Anal. Chem. 1984, 56,899-903 (15) Ottaway, J. M.; Bezur, L.; Marshall, J. Analyst (London) 1080, 105, 1130. (16) Harnly, J. M.; O’Haver, T. C.; Golden, B. M.; Wolf, W. R. Anal. Chem. 1070, 51, 2007. (17) Harnly, J. M.; O’Haver, T. C. Anal. Chem. 1081, 53, 1291. (18) Marshall, J.; Llttlejohn, D.; Ottaway, J. M.; Harnly, J. M.; Mlller-Ihll, N. J.; O’Haver, T. C. Analyst (London) 1083, 708, 178. (19) Harnly, J. M. Anal. Chern. 1082, 5 4 , 1043. (20) IUPAC Commlsslon on Spectrochemical and Other Optical Procedures, Nomenclature, Symbols, Units, and Thelr Usage In Spectrochemical Analysis. I1 Data Interpretatlon Pure Appl. Chem. 1078, 45, 99. (21) Herrrnann, R.; Alkemade, C. T. J. “Chemical Analysls by Flame Photometry”; Intersclence: New York, 1983; p 130. (22) O’Haver, T. C.; Harnly, J. M.; Zander, A. T. Anal. Chem. 1078, 50, 1221. (23) Harnly, J. M. Anal. Chem. 1082, 5 4 , 876.
899
(24) Bezur, L.; Marshall, J.; Ottaway, J. M.; Fakhrul-Aldeen, R. Analyst (London) 1083, 708, 553. (25) de W a n , L.; Wlnefordner, J. D. Spectrochlm. Acta, Part 8 1088, 238,277. (26) Kellher, P. N.; Wohlers, C. C. Anel. Chem. 1074, 4 6 , 682. (27) “Analytlcal Methods for Atomlc Absorption Spectrophotometry”; Perkin-Elmer Corp.: Norwalk, CT, Jan 1982.
RECEIVED for review June 2, 1983. Resubmitted November 23, 1983, Accepted janUary 19, 1984. Mention of trademark Or proprietary products does not constitute a guarantee or warranty of the product by the US. Department of Agriculture to the exclusion of other and does not imply their products that may also be suitable.
Comparison of Pneumatic Nebulizers in Current Use for Inductively Coupled Plasma Atomic Emission Spectrometry Frans J. M. J. Maessen,* Pieter Coevert, and Johannes Balke
Laboratory of Analytical Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Two concentric and three cross-flow nebulizers operated wlth and without a pump are examlned. The nebulization systems are prlmarlly assessed on the bask 01 thelr suitability for simultaneous multlelement analysls, considering the slgnalto-background ratlo (S/6) as a principal analytical parameter and the pressure drop of the nebuiizlng gas across the gas capillary (AP) as the prlnclpai experimental variable. The relatlons between AP and S/6 have been establlshed at 14 analysis wavelengths. Compared to natural liquid uptake, forced nebulizer feeding leads to less pronounced peaks in the AP V.I S / B curves. The transport efflclency ( e ) measured by direct aerosol collectlon is determlned as a function of the flow rate of the nebulizing gas (0,) over ranges of QQ of speclflc Interest for the Individual nebulizers. The moderate varlabliity of analytical performance observed for the nebuilzers examlned Is discussed on the basis of the applicable values of E, QL, and Ow
Over the last few years, papers regularly appeared dealing with the performance of nebulizers for ICP analysis (1-9). In these studies the efficiency of the nebulizing process was established either indirectly by measuring the difference between the uptake and waste (1-5) or directly by determining the increase of weight of adsorption columns connected in series to the plasma exit of the spray chamber (6-9). Since the aerosol concentration in the plasma can be calculated only if the efficiency of the nebulization process is known, accurate determination of the latter quantity is of the utmost importance for assessing nebulizer-spray chamber performance. Smith and Browner (10) recently examined the relative merits of several direct and indirect methods for determining the nebulization efficiency or, better, the transport efficiency, as defied by the authors. Their results showed that the indirect method is prone to severe systematic and random error whereas of the considered direct methods, viz., filter, cascade impact, and adsorption by silica gel, the silica gel collection method appeared to be affected with positive bias. However, for low gas flow systems, such as the nebulizer described by 0003-2700/84/0356-0899$01.50/0
Ripson and De Galan (7), this bias might be small enough to make results from solvent adsorption procedures acceptable as an indication of system performance (11, 12). But only efficiencies based on solute transport measurements, e.g., cascade impact or filter system, should be adequate to allow meaningful interlaboratory comparison of transport efficiency data (10). Starting from this conclusion and considering the large variety of nebulizers presently available for ICP analysis (13)) a study was undertaken to contribute to the performance evaluation of various types of nebulizers and to facilitate nebulizer selection in analytical practice. In the present work five pneumatic nebulizers were examined, viz., two concentric, two cross-flow, and one Babington type. The concentric and cross-flow nebulizers were operated with both free and forced feeding. Transport efficiencies were measured employing an aerosol collection system using filters. Line and background measurements were performed at 14 analysis wavelengths. The nebulization systems examined were primarily assessed on the basis of their suitability for simultaneous multielement analysis, considering the signalto-background ratio as a principal analytical parameter (14-16). The pressure drop of the nebulizing gas across the gas capillary (hp)was considered as the principal experimental variable because in commercial ICP instruments hp is frequently the only precisely controllable nebulizer parameter and, further, because AP is a fundamental parameter for nebulizer modification, e.g., the high-pressure “MAK” nebulizer (17)) and nebulizer design (18),including the calculation of nozzle shapes (19).
EXPERIMENTAL SECTION Nebulizer Types. The following nebulizers were tested. Two concentric glass nebulizers, Models TR-20-B1and TR-SO-B1,from J. E. Meinhard Associates, Santa Ana, CA (ZO), and three cross-flow nebulizers, viz., one adjustable, one fixed, and one of the high solids type, all three from Jarrell-Ash, Waltham, MA. The adjustable cross-flow nebulizer (Catalog No. 90-974) is essentially the same as the pneumatic nebulizer developed by Knisely et al. (21) for use at low gas flow rates. The fixed cross-flownebulizer (Catalog No. 90-790) is based on the design 0 1984 American Chemical Soclety
900
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
Table I. Nebulizing Systems Examined and the Corresponding Codes Used for Reference nebulizer
a
model
type
solution uptake
Jarrell-Ash, 90-974
cross-flow, adjustable
Jarrell-Ash, 90-7 90
cross-flow, fixed
Jarrell-Ash, 90-791
high solids
without pump with pump without pump with pump with pump
Meinhard, TR-20-B1
concentric
sol code
without pump with pump Meinhard, TR-30-B1 concentric without pump with pump (low c.) and (high c.) refer the composition of the test solutions used; see Experimental Section.
Table 11. Nebulizer Characteristics Considered pressure drop ( AP) gas flow rate ( QG ) uptake rate ( QL ) aerosol delivering rate ( Q A ) transport efficiency
(E)
pressure drop of the nebulizing gas across the gas capillary (Pa) flow rate of the nebulizing gas ( L min-’) forced or natural solution flow rate through the liquid liquid capillary (g min-’) mass of aspirated solution entering the plasma (g min--l) ratio of QA to QL (%)
described by Novak et al. (22) and consists of a body made of a Teflon-like material with a sapphire jewel orifice as the gas jet and platinum tubing as the sample uptake. The high solids nebulizer (Catalog No. 90-791) is of a type similar to that as described by Wolcott and Sobel (23)and by Suddendorf and Boyer (2). This nebulizer consists of a body identical with that used for the fixed cross-flow and has the same type of sapphire jewel orifice. It has no sample uptake tube but instead the sample reaches the gas orifice from the top via a V-shaped groove. Nebulizers of that type can be operated by forced sample feeding only. More detailed technical information of the cross-flow and high solids nebulizers considered in the present study can be found in ref 24. Table I presents an overview of the nebulization systems examined and the codes used for reference. All nebulizers were operated in combination with similar spray chambers consisting of simple glass cylinders (31 mm inner diameter and 150 mm length) with no baffles ( I ) . Forced nebulizer feeding was accomplished by using a Gilson Minipulse I1 ten-roller peristaltic pump. Nebulizer and Analytical Performance Characteristics. Table I1 lists the nebulizer characteristics considered. The pressure drop of the nebulizing gas across the gas capillary, AP, was adjusted with a precision manometer, 0 to 2.75 X lo5 Pa, Econosto N.V., Rotterdam, The Netherlands. The flow rate of the nebulizing gas, QG,was measured employing a high rate flow meter, tube type R 2-15-A, Brooks Instruments B.V., Veenendaal, The Netherlands. In the case of forced nebulizer feeding, the solution uptake rate, QL,was set with the peristaltic pump whereas the natural uptake was established with a calibrated measuring cylinder. Transport efficiencies were determined according to the direct filter collection method. The system used is essentially the same as that described by Smith and Browner (IO). The spray chamber was positioned upright with the filter holder vertically in line above the exit tube. The aerosol was made clearly visible by illumination. Air was sucked through the filters at such a rate that the original aerosol streaming pattern in the exit tube was not affected. Nuclepore polycarbonate membrane filters developed for aerosol filtration (25) were employed (0.4 pm pore size, 47 mm diameter). A 1 mm mesh Teflon wire netting was used to support the filters. A solution containing 50 pg mL-’ each of Cd, Cr, Fe, and Mn was used for the efficiency determinations. The filters were extracted with hot 0.1 M hydrochloric acid and the element content was determined by ICAP-AES. As a check on collection efficiency two filters were operated in series. The
CF, Adj.(-) CF, Adj.(t) CF. F(-) CF; Fit’) v (low c.)= V (high c . ) ~ Conc. 20(-) Conc. 20( + ) Conc. 30(-) Conc. 30(+)
Table 111. Wavelength and Excitation Potential of the Analysis Lines Used and Ionization Potential of the Relevant Species species
wavelength, nm
A1 I Ba I1 Ca I1 Cd I1 Cr I1 cu I Fe I1 Li I Mg I1 Mn I1 Na I Pb I1 Ti I1 Zn I
308.21 5 455.403 393.366 226.502 267.716 324.754 238.204 670.784 280.270 257.610 588.995 220.353 337.280 213.856
excitation ionization potential, potential, V V 4.02 2.72 3.15 5.47 6.15 3.82 5.20 1.85 4.42 4.81 2.10 7.37 3.69 5.79
5.98 9.95 11.87 16.84 16.49 7.72 16.16 5.39 15.03 15.46 5.14 14.96 13.63 9.39
results showed that with the first fiiter a collection efficiencybetter than 97% was obtained. Therefore, the definite measurements were performed with a single filter. For any combination of experimental conditions examined, aerosol transport efficiency measurements were carried out by aspirating under corresponding conditions 10 mL of the solution containing four elements. In order to avoid filter saturation, the volume of the aspirated solution was reduced to 6 mL in the case of efficiencies greater than 2%. The ICP-AES instrument used was a Jarrell-Ash Model 975 Plasma AtomComp direct reading spectrometric system. Specifications of the instrumental components and facilities have been described elsewhere (26). Analytical performance characteristics were established by employing an aqueous solution containing 14 elements with the following concentrations: 5 pg mL-l each of Al, Na, and Pb, 1pg mL-’ each of Cd, Cr, Cu, Fe, Li, Ti, and Zn, and 0.2 pg mL-’ each of Ba, Ca, Mg, and Mn. In order to test the high solids nebulizer also under conditions for which this type especially was developed, a special test solution was prepared containing 30 mg mL-l of Na in addition to the 14 elements of the solution used for testing all nebulization systems, including the high solids nebulizer. Consequently, the latter solution, which was exclusively used for the high solids nebulizer, dispenses with Na as a test element. It should be noted that the “system” in Table I referred to as “V (high c.)” is not really a specific nebulization system characterized by nebulizer design and way of sample introduction but rather a special condition, viz., the aspiration of solutions containing analytes in the presence of a large excess of concomitant salt. The analytical performance characteristics considered were the net line intensity ( S ) ,the background intensity (B)measured at the peak wavelength of the analysis line when a blank solution was nebulized, the signal to background ratio ( S I B ) ,and the medium term stability of the gross line intensity S + B (stability). Table I11 lists the used analytical wavelengths of the test elements employed. The applied ICAP operation conditions are summarized in Table IV.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
Table IV. Compromise ICAP Operating Conditions for Simultaneous Analysis of the Test Elements Considered 19 L min-’ outer gas flow rate aerosol carrier gas flow ratea solution uptake ratea 1.05 kW forward power 5 0.65 1.24 3.09 1.40 1.00 1.26 1.03
B ) of 14 test elements based on six groups of four
can be anticipated (32) giving similar S I B values at the same observation height (29). To summarize, the identified moderate variability of analytical performance of nebulization systems is due to the fairly constant amount of aerosol as well as to the similar droplet size distribution of the aerosol reaching the plasma per unit of time.
ACKNOWLEDGMENT We wish to thank Piet J. H. Scheeren for his software support concerning storage, correction, and interpolation of the data.
LITERATURE CITED Wohlers. C. C. ICP Inf. Newsl. 1977, 3 , 37. Suddendorf, R. F.; Boyer, K. W. Anal. Chem. 1978, 50, 1769. Gustavsson, A. ICP Inf. Newsl. 1979, 5 , 312. McKlnnon, P. J.; Giess, K. C. I n “Developments in Atomic Spectrochemical Analysis”; Barnes, R. M., Ed.; Heyden: London, 1981; p 287. Magyar, 8.; Aeschbach, F. Spectrochlm. Acta, Part B 1980, 358, 839. Schutyser, P.; Janssens, E. Spectrochim. Acta, Part B 1979, 348, 443. Rlpson, P. A. M.; De Galan, L. Spectrochim. Acfa, Part B 1981, 368, 71. Ebdon, L.; Cave, M. R. Analyst (London) 1982, 707, 172. Janssens, E. Ph.D. Thesls, State University of Ghent, Belgium, Nuclear Sciences College, 1983. Smith, D. D.; Browner, R. F. Anal. Chem. 1982, 54, 533. Ripson, P. A. M.; De Galan, L. Anal. Chem. 1983, 55, 372. Browner, R. F.; Smith, D. 0.Anal. Chem. 1983, 55, 373. Barnes, R. M. ICP Inf. Newsl. 1981, 6, 459. Allemand, C. D.; Barnes, R. M.; Wohlers, C. C. Anal. Chem. 1979, 57, 2392. Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33, 206. Boumans, P. W. J. M.; McKenna, R. J.; Bosveld, M. Spectrochlm. Acta, Part B 1981, 368, 1031. Anderson, H.; Kalser, H.; Meddlngs. B. “Developments in Atomic Plasma spectrochemical Analysis”; Barnes, R. M., Ed.; Heyden: London, 1981; p 251. Meinhard, J. E. Preprints, 5th Annual Meeting FACSS, Boston, MA, 1978, Abstract No. 55. Gustavsson, A. Spectrochim. Acta, Part B 1983, 388, 995. Melnhard, J. E. “Applications of Plasma Emlsslon Spectrochemistry”; Barnes, R. M., Ed.; Heyden: Philadelphia, PA, 1979; p 2. Knisely, R. M.; Amenson, H.; Butler, C. C.; Fassel, V. A. Appl. Spectrosc. 1974, 28, 285. Novak, J. W., Jr; L i b , D. E.; Boorn, A. W.; Browner, R. F. Anal. Chem. 1980, 52, 576. Wolcott, J. W.; Sobel, C. B. Appl. Spectrosc. 1978, 32, 591. Wohlers, C. C.; Hoffman, C. J. ICP I n f . Newsl. 1981, 6, 500. Spurny, K. R.; Lodge, J. P., Jr.; Frank, E. R.; Sheesley, D. C. Environ. Sci. Techno/. 1969, 3 , 453. Maessen, F. J. M. J.; Balke, J. Spectrochlm. Acta, Part B 1982, 378, 17
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(27) Boumans. P. W. J. M. ICP I n f . News/. 1978, 4, 89. (28) Blades, M. W.; Horiick, 0. Spectrochlm. Acta, Part B 1981, 368, 861. (29) Browner, R. F.; Boorn, A. W.; Smith, D. D. Anal. Chem. 1982, 54, 1411. (30) Kawaguchi, H.; Ito, T.; Mizulke, A. Spectrochim. Acta, Part B 1981, 368. 615. (31) Furuta, N.; Horiick, G. Spectrochlm. Acta, Part B 1982, 378, 53. (32) Novak, J. W., Jr.; Browner, R. F. Anal. Chern. 1980, 52, 792.
RECEIVED for review August 29,1983. Accepted January 16, 1984.
