ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1853
CORRESPONDENCE Dual-Discharge Ionization Source Sir: In recent years, the glow discharge has been studied as a possible alternative source t o the pulsed rf spark ion source in mass spectrometry (1-4) and to electrothermal atomization in atomic absorption ( 5 , 6). Others have investigated glow discharges and low pressure plasmas as surface analysis probes (7-9). For trace element analysis by mass spectrometry, the glow discharge has been particularly effective in providing a stable ion beam of high intensity. T h e advantages of cost, simplicity, and stability, relative to the spark source, have led us to couple this source to a quadrupole mass filter to produce a compact solids mass spectrometer (10, 11). T h e glow discharge relies on ion bombardment sputtering (12,13)to remove sample atoms from the cathode surface for subsequent ionization-chiefly by electron impact or Penning effects-in t,he active discharge plasma. The fraction of the sputtered atoms which become ionized in the discharge has been estimated a t 1-10% (14) depending upon conditions. These ions exit the source as part of a n expanding beam, comprised mainly of neutral atoms of the discharge gas and the sputtered sample. While the sensitivity of ion detectors allows use of the small ion population, an enhancement of the ion-to-neutral ratio would clearly be advantageous. We have improved this ratio significantly by adding a second discharge within the ionization chamber of a coaxial cathode ion source (15). The modified unit now consists of a primary high current discharge onto the analytical sample cathode and a secondary low current discharge directed just across the ion exit orifice. T h e primary discharge serves the same sputter release and ionization role as previously described (10)with the secondary discharge acting to post-ionize a fraction of the neutral species in t h e exiting atomic beam. Secondary discharges of a different type have been previously used by others to produce high intensity hollow cathode lamps for atomic absorption (16, 17) and atomic fluorescence (18). Similar units have been used to enhance optical emission for multielement analysis (19, 20). This report describes a dual-discharge ionization source which appears to offer significant advantages over our previous single discharge mode. Not only is sensitivity enhanced, but the population of certain molecular species has been reduced, a n important factor in reducing spectral interferences. EXPERIMENTAL Source Design. Figure 1 illustrates the dual-discharge ionization source. The primary discharge features a stable, dc diode abnormal glow discharge (22,22) localized between concentric cylindrical electrodes (231,a design readily adaptable to our quadrupole instrument ( I O , 11). The analytical sample cathode, (l),2.54-cm length and 0.61-cm id., was mounted on a 0.40-cm diameter Cu rod shielded with a 7-mm a d . Pyrex tubing and connected at the end of the source glass housing (2) to a brass rod through a ceramic insulator and a nylon 1.90-cm Cajon Ultra-Torr fitting. A 1.52-cm i.d., 2.03-cm 0.d. stainless steel cylinder (3), lined with a removeable tantalum tubular foil, was mated to the source entrance flange with machine screws to serve as a cylindrical anode. A 0.90-cm long, 3.05-cm 0.d. end collar (4) allowed (a) securing a 0.13-mm thick Ta disk with a 0.5-mm diameter anode ion exit orifice to the stainless steel anode body and (b) inserting two secondary discharge electrodes into the negative glow plasma region established by the coaxial cathode 0003-2700/79/0351-1853$01 0010
discharge. The two 1.0-mm diameter secondary discharge electrodes (5, 6) entered the anode cylinder through 0.5-cm diameter diametrically opposed holes drilled in the steel end collar and were mounted in machinable glass ceramic MACOR, Corning Glass Works, Corning, N.Y.), which provided electrical isolation from the anode. A 1.5-mm electrode gap and 3.8-mm secondary electrode/exit orifice separation were used; both these distances are individually variable. Electrical leads for the dc potential to the secondary electrodes were made through two MHV vacuum/electrical feedthroughs (Ceramaseal, Inc., New Lebanon, N.Y.) welded into the source entrance flange. Analytical cathodes were prepared from OFHC copper and stainless steel stock. Secondary electrode materials were W-1 % Th, Ta, and high density graphite. Procedures. Analytical sample cathodes and secondary electrodes were surface cleaned by rinses in ethanol, dilute HNO,, and deionized-distilled water. Argon was used as the inert fill gas. With the source envelope pressure in the 1-2 Torr range, the coaxial cathode source typically operated at 3O-mA constant current regulation and dc voltages of 600-800 V supplied by a programmable power supply (Hewlett-Packard Model 6521A). A Kepco Model BHK power supply in the constant current mode drove the secondary discharge, with 2-10 mA secondary discharge currents and 900-1200 V dc typically delivered
RESULTS T h e coaxial cathode mode was selected to couple with the secondary discharge because of its convenient geometry which allows the use of cathodes of various sizes and configurations, depending upon the analytical sample needs. It also allows easy positioning of the secondary electrodes near the exiting atomic beam where additional ionization effects can be significant. T h e analytical sample is made the cathode in the primary discharge, thus acting as the site of ion bombardment sputtering, causing release of the sample, mostly as neutral atoms, into the plasma discharge where subsequent ionization of these neutral species can occur ( 1 4 , 2 4 , 2 5 ) .'The role of the secondary discharge is to enhance ionization of the sample sputtered neutral atoms by injecting electrons or other sufficiently energetic species (such as argon met astable atoms, Arm*) to promote collisional ionization of neutral atoms which enter the discharge sphere of influence. T h a t enhanced ionization occurs can be seen from Figure 2, which shows comparison spectra of copper isotopes taken with and without the secondary discharge on. In general, ion current increases of 2- to 20-fold are observed, with these effects varying somewhat from element to element. Several factors can affect the resultant ion enhancement, including secondary discharge voltage and current, electrode composition and shape, and electrode placement. In addition to the predominant atomic species produced in the glow discharge, molecular ions are also generally observed (3, 10) and are affected by the secondary discharge. Figure 3 shows a comparison of a mass spectral region where the ion a t m / e = 19 (presumably H,O'), is almost entirely eliminated. Other molecular ions, such as Ar2+ and NO+, also show consistent reductions. Still other molecular ions show the property of increasing, but a t a much lower rate than d o atomic ions. Is the energy of the secondary discharge causing breakup of molecular ions? In several cases, this appears to be so. However, molecular ions containing the sputtered atom (e.g., CuAr+) exhibit enhancements of 3-4X with initiation C 1979 American Chemical Society
1854
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Single Dischorgs
fig
7
5
Dual
Discharge
Chamber
12
Figure 1. Dual-discharge ionization source/quadrupole mass spec-, trometer. (1) Primary (analytical) cathode, (2) source glass housing, (3) concentric anode, (4)anode end collar, (5, 6) secondary discharge electrodes, (7) skimmer, (8) quadrupole mass filter, (9) energy analyzer, (10) viewport, (11) discharge gas inlet, (12) source thermocouple gauge. See Ref 10 for more details of 7-12
63
I
Single
Discharge IOX
63
I
Sens.
y
Dual isc charge
-
x
.-
Y)
c ly
e
-c
-u
!
P
II
a
Flgure 2. Ion enhancement observed with the dual discharge using a copper coaxial cathode. (a) Primary discharge conditions: 30 mA, 593 V dc, 1.5-Torr source pressure. (b) Secondary discharge conditions: 10 mA, 1000 V dc. Ta electrodes used. Copper signal enhancement: 15.8. m l e 68 = ArN,'
of the secondary discharge. probably due to an increase in argon metastable species which can influence both Cu+ and CuAr+ populations. Electron impact ionization does not generate CuAr+, in that there are no CuAr neutral species in the discharge (26). Rather, an associative ionization channel involving Arm* and Cu neutral atoms produces CuAr+. By contrast, the Cu,+ species shows a decrease with the secondary discharge on, suggesting t h a t neither Cu" nor Arm*haa any significant role in its formation or t h a t the bond in Cu2+is easily broken by the secondary discharge. The secondary electrodes produce characteristic component ions also, making the selection of electrode material important. High atomic weight, monoisotopic metals are desirable to minimize spectral interference. T a has been quite satisfactory, interfering only with Au a t m / e = 197 from TaO+. W - l % T h electrodes have also worked well, but with more spectral interferences. Perhaps the best electrode material, based on entirely different characteristics, has been high density graphite. This material yields a stable discharge but sputters poorly and thus acts as a more inert electrode with small spectral contribution. A strong l2Ct peak is present, but no C, clusters are observed. The high resistance of the secondary discharge produces a more sparklike spectrum for the metal
Q b Figure 3. Secondary glow discharge effects on low mass ion species. (a) Secondary discharge off. Same conditions as in Figure 2a. (b) Secondary discharge on. Same conditions as in Figxe 2b. Full scale sensitivity same in (a) and (b)
electrodes, with strong doubly ionized species appearing for W, T h , and Ta. It is possible t h a t this type discharge has analytical possibilities of itself t h a t will be explored. Two adjacent discharges within the same envelope are somewhat interactive, as noted by other workers (13-203. Turning the secondary discharge on reduces the voltage of the primary current regulated discharge by about 10-1570. In principle, this should cause lower energy sputtering ions and reduce the sputter yield of the analytical cathode, in which case the ion population enhancements are really larger than the apparent measurements. Simply increasing the electron population in the ion source by addition of a hot filament discharge does not produce ion enhancements as large as those shown by the secondary glow discharge. Evidently such low voltage, high current secondary discharges are quite efficient for atomic excitation (20, 27). However, the high voltage, low current glow discharge has been more effective for our ion source in enhancing ion populations. We have used a filament source secondary discharge with our primary glow discharge and observed ion enhancements of only 2- to 3-fold. This lower ion gain, coupled with the more inconvenient and less durable nature of the filament relative to the graphite rod secondary electrodes, caused us to favor the dual glow discharges. At this point, the dual-discharge mode appears to offer significant sensitivity enhancement under preliminary conditions which are probably not optimum, thus giving expectation of further improvement. The addit.iona.1 set of electrodes requires a second power supply, but a relatively inexpensive unit will suffice. Many aspects o f this source remain to be studied. For example, the possibility of discriminating against molecular interference species hy means of a high voltage secondary discharge is of considerable interest to US.
LITERATURE CITED ( 1 ) Harrison, W. W.; Magee, C. W. Anal. Chem. 1974, 4 6 , 461-464. (2) Colby. B. N.; Evans, C. A , . Jr. Anal. Chem. 1974, 4 6 , 1236-1242. (3) Daughtrey,E. H., Jr.; Harrison, W. W. Anal. Chem. 1975, 47, 1024-1028. (4) Wallace, J. I?.;Natusch, D. F. S.; Colby. B. N.; Evans, C. A,, Jr. Anal. Chem. 1976, 4 8 , 118-120. (5) Bruhn, C. G.; Harrison, W. W. Anal. Chem 4978, 50, 16--21. (6) Gough, D. S. Anal. Chem. 1976, 4 8 , 1926-1930. (7) Coburn, J. W.; Kay, Eric. Appl. Pbys. Lett. 1971, 19, 3.50-352. (8) Oechsner, H.; Sturnpe, F. Appl. Phys 1977, 1 4 , 43-47. (9)Cobvn, J. W.; Tagbuer, E.; Kay, Eric. J. Appl. Fbys. 1974, 45, 1779--1766. (10) Bentz, B. L.; Bruhn, C. G.; Harrison, W. W. Int. J . A4ass Specborn. Ion Phys. 1978. 2 8 , 409-425. (11) Bruhn, C. G.; Bentz, B. L.; Harrison, W. W. Anal Cbem. 1878. 5 0 ,
373-375.
ANALYTICAL CHEMISTRY, VOL. 51, (12) Stuart. R . V.: Wehner, G. K. J. Appl. Phys. 1964, 3 5 , 1819-1824. (13) Wehner, G. K.: Anderson, G. S. In "Handbook of Thin Film Technology"; Maissel, L. I., Glang, R., Eds.; McGraw-Hill: New York, 1970; Chapter 3. (14) Westwood, W. D. Progr. Surf. Sci. 1976, 7, 71-109. (15) Mattson, W. A.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1976, 48, 489-491. (16) Sullivan, J. V.; Walsh. A. Spectrochim. Acta 1965, 21, 721-726. (17) Sullivan, J. V.; VanLoon, J. C. Anal. Cbim. Acta 1978. 702, 25-32. (18) Lowe, R. M. Spectrochim. Acta, Part B 1971, 26, 201-205. (19) Gough, D. S.;Sullivan, J. V. Analyst(London) 1978, 103, 887-890. (20) Lowe, R. M. Spectrochim. Acta, Part 5 1976, 37, 257-261. (21) W i r a a , J. D. "GaseousConductors, T k n y and EngineeringApplications"; Dover: New York. 1941. (22) Howatson, A. M. "An Introduction to Gas Discharges", 2nd ed.;Pergamon Press: Oxford, 1965. (23) Loeb, L. B. "Fundamental Processes of Electrical Discharges in Gases"; Wiley, Chapman and Hall: London, 1939. (24) Gerhard, W.; Oechsner, H. Z. Physik., Part B 1975, 22, 41-48.
NO. 11, SEPTEMBER
1979
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(25) Coburn, J. W.; Kay, Eric. Appi. Phys. Lett. 1971, 7 6 , 435-438. (26) Coburn, J. W.; Eckstein, E. W.; Kay, Eric. J. Vac. Sci. Techno/. 1975, 12 . - , 151-154 - . . - .. (27) Human, H.G.C.; Zeegers, P. J. Th.; van Elst, J. A. Spectrochim. Acta. Part B 1974, 29, 111-1 19.
W. W. Harrison* B. L. Bentz Department of Chemistry University of Virginia Charlottesville, Virginia 22901
RECEIVED for review March 12, 1979. Accepted May 21,1979. Financial support from the National Institutes of Health Grant GM-14569 is gratefully acknowledged.
Correlation of Alkane Size with Liquid-Gel Permeation: Gauche Conformational Isomer Effect Sir: Previously, this laboratory reported a study ( I ) of stationary phase porosimetry based on the calibration of liquid-gel permeation. Various materials were examined in tetrahydrofuran using a variety of solute sizes from 4 8, (methane) to IO38, (polystyrene). The sizes of normal alkane molecules smaller than hexadecane were taken to be linearly related to their mass, while polymeric solutes were referred t o their equivalent hydrodynamic radius. A significant contribution, the effect of molecular conformation, was reported by Schultz ( 2 ) . The present work examines the relationship between conformation and molecular size of normal and branched alkanes. Stationary phase permeability regulates liquid-gel chromatographic retention. The extent of the permeation varies directly with the pore size and with the pore volume, but oppositely with solute size. I t is implicit in Schultz's measurements that the permeation is also enhanced by the conformational compactness of the solute molecule. Giddings' equation (3) provides a convenient basis for calibration -In D = go
+ g,L
(1)
where D is solute distribution coefficient and L is the solute size. Several workers (see Ref. 1) have found for normal hydrocarbons of low molecular weight (MW) that
L = L o + L1 M W
(2)
Note here t h a t Lo indicates the apparent size of a molecule of zero mass. This may be associated with the thermal energy, 312 kT. Schultz (2)has reported that gel permeation of hydrocarbon molecules varies oppositely with the average number of gauche arrangements (Zg),However, this contribution has yet to be treated analytically. Schultz showed that a more unified correlation could be obtained from an arbitrary parallel shifting of the results on grouped isomeric homologues. We have analyzed Schultz's results and we find that the permeation is oppositely related to molecular weight or molecular volume, and directly related to the number of gauche conformations, Zg. I t follows that the 2, term indicates an effective diminishing of the molecular size. It therefore describes the molecular compactness. An explicit treatment was approached by assuming the following modification to Equation 2:
L = Lo'
+ L1'
( M W - aZ,)
(3)
0003-2700/79/035 1-1855$01.OO/O
The values of L,,', L1' and a need to be determined experimentally. Equation 3 was applied to Schultz's data in the following way. T h e manufacturer of Schultz's apparatus was able to provide an estimate ( 4 ) of the empty column volume ( V , = 200 mL), the estimated extra-column dead volume (V, = 2.5 mL) and the volume corresponding to one "count" (V, = 5.0 mL). The exclusion volume was calculated using Vo = 78 mL. This estimate is based on other findings of 0.38-0.40 as the void fraction of uncompressed settled chromatographic beds packed with spherical beads (5, 6). These were applied to Schultz's data to obtain distribution coefficients using the following expression:
D = (5.0 VE - Vo)/V,
(4)
where the stationary phase volume Vs is taken ( I ) from V, -
(Vo - V,).
The results obtained from Equations 2 and 3 were compared in the following way. Equations 1 and 2 lead ( I ) to the following: -In D = a
+ b MW
(5)
However, by combining Equations 1 and 3 we have the basis for a refined approach:
+ b'
-In D = a'
(MW
-
02,)
(6)
These latter two equations were fitted separately using linear regression analysis of Schultz's data. The following results were obtained: for Equation 5 ( a = 1.11, b = 0.00398) and for Equation 6 (a' = 1.10, b' = 0.00440, (Y = 4.95). The deviations for these two equations are plotted in Fikwre l. The dashed lines show a substantial reduction in the standard deviation s from 0.0173 (Equation 5) to 0.0074 (Equation 6).
s = [X{ln D (calcd)
-
In D ( o b ~ d ) ] ~ ] ' / ~ (7) / \ ~
where n is the number of data points. Figure 1 illustrates this graphically by showing the difference 6;
6; = In D (calcd)
-
In D (obsd)
(8)
for individual points and comparing In D (calcd) by Equations 5 and 6. The results so far support two conclusions, (a) that Giddings' equation can accommodate the permeation effect due to small molecule conformation and (b) that the con0 1979 American Chemical Society
