Anal. Chem. 1985, 57, 1253-1255
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Determination of Metals in Aqueous Solution by Direct Nebulization into an Air Microwave Induced Plasma J o h n J. U r h a n d J o n W. C a m a h a n * Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115
A system for the malntenance of an air mlcrowave Induced plasma (Alr-MIP) at atmospheric pressure Is presented. The Alr-MIP Is malntalned with a slotted torch design and Is operated by applylng 300-500 W at 2.45 GHz. Analytical characterlstics are deflned for pneumatlcally nebulized metal contalnlng aqueous solutions. The system exhlblts detectlon llmlts In the 0.1 ppm range for Na, Cu, and Cr and 0.5 ppm for Pb, Mo, and Ca. Llnear ranges are greater than 3 orders of magnitude. Detectabllltles are poor for elements with metal-oxygen bond strengths greater than 6 eV.
Microwave induced plasmas utilizing various plasma support gases have been reported and reviewed (1-5). Most analytical microwave induced plasma work has been restricted to argon and helium plasma gases. Deutsch and Heiftje (6) have recently described the analytical performance of an atmospheric pressure nitrogen MIP (N,-MIP) with promising results. Development in our labs of a system capable of maintaining plasmas of air, Nz, Ar, and He resulted in the present study of the analytical characteristics of an Air-MIP. Certain analytical advantages may be attained from the use of molecular gas plasmas. Barnes et al. (7,8) have shown and calculated that oxygen and nitrogen inductively coupled plasmas will more efficiently decompose refractory compounds such as A1,03 than argon plasmas. This factor may serve to enhance the amount of excited analyte in the plasma, thus improving detection limits. A second important reason to consider the Air-MIP is it’s potential cost effectivness. An Air-MIP is potentially less expensive in terms of original purchase and operating costs than many other systems currently available. This work is a report of the analytical characteristics of the Air-MIP. Linear response of greater than 3 orders of magnitude of concentration is shown for several representative elements. Detection limits for 6 of the 11 elements investigated are below 1ppm. High detection limits in the Air-MIP are reported for elements with metal-oxygen bond strengths greater than 6 eV. EXPERIMENTAL SECTION Instrumentation. The torch, cavity, and microwave generator system used for this work has been described previously (9). The Pyrex cooling jacket of the torch was replaced with a quartz jacket to provide better torch thermal stability. For detection limit and linear range studies,the 0.35-m holographic scanning spectrometer was replaced with a 0.75-m Echelle spectrometer (Spectraspan 111, Spectrametrics, Andover, MA). Plasma Ignition. The Air-MIP was difficult to ignite before the impedance of the Beenakker cavity was matched with the nominal impedance of the microwave generator. Tuning was accomplished by adjusting the length of the tuning stubs on the internally tuned cavity until the reflected power was minimized as indicated at the generator. The optimum positions for the tuning stubs were determined after the air plasma was ignited and the system operated continuously for about 5 min. A number of plasma ignition methods were investigated. A common method used for initiating microwave plasmas has been
to insert a copper wire, which heats in the microwave field and presumably acts as an electron source igniting the plasma. The method used was to adjust the generator to approximately 400 W of output power with a copper wire inserted into the plasma region. The Air-MIP proved to be difficult to ignite with this method. However, when the Cu wire was trimmed to a sharp point, exposing unoxidized metal prior to each ignition, the plasma could be ignited with relative ease. Once the system was properly tuned, ignition became a matter of trimming the wire, inserting it into the plasma region, and turning the generator on. To minimize the microwave antenna effect of the inserted wire, the amount of wire extending beyond the plasma region was kept to a minimum. It should be noted that another method of plasma ignition was also used. A spark introduction apparatus small enough to be inserted into the torch was devised with a copper wire coiled about a Tesla coil and extending away far enough to be inserted into the plasma region. This technique proved to be successful but clumsy. RESULTS AND DISCUSSION Optimization. Spectral scans reveal a number of diatomic species. The spectrum of the Air-MIP from 200 to 800 nm is shown in Figure 1. Molecular bands observed arose from Nz,Nz+, NO, CN, and Cz. Avoiding these spectral features became a criteria in selecting the emission lines to be observed for analytical purposes. The method previously reported for determining the limits of the power and flow settings (9) has since been redefined. Lower plasma gas flows than previously considered to yield a stable plasma gave superior analytical results with many of the elements analyzed. This region of performance was first explored while optimizing for Fe. By use of the simplex method (10) for optimization, it was found that the optimum flow rate for Fe was lower than previously thought to be acceptable a t 400 W applied power. A study was undertaken to determine the optimal viewing position. Figure 2a shows an axial profile of signal, background, and S I B for a 1 ppt solution of Fe observed a t the 371.994-nm atomic emission line. Spectrometer slit dimensions were 200 pm x 100 wm for both the entrance and exit slits. The distance units along the x axis indicate the position of the entrance slit in relation to the plasma image. The center of the plasma is at 2.5 mm. Figure 2b shows a plot of the same variables, radially viewing the image of the plasma tailflame. In this plot, zero corresponds to the base of the afterglow as it exits from the torch opening at the cavity faceplate. Both plots were generated with the plasma at optimal conditions for the particular viewing position. These plots proved to be inconclusive in determining the optimal viewing position since the greatest signal to background ratio was not observed in the region of the most intense analytical signal. Therefore, detection limits were determined in three analytically promising observation modes. These observation modes were (1) viewing the plasma axially (end on), with the fireball of the plasma focused on the entrance slit, (2) axially but 1.25 mm off-center, and (3) radially (from the side) in the region of the tailflame where the signal-to-background ratio was maximum. These detection limits are listed in Table I. From a review of this table, it may be concluded that axial on-center viewing
0003-2700/85/0357-1253$0 1.50/0 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
e >I
C
-C
u NH
U
u
OH
L
NO I 0 0
a
I
I
I
0
e
I 0 0
I
I
I
I
0
0 0 d
0
in
(0
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I
I
0 m 0
0 N 0
Wavelength
Flgure 1. Spectrum of the Air-MIP
Table I. Iron Detection Limits in Various Plasma Viewing Positions with a 0.35-m Spectrometer viewing position
power,
W
flow, L/min
detection limit, ppm
axial, on-center axial, 1.25 mm from center radial
400 400 400
2.9 2.9 5.8
2.1 4.9 13
Table 11. Comparative Line Intensities" element
Cr
Sr Ca Position (rnrn)
cu Mo Pb
B Na La
W
wavelength, nm
axial
radial
360.533 I 359.349 I 357.869 I 460.733 I 421.552 I1 407.771 I1 393.366 I1 396.847 I1 422.673 I 327.396 I 324.754 I 390.296 I 386.411 I 379.825 I 405.781 I 368.346 I 283.306 I 249.677 I 249.733 I 589.592 I 588.995 I 394.910 I1 400.875 I 407.436 I
6.50 7.70 10.00 10.00 5.55 7.75 10.00 7.60 9.40 7.43 10.00 6.56 7.65 10.00 9.50 10.00 8.50 10.00 9.38 6.60 10.00 10.00 10.00 10.00
5.98 8.14 10.00 10.00 1.70 3.45 5.03 2.62 10.00 5.62 10.00 6.37 8.92 10.00 N/O 10.00
N/O N/O N/O 6.85 10.00 N/O N/O N/O
" N / O = Line was not observed.
Position (rnm) Flgure 2. Iron emission profiles for the Alr-MIP.
yields the best detection limits for Fe. This conclusion was corroborated after spectra for the various elements were obtained. It may be noted that the detection limit for the optimal viewing position is higher than the detection limit reported for Fe in the detection limit discussion. This is
because the spectrometer used in the plasma viewing position study was of low resolution while final detection limit data were gathered with a higher resolution spectrometer. Table I1 is a listing of the elements considered in this study. Listed are the elements, the most intense wavelengths observed, and the normalized intensities of those lines. The most intense emission lines for each element were assigned a relative intensity of 10.00. This table shows that several elements at a concentration of 1ppt were seen when viewed axially but were undetected when viewed radially (W, La, and B). Therefore, the plasma was observed axially on-center throughout the remainder of the experiments. The plasma torch was moved from the low-resolution 0.35-m McPherson EU-700 to the high-resolution 0.75-m echelle spectrometer. The system was configured for axial on-center
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T a b l e 111. O p t i m i z e d C o n d i t i o n s and F i g u r e s o f M e r i t f o r t h e Air-MIP wavelength,
power,
flow,
element
nm
W
L/min
linear range, PPm
Na
588.995 I 324.754 I 357.869 I 379.825 I 393.366 I1 368.346 I 371.994 I 249.677 I
500 400 300 350 300 300 400 500
8.64 5.18 4.32 4.32 4.32 5.76 4.32 5.76
1000-1 1000-1 1000-1 1000-1 1000-1 1000-1 1000-3 1000-100
cu
Cr Mo Ca
Pb Fe
B Zn La
W
detection slope
intercept
0.888 f 0.025 0.442 i 0.045 0.870 f 0.083 0.504 f 0.059 0.946 f 0.013 0.190 f 0.022 1.002 f 0.021 0.034 i 0.038 0.952 f 0.009 0.160 f 0.020 0.861 f 0.042 0.346 f 0.081 0.943 f 0.052 0.178 f 0.109 1.117 f 0.110 -0.321 f 0.279
0.082 0.089 0.090 0.42 0.43 0.59 1.4 93
loo+ loo+ loo+
481.053 I 394.910 I1 400.875 I
viewing. With a magnification factor of 2 for the primary optic, the fireball was focused on the entrance slit of the spectrometer using identical entrance and exit slit openings of 200 X 100 wm. By use of the simplex procedure, the optimum power applied and carrier gas flow rate for each element was obtained. Boundaries for the simplex were set at minimum stable plasma gas flows and the maximum power output of the microwave generator (500 W). Throughout these studies coolant and nebulizer gas flow, viewing position, reflected power, nebulized solution uptake rate, and nebulization system were all kept constant, at the optimal conditions for Fe. Table I11 shows that almost every element has unique optimal conditions. The detection limits for Na, Ca, and Cu are relatively low (CO.09 ppm) and would be acceptable for a large number of routine analyses. There is a correlation between high metal-oxygen bond strengths and high detection limits. The oxygen of the plasma gas (ca. 21%) does seem to effect the detection limit of those elements with metal-oxygen bond strengths of greater than 6.0 eV. Zinc, which exhibits a poor detection limit, has a Zn-0 bond strength below 6.0 eV but has a high excitation energy (6.66 eV). Similarly poor zinc detection limits were reported by Deutch and Heiftje (6) using a N2-MIP. A second factor contributing to the high detection limits may be the high gas flow rates required. Better detection limits have been reported for metals in solution with an argon-MIP (11). The total gas flow of the system in ref 11 was only 0.45 L/min. Typical gas flows for the Air-MIP are 4-8 L/min. We are currently exploring lower flow torch designs. Optimum conditions are not reported for La, W, and Zn. This is because the emission intensity of these elements was not sufficient to warrant analytical investigations. Spectroscopic Temperature. Analyte excitation temperature was determined by using the nine-point slope method of Reif, Kneiseley, and Fassel (12). Optics were arranged with a magnification factor of 6X. Background and iron spectra were obtained, the background was subtracted from the iron spectra, and the integrated intensity of each iron line was calculated. Utilization of the transition probabilities from the data of Reif et al. (12) yielded a better correlation coefficient
limit, ppm
eV
metal-oxide bond dissociation energy, e V
2.11 3.89 3.46 3.26 3.15 4.34 3.33 4.96
2.8 4.9 4.4 5.0 4.8 3.9 4.3 8.1
6.66 3.54 3.40
4.0 8.1 6.9
E,,
(0.98) than the use of probabilities tabulated by either Banfield and Huber (13)or Huber and Parkinson (14). The temperature obtained was 5670 f 80 K at the 95% confidence interval. This temperature is above that of flames (15)and very near that of the inductively coupled plasma (16).
CONCLUSION The Air-MIP produces reasonably low detection limits for a number of elements. With improvements in the nebulization system and torch design and the application of larger amounts of power, it is likely that the detection limits reported here can be improved. However, these improvements in system performance are necessary if the Air-MIP is to become competitive with other plasma emission sources. R e g i s t r y No. Na, 7440-23-5; Cu, 7440-50-8; Cr, 7440-47-3; Mo, 7439-98-7; Ca, 7440-70-2; Pb, 7439-92-1; F e , 7439-89-6; B, 744042-8.
LITERATURE CITED Zander, A. T.; Heiftje, G. M. Appl. Spectrosc. 1981,35, 357-371. Carnahan, J. W. Am. Lab. (Fairfield, Conn.) 1983, 15(8),31-36. Greenfield, S.; McGeachin, H. McD.; Smith, P. B. Talanta 1975,22, 1-15. Greenfield, S.; McGeachin, H. McD.; Smith, P. B. Takanta 1975,22, 553-562. Greenfield, S.; McGeachin, H. McD.; Smith, P. B. Talanta lg76, 23, 1-13. Deutsch, R. D.; Heiftje, G. M. Anal. Chem. 1984, 56, 1923-1927. Barnes, R. M., Kovacic, N. 1Ith Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, Sept 16-21, 1984; Paper 76. Barnes, R. M.; Nikdel, S. Appl. Spectrosc. 1978,30,310-320. Michlewicz, K. G.; Urh, J. J.; Carnahan, J. W. Spectrochim.Acta, Part 8 ,in press. Deming, S. N.; Morgan, S. L. Anal Chem. 1073, 4 5 , 278A-283A. Haas, D. L.;Caruso, J. A. Anal. Chem. 1984,56, 2014-2019. Reif, I.Ph.D. Thesis, Iowa State University, Ames, IA, 1971. Banfield, F. P.; Huber, M. C. E. Astrophys. J . 1973, 786, 335-346. Huber, M. C.E.; Parkinson, W. H. Astrophys. J . 1972, 172, 229-247. Parsons, M. L.; Smith, B. W.; Bentiey, G. E. "Handbook of Flame Spectroscopy"; Plenum Press: New York, 1975. Kalnicky, D. J.; Kniseley, R. N.; Fassel, V. A. Spectrochim, Acta, Part 8 1975,308,511-525.
RECEIVED for review December 14,1984. Accepted February 22, 1985.
