very useful technique to increase secondary ion emission and the thermal equilibrium model can be applied to this case as well as the normal operation in high vacuum. The analyses have further indicated that similar samples analyzed under identical bombardment conditions appear to establish similar temperature and electron densities. This makes it possible to apply a quantitative correction procedure which requires no internal standards when similar samples are being analyzed. Applicability of this approach for other metals will be discussed in a future communication. As another test of the model, the thermodynamic model has also been applied t o the calibration curve method and it has been found that the theory describes well the small changes in the slope of the calibration curve with the changes of p ( 0 2 )
ACKNOWLEDGMENT The authors wish to express their appreciation to C. A. Evans and D. Simons for helpful discussion and comments in addition to also making corrections in the manuscript.
LITERATURE CITED (1) H. E. Beske. 2. Naturforsch.. TeilA, 22 459 (1967). (2) C. A. Evans, Jr., Anal. Chem., 44 (13), 67A (1972). (3) C. A. Andersen, Sixth, and Seventh Natl. Electron Microprobe Conf.. Pittsburgh, 1971; San Francisco, 1972. (4) K. Tsunoyama, Y. Ohashi, T. Suzuki. and K. Tsuruoka, Tetsu To Ha@ne. 13, 253 (1974). (5) R . Shimizu, T. Ishitani, and Y. Ueshima, Jpn. J. Appl. Phys., 13, 249 (1974). (6) R. Shimizu, K. Kato, and T. Ishitani, Jpn. J. Appl. Phys., 13, 1477 (1974). (7) H. Tamura, T. Kondo, and T. Hirano, "Proc. 6th Int. Conf. X-ray Optics and Microanalysis", G. Shinoda. K. Kohra, and T. Ichinokawa. Ed., University of Tokyo Press, Tokyo, 1972, p 423. (8) H. Tamura, T. Kondo, and H. Doi, Adv. Mass Specfrom., 5, 441 (1968). (9) L. de Galan, R . Smith, and J. D. Winefordner, Specfrochim. Acta, Par7 B, 23, 521 (1968). (10) H. W. Drawin and P. Felenbok. "Data for Plasmas in Local Thermodynamic Equilibrium", Gauthier-Villars, Paris, 1965. (1 1) C. A. Andersen and J. R. Hinthorne, Anal. Chem., 45, 1421 (1973). (12) V. Leroy, J. P. Servais. and L. Habraken, C. R. M., No. 35, 6 9 (1973). (13) K. Tsunoyama, Y. Ohashi, T. Suzuki, and K. Tsuruoka, Jpn. J. Appl. Phys., 13, No. 6 (1974).
RECEIVEDfor review October 23, 1974. Accepted January 20, 1975. This paper was presented a t the Seventh International Conference on X-ray Optics and Microanalysis in Moscow and Kiev, July 9-16, 1974, under the title "A quantitative approach for the ion probe microanalyzer".
Critical Parameters Affecting the Hollow Cathode Ion Source E. H. Daughtrey, Jr., and W. W. Harrison D e p a r t m e n t of Chemistry, University of Virginia, Charottesville, VA
22903
A hollow cathode ion source was studied for the analysis of solids and solutions. Cathodic sampling was shown to have certain advantages over anode sampling. The effects of pressure, current, and discharge gas were studied. Temperature measurements of bulk cathode temperatures were mqde with and without cathode cooling. The precision of trace metal measurements was f17-22 YO RSD. Sensitlvities were in the sub-ppm range for solutions and ppm range for solids.
The recent development of the hollow cathode ion source (HCIS) has indicated its considerable promise as a technique for the analysis of solids (1-3), and of surface films deposited from solution ( 4 , 5 ) . The hollow cathode discharge has been studied extensively by physicists (6, 7) for many years. It is widely used as an atomic absorption line source and, less widely, as an atomic emission source (810). However, there have been relatively few reports concerned with ion sampling of the hollow cathode discharge. Pahl and coworkers (11-13) have investigated many fundamental aspects of the discharge in their plasma diagnostics measurements. Recent reports from this laboratory have shown that both dc and rf potentials are useful for HCIS applications. Coburn and Kay have reported the use of a planar glow discharge as an ion source (14, 1 5 ) . The aims of this investigation are to illustrate an improved HCIS design, to more rigorously evaluate and optimize the analytical conditions for operation, and to demonstrate its feasibility as an analytical technique. 1024
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
EXPERIMENTAL Source Modifications. An AEI MS-702 spark source mass spectrometer with electrical detection was used with previously described modifications (2, 16). T h e pumping and source chamber modifications provided extra ports through which electrical leads (for temperature and pressure measuring devices for the hollow cathode), t h e discharge gas feedthrough, and a liquid nitrogen cold finger were introduced. T h e discharge gas flows from t h e tank through a molecular sieve trap, followed by a 6-inch length of glass tubing (to isolate the t a n k from the accelerating voltage), a Nupro metering valve, and a Dry Ice-acetone cold trap. Inside t h e source, the gas supply tubing is coupled t o the hollow cathode tube with a 6-inch length of Cajon flexible tubing and a Swagelok fitting. T h e dc potential to produce the hollow cathode discharge was taken into the source through the internal rf spark connections from a Kepco Model H B power supply. This unit was operated in the constant current mode and floated a t t h e acceleration potential by isolation from earth with a 30-kV isolation transformer (Del Electronics Corp., Mt. Vernon, NY). Hollow Cathode Tube. Initial investigations were performed using the HCIS previously described (2) which features ion sampling out of t h e anode region of the discharge. An improved design, in which the discharge is sampled through the cathode, is shown in Figure 1. Brass was used for all metal parts except the cathode. T h e gas enters through a Swagelok-Cajon Ultra Torr fitting coupled t o the Pyrex body of t h e tube a t 5 . T h e pressure can be monitored by attachment of a thermocouple gauge a t port 6; this port can alternatively be plugged with an Ultra T o r r fitting. T h e ring anode 11 is connected t o contact screw 3 through an Ultra Torr fitting. T h e glass body 4 is supported by a press fit into the cathode block 2 and by t h e gas connection a t 5. T h e cathode 1 is held by a contact screw t o t h e cathode block, which is attached t o the MS702 back plate. Alignment of the 0.040-in. cathode slit with t h e ion axis is achieved by optical alignment with a telescope mounted on t h e ion axis, by adjusting the position of t h e cathode block. T h e
cathode is made negative relative t o the anode and held a t a 10-kV acceleration potential through the cathode block and back plate; the positive anode connection is made by the other lead onto contact screw 3. Also shown in Figure 1 is the coupling of the cathode block to the mass spectrometer and liquid nitrogen cold finger. Thermal contact between the block and the cold finger is achieved with fine metal filings. T h e reservoir volume of the cold finger is about 250 cm3.
RESULTS AND DISCUSSION Cathodic vs. Anodic Sampling. In first designing a HCIS ( 1 , 21, it was experimentally more convenient to construct a unit which allowed ion extraction through the anode, which acted as the entrance slit of the mass spectrometer. The anode region, however, is some distance from the analytical sample located in the hollow cathode cavity. After sputter release from the cathode surface into the negative glow, atoms must diffuse through an essentially field free region toward the anode for subsequent ionization and extraction. Sampling instead from the cathode fall region adjacent to the cathode surface appeared to offer more promise, particularly for thin sample films which are stripped rapidly away by ion bombardment. Comparisons will be made to the anode sampler (AS) design while describing our current, work with the cathode sampler (CS) unit. The CS produced significant improvements in spectral purity, current capability, discharge stability, demountability, and general experimental versatility. The AS exhibited chronic spectral complications (protonated, hydrated, oxide, and hydroxide forms of the major sputtered species) due in large part to absorption and subsequent release in the discharge of air and water vapor by the Lava insulator. The glass and metal construction (Figure 1) of the CS has significantly reduced these problems. Higher current capability is provided for the CS because of the addition of the large metal block heat sink. Connection to the liquid nitrogen cooling finger further extends the CS current range. Perhaps because of the lower operating temperatures, the discharge is very stable, even with the addition of sample salts in the hollow cathode cavity. Also contributing to stability is the larger buffer volume of the CS body, so that outgassing and discharge bursts have less effect on the net operating pressure. The AS required complete removal from the source for cathode changing, resulting in a long turn-around time, while t,he CS allows rapid in-place cathode exchange by retraction of the glass body. The new design also has built-in versatility which allows convenient temperature and pressure measurements, as well as advantageous visual observation of the discharge. The optical emission may be coupled through a quartz front window in the mass spectrometer face plate to record discharge emission spectra. The CS is now used for all our HCIS studies. Choice of Discharge Gas. Helium, neon, argon, nitrogen, and oxygen were investigated as discharge gases. Selection of the most suitable gas is dependent upon the sputtering and ionizing properties of the gas and also upon its spectral contribution. Each of the gases produced characteristic spectra from a copper cathode matrix, although helium clearly produced the least net sputter yield ( 1 7 ) .I t is difficult to completely compare discharge conditions from one gas to another because of the differences in ion gauge response to the several gases, but the matrix contribution to the net ion flux ranged from 0.01 to 10%. Factors controlling these ratios will be described later in this report. Standard conditions of 20-mA tube current and 2.5 x source pressure (corresponding to about 1 Torr in the discharge) were selected as a compromise covering ion flux, discharge stability, and sputter action.
Figure 1. Hollow cathode ion source with cathodic sampling (1) Hollow cathode, (2) cathode block, (3) anode connection. (4) glass body, (5) gas inlet port, (6) auxiliary port, (7) quartz window, (8) heat transfer block, (9) glass cold finger, (10) MS-702 insulators. (11) anode ring
The molecular gases, nitrogen and oxygen, produced very interesting but cluttered spectra. Many molecular species arising from the gas itself and also from combinations with the sputtered matrix, predominated in the spectra and made analytical utility questionable. The discharge was quite stable and, in fact, ran well on room air leaked through the control needle valve. Oxygen might find some specific application where a surface reactive reagent is desired, but the extensive formation of polyoxides elimated its use for survey purposes. Of the “inert” gases, helium is of limited application because of its low sputtering power, and neon offers no significant advantage to justify its high cost. The best general discharge gas has been argon which presents a suitable compromise between sputter yield and ionization energy while exhibiting acceptable spectral characteristics. For an argon discharge in a copper cathode, the major species observed (beyond mass 20) were from Ar+, ArH+, Cu+, and Ar2+, the relative magnitude of which varied with discharge conditions. CuAr+, C U ~ +and , CuO+ were present in smaller amounts. Argon was the fill gas used for the rest of this study. Cathode Material. A series of hollow cathodes was prepared, comprised of over 20 different metals, alloys, and graphites. HCIS spectra were taken for each. Of interest was the analytical capability for direct analysis of each cathode for its matrix and impurity components, but we also wished to investigate the types of species formed in the discharge, as well as determine the suitability of each matrix as a substrate for the analysis of surface films deposited from solution. Representative high gain spectra for the metals are shown in Figure 2 (copper) and Figure 3 (zinc). Traces of metal impurities (-100 ppm) appear in the spectra. Both metals sputter especially well, with zinc yielding the highest matrix-to-argon ion ratio of all the materials tested. Singly charged atomic species, along with some matrix molecules, exemplify the metal spectra. The absence of doubly charged species, except for the matrix and fill gas, simplifies the spectra but hinders some qualitative identification, particularly for monoisotopic elements. Typical alloy spectra are shown in Figure 4 (brass) and Figure 5 (stainless steel). The brass, which shows a very clean region above mass 80, has been useful for the analysis of solutions. Boumans (28) has shown that the relative sputtering rates for the components of an alloy generally follow the sputtering rates of the pure materials. In stainless steel, most of the components have similar sputter rates in argon, with the exception of silver, which has a very high sputter efficiency. The trace of silver shown in Figure 5 , for example, is not detected by spark source mass spectrometry a t a comparable overall sensitivity. ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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I 4
\
/ \
r
\
Figure 2. HClS spectrum of a copper cathode in argon at 20 mA
93-
,
Pd
lo
Z-A-
C6
34 3resLre
38
.@rr
Figure 6. Effect of pressure on matrix and discharge gas ion intensities r
201
Flgure 3. HClS spectrum of a zinc cathode in argon at 20 mA
I"' 33
0 cz 0 Pg'
si
d 20
4'0
Current
Figure 4. HClS spectrum of a brass cathode in argon at 20 mA
Figure 5. HSiC spectrum of a stainless steel cathode in argon at 20 mA
1026
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
60
80
100
'
5
33
ma
Figure 7. Effect of tube current on matrix and discharge gas ion intensities
Effect of Pressure. The fill gas pressure is known to be a critical parameter in hollow cathode emission studies. Because pressure is a factor in sputtering processes, it should also be expected to have an effect on ion intensities obtained from the hollow cathode discharge. With the CS design, pressure measurements via a thermocouple attached a t port 6 (Figure 1) showed that the discharge operated in the approximate pressure range used in emission studies. Pressure measurements were taken, in situ, with the acceleration voltage off. Correlation of tube pressure with the source ion gauge readings allowed subsequent indirect tube pressure determinations The effect of pressure on the 6 3 C ~ and f joArf ion intensities and on the 63/40 ion ratio is shown in Figure 6. The increase in the 63/40 ratio would imply that it is advantageous to operate a t the highest pressure that the pumping
capacity of the mass spectrometer will tolerate. However, the individual ion intensities are dropping off rapidly with increasing pressure, indicating that a compromise must be reached. Pressures of 0.4-0.5 Torr were generally selected. The maximum ion ratio was greater when sampling out the cathode rather than from the anode, indicating a greater population of sputtered ions in the bottom of the cathode. Effect of Current. The CS had an extended current range (-200 mA) due to the cold-finger attachment. The effect of the most useful current region on s 3 C ~ and f 40Ar+ ion intensities and on the 63/40 ratio is shown in Figure 7. Both ion intensities and the ion ratio increase with increasing current. The higher current capability can thus be used to increase both the ion flux and the sputtered-to-argon ion ratio to increase analytical sensitivity. The chief limitation of high current operation appears to be the accelerated sputtering of cathode material onto the glass body between the anode and cathode. For sample films analyzed by scanning methods, the current is held a t reduced levels to avoid too abrupt a removal of the sputtered sample. Cathode Temperature. Temperature measurement was achieved by insertion of a thermistor probe through a hole in the cathode block to f i t flush against the side of the cathode allowing recorder display of time-temperature plots. In this way, the bulk temperature of the cathode could be determined and the effectiveness of the liquid nitrogen cooling evaluated. As shown in Figure l, the cooling relies on conduction through a metal heat sink. That, plus the imperfect contact of the glass cooling finger to the metal conductor, reduces considerably the efficiency of the liquid nitrogen cooling. At a tube current of 65 mA, an equilibrium cooled cathode temperature of 75 OC was indicated. By contrast, without cooling, several types of metal cathodes can melt under these same discharge conditions. At 100 mA, the highest current normally useful for analytical purposes, the temperature continued to rise slowly, even after 30 minutes operation, a t which time it had achieved only 200 "C. Operation a t higher currents accelerates this effect and is presently limited to relatively short on-times. A full range mass scan can be made, however, in about 8 minutes. Resolution. A comparison was made of the relative resolution attainable from the anode vs. cathode sampling modes. The resolution was determined by the separation of half height method ( 1 9 ) , using the formula, RP = dM/w, where RP is the resolving power, d is the separation between two peaks one mass unit apart, w is the peak width, and M is the mass. For the AS, the resolution (under less than optimum conditions) was 600, and that of the CS was 300. This difference in resolution is probably due to the different discharge conditions in the two areas. Ions sampled a t the bottom of the cathode are extracted from the cathode fall, where they may have been formed with any of a range of energies, depending upon where in the cathode fall they were formed. This would be especially true of sputtered atoms ionized in or a t the edge of the cathode dark space, giving rise to different kinetic energies as they are accelerated across the fall. In the case of the anode extraction mode, sampling is from an essentially field-free region. Ionization in this region is thought to be mainly by Penning processes, resulting in an energy spread which would be expected to be less than in sampling from the cathode. The greater resolution offered by the anode sampler can be useful in resolving molecular from atomic ions, but the lower population of sputtered ions near the anode makes cathode sampling preferable for our work. Reproducibility. The HCIS produces an ion flux which is extremely stable, as indicated by the ion heam monitor. The short term fluctuation in beam intensity (90-99%
Table I. HCIS Reproducibility Study for Five Consecutive Mass Scans Using an NBS 810a Steel Cathode m / e So.
R c l std d e v , %
27 28 29 30 32 35 38 39 41 44 45 50 52
32 9 33 37 25 32 4 30 47 30 6 11 6
m / e No.
Re1 std de",%
54 55 57 58 63 65 72 76 80 116 118 120 AV RSD =
15 21 15 16 17 15 45 30 3 26 13 25 21.7
Table 11. HCIS Reproducibility Study For Four Consecutive Mass Scans for Elements Deposited from Solution onto a Copper Cathode m J e No.
Re1 std de",;?
m l e No.
38 50 51 52 53 54 55 56 57 58 59 60 61 62 64 66 67 68 70 71 72 74 75 76 78 86 87 88 89 90 91 92
11 39 13 40 15 23 11 7 12 13 32 15 49 37 28 26 18 11 8 4 17 37 19 16 26 17 16 7 4 7 11 18
102 103 104 105 106 107 109 110 111 112 113 114 115 126 128 130 138 139 147 152 154 155 158 160 164 166 168 174 176 178 180 208
Re1 std dev, 36
8 35 10 20 37 41 32 19 22 29 18 23 3 2 12 22 16 9 10 11 5 6 9 10 15 3 8 7 14 7 6 6 Av RSD = 16.9%
argon species) is within 2%. However, such stability does not preclude significant variation in sputtered sample ions which would not be evident on the monitor. Table I shows that for NBS steel 810a, the elemental precision is only modest. For spark source mass spectrometry scans, a precision of 30-35% is typically obtained for this sample type. The HCIS shows some improvement to 21.7% RSD. Some of the worst offenders appear to result from a drift in response, but no overall pattern emerges. Closer control of discharge parameters is required. Better precision has been achieved using an rf potential with the HCIS ( 5 ) , where 8.7% RSD was obtained for a similar steel sample. ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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:03
m /e
Figure 8. HCIS spectrum of 0.2-ml omnibus sample solution deposited onto a copper cathode. Elements present at 200 ppm in the solution
Figure 9. HCIS spectrum of rare earths deposited onto a copper cathode
A second precision study involved a 0.20-ml omnibus solution of thirty elements, each a t a concentration of 200 ppm deposited onto a copper cathode. The % RSD results of four scans are given in Table 11. The overall RSD for 64 masses avcraged 16.9%. Again, for certain peaks exhibiting large deviations, a drift in the peak heights was observed, rather than random fluctuations. The drift may be in either direction, and may be caused by inhomogeneity of the deposited salt or changes in the mode of entry into the discharge. The net result still represents a significant improvement over analogous spark source mass spectrometry electrical scanning. Sensitivity. A discussion of the sensitivity of a mass spectrometric survey technique can be divided into two basic considerations: 1) the ultimate sensitivity obtainable for the technique; and 2) the relative differences in sensitivity [relative sensitivity factors, RSF (20)] for different elements in the same matrix. These considerations will most likely be different for elements present in a cathode cavity surface film. The sensitivity of the technique will be dependent upon the ease of sputtering and ionization and, in some cases, the volatility of the element of interest. A cooled cathode should yield ions mainly by sputter processes, but if the temperature rises to the point where thermal ionization is significant, relative sensitivities can change. For metal alloys, and particularly for deposited surface films, the intrinsic sputter rate of the matrix material may play a key role in transporting the elements of analytical interest into the discharge. For metal cathodes run a t high gain conditions, HCIS sensitivities for trace elements can be generally considered to be in the 1-10 ppma range for the CS depending upon individual sputter yield. The RSF's between elemental components generally are within a factor of 3, in line with the variation in sputter yields (17). An exception is found in the more volatile components, such as zinc in brass, where the apparent zinc concentration by HCIS can be a factor of 10 higher than that indicated by the spark source. Lead and tin also show such anomalies. The sensitivity of an element in solution, deposited and dried to a residual film in the cathode cavity, is greater than that of cathode constitutents. From spectra of the omnibus solution, an average elemental sensitivity can be estimated a t about 0.1 ppm or less in solution for many of the elements even at low sputtering conditions. Higher discharge current and integration of the total signal of the element from the HCIS may improve this further. The CS ion
source showed greater sensitivity (-100 X) for elements deposited from solution than did the AS model. Figure 8 shows a HCIS spectrum of the omnibus sample solution which was deposited onto a copper cathode. Figure 9 shows a high mass region of a similar mass spectrum after the addition of a series of rare earths (each a t 200 ppm in solution). The near uniform response of the rare earth ions is evident. In general, the RSF's are within a factor of 3-5 of yttrium, approximately that obtained by spark source, a range generally considered good in instrumental analytical methods. Atomic and molecular interferences from the cathode species are not as serious a problem as we had anticipated, because the salt deposited on the cathode appears to significantly reduce substrate contribution. The relatively uniform RSF's obtained for solutions deposited as a residual film, taken with the good sensitivity obtained, make the HCIS technique of considerable interest to us for the survey analysis of solutions by mass spectrometry.
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LITERATURE CITED W. W. Harrison and C. W. Magee, 21st Annual Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, May 1973. W. W. Harrison and C. W. Magee, Anal. Cbem., 46,461 (1974). 8 . N. Colby and C. A. Evans, Anal. Chem., 46, 1236 (1974). E. H. Daughtrey, Jr.. and W. W. Harrison, 22nd Annual Conference on Mass Spectrometry and Allied Topics, Philiadelphia, PA, May 1974. D. L. Donohue and W. W. Harrison, 22nd Annual Conference on Mass Spectrometry and Allied Topics, Philadelphia, PA, May 1974. F. Paschen, Ann. Pbys., 50, 901 (1916). H. Schuler, Z. Pbys., 22, 264 (1921). F. T. Birks. Spectrochim. Acta, 6, 169 (1954). G. Milazzo. Appl. Spectrosc., 21, 185 (1967). E. H. Daughtrey and W. W. Harrison, Anal. Cbim. Acta, 67, 253 (1973). F. Howorka and M. Pahl, 2. Naturforscb., Teil A. 27, 1425 (1972). H. Helm, F. Howorka, and M. Pahl, Z.Naturforscb., Teil A, 27, 1417 (1972). F. Howorka, W. Lindinger, and M. Pahl, lnt. J. Mass Spectrom. /on Pbys.. 12, 67 (1973). J. W. Coburn and E. Kay, Appl. Phys. Lett., 18, 435 (1971). J. W. Coburn and E. Kay, Appl. Pbys. Lett., 19, 350 (1971). C. W. Magee, D. L. Donohue, and W. W. Harrison, Anal. Chem., 44, 2913 (1972). G. Carter and J. S. Colligon, "Ion Bombardment of Solids", American Elsevier, New York. NY, 1968. P. W. J. M. Boumans, Anal. Chem., 44, 1219 (1972). MS-702 Instruction Manual, Associated Electrical Industries, Manchester, England. H. Farrar IV in "Trace Analysis by Mass Spectrometry", A. J. Ahearn, Ed., Academic Press, New York, NY, 1972, Chapter 8, p 265.
RECEIVEDfor review October 18, 1974. Accepted February 4, 1975. This research was supported by NIH Grant No. GM-14569.
