Anal. Chem. 2000, 72, 1885-1893
A Large Bore-Direct Injection High Efficiency Nebulizer for Inductively Coupled Plasma Spectrometry Billy W. Acon, John A. McLean, and Akbar Montaser*
Department of Chemistry, George Washington University, Washington, D.C. 20052
A large bore-direct injection high efficiency nebulizer (LBDIHEN) is introduced that is less prone to capillary blockage and optimally operates at low nebulizer gas pressures compared with the conventional DIHEN used for inductively coupled plasma (ICP) spectrometries. The aerosol quality is examined using a two-dimensional phase Doppler particle analyzer (2D PDPA), and analytical figures of merits are acquired by ICP mass spectrometry. Compared with the DIHEN, the LB-DIHEN produces larger droplets, but the velocity distributions and mean droplet velocities are narrower and lower, respectively, providing longer residence times for the droplets in the plasma. High RF power (1500 W), low nebulizer gas flow rates (0.25-0.35 L/min), and low solution uptake rates (80-110 µL/min) are required to operate the LB-DIHEN at optimum conditions for ICPMS. Detection limits and sensitivities measured with the LB-DIHEN are superior to those of a conventional nebulizer-spray chamber combination, but precision is inferior. The performance of the LB-DIHEN is further explored in the determination of trace elements in an herbal extract. Inductively coupled plasma mass spectrometry (ICPMS) and ICP atomic emission spectrometry (ICPAES) are established techniques for elemental and isotopic analysis.1,2 However, introduction of sample into ICPs or other plasmas is a critical link, with liquid sample introduction being the most common.3,4 The typical pneumatic nebulizer spray chamber arrangement is disadvantageous because of large sample consumption (typically 1-2 mL/min) and low analyte transport efficiency (1-20%) to the plasma. A reduction in the sample volume requirement and an increase in the analyte transport efficiency is desirable, particularly in the analysis of limited, expensive, or hazardous samples or for * To whom correspondence should be addressed: Telephone: 202-994-6480. Fax: 202-994-2298. Email:
[email protected]. (1) Montaser, A., Ed. Inductively Coupled Plasma Mass Spectrometry; Wiley: New York, 1998. (2) Montaser, A., Golightly, D. W., Eds. Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Wiley-VCH: New York, 1992. (3) Montaser, A.; Minnich, M. G.; McLean, J. A.; Liu, H.; Caruso, J. A.; McLeod, C. W. Sample Introduction in ICPMS. In Inductively Coupled Plasma Mass Spectrometry; Montaser, A., Ed.; Wiley: New York, 1998. (4) Montaser, A.; Minnich, M. G.; Liu, H.; Gustavsson, A. G. T.; Browner, R. F. Fundamental Aspects of Sample Introduction in ICP Spectrometry. In Inductively Coupled Plasma Mass Spectrometry; Montaser, A., Ed.; Wiley: New York, 1998. 10.1021/ac9911589 CCC: $19.00 Published on Web 03/18/2000
© 2000 American Chemical Society
interfacing microbore high-performance liquid chromatography (µHPLC) and capillary electrophoresis with ICPMS and ICPAES detection in speciation studies.3 Recently, we introduced a direct injection high efficiency nebulizer (DIHEN)5-8 for the introduction of 1-100 µL test solutions into high-temperature plasmas. The DIHEN is a simple micronebulizer that requires no spray chamber and provides 100% analyte transport to the plasma for ICPAES and ICPMS detection. In contrast to the direct injection nebulizer (DIN),9-13 the DIHEN is simple to use, less expensive, and requires no high-pressure pump for solution delivery. Compared with the conventional nebulizer spray chamber arrangement, the direct injection of sample into the plasma currently offers a number of benefits and drawbacks. The primary advantages include: (1) a low internal dead volume and thus rapid response times and reduced memory effects; (2) no volatile analytes lost in the spray chamber; (3) improved precision by eliminating noise sources attributed to the spray chamber; and (4) similar or improved sensitivity and detection limits when operated at microliters per minute compared with conventional nebulizer-spray chamber combinations operated at milliliters per minute sample consumption rates. One drawback of many micronebulizers is their smaller critical dimensions compared with conventional pneumatic nebulizers. This can lead to a higher incidence of nebulizer blockage when samples of high total dissolved solids or slurries are to be analyzed. Relatedly, micronebulizers are typically more difficult to construct and may require a high nebulizer gas pressure for optimal operation. The aim of this work was to explore a large bore-direct injection high efficiency nebulizer (LB-DIHEN) that minimizes capillary blockage and optimally operates at low nebulizer gas pressures (5) McLean, J. A.; Zhang, H.; Montaser, A. Anal. Chem. 1998, 70, 1012-1020. (6) McLean, J. A.; Minnich, M. G.; Iacone, L. A.; Liu, H.; Montaser, A. J. Anal. At. Spectrom. 1998, 13, 829-842. (7) Singh, J.; McLean, J. A.; Pritchard, D. E.; Montaser, A.; Patierno, S. R. Toxicol. Sci. 1998, 46, 220-226. (8) Becker, J. S.; Dietze, H.-J.; McLean, J. A.; Montaser, A. Anal. Chem. 1999, 71, 3077-3084. (9) Greenfield, S.; Jones, I. L.; Berry, C. T.; Spash, D. I. Improvements Relating to Spectroscopic Methods and Apparatus. UK Patent 1,109,602, 1968. (10) Lawrence, K. E.; Rice, G. W.; Fassel, V. A. Anal. Chem. 1984, 56, 289292. (11) Wiederin, D. R.; Smith, F. G.; Houk, R. S. Anal. Chem. 1991, 63, 219-225. (12) Shum, S. C. K.; Johnson, S. K.; Pang, H.-M.; Houk, R. S. Appl. Spectrosc. 1993, 47, 575-583. (13) McLean, J. A.; Huff, R. A.; Montaser, A. Appl. Spectrosc. 1999, 53, 13311340.
Analytical Chemistry, Vol. 72, No. 8, April 15, 2000 1885
Figure 1. (A) Diagram of the large bore-direct injection high efficiency nebulizer (LB-DIHEN) and the DIHEN. (B) Scaled section view of the nebulizer tip for the LB-DIHEN design (left) and DIHEN (right). Table 1. Critical Dimensions and Parameters for the LB-DIHEN and DIHEN in Comparison with the Nominal Ranges for a Conventional TR-30 Nebulizer and a HEN LB-DIHEN (DIHEN-30-AA)
conventional nebulizera (TR-30-AA)
DIHEN (DIHEN-170-AA)
HENa (HEN-170-AA)
318 16 412 0.0794 0.0371 36 5
220-320 15-40 350-450 0.05-0.10 0.03-0.04 25-40 4-7
104 20 173 0.0085 0.0094 155 30
70-110 15-40 150-200 0.0038-0.0095 0.007-0.01 150-180 25-60
solution capillary i.d. (µm) capillary wall thickness (µm) gas orifice i.d. (µm) capillary annulus area (mm2) gas annulus area (mm2) gas operating pressure (at 1 L/min Ar, psig) gas operating pressure (at 0.2 L/min Ar, psig) a
Nominal values for this class of nebulizer (Courtesy of J E Meinhard Associates, Inc.).
(5-10 psig) compared with the conventional DIHEN (30-60 psig). This report includes design details for the LB-DIHEN, analytical figures of merit using ICPMS, key aerosol parameters acquired with a two-dimensional phase Doppler particle analyzer (2D PDPA), and application of the LB-DIHEN to the ICPMS determination of trace elements in a commercially available herbal medicine (containing eight herbs, extracts, and other compounds) for treating blood pressure. Results are also compared with the data acquired for the DIHEN. EXPERIMENTAL SECTION Large Bore-Direct Injection High Efficiency Nebulizer. The LB-DIHEN (model DIHEN-30-AA, J E Meinhard Associates, Inc., Santa Ana, CA) structure is similar to that of a DIHEN (model DIHEN-170-AA, J E Meinhard Associates, Inc.). Figure 1A illustrates the typical components of a LB-DIHEN and a DIHEN designed to replace the injector tube of a demountable-type ICP torch. The critical tip dimensions for both micronebulizers are 1886
Analytical Chemistry, Vol. 72, No. 8, April 15, 2000
shown to scale in Figure 1B, and values are listed in Table 1. The critical dimensions of the LB-DIHEN and the DIHEN are similar to those for a conventional concentric nebulizer (e.g., TR-30-AA type) and a high efficiency nebulizer (HEN), respectively.14-16 The LB-DIHEN was interfaced with the demountable torch of the ICPMS instrument in the same manner used for the DIHEN.5 Briefly, the LB-DIHEN was inserted into a Delrin adapter (DIHEN Adapter, J E Meinhard Associates, Inc.) and positioned 2 mm below the intermediate tube of the plasma torch. Nebulizer gas for the LB-DIHEN was supplied via the mass flow controller of the ICPMS instrument (model 1179 A, MKS Instruments, Andover, MA). For the DIHEN, nebulizer gas flow rate was controlled using an external mass flow controller (model 8200, (14) Nam, S. H.; Lim, J. S.; Montaser, A. J. Anal. At. Spectrom. 1994, 9, 13571362. (15) Liu, H.; Montaser, A. Anal. Chem. 1994, 66, 3233-3242. (16) Liu, H.; Montaser, A.; Dolan, S. P.; Schwartz, R. S. J. Anal. At. Spectrom. 1996, 11, 307-311.
Table 2. Operating Conditions for the Ar ICPMS Instrumenta ICPMS system RF power, W nominal frequency (MHz) RF generator type induction coil circuitry sampling depth (above load coil) (mm) sampler (orifice diameter, mm) skimmer (orifice diameter, mm) outer gas flow rate (L/min) intermediate gas flow rate (L/min)
solution flow mode data acquisition parameters scan mode points/mass resolution (amu) sweeps/reading readings/replicate replicates dwell time/mass (ms) integration time (ms) a
PE-Sciex Elan 6000 1500 40 free-running 3-turn coil, PLASMALOK 11 nickel, 1.1 nickel, 0.9 15 1.2
figures of merit
microscale flow injection
continuous
injection
peak hopping 1 0.7 10 5 11 20 1000
peak hopping 1 0.7 15 1 1000 20 300
Unless otherwise indicated.
Matheson Gas Products, East Rutherford, NJ). To maintain a nebulizer gas flow rate of 0.3 and 0.2 L/min for the LB-DIHEN and the DIHEN, back pressures of 7 and 30 psig were required, respectively. Thus, in principle, both nebulizers can be easily used with all commercially available ICP based instruments. In acquiring the figures of merit in ICPMS, solutions were delivered to the nebulizers either in a natural aspiration mode (no solution pump was used) or in a continuous-flow mode using a four-channel peristaltic pump (model Rabbit, Rainin Instrument Co., Inc., Woburn, MA). In the latter case, narrow-bore Tygon tubing (0.015-in i.d., Astoria-Pacific Inc., Clackamas, OR) was utilized to reduce peristaltic-related noise in the solution delivery. To reduce memory effects, in both cases the dead volume of the nebulizers was reduced to
