Anal. Chem. 1990, 62,1239-1241
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Inductively Coupled Plasma Axial Viewing Absorption Technique Gary D. Rayson* and Daniel Y. Shen Chemistry Department, New Mexico State University, Box 30001, Las Cruces, New Mexico 88003
A technlque has been developed for the measurement of atomic absorption resulting from atoms generated by an Inductively coupled argon plasma. The plasma Is oriented akng the optical path by the lncorporatlon of a window in the base of a conventlonal torch and placing the torch In a horlzontal conflguratlon. The analytical performance of this technique has been characterized for the determlnatlon of Ag, Cu, and Mg. The lknlts of detectlon determlned uslng atomic absorp tlon with this conflguratlon were found to be comparable to those determined uslng atomic emission.
INTRODUCTION Early in the development of the inductively coupled plasma (ICP), Wendt and Fassel (I) and Greenfield et al. (2) investigated the possibility of using atomic absorption to detect the atoms generated by the discharge. Additionally, Veillon and Margoshes (3) and Mermet and Trassy ( 4 ) have investigated the feasibility of an ICP as an atom reservoir for atomic absorption spectrometry (AAS). Also, Ng et al. (5) described the utilization of an atmospheric pressure microwave-induced plasma (MIP) for AAS. These preliminary studies displayed relatively poor figures of merit for the elements investigated with respect to the determination of those elements by atomic emission spectrometry with an ICP (ICP-AES). More recently, Liang and Blades (6) have described the utilization of a capacitively coupled plasma discharge as an atom source with atomic absorption detection. Their results were encouraging for the use of this type of discharge. Unfortunately, sample introduction appears to be limited to gases and dry aerosol, which produce transient signal schemes (pulsed laser ablation and electrothermal vaporization). Wendt and Fassel (I) used a multipass assembly, which directed the primary radiation (passed three times through the discharges) as a parallel beam, for atomic absorption spectrometry with an inductively coupled plasma. The problems associated with their assembly was the complexity of the optical setup. Greenfield et al. (2) used a T-shaped torch, the exit of which opened into the side wall of a tube that was perpendicular to the torch, to change the direction of the “tail flame” of the plasma and used the tail flame as the absorption source. The configuration used by Veillon and Margoshes (3) consisted of a single pass of the incident radiation across the radius of the discharge. The plasma used in those studies was operated at 4.8 MHz. Their conclusions indicated that the ICP is not significantly better than flames except for the determination of refractory metals. Mermet and Trassy (4) described a torch assembly that was oriented along the optical axis. Absorption measurements were accomplished by placing a lens a t the base of their demountable torch assembly. The capacitively coupled plasma atomic absorption system described by Liang and Blades (6) oriented the discharge along
* Author to whom all correspondence should be addressed.
the optical axis. This arrangement provided a longer effective path length with maximum residence of the atoms within the optical path. The present study describes the development of a system that enables the detection of atoms generated within an ICP by atomic absorption using axial viewing of the discharge in a manner similar to those described by Mermet and Tracy for an ICP ( 4 ) and Ng et al. for an MIP (5).
EXPERIMENTAL SECTION Optical System. A block diagram of the experimental assembly used in these studies is shown in Figure 1. The output of a hollow cathode lamp was directed through the base of a ”see-through”ICP torch by a 24-cm focal length lens. An image of the hollow cathode was focused at the object plane of an over-and-under symmetrical-arm-pair off-axis spherical mirror image transfer configuration for coma correction (7-9). The magnification of the image of the cathode was 1.5. The mirrors were each 11.4 cm in diameter with focal lengths of 114 cm. A light-limitingaperture, consisting of two razor blades placed 0.18 mm apart, was placed at the tangential image plane of the mirror system. The sagittal image plane was positioned at the entrance slits of the monochromator. Wavelength isolation was achieved by a 0.85-m focal length cross-dispersion, Echelle monochromator typically used with a Spectrospan V plasma emission spectrometer (Applied Research Laboratories, Valencia, CA). The spatial resolution of this optical system was 560 Mm. This measurement was made by placing a fine screen at the object plane of the mirror system where the image of the hollow cathode lamp was focused. The screen was then moved perpendicular to the optical axis in 125-rm increments, and the intensity of the hollow cathode output was recorded at each step. The achieved level of spatial resolution enabled the rejection of the annular portion of the plasma and the selective viewing of the central axis of the discharge. A 27.12-MHz quartz-controlled radio-frequency (rf)generator and an impedance matching network (Plasma-Therm, Inc., Kresson, NJ) were used with a three-turn load coil to sustain the discharge. The entire impedance matching network and torch assembly was placed on a three-dimensional translation stage to enable precision movement of the torch with respect to the optical axis. Following the monochromator assembly described previously, the output signal from the photomultiplier tube (PMT) was directed to a lock-in amplifier (Model SR510, Stanford Research Systems, Palo Alto, CA). The lock-in application was used to discriminate against background emission of the plasma and enhance the signal-to-noiseratio (SIN).The signal was further processed and digitized at a rate of 100 points s-l by a data acquisition system (Models SR 245 and SR 235, Stanford Research System). The resulting signal was then processed and analyzed by a dedicated microcomputer system (Whole Earth Computers, Berkeley, CA). The torch was a modification of a commercially available ICP torch (Plasma-Therm). As indicated in Figure 2, the base of the torch normally used for sample introduction was removed. An enlarged end was added, which included an optical window (1.8cm diameter) that was located 3.5 cm from the plasma gas inlet tubing. The sample inlet was positioned perpendicular to the axis of the torch and tangential to the torch in a manner similar to that of the intermediate and coolant gas flow inlets. The sample aerosol was introduced to the discharge along the same path as the
0003-2700/90/0362-1239$02.50/00 1990 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
0.1 6 u
9
0.12
g
0.08
n c
4 0.04
0.00 400
640
880
I120
1360
1600
COU.PYtW ].I.*-
Applied PowerIW
Figure 3. Measured absorbance (lateral viewing) as a function of applied rf power for 100 mg of Ag L-I at vertical positions of 2 (m), 7 (@), 12 (A),17 (V),and 22 (*) mm above the load coil.
Sample 'Iasma
Coolant
7 A ' Figure 2. Schematic diagram of ICP torch used for absorption measurements. A = 35 mm, B = 18 mm.
I
1
500
700
I
".VU
Table I. Operation Conditions of the Data Acquisition System
900
1100
1300
I500
Applied P o w e r / W
wavelength of detection, nm sensitivity of lock-in amplifier, mV phase of lock-in time constant (Dre/Dost).s
Ag
Cu
Mg
328.1 2 0.9 0.1/0.1
324.8 2 8.3 1/0.1
285.2 5 179.3 0.3/0.1
incident radiation from the hollow cathode lamp passing through the base of the torch. The torch was positioned horizontally along the optical path of the system. The incident radiation was transmitted along the central portion of the ICP where the sample aerosol is vaporized and atomized. The subsequently attenuated radiation was then transmitted through the image transfer system to the monochromator and photomultiplier tube detector. Samples were introduced to the ICP atomic absorption system by a concentric-glassnebulizer (Plasma-Therm) with a Scott-type, double-pass spray chamber. All solutions were delivered to the nebulizer by a peristaltic pump with a flow rate of 0.78 mL min-'. Except when indicated otherwise, the plasma was operated at a rf power input of 600 W with coolant gas flow at 16.3 L min-' and nebulizer gas flow of 0.85 L min-'. Additional data acquisition system conditions are listed in Table I. Solutions. Stock solutions of lo00 mg of silver, copper, and magnesium L-' were prepared by dissolution of the reagent-grade nitrate salts in doubly distilled deionized water. All sample solutions were prepared daily by serial dilution with doubly distilled deionized water. Atomic Absorption. As indicated in the experimental assembly block diagram (Figure l),the incident light from the hollow cathode lamp was imaged through the central channel of the annular plasma discharge. The attenuated transmitted light was then imaged onto the entrance slits of the monochromator for detection. The intensity of the transmitted radiation was measured as the 100% transmittance level with the introduction of a blank. Measurement of the transmitted radiation intensity resulting from the introduction of the anal-containing sample enabled the determination of the attenuated light level. Absorbances were calculated by using the Beer's law relation. Atomic Emission. In order to provide an accurate assessment of the analytical figures of merit for the atomic absorption con-
Figure 4. Measured absorbance as a function of applied rf power for 10 mg of Ag L-' with axial viewing.
figuration described in this study, atomic emission experiments were carried out using the same optical and data acquisition systems. However, the torch was oriented vertically for these measurements in order to better approximate the normal analytical configuration.
RESULTS AND DISCUSSION In an effort to determine the best set of operating conditions for atomic absorption measurements, the magnitude of the applied rf field and the current of the hollow cathode lamps were varied systematically. Because of a reported dependence of the population of ground-state atoms within the ICP on the distance from the discharge within the load coils, absorption measurements were recorded as a function of distance from the load coil at several power levels. Figure 3 displays these lateral absorption measurements for a sample of 100 mg of Ag L-' with the torch oriented vertically and the incident radiation directed across the diameter of the discharge. A maximum absorbance was observed at an operating forward power of 500-600 W. Decreasing the sample concentration to 10 mg L-* indicated no significant loss of sensitivity at that operating power. Thus, 600 W was selected as the level of the applied rf power for all subsequent absorption measurements. The results of a similar study involving absorbance measurements obtained with axial viewing of the discharge are shown in Figure 4. A maximum absorbance was observed at an operating power of 500-600 W with the introduction of 10 mg of Ag L-l. The magnitude of the measured absorbance with axial viewing was observed for 10 mg L-' to be approximately the same magnitude as the largest measurement recorded with lateral viewing of the atoms generated within the discharge for a sample of 100 mg L-'. Working curves for silver, copper, and magnesium employing the inductively coupled argon plasma axial viewing
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
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.
0
20
40
60
80
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Flgure 5. Working curves for Ag at 328.1 nm (W), Mg at 285.2 nm (A),and Cu at 324.75 nm (0)with axial viewing and a lamp current of 10 mA.
Table 11. Comparison of Detection Limits for This Study detection limits, ppm elements
ICAPAVAT
ICP-AES
0.76
2.2 2.4 1.3
0.068
0.66
absorption technique (ICAPAVAT) are displayed in Figure 5. Linear dynamic ranges of 1-2 orders of magnitude are demonstrated for each of these elements. This range of concentrations for which an adherence to Beer's law is observed is comparable to the linear dynamic range typically observed in flame atomic absorption. The detection limits for silver, copper, and magnesium determined by using the ICAPAVAT are listed in Table 11. The detection limits were determined at a S I N of 3. The blank noise levels for SIN calculation were obtained by using the following formula (IO):
where SA, s, so, sB, and T represent the standard deviations of calculated absorbance, the sample transmitted intensity, the 100% transmittance signal, and the intensity of light measured with the hollow cathode lamp output blocked and
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the calculated transmittance, respectively. The calculated transmittance was obtained by dividing the measured sample transmitted intensity by the intensity of the measured signal with the introduction of a blank. The calculated standard deviation of the absorbance for this study was 0.0295. In comparison, the measured standard deviations of repeated blank absorbance measurements ranged from 0.0354 to 0.0549. The similarity between these values reenforces the applicability of the calculated standard deviations used throughout this study. Because of the relatively poor optical throughput of the imaging system used in this study (with the incorporation of a horizontal slit at the tangential image plane), the detection limits for ICP-AES were determined by using the same optical configuration for silver, copper, and magnesium in order to provide a comparison for the performance and quality of the system, and the results are displayed in Table 11. The detection limits for these atomic emission measurements were determined as the concentration of analyte that resulted in a signal that was 3 times the blank noise level. Lateral viewing was used for all emission measurements in which the same operating parameters were used as those employed for absorption measurements, and the positions were reoptimized. Readily apparent are the similarities in the detection limits for these metals when the same optical system was used with these two detection techniques. It should be noted, however, that these emission detection limits are significantly poorer than those commonly reported for the ICP.
LITERATURE CITED (1) Wendt. R. H.; Fassel, V. A. Anal. Chem. 1968, 38, 337; 1968, 4 0 , 385. (2) Greenfield, S.; Smith, P. 8.; Breeze, A. E.; Chiton, N. M. D. Anal. Chim. Acta 1968, 4 1 , 385. (3) Veillon, C.; Margoshes, M. Spectrochim. Acta, Part 8 1968, 23, 503. (4) Merrnet, J. M.; Trassy, C. Appl. Spectrosc. 1977, 31, 237. (5) Ng, K. C.; Jensen, R. S.; Brechrnann, M. J.; Santos, W. C. Anal. Chem. 1988, 6 0 , 2818. (6) Liang, D. C.; Blades, M. W. Anal. Chem. 1988, 60, 27. (7) Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1978, 50, 1714. (8) Walters, J. P. Anal. Chem. 1968, 4 0 , 1540. (9) Klueppel, R. J.; Coleman, D. M.; Eaton. W. S.; GoMstein, S. A.; Sacks, R. D.; Waters, J. P. Spectrochim. Acta, Part B 1978, 33, 1. (10) Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis; Prentice-Hall: Englewood Cliffs, NJ, 1988; p 169.
RECEIVED for review December 18,1989. Accepted March 26, 1990. We acknowledge the financial support of Sandia National Laboratories for this research.
