3838
Anal. Chem. 1993, 65, 3636-3643
Line Selection and Evaluation of Radio Frequency Glow Discharge Atomic Emission Spectrometry for the Analysis of Copper and Aluminum Alloys Tina R. Harville and R. Kenneth Marcus* Department of Chemistry, Howard L. Hunter Chemical Laboratories, Clemson University, Clemson, South Carolina 29634-1905
A methodical approach to line selection based on signal-to-background (S/B) for bulk metals and signal-to-noise (S/N) measurements for trace components in both copper and aluminum alloys has been carried out using radio frequency glow discharge atomic emission spectrometry (rf-GDAES). The analytical lines selected were used to evaluate the figures of merit of accuracy, precision, and limits of detection. Accuracies within the certified error were achieved without the use of an internal standard for the copper alloys. For the aluminum alloys, the use of an internal standard was necessary in order to achieve this level of accuracy. The figures of merit also include internal (short-term)precisions of 0.1-0.6% RSD, external (sample-to-sample)precisions of 4 % RSD, plasma stabilization times of less than 3 min, and limits of detection on the order of tens of parts per billion. The analytical figures of merit presented suggest that rf-GD-AES is a viable alternative to traditional arc and spark emission techniques.
INTRODUCTION For over 50 years, the direct elemental analysis of metals and alloys has been dominated by arc and spark emission spectroscopies. These methods are as well characterized in terms of their analytical application as any in analytical chemistry. In addition to having a wealth of background applications information, emission spectroscopy has the advantage of providing the rapid, multielement analysis which is required for metallurgical foundry support. Despite these successes, however, the traditional solids emission methods suffer from a limited dynamic range, matrix effects, and limits of detection that are not sufficient for many emerging applications, as well as a general lack of continuing methodology development.' Glow discharge atomic emission spectroscopy (GD-AES) has long been suggested to be a viable alternative to arc and spark emission techniques. Since the design of Grimm,2the method has proven itself to be as sensitive and as precise and to have a larger dynamic range than its solids-analysis counterpart^.^-^ Additionally, the sources have been shown to be a powerful means of performing quantitative depth profiling.68 Despite these benefits, the limited use of glow (1) Boumans, P. W. J. M. Spectrochim. Acta 1991, 46B, 725-739. (2) Grimm, W. Spectrochim. Acta 1968, 23B, 443. (3) Marcus, R. K. Spectroscopy 1992, 7 , 12-21. (4) Kruger, R. A,; Butler, L. R. P.; Liebenberg, C. J.; Bohmer, R. G. Analyst 1977, 102, 949-954. (5) KO,J. B. Spectrochim. Acta 1984, 39B, 1405-1423. (6) Bengston, A,; Eklund, A.; Lundholm, M.; Saric, A. J Anal. At Spectrom. 1990, 5, 563.
discharge devices, especially within the United States, has not provided enough evidence of definitive advantages for the glow discharge to supplant the traditional arc and spark methods for bulk metal analysis. In this laboratory, we have focused our research efforts on the development and evaluation of radio frequency (rf) powered glow discharge devices. Although the ability of these devices to directly sputter-atomize nonconductive samples is a definite advantage, they have also proven to be powerful atomization and excitation sources for metals and alloys as well. Previous studies have been performed in efforts to characterize the parametric dependencies and the effects of source anode geometry on analyte e m i s s i ~ n . ~More J ~ recently, the use of directed discharge gas flow has been studied and compared to other glow discharge configurations." Having determined an optimum source geometry and set of discharge operating conditions, the focus of this study is the methodology development and evaluation of the rf-GD-AES technique for analytical applications. The majority of metal analyses that have been conducted in the glow discharge area involve atomic emis~ion~,~J~-16 (GDAES) and mass spectr~metries'~J~ (GD-MS), aimed principally a t the alloying and minor elements. Most AE quantitative depth-profiling analyses in the glow discharge field are used for the surface analysis of major constituentsof binary alloys and some minor e l e m e n t ~ . ~Trace ~ ~ J ~metal analyses in the glow discharge area seem to be dominated by MS,20-22 which has demonstrated very good sensitivity and also has yielded detection limits (sub-ppblevel) which are much lower than arc and sparkmethods. However, spectral interferences, resulting primarily from the combination of polyatomic species of the matrix element and the discharge gas,lBcan present problems a t certain detection levels, particularly for quadrupole spectrometer systems. Analysis times may be up to 10-15 min for the quadrupole GD-MS and as long as 1 h for sector GD-MS systems. Along with the complexity and high costs of operation for sector-based MS systems, these factors would support that an alternative method which (7) Pons-Corbeau, J. Surf. Inter/. Anal. 1985, 7, 169-176. ( 8 ) Tsuji, K.; Hirokawa, K. Surf. Interf. Anal. 1990, 15, 223-228. (9) Winchester, M. R.; Lazik, C.; Marcus, R. K. Spectrochim. Acta 1991,46B, 483-499. (10) Lazik, C.; Marcus, R. K. Spectrochim. Acta 1992, 47B, 13091324. (11) Lazik, C.; Marcus, R. K. Spectrochim. Acta 1993,48B, 863-875. (12) Banks, P. R.; Blades, M. W. Spectrochim. Acta 1989,44B, 11171124. (13) De Marco, R.; Kew, D. Spectrochim. Acta. 1986, 41B, 591-595. (14) Broekaert, 99% confidence that the signal is due to the analyte and not random error. The LOD values obtained from this method are listed in Table X for the four elements in copper and aluminum matrices. The second method has been given by Boumans et al. and is described by the equation LOD = (0.01)3(RSDB)m (4) S/B where RSDB is the relative standard deviation in the background, S/B is the signal-to-backgroundratio, and m is the concentration of the anal* used in the determination, with a value of 3 being used to give a >99 % confidence level.30 Again, since there is no way to measure the background of a blank in GD-AES, this method assumes that the lowest (29) Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis; Prentice Halk New Jersey, 1988. (30)Boumans, P. W. J. M.; Vrakking, J. J. A. M. Spectrochim. Acta 1987,42B, 819-840.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 24, DECEMBER 15, 1993
Table X. Limits of Detection for Trace Elements in Copper and Aluminum Alloys calibration curve RSD of background element (PP4 (PP4 Copper Matrix Cr 360.53 nm 0.24 (2.8 ppm certified in Cu) Ni 346.16 nm 0.46 (22 ppm certified in Cu) Fe 385.99 nm 1.9 (35 ppm uncertified in Cu) Zn 334.50 nm 5.4 (8 ppm certified in Cu) Aluminum Matrix Cr 360.53 nm 0.30 (11ppm certified in Al) 1.4 Ni 346.16 nrn (6 ppm certified in Al) Fe 385.99 nm 3.2 (790 ppm certified in Al) Zn 334.50 nm 7.1 (510 ppm certified in Al) ~
0.018 0.086 0.12 0.12
0.085
Limit of detection values for zinc and iron (which have higher concentration values) in aluminum are higher by both methods. Interestingly, LOD values obtained by arc optical emission spectrometry are also higher for zinc and iron in aluminum,20indicating that perhaps the sputtering yields are lower in aluminum than in copper, and, as supported by their BEC values, that indeed the background is higher for the zinc and iron lines in aluminum. In an effort to demonstrate the detectability of the rf GDAES technique, Figure 4 depicts wavelength scans over both the Cr(1) 360.53-nm and Ni(1) 346.16-nm peaks at trace levels in the NIST 1251 high purity copper standard. The scans were taken at the same conditions as those used for measuring BEC and LOD values, and it is evident from the figure that the high resolution obtained is sufficient for trace analysis.
0.056
CONCLUSIONS
2.2 2.0
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background value measured on either side of the peak is the “true” background. Therefore, there is probably some error associatedwith this method as well. The LOD values obtained from the second method are also listed in Table X. The values for chromium and nickel are on the order of tens of ppb in both matrices, illustrating the greater sensitivity of these elements, while the LOD values of the less sensitive iron and zinc are higher. The LOD values obtained from the first method are higher than those given by the second method, showingthe influence of sample-to-sample error associated with the slope of the line, which is based on a three-point curve using the peak signal only. The standard deviations of the peak signals themselves are generally higher than those of the respective background values. This explains why the LOD values are higher with the first method, which is dependent upon the standard deviation of the peak signal, while the second method is dependent upon the standard deviation of the background of the signal. The LOD values are lower with the RSDB method, indicating that the stability of the background in the glow discharge environment is very good, with the SIB ratios being consistently high for the various lines, and therefore this method likely gives a more accurate representation of the true limits of detection.
A methodical approach to analytical line selection beginning with the bulk metal to the find selection of lines at the trace level is an important step in analysis. Line selection based on S/B and S/N measurements ensures minimal interference from other lines and identifies those lines which exhibit better calibration quality and ultimately provide higher accuracy in a real analysis. Calibration curves for elements in both copper and aluminum alloys show excellentlinearity without internal standardization, with correlation coefficients above 0.98 and intercepts passing acceptably close to the origin. As demonstrated by the differences in the percent concentrations of copper and aluminum in the host matrices, the linearity indicates that strict matrix matching is not necessary in calibration. This study supports a conclusionmade by Ulgen and Dogan that matrix and interelement effects are not significant with the glow discharge source.31 Analytical figures of merit such as precision and accuracy were evaluated in both copper and aluminum alloys. Internal precisions on the order of 0 . 1 4 6 % RSD were achieved for trace elements in both matrices along with external precisions of about 4% RSD, indicating good plasma stability and reproducibility. In evaluating accuracy, the percent error associated with each element in both matrices was well within the certified error. For the aluminum alloys, it was found that the use of an internal standard is necessary in order to achieve this level of accuracy. It cannot be overly stressed that all of the measurements reported here were performed without the benefits afforded by siniultaneous detection as
Cr 2.8 ppm
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346.13
346.18
346.25
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Wavelength scans of Cr(1) 360.53 nm and Ni(1) 346.16 nm present at trace levels in the NIST C1251 high purity copper,
ANALYTICAL CHEMISTRY, VOL. 65, NO. 24, DECEMBER 15, lQ93 3643
employed in arc and spark spectroscopies. In that regard, the rf-GD-AES is not shown in its best possible case and even better levels of precisionand accuracy can be easilyenvisioned. The majority of the total BEC values for elements in copper arise from the matrix-independent BEC, while those in aluminum are due to mainly the matrix itself, indicating that LOD values in aluminum are limited by a higher background. Limits of detection were on the order of tens of ppb for chromium and nickel in copper and aluminum, by utilizing the RSDB method for calculation. Limits of detection were somewhat higher in the aluminum alloys, especially for zinc and iron, as is the case in arc optical emission spectrometry as well.20 The comparison of limits of detection of various techniques is somewhat difficultdue to the numerous methods of calculation,which are usually not describedwhen reporting LOD values. General LOD values reported for GD-AES are in the single ppm For the traditional arc and spark emission techniques, generalized LOD values are typically about or slightly below 1ppm.ms21 For example, Kudermann has reported LOD values for chromium (0.4ppm), nickel (0.6 ppm), iron (0.7 ppm), and zinc (0.7ppm) as well as other elements in The rf GD-AES LOD values have demonstrated an improvement of 1-2 orders of magnitude over dc glow discharge and arc and spark analytical methods of detection. (31) Ulgen, A,; Dogan, M. Anal. Lett. 1990,23, 1897-1906.
This study indicates that quantitative analysis using rfGD-AES is a viable alternative to those utilizing traditional arc and spark techniques, with good working calibration quality, competitive analytical figures of merit, and much potential for future work. As this evaluation of the rf-GDAES source has shown promising analytical results which are comparable to and even surpassingfigures of merit associated with dc glow discharge sources and has the inherent nonconducting analysis capabilities, this source may prove an excellent choice as a single source sufficient for the elemental analysis of all bulk solid materials.
ACKNOWLEDGMENT
This work has been made possible by financial and instrumentation support from Jobin-Yvon, Division of Instruments SA, along with many helpful discussionswith Alain Le Marchand from the Jobin-Yvon, Emission Division. Financial support from the National Science Foundation (CHE-9117152) is also greatly appreciated. Helpful discussions with Professor Ron Williams (Clemson University) are also acknowledged. RECEIVEDfor review June 1, 1993. Accepted September 21, 1993. *Abstract published in Advance ACS Abstracts, November 1,1993.
