Use of Internal Standards for Atomic Absorption Spectrometric Analyses of Samples Atomized by Sputtering D. C. McDonald Division of Chemical Physics, CSIRO, P. 0. Box 160, Clayton, Victoria, Australia
Concentrations of iron, chromium, nickel, and copper In a number of different alloys and In powdered metallic and nonmetallic samples were determined by atomic absorption measurements of atomic vapors produced by cathodic sputtering in a low-pressure, raregas discharge. The ratlo of the analyte absorbance to the absorbance of an internal standard element In the sample was compared with the corresponding ratio in a reference sample. This allowed accurate determinations to be made even when the sputtering rate of the sample was quite different from that of the reference sample.
Up to the present, work on the analysis of metals and alloys by atomic absorption and atomic fluorescence measurements of vapors produced by cathodic sputtering has been restricted to samples which have similar sputtering rates (1-5). Changes in sputtering rate may occur when different matrices are sputtered or when there is a change in the energy or flux of the bombarding ions (6). The energy and flux of the bombarding ions depend upon the discharge current, discharge voltage, and the pressure in the sputtering cell. This paper extends the work of Gough (5) to series of metallic and nonmetallic samples which sputter a t different rates. It describes a simple and effective method of correcting for changes in sputtering rates. The method involves the use of an internal standard, which may be either a constituent element of known concentration in the sample or another element with which the sample is mixed in a known proportion. A single reference sample is used to determine concentrations of the analyte in the samples. When alloys are to be analyzed, the approximate concentration of the major constituent element is often known. With low-alloy steels, for example, only a small error is introduced by simply assuming that the concentration of iron in the sample is 94% and using iron as an internal standard. Accurate analyses may also be made using a minor constituent element of known concentration as the internal standard. A different approach (7) is to mix a powdered sample in known proportion with, for example, copper powder, press the mixed powders into a disk, and sputter the disk using copper as the internal standard. Copper is a particularly useful internal standard because it has many resonance lines with absorbance sensitivities spanning a very wide range (1.00 to 0.00017, see Table I). Copper is readily pressed into disks, and it also sputters very efficiently.
THEORETICAL T h e absorbance, A , of an atomic vapor, for values in the linear absorbance region, can be expressed (8), as
(Iz,l) @ where ko is the peak absorption coefficient for a single, Doppler-broadened line, 1 is the absorption path length, and A
a
is a line-profile factor (8) expressing the departure of the actual absorbance from that of the ideal case of a monochromatic line source and a single, unshifted, Dopplerbroadened absorption line. The absorption coefficient, ko, is proportional to the steady-state concentration of absorbing 1336
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
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Table I. Absorbance Sensitivities, Line Profile Factors ( a ) and Oscillator Strengths (f)for Copper (8) Resonance Relative absorbance line, nm Sensitivitya ( @ h i ) Q, f b , absolute 0.32 0.43 324.7 1.00 327.4 0.44 0.28 0.215 * 0.009 217.9 0.123 0.20 0.127 ?: 0.010 216.5 0.092 0.25 0.077 ?: 0.004 218.2 0.088 0.25 0.071 i 0.004 222.6 0.048 0.30 0.033 ?: 0.002 202.4 0.018 0.22 0.018 i 0.002 249.2 0.0077 0.27 0.0052 * 0.0004 224.4 0.0045 0.29 0.0031 i 0.0002 244.2 0,0018 0.19 0.0018 i 0,0001 236.3 0.000 1 7 0.30 0.00011 i 0.00004 For a hollow-cathode lamp source (low current) and a sputteringtype absorption cell. Normalized relative to f,,,., = 0.43. atoms in the sputtered vapor, and hence to the concentration of those atoms in the sample (9). The absorbances of two resonance lines of the same element, measured under the same conditions, are related (8) by
where f, is the oscillator strength of the resonance line of wavelength A,, and the product @Af can be referred to as the absorbance sensitivity. Thus the measured absorbance of one resonance line can accurately be converted to the absorbance of another resonance line if the @-factorsand the oscillator strengths are known. Determinations of the absorbance sensitivities have recently been made for 11resonance lines of copper using a hollow-cathode source and a sputtering absorption cell (8), and these results are included in Table I, together with the measured f-values and the calculated @-values. When an internal standard is used together with a reference sample of known analyte concentration, the w/w concentration, Cas,of the analyte element in the analytical sample can be determined using
(@An
(3) where the superscripts S and R denote the unknown and reference samples, respectively, and the subscripts a and i denote the analyte element and internal standard element, respectively. This is the equation used t o calculate the concentrations reported in this paper; it is based upon the assumption that the ratio of the absorbance of an element in the vapor phase to its concentration in the solid remains constant from sample to sample for a given element. The validity of this assumption is discussed later.
EXPERIMENTAL Apparatus. The absorption measurements were made using a Varias-Techtron Model AA-6 atomic absorption spectrophotometer, in which a Pyrex sputtering cell was substituted for the
burner. The sputtering cell and associated gas-control system were the same as those described by Gough ( 5 ) . The flow-rate of argon was 0.25 L/min and the pressure was typically 4 Torr. The sputtering-discharge power supply was a regulated dc supply capable of delivering 100 mA at 800 V. Current-regulation was used to maintain a constant current-density on the restricted, sputtered surface area of the specimen. A four-digit meter was used to display the value of the discharge voltage. The changes of voltage during the establishment of steady-state sputtering conditions gave some indication of the state of the sputtering surface. Samples. Alloys. The following certified materials were used. Low-alloy steels SS404, SS406, SS409, SS410 (e.g., SS410, Fe 92.96, Ni 2.04, Cr 1.72, Si 1.00, Cu 0.47, V 0.46, Mn 0.43, Mo 0.41, C 0.39, P 0.066, S 0.053%) from Bureau of Analysed Samples Ltd., U.K.; H 1168 (Fe 96.7, Ni 1.03, Cr 0.54, Mn 0.47, C 0.26, Cu 0.26, Mo 0.20, V 0.17, Co 0.16, W 0.077, Si 0.075%) from NBS, Washington. High-speed steel SS486 (Fe 80.66, W 6.48, Mo 5.23, Cr 4.53, V 1.92, C 0.74, Si 0.14, Co 0.13, Mn 0.12, S 0.029, P 0.021%) from Bureau of Analysed Samples Ltd. Cartridge Brass C l l l O (Cu 67.4, Zn 32.2, P b 0.106, Fe 0.072, Sn 0.055, Ni 0.052, Ag 0.019, As 0.019, Sb 0.018, Cd 0.013, P 0.010%) and Commercial Bronze C1115 (Cu 87.9, Zn 11.7, Fe 0.13, Sn 0.10, Ni 0.074, Pb 0.013, P 0.005%) from NBS. Analyzed alloys used were Product Steel No. 5 (Fe 88.61, Ni 4.28, C 3.51, Cr 1.28, Mn 0.99, Si 0.93, Mo 0.26, P 0.098, S 0.046%) from Broken Hill Proprietary Ltd., Australia. Aluminum SAC 1576 (A193.3, Si 5.13, Fe 1.13, Cu 0.16, Mn 0.11, Ni 0.10, Mo 0.025, Cr 0.008%) from Alcoa, Australia. The alloys were required to have a flat surface (5) of 3.5 cm minimum diameter to seal against an O-ring. The sputtered area was only 0.4 cm2 in the center of the flat surface, Le., 4% of the minimum prepared surface area. The alloys were supplied in the form of short (23 cm length) cylinders. They are not guaranteed to be homogeneous, and the part most likely to differ from specification is the central, most conveniently sputtered, region. Nonmetallic Materials. A number of copper smelting and refining slags that had been obtained using submerged combustion (IO) and which had been analyzed chemically, were studied, as also were seven nickel salts of laboratory or reagent grade purity. They were ground to -200 mesh ( < 7 5 - ~ mdiameter), mixed with copper, silver, nickel, or iron powder of this particle size or smaller, and pressed at about 1000 MPa (10 tonne cm-') into disks of 2.5-cm diameter and about 0.2 cm thick. A more detailed description of the preparation of disks incorporating powdered samples will be given in a separate paper devoted to the analysis of powdered samples. Procedure. The hollow-cathode lamp currents were sufficiently low that self-absorption did not occur for the selected resonance lines (=2-4 mA for strong lines and up to 8 mA for resonance lines of low oscillator strength). The sputtering-discharge currents were limited to values for which the ratio A J A , remained constant. This required that the absorbances of the selected resonance lines never exceeded 0.15. Nonatomic absorbance, due to the presence of agglomerates (5, 11, 12), was measured using a hydrogen or deuterium lamp and subtracted from the measured absorbance of the resonance line. Typical values measured at 324.7 nm, for a sputtering current of 40 mA and a discharge voltage of 470 V, were 0.004-0.006 for silver disks, and 0.001-0.002 for copper disks. For the samples with major constituents other than silver and copper, the nonatomic absorbance was found to be smaller still. The monochromator slits were set sufficiently narrow to eliminate spectral interference; for Fe (248.3 nm) and Ni (232.0 nm), this required a spectral bandpass of only 0.07 nm. In some cases (Tables 111, V and VIII), absorbances were measured at one copper wavelength and corrected to absorbances at another wavelength using the absorbance sensitivities given in Table I.
RESULTS Solid Alloys. The time taken to reach equilibrium sputtering conditions for these samples was 5-6 min. Chromium in Steel. For this analysis the discharge voltage varied only between 425 and 460 V. Table I1 shows that the
Table 11. Analysis of Solid Alloy Samples' Analyte, % Sample SS404
Element Fe Cr Fe Cr Fe Cr Fe Cr Fe
h , nm Certificate Measured 95.83 386.0 0.68 0.66 357.9 92.91 SS409 386.0 1.24 357.9 1.22 SS486 386.0 80.66 4.22b 357.9 4.53 B.H.P. No. 5 386.0 88.61 1.28 1.30 357.9 SS406 386.0 93.66 1.69 1.65 Ni 352.5 Fe 386.0 93.16 SS408 4.58 4.54 Ni 352.5 SS4 1 0 Fe 386.0 92.96 Ni 352.5 2.04 2.06 B.H.P. No. 5 Fe 386.0 88.61 4.28 4.37 Ni 352.5 a Internal standard: iron. Reference samples: SS407 (3.00% Cr); SS409 (3.14% Ni). Sputtering discharge conditions: Cr: 12 mA, 425-460 V, 3.7 Torr Ar; Ni: 1 6 Chromium was determA, 400-500 V, 3.7 Torr Ar. mined as 4.23 percent by flame atomic absorption analysis of scrapings taken from the sputtered area.
Table 111. Analysis of Solid Alloy Samples' Analyte, % EleSample ment h , nm Certificate Measured SS406 Cu 324.7 0.32 [low alloy steel) Ni 352.5 1.69 1.75 1168 Cu 324.7 0.26 1.03 0.94 (low alloy steel) Ni 352.5 SAC 1576 Cu 324.7 0.16 (aluminum) Ni 352.5 0.10 0.097 CllOO Cu 224.4b 67.43 (cartridge brass) Ni 352.5 0.052 0.050 C1115 Cu 224.4b 87.96 (commercial bronze) Ni 352.5 0.074 0.076 a Internal standard: copper. Reference sample: $23407 (0.43% Cu, 0.61% Ni). Sputtering discharge conditions: Acu is measured at 224.4 60 mA, 480 V, 4-6 Torr Ar. nm and corrected to 324.7 nm. measured concentrations of chromium are in good agreement with the certificate values, except for sample $3486. Some metal was scraped from the sputtered area of this sample, dissolved in hydrochloric acid, and analyzed by flame atomic absorption spectrometry, which gave a result in very close agreement with that determined by sputtering. This appears to be a n example of the central region of a standard sample differing from the specification for the bulk. Nickel in Steel. The discharge voltages for these samples varied between 400 and 500 V. The measured nickel concentrations are also shown in Table 11. Nickel in Different Alloys Containing Copper. For these very different matrices, the argon pressure in the sputtering cell was varied ( 4 4 Torr) from sample to sample to maintain a constant discharge voltage (480 V) a t a constant sputtering current (60 mA). This was necessary to obtain an adequate sputtering rate under suitable discharge conditions for all samples. The measured concentrations of nickel are shown in Table 111. Powdered S a m p l e s Incorporated i n t o Disks. Equilibrium sputtering conditions for the disk specimens were achieved in a maximum of 10 min for newly-pressed disks. Nickel and Iron in Low-Alloy Steel Incorporated into a Copper Disk. A needle file was used to obtain 100 mg of fine filings from a piece of SS408. These filings were thoroughly mixed with 1 g copper powder, and the resulting mixture was ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
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Table IV. Samples Incorporated in Copper Disksa Analyte, % EleCertiSample ment h , nm ficate Measured SS408 Cu 249.2 Fe 248.3 93.16 93.9 Ni 232.0 4.58 4.58 By Solution AAS Nickel compounds: CU 249.2 98-9gb hydroxide Ni 232.0 60.2 57.6 oxide (green) Ni 232.0 68.6 67.2 carbonate Ni 232.0 45.5 44.2 sulfate (heated to 180 "C) Ni 232.0 33.4 32.3 fluoride Ni 232.0 36.9 37.2 chloride (hydrated) Ni 232.0 24.3 24.4 sulfide (precipitated) Ni 232.0 47.8 46.4 a Internal standard: copper. Reference specimens: For SS408, a Cu disk containing 0.481%Ni, 1.29%Fe. For Ni compounds, a Cu disk containing 0.309%Ni. Sputtering discharge conditions: SS408: 50 mA, 470 V, 4.5 Torr Ar; Ni compounds: 30 mA, 450-480 V, 4.5 Torr Ar. From composition of disk as prepared. made up to 5 g by mixing with more copper powder. The discharge voltage was 470 V for the specimen containing SS408 and 490 V for the reference specimen (known amounts of nickel and iron powders pressed into a disk with copper powder). The measured concentrations of nickel and iron in SS408 are shown in Table IV. Nickel in Nickel Compounds Incorporated into Copper Disks. The disks were made up to contain 1%by weight of nickel, irrespective of the nature of the salt. The discharge voltages for these specimens varied only between 450 and 480 V although the sputtering rate of the nickel carbonate specimen was found to be 11%higher than that of the nickel oxide specimen. Copper was used as the internal standard. Measured concentrations for nickel in the different compounds are also shown in Table IV. Copper in Copper Slags Incorporated into Silver Disks. Silver has only two usable resonance lines (328.1 and 338.3 nm), each of which is almost totally absorbed when high concentrations of silver are sputtered, and so it is unsuitable as an internal standard. Nickel was chosen instead for this purpose, and disk specimens of 1%nickel, 30% copper slag, and 69% silver were pressed. One slag sample was known to contain 0.1 % Ni but the others contained no significant nickel concentrations. The discharge voltages varied over a wide range from 320 to 500 V, but the measured copper concentrations are in good agreement with the values determined chemically (Table V).
DISCUSSION The advantages of the method outlined in this paper are best illustrated by the results obtained for the copper slags in the silver disk specimens. Although a very wide range of sputtering rates was evident (Table VI), the measured concentrations do not reflect the substantial errors (as much as 500%) which would have arisen without the use of internal standards. In addition, the time taken to weigh, mix, press, clean-up by sputtering, and measure the absorbances for each of these slag samples was only 15 min. This time is to be compared with the 3-4 h taken to determine the concentrations by conventional chemical methods. Further, Table I11 shows that, when the absorbance sensitivities are known for several resonance lines of an element, accurate analyses may be made over a wide range of concentrations without adjustment of the sputtering conditions. 1338
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9 , AUGUST 1977
Table V. Slag Samples Incorporated in Silver Disksa Analyte, % Chemical Sample Element h , nm analysis Measured CR8 Ni 23 2.0 Cu 249.2' 73.2 72.1 CR7lDip 1 Ni 232.0 Cu 327.4c 14.9 14.9 E.R. & S. Ni 232.0 Cu 327.4e 5.3 5.2b cc 12 Ni 232.0 1.24 Cu 324.7 1.19 CC 14 Ni 232.0 Cu 324.7 0.44 0.42 a Internal standard: nickel, 1.00%incorporated in disk. Reference specimen: Ag disk containing 5.00%Cu, 5.00% Ni. Sputterin discharge conditions: 40 mA, 320-500 V, 4 . 5 Torr Ar. Corrected for 0.1% nickel in the slag sample. e A , is measured at this wavelength and corrected to 324.7 nm. Table VI. Relative Sputtering Rates for Copper Slag Samples Incorporated in Silver Disksa Sample Relative sputtering rate Discharge voltage, V E.R. & S. 1.00 490 CR8 0.87 500
cc 12
0.37
500
C R ~ / D ~1 D 0.24 320 CC 14 0.22 470 a Measured by the absorbance for 1.00% nickel incorporated in each disk. Sputtering discharge conditions: 40 mA, 4.5 Torr Ar. Table VII. Variation of Ratio of Absorbances of Analytes and Internal Standard in Copper Disksa Discharge A N i 232.0 conditions, mA A c U 244.2 A c U 244.2 Voltage, 470 V. Pressure, 6.5-4.0
Torr Ar Voltage, 510-425 V.
248.3 244.2
50 45 40 35 30 25
0.040 0.043 0.045 0.042 0.038 0.032
2.15 2.19 2.11 2.14 2.13 2.25
1.73 1.79 1.78 1.76 1.76
55 50 45 40
0.048 0.047 0.040 0.032
2.19 2.11 2.13 2.13
1.77
1.78
1.70
Pressure, 1.73 6 Torr Ar 1.78 a Specimen: Cu 98.82%,Ni 0.309%,Fe 0.874%. The interesting possibility of analysis without the use of a reference sample, first proposed by Walsh (13) in 1955, can again be raised. For small absorbances, the absorbance ratio for two elements in any sample is given by
(4) where M is the atomic weight, N the steady-state number density of sputtered atoms, and F the fraction of those sputtered atoms in the ground state. In the absence of preferential effects, such as preferential sputtering, preferential ionization, or the preferential agglomeration of one element, then
(5) where D is the diffusion coefficient of the sputtered atoms
Table VIII. Proportionality of Concentration Ratios to Absorbance Ratios for Solid Alloy Samples' Sample R, = c N i / c c u RA = ANiIAcu RJRA 5.28 0.851 6.2 SS406 3.96 0.572 6.9 H 1168 0.625 0.0952 6.6 SAC 1576 0.000771 0.000116b 6.6 c 1100 0.000841 0.000136b 6.2 C 1115 a Copper measured at 324.7 nm and nickel measured at Acu is measured a t 224.4 nm and corrected 352.5nm. to 324.7 nm.
knowing the concentration of one constituent of a sample, to determine that of any other constituent without the need for a reference sample.
ACKNOWLEDGMENT The author thanks A. Walsh for suggesting this study and for his continued encouragement and interest, J. M. Floyd for supplying the analyzed copper slag samples, J. B. Willis for analyzing the nickel compounds and incorporating them into copper disks, and P. Hannaford for many valuable suggestions.
LITERATURE CITED
i
in the sputtering cell. If the constants in Equations 4 and 5 are known, it is possible to determine the concentration of any element relative to any other in the sample. The results in Table VI1 show that the absorbance ratios ANi/AcUand AFe/AcU for a copper disk specimen are not affected by changes in discharge current, voltage, or pressure over the ranges investigated. Furthermore, Table VI11 shows that for several very different alloys the ratio ANi/Acu for the sputtered vapor is directly proportional to the ratio of the concentrations, CNi/CCu, in the solid. The results shown in both these Tables indicate that preferential sputtering, ionization, and agglomeration, if present, remain constant over a wide range of conditions. This suggests that preferential effects do not occur at all in the type of discharge used in this investigation, and hence that Equation 5 is valid. Thus it may be possible,
(1) B. J. Russell and A. Walsh, Spectrochim. Acta, 15, 883 (1959). (2) B. M. Gatehouse and A. Walsh, Spectrochim. Acta, 16, 602 (1960). (3) A. Walsh, Proceedings of the Xth Colloquium Spectroscopicurn Internationale, Washington, 1962, p. 127 (4) D. S.Gough, P. Hannaford, and A. Walsh, Spectrochim. Acta, Part E, 28. 197 11973). (5) D:'S.G&gh, Anal. Chem., 48, 1926 (1976). (6) G. M. McCracken, Rep. Prog. Phys., 36, 241 (1975). (7) M. Dogan, K. Laqua, and H. Massrnan, Spectrochim. Acta, Parts, 26, 631 (1971). (8) P. Hannaford and D. C. McDonald, unpublished work. (9) H. Jager and F. Blurn, Spectrochim. Acta, Part B , 29, 73 (1974). (IO) J. M. Floyd, Institute of Fuels Conference, Adelaide, Australia, 1974. (11) K. Kimoto, Y. Kamiya, M. Nonoyarna, and R. Uyeda, Jpn. J. Appl. Pbys., 2, 702 (1963). (12) Yu. 1. Petrov, Opt. Spectrosc. (USSR), 27, 359 (1969). (13) A. Walsh, Spectrochim. Acta, 7, 108 (1955).
RECEIVEDfor review February 22, 1977. Accepted May 10, 1977.
Laser-Vaporized Atomic Absorption Spectrometry of Solid Samples Toshio Ishlzuka, * Yoshinori Uwamino, and Hlroshi Sunahara' Government Industrial Research Institute, Nagoya 1- 1, Hirate-machi, Kita-ku, Nagoya, 462 Japan
A Q-switched ruby laser was used to produce the atomic vapor from the solid samples, such as brass, steel or aluminum alloy. The atomlc vapor produced wlth the laser beam was Introduced Into the absorption path by the flow of Ar gas. The absorption signals of AI, Cr, Cu, Fe, Mn, Mo, NI, and V were traced wlth an oscllloscope, and those elements were determlned wlth the peak-helght or integratlon method. Preclsbn (reiatlve standard devlatlon) was 2.1 to 10% for the peakhelght method, and 1.0 to 12% for the Integration method. The analytlcal curves prepared wlth both methods were hear for all the elements studled. The detectlon llmlts obtalned wlth the peak-helght method were lower than those obtalned wlth the Integration method.
Flames or graphite furnaces are normally used as an atomizer for solution samples in atomic absorption spectrometry (AAS). In the case of solid samples, they are converted into solution samples, and the analyses are performed. In order to analyze directly various elements in solid 'Present address, The Faculty of Engineering, Hiroshima University, Hiroshima, Japan.
samples, some methods for the atomization of solid samples have been investigated (1). Laser beam has been used as the atomizer for the atomic absorption of solid samples by several investigators (24). In these studies, the plumes produced on the sample surfaces by the laser beam were used as an absorption path. A xenon flash lamp (21, the continuum at the base of the laser plume (3), pulse-radiated hollow-cathode lamps ( 4 , 5), and dc-operated hollow-cathode lamps (6) were used as the primary light source for AAS. The instantaneous emission from the laser plume is significantly intense compared with the light from the hollow-cathode lamps of conventional light source for AAS. In the laser plume, the line spectra from excited atoms or ions and the continuous spectrum from bremsstrahlung of electrons are observed. When the laser plume is used as the absorption path in AAS, those emission spectra may interfere with atomic absorption measurement. Then compensation for background emission must be performed. The authors designed and constructed an atomizationabsorption cell in which the chamber for atomizing solid sample was separated from the absorption path. An atomic absorption spectrophotometer with conventional hollowcathode lamps and monochromator was used. Laser-vaporized ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
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