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L., Atomic line profile measurements on hollow-cathode and electrodeless discharge lamps using a high-resolution Cchelle monochromator, Spectrochim. Acta, Part B, 1985, 40, 1585. (CSIRO Division of Chemical Physics, PO Box 160, Clayton 3168, Victoria, Australia). Greenfield, S., *Hieftje, G. M., Omenetto, N., Scheeline, A., Slavin, W., Twenty-five years of analytical atomic spectroscopy, Anal. Chim. Acta, 1986, 180, 69. (Dept Chem., Indiana Univ., Bloomington, IN 47405, USA). Bain, D. C., Berrow, M. L., McHardy, W. J., Paterson, E., Russell, J. D., Sharp, B. L., *Ure, A. M., West, T. S., Optical, electron and X-ray spectrometry in soil analysis, Anal. Chim. Acta, 1986,180,163.(Macaulay Inst. for Soil Res., Craigiebuckler, Aberdeen AB9 2QJ, UK). Marshall, J., Ottaway, B. J., Ottaway, J. M., Littlejohn, D., Continuum-source atomic absorption spectrometry- new lamps for old?, Anal. Chim. Acta, 1986, 180, 357. (Dept. Pure and Appl. Chem., Univ. Strathclyde, 295 Cathedral St., Glasgow G1 lXL, UK). Cedergren, A., Lindberg, I., Lundberg, E., Baxter, D. C., Frech, W., Investigation of reactions involved in graphite furnace atomic absorption procedures. Part 12. A study of some factors influencing the determination of selenium, Anal. Chim. Acta, 1986, 180, 373. (Dept. Anal. Chem., Univ. Umel, S-901 87 Umel, Sweden).33R JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 8711 12. 871113. 871114. 8711 15. 871116. 871117. 87/118.8711 19. 871120. 871121. 871122. 871123. 871124. 871125. Berndt, H., Baasner, J., Messerschmidt, J., Vergleichende atomabsorptionsspektrometrische Untersuchungen zur elementspurenbestimmung in urin, Anal. Chim. Acta, 1986, 180, 389. (Institut fur Spektrochemie und Ange- wandte Spektroskopie, Bunsen-Kirchoff-Strasse l l, D-4600 Dortmund 1, FRG). Diamy, A. M., Gonzalez Flesca, N., Legrand, J. C., Formation et dtsactivation par I’oxyghe moltculaire de I’atome metastable O(5S) dans une dtcharge oxyg&ne - htlium, Spectrochim. Acta, Part B, 1986,41,317. (Labor- atoire de Chimie Gtntrale, Unit6 Associke au CNRS NO. 870, UniversitC Pierre et Marie Curie, 4, Place Jussieu, 75230 Paris Cedex 05, France). McGeorge, S. W., Salin, E. D., Spatial resolution enhance- ment for linear photodiode array atomic spectrometry, Spectrochim.Acta, Part B , 1986, 41, 327. (Dept. Chem., McGill Univ., 801 Sherbrooke St. W., Montreal, Quebec H3A 2K6, Canada). Marasinghe, P. A. B., Lovett, R. J., An evaluation of methods for estimating the electron Stark widths of atomic spectral lines, Spectrochim. Acta, Part B , 1986, 41, 349. (Dept. Chem., North Dakota State Univ., Fargo, ND 58105, USA). Webb, B. D., Denton, M. B., Comparison of a very high frequency 148 MHz inductively coupled plasma to a 27 MHz ICP, Spectrochim. Acta, Part B, 1986, 41, 361. (Dept. Chem., Univ. Arizona, Tucson, AZ 85721, USA). Rettberg, T. M., Holcombe, J. A., Interference minimisa- tion using second surface atomiser for furnace atomic absorption, Spectrochim. Acta, Part B, 1986, 41, 377.(Chem. Dept., Univ. Texas at Austin, Austin, TX 78712, USA). Albers, D., Sacks, R., Radiative properties of an electric- ally vaporised thin film plasma in an external magnetic field, Spectrochim. Acta, Part B , 1986, 41, 391. (Dept. Chem., Univ. Michigan, Ann Arbor, MI 48109, USA). Lanauze, J. A., Winefordner, J. D., Some observations on the use of photoacoustic spectroscopy for flame elemental analysis, Spectrochim. Acta, Part B, 1986,41,407. (Dept. Chem., Univ. Florida, Gainesville, FL 32611, USA). Matousek, J. P., Orr, B. J., Selby, M., Interferences due to easily ionised elements in a microwave-induced plasma system with graphite furnace sample introduction, Spectro- chim. Acta, Part B , 1986,41,415. (Sch. Chem., Univ. New South Wales, PO Box 1, Kensington, NSW 2033, Aus- tralia).Gillson, G., Horlick, G., An atomic absorption study of ground state neutral atoms and ions in the inductively coupled plasma, Spectrochim. Acta, Part B , 1986,41,431. (Dept. Chem., Univ. Alberta, Edmonton, Alberta T6G 2G2, Canada). Rademeyer, C. J., Human, H. G. C., Faure, P. K., The dynamic wall and gas temperature distributions in a graphite furnace atomiser, Spectrochim. Acta, Part B , 1986, 41, 439. (Technikon Pretoria, 420 Church St., Pretoria 0002, South Africa). Miller, M. H., Zander, A. T., Thermal pinch effect in the argon d.c. plasma, Spectrochim. Acta, Part B , 1986, 41, 453. (Maryland Dept. Legislative Reference, 90 State Circle, Annapolis, MD 21401, USA). Patel, B. M., Winefordner, J. D., Glow discharge source atomisation for the laser-excited atomic fluorescence spectrometric studies of indium, Spectrochim.Acta, Part B, 1986, 41, 469. (Dept. Chem., Univ. Florida, Gaines- ville, FL 32611, USA). Edelson, M. C., DeKalb, E. L., Winge, R. K., Fassel, V. A., Analytical atomic spectroscopy of plutonium-I. High resolution spectra of plutonium emitted in an inductively coupled plasma, Spectrochim. Acta, Part B, 1986,41,475. (Ames Lab.-US DOE, Iowa State Univ., Ames, IA 50011, USA). 8711 26, 871127. 8711 28. 871129. 871130. 871131. 871132. 871133. Bolshov, M. A., Zybin, A. V., Koloshnikov, V. G., Mayorov, I. A., Smirenkina, I. I., Laser excited fluores- cence analysis with electrothermal sample atomisation in vacuum, Spectrochim. Acta, Part B , 1986, 41, 487. (Inst. Spectroscopy, Acad Sci., 142092 Moscow obl., Podolskii r-n, Troitzk, USSR).Belchamber, R. M., Betteridge, D., Wade, A. P., Cruick- shank, A. J., Davison, P., Removal of a matrix effect in ICP-AES multi-element analysis by simplex optimisation, Spectrochim. Acta, Part B , 1986, 41, 503. (BP Research Centre, Chertsey Rd., Sunbury-on-Thames, Middlesex TW16 7LN, UK). Butler, L. R. P., Laqua, K., Strasheim, A., International Union of Pure and Applied Chemistry. Analytical Che- mistry Division, Commission on Spectrochemical and Other Optical Procedures for Analysis. Nomenclature, symbols, units and their usage in spectrochemical analysis -V. Radiation sources, Spectrochim. Acta, Part B , 1986, 41, 507. (Information and Research Services, CSIR, Pretoria, South Africa). Strasheim, A., Bohmer, R.G., A study of a high repetition condensed arc source for the analysis of low alloy steels using different counter electrode sample configurations- 11. A comparison between flat and dimple samples using a tungsten pin counter electrode, Spectrochim. Acta, Part B, 1986, 41, 547. (Dept. Chem., Univ. Pretoria, Pretoria 0002, South Africa). Welz, B., Schlemmer, G., Ortner, H. M., Scanning electron microscopy studies on surfaces from electrother- mal atomic absorption spectrometry-11. Total pyrolytic graphite platforms in pyrographite coated polycrystalline electrographite tubes, Spectrochim. Acta, Part B, 1986,41, 567. (Dept. Appl. Res., Bodenseewerk Perkin-Elmer & Co. GmbH, D-7770 Uberlingen, FRG). De Marco, R., Kew, D., Sullivan, J. V., Determination of major constituents in metal samples by emission spec- trometry using a demountable hollow cathode source and internal standardisation, Spectrochim.Acta, Part B, 1986, 41, 591. (Royal Melbourne Inst. Technol., GPO Box 2476V, Melbourne 3001, Victoria, Australia). de Loos-Vollebregt, M. T. C., de Galan, L., Stray light in Zeeman and pulsed hollow cathode lamp atomic absorp- tion spectrometry, Spectrochim. Acta, Part B, 1986, 41, 597. (Laboratorium voor Analytische Scheikunde, Tech- nische Hogeschool Delft, Jaffalaan 9, 2628 BX Delft, The Netherlands). Gillson, G., Horlick, G., An atomic fluorescence study of easily ionisable element interferences in the inductively coupled plasma, Spectrochim. Acta, Part B , 1986,41,619. (Dept. Chem., Univ. Alberta, Edmonton, Alberta T6G 2G2, Canada).Papers 871C13487/C230 were presented at SAC 8613rd BNASS held at the University of Bristol, UK, 20-26 July, 1986. 87lC 134. 87lC135. 871C136. 87/C137. Tolg, G., Extreme trace analysis of the elements-the state of the art today and tomorrow, (Institut fiir Spektrochemie und angewandte Spektroskopie in Dortmund und Labora- torium fiir Reinststoffanalytik des Max-Planck-Institutes fur Metallforschung in Stuttgart, Bunsen-Kirchhoff-Str. 11/13, D-4600 Dortmund 1, FRG). Greentfeld, S., Thomsen, M., Spectral interference in atomic spectrometry: the advantage of the atomic fluores- cence technique, (Loughborough Univ. Technol., Lough- borough, Leicestershire LEll 3TU, UK). Hutton, R. C., Cantle, J. E., Eaton, A. N., Blair, P. D., Fundamental performance criteria in ICP-MS, (VG Iso- topes Ltd., Ion Path, Road Three, Winsford, Cheshire CW7 3BX, UK).Koch, K. R., Polyurethane foams in selective separation and pre-concentration of the platinum group metals, (Dept. Anal. Sci., Univ. Cape Town, P. Bag Rondebosch, 7700, South Africa).34R 87lC138. 87lC 139. 87lC140. 87lC141. 87lC142. 87lC 143. 87lC144. 87lCl45. 87lC146. 87lC147. 871C148. 87lC149. 87lC150. 87lC15 1. 87lC152. 87lC153. JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Horwitz, W., Albert, R., The standardisation of analytical methods, (Food and Drug Administration, Center for Food Safety and Applied Nutrition, HFF-7, Washington, DC 20204, USA). Ripley, B. D., Thompson, M., Regression techniques for the detection of bias, (Dept.Mathematics, Univ. of Strathclyde, Glasgow G1 lXH, UK). Sychra, V., KolihovB, D., HlavBt, R., Dolefal, J., Piischel, P., Formhek, Z., Ortner, H., Advances in metal-based electrothermal atomisers, (Inst. Chem. Tech., Depart- ment of AAS, 166 28 Prague 6, Czechoslovakia). Shand, C. A., Ure, A. M., Mitchell, M. C., Graphite furnace atomic absorption determination of selenium in plant materials following combustion in a stream of oxygen, (Dept . Spectrochemistry, Macaulay Inst. Soil Res., Craigiebuckler, Aberdeen AB9 2QJ, UK). Schlemmer, G., Weltz, B., The use of alternative gases in graphite furnace atomic absorption spectrometry, (Dept. Appl. Res., Bodenseewerk Perkin-Elmer and Co. GmbH, D-7770 Uberlingen, FRG). Lee, M., Green, P., Brown, A. A., Methods to overcome interferences in the determination of selenium in clinical materials, (Pye Unicam, York St., Cambridge CB1 2PX, UK) .Dittrich, K., The use of lasers and other non-thermal excitation in atomic spectroscopy for trace analysis, (Karl-Mam-University Leipzig, Dept. Chem., Talstrasse 35, DDR 7010 Leipzig, GDR). Harnly, J. M., Holcombe, J. A., A method for indentifying and quantifylng background correction errors, (US Dept. Agriculture, Agricultural Res. Service, Nutrient Composi- tion Lab., Building 161, BARC-E, Beltsville, MD 20705, USA). Andersen, J. R., Aluminium in man as determined by Zeeman-corrected atomic absorption spectrometry, (Dept. Chem, Royal Danish Sch. Pharmacy, DK-2100 Copenhagen, Denmark). Carroll, J., Egila, J. N., Littlejohn, D., Marshall, J., Ottaway, J. M., Stephen, S.C., Efficiency of microcom- puter controlled background correction for ETA - conti- nuum source AAS in comparison with Zeeman-effect and D,-lamp background correction, (Dept. Pure and Appl. Chem., Univ. Strathclyde, Cathedral St., Glasgow G1 lXL, UK). Carroll, J., Littlejohn, D., Ottaway, J. M., Quinn, A.-M., Furnace atomic non-thermal excitation spectrometry-a novel electrothermal emission source, (Dept. Pure and Appl. Chem., Univ. Strathclyde, Cathedral St., Glasgow G1 lXL, UK). Thorne, A., Fourier transform atomic spectroscopy, (Blackett Lab., Imperial Coll. of Sci. Technol., London SW7, UK). Dawson, J. B., Kersey, A. D., Hajizadeh-Saffar, M., Duffield, R. J., Fisher, G. W., Measurement of magnetic- ally induced optical rotation in atomic vapours, (Dept. Medical Physics, General Infirmary, Leeds LS1 3EX, UK).Faires, L. M., Brault, J. W., Applications of high resolution Fourier transform spectrometry to studies of the inductively coupled plasma source, (Los Alamos Natl. Lab., Los Alamos, NM 87545, USA). Fielding, R. J., Steers, E. B. M., Charge transfer excitation in glow discharges, (Dept. Electronic and Communica- tions Engineering and Appl. Physics, Polytech. of North London, London, UK). Snook, R. D., Torch configuration and designs for induc- tively coupled plasma atomic emission spectrometry, (Chelsea Instruments Ltd., Epirus Rd., London SW6 7UR, UK). 87tC 154. 87lC155. 87tC156. 87lC 157. 87lC158. 87lC159. 87lC 60. 87/C 61. 87lC162. 87tC163. 87lC164. 87lC165. 87lC 166. 87lC167. 87lC168.87lC169. 871C170. LeWin, K., Walsh, J. N., Miles, D. L., Determination of sulphide in groundwaters by ICP-AES, (Geology Dept., King’s College, Strand, London WC2R 2LS, UK). Ramsey, M. H., Thompson, M., High-accuracy analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES) using the PRISM correction for noise, drift and matrix effects, (Appl. Geochem. Res. Group, Dept. Geology, Imperial College of Sci. Technol., London SW7 2BP, UK). De Marco, R., Kew, D., Sullivan, J. V., Use of a hollow cathode emission source for determination of major constituents in metal samples, (Royal Melbourne Inst. Technol., GPO Box 2476V, Melbourne, 3001 Victoria, Australia). Burns, D. T., Harriott, M., McArdle, S., Maxwell, T. H., Determinations of inorganic and butyltin in sea water over oyster beds around Strangford Lough, (Dept.Anal. Chem., The Queen’s Univ. Belfast, Belfast BT9 5AG, UK) . Ebdon, L., Evans, K. M., Hill, S. J., Munro, S., Walton, A. P., Crews, J. M., Recent advances in trace metal speciation by coupled chromatography - atomic spectro- scopy, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Walton, A. P., Ebdon, L., Millward, G. E., Metal methylation and its significance in estuarine waters, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Cresser, M., Pneumatic nebulisers-poor pumps and inferior sub-samplers?, (Dept. Soil Sci., Univ. Aberdeen, Aberdeen AB9 2UE, UK) . Jackson, K. W., Karwowska, R., The atomisation of trace metals from refractory matrices by slurry - electrothermal atomic absorption spectrometry, (Dept.Chem., Univ. Saskatchewan, Saskatoon, Sask. S7N OWO, Canada). Ebdon, L., Foulkes, M. E., Norman, P., Sparkes, S. T., Plasma emission spectroscopy using slurry atomisation-a dream come true?, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Norman, P., Ebdon, L., Analysis of zeolites and other minerals by ICP-OES using slurry atomisation, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Hutton, R. C., Cantle, J. E., Blair, P., Gordon, J., Factors affecting isotope ratio measurements in ICP-MS, (VG Isotopes Ltd., Ion Path, Road Three, Winsford, Cheshire CW7 3BX, UK). Miles, D. L., Cook, J. M., Cheung, Y. Y., Date, A. R., A comparison between ICP-MS and ICP-OES for the analysis of groundwaters, (British Geological Survey, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK).Date, A. R., ICP-MS: the best thing in analytical chemistry since chopped light?, (British Geological Survey, 64 Gray’s Inn Rd., London WClX 8NG, UK). Oliver, G. J., X-ray fluorescence analysis in the ceramic and allied industries, (British Ceramic Res. Assoc. Ltd., Queen’s Rd., Penkhull, Stoke on Trent ST4 7LQ, UK). Adamson, B. W., Price, B. J., Developments in crystals and excitation systems for XRF analysis, (Appl. Res. Lab. Ltd., Wingate Road, Luton, Bedfordshire LU4 8PU, UK). Warren, P. L., Humber, J., Horton, M., The role of energy dispersive XRF in process analysis, (Wilton Cen- tre, ICI Petrochemicals and Plastics Div., PO Box 90, Wilton, Middlesbrough, Cleveland TS6 8JA, UK).Uden, P. C., Perpall, H. J., Yoo, Y. J., Empirical formula determination in pyrolysis - gas chromatography by plasma atomic emission spectrometry, (Dept. Chem. Lederle Graduate Res. Cent., Univ. Massachusetts, Amherst, MA 01003, USA).JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 35 R 87lC171. 87lC172. 87lC 173. 87lC174. 87lC175. 87lC176. 87lC177. 87lC 178. 87K179. 871Cl80. 87lC181. 87lC 182. 87lC183. 871C184. 87JC185. 87lC186. Greenway, G. M., Barnett, N. W., The production of vapour standards for the trace analysis of organometallic compounds in air using microwave plasma detection, (Dept. Sci., Humberside College of Higher Education, Hull, UK). Sanz-Medel, A., Rodriguez Roza, R., Diaz Garcia, M.E., Ujaimi, A. L. R., Barnett, N. W., New instrumental approaches to the problem of the determination of aluminium at the p.p.b. level in biological fluids, (Dept. Anal. Chem., Fac. Chem., Univ. Oviedo, Oviedo, Spain). Hickman, D. A., Analysis in forensic science using atomic spectroscopy, (Metropolitan Police Forensic Sci. Lab., 109 Lambeth Rd., London SEl7LP, UK). Nerin, C., Garnica, A., Cacho, J., Formation and extrac- tion of ion pairs and indirect determination by atomic absorption spectrometry, (Dept . Quimica, Escuela Tkc- nica Superior Ingenieros Industriales, Universidad de Zaragoza, Spain). Davies, J., Low power laminar flow torch for inductively coupled plasma atomic emission spectroscopy, (Trace Anal. Lab., Dept.Chem., Imperial College Sci. Technol., London SW7 2AY, UK). Mitchell, M. C., Shand, C. A., Berrow, M. L., The direct graphite furnace atomic absorption determination of acetic acid extracts of soils, (Dept. Spectrochem., Macaulay Inst. for Soil Res., Craigiebuckler, Aberdeen AB9 2QJ, UK). Ayodele, J. T., Essiet, E. U., Determination of trace metals in Cassiutoru seed and oil, (Dept. Chem., Dept. Geography, Bayero Univ., PMB 3011, Kano, Nigeria). Greenfield, S., Thomsen, M., Non-resonance atomic fluorescence spectrometry with a dual plasma system, (Loughborough Univ. Technology, Loughborough, Leicestershire LEll 3TU, UK). Hall, G. E. M., Park, C. J., The determination of tungsten in geological materials by inductively coupled mass spec- trometry, (Geological Survey of Canada, 601 Booth St., Ottawa, Ontario KIA OE8, Canada).Bustos, A., Shchez Rojas, F., Bosch Ojeda, C., Garcia de Torres, A., Can0 Pavhn, J. M., The use of 1,5-bis(di-2- pyridylmethy1ene)thiocarbonohydrazide as extracting reagent for the determination of some transition metal ions by atomic absorption spectrophotometry, (Dept. Anal. Chem., Univ. Malaga, Mdlaga, Spain). Brajter, K., Shnawska, K., Determination of gold in platinum alloys by an AAS method, (Dept. Chem., Univ. Warsaw, Pasteura 1,OZ-093 Warsaw, Poland). Brown, A. A., Morton, S. F., Wassal, M. P., Applications of probe atomisation in graphite furnace atomic absorp- tion spectrometry, (Pye Unicam, York St., Cambridge CB12PX, UK). Sheikh, S. U., Afshan, A. S., Jaf€ar, M., Ashraf, M., Trace metal analysis using atomic absorption and polarography , (Dept.Chem., Quaid-i-Azam Univ., Islamabad, Pakis- tan). Bysouth, S. R., Tyson, J. F., Current calibration practices for flame atomic absorption spectrometry, (Dept. Chem., Univ. Technol., Loughborough, Leicestershire LEll 3127, UK). Bysouth, S. R., Tyson, J. F., Atomic absorption calibra- tion using flow injection concentration gradient and dilution techniques, (Dept. Chem., Univ. Technol., Loughborough, Leicestershire LEll 3TU, UK). Littlejohn, D., Stephen, S. C., Ottaway, J. M., Application of a slurry technique for the trace element analysis of foodstuffs by electrothermal atomic absoprtion spec- trometry, (Dept. Pure and Appl. Chem., Univ. Strathclyde, Cathedral St., Glasgow G1 lXL, UK). 87lC187. 871C188. 87lC189.871C190. 87lC191. 87lC192. 87lC193. 87lC194. 87lC195. 87lC196. 87lC197. 87lC198. 87lC199. 87lC200. 87lC201. 87lC202. Carroll, J., Corr, S., Littlejohn, D., Marshall, J., Quinn, A.-M., Ottaway, J. M., Recent developments in automatic probe atomisation in ETA-AAS, (Dept. Pure and Appl. Chem., Univ. Strathclyde, Cathedral St., Glasgow G1 lXL, UK). Hutton, R. C., Eaton, A. N., Cantle, J. E., Analysis of organic solutions by ICP-MS, (VG Isotopes Ltd., Ion Path, Road Three, Winsford, Cheshire CW7 3BX, UK). Nayler, R., Brown, R., Jackson, P., Sanderson, N. E., The analysis of trace elements in metals by high resolution glow discharge mass spectrometry, (VG Isotopes Ltd., Ion Path, Road Three, Winsford, Cheshire CW7 3BX, UK). Mendes-Bezerra, A. E., Mota, R. G. S., Martins, M.M. C., Effects of some ions on lithium atomic absorption, (Departamento de Quimica-Analitica, Universidade Federal do Ceard, Caixa Postal 3010, Fortaleza-Ceari, Brazil). Mendes-Bezerra, A. E., Paiva, M. E. D., Martins, M. M. C., An atomic absorption study of strontium, (Depart- mento de Quimica-Analitica, Universidade Federal do CearA, Caixa Postal 3010, Fortaleza-CearA, Brazil). Valchrcel, M., Gallego, M., Martinez, P., Indirect atomic absorption methods based on continuous precipitation in flow injection analysis, (Dept. Anal. Chem., Fac. Sci., Univ. Cbrdoba, CBrdoba, Spain). Steers, E. B. M., Zendehnam, A., Electrical and spectral characteristics of the positive column of a low pressure discharge in inert gases, (Dept. Electronic and Communi- cations Engineering and Appl.Physics, Polytech. North London, London, UK). D’hnocenzio, F., Ottaviani, M., De Biasi, A., Indirect determination of sulphate in rain-water by atomic absopr- tion spectrometry, (Istituto Superiore di Sanitd, Labora- torio di Igiene del Territorio, Wale Regina Elena 299, 00161 Rome, Italy). Parry, H. G. M., Ebdon, L., Direct analysis of powdered whole coal by ETA-AAS without sample dissolution, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Sparkes, S. T., Ebdon, L., Agricultural sample analysis by slurry atomisation - plasma emission spectroscopy, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Sparkes, S. T., Ebdon, L., Slurry atomisation by DCP- some theoretical considerations, (Dept.Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Hill, S. J., Ebdon, L., Jones, P., Interfacing HPLC - FAAS for trace metal speciation, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Cheung, Y. Y., Date, A. R., Shepherd, T. J., Miller, M. F., The application of ETV ICP-MS to fluid inclusion analysis, (British Geological Survey, 64 Gray’s Inn Rd., London WClX 8NG, UK). Barnett, N. W., Spillane, D. E. M., Taobi, A. A. H., The analysis of some noise sources in inductively coupled plasma optical emission spectroscopy, (Dept. Environ. Sci., Plymouth Polytech., Drake Circus, Plymouth PL4 8AA, UK). Jasim, F. H., Awad, N. A. N., ETA-AAS investigations of Sc, Tb, Eu and Yb using different metal carbide coated pyrographite atomisers and matrix modifiers, (Dept.Chem., Coll. Sci., Univ. Baghdad, Jadiriya, Baghdad, Iraq). Welz, B., Schubert-Jacobs, M., Use of the trapping technique to investigate atomisation mechanisms in hydride generation atomic absorption spectrometry, (Dept. Appl. Res., Bodenseewerk Perkin-Elmer & Co., GmbH, D-7770 Uberlingen, FRG).36R 87lC203. 87lC204. 87lC205. 87lC206. 87IC207. 87lC208. 87lC209. 87lC210. 87lC211. 871C212. 87IC2 13. 87lC214. 87IC215, 87/C216. 87lC217. 87lC218. 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Chem., Univ. Oulu, SF-90570 Oulu, Finland). Barton, H. N., Emission spectrographic determination of volatile trace elements in geological materials by a carrier distillation technique, J. Geochem. Explor., 1986,25,367. (Fed. Cent., US Geol. Surv., Denver, CO 80225, USA). Zolotareva, N. I., Kuzyakov, Yu. Ya., Khlystova, A. D., Chernova, N. A., The effect of additives on the volatility of elements in d.c. arc sources during atomic emission analysis of nickel(I1) oxide, Zh. Anal. Khirn., 1986, 41, 805. (M. V. Lomonosov Moscow State Univ., Moscow, USSR). Nakamura, E., Namiki, H., Determination of cadmium in the presence of large amount of sodium chloride by flame atomic absorption spectrometry. Simple correction method for background absorption with sodium chloride, Kogyo Yosui, 1986,329,14. (Fac. Educ., Yokohama Natl. Univ., Yokohama 240, Japan).

ISSN:0267-9477    

DOI:10.1039/JA987020029R

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 33 Self-matrix Effects as a Cause of Calibration Curvature in Inductively Coupled Plasma Atomic Emission Spectrometry Michael H. Ramsey and Michael Thompson Applied Geochemistry Research Group, Department of Geology, Imperial College of Science and Technology, London S W7 2BP, UK Stephen J. Walton Applied Research Laboratories, Wingate Road, Luton, Bedfordshire, LU4 8PU, UK A new cause of calibration curvature in inductively coupled plasma atomic emission spectrometry is proposed, the self-matrix effect. For a series of calcium lines, the progressive decrease in sensitivity with increasing calcium is shown to be similar to that of a number of concomitant trace element lines undergoing a matrix effect. The magnitude of the effect depends on the excitation energy of the line, for both calcium and trace element lines.These observations are consistent with a fall in excitation temperature as a common cause. An empirically derived functional relationship gave a good fit to the calibration data of the calcium lines. Keywords : Inductively coupled plasma; atomic emission; calibration curvature; calibration equation; self- matrix effecr The relationship between analyte concentration in the test solutions and the intensity of an appropriate spectral line is nearly linear in inductively coupled plasma atomic emission spectrometry (ICP-AES) over an extended concentration range. This feature is one of the important advantages of ICP-AES, and is essential for genuine simultaneous multi- element analysis.Deviations from linearity at high analyte concentrations are well known, but the onset of non-linearity can be detected at much lower concentrations, usually within two orders of magnitude of the detection limit for most lines in regular use. However, there is often an effectively linear concentration range where deviations have little effect on accuracy, and this may extend to about 4-5 orders of magnitude. No comprehensive account of the curvature of ICP-AES lines has yet been produced. Possible causes of non-linearity that have been suggested fall into two broad categories: (i) causes due to classical line broadening mechanisms, i. e., natural, Doppler, Lorentz, Holtzmark, Stark, Zeeman, quenching, self-absorption and hyperfine structure’; and (ii) other causes including ionisation interference ,2 nebuliser efficiency, sample transport in the plasma and spectrally unresolved stray light.The most important of these mechan- isms in ICP-AES is generally supposed to be self-absorption and, in certain instances, ionisation interference. Human and Scott3 reported both of these effects to be minimal at viewing heights low in the inductively coupled plasma (ca. 10 mm above the load coil) but at their maximum at high levels of the plasma (ca. 25 mm). They also reported that no quantitative method was available that could predict the onset and extent of self absorption. Self absorption in flames has been studied in detail,4 but it is unclear to what extent the conclusions apply to the optically thin ICP.In the absence of a definitive study on linear ranges of ICP-AES calibration lines, either of two contradictory assumptions have been occasionally invoked. One such assumption maintains that there is a constant “orders of magnitude” linear range above the detection limit for all lines of a particular element. This implies that the selection of a less sensitive line would automatically provide linearity to higher concentrations. The second assumption suggests that all calibration lines of an element begin to curve at the same concentration, and thus the longest linear ranges should be found among lines with the highest sensitivities (or lowest detection limits). These two assumptions are examined in this study. The main purpose of the work described here, however, was to substantiate a proposed new mechanism for the non- linearity of calibration lines in ICP-AES, the self-matrix effect.This process would be independent of and additional to any other causes of curvature. The idea of this mechanism arose from a previous study of matrix effects from elements such as calcium on the lines of a range of analytes.5 It was noticed that the progressive fall in the sensitivity of the calcium in its calibration line corresponded well with the matrix effects on analyte line intensities as the concentration of the calcium was increased. Studies with many matrix elements and lines of many analytes6 supported the hypothesis that the matrix effects were due to a fall in excitation temperature caused by the dissipation of energy in the plasma by the matrix.Such a fall in excitation temperature should also affect the intensity of the emission lines of the matrix element itself, and this would become evident as a non-linearity in the graph of intensity against concentration for those lines. The objectives of this study were to seek confirmation of this hypothesis, and to assess the magnitude of a possible self-matrix effect in relation to other causes of curvature. Initial Observations with the Ca I1 317.93-nm Line The first stage in testing the hypothesis was a re-examination of the data collected during the investigation of matrix effects due to calcium on various analytes.5 In that study, the effects of calcium concentrations at nine levels, ranging from 100 to 10000 pg ml-1, on the sensitivity of 15 analyte lines were presented. The matrix effects were expressed in terms of “relative sensitivity,” which is the sensitivity measured in the presence of calcium ratioed to that measured in its absence, and are shown in Fig.1. The relative sensitivity values for a Ca I1 line recorded simultaneously (317.93 nm) have been added for comparison and show a marked similarity in curve shape to the previous data. In this instance the response of the calcium line was roughly converted into relative sensitivity by expressing the sensitivity at each concentration relative to the sensitivity at the lowest concentration. Sensitivity values were calculated as the slope of the line connecting the appropriate point on the calibration34 0.1 0.0 - > v, .- c -0.1 .- tl .- JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 . \ \ Li \ - \\ \ \ cu '\ \ - \ 1 .I 1 .a > 4- '5 0.9 .- 4- .- 0) a a .E' 0.8 a CK - 0.7 0.6 L e, > -0.2 .- tl - 2 - -0.3 -0.4 Li - - - I I I I 0 2000 4000 6000 8000 10000 Ca concentratiordpg ml-' Fig. 1. Effect of increasing Ca concentration on the relative sensitivities of concomitant analytes and the Ca I1 317.9 nm emission line. The similarity in curve shape suggests a common cause i I I I I I 1 C Concentration - Fig. 2. Schematic calibration graph. The relative sensitivity at concentration c is the slope of the cord A divided by the slope of the tangent at zero concentration B (i.e., the reference sensitivity) curve to the response from the blank solution. This value represents the chord rather than the tangent of a traditional calibration curve (Fig.2). These chords represent hypotheti- cal calibration curves at various constant excitation tempera- tures, which, according to the model, would be straight lines (between that point and the zero point) in the absence of other causes of curvature. Visual comparison of the curve shapes in Fig. 1 shows the similarity between that for calcium and those of barium and strontium. A quantitative investigation confirmed this. The relative sensitivity (S,) data for calcium at different concentra- tions (c) were fitted to the empirical exponential equation used previously for the analyte line data,S namely S,=l+A(l-e-Bc) . . . . . . (1) The adjustable parameters found were A = -0.387 and B = 1.7 x 10-4.The value of B is intermediate between that for beryllium (3.6 x 10-4) and those for strontium (9.8 x 10-5) and barium (5.8 X 10-5). This suggests that the behaviour of the matrix element line is exactly analogous to that found for the analyte lines. The fitted value of the A parameter is Zn ', 1 I I I "-0 4 8 12 16 - ~ -7 Total excitation potential of analyte/eV Fig. 3. Relative sensitivities of lines of various elements in a matrix of 10 OOO pg ml-1 of Ca plotted against total excitation potential. The curvature of a matrix line (encircled symbol) can be represented by an S, value that falls on the same trend relatively high compared with most analytes, but this reflects the high total excitation potential ( E ) of the calcium line ( i e . , the sum of the excitation potential and the ionisation potential, 13.16 eV). When the relative sensitivity of the calcium line at 10000 yg ml-1 (0.677) is plotted against total excitation potential together with the corresponding data for the analyte lines, the point falls on the same linear trend, close to that of beryllium (Fig.3). The linear trend has been shown to be the expected result5 of a change in excitation temperature from T to T + AT, according to the equation where k is the Boltzman constant and E is the total excitation potential. These initial findings supported the idea that the calibration curvature of the calcium line might be due to the mechanism of a self-matrix effect. Further experiments were designed to test the more general applicability of this hypothesis to lines of calcium.Experimental The study was restricted to one element (calcium) to preclude complications such as variable susceptibility to ionisation interference. Fourteen calcium lines (Table 1) were selected to include both atom and ion lines, and to span a wide range of excitation potentials, wavelengths and detection limits. Fif- teen test solutions were prepared in molar hydrochloric acid with calcium concentrations ranging from 0.1 to 10000 pg ml-1. Sequential measurements of the corresponding calcium line intensities were performed using the instrumental operating conditions described in Table 2. Comparable data on the matrix effects from calcium on 13 analyte lines were also obtained on the same instrument under identical condi- tions, using the previously quoted wavelengths.5 The relative sensitivities of the analytes were measured at analyte concen- trations of 5 or 0.5 pg ml-1 (for Be, Cd and Mo).Results and Discussion Calibration graphs for the calcium lines studied are shown in Fig. 4 plotted on logarithmic axes to show the linear ranges.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 35 Table 1. Description of 14 calcium lines studied Line number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Wavelengt Wnm 239.856 272.165 299.496 299.73 1 315.887 317.933 370.603 373.690 393.366 422.673 504.163 616.644 657.278 732.615 Atom/ Order ion 2 I 2 I 2 I 2 I 2 I1 2 I1 2 I1 2 I1 2 I1 1 I 1 I 1 I 1 I 1 I Total excitation potential/ eV 5.17 4.56 6.02 6.02 13.16 13.16 12.58 12.58 9.26 2.93 5.17 4.53 1.89 4.63 Oscillator strength 0.043 0.001 0.148 0.054 0.910 0.820 0.173 0.173 0.690 1.750 0.076 0.042 (R 5 x 10-5 - P 10.0 44.0 16.0 290.0 260.0 64.0 65.0 270.0 740.0 38.0 26.0 0.20 0.03 - Table 2.Description of ICP-AES instrumentation and operating conditions Applied Research Laboratories 1-m vacuum spectrometer . . . . . . Model 3520 Primary slit/pm . . . . . . . . . . . . 20 Secondary slit/pm . . . . . . . . . . 50 Gratingilines mm-1 . . . . . . . . . . 1080 Dispersiodnm mm-1 . . . . . . . . . . 0.92 (in first order) Forward powerkW . . . . . . . . . . 1.20 Viewing height above load coiYmm . . . . 15 Viewing window, square of side/mm . . . . 4 Determination . . . . . . . . . . . . Sequential Computer . . . . . . . . . . . . . . PDP 11/23 FrequencyMHz . .. . . . . . . . 27.12 Torch type . . . . . . . . . . . . Fassel Gas flow-ratedl min-1: Coolant . . . . . . . . . . . . . . 12 Auxiliary . . . . . . . . . . . . 0.8 Injector (humidified) . . . . . . . . 1.00 Spray chamber . . . . . . . . . . . . Conical single pass Nebuliser, concentric glass Meinhard . . . . TR-30-3A Solution uptake rate (unpumped)/ml min-1 . . 2.40 Uptake tube (polyethylene)/mm . . . . . . 400 x 0.8 i.d. Nebuliser tip wash/ml . . . . . . . . . . 0.5 Pre-flush time, number of and time of integratiods . . . . . . . . . . . . 30,3 x 3 2 1 I I I I 1 I I - 4 - 3 - 2 - 1 0 1 2 3 4 5 Log (Ca concentrationlyg ml-1) Fig. 4. Calibration graphs of the 14 calcium lines plotted between their detection limits and the concentration at which S, is equal to 0.95.Marked on each line is also the background equivalent concentration (e) and the concentration at which S, becomes 0.99 (@) Lines are plotted with their true relative intensities, i.e., the most sensitive line is number 9, at the top. Each line is plotted over its “effectively linear range,” viz. from its detection limit (q) to the concentration cggY0 at which the relative sensitivity has fallen to 0.95, and therefore where the deviation from linearity equals 5% of the intensity. The non-linearities are visually imperceptible at the scale of this diagram. Estimation of Relative Sensitivities If the postulated self-matrix effect were the sole cause of curvature of calibration lines, equation (2) would predict a linear relationship between the relative sensitivities of the calcium lines at any particular concentration, and their total excitation potentials.The slope of this line should correspond to the same fall in excitation temperature as that implied by matrix effects on analyte lines. To test this prediction, accurate estimates of the sensitivity of each line at zero concentration and at other points on the calibration curve were needed to calculate relative sensitivi- ties. Therefore the gross intensity data for each calcium line were fitted to a specially derived equation that allowed an objective quantification of intensity of the blank (lo) and the initial (or reference) sensitivity (So). The equation was derived by integration of equation (1), the empirical relationship developed to describe matrix effects from calcium on a series of analytes.5 The relative sensitivity (S,) is equal to the sensitivity (dlldc) at a given concentration (c) divided by the reference sensitivity (So), so from equation (1) we obtain Integrating we get I = So (K + c + Ac + - A e-Bc) B where K is the integration constant.When c = 0 then I = lo and . . . . (4) Hence . . . (6) Substituting equation (6) into equation (4) we have36 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 ~~ Table 3. Parameter for equation (8) fitted to the calibration data of the calcium lines Equation fitting parameters B Line Sd number Wavelengthhm ZdmV mV PLg A 1 239.856 15.37 0.2505 -0.3584 0.5333 X 10-4 2 272.165 147.70 0.0595 -0.2862 5.9280 x 10-4 3 299.496 174.07 0.4078 -0.3977 0.5243 X 10-4 4 299.73 1 178.94 0.5669 -1.2538 0.2561 x 10-4 5 315.887 0.96 1 .W31 -0.7532 0.6411 x 10-4 6 317.933 2.95 5.0560 -1.3526 0.5770 x 10-4 7 370.603 4.05 0.8342 -2.9471 0.1006 X 10-4 8 373.690 4.38 1.0863 -0.6508 0.5697 x 10-4 9 393.366 3.54 90.0660 -0.4228 95.3300 x 10-4 10 422.673 4.50 2.2775 -0.2296 6.0899 x 10-4 11 504.163 392.10 0.6148 -0.1986 1.0661 X 10-4 616.644 258.73 0.2090 -0.6799 0.2456 X 10-4 12 13 657.278 551.00 0.0144 14 732.615 149.00 0.4402 -0.5241 0.2843 x 10-4 - - Table 4.Example of goodness of fit between experimental data from Ca I 370.6 nm and equation (8). The scaling factor for the residuals was 0.008 of the gross intensity Intensity/mV Ca concentration/ ResiduaY Scaled Experimental Fitted mV residual !% d-' 0.1 4.16 4.12 0.04 1.15 0 3.99 4.05 -0.06 - 1.91 1 4.95 4.87 0.08 1.90 10 12.33 12.40 -0.07 -0.74 25 24.68 24.89 -0.21 - 1.05 50 45.78 45.73 0.05 0.15 75 66.86 66.55 0.31 0.58 100 86.93 87.36 -0.43 -0.61 250 211.53 211.81 -0.28 -0.17 500 417.97 418.05 -0.08 -0.02 750 628.87 622.75 6.11 1.22 1000 830.40 825.89 4.51 0.68 2 500 2001.00 2012.86 -11.86 -0.74 5000 3845.00 3870.95 -25.95 -0.84 7 500 5649.33 5582.12 67.21 1.49 10 000 7102.76 71 49.97 -47.30 -0.83 giving the final equation as A so B I = 10 + (1 + A)S$ - - (1 - e-Bc) - Equation (8) is a calibration curve giving I as a function of c and empirical constants.The values of the four fitting parameters I, So, A and B were estimated for each calcium line using a numerical method based on weighted non-linear fitting,' and are given in Table 3. One exception was line number 13 (Ca 1657.3 nm) where the deviation from linearity was so slight as to make significant fitting of equation (8) impossible.Linearity was assumed for this line. The goodness of fit of equation (8) to the experimental data over six orders of magnitude was generally excellent. This can be judged from the detailed information of the fitting of one typical line given in Table 4. The fit was assessed by scaling the residuals to an estimate of the precision of the intensity measurement between nebulisations, found to be 0.8% of the intensity. The scaled residuals have a mean of 0.02, a standard deviation of 1.07 and show no trends. These features indicate that the equation has no lack of fit but only deviations due to random error. As the empirical equation described the experimental data very well, it could be used to estimate the reference sensitivity (So), which is equal to the limiting value of dlldc as c approaches zero.From the reasoning given above the curvature of each calcium line at concentration c could then be expressed as the relative sensitivity (SJ, calculated from the experimental intensity I, using the equation &= (4 -1o)lSlJC . . . . . . (9) Matrix Effects and Calibration Curvature Related to Excita- tion Potential Matrix efseects The matrix effects of calcium on lines of 13 other elements were measured in this study for comparison with the proposed self-matrix effects on the calcium lines. The concentration of the calcium was held at 5000 pg ml-1. The relative sensitivities obtained are shown in Fig.5(a) plotted against the total excitation potentials. A linear relationship between In& and E is evident in this new data, similar in nature to that shown in Fig. 3. The regression statistics (with parenthesised standard errors) are: slope -0.0124 (0.0024) and intercept 0.004 (0.028). The fall in excitation temperature implied by this slope is calculated from equation (2) as AT = -60 f 12 K, assuming that T = 7500 K. The intercept is not significantly different from zero, implying that matrix effects would not be observed in a line of E = 0. The shaded region in Fig. 5(a) shows the 95% confidence band for predicted values of Ins,. This trend can be taken as a model for plasma matrix effects. Calibration curvature caused by the proposed self- matrix effect should comply with this model, and result in experimental points falling within the 95% confidence band delineated in Fig.5(a). Calibration curvature Fig. 5(b) shows the experimental relative sensitivities of the calcium lines at a concentration of 5000 pg ml-1, as calculated from equation (9), plotted against their total excitation potentials. This particular concentration of calcium wasJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 1.2 1.1 1 .o > > cn C Q, (c1 Q1 c .- .- c .- 2 0.9 .- 4- - a 0.8 0.7 -0.4 ’ 1 1 I I L I 1 0 5 10 15 20 0 5 10 15 Total excitation potential /eV Fig. 5. (a) Relative sensitivities of lines of 13 elements in a matrix of 5000 pg ml-1 of Ca plotted against total excitation potential.The straight line is the regression of lnS, on E. The shaded area represents the 95% confidence region for predicted values of Id,. (b) Relative sensitivities of 13 calcium lines at 5000 pg ml-1 of Ca, superimposed on the confidence region shown in (a) selected to give the maximum number of lines with simul- taneously measurable curvatures. The points are shown superimposed on the regression confidence band derived from the matrix effects shown in Fig. 5(a). Of the 14 points 11 fall close to the linear trend predicted for self-matrix effects, and are within the confidence band for predictions from the model. These points represent lines conforming to the self-matrix effect model. The regression coefficients and standard errors derived from the 11 conforming points are: slope, -0.0095 (0.0024); intercept, -0.007 (0.021).These are not significantly different from the corresponding statistics derived from matrix effects. The intercept value is not significantly different from zero which implies that perfect calibration linearity is approached as the excitation potential of the lines tends to zero. The similarity in the slopes of the two regression lines suggests that no other systematic cause of line curvature is producing a measurable effect. A hypothetical ionisation interference for example would affect the S, values of all of the ion lines of calcium equally, rather than as a function of E. The atom lines also would be affected identically, but in the opposite sense. However, no such effect is apparent. Trans- port effects, such as aerosol ionic re-distribution8 would, if present, affect all of the calcium lines to the same extent, and not as a function of E.Other causes of curvature such as self absorption and line broadening are specific to particular lines. Any effects of these superimposed on the self-matrix effect would become appar- ent as a random scatter of points about the regression line or random outlying points. Both of these features are seen in Fig. 5(b). However, because of the significant trend, it seems that the self-matrix effect must be the dominant factor for the 11 conforming points. Lines Showing “Anomalous Curvature” Any causes of curvature additional to the proposed self-matrix effect would be superimposed on the trend described above. In fact three of the 14 calcium lines studied gave relative sensitivity values that were lower than the trend of the other values and outside the 95% confidence region.These three calcium lines are more curved than can be accounted for by self-matrix effects alone (“anomalous curvature”). Line number 10 (Ca I 422.7 nm) is a resonance line with an extremely high value of oscillator strength 0. The propensity to self absorption is given by the absorbance31 in the equation Absorbance a NfAvD . . . * (10) where N is the concentration of the absorbing analyte species and AvD is the Doppler half-width, which may in turn be calculated from AvD = 7.162 X 1 0 - 7 ~ ~ a . . (11) where vo is the frequency (cm-I), T the temperature (K) and M the relative atomic mass.1 In this instance N, Tand Mare all constant and for a wavelength h we have Absorbance m f i .. . . . . (12) The product fi is given for each line in column 7 of Table 1 and the value for line number 10 is more than twice that of any other line. This line is therefore seemingly the most prone to self-absorption effects, which may account for the lower value of relative sensitivity. Line number 9 (Ca I1 393.4 nm) is seven times more sensitive than any other line studied (data in column 3 of Table 5). The value of fi for this line is relatively high but similar to other lines that are not anomalous in curvature (numbers 5 and 6). Self absorption however, has been reported for this line,3 and this seems to be the most probable explanation for the earlier onset of curvature. The third line to show significantly greater curvature than predicted is line number 2 (Ca 1272.2 nm).A wavelength scan of this line suggested a possible hyperfine structure with two peaks, a circumstance in itself likely to give rise to non- linearity. . . Conventional Views of ICP-AES Calibration Curvature Refuted The data obtained can also be used to check the applicability of the two previously mentioned assumptions about the effectively linear ranges of analytical calibrations in ICP-AES.38 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Table 5. Curvature data for the calcium Line number Wavelengtidnm 1 239.856 2 272.165 3 299.496 4 299.731 5 315.887 6 317.933 7 370.603 8 373.690 9 393.366 10 422.673 11 504.163 12 616.644 13 657.278 14 732.615 lines Sensitivity related to line 13 (log,,) 1.50 0.65 1.70 1.84 3.86 4.67 3.91 3.57 5.52 4.11 1.79 1.41 1.97 0.00 Relative sensitivity at 5000 pg ml-1 Ca 0.952 0.816 0.946 0.903 0.886 0.822 0.918 0.908 0.840 0.934 0.943 1 .Ooo 0.949 - Concentration of Ca at which 5% curvature occurs pg ml-1 5 800 700 5 200 3 200 2 200 1300 3 200 2900 40 800 5 700 6 300 >loo00 7 200 (‘95%)’ Detection limit (2a) pg ml-1 1 .o 20.0 4.0 2.0 0.03 0.004 0.04 0.06 0.0005 0.03 5 .O 10.0 200.0 5.0 (CLY “Linear range’’ orders of magnitude, 3.8 1.5 3.1 3.2 4.9 5.5 4.9 4.7 4.9 4.4 3.1 2.8 >1.7 3.2 log (cg5%/cL) The assumption that all lines of the same analyte curve to the same extent at any one concentration is clearly refuted by the variability of relative sensitivity at 5000 pg ml-1 of calcium [Fig.5(b)]. Failure of this assumption is also clear in Fig. 4, which shows the concentrations at which the calcium lines display specific values of relative sensitivity, arbitrarily set at 0.99 and 0.95. The second assumption was that for all lines of a particular element there is a constant ratio between the onset of curvature and the detection limit. This can be tested by reference to the data given in columns 5 and 6 of Table 5. The detection limit (q) was here quantified as the concentration equivalent to twice the standard deviation of the blank response. The ratios cg5yolcL given in column 7 of Table 5 vary considerably: even excluding line number 2 (Ca I 272 nm) they range from 102.8 to 105.5. A tentative observation from this limited data is that lines of high excitation potential, although most susceptible to curvature at a given concentration of calcium, often have the longest linear ranges, simply by virtue of their high sensitivities and resultant lower detection limits.Conclusions This investigation into the causes of calibration curvature of calcium lines in ICP-AES, although clearly limited in its scope, has found support for a new mechanism producing non-linearity . This postulated mechanism is a self-matrix effect whereby the increasing concentration of an element affects the sensitivity of its own emission lines. The self-matrix effect has been found to be quantitatively similar to the ordinary matrix effect, when both are studied as a function of the matrix element concentration, and also when both are studied as a function of the excitation potential of the emission lines.The characteristics of these effects are thus consistent with a progressive reduction in excitation temperature caused by increasing concentrations of the element. The experimental results suggest that, for calcium, the self-matrix effect is the dominant cause of curvature in the majority of the emission lines studied. This conclusion applies to the compromise operating conditions typical of routine analysis used in this study, and to the limited concentration range covered. At high plasma viewing heights, however, increased self-absorption and ionisation interference would be expected to play an increased role. For elements other than calcium, the proposed mechanism would give rise to predic- tions of greatest curvature among elements with a high capacity to cool the plasma6 and for lines of high total excitation potential.Inespective of the actual cause of curvature, a new empirical calibration equation, derived from the description of ordinary matrix effects, gave an excellent fit for the data for the calcium lines. The use of this equation in routine analysis would reduce bias due to lack of fit between the calibration points and the mathematical line, as it seems to be inherently the correct shape. In addition, such an equation would be of great asssistance in identifying “outliers” among the calibra- tion points. The splined polynomial curves that are normally used in ICP-AES software packages offer neither of these advantages. Thus the use of the equation could provide a further small improvement to ICP-AES accuracy. Certain methods described for removal of matrix effects on analyte lines, such as the parameter-related internal standard method (PRISM), in theory could be utilised to remove the curvature of calibration lines caused by the self-matrix effect and thus extend their useful range. This preliminary study suggests that a detailed survey of the curvature of calibration lines of a wide range of elements should be undertaken. Such a study may reveal a predictable pattern which would be valuable in the selection and fitting of the most appropriate line for any analytical problem. It would also serve to substantiate or refute the proposed self-matrix effect . 1. 2. 3. 4. 5. 6. 7. 8. References Kirkbright, G . F., and Sargent, M., “Atomic Absorption and Fluorescence Spectroscopy,” Academic Press, London, 1974, Boumans, P. W. J. M., Wagenaar, H., and De Boer, F. J., paper presented at the 17th Colloquium Spectroscopicum Internationale, Florence, 1973, Vol. 1, p. 114. Human, H. G. C., and Scott, R. H., Spectrochim. Acta, Part B, 1976,31,459. Alkemade, C. T. J . , Hollander, T., Snellerman, W., and Zeegers, P. J. T., “Metal Vapours in Flames,” Pergamon Press, Oxford, 1982, pp. 182-187. Thompson, M., and Ramsey, M. H., Analyst, 1985,110,1413. Ramsey, M. H., and Thompson, M., J . Anal. At. Spectrom., 1986, 1, 185. Head, J. H., USAir Force Acad. Tech. Rep., No. 70-5, 1970, Boraviec, J. A., Boorne, A. W . , Pillard, J. H., Cresser, M. S . , and Browner, R. F., Anal. Chem., 1980, 52, 1054. pp. 34-43. 49 PP. Paper 56/38 Received May 16th, 1986 Accepted August 26th, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200033

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 39 Direct Atomic Spectrometric Analysis by Slurry Atomisation Part 1. Optimisation of Whole Coal Analysis by Inductively Coupled Plasma Atomic Emission Spectrometry Les Ebdon and John R. Wilkinson" Department of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth, Devon PL4 8AA, UK The parameters that affect the slurry atomisation of whole coal in an inductively coupled plasma for atomic emission spectrometry have been investigated and optimum conditions for quantitative analysis using calibration by simple aqueous standards established. A high solids nebuliser and both argon and nitrogen cooled plasmas were used. Of the parameters, particle size, slurry concentration (up to 25% mN) and sample pumping rate, the first was shown to have most effect on atomisation efficiency. The operating parameters of each plasma were optimised using the simplex technique and manganese determined in whole coal slurries with an average recovery of 95%.Keywords: Slurry atomisation; whole coal analysis; solid sampling; inductively coupled plasma atomic emission spectrometry The direct atomisation of solid or slurried samples would appear to offer a rapid method for the analysis of intractable matrices that might normally require pre-treatment using lengthy digestion, fusion or combustion procedures. Tra- ditionally arc and spark atomic emission spectrometry have been important techniques for the analysis of solid samples. However, matrix interferences and irreproducible arc con- ditions may cause poor precision while the small amount of sample volatilised in spark methods may result in poor sensitivity, It is usually necessary to calibrate using standards with compositions closely resembling those of the samples.These techniques are better suited to the analysis of bulk solids than powders. In contrast, atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry (ICP-AES) offer good sensitivity, precision and relative freedom from chemical interferences. A comprehensive review by Langmyhrl described the various approaches to the direct analysis of solid samples using atomic absorption spec- trometry and it contains many references to both theoretical considerations and practical applications.Another review ,2 specifically concerning the direct analysis of solids, dealt more generally with atomic emission and atomic fluorescence as well as atomic absorption spectrometry. The ready availability of plasma emission spectrometers and their ability to perform simultaneous or rapid sequential multi-element analysis has focused attention on the desirabil- ity of introducing solid samples into plasma sources. Currently sample preparation time may exceed instrument time by orders of magnitude and sample preparation may be prone to errors through contamination or sample loss, or be hazardous given the vigorous dissolution procedures using concentrated acids (especially HF and HC104) sometimes required. Coal is a good example of a material that is tedious to bring into solution.For these reasons several workersslo have con- sidered the possibility of introducing suspensions of powders into plasmas for atomic emission spectrometry. This process, termed slurry atomisation, has advantages in that it involves minimum modification to existing instrumentation and is potentially capable of calibration with aqueous standards. While slurry atomisation has been applied for some years in flame spectrometryll-14 it is considered that the high tem- * Present address: EDT Research, 14 Trading Estate Road, London NWlO 7LU, UK. perature and longer residence time of the inductively coupled plasma make it a preferable atomiser for solid samples. Clearly the efficiency of atomisation, defined by Willis14 as the ratio of the signal from the slurry sample to that from an aqueous solution containing an equivalent concentration of analyte, may be controlled by a variety of parameters.This paper describes the investigation of these parameters for coal slurry analysis in the ICP and their optimisation for the determination of manganese in coal. The development and characterisation of a unique nebulisa- tion principle by Babingtonls has provided a means of sample introduction that is well suited to slurried samples. The advantage of this type of nebuliser is that the sample is no longer required to pass through a very narrow capillary (cu. 0.35 mm). Instead the sample flows through a relatively broad tube (up to a few mm) and is delivered to a narrow orifice through which argon issues at close to supersonic velocity.Shattering of the sample stream by the argon allows direct generation of high-density , finely dispersed aerosols from a variety of complex materials. Several workers have reported modified versions of this nebuliser.l"l8 In the nebuliser developed in our group19 the argon used for nebulisation is discharged from a narrow orifice located in a V groove that had been cut along one end of the nebuliser body. Sample is allowed to trickle down the groove and the greater contact established between sample and gas results in efficient nebulisation. Experimental The Radyne R50P free-running plasma generator and optical system used have been described previously,20 as has the P"E high-solids nebuliser used,l9 which is a one-piece Babington-type with a gas orifice of 0.35 mm and a solution orifice of 0.8 mm.The nebuliser was force-fed by a small peristaltic pump (Schucho Mini Pump Mark IV, 60 rev min-l, Schucho Scientific, Halliwick Court Place, London, UK). The aerosol produced was transported to the central injector tube of the plasma torch via a laboratory constructed double-pass spray chamber.19 In later experiments, when high pumping rates were used to deliver coal slurries to the nebuliser a recycling spray chamber (Fig. 1) was used to conserve sample solution. A modified Greenfield torch was used.20 An inverted image of the plasma was projected 1 : 1 on to the 25-pm entrance slit40 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 of the monochromator using a quartz lens, 7.5-cm focal length.The spectroscopic emission lines of interest were isolated using a 0.5-m Ebert scanning monochromator [Jarrell Ash (Europe), Le Locle, Switzerland]. The radiation was then focused via a 25-pm exit slit on to a photomultiplier tube (Hamamatsu R106, operated at 500 V). The signal from the photomultiplier was amplified using a linear picoammeter (LM 10, Chelsea Instruments Ltd., London, UK), and fed into a three-pen potentiometric chart recorder (Type MC 641-32, Watanabe Instrument Corporation, Japan). Coal samples for preliminary slurry investigations were ground using a Tema Laboratory Disc Mill (Model T100, Siebtechnick, Mulheim, FRG). Subsequent samples were ground using a McCrone Micronising Mill (McCrone Research Associates Ltd., 2 McCrone Mews, Belsize Lane, London NW3, UK).Coal ash solutions were analysed for manganese using an Instrumentation Laboratory IL 151 atomic absorption spec- trometer (Instrumentation Laboratory, Lexington, MA, USA). Particle size distributions were determined using a Coulter Counter Model T A I1 Multichannel Particle Counter (Coulter Electronics, Northwell Drive, Luton, Bedfordshire, UK). Proprietory Coulter electrolyte and dispersant were used to suspend the samples that were sized using a counting tube with a 140-pm orifice. Reagents Manganese calibration standards for both the atomic absorp- tion and inductively coupled plasma methods were prepared by serial dilution of a stock 1000 pg ml-1 solution (BDH Chemicals, Poole, Dorset, UK). For slurry analyses dilutions were made using a stock diluent of Triton X-lo0 solution (BDH Chemicals, 1% VIV).This was added carefully as a wetting and dispersing agent, taking care not to cause excess foaming. The following analytical-reagent grade reagents were used in the coal ash dissolution procedure. Concentrated hydrochloric acid. Concentrated nitric acid. Concentrated hydrofluoric acid. Saturated boric acid solution (6% mlV in distilled, de-ionised water). Procedures Analysis of coal using frame atomic absorption spectrometry Coal samples were ashed and the ash dissolved using the American National Standards InstituteIAmerican Society for Testing Materials method ANSUASTM D 3683-78.21 Using this method coal ashed at 500 "C was dissolved with aqua regia and hydrofluoric acid. Saturated boric acid solution was used to neutralise excess of hydrofluoric acid.The solutions obtained were analysed for manganese, using atomic absorp- tion spectrometry. The spectrometer operating conditions used were those recommended by the instrument manufac- turer. Grinding of coal samples For the particle size investigations 40 g of one coal sample were ground using the Tema Laboratory Disc Mill. After grinding the whole of the sample for approximately 2 min, the sample was tipped into a series of sieves and the different size fractions isolated by manual agitation of the sieves. The particle size ranges were 250-125, 125-106, 106-63, 63-53, 53-38 and less than 38 pm. Portions of each fraction were used to prepare the coal slurries (4% mlV) that were used to assess the effect of particle size on emission signal.For the slurry simplex optimisation procedures a second 40-g portion of the same coal sample was ground in four 10-g portions using the Tema Laboratory Disc Mill. After screen- ing the powder produced, those particles not passing through a 38-pm sieve were combined and ground repeatedly until all of the sample had been reduced to less than 38 pm in diameter. All the particles were combined and a coal slurry (4% mlV, 1 1 total volume) prepared using the whole of the sample. This slurry was used during optimisation of the all-argon and nitrogen-cooled plasmas. In later experiments coal was prepared for quantitative analysis using the McCrone Micro- nising Mill. Typically 5 g of sample were dry-ground for approximately 30 min and the sample mixed thoroughly prior to preparation of the slurry (10% mlv).Optimisation studies The variable step-size simplex method, as previously applied to the inductively coupled plasma by Ebdon et ~1.22 was used to optimise plasma performance. Operating conditions were optimised for manganese in coal slurries in both all-argon and nitrogen-cooled plasmas using the manganese 403. l-nm atom line. Reasons leading to the selection of this line instead of the more sensitive and more commonly reported 257.6-nm ion line are discussed later. Manganese was selected due to the availability of a great deal of analytical information23 on the determination of managanese in aqueous samples in both types of plasma. Quantitative emission measurements were made using an all-argon plasma only, using the optimised operating condi- tions given in Table 1.The emission line was repetitively F Fig. 1. Recycling nebulisation system. A, Peristaltic pump, deliver- ing 15 ml min-1 of slurry; B, PTFE high solids nebuliser; C, double-pass spray chamber; and D, slurry reservoir Table 1. Simplex optimised operating conditions for the determination of manganese by direct atomisation of whole coal slurries. SBR values measured at the manganese 403.1-nm atom line, using a 4% m/V coal slurry (particles smaller than 38 pm), pumped at 2.0 ml min-1 Plasma type Parameter All argon Nitrogen cooled Coolant gas flow-rate*/l min-1 2.65 (Ar) 4 -0 (N2) Plasma gas flow-rate t/l min-* 13.3 13.3 Injector gas flow-rateA min-1 0.35 0.38 Power $/kW . . , . . . .. 0.29 0.40 Observed heighvmm above 33.5 loadcoil . . . . . . . ; 34 * Defined in this paper as the outer gas flow. t Defined in this paper as the intermediate gas flow. $ Defined in this paper as the power coupled into the plasma and measured calorimetrically (see reference 22), the power supplied by the generator is likely to be a factor of three greater.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 A ( a ) 41 2.5 2.0 1.5 1 .o 0.5 0 1.0 2.0 3.0 4.0 5.0 0 5.0 4.0 3.0 2.0 1 .o , I I I I I I I I I I 1.0 2.0 3.0 4.0 5.0 0 1.0 2.0 3.0 4.0 5. Pumping rateiml rnin-1 12.0 10.0 8.0 6.0 4.0 2.0 0 I I 1 1 I 1.0 2.0 3.0 4.0 5.0 Fig. 2. Signal to back round ratio as a functicn of sample pumping rate for: A, all-argon plasma; and B, nitrogen-cooled plasma.Coal slurries of: (a) 1% mlV; (b) 4 8 mlV; ( c ) 10% mlV; and ( d ) 25% mlV 2.5 .- c h T3 2.0 g 2 1.5 n 2 1.0 3 0 - m rn 0.5 0 Slurry concentration, O/O m/V Fig. 3: Signal to background ratio as a function of slurry concentration, for: A, all-argon plasma; and B, nitrogen-cooled plasma. Sample pumping-rates of (a) 0.67 ml min-1; ( b ) 1.4 ml min-1; ( c ) 2.0 ml min-I; ( d ) 3.0 ml min-1; and ( e ) 5.0 ml min-l scanned to allow background correction because of a matrix peak adjacent to the analytical line. Results and Discussion Optimisation of Plasma Performance The initial operating conditions selected were those reported previously23 when this instrumentation had been used to determine manganese in aqueous solutions of metallurgical samples. The most sensitive manganese analytical line, the 257.6-nm ion line, when observed using the reported optimum operating conditions, failed to yield a measurable signal to background ratio (SBR) for manganese when coal slurries were introduced.Changes in these conditions, designed to increase the residence time of the coal particles in the plasma fireball, failed to produce the desired increase in SBR. Possible reasons are discussed later. As previous studies had also provided much information on the manganese 403.1-nm atom line, the reported optimum operating condi- tions for this line were used and a measurable SBR for manganese was observed at this line when atomising coal slurries. Consequently this line was used throughout subse- quent experiments. The parameters investigated were those expected to exert the greatest effect on the SBR, i.e., particle size, slurry concentration and sample pumping rate.Particle sizes were investigated in the ranges 250-125, 106-63,63-53, 53-38 and <38 pm. Slurry concentrations and sample pump- ing rates were varied in the ranges 1-25% mlV and 0.67-15 ml min-1, respectively. Both types of plasma were used and the results are shown in Figs. 2-7. From these results the following general conclusions were drawn. Particle size was the critical factor in determining the 0 .- c 1.0 ' U 3 rn Y m P 0 a 2 c - & 0.5 . v) .- I 1 I 0 50 100, 150 200 Particle size/pm Fig. 4. Si nal to background ratio as a function of particle size, pumping a f% mlVslurry at 2.0 ml min-1: A, all-argon plasma; and B, nitrogen-cooled plasma magnitude of the emission signal.Only particles of less than 38 ym produced significant signals, while those in the range 38-53 ym produced signals at least twice the size of particles in the range 53-63 pm. Particles greater than 70 pm contributed virtually nothing to the over-all SBR. Subsequent experiments suggest that large particles are excluded by the spray chamber and do not reach the plasma.42 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 0 6.0 I .- .I- I I I I I 0 50 100 150 200 Particle sizeipm Fig. 5. Si nal to background ratio as a function of particle size, pumping a fo% rnlvslurry at 2.0 ml min-1: A, all-argon plasma; and B, nitrogen-cooled plasma 0 50 100 150 200 Particle size/pm Fig. 6. Signal to background ratio as a function of particle size, pumping a 1% rnlvslurry at 5.0 ml min-1: A, all-argon plasma; and B, nitrogen-cooled plasma 12.0 - 0 .- c z 5 8.0 - 2 -0 rn Y m a 0 c - m .- 5 4.0 v) 0 50 100 150 200 Particle size/Vm Fig.7: Signal to background ratio as a function of particle size, pumping a 4% rnlVslurry at 5.0 ml min-1: A, all-argon plasma; and B, nitrogen-cooled plasma To ensure that the reponses from different particle size fractions were not due to the association of the manganese with different particle size fractions, samples from each size fraction were analysed for manganese content using flame atomic absorption spectrometry. The results showed no significant differences between the various size fractions. An average value of 90 k 1.5 pg g-1 was obtained and this agreed fairly well with an independent analysis figure of 99 pg g-1, determined by neutron activation analysis.For the all-argon plasma, SBR increased rectilinearly with increasing slurry concentration up to approximately 20% mlV. The deviation from linearity was ca. 5% at 25% mlV. For the nitrogen-cooled plasma SBR increases linearly (slope = 0.4) up to slurry concentrations of ca. 10% mlV. While the absolute emission signal was greater in the nitrogen-cooled plasma, the increased background level again resulted in inferior SBR and this is reflected in Figs. 2-7. The signal to background ratio increased with increase in sample pumping rate, more so for the all-argon plasma than for the nitrogen-cooled plasma, which showed little change in SBR at sample pumping rates greater than 2.0 ml min-1.For the all-argon plasma, as the slurry concentration was increased, the range of linearity of response was extended to higher pumping rates. The greatest SBR was obtained with a sample pumping rate of ca. 15 ml min-1. However, the improvement in SBR obtained on increasing the sample pumping rate from 5 to 15 ml min-1 was only 10-15%. While excessive sample consumption at such elevated pumping rates was avoided by using the recycling nebulisation system, prolonged pumping and recycling of residual slurry was clearly inadvisable. Plasma performance for slurry atomisation was then opti- mised using the variable step size simplex procedure. The SBR was measured at the 403.1-nm manganese atom line in both the all-argon and the nitrogen-cooled plasmas.A slurry concentration of 4% mlV and a sample pumping rate of 2.0 ml min-1 were chosen in order to minimise consumption of slurry while producing an acceptable signal size. The coal sample was ground and sieved repeatedly until all the particles passed through a 38-pm sieve. Particle size analysis showed that ca. 4% (by number) of the particles were smaller than 10 pm. The optimum conditions established for both plasmas are given in Table 1. Again the all-argon plasma gave better results in terms of SBR because of the lower background observed. Under the optimum conditions the apparent concentration of manganese in the coal, as determined using aqueous calibration standards in the all-argon and nitrogen- cooled plasma, was 48.6 and 36.6 pg g-1, respectively (compared with 90 pg g-1 by AAS).These concentrations represent atomisation efficiencies (relative to aqueous solu- tions) of 60 and 40% for the all-argon and nitrogen-cooled plasmas, respectively. It was decided at this point to use the all-argon plasma for all subsequent experiments because of the better SBRs available relative to the nitrogen-cooled plasma. A comparison of the optimum conditions for the determina- tion of manganese in coal slurries and aqueous solutions using an all-argon plasma is shown in Table 2. Several significant and inter-related features were apparent. The height of observation above the load-coil was somewhat higher for slurries than for aqueous solutions. Although the plasma (intermediate) gas flows were similar, the coolant (outer) and injector gas flows were markedly lower for slurries. The outcome of a combination of these factors was a shift towards longer residence times in the atom cell.This effect would be expected to promote more efficient matrix destruction and analyte atomisation. That lower power was preferred for slurry atomisation is perhaps surprising. It was thought that this may have been the outcome of having optimised the operating conditions for signal to background. Whereas theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 43 Table 2. Comparison of simplex optimised operating conditions for the determination of manganese in aqueous solution and in whole coal slurries using an all-argon plasma. SBR values measured at the manganese 403.1-nm atom line Sample type Parameter Aqueous Whole coal solution* slurry Coolant gas flow-rate/l min-1 8.7 2.65 Plasma gas flow-rate/l min-1 .. 14.0 13.3 Injector gas flow-rate4 min-1 0.58 0.35 Power/kW . . . . . . . . 0.50 0.29 Observation height/mm . . aboveloadcoil . . . . . . 26 34 * From reference 23. .! I80 I 1 c n 2.0 5 9 n 01 Y 1.5 0 4- 1.0 m C 01 .- 0.5 0 PowerikW Fig. 8. Univariate search for optimum power: slurry atomisation using simplex optimised conditions. Measurements taken at the manganese 403.1-nm atom line, pumping a 4% m/V slurry at 2.0 ml min-1: a, signal; b, background; and c, signal to background ratio. A, “Optimum” value as identified by simplex procedure background increases linearly with increasing power, this may not be so for the analyte emission signal.If this is so then a point will be reached when the background is increasing more rapidly than the signal and for slurry atomisation this point appears to be reached at relatively low power. This was confirmed by performing a univariate search to identify the optimum power (Fig. 8). Quantitative Determination of Manganese When coal slurries were analysed with a sample pumping rate of 2 ml min-1 and a 4% m/V slurry containing particles in the range 20-40 pm an apparent atomisation efficiency of 6O%, relative to aqueous solutions was obtained. In order to maximise the SBR a sample pumping rate of 15 ml min-1 and the recycling nebuliser were used. The apparent atomisation efficiency was increased to 70-75%. It was felt that this figure could be further improved if the proportion of particles in the sample smaller than 10 ym could be increased.Consequently a portion of the coal sample was re-ground using the micronising mill. The number of particles smaller than 10 ym was increased from ca. 4 to 40% and more than 95% of the particles were smaller than 25 pm. Sample preparation time was about 30 min. Wet grinding of the sample using aqueous or organic solvents might be expected to produce even more small particles. However, this may also result in the loss of soluble inorganic and/or organic trace metal components from the coal or introduce contamination. The ground coal sample was used to prepare three slurries (10% m/V dispersed in aqueous 1% V/V Triton X-100) and attempts were made to determine manganese by direct atomisation of these sIurries in an all-argon plasma.The operating conditions used were those previously identified by Table 3. Direct determination of manganese in coal* by slurry atomisation in an all-argon inductively coupled plasma? Sample No. Mn found/pg g-1 Recovery, YO 1 82.8 f 9.9 92 k 12 2 95.4k 11.4 106 f 12 3 79.2 k 11.9 88 k 15 Mean 86.0? 11 95k 13 Mn found by AAS: . . 90 k 1.5 (5 replicates) * 95% of particulates in the slurry <25 pm diameter. t Errors refer to +2 standard deviations. . . . . . . . . the simplex procedure. The apparent atomisation efficiency was again increased and found to be in the range 88-106Y0. The three replicate coal slurries were analysed for manganese content and the results are given in Table 3.Conclusion The atomisation of slurried samples using the high tempera- ture atom cell of the inductively coupled plasma has been investigated. Traditionally, problems of nebuliser and burner blockage are encountered when samples containing suspen- ded solids or high salt concentrations are analysed using conventional flame techniques. A significant contribution to the avoidance of such problems has been made by the modified Babington-type nebuliser used throughout this work. Blocking of this nebuliser is avoided by pumping the slurry into the nebulising gas stream via a relatively wide diameter tube (ca. 0.8 mm). With this arrangement the nebuliser has been found to give excellent service over a wide range of slurry concentrations (up to 25% mlv), introduced at high sample pumping rates (10-20 ml min-1) and containing relatively large particles (up to 250 pm with a pumping rate of 4 ml min-1 and a 10% m/V slurry).Excessive consumption of slurry when using elevated pumping rates was avoided using a gas-tight recycling system. Owing to the much higher background of the nitrogen- cooled plasma, the all-argon plasma offered better signal to background ratios and appears to be better suited to routine analytical applications. Additionally, the SBR was found to be dependent on slurry concentration, sample pumping rate and particularly particle size. The feasibility of slurry atomisation and its application to the determination of trace metals is clearly demonstrated and the method shows considerable promise for the routine analysis of powdered coal and related materials. 1.2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. References Langmyhr, F. J., Analyst, 1979, 104, 993. Van Loon, J. C., Anal. Chem., l980,52,955A. Fuller, C. W., Hutton, R. C., and Preston, B., Analyst, 1981, 106, 913. Wilkinson, J. R., Ebdon, L., and Jackson, K. W., Anal. Proc., 1982, 19, 305. Sugimae, A,, and Mizoguchi, T., Anal. Chim. Acta, 1982,144, 205. Spiers, G. A., Dudas, M. J . , and Hodgins, L. W., Clay Clay Miner., 1983, 5, 31. Watson, A. E., and Moore, G . L., S. Afr. J. Chem., 1984,37, 81. Mohamed, N., and Fry, R. C., Anal. Chem., 1981, 53,450. Mohamed, N., Brown, R. M., Jr., and Fry, R. C., Appl. Spectrosc., 1981, 35, 153. Wichman, M. D., Fry, R. C., and Hoffman, M. K., Appl. Spectrosc., 1986, 40, 351. Gilbert, P. T., Jr., Anal. Chem., 1962, 34, 1025. Mason, J. L., Anal. Chem., 1963,35,824. Lebedev, V. I., Zh. Anal. Khim., 1969,24,337.44 14. 15. 16. 17. 18. 19. 20. JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Willis, J. B., Anal. Chem., 1975,47, 1752. Babington, R. S., Pop. Sci., 1973, May, 102. Thelin, B., Analyst, 1981, 106, 54. Wolcott, J. F., and Sobel, C. B., Appl. Spectrosc., 1978, 32, 591. Suddendorf, R. F., and Boyer, K. W., Anal. Chem., 1978,50, 1769. Ebdon, L., and Cave, M. R., Analyst, 1982, 107, 172. Ebdon, L., Mowthorpe, D. J., and Cave, M. R., Anal. Chim. Acta, 1980, 115, 171. 21. 22. 23. American Society for Testing and Materials, ANSUASTM D3683-78, ASTM Headquarters, 1916 Race Street, Philadel- phia, PA, USA, 1978. Ebdon, L., Cave, M. R., and Mowthorpe, D. J., Anal. Chim. Acta, 1980, 115, 179. Cave, M. R., PhD Thesis, CNAA, Sheffield City Polytechnic, 1980. Paper J6/48 Received June 30th, 1986 Accepted September 3rd, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200039

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 45 Investigations of a Reduced Palladium Chemical Modifier for Graphite Furnace Atomic Absorption Spectrometry Lucinda M. Voth-Beach and Douglas E. Shrader Varian Instrument Group, AA Resource Center, 205 W. Touhy Avenue, Park Ridge, IL 60068, USA Palladium is a very useful chemical modifier for graphite furnace atomic absorption spectrometry. It can be used to stabilise many elements several hundred degrees higher than is possible with current methods. Its performance as a modifier is strongly affected by the sample matrix. The addition of a reducing agent provides for more consistent performance. The main purpose of the reducing agent is to "modify" the form of the palladium, guaranteeing that palladium is reduced to the metal early in the temperature programme.Differences were seen in the behaviour of the palladium modifier depending on the reduction method used. A comparison of palladium modifier methods in spike recovery studies of thallium was carried out to investigate differences between the reduction methods. It was suspected that differences in the physical form of palladium contributed to the variability in results. Scanning electron micrographs were used to investigate the physical form of palladium on the graphite surface. A method using the palladium modifier for the determination of selenium was compared with a commonly used procedure in spike recovery studies. Keywords: Graphite furnace atomic absorption spectrometry; chemical modification; palladium; reducing agent; scanning electron micrograph Chemical modification techniques are widely used in graphite furnace atomic absorption spectrometry (GFAAS) .Pallad- ium is a very effective chemical modifier and can be used to stabilise many elements to several hundred degrees higher than the temperatures possible with current methods.1-8 Of the elements tested, the greatest temperature shifts are achieved for the semi-metallic elements such as As, Se, Te, Bi, Sb, Pb, Tl, Ga, Ge and P. Ash temperatures can be raised 400-800 "C higher than current methods allow. Temperature shifts are somewhat less for the transition metals and ash temperatures can be raised 200-500 "C higher than allowed with current methods. Palladium has no effect on elements in Groups I and I1 of the Periodic Table.The change in appearance time is believed to be due to the formation of an intermetallic species. Palladium may act as a "solid solution'' retaining the analyte element to a higher temperature due to metallic interactions, Palladium metal acts as the chemical modifier, although palladium would normally be introduced into the furnace as a chloride or nitrate salt. At some point in the temperature programme, palladium metal is obtained either through thermal decomposition or reduction of the palladium com- pound present. In the authors' early investigations of pallad- ium as a chemical modifier, it was found that its performance was strongly affected by the sample matrix. For example, palladium appeared to be an excellent modifier for a variety of elements in high sodium chloride matrices, however, it performed poorly in samples containing high concentrations of strong oxidisers (such as HN03, H2S04 and Na2S04). The modifier solution used was also a source of variability in performance.Modifier solutions prepared from palladium chloride behaved differently from modifier solutions prepared from palladium foil dissolved in aqua regia. Modifier solutions containing 1-2% nitric acid gave much poorer performance for a variety of elements than palladium chloride solutions. For example, when a palladium modifier solution containing nitric acid was introduced into the furnace with a lead standard solution, no lead signal was obtained. Possibly palladium metal is not obtained soon enough in the temperature programme to interact with the relatively volatile lead.However, not only was no thermal stabilisation achieved, but the palladium compound present appeared to be a severe interferent. If the ash temperature was lowered to below what would normally be used for lead in an aqueous solution, a very small irregular atomisation signal was obtained. This behavi- our occurred with platform as well as wall atomisation. If the inert gas was changed from 100% argon to 5% hydrogen in 95% argon, the expected lead signal was recovered. It is believed that the hydrogen promotes the formation of palladium metal earlier in the temperature programme. Steps taken to guarantee that palladium is present as the reduced metal as early as possible greatly improve perfor- mance of the modifier.The palladium modifier solution can be pre-injected and the graphite tube heated to 1000 "C. Such a method has been used to stabilise mercury.9 It is assumed that at this temperature palladium metal is present on the graphite surface. The sample can then be introduced. The addition of a reducing agent such as 5% hydrogen in 95% argon, ascorbic acid, or hydroxylamine hydrochloride also appears to guaran- tee that the palladium is present as the metal early in the temperature programme. The reducing agents can be intro- duced along with the sample and pre-injection of the modifier solution is not necessary. With the addition of a reducing agent, palladium becomes a consistent modifier for samples containing high concentra- tions of strong oxidisers. It might initially appear that the reducing agent simply reacts with the nitric acid.However, supplementary investigations showed that this is not the most important effect. Experiments were carried out using pre- injection of modifier solutions. For example, a modifier solution of palladium chloride and hydroxylamine hydro- chloride (1500 mg 1-1 of Pd and 2% mlV hydroxylamine hydrochloride) was pre-injected into the graphite furnace and the furnace was heated to 1000 "C for 10 s. At this temperature it was assumed that the major portion of the hydroxylamine hydrochloride was decomposed and removed from the graph- ite tube and a surface of palladium metal remained. A standard containing 50% nitric acid was then injected on to this surface and the graphite tube was taken through the temperature programme.Thermal stabilisation of the analyte element did occur and the analyte signal obtained was similar to that of an aqueous standard. (Thallium and lead were used for these experiments.) However, if the standard solution containing 50% nitric acid was introduced into the furnace along with the modifier solution (1500 mg 1-1 of Pd and 2% mlV hydroxylamine hydrochloride) no signal was obtained. Certainly the excess of nitric acid would be expected effec- tively to "destroy" the hydroxylamine hydrochloride. [The expected analyte signal was obtained from standards with lower concentrations of nitric acid (<3%) .] However, pre- injection studies showed that the hydroxylamine hydro- chloride was not necessary and the presence of palladiummetal on the graphite surface resulted in thermal stabilisation.Therefore, the most important effect of the reducing agent appears to be in “modifying” the form of the palladium. Palladium is an easily reducible element and almost any reducing agent will give some beneficial effect. This paper describes work done with ascorbic acid, hydroxylamine hydrochloride and hydrogen. Ascorbic acid is a powerful enough reducing agent to precipitate palladium out of solution rather quickly. Therefore, it is impractical to combine palladium and ascorbic acid in the same solution. This combination must be added to the graphite furnace as separate solutions. Hydroxylamine hydrochloride can be added to the palladium solution, but it produced poorer results for a number of elements.Methods using hydroxylamine hydro- chloride are sensitive to very high concentrations of nitric acid. Nitric acid may simply oxidise the reducing agent, making it less effective in palladium reduction. The results from spike recovery studies listed in Table 1 show that for thallium the use of hydroxylamine hydrochloride resulted in no signal when very high concentrations of nitric acid were present. (For this study the interfering solutions were introduced directly into the fwnace along with standard and modifier solutions.) This severe suppression of the analyte signal was also seen for lead when hydroxylamine hydrochloride was used. Other elements such as selenium did not show the extreme sensitivity to nitric acid. Therefore, oxidation of the reducing agent may not be the only mechanism of the nitric acid interference. The use of hydrogen as a reducing agent appears to be promising for a number of reasons.It is cleaner, leaving no residue, and is less subject to contamination; it is also easy to use. A pre-mixed gas of 5% hydrogen in 95% argon can simply be introduced into the furnace. More importantly, the problem encountered with high concentrations of nitric acid is eliminated with the use of hydrogen. There was considerable variability in the performance of the palladium modifier depending on the reduction method used. The chemical reducing agents are very different compounds and the species produced in their decomposition in the graphite furnace may or may not be beneficial during the analyte atomisation process.It is also possible that differences in the physical form of the palladium on the graphite surface may influence the performance of the modifier. Scanning electron micrographs of the graphite surface with palladium deposits obtained by different reduction methods were obtained to investigate whether the physical form of palladium influenced the modifier behaviour. Experimental Equipment A SpectrAA-40 atomic absorption spectrophotometer and GTA-96 graphite tube atomiser with sample dispenser (Varian Techtron Pty. Ltd., Melbourne, Australia) were used to evaluate the effectiveness of the various palladium modi- fiers. Manufacturer’s recommendations for wavelength, spec- tral band width and lamp current parameters were followed. The graphite furnace temperature parameters were optimised for individual elements.The scanning electron micrographs were obtained with a Hitachi S-800 field emission scanning electron microscope (SEM) . Procedures Palladium modifier solutions were prepared by dissolving palladium foil (Aldrich Chemical Company, Milwaukee, WI, USA) in a minimum volume of aqua regia and diluting with de-ionised water. Analytical-reagent grade hydroxylamine hydrochloride (J. T. Baker Chemical Company, Phillipsburg, NJ, USA), ascorbic acid and glycerol (Mallinckrodt, Inc., Paris, KY, USA) were used to prepare modifier solutions. Solid pyrolytic platforms (Varian Techtron) were used as base material on which the palladium modifiers were added for subsequent viewing by the scanning electron microscope.The modifier solutions were deposited on to the platform and the platform was then heated in the graphite furnace to 1000 “C and held at that temperature for 15 s. Four palladium surfaces were prepared: (1) 50 pg of Pd, 500 pg of hydroxyl- amine hydrochloride; (2) 50 pg of Pd, 250 pg of ascorbic acid (added from separate solutions); (3) 50 pg of Pd, solution dried and then reduced with 5% hydrogen in 95% argon (this gas mixture was used during the entire heating cycle); and (4) 50 pg of Pd, 350 pg of glycerol, solution dried and then reduced with 5% hydrogen in 95% argon in the same manner 46 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 (4 (b) Fig. 1. Palladium de osited with hydroxylamine hydrochloride on pyrolytic graphite. Graphite heated to 1000 “C.(a) 2000 times magnification; and (b) ~OoooO times magnificationJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 47 as surface (3). The amounts of palladium and other reagents used to obtain these surfaces were 2-3 times greater than the amounts typically used for chemical modification in an analytical method. This increase was to ensure that the palladium was easily visible by scanning electron microscopy. Micrographs of 2000 and 20 000 times magnification are shown in this paper. * Results and Discussion Shown in Fig. l(a) is the palladium deposit obtained when palladium was added with hydroxylamine hydrochloride [surface (l)]. As indicated in this micrograph, there is considerable variation in palladium particle size. Some of the palladium globules are several micrometres in diameter.A portion of the same surface at 20000 times magnification is shown in Fig. l(b). The palladium droplets visible in this micrograph are approximately 1 pm in diameter. [The scale is indicated at the bottom of each micrograph. * Each single scale division in the lower right of Fig. l(a) represents 1.5 pm in the 2000 times magnification micrographs. Each single scale division in Fig. l(b) represents 0.15 pm in the 20000 times magnification micrographs.] While hydroxylamine hydro- * The magnifications given refer to the original micrographs, which have all (Figs. 1-4) been reduced by 20% for publication purposes. chloride was used with palladium for this micrograph, pre-injecting a palladium solution with no reducing agent and heating to 1000 "C produced a very similar type of distribution of palladium.A 2000 times magnification of the deposit on the graphite surface when palladium was introduced with ascorbic acid is shown in Fig. 2(a), surface (2). The palladium particles of lighter intensity appear to be embedded in the carbon residue from the ascorbic acid. Ascorbic acid will reduce the pallad- ium to the elemental form relatively rapidly to form a precipitate, thus, it is likely that the palladium is present as the metal before the solution completely dries. Shown in Fig. 2(b) is a 20000 times magnification of a cluster of palladium particles. The darker carbon residue from the ascorbic acid is visible. Apparently when ascorbic acid is used as the reducing agent, relatively small particles of palladium are obtained (0.1-0.15 pm), however, they are not evenly distributed. In general, ascorbic acid produces better results than hydroxyl- amine hydrochloride. A comparison of the two methods in spike recovery studies of thallium produced the results listed in Table 1.Although the ascorbic acid method appeared to be a good method for a variety of elements including thallium, lead and tin, in selenium determinations, ascorbic acid methods gave poorer results than hydroxylamine hydro- chloride methods. Shown in Fig. 3(a) is a 2000 times magnification of a palladium deposit when 5% hydrogen in 95% argon was Table 1. Determination of thallium. Spike recoveries (YO) in interfering matrices, palladium chemical modification 20 pg Pd + 200 pg 20 pg Pd + 60 pg 20 pg Pd + 100 pg hydroxylamine hydrochloride ascorbic acid 20 pg Pd + H2 glycerol + H2 solution Peak height Peak area Peak height Peak area Peak height Peak area Peak height Peak area 5 @2S% NaCl .. 90 96 93 93 95 103 93 96 5 p15.0% NaCl . . 75 86 91 90 75 84 89 93 5plseawater . . 32 20 87 69 65 41 103 100 5 pl concentrated HCl . . . . 95 97 86 99 101 87 99 101 5 pl concentrated HN03 . . . . 0 0 88 87 103 101 98 103 5 ~120% H2S04 . . 65 67 85 87 84 61 81 85 5 p11 .O% Na2S04 70 69 65 78 75 65 86 89 Fig. 2. 20 OOO times magnification Palladium deposited with ascorbic acid on pyrolytic graphite. Graphite heated to 1000 "C. ( a ) 2000 times magnification; and (b)48 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 (b) - ~- Fig. 3. Palladium solution dried and then Pd reduced with 5% hydrogen in95% argon. Graphite heated to-1000 "C. (a) 2000 times magnification; and ( b ) 20 000 times magnification introduced into the furnace and no chemical reducing agent was used [surface (3)]. The contrast between the palladium and the graphite is poorer than in the previous micrographs. This may not be significant, but simply reflects different SEM instrument parameters. Some spherical particles of palladium are apparent in this micrograph, however, in many instances the particles appear to be clustered together. This is more obvious at 20 000 times magnification as shown in Fig. 3(b). When the palladium-hydrogen method was used as a chemical modifier in analytical procedures, atomisation sig- nals were typically broad and irreproducible, particularly on new graphite tubes.Absorbance profiles with shoulders or more than one peak maxima were seen for a variety of elements. This type of behaviour was also seen for some elements when modifier solutions containing both palladium chloride and hydroxylamine hydrochloride were used. Both methods appear to produce a surface of palladium on graphite characterised by a high population of large particles of palladium as well as some smaller particles. Possibly the "sticking" of the analyte element to the palladium and the release of the analyte element from the palladium are affected by the mass and surface area of the palladium particle. It is likely that the irregular atomisation peak shapes are reflec- tions of the uneven rate of release of the analyte element. Irregular peak shapes such as peaks with two peak maxima and pronounced shoulders, as well as very different peak shapes for standards and samples made it difficult to evaluate the palladium hydrogen method in interference studies.For example, on new graphite tubes it was possible to encounter differences in peak shape between standards and samples. When this occurred peak-area measurements were preferred and it was not uncommon to obtain peak-height recoveries greater than 100% in spike recovery studies. This behaviour was encountered more often with the less volatile elements such as tin. More volatile elements such as lead and thallium did not show such variability in peak shape. The surface shown in Fig.4(a) at 2000 times magnification was obtained by using 5% hydrogen in 95% argon with a palladium solution containing 1% glycerol [surface (4)]. There are some larger particles of palladium, however, it is obvious that the glycerol has had a pronounced effect on palladium particle size and distribution. A portion of the same surface at 20 000 times magnification is shown in Fig. 4(b). The particles are considerably smaller and better dispersed than those seen earlier. An average particle diameter is 0.05-0.15 pm. The smaller particles result in a great increase in palladium surface area. In the upper portion of this micrograph the palladium particles appear to be piled on top of each other without coalescing into a larger droplet. This modification method of introducing palladium with glycerol and reducing with hydrogen resulted in atomisation peaks that were better shaped, appearing sharper and more Gaussian. For some elements such as tin the improvement was obvious even with standard solutions.For other more volatile elements such as lead and thallium the improvement in peak shape was apparent on brand new graphite tubes but less evident on used graphite tubes. For a typical analytical method a solution of 1000-1500 mg 1-1 of palladium containing 1% glycerol was introduced into the graphite furnace along with the sample. The solution was dried and 5% hydrogen in 95% argon was introduced at 3 1 min-1 (usually for 40-60 s after the drying stage during a temperature ramp from 90 to 300 "C). Glycerol does not decompose until ca.290 "C, so the reduction of palladium essentially takes place after the water has evaporated and with the glycerol still present. The 5 YO hydrogen can be used during the rest of the temperature programme. It did not appear to be either detrimental or beneficial. Table 1 lists the results of a study comparing different palladium modification methods. Spike recoveries of thallium were carried out for each interferent solution. Wall atomisa- tion was used with an ash temperature of 900 "C and an atomisation temperature of 2600 "C. The interfering solutions were introduced into the furnace along with standard and modifier solutions. Recoveries were calculated in both peak height and peak area, compared with interferent free standard and modifier absorbance signals. These results were obtained with wall atomisation, the platform appears to be unneces- sary.Palladium delays analyte volatilisation until a higher gas phase temperature is achieved; and like platform atomisation, this atomisation delay would be expected to minimise gas phase interferences. Thus, the improved accuracy realised with the palladium chemical modifier probably reflects the retention of the analyte element in the condensed phase until a higher gas phase temperature is achieved. The results listed in Table 1 show that in the determinationJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 49 ( a Fig. 4. Palladium solution containing 1% glycerol dried and then Pd reduced with 5% hydrogen in 95% argon. Graphite heated to 1000 “C.(a) 2000 times magnification; and ( b ) 20 OOO times magnification of thallium the palladium - hydroxylamine hydrochloride method was sensitive to high concentrations of nitric acid. This behaviour was also seen for other elements. The palladium - ascorbic acid method gave better recoveries than the pallad- ium - hydroxylamine hydrochloride method. With the pallad- ium - hydrogen methods good recoveries were obtained in the presence of high concentrations of nitric acid. These results also show that recoveries improved in several instances with glycerol added to the palladium solution when hydrogen was used. The surface of the graphite tube is another parameter affecting performance of the palladium modifier. The results in Table 1 were obtained on used pyrolytically coated graphite tubes (50-100 previous injections).Brand new pyrolytically coated tubes resulted in somewhat poorer recoveries in these types of recovery studies. It is likely that palladium particle size and distribution are influenced by the chemistry of the graphite surface. Interference performance on new pyrolytic- ally coated tubes improves with repeated injections of sample solutions. For more volatile elements such as lead and thallium, the “break-in” time of a new tube is shorter than that necessary for a less volatile element such as tin. It is also possible to use uncoated graphite tubes for the palladium methods. In the authors’ experience, the results reported in this paper could not always be exactly reproduced and it appeared that the variation in results was due to differences in the graphite tubes.More investigations of the interaction of palladium and the graphite surface are needed to optimise the palladium methods. The palladium - glycerol - H2 method can be used for many elements including Pb, T1, Sn, Cd and As. However, one element that appeared to behave differently was selenium. The palladium - hydroxylamine hydrochloride method appeared to give better results (20 pg of Pd, 200 pg of hydroxylamine hydrochloride). The organic material intro- duced into the atomisation process from the ascorbic acid or glycerol may be detrimental, or, possibly, hydroxylamine hydrochloride has a beneficial effect on the analyte element selenium. The effect of reducing agents on the analyte element was not studied in this work.Selenium compounds can be reduced to elemental selenium with a mild reducing agent such as hydrazine. 10 Hydroxylamine hydrochloride produces a Table 2. Interference studies for selenium Spiked recoveries, YO Nickel method* Palladium methodt Peak Peak Peak Peak Interferent solutions height area height area 5p15.0% NaCl . . . . 89 111 94 88 5plseawater , . . . 55 69 87 78 5 p1 concentrated HCI 66 69 102 108 5 p1 concentrated 5420%H2S04.. . . 0 0 79 84 542.0YoH2S04 . . 0 0 NA NA 5 p10.5% Na2S04 . . 41 48 91 90 5 p11.0% Na2S04 . . 32 31 76 77 5p12.5Y0NaCl . . . . 91 112 100 93 HN03 . . . . . . 106 100 75 94 * Platform atomisation 20 pg of Ni + 25 pg of Mg(NO&. f Wall atomisation, 20 pg of Pd + 200 pg of hydroxylamine hydrochloride. similar effect, which can be demonstrated by adding it to a selenium solution of 1000 mg 1-1 of selenium.A deep red precipitate of amorphous selenium is produced. This reduc- tion of the analyte element would be expected to be beneficial for the formation of an intermetallic species with reduced palladium. The interference performance of a palladium - hydroxyl- amine hydrochloride method for selenium was compared with the performance of a commonly used procedure. The com- monly used procedure requires the use of the platform and chemical modifiers of nickel nitrate and magnesium nitrate. Wall atomisation was used for the palladium method. The results of this study are listed in Table 2. While both methods perform reasonably well in NaCl solutions, sea water is more complex and results were poorer.As shown, the palladium method is tolerant of very high concentrations of hydrochloric acid but is sensitive to very high concentrations of nitric acid and the platform nickel - magnesium nitrate method for selenium gave a better recovery. However, the palladium method performed significantly better in both sulphuric acid and sodium sulphate matrices.50 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Conclusion Palladium is an effective chemical modifier and can be used to stabilise many elements to several hundred degrees higher than the temperatures possible with current methods. Palladium metal acts as the chemical modifier retaining the analyte element until a higher gas phase temperature is achieved.This appears to give many of the advantages normally associated with platform atomisation. The addition of a reducing agent promotes the formation of palladium metal earlier in the temperature programme resulting in more consistent performance of the modifier. The scanning electron micrographs illustrated that size and distribution of the palladium particles on the graphite surface were different depending on the reduction method used. This is believed to influence the efficiency of the modifier. There are also other factors involved, such as the graphite surface. When palladium was used as a chemical modifier in analytical procedures, methods resulting in smaller, more evenly distributed particles gave better recoveries in interference studies. The authors gratefully acknowledge Cliff Nishimoto (Varian Central Research) for his assistance in obtaining the scanning electron micrographs. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. References Xiao-quan, S. , Zhe-ming, N., and Li, Z. , At. Spectrosc., 1984, 5 , 1. Xiao-quan, S., Zhe-ming, N., and Li, Z., Tuluntu, 1984, 31, 150. Xiao-quan, S., and Kaijin, H., Tuluntu, 1985, 32, 23. Xiao-quan, S., and Dian-Xun, W., Anal. Chim. Actu, 1985, 173,315. Ping, L., Fuwa, K., and Matsumoto, K., Anal. Chim. Actu, 1985. 171. 279. Schlemmer, G., and Welz, B., Poster 077, presented at Colloquium Spectroscopicum Internationale XXIV, Garmisch- Partenkirchen, FRG, September, 1985. Niskavaara, H., Virtasalo, J., and Lajunen, L., Spectrochim. Actu, Part B, 1985,40, 1219. Voth-Beach, L., and Shrader, D., Spectroscopy (Springfield, Oreg.), 1986, 1, (lo), 49. Grobenski, Z., Erler, W., and Voellkopf, U., At. Spectrosc., 1985,6, 91. Cooper, W., and Westbury, R., in Zingaro, R., and Cooper, W., Editors, “Selenium,” Van Nostrand Reinhold, New York, 1974, p. 109. Paper J6l10.5 Received November 5th, 1986 Accepted November 18th, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200045

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 51 Spray Deposition versus Single-drop Deposition for Calibration of an Electrostatic Accumulation Furnace for Electrothermal Atomisation Atomic Absorption Spectrometry Giancarlo Torsi and Francesco Palmisano Laboratorio di Chimica Analitica, Dipartimento di Chimica dell‘ Universita, Via G. Amendola, 773-70126 Bari, Italy Spray deposition, e.g., deposition by electrostatic capture of aerosol droplets (aerodynamic diameter of 1-2 pm) generated from a standard solution of the analyte is proposed for the calibration of the electrostatic accumulation furnace for electrothermal atomisation atomic absorption spectrometry. Deposition of the analyte can be achieved with a relative standard deviation of ca. 2%. The spray deposition mode has been found to be insensitive to the ageing and surface conditions of the furnace but not to the nature of the furnace material. The different behaviours observed for different materials are explained in terms of different atom loss mechanisms.Keywords: Electrostatic capture; electrothermal atomisation; atomic absorption spectrometry; electrostatic aerosol deposition The electrostatic accumulation furnace for electrothermal atomisation atomic spectrometry (EAFEAS) is a new method’-3 for elemental analysis of particulate matter. Its main features include: high selectivity, high sensitivity, high speed of analysis and absence of sample manipulation. This performance arises from the possibility of electrostatically capturing a given analyte with high efficiency and high spatial resolution4 in the graphite tube of an electrothermal atomiser.The method has been applied to the determination of metals in air, and it should give equivalent results in other gases provided they can sustain a corona discharge with a suffi- ciently high space charge.5 A solution, if previously nebulised, can be electrostatically captured. A sample introduction mode in which a nebulised solution is sprayed into a graphite atomiser has been tested with reduced interference effects.613 A commercial apparatus based on this principle is available. For the direct analysis of particulate matter, a suitable calibration procedure must be used because gaseous standards containing particulate matter of known composition and particle size distributions are not available.In previous papersl-3 the calibration problem was overcome by the use of a standard solution deposited as a single drop in the centre of the graphite tube. Many assumptions are implicit in this procedure, the most important being: (i) the analyte is electrostatically captured in the same section of the graphite tube as that where the calibration solution is deposited (different heating rates and gas loss kinetics are encountered along the graphite tube); (ii) the efficiency of the electrostatic precipitation step is almost quantitative; (iii) the atomisation of the analyte is independent of the chemical composition and physical state of the sample; and (iv) the atomisation step is not influenced by the deposition mode. The validity of assumptions (i) and (ii) have been proved experimentally.5 Assumption (iii) is related to the matrix and for particulate matter interferences appear to be less severe than for solutions.Research on this subject is beyond the scope of this paper, which deals mainly with the verification of assumption (iv) . As a particulate matter standard is very difficult to obtain, we employed a solution aerosol, which can be easily obtained in a controlled and reproducible way. A solution aerosol with very small droplets is, for the present purposes, a convenient method of simulating an air sample with a certain particulate matter content. A comparison between the “solution deposition” mode (e.g., the deposition of a single drop of ca. 3 pl) and the “spray deposition” mode (e.g., the electrostatic capture of a nebu- lised solution in the same device and in the same position as that where the solution drop was deposited) has been made with three graphite materials and lead and zinc analytes.The two metals were found to behave in a similar manner, and only results for lead will be reported and discussed. Experimental Reagents and Materials Either argon or nitrogen may be used as the inert gas, as they give practically indistinguishable results. All chemicals used were of analytical-reagent grade. Stock solutions (100 pg ml-1) of Pb” and ZnII were prepared from the nitrate salts in doubly distilled water and adjusted to pH 2 with concen- trated nitric acid. Dilute solutions were prepared daily just prior to use. The graphite tubes used were: (i) prepared from graphite rods, density 1.52 g cm-3 (Ultra-Carbon U-2, Bay City, MI, USA), machined to the required dimensions in our workshop; (ii) Carbon-Lorraine (Gennevilliers, France) manufactured according to our design with a proprietary method (density 1.7 g cm-3); (iii) Ringsdorff GmbH (Bonn-Bad-Godesberg, FRG) manufactured according to our design and covered with a layer of pyrolytic graphite by the same company.Apparatus The basic instrumentation for atomic absorption measure- ments has been described elsewhere.1-3 The lines used were 283.3 and 213.9 nm (0.2 nm band pass) for lead and zinc, respectively. The furnace power supply was a Perkin-Elmer HGA-2100 Controller slightly modified in order to obtain complete gas stop during the atomisation step and to give a trigger signal to an oscilloscope (Gould Advanced Oscilloscope Model 4000) from which hard copies of the absorbance as a function of time could be obtained by means of a Gould 4001 option and a strip-chart recorder.Peak heights were usually measured. Some sets of data for peak area were obtained by graphic integration of the absorbance profiles. As different graphite materials and different furnaces were used, the same atomisa- tion temperature would have given different peak heights for52 1 C JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 - To the furnace (a’ Waste f B A - I Tank air 3 cm Fig. 1. (a) Block diagram of the aerosol generator. A, Nebuliser (concentnc flow type from Instrumentation Laboratory); B, impact chamber (Plexiglass block ; C, chimney (glass tube 50 x 2.5 cm i.d.); above the top or)the impact chamber.(b) 8etaiLd view of parts A, B and C in Fig. l(a) D, syringe pum ; and E: h ow meter. Sam ling oint is located 20 cm the absorptions. To circumvent this problem the temperature of the nominal atomisation step of the HGA-2100 Controller was adjusted in order to produce a peak for a given metal at a fixed time (3.0 f 0.2 s). The peak time was taken as a measure of the heating rate assuming that the atomisation mechanism was not influenced by the graphite material. The aerosol generator, assembled by combining a commer- cially available nebuliser (Instrumentation Laboratory) with a laboratory-made impact chamber and transport device, is given schematically in Fig.1. The impact chamber, the chimney and the lateral sampling point were designed in order to provide an air stream with a reproducible analyte concen- tration, reproducible drop-size distribution and a cut-off effect for large particles. The gas flow and solution feed were controlled to &I%. A Particle Monitor Model 225 (Royco Instruments Inc., Menlo Park, CA, USA) was used5 to evaluate the drop-size distribution in the aerosol entering the electrostatic furnace. It could count particles on five channels in the following ranges 75 m I 6 50 m z . 4- 2 0 0 25 I I I I L- r - I - 1 1 1 I I I I 0 1 2 3 Aerodynamic diameterlpm Fig. 2. Droplets counts versuS aerodynamic diameter. Solid line, 0.01 M HN03 solution, and broken line, 0.01 M HN03 + 0.5 M NaCl solution.Solution flow-rate, 0.63 ml min-1; and gas flow-rate, 3.2 1 min-1 of aerodynamic diameter: 0.3-0.5; 0.5-0.7; 0.7-1.4; 1.4-3; and >3 pm. A PAR 174A Polarographic Analyzer (EG&G Princeton Applied Research, NJ, USA) coupled to a conventional three-electrode system was used for polarographic measure- ments. Results and Discussion Aerosol Droplet Size Distribution The size distribution of the droplets sampled from the aerosol generator was determined with the aid of a particle monitor connected to the sampling point of the aerosol generator by a Tygon tube. Because the gas flow normally used for electros- tatic capture of the aerosol differs from the fixed calibration flow of the particle monitor, an additional source of filtered air was used. Typical size distribution histograms for aerosol droplets generated from a solution of 0.01 M HN03 with and without the addition of NaCl are presented in Fig.2. As can be seen, the addition of 0.5 M NaCl produced a shift in the aerodynamic particle size towards higher values. According to Cresser and Browner14 and Skogerboe and Freeland15 the droplet evapor- ation during the aerosol transport may be a plausible and at least a partial cause of the effect observed. Even in the worst instance (e.g., a high salinity solution)? only a small fraction of the droplets produced by our aerosol generator had an aerodynamic diameter exceeding 3 pm. Most work on nebulisers has shown a bimodal distribution with a second peak at ca. 7-10 pm; the same situation could be verified in the present investigation, even though the presence of an impact chamber in the aerosol producing device greatly reduces this possibility.However, the contribution to the total mass of the aerosol produced in a given time of droplets with an aerodynamic diameter exceeding 3 pm should be negligible because, as already stated, the aerosol generator has been designed to eliminate very large droplets. Spray Deposition versus Solution Deposition The amount of analyte electrostatically captured from the aerosol in the spray deposition mode could not be directly and easily evaluated (as it could for single drop deposition), because of the small amounts involved (generally less than 1 ng). An indirect method of measurement was employed in which a known excess of a foreign ion was added in order to give an exactly known value for the analyte to foreign ion concentration ratio.Cadmium(I1) was chosen as the foreign ion because (i) it could be easily determined by an indepen-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 53 dent technique such as differential-pulse polarography (DPP); (ii) it did not interfere, even at relatively high concentrations (200 wg ml-l), in the atomic absorption measurements of lead and zinc and (iii) it was not detectable in blank samples (untreated furnaces). Before, during and after a series of spray depositions, some randomly selected furnaces were not used in the atomic spectrometer but were carefully dismantled and their graphite tubes soaked in 0.1 M HN03. After 2 h, aliquots of these solutions were transferred into a polarographic cell, and the Cd" contents determined by DPP with the standard additions technique.The amount of cadmium electrostatically captured from the aerosol could be determined and, from the Pb11:CdII ratio in the test solution, the amount of lead deposited in the electrostatic furnace could be calculated. The efficiency of the recovery of the Cd" from the graphite surface was previously established by depositing a known volume of a CdII standard solution on graphite tubes, which were then processed as previously described. The day-to-day relative standard deviation (RSD) for the spray deposition mode, as evaluated by the indirect polaro- graphic measurements described, was 2.1% (n = 12 over 3 d). The RSD value for the solution deposition mode, evaluated by repetitive weighing of the drop produced by our solution depositor,3 was typically ca.0.3%. Note that most of the variance associated with the spray deposition really arises from the procedure followed for polarographic measure- ments, so that spray deposition is not really much worse than drop deposition, contrary to what is suggested from a comparison of the two RSD values. The RSD values for atomic absorption measurements (peak heights) for 0.32 ng of lead deposited by the spray and solution modes were found to be quite similar (5.2 and 6.3%, respectively) but considerably higher than those given above. The RSD values associated with solution and spray deposition suggest that variance in the atomic absorption measurements arises mostly from factors not related to the mode of deposition.The major source of variance may be irreproducibility in making the electrical contacts and therefore in the heating rates of the furnace used. Analytical Responses versus Furnace Lifetime For each type of graphite used, the atomic absorption response (for a given amount of analyte) was found to be independent of the deposition mode only for furnaces that 0.50 0) u cu e cn 2 0.25 0.5 1 .o Amount of Pbing Fig. 3. Calibration plots (peak heights) by solution deposition mode on Ultra Carbon U-2 furnaces for different ageing times. A, No firing; B, 200 firings; and C, 300 firings. Circles represent average values of six determinations on different furnaces. Solid lines were calculated by second-order least-squares regression analysis.Calibration plot by spray deposition mode gave regression lines practically overlapping line A even for furnaces that had undergone ca. 500 firings have undergone a limited small number of firings. As the number of firings increased, the slope of the calibration plot decreased (Fig. 3). However for a given furnace the decrease is much more rapid for the solution than for the spray deposition mode. From Fig. 3 it can be seen that for Ultra Carbon U-2 graphite and with the solution deposition mode a considerable loss of sensitivity occurred after only 200 firings. Further ageing of the furnace produced not only a further reduction in sensitivity but also a worsening in the reproduci- bility of the measurements. 16 The spray deposition mode did not produce any appreciable variation in the reproducibility even after 500 firings; only a slight decrease in sensitivity was observed which was more likely to be due to an increase in the graphite resistance, caused by successive firings, rather than to surface phenomena.The same trend was observed with the two other graphite materials used. However, the number of firings necessary to observe similar effects increases in the order Ultra Carbon > Carbonne-Lorraine >> Ringsdorff. Sensitivity versus Graphite Material The analytical response of different new furnace materials is not influenced by the deposition mode. The sensitivity, however, was found to be lugher for pyrolytic graphite than for untreated graphite (Fig. 4). Moreover, comparison of the experimental results summarised in Figs.3 and 4 reveal that spray deposition is quite insensitive to repetitive firings but not to the nature of the furnace material. Atom loss mechanisms involve diffusion and convection along the tube axis and diffusion across the tube walls.17-22 It is well known that a good pyrolytic coating is a barrier to the diffusion of atoms. Diffusion along the axis for a given furnace geometry should be independent of the nature of the furnace material, as long as the heating rate is the same. Convection along the furnace axis is due mainly to: (i) thermal expansion of the inert gas, which should be the same whatever the furnace material, and (ii) the release of gases desorbed during furnace heating, which could be rather different according to the different surface conditions, the nature of the inert gas, the nature of the test solution and its deposition mode.These different convection effects and the possibility of diffusion across the walls of untreated graphite could account for the large difference in sensitivity found experimentally. The different behaviour between solution and spray deposition observed when the surface conditions evolve, with the 0.50 al c lu + a 2 0.25 A I I I 1 0.4 0.8 1.2 1.4 Amount of Pb/ng Fig. 4. Calibration plots (peak heights) by spray deposition mode relevant to new furnaces: A, Ringsdorff; B, Carbonne-Lorraine; and C, Ultra Carbon U-2. The same results were obtained by solution deposition mode54 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 formation of pores and cracks resulting from the loss of surface materials, can be ascribed to solution creeping into the tube walls. The large scattering of results observed after a large number of firings can also be explained by assuming that the creeping is irreproducible. Conclusions From the data presented, spray deposition is preferred for the calibration of the EAFEAS method, because it is less sensitive to surface conditions and ageing of the furnace. Only when the surface is well characterised could the solution deposition mode be accepted. However, the similarity in the behaviour of the aerosol droplets and particulate matter remains to be verified experimentally even if the origin of the different behaviour between spray and solution deposition, e.g., the creeping of the solution in the pores and cracks of the surface, strongly suggests such a similarity.This work was carried out with financial assistance from the Minister0 della Pubblica Istruzione. References 1. Torsi, G., Desimoni, E., Palmisano, F., and Sabbatini, L., Anal. Chem., 1981,53,1035. 2. Torsi, G., and Palmisano, F., Analyst, 1983, 108, 1318. 3. Torsi, G., Palmisano, F., Desimoni, E., and Rinaldi, R., Ann. Chim. (Rome), 1982,72, 365. 4. Torsi, G., and Palmisano, F., Spectrochim. Acta, Part B, 1986, 41, 257. 5. Nasser, E., “Fundamentals of Gaseous Ionization and Plasma Electronics,” Wiley Interscience, New York, 1971, Chapter 12. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. Matousek, J. P., Talanta, 1977,24, 315. Chamsaz, M., Sharp, B. L., and West, T. S., Talanta, 1980,27, 867. Fazakas, J., Anal. Lett., 1982, 15(A7), 573. Del Rosario, A. R., Guirguis, G. N., Perez, G. P., Matias, V. C., Li, T. H., and Flessel, C. P., Znt. J . Environ. Anal. Chem., 1982,12,223. Wei, F., Yang, J. T., and Yin, F., Anal. Lett., 1983, 16(B7), 501. Shabushning, J. G., and Hieftje, G. M., Anal. Chim. Acta, 1983, 148, 181. Sotera, J. J., Cristiano, L. C., Conley, M. K., andKahn, H. L., Anal. Chem., 1983, 55,204. Tapia, A. T., Combs, P. A., and Sneddon, J., Anal. Lett., 1984, 17(A20), 2333. Creder, M. S., and Browner, R. F., Spectrochim. Acta, Part B, 1980, 35,73. Skogerboe, R. K., and Freeland, S. J., Appl. Spectrosc., 1985, 39, 925. Littlejohn, D., Duncan, I., Marshall, J., and Ottaway, J. M., Anal. Chim. Acta, 1984, 157, 291. L‘vov, B. V., “Atomic Absorption Spectrochemical Analysis,” Adam Hilger, London, 1970, Chapter 5. Dymott, T. C., Wassall, M. P., and Whiteside, P. J., Analyst, 1985,110,467. Slavin, W., and Carnrick, J. R., Spectrochim. Acta, Part B, 1984, 39,271. Van den Brock, W. M. G., and de Galan, L., Anal. Chem., 1977,49,2176. Sabbatini, L., and Tessari, G., Ann. Chim. (Rome), 1984,74, 779. Paveri Fontana, S. L., and Tessari, G., Prog. Anal. At. Spectrosc., 1984,7,243. Paper J6168 Received July 29th, 1986 Accepted October lst, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200051

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Atomic Absorption Spectrometric Determination of Lead in Gasolines by Generation of its Covalent Hydride 55 Jose Aznarez, Juan Carlos Vidal and Rafael Carnicer Department of Analytical Chemistry, Faculty of Sciences, University of Zaragoza, Zaragoza, Spain This paper describes a method for the atomic absorption spectrometric determination of lead in gasolines by hydride generation directly in the sample diluted with N,N-dimethylformamide (DMF) and with the addition of NaBH4 solution in a 2% mNDMF solution. The determination is carried out with an electrothermal atomiser (an electrically heated quartz tube). The gasolines were initially standardised using two ASTM methods, and the values for concentrations of lead obtained were used for studying the best conditions for the hydride generation and the calibration.Beer's law is obeyed for up to 0.40 pg of lead, and the sensitivity obtained is ca. 10 ng ml-1. The method has been applied to the determination of lead in commercial gasolines, with good accuracy and precision. Keywords: Lead determination; h ydride generation; atomic absorption spectrometry; gasolines Organic lead anti-knock additives containing tetraalkyllead (TAL) compounds [methyl and/or ethyl groups, primarily tetramethyllead (TML) and tetraethyllead (TEL)] have been added to commercial gasolines and fuels since 1923 in order to improve the octane rating of these gasolines. Such additives are a convenient and economic way of increasing the octane rating of gasolines for use in high-compression internal combustion engines and for maximum anti-knock effective- ness.In recent years, special attention has been given to the contribution of motor vehicles exhaust to environmental lead pollution. Owing to the thermal instability of the TAL compounds and the addition of halide scavengers to petrol, considerable amounts of organic and inorganic lead pollutants are emitted into the atmosphere. In fact, the major source of airborne lead pollution is the incomplete combustion of leaded petrol.1 Other causes of lead pollution in urban atmospheres by vehicles are the spillage of leaded petrol or evaporation from fuel tanks and carburettors. Owing to the toxicity of TAL and inorganic lead halide compounds, there has been a great deal of debate over the adverse effects arising from exposure of people, especially children, to the present environmental concentrations of lead.2J There is no doubt that lead in gasolines is an important contributor to environmental pollution, and owing to its toxicity, in recent years many countries have given special attention to reducing the lead content in gasoline or of replacing TAL compound additives in petrols with other chemicals.4 Methods for determining the total amount of lead in gasolines are therefore especially important because of the current concern about the potential health and environmental effects accompanying these lead compounds.Some of the instrumental techniques used are based on atomic absorption spectrometry (AAS), neutron activation analysis, X-ray fluorescence and atomic emission spectrometry.* Determination of lead in petrol by AAS was first reported by Robinson using TEL in isooctane as a standard and dilution of the gasoline sample with this solvent .6 Atomic absorption spectrometric methods, because of the combination of their sensitivity, speed, precision and accuracy are the most attractive approach to the determination of lead in gaso- lines.7-10 These methods involve a chemical pre-treatment of the gasoline, the reaction of the TAL compounds with iodine to convert alkylleads into lead iodide and calibration with organic or inorganic lead standards. Polo-Diez et al.11 carried out the determination of lead in gasolines using emulsions. The method involved the formation of an oil - water emulsion from a small amount of the gasoline and a larger amount of water in the presence of a suitable emulsifier and subsequently the direct aspiration of this emulsion into the flame.Calibration was carried out in benzene using the standard additions method. Holding et al. 12 diluted the gasolines with 4-methylpentan-2-one and reacted the TAL with a solution of iodine in toluene. The dialkyllead iodides formed were stabilised by the addition of a quaternary ammonium salt (Aliquat 336) and the total lead content was determined by AAS using standards prepared from lead(I1) chloride. In two ASTM methodsl3J4 the determination of lead in gasolines is based on an initial reaction of the TAL compounds with iodine monochloride, extraction into an aqueous phase as dialkyllead compounds and mineralisation of the organolead compounds by evaporation and oxidation with nitric acid by boiling the solution or destroying the excess of iodine monochloride with sodium sulphite.The determination of lead is carried out titrimetrically with EDTA using Xylenol Orange as the indicator ,I3 or spectrophotometrically by extraction of lead dithizonate into chloroform. These methods are time consuming, require special atten- tion and there are associated problems, such as the AAS responses with the different lead standards and alkylleads or with the solvents used for the dilution of gasolines, and later with nebulisation and atomisation. In this work, a simple method has been developed for the determination of lead in gasolines by AAS, by diluting the sample with N,N-dimethylformamide (DMF) and then hydride generation by direct injection of a NaBH4 - DMF solution.Lead has been determined previously by flame AAS using hydride generation from a non-aqueous medium15 in BCS standard steels and airborne particles. Following a similar procedure, antimony has also been determined using hydride generation from a non-aqueous medium by flame AAS in BCS standard steels16 or in polyl(viny1 chloride) polymers using an electrically heated silica tube as the atomisation device.17 In this work, the determination of lead in gasolines is also based on the hydride generation from a non-aqueous medium, the sample being diluted with DMF, followed by AAS, using an electrically heated quartz tube. The method is simple, rapid and shows good accuracy.Experimental Apparatus Atomic absorption spectrometer. Pye Unicam SP-9, with an electrically heated quartz tube and temperature control by a platinum - rhodium thermocouple. Background correction56 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 was not used owing to the low noise signal of the blanks and samples. Lead hollow-cathode lamp. Perkin-Elmer , with maximum current 15 mA and wavelength 283.3 nm. Rotameter. Messrohr FT-1, Fischer and Porter, for measur- ing the nitrogen flow-rate. Reagents and Solutions All chemicals were of analytical-reagent grade and were obtained from E. Merck. Standard lead solution, lo00 pg ml-1. Prepared by dissol- ving the required mass of lead salt (nitrate, chloride or acetate) and diluting to 100 ml with N,N-dimethylformamide (DMF).N,N-Dimethylfomzamide. Purified by distilling over NaHC03 and collecting the fraction boiling between 148 and 150 "C. Hydrochloric acid solution in DMF, 12% VlV. Sodium tetrahydroborate(III) solution, 2% mlV in DMF. Anhydrous acetic acid, sulphuric acid and nitric acid Isooctane. Commercial gasolines. 92, 96 and 98 octane ratings, Non-leaded gasoline. Obtained from ENPETROL solutions, in DMF, containing TEL and TML in unstated ratios. (Empresa Nacional del Petroleo, Spain). Procedure Place 1 ml of the gasoline diluted with DMF into the hydride generator, described previously.18 Add 3 ml of the 12% V/V (1.5 M) HCl - DMF solution and connect the nitrogen gas stream (flow-rate 484 cm3 min-1) for 45 s to expel any air.Inject 3 ml of the 2% mlV NaBH4 in DMF solution through the septum membrane of the generator and record the atomic absorption signal of lead at 283.3 nm. Prepare a calibration graph using the standard additions method (by adding standardised leaded gasoline containing known concentrations of lead to the sample) and follow the proposed procedure. The optimum instrument parameters are listed below in Table 5. Results and Discussion Generation of the Covalent Hydride of the Lead in Gasoline Preliminary tests on the lead hydride generation were carried out on an unleaded gasoline with added lead salts (nitrate, acetate or chloride). Standard lead in gasoline solutions were prepared by diluting with DMF and isooctane. The results obtained for the hydride formation by the addition of various acid solutions (acetic, hydrochloric or sulphuric acid in DMF) to the gasoline at different concentrations and later injection of a 3% mlV NaBH4 - DMF solution were poor, and the atomic absorption signal was very low.The use of isooctane as a diluent of the gasolines was also a disadvantage because of the molecular absorbance of the vapour transported to the atomisation tube by the nitrogen, and its tendency to ignite spontaneously at the ends of the open quartz tube. Addition of organic solvents, such as carbon tetrachloride, formamide or benzene, to the acidified gasoline to act as electron acceptors was investigated in order to improve the reduction to lead hydride in the presence of DMF, which is a strong basic and solvating reagent.19 Using a 25% WV HC1 acid solution in DMF, these results were no better.Owing to the impossibility of obtaining tetraethyllead (TEL) for the preparation of gasoline standards, commer- cially available gasolines with a known lead content were used for the calibration, i.e., ones that had been previously standardised using two ASTM methods.13J4 These two methods involve the attack of tetraalkyllead with IC1 and extraction into the aqueous phase as dialkyldibromide lead. The first method13 was titrimetric, using EDTA solution at pH 5 and Xylenol Orange as the indicator. The second14 was a spectrophotomehic method, and involved the extraction of lead dithizonate into chloroform. The results obtained for the lead concentration in various types of commercial Spanish gasolines by the two ASTM methods are presented in Table 1.These standardised gasolines were used for the study and optimisation of the lead hydride generation and for the calibration, The concentration of lead in these standardised gasolines remained constant at least for two weeks if kept in a plastic vessel in a refrigerator. The proposed procedure involves the dilution of commer- cial gasoline with DMF, the addition of an acid solution in Table 1. Concentration of lead in various types of Spanish commer- cially available gasolines using two ASTM methods Lead contenumg ml-1 ASTM ASTM Octane rating D-3341-80* D-3116-82* Mean value 92 339.6 330.7 335.1 96 535.7 541.6 538.6 97 386.4 398.8 392.6 * Average concentration of three determinations.Table 2. Precision for the determination of 0.251 pg ml-1 of lead with different HC1 in DMF concentrations (n = 10) HC1 Mean concentration/ absorbance Standard Coefficient of M value deviation variation, % 1 0.245 0.021 8.57 1.2 0.296 0.017 5.74 1.5 0.323 0.010 3.10 2 0.404 0.028 6.93 0 1.50 3.00 4.50 6.00 [H~S~L&M [HNO~]/M HCI ]/M Fig. 1. Atomic absorbtion obtained for the determination of 0.861 pg of Pb (0.215 pg ml-1) with different acid concentrations in DMF: (a) sulphuric acid; (b) nitric acid; and (c) hydrochloric acidJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 57 0.294 0.245 0 1.50 3.00 4.50 6.00 NaBH4 in DMF concentration, '/o m/V Fig. 2. Atomic absorbtion for the determination of 0.864 pg of Pb by the hydride generation method for different concentrations of NaBH4 in DMF 0.400 0.300 a C rn e 0.200 $ 2 0.100 0 Time- Fig. 3.Atomic absorption signals obtained during the determination of Pb by AAS and direct hydride generation from a 96 octane rated gasoline DMF and the direct injection of the NaBH4 in DMF solution to the resulting gasoline solution. Atomic absorption signals obtained with this procedure were fast, as the hydride evolution takes about 30 s. The sensitivity obtained is high, and this will be discussed later. Acids for Hydride Formation The acid used for the hydride evolution must be miscible with the DMF and the gasoline, and its presence is essential for the hydride generation and simultaneous evolution of hydrogen in the presence of the NaBH4 in DMF solution. The acids studied were nitric, sulphuric and hydrochloric acids.In all instances, acid cogcentrations of less than 1 M in the generator vessel produced lower atomic absorption signals owing to insufficient hydrogen being evolved for the hydride generation. However, if the concentration of the acid is more than 4 M, the extra hydrogen produced not only dilutes the lead hydride, but also ignites spontaneously at the ends of the open quartz tube. The best signals were obtained with an acid concentration of ca. 3 M. However, in spite of the improved sensitivity, reproducibility in this instance was worse owing to the larger amount of hydrogen being generated. Hence, lower acids concentrations were in fact used. Results for the determination of 0.861 pg of lead (0.215 pg ml-1 of Pb) at different concentrations of HC1, H2S04 and HN03 in DMF are shown in Fig.1. The absorbances are the averages of three determinations. The best sensitivities were obtained with 3 M H2SO4 (0.540 A), 2 M HN03 (0.482 A) and 3 M HCI (0.486 A) in the generator flask. However, as was indicated above, the precision in these three situations was inadequate, and the coefficient of variation was greater than Table 3. Atomic absorption results obtained at different atomisation temperatures for the determination of 0.861 pg of lead TemperaturePC Absorbance* 520 No peak 610 0.316 730 0.348 850 0.440 950 0.760 * Average of three determinations. Table 4. Atomic absorption results obtained with different nitrogen flow-rates for the determination of 0.692 pg of lead (0.173 pg ml-l) Nitrogen flow-rate/ cm3 min- Absorbance* 242 0.176 315 0.268 400 0.296 484 0.294 564 0.297 650 0.330 * Average of three determinations.Table 5. Optimum instrument parameters for the determination of lead by the hydride generation method LampcurrenUmA . . 8 Spectral band widthhm 0.5 Wavelengthhm . . . . 283.3 Nitrogen flow-rate/ cm3min-1 . . . . 484 Gain . . . . . . . . 1 Expansionscale . . . . 1 Temperature of quartztubePC . . . . 850 5% for ten consecutive determinations. Optimum precision was obtained with 1 M H2SO4 (0.400 A), 1 M HN03 (0.408 A) and 1.5 M HCl(O.320 A) giving a coefficient of variation of less than 5% (n = 10). The hydrochloric acid shows the best results because of faster hydride generation (15-30 s) and so finally, an HCl in DMF concentration of 1.5 M was chosen.Table 2 shows the precision for the determination of 0.861 pg of lead (0.215 pg ml-1) with different concentrations of HCl (n = 10). All of the results for the investigations given above were carried out with a 96 octane rated gasoline. Studies on the optimum acidities for 92 and 98 octane rated gasolines gave similar results. The NaBH4 solution was of 3% m/V in DMF. Optimisation of Hydride Generation in the Gasoline The volume of gasoline added to the generator flask was 1 ml; greater volumes were not used because the AA peaks were broadened. This volume of sample was diluted with 3 ml of 1.5 M HC1 in DMF solution. Then 3 ml of the NaBH4 in DMF solution were injected; the optimum concentration necessary for the reduction was 2% m/V NaBH4 in DMF.The atomic absorptions (average of three determinations) obtained with different concentrations of NaBH4 are shown in Fig. 2. The lead concentration in the final solution was 0.173 pg ml-1 (0.692 pg of Pb). From the results obtained, it can be seen that there was dilution of the lead hydride when an excess of hydrogen was produced when a concentration of greater than 2% mlV NaBH4 in DMF was used. Concentrations of less than 2% m/V produced a decrease in the peak heights of the atomic absorptions because of the lower efficiency of the lead hydride generation.58 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Table 6. Results obtained for the determination of lead in various commercially available gasolines by hydride generation and comparison with the results obtained by the two ASTM methods Hydride generation method ASTM methods Octane Standard Coefficient rating Pb found*/ deviation/ of variation, ASTM-801 ASTM-82/ of gasoline mg 1-1 mg 1-1 Yo mg 1-1 mg 1-1 92 312.9 13.1 4.19 317.9 322.3 96 391.5 12.7 3.33 400.3 392.2 97 380.9 15.7 4.12 385.1 392.6 * Average of five determinations. -F Average of three determinations.The optimum atomisation temperature was 850 “C. The higher was the atomisation temperature the larger were the atomic absorption signals, as can be seen in Table 3, but there is the disadvantage of higher background absorption and, consequently, a worse detection limit. The atomic absorption signals for the determination of 0.861 pg of lead were constant for nitrogen flow-rates over the range 400-564 cm3 min-1, as is shown in Table 4.Higher nitrogen flow-rates produced larger atomic absorp- tion peaks, but the reproducibility was poor. The optimum instrument conditions for the determination of lead by direct generation of its hydride from the gasoline are summarised in Table 5. Reproducibility and Precision Using the 96 octane rated gasoline, the mean absorbance value is 0.378 (ten determinations), corresponding to a lead content of 0.603 pg (0.150 pg ml-1 of lead). The standard deviation and the coefficient of variation were 0.012 and 3.1;%, respectively, with this type of commercial gasoline. Fig. 3 shows the AAS peaks obtained for the determination of 0.150 pg ml-1 of lead (0.603 pg) in the 96 octane rated gasoline using the optimised conditions (n = 10).With the 92 octane rated commercial gasoline, the mean absorbance value (ten determinations) with a lead concentra- tion of 0.151 pg ml-1 (0.603 pg) was 0.528, with a standard deviation of 0.014 and a coefficient of variation of 2.65%. With the 97 octane rated commercial gasoline, the mean absorbance value was 0.375 (n = lo), with a lead concentra- tion of 0.150 pg ml-1 (0.603 pg). The standard deviation and the coefficient of variation were 0.018 and 4.80%, respec- tively. Calibration Graph, Sensitivity and Detection Limit Calibration graphs were prepared for two types of gasoline, 92 and 97 octane. The results obtained were as follows: 92 octane, Beers law was obeyed over the range 0.014-0.081 pg ml-1 of lead (0.054-0.324 pg), R = 0.9991; and 97 octane, Beers law obeyed over the range 0.015-0.094 pg ml-1 of lead (0.063-0.379 pg), R = 0.9985.The sensitivity (1% absorbtion) and detection limit (twice the background signal) were: 92 octane, 0.010 pg of lead (2.5 ng ml-1) and 0.042 pg of lead (10 ng ml-I), and 97 octane, 0.008 pg of lead (2.0 ng ml-1) and 0.039 pg of lead (9 ng ml-I), respectively. Application The proposed method has been applied to the determination of lead in commercially available gasolines of various octane ratings using the standard additions method. Each type of gasoline was analysed five times, and the results obtained are given in Table 6. Dimethylformamide was used as solvent to dilute the sample and to dilute the standardised gasolines used in the standard additions method.The gasoline, sample and standard used were of the same octane rating. The results obtained by the standard additions method were compared with those obtained by the two ASTM methods and show good accuracy. Conclusions The determination of lead in commercially available Spanish gasolines containing TAL additives by the proposed method is a simple, rapid and economic means of monitoring the total amount of lead in these samples. The method is sensitive and incorporates a high dilution of the sample, but as a result, and owing to the low detection limit, it can be used to monitor the total amount of lead in an unleaded gasoline. Satisfactory results were obtained when the method was applied to gasolines of different octane ratings and it showed good accuracy when compared with the results obtained from two ASTM methods.1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. References Harrison, R. M., and Laxen, D. P., Chem. Br., 1980,16,308. Turner, D., Chem. Br., 1980, 16, 312. Boeckx, R. L., Anal. Chem., 1986, 58, 275A. Dartnell, P. L., Chem. Br., 1980, 16, 308. Gonzalez, M. C., and Rodriguez, A. R., Rev. Inst. Fr. Pet., 1983,38,221. Robinson, J. W., Anal. Chim. Acta, 1961, 24, 451. Kashiki, M., Yamazoe, S., and Oshima, S., Anal. Chim. Acta, 1971,53, 95. Lukasiewick, R. J., Berens, P. H., and Buell, E. E., Anal. Chem., 1975,47, 1045. McCorriston, L. L., and Ritchie, R. K., Anal. Chem., 1975,47, 1137. Scott, D. R., Holboke, L. E., and Hadeinshi, T., Anal. Chem., 1983,55,2006. Polo-Diez, L., Hernindez-MCndez, J., and Pedraz-Penalva, F., Analyst, 1980, 105, 37. Holding, S. T., and Palmer, J. M., Analyst, 1984, 109, 507. “Annual Book of ASTM Standards,” American Society for Testing and Materials, Philadelphia, 1983, Vol. 05.03. Pet- roleum Products and Lubricants, Method D 3341-80. “Annual Book of ASTM Standards,” American Society for Testing and Materials, Philadelphia, 1983, Vol. 05.03. Pet- roleum Products and Lubricants, Method D 3116-82. Aznirez, J., Palacios, F., Vidal, J. C., and GalbBn, J., Analyst, 1984, 109, 713. AznBrez, J., Palacios, F., Ortega, M. S., and Vidal, J. C., Analyst, 1984, 109, 123. Aznarez, J., Vidal, J. C., and Gascon, J. M., At. Spectrosc., 1986,7, 59. Aznarez, J., Castillo, J. R., Bonilla, A., and Lanaja, J., At. Spectrosc., 1981, 2, 125. Burger, K. , “Solvation, Ionic and Complex Formation Reac- tion in Non-aqueous Solvents,” Elsevier, Amsterdam, 1983. Paper J61.56 Received July 25th, 1986 Accepted October 9th, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200055

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 I I PI p2 + s 1 59 PAR X- Yplotter O- -4 162 Population Distribution of Atomic Uranium in the Afterglow of a Pulsed Hollow-cathode Discharge 0 0 PAR PAR 164 164 Yves Demers, Jean-Marie Gagne and Piero Pianarosa Laboratoire d'Optique et de Spectroscopie, Departernent de Genie Physique, Ecole Polytechnique, Case Postale 6079, Succursale "A", Montreal H3C 3A7, Canada From laser absorption measurements we have deduced the time evolution of the population distribution of atomic uranium in the afterglow of a pulsed hollow-cathode type discharge. The vapour generator operates with xenon as the discharge sustaining gas a t a pressure of 280 Pa (2.1 Torr). The current pulse characteristics are width 250 ps and height 1.5 A.The pulse repetition frequency is 100 Hz. It is shown that the populations in the three metastable levels at 6249, 3868 and 3800 cm-1 decrease almost exponentially in a time interval between 150 and 300 ps. From 400 ps onwards in the afterglow, the atom population is essentially shared between the ground and the first metastable (620 cm-1) levels. Furthermore, starting from 9 ms in the afterglow more than 80% of the U atoms are found in the ground level. Keywords: Pulsed hollow-cathode discharge; afterglow; uranium; population distribution Hollow-cathode type discharges, widely used in the past essentially as sources of narrow spectral lines for spectroscopic investigations, are now finding applications as generators of metal atoms in the vapour phase.'-10 Our laboratory has been using d.c.and pulsed vapour generators of a hollow-cathode design for a number of years. They were used for spectroscopic studies of uranium,ll-15 for the isotope analysis of uranium samples16 and, more recently, to investigate the production of atomic vapours of Th,17 Mo18 and Zr.19 Despite the wide range of applications cathode sputtering affords, many of the physical phenomena and parameters occurring in and characterising the discharge are still not fully understood. Among the latter, the temperature and popula- tion distributions in the various atomic levels are important for the characterisation of this type of vapour generator. While excitation or spectroscopic temperatures and the ensuing population distribution in the atomic level apparent in d.c.powered hollow-cathode discharges have been exten- sively studied,2@-23 the same parameters and the correspond- ing population distribution in the afterglow of pulsed dis- charges are much less well understood. In the afterglow of a pulsed discharge, excitation of atoms by the discharge and their interaction with the radiation in the hollow-cathode cavity no longer exist. As a consequence, at r 1 the end of the current pulse, radiative levels decay rapidly to the ground state or to any one of the low-lying metastable levels. These metastable states decay mainly by collisions with low-energy electrons or foreign gas atoms and their lifetime can be considerable indeed. It would then be quite feasible to have an appreciable fraction of sputtered atoms in the low-lying levels well after the end of the current pulses.To improve the characterisation of our pulsed discharge tubes we therefore deemed it important to investigate the distributian of U atoms between the ground and the first few excited levels using laser absorption spectroscopy. Experimental The reservoir of U atoms used in this work, a laboratory-made hollow-cathode discharge cell, has already been described elsewhere (see, for example, references 13-15). The pulsed power supply built in our laboratory generates current pulses with intensity and width fixed specifically for this investiga- tion, at 1.5 A and 250 ps, respectively. Pulse rise and fall times are less than 5 ps. The pulse repetition rate is fixed at 100 Hz. For this study the discharge sustaining gas is xenon [Xe pressure 280 Pa (2.1 Torr)].These experimental conditions have been found to correspond to a maximum U I ground state density. 13 Fig. 1. Laser spectrometer, see text for details60 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 The laser spectrometer used for this investigation is shown in Fig. 1. The laser, a Coherent Radiation 599, is monomode and frequency stabilised; its line width is less than 7 MHz. The dye is Rhodamine 6G. A 0.5-m monochromator is used to tune the laser wavelength to the atomic transitions under study. The incident laser beam is divided by a beam splitter M1, whose transmission efficiency is such that the beam intensity incident at the generator is less than 6 pW cm-2. Such a low value is needed in order to have an absorption that is independent of the incident intensity,13 hence saturation effects will not plague the measurements.Two photodiodes, P1 and P2, (rise time ca. 20 ns) provide signals proportional to incident, Io(v), and transmitted, Itr(v,t), light intensities, respectively. These signals are fed into the sampling heads of a boxcar integrator (Princeton Allied Research Models 164 and 162). The boxcar is triggered by the leading edge of the current pulse. Its output provides a voltage signal that is proportional to -log [It,(v,t)/I0(v)] = absorbance and drives the Y axis of a chart recorder. The Y axis has been calibrated to give directly the value of the integral of the absorption coefficient over the optical path, i.e., With this experimental configuration two different types of measurements are possible.1. We can fix the sampling windows of the boxcar at a definite time ti measured from the end of the current pulses. By scanning the laser frequency synchronously with successive current pulses we obtain the profile of the absorption coefficient over the optical path at the predetermined time ti. In this mode of operation the frequency resolution is approximately 15 MHz and the temporal resolution is equal to the width of the sampling windows (5 p). 2. Alternatively, we can fix the laser frequency at the centre of an absorption line (YO) and, with each current pulse, shift the boxcar sampling windows an amount At. We then obtain the time evolution of the absorption coefficient at the line centre Jb k,,(t, <)dc.In this mode of operation the frequency resolution is ca. 50 MHz and the temporal resolution 50 ys. Results We investigated the atom density in the 5L:.ground state and in four low-lying metastable levels of uranium. These levels are the 620 cm-1 (5K3, the 3800 cm-1 (5L:), the 3868 cm-1 (53 belonging to the Pds2 configuration and the 6249 cm-1 (7 ) level belonging to the f3d2s configuration. The para- meters associated with the transitions that were studied are listed in Table 1. Measurements of Iik(v, t = ti, c)d< The integral k(v, t = ti, 5)dt; over the vapour phase length was measured at different ti between 0 and 400 ps. We assumed that in this time interval there is no diffusion of the atomic vapour outside the cavity of the cathode.The absorbing path length is then equal to the cathode length Z (Z = 3 cm). Assuming the vapour to be homogeneous, we have Zk(v, t = ti) = JLk(v, t = t i , c)dc . . . . (2) The population level can then be evaluated using24 N = - 8ng1 $” k(v,t=ti)dv . . . . (3) ho2Ag2 -” In this expression gl and g2 are the statistical weights of the lower and upper level of the transition, A is the Einstein transition probability and the wavelength of the transition at the line centre. The total absorption coefficient Jwk(v, t = tJdv used in equation (3) is obtained by simply taking the area under the experimentally measured curve of k(v, t = ti) versus frequency. Fig. 2 is an example of the profile of an absorption coefficient k(v, t = ti) versus v. The profile deviates slightly from a purely Gaussian shape.This could be due to the presence, in the early afterglow, of spatial inhomogeneities in the kinetic temperature values and, as pointed out by Leblanc et aZ.,25 of motion of the vapour as a whole. The experimental results are summarised in Table 2. Fig. 3 is a plot of atom density in the three levels at 3800, 3868 and 6249 cm-1 as a function of the time in the afterglow. There are two main sources of error in the density values measured. The first one is the uncertainty in the published values of the transition probabilities. The second one is the difficulty in estimating the length Z of the volume occupied by the vapour inside the cavity of the cathode. We estimate that the population densities given here are accurate to within a factor of two.- W 0.5 1 0.6 0 1 2 3 4 5 6 dGHz Fig. 2. Profile of the absorption coefficient k(v) at ti = 0 for the transitions at 598.6 nm: continuous line, experimental profile; Gaussian line-shape of equivalent width ~~ Table 1. Wavelengths, upper and lower energies and classifications and transition probabilities Lower energy level Upper energy level WavelengtWnm Elcm-1 Classification Elcm-1 Classification All06 s-1 575.81 0 5L: 17361 395 7L6 0.21 f 0.04* 5Kz 17361 395 7L6 0.59 k 0.18.t 597.15 620.323 597.63 3800.829 5G 20528.898 ’M8 2.1 k 0.93 598.61 3868.486 5H: 20569.228 J = 4 1.1 k 0.4$ 599.73 6249.029 ’@ 22918.555 5L7 1.9 k 0.7$ * From reference 13. t From reference 14. t From reference 32.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 61 Table 2. Experimentally determined populations of the energy levels (1012 cm-3) tips 0 17 34 50 100 150 200 250 350 NO 24 - - 31 30 29 27 27 27 N620 N3800 21 4.7 - - - - 3.0 2.1 11 1.4 - 0.85 - 0.47 9 0.24 - - N3868 3.5 - - 2.1 1.5 0.95 0.60 0.35 0.21 N6249 4.4 2.3 1.8 1.4 0.75 0.35 - - - 0 100 200 300 400 U P Fig. 3. Time evolution of the population of the levels at 3800 cm-l (A), 3868 cm-1 (B) and 6249 cm-1 (C) at the beginning of the afterglow Time Evolution of 1; kv,(t, <)d& In a subsequent study we monitored the absorption coefficient time evolution at the line centre, i. e., the quantity sb kvo(t, l;) dl;. The measurements were performed for the transitions at 575.81 nm (0 cm-1 + 17361 cm-I), 597.1 nm (620 cm-1 -+ 17361 cm-1) and 597.6 nm (3800 cm-l-+ 20528 cm-1).The results obtained are presented in Figs. 4-6, respectively. At the end of the current pulse the atomic vapour starts diffusing out from the cathode cavity and, in the time span of a few milliseconds, it fills the entire volume of the generator. The quantity k(v, t, 0) becomes an unknown function of 5 and this fact makes the use of equation (3) impossible. As a consequence, the absolute population density in the different levels cannot be evaluated any longer. However, we can estimate the population ratio of two levels if we make the reasonable assumptions that this ratio is constant anywhere inside the generator and that the line shapes of the different transitions are the same. This ratio can be evaluated from Using equation (4) and the experimental values for the integrated absorption coefficients from Figs. 4 and 5 we evaluated the population ratios in the ground (5Lz) and first metastable (620 cm-1, 5K:) levels.The results are shown in Fig. 7. From the Doppler width of the h = 575.8-nm line we have deduced that the temperature of the U atoms, fort > 700 ps, is almost constant and equal to 380 K. For local thermodynamic equilibrium, for a temperature Tk = 380 K, the Boltzmann distribution would give a value 10.5 for the &&2dN62&0 ratio. As can be seen from Fig. 7 the exponential decay constant is ca. 4 ms, and at t = 9.75 ms the vapour has not yet reached the condition of equilibrium. 1 I I I 1 I 1 I I 0 1 2 3 4 5 6 7 8 9 10 tlms Fig. 4. Evolution of Jbkv,,(f, f)df for the h = 575.8-nm line Discussion and Conclusions From the values presented in Table 2 and from Figs.3-6 it appears that the time evolution of the metastable levels at 6249, 3868 and 3800 cm-1 is quite different from that of the 620 cm-1 level and of the ground level. The first three levels are characterised by a rapid decrease in population in the very early afterglow (WOO ps). This is followed, however, by a slight, but reproducible, population increase at t = 700 ps (see Fig. 6). This rapid destruction of the metastable levels is quite a common feature of gas discharges in general.2G28 Two facts lead us to believe that this rapid destruction of the metastable levels may proceed via superelastic collisions with electrons. Firstly, the slow decay (z = 0.5 ms) of the U I1 density in a similar discharge29 implies that the electron density is still rather high during the first few milliseconds in the afterglow.Secondly, in the afterglow of a pulsed discharge the electron gas temperature tends very rapidly (50 ps) to the carrier gas kinetic temperature.= In our case this corresponds to a drastic cooling of the electron gas from a temperature of the order of 3500 K30 to ca. 850 K in the first 50 ps of the afterglow regime.25 At this electron temperature, and for the levels under investigation, de-excitation phenomena are clearly predominating over excitation. The second trend typical of the time evolution of the population common to these three levels is the slight increase observed at ca. 700 p. This increase reaches a maximum at t = 1.2 ms (see Fig.6) then slowly diminishes and is zero again at t ca. 5 ms in the late afterglow. This population variation seems to proceed via ion recombination effects followed by radiative transitions to lower metastable levels.26 The 620 cm-1 and the ground levels, on the contrary, decrease more slowly in the discharge afterglow when compared with the previous three levels. One characteristic62 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 5 o 1 2 3 4 5 6 7 a 9 10 tlms Fig. 5. Evolution of $,kv0(t, 5)dc for the li = 597.1-nm line - 0 3 3 3 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Urns Fig. 6. Evolution of J$vo(t, 5)dC for the li = 597.6-nrn line 2t 0 2 4 6 8 1 0 t/ms Fig. 7. Time evolution of the ratio NOg62dN620g0 in the discharge afterglow.The broken line represents the equilibrium value of this ratio feature of the ground level is its increase in population in the first few instants of the afterglow regime (see Fig. 4). This increase follows from radiative and collisional de-excitation processes of the high lying levels populated by the glow discharge. By comparison, the increase in population of the 620 cm-1 level in the same time span is much less pronounced. This difference can be explained by the fact that, while ground- state atoms are destroyed in part by diffusion processes and in part by Penning type reactions,” the 620 cm-* atoms decay by collisional de-excitation to the ground state, further contribut- ing to this latter population, Note also that the small increase in lh k (Y, t , c)dc observed in Fig.5 could be due to a reduction in the afterglow regime of the Doppler line width. From our study it appears that, when the vapour generator operates in a pulsed mode, the populations of the levels at 6249, 3868 and 3800 cm-1 are, for t > 400 ps, approximately two orders of magnitude lower than the ground-state popula- tion. It stands to reason that the same temporal behaviour could be attributed to the other U metastable levels as they are higher in energy than the 3800 and 3868 cm-1 levels. Hence we conclude that in the late afterglow the majority of sputtered atoms are distributed between the ground level and the metastable state at 620 cm-1. Finally, in the very late afterglow ( t > 9 ms), at least 80% of the atoms are in the ground state.The technical assistance of P. A. Dion and Y. Lemire is greatly appreciated. This work has been supported in part by the Natural Sciences and Engineering Research Council of Canada and the Ministkre de 1’Education du Quebec. 1. 2. 3. 4. 5 . 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. References Russell, B. J., and Walsh, A., Spectrochim. Acta, 1959, 15, 883. Gatehouse, B. M., and Walsh, A., Spectrochim. Acta, 1960, 16,602. Gough, D. S., Hannaford, P., and Walsh, A., Spectrochim. Acta, Part B, 1973, 28, 197. Gough, D. S., Anal. Chem., 1976,48, 1926. Gerstenberger, D. C., Solanki, R., Collins, G. J., ZEEE J. Quantum. Electron., 1980, QE16, 820 and references cited therein. Goleb, J.A., and Brody, J. K., Anal. Chim. Ada, 1963, 28, 457. Goleb, J. A,, Anal. Chem., 1963, 35, 1978. Goleb, J. A., Anal. Chim. Acta, 1966, 34, 135. Goleb, J. A,, and Yokoyama, Y., Anal. Chim. Acta, 1964,30, 213. Goleb, J. A., Anal. Chim. Acta, 1966, 36, 130. GagnC, J.-M., Mongeau, B., Leblanc, B., Saint-Dizier, J.-P., Pianarosa, P., and Bertrand, L., Appl. Opt., 1978, 17, 2507. GagnC, J.-M., Leblanc, B., Mongeau, B., Carleer, M., and Bertrand, L., Appl. Opt., 1979, 18, 1084. GagnC, J.-M., Carleer, M., Leblanc, B., Demers, Y., and Mongeau, B., Appl. Opt., 1979, 18,2107. GagnC, J.-M., Mongeau, B., Demers, Y., and Pianarosa, P., J. Opt. SOC. Am., 1981,71, 1140. Dr&ze, C., Demers, Y., and GagnC, J.-M., J. Opt. SOC. Am., 1982,72,912. Pianarosa, P., Demers, Y., and GagnC, J.-M., J. Opt. SOC. Am., 1984, B11, 704. Pianarosa, P., Demers, Y., and GagnC, J.-M., Spectrochim. Acta, Part B, 1984, 39, 761. Chevalier, G., unpublished results, Ecole Polytechnique, Montreal, Canada, 1983. GagnC, J.-M., and Chevalier, G., Internal Report, Ecole Polytechnique, Montreal, Canada, 1985. Mehs, D. M., and Niemczyk, T. M., Appl. Spectrosc., 1981,35, 66. Palmer, B. A., Keller, R. A., and Engleman, R., Jr., LA 8251 Report, Los Alamos National Laboratory, Los Alarnos, NM, USA, 1980. Palmer, B. A., and Engleman, R., Jr., LA 9615 Report, Los Alamos National Laboratory, Los Alamos, NM, USA, 1983. Keller, R. A., Englernan, R., Jr., and Zalewski, E. F., J . Opt. SOC. Am., 1979, 69, 738. Mitchell, A. C. G., and Zemanski, M. W., “Resonance Radiation and Excited Atoms,” Cambridge University Press, New York, 1971. Leblanc, B., Carleer, M., Demers, Y., and GagnC, J.-M., Appl. Opt., 1980, 19,463. Phelps, A. V., and Molnar, J. P., Phys. Rev., 1953,89, 1202. Phelps, A. V., Phys. Rev., 1955,99, 1307. Cherrington, B. E., “Gaseous Electronics and Gas Lasers,” Pergamon Press, New York, 1979. Bouchard, P., PhD Thesk, Ecole Polytechnique, 1985. Demers, Y., MSc Thesis, Ecole Polytechnique, 1980. GagnC, J.-M., Bouchard, P., and Gleizes, A., Can. J. Phys., 1985,63, 1506. Corliss, C. H., J. Res. Nutl. Bur. Stand., Sect. A, 1976, 80, 1. Paper JA.512 Received October 4th, I985 Accepted August 4th, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200059

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 63 Laser-excited Atomic Fluorescence Spectrometry as a Practical Analytical Method Part 2. Evaluation of a Graphite Tube Atomiser for the Determination of Trace Amounts of Indium, Gallium, Aluminium, Vanadium and Iridium by LAFS Klaus Dittrich and Hans-Joachim Stark Sektion Chemie, Karl-Marx-Universitat, WB Analytik, Talstr. 35 DDR-70 10, Leipzig, GDR ~~ The performance of a new tube atomiser design for the laser-excited atomic fluorescence (LAFS) technique was evaluated and compared with a laboratory-made carbon rod atomiser for the determination of the elements indium, gallium, aluminium, vanadium and iridium. The tube atomiser is a modified commercial HGA-500/EA 3. The atomic fluorescence is measured within the tube.There was a considerable improvement in the detection limits, sensitivities and reproducibilities obtained with the tube atomiser in comparison with the rod atomiser, especially for elements of low volatility. The influence of halide matrices (F-, CI- and Br-) on the fluorescence intensity of indium was investigated for the rod atomiser, tube-wall atomisation and tube-platform atomisation where the L'vov platform technique was used. With the tube atomisation the influence of the matrix was reduced further by using the L'vov platform technique. Keywords: Laser-excited atomic fluorescence spectrometry; trace analysis; graphite tube atomiser Laser-excited atomic fluorescence spectrometry (LAFS) is, as both theory and practice have shown, by far the most sensitive atomic spectrometric technique.Problems in the realisation of the theoretically predicted detection limits in practice (es- pecially with real samples of complex composition) often arise for the following reasons: (i) a lack of power and an unstable laser system; (ii) stray light problems; and (iii) ineffective atomiser systems. In the current research programme we were particularly concerned with the third of these points, the atomiser problems. In previous work with LAFS, only open atomisers have been used; chemical flames, physical flames (e.g., the ICP) and open electrothermal atomisers (e.g., the West type carbon rod atomiserl>2). We investigated whether the semi- enclosed carbon tube atomiser (CTA), which is well known for its use in AAS with electrothermal atomisation, could overcome the disadvantages caused by the thermal inho- mogeneity in open atomiser systems.For this purpose we modified commercially available graphite tube atomisers (the HGA 500EA 3 and Beckman Type 1268 atomisers) for use with the LAFS-ETA system.3 We have already discussed the spectrometer system that was used and the modifications of the atomiser in our previous paper.3 Our first impressions of the advantages of the new enclosed tube atomiser design for LAFS-ETA when dealing with the volatile element lead were encouraging, but we expected to find the greatest improvements in the analytical results for elements of particularly low volatility. We therefore extended our investigations to some elements with medium and low volatility (i.e., with medium and high atomisation temperatures).In this paper we present and discuss the results of investigations on the determination of the elements indium, gallium, aluminium, vanadium and iridium by LAFS-ETA. Experimental Choice of Elements Investigated The choice of the investigated elements was made firstly according to their volatility (boiling-points) and secondly according to the parameters of the laser - spectrometer system (wavelengths attainable), Direct line fluorescence was predominantly used to prevent stray light and scatter problems. Table 1 shows the selected elements, their boiling-points, the wavelength pairs used and the type of fluorescence used. Only for aluminium did we have to use resonance fluor- escence, because it was not possible to separate the exciting and fluorescence wavelengths due to the poor resolution of the spectrometer system (> 9 nm).Lead, which we investigated previously, is an element of high volatility with no atomisation problems3; however, contamination problems are considerable. Indium has similar Table 1. Characterisation of the investigated elements Boiling- Wavelengthlnm point/ Element "C Excitation Fluorescence In . . . . . . . . 2300 303.936 325.556 Ga . . . . . . . . 2344 287.424 294.418 294.364 A1 . . . . . . . . 2467 308.216 308.21 6 309,271 V . . . . . . . . 3500 264.771 354.350 Ir . . . . . . . . 4800 284.972 357.372 * DLF, direct line fluorescence; and RF, resonance fluorescence. Type of fluorescence* DLF DLF RF DLF DLF DLF Maximum energy of dye laser/ PJ 5 3 3 1 764 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 volatilisation properties to lead but gallium and aluminium are elements of medium volatility. These elements have a tendency to form thermally stable diatomic molecules (e.g., oxides and halides) in the gaseous phase and relatively high atomisation temperatures are required to dissociate these molecules. Vanadium and iridium are elements of low volatility, which require extremely high atomisation temperatures. Vanadium forms a thermally stable carbide with the graphite of the atomiser in the charring phase, which is difficult to atomise and the boiling-point of iedium is very high. Hence, the elements selected give a broad spectrum of atomisation efficiency , covering most situations of analytical interest and therefore provide a true test of the new atomiser.Solutions Stock solutions (0.2 mg ml-1) of In, Ga and A1 were prepared from the metal nitrates in 1 M HNO3. Stock solutions (1 mg ml-1) of V (from VOS04) and Ir (from H2IrCk) were also prepared in 1 M m03. The matrix solutions used were 1 M solutions of the sodium halides. Stock solutions were diluted with 0.01 M HN03. All reagents were of analytical-reagent or Suprapur grade and the water used was distilled three times in a quartz apparatus. Conditions and Apparatus The laboratory-made atomic fluorescence spectrometer, which used components from Carl Zeiss, Jena, GDR, has been described previously. The carbon rod atomiser (CRA) built by us was run at an argon gas flow-rate of 60 1 h-1.When investigating the carbon tube atomiser (CTA) the modified HGA-5OOEA 3 cuvette was used3 [HGA-500 (Perkin-Elmer Ltd., Norwalk, CT, USA), EA 3 (Carl Zeiss, Jena, GDR)]. The internal gas flow was stopped during the atomisation step. The cuvette was enclosed by two quartz windows. Fluorescence was measured along the tube axis and the laser beam was aligned perpendicular to the tube axis. The holes for the laser beam were made as small as possible (rectangular, 1.5 X 4.0 mm) and slightly removed (2 mm) from the atomiser centre. In this position the sensitivity was best, due to the gas dynamics and evaporation in the tube. We have not investi- gated this fact in detail. These conditions were found to be the optimum for the HGA 500EA 3 atomiser for LAFS.Rectangular platforms made from pyrolytic graphite measuring 15 x 4 x 1 mm with a cavity for holding the sample droplet were used for the L'vov platform technique. Results Determination of the Sensitivity and Detection Limits of the Elements Investigated The correlation between the intensity (I) and concentration (C), which was measured at many points over 3 4 orders of concentration, was not linear, but the log transformed values log Zllog C gave a linear dependence in this range. For the three lowest concentrations, about one order of concentration, we also found a linear dependence for IIC. Therefore we calculated, for these three points, a linear calibration function. The detection limits were determined by extrapolating the calibration function and calculating the 3a level of the background noise.The sensitivity levels were calculated as reciprocal sensiti- vities at the 100-mV signal level. A 100-mV signal was generally equivalent to the 3a level of the background noise, i. e., the reciprocal sensitivity corresponds approximately to the detection limit. Table 2 shows the detection limits and the reciprocal sensitivities at 100 mV for both atomisers (CTA and CRA) and also the improvement factors using the CTA (CWCTA). Table 2 shows that in all instances both the reciprocal sensitivities and the detection limits are improved by using the CTA. The greatest improvements were obtained for the low volatility elements vanadium and iridium; the detection limits for these elements were therefore improved to a greater extent than the reciprocal sensitivities.This effect is due to the unavoidable high noise level caused by the glowing rod, in particular because of the short distance between the laser beam and the top of the rod necessary in order to observe any fluorescence at all for vanadium and iridium with the CRA. The influence of the use of the L'vov platform on the sensitivities and detection limits was tested for indium, gallium and aluminium. No significant influence was found. However, with platform atomisation there were more problems with the continuum light emitted by the glowing platform. We there- fore had to be extra careful in order to prevent the thermal background radiation from entering the monochromator entrance slit. It should however be noted that the rectangular platforms that were used were not the optimum for use with LAFS. Better results could be obtained by using smaller, square platforms, which would concentrate the sample more in the analytical zone and achieve a greater distance between the analytical zone and the glowing platform.Reproducibility The reproducibility of atomisation was tested for indium by atomising 500 pg of indium per 10-pl sample volume 150 times. This sample volume was possible for both rods and tubes. The diameter of the rod was 5 mm with a small cavity suitable for the 10-p1 sample. The reproducibility of the CRA was found to be CCL. 0.18 relative standard deviation (RSD) and that of CTA was cu. 0.05. The reproducibility for hand dosage using a 10-pl pipette was also ca. 0.05. This shows that the reproducibility of atomisation of the CTA is much better than that of CRA.Lifetime of the Tubes and Rods The lifetime of the rods when used for low volatility elements (vanadium and iridium) was found to be only 10-15 atomisa- Table 2. Results for LAFS using the CRA and CTA: a comparison Detection limit/pg Improvement Element CRA CTA factor In . . . . . . . . 5.5 0.14 40 Ga . . . . . . . . 70 10 7 A1 . . , . . , . . 45 4 11 v . . . . . . . . 53oooo 2200 240 Ir . . . . . . . . 600000 475 1660 Reciprocal sensitivities/ pg per 100 mV Improvement CRA CTA factor 25 0.5 50 200 - 22 9 55 6 9 67000 860 80 1OOOOO 740 135JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 65 Log (concentrationh) Fig. 1. Interference of sodium halides [ a) NaF; (b) NaCl; and (c) NaBr] on In direct line fluorescence (LAFS-ETA) using different atomisers and atomisation procedures: A, &RA; B, CTA; and C, CTA with L’vov platform Table 3.Dissociation energies (ED) of InX molecules4 Molecule EdeV InF 5.25 InCl 4.42 InBr 3.99 tion cycles. The tubes were usually better, providing 150 or more atomisation cycles. For the elements of higher volatility the lifetimes were of course longer (ca. 500 atomisation cycles for one tube) and there was not much difference between the rods and the tubes. Influence of Halide Matrices on the Indium Direct-line Fluorescence In order to obtain further information about the influence of chemical matrices in the different atomiser systems, the influence of fluoride, chloride and bromide salts on the indium direct-line fluorescence (DLF) using the CRA, the CTA and the CTA with the L‘vov platform was examined.The particular matrices were chosen because the depressive influence of halide matrices on the indium atomic absorption signal is well known, as shown by the reaction In + X InX where X = halide, which lowers the concentratioh of free In atoms in the gaseous phase. The depressive effect should be increased with an increase in the stability of the molecules and as the temperature in the analytical zone of the atomiser is reduced. An indication of the stability of these molecules is revealed by their dissociation energies. Table 3 shows the dissociation energies of the indium monohalides of interest. We expected to find the largest effect for the fluoride matrix and the open-type CRA.The results obtained are given in Fig. 1, which show that the influence of the halides on the indium DLF was depressive, as with indium AAS. In all instances the CRA was influenced most by the matrices. The results for the CTA by wall atomisation were better; but the best results were obtained with the CTA and the L’vov platform. We noted that the interference from the matrix using the CTA with the L’VOV platform was 100 times less than for the CRA and using the CTA without the platform was ten time less than for the CRA. Fig. 1 further shows that the influence of the chloride matrix is the strongest. This does not seem to agree with the values for the thermal stability of the molecules. Owing to the higher stability of the InF molecules, fluoride should have the strongest influence.However, the change is caused by the removal of the slightly volatile and weak H2F2 by the stronger acid in the diluent during the drying and charring steps (thermal hydrolysis). In this way some fluoride is removed from the sample before the atomisation phase. Hence the effect of the fluoride matrix is less than that of the chloride matrix. The influence of bromide is less than that of chloride, which is as expected from the thermal stability of the InX molecules. Discussion of the Results The experiments carried out in order to test the sensitivities, detection limits, reproducibility and the influence of matrices show that in all instances the semi-enclosed CTA gives better analytical results than the open CRA.These results are due to the better thermal homogeneity of the analytical zone of the semi-enclosed tube atomiser. We noted a strong negative temperature gradient above the heated carbon rod, because the plasma is heated from only one side, from the surface of the glowing rod. In the semi-enclosed tube type atomiser such a strong temperature gradient does not exist, due to the heating of the plasma of the analytical zone from all directions (tube walls). L’vov platform atomisation, however, provides better isothermal conditions because of the heating delay of the platform in relation to the tube walls and therefore allows the vaporisation of the sample into a hotter, more thermally homogeneous plasma. This more isothermal, hotter plasma environment provides better atomisation of the vaporised particles or molecules, less recombination and condensation of free atoms and so a higher sensitivity, and less matrix influence by molecule formation.Platform atomisation is only possible for the CTA not for the CRA. Therefore when considering the influence of the matrix, the best results were obtained by using the CTA with the L’vov platform, followed by the CTA with wall atomisation. In spite of the fact that the glowing area of the CTA is greater than that of the CRA and that the laser beam must pass through the holes in the tube wall, stray light and thermal background radiation did not occur to a greater extent in the tube atomiser than in the open-type CRA. Hence the66 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 detection limits of the tube atomiser were found to be improved as well as the sensitivities. Moreover, the detection limits of the low-volatility elements (vanadium and iridium) are improved to a greater extent than the reciprocal sensitivi- ties by using the CTA instead of the CRA. This is due to the fact that the atomisation efficiency of the CTA is relatively constant across the tube diameter (heating from all direc- tions), and there is no strong negative temperature gradient as in the CRA. Therefore, the analytical zone of the CTA need not be constrained close to the glowing surface of the tube in the case of the low-volatility elements. With the CRA we could not observe fluorescence of iridium or vanadium without bringing the analytical zone (the laser beam) extremely close to the surface of the rod, because the highest concentration of free atoms is found directly above the top of the rod (the highest temperature zone).The distance was then less than 1 mm. However, the shorter this distance, the more difficult it was to prevent background radiation from entering the monochromator entrance slit. Thus greater background radiation noise was observed with the CRA compared with the CTA for the low-volatility elements and the detection limits were improved further by using the CTA. The better reproducibility of atomisation in the CTA is also caused by improved thermal homogeneity of the plasma in this atomiser. The results obtained are correct for both direct line and resonance line (compare with the A1 results) atomic fluorescence.Of course the tube atomiser may have a higher stray light following the higher particle concentration in general. However low-volatility elements can be measured using rods only very near to the bottom of the rod, where also high matrix densities exist. Conclusions Electrothermal atomisation can be a tool to attain extremely high sensitivity in LAFS-ETA (detection limits down to 10-15-10-16 g). All types of electrothermal atomisers can be used for LAFS. We have shown that a modified CTA leads to better analytical results than an open CRA. In conclusion: 1. The CTA improved the sensitivity of LAFS-ETA in comparison with CRA. 2. Better detection limits were obtained in the CTA than in the CRA, especially for low-volatility elements. 3. The CTA leads to better reproducibility of atomisation (CTA, 0.05% RSD; and CRA, 0.18% RSD). 4. The CTA is not as susceptible to matrix interference as the CRA. 5. The CTA gives the possibility of using the L’vov platform technique, making the atomiser thermally more homogeneous and depressing the matrix interferences further. The advantages of the CTA are mostly due to the thermally more homogeneous plasma of the semi-enclosed CTA in comparison with the CRA. Therefore in the CTA the efficiency of atomisation is improved. The modified CTA represents an excellent electrothermal atomiser for LAFS- ETA and should be used in preference to other systems. References 1. Bolshov, M. A., Zybin, A. V., and Smirenkina, I. I., Spectrochim. Acta, Part B, 1981, 36, 1143. 2. Omenetto, N., andHuman, H. G. C., Spectrochim. Acta, Part B, 1984,39, 1333. 3. Dittrich, K., and Stark, H.-J., J. Anal. At. Spectrom., 1986,1, 237. 4. Krasnova, K. S., Editor, “Molecular Constants of Inorganic Compounds,’’ Khimiya, Leningrad, 1979. Paper J6/23 Received March 18th, 1986 Accepted July 28th, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200063

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 67 Silicate Rock Analysis by Energy-dispersive X-ray Fluorescence Using a Cobalt Anode X-ray Tube Part 2.” Practical Application and Routine Performance in the Determination of Chromium, Vanadium and Barium Philip J. Potts, Peter C. Webb, John S. Watson and David W. Wright Department of Earth Sciences, Open University, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK This paper investigates the practical determination of the elements Cr, V and Ba by energy dispersive X-ray fluorescence using the excitation conditions dedcribed in Part 1. Particular attention was paid to the selection of a reliable set of geochemical reference materials to form a coherent calibration set. Discrepancies caused by the overlap of calcium - aluminium and calcium - silicon sum peaks on Ba Land Cr K lines in those samples containing more than 10% CaO were eliminated by reducing spectrum data accumulation rates.New data are presented for the elements Cr, V and Ba (and Ti and Mn) in nine reference materials recently distributed by the Geological Survey of Japan and these results show satisfactory agreement with other published values. Keywords: Silicate rock analysis; energy-dispersive X-ray fluorescence; cobalt anode X-ray tube; chromium, vanadium and barium determination The elements chromium, vanadium and barium are important in many geochemical studies, for which determinations are required down to concentrations of a few p.p.m. Several techniques are capable of achieving this goal, including wavelength-dispersive X-ray fluorescence (WD-XRF).However, this method is not without some analytical difficul- ties. In particular, although conventional wavelength-disper- sive X-ray spectrometers have high nominal resolution, several serious spectrum overlap interferences affect the elements of interest, e.g., Ti KP on V Ka, V KP on Cr Ka and Ti Ka on Ba La. Indeed the latter interference is relatively severe in silicate rock analysis applications, causing some workers to prefer the less intense Ba Lp line for analysis.’ Recent work in this laboratory2J has established that energy-dispersive X-ray fluorescence (ED-XFW) is as effec- tive as the wavelength-dispersive technique in determining routinely a wide range of major and trace elements in silicate rocks. Detection limits, particularly for the heavier trace elements (Rb, Sr, Y, Zr, Nb, Pb and Th), were found to be equivalent to those encountered in routine WD-XRF schemes of analysis.3 However, analytical data for the trace elements Cr, V and Ba were unsatisfactory and not reported.The reasons for this were because (i) K lines of these elements were not adequately excited using the general purpose silver tube then available and (ii) the resolution response of an energy-dispersive detector compares unfavourably with that of a wavelength-dispersive spectrometer in this region of the X-ray spectrum. Data presented in Part 1 4 show that the difficulties in determining Cr, V and Ba by ED-XRF analysis are largely overcome by exciting samples with a cobalt anode X-ray tube. Not only are the trace elements of interest excited efficiently by the characteristic tube lines, but in combination with a 12.5-pm iron primary beam filter, the excitation of these elements is selectively enhanced relative to that of iron.As the abundance of iron in silicate rocks is normally several orders of magnitude higher than that of the trace elements in question, this selective excitation has beneficial analytical consequences as discussed previously.4 In this paper, the practical application of selective cobalt * For Part 1 of this series see reference 4. tube excitation is examined in the ED-XRF analysis of silicate rocks. In particular, the choice of reference materials and the reliability of the resultant calibration data set is examined critically.The quality of refined calibrations is assessed through determinations of the elements Cr, V and Ba (and additional data for Ti and Mn) in nine new silicate reference materials recently distributed by the Geological Survey of Japan. Instrumental Procedures Analytical data were obtained from an energy-dispersive X-ray fluorescence spectrometer (Link Systems MECA 10- 44) fitted with a cobalt anode side-window X-ray tube. Samples were excited at 20 kV, 0.1 mA using a 12.5-pm iron primary beam filter. The nominal resolution of the Si(Li) detector was 165 eV at 5.9 keV and the X-ray tube was operated in the pulsed mode to permit data acquisition rates of 10000 counts s-1 at only 3&35% dead time. Spectra were counted for 800 live seconds and peak areas were quantified using a proprietary filtered least-squares deconvolution pro- gram.5 This deconvolution program has been shown to calculate the areas of overlapping spectrum lines reliably in ED-XRF applications,2 and is an important factor that contributed to the success of the present study.Proceedings for deriving the X-ray profiles used in deconvolution and other experimental details have been described earlier.2 The instrument was calibrated using the wide range of international reference materials listed in Table 1, prepared as pressed powder pellets. Calibration equations were obtained for individual elements by linear regression of X-ray count data against apparent fluorescence concentrations6 calculated using the “usable” compositions of Abbey.7 In routine analyses, apparent concentrations were derived from the calibration equation.“True” concentrations were then calcu- lated from an iterative mass attenuation correction procedure using data for the major elements determined from photo- peaks in the same spectra that were recorded for the measurement of Cr, V and Ba count data. The mass attenuation correction procedure utilised the coefficients of Leroux and Thinh8 and was adapted to account for structural water as estimated from the loss on ignition of a separate aliquot of each sample.68 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 Results and Discussion The success of any calibration based on international refer- ence materials depends on the reliability of “usable” values taken from the literature.As discussed by Abbey,9 geochem- ical reference material compositions cannot be adopted uncritically when choosing a calibration data set. Of particular concern in this work were the data used for the calibration of chromium. Most silicate reference materials contain less than 400 p.p.m. of chromium. However, three widely used reference samples contain significantly higher concentrations, namely: peridotite USGS PCC-1 (2800 p.p.m.), dunite NIM-D (2900 p.p.m.) and pyroxenite NIM-P (24000 p.p.m.). Preliminary examination of the ED-XRF calibration data set indicated that significant discrepancies could affect the fit of the calibration line at low concentrations if these exceptionally high values were included. This effect is demonstrated in Fig.1 , which plots the ED-XRF calibration data for ten reference materials selected arbitrarily to cover the range 0-400 p.p.m. of chromium together with data for PCC-1. The complete data set is plotted in Fig. l(a). In Fig. l(b) and (c), a linear regression for these data is plotted over the range 0-0.6 (apparent fluorescence values as YO oxide). All ten data points were included in the linear regression plotted in Fig. l(b). The datum for PCC-1 was excluded from the regression plotted in Fig. l(c). Examination of these graphs shows that bias is introduced into the regression line plotted in Fig. l(b) characterised in this instance by a small but significant positive intercept with the intensity axis. This bias is virtually eliminated when PCC-1 is excluded from the data set.This phenomenon could be caused by a non-linear response from the instrumentation at high chromium count rates. Alterna- tively, rounding errors (+ 50 p.p.m.) in the quoted composi- tion could also contribute to this discrepancy. These results thus highlight the unreasonably high weighting that may be placed on a single datum by a linear least-squares regression procedure, applied to a calibration data set which includes one point that is significantly isolated from the main body of data. To avoid such errors, Thompson10 recommended that the simple linear regression procedure was likely to give satisfac- tory fit only if ten or more data points were used and if these data were more or less uniformly spread over the entire concentration range from zero upwards.This latter criterion is not upheld when PCC-1 is included in the calibration set. This sample was, therefore, excluded from the calibration used in this work. By similar reasoning, the reference samples Mica-Mg (4000 p.p.m. Ba) and NIM-S (2400 p.p.m. Ba) were not included in the barium calibration. Table 1. Cr, V and Ba compositions of reference materials (p.p.m.). Data in italics were used in the final calibration. All data are abstracted from Abbey.’ Data marked with ? are “less reliable” values; the remainder are “usable” values Reference material AGV-1 . . . . . . BCR-1 . . . . . . G-2 . . . . . . . . GSP-1 . . . . . . PCC-1 . . . . . . BHVO-1.. . . . . QLO-1 . . . . . . RGM-1 . . . . . . SDC-1 . . . . . . sm-1 . . . . . . BIR-1 . . . . .. DNC-1 . . . . . . w-2 . . . . . . BCS-375 . . . . . . BCS-376 . . . . . . G A . . . . . . . . GH . . . . . . . . BR . . . . . . . . Mica-Fe . . . . . . Mica-Mg.. . . . . USGS (USA)- BCS ( U K j CRPG (France)- ANRT (France)- DR-N . . . . . . FK-N . . . . . . GS-N . . . . . . AN-G . . . . . . BE-N . . . . . . MA-N . . . . . . GSJ (Japan)- NIM (South Africa)- JB-1 . . . . . . JG-1 . . . . . . NIM-D . . . . . . NIM-G . . . . . . NIM-L . . . . . . NIM-N . . . . . . NIM-P . . . . . . NIM-S . . . . SY-2 . . . . . . SY-3 . . . . . . CCRMP (Canada*)- Cr V Ba 10 15 8 12 2800 300 4.2? 4? 66? 4? 370? 270? 92 125 420 36 54 29 320? 61 14? I05? 31 0 150 260 - 1200 680 1900 1300 135 1400 800 650 560 120 175 4? 6. l? . . . . . . . . . . . . . . . . . . . . . . . . . . 25 - 90? 450? .. . . 12 6 380 90 100 38 5? 240 135? 90? 850 22 1050 145 4000 . . . . . . . . 230 - 62? 70 240 4.6? 390 21 O? 1400 34 1050 42 42 55 50 360 3? 3? * . . . . . . . . . . . 400 53 21 0 24 490 460 . . . . lo? 120? 450 100 2400 46? 2900 12 lo? 30? 24000 12 40 81 220 230 10 2? . . . . . . . . . . 12 10 52 51 460 430 . . . . x104 125 v) U 5 100 8 . > C .z 75 +I - 50 25 0 xi04 12 8 4 x104 ( b ) PCC-1 included 12 - 8 - 4 - 0 I (c) PCC-1 excluded I (a) all data PCC-1 0 z I 1 0.2 0.4 0.6 0 0.02 0.04 0.06 0 0.02 0.04 0.06 Apparent fluorescence values, O/O oxide Fig. 1. ED-XRF calibration lines for chromium. (a) Original data set (X-ray count intensity versus apparent fluorescence concentration in % oxide) for ten reference materials having Cr com ositions between 0 and 600 p.p.m. (AGV-la, JG-1, SDC-1, GS-N, BCS-375, JB-1, BE-N, BHVO-1, BR, DR-N) and PCC-1 (2800 p.p,m.C$. (b) Linear regression for data set including PCC-1. (c) Linear regression for data set excluding PCC-1. The lower part of the calibration line only is plotted in Figs. (b) and (c)JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 69 In order to select reference materials suitable for setting up a reliable calibration, as wide a range of silicate reference samples as possible were analysed, the resulting data being examined carefully for internal consistency. In addition to the potential calibration bias arising from samples with unusually high elemental compositions, as discussed above, it was found that fitting errors were also large if all other values were included uncritically in the calibration.Data were, therefore, examined carefully to identify outliers both in calibration data plots, (that is X-ray counts plotted against apparent fluores- cence values) and in plots of expected versus analysed compositions. After exclusion of samples that either plotted as outliers or had unusually high concentrations, the reference values accepted for the final calibration data set are listed in Table 1. The self-consistency of the resultant calibration is demonstrated in Figs. 2 (Cr), 3 (V) and 4 (Ba). In some instances the values discarded from the calibration data set correlated with spectral overlap interferences caused by sum peaks. These are considered further below. In other instances, discrepancies could not be correlated with any identifiable source of systematic bias.BIR-1 (13.33% CaO), W-2 (10.89y0 CaO), AN-G (15.92% CaO), DNC-1(11.52% CaO) andNIM-N (11.50% CaO). The magnitude of this effect was particularly exaggerated when the abundance of coexisting Cr or Ba was low, as may be judged by the data for these samples identified separately in Figs. 2 and 4. To minimise the influence of these sum-peak interferences in analytical results reported in this work, excitation con- ditions were modified from those in the original proposal.4 Because of design limitations in our instrumentation, it was not possible simply to reduce the tube current below the recommended value of 0.1 mA. As an alternative, it was found effective to double the thickness of the iron primary beam filter to 25 pm to reduce significantly the rate of data accumulation and so eliminate sum-peak interferences.Com- pensation for the expected reduction in analytical precision was achieved by increasing spectrum count times from 800 to 1200 s. In consequence, detection limits for Cr, V and Ba are expected to be similar to those originally reported.4 Sum-peak Interferences The analysis conditions derived in the previous paper4 were selected for materials having relatively low calcium contents. However, examination of ED-XW data for a wide range of silicate reference materials showed that unacceptably high count rates were observed for samples containing more than about 10% CaO. This major element is efficiently excited under the conditions optimised for trace chromium determi- nations and sum peaks arising from coincident detection of calcium Kar and both the aluminium and silicon K lines then become significant.These sum peaks were not taken into account by the spectrum deconvolution software used here and caused significant spectrum interferences on Ba Lp2 and Cr Ka as follows: Ba Lp2 (5.16 keV) is overlapped by Ca Kar + A1 Ka (5.18 keV); and Cr Ka (5.41 keV) interfered with by Ca Kar + Si Ka (5.43 keV). No comparable interference was detected on the vanadium K lines. The effect of these sum-peak interferences is to increase the apparent intensity of the trace element line causing systematically high determinations of Cr and/or Ba in 400 E 300 c- al C 2 6 200 c 8 % -0 z P - 100 I 1 I I 100 200 300 400 Expected Cr content, p.p.rn.Fig. 2. Analysed versus expected content of _chromium in silicate reference materials between 0 and 400 p.p.m. 0, Values accepted in the final calibration; 0, values discarded from final calibration; and + , values subject to significant sum peak interferences (see text) 400 300 2 c al c 8 > 200 U v) z m - 2 100 100 200 300 400 Expected V content, p.p.m. Fig. 3. Analysed versus expected content of vanadium in silicate reference materials between 0 and 400 p.p.m. See Fig. 2 for symbol identification 1500 E 2 w- al c = 1000 8 m P U rn > m 4 500 - 1 I I 500 1000 1500 Expected Ba content, p.p.m. Fig. 4. Analysed versus expected content of barium in sificatc reference materials between 0 and 2000 p.p.m. See Fig. 2 for symbol identification70 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL.2 Table 2. Analytical results for reference materials from the Geological Survey of Japan. Analyses carried out on two powder pellets (1 and 2); x, mean determination; CV, coefficient of variation of the four analyses listed for each element = 100 x standard deviatiodx. All data listed in p.p.m. JA-la JB-la Element 1 2 R CV,% 1 2 x CV,% Cr V Ba Ti Mn Cr V Ba Ti Mn Cr V Ba Ti Mn Cr V Ba Ti Mn Cr V Ba Ti Mn 6 5 107 117 402 434 5080 5024 1162 1149 5 5.5 10.5 6 117 114.8 4.5 118 399 414.8 4.1 424 5099 5073 0.66 5090 1162 1157 0.55 1154 JB-2 366 371 198 201 660 592 7823 7879 1119 1129 372 369.2 0.76 368 204 203.5 2.6 21 1 579 635.5 9.7 711 7906 7857 0.54 7821 1132 1124 0.69 1116 JB-3 1 27 22 546 546 295 339 7047 6992 1752 1732 2 x CV,% 30* 26.5 12.5 27 551* 544.0 1.4 533 410* 315.8 25.3 219 6984* 7013 0.43 7029 1753* 1742.5 0.66 1733 JG-la 1 50 48 349 364 405 343 7455 7445 1348 1346 2 x CV,% 45 * 51.0 13.6 61 373 * 354.5 5.1 332 855*,f 355.3 12.6 318 7249 * 7382 1.3 7380 1352* 1345.3 0.54 1335 JGb-1 1 18 16 24 28 482 437 1593 1603 437 490 2 x CV,% 26 22.5 29.4 30 19 21.8 24.4 16 453 461.0 4.3 472 1597 1598.5 0.28 1601 432 446.5 6.6 427 1 65 71 614 613 c95 190 9587 9588 1553 1549 2 x CV,% 67 66.3 5.7 62 616 616.3 2.6 622 c94 169 9618 9575 0.5 9507 1559 1551.5 0.69 1545 - - JP-1 JR-1 1 2 B CV,% 1 2 x CV,% 2722 2745 18 13 < 12 < 12 141 143 987 983 3001 2868.8 5.5 3007 17 15.5 15.4 14 <12 <12 - < 12 142 142.0 0.57 142 994 990.5 0.68 998 JR-2 1 <3 (3 <4 <4 <23 <23 491 498 790 794 2 x CV,% - - <3 5 <4 <4 - <4 <23 45 502 493.0 1.9 48 1 789 791.0 0.27 791 - - 12 11 8 7 53 48 737 740 710 717 - - <3 5 <4 <4 45 48.0 7.4 46 728 734.0 0.75 73 1 699 707.3 1.1 703 - - * These analyses were for pellet 1.7 Determination not included in the calculation of the mean.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 71 Table 3. Comparison between analytical results and published values for the new GSJ reference materials (p.p.rn.). Values in parentheses may be subject to additional uncertainty in extrapolating the calibration (see text) Ti V Reference material Ja-landesite . . . . JB-labasalt . . . . JB-2basalt . . . . JB-3basalt . . . . JG-la granodiorite .. JGb-lgabbro . . . . JP-lperidotite . . JR-lrhyolite . . . . JR-2rhyolite . . . . This work . . 5073 . . 7857 . . 7013 . . 7382' . . 1559 . . 9575 . . 142 . . 734 . . 493 Bower And011 et al. 13 5220 5100 k 120 8030 - 7130 7070 f 120 8690 - 1560 - 9710 - - - 600 660+60 540 - Cr This work 115 204 544 354 22 616 16 <4 - Ando" 103 21 1 540 24 - - - - - Mn Bower et al. 13 111 f 6 590f40 - - - - - 7 + 2 - Ja-1 andesite . . . . JB-labasalt . . . . JB-2basalt . . . . JB-3 basalt , . . . JG-la granodiorite . . JGb-lgabbro . . . . JP-lperidotite . . JR-lrhyolite . . . . JR-2rhyolite . . . . This work . . 5.5 . . 369 . . 27 . . 51 . . 22 . . 66 . . (2869) - . . - . . Ando 6 405 28 53 - Bower et al. 13 7 2 2 28 f 2 - - - - - 4 f 2 - Potts and Rogers12 10 430 31 62 20 62 (2942) 4 6.7 This work 1157 1124 1742 1345 446 1552 990 707 79 1 Bower Ando" et al.13 1160 1180 f 20 1240 - 1550 1780 f 40 1240 - 490 - 1320 - - - 770 770 f 15 850 - Ba Ja-landesite . . . . JB-labasalt . . . . JB-2 basalt . . . . JB-3basalt . . . . JG-lagranodiorite . . JGb-lgabbro . . . . JP-lperidotite . . JR-1 rhyolite . . . . JR-2rhyolite . . . . This work . . 415 , . 636 . . 316 . . 355 . . 461 . . <12 . . 48 - . . - . . And011 307 490 208 462 - - - 40 - Bower et al. 13 370 k 40 290 f 40 - - - - - 64 k 40 - Potts and Rogers12 303 509 246 238 484 - - 107 105 Analysis of Reference Materials from the Geological Survey of Japan The value of the proposed scheme was tested by analysing, as unknowns, nine reference materials recently distributed by the Geological Survey of Japan.These sampl'es were not included in the calibration data set although granite JG-la and basalt JB-la appear to be re-issues of earlier samples, JG-1 and JB-1 , which were used as calibration reference materials. Duplicate samples of pressed pellets of each reference material were each analysed twice using the reduced excita- tion conditions outlined above. The results for each determi- nation are listed in Table 2 together with the mean values and coefficients of variation. Data for the elements Ti and Mn are included in this table as these are also efficiently excited under the conditions adopted here. The precision of the results listed in Table 2 is considered to be satisfactory for all elements, but with some reservations about the barium data, an element for which interferences in ED spectra are expected to be most severe.In Table 3, average results are compared with the proposed values of Ando" and analysed data of both Potts and Rogers12 (neutron activation data for Cr) and Bower et ul. 13 (data averaged from XRF, neutron activation and ICP atomic emission determinations). Data presented here gener- ally lie within the spread of results reported by other workers, thus justifying the use of this ED-XRF method for the routine determination of Cr, V and Ba in silicate rocks. In view of the uncertainties described earlier in analysing reference materials of unusually high chromium content, some reserva- tion must be expressed in the reliability of extrapolated data for JP-1 (2869 p.p.m. Cr average) presented in Table 2.Conclusion Energy-dispersive X-ray fluorescence analysis using cobalt anode X-ray tube excitation may be employed successfully to determine the geochemically important trace elements Cr, V and Ba in silicate rocks. The accuracy and reliability of results depends on a critical evaluation of reference material values incorporated in the calibration data set. It was necessary to remove data for reference samples containing unusually high concentrations of chromium and barium from the calibration set to prevent bias at low concentrations in the corresponding calibration line. If samples containing more than cu. 10% CaO are to be analysed, care must be taken to avoid excessively high data accumulation rates and so prevent sum-peak interferences on the elements Cr and Ba. In the analysis of nine reference materials distributed by the Geological Survey of Japan, results lie within the spread of data reported . previously.72 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL, 2 The authors gratefully acknowledge Link Systems Ltd. for loan of the cobalt X-ray tube used in this work and John Taylor for assistance in preparing this manuscript. 7. Abbey, S., Geol. Surv. Can. Pap., 83-15, 1983, 114 pp. 8. Leroux, J., and Thinh, T. P., “Revised Tables of X-ray Mass Attenuation Coefficients,” Corporation Scientifique Claisse, Quebec, 1977,46 pp. 1. 2. 3. 4. 5. 6. References Nomsh, K., and Chapell, B. W., in Zussman, J., Editor, “Physical Methods in Determinative Mineralogy,” Academic Press, London, 1977, pp. 201-272. Potts, P. J., Webb, P. C., and Watson, J. S., X-Ray Spectrom., 1984, 13, 2. Potts, P. J., Webb, P. C., and Watson, J. S. ,Analyst, 1985,110, 507. Potts, P. J., Webb, P. C., and Watson, J. S., J. Anal. At. Spectrom., 1986, 1, 467. Statham, P. J., Anal. Chem., 1977, 49,2149. Nornsh, K . , and Hutton, J. T., Geochim. Cosmochim. Acta, 1969,33,431. 9. Abbey, S., Anal. Chem., 1981, 53,529A. 10. Thompson, M., Analyst, 1982, 107, 1169. 11. Ando, A., in Govindaraju, K., Editor, Geostand. Newsl. Spec. Issue, 1984, 8 , Appendix I. 12. Potts, P. J., and Rogers, N. W., Geostand. Newsl., 1986, 10, 121. 13. Bower, N. W., Gladney, E. S., Hagan, R. C., Trujillo, P. E., and Warren, R. G., Geostand. Newsl., 1985,9, 199. Note-Reference 4 is to Part 1 of this series. Paper J6l49 Received July 2nd, 1986 Accepted October 15th, 1986

ISSN:0267-9477    

DOI:10.1039/JA9870200067

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 73 KPlKa Intensity Ratios for Rare Earth Compounds Using Radioisotope Induced X-ray Fluorescence Kenneth J. Borowski, Fook S. Tham and lvor L. Preiss Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N Y 12 180, USA Measurements of the KP to K a intensity yield for the rare earth elements in various chemical compounds and as the pure elements have been made utilising radioisotope induced fluorescence and a hyperpure germanium (HpGe) detection system. Deviations from the ratios anticipated from theoretical considerations or from existing compilations have been found for LaB6, Ce(NH4)2(N03)6, Pr6O11, Gd (citrate solution), Gd (EDTA solution), Tb407, Lu203, Lu (foil) and Eu in acid solution. Keywords: X-ray fluorescence; intensity ratios; rare earth elements Energy shifts associated with the K and L levels of copper and its oxides were first noted by Nordling et al.1 in 1958. Citrin et al.,2 Keski-Rahkonen et ul.3 and Yin and co-workers475 reported changes in the line widths for X-rays emanating from elements in various chemical forms. Consideration of both of these effects of chemical state leads to the conclusion that the Weisskopf - Wigner transition probabilities should also present deviations from the free atom transition probabilities presented theoretically by Scofield.6.7 That is, the transition probabilities as exemplified by the KP/Ka ratio should demonstrate changes that are influenced by chemical bonding. Indeed, intensity shifts for both Gd and Tb have been eluded to by Borchert et aZ.8 in their 1982 report.We have conducted a series of measurements of the KPKa intensity ratios on a variety of rare earth elements in various chemical forms. Fluorescence in each instance was with a monochromatic radioisotope source using thin, transparent samples and a hyperpure germanium (HpGe) detector system. The high X-ray energies minimise the effects of sample self-attenuation in the thin samples as well as air-path effects. The use of HpGe with 370-eV resolution for the "1Am 59.5-keV y-ray ensures both efficiency and resolution sufficient to resolve clearly the lines in question. The rare earth elements were chosen for these initial studies because their chemical bonding incorporates d as well as f sub-shell orbital electrons. Thus, effects on inner shell (namely L) orbitals could be of significance. Experimental Sample Preparation Pure metal foils, metal oxides and other high-purity com- pounds (>99.9%) were obtained from regular commercial suppliers (Alpha Inorganics and American Potash and Chem- ical Corporation).Materials were used as obtained from the suppliers without further purification. As lanthanide metal foils are reactive to both atmospheric oxygen and water vapour, precautions were taken to maintain the chemical purity of the foil surfaces by storing them in a nitrogen purged desiccator and periodically polishing them with a mild abrasive under absolute ethanol. The lanthanide oxides and other high-purity compounds were also stored in the desicca- tor to limit the incorporation of water.Powdered samples were prepared for the X-ray fluores- cence (XRF) study by uniformly distributing cu. 100 mg of the powder between layers of adhesive Cellophane in a 19.0 mm diameter hole cut into a 0,225 mm thick card mount. The card mount was trimmed to reduce the surface area of the sample and holder, as it is the card mount (i.e., the sample support) along with the sample itself that contribute predominantly to the observed spectrum background via scattering of the incident photons. The metal foils are self-supporting and required no additional mounting for support. Sample solu- tions were prepared and fluoresced in 75 mm long, thin walled (1.2 mm o.d., 1.0 mm i.d.) polycarbonate test-tubes. The solutions were prepared from analytical-reagent grade glacial acetic acid and triply distilled water.Ionisation Sources Two different sealed radioisotope sources were used in this study. These were 153Gd and 241Am, each with activities of up to 30 mCi. The 153Gd source, which presents excitation lines at 97.5 and 42.8 keV, were used for the elements terbium to lutetium. The 241Am with a 59.5-keV gamma source was used for the elements barium to gadolinium. These sources were supplied by Amersham International (153Gd) and New England Nuclear (241Am). The radioisotope sources were placed in shielding colhma- tor assemblies. The collimators were designed such that an almost parallel beam of exciting photons was incident on the samples. A shield configuration was placed between the collimated source and the detector to reduce background radiation from the source which could affect the instrumental dead time.Details of this type of assembly have been published previously.9 Spectrometer - Data Acquisition System The detector used in this study was a HpGe detector supplied by Canberra Industries. The detector system had a resolution of 154 eV at 5.9 keV and 500 eV at 122 keV. The active diameter was 5.6 mm with a crystal thickness of 5 mm and a beryllium window thickness of 0.051 mm. The detector was coupled to a pulsed optical feedback pre-amplifier with a Energy - Fig. 1. X-ray spectrum of Sm203 using "'Am and a HpGe detector74 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 cooled field effect transistor (FET). A Canberra Model 1413 spectroscopic amplifier was used and its signals were d.c.coupled to a Northern Scientific, 100-MHz analogue to digital converter. This system was shown to be stable to within one part in 8 X lo3 over a 5-day period. The digitised signals were stored in a Northern Scientific Model 636 memory unit and transferred for data reduction to an Apple I1 Plus microcom- puter. Data Reduction The K X-ray fluorescence spectrum for a particular sample was accumulated in the Northern Scientific memory unit. The data were then transferred to the Apple I1 Plus microcom- puter and stored on a floppy disk. Fig. 1 shows a typical X-ray fluorescence spectrum of samarium oxide. The spectrum was accumulated for 5000 s live counting time, using the 59.5 keV gamma radiation from 241Am as the ionisation source.The first stage in the data manipulation was a simple background stripping operation that subtracted from each spectrum, channel for channel, a background spectrum accumulated for an equal live counting time with a “blank” sample mount, ie., Cellophane sealed powder mount or solvent filled test-tube, without any sample. This stripping operation primarily accounted for background counts in the spectral region of interest due to the scattering of incident photons off the sample mounting and also accounted for background counts due to any naturally occurring decay products in the environment of the detector system. After subtracting the background contributions due to scattering from the blank and the surroundings, a second background subtraction was performed.This second routine subtracts, from the net spectra, the contribution to the peak from the actual sample scattering. This routine, using the spectra with gross background subtraction, involves averaging the values 15 channels above and below the tenth maximum point of the X-ray peak, integrating this average value over the peak width, and subtracting from the peak area those events now associated with sample scattering. The new net count is thus corrected for the background due to general as well as background scattering for the sample itself. These values are then used for computing KPIKa intensity ratios. The intensities are, therefore, corrected for both general and sample scattering into the region of interest. No effort was made to deconvolute the Ka or KP multiplets.Thus, the integrals obtained and the ratios compared are for the total peak intensity for Ka and KP. The errors reported are the normal statistical errors for background subtraction and peak integration. 10 Corrections for self-attenuation of the X-rays in the samples and the air path are minimal at these energies when compared with the counting statistics. We have not corrected for self-attenuation within the samples, nor attenuation by the air path and detector window. At the typical X-ray energies used for the rare earths these cummulative errors are of the order of 5%. Therefore, these corrections were not made to the present data as precision to this level is not the intent. Results Table 1 lists the relative K P K a intensity ratios of the elements from 2 = 56 (barium) to 2 = 71 (lutetium) inclusive, except for 2 = 61 promethium, as measured in this work.The precision values given are the result of at least 12 measure- ments. Also listed in Table 1 are the theoretical values for the KPKa ratios as calculated by Scofield6.7 in 1974 using the Hartree - Fock model and the “most probable” values as compiled by Salem et al., 11 derived from previous experimen- tal work, or as extrapolated from the experimental data points. Fig. 2 shows the K P K a intensity ratios versw 2, as measured in this work, compared with the values predicted by Table 1. Calculated and experimental KB/Ka intensity ratios versus 2 z 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Calculated Literature Element values* datat Ba 0.2433 0.2372 La 0.2455 0.2396 Ce 0.2464 0.2419 Pr 0.2481 0.2442 Nd 0.2504 0.2473 Pm 0.2521 0.2482 Sm 0.2537 0.2500 Eu 0.2549 0.2523 Gd 0.2570 0.2551 7% 0.2575 0.2561 DY 0.2583 0.2571 Ho 0.2599 0.2580 Er 0.2612 0.2602 Tm 0.2626 0.2618 Yb 0.2634 0.2642 Lu 0.2651 0.2656 Z l 56 57A 57B 58A 58B 59 60 61 62A 62B 63A 63B 63C 64A 64B 64C 65A 65B 66 67 68 69 70 71A 71B Sample BaBr2 La2( S0)49H20 Ce02 LaB6 Ce(NH4)2(N03)6 pr601 1 Nd203 - Sm foil Sm203 Eu203 Eu203 Eu203.(water quenched) (acid solution) Gdz03 (citrate solution) (EDTA solution) 7% foil 7%407 Gd203 Gd203 Dy2°3 Ho203 Er203 Tm203 n2q3 Lu f0ll Lu203 Present data 0.2450 k 0.0084 0.2959 t- 0.0023 0.2433 t- 0.0037 0.2624 t- 0.0017 0.2349 t- 0.0037 0.2571 k 0.0007 0.2533 k 0.0021 0.2632 k 0.0036 0.2610 f 0.0024 0.2555 k 0.0028 0.2503 f 0.0026 - 0.2122 f 0.0023 0.2516 t- 0.0017 0.2408 f 0.0078 0.2401 t- 0.0058 0.2644 f 0.0117 0.3951 k 0.0740 0.2452 k 0.0112 0.2624 f 0.0186 0.2364 k 0.0313 0.2545 f 0.0314 0.2625 t- 0.0105 0.2483 f 0.0049 0.1821 f 0.0137 * Scofield’s Hartree - Fock calculated values.6.7 t “Most probable” values compiled by Salem et al.11 $ See Fig. 2.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1987, VOL. 2 T 75 :: 65A & 0.460 0.440 0.420 0.400 0.380 0 0.360 .- 4- 2 r 0.340 0.320 $ 0.300 h Y 0.280 0.260 0.240 0.220 0.200 0.180 0.160 4- .- 03 C .- 57 A A 1 1 1 1 I l I 1 1 I I I l I 1 1 1 1 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 z Fig. 2. 0, data from Scofield6y7; and a, data from Salem et al.11 KP/Ka intensity ratios as a function of 2.Compound identification is indicated as in Table 1, column 5. X , Present data; Scofield’s Hartree - Fock calculations and the set of “most probable” values compiled by Salem et aZ.11 Discussion It is evident that the chemical form of elements such as lanthanum, cerium, europium, terbium and lutetium has a profound influence on the measurement of KfVKa intensity ratios. The theoretical values given for KPIKa intensity ratios are those calculated by Scofield6.7 in 1974. The Hartree - Fock wave functions for the electronic states are those of neutral free atoms unperturbed by molecular orbital distortion of d or f orbitals or by ionisation. The “most probable” values for KPKa intensity ratios, as reported by Salem et al. ,I1 are the weighted averages of the available experimental values and an extrapolation for those elements where no experimental data exists, and thus do not reflect any effects of the chemical form.Any fluctuations associated with the chemical form could be averaged out and would not be evident in the supposed “most probable” values. To a large extent, experimental values were obtained from electron capture isotopes and not via a photoionisation fluorescence process. The chemical form of the radioisotopes is generally not well documented in the primary references. However, one may assume that because of the chemical similarities of the rare earths as a group, that these isotope samples are likely to be substantially in the form of oxides. Neither the theoretical nor the averaged experimen- tal values reflect changes in Auger yield, which in itself could affect the X-ray intensities. It is not our intent at this time to suggest all of the various corrections that might be imposed on Scofield’s calculations in order to predict better experimental KPKa intensity ratios as a function of chemical form.However, it is a fairly straight- forward process to point out the large deviations in our experimental values from the theoretical ones, and to suggest a basis for the perturbation from the neutral free atom model. We wish to alert the experimentalist to the fact that large deviations in the KP/Ka intensity ratios exist in the rare earths and therefore, may also be observable in other elements whose compounds exhibit similar electronic struc- ture anomalies.One of the most striking deviations from the anticipated values is to be found in the case of lanthanum hexaboride. This unusual compound, in thin film deposition, presents an instance where the equivalent of as many as 20 outer shell electrons of lanthanum are associated with the surrounding borons.12 This instance clearly does not satisfy the neutral free atom model nor is it similar to the crystalline form associated with lanthanum sulphate. Similarly, the values measured for europium show a deviation when one examines the +3 ion in acid solution. In this instance the unperturbed f orbital configuration presents a large number of unpaired electrons. The complex nature of the structure of the cerium complex ion and the chelate of gadolinium when in solution also present electronic configurations and/or molecular orbital structures that are dissimilar to the structures found in either the neutral free atom or the oxide.On the other hand, the oxide of terbium (Tb407) itself presents a complex electronic structure with the probability of a large number of unpaired electrons associated with terbium. Finally, the complex electronic structure associated with the free lutetium atom is undoubtedly exacerbated when it is chemically bound. An example of deviation from expected values is found in the unusual metalloid Nb3Sn.13 Preliminary studies have shown that the KPKa intensity ratio exhibits a 15% deviation for Nb (theoretical 0,195; experimental 0.225) while the value of the ratio for Sn agrees with the theoretical value well within experimental error. It is evident therefore that the analyst should be aware of such deviations and should make appro- priate corrections.Such corrections could be correlated with similar effects noted in Auger yields and associated with deviations in both line width and energy shifts.14J576 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 19 References 1. Nordling, C., Sokolowski, E., and Siegbahn, K., Ark. Fys., 1958, 13,483. 2. Citrin, P. H., Eisenberger, P. M., Marra, W. C., Aberg, T., Utriainen, J., and Kallne, E., Phys. Rev. B, 1974, 10, 1762. 3. Keski-Rahkonen, O., and Krause, M. D., Phys. Rev. A , 1977, 15,959. 4. Yin, L. I., Adler, I., Chen, M. H., and Crasemann, B., Phys. Rev. A, 1973, 7 , 897. 5. Yin, L. I., Adler, I., Tsang, T., Chen, M. H., Ringers, A. D., and Crasemann, B., Phys. Rev. A, 1974,9,1070. 6. Scofield, J. H., Phys. Rev. A , 1974,9, 1041. 7. Scofield, J. H., At. Data Nucl. Data Tables, 1974, 14, 121. 8. Borchert, G. L., Rose, T., and Schult, 0. W. B., in Crasemann, B., Editor, “X-Ray and Atomic Inner-Shell Physics-1982,” American Institute of Physics, New York, 1982, pp. 115-120. 9. Preiss, I. L., Ptak, T., and Frank, A. S., Nuc Methoa3 A , 1986,242,539. 10. Tsoulfanidis, N., “Measurement and Detection of I McGraw-Hill, New York, 1983, pp. 19-74. 11. Salem, S. I., Panossian, S. L., and Krause, R. A Nucl. Data Tables, 1974, 14,91. 12. Ryan, J., personal communication, 1984. 13. Yennello, S., B.Sc. Thesis, Physics Department, Polytechnic Institute, 1986. 14. Fuggle, J. C., and Alvarado, S. F., Phys. Rev. A 1615. 15. Crasemann, B., Chen, M. H., and Mark, H., J . Op B, 1984,1,224. P6 Received April Accepted August

ISSN:0267-9477    

DOI:10.1039/JA9870200073

出版商:RSC    

年代:1987

数据来源: RSC