Anal. Chem. 1993, 65,2784-2790
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Investigation of Helium Inductively Coupled Plasma-Mass Spectrometry for the Detection of Metals and Nonmetals in Aqueous Solutions Sang-Ho Nam, Wellington R. L. Masamba, and Akbar Montaserl Department of Chemistry, T h e George Washington University, Washington, D.C. 20052
Aqueous solutions are introduced into a helium inductively coupled plasma mass spectrometer (He ICPMS) for the detection of metals and nonmetals. The effect of four approaches for reducing the extent of secondary discharge in He ICPMS is examined. T h e s e approaches include the following: (i) the use of a center-tapped load coil, (ii) modified load coils, (iii) electrostatic shields, and (iv) application of samplers made from nonconductive materials and aluminum. Analytical characteristics of the system such as the background mass spectrum for injection of nitric acid solution, effects of the length of the plasma confinement tube, forward power and injector gas flow rate on ion signal intensities, and the level of oxide and doubly charged ions are determined. Isotope ratios for 63Cu/66Cu,6sNi/60Ni,and 80Se/ We are measured. The limits of detection, obtained with an analogue detector and a prototype spectrometer, for the metals studied are in the range 0.2-9 ng/mL while those for As, Br, Se, and I are 0.2,1,0.2, and 0.04 ng/mL, respectively. Linear dynamic ranges of over 4 orders of magnitude are achieved. The system allows the determination of certain isotopes that suffer from spectral interference in Ar ICPMS. INTRODUCTION The concept of plasma source mass spectrometry was first demonstrated by Gray' in 1975 using a capillary arc plasma. Subsequently, Houk et a1.2 coupled an argon inductively coupled plasma (Ar ICP) to a mass spectrometer (MS). Since then work has extended to other plasma sources, but the Ar ICPMS offers the best analytical performance in terms of high detection power, wide linear dynamic range, multielement capability, and isotopic ratio analysis.3 These characteristics are due to the unique properties of the Ar ICP, namely, relatively high gas and electron temperatures, high electron number density, and annular c~nfiguration.~ The use of Ar as a plasma gas, however, has two major limitations: 4-a (i) the argon plasma is not an efficient ioniza-
* To whom correspondence should be addressed.
(1)Gray, A. L. Analyst 1975,100,289-299. (2)Houk, R.S.;Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980,52, 2283-2289. (3)Jarvis, K. E.; Gray, A. L.; Houk, R. S. Handbook of Inductively Coupled Plasma Mass Spectrometry; Chapman and Hall: New York, 1992;Chapter 2. (4)Montaser, A. Assessment of Potentials and Limitations of Plasmas Sources Compared to ICP Discharges in Analytical Spectrometry. In Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed.; Montaser, A., Golightly, D. W., Eds.; VCH: New York, 1992. (5) Montaser, A.; Chan, S.; Koppenaal, D. W. Anal. Chem. 1987,59, 1240-1242. 0003-2700/93/0365-2784$04.0010
tion source for high-ionization-energy elements such as the nonmetals, and (ii) the three isotopes of Ar (3'3Ar+,sAr+, 40Ar+)as well as polyatomic species containing Ar such as 40Ar160+,40Ar40Ar+, and 40Ar35C1+interfere with the determination of certain isotopes, e.g., W a + , MFe+, We+, and 75As+. These drawbacks are troublesome because, for example, the determinations of As and Se are of particular interest in the fields of environmental and clinical ~hemistry.~ From a fundamental viewpoint, the use of He (ionization energy 24.6 eV) instead of Ar (ionization energy 15.8 eV) has the benefit of enhancing the degree of ionization of difficultto-ionize elements including nonmetals.5~~The detection limits for nonmetals in He ICP atomic emission spectrometry (He ICPAES) have been shown to be better than those in Ar ICPAES.l0 Also, He essentially is monoisotopic and occurs at a low mass; thus, the high-mass polyatomic interferences experienced with Ar plasmas are e1iminated.w From a practical perspective, however, He plasmas have the disadvantages of possessing lower electron number densities and gas temperatures than the Ar ICP.lo Relatedly, the strengths of both the electric and magnetic fields predicted by computer simulation are approximately 1order of magnitude larger for the He ICP than those in the Ar ICP." In short, interfacelinked discharges are very strong when the He ICP is used instead of Ar ICP as an ionization source for mass spectrometry. Earlier experimental results, even when gaseous samples were injected into the plasma,5J2have documented the validity of this general prediction for He ICPMS. The cited fundamental advantages of He plasmas over Ar plasmas have prompted a number of investigators to explore He or Ar-He mixed gas ICPs,5>6J2-l7helium microwave(6)Montaser, A,; Ohls,K. D.; Golightly, D. W. Inductively Coupled Plasmas in Gases Other Than Argon. In Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed.; Montaser, A., Golightly, D. W., Eds.; VCH: New York, 1992. (7)Satzger,R. D.; Fricke,F. L.;Brown,P. G.; Caruso,J. A. Spectrochim. Acta 1987,42B,705-712. (8) Creed, J. T.; Davidson, T. M.; Shen, W.; Brown, P. G.; Caruso, J. A. Spectrochim. Acta 1989,44B,909-924. (9)Evans, E.H.; Ebdon, L. J. Anal. At. Spectrom. 1989,4,299-300. (10)Ishii, I.; Tan, H.; Chan, S.; Montaser, A. Spectrochim. Acta 1991, 46B, 901-916,and references therein. (11)Cai, M.; Montaser, A.; Mostaghimi, J. Spectrochim. Acta 1993,
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48B.789-807. (12)Koppenaal, D. W.; Quinton, L. F. J.Anal. At. Spectrom. 1988,3,
667-672;1988,3, 1144. (13)Nam, S.;Tan, H.; Montaser, A. Helium Inductively Coupled Plasma-Mass Spectrometry. Presented at the 1991 FACSS Meeting, Anaheim, CA. (14)Nam,S.;Masamba, W.;Zhang,H.;Hsiech,C.,Montaaer,A.Recent Studies in Helium Inductively Coupled Plasma Mass Spectrometry. Presented at the 1992 FACSS Meeting, Philadelphia, PA. (15)Koppenaal, D. W.; Pinkston, T. L.; Tweedy, S. W. Presented at the 1988 Winter Conference on Plasma Spectrochemistry, San Diego, CA. (16)Sheppard, B. S.;Shen, W. L.; Caruso, J. A. J. Am. SOC. Mass Spectrom. 1990,2,355-361. (17)Sheppard, B. S.;Shen, W.; Davidson, T. M.; Caruso, J. A. J.Anal. At. Spectrom. 1990,5, 697-700. 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
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induced plasmas (He MIPS), and helium-argon M I P S ~ ~ ~Table J ~ I. ~ Experimental ~ Facilities as ionization sources for mass spectrometry. Most studies i n I. Mass Spectrometer He ICPMS and He MIPMS have focused on introduction of PLASMASS (Delsi-Nermag, Argenteuil, France) ICPMS system gaseous samples into the plasmas for applications in gas (mass range, 0-300 amu; adjustable resolution between 0.5 chromatographic detection. Work o n solution introduction and 4 amu; scan speed, 2000 amu/s) consists of the following: in He MIPMS has been conducted b y Caruso and co(1) an aluminum sample (0.9-mm orifice, inside and outaide w0rkers892~and Park et al.20 No studies have been published cone angles of 90" and 120°, respectively) and a nickel skimmer (0.6-mm orifice, inside and outaide cone angles of for injection of aqueous samples into He ICPMS. Montaser 100O and 120°, respectively) with ita tip located 8 mm et al.6 described the first coupling of an atmospheric pressure behind the sampler He ICP to a commercial ICPMS instrument. Gas mixtures (2) an analyzer quadrupole (35 cm long) and a prefilter of helium and compounds containing the halogens were quadrupole (12 cm long) with a rod diameter of 15.6 mm introduced into the He ICPMS. Koppenaal and Quinton12 (3) an input lens system made of a cylindrical lens to focus presented theoretical considerations for the design of a He the ion beam from the plasma, a quadrupole used as a transmission lens, and an exit plate ICPMS interface and investigated the severe nature of the (4) a Coniphot detector (see ref 28) secondary discharge formed by the H e plasma on the sampler. (5) a three-stage, differentially pumped vacuum system In this work, aqueous solutions of both metals and (6) a PC-AT Type 80286-based computer (WYSEpc 286, nonmetals are introduced into a He ICPMS by ultrasonic WYSE Technology, San Jose, CA) with a typical VGA nebulization. Also, t h e efficacy of four approaches for reducing the extent of secondary discharge in H e ICPMS is examined. Analytical characteristics of the system such as the background mass spectrum for injection of nitric acid solution, the e f f e c b of t h e length of the plasma confinement tube, forward power, and injector gas flow rate on ion signal intensities are determined along with the levels of oxide ions and doubly ionized species, linear dynamic ranges, and detection limits for metals and a number of difficult-to-ionize
nonmetals.
EXPERIMENTAL SECTION Details of the experimental system are summarized in Table I. The operating conditions are given in Table 11. T h e crystalcontrolled, 40.68 MHz ICP system and Delsi-Nermag mass spectrometer are the same as those described elsewhere,%except that a different torch and operation conditions are used. A demountable, MACOR-based, low-gas-flow torch was used in this work to form a He ICP. T h e original torch designB was modified slightly by using two O-rings to fit the plasma confinement tube to the MACOR torch base. The length of the plasma confinement tube was varied from 60 t o 120 mm t o get the highest net ion signals. In order to form a stable H e ICP, the impedance matching network was modified as described elsewhere." T h e Delsi-Nermag mass spectrometer used in this work is a prototype instrument that has three major differences with the commonly used ICPMS instrumentd2 (i) two quadrupoles are (18) Brown,P. G.;Davidson,T.M.;Caruso, J.A. J.Anal.At.Spectrom. 1988,3, 763-769. (19) Creed, J.T.;Mohamad,A. H.;Davidmn,T.M.;Ataman, G.; Caruso, J. A. J. Anal. At. SDectrom. 1988.3.923-926. (20) Park, C. J.; Pak, Y. N.;Lee, K.W. Anal. Sci. 1992,8, 443-448. (21) Chambers, D. M.; Carnahan, J. W.; Jin, 4.;Hieftje, G. M. Spectrochim. Acta 1991,468, 1745-1765. (22) Story,W. C.; Olson, L. K.; Shen, W. L.; Creed, J. T.; Caruso, J. A. J. Anal. At. Spectrom. 1990,5, 467-470. (23) Satzger, R. D.; Brueggemeyer, T. W. Mikrochim. Acta 1989, 3, 239-246. (24) Heitkemper, D.; Creed, J. T.; Caruso, J. A. J. Chromatogr. Sci. 1990.28.175-181. ~. .. - ~-~ (25) Creed, J. T.; Davidson, T. M.; Shen, W. L.; Caruso, J. A. J.Anal. At. Spectrom. 1990, 5, 109-113. (26) Suyani, H.; Creed, J. T.; Caruso, J. A.; Satzger, R. D. J.Anal. At. Spectrom. 1989, 4, 777-782. (27) Mohamad, A. H.: Creed, J. T.; Davidson, T. M.; Caruso, J. A.; Appl. Spectrosc. 1989,43, 1127-1131. (28) Montaser, A,; Tan, H.;Ishii, I.; Nam, S.; Cai, M. Anal. Chem. 1991,63, 2660-2665. (29) Chan, S.; Van Hoven, R. L.; Montaser, A. Anal. Chem. 1986,58, 2342-2343. (30) Montaser, A.; Ishii, I.; Clifford, R. H.; Sinex, S. A.; Capar, S. G. Anal. Chem. 1989,61,2589-2592. (31) Douglas, D. J.; French, J. B. Spegtrochim. Acta 1986,418, 197204. (32) Horlick, G.; Shao, Y. Inductively Coupled Plasma-Mass Spectrometry for Elemental Analysis. In Inductively Coupled Plasmas in
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Analytical Atomic Spectrometry, 2nd ed.; Montaser, A., Golightly, D. W., Eds.; VCH New York, 1992; and references therein.
color display and a laser jet printer; menu-driven software 11. ICP and Sample Introduction Systems (1) a solid-state, 1.6-kW, 40.68-MHz crystal-controlled generator (Model ICP 16, RF Power Products, Inc., Voorhees, NJ) with an automatic phase and magnitude matching network" (2) a five-turn load coil used with a demountable, low-gasflow torch10,B (3) the torch housing assembly and the impedance matching network" mounted on grounded X-Y-Z translation stages (4) a mass flow controller (Model 8240, Matheson Gas Co., East Rutherford, NJ) used for the nebulizer injector gas line and a gas flow meter (Matheson Gas Co., East Rutherford, NJ) used for the plasma gas. The purity of helium was 99.997% (Roberta Oxygen Co., Rockville, MD) (5) sample delivery unit for the ultrasonic nebulizer: a peristaltic pump (Minipuls 2, Rainin Instrument Co., Woburn, MA) (6) an ultrasonic nebulizer and the associated desolvation unit (Model U-5000, CETAC Technologies, Inc., Omaha, NE) Table 11. Operating Conditions for the He ICPMS System ICP System forward power, W 800 reflected power, W
