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Microdetermination of phosphorus in organic materials from polymer industry by microwave-induced plasma atomic emission spectrometry after microwave digestion
Microchemical Journal 70 Ž2001. 41᎐49
Microdetermination of phosphorus in organic materials from polymer industry by microwave-induced plasma atomic emission spectrometry after microwave digestion Krzysztof JankowskiU Department of Analytical Chemistry, Faculty of Chemistry, Warsaw Uni¨ ersity of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland Received 13 December 2000; received in revised form 6 June 2001; accepted 8 June 2001
Abstract Microwave digestion with nitric acid precedes the determination of phosphorus in different materials from the polymer industry by microwave-induced plasma atomic emission spectrometry ŽMIP-AES.. Atomic emission is measured at the P I 213.618-nm line. The experimental detection limit ŽDL. is 150 ng mly1. The accuracy of the method was evaluated with the analysis of certified reference material NIST SRM 1568A Rice Flour. For comparison, the spectrophotometric determination of phosphorus by the phosphomolybdenum blue method was performed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: materials
Atomic emission spectrometry; Microwave-induced plasma; Microwave digestion; Phosphorus; Polymer industry
1. Introduction The phosphorus content determination in materials of organic origin is widely investigated w1,2x. The analytical procedure consists most often
U
Fax: q48-2-628-33-39. E-mail address: [email protected] ŽK. Jankowski..
of the decomposition of the sample during which organic substances causing interferences in phosphorus determination are removed, and moreover, the element determined is converted from the organophosphorus compound to the orthophosphate ion form and as such is determined by various methods. Oxygen-flask combustion w3x is the most common applied decomposition method of phospho-
0026-265Xr01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 . 0 0 0 9 7 - 2
42
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
rus-containing samples, where the residue after combustion is absorbed in a mineral acid solution. Microwave digestion with the use of oxidizing acids is the other common sample pretreatment technique w4x. In many cases phosphorus occurs in the form of volatile compounds or the intermediates formed during decomposition are volatile, which may lead to considerable losses. Therefore, closed vessels are recommended for the digestion of the substances studied. The necessity of quantitative conversion of phosphorus into orthophosphates before measurement to obtain an exact result of phosphorus content determination is usually highlighted. Some analytical procedures contain additionally a step of phosphorus oxidation in the digest w3,5x. A number of spectroscopic methods for the determination of phosphorus have been elaborated. The spectrophotometric methods consist in the formation of colored phosphate heteropolyacids or corresponding blues w6x. However, silicon, arsenic and germanium can also form heteropolyacids under the same conditions. Moreover, the formation of heteropolyacids may be affected by the presence of metals forming sparingly soluble phosphates, e.g. of barium, lead and zinc. The determination selectivity can be increased by separating phosphorus by extraction, but this complicates and prolongs the course of analysis. However, the spectroscopic method described is exact and sensitive Žthe lowest phosphorus concentration that can be determined is approx. 0.3 g mly1 ., and therefore readily applied. Saliman w7x applied the phosphomolybdenum blue method for phosphorus determination in organic materials. The fact that the presence of phosphates suppresses calcium emission is utilized in the indirect phosphate determination by flame emission spectrometry. The presence of other metals, e.g. barium and zinc have a great effect on the emission intensity measured w8x. Direct phosphate determination by atomic absorption spectroscopy is carried out in the vacuum ultraviolet range, where sensitive phosphorus lines occur. Vigler et al. w9x utilized the HGA-AAS method for the determination of phosphorus in organic samples. The element of interest present in polyester samples
as alkyl-phenyl phosphite was directly vaporized in the atomizer. Lonardo et al. w10x determined phosphorus in polymer samples by laser excited atomic fluorescence spectrometry, and for comparison carried out the same determination employing the ETAAS and ICP-AES methods after sample digestion. For AFS, AAS and AES they obtained detection limits ŽDLs. of 0.4, 550 and 2 ng mly1 , respectively. However, the linear dynamic range for the AFS method extended over approximately five orders of magnitude and was the widest in comparison with the other methods. The comparison of sensitivity and linear ranges for other methods of phosphorus determination was also made. In recent years plasma atomic emission spectrometry techniques for the determination of organic phosphorus are widely investigated. They offer detection limits that are far lower than required and permit precise phosphorus determination over a wide concentration range, often with the omission of the sample digestion procedure. Andreau et al. w11x determined phosphorus by ICP-AES ŽDL approx. 70 ng mly1 . in the presence of 10 or so metals in engine lubricating oil by dissolving it by means of kerosene. Nagourney and Madan w12x applied a mixture of 2butoxyethanol and 2-ethylhexanoic acid for dissolving plastics additive samples, in order to determine the content of phosphorus, barium, cadmium and zinc by the ICP-AES method. For comparison, they carried out the analysis of aqueous solutions of samples obtained by ashing and acid dissolution pretreatment by means of the same method. Plaami and Kumpulainen w13x separated phytic acid from cereals and determined phosphorus by ICP-AES, thus eliminating the necessity of applying the difficult acid digestion. Microwave-induced plasma hyphenated to gas chromatography is routinely utilized in the analysis of organic microsamples w14x. The sensitivity of the method is usually reported as 1.5 pg sy1 . Becker et al. w15x determined an empirical formulae for a series of alkylated and arylated phosphites by GC-MIP-AES. For various phosphoruscontaining pesticides an identical measurement sensitivity is reported w16,17x. It suggests that the organic part of the compounds studied does not
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
act as a chemical or physical interference. However, for helium MIP-AES with aqueous solution nebulization a dependence of the calibration curve slope on the chemical composition of the phosphorus-containing compound was observed w18x. In this work the low-power argon MIP-AES method was applied for the first time for the determination of aqueous phosphorus in samples from the polymer industry decomposed by microwave digestion. The phosphorus content was also determined spectrophotometrically with the phosphomolybdenum blue method. The validity of the method was evaluated by analyzing a certified material: NIST SRM 1568A Rice Flour.
2. Experimental 2.1. Apparatus and measurement conditions A MIP 750-MV ŽAnalab Ltd, Warsaw, Poland. atomic emission spectrometer equipped with the vertically positioned aerosol cooled plasma system ŽPlazmatronika-Service, Wroclaw, Poland. previously described w19x and the ultrasonic nebulizer Žlaboratory-made. w20x were used for atomic emission measurements. The plasma operating conditions are listed in Table 1. Spectrophotometric measurements were performed by means of Specord M-40 ŽCarl Zeiss, Jena, Germany. with 1-cm cells. A microwave digestion unit UniClever TM BM-1z ŽPlazmatronika-Service, Wroclaw, Poland. operating in the pressure-control mode was used for the decomposition of samples. Table 1 Instrumental parameters for MIP-AES system Microwave frequency ŽMHz. Microwave power ŽW. Plasma viewing mode Plasma gas Gas flow rate Žml miny1 . Sample flow rate Žl miny1 . Integration time Žs. Analytical wavelength Žnm.
2450 170 Axial Argon, 99.998% 180 30 0.5 P I 213.618
43
2.2. Reagents and standards All solutions were prepared from analytical reagent grade chemicals and de-ionized water. The inorganic phosphorus stock solution Ž1 g ly1 . was obtained by dissolving a weighed amount of disodium hydrogen phosphate ŽDHP. ŽMerck, Darmstadt, Germany. in water. Calibration solutions for emission spectrometry and spectrophotometry measurements were obtained by successive dilution of the stock solution with 2% nitric acid and water, respectively. The organic phosphorus stock solution Ž0.2 g ly1 . was obtained by dissolving a weighed amount of phenyl disodium phosphate ŽPDP. ŽMerck, Darmstadt, Germany. in water. Calibration solutions were obtained by successive dilution of the stock solution with water. For microwave digestion 65% nitric acid GR for analysis and sodium nitrate ACS ŽMerck, Darmstadt, Germany. were used. NIST SRM 1568A Rice Flour was applied as reference material. The following solutions were prepared for spectrophotometric determinations: Potassium periodate, 0.5% solution in 0.1 M sulfuric acid. Hydrazine sulfate, 1% and 0.1% solution. Sulfuric acid, 0.5 M solution. Ammonium molybdate, 1% solution. 2.3. Microwa¨ e digestion procedure A 0.1-g sample of high phosphorus content was weighed into a 110-ml PTFE digestion vessel. Then, 5 ml of HNO3 and 0.05 g of NaNO3 were added and the vessel was covered. The sample was heated according to the program presented in Table 2. After cooling, the solution was evaporated to approximately 2 ml, transferred to a 100-ml volumetric flask and diluted to the mark with water. A 0.1᎐0.2-g sample of low phosphorus content was weighed into a 110-ml PTFE digestion vessel. Then, 5 ml of HNO3 and 0.01 g of NaNO3 were added and the vessel was covered. The sample was heated according to the program presented in
44
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
Table 2 Operating conditions for digestion system Maximum power ŽW. Pressure level Žatm. PTFE vessel volume Žml. Program
300 42᎐44 110 10% powerr5 min 80% powerr15 min 100% powerr20 min
Table 2. After cooling, the solution was evaporated to approximately 0.2 ml, transferred to a 10-ml volumetric flask and diluted to the mark with water. 2.4. MIP-AES measurements Calibration solutions of inorganic as well as organic phosphorus were nebulized with the ultrasonic nebulizer and introduced as wet aerosol into the MIP under operating conditions given in Table 1. The linear dynamic range within 0.5᎐200 g mly1 of phosphorus was evaluated. At least five replicates were determined for each data point in order to calculate the linear dynamic range, detection limit and short-term precision Žexpressed as %R.S.D.. in each case. 2.5. Spectrophotometric determination An aliquot of the solution after decomposition containing up to 80 g of phosphorus was placed in a beaker. It was diluted to the volume of approximately 10 ml, after which 1 ml of the potassium periodate solution was added and the whole was heated at approximately 70⬚C. After 15 min 1% hydrazine sulfate solution was added dropwise, in order to reduce the periodate remaining in the solution, until the moment when the solution did not color after the addition of a consecutive portion of hydrazine. Then the solution was evaporated nearly to dryness. The sample was transferred by means of 10 ml of 0.5 M sulfuric acid solution from the beaker to a 50-ml measuring flask. Then, consecutively, 2.5 ml of ammonium molybdate solution and 2.5 ml of hydrazine sulfate solution Ž0.1%. were added and the whole was filled with water to the mark. The flasks were heated in a boiling water bath for
10 min and then cooled. The absorbance of the solution was measured with respect to the blank at a wavelength of 829 nm w21x. The phosphorus content was determined on the basis of the standard curve in the 0.2᎐1.6-g mly1 range of phosphorus. The solutions for making the standard curve were prepared from inorganic phosphorus stock solution by appropriate dilution.
3. Results and discussion 3.1. Optimization of digestion procedure In order to carry out phosphorus determination in the samples studied it is necessary to apply an efficient decomposition method assuring complete holding of phosphorus in the solution. However, some organic phosphorus compounds are quite volatile and may be easily lost during heating of the sample. In preliminary tests of decomposition of some phosphorus compounds by means of acid digestion in an opened vessel a loss of up to 30% of phosphorus was observed. Microwave digestion for the preparation of the phosphorus-containing sample has advantages compared with oxygen flask combustion. The precision of the latter is lower due to the smaller quantity of sample used, especially at low phosphorus content. Moreover, using oxygen flask combustion the completeness of the destruction of the organic matter is not achieved for some types of polymer materials and solutions. Since decomposition of polymers proceeds generally with difficulty the rapid heating of the sample and the high pressure generated in a closed container can result in the reduction of preparation time. Then a microwave digestion system operating at pressure-control mode was applied for sample decomposition. Nitric and sulfuric acids are the most common reagents for this type of digestion. However, the presence of sulfuric acid in the resulted sample solution caused a significant decrease in the emission intensity of phosphorus. Nitric acid is successfully applied for the digestion of plastic materials w22x. However, during the decomposition of samples of considerable phosphorus content par-
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
tial volatilization of phosphorus compounds occurs Žlosses maximally 5%.. In order to cause complete recovery of phosphorus in the solution, sodium nitrate was applied, which increases the boiling point of the solution and is necessary for forming a phosphate salt. The addition of metal oxides or salts to the sample to hold phosphorus is described in various analytical procedures w7,9,23x. The application of nitric acid with the addition of sodium nitrate permitted complete recovery of phosphorus Ž98.9" 0.9%. in the analyzed samples under digestion conditions presented in Table 2. After cooling, the excessive nitric acid was evaporated to obtain finally 2% concentration of the acid in the diluted solution. A reagent blank was also prepared by the same procedure. 3.2. Determination by the MIP-AES method Atomic emission spectrometry with plasma sources is widely employed for elemental analysis, including determination of phosphorus in various materials, owing to fairly low detection limits and wide linear dynamic range. MIP-AES provides, besides more popular ICP-AES, good precision and accuracy of determination. The lowest DL 4.5 ng mly1 obtained with helium MIP was reported by Jin et al. w24x at the phosphorus 213.6-nm line. This value is comparable with DL 2 ng mly1 obtained with ICP-AES at the most sensitive 178.3-nm line in the vacuum-UV region w10x. In contrast, the DL obtained by Perkins and Long w25x with helium MIP was 400 ng mly1 . The determination of aqueous phosphorus by HeMIP-AES was also reported by Wu and Carnahan w18x. The detection limit was 1400 ng mly1 and linearity in the concentration ranged from 10 to 10 000 g mly1 . However, the helium plasma is not resistant of loading large amounts of water and moderate or high power sources should be used. The argon microwave discharge seems to be much more capable of tolerating the presence of the solvent in the plasma. A limited number of data on the phosphorus emission lines in argon MIP are accessible w26,27x. The spectrum was obtained in the spectral 200᎐260-nm region in which the more intense
45
Table 3 Observed emission lines of phosphorus Žabove 200 nm. in various plasmas Wavelength Žnm.
202.347 203.349 213.547 213.618 214.914 215.294 215.408 253.401 253.565 255.328 255.490
Intensity Ar-MIP this work
He-MIP w26x
Ar-ICP w27x
1 3 8 100 65 4 7 11 55 20 9
2 7 ᎐ 100 65 5 7 15 64 29 13
10 19 22 100 100 9 18 8 28 13 ᎐
lines of phosphorus appear with the Ar-MIP, excluding the vacuum-UV region unavailable in this study. The wavelengths identified and respective relative intensities, normalized with respect to the intensity of the most intense line, are collected in Table 3. The most intense phosphorus emission is observed at 213.618 nm and it is consistent with that reported by Tanabe et al. w28x for He-MIP and Winge et al. w29x for Ar-ICP. The effect of argon flow rate on the relative emission intensity of phosphorus is shown in Fig. 1. The greatest intensity of emission was obtained at a relatively low gas flow of approximately 180 ml miny1 . A similar result was obtained previously in our laboratory for a somewhat different MIP system w19x. The optimal gas flow for a majority of elements studied earlier was approximately 250 ml miny1 w30x. Thus, for efficient phosphorus excitation a longer residence time in the plasma is required. This is connected probably with the necessity of cleaving stable phosphorus᎐oxygen bonds during sample atomization. This is supported by the observed effect of microwave power on relative emission intensity of phosphorus shown in Fig. 2. The signal-to-background ratio sharply increases with the increase of the power level. For the studies of detection limits and linear responses disodium hydrogen phosphate and phenyl disodium phosphate were used for solu-
46
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
Fig. 1. The effect of microwave power on signal-to-background ratio ŽSBR. for phosphorus solution at 25 g mly1 .
tion preparation. The detection limits based on the 3 concept and seven replicate measurements were 85 and 92 ng mly1 for inorganic and organic phosphorus, respectively. The DLs are similar to those obtained for phosphorus with argon ICP-AES at 213.6 nm, namely 76 ng mly1 by Winge et al. w29x and 70 ng mly1 by Andreau et al. w11x. Both molecular forms of phosphorus produced good linearity in the concentration range of 0.5᎐130 g mly1 . A negligible effect of the molecular form on phosphorus emission intensity was found. At the concentrations higher than 100 g mly1 the intensity of phosphorus in organic form was 98% that of the inorganic form. However, the difference in short-term precision Ž%R.S.D.. was more clear: 1.86 for DHP and 2.54 for PDP. From a practical point of view, the observation of the lack of effect of the molecular form of these two substances studied permits to assume that in the samples studied from the polymer industry the measured phosphorus emission in-
tensity would not essentially depend on the chemical form of phosphorus in the solution after microwave digestion. A similar result was obtained earlier when elaborating the method of phosphorus determination by MIP-AES in polymer additives containing phosphites w31x. 3.3. Spectrophotometric determination A common method based on the formation of phosphomolybdenum blue was proposed for the determination of phosphorus in the form of phosphates, by means of which phosphorus can be determined in the quantity of 1᎐80 g w6x. The method is sensitive and reproducible, and in the case when no other elements are present in the sample forming heteropolyacids, also selective and simple in performance. Preliminary investigations show that after performing microwave digestion in the samples studied of high phosphorus content Žsee Table 4. only part of phosphorus is determined Žapprox. 90%..
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
47
Fig. 2. The effect of argon flow rate on signal-to-background ratio ŽSBR. for phosphorus solution at 25 g mly1 .
It has been assumed that organic phosphorus may be non-quantitatively converted into orthophosphate. Potassium periodate was applied for oxidizing phosphorus in the digest. The excess of periodate was reduced with hydrazine, and the excess of hydrazine decomposed during further heating of the sample.
In the case of phosphorus analysis in PDP and tri-isodecyl phosphite ŽTDP. the use of periodate and subsequent reduction with hydrazine caused an increase in absorbance by approximately 10%. Simultaneously, no effect of the oxidation with periodate on the phosphorus content determined in all the samples studied was observed when the
Table 4 Determination of phosphorus in samples and standard reference material NIST SRM 1568A Rice Flour Sample
Phenyl disodium phosphate ŽPDP. Tri-isooctyl phosphite ŽTDP. Modified polyaniline ŽMP. MP solution in m-cresol Polyvinylchloride ŽPVC. SRM 1568A Rice Flour a
For five determinations.
P content Ž%. Expected
Found with MIP-AESa
Found with spectrophotometry a
12.19
11.98" 0.30
11.91" 0.35
6.17
6.07" 0.16 0.116" 0.004 0.020" 0.001 0.067" 0.002
6.10" 0.18 0.113" 0.004 0.021" 0.001 0.062" 0.003
0.149" 0.006
0.148" 0.007
Certified value 0.153" 0.008
48
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
analysis was carried out by the MIP-AES method. This leads to the conclusion that the addition of potassium periodate followed by reduction with hydrazine in the case of the spectrophotometric method of determination has an effect on the colored reaction medium and not on the form of the element determined. In this way the interference by nitric acid on the phosphomolybdenum blue formation is probably eliminated. The other possible explanation is that the chemical form of phosphorus in the solution has a negligible effect on the results obtained by the MIP-AES method. The method elaborated is characterized by good precision Žrelative error approx. 3%.. However, the overall precision of determination is worse due to the large effect of the sample preparation stage, when it is difficult to achieve complete decomposition of the organic compounds without causing simultaneous liberation of part of phosphorus. The results and precision of determinations are presented in Table 4. 3.4. Sample analysis Six samples, including NIST SRM 1568A Rice Flour certified reference material, were analyzed to evaluate the feasibility of the MIP-AES method. The phosphorus content by the spectrophotometric method was additionally determined. Since no reference material of certified phosphorus content appropriate for the analysis of products from the polymer industry was found, a food product was used, as it is an organic material of a similar phosphorus content. As shown in Table 4, the results obtained both by the MIP-AES and spectrophotometric method are comparable. The results obtained for the reference material are also in good agreement with the certified value. Phosphorus content found in PDP is somewhat lower than expected, presumably, as a result of phosphorus losses during digestion. Additionally, the phosphorus content in the PDP sample declared by the manufacturer was at least 98%. For the MP solution sample the accuracy was evaluated by the recovery of phosphorus from the spiked samples. Each sample was weighed into a PTFE vessel for microwave digestion as described
above. Some samples were spiked with a known amount of PDP and decomposed in the same conditions. The spike recovery was 98.1" 2.1%.
4. Conclusion The potential of the Ar-MIP-AES for the determination of phosphorus in organic substances after sample digestion was investigated. Sample pretreatment has an essential effect on the accuracy and precision of the result obtained as well as on the time of performing the determination. The studies carried out show the necessity of performing sample digestion under conditions assuring complete decomposition of the organic part and preventing phosphorus losses from the sample. The utilization of microwave digestion with nitric acid and sodium nitrate at pressure-controlled mode assures complete recovery of the analyte. The proposed MIP-AES method has proven to be a simple and sensitive technique for the determination of phosphorus in organic materials from the polymer industry. The spectrophotometric method based on formation of phosphomolybdenum blue exhibits the similar analytical performance, however, more complicated sample preparation is necessary. References w1x T.R. Crompton, Comprehensive Organometallic Analysis, Plenum Press, New York, 1987. w2x L. Mazor, Methods of Organic Analysis, Akademiai Kiado, Budapest, 1983. w3x L. Maric, ´ M. ˇSiroki, Z. ˇStefanac, Microchem. J. 21 Ž1976. 129. w4x H.M. Kingston, L.B. Jassie, Introduction to Microwave Sample Preparation: Theory and Practice, American Chemical Society, Washington, DC, 1988. w5x V. Zatka, Ropa Uhlie 10 Ž1968. 413. w6x Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Ellis Horwood, Chichester, John Wiley, New York, 1986. w7x P.M. Saliman, Anal. Chem. 36 Ž1964. 112. w8x E. Szebenyi-Gyory, P.J. Slevin, G. Svehla, L. Erdey, Talanta 17 Ž1970. 1167. w9x M.S. Vigler, A. Strecker, A. Varnes, Appl. Spectrosc. 32 Ž1978. 60.
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49 w10x R.F. Lonardo, A.I. Yuzefovsky, K.X. Yang, R.G. Michel, E.S. Frame, J. Barren, J. Anal. At. Spectrom. 11 Ž1996. 279. w11x C. Andreau, K.J. Fredeen, M. Thomsen, D.A. Yates, At. Spectrosc. 16 Ž1995. 79. w12x S.J. Nagourney, R.K. Madan, J. Test. Eval. 19 Ž1991. 77. w13x S. Plaami, J. Kumpulainen, J. Assoc. Off. Anal. Chem. 74 Ž1991. 32. w14x J.J. Sullivan, Trends Anal. Chem. 10 Ž1991. 23. ¨ w15x G. Becker, A. Colmsjo, Anal. Chim. Acta ¨ C. Ostman, 340 Ž1997. 181. w16x C.A. Bache, D.J. Lisk, J. Assoc. Off. Anal. Chem. 50 Ž1967. 1246. w17x B. Riviere, J.M. Mermet, D. Deruaz, J. Anal. At. Spectrom. 2 Ž1987. 705. w18x M. Wu, J.W. Carnahan, J. Anal. At. Spectrom. 7 Ž1992. 1249. w19x K. Jankowski, R. Parosa, A. Ramsza, E. Reszke, Spectrochim. Acta, Part B 54 Ž1999. 515. w20x K. Jankowski, D. Karmasz, A. Ramsza, E. Reszke, Spectrochim. Acta, Part B 52 Ž1997. 1813.
49
w21x N. Obarski, private communication. w22x A. Vollrath, T. Otz, C. Hohl, H.G. Seiler, Fresenius J. Anal. Chem. 344 Ž1992. 269. w23x W.S. Brammell, J. Assoc. Off. Anal. Chem. 64 Ž1981. 808. w24x Q. Jin, H. Zhang, D. Ye, J. Zhang, Microchem. J. 47 Ž1993. 278. w25x L.D. Perkins, G.L. Long, Appl. Spectrosc. 43 Ž1989. 499. w26x Q. Jin, Y. Duan, J.A. Olivares, Spectrochim. Acta, Part B 52 Ž1997. 131. w27x D. Kollotzek, P. Tschopel, G. Tolg, ¨ ¨ Spectrochim. Acta, Part B 39 Ž1984. 625. w28x K. Tanabe, H. Haraguchi, K. Fuwa, Spectrochim. Acta, Part B 36 Ž1981. 119. w29x R.K. Winge, V.J. Peterson, V.A. Fassel, Appl. Spectrosc. 33 Ž1979. 206. w30x K. Jankowski, J. Anal. At. Spectrom. 14 Ž1999. 1419. w31x K. Jankowski, A. Jerzak, K. Bujnowski, A. SernickaPoluchowicz, L. Synoradzki, Method of determination of phosphorus in polymer additives, especially for PVC processing, Pat. PL 172314 Ž1994..
Microdetermination of phosphorus in organic materials from polymer industry by microwave-induced plasma atomic emission spectrometry after microwave digestion Krzysztof JankowskiU Department of Analytical Chemistry, Faculty of Chemistry, Warsaw Uni¨ ersity of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland Received 13 December 2000; received in revised form 6 June 2001; accepted 8 June 2001
Abstract Microwave digestion with nitric acid precedes the determination of phosphorus in different materials from the polymer industry by microwave-induced plasma atomic emission spectrometry ŽMIP-AES.. Atomic emission is measured at the P I 213.618-nm line. The experimental detection limit ŽDL. is 150 ng mly1. The accuracy of the method was evaluated with the analysis of certified reference material NIST SRM 1568A Rice Flour. For comparison, the spectrophotometric determination of phosphorus by the phosphomolybdenum blue method was performed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: materials
Atomic emission spectrometry; Microwave-induced plasma; Microwave digestion; Phosphorus; Polymer industry
1. Introduction The phosphorus content determination in materials of organic origin is widely investigated w1,2x. The analytical procedure consists most often
U
Fax: q48-2-628-33-39. E-mail address: [email protected] ŽK. Jankowski..
of the decomposition of the sample during which organic substances causing interferences in phosphorus determination are removed, and moreover, the element determined is converted from the organophosphorus compound to the orthophosphate ion form and as such is determined by various methods. Oxygen-flask combustion w3x is the most common applied decomposition method of phospho-
0026-265Xr01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 . 0 0 0 9 7 - 2
42
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
rus-containing samples, where the residue after combustion is absorbed in a mineral acid solution. Microwave digestion with the use of oxidizing acids is the other common sample pretreatment technique w4x. In many cases phosphorus occurs in the form of volatile compounds or the intermediates formed during decomposition are volatile, which may lead to considerable losses. Therefore, closed vessels are recommended for the digestion of the substances studied. The necessity of quantitative conversion of phosphorus into orthophosphates before measurement to obtain an exact result of phosphorus content determination is usually highlighted. Some analytical procedures contain additionally a step of phosphorus oxidation in the digest w3,5x. A number of spectroscopic methods for the determination of phosphorus have been elaborated. The spectrophotometric methods consist in the formation of colored phosphate heteropolyacids or corresponding blues w6x. However, silicon, arsenic and germanium can also form heteropolyacids under the same conditions. Moreover, the formation of heteropolyacids may be affected by the presence of metals forming sparingly soluble phosphates, e.g. of barium, lead and zinc. The determination selectivity can be increased by separating phosphorus by extraction, but this complicates and prolongs the course of analysis. However, the spectroscopic method described is exact and sensitive Žthe lowest phosphorus concentration that can be determined is approx. 0.3 g mly1 ., and therefore readily applied. Saliman w7x applied the phosphomolybdenum blue method for phosphorus determination in organic materials. The fact that the presence of phosphates suppresses calcium emission is utilized in the indirect phosphate determination by flame emission spectrometry. The presence of other metals, e.g. barium and zinc have a great effect on the emission intensity measured w8x. Direct phosphate determination by atomic absorption spectroscopy is carried out in the vacuum ultraviolet range, where sensitive phosphorus lines occur. Vigler et al. w9x utilized the HGA-AAS method for the determination of phosphorus in organic samples. The element of interest present in polyester samples
as alkyl-phenyl phosphite was directly vaporized in the atomizer. Lonardo et al. w10x determined phosphorus in polymer samples by laser excited atomic fluorescence spectrometry, and for comparison carried out the same determination employing the ETAAS and ICP-AES methods after sample digestion. For AFS, AAS and AES they obtained detection limits ŽDLs. of 0.4, 550 and 2 ng mly1 , respectively. However, the linear dynamic range for the AFS method extended over approximately five orders of magnitude and was the widest in comparison with the other methods. The comparison of sensitivity and linear ranges for other methods of phosphorus determination was also made. In recent years plasma atomic emission spectrometry techniques for the determination of organic phosphorus are widely investigated. They offer detection limits that are far lower than required and permit precise phosphorus determination over a wide concentration range, often with the omission of the sample digestion procedure. Andreau et al. w11x determined phosphorus by ICP-AES ŽDL approx. 70 ng mly1 . in the presence of 10 or so metals in engine lubricating oil by dissolving it by means of kerosene. Nagourney and Madan w12x applied a mixture of 2butoxyethanol and 2-ethylhexanoic acid for dissolving plastics additive samples, in order to determine the content of phosphorus, barium, cadmium and zinc by the ICP-AES method. For comparison, they carried out the analysis of aqueous solutions of samples obtained by ashing and acid dissolution pretreatment by means of the same method. Plaami and Kumpulainen w13x separated phytic acid from cereals and determined phosphorus by ICP-AES, thus eliminating the necessity of applying the difficult acid digestion. Microwave-induced plasma hyphenated to gas chromatography is routinely utilized in the analysis of organic microsamples w14x. The sensitivity of the method is usually reported as 1.5 pg sy1 . Becker et al. w15x determined an empirical formulae for a series of alkylated and arylated phosphites by GC-MIP-AES. For various phosphoruscontaining pesticides an identical measurement sensitivity is reported w16,17x. It suggests that the organic part of the compounds studied does not
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
act as a chemical or physical interference. However, for helium MIP-AES with aqueous solution nebulization a dependence of the calibration curve slope on the chemical composition of the phosphorus-containing compound was observed w18x. In this work the low-power argon MIP-AES method was applied for the first time for the determination of aqueous phosphorus in samples from the polymer industry decomposed by microwave digestion. The phosphorus content was also determined spectrophotometrically with the phosphomolybdenum blue method. The validity of the method was evaluated by analyzing a certified material: NIST SRM 1568A Rice Flour.
2. Experimental 2.1. Apparatus and measurement conditions A MIP 750-MV ŽAnalab Ltd, Warsaw, Poland. atomic emission spectrometer equipped with the vertically positioned aerosol cooled plasma system ŽPlazmatronika-Service, Wroclaw, Poland. previously described w19x and the ultrasonic nebulizer Žlaboratory-made. w20x were used for atomic emission measurements. The plasma operating conditions are listed in Table 1. Spectrophotometric measurements were performed by means of Specord M-40 ŽCarl Zeiss, Jena, Germany. with 1-cm cells. A microwave digestion unit UniClever TM BM-1z ŽPlazmatronika-Service, Wroclaw, Poland. operating in the pressure-control mode was used for the decomposition of samples. Table 1 Instrumental parameters for MIP-AES system Microwave frequency ŽMHz. Microwave power ŽW. Plasma viewing mode Plasma gas Gas flow rate Žml miny1 . Sample flow rate Žl miny1 . Integration time Žs. Analytical wavelength Žnm.
2450 170 Axial Argon, 99.998% 180 30 0.5 P I 213.618
43
2.2. Reagents and standards All solutions were prepared from analytical reagent grade chemicals and de-ionized water. The inorganic phosphorus stock solution Ž1 g ly1 . was obtained by dissolving a weighed amount of disodium hydrogen phosphate ŽDHP. ŽMerck, Darmstadt, Germany. in water. Calibration solutions for emission spectrometry and spectrophotometry measurements were obtained by successive dilution of the stock solution with 2% nitric acid and water, respectively. The organic phosphorus stock solution Ž0.2 g ly1 . was obtained by dissolving a weighed amount of phenyl disodium phosphate ŽPDP. ŽMerck, Darmstadt, Germany. in water. Calibration solutions were obtained by successive dilution of the stock solution with water. For microwave digestion 65% nitric acid GR for analysis and sodium nitrate ACS ŽMerck, Darmstadt, Germany. were used. NIST SRM 1568A Rice Flour was applied as reference material. The following solutions were prepared for spectrophotometric determinations: Potassium periodate, 0.5% solution in 0.1 M sulfuric acid. Hydrazine sulfate, 1% and 0.1% solution. Sulfuric acid, 0.5 M solution. Ammonium molybdate, 1% solution. 2.3. Microwa¨ e digestion procedure A 0.1-g sample of high phosphorus content was weighed into a 110-ml PTFE digestion vessel. Then, 5 ml of HNO3 and 0.05 g of NaNO3 were added and the vessel was covered. The sample was heated according to the program presented in Table 2. After cooling, the solution was evaporated to approximately 2 ml, transferred to a 100-ml volumetric flask and diluted to the mark with water. A 0.1᎐0.2-g sample of low phosphorus content was weighed into a 110-ml PTFE digestion vessel. Then, 5 ml of HNO3 and 0.01 g of NaNO3 were added and the vessel was covered. The sample was heated according to the program presented in
44
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
Table 2 Operating conditions for digestion system Maximum power ŽW. Pressure level Žatm. PTFE vessel volume Žml. Program
300 42᎐44 110 10% powerr5 min 80% powerr15 min 100% powerr20 min
Table 2. After cooling, the solution was evaporated to approximately 0.2 ml, transferred to a 10-ml volumetric flask and diluted to the mark with water. 2.4. MIP-AES measurements Calibration solutions of inorganic as well as organic phosphorus were nebulized with the ultrasonic nebulizer and introduced as wet aerosol into the MIP under operating conditions given in Table 1. The linear dynamic range within 0.5᎐200 g mly1 of phosphorus was evaluated. At least five replicates were determined for each data point in order to calculate the linear dynamic range, detection limit and short-term precision Žexpressed as %R.S.D.. in each case. 2.5. Spectrophotometric determination An aliquot of the solution after decomposition containing up to 80 g of phosphorus was placed in a beaker. It was diluted to the volume of approximately 10 ml, after which 1 ml of the potassium periodate solution was added and the whole was heated at approximately 70⬚C. After 15 min 1% hydrazine sulfate solution was added dropwise, in order to reduce the periodate remaining in the solution, until the moment when the solution did not color after the addition of a consecutive portion of hydrazine. Then the solution was evaporated nearly to dryness. The sample was transferred by means of 10 ml of 0.5 M sulfuric acid solution from the beaker to a 50-ml measuring flask. Then, consecutively, 2.5 ml of ammonium molybdate solution and 2.5 ml of hydrazine sulfate solution Ž0.1%. were added and the whole was filled with water to the mark. The flasks were heated in a boiling water bath for
10 min and then cooled. The absorbance of the solution was measured with respect to the blank at a wavelength of 829 nm w21x. The phosphorus content was determined on the basis of the standard curve in the 0.2᎐1.6-g mly1 range of phosphorus. The solutions for making the standard curve were prepared from inorganic phosphorus stock solution by appropriate dilution.
3. Results and discussion 3.1. Optimization of digestion procedure In order to carry out phosphorus determination in the samples studied it is necessary to apply an efficient decomposition method assuring complete holding of phosphorus in the solution. However, some organic phosphorus compounds are quite volatile and may be easily lost during heating of the sample. In preliminary tests of decomposition of some phosphorus compounds by means of acid digestion in an opened vessel a loss of up to 30% of phosphorus was observed. Microwave digestion for the preparation of the phosphorus-containing sample has advantages compared with oxygen flask combustion. The precision of the latter is lower due to the smaller quantity of sample used, especially at low phosphorus content. Moreover, using oxygen flask combustion the completeness of the destruction of the organic matter is not achieved for some types of polymer materials and solutions. Since decomposition of polymers proceeds generally with difficulty the rapid heating of the sample and the high pressure generated in a closed container can result in the reduction of preparation time. Then a microwave digestion system operating at pressure-control mode was applied for sample decomposition. Nitric and sulfuric acids are the most common reagents for this type of digestion. However, the presence of sulfuric acid in the resulted sample solution caused a significant decrease in the emission intensity of phosphorus. Nitric acid is successfully applied for the digestion of plastic materials w22x. However, during the decomposition of samples of considerable phosphorus content par-
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
tial volatilization of phosphorus compounds occurs Žlosses maximally 5%.. In order to cause complete recovery of phosphorus in the solution, sodium nitrate was applied, which increases the boiling point of the solution and is necessary for forming a phosphate salt. The addition of metal oxides or salts to the sample to hold phosphorus is described in various analytical procedures w7,9,23x. The application of nitric acid with the addition of sodium nitrate permitted complete recovery of phosphorus Ž98.9" 0.9%. in the analyzed samples under digestion conditions presented in Table 2. After cooling, the excessive nitric acid was evaporated to obtain finally 2% concentration of the acid in the diluted solution. A reagent blank was also prepared by the same procedure. 3.2. Determination by the MIP-AES method Atomic emission spectrometry with plasma sources is widely employed for elemental analysis, including determination of phosphorus in various materials, owing to fairly low detection limits and wide linear dynamic range. MIP-AES provides, besides more popular ICP-AES, good precision and accuracy of determination. The lowest DL 4.5 ng mly1 obtained with helium MIP was reported by Jin et al. w24x at the phosphorus 213.6-nm line. This value is comparable with DL 2 ng mly1 obtained with ICP-AES at the most sensitive 178.3-nm line in the vacuum-UV region w10x. In contrast, the DL obtained by Perkins and Long w25x with helium MIP was 400 ng mly1 . The determination of aqueous phosphorus by HeMIP-AES was also reported by Wu and Carnahan w18x. The detection limit was 1400 ng mly1 and linearity in the concentration ranged from 10 to 10 000 g mly1 . However, the helium plasma is not resistant of loading large amounts of water and moderate or high power sources should be used. The argon microwave discharge seems to be much more capable of tolerating the presence of the solvent in the plasma. A limited number of data on the phosphorus emission lines in argon MIP are accessible w26,27x. The spectrum was obtained in the spectral 200᎐260-nm region in which the more intense
45
Table 3 Observed emission lines of phosphorus Žabove 200 nm. in various plasmas Wavelength Žnm.
202.347 203.349 213.547 213.618 214.914 215.294 215.408 253.401 253.565 255.328 255.490
Intensity Ar-MIP this work
He-MIP w26x
Ar-ICP w27x
1 3 8 100 65 4 7 11 55 20 9
2 7 ᎐ 100 65 5 7 15 64 29 13
10 19 22 100 100 9 18 8 28 13 ᎐
lines of phosphorus appear with the Ar-MIP, excluding the vacuum-UV region unavailable in this study. The wavelengths identified and respective relative intensities, normalized with respect to the intensity of the most intense line, are collected in Table 3. The most intense phosphorus emission is observed at 213.618 nm and it is consistent with that reported by Tanabe et al. w28x for He-MIP and Winge et al. w29x for Ar-ICP. The effect of argon flow rate on the relative emission intensity of phosphorus is shown in Fig. 1. The greatest intensity of emission was obtained at a relatively low gas flow of approximately 180 ml miny1 . A similar result was obtained previously in our laboratory for a somewhat different MIP system w19x. The optimal gas flow for a majority of elements studied earlier was approximately 250 ml miny1 w30x. Thus, for efficient phosphorus excitation a longer residence time in the plasma is required. This is connected probably with the necessity of cleaving stable phosphorus᎐oxygen bonds during sample atomization. This is supported by the observed effect of microwave power on relative emission intensity of phosphorus shown in Fig. 2. The signal-to-background ratio sharply increases with the increase of the power level. For the studies of detection limits and linear responses disodium hydrogen phosphate and phenyl disodium phosphate were used for solu-
46
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
Fig. 1. The effect of microwave power on signal-to-background ratio ŽSBR. for phosphorus solution at 25 g mly1 .
tion preparation. The detection limits based on the 3 concept and seven replicate measurements were 85 and 92 ng mly1 for inorganic and organic phosphorus, respectively. The DLs are similar to those obtained for phosphorus with argon ICP-AES at 213.6 nm, namely 76 ng mly1 by Winge et al. w29x and 70 ng mly1 by Andreau et al. w11x. Both molecular forms of phosphorus produced good linearity in the concentration range of 0.5᎐130 g mly1 . A negligible effect of the molecular form on phosphorus emission intensity was found. At the concentrations higher than 100 g mly1 the intensity of phosphorus in organic form was 98% that of the inorganic form. However, the difference in short-term precision Ž%R.S.D.. was more clear: 1.86 for DHP and 2.54 for PDP. From a practical point of view, the observation of the lack of effect of the molecular form of these two substances studied permits to assume that in the samples studied from the polymer industry the measured phosphorus emission in-
tensity would not essentially depend on the chemical form of phosphorus in the solution after microwave digestion. A similar result was obtained earlier when elaborating the method of phosphorus determination by MIP-AES in polymer additives containing phosphites w31x. 3.3. Spectrophotometric determination A common method based on the formation of phosphomolybdenum blue was proposed for the determination of phosphorus in the form of phosphates, by means of which phosphorus can be determined in the quantity of 1᎐80 g w6x. The method is sensitive and reproducible, and in the case when no other elements are present in the sample forming heteropolyacids, also selective and simple in performance. Preliminary investigations show that after performing microwave digestion in the samples studied of high phosphorus content Žsee Table 4. only part of phosphorus is determined Žapprox. 90%..
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
47
Fig. 2. The effect of argon flow rate on signal-to-background ratio ŽSBR. for phosphorus solution at 25 g mly1 .
It has been assumed that organic phosphorus may be non-quantitatively converted into orthophosphate. Potassium periodate was applied for oxidizing phosphorus in the digest. The excess of periodate was reduced with hydrazine, and the excess of hydrazine decomposed during further heating of the sample.
In the case of phosphorus analysis in PDP and tri-isodecyl phosphite ŽTDP. the use of periodate and subsequent reduction with hydrazine caused an increase in absorbance by approximately 10%. Simultaneously, no effect of the oxidation with periodate on the phosphorus content determined in all the samples studied was observed when the
Table 4 Determination of phosphorus in samples and standard reference material NIST SRM 1568A Rice Flour Sample
Phenyl disodium phosphate ŽPDP. Tri-isooctyl phosphite ŽTDP. Modified polyaniline ŽMP. MP solution in m-cresol Polyvinylchloride ŽPVC. SRM 1568A Rice Flour a
For five determinations.
P content Ž%. Expected
Found with MIP-AESa
Found with spectrophotometry a
12.19
11.98" 0.30
11.91" 0.35
6.17
6.07" 0.16 0.116" 0.004 0.020" 0.001 0.067" 0.002
6.10" 0.18 0.113" 0.004 0.021" 0.001 0.062" 0.003
0.149" 0.006
0.148" 0.007
Certified value 0.153" 0.008
48
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49
analysis was carried out by the MIP-AES method. This leads to the conclusion that the addition of potassium periodate followed by reduction with hydrazine in the case of the spectrophotometric method of determination has an effect on the colored reaction medium and not on the form of the element determined. In this way the interference by nitric acid on the phosphomolybdenum blue formation is probably eliminated. The other possible explanation is that the chemical form of phosphorus in the solution has a negligible effect on the results obtained by the MIP-AES method. The method elaborated is characterized by good precision Žrelative error approx. 3%.. However, the overall precision of determination is worse due to the large effect of the sample preparation stage, when it is difficult to achieve complete decomposition of the organic compounds without causing simultaneous liberation of part of phosphorus. The results and precision of determinations are presented in Table 4. 3.4. Sample analysis Six samples, including NIST SRM 1568A Rice Flour certified reference material, were analyzed to evaluate the feasibility of the MIP-AES method. The phosphorus content by the spectrophotometric method was additionally determined. Since no reference material of certified phosphorus content appropriate for the analysis of products from the polymer industry was found, a food product was used, as it is an organic material of a similar phosphorus content. As shown in Table 4, the results obtained both by the MIP-AES and spectrophotometric method are comparable. The results obtained for the reference material are also in good agreement with the certified value. Phosphorus content found in PDP is somewhat lower than expected, presumably, as a result of phosphorus losses during digestion. Additionally, the phosphorus content in the PDP sample declared by the manufacturer was at least 98%. For the MP solution sample the accuracy was evaluated by the recovery of phosphorus from the spiked samples. Each sample was weighed into a PTFE vessel for microwave digestion as described
above. Some samples were spiked with a known amount of PDP and decomposed in the same conditions. The spike recovery was 98.1" 2.1%.
4. Conclusion The potential of the Ar-MIP-AES for the determination of phosphorus in organic substances after sample digestion was investigated. Sample pretreatment has an essential effect on the accuracy and precision of the result obtained as well as on the time of performing the determination. The studies carried out show the necessity of performing sample digestion under conditions assuring complete decomposition of the organic part and preventing phosphorus losses from the sample. The utilization of microwave digestion with nitric acid and sodium nitrate at pressure-controlled mode assures complete recovery of the analyte. The proposed MIP-AES method has proven to be a simple and sensitive technique for the determination of phosphorus in organic materials from the polymer industry. The spectrophotometric method based on formation of phosphomolybdenum blue exhibits the similar analytical performance, however, more complicated sample preparation is necessary. References w1x T.R. Crompton, Comprehensive Organometallic Analysis, Plenum Press, New York, 1987. w2x L. Mazor, Methods of Organic Analysis, Akademiai Kiado, Budapest, 1983. w3x L. Maric, ´ M. ˇSiroki, Z. ˇStefanac, Microchem. J. 21 Ž1976. 129. w4x H.M. Kingston, L.B. Jassie, Introduction to Microwave Sample Preparation: Theory and Practice, American Chemical Society, Washington, DC, 1988. w5x V. Zatka, Ropa Uhlie 10 Ž1968. 413. w6x Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Ellis Horwood, Chichester, John Wiley, New York, 1986. w7x P.M. Saliman, Anal. Chem. 36 Ž1964. 112. w8x E. Szebenyi-Gyory, P.J. Slevin, G. Svehla, L. Erdey, Talanta 17 Ž1970. 1167. w9x M.S. Vigler, A. Strecker, A. Varnes, Appl. Spectrosc. 32 Ž1978. 60.
K. Jankowski r Microchemical Journal 70 (2001) 41᎐49 w10x R.F. Lonardo, A.I. Yuzefovsky, K.X. Yang, R.G. Michel, E.S. Frame, J. Barren, J. Anal. At. Spectrom. 11 Ž1996. 279. w11x C. Andreau, K.J. Fredeen, M. Thomsen, D.A. Yates, At. Spectrosc. 16 Ž1995. 79. w12x S.J. Nagourney, R.K. Madan, J. Test. Eval. 19 Ž1991. 77. w13x S. Plaami, J. Kumpulainen, J. Assoc. Off. Anal. Chem. 74 Ž1991. 32. w14x J.J. Sullivan, Trends Anal. Chem. 10 Ž1991. 23. ¨ w15x G. Becker, A. Colmsjo, Anal. Chim. Acta ¨ C. Ostman, 340 Ž1997. 181. w16x C.A. Bache, D.J. Lisk, J. Assoc. Off. Anal. Chem. 50 Ž1967. 1246. w17x B. Riviere, J.M. Mermet, D. Deruaz, J. Anal. At. Spectrom. 2 Ž1987. 705. w18x M. Wu, J.W. Carnahan, J. Anal. At. Spectrom. 7 Ž1992. 1249. w19x K. Jankowski, R. Parosa, A. Ramsza, E. Reszke, Spectrochim. Acta, Part B 54 Ž1999. 515. w20x K. Jankowski, D. Karmasz, A. Ramsza, E. Reszke, Spectrochim. Acta, Part B 52 Ž1997. 1813.
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w21x N. Obarski, private communication. w22x A. Vollrath, T. Otz, C. Hohl, H.G. Seiler, Fresenius J. Anal. Chem. 344 Ž1992. 269. w23x W.S. Brammell, J. Assoc. Off. Anal. Chem. 64 Ž1981. 808. w24x Q. Jin, H. Zhang, D. Ye, J. Zhang, Microchem. J. 47 Ž1993. 278. w25x L.D. Perkins, G.L. Long, Appl. Spectrosc. 43 Ž1989. 499. w26x Q. Jin, Y. Duan, J.A. Olivares, Spectrochim. Acta, Part B 52 Ž1997. 131. w27x D. Kollotzek, P. Tschopel, G. Tolg, ¨ ¨ Spectrochim. Acta, Part B 39 Ž1984. 625. w28x K. Tanabe, H. Haraguchi, K. Fuwa, Spectrochim. Acta, Part B 36 Ž1981. 119. w29x R.K. Winge, V.J. Peterson, V.A. Fassel, Appl. Spectrosc. 33 Ž1979. 206. w30x K. Jankowski, J. Anal. At. Spectrom. 14 Ž1999. 1419. w31x K. Jankowski, A. Jerzak, K. Bujnowski, A. SernickaPoluchowicz, L. Synoradzki, Method of determination of phosphorus in polymer additives, especially for PVC processing, Pat. PL 172314 Ž1994..