摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 217 Influence of Operating Conditions on the Effects of Acids in Inductively Coupled Plasma Atomic Emission Spectrometry* Albert0 Fernandez Miguel Murillo and Nereida Carrion Centro de Quimica Analitica Escuela de Quimica Facultad de Ciencias Universidad Central de Venezuela P. 0. 471 02 Caracas 7 04 1 -A Venezuela Jean-Michel Mermet Laboratoire des Sciences Analytiques Bat. 308 Universite Claude Bernard Lyon I 69622 Villeurbanne Cedex France The influence of the operating conditions on interferences from HCI HNO and H2S04 in inductive1 coupled plasma atomic emission spectrometry was studied. The acid concentration was in the 0-2 mol I- range. It was found that the acid interferences were strongly dependent on the operating conditions.Several different plasma excitation conditions were obtained by modifying the aerosol carrier gas and the intermediate gas flow rates. The test element was vanadium. Plasma diagnostics were performed by measuring the excitation temperature the electron number density and the ionic-to-atomic line intensity ratio. In each instance it was found that the excitation temperature was not modified as a function of the acid concentration. For long residence time and efficient energy transfer the electron number density and the ionic-to-atomic line intensity ratio were not modified. Only a minor depressing effect was observed for ionic lines which was attributed to the aerosol formation and transport. In contrast for short residence time and inefficient energy transfer both the electron number density and the ionic-to-atomic line intensity ratio were depressed.The significant depressing effect for ionic lines was attributed to a change in the excitation conditions. It is therefore possible to separate the acid effects originating from the sample introduction system from those originating in the plasma by carefully selecting the operating conditions of the plasma. Keywords Inductively coupled plasma; atomic emission spectrometry; mineral acid interference Y The acidic medium is one of the most commonly used matrices in inductively coupled plasma atomic emission spectrometry (ICP-AES). However use of mineral acids generally leads to interferences. Few investigations on acid effects have been reported so far.'-'' In general increasing the mineral acid concentration causes a significant decrease in the emission signal observed in ICP-AES.The depression of intensity has been attributed to (i) a decrease in the sample aspiration rate as a result of increased viscosity;'.2 ( i i ) a change in nebulizer efficiency and droplet size d i s t r i b ~ t i o n ; ~ . ~ . ~ (iii) a variation in the aerosol transport efficiency; and (iv) a change in plasma excitation condition^.^.^,^ Chudinov et aL8 and Yoshimura et aL9 have reported that the magnitude of the interference due to the acids differs from one line to another. In the instance of ionic lines the change in line intensity is correlated with the excitation energy. Chudinov et aL8 have indicated that the acid effect depends on the type of acid and this effect increases in the following order HC1< HN03 < HC104 << H3P04 < H2S04.They found a decrease in excitation temperature (< 300 K) with increasing concentration of mineral acid in the range of 0-2mol1-'. They concluded that for a low-power plasma and a low acid concentration range (< 1 mol 1-') the depressing effect results from a decrease in excitation temperature rather than a reduction in sample uptake rate. However no explanation was provided to elucidate the mechanisms involved in change in excitation temperature. Marichy et aI." reported the effect of mineral acid at concentrations below 1% v/v. They found an increase in analyte emission signals when HC1 was used with a maximum effect at a concentration of 0.001% in contrast to the depression observed with HC104.No significant effect was reported with HNO in this concentration range. This enhance- ment or depressive effect could not be explained by a change in aerosol characteristic or changes in nebulizer and transport * Presented at the XXVIII Colloquium Spectroscopicum Inter- nationale (CSI) York UK June 29-July 4 1993. efficiency. It was rather a change in the chemical composition of the aerosol with a 50% decrease in acid concentration compared with that of the solution. In the present work the effect of HCl HNO and H,SO was investigated using vanadium as the test element. Both the ionic and atomic line intensities were studied as a function of the operating conditions of the ICP. The acid concentration was varied in the range 0-2 moll-'. In order to evaluate the influence of the operating parameters on the effect of acid a full two-level and three-factor factorial study was designed.Three sets of operating conditions were selected by adjusting the carrier gas flow rate the intermediate gas flow rate and the power. The corresponding plasma excitation conditions were assessed by measuring the excitation temperature the electron number density and the ionic-to-atomic line inten- sity ratio. Experimental Instrumentation A Jobin-Yvon JY-24 ICP emission spectrometer was used. The original gas flow meters for the aerosol carrier and intermediate gas were replaced by mass flow controllers (Brooks 5850E). Solutions were introduced into the plasma by means of a Meinhard nebulizer (TR-20-C2) with a Scott-type spray chamber working at room temperature.Temperature Measurements The Boltzmann plot method was used to measure the excitation temperature. Sixteen vanadium ionic lines were selected. Their wavelength energy level and oscillator strength values were taken from refs. 11 and 12. Electron Number Density The electron number density was determined from the Stark broadening of the 486.12 nm (Hp-line) using Griem's approxi- mation described in ref. 13.218 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Ionic-to-atomic Line Intensity Ratio The intensity ratio of the Mg I1 280.270nm line to the Mg I 285.213 nm line was measured as this is a simple test to verify the plasma ~0nditions.l~ It has been found that values of the ratio of above eight generally lead to robust conditions ie.a plasma not sensitive to matrix effects in contrast to values at less than four where the plasma is significantly sensitive to matrix effects. Chemicals and Emission Lines Vanadium was also selected as the test element to study the effect of acid because stable aqueous solutions of vanadium can be obtained without the addition of acid and it presents well known atomic and ionic emission lines. Wavelengths and excitation potentials are listed in Table 1. Each net intensity line was obtained by subtracting the background from the gross intensity. Relative line intensities (I,) were normalized to the intensity measured in the absence of any acid. Solutions of vanadium were prepared at a concentration of 20mgml-' by dilution from a concentrate prepared from NH,V02 (Merck pro analysi grade).Hydrochloric acid (37% v/v) HN03 (65% v/v) and H2S04 (32% v/v) from Riedel-de Haen were used. Measurements were taken after each increase in acid concentration. An equilibrium time of approximately 5-10 min was found to be necessary between measurements at different acid concentration^.^^^^*'^ At least five replicates with an integration time of 0.3 s were used for each concentration. Full Two-level and Three-factor Factorial Study In order to evaluate the influence of the operating parameters on the effect of acid concentration for atomic and ionic emission intensities a full two-level and three-factor (23) fac- torial study was designed. The three factors considered were power ( P ) aerosol carrier gas flow rate (C) and intermediate (or sheathing) gas flow rate (S).The response used was the I of vanadium at a 2.0 mol 1-1 acid concentration normalized to that obtained without acid. This part of the work was conducted for HNO as the interferent and the results were extrapolated to HC1 and H,SO,. The V I1 294.602 nm and V I 437.924 nm lines were selected for this study. The three factors and their levels are listed in Table 2. Results and Discussion Factorial Study The experimental values of the relative signal of the vanadium lines for each combination of operating parameters are listed in Tables 3 and 4 for the ionic and atomic lines respectively. From the complete analysis of variance for the factorial design conclusions can be drawn for ionic and atomic lines.For the ionic line the F-test showed that the intermediate (or sheath- ing) gas flow rate (S) and the aerosol carrier gas flow rate (C) and their interactions were statistically significant at 1 YO. However the effect and the interaction of the power ( P ) P x S and P x C were not statistically significant at 10%. The effects calculated from the factorial analysis are listed in Table 5. A comparison of the estimates with their standard errors sug- gested that C and S and their interactions were significant whereas the remaining effect ( P ) and their interaction could be generated by noise. The effect of C and S cannot be interpreted separately because of the large interaction. For the atomic line the F-test shows that all effects and their interactions were not statistically significant.A comparison of the estimate with their standard errors (as listed in Table 5) suggested that all the effects could be generated by noise. The results obtained show that the magnitude of the acid interference for ionic lines is strongly affected by the aerosol carrier and intermediate gas flow rates. However the magnitude of the effect of the acid for atomic lines is independent of both flow rates. For the ionic line the effect of the intermediate gas flow rate is very important and the effect of the carrier gas is strongly dependent on the former. As can be seen in Table 3 for the runs with intermediate gas flow rate at a high level the effect of the aerosol carrier gas flow rate is positive increasing the interference from the acid.However in the runs with the Table3 Experimental values obtained for the relative signal (I,) for the V I1 line at 292.402 nm; all values correspond to the average of three replicate runs Aerosol carrier gas flow rate/l min-' Power/kW 0.70* 1 .oo* 0.707 1.007 1.08 0.8753 0.9440 0.8734 0.7050 0.91 0.8983 0.9549 0.8464 0.701 8 *Intermediate gas flow rate 0 1 min-'. ?Intermediate gas flow rate 0.38 1 min-'. Table 4 Experimental values obtained for the relative signal (I,) for the V I line at 437.924nm all values correspond to the average of three replicates runs Aerosol carrier gas flow rate/l min-' Power/k W 0.70* 1 .oo* 0.707 1.oo.t 1.08 0.960 0.9490 0.9649 0.9502 0.9 1 0.956 0.946 0.943 0.9588 Table 1 Wavelengths and excitation potentials of vanadium emission lines examined *Intermediate gas flow rate 0 1 min-'. ?Intermediate gas flow rate 0.38 1 min-'.~~ Excitation Spectral species Wavelength/nm potentiallev v I1 292.402 4.63 v I1 292.464 4.61 v I1 309.31 1 4.40 V I 437.924 3.13 V I 446.029 4.64 Table 2 Factor levels for the two-level and three-factors factorial study Factor Level Power/k W 0.91 1.08 Intermediate gas flow rate/l min-' 0 0.38 Aerosol gas flow rate/l min - ' 0.70 1.00 Table 5 Calculated effects and standard errors for 23 factorial design for V lines Effect Estimate k standard error V I1 292.402 nm Average 0.850 Main efects- C - 0.047 f 0.004 S -0.136f0.004 P 0.000 f 0.004 Two-factor interactions- CXS - 0.1 10 f 0.004 CXP - 0.003 f 0.004 SXP 0.01 6 f 0.004 Three-factor interaction- CXSXP 0.000 * 0.004 V 1437.924nm 0.953 - 0.005 & 0.005 0.001 & 0.005 0.005 f 0.005 0.005 & 0.005 0.005 0.005 0.002 & 0.005 0.007 & 0.005JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 219 Table 6 Operating instrumental conditions and plasma excitation conditions Aerosol carrier gas Intermediate Power/ flow rate/ gas flow Excitation Electronic Condition kW 1 min-' rate/l min - ' temperature/K density/crn- Mg II/Mg I 1 1.08 0.75 2 0.91 0.95 3 0.9 1 1 .oo 0 5100 205 & lo-' 8.5 0.25 4765 95 -t lop1 4.9 0.38 4200 62k lo-' 2.0 intermediate gas flow rate at a low level the effect of the aerosol carrier gas is negative. Influence of Plasma Conditions on the Acid Effects To relate the influence of plasma excitation conditions to the effect of acid concentration three different sets of operating conditions were selected in order to cover a large range of temperatures electron number densities and ionic-to-atomic line intensity ratios.The operating parameters are listed in Table 6 along with the corresponding excitation temperatures electron number densities and Mg I1 to Mg I ratios which were measured without acid. Conditions 1 lead to a high Mg I1 to Mg I line intensity ratio whereas conditions 3 lead to a low ratio. It could be anticipated that the influence of HNO HCl and H2S04 on vanadium atomic and ionic line intensities would be different for each set of conditions. The effect of HNO concentration on the atomic and ionic vanadium lines is shown in Fig. 1 for the three conditions.For conditions 1 [Fig. l(a)] the decrease in the line intensities was less than 5% with increasing HNO concentration. This behav- iour was observed for both atomic and ionic lines. For con- ditions 2 and 3 [Fig. l(b and c) respectively] a strong decrease in the ionic lines was observed in contrast to atomic lines where the effect was less marked. The behaviour of atomic lines was similar to that observed under conditions 1. The effect of acid on the ionic lines was sensitive to the plasma conditions whereas the depressive effects on atomic lines were independent of these conditions. The effect of HC1 concentration on the vanadium line intensities is shown in Fig. 2. The effect of HC1 was found to be similar to that obtained for HNO,. The results obtained for H2S04 are shown in Fig.3. Under conditions 1 [Fig. 3(a)] the magnitude of the H2S04 effect was approximately the same for atomic and ionic lines and more important than that observed with HNO and HCl under the same operating instrumental conditions. This behaviour can easily be explained by the significant change in viscosity of the solution caused by the increase in H2S04 concentration. However when working under conditions 2 and 3 [Fig. 3(b and c) respectively] a further depressing effect was observed for ionic lines compared with atomic lines. This means that besides the viscosity effect 1 .o 0.9 1 I 0.8 0.6 0.5 I I I I 0.5 ' I I I I 1 .o 0.9 0.8 0.7 o.6 t 0.5 ' I I I I 0 0.5 1 .o 1.5 2.0 [HNO,l/mol I - ' Fig. 1 Effect of HNO concentration on the relative emission intensit- ies of vanadium atomic and ionic lines under the three different instrumental operating conditions (a) 1; (b) 2; and (c) 3 (see text for details of 1,2 and 3).. V I1 292.402 nm; A V I1 292.464 nm; + V I1 309.311 nm; A V I 437.924 nm; and 0 V I 446.029 nm 1 .o 0.9 0.8 1 0.7 t O5 0.5 3 5 1.0 C 0 3 C .- 0.9 .- 3 0.8 '- 0.7 t 1 .o 0.9 0.8 0.6 0.7 1 0.5 I I I I 0 0.5 1 .o 1.5 2.0 tHCll/mol 1-l Fig. 2 Effect of HC1 concentration on the relative emission intensities of vanadium atomic and ionic lines. Under the three different instru- mental operating conditions (a) 1; (b) 2; and (c) 3 (see text for details of 1 2 and 3). . V I1 292.402nm; A V I1 292.464nm; + V I1 309.311 nm; A V I 437.924 nm; and 0 V I 446.029 nmJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 220 1 .o 0.9 0.8 0.7 1 .o 0.9 0.8 0.7 0.5 0 3 0.5 1.0 1.5 2 .o [H,SO,l/mol I - ' Fig. 3 Effect of H,S04 concentration on the relative emission intensit- ies of vanadium atomic and ionic lines. Under the three different instrumental operating conditions (a) 1; (b) 2; and (c) 3 (see text for details of 1 2 and 3). H V I1 292.402 nm; A V I1 292.464 nm; + V I1 309.311 nm; A V I 437.924 nm; and 0 V I 446.029 nm a change similar to that obtained for HC1 and HNO was observed. It is therefore important to verify the possible effect of change in acid concentration on the plasma conditions. Effect of Acid on Plasma Excitation Conditions The effect of HC1 concentration in the 0-2 mol 1-1 range on the excitation temperature the electron number density and the Mg I1 280.270 nm to Mg I 285.213 nm ratio under con- ditions 1 and 3 is summarized in Figs.4 5 and 6 respectively. Although conditions 1 and 3 lead to significantly different temperatures it is shown in Fig. 4 that the HCl concentration has no effect on the excitation temperature even under con- ditions 3. The same results were obtained with HN03 and H,S04. Several independent measurements were repeated to 6000 I Q &---4 s 5000 1 I I I I I I I 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 [HCll/mol I-' Fig.4 Effect of the concentration of HCI acid on the excitation temperature under differents instrumental operating conditions 0 1; and A 3 196 194 192 190 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 [HCll/mol I -' Fig. 5 Effect of the concentration of HC1 on the plasma electron density under differents instrumental operational conditions 0 1; and A 3 r 1 0 .- c 2.0 F > v) C c .- 1.8 .- a C .- 1.6 1 0 Zi - - p6.5' I ' ' I ' ' I 1.4 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2 [HCll/mol I -' Fig.6 Effect of the concentration of HCl on the Mg II/Mg I line intensity ratio under different instrumental operating conditions 0 1; A 3 confirm these results These results differ from those obtained by other However a different conclusion is obtained for the electron number density. The density remains the same under conditions 1 whereas it is significantly decreased under conditions 3.The decrease was most significant over the 0-1 moll-' concen- tration range which corresponded to the large decrease in ionic line intensities as shown Fig.5 for HC1. Although no change in the excitation temperature was observed there is a variation in the ion-to-atom equilibrium resulting from the change in the electron number density. This is confirmed by the measurement of the Mg I1 to Mg I line intensity ratio. It is shown in Fig. 6 that the ratio is not modified under conditions 1 whereas it is significantly changed under con- ditions 3. The same experiment based on the use of several elements vanadium manganese lanthanum barium and magnesium which were selected because of the possibility of having a stable solution without acid (see Fig. 7). Two acid concen- trations are plotted 0.5 and 2 mol l-l respectively. Conditions 3 were used. Lines and energies are listed in Table 7. Results were normalized to those obtained without acid and plotted as a function of the sum of ionization and excitation energy.The higher the energy sum the stronger the effect of acid. This confirms that there is a change in the excitation conditions under conditions 3. It is interesting to note that the manganese lines exhibited a different behaviour. This is because their sum of energies is close to the ionization energy of argon. A charge transfer process is involved in the ionization and excitation of the manganese lines.16 A behaviour similar to that of manga- nese has also been reported near 16eV for copper ionic lines with HN0,.9 Clearly at least two effects can be attributed to the presence of acids such as HC1 and HNO,. These effects can be summar- ized by plotting the behaviour of the V I1 292.402nm lineJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 22 1 Table 7 Wavelength excitation and ionization potential of different elements studied Spectral species v I1 v I1 v I1 Mn I1 Mn I1 Mn TI Mn TI Mn I1 La I1 La I1 La I1 Ba IT Ba TI Mg Wavelengt h/nm 292.402 292.464 309.31 1 259.373 261.020 263.984 343.987 344.198 394.910 408.672 412.323 455.403 493.406 280.270 Excitation poten tial/eV 4.63 4.61 4.40 4.77 8.16 8.75 4.78 5.37 3.54 3.03 3.32 2.72 2.51 4.42 - Ionization potentiallev 6.74 6.74 6.74 7.432 7.432 7.432 7.432 7.432 5.61 5.61 5.61 5.21 5.21 7.644 1.0 I t $ 0.7 9 0.6 0. 0 .- c 0 z 0.5 I CT 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 I I I I I I 1 Energy sumlev Fig. 7 Relative emission intensity for differents elements under instru- mental operating conditions 3.HC1 concentration W 0.5; and O 2 moll-' l.05 4 I 1 .oo - I Aerosol I 0.85 z 0.80 II Plasma 0'75 0.70 \ 0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 [HCll/mol I - ' Fig.8 Effect of the HCI concentration on the relative emission intensity for the V I1 292.402 nm line under instrumental operating conditions 0 1 and A 3 (see text for details of 1 and 3) intensity against the HCl concentration under conditions 1 and 3 (Fig. 8). The ideal behaviour would be no influence of the acid which is represented by a straight line in Fig. 8. Under conditions 1 as there is no change in the excitation temperature and the electron number density the slight decrease in the line intensity can be attributed to a change in the aerosol formation and transport.Under conditions 3 the significant decrease can be mainly attributed to the change in the plasma conditions. Conclusions A study of mineral acid influence in ICP-AES is complex due to the combination of several possible causes leading to the depressing effect. Study of a given cause can be made possible by careful selection of the appropriate ICP operating con- ditions. For instance the influence of the sample introduction system can only be easily studied under operating conditions that lead to a high ionic-to-atomic line intensity ratio e.g. above eight when the Mg I1 280 nm to Mg I 285 nm line intensity ratio is used for the experiment. In contrast the influence of the excitation conditions can be enhanced when working with operating conditions resulting in a low ionic-to- atomic line intensity ratio e.g.less than four. The influence of the generator remains to be verified as different conclusions have been reported.8 More importantly an understanding of why a change in the electron number density was observed without a similar change in the temperature is required. The authors acknowledge the support of the Consejo de Desarrollo Cientifico y Humanistic0 de la Universidad Central de Venezuela for this research Grant No. 03.12.2620/91. References 1 Greenfield S. McGeachin H. McD. and Smith P. B. Anal. Chim. Acta 1976 84 67. 2 Dahlquist R. L. and Knoll J. W. Appl. Spectrosc. 1978 32 1. 3 Maessen F. J. Balke J. and De Boer J. L. Spectrochim. Acta Part B 1982 37 517. 4 Ishii H. and Satoh K. Talanta 1983 30 111. 5 Shen X.-e. and Chen Q.4 Spectrochim. Acta Part B 1983 38 115. 6 Wandt M. A. Pouget M. A. and Rodgers A. Analyst 1984 109 1071. 7 Farino J. Miller J. R. Smith D. D. and Browner R. F. Anal. Chem. 1987 59 2303. 8 Chudinov E. G. Ostroukhova I. I. and Varvanina G. V. Fresenius' 2. Anal. Chem. 1989 335 25. 9 Yoshimura E. Suzuki H. Yamazaki S. and Toda S. Analyst 1990 115 167. 10 Marichy M. Mermet M. and Mermet J.-M. Spectrochirn. Acta Part B 1990 45 1195. 11 Corliss C. and Bozman W. Experimental Transition Probabilties for Spectral Lines of Seventy Elements NBS Monograph 53 Washington D.C. 1962. 12 Jarosz J. Mermet J.-M. and Robin J. Spectrochim. Acta Part B 1978 33 55. 13 Mermet J.-M. in Inductively Coupled Plasma Emision Spectroscopy Part 2 ed. Boumans P. W. J. M. Wiley New York 1987. 14 Mermet J.-M. Anal. Chim. Acta 1991 250 85. 15 Ivaldi J. C. Vollmer J. and Slavin W. Spectrochim. Acta Part B 1991 46 1063. 16 Goldwasser A. and Mermet J.-M. Spectrochim. Acta Part B 1986 41 725. Paper 3/04 703 B Received August 4 1993 Accepted September 3 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900217

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Analysis of Electrical Arc Furnace Flue Dusts by Spark Ablation Inductively Coupled Plasma Atomic Emission Spectrometry* A. G. Coedo M. T. Dorado and 1. G. Cob0 Centro Nacional de Investigaciones Metalurgicas Consejo Superior de lnvestigaciones Cientificas Gregorio del Amo. 8,28040 Madrid Spain 223 A medium-voltage spark was used for the direct nebulization of electric arc furnace (EAF) flue dust. In order to attain the necessary sample conductivity powder pellets are briquetted after mixing the sample 1 +1 with graphite. The elutriated material was excited in an argon inductively coupled plasma (ICP). The use of cellulose as binder provides better results in terms of reproducibility. After optimization of the spark parameters (voltage 500 V; repetition rate 400 s-'; and resistance 2.2 Q) the carrier gas flow rate (2.1 I min-' of argon) and the operating power of the ICP (1.2 kW) precisions (relative standard deviation) for zinc lead cadmium and iron range from 0.8 to 2.0%.The stability of the spark sampling during a complete spark ablation (SA) ICP process ( ~ 9 0 s) was tested by plotting emission intensity versus time profiles. The similarity between the amounts of analyte obtained from different pellets was proven by collecting the spark-eroded particles and analysing their carbon contents. Five steelmaking EAF flue dusts were selected for this study using the two samples with extreme contents of the elements considered for calibration. The results obtained by SA-ICP matched the results obtained by ICP from nebulized solutions.Keywords Spark sampling; non-conductive powder; steel making; dust analysis; spark ablation inductively coupled plasma atomic emission spectrometry The environmental pollution caused by residues from electric arc furnace flue dusts which are considered to be toxic and hazardous products poses a real problem for the iron and steel industry. The use of galvanized sheets is becoming more widespread and when this material is recycled in electric arc furnaces it produces dusts containing Zn Pb and Cd which can be leached into the natural environment and can therefore accumulate in soil and water. Technological processes for treating these powders are being developed intensely at the present time.'y2 Optimization of these processes requires appli- cation of analytical control methods that ensure high sensitivity and selectivity combined with acceptable accuracy and speed. Inductively coupled plasma atomic emission spectrometry (ICP-AES) meets these requirements making possible the analytical follow-up of such processes.The system normally used is a sample dissolution step followed by ICP-AES yet this procedure is hampered by the dissolution of some of the refractory oxides that can be present. Acid digestion and alkali fusion of the insoluble residue is the conventional sample dissolution procedure. This dissolution procedure reduces the power of detection as a result of analyte dilution and high saline concentration and is time-consuming and labour inten- sive.Spark ablation has been applied for bulk analysis of conducting solids3 and powder samples4 and can simplify the ICP analytical process by eliminating the dissolution step. One possible form of preparing non-conductive powder for spark ablation (SA) sampling is to briquette the powder into pellets with a metallic powder to achieve the necessary cond~ctivity.~*~+~ This study aims to ascertain the feasibility of the SA-ICP technique for the analysis of steelmaking flue dusts. Different pellet preparation procedures were investigated to obtain physically stable test samples with adequate conductivity and the optimum proportion of the components dust conducting powder and binder. The influence of sparking operating param- eters (voltage resistance and repetition rate) was studied the total amount of eroded material was controlled and the analytical performance of the technique evaluated.A precision test was conducted to ascertain the repeatability of the ICP measurements and the reproducibility of the sample prep- * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993. aration system. Finally the results obtained for the selected samples by using the proposed method and the corresponding standard deviations (SDs) are presented. Experiment a1 Instrumentation A JY-SAS sparking unit was used as the solid sampling source. The ablated material was excited in a JY 24 ICP spectrometer. Instrumental details and working conditions are listed in Table 1. Samples Representative samples of electric arc furnace (EAF) flue dust were selected for the development of the SA-ICP analytical procedures. Consideration was first given to the information provided by different steelmaking companies on the types of steel they manufacture their steel production and the flue dust generated during the steelmaking process.These samples were initially analysed by ICP spectrometry from nebulized solu- tions obtained with the following dissolution procedure 0.200 g of sample was fused at 1200°C in a Pt crucible with 1 g of the flux mixture consisting of borax and sodium carbonate (1 1). Table 1 Optimized working conditions for SA-ICP Sparking parameters Voltagep Capacit ance/pF Inductance/pH Resistance/Q Repetition rate Electrode (cathode) Permanent carrier gas/l min- ' Analysis carrier gas/l min-' Carrier gas pressure/bar Distance/m Diameter of the transport tube/mm Power/kW Plasma gas/l min - Sheath gas) min-' Transport ICP 500 1 20 2.2 400 s-' Tungsten rod 0 = 2 mm 2.1 0.8 3 1 5 1.2 14 0.2224 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 Table 2 ComDosition of EAF flue dusts (ICP pneumatic nebulization; n := 6) Zn (YO) Pb (Yo) CD (Yo) Fe (YO) - Sample X d X 0 X d X 0 DUST 1 12.52 0.07 2.76 0.05 0.04 1 0.00 1 34.75 0.18 DUST 2 24.6 1 0.15 5.33 0.112 0.47 0.005 15.51 0.15 DUST 3 36.00 0.20 11.21 0.1,o < 0.01 8.55 0.09 DUST 4 44.22 0.2 1 9.60 0. II 1 2.001 0.019 4.92 0.06 DUST 5 46.18 0.23 15.14 0.20 < 0.01 0.88 0.05 The melt was digested with water and dissolved by adding 3 ml of 65% HNO,+ 1 ml of 37% HCl.The final volume of the resulting solution was adjusted to 200ml with water. The samples selected and their composition presented as the average of six values obtained by two different operators with ICP-AES from nebulized solutions are listed in Table 2. Pellet preparation A 4 g portion of sample + 4 g of graphite + 2 g of cellulose were mixed and homogenized in a ball mixer/mill and pressed into a pellet (0=4 cm; thickness= 10 mm) with a load of 40 t cm-2 for 30 s in a hydraulic press. Results and Discussion Sample Preparation Various tests were performed to define suitable procedures for obtaining stable pellets with the necessary conductivity. The main advantage of flue dusts is their extremely fine particle size (95% have a particle size of less than 0.5 pm) which makes these products suitable for direct compaction.The matrix was modified by mixture with a conducting host material. The conducting powders tested were copper alu- minium and graphite. The criteria used to select the most appropriate diluent powder were high conductivity ease of handling low cost acquisition of bulk samples providing stable and reproducible responses to electric discharges and absence of inter-elemental spectral interferences. Copper produces spec- tral interferences in the two most sensitive analytical Zn lines (Zn 213.856nm is interfered with by Cu 213.853 and Zn 202.548 nm is interfered with by Cu 202.434-202.555 nm) and the presence of aluminium powder results in deep erosion and consequently to a massive input of sample even when using very low voltages which decreases the plasma stability.The use of graphite as diluent provides the best results and conse- quently this conductive powder was selected for further tests. The addition of a binder improves the mechanical properties of the pellets enables a better ablation to be carried out and increases the reproducibility of the eroded material. The two binders tested were N-butyl methacrylate (Elvacite) and cellu- lose. Binding with cellulose provides the best results in terms of precision of the SA-ICP results. Another essential aspect of pellet preparation is the ratio of sample to conductive powder and binder. Different proportions of the components were tested 2 + 1 + 0.5; 2 + 1 + 1; 1 + 1 + 0.5; 1 + 2 +0.5 and 2+2 +0.5. With the dilution ratio 1 + 1 +0.5 for sample graphite cellulose the optimum mechanical properties of the pellets and good conductivity were achieved. Unlike spark emission the conductivity of the pellets is not of primary importance because the spark in this case only per- forms ablation while excitation is achieved by the ICP.Consequently the pellet preparation procedure described above was adopted. The analytical line sensitivity and spectral interferences were studied by using solutions containing approximately the back- ground equivalent concentration (BEC) of the element being investigated and the maximum expected concentrations of all the others. This study showed that by using graphite as conductive material the most sensitive lines listed in ref. 8 can be used for the determination of the elements being studied using an ICP Zn=213.856 nm Pb=220.353 nm Cd= 214.438 nm and Fe = 259.940 nm.Optimization of Spark Sampling Spark ablation conditions are defined by the following three parameters voltage (V) resistance (R) and repetition rate (f). The mildest practical condition for SA is obtained with the following parameters I/= 350 V R = 2.2 R and f= 400 Hz no application work was performed for lower frequencies owing to the poor sensitivity and repeatability obtained. Harsher conditions are obtained by decreasing resistance and increasing voltage. Lower resistance values increase the amount of analyte significantly causing material to be deposited in the injector tube of the plasma torch and signal instabilities. Consequently the sparking conditions were optimized by keeping the maxi- mum repetition rate and highest resistance available (400 s-l and 2.2Q respectively) and varying the voltage between 350 and 700 V.By increasing the condenser voltage it is possible to intro- duce more material into the ICP. This was shown by measuring the amounts of carbon from the released aerosol trapped on 47mm diameter glass microfibre filters with a pore size of 0.3 pm. Taking into consideration the intensities and precisions of the ICP measurements the voltage selected was 500V. At 350 and 400V the sensitivity and precision are poorer than those attained at 500 V; at 600 V the analytical signal becomes unstable as flickering occurs in the ICP as a result of an important increase in the amount of sample ablated which causes incomplete particle volatilization in the plasma. Table 3 shows the amount of graphite eroded at various voltages and the corresponding Zn emission intensities obtained from pellets of the sample ‘DUST 2’. The intensity uersus time curves Fig.1 show the stability of the emission for the elements considered from 10 s (pre-spark time) to at least 90 s. This is sufficient time to analyse the four elements considered in the same analytical programme. In an effort to minimize sparking times peak intensities were meas- ured employing a ‘three-point’ mode. In this mode the windows include three points with a distance between them of 0.0003 nm and the intensity value is a weighted average of these three points. Table 3 Eroded graphite and Zn emission intensities (arbitrary units) at various condenser voltages (pellets from sample ‘DUST 2’) Zn Vlv C*/mg min-’ Intensity x lo3 RSD 350 1.8 400 2.8 500 3.5 600 4.3 200 2.6 220 2.5 250 1.9 310 3.4 * Carbon determination was performed by infrared absorption after combustion in an induction furnace.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 225 - ( d ) f I I I I /Dust 1 I I I I I I I Table 4 Analysis of EAF flue dusts by SA-ICP; n=6 I I I I I I I I I Zn (YO) I Dust 4 Sample DUST 1 DUST 2 DUST 3 DUST 4 DUST 5 ECRM 876-1 d X 0 Calibration ‘low’ 24.40 0.50 35.81 0.49 43.65 0.85 Calibration ‘high’ Certified Found 23.29 23.58 0.32 0.45 Pb (%) X n Calibration ‘low’ 5.00 0.11 11.52 0.25 9.43 0.10 Calibration ‘high’ Certified Found 7.82 7.54 0.23 0.20 Cd (Yo) Fe (YO) X fl 0.038 0.001 0.46 0.01 < 0.01 Calibration ‘high’ Calibration ‘low’ Certified Found 0.13 0.11 0.0 1 0.01 ~~~ ~ ~ X n Calibration ‘high’ 15.21 0.20 8.33 0.20 4.70 0.10 Calibration ‘low’ Certified Found 24.85 25.12 0.17 0.22 Fe Zn t I 1 I I I 15 30 45 60 75 Time/s Fig.1 Sparking curves (intensity uersus time) ICP Calibration After verifying the linearity of the emission intensities uersus concentrations of the elements studied within the intervals of contents considered only two of the DUST samples were used to obtain the calibration graphs ‘DUST 1’ and ‘DUST 5’ for Zn Pb and Fe determination and ‘DUST 1’ and ‘DUST 4’ for Cd analysis. The remaining DUST samples were analysed as unknowns. The scans around the selected analytical lines of the two calibration samples are shown in Fig.2. The relative intensities and the absence of spectral interferences can be appreciated. The aforementioned calibrations were used to analyse the other selected steelmaking dusts. The accuracy of the proposed method was tested by analysing one electrical furnace dust reference material Euronorm Certified Reference Material ECRM 876-1. The calculated mean values obtained from the three measurements of each of three identically prepared samples and the corresponding SDs are shown in Table 4. Precision tests were performed using pellets from ‘DUST 1’ and ‘DUST 5’. The procedure described above was used to prepare six pellets of each sample. The instrumental variability was tested by measuring one pellet of each sample ten times; RSD < 1.5% for both samples with the majority of the values lying between 0.7 and 1.2%.The total variance (influenced by the difference between samples) was assessed by measuring the six pellets of each sample ten times also; RSDs ranged from 0.75-1.9 and 0.80-2.0 for DUST 1 and DUST 5 respectively. The corresponding RSD values obtained by ICP-AES from nebulized solutions were < 0.5 and < 0.7%’ respectively for DUST 1 and DUST 5. The analytical performance expressed in terms of back- ground equivalent concentration (BEC) and detection limit (DL) are given in Table 5. The DLs were calculated as the concentration of a solution giving an absorbance equal to three times the SD of the blank. Because there was no EAF flue dust that did not contain the elements being studied and 165 104 A y 44 f 2 + > 101 .- 8 3 c .- - + .- v) C Q .w - 51 2 ( a ) Dust 5 I w 0.01 nm I I I I 213.859 51 30 10 2 2 0.3 5 0 2 43 131 20 259.9 40 214.438 Wave1 eng th/n rn Fig.2 Scans for sample calibration for the analytical lines of (a) Zn; (b) Pb; (c) Cd; and ( d ) Fe. The scale given in (a) also applies to (b) (c) and ( d ) Table 5 Analytical performance Element Zn Pb Cd Fe BEC (Yo) 1.5 1 .o 0.05 1.2 DL (‘/o) 0.075 0.080 0.003 0.080 containing 100% of the remaining elements off-peak measure- ments were estimated as on-peak values. These BEC and DL values are of the same order of magnitude as those provided by ICP-AES using nebulized solutions obtained from an alkaline fusion of the sample. Conclusion With the pellet preparation procedure described it is possible to directly analyse EAF flue dusts by using SA as the solid sampling system for ICP.The method clearly simplifies the226 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 conventional analytical ICP process by removing the dissolu- tion step. By mixing the sample with graphite in a proportion of 1+1 good conductivity is achieved and the addition of cellulose permits better ablation of the material and also improves the mechanical stability of the pellets. The amounts of carbon in the released aerosol were evaluated in order to test the stability and the efficacy of SA by analysing the aerosol trapped at the end of the 0.75m long plastic tube (i.d.=5 mm) that connects the spark cell with the bottom of the torch.Transport losses take place along this tube and in order to improve the precision it is necessary to clean the plastic tube periodically using an argon or air stream in particular when a sample with much lower contents than the previous one must be analysed. The BEC and DL values are comparable to those obtained after sample dissolution ( 1 g I-' of sample) moreover the analytical precision is sufficient for application in metal recovery processes. Consequently it can be concluded that for these materials the proposed analytical procedure is a viable alternative to ICP-AES following sample dissolution. 4 5 6 7 8 References Lopez F. A. Balcazar N. Formoso A. Medina F. and Jimenez R. Rev. Metal. (Madrid) 1990 26 386. Cuadra A. and Limpo J. L. Quim. Ind. (Madrid) 1992 38 27. Gomez Coedo A. Dorado Lopez M. T. Jiminez Seco J. L. and Gutierrez Cobo I. J . Anal. At. Spectrom. 1992 7 11. Aziz A. Broekaert J. A. C. Leis F. and Laqua IS. Spectrochim. Acta Part B 1984 39 1091. Ohls K. and Sommer D. Fresenius' Z . Anal. Chem. 1979,296,241. Scott R. H. Spectrochim. Acta Part B 1978 33 123. Steffan I. ICP In$ Newsl. 1991 16 10 564. Boumans P. W. J. M. Line Coincidence Tables f o r ICP-AES Pergamon Press Oxford 2nd edn 1984. Paper 3/045 78 A Received July 30 1993 Accepted September 14 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900223

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 227 Determination of Aluminium Barium Magnesium and Manganese in Tea Leaf by Slurry Nebulization Inductively Coupled Plasma Atomic Emission Spectrometry* Colin K. Manickum and Alistair A. Verbeekt Department of Chemistry University of Natal P. 0. Box 375 Pietermaritzburg 3200 South Africa Samples of commercially available tea were ground in an agate pestle and mortar. Particles having diameter greater than about 80 pm were removed from this initial grind material by transport of the finer material in a flow of carbon dioxide and trapping the flow in a tube cooled with liquid nitrogen. Analyses of both the initial grind and the finer material by slurry nebulization using aqueous solutions for calibration purposes and by conventional solution nebulization after dry ashing and acid dissolution were carried out.While the initial grind material gave results for Al Ba Mg and Mn which were up to 4.3% lower than those obtained when using conventional solution analysis slurry nebulization of the finer material gave results having relative standard deviations between 0.3 and 1.8% and not significantly different (95% confidence level) from those of conventional analysis. Keywords inductively coupled plasma atomic emission spectrometry; slurry nebulization; tea leaf Dissolution of solid samples for pneumatic nebulization of the solution into an inductively coupled plasma requires the use of reagents and procedures which are often hazardous and usually lengthy. During these procedures the undesirable risks of analyte contamination and loss of more volatile analyte material are always present.In addition incomplete dissolution of some sample constituents can lead to low results if further sample treatment is not carried out on residues left by the initial attack. The nebulization of finely divided sample mate- rial suspended in a liquid by using a high-solids pneumatic nebulizer provides a convenient method of sample introduc- tion into the plasma and usually requires no alteration to the existing spectrometer except the exchange of the nebulizer itself. This procedure of slurry nebulization is not without problems which can be serious enough to invalidate the use of the technique in some cases.1y2 The sample types that have been investigated using slurry nebulization cover a wide spectrum from plant material and coal to refractory samples such as firebrick and recently an abstract has appeared reporting the successful determination of some elements in plant leaves including tea leaf mussels and soil and rock samples3 using aqueous calibration stan- dards. There have been a number of other reports of successful determinations using slurry nebulization and such calibration standards the work of Ebdon and Goodall' and of McCurdy and Fry4 being particularly noteworthy.Much attention has been paid in these investigations to the importance of ensuring a small particle size of the solid material in the slurry and there is considerable evidence that particle sizes of less than about 6 pm are necessary if inefficiencies in sample transport and atomization are to be a ~ o i d e d .~ - ~ Relatively less attention has however been paid to the use of slurry nebulization into an ICP for plant material analysis as is clear from recent in this journal. This must partially be owing to the relative ease of achieving complete dissolution of such samples for analysis compared with for example geological samples. Nevertheless the problem of contamination and loss of material during the dissolution process remains and it might still be worthwhile to approach the analysis using a slurry rather than a solution nebulization procedure. The work described in this paper concerns the determination of Al Mg Mn and Ba in tea leaf using slurry nebulization ~~ ~ * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993.t To whom correspondence should be addressed. into an argon ICP with calibration by means of aqueous standards. The use of aqueous solutions of the elements for calibration purposes is the method of choice since it avoids the pitfalls that can be associated with other methods of calibration' and which lead to analytical inaccuracies. In addition the method of standard additions is not particularly suitable since the addition cannot necessarily be made in the same solid matrix as the sample which would ensure that it had the same transport and atomization properties as the sample slurry. Even if standard reference materials were to be used a very large range would need to be available to deal with all of the types of samples likely to be encountered and in many cases the elements of interest are not certified so that large uncertainties could exist in the mean values used. Experimental Instrumentation A sequential ICP emission spectrometer (Instrumentation Laboratory Plasma-100) in standard form except for the removal of the in-line sample filter was used for the calibration and analysis of samples that had been dissolved.For the slurry samples and their associated aqueous calibration standards the standard nebulizer in its end-cap to the mixing chamber was replaced by a van den Plas V-groove nebulizer in an end- cap. The spoiler cone in the mixing chamber was not removed. Particle size distributions were measured using a laser scattering particle sizer (Malvern Instruments Mastersizer) while sample grinding was performed with an agate electrically driven mortar and pestle (Glen Creston).Sample Treatment Separation of residual coarse-grained (> 100 pm) sample was achieved by transport of the material using carbon dioxide through a baffled glass tube at a flow rate of 1-2mlminV1 with subsequent collection by freezing out in a tube cooled in liquid nitrogen. The finer sample material was recovered following removal of the carbon dioxide by sublimation while the collecting tube was warmed to room temperature. A sample (approximately 0.5 g) for conventional analysis was ashed by slow heating in a furnace from room temperature to 480-490 "C. The residue was treated with concentrated (16 mol 1-') HNO (0.6ml) and evaporated to dryness before being heated at 490 "C for 15 min.The cooled residue was moistened with H 2 0 and dissolved by warming with concen-228 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 trated (10 mol 1 - I ) HC1 (0.25 ml) and H20 (10 ml). Triton- XlOO (25 pl) was added and the solution was diluted to 50 ml. For slurry nebulization ICP-AES sample (approximately 0.1 g) was mixed in an ultrasonic bath for 5 min with 10 ml of a solution containing 10 mol 1-1 HCl(1 ml) and Triton X-100 (0.1 ml) in H 2 0 (200ml). The slurry was then stirred using a magnetic stirrer whilst being nebulized. The wavelengths of the lines used for analysis are presented in Table 1. Each line was corrected for background; the lines for A1 and Mg were corrected on both sides and those for Mn and Ba on the high wavelength side only.Five 1 s counts were made at each wavelength. The optimum observation height for each line was checked for both aqueous solutions and slurries and each line was observed at this height. No difference in observation height was found except in the case of Ba which was measured 2 mm higher in the plasma when introduced as a slurry although this height increase may not be significant. It is only possible to vary the observation height of the Plasma-100 in 2 mm increments so a compromise value of 1 mm higher could not be chosen and consequently during the calibration procedure for the slurry measurements the aqueous standards were observed at the height found to be optimum for slurries.Conditions for the different nebulizer flow rates and the sample pumping rates were optimized by the fixed size simplex method and are noted in Table 1. Calibration of the spec- trometer was by means of the same set of three aqueous mixed element standards containing HC1 and Triton-X regardless of the nebulizer being used. Results and Discussion Any attempt to separate finer from coarser material in a sample introduces the possibility of discrimination between particles of a non-homogeneous mixture. This discrimination becomes less important as the homogeneity of the sample material increases. While such problems are known to be of great importance in for example geological samples where the different minerals present have differing degrees of hardness and resistance to grinding they are not necessarily a source of error in the analysis of relatively soft samples such as biological materials.Nevertheless it is known that leaves and bark or stalks of plants contain differing amounts of some elements as has been confirmed by analysis carried out in these labora- tories on tree samples and vegetable samples such as cabbage. Should a grinding process therefore introduce a particle size discrimination based on the different degrees of hardness of leaves and stalks of tea any subsequent separation of particle sizes would introduce a discrimination factor into the results. In order to confirm that the separation of a relatively small number of larger particles (but a larger proportion of the total volume and thus mass of the sample) would not introduce such errors into the analysis samples of the initial grind material and the finer separated material were analysed using a method based on one which had been shown to be suitable for the analysis of tree leaves and other botanical and animal Table 1 Instrumental parameters Element Wavelength/nm A1 396.15 Ba 455.40 293.65 257.61 Mn Mg Standard IL crossflow nebulizer Pressure 24 lb in-2 Sample pump rate 1.5 ml min-' Pressure 32 Ib in-' Sample pump rate 1.5 ml min-' van Den Plas V-groove nebulizer ,samples." The pertinent properties of the particle size distri- butions and the results obtained for the analysis of these samples are shown in Table 2.The results show that there has been no elemental discrimination in the separation process and the average value for all determinations for each element was used as a basis for comparison of the results obtained by slurry analysis.Table 3 shows the results obtained from the analysis of slurries prepared from both the initial grind and the separated finer material. While the determinations using initial grind material for Al Ba and Mg were significantly lower (95% confidence) than the control values for these elements the results from the separated material were not significantly different at the 95% confidence level as determined by use of the t-test. The results obtained for Mn from the initial grind material however were not significantly different from those of the controls; yet those obtained from the finer material were significantly different (calculated t = 2.89; 6 degrees of freedom) at the 95% but not at the 98% level of confidence. The correspondence between the values can be considered good.The fact that some of the slurry particles were consider- ably larger than the size of the cut-off diameter for transport through some sample introduction system^^,^-^ and yet did not lead to seriously low recoveries indicates that it may not be essential to obtain a very small particle size for slurry nebulization. The maximum particle size in the slurries of Liu and Li3 was determined by the 300 mesh sieve used and was shown to be 44 pm by particle size analysis. These workers also obtained good correspondence between their slurry analy- sis and the values for the certified reference materials they used. Whilst very few minerals have a density which is less than Table 2 of tea leaves (all results as pg g-' of dry leaf) Particle size data and analytical results for control analyses Sample A1 Ba Mg Mn D(v 0.9) = 51.3 pm* 826 39.2 2250 641 D(4,3) = 20.6 pmt 804 39.4 2244 639 D(u 0.5) = 8.1 pm$ Separated finer material D(u 0.9) = 36.1 pm 824 39.6 2269 645 D(4,3) = 13.4 pm 809 39.4 2255 649 D(u 0.5 j = 7.1 pm Mean of all results 816 39.4 2255 644 Initial grind * D(q0.9) is the diameter exceeded by 10% of the volume t D(4,3) is the mean diameter over the volume distribution. This is 4 D(u 0.5) is the median diameter of the distribution.distribution. also known as the Herdan or the Brouckere diameter. Table 3 Results from slurry analysis of tea leaves (all results in pg g-' of dry sample) Sample Initial grind material Separated finer material Mean of separated material results Mean of all conventional analysis results A1 776 & 12* 782 & 28 803 f 13 796 & 16 782 f 8 815 f 14 799 & 141- 816 f llt Ba 37.9 & 0.1 38.2 f 0.1 39.2 0.1 38.3 +_ 0.1 39.4 f 0.1 39.2 k 0.1 39.0 f 0.5 39.4 f 0.2 ~~~ Mg 2249 f 28 2249 & 65 2276 f 54 2276 +_ 40 2260 & 59 2267 & 41 2270 f 8 2255 f 11 Mn 650 k 8 642 k 9 651 _+ 6 648 f 8 656 & 6 651 f 8 652 k 3 644$.4 * Standard deviation within the set of intensities measured for each t Standard deviation of the set of four mean values.sample (n = 5).JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 229 2gcm-3 and the lowest value listed in a table of common mineral densitiesll is 1.6 g cmP3 the density of dried biological material can be much less.Thus transport of fine material of this type by a gas flow might be expected to be much more efficient than transport of denser materials. It would seem more reasonable to suggest that density differences also play an important role in the efficiency of sample transport than to suggest that differences in nebulizer and spray chamber design could account for the apparently much more efficient sample transport of larger particles despite the improvements noted4 in the efficiency of transport when tortuous paths through the system are eliminated. It is also known that the leaching of analytes from the sample occurs during the dispersion pro- cedure. Between 50 and 60% of the total A1 and Ba and between 70 and 77% of the total Mg and Mn were found to be extracted during this 5 min dispersion period.Thus any influence of particle size on the recovery of analyte from the slurry might be diminished considerably by this fact. However in order to achieve good correspondence between conventional and slurry analysis results it is still necessary that slurry particles even if partially leached of analyte must be efficiently delivered to and atomized in the plasma. It is interesting to note that similar amounts of analyte elements are leached in the usual 5 min brewing of tea made with boiling water. The authors are grateful to Dr. A. Pitchford of Hulett Aluminium Pietermaritzburg for provision of the particle size analyses. 1 2 3 4 5 6 7 8 9 10 11 References Ebdon L. and Goodall P. J . Anal. At. Spectrom. 1992 7 111 1. Verbeek A. A. and Brenner I. B. J . Anal. At. Spectrom. 1989 4 23. Liu W.-y. and Li J.-x. ICP Inf. Newsl. 1992 18 237. McCurdy D. L. and Fry R. C. Anal. Chem. 1986 58 3126. Ebdon L. and Collier A. R. J. Anal. At. Spectrom. 1988 3 557. Ebdon L. Foulkes M. E. and Hill S. J . Anal. At. Spectrom. 1990 5 67. Saba C. S. Rhine W. E. and Eisentraut K. J. Anal. Chem. 1981 53 1099. Cresser M. S. Armstrong J. Cook J. Dean J. R. Watkins P. and Cave M. J. Anal. At. Spectrom. 1993 8 1R. Cresser M. S. Armstrong J. Dean J. Watkins P. and Cave M. J. Anal. At. Spectrom. 1992 7 1R. Verbeek A. A. Spectrochim. Acta Part B 1984 39 599. Handbook of Chemistry and Physics ed. Weast R. C. The Chemical Rubber Company Cleveland OH 50th edn. 1970. Paper 3 fO3931 E Received July 7 1993 Accepted October 21 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900227

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 23 1 Sensitive Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Cadmium by Continuous With Sodium Tetraethylborate* M. C. Valdes-Hevia y Temprano M. I?. Fernandez de la Campa and A. Sanz-Medelt Department of Physical and Analytical Chemistry Faculty of Chemistry University of Oviedo clJulian Claveria 8 33006-Oviedo Spain Al ky I at i on Continuous flow generation of volatile Cd species using NaBEt as a means of gaseous sample introduction into an inductively coupled plasma atomic emission spectrometry (ICP-AES) instrument has been investigated in detail. The assumed generation of CdEt2 is discussed and critically evaluated in terms of the sensitivity selectivity and accuracy of the corresponding ICP-AES determination of low levels of Cd.The proposed method for the determination of Cd by ICP-AES using NaBEt provided detection limits of 0.4 ng ml-’ which are ten times better than those of conventional nebulization ICP-AES. A precision of & 1.4% at the 50 ng ml-’ level of Cd was observed. Interference studies have been carried out which demonstrated high selectivity of the proposed method. This method has been applied to the determination of low levels of Cd by ICP- AES in samples of sea-water and tea infusions. Satisfactory validation of the results obtained has been provided by electrothermal atomic absorption spectrometric analysis of the same samples. Keywords Cadmium; inductively coupled plasma atomic emission spectrometry; continuous vapour generation; sodium tetraethylborate(iti) The most important feature that distinguishes heavy metals from other toxic pollutants is that metals as such are not biodegradable even though their potential toxicity in the environment as with other toxic metals is controlled largely by their physico-chemical form.Cadmium is therefore charac- terized by a long persistence in the environment and biological ‘half-life’ which accounts for its bioaccumulation in individuals.’ In recent years several cases of Cd poisoning have been reported2 and experimental results have confirmed its high toxicity. In fact Cd is considered together with Hg by national and international legislation to be the most toxic of metals. Consequently the maximum permissible concentrations of Cd in environmental food and freshwater samples3 are always extremely low.Therefore reliable control of this element in those samples requires very sensitive analytical techniques and the detection power of inductively coupled plasma atomic emission spectrometry (ICP-AES) with conventional nebuliz- ation is insufficient to determine the concentrations of Cd usually found in such samples. Although determinations of Cd using the generation of the volatile chloride have been reported the generation of the volatile hydride from aqueous solutions at room temperature which is so useful to increase the sensitivity for other elements in atomic absorption spectrometry (AAS) and ICP-AES is very difficult with Cd2+ owing to the instability of its hydride at temperatures above those of liquid nitrogen.’ In spite of such instability CdHz has been proposed as the volatile species of Cd formed when treating Cd2+ solutions with NaBH in an organic medium of dimethylformamide6 or in a vesicular organized medium of didodecyldimethylammonium bromide ( DDAB).7 A survey of the organometallic literature’ shows that the alkylboranes are able to alkylate several cationic compounds of metals such as Pb Hg Sn T1 Cd Zn and Cu to give fairly volatile organometallic species. Thus a possible alternative route to the more stable volatile Cd species is alkylation of the metal using alkylboranes.Honeycutt and Riddle,’ first reported the use of sodium tetraethylborate (NaBEt,) in aqueous solutions to produce volatile organometallic species * Presented at the XXVIIl Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993.t To whom correspondence should be addressed. of Pb and Hg from their inorganic salts and this approach has already been applied to the determination and speciation and their alkyl ions. D’Ulivo and Chen25 have reported the use of such alkylation reactions for the atomic fluorescence spectrometric (AFS) determination of very low amounts of Cd even if no conclusive identification of the exact Cd species formed could be achieved. In the present paper the formation of such volatile Cd species with NaBEt and its application to increase the sensi- tivity of Cd in determinations by ICP-AES have been investi- gated thoroughly. The effect of the nature of organized media and temperature on the efficiency of generation of the Cd volatile species have been studied.A new highly sensitive vapour generation (VG) ICP-AES method for the determi- nation of Cd is proposed which has been succesfully applied to the determination of low levels of this toxic metal in samples of sea-water and tea infusions. of pb,1&13 ~~,10,14-18 Sn 10.19-22 Sb and Ge,10,23 Se 24 7 Experimental Instrumentation An inductively coupled plasma atomic emission spectrometer Philips Model PU7000 was used for detection by ICP-AES. An atomic absorption spectrometer Perkin-Elmer Model 2280 equipped with a hollow cathode lamp (Perkin-Elmer) and a recorder (Perkin-Elmer Model 56) was used for AAS detection in connection with a laboratory-made batch hydride generator. The gas-liquid separation interface used was a grid- type nebulizer and spray chamber provided with the ICP instrument (see below). The Gilson Minipuls 2 peristaltic pump and the experimental flow system used are shown diagramat- ically in Fig.1. Reagents A 1000 pg ml-’ Cd” stock standard solution was stabilized in 0.5 mol 1-1 HN03 (Merck Darmstadt Germany). Working solutions were freshly prepared daily by diluting appropriate aliquots of this stock solution with ultrapure water. Sodium tetraethylborate(rI1) solutions were prepared just before use by dissolving the solid reagent (Alfa Ventron Danvers MA USA) in ultrapure water (Milli-Q Millipore Milford MA USA) stabilized in a 1% m/v NaOH solution. The reagent was stored in the dark in poly(tetrafluoroethy1ene)232 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 n ICP-AES r- 1 - 1 I I +111 A Per is t a I t i c Pump I I Ar nebulizer Fig. 1 Continuous vapour generation flow system (PTFE) vessels. Aqueous NaBEt solutions were stable for about 1 week when stored at 4 "C in darkened capped bottles. Cetyltrimethylammonium bromide (CTAB) solution (1 x lop2 moll-') was prepared by dissolving the surfactant powder (Fluka Buchs Switzerland) in water by gentle warm- ing. Other surfactants such as sodium lauryl sulfate (SLS) solution (1 x lop2 mol l-l Sigma St. Louis MO USA) Triton X-100 (TX-100) solution (2% v/v Merck) Zwittergent-3.16 (ZW-3.16) solution (1 x mol I-' Carbiochem-Behring La Jolla CA USA) were prepared in a similar way. The didodecyl- dimethylammonium bromide (DDAB) and the dihexadecyl phosphate (DHDP) vesicles solutions (1 x lo-' moll-') were prepared by dissolving the surfactant powders (Kodak Rochester NY USA and Aldrich Milwaukee WI USA respectively) in water and sonicating at room temperature (DDAB) or at 90 "C (DHDP) at a power of 60 W for about 12min with the tip of a high-intensity ultrasonic processor (Sonics & Materials Danbury CT USA).All mineral acids and metal salts used were of analytical- reagent grade and ultrapure water (Milli-Q) was used throughout. General VG-ICP-AES Procedure Continuous generation of volatile diethylcadmium In the flow system shown schematically in Fig. 1 the Cd sample dissolved in 0.2 mol I-' HCl was pumped continuously through one of the channels of the peristaltic pump at a rate of 0.75mlmin-' and merged with a 1.2% m/v solution of NaBEt at the same flow rate.This mixed solution feeds the grid nebulizer of the ICP instrument detuned in order to allow for separation of the volatile species that would eventually reach the torch of the plasma while the liquid phase goes to drain. Cadmium is measured at the 214.440 nm emission line and the instrumental conditions are detailed in Table 1. Background correction using two off-the-line points at 214.408 and 214.468 nm was used throughout. Samples Samples of sea-water from Villaviciosa (Asturias Spain) and infusions of commercial tea were analysed without any pre- treatment simply by following the continuous flow generation of volatile Cd species and the general ICP-AES procedure described above.The infusions of tea were prepared by extrac- tion of 2 g of tea in 250ml of Milli-Q water with gentle warming. Results and Discussion Optimization of Instrumental and Chemical Parameters Using the procedure outlined above for continuous gas-liquid separation of volatile Cd and ICP detection the effect on the Table 1 Optimum conditions for generation of CdEt (a) Generation by continuous VG-ICP-AES- Optimum plasma experimental conditions 'Wavelength 214.440 nm 1h.f. forward power 1.0 kW Aerosol gas pressure Outer gas flow htermediate gas flow :Final sample introduction [ntegration time 3 s 40 psi* 13 1 min-' 0 1 min-' flow rate 1.5 ml min Optimum chemical parameters HCl NaBEt (b) Generation by batch VG-AAS- 0.2 mol 1-' (flow rate 0.75 ml min-') 1.2% m/v in 1% m/v NaOH (flow rate 0.75 ml min-') Optimum AAS experimental conditions Wavelength 228.8 nm Lamp intensity 7 mA Slit 0.7 nm Gain 75 v Air flow rate C2H flow rate Argon flow rate Total volume 5 ml 15.5 1 min-' 2 1 min-' 1.5 1 min-' Optimum chemical parameters HCl 0.2 rnol I-' NaBEt Injection of 2 ml of 1.2 O/O m/v in 1% m/v NaOH * 1 psi = 6894.8 Pa.Cd signals of plasma instrumental variables such as nebulizer gas pressure forward r.f. power and coolant gas flow were studied. The variables in the chemical generation of the volatile species such as concentration of reagents and flow rates were investigated by following a univariant-type experimen- tal search. Maximum signal-to-background ratio at the 214.440 nm line of Cd was always the optimization criterion. The optimum ICP instrumental values observed in such experi- ments are summarized in Table 1 (a).For optimum instrumental settings chemical generation parameters were then investigated (concentration flow and final pH). The results observed have been plotted in Fig. 2. As can be seen in Fig. 2 A the effect of HCl concentration is fairly critical with an optimum pH of 2.1 for generation. Fig. 2 B shows the effect of the concentration of NaBEt,. This was not as critical as pH and a concentration PH .- - +d v) 1.0 1;2 1.; 2;OA c S i- e m- 200 2 250 *.I .- 0 [NaBEt,] (YO m/v) Fig. 2 in the Cd signal by VG-ICP-AES Effect of A sample acidity (HCl) and B NaBEt concentrationJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 233 of 1.2% m/v was eventually selected to save on this expensive reagent without important losses in sensitivity.The influence of sample flow rate was also studied between 0.25 to 5 ml min-'. As expected the signal increased with the sample flow rate then a plateau was reached for stationary vapour generation-excitation conditions after a flow of 1.5 ml min-' which was the final value selected for subsequent experiments. Optimum values selected for ICP instrumental plasma con- ditions and for continuous chemical generation of CdEt are summarized separately in Table 1 (a). Studies of the influence of temperature and efficiency of the generation-volatilization of Cd were carried out by batch AAS because of the impossibility of obtaining these data by continu- ous VG-ICP-AES. Optimum parameters for CdEt generation by batch AAS are given in Table l(b) the acidity of the solution and the flow rate of the carrier gas being the most critical parameters affecting the Cd AAS signal.Analytical Performance Characteristics Using all the experimental conditions given in Table l(a) the analytical performance characteristics of the corresponding VG-ICP-AES method were evaluated. The observed slope of the calibration lines detection limits and precision of the determinations of Cd by the proposed VG-ICP-AES method are summarized in Table 2. Characteristics observed for conventional nebulization are also given for comparison. The ICP-AES signals observed for conventional nebulization ( 100 ng ml- ' of Cd) CdH generation from DDAB7 and CdEt generation with NaBEt are shown for comparison in Fig. 3.The calibration graph in the last case was linear up to 1 pg ml-' of Cd and the detection limit (3sb IUPAC criterion) was 0.4 ng ml-I of Cd (10-fold better than the value observed Table 2 Analytical performance characteristics for determination of Cd by ICP-AES Method Detection limit*/ Slope/ Precision ng ml-' ml ng-' (%)? Conventional Vapour generation nebulization 5 2.66 x 105 1.3 with NaBEt 0.4 2.92 x lo6 1.4 *Detection limit = 3sb/slope. ?Precision (RSD %)=relative standard deviation at the 50 ng ml-' level. Precision values are within-run results (ten replicates). 350 A .g 300 3 > 2 250 L c .- + 2 200 g 100 .- fn .- 50 I I 214.41 214.44 214.47 Wavele ng t h/nm Fig.3 Emission spectra of 100ngmlF' of Cd. A Generation of CdEt from water; B generation of CdH from DDAB vesicles; and C conventional nebulization using conventional nebulization and twice better than CdHz generation from DDAB).The within-run precision evaluated by analysing ten independent replicates of a solution containing 50 ng ml-' of Cd was & 1.4%. Effect of 'Organized Media' on the Generation of CdEt It has been shown previously by this group that 'organized media' such as micelles and vesicles can improve the sensitivity of hydride generation because of the new microenvironment created in the solution. The kinetics of the generation of volatile species from the desired analyte can be improved,26 and this effect has been applied to improve the determination of Cd As and Pb by ICP-AES via 'cadmium hydride' arsine and plumbane generation in organized media.7-26.27 Several types of organized assemblies were assayed including cationic (CTAB) anionic (SLS) zwitter-ionic (ZW-3.16) and non-ionic (Triton X-100) micelles and also anionic (DHDP) and cationic (DDAB) vesicles.Using the experimental con- ditions given in Table l(a) the analytical parameters of the corresponding VG method for the different reaction media were evaluated. The observed influence of different organized media on the slope detection limits and precision of the determination of Cd by continuous VG-AAS and by VG-ICP- AES (in this latter case for those two surfactants which had shown the best behaviour in AAS i.e. TX-100 and DDAB) is summarized in Table 3. As can be seen unexpectedly the effect of organized media on the generation of volatile CdEt proved not to be as benefitial as in the case of volatile CdH (poss- ible specie^).^ Interference Studies Using the selected optimum conditions given in Table 1 (a) the effect of the presence of foreign elements on the VG-ICP-AES signal of Cd was investigated.All of the potentially interfering elements tested and the levels of tolerance observed in the corresponding determi- nations of Cd are summarized in Table4. Hydride forming elements and high levels of alkali alkaline earth metals or common anions were found not to affect generation of CdEt,. Only Zn and Ni could be a problem but only if present at relatively high excesses ( 1 500 = Cd interferent). In other words the proposed method is fairly selective for Cd. Effect of Mineral Acids and Their Concentration on the Cd Signal The different mineral acids presently used in many sample digestions including HC1 HN03 H2S04 HClO and HF were tested in the alkylation of Cd with NaBEt,. All these mineral acids (except HF which was tested in connection with H3BO3) showed similar behaviour to that illustrated for HCl and HNO in Fig.4 showing a maximum signal at a concen- tration of 0.2 mol I-' (pH 2.1). The use of HCl provided the highest signal for Cd and was selected for use in further experiments. In order to clarify this behaviour of HCl in the generation of volatile Cd species the effect of chloride as NaCI was also studied using HNO for sample acidification. Concentrations of NaCl of between 0.01 and 1 moll-' were found not to affect the observed Cd signals.Thus it seems that it is HCl that favours the metal alkylation that is observed. Influence of Generation Temperature The effect of the reaction vessel temperature where alkylation takes place was studied between 0 and 65°C using AAS measurements of Cd. Results showed that temperature greatly affects the kinetics and also the observed efficiency of the reaction for generation of the volatile species. During these234 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 3 AES detection Comparison of different organized media for volatile species of Cd using NaBEt with continuous VG-AAS and continuous VG-ICP- (a) AAS detection- Media Water DDAB SLS TX-100 CTAB DHDP ZW-3.16 (b) ICP-AES detection- Media Water DDAB TX-100 Surfactant concentration/ mol I-' 1 x 1 x - 1 x 10-3 1 x 10-3 0.15 1 x lo- Surfact ant concentration/moll-' - 10-3 Slope/ ml ng-' 1.668 1.990 0.359 1.888 1.449 1.751 1.749 Detection limit?/ ng ml-' 0.4 0.62 0.36 A -k sb* 0.028 f 0.002 0.03 1 & 0.002 0.020 & 0.004 0.039 k 0.002 0.028 & 0.003 0.041 f 0.005 0.028 k 0.003 Slope/ ml ng-' 2.92 x lo6 3.30 x lo6 3.48 x lo6 DLt/ ng ml-' 3.6 2.4 3.8 6.2 8.6 5.1 35 Precision 1.4 2.3 1.6 W)$ Precision W)$ 2.3 2.5 3.9 2.8 2.4 3.7 2.5 * Ab (mean value of absorbance for ten replicates of the blank); and sb standard deviation for ten replicates of the blank. ?Detection limit = 3sb/slope.1 Precision =relative standard deviation (RSD %) on 50 ng ml-I (n = 10). Table 4 Interference studies on 50 ng ml-' of Cd by generation of CdEt and ICP-AES ~~ Interferent As"' Zn" CU" M n" Ni" Fe"' Hg" Pb" M g" Ca" Na' K' Nitrate Chloride Cd interferent mass ratio 1:lO 1 100 1 500 1 10 1 100 1 500 1 10 1 100 1:500 1:lO 1:100 1:500 1 10 1 100 1 500 1 10 1:100 1 500 1 10 1 100 1:500 1 10 1 100 1 500 1:lO 1 100 1:500 1 10 1 100 1 500 1 500 1 1000 1 500 1 1000 1 1000 1 10000 1 1000 1 10000 Amount of interferent Recovery pg ml-' (%)* 0.5 99 5 103 25 96 0.5 99 5 101 25 93 0.5 100 5 99 25 96 0.5 102 5 99 25 100 0.5 99 5 93 25 91 0.5 98 5 99 25 98 0.5 99 5 99 25 99 0.5 100 5 99 25 99 0.5 99 5 102 25 101 0.5 99 5 100 25 98 25 98 50 99 25 98 50 99 50 99 500 100 50 99 500 101 *The precision observed in each case (expressed as RSD) were within +3%.0 0.05 0.10 0.15 0.20 0.25 0.30 Acid concentratiordmol r' Fig.4 Effect of A HCI B HNO and C HF on the ICP-AES signal of Cd by VG-ICP-AES temperature experiments the generation was carried out using a Cd concentration of 25 ng ml-' in a final volume of 5 ml. An adequately thermostated typical batch AAS hydride gener- ation system and the conditions specified in Table l(b) were used. The results obtained have been plotted in Fig. 5 for peak height and peak area of the observed AAS transient signals. They show that with increasing reaction temperatures forma- tion of Cd and transport to the atomizer increases. At tempera- tures around 60°C the reaction for generation of the volatile species of Cd and transport is very fast. In other words at temperatures higher than room temperature for the vapour generation a further decrease in the detection limit for Cd as compared with the values given in Table 2 could be obtained.Efficiency of the Generation-Volatilization of the Cd Species The efficiency of the generation of volatile Cd using NaBEt was evaluated. To do so the amount of Cd in the residual aqueous solutions after the corresponding tetraethylborate reaction in batch was determined by electrothermal (ET) AAS. Six independent standard solutions of 100 ng ml-' of Cd were treated with tetraethylborate at room temperature under the optimized experimental conditions [Table 1 (b)] and theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 235 Oa7* 25 Table 5 Determination of Cd in real samples by generation of CdEt and ICP-AES 0 10 20 30 40 50 60 U Temperatu rePC Fig.5 Temperature effect on Cd signal by batch-VG-AAS remaining Cd was determined in the residual aqueous solu- tions. This was repeated at a reaction temperature of 60°C. The results obtained showed that the efficiency of analyte volatilization was (79.5 f 1.9)’?40 at room temperature and (98.8 +_ 1.2)% at 60 “C. Preliminary efforts to characterize the exact nature of the observed volatile species of Cd measured in the ICP (formed when the analyte and NaBEt solutions merge in the continu- ous VG system of Fig. 1) have failed so far [attempts to generate the possible ‘CdEti2’ in the batch mode and analyse it by gas chromatography-mass spectrometry (GC-MS) for element identification have been unsuccessful so far but studies of alternative techniques are in progress]. However it is worth noting that the VG (volatile species generation) technique proposed here offers ten times better detection limits for Cd than conventional nebulization ICP- AES.Analysis of Real Samples Once the best conditions for the generation of diethylcadmium had been established the recommended VG-ICP-AES pro- cedure was applied to the direct determination of low levels of Cd in sea-water and tea infusions. Background correction (at 214.408 and 214.468nm) was employed and the other con- ditions used were as detailed in Table l(a). For sea-water samples known amounts of Cd were spiked into the real samples as their Cd contents were undetectable and the samples were then analysed using the recommended procedure without any pre-treatment.Tea samples were ana- lysed directly and the results obtained by the proposed method were compared with those by ETAAS for the same samples obtained in our laboratory. The values observed in all cases are summarized in Table 5. As can be seen very good agreement between the expected and the obtained results was observed. Therefore the validity of the new VG-ICP-AES method proposed for the determi- nation of low levels of Cd has been shown in the analysis of sea-water and tea infusions. Conclusions It has been demonstrated that the generation of alkylated volatile species of Cd (probably CdEt2) using NaBEt as the reducing agent can be applied to the determination of low levels of Cd. This method has proved to be much more sensitive than conventional nebulization.The selectivity is also Sea-water- Cd concentration*/ng ml-’ Sample 1 2 3 4 5 6 Tea infusion- ~ ~~~ ~~~ VG-ICP Cd added 4.1 f0.9 4 7.3 k 0.8 7 9.8 & 0.5 10 5.2 & 0.8 5 10.6 f 0.6 11 3.3 f 0.9 3 Cd concentration*/ng ml-’ Sample VG-ICP ETAAS Normal tea 1 5.3 f0.8 5.1 k0.3 Normal tea 2 22.0 f 0.5 22.8 f 0.9 Jasmine tea 6.0 f 0.9 6.1 rfi 0.2 Grey tea 1 17.4f0.6 17.1 k0.8 Grey tea 2 10.9f0.7 10.7k0.2 Orange tea 8.2 k 1.0 7.9 0.4 *Mean f SD (n = 3). Analysis using calibration line. high and so it is more appropriate for the determination of the metal in environmental and food samples as demonstrated here for sea-water and tea infusion samples. The sensitivity of the proposed method (measured by the slope of the calibration graphs) is about ten times higher than that obtained with conventional nebulization ICP-AES.Moreover ten times lower detection limits (DL=0.4 ng ml-1 at room temperature versus 5 ng ml-I) as compared with conventional nebulization can be achieved via continuous VG with NaBEt,. Using AFS measurements and similar VG D’Ulivo and Chen2’ have reported a DL of 0.2 ng ml-I for Cd and using AAS measure- ments the DL was 1 ng ml-’. As the volatile species formed probably has two alkyl groups2’ the presence of organized media such as micelles of vesicles could improve the efficiency of the generation at room temperature. The hydrocarbon layers of the micelles and could exert ‘concentration effects’ on the reagents and so a higher efficiency of alkylation (VG) could be expected. Unexpectedly the organized media tested proved not to be so beneficial for alkylation at Cd and generation of CdEt probably because the efficiency of the volatile alkylated species formed seems to be very high in aqueous media (80-100%).The temperature has a great effect on the kinetics (the peaks are narrower) and the efficiency (80% at room temperature while at 65°C it was 99Y0) of the volatile Cd compound evolved. The definitive nature of the volatile compound formed reaching the atomizer most probably CdEtz by analogy with Hg,” has yet to be positively confirmed. However a new ICP- AES method for the determination of Cd in real samples has been established based on the generation of such volatile species. Financial support from FICYT (Fundacion para el Foment0 en Asturias de la Investigacibn Cientifica Aplicada y la Tecnologia) and CICYT (Comision Interministerial de Ciencia y Tecnologia) and also the FICYT grant to M.C. Valdes- Hevia y Temprano is acknowledged. The loan of the ICP PU7000 instrument by Unicam Analytical Systems (Cam- bridge UK) is also gratefully acknowledged. References 1 Page A. L. Bingham F. L. and Chang A. C. in E’ect of Heavy Metal Pollution on Plants ed. Lepp N. W. Applied Science Publishers London 1981 1 pp. 77-109.236 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Yamagata N. and Shigematsu I. Bull. Inst. Public Health (Tokyo) 1970,19 1. Ravera O. Experientia 1984 40 2. Skogerboe R. K. Dick D. L. Pavlica D. A. and Lichte F. E. Anal. Chem. 1975 47 568. Barbaras G.D. Dillard C. Finholt A. E. Wartik T. Wilzbach K. E. and Schlesinger H. I. J. Am. Chem. SOC. 1951 73,4585. Cacho J. Beltran I. and Nerin C. J. Anal. At. Spectrom. 1989 4 661. Valdes-Hevia y Temprano M. C. Fernandez de la Campa M. R. and Sanz-Medel A J. Anal. At. Spectrom. 1993 8 847. Negishi E. Comprehensive Organometallic Chemistry Pergamon Press Oxford 1982 vol. 7 pp. 276-277. Honeycutt J. B. and Riddle J. M. J. Am. Chem. SOC. 1961,83,369. Ashby J. Clark S. and Craig P. J. J. Anal. At. Spectrom. 1988 3 735. Rapsomanikis S. Donard 0. F. X. and Weber J. H. Anal. Chem. 1986 58 35. Blais J. S. and Marshall W. D. J. Anal. At. Spectrom. 1989,4,641. Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1989 61 1867. Rapsomanikis S. and Craig P. J. Anal. Chim. Acta 1991,248 563. Bloom N. Can. J. Fish. Aquat. Sci. 1989 46 1131. AL-Rashdan A. Vela N. P. and Caruso J. A. J. Anal. At. Spectrom. 1992 7 551. 17 18 19 20 21 22 23 24 25 26 27 Fischer R. Rapsomanikis S. and Andreae M. O. Anal. Chem. 1993 65 763. Craig P. J. Mennie D. Osteah N. Donard 0. F. X. Martin F. Analyst 1992 117 823. Cai Y. Rapsomanikis S. and Andreae M. O. J. Anal. At. Spectrom. 1993 119. Cai Y. Rapsomanikis S. Andreae M. O. Anal. Chim. Acta 1993 274 243. Ashby J. R. and Craig P. J. Appl. Organomet. Chem. 1991,5 173. Ashby J. R. and Craig P. J. Sci. Total Environ. 1989 78 219. Yan D. Yan Z. Cheng G. Li A. Talanta 1982 29 519. Clark S. and Craig P. J. Mikrochim. Acta 1992 109 141. DUlivo A. and Chen Y. J. Anal. At. Spectrom. 1989 4 319. Valdes-Hevia y Temprano M. C. Aizpun Fernandez B. Fernandez de la Campa M. R. Sanz-Medel A. Anal. Chim. Acta. 1993 283 175. Aizpun Fernandez B. Valdes-Hevia y Temprano M. C. Fernandez de la Campa M. R. Sanz-Medel A. and Neil P. Talanta 1992 39 1517. Paper 3/03897A Received July 6 1993 Accepted October 5 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900231

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 237 Use of Emulsion Systems for the Determination of Sulfur Nickel and Vanadium in Heavy Crude Oil Samples by lnductively Coupled Plasma Atomic Emission Spectrometry* Miguel Murillo and Jose Chirinost Centro de Quimica Analitica Facultad de Ciencias Universidad Central de Venezuela P. 0. Box 47702 Caracas 704 I -A Venezuela A useful and rapid procedure is described for the determination of sulfur nickel and vanadium in crude oil by inductively coupled plasma atomic emission spectrometry. Samples were prepared by emulsifying crude oil in water. Aqueous inorganic solutions with the same amount of emulsifier and solvent were used as calibration standards. Heavy crude oils were analysed and the results were compared with those obtained by digestion methods.To evaluate the accuracy of the method National Institute of Standards and Technology Standard Reference Materials 1622c Sulfur in Residual Fuel Oil and 1634a Trace Elements in Fuel Oil were analysed. No statistically significant differences were observed between the results obtained by this method and the certified values. The precision of the method was in the range from 1 to 3% relative standard deviation. Keywords lnductively coupled plasma atomic emission spectrometry; sulfur; trace metals; crude oil; emulsion systems There is a need in the petroleum industry to quantify sulfur nickel and vanadium in crude oil samples as they are catalyst poisons and cause corrosion in furnaces and boilers. Also sulfur contributes to environmental pollution through acid rain.' For this reason sensitive rapid and precise methods for the determination of sulfur in crude oils are essential.In this respect inductively coupled plasma atomic emission spec- trometry (ICP-AES) is an attractive technique for the determi- nation of sulfur and trace metals because of its applicability in multi-element determinations and the low detection limits obtainable. Sulfur nickel and vanadium can be determined by ICP-AES using different sample preparation methods. These methods are discussed below. Digestion treatments are used to avoid the matrix problems associated with the analysis of crude oil^.^-^ The physical and chemical properties of samples treated in this manner are similar to those of aqueous standards therefore the determi- nation of analytes in various different matrices is possible with the use of a single calibration curve.Digestion processes have some disadvantages such as long analysis times and trace metal contamination from acid reagents digestion vessels and airborne particulates which can affect the accuracy of the analytical results. Some workers have determined sulfur nickel and vanadium in crude oils by direct nebulization of the samples dissolved in organic Direct dilution is simple can be automated is less time consuming than alternative procedures such as digestion and is applicable to a wide range of petroleum products. In routine analyses agreement between values obtained by direct dilution and those obtained by digestion of the sample is very good! Care must be taken to ensure that the chemical characteristics of the standards match those of the samples as closely as possible.This problem can be overcome by selecting a more appropriate standard material. An evaluation of the effects of a variety of common organic solvents on the analytical signal indicates that problems are experienced with organic solvents because of their volatility.'' Solvents with greater volatility than water require changes to the normal ICP-AES operating conditions. Some of these changes include an increase in the forward power together * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993. t To whom correspondence should be addressed. with additional plasma gas to protect the torch and the addition of oxygen to the plasma to achieve the combustion of the solvent vapour.Also addition of oxygen prevents the build-up of carbon.",12 As an alternative to the use of organic solvents the oil samples were emulsified in water and the emulsions were introduced into the ICP. The use of emulsions can reduce the organic content of the sample solution to less than 95% m/m. Emulsion techniques have been reported in the literature for atomic absortion spe~trometry,'~~'~ flame photometry15 and direct current plasma emission spectrometry,16 however there are only a few references about the use of emulsion systems for sample introduction into an ICP. Determination of sulfur and trace metals in crude oil and oily products by ICP mass spectrometry (ICP-MS) and ICP-AES have been reported by Lord17 and Borszeki et a1.18 They obtained good accuracy and precision in their methods.Previous studies did not involve the analysis of heavy crude oils [crude with American Petroleum Institute (API) gravity less than 141 as these samples are difficult to handle because of their viscosity and stable emulsions are difficult to obtain because of the high asphaltene content of these oils. In this paper an ICP-AES method is described and evaluated for the determination of sulfur nickel and vanadium in heavy crude oils by using the emulsification technique for sample preparation. Experimental Instrumentation The inductively coupled plasma spectrometer employed for this study was a Jobin-Yvon Model JY24. Details of the instrument and the operating conditions used throughout this work are listed in Table 1.Reagents Sulfur solutions were prepared by diluting a 1000 pg ml-' standard prepared from pro analysi sodium sulfate (Merck). Nickel and vanadium solutions were obtained from 1000 pg ml-' stock solutions (BDH). Ethoxy nonilphenol (Etoxyl) and sulfur free xylene (BDH) were used as emulsifier and solvent respectively for emulsion preparation. De-ionized water (MilliQ grade Millipore) was used throughout this work.238 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 Instrumental and experimental parameters Spectrometer Jobin-Yvon Model JY24 Grating Holographic grating having 3600 R.f. generator/MHz 40 Nebulizer Spray chamber Scott type Gas flow rates/l min-’ grooves mm-’ Forward power/W 900 Meinhard C type (with peristaltic Pump) Plasma 12 Intermediate 2 Sheathing 0.1 Carrier 0.9 Purge 10 Observation height Measurement time/s 0.5 Working wavelengthtnm 15 mm above load coil Sulfur 181.9 Vanadium 309.3 1 1 Nickel 23 1.604 Samples The samples tested were various Venezuelan crude oils having an API gravity of less than 14 (heavy crude oils).To assess the accuracy of the proposed method National Institute of Standard and Technology (NIST) Standard Reference Materials (SRM) 1622c Sulfur in Residual Fuel Oil and 1634a Trace Elements in Fuel Oil were tested. Sample Preparation Portions (G0.2500 g) of crude oil are weighed into a tared 30ml glass bottle. A 0.5000g portion of xylene is added to each sample and the contents are mixed until a homogenous solution is obtained.Each sample is then mixed with 0.40 g of emulsifier and the mixture is mechanically agitated until a homogenous solution phase is produced again. The mechanical agitation can be accomplished by use of an ultrasonic bath. De-ionized water is added with continual agitation until a final mass of 20g is obtained. The time required to prepare an emulsion is approximately 10 min per sample. Results and Discussion Emulsion Preparation A number of factors such as concentrations of crude oil surfactant and water; chemical composition of the crude oil; temperature; and the technique used to mix and homogenize the components can influence the stability of an emulsion. These variables are too numerous to evaluate their effect on the stability of the emulsions experimentally.Becher” reported that a non-ionic emulsifier with an hydrophile-lipophile bal- ance (HBL) of 8-18 should be used in order to form a stable oil-in-water emulsion. In this sense 10 mol of ethoxylation nonilphenol (HBL = 14) were selected as an emulsifier. This nonilphenol is a more efficient emulsifier for heavy crude oils than others used for crude oils and some of their product^.'^*^* The optimum concentration of surfactant required to obtain a stable emulsion was obtained experimentally by varying the amount of emulsifier. Homogenization of the emulsion was evaluated visually. The best surfactant concentration obtained was 2.00% m/m for emulsions with a 1.25% m/m oil phase. A little xylene was used for emulsion preparation to reduce the viscosity of heavy crude oils.This pre-treatment facilitates effective mixing and interaction between the oil and surfactant that must be accomplished prior to the addition of water. Analytical Conditions for Sulfur Nickel and Vanadium Determination by ICP-OES. Ionic to atomic line intensity ratios were used to optimize the operating parameters of the plasma.20 The magnesium lines Mg I1 (280.270 nm) and Mg I (285.213 nm) were selected for the optimization of the following parameters carrier plasma interme:diate and sheathing gas flow rates forward power and purge gas flow rate (for sulfur determination). The operating parameters are presented in Table 1. Calibration Mode To choose the correct calibration method for the determination of sulfur nickel and vanadium the slopes of calibration curves obtained by using inorganic aqueous solutions emulsified crude oil solutions and emulsified inorganic aqueous solutions (with the same amount of emulsifier as used in emulsified crude oil solutions) were compared.These calibration curves were obtained by using the single off-peak background correc- tion method. The results are presented in Figs. 1-3. For sulfur and nickel (Figs. 1 and 2) the slopes of the emulsified crude oil and emulsified inorganic aqueous solutions were the same. This means that emulsified inorganic aqueous solutions can be used for calibration purposes under the working conditions selected. This conclusion cannot be applied to aqueous solu- tions because the slopes have a different behaviour when compared with the slopes obtained for emulsified solutions.In the case of vanadium (Fig. 3) all the calibration curves exhibit similar behaviour. Therefore emulsified inorganic aqueous solutions were chosen for calibration purposes. 0 0 6 Q 200 300 400 500 600 700 800 900 [Sllpg mi-’ Fig. 1 Calibration curves for sulfur obtained by using 0 aqueous solutions; 0 emulsified aqueous solutions and + emulsified crude oils solutions ;;i 160 0 - a Q .t n I 1 1 I 1 I L u u 0.4 0.6 0.8 1 .o 1.2 1.4 1.6 [Nil/pg ml-’ Fig.2 Calibration curves for nickel obtained by using 0 aqueous solutions; 0 emulsified aqueous ‘solutions; and + emulsified crude oils s’olutionsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 239 A in 4- .- 5 4500 - 2500 m in .- 6 1500 E .- v) .- w 500 8 8 8 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 [VI/pg mi-' Fig.3 Calibration curves for vanadium obtained by using 0 aque- ous solutions; 0 emulsified aqueous solutions; and + emulsified crude oils solutions Table 2 Detection limits for sulfur nickel and vanadium ~ Aqueous Emulsified aqueous Emulsified crude oil solution/ solution/ solution/ Analyte pg ml - ' pg ml-' pg ml-' Sulfur 0.04 0.5 0.6 Nickel 0.03 0.06 0.08 Vanadium 0.01 0.02 0.02 Table 3 Determination of sulfur nickel and vanadium in different SRMs using the proposed method Sulfur (YO m/m) SRM Experimental* Certified NIST 1622c 2.2 + 0.1 2.012 f 0.025 NIST 1634a 2.7 f 0.1 2.85 f 0.05 Nickel/pg ml-' NIST 1622c NIST 1634a Experimental* Certified - 27+2 Vanadium/pg ml-I - 29+ 1 NIST 1622c NIST 1634a Experimental* Certified - - 54f2 56f2 * Mean of five determinations and f RSD at the 95% confidence level.Detection Limit Detection limits (expressed as the concentration associated with the smallest signal that can be distinguished with a pre- determined change from the random fluctuations of the back- ground) were determined for sulfur nickel and vanadium in aqueous emulsified inorganic aqueous and emulsified crude oil solutions by the method of Miller and Miller.21 The detection limits are presented in Table 2. Results show that the detection limits for emulsified crude oil and emulsified aqueous solutions are the same. Also these detection limits are higher than those obtained for aqueous solutions. These differences were mainly due to the different background emis- sion signals obtained for aqueous solutions and emulsified solutions.Accuracy and Precision of the Method To assess the accuracy of the method two Standard Reference Materials NIST SRM 1622c and NIST SRM 1634a were analysed. The results are presented in Table 3. Significance tests2' (t- and F-tests) at a confidence level of 95% indicated that the results obtained were in good agreement with the certified values. The precision of the method was in the region of 2% expressed as relative standard deviation (RSD). Reliability of the Method In order to test the applicability of the proposed method to the analysis of real samples several heavy crude oils were analysed. The results obtained by ICP-AES for emulsion samples were compared with those obtained by digestion methods (sulfate ash2 and bomb digestion3).The different sulfur nickel and vanadium contents obtained are listed in Table 4. Results show that ICP-AES values are in agreement with digestion method values when t- and F-tests2' were applied to the results at a confidence level of 95%. Conclusions The direct determination of sulfur nickel and vanadium in heavy crude oil was successfully achieved by using emulsion systems for sample introduction in ICP-AES. Good precision and accuracy were obtained using emulsified inorganic aqueous solutions for calibration purposes with no internal standard. Emulsion sample preparation is extremely quick and the problems experienced with organic solvents such as carbon build-up are avoided because of the reduction of the organic content of the sample solution.This work was supported in part by the Consejo de Desarrollo Cientifico Humanistic0 de la Universidad Central de Venezuela (Research Grant 03.12.2136.89) and in part by the Consejo Nacional de Investigaciones Cientificas y Tecnologicas (Research Grant 184-93) Table 4 Determination of sulfur nickel and vanadium in various crude oils using the proposed method; results given as mean k RSD n = 3 at the 95% confidence level Sulfur (Yo m/m) Nickel/pg ml- ' Vanadium/pg ml - ' Gravity R.M.( B) A 8 2.8 f 0.1 2.54 f 0.05 81+5 81f2 280 A 25 249 f 5 B 9 4.7 & 0.1 4.78 0.07 99+7 102f3 468 + 50 478 f 9 C 10 5.7 * 0.2 5.5 1 f 0.08 107 & 7 D 12 3.8 k 0.1 3.66 2 0.07 88k7 87+3 415 f 50 415 + 8 E 13 3.0f0.1 2.80 & 0.06 54f7 57f2 451 f 37 439 f 8 Sample (API*) I C P - A E S R.M.(A)T ICP-AES R.M.( B)S ICP-AES 107 3 1200 f 62 1188 & 30 * API =American Petroleum Institute.7 R.M.(A) = Reference method values obtained using combustion bomb digestion m e t h ~ d . ~ $ R.M.( B) = Reference method values obtained using sulfate ash method.*1 2 3 4 5 6 7 8 9 10 11 12 13 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 References Milner 0. J. Analysis of Petroleum for Trace Elements Pergamon Press New York 1st edn. 1963 pp. 68 97-98. Milner 0. J. Anal. Chem. 1952 24 1728. Murillo M. Carrion N. and Chirinos J. J. Anal. At. Spectrom. 1993 8 493. Hausler D. and Carlson R. Spectrochim. Acta Reo. 1991,14 97. Merryfield R. N. and Runnels J. H. Developments in Atomic Plasma Spectrochemical Analysis ed. Barnec R. M. Heyden Philadelphia PA 1981 pp. 396-403. Wallace G. F. and Ediger R. D. At. Spectrosc. 1981 2 169. Barret P. and Pruszkowski E. Anal. Chem. 1984 56 1927. Fabec J. and Ruschak M. Anal. Chem. 1985 57 1853. Jansen E. B. M. Knipscheer J. H. and Nagtegaal M. J. Anal. At. Spectrom. 1992 7 127. Boorn A and Browner R. Anal. Chem. 1982,54 1402. Hutton R. C. J. Anal. At. Spectrom. 1986 1 259. Hausler D. Spectrochim. Acta Part B 1987 42 63. Berenguer V. Guinon J. L. and de la Guardia M. Anal. Chem. 1979 294 416. 14 Polo-Diez L. Hernandez-Mendez J. and Pedraz-Penalva F. Analyst 1980 105 37. 15 De la Guardia M. Salvador A. and Berenguer V. Analusis 1980 8 448. 16 De la Guardia M. Salvador A. and Berenguer V. Analusis 1981 9 74. 17 Lord C. Anal. Chern. 1991,63 1594. 18 Borszeki J. Knapp G. Halmos P. and Bartha L. Mikrochim. Acta 1992 108 157 119 Becher P. Emulsiones Teoria y Practica Blume Madrid 1972. 20 Mermet J. Anal. Chim. Acta 1991 250 85. 21 Miller J. E. and Miller J. N. Statisticsfor Analytical Chemistry Wiley New York 1984. Paper 3103885 H Received July 6 1993 Accepted November 12 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900237

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Determination of Rare Earth Elements in Mineral Waters by Inductively Coupled Plasma Atomic Emission Spectrometry* Jana Kubova Faculty of Natural Sciences Comenius University 842 15 Bratislava Slovakia Vladislav Nevoral Reference Laboratory for Natural Healing Springs of the Ministry of Health 353 01 Marianske Lazn6 Czech Republic Vladi mir StreS ko Faculty of Natural Sciences Comenius University 842 15 Bratislava Slovakia 241 A procedure for selective preconcentration of all rare earth elements (REE) in mineral waters on a Dowex cation exchanger has been developed. The precision and accuracy of the procedure were checked on a synthetic standard solution as well as on real samples using inductively coupled plasma atomic emission spectrometry (ICP-AES).The efficiency of the preconcentration procedure was checked by recovery tests and the reliability of the ICP-AES determination of REE established by comparing the results obtained with those from spectrophotometric analysis. Keywords Mineral waters; rare earth elements; preconcentration; cation exchange; inductively coupled plasma atomic emission spectrometry Information about the rare earth elements (REE) contents in solid natural materials (minerals and rocks) enable some important geochemical problems to be solved.' However obtaining reliable results presents a complex analytical task. Instrumental neutron activation analysis ( INAA),2-4 spark source mass spe~trometry,~ isotope dilution mass spec- trometry,' inductively coupled plasma atomic emission spec- trometry ( ICP-AES)7-'o and more recently inductively coupled plasma mass spectrometry ( ICP-MS)11-13 are the most important methods that have been used for the determination of low levels of REE.Owing to its selectivity and the comparatively low initial cost and subsequent running costs as compared with for example ICP-MS or INAA ICP-AES was chosen for the present determination of REE in mineral waters. The REE contents in mineral waters are extremely low as compared with the main components and also show a large variation in their chemical composition. Therefore for the deter- mination of REE in mineral waters the use of a separation- preconcentration procedure is inevitable. Coprecipitation of Fe(OH)3,14 A1(OH)3,15,16 CaC204 or CaF2,17*18 separation on ion exchangers such as AG 50W'9,20 or Dowex 50W21-23 and liquid-liquid extraction2&26 are the most frequent procedures that have been described for the preconcentration of REE.The proposed procedure for the determination of REE in mineral waters uses a separation-preconcentration procedure with an ion exchanger Dowex 50W X12. Experimental Apparatus The instrumentation used for the AES measurements was as follows sequential atomic emission spectrometer (Plasmakon S 35 Kontron Germany) with a grating of 2400 lines mm-'; a concentric glass nebulizer (Type B Meinhard); an Ar-Ar plasma power 1.5 kW frequency 27.12 MHz flow rate of outer gas 14.5 dm3min-' of intermediate gas 1.0 dm3min-' of aerosol carrier gas 1.0 dm3rnin-l; sample uptake rate 1.5 cm3 min-' controlled by a peristaltic pump; and inte- gration time 5 s.For the spectrophotometric measurements a prism spectro- photometer (Spektromom 204 MCM Hungary) was used. Analytical Lines The wavelengths of the spectral lines and positions for the measurement of the background chosen in order to ensure the lowest mutual spectral interference^,^^ are listed in Table 1. All measurements were performed with blank and background correction. Chemicals Standard solutions of the REE were prepared by appropriate dilution of 1000 ppm atomic absorption spectrometric (AAS) standards (Alfa Products Ventron). The acids used were of analytical-reagent grade (Merck). The cation exchanger (Fluka Bucks) Dowex 50W X12 (50-100 mesh) was washed with 1moldm-3 NH4Cl and Table 1 Measured spectral lines Detection limit (3s) Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Spectral line/nm 361.38 371.03 333.75 413.76 390.84 430.36 359.26 381.97 342.25 350.92 353.17 345.60 349.91 346.22 328.94 261.54 Background/nm 361.50 370.90 333.84 413.87 390.90 430.41 359.40 382.06 342.35 350.96 353.21 345.69 349.95 346.26 328.99 26 1.42 A*/ Btl pg drn-3 ng dm-3 0.5 0.3 0.8 0.4 2.6 1.3 10.1 5.1 5.6 2.8 5.9 2.9 6.1 3.1 1 .o 0.5 3.1 1.6 3.9 1.9 2.0 1 .o 1.2 0.6 2.5 1.2 1.8 0.9 0.4 0.2 0.4 0.2 * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993.*A Without the preconcentration procedure. ?After the preconcentration procedure (from 50 dm3 to 25 cm3).242 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0.2 mol dm-3 ammonium citrate [(NH4),HC6H307] solu- tion pH 4.2 and then converted into the Hf form with 6 mol dmP3 HCl.Preconcentration of REE by Cation-exchange Separation Aliquots of 25-100 dm3 of mineral water were taken in polyethylene vessels (pre-washed with hot 3 mol dmd3 HC1) and immediately acidified with 6moldm-3 HCl to a pH The same amounts of each sample were placed in glass vessels ( 5 dm3) and boiled to remove CO followed by any volatile organic compounds that could be present. After partial cooling the sample was allowed to pass (at a flow rate of 4-5 cm3 min-') through a 16 mm diameter column packed with 302 mm of Dowex 50W X12 (50-100 mesh). The empty sampling vessels were washed with 250 cm3 of hot 3 mol dm-3 HC1 and SO0 cm3 of de-ionized water and the washings were added to the remaining 4-5 dm3 of sample.After the exchange process the cation exchanger was washed with 200cm3 of 40% acetone and 200 cm3 of 80% acetone which removed the majority of the organic compounds retained from the sample. Through the cation exchanger were then passed 100cm3 of 40% acetone 100 cm3 of distilled water and the main compo- nents of the mineral waters i.e. Na+ K' Mg2+ Ca2+ and Fe3+ were then eluted with 1500 cm3 of 1.6 mol dm-3 HC1 (at a flow rate of 1 cm3min-'). The REE were eluted with 1100 cm3 of 6 mol dm-3 HCl (at a flow rate of 0.5 cm3 min-I). The eluate was then evaporated to dryness in a quartz dish. To the remaining portion (about 100 cm3) 10 cm3 of concen- trated. HN03 were added to decompose any organic material present due to the ion exchangers.To the dry residue 10 cm3 of 6moldm-3 HC1 were added and again evaporated to dryness. The final residue was diluted with 2 cm3 of 6 mol dm-3 HCl the solution was transferred into a 25 cm3 calibrated flask diluted to the mark with de-ionized water and then used for the ICP-AES measurements. For the spectrophotometric determinations the dry residue was treated according to the procedure described in detail in ref. 21 i.e. after its dissolution in 0.1 mol dm-3 HCI it was passed through a chromatographic column and the separated REE were eluted with a-hydroxyisobutyric acid of various concentrations. The spec- trophotometric determinations were performed at 540 nm using XyIenol Orange in the presence of cetyfpyridinium bromide.of 1.5-1.8. Results and Discussion Precision and Accuracy of the Procedure The precision and accuracy of the proposed procedure were determined by the analysis of a synthetic standard solution spiked with known contents of the macrocomponents and REE. The composition of the synthetic sample without the REE data is given in Table 2. To the synthetic sample (as shown in Table 2) the REE were added in the amounts given in Table 3. A 25 cm3 aliquot of the synthetic sample with known REE contents (Table 3) was diluted to a volume of 12 dm3 using the synthetic sample free from REE (Table 2) and acidified with 60cm3 of 6moldm-3 HC1. The REE were preconcen- trated by the described procedure to the original volume of 25 cm3 and determined by ICP-AES.A total of 15 synthetically prepared samples of the same composition were preconcentrated by this procedure. Their mean REE contents as well as relative standard deviations (RSDs) are listed in Table 3 (column A). The data in column B of Table 3 are the results obtained for the same original synthetic samples containing the REE but which were analysed without the application of the separation-preconcentration procedure. It is evident from Table 3 that results obtained by Table 2 Composition 01' the synthetic mineral water Component Li + Na+ K+ NH Rb+ c s + cu2 + Mg2+ Ca2 + Sr2 + Ba2 + Zn2+ Cd2+ ~ 1 3 + Concentration/ mg dm-3 1.632 589.7 56.2 2.40 1.65 1.58 0.016 38.42 74.67 0.360 0.040 0.008 0.006 0.008 Component Ti4 + Pb2+ v4 + Cr3 + Mo6+ Mn2+ Fe2+ Fe3+ co2 + Ni2 + F- Br- I- HB02 Concentration/ mg dm-3 0.0008 0.0008 0.0008 0.0008 0.0008 0.024 2.173 7.120 0.016 0.016 0.156 1.546 0.193 28.900 Table 3 Recoveries of REE from synthetic sample Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Added/ mg dm-3 4.8 8.0 4.8 8.0 4.8 8.0 4.8 1.6 4.8 1.6 4.8 1.6 4.8 1.6 4.8 1.6 Found RSD A*/ Bt/ (procedure A) mgdm-3 mgdm-3 (%) 3.8 4.9 9.0 8.2 8.2 2.0 4.8 4.9 7.7 8.5 8.5 6.1 4.9 4.8 5.1 8.7 8.8 4.4 4.9 5.0 4.7 1.8 1.8 7.8 4.9 4.9 3.2 1.7 1.7 3.5 4.8 5.0 4.6 1.7 1.7 3.6 4.9 5.0 2.8 1.8 1.8 3.4 4.7 4.7 1.8 1.5 1.5 4.8 ~~~ - *A With cation exchanger (n = 15). B Without cation exchanger (n= 10).both procedures are in good agreement (with the exception The blank valiies were [with the exception of Eu (0.012 pg dm-3) and Nd (0.018 pg dm-3)] below the detection limit and were determined after passing the synthetic standard solution free of FLEE through the cation exchanger (ten replicates).The accuracy of the proposed ICP-AES method was checked spectrophotometrically with real samples of mineral waters. As an illustration examples of the determination of REE with the use of the both methods are presented in Table 4. of SC). Conclusion The method described enables a reliable ICP-AES determi- nation of all REE in mineral waters to be carried out. The possibility of treating a fairly large volume of mineral water ensures that very low REE contents can be determined. In order to obtain the high preconcentration factors necessary for the reliable determination of all REE present in the mineral waters at extremely low concentrations the cation exchanger Dowex was chosen.The separation-preconcentration pro- cedure is fairly timt consuming but passing the sample through the ion-exchange column does not need any particular atten- tion. Owing to the low abundance of REE in the laboratory environment the danger of contamination during the whole procedure can be neglected. The reliability of the separation-preconcentration procedureJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 243 Table 4 Accuracy of REE determination in natural mineral waters; concentrations are expressed in pg dm-3 n = 3 Element sc Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu c Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu c Element s c Y La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Sample 1 SP* ICP-AES 0.385 0.338 3.600 3.210 0.360 0.413 1.160 1.410 0.190 0.220 1.305 1.280 0.400 0.458 0.122 0.120 0.578 0.598 0.081 0.086 0.500 0.460 0.120 0.130 0.387 0.331 0.057 0.056 0.466 0.413 0.073 0.072 9.784 9.595 Sample 4 Sample 2 Sample 3 SP ICP-AES 0.280 0.243 2.750 2.860 0.191 0.188 0.565 0.550 0.099 0.091 0.740 0.710 0.219 0.210 0.068 0.069 0.318 0.328 0.056 0.060 0.318 0.320 0.081 0.081 0.281 0.282 0.049 0.044 0.380 0.386 0.058 0.054 6.453 6.476 Sample 5 SP ICP-AES 0.560 0.496 3.030 2.990 0.224 0.228 0.780 0.765 0.122 0.106 0.960 0.986 0.290 0.263 0.082 0.082 0.433 0.397 0.062 0.065 0.382 0.386 0.100 0.090 0.320 0.302 0.049 0.050 0.380 0.400 0.070 0.066 7.844 7.672 Sample 6 SP ICP-AES 0.148 0.115 1.004 1.053 0.451 0.487 0.671 0.719 0.091 0,095 0.231 0.257 0.116 0.124 0.030 0.031 0.114 0.131 0.019 0.021 0.130 0.136 0.027 0.020 0.078 0.069 0.012 0.012 0.083 0.079 0.012 0.012 3.217 3.361 Sample 7 SP ICP-AES 0.125 0.103 0.370 0.336 0.054 0.064 0.088 0.085 0.012 0.016 0.034 0.047 0.010 0.012 0.005 0.006 0.020 0.028 0.007 0.007 0.026 0.030 0.007 0.008 0.020 0.024 0.002 0.003 0.020 0.028 0.002 0.002 0.802 0.799 Sample 8 SP ICP-AES 1.010 0.983 2.080 1.960 0.099 0.101 0.339 0.334 0.060 0.058 0.477 0.476 0.166 0.141 0.050 0.048 0.246 0.229 0.045 0.043 0.316 0.309 0.047 0.047 0.190 0.174 0.028 0.026 0.224 0.219 0.031 0.029 5.408 5.177 Sample 9 SP ICP-AES 0.037 0.965 0.282 0.652 0.064 0.265 0.065 0.021 0.080 0.01 5 0.079 0.014 0.089 0.012 0.068 0.010 0.039 0.970 0.280 0.652 0.065 0.308 0.067 0.020 0.09 1 0.012 0.083 0.015 0.065 0.013 0.072 0.009 SP ICP-AES 0.562 2.720 0.075 0.029 0.042 0.226 0.101 0.048 0.330 0.056 0.404 0.085 0.210 0.03 1 0.162 0.017 0.520 2.690 0.080 0.03 1 0.030 0.260 0.114 0.040 0.380 0.060 0.380 0.088 0.220 0.029 0.170 0.016 c 2.718 2.761 5.098 5.108 Sample Source Locality Ambroi I Ambroi I1 Ambroi 111 Excelsior Lesni Karolina 11-sano Vincen tka Richard Marien bad Marien b ad Marienbad Marien bad Marienbad Marienbad Dolni Kramolin LuhaEovice Lazne Kynivart SP ICP-AES 0.092 0.081 0.820 0.862 1.980 1.933 0.518 0.559 0.066 0.066 0.427 0.416 0.102 0.112 0.035 0.033 0.122 0.118 0.022 0.026 0.080 0.082 0.036 0.033 0.060 0.060 0.031 0.023 0.068 0.066 0.009 0.009 4.468 4.479 Original volume of sample/dm3 38.0 65.0 90.0 62.0 62.0 62.0 44.0 63.0 105.0 50.0 50.0 50.0 42.0 42.0 42.0 65.0 69.0 69.0 100.0 94.0 92.0 25.0 25.0 25.0 99.0 105.0 100.0 was checked by a recovery test and by comparison of the results obtained with ICP-AES with those obtained by spectro- photometry.The REE contents in the mineral waters deter- mined by the described procedure correspond approximately to their contents in the Earth’s crust but the light REE are less abundant in the waters analysed. The total REE contents in the analysed mineral water samples were between 2 x lo-’ and 3 x g dm-3. The results obtained were used for the characterization of the mineral water springs and the mutual correlations of the REE contents served as a basis for hydrogeo- chemical evaluation of water-rock interactions. 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 References Henderson P.Rare Earth Element Geochemistry Elsevier Amsterdam 1984. Smakhtin L. A. Mekhryusheva L. I. Filippova N. V. Miglina N. V. and Sinitsyna T. S. J. Radioanal. Nucl. Chem. 1991 154 293. Honda T. 01 T. Ossaka T. Nozaki T. and Kakihana H. J. Radioanal. Nucl. Chem. 1989 133 301. Honda T. 01 T. Ossaka T. Nozaki T. and Kakihana H. J. Radioanal. Nucl. Chem. 1989 134 13. Bacon R. and Ure A. M. Analyst 1984 109 1229. Thirlwall M. F. Chem. Geol. 1982 35 155. Roelandts I. At. Spectrosc. 1988 9 49. Nakamura Y. Murai Y. Ni D. and Liu Y. Bunseki Kagaku 1991 40 T125. Kuban V. Jancarova I. and Otruba V. Anal. Chim. Acta 1991 254 21. Efremova L. B. and Sorokina N. A. Zh. Anal. Khim. 1991 46 2259. Kawabata K.Kishi Y. Kawaguchi O. and Watanabe Y. Anal. Chem. 1991,63 2137. Moeller P. Dulski P. and Luck I. Spectrochim. Acta Part B 1992 47 1379. Shibata N. Fudagawa N. and Kubota M. Anal. Chem. 1991 63 636. Greaves M. J. Elderfield H. and Klinkhammer G. P. Anal. Chim. Acta 1989 218 265. Honda T. 01 T. Ossaka T. Nozaki T. and Kakihana H. J. Radioanal. Nucl. Chem. 1989 130 81. 01 T. Kikawada Y. Monda T. Ossaka T. and Kakihana H. J. Radioanal. Nucl. Chem. 1990 140 365. Roychowdhury P. Roy N. K. Das D. K. and Das A. K. Talanta 1989 36 1183. Kubova J. Hvoidara P. PlSko E. and PolakoviEova J. Geol. Carpathica 1988 39 569. Zachmann D. W. Anal. Chem. 1988,60 420. Strelow F. W. E. and Victor A. M. Talanta 1990 37 1155. Nevoral V. Coll. Czech. Chem. Commun. 1978 43 2274. Matherny M. and Macejko G. Fresenius’ J. Anal. Chem. 1991 340 178. Sanchez-Ocampo A. Lopez-Gonzalez H. and Jimenes-Reyes M. J. Radioanal. Nucl. Chem. 1991 154 435. Kopunec R. and Benitez J. C. J. Radioanal. Nucl. Chem. 1991 150 269. Haraguchi K. Yamazaki Y. Saitoh T. Kamidate T. and Watanabe H. Anal. Sci. 1990 6 877. Shabani M. B. and Masuda A. Anal. Chem. 1991 63 2099. PlSko E. The Proceedings of Analytiktrefen 1986 Wiss. Beitrage K. Marx Univ. Leipzig 1987 vol. 11 pp. 277-292. Paper 3/04907H Received August 13 1993 Accepted November 1 1993 * SP = spectrophotometric determination.

ISSN:0267-9477    

DOI:10.1039/JA9940900241

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 245 Analysis of Some Low Silicon Content Alloys by Inductively Coupled Plasma Atomic Emission Spectrometry* I. HlavaCek and 1. HlavaCkova Analytika Company Limited U Elektty 650 194 05 Prague 9 Czech Republic Inductively coupled plasma atomic emission spectrometry procedures have been developed for the analysis of some low silicon content alloys such as low-alloy steels nickel metal FeMo high-alloy steel cobalt alloy FeNb and FeV. In most cases the special emphasis has been placed on the determination of silicon together with the other analyte elements. Keywords lnductively coupled plasma atomic emission spectrometry; multi-element analysis; silicon; alloys; sample decomposition; phosphoric acid For the determination of low silicon contents of up to about 0.3% Si (sample mass of 0.500 g) it is possible to replace molecular absorption spectrometry (MAS) originally used i.e.UV/VIS spectroscopy with inductively coupled plasma atomic emission spectrometry (ICP-AES) or flame atomic absorption spectrometry (FAAS) for some metallic materials. The sample decomposition procedure (sample dissolution with hydro- chloric acid nitric acid or their mixture) is sufficient for the dissolution of silicon and some other elements. The dissolution conditions i.e. heating at a temperature of about 60°C does not result in the exclusion of silicon from the sample solution. In this way it is possible to dissolve metallic samples with matrices containing largely iron nickel chromium or manga- nese. The silicon determination in FeMo however results in incorrect values of silicon content because molybdenum sili- cides such as MoSi MoSi and possibly Mo,Si are not decomposed with hydrochloric and nitric acids.It was found that sufficient decomposition of molybdenum silicides is ensured by dissolving them with a mixture of sulfuric phos- phoric and if necessary nitric acids. The sample solution must be evaporated until white fumes of sulfur trioxide appear. Therefore ferroalloys containing low silicon content ( FeV and FeNb) Co alloys (Real) and steel Poldi AKRB samples were also decomposed with a mixture of sulfuric and phosphoric acids in a study of the hardly soluble components. Experimental Instrumentation An ARL 33000 LA sequential atomic emission spectrometer with an inductively coupled argon plasma was used for the determination of analyte elements under compromise instru- mental conditions i.e.at an observation height of 18 mm and an argon carrier gas flow rate of about 1.2 1 min-l. A Perkin- Elmer 503 flame atomic absorption spectrometer was used for comparative analysis by FAAS. Sample Preparation The average chemical composition of some alloys containing low and medium silicon contents is presented in Table 1. Table 2 summarizes the sample dissolution procedures. Essentially the alloy samples in chip (alloys and steels) or the fine powder form (ferroalloys especially the FeNb particle sizes should be less than 0.05mm without separating the various size fractions) are dissolved with phosphoric and sulfuric acids (except for low-alloy steels and nickel metal) in polytetrafluoroethylene (PTFE) vessels at a temperature of between 150 and 200°C without the addition of hydrofluoric acid.The decomposition time has to be prolonged for about 10-15 min after the sample dissolution. In some instances it is appropriate to start the sample decomposition with hydro- chloric and nitric acids. The low-alloy steel and nickel metal samples are dissolved only with hydrochloric and nitric acids in glass beakers. In this way the alloy samples are quantitatively converted into a soluble form. If the FeNb samples are analysed the sample solutions have to be stabilized with the addition of oxalic acid. In the analysis of FeV it was found that the aluminium content values obtained are lower.Therefore the residual aluminium content was determined by FAAS after fusing insoluble residues with potassium disulfate see Table2. The sample decomposition of some alloys was also performed in a microwave oven using practically the same procedures as in PTFE beakers under atmospheric pressure. The decomposition time was reduced from 2-3 h to about 10-15 min for all the samples analysed. Blank and synthetic samples were used for calibration and were prepared by the same procedures as the real alloy samples. The matrix element composition was simulated using high- purity metals. The synthetic calibration samples also contained known additions of the analyte elements in the concentration range to be considered. For practical purposes the matrix composition of the calibration samples can also be simulated using suitable certified or internal reference materials with known additions of some analyte elements.Table 1 Chemical composition of some low and medium silicon content alloys (YO) Alloy Low-alloy steel Ni metal FeMo High-alloy steel AKRB Cobalt alloy Real FeNb FeV t Some cobalt alloy Real types also contain Mo and Ni. * Presented at the XXVIII Colloquium Spectroscopium Internationale (CSI) York UK June 29-July 4 1993.246 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 2 Sample preparation Sample Evaporation Final Alloy mass/g Dissolution to white fames volume/ml Low-alloy 1 .ooo 10 ml HN03 conc. - 100 1 .ooo 10 ml HNO conc. - 100 steel Ni metal FeMo 0.500 10 ml HNO conc. 10 ml H2S04 (1 + 1) 250 High-all0 y 0.500 10ml HN03 ( l + l ) 10 ml H,SO (1 + 1) 250 Co-alloy 0.500 20 ml HCl (1 + 1) 10 ml HNO conc. 250 10 ml HCl (1 + 1) 10 ml HCl (1 + 1) 10mlHC1(1+1) 25 ml H,PO conc.steel AKRB 10ml HC1 (1+1) 25 ml H,PO conc. Real 10 ml HzS04 (1 + 1) 25 ml H3P0 conc. 25 ml H3P0 conc. 25 ml H3P04 conc. 2 g NH,OII*HCl FeNb* 0.200 - 10 ml H,SO (1 + 1) 250* FeV7 0.500 10 ml HC1 conc. 10 ml H2S0 (1 + 1) 2507 5ml HNO (l-kl) * FeNb sample solution is stabilized with addition of 2.5 g of oxalic acid (COOH),*2H20. -f Aluminium contained in FeV sample was not quantitatively converted into a solution by the described procedure. Insoluble residues were analysed after filtration of a sample solution washed with warm HC1 (1 -t 9) and water ashed and fused with 2 g of potassium disulfate in a quartz crucible.After dissolution of the melt in water and dilution to 100 ml with water in a calibrated flask the determination of aluminium was performed by flame atomic absorption spectrometry using the spectral line 309.3 nm in the fuel rich N,0-C2H2 flame. The total aluminium content in FeV is given by the sum of aluminium contents found by ICP-AES and FAAS. Results and Discussion The analyte elements were determined using the spectral lines recommended by the manufacturer see Table 3 except for cobalt tantalum titanium and tungsten. The original Co I1 238.892 nm line was used only for the analysis of nickel metal where a low iron content was found because the line was severely affected by the nearby Fe I1 238.863 nm line. In the Table 3 Analyte elements and analytical spectral lines for ICP-AES Element and line A1 I B I Cd I1 c o I1 Cr I c u I Fe I1 Mn I1 Mo I Ni I1 Si I Ta Ti I v I1 W I Zn I1 Wavelength/nm 394.401 249.678 226.502 238.892 360.533 324.754 259.940 257.610 317.035 23 1.604 251.611 265.327 363.546 311.071 400.875 202.548 Secondary slit- widt h/pm 75 50 75 75 75 75 75 75 75 75 75 75 75 75 50 75 Table 4 Interferences from added elements at a concentration of 1 % for different Ni lines (nm) Interfering element A1 c o Cr c u Fe Mn Mo Si Ti v W Background equivalent concentration (YO Ni) Ni TI 221.647 nm Ni I 232.003 nm 0.0018 0.005 < 0.005 0.0004 0.047 < 0.0005 - - < 0.0005 0.00007 0.0009 o.Ooo1 0.00016 0.001 1 0.0002 0.0004 0.007 0.0002 0.044 o.Ooo1 0.0002 - - 0.0002 0.0085 0.0013 0.0004 Ni I1 231.604 nm < 0.00005 - - - 0.00005 - Table 5 Interferences from added elements at a concentration of 1% for different tungsten lines (nm) Background equivalent concentration (YO W) Interfering element A1 c o Cr cu Fe Mn Mo Ni Ti v w I1 207.91 1 0.0017 0.00018 0.00027 0.0012 0.00012 0.00014 0.00075 0.00012 0.0003 0.00 12 w I1 w I1 w I1 W I 224.875 218.936 239.709 400.875 - < 0.0002 - 0.0002 - 0.0001 - < 0.001 - - - - - - - - 0.0037 0.0012 0.0042 0.00025 - - - 0.0003 - 0.001 - 0.00005 - - - - - 0.09 - - - - - 0.0004 Table 6 Interferences from added elements at a concentration of YO Interfering element c o Cr c u Fe Mo Nb Ni Ti V W Background equivalent concentration (%) < 0.005 Ni co 0.0001 3 0.008 Zn 0.01 Zn 0.0005 Cd 0.03 c o 0.005 Cr < 0.0005 Si 0.00 15 Ta 0.0004 Zn 0.0004 c o 0.0075 Si < O.oO15 V 0.0007 A1 0.0055 Cu 0.0005 Si 0.0005 Ta 0.007 Ti 0.001 A1 0.0005 co 0.001 Ti 0.01 1 v 0.0003 Cr 0.00014 co 0.0016 Si Note Co I 350.228 nm Co I1 238.892 nm Ta I1 240.063 nm Co I 350.228 nm Ta I1 240.063 nm Ti I 363.546 nm Co I 350.228 nm Ti I1 323.452 nm Co I 350.228 nmJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 247 other instances the determination of cobalt was performed using the Co I 350.228 nm line at an observation height of 26 mm. Similarly the determination of tantalum in FeNb was performed using the Ta I1 240.063 nm line at an observation height of 18 mm instead of the Ta 265.327 nm line of low intensity which is severely influenced by the presence of chromium. For the determination of titanium in steel Poldi AKRB the Ti I1 323.452 nm line at an observation height of 18 mm was selected instead of the Ti I 363.546 nm line which is affected by the presence of molybdenum.For the determi- nation of nickel and tungsten in the cobalt alloy Real more suitable spectral lines were investigated. Besides the original Ni I1 231.604 nm line the Ni I1 221.647 and Ni I 232.003 nm lines were examined. For comparison the spectral interferences found at the optimal observation height of 18 mm are presented in Table 4. Finally the determination of nickel was performed using the Ni I1 231.604nm line because it was the most significant spectral line. The determination of tungsten was originally performed using the W I 400.875 nm line of very low intensity.Therefore the W I1 207.911 W I1 224.875 W I1 218.936 and W I1 239.709 nm spectral lines were studied. The Table 7 Comparison of results for silicon content (YO) in real and certified reference materials (CRMs) low-alloy steels Sample 871 -CRM BCS-CRM 149 BCS-CRM 270 R 42616 R 42716 R 42816 R 42916 R 43016 R 43116 Certificate value MAS* 0.0095 0.013 0.0025 < 0.002 0.050S 0.05 5 0.052 - 0.043 0.053 - 0.057 - 0.045 - 0.046 - - ICP-AES 0.0105 0.001 5 0.056 0.056 0.051 0.057 0.064 0.05 1 0.053 * MAS Molecular absorption spectrometry. t Correlation coefficient for determination of silicon (MAS and ICP- AES) was found to be 0.993. Table 8 Comparison of results (%) for CRMs of nickel metal W I1 207.911 nm line appeared to be the most advantageous spectral line compared with the original line at an observation height of 18mm.The most important interferences from accompanying elements found for tungsten are shown in Table 5. The spectral interferences of the matrix and the other accompanying elements of the spectral line intensities of the analyte elements are reported in Table 6. The contingent interferences were corrected for the increase or decrease in the spectral background due to differences in the sample matrix composition if necessary. The proposed procedures were verified by means of Czechoslovakian British and German certified reference materials. For comparison and verification of accuracy the samples were also analysed by other analytical methods such as gravimetry titrimetry MAS and FAAS. The following tables present results obtained for some metallic materials analysed by ICP-AES.Comparison of the ICP-AES and MAS results for the low silicon contents in low-alloy steels is given in Table 7. Table 8 presents the analytical results of ICP-AES and FAAS and certified values for nickel metal. Similarly the analytical results obtained by ICP-AES FAAS and other analytical methods are shown in Tables 9-13 for ferromolyb- denum high-alloy steel Poldi AKRB cobalt alloy Real FeNb and FeV respectively. The sample decomposition of FeNb with sulfuric and phos- phoric acids was also used for the determination of nitrogen by titrimetry after distillation. The results for the nitrogen contents in FeNb are compared with those found by vacuum extraction (analyser Balzers EAN 202) are given in Table 14.The precision of the determination of analyte elements characterized by means of the basic statistical data is presented in Tables 15-17 for FeMo FeNb and FeV respectively. The limits of determination defined as ten times the standard deviation of the background noise were mostly within the range 0.002-0.02% for a sample mass of 0.500 g in 250 ml of solution i.e. within the range 40-400 ng ml-I for the analyte elements. The limit of determination achieved also depends on the sample matrix composition the other accompanying elements and their mutual spectral influence. Sample Method Cd c o Cr cu Fe Mn Si Zn &AN-CRM Certificate 0.0003 0.078 - 0.01 1 0.0 16 0.105 0.035 0.0057 5- 15-01 5 FAAS 0.0004 0.064 < 0.005 0.01 1 0.0 15 0.106 - 0.005 ICP-AES 0.001 0.064 < 0.005 0.012 0.014 0.109 0.035 0.006 &AN-CRM Certificate 0.002 0.21 0.079 0.101 0.0138 0.163 0.0 19 5-51-016 FAAS 0.0017 0.193 < 0.005 0.083 0.108 0.014 - - - ICP-AES 0.00 15 0.195 0.005 0.084 0.107 0.014 0.155 0.0195 Table 9 Comparison of results (YO) for real and CRM samples of ferromolybdenum Sample 02 03 41 42 43 90 & AN-CRM 4-4-02 21 1J16A Method FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others Certificate FAAS Certificate ICP-AES ICP- AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP- AES A1 - < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 - - - - - - - < 0.02 < 0.02 - Cr < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 < 0.02 0.02 < 0.02 0.02 0.07 0.04 0.02 < 0.02 < 0.02 - - c u 0.24 0.26 0.49 0.53 0.18 0.185 0.15 0.15 0.20 0.205 0.36 0.37 0.52 0.495 0.50 0.192 0.02 Mn 0.01 < 0.01 0.01 <0.01 0.01 < 0.01 0.01 <0.01 0.01 < 0.01 0.08 0.08 < 0.01 < 0.01 < 0.01 - 0.004 Ni 0.04 0.05 0.04 0.05 0.07 0.06 0.05 0.05 0.05 0.06 0.05 0.05 0.045 0.04 0.02 - - Si 0.03* 0.25 0.04* 0.03 0.03* 0.03 0.02* 0.035 0.04* 0.03 0.60* 0.63 0.091 0.10 0.095 0.226 0.22 v < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 <0.01 <0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.01 - - * Gravimetry.248 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 10 Comparison of results (%) for real samples of high-alloy steel AKRB. Correlation coefficients for determination of manganese molybdenum silicon titanium and tungsten (chemical methods and ICP-ALES) were found to be 0.999 0.999 0.999 0.998 and 0.999 respectively c o Cr c u Mn Mo Si Ti v W Sample Method A1 B* B7 1608 P1 Chemical - - - - 12.691 - 0.271 0.5155 0.109 0.5259 - 1.9755 - - 0.52 0.003 - FAAS 2.01 - - 0.00 1 0.039 0.235 0.55 ICP-AES 2.00 0.065 0.063 0.002 12.53 0.037 0.245 0.52 0.08 0.535 0.002 1.955 1608 P2 Chemical - FAAS 2.35 ICP-AES 2.31 0.05 0.55 0.002 13.04 0.087 0.26 0.76 0.24 0.735 0.003 2.22 1608 P3 Chemical - - - - 13.741 - 0.441 1.0135 0.41 0.899 - 2.619 0.005 - ICP-AES 2.61 0.085 0.090 0.0025 13.66 0.037 0.42 1.04 0.385 0.90 0.007 2.62 1608 P4 Chemical - - - - 14.825 - 0.581 1.X$ 0.5% 1.0% - 2.999 - - 0.006 - FAAS ICP-AES 2.87 0.10 0.095 0.0025 14.47 0.038 0.56 1.295 0.46 1.09 0.005 3.03 1608 P5 Chemical - - 0.006 - FAAS ICP-AES 3.18 0.12 0.113 0.005 15.22 0.041 0.66 1.56 0.61 1.195 0.006 3.41 3102 X1 Chemical - FAAS - - - 13.295 - 0.301 0.755 0.269 0.735 - 2.21* - 0.090 0.255 0.79 0.72 0.004 - - 0.0015 - - - - - FAAS - - 0.0015 - 0.040 0.405 - - - 0.002 - 0.042 0.54 - - - - - 15.841 - 0.68j 1.561 0.64 1.174 - 3.329 - - - 14.401 - 0.311 1.19$ 0,199 0.8oEj - 2.879 - - I 0.002 - 0.043 0.64 - - - 0.031 0.28 - - - 0.003 0.002 - - - - ICP-AES 2.71 0.02 0.018 0.002 14.10 0.030 0.29 1.22 0.18 0.805 0.003 2.95 3102 X Chemical - - - - 15.121 - 0.325 122$ 0.229 0.995 - 2.8% - 0.004 - ICP-AES 3.16 0.09 0.095 0.003 14.76 0.030 0.29 1.20 0.21 1.04 0.004 2.88 - - - - 0.0015 - 0.032 0.28 - FAAS * Boron content determined directly without separation.7 Boron content determined after hydroxide separation. $ Titrimetry. 9 MAS. Table 11 Comparison of results (YO) for real samples of cobalt alloy Reail ~~ Method FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others ICP-AESl I C P - A E S ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES A1 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 c u 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Fe 2.22* 2.13 3.04* 2.85 2.18* 2.00 2.22* 2.07 2.10* 1.98 0.33* 0.32 0.38* 0.37 0.15* 0.14 0.18* 0.18 Mn 0.207 0.175 0.20t 0.17 0.21t 0.175 0.197 0.15 0.207 0.16 0.497 0.50 0.507 0.50 0.527 0.54 0.50t 0.485 Mo Ni Si 1.939 1.925 1.94 1.85 1.924 1.925 1.899 1.87 1.9% 1.91 0.97 1.169 1.17 1.279 1.26 1.125 1.16 1.025 v 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 w 4.85* 4.90 4.72* 4.77 4.60* 4.71 4.79* 4.88 4.68* 4.81 4.35" 4.33 4.78* 4.65 4.35* 4.35 4.54* 4.63 Sample 1638 P 1640 P 1641 P 1642 P 1643 P - 0.015* 0.02 0.70* 0.70 0.025 0.74" 0.73 - 1734 P1 1734 P - 0.05 2.19 2.20 0.075 1.89 1.93 - 1735 P1 1735 P * MAS. j.Titrimetry. 8 Gravimetry. j Sample decomposition for ICP-AES was performed by means of a microwave oven. Table 12 Comparison of results (%) for real and CRM samples of ferroniobium Sample 5 S Method FAAS and others FAAS and others FAAS and others Certificate value ICP-AES ICP-AES ICP-AES A1 Cr 0.04 0.04 0.01 < 0.02 0.14 0.14 c u 0.02 0.02 0.02 0.03 0.0 1 < 0.02 Mn 1.71 1.67 5.46 5.20 0.27 0.34 Mo 0.03 0.03 0.02 < 0.02 0.02 0.02 Ni 0.06 0.04 0.02 < 0.02 0.02 < 0.02 Si Ta Ti 4.98 0.23 5.02 0.25 0.40 0.63 0.32 0.63 1.047 4.3 1.03 4.76 4.9 0.47 v 0.05 0.04 0.03 0.025 0.06 0.085 - W - 0.96 1.59 - - 0.285 0.97 10.77* 10.6 0.72 - - 0.24 < 0.02 NHKG 1 .oo 1.70 < 0.02 0.24 BCS-CRM 362 FAAS and others Certificate value Certificate value ICP-AES ICP-AES ICP-AES 0.195 0.195 0.01 < 0.02 1.88 1.81 0.01 < 0.02 0.01 < 0.02 - 0.045 0.055 - 0.73" 0.73 1.79 1.86 1.03 1.055 4.757 0.44 4.76 0.465 0.306 1.32 0.30 1.12 3.85 0.567 3.29 0.515 0.04 0.045 0.075 0.355 - - - 1.605 2.53 2.42 1.86 1.765 0.24 - CRM 576-1 0.20 0.19 - 0.055 0.055 - 0.275 1.45 - CRM 579-1 * Titrimetry (after separation and hydrolysis of K,SiF,).t MAS (with Malachite Green).249 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 13 Comparison of results (YO) for real and CRM samples of FeV Sample 58 D 4087 D 4088 D 4089 D 4130 D 4131 D 4132 D 4133 BCS-CRM 20512 BAM-CRM 531-1 Method FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others FAAS and others Certificate value FAAS Certificate value ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES A1 total 1.80 1.77 1.76 1.70 1.88 1.85 1.66 1.58 1.75 1.74 1.80 1.70 1.10 0.97 1.60 1.69 2.0* 1.95 1.92 1.59 1.61 Cr 0.15 0.11 0.08 0.08 0.135 0.14 0.10 0.10 0.12 0.12 0.10 0.11 0.15 0.14 0.11 0.11 0.19 0.205 0.365 c u 0.01 0.01 0.025 0.025 0.025 0.025 0.025 0.025 0.03 0.03 0.05 0.05 0.04 0.04 0.12 0.12 0.247 0.258 0.019 0.023 Mn 0.18 0.17 0.18 0.175 0.17 0.17 0.16 0.16 0.16 0.15 0.16 0.16 0.20 0.19 0.17 0.17 0.25 0.255 0.15 0.14 Ni 0.06 0.05 0.065 0.075 0.10 0.105 0.07 0.075 0.05 0.06 0.05 0.05 0.04 0.05 0.11 0.1 1 < 0.02 0.045 - 0.02 Si 0.70t 0.66 - 0.71 - 0.76 - 0.73 0.72t 0.72 0.72t 0.74 0.83t 0.82 0.70T 0.62 1.02* 1.03 1.005 0.69 0.685 * Information value not certified. t Gravimetry.Table 14 Comparison of results for nitrogen content (%) in real samples of FeNb Vacuum extraction Sample (Balzers Ni-bath) Distillation* S 0.010 0.006 0.01 1 45 0.163 0.168 0.162 59 0.112 0.106 0.112 72 0.123 0.123 0.118 * Parallel determinations. Table 15 Precision of determination of analyte elements (%) in FeMo eSAN-CRM 4-4-02 samples Parameter A1 Cr c u Mn Ni Si v n* 10 8 15 13 13 16 8 Average < 0.02 < 0.02 0.50 < 0.01 0.04 0.095 < 0.01 SDt - - 0.012 - 0.004 0.012 - - - 2.4 - 10.0 12.6 - RSDS * n =Number of analyses.t SD = Standard deviation. $ RSD = Relative standard deviation. Table 16 Precision of determination of analyte elements (YO) in FeNb BCS-CRM 362 samples Parameter A1 Cr cu Mn Mo Ni Si Ta Ti v W n 5 5 5 4 4 4 6 6 6 4 4 Average 1.605 0.195 ~ 0 . 0 2 1.81 <0.02 <0.02 0.73 4.76 0.465 0.045 0.24 SD 0.026 0.021 RSD 1.6 10.8 - 4.4 - - 4.1 1.9 5.3 24.4 9.2 - 0.080 - - 0.030 0.089 0.025 0.011 0.022250 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 17 Precision of determination of analyte elements (%) in real and CRM samples of FeV Sample Parameter A1 total Cr c u Ian Ni Si FeV 80 n Average SD RSD BAM-CRM n 531-1 Average SD RSD 33 2.715 0.069 2.5 1.61 0.038 2.4 25 33 0.19 0.012 6.3 0.365 0.013 3.6 24 34 0.0065 0.001 1 16.9 25 0.023 0.0024 10.4 3 4. 0.21 0.005 2.2 0.14 0.004 2.7 2 5 33 0.075 0.005 6.5 0.02 0.003 25 13.7 33 0.68 5 0.017 2.5 0.685 0.019 2.8 24 Conclusion It was found that the described ICP-AES procedures can be used successfully in metallurgical laboratories for the multi- element analysis of some low silicon content alloys. Silicon can be determined together with the other analyte elements without the application of hydrofluoric acid. In some instances the microwave oven was used to reduce the digestion time. The described ICP-AES procedures have been used for the multi-element analysis of high silicon content ferroalloys in everyday laboratory practice for the past years. Paper 3/03933A Received July 7 1993 Accepted October 12 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900245

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 25 1 Multi-element Analysis of Some High Silicon Content Ferroalloys by Inductively Coupled Plasma Atomic Emission Spectrometry* 1. HlavaCkova and I. HlavaCek Analytika Company Limited U Elekfry 650 194 05 Prague 9 Czech Republic ~ ~~ ~ ~~ Procedures using inductively coupled plasma atomic emission spectrometry (ICP-AES) have been developed for both the determination of matrix and minor and trace elements in some FeSi alloys such as FeSi 45 FeSi 75 FeCrSi FeMnSi FeMnCaSi FeTiSi and FeZrSi. Keywords Inductively coupled plasma atomic emission spectrometry; multi-element analysis; ferroalloys; sample decomposition; silicon; microwave digestion In the steel producing industry ferroalloys with high silicon content are used for de-oxidation and final modification of chemical composition of steels before casting.Sample decomposition and the quantitative sample conver- sion into a soluble form are relatively difficult. The FeSi alloys are usually decomposed by fusing with sodium peroxide or by dissolution with hydrofluoric acid. For FeSi alloys analysis using inductively coupled plasma atomic emission spec- trometry (ICP-AES) the solid samples were dissolved using phosphoric acid. Experimental Instrumentation The analyte elements were determined by means of an ARL 33000 LA sequential emission spectrometer with an inductively coupled argon plasma under compromise instrumental con- ditions i.e. at an observation height of 18 mm and an argon carrier gas flow rate of about 1.2 1 min-'. A Perkin-Elmer 593 flame atomic absorption spectrometer was used for compara- tive analysis by flame atomic absorption spectrometry.A commercial domestic microwave oven Philips M 704 with a timer and variable power settings equivalent to outputs of 210 330 450 and 600 W was used. Sample Preparation It was found that phosphoric acid is suitable for the decom- position of carbides nitrides and silicides contained in steels. Gelatinous silicic acid is converted into a solution with phos- phoric acid which also attacks so-called 'metallic' silicon. It was observed that the dissolution of silicic acid gel is relatively faster (about 15 min) than that of 'metallic' silicon. The process rate can be substantially increased if the 'metallic' silicon particle size is about 10 pm. The conversion of silicon into a soluble form with phosphoric acid is very important in the sample preparation of complex high silicon content ferroalloys. In addition silicon can be determined together with the other analyte elements. The general working procedure with phos- phoric acid for the decomposition of metallic samples can be described as shown in Scheme 1.The systems Fe-Si Cr-Si Mn-Si Ti-Si and Zr-Si represent various silicides occurring in high silicon content ferroalloys. So-called 'metallic' silicon is found in alloys containing more than 50% Si. It is similar in the ferrosilicon alloys see Table 2. The metal silicides are relatively soluble in phosphoric acid. The presence of 'metallic' silicon in some FeSi alloys results in a slowing of sample decomposition because sample particles can be coated with insoluble reaction products if the decompo- sition takes too long.Therefore in such cases the sample decomposition was started with sodium hydroxide solution. In principle the FeSi alloy samples in the fine powder form (the sample particle sizes should be less than 0.05 mm without separating the various size fractions) are digested with phos- phoric acid in polytetrafluoroethylene (PTFE) vessels at a temperature of between 150 and 220 "C without the application of hydrofluoric acid. The decomposition time has to be pro- longed for about 10-15 min after sample dissolution. In some instances it is necessary to carry out sample decomposition with sodium hydroxide solution as in the case of FeSi FeCrSi and FeZrSi alloys. After sample dissolution the ferrosilicon alloy sample solutions are oxidized and stabilized with nitric acid or hydrogen peroxide and immediately diluted with de-ionized water to a final volume of 500ml.The sample dissolution procedures should be strictly adhered to see Table 3. In this way the ferrosilicon alloy samples are quanti- tatively converted into a soluble form. Sample decomposition was also carried out in a microwave oven using the same procedures as in the PTFE beakers under atmospheric press- ure. The decomposition time was reduced from 2-3 h to about 10-15 min for all ferrosilicon alloy samples see Fig. 1 and Table 4. A blank and calibration samples were prepared by the same procedures as for the real ferrosilicon alloy samples using certified and internal ferrosilicon alloy reference materials with 150-200 "C I I t not diluted immediately a gel can arise in about Sample solution (syrup) contains a mixture of H3P04 and H4P207 (so-called 'condensed' or 'strong' phosphoric acid) after evaporation I to white fumes of sulfur frioxide to white fumes of sulfur trioxide again Clear and stable sample solution is obtained if it is diluted with water immediately after cooling Hydrated silicon dioxide Si02*x H20 is usually excluded from sample so- lution after dilution * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) York UK June 29-July 4 1993.Scheme 1252 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 200 9 h' 100 0 5 10 Time/min Fig. 1 Dependence of temperature (7) in a sample solution on heating time in the microwave oven Philips M704 at power outputs of A 600; B 450; C 330 and D 210 W.Measured solution 50ml of phosphoric acid (85% m/m) in a PTFE beaker known additions of some analyte elements in the concentration range to be considered see Table 5. Results and Discussion With regard to the possible presence of non-metallic compo- nents (especially SiO and A1,03 present in slags) in the ferrosilicon alloy samples it was necessary to analyse contin- gent insoluble residues. The residues were analysed after separ- ation by membrane filtration and fusion with sodium carbonate in a platinum crucible. The aluminium and silicon contents determined in the residues were practically negligible (< 0.02% A1 and <0.05% Si).The analyte elements and the spectral lines used are pre- Table 1 Chemical composition of ferrosilicon alloys (%) Alloy FeSi 45 FeSi 75 FeCrSi FeMnSi FeMnCaSi FeTiSi FeZrSi Fe Si Ca Cr 40 40-48 20 72-78 45-55 28-32 15-30 40-50 15-30 15-35 25-50 Mn Ti Zr 60-70 15-30 20-30 30-40 Table 3 Sample preparation sented in Table 6. The spectral interferences of the matrix and the other accompanying elements on the spectral line intensities of the analyte elements were investigated. The more significant effects were found only for iron on chromium manganese on nickel and zirconium on hafnium i.e. the interfering element at a concentration of 1% gives a background equivalent concentration (the concentrations are related to solid samples) of as follows 1% Fe ...0.0005% Cr 1% Mn ... 0.0002% Ni 1% Zr ... 0.0007% Hf. In practice the interferences were corrected if necessary for the increase or decrease in the spectral background due to differences in the sample matrix composition. Special emphasis was placed on the reliability of high matrix element content determination. For comparison and verifi- cation of accuracy the ferrosilicon alloy samples were also analysed by other analytical methods such as gravimetry for Si and Zr (with cupferron) titrimetry for Cr (with potassium permanganate after oxidation with potassium peroxodisulfate in the presence of silver nitrate) Mn (with potassium permanga- nate after separation with zinc oxide) Ca and Fe (with complexone) and Ti [with iron(II1) chloride]. The high matrix element content determination was also examined by means of internal standardization (Ru) for the purpose of precision enhancement.The contribution of internal standardization however was insignificant. Tables 7-11 give a comparison of the ICP-AES results to those of other analytical methods for the ferrosilicon alloy matrix elements. Similarly the results of ICP-AES and other analytical methods such as gravimetry titrimetry molecular absorption spectrometry (UV/VIS spec- troscopy) neutron activation analysis and flame atomic absorption spectrometry (FAAS) are shown in Tables 12 and 13 for both high and low element contents. The ICP-AES analytical procedures were also verified by means of Czechoslovakian and British ferrosilicon alloy certified and internal reference materials (measured against synthetic calibration samples) see Table 5.The precision of determination of analyte elements characterized by the basic statistical data is presented in Tables 14 and 15 for FeSi 45 Table 2 Specification of silicides occurring in ferrosilicon alloys Ferrosilicon alloy Silicide FeSi FeCrSi CrSi CrSi FeMnSi Mn,Si MnSi Mn,Si3 FeTiSi Ti2% TiSi FeZrSi ZrSi Fe3Si FeSi Fe& 'metallic Si' Alloy FeSi 45 FeSi 75 FeCrSi FeMnSi* FeMnCaSi FeTiSi FeZrSi Sample mass/g Pre-decomposition Dissolution 0.300 - 50 ml H3PO4 conc. 0.100 10 ml10Y0 (m/m) NaOH 10 ml HNO conc. 50 ml H3PO4 conc. 0.300 5 ml 10% (m/m) NaOH 50 ml H3PO4 conc. 0.300 - 50 ml H3PO4 conc. 0.300 50 ml H3P04 conc. 0.200 5 ml 10% (m/m) NaOH 10 ml HNO conc. 50 ml H3P04 conc. Oxidation and stabilization 10 ml HNO conc.10 ml HNO conc. 2mlH,02(1+1)or 10 ml HNO conc. +0.2 g NaNO 5ml H,02 (1+1) * Sample preparation the same for FeMnSi and FeMnCaSi.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 253 Table 4 Comparison of the decomposition times for some alloy types Solution Sample type temperaturePC FeCrSi FeMnSi 200-230 FeTiSi A1 150-170 FeTi Ti-alloys 150-170 Ni-steel Co-alloy Real 170-200 ‘Hard’ FeCr (8%C) 200-230 A1 alloys (< 10% Si) 170-200 Decomposition time/min Electrical Microwave heating-plate oven* 120-180 12-15 60- 120 10-12 45-60 10-12 120 10 120-1 80 12-15 60-90 10 * Power output of the microwave oven was 330 W Table 5 Specification of used reference materials Table 7 Comparison of results for silicon content (%) in real and CRM samples of FeSi 75 Si Si ICP-AES ICP-AES Si (Si Ru ratio)? (without Ru) Sample gravimetry* (internal standard) (directly)* C 5619 74.70 74.45 74.60 C 5620 75.20 76.15 75.15 C 5621 76.40 75.80 75.95 77.45 77.35 77.40 C 5624 C 5626 67.70 69.05 69.10 C 5629 76.00 76.35 76.30 C 5630 74.45 76.25 75.90 C 5631 72.85 73.10 72.95 4-1-01 76.98$ 76.80 77.10 BCS-CRM 305 76.00$ 76.10 75.80 CSAN-CRM ~ ~~ Ferrosilicon alloy Reference material FeSi 45 ~SAN-CRM* 4-1-02 FeSi 75 FeCrSi GSAN-CRM 4-5-03 FeMnSi CSAN-CRM 4-5-02 TSAN-CRM 4-1-01 BCS-CRMt 305 FeTiSi Internal RM FeZrSi Internal RM 1st I * Correlation coefficient for gravimetric and ICP-AES (Si measured t Correlation coefficient for gravimetric and ICP-AES (Si Ru ratio) 1 Certified value.directly without Ru) methods was found to be 0.980.methods was found to be 0.967. * CSAN Czechoslovakian Analytical Normal. t BCS British Chemical Standard. Table 6 Analyte elements and analytical spectral lines Table9 Comparison of results for manganese content (YO) in real samples of FeMnSi; correlation coefficient for determination of manga- nese (titrimetry and TCP-AES) was found to be 0.982 Element and line A1 I Ca I Cr I c u I Fe I1 Hf I1 Mn I1 Ni I1 Ru* I Si I Ti I1 Zr TI Wavelength/nm 394.401 422.673 360.53 3 324.754 259.940 264.14 1 257.610 231.604 349.894 251.61 1 323.452 343.823 Secondary slit-width/pm 75 75 75 75 75 75 75 75 75 75 75 75 Sample C 5611 C 5614 C 5617 C 5658 C 5693 C 5694 C 5696 C 6004 C 6005 C 6010 C 6020 Mn titrimetry 73.55 73.20 67.65 68.50 75.65 74.10 73.80 72.80 69.35 68.35 73.50 Mn 73.10 73.65 68.10 68.65 75.80 74.45 73.25 72.75 70.65 69.10 72.90 ICP-AES * Measured on reference photomultiplier (internal standardization). Table8 Comparison of results for silicon and zirconium contents (YO) in real samples of FeZrSi; correlation coefficient for determination of silicon (gravimetry and ICP-AES) was found to be 0.989 and for determination of zirconium (gravimetry and ICP-AES after decomposition of FeZrSi samples with H,PO,) was found to be 0.989 Sample C 5566 c 5574 C 5576 C 5589 C 5591 C 5603 C 5604 C 5605 C 5636 1st I1 F 39 Si gravimetry 47.90 52.40 48.10 5 1 .OO 55.00 56.15 54.50 52.25 43.40 51.20 43.90 Si 48.60 53.60 48.30 52.30 54.60 56.10 54.90 52.50 44.3 5 52.50 43.70 ICP-AES Zr gravimetry 29.15 29.50 37.70 33.45 32.5 5 35.45 36.40 39.55 34.90 35.30 23.70t Zr ICP-AES (H3PO4) 29.10 29.00 37.30 32.70 32.00 35.70 36.20 39.80 34.90 35.60 25.60 Zr ICP-AES (HF)* 29.20 29.45 37.15 33.00 35.20 36.30 39.25 - * Determination of zirconium was performed after decomposition of FeZrSi samples with HF.Determination of zirconium was performed by molecular absorption spectrophotometry with Xylenol Orange after hydroxide separation.254 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 10 Comparison of results for chromium content (YO) in real samples of FeCrSi; correlation coefficient for determination of chromium (titrimetry and ICP-AES) was found to be 0.925 Sample Cr titrimetry Cr ICP-AES Sample Cr titrimetry Cr ICP-AES Sample Cr titrimetry Cr ICP-AES C 5590 C 5609 C 5622 C 5623 C 5627 C 5628 C 5634 C 5635 3 1.90 3 1.90 30.75 31.55 29.20 31.20 29.00 29.60 32.20 32.50 3 1 .OO 31.15 29.15 3 1.50 28.95 29.70 C 5642 C 5644 C 5648 C 5653 C 5654 C 5659 C 5664 C 5666 29.60 29.45 28.75 31.80 27.50 31.40 29.85 30.65 28.90 28.60 28.25 30.70 27.05 30.45 29.95 30.45 C 5667 C 5669 C 5670 C 5671 C 5672 C 5675 C 5676 30.30 30.30 30.45 30.00 30.45 30.75 28.55 30.80 30.20 29.70 29.95 30.30 31.30 28.70 Table 11 Comparison of results for manganese silicon and calcium contents (YO) in real samples of FeMnCaSi Mn Mn Si Si Ca Ca Sample titrimetry ICP-AES gravirnetry ICP-AES titrimetry ICP-AES C 5606 13.70 13.60 52.35 51.90 32.05 31.95 C 5660 23.75 24.00 48.95 49.20 19.40 18.60 C 5681 24.50 24.85 44.90 45.30 18.75 18.05 C 6000 30.70 30.50 41.35 41 .OO 16.85 16.70 Table 12 Comparison of results (%) for real and CRM samples of FeCrSi; correlation coefficients for determination of chromium and silicon were found to be 0.960 and 0.945 respectively Sample C 5562 C 5563 C 5564 C 5583 C 5587 C 5590 C 5609 C 5622 C 5623 CSAN-CRM 4-5-03 Method FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others Certificate value FAAS others ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES TCP-AES ICP-AES TCP-AES A1 0.60 0.64 0.80 0.84 1.05 1.02 - 1.04 1 .00 0.76 1.02 1 .oo - 0.99 0.98 Cr 3 1.30* 31.35 30.30* 29.75 32.75* 32.60 29.15* 29.35 29.60* 29.95 31.90* 32.20 31.90* 32.50 30.75* 3 1 .OO 31.55* 31.15 32.62 32.80* 32.70 c u 0.02 0.025 0.03 0.025 0.02 0.02 0.03 0.03 0.03 0.02 0.015 0.03 0.03 0.025 0.03 0.025 0.05 0.045 0.015 0.02 Mn 0.14 0.13 0.13 0.13 0.13 0.13 0.20 0.18 0.19 0.18 0.10 0.11 0.28 0.28 0.19 0.26 0.25 - 0.08 0.085 Ni 0.175 0.18 0.17 0.17 0.17 0.17 0.17 0.175 0.16 0.16 0.16 0.15 0.17 0.17 0.16 0.17 0.18 0.18 0.15 0.175 - Si 49.35t 49.80 5 1.20t 50.10 50.15t 49.70 47.501- 48.45 49.609 50.20 51.00t 50.80 48.751- 48.80 53.251- 53.05 49.157 49.20 51.60 5 1.40 - Ti 0.0951 0.11 0.081 0.10 0.081 0.085 0.091 0.08 0.091 0.10 0.151 0.13 - 0.14 0.13 0.06$ 0.085 - - * Titrimetry.t Gravimetry. 1 Photometry (with H,O,). Table 13 Comparison of results (YO) for real ferrosilicon alloy samples; sample decomposition for ICP-AES was performed by means of the microwave oven Sample FeSi 75 Fe M n Si/ A FeMnSi/B FeTiSi/A FeTiSi/B FeZrSi/A FeZrSi/B FeZrSi/C Method FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others FAAS others ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES TCP-AES ICP-AES JCP-AES A1 2.00 2.06 0.01 < 0.03 0.02 < 0.03 14.35 14.45 7.00 7.20 1.68 1.65 0.68 0.63 0.70 0.74 Cr 0.13 0.12 - 0.05 0.05 0.335 0.32 0.16 0.19 0.40 0.35 0.46 0.51 0.42 0.38 c u 0.06 0.06 0.025 0.03 0.03 0.035 0.23 0.24 0.03 0.03 0.20 0.19 0.20 0.20 0.12 0.12 Fe - 11.os-f 12.05 7.40t 7.301 - 10.657 10.60 - Hf* Mn 0.12 0.13 - 67.80 - 73.50t - 73.55 - 0.95 - 0.96 0.54 - 0.53 0.44$ 0.24 0.46 0.22 0.53$ 0.24 0.51 0.23 0.22 - 0.27 - - - 67.301- - - Ni 0.04 0.04 0.04 0.06 0.07 0.09 0.04 0.05 0.025 0.02 0.03 0.02 0.03 0.04 0.04 0.05 Si 78.3% 78.25 15.90s 15.95 15.65s 16.05 25.8% 26.20 34.30 42.45s 42.40 52.4@ 53.60 51.00s 52.30 33.75s Ti 0.077 0.11 0.1q 0.22 0.2q 0.22 22.80t 23.20 19.907 19.40 1.2q 1.25 1.2017 1.31 0.2017 0.21 Zr - 33.854 34.20 29.5% 29.00 32.70 33.45s * Determination of hafnium was performed after decomposition of FeZrSi samples t Titrimetry. $ Neutron activation analysis.7 Photometry (with H,O,). with HF. 8 Gravimetry.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 255 Table 14 Precision of determination of analyte elements (YO) in FeSi CRM samples Sample CSAN-CRM 4- 1-02 FeSi 45 CSAN-CRM 4-1-01 FeSi 75 BSC-CRM 305 FeSi 75 Parameter* Certificate value n Average SD RSD Certificate value n Average SD RSD Certificate value n Average SD RSD A1 0.49 0.435 0.047 11 10.9 19 1.86 1.905 0.050 2.6 1.25 6 1.27 0.088 6.9 Cr 0.32 0.325 0.007 2.1 0.16 0.155 0.012 7.4 7 0.085 0.004 4.6 12 19 - c u - 15 0.108 0.004 3.5 - 14 0.038 0.004 10.5 8 0.103 0.004 3.7 0.107 Fe - 16 56.91 0.665 1.2 - 18 20.74 0.595 2.9 2 1.4t 6 21.91 0.360 1.6 Mn 0.32 0.325 0.007 2.0 0.11 0.105 0.009 8.1 14 19 - 7 0.135 0.00 1 1 .o Ni - 14 0.045 0.003 6.1 - 18 0.035 0.017 53.5 7 0.075 0.002 2.0 - Si 41.05 12 41.21 0.289 0.7 76.98 15 76.81 0.375 0.5 76.0 6 76.01 0.127 0.2 Ti - 13 0.05 0.006 13 13 - 0.115 0.022 19.4 - 7 0.115 0.002 1.5 * n =No.of analyses; SD = standard deviation; RSD =relative standard deviation. f Informative value not certified. Table 15 Precision of determination of analyte elements (Yo) in CRM samples of FeCrSi and FeMnSi and real samples of FeTiSi and FeZrSi Sample CSAN-CRM 4-5-03 FeCrSi CSAN-CRM 4-5-02 FeMnSi FeTiSi FeZrSi Parameter Certificate value n Average SD RSD Certificate value n Average SD RSD n Average SD RSD n Average SD RSD A1 - 19 0.98 0.015 1.5 - 13 < 0.03 - - 6 14.22 0.18 1.3 7 1.57 0.045 2.9 Cr 32.62 18 32.68 0.327 1 .o - 15 0.3 1 0.049 15.8 - - - - 7 0.525 0.010 1.9 c u - 18 0.019 0.002 9.5 - 17 0.020 0.00 1 6.2 - - - - 7 0.126 0.004 3.2 Fe - 18 14.95 0.109 0.7 - 15 3.89 0.106 2.7 6 35.47 0.29 0.8 Zr 8 34.22 0.28 0.8 Mn - 18 0.085 0.007 7.8 74.08 15 73.98 0.384 0.5 - - - - 7 0.15 0.004 2.7 Ni - 19 0.175 0.025 14.5 16 - 0.045 0.008 17.0 - - - - 7 < 0.02 - - Si 5 1.60 17 51.39 0.455 0.9 19.95 18 19.95 0.286 1.4 6 26.23 0.21 0.8 8 42.39 0.22 0.5 Ti - 17 0.085 0.014 15.8 16 - 0.295 0.028 9.6 6 23.19 0.13 0.6 7 1.79 0.03 1.7 (containing 45% Si) FeSi 75 (containing 75% Si) FeCrSi FeMnSi FeTiSi and FeZrSi.The limits of determination found for ferrosilicon FeSi 75 and the other ferrosiliconalloys are reported in Tables 16 and 17 respectively. Conclusion The ICP-AES procedures can be applied effectively to the multi-element analysis of some ferrosilicon alloys. The pro- cedures proposed are reliable and relatively simple. Silicon can be determined together with the other analyte elements. The application of hydrofluoric acid is not required and therefore Table 16 Limits of determination (defined as ten times the standard deviation of the background noise) for FeSi 75 Limit of determination Element % ng ml-' A1 0.10 200 Cr 0.05 100 c u 0.02 40 Mn 0.02 40 Ni 0.02 40 Ti 0.02 40 Table 17 Limits of determination (defined as ten times the standard deviation of the background noise) for FeSi 45 FeCrSi FeMnSi FeTiSi and FeZrSi Limit of determination Element YO ngml-' A1 0.03 180 Cr 0.02 120 c u 0.01 60 Mn 0.01 60 Ni 0.02 120 Ti 0.01 60 a quartz plasma torch and a glass nebulizer can be used. If a microwave oven is used for the sample decomposition the digestion time is substantially reduced. The described ICP-AES procedures have been used for the multi-element analysis of high silicon content ferroalloys in everyday laboratory practice for the past 5 years. Paper 3/03932C Received July 7 1993 Accepted October 12 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900251

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 257 Direct Solid Sample Analysis of Silicon Carbide Powders by Direct Current Glow Discharge and Direct Current Arc Emission Spectrometry* K. Florian TU Kosice Department of Chemistry Letna 9 SK-0400 I Kosice Slovakia W. Fischer and H. Nickel Research Centre Jiilich GmbH Institute of Materials for Energy Systems P. 0. Box 19 13 0-52425 Julich Germany Glow discharge atomic emission spectrometry (GD-AES) and the classical spectrometric method with a d.c. arc source (D.c.-arc-AES) were applied to the direct solid sample analysis of SIC powders. The homogenity of pellets prepared from mixtures of Sic with copper powder and used for the GD experiments was investigated in detail. Various methods were tested for the analytical calibration and applied to the direct analysis of technical Sic powders. The experimental data were evaluated by using chemometric procedures.The results of the two methods were compared. Keywords Glow discharge atomic emission spectrometry; direct current arc emission spectrometry; direct solid sample analysis; silicon carbide powder; calibration with model standards Ceramic materials based on silicon carbide are used for such applications as slidings seals soot filters heat exchangers and for other high temperature applications. The mechanical behav- iour of the components sintered from Sic powders are influ- enced by the purity of the powder itself. Impurities of technical importance in Sic powders are Al Fe V Ti Ni Ca Cr and B. They are introduced by the starting materials as well as in the production process.There is a substantial need for the Sic manufacturers to check the evolution of contamination during the production and to certify the products by a simple and quick multi-elemental routine analysis with good precision accuracy and limit of detection (LOD). Spectrometric analytical methods have been used for the quantitative determination of the impurities mentioned in ceramic materials.’-3 The application of a conventional dissolu- tion method for sample preparation is limited for refractory compounds like Sic owing to their resistance to chemical attack. This resistance increases the potential danger of intro- ducing additional impurities during sample preparation. Therefore techniques for the direct analysis of the solid sample are preferred.Direct current glow discharge atomic emission spectrometry (GD-AES) utilizing the sputtering process for the destruction of the sample and d.c. arc emission spectrometry (d.c.-arc- AES) were applied for the direct analysis of Sic powders. Both methods need a certified reference material (CRM) for analyt- ical calibration. In general such CRMs are not available. Therefore two types of calibration samples were prepared (i) the matrix was modelled as an equimolar mixture of silicon and graphite powders (Si+C); and (ii) superpure silicon car- bide powder was used as a matrix. The analytes were added to both matrices as oxides of spectral purity. Furthermore a set of technical Sic powders and the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 112b Silicon Carbide all well characterized by various methods including wet chemical analysis were used for checking the accuracy of both spectro- metric methods.A test of the homogeneity of the calibration samples com- parison of the results of the calibration of GD and d.c.-arc- AES and calculation of the precision accuracy and LOD values are the subject of this study. * Presented at the XXVIII Colloquium Spectroscopicum Inter- nationale (CSI) York UK June 29-July 4 1993. Experimental Samples and Sample Preparation Two types of matrices were prepared for analytical calibration (i) an equimolar mixture of Si powder (Merck No. 12497 > 99% Si < 120 pm) and graphite (Ringsdorff type RW-A/T >99.9999% C <60 pm); and (ii) superpure Sic powder a special product of the Elektroschmelzwerk Kempten Germany (made as a reference material for a round robin test of the purity of Sic used for technical purposes; the concentrations of the impurities of interest were kept some orders of magnitude lower than usual).The analytes were added to these matrices as oxides of spectral purity in the concentrations given in Table 1. The mixtures were homogenized by grinding in an agate mortar for 5 min. For the GD-AES measurements the calibration mixtures were diluted with copper powder (Merck No.2704 3 99.9% 60 pm) in a ratio of 3:7. In the next step they were compacted into pellets in a hydraulic press at 800 MPa for 120 s together with pure copper powder.’ The arrangement of the calibration mixture and the surrounding copper powder in the pressing tool was such that the calibration mixture formed a centered inlet (10 mm diameter 1 mm in thickness) on one side of the pellet surrounded and mechanically stabilized by a copper ‘holder’. For the d.c.-arc-AES investigations graphite powder and the spectrochemical additive CoF2 + Ba(N0,)26 were added to the calibration mixture in the ratio 1 1 1.A 15 mg portion of the mixture was used for each analysis after intense homogenization. Table 2 lists the impurity concentrations in the set of techni- Table 1 Analyte concentrations (YO) in calibration samples si+c Sic Sample No. Al Fe V Ca B Cr Al Fe V B Cr Ca 1 1.0 0.3 16 1 .o 2 0.7 0.2 0.7 3 0.316 0.1 0.316 4 0.1 0.0316 0.1 5 0.03 16 0.01 0.05 6 0.01 0.03 16 -258 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 2 Set of technical Sic powders used for check of the calibration accuracy (RSDs not available) Concentralion (%) Analyte A1 Fe V B Cr Ca Grain size/pm F-400 0.12 0.31 0.1 1 0.01 0.03 1 0.16 17 F-320 0.025 0.030 0.060 0.008 Unknown 0.003 1 27-30 ‘EXTRA’ 0.128 0.0364 0.0346 Unknown 0.0005 0.003 7 z 60 NMP-1 0.010 0.0271 0.012 Unknown 0.0005 0.0005 < 15 NMP-2 0.034 0.007 0.0185 Unknown Unknown 0.0006 < 35 NIST 0.34 0.1 1 0.019 Unknown Unknown 0.17 Unknown cal SIC powders and NIST SRM 112b. These data have been used to check the accuracy. GD-AES Measurements The experiments were performed with the commercial equip- ment SPECTRUMAT 1000s (Leco Germany); some technical details are summarized in Table 3.For data on the analytical lines used see Table 5. To avoid fluctuations of the emission intensity caused by air leakages at the sample gasket or by the porosity of the pellet itself an evacuated cup was used.2 Fig. 1 shows a schematic diagram of the slice-by-slice removal of the centered inlet of a pellet during each GD measurement. During the compacting the hard constituents of a calibration mixture are pressed into the softer components Table 3 GD-AES equipment Source Discharge tube Discharge parameters Discharge atmosphere Spectrometer Polychromator Entrance slit Exit slits Grating Dispersion range Detectors Grimm-type 8 mm diameter Constant voltage mode; U = Ar 99.99%; mass flow 300 water-cooled cathode plate lo00 V; Current z 60 mA ml min-’ (pAr ~ 0 .9 hPa) Paschen-Runge configuration; focal length 1000 mm evacuated chamber 50 pm 10 pm Holographic 2 160 grooves mm - 130-430 nm (first order) D= Hamamatsu photomultiplier 0.34 nm mm-’ special entrance windows for vacuum UV region Pellet weighed Fig. 1 Schematic diagram of the slice-by-slice removal of a GD pellet of the pellet so the first slices near to the surface of a new pellet are not representative of the whole inlet. Therefore these upper slices were removed during the first three unevaluated measurements from M(-2) to M(0). The sputtering time for each slice M(i) was fixed to 30 s. Nine slices i.e. nine measure- ments were made on each pellet. Integrated intensities were used for all further analytical evaluations. The nine measurements are arranged into three groups (see Fig.1) the upper group (U) representing the mean of the first three slices M(l)-M(3) the middle (M) for the next three and the lower group (L) for the last three slices. These mean values U M and L were used for the statistical test of the homogeneity of the pellets. D.c.-arc-AES Measurements The experimental conditions for the d.c.-arc experiments are given in Table 4. The analytical lines used for the calibration (Table 5 ) are not identical with those of the GD-AES measure- Table 4 D.c.-arc-AES spectrograph Source Arc discharge Excitation Electrodes Spectrograph Grating spectrograph Entrance slit Grating Dispersion range Detection/evaluation Photographic plate Exposition time Spectra evaluation UBI-1 (Zeiss Jena) Free burning d.c.arc; 4 mm electrode gap; carrier electrode as anode; 1=8.5 A High resistance carbon electrodes in water-cooled holders carrier electrode type SW 380 counter electrode type SW 202 (Elektrokarbon Topolcany Slovakia) PGS-2 (Zeiss Jena) focal length 20 pm 651 grooves mm-’ 250-330 nm (second order) 2000 mm D=0.36 nm mm-’ ORWO WU-3 80 s (total evaporation) Modernized 4D densitometer 0 < S < 4 1 transformation Table 5 Analytical lines (wavelengths in nm) Analyte A1 Fe V B Cr Ca ~ ~~~ GD-AES D.c.-arc- AES A1 1396.15 A1 1308.22 Fe I1 249.32 Fe I 302.11 V 1411.18 V 1318.34 B 1208.94 B 1249.68 Cr I 302.16 No sensitive UV line Cr T 425.43 Ca I1 393.37JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 259 ments. The two devices are equipped with spectrometers with different dispersion ranges.Calibration was provided on the basis of a 5-fold measure- ment of each calibration sample. The precision was calculated from ten replicate measurements for two selected calibration samples (see Table 6). Results and Discussion Homogeneity Tests The first test concerns the homogeneity of the elemental distribution within a GD pellet by comparison of groups of slices. The statistical check was performed according to Lord‘s ~riterion.~ This test utilizes the largest difference AI, of the emission intensities within each group of slices instead of the standard deviations. This type of testing is useful if only a small number of repeated measurements is available (in the present case n = 3). The test criterion is the value u calculated from eqn.(1 ) where iA and IB are the averaged emission intensities for any analyte in the groups tested. The value of u has to be compared with its counterpart UTab (ref. 7) UTab = 0.636 for a statistical precision of 95% and n = 3; if u < UTab then the groups compared can be considered homogeneous. The test results of one arbitrary GD calibration pellet are given in Table 7. The test results confirm the homogeneity of the pellet in the first two groups U and M for most analytes. It reveals a general inhomogeneity of Fe and B. Therefore correct analyt- ical results can be expected only if the evaluation is limited to the first three slices i.e. to the group U. The homogeneity of different pellets prepared from the same calibration mixture was tested in the same manner.The mean value of the first groups U of the different pellets have been used for the comparison. The results are presented in Table 8. The numbers 1 2 and 3 stand for arbitrary identifiers of different pellets. Table 6 RSD (YO) values for two arbitrarily selected calibration samples (n = 10) sample 1 sample 2 Analyte c (%) GD D.c.-arc c (%) GD D.c.-arc A1 0.316 1.2 20 0.1 5.2 5.7 Fe 0.316 9.7 24 0.1 30 6.6 V 0.316 2.3 32 0.1 3.4 7.3 B 0.1 10 15 0.0316 16 7.6 Cr 0.1 1.8 27 0.0316 5.8 20 Ca 0.316 16 - 0.1 19 - Table 9 RSDs (YO) of repeated d.c.-arc measurements Calibration sample Analyte 1 2 3 4 5 6 A1 24 20 5.7 10 30 1 1 Fe 25 24 6.6 17 10 25 31 32 7.3 29 19 - V B 19 15 26 7.5 6.4 12 Cr 31 27 20 20 8.8 20 The results of this second test do not show a clear tendency.Therefore it is assumed that the pellets are equivalent to each other. Owing to the total evaporation of a calibration sample in a d.c.-arc measurement a similar test to that used for GD is excluded. Consequently the relative standard deviations (RSDs) of repeated measurements must be compared. These RSD data are in good agreement with the data given in Table 6 for the precision of the d.c.-arc method. It can be concluded therefore that the d.c.-arc calibration mixtures are homogeneous. Analytical Calibration Owing to the limitation of the dynamic range of the detection system of the SPECTRUMAT 1000s to two orders of magni- tude the intensity ratios y = IJIsi have been plotted directly against the concentration (c) of the analytes.A straight line [eqn. (2)] was fitted to these data by linear regression Y=A+Bxc (2) For the d.c.-arc measurements with photographic registration and the evaluation of the plate blackening over four orders of magnitude a double logarithmic plot of log(y) uersus log(c) is more convenient [eqn. (3)] (3) The results of the analytical calibration are listed in Table 10 together with the so-called ‘expected precision’ RSD (c)(%) of the concentration calculated according to eqn. (4) log(y) = A’ + B’ x log(c) RSD(c) = (sreS/B) x J( 1/N + l/n) x 100 (4) where s, is the residual standard deviation after linear regression N the number of calibration samples and n the number of repeated measurements. Table 11 gives the LOD cL calculated using the 3s criterion from blank measurement data (15 repetitions).Figs. 2-12 show the calibration plots. Calcium could not be determined with the d.c.-arc owing to the absence of a sensitive emission line within the dispersion region of the PGS-2 spectrometer. Table 7 Results of the homogeneity test within a GD pellet (U M and L indicate the groups of slices compared + identical; - different; and x irrelevant comparison) ~~ A1 Fe V B Cr Ca U M L U M L U M L U M L U M L U M L U x + - x - - x + + x - - x + - x + - x - + x - + x - M + x - X X X X + - L X - x - + x - - - - - - - - - - - - Table 8 Pellet A1 Fe V B Cr Ca Results of the homogeneity test of different GD pellets (1 2 and 3 are arbitrary pellet identifiers); symbols as in Table 7 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 x - + x + + x - + x + - x - + x + + x - + x - 2 - x + + x + 3 + + x + + x + + x - - X + - X + - x - x + + x - -260 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 10 Numerical results of the analytical calibration GD-AES Si+C Sic Analyte A B RSD (c) (%) A B RSD (c) (Yo) A1 0.060 1.870 2.3 0.023 2.763 2.9 Fe 0.052 0.323 5.4 0.042 0.366 1.5 v 0.023 0.480 2.1 0.038 0.492 0.7 B 0.008 2.158 0.7 Cr 0.117 5.852 1.4 0.145 6.098 0.7 Ca 0.104 1.828 2.4 0.061 2.823 0.6 - - - D.c.-arc-AES ~ ~~~ s i + c Sic Analyte A' B' RSD (c) (YO) A' B' RSD (c) (YO) A1 0.84 0.65 8.8 0.60 0.48 27 Fe 0.35 0.53 18 0.38 0.76 29 v 0.74 0.89 20 0.63 1.08 18 - 0.38 0.43 25 B Cr 0.416 0.66 18 0.57 1.07 7.8 - - Table 11 Limits of detection (cL) in pg g-' Analyte A1 Fe V B Cr Ca GD-AES - Si+C Sic 50 98 557 765 646 325 44 8.5 10 27 106 - 0.4 1 + 0.3 -* 3 0.2 0.1 LOD -* I I I I I I 0 0.2 0.4 0.6 0.8 1 .o LFel (YO) Fig.3 GD calibration graph of Fe in B Si+ C; A Sic; 0 Sic F320; and + SIC F400 matrices 0.6 I 1 0.5 D.c.-arc- AES 0.4 ~~ Si+C Sic 0.3 0.3 1 .o 15 4.6 23 5.1 56 - - - - < 0.3 -2' 0.2 0.1 LOD + ~~ 0 0.2 0.4 0.6 0.8 7 .O [VI (%I Fig. 4 GD calibration graph of V in Si + C; A Sic; 0 Sic F320; and + SIC F400 matrices A 1.0 1 0 0.2 0.4 0.6 0.8 1 .o [All (%I Fig.2 GD calibration graph of A1 in B Si+C; A SIC; 0 Sic F320; and + Sic F400 matrices The differences in the parameters A and B of the GD calibration curves of the two matrices are small (Table 10). Further conclusions from the values of A should not be made since they correspond to an unallowed extrapolation below the lower calibration limit.They include the influence of the impurities in the diluent and the substances used for matrix modelling as well as the error of the linear regression. A more detailed discussion of this low concentration range requires further experiments to be carried out. The calibration results confirm the general experience that in GD-AES the matrix effect is either small or completely absent. Boron is the only exception its calibration curve being non-linear. Neither additional contamination of the pellets introduced during the preparation nor a line overlap can be the reason for this deviation. It may be that chemical reactions in the discharge 0 0.1 0.2 0.3 0.4 [Bl (Yo) Fig. 5 GD calibration graph of B in .Si +C; A Sic; 0 Sic F320 matrices plasma e.? the formation of B-C compounds are responsible. In contrast the d.c.-arc calibration results show a pro- nounced influence of the sample matrix (see results for Fe V or Cr). The slope of the calibration curves becomes stronger when the Si+C matrix is substituted by Sic. The changes in the A' value are negligibly small. The stronger slopes of the SIC calibration curves are important especially in the case of higher analyte concentrations. This matrix effect calls for26 1 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0 0.2 [Crl (%) 0.4 Fig. 6 GD calibration graph of Cr in . Si+C; A Sic; 0 Sic F320; and + Sic F400 matrices 2 2 ,u . 1 LOD -+ 0 0.2 0.4 0.6 0.8 1 .o [Cal (%I Fig.7 GD calibration graph of Ca in W Si+C; A Sic; 0 Sic F320; and + Sic F400 matrices 1 -1 1 I I 1 -3 -2 -1 0 Log [Cn (%)I Fig.8 D.c.-arc calibration graph of A1 in m Si +C; A Sic; x Sic NMP-1; 0 Sic F320; A SIC NMP-2; + Sic F400; 0 Sic NIST; and 0 Sic 'extra' matrices further investigation of the evaporation behaviour of the calibration samples. To date the spectrochemical additives have been optimized only with respect to a quick and complete destruction of the Sic crystal lattice3 and total sample evapor- ation necessary for quantitative analysis. The additives were not optimized from the point of view of the matrix effect. The fit of a straight line to the GD calibration data is better than that to the d.c.-arc data. In d.c.-arc the correlation I I J -3 - 2 -1 0 Log [CFe (%)I Fig. 9 D.c.-arc calibration graph of Fe in 1 Si + C; A Sic; A Sic NMP-2; x Sic NMP-1; 0 SIC F320; + Sic F400; 0 SIC NIST; and 0.Sic 'extra' matrices -3 - 2 - 1 Log [C" (%)I 0 Fig. 10 D.c.-aRC calibration graph of V in m Si +C; A Sic; x SIC NMP-1; A Sic NMP-2; 0 Sic NIST; 0 SIC 'extra'; + Sic F400; and 0 Sic F320 matrices 0.5 I - 0.5 -1 I I I -3 - 2 -1 0 Log Ice (%)I Fig. 11 matrices; the arrow indicates the B contamination level Attempt at d.c.-arc calibration of B in . Si+C and A Si coefficients of the linear regression ranged from 0.95 (B) to 0.99 (Al Fe) compared with >0.99 for all analytes in GD. The 'expected precision' of the determination of the concen- trations in GD is significantly better than in d.c.-arc. Possible reasons for this finding are the more even sample atomization in the sputtering process combined with the photoelectric detection of the analytical signal.The LODs for the two262 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 -2.0 L I I I - 3 -2 - 1 0 Log [cc (%)I Fig. 12 D.c.-arc calibration graph of Cr in W Si+C and A Sic matrices 0.05 - s - -0 0.04 .- E 0.03 -0 z 0.02 0 .- .I- E E 0.01 a u 0 u 0 Fig. 13 0.75 (a) N M P-2 I I I I 0.5 0.25 0.01 0.02 0.03 0.04 0.05 0 0.25 0.5 0.75 Given concentration of Al (YO) Comparison of the accuracy of the analytical d.c.-arc results ofvarious Sic powders using calibration data of both the Si+C (0) and the Sic (+ 1 matrices methods (see Table 11) show opposite tendencies of the detect- ibility of an analyte and the precision of its determination. Obviously the precision of the d.c.-arc method is worse than that obtained with GD but its advantage is the low LOD.First experiments with improved equipment where the photo- graphic plate is substituted by a photoelectric detection system indicate that a higher precision might be achievable with the d.c.-arc also.g The accuracy of the GD analyses could be tested only for the Sic powder F-400 because of the relatively high consump- tion of material in the preparation of pellets. The RSDs between the determined and the given concentrations ranged from -44% for Fe to +50% for Al. The sample consumption of d.c.-arc measurements (15 mg per measurement) is considerably lower than in GD. Therefore six Sic powders were tested. The Yuden plot7 of Al shown in Fig. 13 is representative of the other analytes too.In the worst case the deviation between the determined and the given value increases to 80%. Conclusions The present study demonstrates that both GD-AES and d.c.- arc-AES can be used for the direct solid sample analysis of impurities in Sic powders. The limitation of the investigation to the impurity elements discussed does not reflect a limitation of the method but rather of the spectroscopic equipment available. Samples for calibration can be prepared from powder mixtures consisting of Si+C or Sic with additions of the analytes. The homogeneity of the calibration samples is ensured if the preparation procedure described is applied. Both methods differ in their precision and their LODs; the precision of the d.c.-arc method is not as good as that achieved with GD-AES but the LODs are 1-2 orders of magnitude lower.No matrix effect was observed in the GD measurements. The pronounced matrix effect in the d.c.-arc measurements requires further studies especially in relation to the influence of chemical modifiers. The accuracy of the analyses is dependent on the analyte and its concentration level. It is believed that the accuracy achieved is acceptable for the routine certification of technical Sic powders. The present investigation was carried out as a sub-task of the contract on scientific-technical cooperation between Germany and Czechoslovakia. The authors thank the ministries of both countries for financial support. Furthermore the authors thank Mr. J. Hassler (Elektroschmelzwerk Kempten Germany) and Mr. Dr. G. Wolff (Research Centre Julich GmbH Germany) for making available the Sic materials investigated. References Broekaert J. A. C. and Tolg G. Mikrochim. Acta (Wien) 1990 11 173. Ehrlich G. Stahlberg U. and Hoffmann V. Spectrochim. Acta Part B 1992 46 115. Florian K. Nickel H. and Zadgorska Z. Fresenius J. Anal. Chem. 1993,345,445. Hassler J. personal communication. Guntur D. S. Fischer W. Mazurkiewicz M. Naoumidis A. and Nickel H. Reports of the Research Centre Julich Jiilich Germany Jul-Report No. 2592 1992. Nickel H. Zadgorska Z. and Wolff G. Spectrochim. Acta Part B 1993 48 25. Eckschlager K. Horsack I. and Kodejs Z. Evaluation of Analytical Results and Methods SNTL Publishing Company Prague 1980 p. 43 (in Czech). Fischer W. Naoumidis A. and Nickel H. paper presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post- Symposium on Analytical Applications of Glow Discharge in Optical and Mass Spectrometry York UK July 4-7 1993. Florian K. Fischer W. Hassler J. and Nickel H. paper presented at the XXXVI Hungarian Annual Conference on Spectral Analysis Lilafiired Hungary August 24-27 1993. Paper 3103941 B Received July 7 1993 Accepted November 15 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900257

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 263 I Direct Solid Sampling for Analysis with Inductively Coupled Plasma Using a Novel Electronic Spark Source* C. Webb C. B. Cooper 111 A. T. Zander J. T. Arnold and E. S. Lile Varian Associates Ginzton Research Center Palo Alto California 94303 USA S. E. Anderson Va rian Op tical Spectroscopy Ins trum en f s Melbourne A us tralia A solid sampling accessory using a spark discharge for introduction of analyte material into an inductively coupled plasma (ICP) torch has been developed. The device uses a novel electronic design which results in a dramatic simplification of the hardware compared with previously described arrangements. A simple cell consisting of a tungsten electrode adjacent to the sample in a flowing inert gas atmosphere completes the unit as an extremely compact benchtop accessory.This device has been optimized for stable operation over a minimum of 12 min; relative standard deviations for signals over that period are typically <3%. Precisions of 6 4 % were found for the determination of trace elements in National Institute of Standards and Technology Standard Reference Materials with limits of detection in the single digit ppm range. Keywords Solid sampling; spark; inductively coupled plasma Inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) are widely employed analytical techniques. The most common method of sample introduction is by nebuliz- ation of liquids but this requires that solid samples must first be subjected to some dissolution procedure and methods of direct solid sample introduction are of obvious appeal.Additionally solvents can result in polyatomic interferences in ICP-MS.l Spark ablation has previously2+ been shown to be a useful technique for solid sampling and although it is limited to conducting samples techniques have been developed for mixing finely dispersed samples with conducting powders such as g r a ~ h i t e . ~ In this paper results obtained using a novel implementation of spark ablation as a solid sampling accessory (SSA j for ICP is reported. Developments in modern electronic components have permitted the design of a small simple circuit’ which is incorporated into a compact benchtop SSA unit representing a dramatic reduction in hardware compared with previous devices.The present emphasis is on the optimization of the operation of this device and the analytical results obtained from it. Experimental A schematic diagram of the cell is shown in Fig. 1. The sample is sealed to the cell (volume = 1 ml) by means of an O-ring and the spark source is connected to a tungsten electrode (diameter 1.5 mm) the height of which can be adjusted. Argon flows through the cell at a flow rate of z 11 min-’ and transports ablated material to the ICP torch; the argon gas flow rate was not found to be an important parameter. Sample cooling is not necessary. The spark source allows various parameters to be set allowing the adjustment of pulse-width frequency and current. In addition as discussed below separate pre-burn conditions can be set along with the duration of the pre-burn step.The samples have so far consisted of steel [National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) 1261 1263 1264 1265 1269 and 12231 Fe-Ni alloy (NIST SRM 1159) and brass (NIST SRMs 1104,1107 and 1108). Typically for steel samples the analytical settings used were 3 mm tip gap 8 ps pulse-width 60 A current and 500 Hz frequency. The pre-burn settings differed in having * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) York UK June 29-July 4 1993. Tungsten elect rode t- SSA power supply Fig. 1 Schematic diagram of sample cell and spark source. The electronics allows selection of pulse parameters as follows pulse width 4-10 ps; current 20-80 A; and frequency 250-1200 Hz.A timed pre- burn with independent parameter settings is permitted. The tip gap is adjustable in the range 0-10 mm 80 A current and 1000 Hz frequency for the first 40 s. All of the data shown here were obtained with ICP-AES systems. Normal ICP operating conditions were 1.2 kW power input outer argon flow and intermediate flow rates were 13.5 and 1.5 1 min-l respectively. Integration times were 5 s; RSD infor- mation is based on ten readings. Sample preparation simply consisted of abrasion with sandpaper and an acetone rinse. The dry aerosol was transported to the ICP via a plastic (‘Tygon’) tube. The analytical lines used were Fe 240.488 nm (analytical data) or 259.940 nm (optimization data) Ni 231.604 Mn 257.610 V 292.402 Cr 267.716 Sn 242.949 Zn 206.200 and 220.353 nm.Results and Discussion Optimization The criterion for stable operation was that a steady signal could be maintained for 12 min while the signal stabilization264 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 time was minimized; 12min was chosen as a plausible upper bound for analysis time using a sequential spectrometer. This required proper selection of pulse parameters and in particular the tip gap was found to be critical. Craters on sample surfaces were examined using a stylus profilometer and were found to vary essentially as would be expected with respect to tip gap as shown in Fig. 2 where tip gap is a parameter. It should be noted that the 1 mm gap results in a crater which is already several percent of the gap after only 80s.Perhaps more importantly significant build-up of ablated material was also observed on the tip at the 1 mm setting. Accordingly an ICP signal monitored for example from the matrix Fe 260 nm line exhibited noise and drift at a 1 mm gap. This was not a problem at either 3 or 5mm however and the 3mm setting was considered more satisfactory for the remainder of this work since it gave a larger signal and shorter signal stabiliz- ation time than the 5 mm setting. The time required for signal stabilization is obviously important since it represents dead time during which no analysis can be performed. With the analytical conditions described above and no pre-burn the time required for signal stabilization was about 200s.It was demonstrated that this depended upon the sample surface since shutting off the spark once a stable signal was achieved and then re-starting after a 40 s delay resulted in a much faster signal rise with sparking occurring on to the pre-existing crater. It appears that the stabilization is only achieved after some initial conditioning of the surface. Suitable pre-burn conditions to allow accelerated surface conditioning were determined and it was found that an increased frequency was particularly effective in enhancing the rate of erosion. Whereas the crater depth reached= 13 pm in 160 s with the usual analytical settings mentioned above a crater of depth= 17 pm was achieved in only 40 s with the pre- burn settings. Based upon the previous observation that signal stabilization requires ~ 2 0 0 s with the analytical settings it is considered that a crater about this size corresponds to adequate surface conditioning.In Fig. 3 the signal rise characteristics both with and without a 40s pre-burn are compared and it can be seen that the signal reaches its steady-state value within about 80s with the pre-burn compared with about 200s without. Although the pre-burn condition used includes an increase in current various parametric studies have shown that it is the increase in frequency that is most effective in enhancing the rate of signal rise. The results of one such experiment for NIST SRM 1223 steel are shown in Fig. 4 where only the frequency the is varied. The signal increases initially much faster at higher frequencies than can be explained on a linear (effective time-compression) basis.Microscopy of craters sug- gests that melting is occurring and the supra-linear dependence on frequency is probably due to thermal effects. Comparison of integrated signals with crater depth (used for simplicity h i \ I I I W I 40 t 50 -3 -2 - 1 0 1 2 3 Surface positi on/m m Fig. 2 Sections obtained with a stylus profilometer of craters pro- duced on sample surfaces as a function of tip gap (a) 5 mm (160 s); (b) 3 mm (160 s); and (c) 1 mm (80 s). No pre-burn was employed here 0 100 200 300 400 500 600 Time/s Fig. 3 ICP signal intensity for Fe 260 nm is shown as a function of time (zero is spark-on time) A with and B without a 40 s pre-burn as described in the text 0 50 100 150 200 Time/s Fig.4 ICP signal intensity for Fe 260 nm is shown as a function of time. The effect of spark frequency A 244; B 325; C 500; D 1000; and E 1200 Hz on signal rise characteristics is illustrated in lieu of volume since width does not vary much with depth) shows good linearity which would seem to rule out expla- nations dependent on particle size variations. It should be emphasized that the time required for initial signal stabilization is strongly dependent upon requirements with respect to the length of time for which a stable signal must be held i.e. for the present purposes 12min. The 80s initial stabilization time is only 10Y0 of the total spark-on time. If analysis times shorter than 12 min are required then it would be possible (e.g. by using shorter tip gaps) to reduce the time required for signal stabilization also. As discussed above a basic dilemma is that it is desirable to keep the crater diameter small in order to minimize the time required for surface conditioning while keeping the tip gap large to maintain a stable signal.Although we believe that a good compromise utilizing the pre-burn described has been found a more direct approach involving a modification to the tip was also tested. It was noted that a concave tip where sparking occurs from the perimeter produces a ring shaped crater whose diameter is approximately twice that of the electrode. This shows that a spark from one point on the circumference is influenced by the electrostatic field produced by the overall shape of the tip. In order to take advantage of this the conventional pointed tip was used with a collar as illustrated in Fig.5. The crater size is very dependent upon the setback as defined in the figure but a range of setbacks can be found for which the crater diameter is decreased and the depth increased compared with a simple pointed electrode. The magnitude of the effect is small (% 10-15%) however and €or simplicity a simple pointed electrode was used to obtain the analytical data described below.265 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 - - ’\ Setback Fig.5 The ‘collared-tip’ arrangement used as an alternative to the more usual simple point arrangement Analytical Performance The analytical performance was evaluated using a Varian Liberty ICP atomic emission system during a relatively short period of time and it is possible that the ultimate performance will be improved.However the results obtained here are already very encouraging. In Fig. 6 stability data for several elements obtained from an NIST SRM 1223 steel sample are shown. The intensity information is plotted on a reduced scale; i.e. for each element the maximum is set equal to unity and the other data points scaled accordingly. The relative standard deviation values were calculated from the sets of data in Fig. 6 for each element taken over the 12 min period and are given in Table 1. Here the ‘norm’ suffix refers to normalization by the matrix Fe; both stability and precision data are shown. The precision data in Table 1 refer to a data set from ten sequential craters located at different positions on an NIST SRM 1261 steel sample; the sample was repeatedly re-surfaced (using sandpaper and an acetone rinse) and following the pre- burn procedure outlined above intensity information was obtained. Normalization results in an appreciable overall 1.2 r 1 2 0.8 I U i I 0.6 I I I 0 5 10 15 Ti melmin Fig.6 Stability data obtained for various elements in NIST SRM 1223 steel D Fe; 0 Ni; 0 Mn; and x V.The intensity information is plotted on a reduced scale i.e. for each element the maximum is set equal to unity and the other data points scaled accordingly Table 1 Relative standard deviations for stability and precision. The stability values refer to the data in Fig. 6 and are for a single crater over a period of 12 min.The precision values are for 10 separate craters on one sample. The suffix ‘norm’ refers to normalization by Fe matrix Element Fe Ni Mn V Ni norm Mn norm V norm Stability 2.6 3.3 1.9 1.5 1 .o 3.1 2.8 Precision 3.6 4.0 3.7 3.2 1.5 1.5 1.6 improvement for the precision data but not for the stability data. This could reflect small but significant changes in the presentation of the sample to the cell; e.g. variations in surface roughness could cause minute but variable leaks perturbing gas flow patterns in the cell. However all RSDs are 4% or better which compares well in the context of solid sampling generally.’ Analytical accuracy was tested by running a number of both steel and brass SRMs in two series. Fig. 7 shows the comparison of the measured values with the stated values for Cr in steel; other elements behave similarly.Some of the steels were run more than once and in those cases duplicate points are plotted. In the first run of the series NIST SRM 1264 was arbitrarily taken as a ‘standard’ and the sensitivity factors derived from this sample were applied to the data from the remaining runs. General agreement is good over a wide range of concentrations though deviation occurs at lower concentrations because back- ground corrections other than subtraction of the spark-off background were not made. There was generally little need to use matrix normalization except for NIST SRM 1263 (points at stated concentration 1.31 %) which exhibited a noticeably lower erosion rate than the other samples under fixed conditions.Since only three brass samples were used data obtained using the highest concentration for each element are plotted in Fig. 8 to derive the sensitivity factor and the highest point for each element is effectively constrained to lie on the line while the two lower points verify the accuracy of the method. I I I I J I X ~ O - ~ i x i o - * I X I O - ’ I 10 1 x 1 0 2 SRM concentration (%) Fig.7 Calculated analysis data for Cr in steels are plotted against SRM data sheet values A Cr and; B Cr (normalized). ‘Normalized’ refers to data which have been normalized by the matrix (Fe). These data refer to a series of runs with the various steel samples NET SRM 1261 1263 1223 1269 1265 1159 (Fe-Ni alloy) where the first run of the series with NIST SRM 1264 steel was used as the ‘standard’ to derive sensitivity factors for the remaining runs RSD (Yo) I lo-‘ 1 x 10-2 1 1 XI02 SRM concentration (%I Fig.8 Calculated analysis data for various elements in three brass samples are plotted against SRM data sheet values 0 Ni; 0 Sn; 0 Mn; x Fe;+ Zn; and A Pb (see text for further explanation). Since there was no information on Mn at the two lower concentrations these points were plotted at an assumed detection limit of 1 ppm266 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Detection limits were calculated for one of the steel samples and found to be generally in the single digit ppm range. Preliminary data using the solid sampling device for ICP-MS analysis indicate that low concentrations of La (700 ppb) and Nd (700 ppb) in steel are readily measured.Additional detail will be the subject of a future publication." Conclusion A new compact solid sampling accessory for ICP based upon a spark source which utilizes novel electronics has been developed. The optimization of this device has been described noting considerations in the compromise choice of tip gap and the advantage of using a pre-burn at higher pulse frequency for rapid conditioning of the sample surface. Analytical data have been obtained for a number of steel and brass SRMs showing good performance with respect to stability precision detection limits and analytical accuracy. References 1 Alves L. C. Wiederin D. R. and Houk R. S. Anal. Chem. 1992 64 1164. 2 3 4 5 6 7 8 9 10 Routh M. W. and Tikkanen M. W. in lnductively Coupled Plasmas in Analytical Atomic Spectrometry eds. Montaser A. and Golightly D. W. VCH New York 1987 p. 455-468. Human H. G. C. Scott R. H. Oakus A. R. and West C . D. Analyst 1976 101 265. Aziz A. Broeckaert J. A. C. Laqua K. and Leis F. Spectrochim. Acta Part B 1984 39 1091. Jakubowski N. Feldmann I. Sack B. and Stuewer D. J. Anal. At. Spectrom. 1992 7 121. Broekaert J. A. C. Leis F. Raeymaekers B. and Zaray Gy. Spectrochim. Acta Part B 1988 43 339. Steffan I. and Vujicic G. Spectrochim. Acta Part B 1991 47 61 Arnold J. A. Zander A. T. Lile E. S. and Cooper C. B. in the press. Routh M.H. and Tikkanen M. H. in Inductively Coupled Plasmas in Analytical Atomic Spectrometry eds. Montaser A. and Golightly D. W. VCH New York 1987 pp. 441 and 463 Webb C. Plantz M. Cooper C. B. paper presented at the 1994 Winter Conference on Plasma Spectrochemistry San Diego CA USA January 10-14 1994. Paper 3104821 G Received August 10 1993 Accepted October 21 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900263

出版商:RSC    

年代:1994

数据来源: RSC