Anal. Chem. 1998, 70, 1012-1020
A Direct Injection High-Efficiency Nebulizer for Inductively Coupled Plasma Mass Spectrometry John A. McLean, Hao Zhang, and Akbar Montaser*
Department of Chemistry, The George Washington University, Washington, D.C. 20052
A simple, relatively low-cost direct injection high-efficiency nebulizer (DIHEN) is introduced for argon inductively coupled plasma (Ar ICP) spectrometry. The DIHEN may be operated at solution uptake rates of 1-100 µL/min. Analytical performance indexes for the DIHEN and fundamental characteristics of the aerosol produced are obtained using an ICP mass spectrometer (ICPMS) and a two-dimensional phase Doppler particle analyzer (2D PDPA), respectively. Results are compared to those obtained with a conventional crossflow pneumatic nebulizer (PN), equipped with a Scott-type spray chamber. Droplet sizes and velocities produced with the DIHEN are smaller than those reported for the direct injection nebulizer (DIN). The DIHEN offers optimal sensitivity at low injector gas flow rates (∼0.25 L/min) and high rf power (∼1.5 kW). For the 17 elements tested, detection limits (ppt) and sensitivities achieved with the DIHEN (at 85 µL/min) are similar to, or better than, those obtained on the same instrument using the PN (at 1 mL/min). However, because the primary aerosol is injected directly into the plasma, oxide-to-metal ion ratios (MO+/M+) are high, as in the case of the DIN. The utility of the DIHEN for the analysis of small-volume samples is demonstrated by microscale flow injection analysis (µFIA) of Cr bound to human lung DNA. Detection of Cr at the femtogram level is feasible.
characteristics. Recently, several microflow nebulizers have been investigated. They include pneumatic devices such as the highefficiency nebulizer (HEN),5-10 the microconcentric nebulizer (MCN),11,12 the direct injection nebulizer (DIN),13-31 and the oscillating capillary nebulizer (OCN).32,33 Also, piezoelectrically
* Corresponding author. Telephone: 202-994-6480. Fax: 202-994-2298. Email:
[email protected]. (1) Montaser, A., Ed. Inductively Coupled Plasma Mass Spectrometry; Wiley: New York, 1998 (in press). (2) Montaser, A., Golightly, D. W., Eds. Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed.; VCH: New York, 1992. (3) Jarvis, A. E.; Gray, A. L.; Houk, R. S. Handbook of Inductively Coupled Plasma Spectrometry; Chapman and Hall: New York, 1992. (4) Sharp, B. L. J. Anal. At. Spectrom. 1988, 3, 613-652.
(5) Nam, S.-H.; Lim, J.-S.; Montaser, A. J. Anal. At. Spectrom. 1994, 9, 13571362. (6) Liu, H.; Montaser, A.; Dolan, S. P.; Schwartz, R. S. J. Anal. At. Spectrom. 1996, 11, 307-311. (7) Liu, H.; Montaser, A. Anal. Chem. 1994, 66, 3233-3242. (8) Liu, H.; Clifford, R. H.; Dolan, S. P.; Montaser, A. Spectrochim. Acta 1996, 51B, 27-40. (9) Olesik, J. W.; Kinzer, J. A.; Harkleroad, B. Anal. Chem. 1994, 66, 20222030. (10) Pergantis, S. A.; Heithmar, E. M.; Hinners, T. A. Anal. Chem. 1995, 67, 4530-4535. (11) Vanhaecke, F.; Van Holderbeke, M.; Moens, L.; Dams, R. J. Anal. At. Spectrom. 1996, 11, 543-548. (12) Augagneur, S.; Medina, B.; Szpunar, J.; Lobinski, R. J. Anal. At. Spectrom. 1996, 11, 713-721. (13) Greenfield, S.; Jones, I. Ll.; Berry, C. T.; Spash, D. I. Improvements Relating To Spectroscopic Methods and Apparatus. U.K. Patent 1,109,602, 1968. (14) Greenfield, S.; Jones, I. Ll.; Berry, C. T. Plasma Light Source for Spectroscopic Investigation. U.S. Patent 3,467,471, 1969. (15) Lawrence, K. E.; Rice, G. W.; Fassel, V. A. Anal. Chem. 1984, 56, 289292. (16) LaFreniere, K. E.; Rice, G. W.; Fassel, V. A. Spectrochim. Acta 1985, 40B, 1495-1504. (17) LaFreniere, K. E.; Fassel, V. A.; Eckels, D. E. Anal. Chem. 1987, 59, 879887. (18) Avery, T. W.; Chakrabarty, C.; Thompson, J. J. Appl. Spectrosc. 1990, 44, 1690-1698. (19) Wiederin, D. R.; Smith, F. G.; Houk, R. S. Anal. Chem. 1991, 63, 219-225. (20) Wiederin, D. R.; Smyczek, R. E.; Houk, R. S. Anal. Chem. 1991, 63, 16261631. (21) Smith, F. G.; Wiederin, D. R.; Houk, R. S.; Egan, C. B.; Serfass, R. E. Anal. Chim. Acta 1991, 248, 229-234. (22) Shum, S. C. K.; Pang, H.-M.; Houk, R. S. Anal. Chem. 1992, 64, 24442450. (23) Shum, S. C. K.; Neddersen, R.; Houk, R. S. Analyst 1992, 117, 577-582. (24) Powell, M. J.; Quan, E. S. K.; Boomer, D. W.; Wiederin, D. R. Anal. Chem. 1992, 64, 2253-2257. (25) Shum, S. C. K.; Houk, R. S. Anal. Chem. 1993, 65, 2972-2976. (26) Powell, M. J.; Boomer, D. W.; Wiederin, D. R. Anal. Chem. 1995, 67, 24742478. (27) Zoorob, G.; Tomlinson, M.; Wang, J.; Caruso, J. J. Anal. At. Spectrom. 1995, 10, 853-858. (28) Liu, Y.; Lopez-Avila, V.; Zhu, J. J.; Wiederin, D. R.; Beckert, W. F. Anal. Chem. 1995, 67, 2020-2025. (29) Wiederin, D. R.; Houk, R. S. Appl. Spectrosc. 1991, 45, 1408-1412. (30) Shum, S. C. K.; Johnson, S. E.; Pang, H.-M.; Houk, R. S. Appl. Spectrosc. 1993, 47, 575-583. (31) Christodoulou, J.; Kashani, M.; Keohane, B. M.; Sadler, P. J. J. Anal. At. Spectrom. 1996, 11, 1031-1035. (32) Browner, R. F. 1996 Winter Conference on Plasma Spectrochemistry, Abstracts Book; Fort Lauderdale, FL, p 41, IL1. (33) Wang, L.; May, S. W.; Browner, R. F.; Pollock, S. H. J. Anal. At. Spectrom. 1996, 11, 1137-1146.
1012 Analytical Chemistry, Vol. 70, No. 5, March 1, 1998
S0003-2700(97)00855-X CCC: $15.00
In inductively coupled plasma (ICP) spectrometries, test solutions are typically introduced into the plasma in the form of an aerosol.1-4 The combination of pneumatic nebulizer-spray chamber is primarily used in ICP spectrometries because of its simplicity and low cost. However, this arrangement suffers from low analyte transport efficiency (typically 1-2%) and high sample consumption (typically 1-2 mL/min). A simple, low-consumption, highly efficient nebulizer is often required in chromatographic applications or for the direct analysis of semiconductor, biological, forensic, or toxic materials. In these and other cases, the sample is limited, expensive, or hazardous, and it may contain a large fraction of organic solvents that can substantially alter the plasma
© 1998 American Chemical Society Published on Web 01/27/1998
Figure 1. Schematic diagram of the direct injection high-efficiency nebulizer (DIHEN) and an enlarged section view of the nebulizer tip.
driven devices such as the microflow ultrasonic nebulizer (µUSN)34 and the monodisperse dried microparticulate injector (MDMI)35-41 have been investigated. Among these devices the DIN has received significant attention, partly because the nebulizer is an integral part of the ICP torch, and thus 100% of the aerosol is presented to the plasma. This attribute, along with the low internal dead volume (
