Inductively Coupled Plasma-Optical Emission Spectrometry: Application to the Determination of Alloying and Impurity Elements in Low and High Alloy Steels Constance C. Butler, Richard N. Knlseley, and Velmer A. Fassel Ames Laboratory-USAEC and Department of Chemistry, Iowa State University, Ames,
Three different Inductively coupled plasma-optical emission spectrometry (ICP-OES) systems have been used for the determination of residual Impurities and alloying constituents in solutions of high and low alloy steels of widely varying composition. The results of these studies show that the presence of an Iron matrix does not Influence the detection limits for the 12 elements studied and that these elements can be determined at fractional ppm levels wlthout prior chemlcal concentration. The analytical callbrations were established with synthetic reference solutions and the resulting analytical curves exhiblted linearity over three to four orders of magnitude. The relative standard deviations determined over an extended period of time ranged from -0.01 to -0.07.
Extensive studies on argon-supported, inductively coupled plasmas (ICP) have clearly shown that these excitation sources offer great promise for the simultaneous determination of the metals and metalloids a t the ultratrace level in solution. The physical and spectrometric properties of these plasmas and the progress made to date on these analytical applications have been reviewed recently (1-3). In this communication, we report on a critical assessment of the potentialities of this excitation source for the determination of residual impurities and alloying constituents in solutions of low and high alloy steels of widely varying compositions. The data collected in this study show that: a) detection limits of the twelve elements investigated do not deteriorate in the presence of an iron matrix; b) the lowest concentrations quantitatively determinable in a steel matrix range downward from 0.5 ppm for Ni to 0.1 ppm for A1 without any prior chemical preconcentration of the analytes; c) analytical calibrations established with synthetic reference solutions [Symbols, terminology, and usage in this manuscript are in conformity with IUPAC recommendations listed by the Analytical Chemistry Division, Commission on Spectrochemical and Other Optical Procedures for Analysis ( 4 ) . ]prepared by the incremental addition of the analytes to pure iron solutions provide accurate results for the analysis of high alloy steels containing as low as 3.0 wt % iron; and d) analytical calibration curves exhibited linearity over concentration ranges of three to four orders of magnitude.
EXPERIMENTAL To illustrate the versatility of this excitation source, quantitative calibrations and analyses were performed with facilities providing capabilities for either the sequential or the simultaneous determination of a number of constituents. Two different inductively coupled plasma excitation systems differing in their operating characteristics were used in this study. These were used in conjunction with three different spectrometers and readout systems. The characteristics of the three experimental facilities are summarized in Table I. The Applied Research Laboratories (ARL) Quantoscan and the Hilger-Engis spectrometer were mounted a t a 180° angle so that both instruments could observe the same ICP simul-
Iowa 500 10
taneously. Analyte signal measurements with the two instruments were made by peaking the spectrometers on the spectral lines of interest. For the ARL Quantovac, the exit slits that isolated the spectral lines along the focal curve were fixed in position and thus measurements were restricted to those lines programmed on the instrument. In all instances, the signals from the blank reference solutions were measured alternately. With Systems I and 111, the net measures were obtained by subtracting the blank measure from the total signal. No internal references lines were utilized. The analyte signals from the Quantoscan were ratioed to a portion of the background radiation isolated by an optical filter. Approximately 3 ml of solution were required for each intensity measurement. The plasma generated by the high frequency supply used in systems I and I1 was less tolerant of the presence of water vapor than the plasma generated by system 111; hence, partial desolvation was employed for this plasma. Although not demonstrated unequivocally, the -50% peak-to-valley 120-Hzripple on the radiofrequency envelope appears to be responsible for the loss of stability when the plasma is overloaded with water vapor. It should be noted that this power supply operates from a single-phase input and that the high voltage dc power delivered to the oscillator power-amplifier tube is poorly filtered. In contrast, the system I11 generator has a three-phase input and additional filtering is provided for the dc power from the rectifiers. The plasma generated by this supply operates in a stable manner when the aerosol is injected directly into the plasma. Dissolution of Samples. The samples (1.000 g) were dissolved in a mixture of 30 ml of 1:l HC1 and 5 ml of HNO3 following the procedure described by Fassel et al. (8). The resultant solutions were diluted to yield a solution containing 5.00 mg of sample per milliliter. For more concentrated solutions, a buildup of salt occurred a t the tip of the aerosol injection tube causing the plasma to become unstable. The synthetic reference solutions were prepared by additions of the analyte elements to pure Fe solutions (Glidden A-104 Iron Melting Stock, Lot No. 29583); no attempt was made to match the overall composition of the steel samples and the synthetic reference solutions employed for calibration purposes. A small amount of residue (12 to 20 mg of the original 1-gram sample) sometimes remained after the samples were dissolved. These residues were filtered and examined by dc arc emission spectrometry. These examinations showed insignificant or notdetectable levels of the elements of interest.
RESULTS AND DISCUSSION Detection Limits. When steel samples are dissolved and ultimately diluted to the 0.5 wt % in solution level, all residual impurities as well as other alloying constituents, which may also occur in steels at low concentrations, are diluted 200-fold. Thus, the quantitative determination of some residual impurities and alloying constituents requires the ultimate in powers of detection. The overall performance of the inductively coupled plasma with reference to this figure of merit has been adequately documented (1-3); the observed detection limits (the concentration required to produce a line signal of twice the standard deviation in background fluctuations) for the elements of interest in the present study are summarized in column 3 of Table I1 for reference purposes. A valid question, often asked, is whether these values, observed under idealized conditions are transferable to “real-life” situations, Le., in the presence of the sample matrix such as steels in solution. The measureANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
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Table I. Experimental Facilities a n d Operating Conditions System I
System !I
Plasma power supply
Lepel High Frequency Laboratories Model T-2.5-1MC2-J-B generator with attached impedance matching network, 2.5 kW, frequency -29 MHz. Input power was 80% of total power. The load coil was two turns of 5-mm 0.d. copper tubing, i.d. of coil was 27 mm.
Same as System I.
Nebulizer
Pneumatic type described by Kniseley et al. (5). Aerosol was desolvated by a system described by Kniseiey e t al.
Same as System I.
(6).
Gas flows
Spectrometer
Slits Detector electronics
Average sample ne bu 1i z at i on rat e Observation height
Argon used throughout with 10 1./min through the coolant tube and 1.1 l./min through the aerosol tube. No (6 plasma gas" flow was used. Kniseley e t al. have described the gas flow system (5) and the plasma torch assembly ( 6 ) adapted from a design by Greenfield (7). Hilger-Engis Model 1000 1meter Czerny-Turner mounting scanning spectrometer with grating (1200 ruling/ mm) blazed for 500 nm. Reciprocal linear dispersion of 0.8 nm/mm in the first order, First-order spectra were used throughout. 25-pm entrance and exit slits. The photocurrent from an EM1 E256B photomultiplier was amplified with a Princton Applied Research Model 13 electrometer and r e corded with a Leeds and Northrup Speedomax r e corder. Time constant was -3 sec.
3.1 m1/1nin. Observation restricted to a zone 4 m m high centered 18 nim above top of load coil.
Same as System I.
System
Lepel High Frequency Laboratories Model T-5-3 DFl-2-J-S generator with attached impedance matching network, 5 kW, frequency -30 MHz. Input power was 35% of total power. The load coil was two turns of 5-mm 0.d. copper tubing, i.d. of coil was 25 mm. Same as System I except desolvator was not used. Aerosol from spray chamber ( 6 ) was conducted directly into the plasma via a 11-cm long, 20-mm diameter glass tube bent to a 90" angle and connected directly to the aerosol tube of the torch with a ball joint connector. Same a s System I.
Applied Research Laborator i e s (FICA-France) Quantoscan 35 1-meter CzernyTurner mounting scanning spectrometer with grating (1800 ruling/mm) blazed for 210 nm. Reciprocal linear dispersion of 0.5 nm/mm in the first order. First-order spectra were used throughout. 20-pm entrance and 60pm exit slits. The photocurrent from a photomultiplier (type unknown) was integrated by an Applied Research Laboratories capacitor -type integration system and the nieasure was read on a digital voltmeter. Provision was made to ratio the analyte intensities to the relative meas u r e of a region of the background isolated by an optical filter. The background channel was a duplicate of the signal channel. Integration time was time required for reference channel capacitor to charge to preset level (-20 sec). 3 .I nil/niin.
Applied Research Laboratories QVAC 127 multichannel direct reading 1-meter Pachen-Runge mounting spectrometer with grating (2160 rulings/mm blazed for 170.0-215.0 nm region. Reciprocal linear dispersion of 0.46 nm/mm in the first order. First-order spectra were used throughout. 20-bm entrance and 50-pm exit slits. The photocurrents from RCA 1P28 and 1P21 photomultipliers were integrated (10sec integration time) on capacitor type integrators designed and built at the Ames Laboratory. The measures were printed by a Teletype Model 33.
Same as System I.
Same a s Svsteni I.
2.5 ml/niin.
Table 11. Comparison of Detection Limits Obtained in the Presence and Absence of a Matrix Material H?O
Element
nm
"g/ml
ug/ml
A1
396.1 456.2 357.8 324.7 398.8 403.0 351.5 405.8 405.7 417.9 400.8 349.6
0.002 0.007 0.001 0.001 0.003 0.003 0.06 0.01 0.01 0.008 0.002 0.02
0.004 0.013 0.002 0.0003 0.003 0.003
Ce Cr Cu La Mn Ni Nb
Pb Pr W Zr
Wt %
0.07 0.02 0.01 0.01 0.003 0.02
.
Steel
0.008% 21 in
Steel
quantitatively
0.5% Fe solution
solution
in
Lowest
Detection limits
Wavelength
0.005?4 Pb
0.00008 0.0003 0.00004 0.000006 0.00007 0.00007 0.001 0.0004 0.0002 0.0002 0.00006 0.0005
deteminable concn inFe, w t %
0.0004 0.0015 0.0002 0.00003 0.0004 0.0004 0.005 0.002 0,001 0.001 0.0003 0.002
Figure 1. Signals obtained for 0.005% Pb and 0.008% Zr in steel
Table 111. Statistical Data on Analytical Curves Obtained by Linear Least-Squares Fit to a Log-Log Plot Re1 Concn rarge,
Element
AI Cr CU Mn Ni Nb W
Zr Ce La Pr AS Pb
Re1 std dev
wt % in steel
Slope
0.002-1.0 0.002-20 0.002-2.00 0.002-2.0 0.01-1 0 0.005-0.2 0.005-0.2 0.005-0.2 0.005-0.1 0.005-0.1 0.005-0.1 0.002-0.1 0.001-0.02
1.03 1.oo 0.990 1.04 1.04 0.986 0.991 0.971 1.01 0.970 0.980 0.985 0.971
std dev of
of slope, % intensities, %
1.60 1.31 1.22 1.07 2.03 2.31 2.86 3.70 3.88 0.903 1.47 2.73 1.86
4.61 3.57 3.13 3.23 4.37 3.15 2.43 5.05 2.34 0.943 1.53 7.10 1.79
ment of these detection limits in the presence of a 0.5 wt % iron matrix under standard operating procedures showed no significant deterioration in the values. These results, which are summarized in column 4 of Table 11, demonstrate the transferability of the detection limit data. It should be noted that there is at least a factor of 2 to 3 uncertainty in measuring detection limits; hence, the differences noted in columns 3 and 4 of Table I1 are not significant. Lowest Quantitatively Determinable Concentrations. The detection limits shown in column 4 of Table I1 may be referred to the steel matrix in solution; these values are shown in column 5 . If the reasonable assumption is made that a signal at least ten times greater than the standard deviation of the background scatter (noise) is desirable for quantitative determinations, then the multiplication of observed detection limits by a factor of five leads to a quantity defined as the lowest quantitatively determinable (LQD) concentration. These values are shown in the last column in Tahle 11. Previous work has already shown that the LQD concentrations for Ti, Ce, and Nb (0.0003, 0.0007, and 0.001 wt 90,respectively) were readily attained in practice (9). Figure 1 shows tracings of Zr in steel (NBS Standard Reference Material No. 1261) and of P b in steel a t 0.008 and 0.005%, respectively. The Zr 349.6-nm line used for this tracing is not the most sensitive Zr line (Zr 343.82-nm); the latter cannot be used because of a spectral
Figure 2. Analytical curves for the determination-ofCr and Ni in steel
interference from the Fe 343.83-nm line. LQD concentration for Zr 349.62-nm is 0.002 wt %. The signals seen in Figure 1 are easily measurable and demonstrate that the calculated values for the determination limits are conservative. The detection limit for P b is 0.01 wg/ml; hence, the LQD should be 0.001 wt % in Fe if a solution containing a sample concentration of 0.5 mg/ml is used for analysis. It is interesting to note that the flame atomic absorption detection limit for P b is essentially the same (0.01 pg/ml) but the LQD in a 0.5 mg Fe/ml solution has been reported to be 6 to 10 times higher (0.006-0.010) (10) than that obtained with the inductively coupled plasma system. Analytical Calibrations. The analytical curves were generated by measuring the relative intensities of selected analysis lines (see Table 11) emitted when synthetic reference solutions were run under the conditions defined for each of the spectrometer systems. Statistical data for the analytical curves are shown in the upper portion of Table 111. The data for these curves were collected over a period of one year. Data from day to day were normalized to a single analytical curve by using one of the standards as a reference intensity. The analytical curves are linear over a wide concentration; for example, curves for Cr and Ni (see Figure 2) showed no departure from linearity over a concentration range of four or more orders of magnitude. As a result, both the major and trace constituents in a sample can be determined at a single dilution level. In contrast, analytical curves obtained by flame atomic emission and absorption are generally linear only over a 11/2 to 2 orders of magnitude range ( l l ) ,and thus several sample dilutions are often necessary for the determination of both major and minor constituents. Interelement Effects and Spectral Interferences. The determination of common alloying constituents in ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
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Figure 3. Wavelength scans of the analytical lines used for the determination of common alloying elements in steel
steels of widely varying compositions raises the possibility that interelement effects of various types (12) or spectral line interferences may introduce errors in the analytical determinations. Observation on several classical solute vaporization interference systems ( I , 13, 1 4 ) and other systematic studies now in progress (15) have shown that most interelement interference effects are reduced to negligible proportions in the ICP. The analytical results on a series of National Bureau of Standards low and high alloy steel Standard Reference Materials (discussed below) confirm the earlier observations. Because the spectra emitted above the hot central core ( 3 ) are relatively simple, adequate spectral resolution is provided by low-cost, table-model spectrometers. Wavelength scans of the Al, Cr, Cu, Mn, and Ni lines recorded with the Hilger-Engis spectrometer, which possesses a spectral slit width of -0.02 nm, are shown in Figure 3. These traces were made when 0.5% iron solutions containing the equivalent of 0.1% of each of the analytes in iron were nebulized into the plasma. The background tracings were made by scanning the same spectral region of each analytical line when a 0.5% Fe solution was nebulized into the plasma. The iron metal used to prepare the 0.5% iron solution contained residual amounts of Al, Cu, Mn, and Ni but the concentrations of these impurities were negligible compared to the levels at which these metals are normally determined in steels. Refractory Metals. The refractory metals Nb, W, Ta, and Zr are present in certain modified steels a t relatively low concentrations and represent some of the most difficult elements to determine by flame atomic absorption or emis-
sion techniques. For example, Thomerson and Price (16) and Schiller ( 1 7 ) , respectively, have reported that the lowest concentrations directly determinable in steels by atomic absorption were 0.1 and 0.3 wt % for W and Nb; thus, prior chemical concentration procedures were a necessity. Preliminary separations were also required for a flame emission method for Nb reported by Eskew et al. (18).Nakashima et al. (19) have analyzed steels for Nb, Ti, and W utilizing a microwave (2.45 GHz) argon plasma emission system. However, plasmas generated at these high frequencies generally exhibit severe interelement interferences and, therefore, close matching of sample and standard composition is required. For example, these authors report that the addition of 4000 ppm of Fe causes a factor of 3 increase in the intensity of the T i 499.96-nm line and further addition of Fe then caused an abrupt decrease in the intensity. Only Ti could be determined directly; hence, a separation of the Nb, Ti, and Zr with cupferron was necessary prior to the analysis. The statistical data are reported in Table I11 for the analytical curves for the direct determination of Nb, W, and Zr down to concentrations of 0.005 wt % in dissolved steel: no prior chemical concentration of the analyte elements was employed. The major problem involved in refractory metal determinations is the sample dissolution process. The NBS (20) has recommended dissolution in a HNOS-HCl mixture and filtering to remove the residue. This residue is then dissolved in an HF-HNOs mixture in a "Teflon-lined" bomb (21, 22), and the resulting solution is combined with the previous one prior to analysis. The procedure is lengthy and the resulting solution may contain considerable fluoride ion which is not compatible with the glass needles used in our nebulizers. Determination of R a r e Earths. Mixtures of the rare earths or the individual elements are now added to steels a t the trace level to obtain certain desirable properties (23, 24). The three major constituents in the mixture additions are usually Ce, La, and P r and their concentrations in steel range downward to 0.008, 0.005, and 99 wt % (NBS--19 g) down to (1 wt % (NBS--169 9). The average values of three observations were used to establish the concentration of each NBS standard reference sample. Several of the standard samples were run 18 times over a four-week period, and the standard deviations obtained are also included. The results for A1 are particularly interesting since the determination of A1 by atomic absorption spectrometry requires that the reference blank solution contain approximately the same Fe content as the test solution (31).It is apparent from this table that this is not a necessary requirement when an ICP source is used;.for example, SRM 169 has a matrix containing 77% Ni and 20% Cr. Thus, the use of a pure Fe solution as a matrix for preparing reference solutions as well as a blank solution for background correction is apparently valid.
The data in Table IV also illustrate that the performance of the three different inductively coupled plasma-optical emission spectrometry systems is essentially the same. In particular, these data illustrate that the results obtained from a multichannel direct reading spectrometer are equivalent to those obtained with single channel instruments. Precision. Table V shows the results of a precision study with System I conducted over a period of four months. These data reflect the influence of normal changes in the analytical system such as the cleaning and replacement of the torches and repair of nebulizers. All of the data were net emission signals; no internal reference intensities were used.
ACKNOWLEDGMENT We thank Applied Research Laboratories (FICAFrance) for the loan of the Quantoscan 35 scanning spectrometer used in this study. LITERATURE CITED (1) V. A. Fassel, "Electrical Plasma Spectroscopy,'' XVI Colloquium Spectroscopium Internationale, Adam Hilger. London, 1973. (2) V. A. Fassel and R. N. Kniseley. Anal. Chem., 46, 1110A (1974). (3) V. A. Fassel and R. N. Kniseley, Anal. Chem., 46, 1155A (1974). (4) "Nomenclature, Symbols, Units and Their Usage in Spectrochemical Analysis-Ill Analytical Flame Spectroscopy and Associate Procedures." Available from IUPAC Secretariat, Bank Court Chambers, 2/3 Pound Way, Cowley Centre, Oxford OX4 3YF, U.K. (5) R. N. Kniseley, H. Amenson, C. C. Butler, and V. A. Fassel. Appl. Spectrosc., 28, 285 (1974). (6) R. N. Kniseley, V. A. Fassel, and C. C. Butler, Clin. Chem., 19, 807 11Cl7RI \.-.-,. (7) S. Greenfield, I. Li. Jones, C. T. Berry, and D. J. Spash, British Patent No. 1, 109. 602, April 10, 1968. (8) V. A. Fassei, R. W. Slack, and R. N. Kniseley, Anal. Chem., 43, 186 11971). (9) G.~W.'Dickinson,"Application of the Induction Coupled Plasma to Analytical Spectroscopy", PhD. Thesis, Iowa State University, 1969. (10) N. G. Sellers, Anal. Chem., 44, 410 (1972). (11) P. J. T. Zeegers, R. Smith, and J. D. Winefordner, Anal. Chem.. 40 (13),
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(1 2) J. Ramirez-Munoz "Atomic Absorption Spectroscopy," Elsevier Publishing Co., Amsterdam, 1968. (13) S.Greenfield, Proc. SOC.Anal. Chern., 2, 111 (1985). (14) R. H. Wendt and V. A. Fassel, Anal. Chem., 37, 920 (1965). (15) G. F. Larson, V. A. Fassel. R. H. Scott, and R. N. Kniseley, Anal. Chern., 47, 238 (1975). (16) D. R. Thomerson and W. J. Price, Analyst(London).96, 825 (1971). (17) R. Schiller, At. Absorp. News/., 9, 111 (1970). (18) J. B. Eskew, E. A. Jennings, and J. A. Dean, Anal. Left., 1, 947 (1968). (19) R. Nakashima, S. Sasaki, and S. Shibata, Anal. Chim. Acta, 70, 265 (1973). (20) T. C. Rains, National Bureau of Standards, Washington, D.C.. private communication, 1971. (21) B. Bernas, NASA Tech Brief, 68-10104, 20 March, 1968. (22) 9. Bernas, Anal. Chem., 40, 1882 (1968). (23) N. Kippenhan and K. A. Gschneidner, Jr.. "Rare-Earth Metals in Steels," Report No. 1s-RIC-4, Rare Earth Information Center, EMRRI, Iowa State University, Ames, Iowa 50010, 1970. (24) W. G. Wilson and R. G. Weiis, Met. Progr., 104, 75 (1973). (25) H. A. Tucker, R. T. Coulehan, and W. G. Wilson, U.S. Bur. Mines, Rep. Invest., 7153 (1968)unpublished. (26) L. C. Paszton and C. R. Hines, Anal. Chem., 41 (5), 91R (1969). (27) C. R. Hines and D.R. Dulski, Anal. Chem., 43 (5), lOOR (1971). (28) W. Koch and H. Bosch, Mikrochirn. Acta, 1971, 593 (1971). (29) W. R. Nall. Analyst (London), 96, 398 (1971). (30) G. Kisfaludi and M. Lenhof, Anal. Chim. Acta, 54, 83 (1971). (31) P. Konig, K. H. Schmitz. and E. Thiemann, Fresenius' 2. Anal. Chem., 244, 232 (1969).
RECEIVEDfor review August 19, 1974. Accepted December 23, 1974.
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