Solution nebulization of aqueous samples into the tubular-electrode torch capacitatively-coupled microwave plasma

Talanta, Vol. 35, No. 8, pp. 641-645, 1988 Printed in Great Britain. All rights reserved

0039-9140/88 $3.00+0.00 Copyright © 1988 Pergamon Press plc

SOLUTION NEBULIZATION OF AQUEOUS SAMPLES INTO THE TUBULAR-ELECTRODE TORCH CAPACITATIVELY-COUPLED MICROWAVE PLASMA B. M. PATI/L*, J. P. DEAVOR'j"and J. D. WINEFORDNERJ~ Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. (Received 23 September 1987. Revised 26 January 1988. Accepted 5 February 1988) Summary--This work shows the feasibilityof using nebulization for introduction of aqueous samples into the tubular-torch capacitatively-coupledmicrowave plasma (CMP). Previously, solid electrodes were used with this type of plasma, in which analyte carrier and plasma support gases are premixed and swept around the electrode tip. With the new design, the analyte carrier gas passes through the centre of the hollow tubular electrode and mixes with the plasma support gas at the tip of the electrode where the plasma is formed. Sample solutions are nebulized with a Meinhard nebulizer and a laboratory-constructed spray chamber and desolvation system. The tubular torch is made of tantalum. Plasma gases investigated include argon, helium and nitrogen. Typical operating powers are 300-350W. Elements studied include Ag, AI, Ba, Ca, Cd, Cr, Cs, Cu, K, Li, Na, Pb, Pd, Sr and Zn.

There are two main types of microwave plasmas employed in analytical chemistry. By far the most commonly used is the microwave-induced plasma (MIP)) -4 The MIP is confined within a silica or alumina tube and sustained by coupling microwave power to the gas within such a tube by means of a resonant cavity. The second type of microwave plasma, the focus of this paper, is the capacitativelycoupled microwave plasma (CMP), also referred to as the single-electrode plasma (SEP). This discharge features a plasma produced at the tip of a metal electrode to which microwave power is coupled by a coaxial waveguide. Dahmen 5 gave an extensive bibliography of the CMP in his 1981 review and has annually updated it. 6-l° Much work has since been accomplished by several groups in Japan, Europe and the United States. Akatsuka and Atsuya have used the CMP to analyse steels for manganese,~t molybdenum,t2 nickeP 3 and vanadium, 14and also arsenic by hydride generation) 5 They have optimized the procedures and looked for possible interferences. A high concentration of iron was found to provide a convenient spectrochemical buffer. Wunsch and co-workers have performed analysis for tungsten ~6-~8 and ten other elements./9 Feuerbacher2° introduced a 120-mm long plasma Research supported by EPA-CR-813017-01-1. *On leave from the Atomic Energy Commission, India; Department of Atomic Energy, Bhabha Atomic Research Centre, Radiochemistry Division, Bombay 400085, India. ?On leave from Department of Chemistry, College of Charleston, Charleston, SC 29424, U.S.A. ~Author to whom correspondence should be addressed. TAL. 3 5 / ~ D

torch and minimized condensation problems by having the nebulizer very close to the plasma. Disan et al. 2~ determined 21 metals in trace concentrations in aqueous solutions, using nitrogen as a second mantle gas to stabilize the plasma and improve excitation conditions, and also made an interference study. Winefordner and co-workers have performed both applied and fundamental studies of their CMP. Inorganic and organometallic mercury was determined by vaporizing these species thermally from standard reference orchard leaves and tuna and detecting them by atomic emission as a function of vaporization appearance temperature for various chemical species. 22 A platinum-clad tungsten electrode was found to be thermally stable and chemically inert, giving longer electrode life.23 The CMP was characterized with regard to spectroscopic and electron temperatures and density for various plasma gases. 24 In another study, the CMP was optimized to determine trace water in evolved-gas samplesfl5 Trace hydrogen and oxygen in NBS/SRM titanium was determined by heating the metals in a helium atmosphere at reduced pressure and sweeping the evolved gas into the CMP. 26 An air plasma using a solid electrode was optimized for use with aqueous solutions for determination of calcium, potassium and sodium in SRM oyster tissue and glass. 27 The total lead content of gasoline engine exhaust gases was determined and compared with the amount of particulate leadfls Tin has also been determined by means of hydride generation and the tubular electrode CMP. 29 The tubular electrode facilitated mixing of the analyte with the plasma support gas. The increase in sensitivity was over two orders of magnitude for tin. The present study was performed to show the possibilities of nebulizing solutions into the tubular-torch CMP.

641

642

B. M. PATEL et al. Table 1. Instrumentation used for solution nebulization into the tubular electrode torch microwave plasma emission spectrometer system Apparatus

Model

Manufacturer

Magnetron output frequency 2.450 GHz; Plate voltage (max) 4.5 kV; Plate current (max) 350 mA; Output power, 885 W Power supply (for magnetron) constant current, regulation 15%; output voltage (max), 3 kV; output current (max), 400 mA Plasma torch

H 3032 L (for microwave oven)

Hitachi Ltd., Tokyo, Japan

803-330

Hipotronics Inc., Brewster, NY

Nebulizer

Meinhard concentric glass nebulizer, TR-30-A3

Spray chamber and desolvation system Monochromator 0.35 m focal length, 1200 grooves/mm grating scanning spectrometer Photomultiplier tube

EU-700

R-955

High-voltage power supply (for photomultiplier tube) Differential amplifier

226

Recorder

Series 5000

Hamamatsu Corp., Bridgewater, NJ Pacific Instruments, Concord, CA Tektronix Inc., Beaverton, OR Fisher Sci. Co., Pittsburgh, PA

26A2

EXPERIMENTAL Apparatus The single-channel electrode and the electrode-holder assembly have been described elsewhere. 29 The central electrode is made of tantalum and is held within an aluminium electrode holder which screws into a brass tube. Carrier gas passes through the central channel and sheath gas flows around the electrode but within the silica tube. The electrode assembly is water-cooled, and the magnetron is air-cooled. The instrumental components are listed in Table 1. The plasma is attached to the tip of the tubular tantalum electrode. When helium is used as both carrier and sheath gas, the plasma is self-igniting. Other gases or combinations of gases require the insertion of the metal tip of an insulated screwdriver to cause arcing and initiate the plasma. The gas flow-rates have been optimized to yield the

Laboratory constructed 29 J. E. Meinhard Associates, Santa Ana, CA Laboratory constructed GCA/MePherson Instrument Corp., Acton, MA

best signal. Use of argon as the analyte carrier gas and nitrogen as the plasma sheath gas gives a stable plasma, without the plasma-displacement found when an all-helium or helium-nitrogen plasma is used. A schematic diagram of the system is given in Fig. l and the operating conditions are given in Table 2. Procedure Standard solutions of various metals were used to determine analytical figures of merit. They were prepared by dissolution of the metals in acid, or from the metal halides. Blank determinations were made with demineralized distilled water. A Meinhard C-2 concentric nebulizer was used to generate an aerosol in a laboratory-built spray chamber, and the aerosol was carried through a water-cooled condenser to the plasma. The connection between the condenser and the plasma was kept warm with a heating tape. Blank

Direct reading spectrometer

I~1onochromator

Power supply for magnetron SampLe and carrier gas

NebuLizer system

Fig. 1. Block diagram of the experimental system.

Nebulization of aqueous samples Table 2. Operating conditions of spectrometric system Microwave frequency Magnetron anode voltage Magnetron anode current Microwave power output Plasma viewing mode Plasma viewing position Plasma gas (N2) flow-rate Sample nebulizer gas (Ar) flow-rate Solution uptake rate Slit width Slit height

.2.450 GHz

2.1 kV dc 155 mAdc 325 W radial 3 mm above the electrode tip 6.51/min 0.61/min 1.75 l/min 0.35 mm 10 mm

643

90

.....o

80 70 60

~ 50 i~ 40 3O 20 10 260

I 300

I 340

I 380

I 420

I 460

I 500

I 540

Power (W)

and sample solutions were run alternately, each being sampled for a minimum of 30 sec. The plasma was observed by means of a 0.35-m Czerny-Turner monochromator/ photomultiplier system with focusing of an unmagnified image on the entrance slit. A current-to-voltage amplifier/filter system was optimized for each element to yield the maximum signal-to-noise and signal-tobackground ratios. The observation zone was 2-6 mm above the tip of the electrode. Molecular-band emission in certain regions of the spectrum added to the background. The plasma was operated at a power yielding the optimum signal-to-background ratio. RESULTS AND DISCUSSION

Effects o f microwave power on emission intensity An increase in microwave power increased the emission signal for chromium (Fig. 2) but also increased the background emission signal. Similar effects were found for the other elements studied. For each element, an optimum power level was established, based on the signal-to-background ratio, as depicted in Fig. 3 for chromium. Because the sheath or plasma gas and nebulizer gas flow-rates had minimal effect on the signal-to-background ratios for all the element lines studied (see below), univariate searches could be made for finding the operating (not optimized) parameters given in Table 2. Effects o f different gases and flow-rates Variation of the type of plasma-sheath or carrier gas had little effect on the emission intensity when the tubular-torch configuration was used with desolvation. Large changes in the plasma-sheath gas flow changed the overall plasma size and influenced the emission signal. Additionally, changes in this parameter cause an upward shift of the plasma, necessitating a change in observation height. Linearity and detection limits Linear analytical ranges and detection limits were determined, under the experimental conditions given in Table 2, for several metal ions in aqueous solutions. Standard solutions (0.01-1000#g/ml) of several elements were used to measure the emission intensities. Limits of detection were calculated as the concentration of the element in solution which gave a signal three times the standard deviation of the blank 3° (16 replicate measurements of the signal from

Fig. 2. Plot of emission signal intensity vs. microwave power, for 100-/~g/ml Cr(III) solution.

aspiration of demineralized distilled water). Solutions were analysed in triplicate, blanks being alternated with samples, and a cleaning period was allowed before each measurement. Detection limits, useful analytical ranges, and precision (relative standard deviation, RSD) for each element are listed in Table 3. The precisions depend on the spectral background in the region of each line. Table 4 lists the statistics for log-log calibration plots over the linear range for several elements, including the linear dynamic range (as orders of magnitude) from the detection limit to the concentration at which the slope is reduced to 0.95. The limits of detection obtained with the system are compared in Table 5 with those previously reported 27 for use of the W - P t solid-electrode torch. Although the microwave power, nebulizer flow-rate and aspiration efficiency used in the present work are lower, the detection limits are better and lower by factors of 2-10 for the elements studied (except for Sr for which the detection limit is lower by a factor of 100). The large improvement factors for Sr and AI are probably a result of the greater efficiencies of sample introduction into the plasma core, which would be expected to increase the dissociation of stable molecular species such as monoxides. However, the detection limit for Ca with the W - P t solid-electrode

4.4 4.0 3.6 3.2 n- 2.8 2.4

2.0 1.6

~

1.2 260

I 300

I 340

I 380

I 420

I 460

I 500

I 540

Power (W)

Fig. 3. Plot of signal-to-background ratio vs. microwave power, for 100-/ag/ml Cr(III) solution.

644

B . M . PATEL et al. Table 3. Detection limits, analytical ranges and precision (relative standard deviation, RSD) for several elements determined with the use o f solution nebulization into the microwave plasma

Element

Wavelength, nm

Ag AI Ba Ca Cd Cr Cs Cu K Li Na Pb Pd Sr Zn

328.07 396.15 553.56 422.67 228.80 425.43 852.11 324.75 766.49 670.78 588.99 405.78 342.12 460.73 213.86

Linear analytical range, #g /ml

Limit of detection,* #g /ml

2-100 3-500 10-100 2-200 4-250 1-200 10-100 0.5-200 2-500 0.03-20 0.05-15 20-1000 10-200 0.05-100 10-100

0.47 0.5 3.1 0.65 0.62 0.26 4.0 0.09 0.26 0.005 0.01 2.9 2.1 0.01 5.0

RSD, % 1.6 I. 1 2.0 6.5 1.2 1.7 2.8 6.0 3.5 1.7 1.8 1.0 2.1 1.9 4.3

*Limit of detection is defined as the concentration giving a signal equivalent to 3 times the standard deviation of 16 repetitive measurements of the background when demineralized water is nebulized into the microwave plasma.

Table 4. Statistics for log-log calibration plots

Element

Slope

Ag Cd Cr Cu Li Na K Pb

1.03 0.97 0.97 0.98 1.02 1. I 1 1.13 1.05

Std. devn. of slope 4.40E 4.21E 9.96E 1.18E 1.80E 9.66E 3.73E 3.45E

-

04 04 05 03 04 04 03 04

Std. devn. of intercept

Intercept 1.32E - 02 -0.38 -0.29 0.55 0.89 1.14 -0.14 -0.91

4.10E 4.50E 1.07E 1.41E 1.72E 1.22E 2.91E 3.10E

-

04 04 04 03 04 03 03 04

Corr. coeff,

Linear range orders o f magnitude

0.997 0.996 0.999 0.997 0.999 0.998 0.994 0.998

2.5 3 2 2 3 2.5 2 3

Table 5. Limits of detection for several elements measured by use o f solution nebulization for Ta tubular-torch and W - P t solid-electrode torch microwave plasma emission spectrometry Limit o f detection, ppm Element

Wavelength, nm

AI Ca Cd K Li Na Pb Sr Zn

396.15 422.67 228.80 766.49 670.78 588.99 405.78 460.73 213.86

Ta tubular-torch (present work)

Other experimental conditions: Microwave power, W Plasma type Nebulizer gas flow-rate, l./min Solution uptake rate, ml/min Aspiration efficiency, %

W - P t solid-electrode torch 27

0.5 0.65 0.62 0.26 0.005 0.01 2.9 0.01 5.0

4.9 0.4 3.2 0.9 0.008 0.03 7.6 1.0 6.8

325 N 2 - A r plasma 0.6 1.65 3.5

520 air-plasma 2.2 1.2 12.0

Improvement factor 9.8 x 0.6 x 5.2 x 3.5 x 1.6 x 3.0 × 2.6 x I00 x 1.4 x

Nebulization of aqueous samples torch was about 40% lower than that obtained with the tubular torch. CONCLUSION Direct nebulization of aqueous sample solutions into the tubular-electrode torch capacitativelycoupled microwave plasma emission-spectrometer provides low detection limits and wide linear responses for a broad range of metals. The technique appears particularly attractive in view of its lower power requirements, reduced gas consumption, more robust character and lower cost when compared to the ICP. Further studies are in progress to evaluate interference effects, including those from easily ionized elements and of phosphate and aluminium on Ca. REFERENCES

1. R. K. Skogerboe and G. N. Coleman, Anal. Chem., 1976, 48, 611A. 2. C. I. M. Beenakker, P. W. J. M. Boumans and P. J. Rommers, Philips Tech. Rev., 1980, 39, 65. 3. A. T. Zander and G. M. Hieftje, Appl. Spectrosc., 1981, 35, 357. 4. J. P. Matousek, B. J. Orr and M. Selby, Prog. Anal. At. Spectrosc., 1984, 7, 275. 5. J. Dahmen, ICP Inform. Newsl., 1981, 6, 576. 6. Idem, ibid., 1982, 7, 441.

7. 8. 9. 10. 11.

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Idem, ibid., 1983, 9, 81. Idem, ibid., 1984, 10, 71. Idem, ibid., 1985, 11, 71. Idem, ibid., 1986, 12, 7. I. Atsuya and K. Akatsuka, Anal. Chim. Acta, 1976, 81, 61. 12. Idem, ibid., 1978, 99, 351. 13. Idem, ibid., 1980, 119, 341. 14. Idem, Banseki Kagaku, 1980, 29, 714. 15. Idem, Spectrochim. Acta, 1981, 3611, 747. 16. G. Wunsch, ICP Inform. Newsl., 1982, 8, 133. 17. G. Wunsch, N. Czech and G. Hegenberg, Z. Anal. Chem., 1982, 310, 62. 18. G. Wunsch and N. Czech, ibid., 1984, 317, 5. 19. G. Wunsch, G. Hegenberg and N. Czech, Spectrochim. Acta, 1983, 38B, 1135. 20. H. Fuererbacher, ICP Inform. Newsl., 1981, 6, 571. 21. A. Disam, P. Tsch6pel and G. T61g, Z. Anal. Chem., 1982, 310, 131. 22. S. Hanamura, B. W. Smith and J. D. Winefordner, Anal. Chem., 1983, 55, 2026. 23. Idem, Can. J. Spectrosc., 1984, 29, 13. 24. B. Kirsch, S. Hanamura and J. D. Winefordner, Spectrochim. Acta, 1984, 3911, 955. 25. S. Hanamura, B. Kirsch and J. D. Winefordner, Anal. Chem., 1985, 57, 9. 26. Idem, Can. J. Spectrosc., 1985, 30, 46. 27. Y. K. Zhang, S. Hanamura and J. D. Winefordner, Appl. Spectrosc., 1985, 39, 226. 28. H. Vermaak, O. Kujirai, S. Hanamura and J. D. Winefordner, Can. J. Spectrosc., 1986, 31, 95. 29. B. M. Patel, E. Heithmar and J. D. Winefordner, Anal. Chem., 1987, 59, 2374. 30. G. L. Long and J. D. Winefordner, ibid., 1983, 55, 712A.