257
Anal. Chem. 1882, 64, 257-260
Direct Determination of Ultratrace Copper and Iron in Lead and Zinc Metal by Inductively Coupled Plasma Atomic Emission Spectrometry Using the Graphite Cup Direct Insertion Technique Masao Umemoto,*J Koji Hayashi,' and Hiroki Haraguchi2 Chemicals Inspection and Testing Institute, 4-1 -1 Higashi-mukojima, Sumida-ku, Tokyo 131, Japan, and Department of Applied Chemistry, School of Engineering, Nagoya Uniuersity, Furo-cho, Chikusa-ku, Nagoya 464, Japan
The graphite cup direct insertion technique in inductively coupled plasma atomic emission spectrometry was applied to the direct determination of trace amounts of copper and iron in lead and zinc metal. A piece of lead or zinc metal, 2-20 mg, was directly inserted into the plasma with a graphite cup insertion device to determine copper and iron at the ng/g or bg/g level. Copper and Iron, which have higher boiling pdnts than those of lead and zinc, began to evaporate after a h m l complete m a t h evaporation. T h k aibws the elimination of chemical and spectroscopic interference due to matrbt elements. Since the tkne-dependent emlssion profiles of copper and iron in metal samples were different from those in standard solutions, quantitative analyses were performed by the peak integration method using standard sdutbns, where relative standard deviations were 5-20% for metal samples. The values for iron and copper obtained by the direct sample analysis were in good agreement with the certified values of reference materials.
INTRODUCTION The graphite cup direct insertion technique for solid samples in inductively coupled plasma atomic emission spectrometry (ICP-AES) has been investigated as a method for the determination of trace elements (1-41, because high sensitivity is achieved when the samples (chip or powder) are directly inserted into the ICP. Several groups have examined the analytical feasibility of this technique for the analysis of refractory oxides (1-3,5-8), biological materials ( 4 , 9 ) ,and alloys (7,lO). Consequently, the following problems have been noticed in the direct insertion technique (2,7,10):(i) Prompt vaporization of trace elements is prevented by the recrystallization of the powder due to heating. (ii) Complicated time-dependent emission profiles are observed. (iii) Peaktailing of analyte signals is often observed. Some attempts have been made to overcome these drawbacks. Sample powder was mixed with graphite powder to prevent recrystallization (7),and thermochemical reagents or carriers were added to promote volatilization of the elements forming refractory carbides (2, 3 , 5 - 7 , l l ) . Unfortunately, in such attempts, impurities contained in the graphite powder and the carriers cannot often be ignored in the analyses of trace elementa (12),and in addition, the analytical procedure becomes rather cumbersome. Even so, this technique has a possibility of ultratrace analysis because of the high efficiency of sample introduction. In the present paper, volatile high-purity metals were chosen as samples and the direct sample insertion technique was Chemicals Inspection and Testing Institute. School of Engineering. 0003-2700/92/0364-0257$03.00/0
Table I. Instrumentation and Operating Conditions spectrometer focal length grating slit additional spectrometer focal length grating slit rf generator frequency max output power photomultiplier applied voltage graphite cup positiona observation height outer and intermediate gas flow rates
Kyoto Kohen Model UOP-1 Mark-I1 0.8 m 79 grooves/" echelle grating blazed at 75O 58' slit width 100 pm, slit height 1 mm Jobin Yvon Model HR320 0.32 m 1200 g/mm, ruled at 250 nm blaze in first order slit width 100 pm, slit height 1 mm Kyoto Koken, supplied with UOP-1 27.12 MHz, crystal-controlled 2.5 kW Hamamatsu Photonics R955 500 v 6 mm for Cu and 4 mm for Fe above the load coil 15 mm above the load coil 15 and 1.5 L/min
'Cup position is defined as the distance of the top of the graphite cup from the load coil.
investigated to develop direct determination of analytes at the trace or ultratrace level. Trace elements which do not form azeotropic mixtures with matrix metal vaporize after the complete matrix vaporization when the boiling points of the trace elements are higher than that of the matrix. By application of this observable phenomenon, the direct determination of ultratrace elements in metallic samples can be carried out without chemical or spectroscopic interference. The direct sample insertion technique proposed here is applied to determine copper and iron in zinc and lead metal. EXPERIMENTAL SECTION Instrumentation and Operating Conditions. The experiments were carried out with an ICP emission spectrometer, Model UOP-1 Mark I1 from Kyoto Koken Co. (Kyoto, Japan), which was composed of a computer-controlled high-resolution echelle monochromator with an oscillating quartz refractor plate for wavelength modulation (13, 14). The wavelength modulation method was used when necessary. The monochromator of this spectrometer was used for the measurements of the analyte emission signals, and a second monochromator and optical system were employed for the observation of matrix emission signals. The latter monochromator, JOBIN YVON HR320 (Cedex, France), was installed in the right angle of the optical path for the echelle monochromator, where a quartz lens (50 mm in diameter) with a focal length of 150 mm was used in order to make a 1:l magnification image of the plasma on the entrance slit. The graphite cup direct insertion device was the same as that described in a previous paper (15). A cup of 0.5 mm in wall thickness was used. A strip chart recorder (time constant 3 Hz) was used for the peak height measurements, and a Chromatopak C-R113 from Shimadzu Seisakusho Co. (Kyoto, Japan) was employed for the peak integration. Analytical lines chosen were 324.754 nm for copper and 238.204 nm for iron. The spectral interference induced by matrix elements was checked with reference to the "Line Coincidence Table for Inductively Coupled Plasma Atomic 0 1992 American Chemical Society
258
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Emission Spectrometry"(16).The instrument specifications and typical operating conditions are summarized in Table I. Reagents and Materials. Commercial standard stock solutions of 1 mg/mL were serially diluted with 0.5 M nitric acid for the preparation of the standard solutions, where nitric acid of supergrade reagent quality and deionized water prepared by a Milli-Q pure water system MQ4OW (Japan Millipore Co., Tokyo, Japan) were used. Certified reference materials used for analysis were BCS (British Chemical Standards) 210e (lead metal, purity 99.99%) and NIST SRM 1951 (zinc metal, purity 99.999%). In addition, zinc metal (purity 99.99%),Japanese primary standard substances for volumetric determination described in JIS K 8005, was used as a reference material. Graphite cups were machined in our laboratory from graphite rods, High Purity Graphite Spectroscopic Electrode Grade S-6,purchased from Tokai Carbon Co., Ltd. (Tokyo,Japan). Each cup was burnt in the plasma at an rf power of 1.8 kW for more than 5 min before any analytical measurementa. Procedure. Solution samples were prepared by dissolving the metals with diluted nitric acid with heating on a hot plate. In the case of solution samples, a 25-pL volume was applied to the graphite cup with a micropipet and then the top of the cup was positioned 8 mm below the lowest load coil. The plasma was ignited and sustained at an rf power of 1.0 kW to dry the sample. After the solvent was evaporated, the cup was rapidly inserted into the plasma. In the case of solid samples, the metals were cut into pieces with a ceramic cutter and then rinsed with diluted hydrochloric acid and water and finally with ethanol. After drying in a desiccator, a piece of metal was put into the cup, and the same procedure as that for solution samples was performed. The time-dependent emission profiles obtained at a time constant of 0.1 s were recorded on the strip chart recorder. The emkaion profiles of the matrices were o b ~ ~ with e d the additional optical system composed of the secondary monochromator mentioned earlier. The background correction was made by the wavelength modulation method. In the case of emission signal measurement, signal recording was continued until the emission intensity of analyte declined within 1% of the maximum in order to make a background correction. The rf power was then increased up to 1.8 kW to heat the cup in order to eliminate the memory effects due to other components. RESULTS AND DISCUSSION Optimization of Operating Conditions. Relationships between the rf power and the signal-to-background ratios (SBRs) for Cu 1324.754-nm and Fe I1 238.204-nm emission signals were examined. For the copper spectral line, the SBR decreased with increasing rf power. On the other hand, the SBR was at a maximum at an rf power of 1.5 kW for the iron spectral line. In the case of iron, the ionic line with an excitation energy of 5.2 eV was used, while in the case of copper, the atomic line with an excitation energy of 3.8 eV was used. It is noted that these line characteristics resulted in the different evaporation phenomena for copper and iron mentioned above. By consideration of less chemical interference and a relatively large SBR, rf powers of 1.0 and 1.5 kW were selected for copper and iron, respectively. As for the cup position, the SBRs for copper and iron increased up to 6 and 4 mm above the load coil, respectively, and then gradually decreased. The background intensity considerably increased when the cup position was close to the coil. Thus,the SBRs of both copper and iron were small with the cup positioned close to the coil. The SBR for iron deteriorated more markedly than that for copper. This is because the ionic line with the larger excitation energy was used for iron, which was more sensitive to the temperature change. The plasma showed considerable axial shrinkage when the cup was inserted at the higher position, and so the zone close to the plasma tail where the temperature is lower was observed. Next, the dependence of the SBRs on the observation height was examined. In the case of the copper line, the SBR increased with an increase in observation height. Since the
r\ 1.0kW
!
I
I
5
0 Time
10
,
0
5 Ttme
( 5 )
10
(s)
r;
j ] 1.OkW
1.5kW
I
I
I
0
5 Time
10 ( 5 )
0
5 Time
10
15
( 5 )
Figure 1. Comparison of timedependent emlssion profiles for matrix and analytes in lead. Cup posltlon: 5 mm.
signal intensities and dynamic ranges of the calibration curves
became small a t the higher observation height (17),the observation height of 14-18 mm above the load coil was considered desirable, and thus 15 mm was selected. In the case of the iron line, the observation height of 15 mm above the load coil was also selected because the largest SBR waa obtained a t this position. Although both line and background intensity decreased with an increase in observation height, the iron ionic line with an excitation energy of 5.2 eV showed a large decrease in line intensity. As a result, the maximum SBR was attained a t 15 mm above the load coil for the iron line. Selective Volatilization of Analytes from the Matrix. Figure 1 shows the time-dependent emission profiles of the analytes (Bi, Ag, Cu, and Fe) and matrix (Pb), which were obtained by the direct insertion of a piece of lead metal. It is noted here that the signal of copper appeared after vaporization of the matrix, and ita peak signal was observed 7.5 s after the cup insertion. As seen in Figure 1, silver began to vaporize before vaporization of lead finished, and bismuth vaporized during the same period as lead. Iron vaporized after lead almost finished vaporization, in a manner similar to that of copper. Thus,copper and iron could be determined without any influences of the lead matrix. These vaporization phenomena observed for copper and iron may be explained as follows: The temperature of the melted metal may be kept almost at the boiling point of lead (2017 K) during the vaporization of lead. Thus, copper (boiling point 2868 K) and iron (boiling point 3023 K) do not vaporize until the vaporization of lead is completed. As seen in Figure 1,silver, with a boiling point of 2485 K, begins to vaporize just before complete vaporization of lead. This is probably because the sample temperature starts to rise near the end of the vaporization of the lead matrix. Thus, during the vaporization of lead, in general, an anal@ with a much higher boiling point does not vaporize from the graphite cup in the plasma. It should be pointed out here that spectroscopicinterference and enhancement of background continuum emission due to the recombination of matrix ions may result when simultaneous vaporization of matrix and analyte occurs. In such a case, the excitation efficiency of the analyte may also be influenced by the matrix element. Therefore, copper and iron,
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
cu
I
I
I
0
5
10
Time
( 5 )
Figure 2. Influence of matrix on timedependent emission profile of Cu: (A) copper solution with matrix Pb(NO&; (B)copper standard solution.
A
I
I
0
10 Time
20
(s)
Figure 3. Comparison of ti-pendent emission profile of Cu for lead (A) and zinc (e) metal and standard solution (C).
Fe
which vaporized separately from the matrix elements, were subjected to the following investigation for quantitative analysis.
Time-Dependent Emission Profiles of Analytes. Matrix in Nitrate Forms. Since sample matrices often influence the evaporation rate of analytes, the time-dependent profiles of the emission signals were measured for copper and iron. The results are shown in Figure 2. The time-dependent emission profile A was observed for a 1g/mL copper solution containing 20 mg of lead, and profile B was for a 1pg/mL copper standard solution. As seen in Figure 2, the slope is sharper and the peak top is higher in profile A than in profile B. This indicates that the existence of matrix Pb(N0J2 in the former solution affected the evaporation rate of copper. Therefore, the standard addition method should be employed when copper contained in high-concentration metal salts is determined by the present technique. A similar phenomenon was also observed for iron. Matrix in Metal Form. Figure 3 shows time-dependent emission profiles of copper obtained by direct metal insertion with the graphite cup. It should be noted that the leading edge of the copper peak for lead and zinc metals provides sharper slopes than in the standard solution, and the appearance time of copper in metals was delayed compared to that in solution. The atomization of copper in the nitrate form may occur through the carbon reduction of ita oxide with subsequent sublimation of copper (18). It is known that the appearance temperature of copper (defined as the temperature a t which atom formation is first observed) in this process obtained in graphite furnace atomic absorption spectrometry is 1310 K (18). On the other hand, the appearance temperature for copper in lead metal should be higher than the boiling point of lead (2017K), because the emission signal of copper appeared after the complete vaporization of the lead matrix, as shown in Figure 1. In other words, since the atomization occurs a t a higher temperature, the atomization rate of copper in lead metal is much larger than that in solution, so that the slope of the emission peak for copper in lead metal is sharper than that in solution. Figure 4 shows the time-dependent emission profiles of iron for the standard solution and lead and zinc metals. As can be seen in the figure, the leading edges of the iron peaks for the metals are more moderate than that for the standard
259
R
C
0
10 Time
20
(5)
Fb~w 4. Comparison of time-resotved emission profile of Fe for lead (A) and zinc (B)metal and standard soltulon (C).
solution, which is attributed to the fact that iron in metals may easily form carbide (19). It is also noticed that the leading edge of lead metal is sharp compared to that for zinc metal. When the melted state of the metal samplea in the cup was observed immediately after turning off the plasma, the zinc metal was spread over the inner surface of the cup, while the lead metal was in the form of a ball. This observation supports the following assumption: Iron simply s t a y in the lead sphere until the lead completely vaporizes, at which point it starts to vaporize. The zinc vaporizes while spreading on the cup surface, which is helped by the large surface area and irregularities of the cup surface. Thus, contact with carbon is sufficient for zinc metal, which reaulta in more perfect carbide formation, to provide a leading edge more moderate than for lead metal. Analytical Figures of Merit. Analytical calibration curvea for Cu I 327.754-nm and Fe II 238.204-nm spedral linea
280
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Table 11. Determination of Trace Amounts of Copper and Iron in Lead and Zinc Metal Fe concn, p g / g
Cu concn, pg/g sample
direct metal insertion method
std addition method
BCS No. 210e (Pb) JIS K8005 (Zn) NIST SRM 728 (Zn)
2.1 f 0.1" 1.3 f 0.1" 5.4 f 0.3"
2.0 1.3 5.6
Standard deviation for six replicates.
certified value 6b
5.7 [5.3-6.3]'
direct metal insertion method
sM addition method
1.1 f 0.2" 5.8 f 0.6" 2.4 f 0.2"
1.1 5.6 2.5
certified value 5b 2.7 [2.1-3.51'
Maximum concentration of copper indicated by BCS. Range of value reported.
obtained by the integration method were linear up to 125 ng for copper and 750 ng for iron. The detection limits estimated in the case of a 25-pL volume of the standard solutions were 0.22 ng/mL for copper and 0.78 ng/mL for iron, respectively. Here, the detection limit was defined as the amount of the signal equivalent to 3 times the standard deviation of background signal. On the other hand, in the case of metal samples, the detection limits as concentration were calculated to be 0.28 ng/g for copper and 0.98 ng/g for iron with 20 mg of lead in weight. These results indicate that ultratrace analysis on the order of ppb is possible using the present technique. It is noted here that the larger the weight of the sample, the lower the detection limit becomes. The relative standard deviations of six repetitive measurements obtained with a 25-pL volume of standard solution (25 ng/mL Cu and 100 ng/mL Fe) were 2.0% for copper and 2.9% for iron by the peak height measurement method. The relative standard deviations obtained for about 2 mg of zinc metal (5.7 pg/g Cu, 2.7 pglg Fe), SRM728, were 4.5% for copper and 18.7% for iron in the peak integration method. Application to Analysis of Zinc and Lead Metal. The results of the direct analyses of three kinds of metal samples are summarized in Table 11. The results for the solutions of decomposed metala were obtained by the standard addition method. The time-dependent emission profiles of the anal* for the metal samples were different from those for the standard solutions as mentioned earlier. Therefore the peak integration method was employed when the analytes in metal samples were determined using the standard solutions as reference materials. The analytical results obtained by the direct metal insertion method and the standard addition method agree well with each other. In addition, the values obtained by the direct analysis are also in good agreement with the certified values of copper and iron in SRM728. Thus, it is concluded that the direct analysis of zinc and lead metal by this direct sample insertion method is quite accurate. However, larger deviations were observed for BCS No. 210e. Here the assigned values are given as estimated maximum concentrations. The values obtained by the present direct sample insertion method are much smaller than the given values. Analysis of High-Purity Zinc. High-purity zinc, SRM682, was analyzed by the present direct sample insertion method. The certified value of copper is 42 ng/g, and the value of iron, which is not certified and given as information only by NIST, is 100 ng/g. Since the concentrationswere very
low, a piece of metal weighing about 10-20 mg was used as a sample, and contamination in sample treatment, e.g., cutting, rinsing, and drying, was avoided as much as possible. Obtained values were 35 ng/g for copper and 270 ng/g for iron, and relative standard deviations for seven replicates were 25 and 20%, respectively. The analytical value for copper is almost in good agreement with the certified value, whilst the obtained value for iron is a factor of -3 larger than the information value.
CONCLUSION Trace amounts of copper and iron in volatile metals such as lead and zinc were determined by directly inserting a piece of metal into an argon ICP with a graphite cup insertion device. This method does not require the separation of analytes from the matrix nor sample decomposition prior to analysis and allow ultratrace analysis with sufficient precision and accuracy. Thus, the graphite cup direct insertion technique in ICP-AES may be a promising method for direct determination of ultratrace metallic elements in volatile metal samples.
REFERENCES Page, A. G.;Godbole, S. V.: Madraswala, K. H.; Kulkarnl, M. J.; Mallapurkar, V. S.; Joshl, B. D. Spectrochim. A& 1984, 398, 551. Lorber, A.; Goldbart, 2. A ~ l y 1985, ~ t 770, 155. Shao, Y.; Horllck, G. Appl. Spectrosc. 1988, 40, 386. Abdullah, M.; Haraguchl, H. AMI. Chem. 1985, 5 7 , 2059. Kirkbright, 0.F.; LCXlng, 2. AMlySt 1982, 707, 617. Page, A. G.; Madraswala, K. M.; Godbole, S. V.; Kulkaml, M. J.; Mallapurkar, V. S.; Joshl, 8. D. Fresenlus' Z . AMI. Chem. 1983, 375, 38. taray, G.; Broekaert. J. A. C.; Leis, F. SpectrocMm. Acta 1988, 438, 241. Brenner, I . B.; Lorber. A.; Goldbart, 2. S p e c t m h h . Acta W87, 428, 219. Monasterlos. C. V.; Jones, A. M.; Salin, E. D. AM/. Chem. 1988, 5 8 , 780. McLeod, C. W.; Clarke, P. A.; Mowthorpe, D. J. Specbochlm. Acta 1988, 418, 63. Karanassios. V.; Abdullah. M.; Horllck, G. Spectrochlm. Acta 1990, 458, 119. Karanasslos, V.; Horllck, 0.Spectrochlm. Acta 1990, 458, 85. UmemOto M.: Kubota, M. Spectrochlm. Acta 1989, 448, 713. Xu, J.: Kawaguchl, H.; Mlzulke, A. Appl. Spectrmc. 1983, 3 7 , 123. Umemoto M.; Kubota, M. Spectrochlm. Acta 1987, 428, 491. Boumans, P. W. J. M. Une CoincMence Tables for Inductlveljr CoupM Atomk Emission Spectromehy;Pergamon Press, Oxford, U.K., 1980. Human H. G. C.; Scott, R,. H. Spectrochlm. Acta 1978. 3 7 8 , 459. Sturgeon, R. E.; Chakrabatti, C. L.; Langford, C. H. Anal. Chem. 1976, 4 8 , 1792. Sneddon, J.: Ottaway, J. M.; Rowston, W. B. Analyst 1978, 703, 776.
RECEIVED for review June 3, 1991. Revised manuscript received October 21, 1991. Accepted October 25, 1991.
