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(20) Jeans, J. Dynamic Theory of Qases, 4th ed.; Dover Publlshing: New York, 1925; ChaDter 13. Bradley, R. S.; Evans, M. G.; Whytlaw-Gray, R. W. R o c . R . SOC.London, Ser. A 1946, 186A, 368-390. Jacobs, P. W.; RusseMones, A. J . W y s . Chem. 1988, 72, 202-207. Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids; Oxford at Clarendon: London, 1959. Glassner. A. “The Therrnochemlcal Properties of the Oxides, Fluorides, and Chlorides to 2500 K,” ANL-5750; U S . Atomic Energy Commisslon Report, 1959. Svehla, R. Report R-132, National Aeronautics and Space Administration: Cleveland, OH, 1962. Barln, 1.; Knacke, 0. Thermochemical Properries of Inorganic Substances; Springer-Verlag: Berlin, 1973. Pak, Y., Indiena University, unpublished data, 1983. Pound, G. M. J . Phys. Chem. Ref. Data 1975, 4(4), 871. Pound, G. M. J . Phys. Chem. Ref. Data 1979, 8(1), 125. Snelleman, W. Ph.D. Dissertation, Unlversity of Utrecht, Utrecht, The Netherlands, 1965. International Crltical Tables, 1st ed.; McGraw-Hill: New York and London, 1926. Rubeska, I. I n Flame Emission and Atomic Absorption Spectroscopy; Dean, J. A., Rains, T. C., Eds.; Dekker: New York, 1969; Vol. I-Theory, Chapter 11. Gaydon, A. G. Dissociation Energies and Spectra of Diatomic Moiecules; 3rd ed.; Chapman and Hall: London, 1968. Herrmann, R.; Alkemade, C. Th. J. Chemical Analysis by Flame Photometry; Wlley-Interscience: New York, 1965; p 34. Clampitt, N. C.;Hieftje, G. M. Anal. Chem. 1972, 4 4 , 1211-1219.
(36) Fotiev, A. A.; Slobodin. B. V. Zh. Fiz. Khim. 1985, 39(12), 3099-3100. (37) Kvande, H. Acta Chem. Scand., Ser. A 1979, A33(6), 407-412. (38) Ewing, C. T.; Stern, K. H. J . W y s . Chem. 1975, 79, 2007-2017. (39) Hirth, J. P. I n The Characterlzatlon of Mgh-Temperature Vapors; Margrave, J. L., Ed.; Wlley: New York, 1967; Chapter 15, pp 453-472. (40) Chen, X. Department of Engineering Mechanics, Tsinghua University, Beijing, People’s Republic of Chlna, personal communlcatlon. (41) Novak, J. W., Jr.; Browner, R. F. Anal. Chem. 1080, 52, 792-796. (42) Chliders, A. G.; Hleftje, G. M., submttted for publlcation in Anal. Chem. (43) Chen, X.; Chyon, Y. P.; Lee, Y. CI; Pfender, E. Plasma Chem. Plasma Process. 1985, 5 , 119. (44) Childers, A. G.; Hleftje, G. M. Appl. Spectrosc. 1987, 4 0 , 939-944.
RECEIVED for review January 20,1987. Accepted August 22, 1987. Supported in part by the National Science Foundation through Grant CHE 83-20053, by American Cyanamid, and by the Office of Naval Research. G.M.H. expresses his appreciation to the Science and Engineering Research Council of the United Kingdom for fellowship support during a portion of this study. R.M.M. expresses his appreciation to the Office of Naval Research and the Royal Society London for travel support.
Determination of Phosphorus in Copper-Based Alloys Using Ion-Exchange Chromatography and Direct-Current Plasma Emission Spectrometry M. S. Epstein* and W. F. Koch Inorganic Analytical Research Division, National Bureau of Standards, Gaithersburg, Maryland 20899 K. S. Epler and T. C. O’Haver Department of Chemistry, University of Maryland, College Park, Maryland 20742
Phosphorus Is determlned dkectly In acid digests of copperbased alloys by uslng dlrect-current plasma emlsslon spectrometry. The spectral overlap Interference of copper and Iron emlsslon on the most senslthre phosphorus llnes Is elknlnated by lon-exchange separatlon carrled out In the sample flow stream to the emission spectrometer. The separatlons Involve elther a cation exchange to retain the Interfering elements on the column or an anion exchange uslng an actlvated alumlna column to retaln and preconcentrate the phosphorus. Phos phorus M e d o n ynlts obtained by uslng the latter method are 20 times better than those obtalned by uslng continuous asplratlon. A detectlon limit of 10 ng/mL Is observed for a 30mln preconcentratlont h e and 2 mL/mln sample f k w rate. Phosphorus Is determined at the 20 Mg/g level In SRM 875 (cupro-nickel, an alloy contalnlng approximately 90 % copper and 10% nickel) and In phosphorized copper SRMs 1251 and 1252 at concentrallons from 80 to 400 pg/g.
Matrix-induced chemical and spectral interferences are major sources of systematic error in analytical measurements. Most approaches that are available to compensate for these interferences, whether they are instrumental (i.e., background correction) or procedural (i.e., standard addition), usually involve compromisesthat may limit the sensitivity, accuracy, detection limit, and speed of measurement. The most
straightforward approach to eliminate matrix-induced interferences, and the one that requires the least number of compromises, is the complete separation of analyte from interfering species. The direct coupling of chromatographic methods to spectrometric methods for on-line separations offers the possibilities of enhanced sensitivity, using preconcentration, and improved accuracy. The determination of phosphorus in iron- and copper-based alloys by plasma emission is severely hindered by spectral interferences. The most sensitive phosphorus lines at 213.618 and 214.914 nm are subject to interference from the copper ionic lines at 213.598 and 214.897 nm and several weak iron lines. The phosphorus line at 253.565 nm overlaps with the very strong iron line at 253.560 nm. Phosphorus lines in the vacuum UV (178.287 and 177.499 nm) cannot be reached with the commercial echelle spectrometer used with the directcurrent plasma (DCP) nor with many spectrometersused with the inductively coupled plasma (ICP). Thus, a separation prior to analysis is the most useful method for the determination of phosphorus in metals containing high concentrations of iron and copper. Several different approaches have been employed for the determination of phosphorus in metal alloys by plasma emission. Ward and Marciello (I)used a direct-reading ICP spectrometer with the 253.565-nm line for copper-based alloys and the 214.914-nm line for iron-based alloys. Spectral line interference corrections were made based on interferent
0003-2700/87/0359-2872$0 1.50/0 0 1987 American Chemical Society
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Flow c o n t r o l s w t c h e i Spectrometric
exch'onge Apple I l t
Activaftd alumina
Waste
P 10 pg/rnL
1 L
Eluent
HN03 worh
SomplelEluent
lb)
(0)
u *
2%
Flgure 1. Block diagrams of (a) cation exchange and (b) anion exchange chromatographic systems coupled to the DCP.
concentrations determined at the copper and iron channels. Their method of calibration required the determination of all elements present in significant concentrations. Detection limits for phosphorus in both copper-based and iron-based alloys varied from 30 to 90 pg/g. Xu et al. (2) used a wavelength-modulated echelle spectrometer with an ICP to determine phosphorus in steel a t the 213.618-nm phosphorus line. A detection limit of 6 pg/g in steel was reported but "suppression effeds" were observed if the copper concentration exceeded a solution concentration of 40 pg/mL. McLeod et al. (3) developed a method for the determination of phosphorus in steel by using an activated alumina column directly coupled to an ICP for preconcentration and separation. The method, based on previous work by Davies ( 4 ) and Nydahl (5),was limited by the system configuration (200-pL sample volume) to an aqueous detection limit of 0.6 pg/mL (60 pg/g in steel), approximately 5 times poorer than the limit for conventional nebulization. Wittmann and Schuster (6) used the ICP and an indirect method employing a vanado-molybdic complex and determination of molybdenum to obtain a detection limit of 0.4 pg/g phosphorus in steel. We investigated three methods for the determination of phosphorus in copper-based alloys: (1) a spectral-nulling procedure in which the zero-crossing of the interferent response function of a wavelength-modulated spectrometer is used to null out the interferent intensity; (2) a cation-exchange procedure in which the interferents are collected on a column and the phosphorus is not retained and flows directly into the DCP; (3) an anion-exchange procedure in which the phosphorus is preconcentrated on an activated alumina column and the interferents are not retained, with subsequent elution of the preconcentrated phosphorus for analysis. EXPERIMENTAL SECTION I n s t r u m e n t a t i o n . A block diagram of the two ion-exchange
systems coupled to the DCP emission spectrometer is shown in Figure 1. The output from both chromatographicsystems flowed directly into the DCP nebulizer, bypassing the DCP peristaltic pump, which was still used to drain the mixing chamber. The wavelength-modulated echelle spectrometer and detection electronics have been described previously (7). Entrance and exit slit widths of 200 wm were used to obtain the optimum signalto-noise ratio (SNR).A lock-in amplifier time constant of 1s was used for noise reduction and caused no loss of temporal resolution in the chromatograms. The DCP emission source was separated from the baseplate of the spectrometerto ensure thermal isolation of the spectrometer and to remove thermal-induced wavelength drift. Data were acquired and processed by an enhanced Apple 11+ microcomputer (3.5-MHz 65C02 coprocessor). Peak height measurements were used for quantitation. The cation exchange system (Figure la) used a Dionex HPIC-CS5cation-separatorcolumn with a Milton-Royminipump operating at 500 psi and a flow rate of 1 mL/min. Sample was introduced onto the column by using a 50-pL sample injection loop. The eluent was a solution 0.050 mol/L in oxalate and 0.095 mol/L in sodium hydroxide (pH 4.8).
Flgure 2. Comparison of instrument response at 213.618 nm for (a) dc detection and (b) wavelength-modulated detection of phosphorus and copper, using 25-pm slits on the echeile spectrometer.
The anion exchange system (Figure lb) used a microcolumn of activated alumina (aluminumoxide, Woelm acid, anionotropic, activity grade I for chromatography, M. Woelm-Eschwage, Germany, lot no. 446) placed directly in the sample introduction tubing line between the peristaltic pump and the DCP nebulizer. The output of the column could be switched between the nebulizer and waste container by using a two-valve manifold (flow injection valves, 30 psi, 12-V dc, Angar Scientific). The column was made from Pyrex glass tubing (0.4-cm o.d., 0.2-cm i.d., 5-cm packing length) with the alumina packing held between Pyrex glass wool plugs. Optimum performance of the column required care in preparation and use to avoid entrained air bubbles and channeling within the packing. The column was prepared in the following manner. An 8-cm length of glass tubing is cut and the ends are fire polished. The tube is rinsed with 0.1 % Triton-X surfactant in water, a 1-cm plug of wet Pyrex glass wool is inserted into the front end of the column,the column is again rinsed with Triton-X solution,and the entire column is vertically submersed in a water bath. A slurry of alumina, previously sized by washing and decantation as described by Davies ( 4 ) ,is slowly injected into the tube with a Pasteur pipet and allowed to settle. When a 5-cm packing length is reached, the column is sealed with a second plug of wet glass wool and kept under water until ready for use. The column is washed with water between preconcentration and elution steps and with 0.5 M nitric acid between the elution step and the next preconcentration. A solution of 1.5 M sodium hydroxide is used to elute the phosphorus collected on the column. Solution flow was controlled by a peristaltic pump (Gilson Minipuls 2, 8 channels) at 2.1 mL/min. Sample Preparation. Samples of SRM 1251and SRM 1252 (phosphorized copper, 1-2 g) were dissolved by using nitric and hydrochloric acids (8) and diluted to either 100 mL or 10 mL to yield sample digests containing either 2% copper and 6% acid or 10% copper and 30% acid, respectively. Samples of SRM 875 (cupro-nickel,approximately 2 g) were dissolved by using nitric and perchloric acids and diluted to 50 mL to yield sample digests containing 5% perchloric acid. Standards were prepared from high-purity ammonium dihydrogen phosphate (SRM 194) and serially diluted to appropriate concentrations. RESULTS AND DISCUSSION
on P h o s phorus. The choice of wavelength-modulation detection rather than dc detection was based on several factors. Figure 2 illustrates the spectral interference of the Cu 213.598-nm line on the P 213.618-nm line when using both dc and wavelength-modulated detection with the highest resolution available with the echelle spectrometer. The theoretical spectral band-pass at 213.618 nm, using 25-pm slits, is approximately 0.002 nm. However, at this band-pass, the total line width is limited by the element line widths in the DCP, rather than by the spectrometer optics, so the total line width is approximately 0.005 nm for elements at low concentrations S p e c t r a l I n t e r f e r e n c e of Copper and I r o n
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Table I. Wavelengths Available for the Determination of Phosphorus Using the DCP“ wave-
length,
interferent response,c wg/mL P Cu ( 2 % ) Fe (0.05%)
nm
SNRb
213.618 2 13.6 18 214.911 253.565
2.5 9.4 5
24 (330)e 48 23
ND ND ND
6.7
ND
28
detectiond WM DC DC DC
Comparison made at 25-pm slits for maximum spectrometer resolution. * Signal-to-noise ratio for 10 pg/mL of phosphorus, where noise = 3/5 the peak-to-peak noise on base line. ‘Apparent P concentration due to presence of interfering element; ND = none detected. dWM = wavelength modulation; DC = dc detection. e Interferent response at 100-pm slit width. (no self-absorption broadening). The SNR of the dc measurement as shown in Figure 2a is approximately 4 times better than the wavelength-modulated measurement shown in Figure 2b. This is a worst-case situation for the comparison of dc detection with wavelength modulation. Such a situation occurs under detector-noise-limited conditions (i.e., photomultiplier dark current noise) (9) when sinusoidal modulation and second harmonic detection are employed. As the spectral band-pass is increased, the SNR of the wavelength-modulated measurement improves relative to that of dc detection, since the plasma background flicker noise makes a greater contribution to the total system noise. With 200-pm slit widths at the P 213.618-nm line, the SNR for 10 pg/mL of phosphorus was 21 for the wavelength-modulated measurement and 12 for the dc measurement. Furthermore, high alkali element concentrations often used in eluent solutions cause significant changes in the plasma configuration and result in a shift in the base-line of a dc measurement. A 1.5 M solution of sodium hydroxide causes a base-line shift equivalent to 6 pg/mL of phosphorus in a dc measurement while no shift is observed when using wavelength modulation. Since transient signals are generated by both ion-exchange systems, the rapid background correction of the wavelength-modulation system reduces measurement bias and imprecision. The copper interference observed with the wavelengthmodulation system is significantly less than that observed with the dc system, since the second harmonic content of the interfering line response function is lower. The negative signal generated by the copper line in Figure 2b results from the interaction of the wavelength-modulation system with the negative curvature of the extended line wing and not the absolute intensity reaching the detector, as in the case of dc detection. This is the basis of the success of Xu et al. (2) who were able to determine phosphorus in the presence of low concentrations of copper in steel. However, as those authors observed, direct determination of phosphorus in the presence of very high copper concentrations is impossible at this phosphorus wavelength, even when using the high-resolution capability of the echelle spectrometer. Table I summarizes the wavelengths available for the determination of phosphorus a t concentrations of less than 10 pg/mL with the DCP and the effect of copper and iron spectral overlaps on these lines. Spectral Nulling of the Copper Spectral Interference. An alternative to spectroscopicallyresolving the emission line overlap is available when wavelength modulation is employed as a detection system. Spectral interferences can be “nulled” by adjusting the modulation parameters (i.e., the wavelength modulation interval and the wavelength position) so that the zero-crossing of the interferent response function coincides with the maximum of the analyte response function. While this has been shown to work well in a flame atomizer (IO),we
were unable to obtain a stable zero-crossing point for the Cu 213.598-nm line. The zero-crossing method is not sensitive to flicker in the intensity of the interferent line but does require a stable shape of the interferent response function (i.e., the spectral line shape). The DCP has been shown to exhibit “self-reversal noise” (7), which is a flicker noise caused by variations in the concentration of absorbing species in the cool region of the DCP between the spectrometer and the excitation zone. Such a noise would destabilize the interfering element line shape (and thus the response function) from the DCP and would result in a fluctuation of the zero-crossing. It appears that the “zero-crossing”method is not applicable to the DCP when the interferent concentration is high enough to result in significant self-reversal. However, it may be successfully applied to the ICP if self-reversal noise is not significant in that source. Cation-Exchange Separation of Phosphorus from Interferents. A 50-pL aliquot of the sample injected onto the cation separator column produced a transient phosphorus signal with a height approximately 20% of the steady-state signal generated by continuous aspiration of the sample. The relative standard deviation of the peak height for 50 replicate 50-pL injections of a 100 pg/mL phosphorus solution at the P 213.618-nm line was 0.95%. The detection limit, defined as 3 times the standard deviation of the base-line noise, was 1.5 pg/mL, approximately an order of magnitude worse than the continuous aspiration detection limit of 0.2 pg/mL based on replicate 10-9 integrations. Any chromatographic separation inherently involves some sample dilution. This fact combined with the limited injection volume mentioned results in a lower signal amplitude than that obtained for steady-state introduction of the analyte into the emission source. Furthermore, the transient signal generated by the chromatographic system requires a faster response time of the detection system than a steady-state signal (1s compared to 10 s), and thus the increased noise bandwidth of the measurement should result in an increase in noise by a factor of 3 (in the case of shot noise). Therefore, an order of magnitude decrease in SNR is to be expected under the instrumental conditions employed. Detection limits at 253.565 nm were 2.1 pg/mL for the chromatographically coupled DCP system and 0.3 p g / d for continuoussample introduction into the DCP. The calibration curves for all measurements were linear to 250 pg/mL. Figure 3 illustrates the separation of copper and phosphorus in (a) an artificial mixture of 70 pg/mL phosphorus in the presence of 1% copper and in (b) SRM 1251 on the CS5 column, using detection at 213.618 nm. At a flow rate of 1 mL/min, phosphorus elutes with the solvent front after approximately 2 min while the copper is retained on the column and elutes in approximately 3 min. Phosphorus was determined in the 2% acid digests of SRM 1251 by using the cation-exchange column and DCP detection at the P 213.618-nm line and in the 10% acid digest a t the P 253.565-nm line. Table I1 summarizes the results, which are in good agreement with other analyses of the same SRM by the molybdivanadophosphoric acid method (ASTM Procedure E62) (11)and by direct DCP analysis (no chromatographic separation) at the P 253.565-nm line. The direct DCP analysis at the P 253.565-nm line required the separate determination of iron (264 pg/g) and correction for a 30% signal enhancement caused by the iron spectral interference at this line. Complete separation of phosphorus and copper was obtained in the 2% acid digest for analysis at the P 213.618-nm line, but the capacity of the cation-exchange column was exceeded by the higher concentration of copper in the 10% acid digest. Therefore, the P 213.618-nm line could not be used for analysis of the 10% acid digest. The less
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
2875
n (a 1
1: preconcentrate
I wash
1 - '1
\ e l u t e phosphorus
k
,
0
2
I
4
6
TIME AFTER INJECTION ( M I N I
Flgure 3. Chromatograms generated by using the cation-exchange system and illustrating (a) the resolution of a mixture of 1 % copper and 70 pg/mL phosphorus and (b) the separation of phosphorus from the copper matrix in the analysis of SRM 1251.
Table 11. Determination of Phosphorus in SRM 1251 wave-
method"
length, nm
% Cub
ion-exchangeDCP
213.618
2
253.565
10
DCP (corrected)e ASTM E-62 method
253.565
P,' mg/g
calibrationd
0.42 f 0.04 direct 0.44 0.05 std addition 0.31 0.04 direct
* *
0.45 f 0.08 std addition 0.44 f 0.02 std addition 0.39 0.01
" Analysis method. Concentration of copper in sample solution analyzed. Uncertainty expressed as 95% confidence limits. dMethod of calibration: direct = from the calibration curve; std addition = method of standard additions. CDCPanalysis corrected for the iron intensity at 253.565 nm. sensitive P 253.565-nm line, which is not affected by copper emission, was successfully used for the analysis. Iron in the sample was retained by the column so that no correction for the iron spectral interference was necessary. However, the method of standard additions was necessary to correct for a suppression of the phosphorus signal caused by coelution of copper at the higher copper concentration. This approach to separation of phosphorus from copper and iron, in which the interferents are retained on the column while the analyte elutes with the solvent front, minimizes band spreading and maximizes chromatographic resolution. However, the sample size is limited by the capacity of the column to retain the interferenta, and on-line preconcentration of the analyte is not possible. Since the detection limit of the cation-exchange system is only 75 pg/g (in the metal alloy samples), a more sensitive method is required to determine phosphorus at the lower concentrations of interest to the metals industry. This led us to investigate anion-exchange procedures for separation and preiconcentration of phosphorus.
Flgure 4. Chromatograms generated by using the activated alumina column preconcentration and Illustrating (a) the analysis cycle for 1 wg/mL phosphorus standard solution preconcentrated for 10 min and (b) the signal generated by 2.5 pglmL phosphorus In the presence of 6% copper, preconcentrated for 3 mln.
Anion Exchange Separation of Phosphorus from Interferents. McLeod, Cook, and their research collaborators (3, 12, 13) have been the leading proponents of the use of activated alumina as a column packing material for adsorption of oxyanions, including phosphate. However, rather than use the fixed sampling loop employed by these authors, we defined the sample volume by using the constant flow rate controlled by the peristaltic pump and flow times controlled by switching valves. During column loading, the sample solution flow through the column was directed to waste while dilute (0.5 M) nitric acid flowed into the DCP. (If necessary, solution flowing through the column could be directed to the DCP to monitor column conditions.) After preconcentration was completed, the column was washed for 4-8 min with water to remove interferents (Cu, Fe) remaining on the column. The length of wash cycle required for a specific interferent concentration and preconcentration time could be determined by directing the column flow stream to the DCP while monitoring the base-line emission signal, as shown in Figure 4b. Completion of the column wash was indicated by disappearance of the negative signal generated by the copper spectral interference. Water, rather than dilute nitric acid, was used for the column wash prior to phosphorus elution by sodium hydroxide, since we observed some removal of preconcentrated phosphorus from the column by dilute acid. No loss was observed for a water wash. Phosphorus was eluted from the column with 1.5 M sodium hydroxide. Figure 4a shows the entire preconcentration/ wash/elution cycle for a 1 pg/mL phosphorus standard solution preconcentrated for 10 min. The signal maximum is reached approximately 30 s after the sodium hydroxide solvent front reaches the plasma; the signal plateaus for 5-10 s and then decreases until it reaches base line in another 3 min. In practice, the column flow is switched to waste after the maximum is reached, as shown in Figure 4b. While the DCP is far less affected by high concentrations of alkali elements than the ICP, continuous introduction of 1.5 M sodium hydroxide will hasten the degradation of torch components,
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Table 111. Determinationof Phosphorus in SRM 1252 and SRM 875 Using Preconcentration on an Activated Alumina Column sample
% Cu"
P,bp g / g
calibrationc
ref valuesd
SRM 1252 phosphorized copper
2
84 f 8
std addition 85 f 2 (ASTM E-62)
