Anal. Chern. 1985, 57,2901-2907
2901
Characterization of Atomic Emission Lines from Argon, Neon, and Nitrogen Glow Discharge Plasmas Kazuaki Wagatsuma and Kichinosuke Hirokawa*
T h e Research Institute for Iron, Steel, and Other Metals, Tohoku Uniuersity, Sendai 980, Japan
Emission lines and the relative intensities of Cu, Ag, Sn, Ai, and Zn are investlgated in the Wavelength range from 200 to 600 nm when excited with a Grimm-type glow discharge light source. Argon, neon, or nitrogen gas is introduced into the discharge tube to create a glow discharge plasma. The spectrum patterns strongly depend on the kind of foreign gas rather than the other discharge conditions such as the gas pressure and the discharge voltage. Especially, the emisslon intensitles from the singly ionized species are drastically changed by choosing various foreign gases even if the other conditions are kept constant. These observations suggest that the energy levels and their excitation energies of each foreign gas play significant roles for determining the ionizatlon and excitation of the analyte atoms.
In recent years, the Grimm-type glow discharge lamp (I, 2) has been employed for the elemental analysis of surfaces (3-6). In these works, argon has been commonly introduced into the discharge tubes as a foreign gas of a few hundred pascals pressure. Argon gas is considered to be the most suitable for a foreign gas, because the efficiency of sputtering is high (7) and a high-purity argon is easily available compared to the other rare gases. In this glow discharge spectrometry, the sample introduction into the plasma is based on cathode sputtering in the discharge tube. The excitation mechanisms of sputtered materials strongly depend on the kind of the foreign gas. The energy distribution of primary electrons, the energy levels and the density of metastable atoms, the density and the kinetic energy of ionized gases, etc. play important roles in determining the excitation and ionization of sputtered atoms. Therefore, the intensities of the atomic lines emitted from sputtered atoms are expected to be considerably different corresponding to the kind of the foreign gas. Furthermore, apart from conventional arc or spark spectral patterns, the spectrum chart recorded using a glow discharge light source is expected to be fairly peculiar because, in a low pressure, low power, glow discharge, the thermodynamic equilibrium conditions are hardly satisfied (8).
In general, rare gases have the energy levels of metastable atoms; they are, 11.55 eV ( 4 ~ ~and / ~11.72 ) eV (4slj2) for Ar, and 16.62 eV ( 3 ~ ~and , ~ 16.72 ) eV ( 3 ~ ~for , ~Ne ) (9). No levels exist below the energy of the lowest metastable level and many excited levels are densely filled in the range from the level up to the ionization limit, 15.76 eV for Ar and 21.56 eV for Ne (9). The energy of the metastable levels and the ionization limit of neon atom are about 5 eV higher than the corresponding values of the argon atom. Because the excitation and ionization of sputtered atoms are performed through atom-atom, atom-ion, or atom-electron collisions, such energy differences of the metastable atom, which is a important particle in collision processes due to its long lifetime, may influence the emission intensities of sputtered atoms. In the case of the nitrogen molecule, many energy levels continuously exist between 0 and ca. 8 eV, which is due to the many vi0003-2700/85/0357-2901$01.50/0
brational and rotational states belonging to each electronic state (10). The physical parameters that control electronic discharge, for example, the voltage-current curves, have been reported for different foreign gases (13) and the sputtering yields of many metals have been also published for various kinetic energies of several rare gases (12, 13). However, few spectrometrical studies have been carried out when the glow discharge lamp is operated with different foreign gases; that is, such studies should include the assignment and classification of the resulting emission lines and also the comparison of the spectral emission lines and also the comparison of the spectral patterns under various discharge conditions. In this paper, we report the characteristics of emission lines for some cathode elements when argon, neon, and nitrogen are used as a foreign gas.
EXPERIMENTAL SECTION The schematic diagram of our glow discharge lamp was described elsewhere (14). The lamp was made according to the original model reported by Grimm ( I ) . The inner diameter of the hollow anode was 8.0 mm and the distance between the anode and cathode was adjusted to be 0.4-0.6 mm. The lamp was evacuated to 4.0-1.3 Pa and then a foreign gas with the purity of more than 99.9995% was introduced. The pressure of foreign gases and the supplied voltage to the lamp were widely changed to control the discharge conditions. The actual experimental conditions were selected as follows: the gas pressure range 4 X lo2 to 1.3 X lo3 Pa and the discharge voltage range 300-800 V for Ar, 4 X lo2 to 1.3 X lo3 Pa and 400-1000 V for Ne, and 6.7 X lo2 to 2.7 X lo3 Pa and 400-1000 V for N2. Plates of pure Ag, Al, Cu, Sn, and Zn were prepared as the cathode materials. The surfaces were mechanically polished with waterproof emery papers and then finished to mirror faces with emery cloth. The assignments of emission lines are estimated from the coupling calculations using the energy level data in ref 9. The selection rules are based not on the strict LS coupling but on the coupling scheme permitting the intersystem transitions among different multiplets (17). RESULTS AND DISCUSSION Emission Lines of Foreign Gas. A large number of emission lines assigned to the atom, ion, or molecule of a foreign gas appears in the emission spectrum when recorded with a glow discharge light source. Therefore, it is significant to distinguish the emission lines of sputtered atoms from such foreign gas emission lines. The major lines of argon, neon, and nitrogen are summarized in Table I. In the case of argon, there are many Ar atomic emission lines (Ar I) which result from the transitions from 3p54s to 3p54p electron configurations in the wavelength range of 700-950 nm. Such transitions correspond to the energy differences between 11.55-11.83 eV and 12.90-13.48 eV. However, Ar I lines in ultraviolet regions, which are assigned to the transitions 3p54s-3p55p (14.46-14.74 eV), provide no clear profiles except Ar I 415.9 and 420.0 nm. Instead of the Ar I lines, the emission lines from singly ionized species (Ar 11) exist in these wavelengths, as shown in Table Ia. Neon gas exhibits similar spectrum patterns; that is, many intense Ne 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57,
NO. 14, DECEMBER 1985
Table I. Observed Emission Lines o f Argon, Neon, and Nitrogen wavelength, nm
assignment, eV upper lower
intensity"
wavelength, nm
(a) Observed Emission Lines of Argon Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I Ar I Ar 1 Ar I Ar I Ar I Ar I1 Ar I Ar I1 Ar I1 Ar I1 Ar 11 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1 Ar I1
294.3 297.9 349.1 354.6 356.0 357.7 358.2 358.9 372.9 376.4 385.1 386.9 394.4 404.3 407.2 410.4 413.2 415.9 419.1 419.8 420.1 426.0 426.6 427.8 430.0 433.1 433.2 434.8 437.1 438.0 440.0 442.6 443.0 448.2 454.5 458.0 459.0 461.0 465.8 472.7 473.6 476.5 480.6 488.0 496.5
21.35 'P3/2 21.43 2P1/2 21.49 2D312 23.26 'F5jz 23.16 'F7/2 23.01 4F7/2 23.07 4F5/z 22.95 4 F ~ p 19.97 4S3/2 22.79 4D5/z 19.97 4s3/z 23.17 4P512 19.55 4D5/2 21.49 'D3/2 21.50 'D5/2 22.51 4D51z 21.43 'D,p 14.53 [2] 14.51 [2] 14.58 [O] 14.50 [3] 14.74 [0] 14.53 [2] 21.35 'P3/2 14.51 [2] 19.61 4D3jz 19.31 4P1/2 19.49 4D7/2 21.49 'D3/z 19.64 4Dl/z 19.26 'P3/2 19.55 4D5/2 19.61 4D3/2 21.50 2D512 19.87'2P312 19.97 'S1/2 21.13 'F5p 21.14 2F712 19.80 'P1/, 19.76 2D3/z 19.26 4P3/2 19.87 'P3/2 19.22 4P5/2 19.68 2D5jz 19.76 2D312
17.14 'P3/2 17.27 'Dip 17.94 2P1/z 19.76 'D3/2 19.68 'D5/2 19.55 4D5/2 19.61 4D3/2 19.49 4D712 16.64 *P5/2 19.49 4D,/2 16.75 4P3/z 19.97 4P3/2 16.41 4F7/2 18.43 2S3/2 18.45 'F5/z 19.49 4D,/2 18.43 'D3/2 11.55 [2] 11.55 [2] 11.62 [ l ] 11.55 [2] 11.83 [ l ] 11.62 [I] 18.45 'D5/2 11.62 [l] 16.75 4P3/2 16.44 4D3/2 16.64 4P,j2 18.66 'D3/z 16.81 4P1/z 16.44 4D3/2 16.75 4P3/2 16.81 4P112 18.73 'D5/2 17.14 2P312 17.27 'P1/2 18.43 'D3/2 18.45 'D5p 17.14 2P3/2 17.14 'P3/2 16.64 4P5j2 17.27 'Pl/2 16.64 4P5p 17.14 'Pal2 17.27 'P1/2
assignment, eV upper lower
intensity"
(b) Observed Emission Lines of Neon
M W
M M S M W
M W M W W W W S S M S W S S W W
vs
W W W S
M W W M W W S M M
vs
S M W
vs M M W
Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I Ne I Ne I Ne I1 Ne I Ne I1 Ne I1 Ne I Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I1 Ne I Ne I1 Ne I1
295.6 300.2 321.7 323.2 331.0 332.0 332.4 333.5 334.6 334.6 335.5 337.8 339.3 341.8 344.8 347.3 348.2 352.1 356.9 357.4 359.4 366.4 369.4 371.3 372.7 376.6 377.7 395.4 438.5 439.2
31.36 4S312 31.36 4S312 34.81 4P1,2 34.38 'D3,z 31.53 2Pl/z 34.28 2P1/2 31.51 2P,j2 30.89 4D,/2 34.63 4D3/2 34.25 2P31z 30.93 4D61z 31.53 2Pl/2 31.51 2P3j2 20.30 [l] 20.21 [2] 20.19 [3] 31.34 2S1/2 20.37 [O] 34.02 'F7/2 34.02 'F5/2 20.30 [2] 30.55 4P3/2 30.52 4P51z 31.12 2D,lz 31.18 2D3jz 30.52 4P5/2 30.55 4P3j2 21.52 [l] 37.67 4F6i2 37.62 4 G ~ / z
27.17 4P5/2 27.23 4P3/2 30.96 4D3/2 30.55 'D5/2 27.78 'P3/2 30.55 'D3/2 27.78 'P,/Z 27.17 4P5/z 30.93 4D5/2 30.55 'D3/2 27.23 4P3/2 27.86 2P1/2 27.86 2P1jz 16.67 [l] 16.62 [3] 16.62 [3] 27.78 'P3/2 16.85 [l] 30.55 'D5/2 30.55 'D5/2 16.85 [l] 27.17 4P5/2 27.17 4P5j2 27.78 2P3/2 27.86 2Pl/2 27.23 4P3/2 27.27 4P1/2 18.38 [l] 34.84 4F5/2 34.80 4F7/z
M W W W W
M
vs
S S S M S M W W
W
M S M W W W M S
M W W W W W
(c) Observed Emission Lines of Nitrogen
N2 N2 N2 N2 N2 N2 N2 N2 N2
N2 N2 NZ+ N2 N2
N2+ NZ+
295.3 296.2 297.6 313.6 315.9 337.1 353.6 357.7 371.0 375.5 380.5 391.4 399.5 405.9 427.7 470.9
C3n [4] C 3 n 131 c3n [2] c3n [2] c3n [l] c3n [O] c3n [ l ] c3n [O] c3n [2] c3II [l]
c3n [O]
B2Z [O] C 3 n [3] c3n [O] B22 [O]
B2Z: [O]
B3n 121 B311 [l] B 3 n [O] B3n [ l ] B311 [O] B3n [O] B311 [2] B311 [l] B 3 n [4] B3n [3] B311 [2] X'2 [O] B311 [6] B311 [3] x2z [l] x2z [2]
W W
W W S
vs
M S W W
M
vs
W W M W
Key: VS, very strong; S, strong; M, medium; W, weak; VW, very weak; ND, not detected.
I lines appear in the wavelength range 600-800 nm, while the intensities of Ne I1 lines are relatively intense compared to those of Ne I lins below 400 nm. The Ne I lines in the long wavelength side of visible regions are assigned to the transitions 2p53s (16.62-16.85 eV) to 2p53p (18.38-18.96 eV) and those in ultraviolet regions to the transitions 2p53s-2p54p (20.37-20.15 eV). Though the energy diagrams of neon are similar to those of argon, the excitation energy of Ne I (or Ne 11) is always ca. 5 eV (or 10 eV) higher than that of Ar I (or Ar 11),respectively, when the corresponding optical transitions are compared between argon and neon. The assignment of Ar I1 (or Ne 11) lines is classified into either the optical transitions among doublet terms (spin multiplicity = 2) or those among quartet terms (multiplicity = 4). Though the intensities of these lines uniformly increase with an increase in input power to the discharge tube, the intensity ratio of the lines related to the quartets to those of the doublets gradually increase with input power (15). However, the overall spectral patterns of Ar or Ne vary with
experimental conditions, for example, gas pressure, discharge voltage, and the kind of cathode materials, in a similar fashion. When nitrogen is employed as a foreign gas, no emission lines derived from the dissociative species such as N or Nf are observed regardless of the discharge conditions, while the molecular band spectra are strongly emitted, which results from the transitions among various vibrational states in different electronic states of N2 or Nz+. Table ICshows the positions of the band heads, and in its assignment column the vibrational quantum numbers for each electronic state are noted in brackets, according to ref 16. Most of such band spectra can be classifid into the so-called second positive spectral series (16). Emission Lines of Copper. Figure 1shows the spectrum of copper monitored with argon, neon, and nitrogen gas in the wavelength range 200-300 nm, respectively. A few gas emission lines appear in these wavelengths; thus, it is found that the intensities of copper emission lines are considerably different when a foreign gas is changed. Even though the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
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Table 11. Observed Emission Lines of Copper wavelength, c u I1 c u I1 cu I cu I c u I1 c u I1 c u I1 c u I1 c u I1 cu I c u I1 c u I1 c u I1 c u I1 cu I c u I1 c u I1 c u I1 c u I1 c u I1 cu I c u I1 c u I1 c u I1 c u I1 cu I cu I cu I cu I cu I cu I cu I cu I cu I cu I cu I cu I
assignment, eV lower
nm
upper
212.3 212.6 216.5 217.9 217.9 219.2 221.8 222.9 224.7 229.4 229.4 237.0 242.4 248.6 249.2 250.6 252.6 254.5 259.0 260.0 261.8 270.1 270.3 271.3 271.9 276.6 282.4 296.1 324.7 327.4 402.3 406.3 510.6 515.4 521.9 570.1 578.3
9.09 'Dz 8.66 3F7 5.72 'Dip 5.69 'P3p 8.66 3F7 8.49 3Fj 8.42 3P1 8.54 3P0 8.23 3P2 6.79 'P3p 8.23 3Pz 13.65 3D1 13.65 3D1 13.65 'D1 4.97 4P3/2 13.43 'Dz 13.39 3D3 13.3g3D3 13.653D1 13.68'Dz 6.12 'P3/2 13.68'Dz 13.65 'D, 13.43 3D, 13.68 'Dz 6.12 'Pip 5.78 'D5/2 5.57 'F71z 3.82 'P3p 3.79 ZP1p 6.87 'D3p 6.87 'D5/2 3.82 'P3/2 6.19 'D3p 6.19 'D5p 3.82 'P3jZ 3.79 'P1/2
3.26 'Dz 2.83 3D2 0.00 2S1-/Z 0.00 'S1/2 2.98 3D1 2.83 3Di 2.83 'Dz 2.98 'D1 2.72 'D3 1.39 'D5/2 2.83 'D2 8.42 3P1 8.543P0 8.66 3Fz 0.00 2s1/2 8.4g3F3 8.49 3F3 8.52 'F4 8.863Dz 8.92'Fg 1.39 'D5p 9.09'Dz 9.06 3D1 8.86 3Dz 9.12'P1 1.64 'DSp 1.39 'D5/z 1.39 'D5/2 0.00 'SljZ 0.00 ZS'/2 3.79 'P1/2 3.82 'P3iz 1.39 'D5/2 3.79 2Pljz 3.82 'P3p 1.64 'D3p 1.64 'D3/2
intensitf Ar
Ne
Nz
ND ND W W W
W W ND
ND ND W W W ND ND ND ND
M M
M
vw
W W VS W W ND ND ND
W W W
w
ND ND ND ND ND
M ND ND ND ND W VS
vw vw vw vw M
ND ND ND
ND
M
M S M S
ND ND ND ND ND W ND ND ND ND
W
vs
vs vw vs VS
S S ND W ND
vw
VW ND ND W W W
ND ND ND W W
M
J
300
280
240
260
220
nm
S
**
vs vs vs vs vs VS
M M VW VW
vw vw vw vw ND vw
4Key: VS, very strong; S, strong; M, medium; W, weak; VW, very weak; ND, not detected; **, not observed due to the overlapping with gas emission lines.
discharge conditions are widely varied as described in the previous section, the characteristics of these spectral patterns, which are unique for the kind of foreign gas used, are principally reserved. The emission lines of copper are summarized when excited with Ar, Ne, and Nz gas and the relative intensities are roughly classified into five stages as shown in Table 11. Regardless of the kind of foreign gas, two Cu I resonance lines, 324.7 and 327.4 nm, are found to be the most dominant of all Cu emission lines. Figure 2a shows a related part of the Cu I energy level diagram. A typical nonresonance transition at 282.4 nm, which is assigned to 2D6/2 (5.78 eV)-2D6jz(1.39 eV), provides a strong emission line for the Ar-excited or Nz-excited glow discharge, but weak for the Ne-excited discharge. The nonresonance emission lines in the Ne-excited spectrum generally give faint profiles compared to those excited by Ar or NP The emission lines in which the excitation energy exceeds ca. 6 eV, such as Cu I 261.8 nm, are easily excited in the Ar rather than the Nzglow discharge plasma. Figure 2b indicates a part of Cu I1 energy level diagram. The ionization potential of copper is reported to be 7.724 eV (9) and the Cu I1 energy diagram is constructed on the energy scale above this potential. No Cu I1 lines exist in the Nzexcited spectrum. There are some Cu I1 lines which result from the transitions 3dg4p (8.23-8.66 eV)-3d94s (2.12-2.98 eV) in the Ar-excited spectrum, while, in the Ne-excited spectrum the transitions 3d95s (13.39-13.68 eV)-3d94p give Cu I1 lines. It is impossible to excite the transitions 3d95s-3dg4p with the
300 I
280
260
240
220 nm
240
220 nm
I
Figure 1. (a) Copper emission spectrum In the wavelength range 200-300 nm recorded with an argon glow discharge light source: gas pressure, 9.3 X 10' Pa: discharge voltage, 400 V; discharge current, 19.5 mA; spectral band-pass, 0.1 nm. The lines due to Ar gas are labeled with asterisks ("). (b) Copper emission spectrum recorded with Ne gas: gas pressure, 9.3 X 10' Pa; dlscharge voltage, 550 V; discharge current, 11.2 mA; spectral band-pass, 0.1 nrn. The lines due to Ne gas are labeled with asterisks (*). (c) Co per emission spectrum recorded wlth N, gas: gas pressure, 9.3 X 10 Pa: discharge voltage, 550 V; dlscharge current, 15.4 mA; spectral band-pass, 0.1 nm. The llnes due to N2 gas are labeled wlth asterisks (").
f
Ar glow discharge plasma; therefore, the occurrence of such transitions is an important characteristic of the Ne-excited spectrum. This effect explains the spectrum difference be-
2904 eV
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
ta
Table 111. Observed Emission Lines of Silver wavelength, nm
0
M'O'S, -
Figure 2. (a) A part of the Cu I energy level diagram. (b) A part of the Cu I1 energy level diagram.
tween Figure l a and Figure l b in the wavelength range 280-210 nm. In the Nz glow discharge plasma, it is found that Cu atoms are not supplied enough energy to overcome the barrier of the ionization potential and to be lifted up to higher excited states of Cu ions. On the other hand, it is possible to get over the ionization potential in the Ar or Ne glow discharge plasma; however, after ionization, the population of Cu I1 energy levels is rather different between Ar and Ne. More energy transfer to copper atoms is performed in the Ne plasma; therefore, copper atoms can be lifted up to 3d95sstates of Cu I1 (ca. 13.5 eV). However, the Cu I1 excitations are restricted in 3d94p states (ca. 8.6 eV) of Cu I1 with Ar gas. The energy differences is estimated to be ca. 5 eV between 3d95s and 3d94p states of Cu I1 and very similar to the difference in excitation energy between Ar and Ne metastable states. This effect suggests that these metastable states may play a significant role in determining the excitation of sputtered atoms. Emission Lines of Silver. Table I11 shows silver emission lines and their assignments when Ar, Ne, and Nz gases are employed. In the case of Ag as well as Cu, two Ag I resonance lines, 328.1 and 338.3 nm, provide very intense profiles. There are only a few Ag I lines and no great differences among their relative intensities depending on the kind of plasma gas. The ioniziation potential of silver (7.574 eV) (9)is almost the same as that of copper; however, the excitation energies in the Ag I1 energy level diagram give slightly higher values compared to those of Cu I1 (see Figures 2b and 3b). The Ag I1 transitions from 4d95p (9.94-11.27 eV) to 4d95s (4.86-5.71 eV) are more than 2 eV higher than the corresponding Cu I1 transitions 3dg4p-3d94s. The Ag I1 emission lines which result from these transitions can be observed with both Ar and Ne gas. However, with Ar gas, these relative intensities are found to be weak, while the Cu I1 transitions 3d94p-3d94s give relatively strong emission lines, which is due to the differences in the required energies between the Cu I1 and the Ag I1 lines. The other Ag I1 transitions 4ds5s2(13.53-14.08 eV)-4d95p can be also excited with the Ne glow discharge plasma but not with the Ar plasma. These phenomena indicate that the height
Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag I1 Ag 1 Ag 1 Ag I1 Ag I1 Ag 1 Ag I1 Ag I1 Ag 1 Ag 1 Ag I
211.4 214.5 222.9 224.6 228.0 231.7 232.0 232.4 233.1 235.8 241.1 243.8 244.8 247.4 271.2 318.1 326.7 328.1 338.3 349.5 368.3 421.1 462.0 478.9 521.0 546.6 547.2
assignment, eV upper lower 10.72 3D3 11.20 3D1 11.27 'Dz 10.37 3F4 11.14 'P1 10.77 3F2 11.05 IF3 10.19 SF, 10.37 SP1 10.68 3P,3 10.56 3D, 9.94 3P2 10.77 3F2 10.72 3D3 14.94 3D3 14.08 'G4 13.73 'D2 3.78 2P3/z 3.66 2P1ji 13.73 'D2 13.73 'D2 6.72 'DSj2 13.73 'D2 13.73 'D2 6.04 'D3/2 6.05 'D5,z 6.04 'D3/2
4.86 3D3 5.42 3D1 5.71 'D, 4.86 3D3 5.71 'D, 5.42 2Dl 5.71 'D, 4.86 3D3 5.05 3D2 5.42 3D1 5.42 3D1 4.86 3D3 5.71 'D2 5.71 'D2 10.37 3F4 10.19 3F3 9.94 3P2 0.00 0.00
2s1/2 2S',
10.19 3F3 10.37 3P1 3.78 'P3Iz 11.05 'F3 11.14 2P1 3.66 'P,/2 3.78 'P3/2 3.78 'P3/2
intensity" Ar Ne N2 ND VW VW VW W VW W VW ND ND VW VW VW VW ND ND ND
VW ND VW ND W ND M ND W ND W ND M ND W ND W ND W ND M ND S ND W ND W ND VW ND W ND M **
**
w **
ND W ND ND
W
vs vs vs vs vs vs
s s w
ND
** **
M S
ND ND
s vs s vs vw w
'Key: VS, very strong; S, strong; M, medium; W, weak; VW, very weak; ND, not detected; **, not observed due to the overlapping with gas emission lines. la
0
i tb
5
0
4d" 'so -
Flgure 3. (a) A part of the Ag I energy level diagram. the Ag I1 energy level diagram.
(b) A
part of
in excitation energy that can be excited in the Ar plasma is limited to the Ag I1 4d95p states. The higher transitions 4dg6s4d95p,whose excitation energies are required to be more than 15 eV, cannot be found even in the Ne-excited spectrum. The energy difference is estimated to be 3-4 eV between 4d85s2 and 4d95p. Emission Lines of Tin. There are ground state electron configurations (5s25p)having triplet terms (3P)and two singlet
ANALYTICAL CHEMISTRY, VOL.
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Table IV. Observed Emission Lines of Tin
wavelength, nm Sn I1 Sn I Sn I1 Sn I Sn I Sn I1 Sn I Sn I Sn I Sn I Sn I Sn I Sn I Sn I Sn I1 Sn I1 Sn I Sn I Sn I1 Sn I1 Sn I1 Sn I1 Sn I1 Sn I1
215.1 235.5 236.8 242.1 242.9 244.9 270.6 284.0 285.0 286.3 300.9 303.4 317.5 326.2 328.3 335.1 380.1 452.5 507.1 533.3 556.2 559.0 559.7 579.8
assignment, eV
intensitya
upper 5.76 4P1/2 5.47 3D2 5.76 4Pl/z 6.19 lF3 5.53 3D3 12.36 'F5p 4.79 3Pz 4.79 3P2 5.42 3F2 4.33 3P1 4.33 3P1 4.29 3Po 4.33 3P1 4.87 'PI 11.07 2F6/2 11.07 'F7/2,5/2 4.33 3P1 4.87 'PI 13.51 'D3jz ('G7/2) 11.19 'D3/2 11.20 2D5/2 11.07 'F5/2 11.19 'D3p 11.07 'F5/2
lower 0.00
2P1jZ
0.21 3P1 0.53 2P3/z 1.07 ID2 0.42 3P2 7.30 'D3jz 0.21 3P1 0.42 3P2 1.07 'D2 0.00 3P0 0.21 3P1 0.21 3P1 0.42 3P2 1.07 'Dz 7.30 'D3p 7.37 'D5/2 1.07 'Dz 2.13 'So 11.07 'F5/2,7/2 8.86 2Pl/2 8.97 2P3j2 8.85 'D3/2 8.97 2P3/z 8.93 'D5/2
aKey: VS, very strong; S, strong; M, medium; W, weak, VW, very weak; ND, not detected; gas emission lines.
terms (IS and lD) in the Sn I energy level diagram. The transitions related to these terms result in many Sn I emission lines. Such Sn I lines, which can be extensively defined as the resonance lines, occupy a major part of the Ar- or N2excited spectrum; however, in the Ne-excited spectrum, it is found that some Sn I1 lines are stronger than such Sn I lines (see Table IV). On the other hand, the Ag I (Cu I) atomic resonance lines provide the most intense emission lines regardless of the kind of foreign gases. Several Sn I1 emissjon lines can be emitted from the Ne plasma source, which result from the transitions 5s24f 2F (11.1eV)-5s25d 2D (8.9 eV), 5s26d 2D (11.2 eV)-5s26p 2P (8.9 eV), and so on. It is difficult to observe these Sn I1 lines with Ar gas because the required energies are not provided from the Ar glow discharge plasma. As was discussed in the previous subsections, the ionization probability of sputtered atoms is relatively high in the Ne glow discharge plasma. However, the degree of ionization is a minor factor for explaining the anomaly of the Sn I1 relative intensities, because the ionization potential of tin (7.342 eV) (9) is little different from that of silver (or copper). While only a ground state term ( 2 S l j 2 )exists in the Ag (or Cu) energy diagram and their resonance transitions fall into one level, the lower states concerning the Sn I resonance lines are divided into several terms and, due to many transition channels, the population of tin atoms cannot be concentrated on one level such as Ag (or Cu) but be distributed to each ground state term. Therefore, the relative intensities of the Sn I resonance lines are considered to be relatively weak, especially when the ionization probability is high and the overall concentration of tin atoms is made low in the Ne plasma. Emission Lines of Aluminum. The ground state of aluminum atom (3s23p)consists of the doublet terms (2P,j2 and 2P3,2).The A1 I resonance emission lines result from the transitions between the doublets and higher excited levels. In the Ne-excited spectrum, the relative intensities of the A1 I lines are rather weak compared to those in the Ar-excited spectrum, as indicated in Table V. Furthermore, several A1 I1 emission lines, for example, 281.6 and 280.0 nm, are much stronger than the A1 I lines. Because of its low ionization potential (5.984 eV) (9), it is reasonable to assume that a
Ar ND
vw ND vw W ND M M
M W M S S
S
vw
ND M W ND W
vw vw ND W
Ne
N2
M
ND
vw ND vw
vw W W W W W
W ND M
S
M W W W M M M S
vw
ND W S S S W
ND ND ND ND ND ND ND
W W S S S ND
** **
vs vw
S
**, not observed due to the overlapping with
Table V. Observed Emission Lines of Aluminum
wavelength, nm A1 I A1 I A1 I A1 I A1 I A1 I A1 I A1 I1 A1 I1 A1 I1 A1 I A1 I A1 I1 A1 I1 A1 I A1 I A1 I1
226.9 236.7 237.3 256.8 257.5 265.2 266.0 280.0 281.6 290.4 308.2 309.3 318.0 358.7 394.4 396.2 466.3
assignment, eV upper lower 5.48 5.24 5.24 4.83 4.83 4.67 4.67 11.85 11.82 11.69 4.02 4.02 15.59 15.30 3.14 3.14 13.26
0.01 2P3j2 0.00 2P1/, 0.01 2P3/2 0.00 2P1/2 0.01 zP3/2 0.00 2P1jZ 0.01 2P3l2 7.42 'PI 7.42 'P1 7.42 lP1 0.00 2P,/, 0.01 2P3l2 11.69 3P2 11.85 3D3,2 0.00 2P1/, 0.01 'P3/2
10.60 ID2
intensity" Ar Ne N2 W
M
ND VW
M S
W M
s
vw
w
vw
M ND
VW S
ND VS VS ND
W M M M
VS ND
M M
vw vs
** vs vs w
"Key: VS, very strong; S, strong; M, medium; W, weak; VW, very weak; ND, not detected; **, not observed due to the overlapping with gas emission lines.
considerable part of A1 atoms can be ionized in the Ne glow discharge plasma. It is also possible to observe A1 I1 358.7 nm, whose excitation energy exceeds 15 eV, by use of Ne gas. Emission Lines of Zinc. The Zn I resonance line at 307.6 nm is assigned to an intersystem transition 4s4p 3P1(4.03 eV)-4s2 lS0(0.00 eV). In the Ar-excited spectrum, Zn 1307.6 nm is found not to be dominant, and the nonresonance lines in which the transitions fall into the triplet states of 4s4p 3P (4.01-4.03 eV) provide a major part of the spectrum (see Table VI). The upper levels of such nonresonance lines have the excitation energy above ca. 7 eV. It is not easy to excite the transitions 4s5d 3D (8.50 eV)-4s4p 3P,496s 3S (8.11 eV)-4s4p 3P,or 4s4d 3D (7.78 eV1-4~4~ 3Pwith Nz gas probably because these excitation energies are too high. The ioniziation potential of zinc is reported t o be 9.391 eV (9) and is ca. 2 eV higher than that of silver. There are only a few Zn I1 lines
2906
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table VI. Observed Emission Lines of Zinc wavelength, nm Zn I1 Zn I1 Zn I1 Zn I Zn I1 Zn I1 Zn I Zn I Zn I Zn I Zn 1 Zn I Zn I Zn I Zn I Zn I Zn I Zn I Zn I Zn I
202.5 206.2 210.2 213.8 250.2 255.8 258.2 260.8 275.6 277.1 280.1 303.6 307.2 307.6 328.2 330.3 334.5 468.0 472.2 481.1
assignment, eV upper lower 6.12 'P3/z 6.01 2Pljz 12.01 'D3/2 5.80 'P1 10.96 2S1/z 10.96 'SljZ 8.83 3Dz,l 8.83 3D3,z,, 8.50 3D, 8.50 3Dz,l 8.50 3D3,z,1 8.11 3S, 8.11 3S1 4.03 3P1 7.78 3D1 7.78 3Dz,l 7.78 3D3,z 6.65 3S1 6.65 3S1 6.65 3S1
Ar
intensity Ne Nz
0.00 2S1/2 VW 0.00 'S1/z VW 6.12 'P3/2 ND 0.00 'So W 6.01 2 P l / ~VW 6.12 'P312 M 4.03 3P1 VW 4.08 3P2 W 4.01 3P0 M 4.03 3P1 S 4.08 3P2 vs W 4.03 3P1 4.08 3Pz W 0.00 'So M 4.01 3P0 VS 4.03 3P1 VS VS 4.08 SP, 4.01 3P0 M 4.03 3P1 VS 4.08 3Pz VS
M M M W W
M
ND ND ND M ND ND ND ND
ND ND ND VW VW W
vw w **
W M M S
**
ND VW
S
inert gas discharge are effective in exciting the emission lines of metallic elements in the plasma (19). By comparison of the occurrence of the ionic emission lines for each plasma gas, it is found that the metastable states of argon or neon play a significant role for the ionization of sputtered atoms. In the nitrogen glow discharge plasma, sputtred atoms cannot be ionized due to the absence of the distinct metastable states. A collision of the first kind, that is, direct energy transfer between primary electrons and sputtered atoms (M)
** W
M M
M S
"Key: VS, very strong; S, strong; M, medium; W, weak; VW, very weak, ND, not detected; **, not observed due to the overlapping with gas emission lines.
even in the Ne-excited spectrum, which may be due to its high ionization potential. The Zn I1 lines that result from the transitions, 4p 2P (6.01-6.12 eV1-4~ 2S (0.00 eV) and 5s 2S (10.96 eV)-4p 2P, are emitted from the Ne glow discharge plasma. However, the relative intensities of such Zn I1 lines are not strong. The Kind of Plasma Gas and Its Influence on Emission Lines of Sputtered Elements. Our investigation is summarized as follows. (1) Nitrogen Discharge Plasma. No emission lines from ionized species are detected under any discharge conditions. The atomic nonresonance transitions whose excitation energies are above 6-7 eV provide only weak emission lines, which is marked especially in the zinc spectrum. A few atomic resonance lines can be observed with the N2-excited glow discharge plasma. (2) Argon Discharge Plasma. The ionic emission lines that can be observed with the Ar plasma occupy a minor part of the spectrum and their intensities are relatively weak compared to the atomic resonance lines. Such ionic lines are caused from the transitions between lower energy levels as shown in the Cu spectrum. It is difficult to populate the ion excited states having excitation potentials of more than 10-11 eV. In addition to resonance lines, a number of nonresonance lines make the Ar-excited spectrum rather complex. (3) Neon Discharge Plasma. The ionic emission lines indicate clear spectrum profiles when excited in the Ne plasma. In some cases, their intensities are found to be the most intense of all the emission lines containing the atomic resonance transitions. Compared to the Ar-excited lines, it is possible to excite the transitions related to the higher energy levels. The energy in a plasma is transferred from the applied electric field to electrons. Such accelerated electrons can excite or ionize neutral atoms through electron collisions. This discharge energy, which is held in neutral or ionized gas in a plasma, is simultaneously lost due to various deexcitation processes. Some of this energy is emitted by radiation but another part is stored in metastable states. The metastable state has a 106-107times longer lifetime than other states (18) and can play a role as an energy reservoir for again exciting or ionizing other atoms through radiationless collisions. I t is known that the ions and metastable atoms present in an
-
M+ + 2e- (slow)
(1) is a minor process for the ionization because this reaction is principally independent of the kind of foreign gases. It is possible to ionize sputtered atoms through a direct collision of metastable atoms of inert gases
+ Arm M + Ne" M
W W
W
+ e- (fast)
M
-
+ eM+ + Ne + e'
(2)
--
(4)
M+ + Ar
(3) where the superscript (m)indicates a metastable state. It is recognized that, in a low pressure discharge, though an important deexcitation process of the metastables is performed through the diffusion to the walls of a discharge tube (20,21), the collisions as indicated in reactions 2 and 3 are also a major energy transfer process. Because the energy transfer of atom-electron collisions is more effective than that of atomatom collisions, electron collisions of the second kind can create energetic electrons with aid of the metastables (22). Such fast electrons, which are given the potential energy of the metastable states, can ionize and excite a neutral atom to an excited state of the ion. Arm
+ e- (slow)
Ne"
+ e- (slow)
+ e- (fast) M + e- (fast) M
--
+ e- (fast) Ne + e- (fast) M+ + 2e- (slow) M+ + 2e- (slow) Ar
(5) (6)
(7) It is assumed that a low pressure glow discharge is favorable in the reactions 4 and 5 through atom-electron collisions because the population of the metastable states is relatively high (23). The kinetic energy of the energetic electrons closely depends on the energy of the metastable states. The electrons related to the Ne metastables have rather higher energies than those of the Ar metastables because the metastable states of the neon atom are about 5 eV higher than those of argon. Therefore, after ionization, the resulting ions can be lifted up to the higher excited states with Ne rather than Ar gas. However, the fast electrons generated by the electron collisions of a second kind (the reactions 4 and 5) are not always deexcited according to the collisions with the analyte atoms (reactions 6 and 7). For example, these electrons can suffer collisions with neutral Ar first. Therefore, the average kinetic energy of the fast electrons should be lower than that expected by reactions 6 and 7. Though according to reaction 4 there should be many electrons with energy of 11.5-11.8 eV in the Ar plasma, our experimental results indicate that the excitation ability of the Ar plasma is limited to a total energy of 9-10 eV. This discrepancy may be cuased by the actual energy distribution of the fast electrons. The result of this investigation seems to indicate that the kind and intensity of emission lines from sputtered materials can be varied with different foreign gases in a glow discharge light source. I t is interesting to recognize that the energy levels of a plasma gas control the emission spectrum patterns of sputtered materials. Registry No. Nzf, 13966-04-6; Ar, 7440-37-1; Ne, 7440-01-9; Nz, 7727-37-9; Ar+, 14791-69-6; Ne+, 14782-23-1; Cu, 7440-50-8;
Anal. Chem. 1985, 57,2907-2911
Cut, 17493-86-6;Ag, 7440-22-4;Agt, 14701-21-4;Sn, 7440-31-5; Sn', 26288-30-2;Al,7429-90-5;Alt, 14903-36-7;Zn, 7440-66-6;Zn', 15176-26-8.
LITERATURE CITED Grimm, W. Naturwissenschaften 1967, 54, 588. Grlmm, W. Spectrochim. Acta, Part B 1988, 238,433. Berneron, R.; Charbonnier, J. C. SIA, Surf. Interface Anal. 1981, 3, 134. Waitievertch, M. E.; Hurwitz, J. D. Appl. Spectrosc. 1976, 30, 510. Belle, C. J.; Johnson, J. D. Appl. Spectrosc. 1973, 27, 118. Takadoum, J.; Pivin, J. C.; Pons-Corbeau, J.; Berneron, R.; Charbonnier, J. C. S I A , Surf. Interface Anal. 1984, 6, 175. Oechsner. H. Appl. Phys. 1975, 8, 185. McDonald, D. C. Spectrochlm. Acta, Part B 1982, 378,747. Moore. C. E. Natl. Bur. Stand. Circ. ( U S . ) 1949. No. 467. Herzberg, G. "Molecular Spectra and Molecular Structure"; Van-Nostrand Reinhold: New York, 1950. Dogan, M.; Laqua, K.; Massmann, H. Spectrochim. Acta, Part 1971, 268,631. Laegreld, N.; Wehner, G. K. J. Appl. Phys. 1981, 32,365. Rosenberg, D.; Wehner, G. K. J . Appl. Phys. 1962, 33, 1842.
2907
(14) Wagatsuma, K.; Hirokawa, K. SIA, Surf. Interface Anal. 1984, 6, 167. (15) Wagatsuma, K.; Hirokawa, K. Anal. Chem. 1984, 56, 2024. (16) . . Pearse, R. W. 6 . ;Gaydon, A. G. "Identification of Molecular Spectra"; Chapman and Hall: London, 1965. (17) Herzbera, G. "Atomic Spectra and Atomic Structure"; Dover Pubiications: N>w York, 1944.' Bochkova, 0. P.; Shreyder, E. Y. a. "Spectroscopic Analysis of Gas Mixtures"; Academic Press: New York, 1965. Mitchell, A. C. G.; Zemansky, N. W. "Resonance Radiation and Excited Atoms"; Cambridge: New York, 1971. Copley, G. H.; Lee, C. S. Can. J . Phys. 1975, 53,1705. Ellis, E.; Twiddy:,N. D. J. Phys. B 1989, 2, 1366. von Engei, A. Ionized Gases"; Oxford University Press: Oxford, 1965. Strauss, J. A.; Ferreira, H. P.; Human, H. G. C. Spectrochim. Acta, Part B 1982. 378,947.
RECEIVED for review May 20, 1985. Accepted July 17, 1985. We are grateful to Nissan Science Fundation for the financial support of our work.
Determination of Trace Metals in Seawater by Inductively Coupled Plasma Mass Spectrometry with Preconcentration on Silica-Immobilized 8-Hydroxyquinoline J. W. McLaren,* A. P. Mykytiuk, S. N. Willie, and S. S. Berman Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
The application of Inductively coupled plasma mass spectrometry (ICP-MS) to the determlnation of eight trace metals In a coastal seawater reference material is described. Accurate calibration has been achleved for Mn, Co, NI, Cu, Zn, Cd, and Pb by standard additions techniques, while stable Isotope dilution has been applied for Cr, Ni, Cu, Zn, Cd, and Pb. I n both cases the trace metals were separated from the seawater and concentrated 50-fold by adsorption on sliicalmmobllized 8-hydroxyqulnoline prior to Instrumental determinatlon. Detection limits are in the range from 0.2 to 2 ng L-', low enough to permit the analysis of even open ocean samples.
Perhaps the single feature of inductively coupled plasma mass spectrometry (ICP-MS) most responsible for the tremendous interest in this new technique, especially among present users of inductively coupled plasma atomic emission spectrometry (ICP-AES), is its remarkable detection power. Current ICP-MS detection limits in dilute aqueous acid solution are in the range from 0.01 to 0.1 Mg L-' for most elements (1, 2), often 1-3 orders of magnitude lower than for ICP-AES. Whether such impressive detection limits can be achieved in solutions of higher dissolved salts concentrations is the subject of much current ICP-MS research. Already, though, ICP-MS is making an impact in applications to samples of low dissolved salts concentrations for which ICP-AES detection limits have previously been found to be inadequate. The example provided in this report concerns the determination of trace metals (in a 1 M HC1/0.1 M "OB acid mixture) separated from seawater by adsorption on silicaimmobilized 8-hydroxyquinoline (I-8-HOQ). Previously reported determinations of trace metals in seawater by ICP-AES have almost invariably involved a prior
separation by solvent extraction (3-6), coprecipitation (7),ion exchange (8), or adsorption (9-11), not only to avoid the difficulties of introducing a 3.5% salt solution to the ICP but, more importantly, to concentrate the trace metals sufficiently to permit their determination. Concentration factors of at least 100, and preferably 200-500, are required for analysis of unspiked and uncontaminated seawater samples (4,6, 7, 11). Such large concentrations are often rather time-consuming, and difficulties in controlling the blank can be severe. Determinations of trace metals in seawater by ICP-AES have been accomplished in our laboratory by two techniques. The first approach involved separation and 25-fold concentration of the trace metals by means of a chelating ion exchange resin, Chelex-100,and introduction of the concentrates to the ICP by means of an ultrasonic nebulizer with an aerosol desolvation system (8). This method permitted the determination of Fe, Mn, Cu, Ni, and Zn in coastal seawater samples but was inadequate for determination of Cd and Pb. Subsequently, we abandoned ultrasonic nebulization and instead used a more efficient separation procedure, developed by Sturgeon et al. ( l o ) ,which involves adsorption of the metals on silica-immobilized 8-hydroxyquinoline (I-8-HOQ) followed by elution with a 1 M HC1/0.1 M HNOBacid mixture. By this means, Cu, Cd, Mn, Ni, V, and Zn were determined in an open ocean seawater sample after a 225-fold preconcentration, but lead could not be determined and the concentrations of several of the other metals were uncomfortably close to the detection limits ( 2 1 ) . The recent development of a coastal seawater reference material, with the acronym CASS-1, provided an ideal opportunity to assess the performance of ICP-MS in an application which remains difficult by ICP-AES because of inadequate detection power, and also to assess its capability for stable isotope dilution analyses, previously accomplished in our laboratory by spark source mass spectrography (22).
0003-2700/85/0357-2907$01.50/00 1985 American Chemical Society
