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Anal. Chem. 1904, 56, 120-122
Optimization of Electrothermal Atomization-Inductively Coupled Plasma Atomic Emission Spectrometry for Simultaneous Multielement Determination Hassan M. Swaidan' and Gary D. Christian*
Department of Chemistry, BG-IO,University of Washington, Seattle, Washington 98195 Inductively coupled plasma atomic emission spectrometry (ICP-US) is an attractive analytical tool for the simultaneous determination of major, minor, and trace elements in various samples (1-5).The use of direct vaporization of the sample has been described previously in various reports (6-14). Recent modifications of commercial electrothermal atomizers have been described for application to ICP-AES. Crabi et al. (15)modified the HGA 500 electrothermal atomizer to be used as an introduction device to the ICP-AES. They used the L'vov platform to improve the tolerance of their system for variations in matrix composition. Aziz et al. (16)used the HGA 74 graphite furnace atomizer with a modified gas system in the analysis of small-volume biological samples. They indicated that a gas circulation system will minimize memory effects in routine application. Recently ( I 7), we reported the use of the HGA 2000 graphite atomizer in combination with an ICP (GA-ICP). The combination of a graphite furnace and microwave induced plasma atomic emission spectrometer for the multielement analysis of small volume liquid samples has been described (18). Grabau and Fassel (19)have demonstrated the ability of the photodiode array detector as an alternative to the polychromators commonly used with the ICP-AES. In all the previous techniques, detection limits have been reported to be several orders of magnitude better than those obtained under similar conditions with conventional ICP. Compromised conditions for simultaneous multielement analysis by ICP have been reported (20). In use of the HGA-ICP system, compromised conditions must be accommodated for both the graphite atomizer and the inductively coupled plasma in order to obtain the lowest relative detection limits for most elements and to minimize interelement effects, both chemical and spectral. In this paper, we describe optimization of parameters for the HGA-ICP system and explore its applicaton for simultaneous multielement analysis of microsamples, utilizing direct readout of the transient signals by the Jarell Ash Model 955 inductively coupled plasma spectrometer. Elements are arranged by groups for simultaneous determination in mixtures, based on their arrival time in the plasma and compromised operation parameters.
EXPERIMENTAL SECTION Apparatus. A modified Perkin-Elmer HGA 2000 graphite furnace was employed as a sample introduction source. The furnace was modified and connected to the ICP torch of a Jarrel Ash Model 955 Plasma Atomcomp inductively coupled plasma spectrometer through a modified spray chamber as previously described (17). Graphite tubes were coated with a thin pyrolytic film using 0.05 L/min mixture gas (10% methane and 90% argon) by heating at the charring temperature of 2100 "C for 10 min. The operating conditions are described in Table I. Signals were monitored with the polychromator of the Model 955, by commanding the computer to look at the specific channel or channels of interest, and were recorded on a teletype as digital readouts. Samples were introduced into the furnace with a 10-pL syringe micropipet with disposable polypropylene tips. After signals were recorded for the blank and the sample, the graphite atomizer was purged by maximizing the temperature and Present address: Chemistry Department, Faculty of Science, King Saud University, Riyadh, Saudi Arabia. 0003-2700/84/0356-0120$01.50/0
I _ _ _ I _
ll_-lll
Table I. Operating Conditions Jarrel-Ash Model 955 Plasma Atomcomp incident rf power reflected rf power coolant argon flow rate plasma argon flow
1 kW G l o w
18 L/min 0.0-0.5 L/min
Direct Reader dispersion entrance slit width (fixed) exit slit width (fixed) observation height
0.55 nm/mm 25 pm 50 pm 10-20 mm
Perkin-Elmer HGA 2000 argon carrier flow rate
1-2.5 L/min
cycle
temp, "C
drying charring atomizing
400 800
2400
time, s 20 2-5 5-10
running the cycle with argon only. Data were treated with an Apple I1 Plus computer with software from Interactive Microware, Inc., State College, PA, for plotting the graphs using scientific plotter and curve fitting programs. Reagents. Stock solutions of 1000 ppm were obtained from Fisher Scientific Co. Working standard solutions were prepared by fresh dilution of the stock solutions with deionized water. Volumes of 10 p L were used throughout the experiment for measurements.
RESULTS AND DISCUSSION Optimization Routine. After the plasma was ignited and stabilized, the optimal conditions of flow rate, height above the coil, and the reflected power for an element or mixture of elements were obtained as follows. The graphite cycle was adjusted for 400 "C drying for 20 s, 800 OC charring for 5 s, and 2400 "C atomizing for 10 s. A 10-pL aliquot of a blank (deionized water) or sample was injected into the atomizer. Studying one element a t a time (either alone or in mixture), we measured the arrival time (the time from the beginning of the atomization cycle of the atomizer to the rise of the galvanometer on the 955 spectrometer) and the lifetime in the plasma (exposure time) with a timer by observing the rise and fall of the galvanometer on the spectrometer. From this information, the computer was programmed to measure the plasma emission signal over a set observation period (exposure time), adjusting for the arrival time to minimize background recording, Le., to observe primarily the duration of the atomic emission signal. A digital readout was recorded on a teletype for each channel. By use of the appropriate exposure times, the operating parameters were optimized for each element from maximum signal-to-noise ratios. A similar procedure was adopted for the mixed groups, selecting the compromized conditions that give the best detection limits for the most elements. Operating Conditions, Simultaneous multielement analysis is best approached by considering the various parameters that affect optimization for each element. The optimal parameter for each element will necessarily be compromised in exchange for a common set of operating conditions 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
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-
~
Table 11. Optimal Conditions for Single Elements optimal height, mm
element
12 12 12 12 12 12 14 14 14 14 14 14 18
Cd
Mn Ni Cr AI Zn Pb Sb Si co Fe Ag
cu Ba Ca
optimal carrier flow rate, L/min
arrival time, s
1.0
1.5 2.5 2.5 2.0 1.0 2.0
18
1.5 2.0 2.5 2.0 2.0 2.5 2.5
16
2.0
exposure time, s
6 4 4
7 7 4
3
6
6 7 3 4 4 3 4 4 3 6 7
4 4 7 7 4 6 5 6 4 10 4
that give the lowest detection limits for the mixture. Several parameters may influence the HGA-ICP system, not only the magnitude of the detection limits but also interelement interferences. These include the power of the ICP, the observatipn height above the coil, the flow rate of the carrier gas which transports the sample aerosol into the plasma, the graphite atomizer temperature and cycle time, the deterioration and aging of the graphite tube, the length of the transport tube to the ICP, and the arrival and the exposure times for the elements of interest. The graphite tube will deteriorate after a limited number of firings and the signal intensity will diminish. A pyroliticdy coated tube provides a barrier between vapor phase atoms and the graphite substrate (21). Hence, we coated the graphite tube with a thin pyrolytic layer to improve aging of the tube and obtain a steady signal for a longer period of firings. The arrival time (the time from the beginning of the atomization cycle of the graphite atomizer to the deflection of the galvanometer on the spectrometer) and the exposure time (the time from the rise to the end of the fall of the galvanometer on the spectrometer) were measured for each of the 15 elements. Results are given in Table 11. The effect of the observation window on the intensity of the recorded signal and the signal-to-noise ratio is very significant for transient signals, and the observation time should be adjusted to correspond to just the peak width to the extent possible. The background reading from the plasma increases in proportion to the observation time. However, the observation window must be sufficiently broad that slight variations in
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sample arrival time will not affect the observed analyte signal or that multiple elements that arrive at slightly different times can be observed. The flow rate and the observation height above the coil were optimized for maximum signal-to-noise ratio for each of 15 elements (Table 11). On the basis of optimal conditions for individual elements, the 15 elements were arranged into three different groups according to their observation heights. Group one consists of cadmium, manganese, nickel, chromium, aluminum, and zinc with a height of 12 mm. Group two consists of lead, antimony, silicon, cobalt, iron, and silver with a height of 14 mm. Group three consists of copper, barium, and calcium with a height of 18 mm. The elements tend to fall into patterns on the basis of “hard” and “soft” lines as noted by Blades and Horlick (22), Le., those whose spatial behavior is relatively insensitive to operating parameters (group one) and those that are sensitive to operating parameters (group three). Blades and Horlick found the spatial behavior of the “soft” line elements to be correlated with plasma temperature. The flow rate dependence, arrival time, and exposure time were determined for each element in the different mixtures, and compromised conditions were selected to provide the best compromised detection limits for simultaneous determinations. The optimal observation heights above the coil, flow rates, arrival times, and exposure times were, respectively, 12 mm, 1.8 L/min, 4 s, and 7 s for group one, 14 mm, 2.0 L/min, 4 s, and 6 s for group two, and 18 mm, 2.5 L/min, 3 S, and 7 s for group three. Detection Limits. Detection limits were determined by using 1ppm concentrations of the analytes and represent the concentrations which give a net signal equal to three times the experimentally determined standard deviation of the blank for each element. Table I11 lists the detection limits obtained for single elements and compares the values obtained via peak height measurement of the recorded signal (17). The present detection limits are poorer due to the substantial background signal included in the observation window. Detection limits for each element were determined in mixtures for the three groups and are also summarized in Table 111. Results are generally comparable to those for single elements, although there is some substantial enhancement in the mixtures, particularly for chromium and iron, possibly due to formation of alloys of enhanced volatility in the mixtures. Table I11 also compares the present detection limits with those of previous workers utilizing combined electrothermal atomizer/plasma spectrometry and various volume samples. The precisions for the various elements in the group mixtures were determined by using 1 ppm concentrations and seven readings. Relative standard deviations were better than 1070,with 5% or less
-
Table 111. Detection Limits (ng/mL)---
element Cd
Mn Ni Cr
A1 Zn Pb Sb Si
co
Fe
Ag
cu Ba Ca
peak height single mixture measurement wavelength, (this work) (this work) ( 1 7) nm (10 PL) (10 CtL) (10 PL) 228.8 50 20 4 257.6 2 3 0.9 231.6 90 50 205.5 20 5 308.2 60 170 213.8 10 7 8 220.3 40 50 217.5 160 250 10 251.6 80 50 228.6 30 20 4 259.9 70 10 328.0 30 10 0.8 324.7 20 10 493.4 70 150 396.8 60 30
HGA74-ICP (16)
(50 ELL) 8
4 (Cr 11) 6 130 (Pb I)
HGA74-MIP HGA500-ICP (18)
(50 PL) 35 7 76 84 (Cr 11)
( 15 )
(20 PL) 2 0.4
0.7 3
10
50 (Pb I)
2
90
10
18
0.1
77 (Ca I)
________I -______-_____-_I_______
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
for most elements.
Dynamic Range and Relative Sensitivities. The net intensity readings for concentrations ranging from 0.1 ppm to 30 ppm for the elements in the three mixtures were determined. Measurements beyond 10 ppm concentrations for group two were difficult due to apparent reaction between the elements or from anion/cation effects in the mixture which resulted in loss of analyte element, for example, through vaporization of molecular halide species (23). In addition, concentrations in excess of 10 ppm require extensive cleaning of the graphite atomizer between readings, necessitating several purgings which in turn will accelerate the decrease in the intensities of signals by aging the graphite tube. Measurements could be made over at least 2 orders of magnitude for all the elements. The correlation coefficient for signal vs. analyte concentration was 0.992 or better for all the elements tested in group two. ACKNOWLEDGMENT The assistance and discussions of Steven Hartenstein are gratefully acknowledged. Registry No. Cd, 7440-43-9;Mn, 7439-96-5;Ni, 7440-02-0;Cr, 7440-47-3; Al, 7429-90-5; Zn, 7440-66-6; Pb, 7439-92-1; Sb, 7440-36-0; Si, 7440-21-3; Co, 7440-48-4;Fe, 7439-89-6; Ag, 744022-4; Cu, 7440-50-8; B a , 7440-39-3; Ca, 7440-70-2.
LITERATURE CITED (1) Kniseley, R. N.; Fassel, V. A.; Butler, C. C. Clh. Chem. (Wlnston-Sa/em, N.C.)1973, 19, 807. (2) Subrarnanlan, K. S.;Meranger, J. C. Scl. Total Envlron. 1982, 24, 147.
Winge, R. K.; Fassel, V. A,; Kniseley, R. N.; DeKalb, E.; Hass. W. J., Jr. Spectrochlm. Acta, Part B 1977, 326,327. Dahlquist, R. L.; Knoll, J. W. Appl. Spectrosc. 1978, 32, 1. Barnes, R. M. “Application of Plasma Emlssion Spectroscopy”; Heyden & Son Inc.: Philadelphia, PA, 1979. Kleinmann, I.; Svoboda, V. Anal. Chem. 1989, 41, 1029. Nixon, D. E.; Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 210. _ .
Human, H. G. C.; Scott, R. H.; Oakes, A. R.; West, C. D. Analyst(Lon-
don) 1978, 101, 265.
Gunn, A. M.; Millard, D. L.; Kirkbright, G. F. Analyst (London) 1978, 103.1066. Millard, D. L.; Shan, H. C.; Kirkbright, G. F. Analyst (London) 1980, 105, 502. Kirkbright, G. F.; Snook, R. D. Anal. Chem. 1979, 51, 1938. Salln, E. D.; Horllck, 0.Anal. Chem. 1979, 51, 2284. Mermet, J. M.; Hubert, J. Prog. Anal. At. Spectrosc. 1982, 5, 1. Kirkbright, G. F.; Walton, S. J. Analyst (London) 1982, 107,276. Crabi, G.;Cavalll, P.; Achilll, M.; Rossi, G.; Omenetto, N. At. Spectrosc. 1982, 3,81. Aziz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim. Acta, Part6 1982, 3 7 6 . 369. Swaidan, H. M.; Christian, G. D. Can. J. Spectrosc., in press. Azlz, A.; Broekaert, J. A.; Leis, F. Spectrochim. Acta, Part 6 1982, 376,381. Grabau, F.; Fassel, V. A. “Book of Abstracts”; 184th National Meeting of the American Chemical Society, Kansas City, Sept 12-17, 1982; American Chemical Society: Washington, DC, 1982. Berman, S.S.; McLaren, J. W. Appl. Spectrosc. 1978, 32, 372. Slavin, W.; Manning, D. C.; Carnrick, G. R. At. Spectrosc. 1981, 2, 137. Blades, M. W.; Horlick, G. Spectrochlm. Acta, Part6 1981, 366, 861. Fuller, C. W. “Electrothermal Atomization for Atomic Absorption Spectrometry”, Billing & Sons Ltd.: Great Britain, 1977.
RECEIVED for review March 21, 1983. Resubmitted June 24, 1983. Accepted September 26, 1983. We appreciate the financial support of King Saud University and the Arabian American Oil Company for this research.
CORRECTION Performance Characteristics of a Continuum-Source Wavelength-Modulated Atomic Absorption Spectrometer
J. D. Messman, M. S. Epstein, T. C. Rains, and T. C. O’Haver (Anal. Chem. 1983,55, 1055-1058). There are several typographical errors in this paper. On page 1056, under “Instrumentation”, a R374 photodetector was used for WM-AAC measurements of potassium and sodium instead of a R372 photodetector. In Table I on page 1057, the WM-AAC detection limit for Cd should read 0.09 mg/L. In the same table, the wavelength for T1 should read 276.787 nm.
