Anal. Chem. 1980, 52, 1523-1525
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CORRESPONDENCE Low-Power Inductively Coupled Nitrogen Plasma Discharge for Spectrochemical Analysis Sir: Inductively coupled plasma (ICP) discharges for spectrochemical analysis are commonly operated a t atmospheric pressure with argon or a combination of argon and nitrogen, air, or oxygen (It.?). For these later ICP discharges, argon supports the discharge and “coolant” nitrogen or other diatomic gas prevents t h e hot discharge from damaging the container walls. Originally, nitrogen-argon ICP discharges were operated a t substantially higher nominal power levels t h a n argon-supported ICP discharges (1-7), but recently, Greenfield a n d McGeachin ( 8 ) , Greenfield and Burns (9), Montaser and Mortazavi ( I O ) , and Ebdon et al. ( I I , 12) demontrated low-power nitrogen-argon ICP discharges for spectrochemical applications. In contrast to these diatomic gas-argon ICP discharges, no reports have appeared previously describing an ICP discharge substained totally in nitrogen for spectrochemical analysis. Based upon preliminary experitmental measurements, this correspondence describes the operation and properties of a nitrogen ICP discharge. In 1976 Barnes and Nikdel (13) calculated the properties of a spectrochemical ICP discharge in nitrogen and compared these results with those predicted for an argon ICP discharge operated under indentical conditions. Their computations indicated higher maximum plasma temperature and larger plasma volume for the argon than for t h e nitrogen ICP discharge. T h e axial gas velocity of the aerosol carrier gas was found to be higher in the nitrogen than in the argon discharge. Furthermore, a comparison of the minimum operating power a n d magnetic flux density for the two discharges indicated that approximately six times more power would be required t o sustain t h e nitrogen ICP than the argon plasma. Barnes a n d Nikdel also predicted that the decomposition of dry alumina particles injected along the centerline of the two discharges would be more rapid and complete in the nitrogen ICP in spite of higher plasma temperature and longer particle residence time in the argon discharge. As an extension of these predictions, Barnes and Nikdel suggested that the resulting signal-to-background ratio of aluminum analyte should be greater in a nitrogen than in an argon ICP operated at identical conditions. Until now, experimental d a t a with which t o test these predictions have not been available. Recently a nitrogen ICP operating a t a minimum power of 1.3 kW was sustained with a conventional spectrochemical system. This new ICP is being investigated presently to establish the validity of Barnes and Nikdel’s calculations. Because this nitrogen ICP discharge potentially may provide unique spectrochemical results, a preliminary description of its operation and properties is reported.
EXPERIMENTAL The discharge was generated in a configuration similar to that described by Scott et al. ( 1 4 ) with commercially available equipment listed in Table I. A manual, fixed-frequency impedance matching network designed by following the guidelines described by Allemand and Barnes (15)for efficient power transfer was constructed for operation with either 27- or 41-MHz generators. The tip of the aerosol tube was placed 5 mm below the 0003-2700/80/0352-1523$01 .OO/O
Table I. Plasma Equipment and Operating Conditions inductively coupled plasma generator induction coil nebulizer nebulizer chamber torch assembly gas flow rates outer, nitrogen intermediate, nitrogen aerosol, nitrogen observation height imaging optics
monochromator grating slit widths slit heights readout detector readout
Plasma Therm Model HFS-5000D, 40.68 MHz 3.5-turn, ’I8-inchcopper, 25-mm i.d. cross-flow ( 2 9 ) , 2 mL/min conical, 30 mL quartz, 18-mm i.d. ( 1 4 ) , 0.9-mm aerosol orifice 25 L/min 3.5 Llmin 1.5 Llmin 5 mm above coil quartz lens, 2-inch diameter, 200-mm focal length, Oriel A-11-661-37, 1:l image 0.7 5-m Czerny-Turner, Spex Model 1 7 0 0 I1 1200 lines/mm 30 pni 2-mm entrance, 25 mm exit RCA 1P28, 650 V picoammeter, Keithley Model 414s;recording photometer, Heath Model E U. 2 0-28
intermediate tube. A similar separation was found by Greenfield et al. (16). The nitrogen ICP discharge was produced by starting the plasma with argon in the outermost annulus (Le., “plasma” or “coolant” argon) under impedance matching conditions best suited to the argon discharge. With the argon discharge sustained, the impedance matching network was retuned to values previously determined for optimum operation of the nitrogen ICP discharge. Nitrogen was introduced into the argon, and its volumetric flow was increased until the discharge remained unchanged. Next, nitrogen aerosol carrier gas was introduced while simultaneously reducing the argon flow in the outermost stream to zero. An annular nitrogen discharge results. Nitrogen was then introduced into the intermediate annulus (auxiliary or plasma gas), and the flow was adjusted to minimize damage to the intermediate quartz tube of the torch. By following this simple procedure with appropriate impedance matching network parameters, the argon discharge was converted to a nitrogen discharge in 15 s. Since the impedance matching network parameters were fixed during this process, the rf power at the load coil was established by the coupling between the discharge and the induction coil (15). Coupling increased progressively during each stage of the conversion from argon to nitrogen. For example, the mismatched argon discharge operated at a forward-to-reverse power ratio of 1000:500 is compared to the fully matched nitrogen discharge with a forward-to-reverse power ratio of 1495:5. The visible plasma volume was reduced as nitrogen was introduced with the argon in the outermost flow, and as the nitrogen content was increased to 75% of the total gas flow, the discharge resembled a nitrogen-cooled discharge ( 2 , 5 , 7 , 8 , 10). When the argon flow was eliminated, the discharge intensity decreased, and the visible plasma elongated and became triangular. C 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 b
T
n
Table 11. Measured Properties of Nitrogen and Argon ICP Discharges nitrogen argon
N2. N;
I
operating power, kW outer gas flow, L/min -5mm aerosol gas flow, L/min observation height, mm calibration function slope, nA. mL/pg Ca I 422.6 nm Ca I1 393.3 nm line intensity ratio Ca I1 393.3 nm/ 450 Ca I1 396.8 nm I Ca I1 393.3 nm/ Ca 1422.6 nm limits of detection, ng/mL" Ca I1 393.3 n m Ca I 422.6 nm signal-to-background ratio Ca I 422.6 nm at 8.4 pg/mL
N2
IxK)W
200
a
250
300
350
400
1.2 17 1.5 15
1.3
25 1.5 5
0.825 i 0.013 0.876 i 0.012 8.82 ? 0.294 54.6 i 0.089 1.89
1.76
8.81
71.4
800
40
0.1 60
55.7
14.6
a Based on three-times standard deviation of background signal.
Figure 1.
Spectra of (a) argon (bottom) and (b) nitrogen (top) ICP
discharges. Observation zone (-5 mm) corresponds to the center of the induction coil
b
' I
RESULTS AND DISCUSSION Barnes and Nikdel (13) predicted that the introduction of t h e aerosol gas would perturb the centerline temperature in the nitrogen discharge substantially less than in the argon ICP. This was apparent experimentally from the "tunnel" or hole created in the two discharges upon introduction of either dry gas or aqueous analyte solutions. Injection of the nitrogen aerosol carrier gas into the nitrogen ICP produced a significantly less apparent hole along the central axis of the discharge than observed in the argon ICP. Furthermore, nebulization of aqueous analyte solutions with nitrogen into the nitrogen ICP did not appear to enlarge the central channel as it did in the argon discharge. This insensitivity to analyte aerosol introduction and apparent minimal central-channel cooling of the nitrogen ICP offers potential for novel sample introduction approaches in spectrochemical analysis with a lowpower ICP. Some of these possibilities include the on-line analysis of ambient air or other polyatomic gas for airborne analytes; the direct injection of slurries and aerosols without a nebulizer spray chamber, and decomposition of high-density aqueous aerosols from high-solids or ultrasonic nebulizers without prior desolvation. They are currently under study. As predicted by Barnes and Nikdel (13),lower total radiation emitted from the nitrogen ICP discharge was confirmed experimentally when the spectra for both argon and nitrogen discharges were compared as illustrated in Figure 1. Although the principal spectral features of the nitrogen ICP discharge resemble those reported previously by Greenfield et al. (1, 6, 17, 18),Montaser and Mortazavi (IO),and Ohls and Sommer ( 2 ) ,the nitrogen continuum was substantially lower than the argon continuum. As Greenfield et al. (3,17) reported for the nitrogen-argon ICP a t high power levels, the addition of nitrogen to the argon ICP reduced the argon background, and t h e nitrogen molecular band emission did not significantly interfere with normal spectrochemical analysis. In addition to the molecular nitrogen emission from the nitrogen ICP, the spectrum (Figure 1)also contained Si I lines as a result of the ablation of the intermediate quartz tube while the spectrum was recorded. Preliminary data evaluating the spectrochemical potential
?
?
SI
I
?
V
I
,caI
200
250
300
350
400
450
500
Figure 2. Spectra of (a) argon (bottom) and (b) nitrogen (top) ICP discharges with 100 pg/mL calcium solution introduced. Observation zones measured from the top of the induction coil
of the nitrogen ICP are summarized for calcium in Table 11. Results are presented for only all nitrogen and all argon discharges. Whereas the minimum operating power for the argon ICP discharge was 0.2 kW, the nitrogen discharge could not be substained at a power level below 1.3 kW. This factor of six corresponds to the ratio predicted by Barnes and Nikdel (13),although the absolute magnitude of experimental and predicted values differed because of dissimilar plasma configuration and generator frequency employed. The observation height determined for maximum Ca I1 signal was found to be 5 mm above the induction coil in the nitrogen ICP and 15 mm in the argon discharge. T h e spectrum in Figure 2 obtained with 100 pg.mL-' calcium solution demonstrates a reduction in the nitrogen molecular band interference a t this observation position compared t o the mid-coil position in Figure l a (bottom).
Anal. Chem. 1980, 52, 1525-1527
Slopes of concentration calibration functions for Ca I and I1 indicate t h a t the calcium emission was more sensitive in the argon discharge than in the nitrogen ICP especially for the Ca I1 393.3-nm wavelength. Although the intensity ratio of two Ca I1 lines in both discharges is similar for the observation zones chosen, the Ca 11-to-Ca I ratio is substantially greater in the argon ICP than in the nitrogen discharge. T o explain the exceptional Ca 11-to-Ca I ratio in the argon ICP, Mermet (19) proposed an hypothesis for excitation in argon ICP discharges in which the argon metastable levels enhanced the population of analyte elements by Penning ionization. Boumans and deBoer (20) extended this hypothesis by attributing t o the metastables also the role of an easily ionizable constituent so as to explain concomitantly the high sensitivity of ionic lines and the low ionization interferences. Although alternative excitation mechanisms have been described by Alder et al. (21),Lovett (22),and Eckert (231, the absence of high-energy metastable levels in the nitrogen ICP discharge may contribute to the lower ion-to-atom emission ratio in the nitrogen discharge. Interpretation of these preliminary results suggests t h a t a nitrogen ICP discharge is closer to local thermodynamic equilibrium (LTE) than an argon ICP. Greenfield and Burns (9) proposed a similar difference for the nitrogen-argon ICP discharge, and Janca and Talsky (24)found previously that with increasing concentration of nitrogen in argon ICP discharges that the plasma became isothermic a t much lower pressures than pure argon discharges. Various important consequences result from the possibility of an ICP discharge in LTE. First, verification of the Barnes and Nikdel predictions for a nitrogen ICP can be performed quantitatively without questioning the LTE assumption made in their model. Second, the spectrochemical stray light interferences (25) and ion-recombination background (26)observed in argon ICP discharges would be expected to be reduced in the nitrogen ICP. If this were the case, then special precautions to minimize stray light and to correct background interferences required for the argon ICP in the presence of high calcium, magnesium, and aluminum analyte concentrations might not be needed for spectrochemical analysis with the nitrogen ICP. Furthermore, spectral intensities may correspond directly t o both atom and ion emission values in established wavelength tables (27, 28). Limits of detection for Ca I1 and Ca I emissions in the two ICP discharges are also reported in Table 11. Operating conditions were not optimized for either discharge in these preliminary experiments. Whereas, the limit of detection for Ca I1 is significantly lower in the argon discharge, a somewhat improved detection limit for Ca I is observed in nitrogen. The latter results from the improved signal-to-background ratio for atomic lines obtained in the nitrogen ICP discharge. From an economic standpoint, operation of a nitrogen ICP discharge is favored when compared to an argon ICP although the gas flow rates were higher for nitrogen in the present experiments. Operation of the nitrogen ICP a t its minimum power of 1.3 kW may not be optimum for multielement spectrochemical analysis when limits of detection and inter-
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element interferences are considered. Studies of these properties are currently under way. Extrapolation of these preliminary results to other spectrochemical possibilities appears encouraging.
ACKNOWLEDGMENT The technical assistance of C. D. Allemand is greatly appreciated.
LITERATURE CITED (1) Greenfield, S.;McGeachin, H.; Smith, P. 8. ICP Inform. News/. 1976, 2, 167. (2) Ohls, K.; Sommer, D. ICP Inform. Newsl. 1979, 4, 532. (3) Greenfield, S.;Jones, 1. L. W.; Berry, C. T.; Bunch, L. G. Proc. Soc. Anal. Chem. 1965, 2. 1 1 1 . (4) Greenfield, S.;Smith, P. B. Anal. Chim. Acta 1972, 59, 341. (5) Greenfleld, S.;Jones, I.L. W.; McGeachin, H.; Smith, P. B. Anal. Chim.
Acta 1975. 74. 225. (6) Greenfield,'S.; Smith, P. 8. Anal. Chim. Acta 1971, 57,209. (7) Zeeman, P. B.; Terblanche, S. P.; Visser, K., Hamm, F. H. Appl. Spectrosc. 1976. 32,572. (8) Greenfield, S.;McGeachin, H. Anal. Chim. Acta 1978, 100, 101. (9) Greenfield, S.;Burns, D. T. Anal. Chim. Acta 1980, 173,205. IO) Montaser. A.; Mortazavi. J. Anal. Chem. 1960, 52,255. 11) Ebdon, L.; Mowthorpe, D. J.; Cave, M. R. Anal. Chim. Acta 1980, 775, 171. 12) Ebdon, L.; Cave, M. R.; Mowthorpe, D.J. Anal. Chim Acta 1980, 775, 179. 13) Barnes, R . M.; Nikdel, S. Appl. Spectrosc. 1976, 3 0 , 310. 14) Scott, R. H.; Fassel V. A.; Kniseley, R. N.; Nixon, D. E. Anal. Chem. 1974, 46,75. 15) Ailemand, C. D.; Barnes, R. M. Spectrochim. Acta, Part B 1978, 33, 513. (16) Greenfield, S.;Jones, I.L.; Berry, C. T. Ana/yst(London)1964, 89,713. (17) Greenfield, S.Metron 1971, 3(8).224. (18)Greenfield, S. Proc. Anal. Div. Chem. Soc. 1976, 79,279. (19) Mermet, J. M. C.R.Acad. Sci, Ser. B 1975, 287,273. (20) Boumans, P. W. J. M.; deBoer, F. J. Spectrochim. Acta, Part 6 1977, 32, 365. (21) AMer, J. F.; Bombelka, R. M.; Kirkbright, G. F. Proc. Anal. Div. Chem. Soc. 1979, 76,21; Spectrochim. Acta, Part B 1980, 35, 163. (22) Lovett, R. J., presented at the Sixth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, Pa.,
1979;Paper 52. (23) Eckert, H. U., presented at the International Winter Conference on
Developments in Atomic Plasma Spectrochemical Analyses, San Juan, Jan. 1980;Paper 3. (24)Janca, J.; Talsky, A. Rir. Fak. Univ. PurQne Brne, Folia Physica 1974, 18,49. (25)Larson. G. F.; Fassei, V. A.; Winge, R. k:.; Kniseley, R. N. Appl. Spectrosc. 1976, 3 0 , 384. (26)Larson, G. F.; Fassel, V. A. A p p l . Spectrosc. 1979, 33,592. (27)Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33,
206. (28) Boumans, P. W. J. M.; Bosveld, M. Spectrochim. Acta, Part B 1979, 34, 59. (29) Kniseley, R. N.; Amenson, H.; Butler, C. C.; Fassel. V. A. Appl. Spectrosc. 1974, 28, 285.
Ramon M.B a r n e s * G e r h a r d A. Meyer Department of Chemistry GRC Tower I University of Massachusetts Amherst. Massachusetts 01003
RECEIVED for review March 3, 1980. Accepted May 9, 1980. This research is supported by the Department of Energy (Office of Basic Energy Sciences) Contract DE-ASOB77ER04471. This paper was presented a t the Sixth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, Pa., September 1979.
Resolution and Signal-to-Noise in Fourier Transform Mass Spectrometry Sir: Fourier transform mass spectrometry ( F T / M S ) can provide ultra-high-resolution mass spectra in short periods of time ( I , 2 ) . Using an instrument constructed in our labo0003-2700/80/0352-1525$01 .OO/O
ratory and described elsewhere ( 3 ) ,we have obtained resolution up to 760000 for the molecular ion of benzene a t m / t 78 (Figure l a ) . This result was obtained using a benzene
C 1980 American Chemical Society
