Sensitivity and Cathode Geometry of the Hollow Cathode Ionization Source J. R. Wallace‘ and D. F. S. Natusch Department of Chemistry, University of Illinois, Urbana, Ill. 6 180 1
B. N. Colby2 and C. A. Evans, Jr.’ Materials Research Laboratory, University of Illinois, Urbana, Ill. 6 180 1
Previous work has shown the hollow cathode ion source to be a highly promising method for the mass spectrometry of inorganic solids. This work details the effect on cathodic ion intensities of cathode geometry. The cathode bore depth and diameter were found to dramatically affect sensitivity with the shallowest and largest bore producing the greatest cathodic ion Intensity.
The hollow cathode is best known among chemists as an intense light source for atomic absorption spectrometry. However, Harrison and Magee (1) and Colby and Evans (2) have shown that mass spectrometric analysis of the ions produced in a hollow cathode discharge offers special advantages as an analytical tool. These advantages include a particularly simple spectrum consisting almost entirely of singly charged, monatomic ions representative of the cathode material, sensitivity in the parts-per-million range, and relatively nonselective ionization of the cathode material. Furthermore, since the analyte is introduced into the discharge by sputtering of the cathode surface, depth profiling of individual elements is a likely possibility (3). Previous investigations of the hollow cathode ionization source (HCIS) have employed cathodes with long, narrow bores similar to those found in hollow cathode lamps. However, such a design is known to minimize optical emissions from charged species ( 4 ) and thus may not be ideal as a source of ionic species. This article examines the relationship between cathode geometry and the sensitivity of the HCIS to determine the optimum geometry for ion production. Cathodes of varying length, bore diameter, and bore depth were run in the HCIS and the resulting ion intensities were measured. Ions were also sampled at varying radial distances from the center of the cathode bore. EXPERIMENTAL The design of the hollow cathode ionization source employed in this study is shown in Figure 1. Ions produced in the discharge region are drawn through the circular anode aperture (0.020-inch diameter by 0.010 inch deep) and are accelerated through a potential drop of about 4 kV to ground potential. The ion beam is focused onto the object slit of a mass spectrometer (Associated Electronics Industries, MS902) by an einzel lens and deflection plates. Gas escaping through the anode aperture is pumped through eight 0.375-inch diameter openings in the ground plate perimeter. The glass shields are required to confine the discharge to the cathode bore. A small spring lodged in the gas inlet holds the glass shield steady and thus ensures an even gas flow. T o provide adequate pumping speed for the gas loads utilized, the standard AEI source housing and vacuum system were replaced with a custom-made housing fitted with a 4-inch diffusion pump. During operation, cooling water was circulated as illusPresent address, Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720. Present address, E. I. du Pont de Nemours, Instrument Products Division, 1500 s. Shamrock Ave., Monrovia, Calif. 91016. 118
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
trated to prevent overheating of the Viton 0 rings and Nylon insulators. The operating cycle was begun by selecting an electrode at random and securing it to the demountable cathode holder with a set screw. The discharge chamber and gas supply line were then evacuated to less than 10-fim pressure. Argon, purified over a Ti/Zr getter alloy, was metered into the discharge region with a Nupro “S” series needle valve. The discharge was initiated by applying 800 V between the anode and cathode and gradually increasing the argon pressure. The discharge, once started, normally operated between 20 and 100 mA corresponding to an anode-to-cathode potential drop of about 350 V. The parameters optimized were the discharge current and pressure, the accelerating potential, the einzel lens potential, and the potentials on the Y and Z deflection plates. (Two sets of deflection plates were adjusted, one in the mass spectrometer proper and one on the source.) Spectra were measured after the discharge had run for a minimum of 10 minutes in order to ensure a stable ion current. A more detailed description of the operating procedures and apparatus is given by Wallace ( 5 ) and Colby and Evans ( 2 ) . Five separate series of stainless steel hollow cathode electrodes were prepared. Series (a). Identical. 0.250-inch outer diameter, 0.125-inch bore diameter (centered in rod), 0.375-inch bore depth, and 1.175-inch length (0.008-inch cathode-to-anode distance). Series (b). Bore position uaried. Identical to (a) but with bore center offset from the center of the rod by 0.005-inch increments. Series (c). L e n g t h uaried. Identical to (a) but with variable length from 1.150 to 1.18 inches. Series (d). Bore diameter uaried. Identical to (a) but bore diameter varying from 0.047 to 0.218 inch. Series (e). Bore d e p t h uaried. Identical to (a) but with bore depth varying from 0.000 to 0.66 inch. T o minimize the number of variables, each series of electrodes was run at a constant discharge current: 25 mA for series (a) to (d) and 20 mA for series (e). After the constant current measurements, the discharge current was also optimized for a limited number of cathodes in order to measure the maximum obtainable ion current. For series (e) 56FeCintensities were read directly from the spectra which were recorded oscillographically at a scan rate of 1.1 middecade. For the other series, the j6Fe+ intensity was measured with the Faraday cage detector in the static mode. The j6Fe+ intensity was monitored since it was the most intense cathodic line in the spectra and, hence, less prone to interferences. The identity of this line was also confirmed by the proper isotopic ratios in the spectra. In addition, examination of spectra obtained with Cu and T a cathodes indicated that no gas phase species (e.g., 40Ar160C) were present with enough intensity to interfere with the strong 56Fe+ line.
RESULTS Relative j6Fe+ intensities of 62, 65, 78, 63, and 71 were measured for 56Fe+for a series of five equivalent electrodes run consecutively during a single day. Assuming a normal distribution of errors, this range implies that a single measurement should be within 50% of the true value at a confidence level of 90% (6). Since none of these electrodes was used more than once, each measurement refers to a freshly machined surface. Offsetting the center of the bore by as much as 0.045 inch generated no significant change in the 56Fe+intensity
Nylon
0
coot ng wotei
-To
I I
Pump
I
I
Deflect on Plates meter
I
I
C rculor
Calhode-
Cathode Ca-hode Holder
Flgure 1. The hollow cathode ionization source
Flgure 3. 5eFef intensity as a function of bore diameter
\\\
_100 O0I
160 -
0 2 5 m A Discharge Current 0 Optim!ied Dtrchorpe Current
140
-$ -
~
120-
P
i
175mA
I 401
i
.
q,,
I
aia
0008
odi2
I
i
O
lGd
IOb
bl' 0
010
Om
030
040
050
060
0
Y 070
Bore Depth (inches)
0016
oozo
0024
adze
Figure 4. 56Fe+ intensity as a function of bore depth
Cathode to Anode Distance (inches)
Flgure 2. 5sFe+ intensity as a function of anode to cathode distance as long as all bores were offset in the same angular direction (Le., no rotation about the cathode axis). However, rotation of the electrode about its axis caused the 56Fef intensity to decrease 100-fold for those bores offset from the center by 0.030 inch or more. Presumably, this was because of incomplete axial alignment of the cathode with the anode aperture. The typical variation in j6Fe+ intensity with cathodeto-anode distance is illustrated in Figure 2, which also includes the effect of optimizing discharge current. The cathode-to-anode distance of 0.005 inch tolerates a maximum discharge current of 25 mA before shorting the cathode to the anode, while larger separations permit stable discharge currents up to 100 mA. Over the range of distances measured, there is little change in jsFe+ intensity except for possibly a slight increase for the shortest separation. The 56Fe+intensity clearly depends more on the discharge current than on the anode-to-cathode separation. Figure 3 illustrates the effect of bore diameter on 56Fe+ intensity, as well as optimized discharge current for two electrodes. As can be seen, the iron intensity increases steadily by a t least a factor of 1000 as the bore diameter increases from 0.0469 to 0.218 inch. Although discharge cur-
rent has a small effect, bore diameter is a far more dominant factor in determining ion intensities. The relationship between bore depth and j6Fef intensity is shown in Figure 4. Decreasing the bore depth from 0.658 to 0.125 inch causes the iron intensity to increase by a factor of IO4. However, for a cathode with no bore (bore depth = 0), zero ion current was detected in the mass spectrometer; after removal from the source, this electrode showed a burned ring around the outside near the edge of the glass shield but showed no evidence of sputtering from the region near the anode aperture. Although increased discharge current does produce a more intense ion beam, bore depth is clearly a far more critical factor. Repeating any of the series with the same cathodes produced the same relative variation in 5sFe+ intensity, although the absolute intensities often varied by more than a factor of two between runs. Although the relationship between discharge current and ion beam intensity was not explored for every electrode, the jsFe+ intensity typically increased with increasing discharge current up to a plateau; further increasing the discharge current often caused the iron intensity to decrease. After this latter mode of operation, the cathodes would appear burned on the outside as well as on the inside of the bore, suggesting that the discharge had begun to move away from the bore. ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
119
For the cathodes described in this study, as well as for Cu and T a cathodes run separately, weak cathodic ion signals correlated with strong residual gas signals (N+, 0+, NHz+, H20+) which in some cases became as intense as the cathode matrix element. DISCUSSION Since the HCIS ionizes most cathode materials with approximately the same efficiency ( I , 2), the 5sFe+ intensity represents the total cathodic ion intensity. Furthermore, since the background noise in most mass regions is independent of mass, the iron intensities shown in Figures 2 to 4 are inversely proportional to the minimum detectable concentration; the exceptions are volatile elements such as Zn or Hg which are selectively evaporated from the cathode and ionized ( 2 ) . It is apparent from Figures 3 and 4 that optimization of the bore diameter and depth is essential for best sensitivity and, indeed, is much more critical than optimizing the discharge current. The anode-to-cathode distance may also have indirect importance in determining the maximum discharge current tolerated before the cathode begins to short to the anode. Extrapolating the results of Figures 3 and 4 suggests that a flat cathode with no bore should produce the most intense ion beam. Instead, such a cathode produced no ion beam, apparently because the discharge did not occur near the anode orifice. The cathodes employed were 0.250 inch wide and separated from the anode by 0.008 inch while the orifice was 0.020 inch in diameter; thus, for the flat cathode, the pressure close to the orifice was significantly less than in other sections of the source, and the discharge occurred preferentially in other sections of the source. To some extent, this situation can be corrected by using a greater anode-to-cathode distance. The data described in this study are entirely consistent with the numerous investigations of the optical properties of the hollow cathode discharge (7). Mitchell, for example, has shown that in a hollow cathode discharge the concentration ratio of charged to uncharged species increases with decreasing bore depth and increasing bore diameter ( 4 ) .To minimize emissions from charged species, hollow cathodes
for lamps are thus drilled to a depth of several bore diameters. With such a geometry, most sputtering occurs near the center of the bore and cathodic material is deposited near the open end (8) suggesting that the mouth of the bore is an area depleted in ions. It is also well known from optical studies that when the negative glow regions corresponding to different parts of the cathode are forced to coalesce (e.g., by cathode geometry), the current density and optical brightness increase manyfold (9). This phenomenon is known as the hollow cathode effect. From the data available in optical studies as well as from the studies described here, it thus appears that the ideal bore geometry for the HCIS is short and wide to enhance the formation of ions but also sufficiently confined to maintain the hollow cathode effect with its associated high current densities. While a check of absolute sensitivity was not undertaken in this study, an increase in sensitivity over that already reported (2) can be expected. The previous authors showed that a detection limit of 10 to 150 ppma is possible for those elements examined. The present studies indicate that a gain of IO2 or more is possible through the use of high bore diameter to depth ratios. Consequently, detection limits for HCIS mass spectrometry should be in the ppba range. LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9)
W. W. Harrison and C. W. Magee, Anal. Chem., 46, 461 (1974). E. N. Colby and C. A. Evans, Jr., Anal. Chem., 46, 1236 (1974). C. J. Belle and J. D. Johnson, Appl. Spectrosc.,27, 118 (1973). K. 8. Mitchell, J. Opt. SOC.Am., 51, 846 (1961). J. R. Wallace, "The Chemical and Physical Characterization of Airborne Particulate Matter", Thesis, School of Chemical Sciences, University of IIlinois, Urbana, Ill., 1974. C. A. Bennett and N. L. Franklin, "Statistical Analysis in Chemistry and the Chemical Industry", John Wiley and Sons, New York, 1954, p 29. R. Mavrodineanu, "Bibliography on Flame Spectroscopy", Nat. Bur. Stand. (US)Misc Pub., 281, U.S. Government Printing Office, Washington, D.C.. 1967. A . D.White, J. Appl. Phys.,30, 71 1 (1959). P. F. Little and A. von Engel, Proc. RoyalSoc. London, 224, 209 (1954).
RECEIVEDfor review June 10, 1975. Accepted October 6, 1975. This work was supported in part by the National Science Foundation Grants DMR 72-03026 and MPS 7405745.
Sequential Determination of Arsenic, Selenium, Antimony, and Tellurium in Foods via Rapid Hydride Evolution and Atomic Absorption Spectrometry John A. Fiorino,' John W. Jones,* and Stephen G. Capar Bureau of Foods, Food and Drug Administration, Washington, D.C. 20204
Analysis of acid digests of foods for As, Se, Sb, and Te has been semiautomated. Hydrides generated by controlled addltlon of base stabilized NaBH4 solution to acid digests are transported directly into a shielded, hydrogen (nitrogen diluted), entrained-alr flame for atomic absorption spectrophotometric determination of the individual elements. The detectlon limits, based on 1 g of digested sample, are apPresent address. C h e m i s t r y D e p a r t m e n t , V i r g i n i a P o l y t e c h n i c I n s t i t u t e a n d S t a t e L'niversity, Blacksburg. V a . 24060.
120
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
proximately 10 to 20 ng/g for all four elements. Measurement precision is 1-2% relative standard deviation for each element measured at 0.10 pg. A comparison is made of results of analysis of lyophilized fish tissues for As and Se by instrumental neutron activation (INAA), hydride generation with atomic absorption spectrometry, fluorometry, and spectrophotometry. NBS standard reference materials (orchard leaves and bovine liver) analyzed for As, Se, and Sb by this method show excellent agreement with certified values and with independent NAA values.
