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Determination of phosphorus in steels and copper metals by vacuum ultraviolet atomic emission spectrometry with inductively coupled plasma
Am, Vol 408,Nos l/Z,pp 293-300.1985. Rimedm GreatBrltam
05844547/85 13.00 + .GU
Specrrochimm
0 1985Pergamon RessLtd
Determination of phosphorus in steels and copper metals by vacuum ultraviolet atomic emission spectrometry with inductively coupled plasma TAKETOSHI NAKAHARA
Department of AppliedChemistry, College of Engineering, University of Osaka Prefecture, Sakai. Osaka 591, Japan (Receiwd 2 January 1984) Abstract-A simple method is described to determine phosphorus in steels and copper metals by inductively coupled plasma atomic emission spectrometry in the vacuum ultraviolet region of the spectrum. For measuring spectral hnes in the vacuum ultraviolet region, the monochromator and optical path between the plasma torch and the entrance slit have been purged with inert gas such as argon or nitrogen to reduce light absorption by oxygen. In the determination of phosphorus by ICP-AES, spectral interferences observed in the vacuum ultraviolet region have been compared with those in the ultraviolet spectral region above 200 nm. A phosphorus atomic emission line at 178.29 nm has been selected as an analytical line because it is free from spectral interferences of iron and copper lines. An analytical working curve obtained under the optimized operating conditions is rectilinear over approximately 4 orders of magnitude in concentration. The best attainable detection limit (3-u criterion) at P(I)178.29 nm is ca 0.05 PgP ml- ’ in solutions, which represents adetection limit of 10 PgP g- ’ for a 0.5-g sample subjected to a final volume of 100 ml. For accurate determination of phosphorus, the use of closely matched standards is recommended for construction of analytical working curves for phosphorus. The present method has been applied to the determmation of phosphorus in several steels and copper metals without prior separation. The results obtained are in good agreement with the certified values.
1.
INTRODUCTION
INDUCTIVELY coupled plasma atomic emission spectrometry (ICP-AES) continues to grow in popularity as a powerful analytical technique for a wide variety of samples owing to its simultaneous multi-element determination capability, low detection limits for most elements, large linear dynamic ranges, high tolerance to matrix effects and relative ease of operation. Spectral interferences in ICP-AES, however, have often become an important problem [ 1,2]. For a typical example, extreme difficulty is encountered in the determination of traces of phosphorus in steels and copper matrix by conventional ICP-AES, since all of phosphorus lines in the normal ultraviolet (u.v.) region above 200 nm suffer serious spectral interferences from iron and copper lines [3]. The emission from iron interferes strongly with the phosphorus determination at P(I)253.57nm, and also from copper severely at P(I)213.62 nm and P(I)214.91 nm. The second order lines of the latter two lines have been often used for the measurements [3,4]. Removal of coexisting copper, a spectrally interfering element, by solvent extraction with sodium dibenzyldithiocarbamate (NaDBDTC) into benzene has been recommended by ANDOet al. [5] who determined phosphorus in low alloy steels by ICP-AES at P(I)214.91 nm. To reduce spectral interferences, considerable attention has been given to a high-resolution spectrometer that uses either a holographic grating with a large number of grooves, e.g. 3600 grooves mm - I, or an echelle grating. Profiles of spectral lines emitted by an ICP source were measured with a Fabry-Perot interferometer [6, 71. Their full widths at half-maximum (FWHM) were sufficiently narrow so that spectral interferences may be effectively minimized or eliminated by the use of the high-resolution spectrometer system. Xv et al. [8] recently
[l] J. M. MERMETand C. TRASSY,Specwchim. Acta 36B, 269 (1981). [2] R. K. WINGE,V. A. FASSEL,V. J. PETERB~N and M. A. FLOYD, Appl. Spectrosc. 36,210 (1982). [3] A. F. WARD and L. F. MARCIELLO,Anal. Gem. 51, 2264 (1979). [4] A. fbZmL4E, Bunseki Kagaku 29, 502 (1980). [5] J. ANDO, H. UCHIDA,K. IWASAKIand K. TANAKA,Anal. Lett. 14, 1143 (1981). [6] H. G. C. HUMAN and R. H. Scorr, Spectrochim. Acfa 31B, 459 (1979). [7] H. KAWAGUCHI, Y. OSHIO and A. MIZUIKE, Spectrochim. Acta 37B, 809 (1982). [S] J. Xv, H. KAWAGUCHI and A. MIZUIKE, Appl. Spectrosc. 37, 123 (1983). 293
TAKETOSHI NAKAHARA
294
employed a computer-controlled, scanning echelle monochromator combined with an ICP source to eliminate spectral interferences from an iron line and the Cu(I1)213.60-nm line with the phosphorus analysis line, P(I)21 3.62 nm in the determination of phosphorus in steels by ICP-AES. Another simpler way of reducing spectral interferences in ICP-AES is appropriate selection of analytical lines that are free from spectral interferences. This is often realized especially for several nonmetallic elements including phosphorus, whose spectral lines are mostly occurred in the vacuum ultraviolet (VUV) region of the spectrum. The use of spectral lines in the VUV region was proposed by various authors, for the first time by KIRKBRIGHT et al. [9, IO] who recommended the use of P(I)185.9-nm line in the determination of phosphorus by ICP-AES. They purged the monochromator and optical path between the plasma torch and the entrance slit with nitrogen, and reported the detection limits of P, S, I, Hg, As and Se. WALLACE et al. [l l--14] determined the detection limits of S and B and the background equivalent concentration (BEC) of P in steels, also for a system with the optical path purged with nitrogen. They recommended the use of P(I)178.28-nm line for the determination of phosphorus in steels. By using a vacuum spectrometer and purging the optical path with helium or argon, HEINEet al. [ 151, LEE and PRITCHARD [16] and HAYAKAWA et al. [ 171 reported the spectral lines of several nonmetallic elements by ICP-AES in the VUV spectral region. To the author’s knowledge little has been described on the determination of trace concentrations of phosphorus in copper-based samples by ICP-AES without separation procedure. This paper deals with the VUV ICP-AES determination of trace phosphorus in copper metals and low alloy steels employing a conventional monochromator with low resolving power. 2. EXPERIMENTAL 2.1. Instrumentation A Nippon Jarrell-Ash Model ICAP-SOSM inductively coupled argon plasma emission spectrometer was used in combination with a Rikadenki Model R-21 chart recorder. Specification of the instrument is given in Table 1. An inert gas (argon or nitrogen) was used to purge the system from the ICP source to
Table 1. Instrumentation
Crystal-controlled type; 27.12MHz; automatic
Generator (rt)
Plasma torch Gas for ICP Nebulizer Spectrometer
Photomultiplier Signal measurement
F. F. F. F. F. F.
power control; automatic impedance matching network; maxlmum output power 2.0 kW All quartz Fassel type Argon for coolant, plasma and carrier gases Pneumatic cross-flow type Ebert 0.5-m mounting; 1180 lines mm-’ grating blazed at 240 nm; reciprocal linear dispetsion (1st order) 1.6 nm mm-’ Plasma source focussed as 1: 1 image onto entrance slit with a &cm focal length quartz lens HTV R-i06 UH D.c. amplitication/lO-s integration; digital voltmeter readout; chart recorder
KIRKBRIGHT, A. F. WARD and T. S. WEST, Anal. Chim. Acta 62, 241 (1972).
[9] (101 [ll] [12] [13] [14]
G. G. G. G. G. G.
[is]
D. R. HEINE, J. S. BABISand
KIRKBRIGHT,A. F. WARD
and T. S. WEST, Anal. Chim. Acta 64, 353 (1973). 1, 38 (1980). HOULT and R. D. EDIGER, Atom. Specrrosc. 1, 120 (1980).
WALLACE, Atom. Spectrosc. WALLACE, D. W.
WALLACE, Atom. Spectrosc.
2,61
(1981).
WALLXE and R. D. EDIGER, Atom. Spectrosc.
[16] J. LEE and M. W. PRITCHARD,S~c~roc~im. [17]
2, 169 (1981).
M. ELDENTON, Appt. Spectrosc. 34, Acta
36B,591
T. HAYAKAWA, F. KIKUI and S. IKEDA, Spectrochim. Acta
595 (1980).
(1981).
37B, 1069 ($982).
295
Phosphorus in steels and copper metals Monochromotor
Gos control r----------1
I : I Inert gas (Ar or Nz)
Exhoust
box
Flow meter
_&_
I I I I
.
, ,ul : I Stop votve ! I____________ A
1
Fig. 1. Schematic diagram of an inert gas purge system for the VUV ICP-AES.
the detector and the monochromator. A simple schematic diagram of the optical path purge system, based on essentially the principle described by KIRKBUGHTet al. [9], can be seen in Fig. 1. The region from the ICP source to the lens and from the lens to‘the monochromator was enclosed by use of two borosilicate glass cylinders 30-mm o.d. and 105 and 90 mm in length (referred to as a light cell). These, as well as the monochromator, were purged with either argon or nitrogen. Two quartz plates (Fig. 1) as windows of the monochromator are 30 x 30 mm and 0.5 mm thick. As shown in Fig. 1,a purge gas was divided into two flows; one purging the light cell and the photomultiplier (PMT) housing box and the other flowing in the monochromator, the flow rates of which were independently monitored and regulated. The purge gas flow rates in the PMT housing box and the region from the lens to the monochromator were maintained constant at 0.5 and 1 1mitt-‘, respectively, while mainly the flow rate of the purge gas in the region from the ICP source to the lens was regulated. The total flow ofa purge gas in the light cell and the PMT housing box is referred to as the light cell purge gas flow. This system permits extension of the operating range to the VUV spectral region between 170 and 200 nm. For comparison, emission intensities of some U.V.lines of phosphorus above 200 nm were measured in an air atmosphere as well. 2.2. Reagents The stock solution of phosphorus (1000 FgPml-‘) was prepared by dissolving analytical reagentgrade potassium dihydrogen orthophosphate. This was diluted as required with distilled water. Phosphorus-free analytical reagent-grade copper and iron metals were selected to prepare matrix solutions. The content of phosphorus in these metals was below the detection limit of the present method. Other solutions of diverse elements were prepared from analytical reagent-grade salts. Acids were added to match the standards with sample solutions. Argon (99.99 “/Apure) or nitrogen (99.99 ‘; pure) was used as a purge gas to reduce light absorption by oxygen. 2.3. Procedure All emission intensities, when stabilized, are integrated at least in triplicate over a fixed period. Background emission intensity, integrated over the same period, is subtracted from all line emission data to obtain the net line emission intensities. The experimental conditions used throughout this work are summarized in Table 2. Some of the optimum operating conditions shown in Table 2 are described below.
3. RESULTSAND DISCUSSION 3.1. Optimization of operating parameters In an attempt to obtain a maximum line-to-background intensity ratio, 1,/I, (I,, net analyte emission intensity; I,, background emission intensity) for phosphorus, various operating parameters were studied and optimized individually while the other parameters were kept at their optimum values. The parameters investigated were rf power, analytical
296 Table 2. Operating conditions for phosphorus determination Rf power forward Coolant gas flow rate Plasma gas flow rate Carrier gas flow rate Observation height Slit widths Slit height Integration time Wavelengths used
1.3 kW (reflected < 1OW) 16.01min-’ O.Slmin-’ 0.5 I min- ’ 16 mm above the load coil Entrance 25 pm; exit 25 pm 2mm 10 s 177.50, 178.29 and 185.94 nm in the VUV region; 213.62,214.91 and 253.57 nm in the U.V. region
wavelength, observation position in the ICP, argon flow rates of coolant, plasma and carrier gases and purge gas flow rate. Solutions with 10 PgP ml-’ were, unless otherwise stated, used to provide the best line-to-background ratios. The emission spectrum is shown in Fig. 2 between 176 and 187 nm obtained for a phosphorus solution with the argon-purged monochromator and optical path. The 185.94nm multiplet line is intense compared with the other three lines, whereas the background level around the P(1) 185.94~nm line is also higher than those at the other three wavelengths. The relative intensities for those lines shown in Fig. 2 are uncorrected for the significant variation in photomultiplier sensitivity with wavelength. The line-to-background intensity ratios (1,/I,) and background equivalent concentrations (BEC) in PgP ml- ’ obtained for major three lines of phosphorus in the VUV spectral region are given in Table 3. Argon purge gas gave considerably larger Z,/l,and smaller BEC at all the wavelengths than nitrogen purge gas. Therefore, if not noted otherwise, argon was used as a purge gas throughout this work. The effect of variation of argon purge gas on the line-to-background intensity ratio obtained for phosphorus at 185.94, 178.29 and 177.50 nm is illustrated in Fig. 3. Similar results were obtained with use of nitrogen as a purge gas. Optimized purge gas flow rates for the determination of phosphorus at three major lines are given in Table 4.
1
186
184
182 Woveiength
180
178
if6
(nmt
Fig. 2. Emission spectrum from the ICP between 176 and 187 nm obtained for nebulI~tion of a IOpgPml-’ solution with use of the argon purged system.
297
Phosphorus in steels and copper metals Table 3. Emission characteristics of three major phosphorus lines in the VUV spectral region Wavelength (nm) P(I)177.50 P(I)178.29 P(I)185.94
Detection limit bgml-‘)
BEC+ (~grn-‘)
I,/&* 15.51$c9.48]~ 11.15 [6.27] 4.75 [3.25]
0.o4oc0.0423 0.055[0.057] 0.049[0.047]
0.65[ 1.063 0.90[ MO] 2.11[3.08]
*With aspiration of 10 ygPml_’ solution. +Background equivalent concentration. *With argon purge. BWith nitrogen purge.
(b)
5
0 14 Argon
flow
rote
16 I” the
I8 monochromotor
!_._14
20
16
18
20
( I mn’l
Fig. 3. Effect of flow rate of argon purge gas on the line-to-background intensity ratio at (a) P(I)l85.94nm, (b) P(I)178.29nm and (c) P(I)177SOnm. Argon flow rate in the light cell: (A) Slmin- ‘; (Cl) 6lmin-I; (0) 8lmin-‘.
Table 4. Purge gas flows for the VUV lines of phosphorus Wavelength (nm) Observation height (mm) Purge gas flow rates Light cell (Imin- ‘) Monochromator (I min - ‘)
P(I)177.50 16
P(I)1 78.29 16
P(I)185.94 15
8*[5]+ 20[16]
8C51 18[12]
8[41 18[10]
*With argon purge. +With nitrogen purge.
3.2. Detection
limit and analytical
working graph
For the optimized operating conditions desc$bed in Tables 2 and 4, the detection limits for the phosphorus lines studied are reported in Table 3. The detection limits reported here correspond to the concentration of the analyte required to give a net signal, i.e. backgroundcorrected line intensity, equal to three times the standard deviation of the background in accordance with IUPAC recommendation [ 183. Argon and nitrogen purge gases gave almost the same detection limits at all the three VUV lines of phosphorus. For comparison, the detection limits for phosphorus in the U.V.region were also obtained in an air atmosphere with the apparatus employed. The detection limits at 213.62, 214.91 and 253.57 nm were found to be 0.032,0.083 and 0.27 pgrnl- ‘, respectively. As a result, the phosphorus 213.62nm line can give the best limit of detection of all the lines investigated. However, the phosphorus 178.29-nm line was selected as the most suitable analytical line for the determination of phosphorus in steel and copper metal as will be described below. Analytical working graphs were established for phosphorus at the three VUV lines [18] Commission on Spectrcchemical and Other Optical Procedures for Analysis, Pure Appl. Chem. 4599 (1976).
298
TAKETOSHI NAKAHARA
investigated. At each of these lines, the linear region was found to extend to beyond 500 pg ml- ‘. The lowest quantitatively determinable concentration (LQD) at each line was found to be approximately 0.05 PgP ml- I.
3.3. Spectral interference Under the optimized operating conditions, spectral and matrix interferences can not always be avoided. Spectral interferences at the phosphorus lines in the VUV and U.V.spectral regions that may be encountered during the determination of phosphorus in various samples of steel and copper metal are listed in Table 5. These have been characterized by scanning in the vicinity of each analyte emission line. A spectral interference can be obtained which is described by the critical concentration ratio (CCR). The CCR is defined as a ratio of the concentrations of interferent to analyte, at which the ratio of the intensities of the interfering line and the analytical line within the spectral window of the analytical line is equal to unity [19]. The smaller the CCR, the more severe the interference. The most suitable analytical line for the determination of phosphorus in steel and copper metal with the present operating conditions is P(1) 178.29 nm. However, even though most of spectral interferences from major elements, i.e. irot in steel and copper in copper metal, can be circumvented by selecting the phosphorus 178.29-nm line, the presence of minor elements such as Co, Cr, Mn, MO and Ni in the steel samples may cause more or less spectral interference with the phosphorus determination. Therefore, the use of closely matched standards or the method of standard additions is recommended to compensate for these interferences which otherwise would produce an error in the analysis of practical samples.
Table 5. Interferences from diverse elements with the determination phosphorus (10 pg ml- ‘) at major phosphorus lines P(I)177.50 nm
Interferent Al co Cr cu Fe Mn MO Ni
Wavelength Wavelength CCR* (nm) (nm)
-t 177.46 177.61 177.60 171.57 177.56 177.58
_t 1.2 0.4 5.6 7.8 5.4 0.6
P(I)213.62 nm
Interferent co Cr cu Fe Mn MO Ni
P(I)178.29 nm
Wavelength (nm) 213.65 213.56 213.60 213.62 213.60 213.53
CCR 16.4 0.05 11.4 2.0 -
Wavelength (nm)
CCR
4.6 6.6
186.00 185.95 186.01
4.4 4.2 --$
3.8 0.3
185.97 185.94 186.01 185.94
5.8 2.6 6.8
P(I)214.91 nm Wavelength (nm)
P(I)185.94 nm
CCR
178.30 178.30 178.26 178.34 178.29
of
CCR
214.90 214.92
0.06 19.8
214.98 214.78
7.6 3.6
P(I)253.57 nm Wavelength (nm)
CCR
253.60
0.4 -
253.56 253.56
0.3 0.1
253.60
-
*Critical concentration ratio. tElement having no interfering line at or near the phosphorus line. tElement showing no interference (CCR > 200).
[ 191 P. W. J. M. BOUMANS, Line Coincidence Tablesfor Inductively Coupled Plasma Atomic Emission Spectrometry. Pergamon Press, Oxford (1980).
299
Phosphorus in steels and copper metals
3.4. Application of the present method to the determination of phosphorus in practical samples of steel and copper metal The accuracy of the present method was established by analysing several practical samples. To this end, on the basis of the observations described above, the following procedure was followed for the determination of phosphorus in some samples of low alloy steel and copper metal. For steel samples, accurately weighed 0.5 g of chipped sample was dissolved in 20 ml of inverse aqua regia (I : 3 mixture of hydrochloric and nitric acids) by heating gently on a hot plate, diluted to 100 ml with distilled water and nebulized into the ICP source. For a copper metal sample, a 0.5-g sample was accurately weighed, dissolved in 5 ml of concentrated nitric acid by heating slowly on a hot plate, diluted with distilled water exactly to 1OOml and nebulized into the optimized ICP source. As shown in Table 5, major spectral interferen~s from iron and copper lines encountered in the U.V.region measurement can be completely eliminated by selecting the VUV line of P(1) 178.29 nm. However, for accurate determination of phosphorus, closely matrix-matched standard solutions were used for construction of the analytical working curves for phosphorus, taking spectral interferences from some minor elements in the samples and other kinds of interferences from the matrices into consideration, instead of the use of the standard additions method which is a time-consuming procedure and can also mask analytical errors. For analysis of steels, minor constituents such as cobalt, chromium, moly~enum and nickel as well as the matrix element, iron were added and matrix-matched in the preparation of a series of phosphorus standard solutions (O.l-lO~gPml_‘), while only copper was matrix-matched for the determination of phosphorus in copper metals. The results from the determination of phosphorus in several samples of low alloy steel and copper metal are shown in Tables 6 and 7, respectively. All the results are in excellent agreement with the certified values. Relative standard deviations (RSD) were obtained to be about 5 % or less on replicate determinations for each sample.
Table 6. Analytical results of phosphorus in low alloy steels
Sample JSS 150-7+ JSS 151-7 JSS 152-7 JSS 153-7 JSS 154-7 BCS 149-3$
Certified value (pgg- ‘) 430 330 270 130 80 50
Present method Found value (~8 g- ‘) RSD* 432.9 f 13.0 (n = 9) 329.7i 9.5 (n = 6) 268.2 f 8.3 (n = 6) 132.4+ 4S(n = 9) 81.0_+ 3.7(n = 6) 51.2+ 2.7(n=6)
3.0 2.9 3.1 3.4 4.6 5.3
*Relative standard deviation ( p<). +Japan Standards of Iron and Steel from The Iron and Steel Institute of Japan. *British Chemical Standards.
Tabte 7. Analytical results of phosphorus in copper metals
Sample
Certified value &gg-‘)
A B c D
50 130 230 500
Present method Found value &gg-‘)RSD* 51.4_+ 2.6(n = 128.6& 5.1 (n = 227.7 + 8.0 (n = 508.3 + 14.2 (n =
*Relative standard deviation (“,).
8) 12) 12) 12)
5.1 4.0 3.5 2.8
300
TAKETOSHI NAKAHARA
4. CONCLUSIONS It has been demonstrated that phosphorus can be determined by inductively coupled plasma atomic emission spectrometry in the vacuum ultraviolet spectral region in combination with a simple inert gas purge system and a relatively low-resolution monochromator. The proposed method has been successfully applied to the determination of phosphorus in some samples of steel and copper metal. Major spectral interferences from iron and copper lines have completely circumvented by selecting P(1) 178.29~nm line as an analytical one. However, some spectral interferences from minor elements in the sample still remain with use of the present low resolution (reciprocal linear dispersion, 1.6 nm/mm) monochromator. These interferences would be avoided or to a great extent reduced by using a higher resolution spectrometer. Acknowledgements-The author wishes to express his appreciation to The Mitsubishi Metal Corporation for supplying the samples of copper metal and to Mr TAKASHI NAKAHARA for his help in the experiment. The present work was supported in part by a Grant-in-Aid for Scientific Research from the Mimstry of Education, Science and Culture, Japan.
05844547/85 13.00 + .GU
Specrrochimm
0 1985Pergamon RessLtd
Determination of phosphorus in steels and copper metals by vacuum ultraviolet atomic emission spectrometry with inductively coupled plasma TAKETOSHI NAKAHARA
Department of AppliedChemistry, College of Engineering, University of Osaka Prefecture, Sakai. Osaka 591, Japan (Receiwd 2 January 1984) Abstract-A simple method is described to determine phosphorus in steels and copper metals by inductively coupled plasma atomic emission spectrometry in the vacuum ultraviolet region of the spectrum. For measuring spectral hnes in the vacuum ultraviolet region, the monochromator and optical path between the plasma torch and the entrance slit have been purged with inert gas such as argon or nitrogen to reduce light absorption by oxygen. In the determination of phosphorus by ICP-AES, spectral interferences observed in the vacuum ultraviolet region have been compared with those in the ultraviolet spectral region above 200 nm. A phosphorus atomic emission line at 178.29 nm has been selected as an analytical line because it is free from spectral interferences of iron and copper lines. An analytical working curve obtained under the optimized operating conditions is rectilinear over approximately 4 orders of magnitude in concentration. The best attainable detection limit (3-u criterion) at P(I)178.29 nm is ca 0.05 PgP ml- ’ in solutions, which represents adetection limit of 10 PgP g- ’ for a 0.5-g sample subjected to a final volume of 100 ml. For accurate determination of phosphorus, the use of closely matched standards is recommended for construction of analytical working curves for phosphorus. The present method has been applied to the determmation of phosphorus in several steels and copper metals without prior separation. The results obtained are in good agreement with the certified values.
1.
INTRODUCTION
INDUCTIVELY coupled plasma atomic emission spectrometry (ICP-AES) continues to grow in popularity as a powerful analytical technique for a wide variety of samples owing to its simultaneous multi-element determination capability, low detection limits for most elements, large linear dynamic ranges, high tolerance to matrix effects and relative ease of operation. Spectral interferences in ICP-AES, however, have often become an important problem [ 1,2]. For a typical example, extreme difficulty is encountered in the determination of traces of phosphorus in steels and copper matrix by conventional ICP-AES, since all of phosphorus lines in the normal ultraviolet (u.v.) region above 200 nm suffer serious spectral interferences from iron and copper lines [3]. The emission from iron interferes strongly with the phosphorus determination at P(I)253.57nm, and also from copper severely at P(I)213.62 nm and P(I)214.91 nm. The second order lines of the latter two lines have been often used for the measurements [3,4]. Removal of coexisting copper, a spectrally interfering element, by solvent extraction with sodium dibenzyldithiocarbamate (NaDBDTC) into benzene has been recommended by ANDOet al. [5] who determined phosphorus in low alloy steels by ICP-AES at P(I)214.91 nm. To reduce spectral interferences, considerable attention has been given to a high-resolution spectrometer that uses either a holographic grating with a large number of grooves, e.g. 3600 grooves mm - I, or an echelle grating. Profiles of spectral lines emitted by an ICP source were measured with a Fabry-Perot interferometer [6, 71. Their full widths at half-maximum (FWHM) were sufficiently narrow so that spectral interferences may be effectively minimized or eliminated by the use of the high-resolution spectrometer system. Xv et al. [8] recently
[l] J. M. MERMETand C. TRASSY,Specwchim. Acta 36B, 269 (1981). [2] R. K. WINGE,V. A. FASSEL,V. J. PETERB~N and M. A. FLOYD, Appl. Spectrosc. 36,210 (1982). [3] A. F. WARD and L. F. MARCIELLO,Anal. Gem. 51, 2264 (1979). [4] A. fbZmL4E, Bunseki Kagaku 29, 502 (1980). [5] J. ANDO, H. UCHIDA,K. IWASAKIand K. TANAKA,Anal. Lett. 14, 1143 (1981). [6] H. G. C. HUMAN and R. H. Scorr, Spectrochim. Acfa 31B, 459 (1979). [7] H. KAWAGUCHI, Y. OSHIO and A. MIZUIKE, Spectrochim. Acta 37B, 809 (1982). [S] J. Xv, H. KAWAGUCHI and A. MIZUIKE, Appl. Spectrosc. 37, 123 (1983). 293
TAKETOSHI NAKAHARA
294
employed a computer-controlled, scanning echelle monochromator combined with an ICP source to eliminate spectral interferences from an iron line and the Cu(I1)213.60-nm line with the phosphorus analysis line, P(I)21 3.62 nm in the determination of phosphorus in steels by ICP-AES. Another simpler way of reducing spectral interferences in ICP-AES is appropriate selection of analytical lines that are free from spectral interferences. This is often realized especially for several nonmetallic elements including phosphorus, whose spectral lines are mostly occurred in the vacuum ultraviolet (VUV) region of the spectrum. The use of spectral lines in the VUV region was proposed by various authors, for the first time by KIRKBRIGHT et al. [9, IO] who recommended the use of P(I)185.9-nm line in the determination of phosphorus by ICP-AES. They purged the monochromator and optical path between the plasma torch and the entrance slit with nitrogen, and reported the detection limits of P, S, I, Hg, As and Se. WALLACE et al. [l l--14] determined the detection limits of S and B and the background equivalent concentration (BEC) of P in steels, also for a system with the optical path purged with nitrogen. They recommended the use of P(I)178.28-nm line for the determination of phosphorus in steels. By using a vacuum spectrometer and purging the optical path with helium or argon, HEINEet al. [ 151, LEE and PRITCHARD [16] and HAYAKAWA et al. [ 171 reported the spectral lines of several nonmetallic elements by ICP-AES in the VUV spectral region. To the author’s knowledge little has been described on the determination of trace concentrations of phosphorus in copper-based samples by ICP-AES without separation procedure. This paper deals with the VUV ICP-AES determination of trace phosphorus in copper metals and low alloy steels employing a conventional monochromator with low resolving power. 2. EXPERIMENTAL 2.1. Instrumentation A Nippon Jarrell-Ash Model ICAP-SOSM inductively coupled argon plasma emission spectrometer was used in combination with a Rikadenki Model R-21 chart recorder. Specification of the instrument is given in Table 1. An inert gas (argon or nitrogen) was used to purge the system from the ICP source to
Table 1. Instrumentation
Crystal-controlled type; 27.12MHz; automatic
Generator (rt)
Plasma torch Gas for ICP Nebulizer Spectrometer
Photomultiplier Signal measurement
F. F. F. F. F. F.
power control; automatic impedance matching network; maxlmum output power 2.0 kW All quartz Fassel type Argon for coolant, plasma and carrier gases Pneumatic cross-flow type Ebert 0.5-m mounting; 1180 lines mm-’ grating blazed at 240 nm; reciprocal linear dispetsion (1st order) 1.6 nm mm-’ Plasma source focussed as 1: 1 image onto entrance slit with a &cm focal length quartz lens HTV R-i06 UH D.c. amplitication/lO-s integration; digital voltmeter readout; chart recorder
KIRKBRIGHT, A. F. WARD and T. S. WEST, Anal. Chim. Acta 62, 241 (1972).
[9] (101 [ll] [12] [13] [14]
G. G. G. G. G. G.
[is]
D. R. HEINE, J. S. BABISand
KIRKBRIGHT,A. F. WARD
and T. S. WEST, Anal. Chim. Acta 64, 353 (1973). 1, 38 (1980). HOULT and R. D. EDIGER, Atom. Specrrosc. 1, 120 (1980).
WALLACE, Atom. Spectrosc. WALLACE, D. W.
WALLACE, Atom. Spectrosc.
2,61
(1981).
WALLXE and R. D. EDIGER, Atom. Spectrosc.
[16] J. LEE and M. W. PRITCHARD,S~c~roc~im. [17]
2, 169 (1981).
M. ELDENTON, Appt. Spectrosc. 34, Acta
36B,591
T. HAYAKAWA, F. KIKUI and S. IKEDA, Spectrochim. Acta
595 (1980).
(1981).
37B, 1069 ($982).
295
Phosphorus in steels and copper metals Monochromotor
Gos control r----------1
I : I Inert gas (Ar or Nz)
Exhoust
box
Flow meter
_&_
I I I I
.
, ,ul : I Stop votve ! I____________ A
1
Fig. 1. Schematic diagram of an inert gas purge system for the VUV ICP-AES.
the detector and the monochromator. A simple schematic diagram of the optical path purge system, based on essentially the principle described by KIRKBUGHTet al. [9], can be seen in Fig. 1. The region from the ICP source to the lens and from the lens to‘the monochromator was enclosed by use of two borosilicate glass cylinders 30-mm o.d. and 105 and 90 mm in length (referred to as a light cell). These, as well as the monochromator, were purged with either argon or nitrogen. Two quartz plates (Fig. 1) as windows of the monochromator are 30 x 30 mm and 0.5 mm thick. As shown in Fig. 1,a purge gas was divided into two flows; one purging the light cell and the photomultiplier (PMT) housing box and the other flowing in the monochromator, the flow rates of which were independently monitored and regulated. The purge gas flow rates in the PMT housing box and the region from the lens to the monochromator were maintained constant at 0.5 and 1 1mitt-‘, respectively, while mainly the flow rate of the purge gas in the region from the ICP source to the lens was regulated. The total flow ofa purge gas in the light cell and the PMT housing box is referred to as the light cell purge gas flow. This system permits extension of the operating range to the VUV spectral region between 170 and 200 nm. For comparison, emission intensities of some U.V.lines of phosphorus above 200 nm were measured in an air atmosphere as well. 2.2. Reagents The stock solution of phosphorus (1000 FgPml-‘) was prepared by dissolving analytical reagentgrade potassium dihydrogen orthophosphate. This was diluted as required with distilled water. Phosphorus-free analytical reagent-grade copper and iron metals were selected to prepare matrix solutions. The content of phosphorus in these metals was below the detection limit of the present method. Other solutions of diverse elements were prepared from analytical reagent-grade salts. Acids were added to match the standards with sample solutions. Argon (99.99 “/Apure) or nitrogen (99.99 ‘; pure) was used as a purge gas to reduce light absorption by oxygen. 2.3. Procedure All emission intensities, when stabilized, are integrated at least in triplicate over a fixed period. Background emission intensity, integrated over the same period, is subtracted from all line emission data to obtain the net line emission intensities. The experimental conditions used throughout this work are summarized in Table 2. Some of the optimum operating conditions shown in Table 2 are described below.
3. RESULTSAND DISCUSSION 3.1. Optimization of operating parameters In an attempt to obtain a maximum line-to-background intensity ratio, 1,/I, (I,, net analyte emission intensity; I,, background emission intensity) for phosphorus, various operating parameters were studied and optimized individually while the other parameters were kept at their optimum values. The parameters investigated were rf power, analytical
296 Table 2. Operating conditions for phosphorus determination Rf power forward Coolant gas flow rate Plasma gas flow rate Carrier gas flow rate Observation height Slit widths Slit height Integration time Wavelengths used
1.3 kW (reflected < 1OW) 16.01min-’ O.Slmin-’ 0.5 I min- ’ 16 mm above the load coil Entrance 25 pm; exit 25 pm 2mm 10 s 177.50, 178.29 and 185.94 nm in the VUV region; 213.62,214.91 and 253.57 nm in the U.V. region
wavelength, observation position in the ICP, argon flow rates of coolant, plasma and carrier gases and purge gas flow rate. Solutions with 10 PgP ml-’ were, unless otherwise stated, used to provide the best line-to-background ratios. The emission spectrum is shown in Fig. 2 between 176 and 187 nm obtained for a phosphorus solution with the argon-purged monochromator and optical path. The 185.94nm multiplet line is intense compared with the other three lines, whereas the background level around the P(1) 185.94~nm line is also higher than those at the other three wavelengths. The relative intensities for those lines shown in Fig. 2 are uncorrected for the significant variation in photomultiplier sensitivity with wavelength. The line-to-background intensity ratios (1,/I,) and background equivalent concentrations (BEC) in PgP ml- ’ obtained for major three lines of phosphorus in the VUV spectral region are given in Table 3. Argon purge gas gave considerably larger Z,/l,and smaller BEC at all the wavelengths than nitrogen purge gas. Therefore, if not noted otherwise, argon was used as a purge gas throughout this work. The effect of variation of argon purge gas on the line-to-background intensity ratio obtained for phosphorus at 185.94, 178.29 and 177.50 nm is illustrated in Fig. 3. Similar results were obtained with use of nitrogen as a purge gas. Optimized purge gas flow rates for the determination of phosphorus at three major lines are given in Table 4.
1
186
184
182 Woveiength
180
178
if6
(nmt
Fig. 2. Emission spectrum from the ICP between 176 and 187 nm obtained for nebulI~tion of a IOpgPml-’ solution with use of the argon purged system.
297
Phosphorus in steels and copper metals Table 3. Emission characteristics of three major phosphorus lines in the VUV spectral region Wavelength (nm) P(I)177.50 P(I)178.29 P(I)185.94
Detection limit bgml-‘)
BEC+ (~grn-‘)
I,/&* 15.51$c9.48]~ 11.15 [6.27] 4.75 [3.25]
0.o4oc0.0423 0.055[0.057] 0.049[0.047]
0.65[ 1.063 0.90[ MO] 2.11[3.08]
*With aspiration of 10 ygPml_’ solution. +Background equivalent concentration. *With argon purge. BWith nitrogen purge.
(b)
5
0 14 Argon
flow
rote
16 I” the
I8 monochromotor
!_._14
20
16
18
20
( I mn’l
Fig. 3. Effect of flow rate of argon purge gas on the line-to-background intensity ratio at (a) P(I)l85.94nm, (b) P(I)178.29nm and (c) P(I)177SOnm. Argon flow rate in the light cell: (A) Slmin- ‘; (Cl) 6lmin-I; (0) 8lmin-‘.
Table 4. Purge gas flows for the VUV lines of phosphorus Wavelength (nm) Observation height (mm) Purge gas flow rates Light cell (Imin- ‘) Monochromator (I min - ‘)
P(I)177.50 16
P(I)1 78.29 16
P(I)185.94 15
8*[5]+ 20[16]
8C51 18[12]
8[41 18[10]
*With argon purge. +With nitrogen purge.
3.2. Detection
limit and analytical
working graph
For the optimized operating conditions desc$bed in Tables 2 and 4, the detection limits for the phosphorus lines studied are reported in Table 3. The detection limits reported here correspond to the concentration of the analyte required to give a net signal, i.e. backgroundcorrected line intensity, equal to three times the standard deviation of the background in accordance with IUPAC recommendation [ 183. Argon and nitrogen purge gases gave almost the same detection limits at all the three VUV lines of phosphorus. For comparison, the detection limits for phosphorus in the U.V.region were also obtained in an air atmosphere with the apparatus employed. The detection limits at 213.62, 214.91 and 253.57 nm were found to be 0.032,0.083 and 0.27 pgrnl- ‘, respectively. As a result, the phosphorus 213.62nm line can give the best limit of detection of all the lines investigated. However, the phosphorus 178.29-nm line was selected as the most suitable analytical line for the determination of phosphorus in steel and copper metal as will be described below. Analytical working graphs were established for phosphorus at the three VUV lines [18] Commission on Spectrcchemical and Other Optical Procedures for Analysis, Pure Appl. Chem. 4599 (1976).
298
TAKETOSHI NAKAHARA
investigated. At each of these lines, the linear region was found to extend to beyond 500 pg ml- ‘. The lowest quantitatively determinable concentration (LQD) at each line was found to be approximately 0.05 PgP ml- I.
3.3. Spectral interference Under the optimized operating conditions, spectral and matrix interferences can not always be avoided. Spectral interferences at the phosphorus lines in the VUV and U.V.spectral regions that may be encountered during the determination of phosphorus in various samples of steel and copper metal are listed in Table 5. These have been characterized by scanning in the vicinity of each analyte emission line. A spectral interference can be obtained which is described by the critical concentration ratio (CCR). The CCR is defined as a ratio of the concentrations of interferent to analyte, at which the ratio of the intensities of the interfering line and the analytical line within the spectral window of the analytical line is equal to unity [19]. The smaller the CCR, the more severe the interference. The most suitable analytical line for the determination of phosphorus in steel and copper metal with the present operating conditions is P(1) 178.29 nm. However, even though most of spectral interferences from major elements, i.e. irot in steel and copper in copper metal, can be circumvented by selecting the phosphorus 178.29-nm line, the presence of minor elements such as Co, Cr, Mn, MO and Ni in the steel samples may cause more or less spectral interference with the phosphorus determination. Therefore, the use of closely matched standards or the method of standard additions is recommended to compensate for these interferences which otherwise would produce an error in the analysis of practical samples.
Table 5. Interferences from diverse elements with the determination phosphorus (10 pg ml- ‘) at major phosphorus lines P(I)177.50 nm
Interferent Al co Cr cu Fe Mn MO Ni
Wavelength Wavelength CCR* (nm) (nm)
-t 177.46 177.61 177.60 171.57 177.56 177.58
_t 1.2 0.4 5.6 7.8 5.4 0.6
P(I)213.62 nm
Interferent co Cr cu Fe Mn MO Ni
P(I)178.29 nm
Wavelength (nm) 213.65 213.56 213.60 213.62 213.60 213.53
CCR 16.4 0.05 11.4 2.0 -
Wavelength (nm)
CCR
4.6 6.6
186.00 185.95 186.01
4.4 4.2 --$
3.8 0.3
185.97 185.94 186.01 185.94
5.8 2.6 6.8
P(I)214.91 nm Wavelength (nm)
P(I)185.94 nm
CCR
178.30 178.30 178.26 178.34 178.29
of
CCR
214.90 214.92
0.06 19.8
214.98 214.78
7.6 3.6
P(I)253.57 nm Wavelength (nm)
CCR
253.60
0.4 -
253.56 253.56
0.3 0.1
253.60
-
*Critical concentration ratio. tElement having no interfering line at or near the phosphorus line. tElement showing no interference (CCR > 200).
[ 191 P. W. J. M. BOUMANS, Line Coincidence Tablesfor Inductively Coupled Plasma Atomic Emission Spectrometry. Pergamon Press, Oxford (1980).
299
Phosphorus in steels and copper metals
3.4. Application of the present method to the determination of phosphorus in practical samples of steel and copper metal The accuracy of the present method was established by analysing several practical samples. To this end, on the basis of the observations described above, the following procedure was followed for the determination of phosphorus in some samples of low alloy steel and copper metal. For steel samples, accurately weighed 0.5 g of chipped sample was dissolved in 20 ml of inverse aqua regia (I : 3 mixture of hydrochloric and nitric acids) by heating gently on a hot plate, diluted to 100 ml with distilled water and nebulized into the ICP source. For a copper metal sample, a 0.5-g sample was accurately weighed, dissolved in 5 ml of concentrated nitric acid by heating slowly on a hot plate, diluted with distilled water exactly to 1OOml and nebulized into the optimized ICP source. As shown in Table 5, major spectral interferen~s from iron and copper lines encountered in the U.V.region measurement can be completely eliminated by selecting the VUV line of P(1) 178.29 nm. However, for accurate determination of phosphorus, closely matrix-matched standard solutions were used for construction of the analytical working curves for phosphorus, taking spectral interferences from some minor elements in the samples and other kinds of interferences from the matrices into consideration, instead of the use of the standard additions method which is a time-consuming procedure and can also mask analytical errors. For analysis of steels, minor constituents such as cobalt, chromium, moly~enum and nickel as well as the matrix element, iron were added and matrix-matched in the preparation of a series of phosphorus standard solutions (O.l-lO~gPml_‘), while only copper was matrix-matched for the determination of phosphorus in copper metals. The results from the determination of phosphorus in several samples of low alloy steel and copper metal are shown in Tables 6 and 7, respectively. All the results are in excellent agreement with the certified values. Relative standard deviations (RSD) were obtained to be about 5 % or less on replicate determinations for each sample.
Table 6. Analytical results of phosphorus in low alloy steels
Sample JSS 150-7+ JSS 151-7 JSS 152-7 JSS 153-7 JSS 154-7 BCS 149-3$
Certified value (pgg- ‘) 430 330 270 130 80 50
Present method Found value (~8 g- ‘) RSD* 432.9 f 13.0 (n = 9) 329.7i 9.5 (n = 6) 268.2 f 8.3 (n = 6) 132.4+ 4S(n = 9) 81.0_+ 3.7(n = 6) 51.2+ 2.7(n=6)
3.0 2.9 3.1 3.4 4.6 5.3
*Relative standard deviation ( p<). +Japan Standards of Iron and Steel from The Iron and Steel Institute of Japan. *British Chemical Standards.
Tabte 7. Analytical results of phosphorus in copper metals
Sample
Certified value &gg-‘)
A B c D
50 130 230 500
Present method Found value &gg-‘)RSD* 51.4_+ 2.6(n = 128.6& 5.1 (n = 227.7 + 8.0 (n = 508.3 + 14.2 (n =
*Relative standard deviation (“,).
8) 12) 12) 12)
5.1 4.0 3.5 2.8
300
TAKETOSHI NAKAHARA
4. CONCLUSIONS It has been demonstrated that phosphorus can be determined by inductively coupled plasma atomic emission spectrometry in the vacuum ultraviolet spectral region in combination with a simple inert gas purge system and a relatively low-resolution monochromator. The proposed method has been successfully applied to the determination of phosphorus in some samples of steel and copper metal. Major spectral interferences from iron and copper lines have completely circumvented by selecting P(1) 178.29~nm line as an analytical one. However, some spectral interferences from minor elements in the sample still remain with use of the present low resolution (reciprocal linear dispersion, 1.6 nm/mm) monochromator. These interferences would be avoided or to a great extent reduced by using a higher resolution spectrometer. Acknowledgements-The author wishes to express his appreciation to The Mitsubishi Metal Corporation for supplying the samples of copper metal and to Mr TAKASHI NAKAHARA for his help in the experiment. The present work was supported in part by a Grant-in-Aid for Scientific Research from the Mimstry of Education, Science and Culture, Japan.