accelerating voltages a change in the accelerating potential power supply would cause a significant change in the ion intensity because the intensity us. voltage curve is peaked at these higher potentials. Biological Ash-Graphite Electrodes. Possibly because Of the complexity of the matrix, a variety of yield us. time behaviors were found in the ash-graphite samples. Figures 1115 give representative curves for the elemental ions and Figures 16 and 17 illustrate the behavior of several molecular ion species. The curves of CoL, Co2+,and Fe' in Figures 11 (18) J. Franzen and H. Hintenberger, 2. Naturforsch., 18% 397409 (1963). VOL. 40, NO. 14, DECEfABER 1968
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Figure 10. Ion intensity us. accelerating potential found by Franzen and Hintenberger (Ref. 18) and 12 are quite representative of the behavior of the metallic elements, and Figure 13 presents the typical time dependence of the rare earth ion yield. The singly-charged metallic ions exhibit a behavior different from that in the iron matrix. A broad maximum was always noted around 20 psec with a second peak sometimes occurring between 40 and 60 psec. Also, the 20-psec maximum was always more intense than the one occasionally found later in the pulse. Although the singly-charged ions demonstrated a different behavior in the iron and biological-ash matrices, the 2+ ions maintained essentially the same time behavior. Except for being much sharper in the ash-graphite matrix, the same 10psec maximum for the 2f ion yield is observed in Figure 11 as in Figures 4 , 5 , and 6. The shape of the ion yield curve for the singly-charged rare earths was found to be different from that of the metallic ele-
Time ( p s e c ) Figure 12. Ion yield us. time for biological ash-graphite electrodes ments. A broad maximum in the yield curve appeared around 40-50 psec with no early maximum as noted with Fe+ and Cof. As is shown in Figure 13, the doubly-charged rare earth ions have a maximum near 10 psec just as the 2f metallic elements did in the iron matrix.
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Figure 11. Ion yield us. time for biological ash-graphite electrodes 2 1 12
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ANALYTICAL CHEMISTRY
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Figure 14. Ion yield us. time for biological ash-graphite electrodes
Figures 14 and 15 show the yield us. time curves for two of the ionic species of K and C1. These elements were not at a concentration suitable for study in the iron matrix and therefore there is no situation for comparison. The 2+ ions both show a behavior similar to that seen for the 2+ metallic elements and the 1+ ions show no behavior more unusual than the other 1+ species in the ash-graphite matrix. Figures 16 and 17 present the yield-time data for several molecular species. The molecular species of Figure 17 are several of the interferences encountered in the analysis of biological materials as discussed by Evans and Morrison (12). The most striking aspect of these time-resolution curves was that ions were produced from the ash-graphite electrodes after the normal “cut-off’ time seen in the iron electrodes. In many instances, singly-charged ions were detected at 105 psec (No ions were observed in the Fe electrodes after 70 psec) and with an unusually wide electrode gap, ions were produced at 120 psec after time-zero. However, the 2+ ions generally were not produced after 70 psec. This increased relative production of 1 ions at later times is illustrated by the variation from unity seen in the 1+/2+ us. time curves of Figures 11 and 13. When the ion yield curves are compared with the oscilloscopic traces, some of thevarious phenomena can be correlated. For instance, the initial maximum in the 2+ ion production observed for the ash-graphite electrodes can be related to the oscilloscopic traces noted for that matrix (Figure 3). As seen in Figure 3(b), the spark-type discharge was generally observed early in the rf pulse and the shorted electrode type signal became more apparent at the later times. At 60-70 psec, the spark “hash” signal decreases in amplitude just as the 2f ion yield becomes undetectable. Even though the low-voltage (shorted electrode type) signal sometimes alternated during an rf pulse with the high-voltage signal (the rf or spark signal), it
+
Time ( p s e c )
Figure 15. Ion yield us. time for biological ash-graphite electrodes seemed a spark discharge always preceded the low-voltage signal. It is possible the occurrence of the initial high voltage discharge induces a very large number of ionic species from the easily volatilized and ionized oxide matrix into the electrode gap. The high current plasma causes additional sputtering of neutrals and their ionization which contribute to the maintenance of a low voltage, high-current discharge, as if the electrodes were shorted. The similarity between this portion of the discharge and the shorted electrode type signal of Figure 2(a) would suggest the presence of a low resistance gap caused by a large number of charge carriers. The volatility and ease of ionization of the matrix allow this process to occur over a longer time for the available energy in the circuit. When the energy in the tank circuit is depleted or the number of charge carriers drops below a critical value, thedischarge stops and the circuit recovers to the energy required for another spark discharge. On a time average, more spark breakdowns occur in the beginning of the rf pulse and their number decreases with an increase in time from time-zero. This would mean there is a greater percentage of spark-like excitation near the beginning of the pulse and more low-voltage type of excitation as the pulse progresses. If this is true, correlation with the ion yield curves of Figure 11 would indicate the enhanced production of 2+ ions by the high-voltage spark discharge while singlycharged ion production is favored during the low-voltage discharge. Honig’s postulate (19) of the predominance of multiply-charged ions during the arc portion of a single rf discharge suggests an unusual type of excitation of the biological ashgraphite electrodes. This reversal of expected behavior when exciting this easily volatilized and ionized matrix has been observed with another type of discharge. The pulsed dc source of Franzen and Schuy (10, 11) generally produces a large (19) R. E. Honig, Twelfth Annual Conference on Mass Spectrometry and Allied Topics, Montreal, Canada, 1964, p 233. VOb. 40, NO. 14, DECEM5ER 1968
e
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20
40
60
80
100
Time ( p s e c ) Figure 16. Ion yield trodes
time for biological ash-graphite elec-
Figure 17. Ion yield us. time for biological ash-graphite electrodes
amount of multiply-charged ions during the arc or low-voltage portion. But Franzen (20) found the production of multiplycharged species during the arc cycle was drastically reduced when a potassium halide-graphite sample was sparked with the pulsed dc source. Both studies using time resolution (this work and that of Franzen and Schuy) suggest a different excitation when sparking an easily volatilized and ionized matrix than when a metallic matrix is under study. This difference can probably be attributed to the increased number of charge carriers in the electrode gap (low gap resistance) which drastically alter the gap parameters. Analytical Applications. In addition to providing information on the spark source excitation, time resolution can be used to improve the analytical method. Jackson, Whitehead, and Vossen (21) have shown that nonsynchronous beam chopping can be used to improve the analytical precision of the spark source. Because their method consumed more sample, the scatter due to sample inhomogeneities was reduced. Synchronous chopping or time resolution can also improve the accuracy of a qualitative or quantitative determination. As an illustrative example, Figure 12 presents the ion yield us. time for T e + and mass 28 which may be 56Fe2+or a singly charged ion of mass 28 [e.g., 28Si" or (12C160)+]. By comparison to the previously established behavior of a 2+ metallic ion (Figure 111, it can be seen that the intensity of mass 28 has very little relation to the yield 5s. time of a 2+ metallic ion. In addition, the time behavior of mass 28 has greater similarity to the 1+ metallic ions of Figures 11 and 12 than the If molecular species shown inFigure 17. This would infer that the
major contribution to the intensity of mass 28 was from zsSi+. So using time resolution data and similar reasoning, added confidence can be placed in a questionable spectral assignment. In addition to providing qualitative evidence, time resolution can be used to improve the spectral purity of an analytical line. If the production of an interfering specie and the ion line of interest are different in time, selection of the proper time segment would give an improved line-to-interference ratio. As seen from the yields presented in Figure 11 and 13, the influence of a 24- metallic ion at a given mass could be greatly reduced by the use of the latter portion of the rf pulse. The yield us. time curves were dependent on many parameters such as tuning and pulse length. Almost certainly the shape of these curves will differ from one instrument to another. Any or all of these parameters could be varied to optimize the line-to-interference ratio to improve the analytical uses of the instrument.
5s.
(20) J. Franzen, private communication. Institut fur Spektrochemie, 46 Dortrnund, Postfach 778, Germany. May, 1968. (21) P. F.S. Jackson, J. Whitehead, and P. 6. T,Vossen, ANAL. CHEM., 39, 1737 (1967).
21 14
0
ANALYTICAL CHEMISTRY
SUMMARY
The application of time resolution to the rf spark source has shown variations on a time scale previously unpredicted. Honig(l9)and Franzen, Schuy, and Hintenberger ( 1 4 1 7 ) have predicted the variable production of ion species within a single rf discharge which is of the order of 0.1 psec in duration and is repeated throughout the length of the rf pulse. This study has shown variations in ion yield existing on a much larger time scale and depending on the element and matrix under consideration. An oscilloscope was an important part of the study. The oscilloscope provided a method for the maintenance of a constant electrode gap and the desired time relationships. In addition, major differences were found in oscilloscopic traces for various matrices. A correlation between time-yields of the
ash-graphite matrix and the oscilloscopic traces was postulated. The unusual placement of the deflector plates used for time resolution allowed a method of data reduction which minimized variations caused by sample inhomogeneity. In addition to presenting time resolution as a possible means of studying the excitation of the rf spark source, two uses were postulated which could improve the analytical uses of the technique.
ACKNOWLEDGMENT
The authors thank S. F. Peterson and R. K. Skogerboe for their assistance and comments during this study. RECEIVED for review May 13, 1968. Accepted September 3, 1968. Financial support was provided by National Science Foundation Grant GP-7461X.
Studies on the Anion Exchange Behavior of oxylic Acids and Phenols James S. Fritz and Akira Tatedal Znstitute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 50010 The uptake of carboxylic acids and phenols by an anion exchange resin is studied from aqueous solutions containing acetone or methanol. Distribution coefficients are higher in basic solutions where the acid anion predominates, but a significant uptake also occurs from acidic solution. A higher proportion of organic solvent in the solution lowers the distribution coefficients. Several practical separations are demonstrated on short columns. Distribution coefficients and capacity measurements are reported for organic acids on a nonionic polystyrene resin and compared with those for the polystyrene anion exchange resin. Also, uptake of organic acids by a quaternary ammonium salt dissolved in cyclohexane is determined and compared with the tw0 resins studied.
nitrophenol by a macroreticular anion exchange resin, Amberlyst A-26, was studied over a wide range of pH in aqueousorganic solvent mixtures. From the data obtained, conditions were selected and successful separations of some of these acids were obtained on a short anion exchange column. Also the sorption behavior of some of these acids on A-26 resin was compared with the sorption on a porous polystyrene-divinylbenzene copolymer, XAD-2, and with extraction by a liquid anion exchanger, Aliquat 336. From these experiments, some conclusions are drawn regarding the ion exchange behavior of carboxylic acids and phenols.
ALTHOUGHanions of carboxylic acids (1-4) and phenols (5, 6) may be taken up and separated by an anion exchange column, a complicating factor is the uptake of some types of nonionic organic compounds by ion exchange columns. Salting-out chromatography and solubilization chromatography are separation techniques based on selective sorption of organic molecules by ion exchange columns. Thus, cation exchange resins have been used for the separation of the lower aliphatic carboxylic acids (7-11), aromatic acids (12), and phenols (13, 14). In the present work, the sorption of some aliphatic carboxylic acids, benzoic acid, o-nitrobenzoic acid, phenol, and o-
Resins. Amberlyst A-26, macroreticular anion exchange resin was obtained from Rohm and Haas Co. The resin was ground and sieved to 150 200 mesh for use in both the batch and column experiments. The resin was washed several times with hydrochloric acid-ammonium sulfate solution-sodium hydroxide solution cycles and, finally converted into chloride form and air dried. Amberlite XAD-2, a macroreticular resin obtained from Rohm and Haas Co., was treated the same as described above. Aliquat 336 liquid anion exchanger was obtained from General Mills. The concentration of this exchanger was determined by nonaqueous titration with perchloric acid in glacial acetic acid
1
On leave from Kyushu University, Fukuoka, Japan.
(1) C. W. Davies and B. D. R. Owen, J. Chem. Soc., 1956, 1681. (2) K. K. Carroll, Nature, 176, 398 (1955). (3) K. Shimomura and H. F. Walton, ANAL. CHEM.,37, 1012
(1965). (4) S. Egashira, Japan Analyst, 15, 1356 (1966). ( 5 ) M. Magda, R. Chwaszcza, and J. Chmielowoki, Gaz., Woda. Tech. Sunit., 36, 313 (1962). (6) L. T. Clark, J. Chromatogr., 15, 65 (1964). (7) D. Reichenberg, Chem. and Ind. (London),1956,958. (8) T. Seki, J. Biochem. (Tokyo),45, 855 (1958). (9) J. Sherma and W. Rieman, Anal. Clzim. Acta, 20, 357 (1959). (IO) G. A. Harlow and D. H. Morman, ANAL.CHEM., 36, 2438 (1964). (11) . , D. J. Patel and S. L. Bafna, Ind. Enn. - Chem., Prod. Res. Deuelop., 4, l(1965). (12) T. Saki, K. Inamori, and K. Sano, J. Biockem. (Tokyo), 46, 1653 (1959). (13) J. Sherma and W. Rieman, Anal. C/zim.Acta, 18,214 (1958). (14) T. Seki, J. Chromatogr., 4, 6 (1960).
EXPERIMENTAL
N
(15).
Distribution CoefPleients. The distribution coefficients of organic acids on resins were measured by a batch method. Twenty milliliters of organic solvent-water mixture which contained about 0.4 milliequivalents of organic acid, inorganic salt and acid, or base to adjust the pH of the solution was added to 1.5 grams of resin in a 125-ml glass-stoppered flask, shaken mechanically for 1-2 hours and allowed to stand for several hours. After the liquid phase was filtered free from resin, an aliquot of the liquid was taken and analyzed for the organic acid present. The distribution coefficient was calculated from the difference in the concentration of organic acid before and after achievement of equilibrium. Analysis of lower aliphatic acids employed titration with sodium hydroxide solution. In the experiment for alkaline pH solution, the carboxylic acid in resin phase was determined as follows: The resin which filtered free from the liquid was washed into a small column with 85% acetone-0.01M HC1 solution, and the same solution was passed through the (15) S. Siggia, “Quantitative Organic Analysis via Functional Groups,” John Wiley and Sons, New York, N. Y . , 1954. VOL. 40, NO. 14, DECEMBER 1968
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