16
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
content. These samples are typical high chloride samples analyzed in this laboratory. Samples 1-12 (Table 11) are estuarine waters obtained from the Moosonee area, Ontario, Canada, where t h e Hudson Bay salt waters mix with local creek waters. Sample 13 is a seawater from t h e New Brunswick coast, Canada. T h e accuracy of the method was further checked by analyzing the NBS (National Bureau of Standards) Standard Reference materials for Mercury in Water No. 1641 and No. 1642. The No. 1642 was run direct and a value of 1.15 ng/mL was obtained compared to the certified value of 1.18 0.04 ng/mL. T h e No. 1641 which is certified at 1.49 pg/mL was diluted 1000-fold and analyzed. A value of 1.53 pg/mL was obtained.
*
ACKNOWLEDGMENT T h e authors thank Y. K. Chau for his comments on the original manuscript and C. Pacenza for her secretarial help.
LITERATURE CITED (1) P. D. Goulden and B. K. Afghan, "An Automated Method for Determining Mercury in Water", in "Advances in Automated Analysis, 1970, Technicon International Congress", Vol. 2, Mediad, Inc., Tarrytown, N.Y. (2) B. W. Bailey and F. C. Lo, Anal. Chem.. 43, 1525 (1971).
(3) T. B. Bennett, Jr., W. H. McDaniel, and R. N. Hemphill, "Advances in Automated Analysis, 1972 Technical International Congress", Vol. 8, Mediad, Inc., Tarrytown, N.Y. (4) A. A. El-Awady, R. B. Miller, and M. J. Carter, Anal. Chem., 48, 110 (1976). (5) "Method fw Chemical Analysis of Water and Wastewater", EPA Publication No. EPA-625/6-74-003, U.S. Environmental Protection Agency, Office of Technology Transfer, Washington, D.C. 20460. (6) "Analytical Methods Manual", Inland Waters Directorate, Water Quality Branch, Ottawa, Canada, 1974. (7) P. D. Goulden and B. K. Afghan, "An Automated Method for Determining Mercury in Water", Technical Bulletin No. 27, Inland Waters Branch, Department of Energy, Mines, and Resources, Ottawa, Canada, 1970. (8) B. K. Afghan, P. D. Goulden, and J. F. Ryan, Water Res., 6, 1475 (1972). (9) E. A. Jenne and P. Avotins, J . Environ. Qualify, 4 , 427 (1975). (IO) John Carron and Haig Agemian, Anal. Chim. Acta, 9 2 , 61 (1977). (11) "The Sea", M. N. Hill (Genl. Ed.), Volume 2, Interscience Publishers, New York, N.Y., 1963. (12) G. L. Baughman, J. A. Gordon, N. L. Wolfe, and R. G. Zepp, "Chemistry of Organomercurials in Aquatic Systems", EPA-660/3-73-012, National Environmental Research Center, Office of Research and Development, U.S. E.P.A., Corvallis, Ore. 97330. (13) I. R. Janasson, "Mercuy in the Environment", Gedogical S w e y of Canada, Papet 70-57, Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, Canada. (14) "The Industrkl Auto Analyzer", Manual TNO-0210-00, Technicon Industrial Systems, Tarrytown, N.Y. 10591 (15) Haig Agemian and A. S.Y. Chau, Anal. Chim. Acta, 7 5 , 297 (1975). (16) J. F. Kopp, M. C. Longbottom, and L. B. Lobring, J . A m . Water Works ASSOC.,64, 20 (1972).
RECEED for review May 16,1977. Accepted October 11,1977.
Sputter-Atomization Studies with a Glow Discharge C. G. Bruhn' and W. W. Harrison* Department of Chemistry, University of Virginia, Charloftesville, Virginia 2290 1
Cathodic sputtering in a glow discharge is studied as a means of atomization for atomic absorption spectrometry. A scanning electron microscope Is used to examine the microstructure arising on the cathode surface during ion bombardment. The effects of discharge gas, sputter time, current, pressure, and cathode material are examined. The analysis of Ca, Mg, Zn, Au, Ni, and Sn in solution residues shows detection limits ranging to a few ng with precisions of 3-8 %
.
Although the glow discharge is one of the oldest spectroscopic sources, both for photons and ions, it has never become a major tool for t h e analytical chemist, other than in its role as a hollow cathode line source for atomic absorption. The cathodic sputtering phenomenon, central to the glow discharge action, can be analytically useful, however, as an atomization ( 1 , 2 ) ,excitation ( 3 , 4 ) ,and ionization (5,6) source. A cursory consideration of the discharge reveals a simple gas diode operation, but this apparent simplicity can be deceptive. No unified theory has evolved to explain fully the complex processes (e.g., excitation, ionization) involved. Indeed, many theoretical treatments ignore completely the aspect of perhaps most interest t o the analytical chemist, that of cathodic sputtering. The glow discharge was early recognized as a possible solids atomization source for atomic absorption. Metals and alloys
' Op leave from Departamento de Anilisis Instrumental, Escuela
de Quimica y
Farmicia, Universidad de Concepcibn,Concepcih, Chile. 0003-2700/78/0350-0016$0 1.OO/O
(2, 7-9) have -een sputtered and anL-jzed directly against metal standards. The analysis of elements in solution residues (10,11) has also been reported. Other workers (12-14) have used atomic absorption t o study ground state atomic populations as part of plasma diagnostics or sputter investigations. Interest in our laboratory has centered around the use of the glow discharge as a source for optical emission (15, 16), more recently for solids mass spectrometry (6, 17, 18), and, in this report, for atomic absorption. Our goal is to learn more about the basic phenomena occurring within the discharge and subsequently to apply this information to analytical problems. The cathodic sputter surface studies described here are followed by preliminary data demonstrating some analytical possibilities of the glow discharge as an atomization source.
EXPERIMENTAL Glow Discharge Source. A low pressure glow discharge
source operating in the abnormal mode (19,20) was used. The design of the source (Figure 1)shows a glass envelope consisting of two 3-cm i.d. glass joints (Kontes Glass Co.) with a Viton O-ring vacuum seal. The source has two vacuum ports. The lateral one (1)acts as the main exhaust port for the glass chamber, both for initial evacuation and for flow operation. The upper port (2) has a brass tubing (3) of 0.15-cm i.d. inserted concentrically through a 0.63-cm Cajon Ultra Torr fitting which is coupled to the Pyrex body of the glass envelope. Through this tubing, the discharge gas can be continuously bled into the source chamber for flow operation. This tubing also serves as the anode, centrally positioned at 2.3 cm above the bottom of the cathode. The cathode (4) is a replaceable copper or graphite hemicylinder of 1.80-cm length, mounted on a holder ( 5 ) of machinable glass ceramic
D 1977 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
P (Torr) : 0.72
1.50
3.00
17
5.00
CDS:*1.5mm
-01 mm
-
-
NG : - 2 0 cm
-1 O c m
*IOmm
-02mm
Figure 2. Negative glow (NG) and cathode dark space (CDS) configurations at four discharge pressures. 30-mA discharge current
mogeneous surface microstructure on each cathode specimen as a result of the sputtering effect. The glow discharge source was operated at pressures ranging from 0.5 to 5.0 Torr in argon, at currents of up to 50 mA, and at voltages up to 740 V. A 1.5-Torr source pressure was selected as optimum, representing a compromise between analytical sensitivity and the discharge stability.
RESULTS AND DISCUSSION Figure 1. Glow discharge atomization source. See text for details
(Macor, Corning Glass Works, Corning, N.Y.). Electrical contact to the holder is made by a small machine screw threaded into a 0.40-cm diameter copper rod (6) which connects at the bottom of the envelope to a brass rod (7) through a ceramic insulator (8) and a Nylon 1.90-cm Ultra Torr fitting. A 7-mm 0.d. Pyrex tubing shields the copper rod, and a disk of lava (9) insulates the machine screw to prevent arcing from the electrode. The cathode-anode separation allows radiation from a hollow cathode lamp t o be passed through the negative glow region of the discharge localized near the cathode surface. The atomic vapor sputtered from solution sample residues deposited on the cathode surface is sampled using a single-beam atomic absorption spectrometer. Two quartz-windowed tubes (10) (4.0-cm long, 1.3-cm i.d.1 provide an optical path for the radiation through the sputtered vapor in the discharge plasma. The windows are sufficiently removed from the sputter site to avoid significant deposition problems. The glass envelope was mounted on an Ealing vertical and transverse motion carrier which provided translation along two axes for optical alignment with the hollow cathode lamp and the spectrometer entrance slit. The radiation from the hollow cathode lamp was modulated a t 260 Hz with a mechanical light chopper, P.A.R. Model 125 (P.A.R., Princeton, N.J.). A Jarrell-Ash 0.5-m spectrometer, a P.A.R. Model 122 Lock-In Amplifier, and a Sargent Model SRG recorder form the basis of the atomic absorption system. The glow discharge source was powered by a Kepco model BHK regulated power supply. The pressure was monitored on the gas inlet line by a thermocouple gauge with a Gas Control Unit (Barnes Engineering Company) modified to allow a flow mode operation of the source. Cathode S u r f a c e Examination. An ETEC-Autoscan Scanning Electron Microscope (SEM) was used for high resolution examination of the cathode surfaces. The SEM was operated in the secondary electron mode, with a 20 kV-accelerating voltage for the electron beam and 45' for the tilt angle of examination. Procedures. Hemicylindrical cathode specimens were prepared from 0.95-cm o.d. copper tubing of 99.9% purity, surface cleaned by rinses in 10% "OB, deionized-distilledwater, acetone, and ethanol to remove surface contaminants, and dried at 110 O C for 2 h. Graphite cathodes were prepared from high density 0.77-cm rods (U-7, Ultra F purity, Ultra Carbon Corp.), drilled, cut into hemicylindrical pieces of 1.75-cm length, and cleaned with distilled water. Standard solutions were prepared from reagent grade chemicals for the elements examined and a 10-pL portion of the test solution was transferred to the cathode surface by a Hamilton microsyringe. After evaporation to dryness under an infrared lamp, the cathode was placed in the glass-ceramic holder, and the system evacuated and flushed with the selected fill gas. The pressure was adjusted by a Nupro needle metering valve before striking the discharge. Blanks were run for each cathode. This also produced a ho-
S u r f a c e Studies. In our previous studies (21) with a hollow cathode discharge (HCD), various types of surface features had been noted to form as a result of sputtering. The extent to which this microroughening occurs can influence analytical results, probably by affecting the net surface are? and the sputter rate of analytical residue films. Over 30 years ago, cone-like formations were noted (22) and with the subsequent availability of the SEM, more investigators have examined the surface effects of ion sputtering (21-29). An examination of the literature shows that there is no unified theory to explain these phenomena, although a recent review (29) attempts to correlate some of the studies. It is generally agreed that the surface microfeatures arise from nonuniform sputter yield, but the cause of this nonuniformity is variously attributed to localized surface precipitates, grains, inclusions, impurities, dirt, scratches, and surface oxides. Most surface studies have used a directed ion beam in a high vacuum situation where such critical parameters as ion energy, ion dose, and bombardment angle can be controlled. T h e higher pressure and complex conditions of a gas discharge make such control impossible. T h a t coupled with the significant redeposition which occurs leads to often unusual surface topography for the sputtered cathode. The characterization of this microrelief can be important for analytical work, as in the present study. Our glow discharge (GD) source. using an open ended, hemicylindrical cathode, has geometrical flexibility which makes it more suitable than the HCD for ari atomization source. As shown in Figure 2, the GD at the 1.5-Torr operating pressure is characterized by a dark space region close to the inner wall of the cathode and a negative glow region which fills the hemicylinder and spreads somewhat above it. Large changes in sensitivity and reproducibility were observed periodically and appeared to correlate with the observation of changes in the appearance of the cathode surface; this was noted most often with copper cathodes. The surface history of the cathode seems to be of great significance, since the cathode is the matrix support for sample films exposed t o subsequent sputtering in the discharge. Cathodic sputtering produced significant cathode surface changes in our GD source. A number of factors affected these surface phenomena: fill gas, cathode material and its recent surface history (preparation, cleaning, and net sputter time), discharge current, and gas pressure. among others. In investigating the GD as an atomization source, a study of these sputter effects on the cathode surface was first carried out. Fill Gas Effect. Copper cathodes were sputtered a t 20 mA and 1.5 Torr with He and Ar under discharge conditions
18
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
a Figure 3.
b
6
Effect of He vs. Ar as a sputter gas. Copper cathode, 2 0 0 0 X . (a) Unsputtered; (b) He, 15 min; (c) Ar, 5 min
known to yield good absorption intensity of film residues deposited on the cathode surface. Figure 3 shows the surfaces observed a t 2000 magnification. With He, after 15 min of sputtering (Figure 3b), a series of growing formations appeared on the cathode surface; fewer striations are evident than on an unsputtered surface (Figure 3a). With Ar after only 5 min of sputtering (Figure 3c), the cathode surface shows formations which are larger and sharper than the ones observed with He. These structures are reported to originate in grains or inclusions (23, 25, 27) which are randomly distributed on the cathode material and have lower sputtering yield than the rest of the cathode surface, thus acting to protect the underlying material. It is clear that Ar produces greater sputtering than does the lighter mass of He. The difference in excitation potential between the two gases is probably not important, because most of the sputtered species are ejected as single ground-state neutral atoms (28) with only 1-2% ions and excited neutrals (30). From 1-10% of the sputtered neutrals may be subsequently ionized by Penning processes (30). N e t Sputter Time Effect. Poor reproducibility in the absorption measurements when sampling from fresh unsputtered copper cathodes disappeared when the cathodes were sputter-cleaned for identical times before a sample solution was deposited. Given the large change in surface area and the dependence of sputter yield on ion bombardment angle (31, 32), it is reasonable to expect cathode surface topography to affect the absorption intensity. We investigated surfaces generated after different lengths of net sputter time and found information useful in formulating a sputter cleaning procedure. Figure 4 shows a sequence of surface changes after various argon sputter times. At 3 min (Figure 4a), sputter etching has reduced the striations observed in a normal cathode surface; a number of surface irregularities become apparent. At 5 min (Figure 4b), sputter etching produces a surface crowded with small cones or incipient hillocks. At 10 min (Figure 4c), the formation of cluster-type structures of small hillocks is evident. At 60 min (Figure 4d), the growing structures appear as hillocks, which seem to be a favored surface configuration. Preferential surface erosion and transport of sputtered material appears to deepen the surface level with the formation of channels; the hillocks then seem to have grown in size relative to the new surface level. In addition, redeposition of sputtered material also contributes a rounding effect on the hillock surface. Continued sputtering produces no further significant change in surface topography except an additional rounding effect of these cones. Correlation of the surface sputter characteristics of the GD to ion bombardment of flat surfaces (25) shows some common features, notably the cones, spires, or hillock structures easily observable with an optical or electron microscope. Naviniek (29) reports that ion bombardment induced surface changes are not evident until a n ion dose of about 1019 ions cm-' is
Figure 4. 5 0 0 X , 20
Effect of increasing argon sputter time. Copper cathode, mA. (a) 3 min; (b) 5 min; (c) 10 min; (d) 60 rnin
accumulated. We find that an argon ion dose of 2.7 X lo" ions cm-' produces SEM detectable surface structure on our copper cathodes. The initial rapid structure growth probably arises from surface areas which have lower sputtering rates than the rest of the matrix surface. For the GD, the net result is that significant microchanges take place within the first several minutes of sputtering, but thereafter the rate of change is slower, reaching an almost steady-state condition after 1 h. This steady-state condition may be due to a lowering of the sputtering rate of the high yield surface regions as they recess deeper below their surroundings because fewer atoms are able to escape the surface (28). Although copper cathodes can be sputter or acid cleaned and reused, there is enough sample residue embedded by redeposition to create a detectable background. Therefore, new cathodes were used each time, and initially treated by a short (1.5 min) sputter cleaning procedure before a solution was deposited on it. Using the same conditions as during analytical sputtering, this provided surface cleaning (removal
A N A L Y T I C A L CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
Q
b
19
6
Figure 5. Effect of higher discharge current. Copper cathode, argon, 30 m A . (a) 1 h, 5 0 0 X ; (b) 2 h, 5 0 0 X ; (c:) 2 h, 2 0 0 0 X
of oxide or other contaminating layers from the cathode surface), reproducible etching, and surface homogeneity. Effect of Discharge Current. An increase in the discharge current enhances electron-atom collisions (fast and slow electrons with rare gas and sputtered atoms) with a net increase in excitation and ionization of both the fill gas and the sputtered species. T h e sputtering rate increases with both ion energy and flux (30). The effect of a higher GD current is significant because of the increase in the number of ions bombarding the surface. Copper cathodes were run at 10, 20, 30, and 40 mA for 10 min and exhibited surface changes ranging from barely perceptible a t 10 mA to extensive a t 40 mA. A comparison of cathodes run at 20 and 30 mA (see Figure 4d vs. Figure 5a), where a 1-h sputter time is used to accentuate the sputter effects, shows similar conical structures, except that they are larger and have sharper tips at the higher currents. After 2 h (Figure 5b), the surface shows crowded, spire-like structures; their height ranges between 6-15 gm. Figure 5c, at higher magnification, shows the tips of these spires to be capped with material which may have been sufficiently sputter resistant as to initiate the formations. The dynamics of surface changes seem to be greater at larger discharge currents as shown in Figure 5, perhaps partially as a result of higher energy ions bombarding the cathode surface. Increasing the current in an abnormal glow discharge also increases the discharge voltage, resulting in higher average acceleration potentials for the Ar ions. However, when raising the current from 20 to 30 mA, the discharge voltage increased only -lCkl5%, making it doubtful that this effect alone caused the difference in surface structure. The large increase in the numbers of ions striking the surface must play a larger role. Effect of Gas Pressure. Normally a glow discharge operates a t a pressure such that the incident ions and sputtered atoms have a small mean free path compared to the dimensions of the tube. Multiple collisions between the gas particles give rise to a large energy spread (with energies up to the cathode fall potential) among the bombarding ions and to an undetermined angle of incidence. Backdiffusion of a large fraction of the sputtered atoms, estimated as high as 90% at 0.1 Torr (30),tends to complicate the cathode surface microstructure. The effect of pressure on surface structure was determined by studying surface effects a t constant current from 0.72 to 5.0 Torr of argon. The sputter effect was much more pronounced at the lower pressures because of' the accompanying rise in discharge voltage. Thus at 0.72 Torr, the 740-V discharge voltage yields higher energy sputter ions than does the 300-V discharge a t 5.0 Torr. Less redeposition would be expected also at lower pressure due to the larger mean free path. As a n illustration of the effect of gas pressure, Figure 6 shows the surface obtained at 1.0 Torr after 1 h of sputtering vs. the 1.5 Torr used with Figure 4d. The cathode surface
Figure 6. Sputter damage on copper c a b c d e from argori bombardment at 1.0 Torr. 5 0 0 X , 20 m A
(Figure 6) shows signs of intensive sputter effects aiid uffers similarities to the spires observed at higher discharge current i show (Figure 5). The conical structures here are 3-t; p ~ i arid some cluster-type tendencies. If one equates sputter rate with sensitivity, it is clear that optiiriurii coriditiuris should be the lowest possible operating pressure at highest feasible currents. While this does yield good sensitivities fur a metal matrix, our interest was in solution residue sariiples where riiilder sputtering conditions proved advantageous, particularly for precision. Therefore, coriipruriiise discharge coriditiuns were selected. Cathode Base Muteriul. 'l'he use uf copper arid spectroshows a sharp contrast scopic graphite as cathode rnateriol>m in sputter characteristics. Copper has one of the highest sputtering yields under bombardment with Ar+ while graphite has one of the lowest (33). Kedepusitiori of sputtered inaterial in the high pressure discharge precludes the deteriiiiiiation of real sputter rates, but the use of' surface changes as indicators of sputtering effect is ont: qualitative approach. Figure 7 shows a sequence of three different graphite cathodes to compare to the previously depicted Cu cathodes. 'I'he first (Figure 7a) correspunds to a norniiil unsputt ered cathode surface, and the other two were $puttered under the same conditions as were the cupper specillleiis, for 1 5 rriin (Figure 7b) and 1 h (Figure 7 c ) , respectively. 'I'his shuws that sputtering does n u t substantially modify the suri'alcx tupogrophy of a graphite cathude. Even after loiig teriii >puttering (1 h), only a slight siiiuothiiig cf'fect uf the burlace becuiiies apparent. 'I'his obsrrvatio~ii.5 i r i wntrast t u the surface observed on a sputtered coppcr spwiinen. The net sputter removal o f cathide material i l l g l o w discharge is much less than would b e c&ulatctl t r o i i i hnuwii sputter yield and ion current considerations because uf the high gas phase collision rate and resultant redeposition (backdiffusion) of sputtered atoms. Copper cathodes were subjected to sputter periods of 30, 60, arid 90 iniii arid the net
20
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
Cl Figure 7.
e
0
Effect of argon sputtering on graphite, 500X. (a) unsputtered; (b) 15 min; (c) 1 h
weight loss was determined. For the conditions used-30 mA and 1.5 Torr-a net average removal rate of 1.2 gg s-l resulted; about 50% of the cathode loss was deposited on the ceramic cathode holder (see Figure 1). Taking into consideration the sputtering rate and total ion dose, an "effective" sputter yield can be calculated (33,34). The average of six determinations was 0.066 atom ion-' with a variation of about 10%. If this figure is compared to sputter yields determined in low pressure ion bombardment experiments (28,33),it becomes clear that only a small percentage of the sputtered ions escape backdiffusion to the cathode. The average ion energy striking our cathode is not known but, because of collisions, is certainly considerably less than the discharge voltage. Assuming a 200-eV average ion energy and taking Wehner's sputter yield for Cu as 1.10 atoms ion-l for 200-eV argon ions (33),our net sputter yield at 1.5 Torr is only 6.0% of this, thus indicating a redeposition of 94 %. This is not out of line with estimations for other discharge sources (35, 36). Similar studies for graphite cathodes gave unexpected results for a series of nine determinations of sputter yield. I t is known that graphite sputters more poorly than copper, as indicated by our average net removal rate of 0.25 wg s-l or 21% of the figure determined for copper. However, our calculated net sputter yield for graphite, given its low atomic mass, is nearly identical to that for copper, 0.065 atom ion-'. If Wehner's figure of 0.12 atom ion-' for 500 eV argon ions (28) on graphite is extrapolated to 0.06 atom ion-l a t 200 eV, it suggests that no redeposition occurs on the graphite cathode. Although the collisional cross section for sputtered carbon atoms would be smaller than for copper atoms, it seems unlikely that backdiffusion is as low as indicated here. T h e paucity of data on graphite sputter yields is a serious limitation for such considerations. For sample residue films deposited on the cathode surface, copper has the advantage of a larger sputtering rate which enhances the transmission of the sample film into the discharge plasma. But this high sputter rate creates possible problems of changing surface geometry, including massive redeposition. Sample atoms may become buried beneath other sputtered and redeposited atoms of the cathode material. Resputtering can contribute to memory effects, if subsequently a second sample film is deposited and sputtered on the same cathode surface without any pretreatment. The use of acid cleaning followed by a sputter cleaning period between two consecutive samples, tends to modify the surface of the copper cathodes, but does not restore completely the original sensitivity and reproducibility. Graphite, with its low sputter yield, does not contribute significantly to the transmission of a sample film into the discharge plasma, hut does provide a simpler matrix surface, with less memory contribution than copper. A further advantage is its relative inertness to acidic sample solutions. Graphite cathodes can be cleaned and reused if desired, because no significant surface microstructure is present to be altered or destroyed by sputter and/or acid
-
I rnin TIME
d
Figure 8. Absorption signals for 60 and 100 ng Mg. 1.5 Torr, 30 rnA. ( A ) Discharge on; (B) discharge off
cleaning. We have not used pyrolytic graphite as a cathode, but its properties suggest attractive possibilities in sputter studies. Analytical Studies. Standard solutions of Mg at the low ppm range were used to optimize experimental parameters including cathode geometry, choice of filler gas, gas pressure, and discharge current. Several types of cathode geometries were studied (flat disk, tubular, planar), but a hemicylindrical cathode provided the best combination of sensitivity and signal-to-background ratio. Figure 8 shows, for optimum discharge conditions, the absorption signals of sputtered magnesium solution residues deposited on copper cathodes. The initial absorption peak is followed quickly by a drop in the absorption intensity as the solution residue film becomes sputtered off the cathode. The total time period for sample release from the cathode surface is from 0.5 to 1.5 min depending on the element and concentration. The analytical element can usually be completely removed (below detection limits) from the cathode surface by allowing a sufficient net sputter time or by increasing the post-analysis discharge current. However, reuse of the same copper cathode after successive sputtering and/or acid cleaning procedures has a deleterious effect on the reproducibility and intensity of the absorption signals. T h e copper cathodes were quite inexpensive and considered disposable. However, because of the low sputter rate previously described, graphite cathodes were reusable after a sputter cleaning time period of 2 min with a high discharge current (50 mA). T h e variation in the absorption intensity of successive samples was within the range observed when a fresh cathode was used each time. A possible advantage of such cathode reuse, which we have not explored, is that an absorption chamber might be designed with a small syringe-accessible sample addition port so that successive samples could be run without removal of the cathode. Both Cu and graphite were used as cathodes in this study with no significant matrix interferences on the absorption signals as determined during blank runs. In selecting a cathode for solution samples, a nonporous material, such as
A N A L Y T I C A L CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
I. Elemental D e t e c t i o n L i m i t s for Glow Discharge Sputter S o u r c e
Table
Element Ca
Mg Zn
ALP Ni Sna
A b s o r p t i o n wavelength,
422.7 285.2 213.9 242.8 232.0 286.3
nm
D e t e c t i o n limit, n g
2.3 2.9 6.4 111 75
13
a G r a p h i t e c a t h o d e used.
Cu, offers some sensitivity advantages in that the sample residue films remain entirely on the surface. Also, the high sputter yield of Cu may enhance the transmission of the sample residue into the discharge plasma. Actually, little information is available concerning the sputtering and dissociation of molecular species in a glow discharge. The solution residue may become sputtered independently of the cathode material. An area calling for further study is the comparison of elemental sensitivities when the analytical element is bound to a variety of other elements or molecular groups. Detection Limits. A series of analytical curves was obtained from standard solutions for Ca, Mg, Zn, Au, Ni and Sn. A linear analytical response was obtained for Sn in which a single graphite cathode was used for the entire series of standards. Some analytical curves (Mg) exhibit curvature at the higher concentrations, as observed by other investigators (10, 1 1 ) . T h e hollow cathode lamp emits a narrow spectral line compared to the absorption line in a flame, but, as pointed out by Gough (8),the absorption line width of the glow discharge will be comparable to the hollow cathode emission line, possibly leading to nonlinearity in working curves. The detection limits (3x std dev bkg (37)) obtained for six elements are shown in Table I. The poorer detection limit for Au was surprising, given its high sputter yield (33). However, it is hazardous to extrapolate sputter values for pure metals to those obtainable for the same element in a molecular matrix. Copper cathodes that were reused-after being acid cleaned and sputter conditioned-showed a drop in detection limit of about 15-25% with respect to the initial run. Omission of the 1.5-min sputter cleaning period for a fresh Cu cathode resulted in up to 50% poorer detection limit. T h e sputter-conditioning will remove surface contamination, such as a thin layer of metal oxide distributed irregularly across the cathode surface. These less conducting oxide areas can create variations in the surface current density, resulting in non-uniform sputtering. Such oxide or contamination layers are eliminated by sputter-etching the fresh cathode surface until the clean metal becomes apparent; usually 1.5 min suffices. Under these conditions, sample residue films deposited on this clean metal surface are transmitted more uniformly into the discharge plasma. Reproducibility. The precision of the glow discharge source for atomic absorption was investigated by sputtering, under optimal conditions of pressure and discharge current, a series of 10 consecutive solution residue films, each containing 0.6
21
p g of Sn. A single graphite cathode was used. Between each sample run, the cathode was sputtered to (,heck for any background contribution. A relative standard deviation (RSD%) of 6.90% was obtained; for a similar experiment carried out with 0.1-pg residue films of Mg, but using a series of sputter-cleaned copper cathodes, the RSD% was 2.80%. These results are in close agreement with precision estimates in previous absorption work with glow type discharges (10, 11).
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RECEIVED for review August 29, 1977 Accepted October 18, 1977. Financial support from the National Institutes of Health Grant GM-14569 is gratefully acknowledged.
