摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 443 Vaporization of Silicon and Germanium as Molecular Species in Electrothermal Atomizers* Paolo Tittarelli Claudio Biff i and Veselin Kmetovt Stazione Sperimenfale per i Combustibili Wale A. De Gasperi 3 20097 San Donato Milanese MI Italy The vaporization of Si and Ge and their respective oxides was recorded using an ultraviolet spectrometer with diode-array detection. The samples were analysed as aqueous slurries using thermal programmes that are normally employed for trace analysis. Both the elements and the respective oxides form the monoxides SiO(g) and GeO(g) during atomization. In the presence of compounds containing S CaSO or FeS the gaseous sulfides SiS and GeS were observed. While in the case of Si it is possible to obtain SiS as a unique species from wall vaporization with Ge the presence of GeS is always associated with that of GeO.The spectral characteristics of the compounds identified are reported. The presence of SiS has also been detected during the atomization of a bituminous coal. Keywords Electrothermal atomic absorption spectrometry; molecular absorption; silicon monoxide and monosulfide; germanium monoxide and monosulfide The formation of molecular species during the vaporization of samples in electrothermal atomic absorption spectrometry (ETAAS) has attracted the interest of several spectroscopists because of the potential advantages and disadvantages as a result of the presence of such species during the atomization step. Among the advantages cited should be the possibility of determining elements that have resonance lines lying in the far ultraviolet (UV) because of the formation of molecules that absorb in the UV range or elements that exhibit relevant thermal stability but that are easily vaporized as molecular compounds.'-2 The main disadvantages can be identified as the loss of analyte as molecular species and the presence of non-specific absorption which in some cases shows structured spectral behaviour. In the analysis of solids the problem due to non-specific absorption is greatly enhanced in comparison with the analysis of ~olutions.~ This is especially the case for solids having a high inorganic matter content such as coal ashes minerals and soils where the presence of molecular species is particularly evident. During the atomization step owing to the fast heating rate the decomposition of the matrix leads to fast evolution of molecular vapo~rs.~ In the materials cited above Si is often the major component present as the oxide or silicate and can reach concentration levels of up to 30-40% m/m; therefore it is a potential source of non-specific absorption in the analysis of these materials. The vaporization characteristics of the Group 14 elements have been examined extensively.Frech and Cedergren have calculated the conditions for the formation of gaseous species of Si and have given evidence for the formation of SiO(g) above 1600K and the dramatic change in distribution of gaseous Si species as a result of small changes in the partial pressure of OF Miiller-Vogt and Wendl' have evaluated the reactions taking place in the condensed phase while Rademeyer and Vermaak6 have followed the formation of SiO(g) and SiC2(g) and deduced from the presence of these species a mechanism for the atomization of Si. The UV absorption spectrum of SiO(g) has already been obtained during the vaporization of Si02.3 In the case of Ge Dittrich et aL7 have reported the spectrum for GeS(g) with evidence to support that it is GeS given by the absorption bands located between 280 and 290nm.Kolb et aL8 have found that Ge is lost as GeO(g) above 1100 K ~~ * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) Post-Symposium on Graphite Techniques in Analytical Spectroscopy Durham UK July 4-7 1993. t On leave from Centre of Analytical Chemistry and Applied Spectroscopy University of Plovdiv Plovdiv Bulgaria.unless high temperatures and reducing conditions are achieved during the atomization. Recently Doidge and McAllisterg have shown that equilibrium calculations indicate the formation of GeO(g) even at 800 K. Various workers have emphasised that the atomization of Ge is strongly affected by the properties of the graphite surface the addition of acids and bases and the partial pressure of 02. In the studies cited above the atomization of Si (at ng levels) was followed through the thermal treatment of aqueous solu- tions. The present work was carried out to investigate the formation in the gaseous phase of molecular species of Si that originate during the atomization of solid samples.Thermal conditions as close as possible to those employed for the determination of trace elements were used. Also the behaviour of Ge was investigated to support the spectroscopic obser- vations made for Si and the identification of molecular species. In this instance Si and Ge were considered as major constitu- ents of solid matrices and the amounts of these elements vaporized were at microgram levels. Sulfates and sulfides were added to the Si and Ge compounds to simulate the presence of S as is found in real samples and to investigate the formation of S species in the vapour phase. In particular FeS2 was used as the sulfide and CaSO as the sulfate. Both salts are found in minerals for instance coals in addition to Si oxides and silicates. The formation and persistence of molecular species was followed using a spectrometer that has already been used for the characterization of the vapour phase originating from the vaporization of sl~rries.~ The instrumental approach" is simi- lar to those proposed by Shekiro et al." and Majidi et all2 While Tittarelli et a1." and Shekiro et al." employed a deuterium lamp Majidi et ~ 1 .' ~ employed a plasma as the UV source. In all instrumentation mentioned a diode array was used as the detector. Owing to the capability of continuously collecting spectra these approaches are suitable for the investi- gation of fast transient phenomena occurring in the vapour phase. Experimental Instrumentation The instrumental set-up (Jasco KS-100M diode-array spec- trometer and Perkin-Elmer HGA-400 atomizer) has already been described and used for the evaluation of the behaviour of ~lurries.~ In the present study the wavelength range from 190 to 340nm was focused on the diode array to provide better information about the presence of molecular species absorbing at around 200 nm in the spectral region where the444 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 analytical lines of As and Se are located. The correction of non-specific absorption is a crucial aspect in the determination of these elements in ~1urries.l~ The time for collection of the spectra was 0.1 or 0.2 s and the time interval between consecu- tive spectra was 0.1 0.2 or 0.5 s. Pyrolitic graphite coated graphite tubes and platforms were used. Reagents Samples of Si SiOz Ge GeO and CaSO4.2H,O with purity above 99.5% were purchased from Johnson Matthey Karlsruhe Germany.Iron sulfide (FeS pyrite) was a mineral sample; its purity (>95%) was checked through the determi- nation of the Fe and S contents. All compounds were ground down to particle dimensions of below 10 pm. The Bituminous Coal examined was Standard Reference Material SRM 1632b Trace Elements in Coal (Bituminous) supplied by the National Institute of Standards and Technology (NIST) Gaithersburg MD USA. Procedure The solid samples were diluted with water to obtain slurries of 0.5% m/m and maintained under magnetic stirring prior to injection of the samples into the atomizer. There was no addition of nitric acid to the slurries of pure compounds to avoid as much as possible the formation of gaseous nitrogen species during the sample treatment.The slurries were prepared daily. A 10 pl aliquot of each slurry was injected into the graphite tube. A 1% m/m slurry of the Bituminous Coal was prepared in nitric acid as previously de~cribed.~ The thermal programmes applied to the samples were similar to those employed for the determination of trace elements in slurries of environmental samples3 and consisted of drying ashing and atomization steps. However as the aim of the work was to examine the formation of molecular vapours the term vaporization will be used in the following discussions instead of atomization. Two distinct ranges of ashing and vaporization temperatures were applied for Si and Ge (Table 1). The vaporization temperatures were reached under pyrometer control and the spectra were collected at the beginning of the vaporization step.The spectra reported in the figures were obtained using vaporization from the platform. Results and Discussion In a previous paper it was shown that the presence of intense and structured absorption bands during the atomization of slurries occurs only at wavelengths below 280 nm.3 This obser- vation was related to the type of samples examined in that particular study (soils sludges and ashes) the band systems of SiO and of A1 species being the most relevant features of the vapour phase. The vapour-phase behaviour of the Bituminous Coal SRM 1632b vaporized according to a thermal programme developed for the determination of Cd (platform atomization) is shown in Fig.1. Two distinct systems of absorption bands were observed and are attributed to gaseous SiS. The first system lying between 265 and 320nm shows bands with the profile degraded to red (transition 1Z-1rI).14 The second system between 210 and 250 nm consists of sharp bands evenly spaced Table 1 Instrumental parameters for the vaporization of Si and Ge compounds Element Ashing/"C Vaporization/"C Ge 700- 1000 16OO-20OO Si 1000-1200 1800-2400 * Ar flow set to 0 during vaporization. 0.3 Q) C a 2 a n a 0.15 (I) -. i i= 0 1 0 200 225 250 275 300 325 350 Wave lengt h/n m Fig. 1 Spectra collected during the vaporization of SRM 1632b. Ashing temperature 800 "C atomization 1700 "C. Addition of 8 pg of Pd and 6 pg of Mg. Collection time 0.2 s interval 0.5 s. Scale marks on the time scale also represent zero absorbance for the correspond- ing spectrum (transition 1Z+X-E).14 The two systems almost extend over the UV range covered by the spectrometer.No other band systems were detectable only the outstanding atomic line of Mg at 285.1 nm added as a modifier. The Bituminous Coal SRM 1632b contains 1.89%m/m S and 1.4% m/m Si hence both S and Si can be considered as major elements present during the atomization. The S com- pounds of coals are divided into three classes sulfates sulfides (pyrite) and organic sulfur. Sulfur species derived from the decomposition of such compounds can react with Si and form SiS. The appearance of SiS occurs at low temperature. This is in agreement with the observations of Frech and Cedergren who calculated that the formation of SiS(g) occurs above 1330 K and at partial pressure of 0 lower than 1 x lo-' atm (1 atm = 101 325 Pa).At higher partial pressure the formation of SiS would be limited by the concurrent formation of SiO. Vaporization of Silicon To evaluate the formation of SiO(g) and SiS(g) Si and SiO were vaporized alone in the presence of FeS considered as a 'supplier' of S species during the vaporization step and in the presence of CaSO considered as 'supplier' of S and 0 species. The behaviour of Si and SiO during vaporization at 2000 "C is shown in Fig. 2. Both compounds form SiO(g) (band system 'C-'II between 205 and 250nm).14 The formation of SiO(g) during the vaporization of SiO has already been shown;3 the spectra obtained from SiO are reported for comparison.In the case of Si the formation of SiO is due to the diffusion of 0 into the tube through the dosing hole.15 From the calcu- lations made by Frech and Cedergren SiO(g) represents the major component at 2000 "C with a sufficient partial pressure of 02. Gilmutdinov et have recently demonstrated that the concentration of 0 in a pyrolitic graphite coated graphite tube is higher than generally accepted. The appearance of SiO during the vaporization of Si is faster than in the case of SiO (Fig. 2); this observation holds for all thermal conditions applied. This behaviour can be explained according to the lower thermal stability of Si (m.p. 1410 "C) in comparison with that of SiO (m.p. 1610°C).'7 Owing to the large amount of Si loaded onto the platform the supply of SiO(g) is continuous and the concentration remains almost constant during the vaporization step.It is proposed that SiO(g) is formed throughJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 445 0.6 al C m 0.3 5 a 0 ",? 1 I A \ 1 .t5 * J Y Y W 4 2 . 5 E M "- 190 215 240 265 290 315 340 Wavelengt h/nm Fig. 2 Spectra collected during the vaporization of A Si and B SiOa at 200°C. Collection time 0.2 s interval 0.5 s (see Fig. 1 for explanation) an heterogeneous reaction between Si(1) and 02. In the case of Si02 the release of SiO(g) occurs after the decomposition of the molten oxide. As already mentioned CaSO and FeS2 were used to intro- duce S species during the vaporization of Si and SO2. The vaporization of CaSO alone shown in Fig.3 indicates a series of consecutive reactions. The sulfate is decomposed on the graphite surface with formation after about 0.2 s of SO2 (there is an even succession of bands between 260 and 340 nm). This event is followed by the appearance of CS ('Z-'II transition with an outstanding band at 257.6 nm). The forma- tion of SO and CS has already been demonstrated and the molecular absorption of CS has been used to determine the S 2 .o 1 .o I- t I - ' - - - . t v ~ ~~ v 190 215 240 265 290 315 340 Wavel engt h/nm content in fuel oik2 The SO2 can easily be dissociated to SO (dissociation energy of SO is 1.4eV). However the presence of SO cannot be confirmed as its absorption bands are diffuse and overlap with the very intense bands of SO,. There is no clear evidence to advance an unambiguous mechanism of CS formation as the heterogeneous reaction between SO(g) and surface carbon or the reduction of CaSO on the surface and release of CS(g) can both be proposed equally.In the last five spectra shown in Fig. 3 (from 1.0 to 1.5 s) the CS bands decrease however this coincides with the appearance of a series of sharp and intense bands below 210nm attributable to the fourth positive system of CO ('S'rI tran~ition).'~ The formation of CO by reaction of the 0 entering the dosing- hole has been studied by Sturgeon and Falk," who found the pco to be 1 x atm at 2200 K in a graphite atomizer similar to that used in this work. However vaporization cycles of CaO performed under the same experimental conditions do not show the presence of such an intense system.Therefore CO forms during the vaporization of CaSO in the vapour phase as a product of the decomposition of the sulfate. It is suggested that the CS molecules (dissociation energy 7.4 eV)I7 form CO (dissociation energy 11.1 eV) by reaction with the carbon sites oxidized by the decomposition of SO,. The evolution of the species is very fast (about 1.5 s for the appearance of the various species) even at 2000 "C and continu- ous acquisition of the spectra with short collection time and time interval between the spectra is required. Different sulfates (such as Na,S04) give rise consecutively to SO2 CS and CO. An increase in the vaporization temperature affects the kinetics of the process and CS can hardly be detected above 2200°C if the interval between the acquisition of consecutive spectra is set to 0.5 s.The acquisition of spectra at fixed times with multiple runs and stepwise increases in the wavelength can lead to some loss of temporal resolution hence different sulfates could show the formation of SO or CS depending on the vaporization temperature. l8 When Si is vaporized in the presence of CaSO the most relevant effect is the appearance of the absorption systems of Si molecular species (Fig. 4). The presence of CS lasts only 1.6 al C m e 5 a 11 t 0 190 215 240 265 290 315 340 Wavelen gt h/n m Fig. 3 Spectra collected during the vaporization of CaSO at 2000 "C. Collection time 0.1 s interval 0.1 s (see Fig. 1 for explanation) Fig. 4 Spectra collected during the vaporization of Si and CaSO at 2000 "C.Collection time 0.1 s interval 0.1 s (see Fig. 1 for explanation)446 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0.2 s and is immediately followed by the appearance of the band system of SiS(g) between 260 and 310 nm and the SiO(g) system between 215 and 250 nm. The absorption bands of SiO overlap with the SiS system at short wavelength (Figs. 1 and 2). The intensity of SO and CS bands decreases when CaSO is vaporized in the presence of Si (Figs. 3 and 4). This behaviour suggests that CaSO partly decomposes to SO2 and CS and partly reacts with Si in the condensed phase to form SiS(s) which sublimates at low temperature (940 "C). Hence SiS(g) can be obtained through the following reactions CaS0 + Si-+SiS(s) ( 1 ) SiS (s)-+ SiS( g) (2) The SiO(g) is obtained through the reaction of Si with 0 as described for the vaporization of Si alone because the appear- ance of SiO proceeds as is shown in Fig.2. During the vaporization of SiO and sulfate as the appearance of the Si molecular species is delayed CS vapours last for 0.5 s and are followed by the appearance of SiS and SiO. The evolution of the species observed in Fig. 4 is represented in Fig. 5 in a different way. The consecutive formation of CS and CO (or SiS) is indicated by the appearance time of CS and CO and the features of the concentration profiles. The formation of SiS(g) after the evolution of SO and CS indicates that both S02(g) and CS(g) derived from the reaction of CaSO with C could be precursors of SiS(g) by direct gas-solid reaction CaSO,+C+SO,(g) or CS(g) ( 3 ) S02(g)+ Si+SiS(s) or SiS(g) (4) CS(g) + Si-+SiS(s) or SiS(g) (5) SiS(s)+ SiS( g) (6) The formation of SiS(s) or SiS(g) indicated in reactions (4) and (5) depends on the temperature. However it is not possible to assess the relative likelihood of the solid-solid and gas-solid mechanisms from the vaporization data.The profile of the SiO evolution is similar to that of SiS and it not reported in Fig. 5. On increasing the vaporization temperature to 2400 "C the bands of SiO are less evident and the presence of Si absorption can be observed at 251.6 nm (Fig. 6). The difference between the reactivity of Si and Si02 in the formation of molecular species is particularly evident in Fig. 6. After 1 s in the case of SiO the molecular absorption of CS is still intense even at high temperature.In both situations the formation of SiS seems slightly favourable in comparison with that of SiO. The vaporization from the wall of both SiO and Si02 does not produce remarkable differences with the platform vaporiz- ation in the temperature range examined. 0.10 r 1 0 0.5 1 .o 1.5 Time/s Fig.5 Evolution of molecular species during the vaporization of Si and CaS04 at 2000°C A SO,; B CS; C Co; and D SiS 0.8 0.4 al o e 2 $ 0.8 0.4 0' 1 I 1 I I 1 190 215 240 265 290 315 340 Wavelengthtnm Fig. 6 Spectra collected during the vaporization of A Si and B SiO after 1.0 s (a) at 2000°C; and (b) at 2400°C The behaviour of the pyrite considered as a supplier of S species only and which is commonly found in minerals can be followed in Fig.7. The only species observed is CS originat- ing in the condensed phase by the reaction of pyrite with the graphite surface. The intensity of the absorption bands is continuously increasing and lasts until the end of the fraction of the step examined (1.5 s). The appearance time of CS is similar to that found for CaSO (about 0.5 s) as the same vaporization temperature was used in both cases. This simi- larity also supports the formation of CS in the condensed phase in the case of sulfate vaporization. The presence of O2 in the tube does not lead to formation of CO. This observation agrees with the hypothesis described above about the formation 1.6 s m ; Ll a 1 .5 v) \ .- E k- I t 0 190 215 240 265 290 315 340 Wave le n g t h/n m Fig.7 Collection time 0.1 s interval 0.1 s (see Fig. 1 for explanation) Spectra collected during the vaporization of FeS at 2000 "C.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 A - A . ~ . - I I I I I 447 of CO through the reduction of SO (or SO) on the graphite and reaction of CS on the oxidized sites. The simultaneous vaporization of Si and FeS leads to the formation of SiS(g) as the main component of the vapour phase (Fig. 8). The system at short wavelengths is slightly perturbed by the presence of some SiO(g). As no CS can be detected the pyrite reacts directly with the high load of Si on the platform. When the vaporization of Si and FeS2 is per- formed from the wall the same behaviour reported in Fig. 8 is observed. The only difference is represented by the absence of perturbations in the appearance of the 'Z-'Z system of SiS which indicates the absence of detectable SiO.The reaction with O2 entering through the dosing hole is probably less favourable in the case of wall atomization in comparison with the platform. The cloud of O2 is in fact mainly distributed around the hole.I6 The vaporization of SiO and FeS leads to some increase in the intensity of the bands of SiO at the end of the vaporization step but the features of the vapour phase are similar to those reported in Fig. 8. Vaporization of Germanium The behaviour of Ge follows the trend observed for Si although the appearance of molecular species occurs about 300 "C below that of Si species (Ge m.p. 940 "C). The irregular behaviour of Ge during atomization has been attributed to the loss of the element as molecular species by various as GeO and GeS are very volatile (b.p.710 and 430 "C respectively). Frech and Baxterlg concluded that the conditions for the quantitative formation of Ge atoms are not readily achieved." The instrumental conditions and the presence of acids affect the formation of GeO and GeS and consequently the efficiency of atomization. The spectrum of GeS(g) has been shown and the evolution of GeS has been followed at a fixed wavelength at one of the most intense bands of the 'C-'lJ ~ y s t e m . ~ The spectral behaviour of Ge vaporized at 1700 "C is shown in Fig. 9. As in the case of Si the monoxide GeO(g) is formed during the vaporization of the element. The GeO spectrum is characterized by the presence of two systems. The one between 230 and 300nm is due to the 'Z-lII transition while the one 1.6 8 0 : m a 4 0.8 1.5 Fig.8 Spectra collected during the vaporization of Si and FeSz at 2000 "C.Collection time 0.1 s interval 0.1 s (see Fig. 1 for explanation) 1.6 0) C m < 0.8 n s a I 0 190 215 240 265 290 315 340 Wavelengthhm Fig. 9 Collection time 0.1 s interval 0.2 s (see Fig. 1 for explanation) Spectra collected during the vaporization of Ge at 1700°C. below 230 nm is attributed to a lZ-'Z tran~iti0n.l~ The features of the GeO spectrum compare well with those of SiO except for the system at short wavelengths which occurs below 190nm in the case of Si. The vaporization of GeO leads to the formation of GeO(g) analogous with the behaviour of Si and SiO,.Above 1700°C the evolution of GeO is very fast and also occurs during the fast heating from the ashing to the atomization step. The presence of CaSO gives rise to the formation of GeS(g) and GeO(g) as already shown for Si (Fig. 10). The spectrum of GeS is characterized by the band system between 280 and 340nm attributable to the 'E-'n transition and the system of sharp bands between 220 and 240 nm ('Z-'C tran~ition).'~ At 1700"C CO SO and CS molecules cannot be detected owing to the low temperature. The spectrum reported by 2.0 .4 a C v) (II -. e E P F a 1.0 .7 r.c" t 0 190 215 240 265 290 315 340 Wave I e n g t h/n m Fig. 10 Spectra collected during the vaporization of Ge and CaSO at 1700°C. Collection time 0.1 s interval 0.2 s (see Fig. 1 for explanation)448 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Dittrich et ~ l . ~ obtained during the atomization of Ge in the absence of sulfuric acid can be attributed to GeO(g) which forms easily during the thermal treatment. The presence of FeS leads to a vapour-phase behaviour very similar to that obtained in the presence of CaSO (Fig. 11). The vapour species appear at the very beginning of the step while in the case of CaSO the appearance of these species is delayed. Although Dittrich et uL7 have suggested that GeS(g) is formed through a gas-phase reaction Doidge and McAllisterg proposed that the formation of GeS occurs in the condensed phase as they detected this species in an electrother- mal vaporization mass spectrometry system operating under vacuum.The formation of GeS in the condensed phase with the fast evolution of the vapours follows the solid-solid mechanism proposed for SiS. The main difference between Si and Ge concerns the pres- ence in the case of Ge of GeO(g) in any of the thermal conditions examined regardless of the type of vaporization (from the platform or from the wall) or the type of S species added while in the case of Si it is possible to obtain SiS(g) as single molecular species from wall vaporization. Spectral Characteristics The spectra of the gaseous oxides and sulfides of Si and Ge are shown in Fig. 12. Spectrum D is the sum of the spectra of GeS and GeO. The band systems for Ge species are shifted to higher wavelengths in comparison with those of the Si species. As already mentioned the 'C.-lC transition for SiO falls below 190 nm.The wavelengths and relative intensities of the bands of the 'Z-'II systems are reported in Table2. The spectral data compare well with those reported by Pearse and Gaydon,' although the relative intensities of the bands are fairly different in some instances. It should also be pointed out that the band systems of the diatomic molecules CS and CO as shown in the previous figures belong to 1C.-'17 transitions and this type of transition is clearly observed in ab~orpti0n.l~ Some of the band systems (SiO and SiS) can be observed in real samples that have appropriate Si contents. During the atomization of trace elements the correction of such non- specific absorption is therefore required. Ohlsson and Frech2* showed the splitting of the bands of PO and the consequent over-correction of the background for some analytical lines in inverse Zeeman-effect ETAAS.When the Zeeman effect is used for the correction of non- 2.0 r 1 .4 u) -. E .7 ir 0 190 215 240 265 290 315 340 Wavelengthlnm Fig. 11 Spectra collected during the vaporization of Ge and FeS at 1700 "C. Collection time 0.1 s interval 0.2 s (see Fig. 1 for explanation) 0.6 a & 0.4 2 u) a d 1 1 B r Y \I' 0.2 A 1 0 190 215 240 265 290 315 340 Wavelengt h/n m Fig. 12 Spectra of A SiO; B GeO; C SiS; and D GeO+GeS Table 2 Spectral data of Si and Ge oxides and sulfides SiO SiS GeO GeS A/nm I,* 213.6 3 215.3 3 217.3 6 219.4 4 221.1 9 223.1 4 225.1 10 229.2 10 234.0 8 236.0 5 238.1 2 240.8 6 248.3 4 250.6 2 256.1 1 258.1 1 Afnm 253.8 255.2 256.6 258.6 260.0 262.0 263.7 265.7 267.8 271.5 273.9 275.6 277.6 282.1 285.8 288.5 295.3 297.4 302.5 304.5 Ir 7 7 9 8 9 7 10 6 10 9 6 8 8 6 3 6 4 3 1 1 A/nm I 234.7 4 238.1 6 241.5 8 243.5 7 244.9 9 247.2 7 248.6 9 25k.O 7 252.7 10 256.8 9 261.2 8 263.2 4 265.6 6 268.0 6 272.7 4 275.1 2 277.5 1 280.2 3 288.0 2 296.2 1 298.9 1 Alnm I 282.4 7 285.1 8 288.2 10 291.2 10 294.6 9 298.0 8 301.5 6 302.8 5 304.8 3 306.6 5 310.3 3 312.0 3 315.9 3 321.8 2 328.0 1 334.1 1 *Relative intensities I refer to the most intense band of each system and are calculated from the baseline of the system.specific absorption the various bands behave in different ways according to the type of transitions involved. The 'C-'Z system of SiS (Fig. 12) although consisting of sharp and symmetrical bands does not show any splitting of the bands in the presence of a magnetic field as the magnetic moment associated with the orbital and spin angular moments of the electrons is zero." The magnetic moment associated with orbital and spin angular moments of electrons represents the major contribution to the total magnetic moment of a diatomic molecule therefore the splitting of bands would be very small even for very intense magnetic fields.In the case of the 'Z-'II transitions (systems of SiO and SiS at high wavelength) the band splitting is given by the magnetic moment of the 'I7 state which corresponds to 1 J T-l. Hence splitting of molecular lines having width similar to that observed for atoms can occur but only for low values of the total angular momentum J.,l Therefore the interference of SiO and SiS with background correction using the Zeeman effect owing to splitting of molecular lines should appear in limited and defined positions of the UV spectrum and only a few lines of analytical interest could be affected.The same observations made for the splitting of Si speciesJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 449 can be extended to Ge. The analysis of slurries having Ge as a major element is not reported however the presence of sulfuric acid in solutions containing Ge at high concentration could be a potential source of over-correction owing to the formation of GeS(g). The presence of S species in slurries could lead to loss of trace elements as sulfides.However it has been shown3 that the addition of a Pd-Mg modifier to a slurry decreases the intensity of the molecular bands of SiO. This effect although not examined in the present work could be extended to SiS. The increase in ashing and atomization temperatures attain- able when chemical modifiers are used further reduces the presence of molecular species in the vapour phase. The authors gratefully acknowledge a research fellowship granted to V.K. by the Commission of the European Communities (contract No. ERB-CIPA-CT-920371). References Dittrich K. CRC Crit. Rev. Anal. Chem. 1986 16 233. Tittarelli P. and Lavorato G. Anal. Chim. Acta 1987 201 59. Tittarelli P. and Biffi C. J. Anal. At. Spectrom. 1992 7 409. Frech W. and Cedergren A. Anal. Chim. Acta 1980 113 227. Muller-Vogt G. and Wendl W. Anal. Chem. 1981 53 651. Rademeyer C. J. and Vermaak I. J. Anal. At. Spectrom. 1992 7 347. Dittrich K. Mandry R. Mothes R. and Judelevic J. G. Analyst 1985,110 169. 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Kolb A. Muller-Vogt G. and Wendl W. Spectrochim. Acta Part B 1987 42 951. Doidge P. S. and McAllister T. J. Anal. At. Spectrom. 1993 8 403. Tittarelli P. Lancia R. and Zerlia T. Anal. Chem. 1985,57,2002. Shekiro J. M. Skogerboe R. K. and Taylor H. E. Anal. Chem. 1988,60 2578. Majidi V. Ratliff J. and Owens M. Appl. Spectrosc. 1991 45 473. Biffi C. and Tittarelli P. Riv. Combust. 1991 45 197. Pearse R. W. B. and Gaydon A. G. The IdentiJcation of Molecular Spectra Chapman and Hall London 3rd edn. 1963. Sturgeon R. E. and Falk H. Spectrochim. Acta Part B 1988 43 421. Gilmutdinov A. K. Chakrabarti C. L. Hutton J. C. and Mrasov R. M. J. Anal. At. Spectrom. 1992 7 1047. CRC Handbook of Chemistry and Physics ed. Weast R. C. CRC Press Boca Raton 64th edn. 1983. Welz B. Bozsai G. Sperling M. and Radziuk B. J. Anal. At. Spectrom. 1992 7 505. Frech W. and Baxter D. C. Spectrochim. Acta Part B 1990 45 867. Ohlsson K. E. A. and Frech W. J. Anal. At. Spectrorn. 1989 4 379. Herzberg G. Spectra of Diatomic Molecules Van Nostrand Princeton 2nd edn. 1955 Paper 31047376 Received August 5 1993 Accepted October 12 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900443

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 45 1 Determination of Silicon in Fine Gold by Solution and Solid Sample Graphite Furnace Atomic Absorption Spectrometry and Inductively Coupled Plasma Atomic Emission Spectrometry* Michael W. Hinds and Valentina V. Kogan Royal Canadian Mint 320 Sussex Drive Ottawa Ontario Canada K1A OG8 Four methods are described for determining silicon in fine gold. These include a solid sample electrothermal atomic absorption spectrometry (ETAAS) method with aqueous calibration standards an ETAAS solution based method with matrix matched standards an inductively coupled plasma atomic emission spectrometry (ICP-AES) method with matrix matched standards and a spark ablation ICP-AES method. The first three methods give comparable results although the solid sample ETAAS method is more error prone.Results from these techniques were used for the characterization of gold reference materials which were used as calibration standards for determining silicon in gold by spark ablation ICP-AES. In general limits of detection were 3 pg g-’ or better for the methods presented. Keywords Silicon; gold matrix; solid sample; electrothermal atomic absorption spectrometry; inductively coupled plasma atomic emission spectrometry The assay of high volumes of fine gold products can be accomplished by spark ablation inductively coupled atomic emission spectrometry (ICP-AES) and laser ablation induc- tively coupled plasma mass spectrometry (ICP-MS). In these methods the concentrations of common trace metal impurities are determined and the purity of gold is calculated by difference.Calibration is carried out using solid gold reference materials manufactured and characterized at the Royal Canadian Mint (RCM).l Silicon is a common low level impurity in gold. This is not surprising since most gold originates in siliceous ore. Consequently calibration standards manufactured to deter- mine impurities in gold by solid sample spectrometry must also contain silicon. Each trace metal in the gold reference materials was determined by at least two different analytical techniques. Two electrothermal atomic absorption spec- trometry (ETAAS) methods were developed for determining silicon in gold a solution method that used matrix matched standards and a solid sample method that used aqueous standards for calibration.Similarly two ICP-AES methods were also developed a solution based method and a spark ablation method for solid samples. The two ETAAS methods and the solution ICP-AES methods are described and com- pared. The spark ablation ICP-AES method is included to demonstrate that this solid sample ICP-AES method can produce acceptable silicon concentration values. Experimental Electrothermal atomic absorption spectrometers Two different atomic absorption spectrometers were used in the course of this work a PE 5000 and a PE 3100 (Perkin- Elmer Norwalk CT USA) both with an HGA 500 atomizer (Perkin-Elmer). Data collection was carried out using a per- sonal computer connected to the spectrometers via a DAS8 12 bit analogue-to-digital converter (Keithley Metrabyte Taunton MA USA).The data collection hardware and software were based on the work of Allen and Jackson2 Background corrected (continuum source) integrated absorbance values were calculated and used throughout this work. The outside wall temperature was measured by an optical pyrometer which was also computer interfaced.’ ~~ ~ * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993. A silicon hollow cathode lamp was operated at 40 mA. Two wavelengths were used the resonance line at 251.6 nm and the less sensitive non-resonance line at 221.1 nm. In both cases a slit-width of 0.2 nm was used. Temperature programmes for wall and platform atomization are listed in Table 1.Inductively coupled plasma atomic emission spectrometer The spectrometer used in this study was an ICAP 9000 inductively coupled plasma atomic emission spectrometer (Thermo Jarrell Ash Franklin MA USA). Samples could be introduced as liquids via a cross flow nebulizer or as solids (conducting solids only) via a spark ablation device (supplied by the manufacturer). An electronically controlled wave form spark source was used to sample solid metals and to generate a metal aerosol. A flow of argon gas swept the aerosol into the ICP torch where the material was excited. The resulting emission was measured by a 0.75 m focal length direct reading polychromator. A sapphire tipped torch was used to minimize background silicon emission.The optimum wavelength at 251.6nm was selected on the basis of sensitivity and freedom from interferences; these parameters were examined theoreti- cally (from a computer based wavelength library) and empiri- cally. The optimized instrumental parameters are listed in Table2. For solution analysis vanadium was used as an internal standard in 2% dissolved gold solutions and concen- trations were determined by the method of standard additions. Reagents Water used in these experiments was distilled and de-ionized by a Nanopure I1 system (Barnstead/Thermolyne Dubuque IW USA). Commercially prepared high-purity hydrochloric and nitric acids were used (Fisher Scientific Ottawa Ontario Canada) for the preparation of samples and standards. Calibration solutions were prepared from 1000 pg g-’ stock solutions (High Purity Standards Charleston SC USA) that are traceable to National Institute of Standards and Technology (NIST Gaithersburg MD USA) Standard Reference Materials.To ensure matrix matching each Cali- bration solution also contained an appropriate amount of dissolved Au 99.999% purity (Metalor USA Refining Corporation North Attleborough MA USA).452 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 Temperature programmes for silicon in gold solutions L'vov platform Wall atomization Step Temperature/"C Ramp time/s Hold time/s Temperature/"C Ramp time/s Hold time/s 500 1 20 200 15 10 Dry Cool-down 20 1 10 20 1 10 Clean-out 2650 1 3 2650 1 3 Pyrolysis 1200 1 15 1400 1 15 Atomize* 2650 0 6 2650 0 6 * Read 1 s before atomization step and gas flow stopped during atomization.Table 2 Optimum instrumental parameters for trace element determi- nation in gold solutions and solid samples by ICP-AES Parameters Solution Spark ablation Forward r.f. power/W 1100 1100 Reflected r.f. power/W < 30 < 30 Outer gas flow rate/l min-' 19.0 19.0 Carrier flow ratefi min-' 0.4 0.4 Solution uptake rate/ml min- ' - 1.8 Sample Preparation So 1 u t io n For determinations by ETAAS gold samples (0.5g) were dissolved in 4 ml of aqua regia [HCl+HN03(3+1)] in a covered Teflon beaker under minimal heat for about 30min. The solution was either transferred into a 50ml plastic cali- brated flask and brought up to volume with distilled de-ionized water or 15 ml of water were added directly to the dissolved gold (evaporation losses were minimal).For determinations by ICP-AES 2% dissolved gold solu- tions were prepared by dissolving 2 g of a gold sample in 20 ml of aqua regia in a closed Teflon vessel. Samples were heated in a microwave oven (Model MDSSlD CEM Matthews NC USA) for 30min at 75% power. To minimize the risk of an explosion from hydrogen released in the dissolution process the vessels were purged with argon just prior to being sealed. A pressure controlling device maintained the pressure at 100 psi (1 psi "N 6.895 x lo3 Pa) but did not permit the pressure in the closed vessel to exceed this value. Upon dissolution samples were transferred into 100 ml plastic calibrated flasks as described above. Solid sample For ETAAS solid pieces of gold were cut from either shavings or chunks by using a fine sharp stainless-steel knife.Pieces between 0.2 and 0.5 mg were placed on a tared weighing pan and the exact mass was recorded. Two different balances were used in this work a Mettler AE163 analytical balance (smallest division 0.01 mg) and a Mettler UM3 electronic balance (small- est division 0.0001 mg). The weighed gold sample was trans- ferred into a 10 ml plastic cup where it could conveniently be picked up by a fine curved-nose steel forceps and placed in the graphite furnace. This was facilitated by the use of a funnel (a pipette tip cut off 7 mm from the tapered end) set in the dosing hole to assist dropping the sample into the atomizer. One could also insert the sample by dropping it directly through the dosing hole.Solid samples for spark ablation ICP-AES were easily prepared by machining a flat surface on a metal lathe and then mechanically polishing the surface with aluminium car- bide paper. Sample sizes ranged from 2 cm diameter and 0.1 cm thickness to 10 cm diameter and 5 cm thickness. Results and Discussion Dissolved Gold Solution Analysis Silicon is not typically soluble in aqua regia and it was thought that it would remain in particulate form once the gold matrix was dissolved. This was investigated by dissolving a gold sample (doped with silicon) in aqua regia diluting with water and then leaving the solution in a plastic centrifuge tube undisturbed. Silicon absorbance was measured by ETAAS when the solution was originally made up and then each day for five days.The absorbance values remained the same throughout the time of the experiment. This indicated that silicon remains in solution either as a soluble species or as a stable colloidal suspension in the dissolved gold solution (12.5 g 1-1 of Au and approximately 15% hydrochloric acid). However in more dilute solutions (4-10-fold dilution) inte- grated absorbance values for silicon decreased after 1 to 2 h. Upon re-agitation the original integrated signal values were obtained. It would appear then that silicon might not be fully dissolved in the solution and forms a stable suspension in concentrated solutions but is unstable in dilute solutions. Electrothermal Atomic Absorption Spectrometry Optimization of parameters Pyrolysis temperature does not greatly effect the silicon analyt- ical signal from the platform up to 1400"C as shown in Fig 1 (line A).The effect of pyrolysis temperature for 10 ng of Si atomized from the wall of the graphite furnace and observed at 221.1 nm was also studied (Fig. 1 line B). Silicon was stable up to 1600°C. This might be due to the larger analyte mass atomized and observation at the less sensitive wavelength which may mask the smaller changes observed with the platform experiment. The effect of the amount of gold deposited within the atomizer on the integrated absorbance measured for 2 ng of Si is shown in Fig. 2. The signal observed decreases as the 500 1000 1500 2000 TemperaturePC Fig. 1 Effect of pyrolysis temperature on the integrated absorbance for A 2ng of Si with 10 pg of Au from platform atomization at 251.6 nm; and B 10 ng of Si (aqueous solution) from wall atomization at 221.1 nmJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 453 0.3 I I I 1 I I 0 5 10 15 20 Mass of gold/pg Fig.2 Effect of mass of gold in the atomizer on the integrated absorbance of 2 ng of Si from platform atomization at 251.6 nm 0.4 I 1 0 5 10 15 20 25 30 HC1 (%I Fig. 3 Effect of hydrochloric acid concentration on the integrated absorbance of 2 ng of Si from platform atomization at 251.6 nm amount of gold increases. This is consistent with experiments performed by Frech et a13. who proposed that this observed attenuation is due to the formation of fine condensed metal (or oxide) particles within the analyte volume. These particles can act as adsorptive surfaces for analyte atoms in the gas phase which leads to the reduction in the observed integrated signal.Also metal matrix condenses at the cooler ends of the atomizer which also acts as an adsorptive s ~ r f a c e . ~ However the former mechanism would probably be dominant because only very small amounts of gold re-condense at the ends of the atomizer (if at all) at the atomization temperature of 2650 "C and therefore would probably have little effect on the silicon atoms in the gas phase. The effect of hydrochloric acid was investigated since it is another major component of dissolved gold solutions. Chloride present as sodium chloride was reported to be an interferent5 The presence of hydrochloric acid did not appear to affect the integrated signal observed for silicon (Fig.3) however increases in acid concentration decreased the lifetime of the platform. Inductively coupled plasma atomic emission spectrometry There did not appear to be any interference from the gold matrix on the silicon wavelength as shown in Fig. 4. Nevertheless background correction was used because rela- tively high levels of emission were observed for both the acid blank and a high-purity gold blank. There was approximately a 30% decrease in the observed silicon emission due to the gold matrix as compared with the same concentration of silicon in water. This effect did reduce sensitivity but did not have a large detrimental effect on the determination. 150 I I 81 12 I I J Wavelengthhm 251.756 251.476 251.616 Fig. 4 Wavelength scan about the Si emission line at 251.616 nm for A a 2% high-purity gold blank; B 2.0ppm Si in a 2% high-purity gold solution; and C 5.0 ppm Si aqueous solution from ICP-AES atomization silicon atomic absorbance peak shapes from an aqueous solution and a solid gold sample were nearly coinci- dent (Fig.5). Temperature measurements indicated that atom- ization of the majority of silicon atoms from both sample types occurred as stabilized temperature conditions were established within the atomizer. It appears that silicon atoms originating from both the solid sample and the aqueous standards were exposed to similar temperature environments. Thus the residence time for each should be similar and the value of integrated absorbance should also be comparable.This means that aqueous silicon can be used as a calibration standard for the analysis of solid samples. Preliminary determi- nations using aqueous standards were in agreement with results for the determination of silicon by ICP-AES. This led to more detailed experiments being carried out as outlined below. Wall atomization Graphite tubes were modified by enlarging the dosing hole from 2 to 3.5mm in diameter in order to observe physical changes in 0.5mg gold samples. No change was observed when the temperature of the drying step was set to 200°C. However at the pyrolysis temperature ( 1400 "C) samples melted and formed small spheres after the pyrolysis step. Observations of the physical effects at atomization tempera- tures were carried out with a transversely heated graphite atomizer (THGA) (after the platform was taken out) because the THGA permitted convenient recovery of the remaining gold sample (compared with the Massmann type atomizer).A maximum temperature of 2600°C could only be used because of software limitations. After one complete atomizer firing (Table l) it was found that the sample was only reduced in mass by about 24%. The non-resonance line for gold at 274.8 nm was monitored during consecutive firings for one Solid Sample Analysis Electrothermal atomic absorption spectrometry Experimentation with direct solid sample introduction into a graphite atomizer was initiated after observing that with wall 0 1 2 3 4 Timels Fig.5 Absorbance peak profiles for long of Si from an aqueous solution (A) and from a 0.4 mg solid gold sample (B); C is the outside tube wall temperature454 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 3 Figures of merit for three different approaches to determining silicon in gold by solid sample ETAAS; value given +1 standard deviation Wall Wall Platform 221.1 nm 251.6 nm 221.1 nm gas stop gas flow gas stop Characteristic mass*/pg 226 44 750 + 110 190 i- 14 Detection limitt/pg g-' 4.6 16 - * Mass of analyte whose absorbance is equal to 0.0044 s. t k = 3 based on estimated 0.35 mg gold sample. gold sample (0.5 mg). High integrated absorbance signals were observed for three consecutive firings. No variation from the baseline was seen after the fourth firing. This was consistent with the observations of Irwin et aL6 for nickel samples who noted that the majority of the nickel sample remained in the atomizer after one heating cycle.As noted previously the peak shapes for silicon from solution and solid sample were nearly coincident (Fig. 5). Temperature measurements showed that heating at maximum power permit- ted the temperature to stabilize just as atomization occurred. This particular situation is mainly due to the refractory nature of silicon because more volatile elements would have atomized as the temperature of the atomizer was rapidly increasing. Two approaches to the determination of solid samples were used gas flow during atomization with detection at the resonance line for silicon (25 1.6 nm) and gas stop flow during atomization with detection at a non-resonance line for silicon (221.1 nm).In both cases peak shapes were quite similar. Sensitivity was fairly low for gas flow rates between 100 and 250mlmin-' and was similar to the value shown in Table 3 for a flow rate of lOOmlmin-'. Characteristic mass values for flow rates of 50 ml min-' approached those obtained from detection at 221.1 nm with gas stop conditions. Platform atomization Platform atomization was tried and the figures of merit are presented in Table 3. Improved sensitivity and reproducibility were observed compared with wall atomization. Unfortunately there was incomplete atomization of silicon from the solid gold samples. Subsequent re-firing of the atomizer after a determination in many cases produced another silicon signal that was about 20-30% of the original signal.This was not acceptable for quantitative determinations and was probably caused from the reduced heating rate of the platform which slowed the release of silicon from the solid sample and ulti- mately led to incomplete atomization. Method comparison Limits of detection (LODs) (Table 3) were based on the integrated absorbance measured for the empty atomizer firing immediately following the determination of silicon in a solid sample. As previously noted about 75% of the sample mass remains after the first atomization cycle. It was estimated that the average amount of gold remaining in the furnace used for the calculation of LODs would be 0.35 mg. Gold remaining in the atomizer did not appear to contain any measurable levels of silicon. High-purity gold was not used as a blank because it contains low levels of silicon (<1 pgg-') which may not be homogeneously distributed in the gold.The estimates of LOD indicate that the method utilizing the non-resonance line at 221.1 nm has a lower LOD than the method involving gas flow during atomization. No reasonable estimate could be obtained for atomization from a platform because of the frequent occurrence of memory effects Monitoring silicon absorbance at the non-resonance line (221.1 nm) was the more favoured technique because of the I I I 251.476 251.616 251.756 Wavel e ngt h/n m Fig. 6 Wavelength scan about the Si emission line at 251.616 nm for 90.pg g-' Si in A gold reference material FAUlO and B 27.8 pg g-' Si in gold reference material FAU8 from spark ablation ICP-AES better sensitivity and better LODs obtained compared with measuring the absorbance at the resonance line (251.6 nm) with a high gas flow during atomization (noted in the second column of Table 3).Spark ablution inductively coupled plasma atomic emission spectrometry Spark ablation ICP-AES is a well established technique for the determination of trace metals particularly for the iron and steel ind~stry.~ The accuracy of trace element determinations by this technique depends mainly on the concentration values assigned to solid sample calibration standards and the homo- geneity of the standards. Gold reference materials have been prepared by the RCM specifically for this purpose. Trace element concentrations have been determined by at least two independent methods.A detailed account of the manufacture and characterization of these reference materials has been outlined by Kogan et al.' The calibration graph was linear for the three standards covering silicon concentrations up to 28 pg g-l with a linear regression coefficient of 0.9964. A scan of the emission about the 251.6 nm silicon emission line confirmed that there was little evidence of interference from the gold matrix (Fig. 6). The peak areas for each of the profiles outlined in Fig. 6 are proportional to the silicon concentrations determined by the other methods described in this paper profile A 9.0k2.2 pg 8-l (k 1 standard deviation) and profile B 27.8k4.8 pg g-'. The average diameter of the ablated craters was 2.4k0.9 mm and the average depth was about 0.25 mm.The amount of gold ablated was about 1 mg determined by volume calculations (assuming a cylindrical crater shape) and by measuring the mass before and after ablation. Comparison of Analytical Results A comparison of analytical results for three of the methods is shown in Table 4. In general there is very good overlap between solution concentration values obtained by ETAAS and ICP- AES. Solid sample ETAAS as one would expect was less Table 4 Comparison of the determination of silicon in different gold reference materials by different methods; values given & 1 standard deviation CSil/Clg g-' Methods FAU8 FAUlO Solid sample ETAAS* (n = 4) 30$4 6.9 & 1.3 Solution ETAAS (n = 4) 9.0 & 1.4 Solution ICP-AES (TI = 5 ) 9.1 f 0.8 27.2 + 1.8 26.4 t_ 1.7 * 221.1 nm 0 gas flow during atomization.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 455 precise and somewhat biased compared with the solution based methods. Nevertheless values obtained through solid sample ETAAS overlapped with the solution methods for FAU8 gold reference material (RCM). The solid sample value for FAUlO gold reference material (RCM) is significantly different from solution methods as determined by a t-test (n= 4 95% confidence level). This value can be considered to be somewhat comparable especially if one takes into account all the potential errors associated with solid sample methods.' Spark ablation ICP-AES was used to determine silicon in a gold reference material undergoing certification. Existing gold reference materials with different silicon concentration levels (determined by at least two of the three other aforementioned techniques) were used for calibration.A concentration value of 45 pg 8-l was obtained. This was in good agreement with the value of 47 pg g-' obtained from determination using solution ICP-AES. The experiment was repeated for another gold reference material. There was good agreement between the value of 29.1 f0.9 pg 8-l (+ 1 standard deviation; n=4) obtained from solution ICP-AES and the value of 29.10+ 1.63 pg 8-l (n = 3) from spark ablation ICP-AES. Figures of merit for the four techniques are compared in Table 5. Sample masses presented to the atom source were compared. Solution ICP-AES used the largest amount of sample whereas the smallest amount was consumed by the solution ETAAS method.The sample uptake rate for ICP- AES was 1.85 ml min-' for 30 s per determination. A factor of 0.02 was applied to account for the 2% nebulizer efficiency. Sample masses consumed by the solid sample methods (ETAAS and ICP-AES) were within a factor of 2 of each other. It is difficult to compare sensitivities between AAS and AES techniques. Comparisons were made between similar methods. As one would expect solution ETAAS is more sensitive than solid sample ETAAS yet both have about the same LOD. The AES methods are less sensitive than ETAAS techniques but appear to be more reproducible as denoted by the lower LODs. Conclusion Four different analytical methods for determining silicon in fine gold have been presented and the method involving the analysis of gold solutions by ETAAS appears to be the most sensitive technique.It was found that the determination of silicon in gold by solid sample ETAAS (wall atomization) with calibration by aqueous standards was reasonably accurate despite the physi- cal differences between the aqueous calibration standards and the solid gold sample. The main reason for this phenomenon appears to be that the atomization of silicon occurs as steady- state temperature conditions are established within the graphite furnace for aqueous solution and solid samples. However for this application solid sample ETAAS has limited utility Table 5 Comparison of figures of merit for solid sample and solution based techniques for the determination of silicon in gold Solid sample Solution methods methods ETAAS ICP-AES ETAAS ICP-AES 0.5 1 0.10 220 Sample mass/mg Sensitivity Characteristic mass*/pg 39 - 226 - Counts (ppm-') - Detection limit/pg g-' 3 1 3 1 30 - 22 * Mass of analyte whose integrated absorbance signal is equal to 0.0044 s.because it is more time consuming and less precise than solution ETAAS. The determination of silicon by solution ICP-AES is con- venient but there is a slight decrease in sensitivity owing to the dissolved gold matrix. Spark ablation ICP-AES is some- what more sensitive and is very convenient for the routine analysis of large numbers of solid samples. However this method requires calibration by solid gold reference materials which up till recently have not been available. The authors thank L. McKay G. Ocampo and G. Valente for their assistance in completing the experiments for this paper. M.H. also thanks K. W. Jackson (New York State Department of Health) for the graphite furnace temperature measurements and V. Luong (National Research Council of Canada) for assistance in computer interfacing the atomic absorption spectrometer used in these experiments. References Kogan V. Hinds M. W. Ocampo G. and Valente G. in Precious Metals 1993 ed. Mishra R. International Precious Metal Institute Allentown PA 1993. Allen E. and Jackson K. W. Anal. Chim. Acta 1987 192 355. Frech W. L'vov B. V. and Romanova N. P. Spectrochim. Acta Part B 1992,47 1461. Frech W. Li K. Berglund M. and Baxter D. C. J. Anal. At. Spectrom. 1992 7 141. Frech W. and Cedergren Anal. Chim. Acta 1980 113 227. Irwin R. Mikkelsen A. Michel R. G. Dougherty J. P. and Preli F. R. Spectrochim. Acta Part B 1990 45 903. Lemarchand A. Labarraque G. Masson P. Broekaert J. A. C. J. Anal. At. Spectrom. 1987 2 481. Bendicho C. and de Loos-Vollebregt M. T. C. J. Anal. At. Spectrom. 1991 6 353. Paper 310561 OD Received December 17 1993 Accepted October 12 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900451

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

457 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Determination of Nickel in Serum of Haemodialysed Patients by Means of Electrothermal Atomic Absorption Spectrometry With Deuterium Background Correction* Marina Patriarcat and Gordon S. Fell Department of Pathological Biochemistry University of Glasgow Royal Infirmary Glasgow G4 OSF UK In this paper an improved method for the determination of Ni in serum by means of electrothermal atomic absorption spectrometry (ETAAS) with deuterium background correction is described. Analysis was performed after a 1 + 1 dilution of the serum samples with a solution containing 1 O/O v/v HN03 and 0.25% v/v Triton X-1 00. Aqueous Ni standard solutions were used for calibration. Sensitivity and accuracy were comparable to those reported for procedures based on Zeeman-effect background corrected ETAAS. The characteristic mass was 14 pg and the average analytical recovery was 101.5+4.8% (n=9).The analysis of Seronorm Trace Element Control Serum yielded a value of 3.21 f 0.1 7 1-19 I-' (n = 10) as compared with the recommended concentration of 3.2 pg I-'. For the Standard Reference Material Bovine Serum SRM 8419 from the National Institute of Standards and Technology a value of 0.46 & 0.05 1-19 I-' (n =4) was determined by this method in agreement with recent findings of other researchers. Improved precision (5.8% within-day 7.5% between- day) and detection limit (0.15 pg I-') in comparison with a previously reported procedure based on ETAAS with deuterium background correction was obtained. Serum Ni levels determined by this method in 25 haemodialysed patients ranged from 0.7 to 4.0 pg I-' [mean & standard deviation (SD) 2.4 f 0.9 pg I-')] and in eight women with normal renal function from 0.3 to 0.9 (mean f SD 0.5 0.2 pg I-').Keywords Nickel; serum; haemodialysis; atomic absorption spectrometry; deuterium background correction Hypernickelaemia has been reported to occur in pathological conditions such as acute myocardial infarction,l- rheumatoid arthritis4 and in subjects undergoing regular haem~dialysis.~-' Nixon et al.' found that the average concentration of Ni in serum of haemodialysed patients was 6.38 & 3.36 pg l-l in comparison with 0.14 k0.09 pg 1-1 in healthy controls. Although the determination of such low levels of Ni in serum required sample preparation and analysis to be carried out in a class 100 environment,' in the experience of the present workers the investigation of the much higher Ni concentrations found in haemodialysed patients can be carried out under less strict conditions.In healthy subjects Ni is rapidly excreted in urine with bile hair and sweat probably playing a minor role." The mechan- ism of Ni clearance from the blood is still unknown although Glennan and Sarkar" have suggested that it could involve an equilibrium between the complexes of Ni with albumin and low relative molecular mass compounds mainly L-hystidine. The observation that patients with chronic renal failure main- tained on haemodialysis have hypernickelaemia indicates that Ni is not completely removed from the body by this treatment and could accumulate in bone and tissues.Nickel is a toxic substance which has been recognized as a carcinogenic agent and a cause of cutaneous and systemic hypersensitivity in man. Acute Ni intoxication and allergic reactions have been reported after dialysis with contaminated fluids.12,13 Hopfer et aL5 highlighted the similarity between a number of disorders observed in patients undergoing long-term haemodialysis and the effects observed in rodents after parenteral administration of NiC1,. These included lipid per~xidation,'~ impaired cellular and humoral imm~nity'~-'~ and hyperpr~lactinaemia.~'*~' At present electrothermal atomic absorption spectrometry (ETAAS) is the simplest and most reliable technique for the determination of Ni in biological fluids.Analytical procedures using Zeeman-effect background correction (Z-ETAAS)'Y~~~~ have shown better performance in comparison with methods * Presented at the XXVIII Colloquium Spectroscopicum Inter- nationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993. 7 On leave from the Laboratorio di Biochimica Clinica Istituto Superiore di Sanita viale Regina Elena 299 00161 Roma Italy. based on ETAAS with deuterium background correction (D,-ETAAS).6 However the determination of Ni in serum by means of D,-ETAAS with instrumentation of more recent design has not been reported. Such information could be useful to routine laboratories for applications in clinical and occupational toxicology.In this paper the performance of a method for the determi- nation of Ni in serum using D,-ETAAS is described and its application to the assessment of serum Ni concentrations in a group of patients with chronic renal failure maintained on haemodialysis is reported. Experimental Instrumentation Determinations of Ni were carried out with a Perkin-Elmer atomic absorption spectrometer Model 1100B with deuterium arc background correction a graphite furnace Model HGA-700 an autosampler Model AS-70 and an Ni hollow cathode lamp. Signals were recorded by a built-in computerized system. Pyrolytic graphite coated furnace tubes were also obtained from Perkin-Elmer. Reagents An Ni stock solution 1 g 1-1 (SpectrosoL grade) Triton X-100 and ultrapure HNO (65% Aristar grade) were all obtained from BDH Poole UK.Nitric acid was further purified by sub-boiling in poly(tetrafluoroethy1ene) bottles. Ultrapure water was obtained by a four-stage purification using ion exchange (Elgastat UHP Elga High Wycombe UK). Working standard solutions containing 0 2.5 5 10 and 20 pg 1-1 of Ni were prepared in 1% v/v HNO,. Contamination Control All plastic-ware (k tubes Pasteurs and AAS cups) were soaked overnight in 20% HNO rinsed thoroughly six times with de-ionized water dried in a laminar flow hood and stored in clean plastic bags until use. Pipette tips were rinsed three times with 20% HNO and de-ionized water before use.458 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Manipulation of samples was carried out in a laminar flow hood.Subjects Blood samples were obtained from 25 patients with end-stage chronic renal failure [ 18 men and seven women aged from 22 to 78 years mean &standard deviation (SD) 53 & 17 years] who had been treated by haemodialysis three times a week at Glasgow Royal Infirmary for an average of 42+59 months (range > 1-290 months). Dialysis was performed with equipment from Gambro Dialysatoren (Munich Germany) and Fresenius AG (Munich Germany) using capillary flow dialysers with Cuprophan mem- branes (Baxter Healthcare Thetford Norfolk UK CF ST15 membrane surface 0.9m2 and CF ST23 membrane surface 1.25 m2) or cellulose acetate hollow fibre dialysers (Baxter CA 150 membrane surface 1.5 m2) Conventional electrolyte concentrate solutions manufactured by Gambro and Fresenius respectively were diluted 1 + 34 with water purified by reverse osmosis.Serum Ni levels were also measured in a control group of eight women with normal renal function aged from 29 to 73 years (mean -t SD 48 f 19 years) who were receiving total parenteral nutrition (TPN) and were being monitored for essential trace element status. Blood Collection Blood samples were obtained before dialysis directly from the intra-arterial cannula. The first 10 ml of blood were collected for routine analyses another 10 ml aliquot was withdrawn in a plastic tube allowed to clot for at least 1 h and centrifuged at 2500 rev min-l for 10 min. Serum was transferred into a clean 5 ml plastic tube using a poly(propy1ene) pipette and stored at - 20 "C. Post-dialysis samples were obtained with the same procedure from 12 patients (nine men three women aged 53_+21 years range 23-78 average time on dialysis 37f20 months range 8-67 months).Blood samples from the TPN subjects were taken using a plastic intravenous (IV) cannula. Procedure Aqueous standards and serum samples were diluted 1 + 1 with a solution containing 1% v/v HNO and 0.25% v/v Triton X-100. A volume of 5 0 ~ 1 was injected into the furnace. All standards and serum samples were analysed in duplicate. The instrumental conditions were wavelength 232.0 nm; slit 0.2 nm; integration time 4 s; and lamp current 15 mA. Signals were measured as peak area. The graphite furnace programme is reported in Table 1. Concentrations of Ni in the samples were obtained by comparison with a calibration curve obtained from the absorbance of the aqueous standard solutions.Table 1 Graphite furnace temperature programme Temperature/ Step "C 1 100 2 150 3 200 4 1200 5 2600 6 2700 Ramp time/ Hold time/ 1 1 50 5 30 5 80 50 0 4 1 3 S S Gas flow/ Read/ ml min" s 300 - 300 - 300 - 300* - 300 0 -0.5 - * Gas flow = 0 for the last 5 s in this step. Additional Analysis Serum albumin was determined by the Bromocresol Green method with an Olympus automatic analyser. Results and Discussion Analytical Performances of Method The plot of absorbance versus Ni concentration was linear within the range 0-20 pg I-'. Using a prolonged ashing time the background signal observed during the analysis of serum samples was maintained below 0.150 A s and could be managed efficaciously by the deuterium background corrector (Fig.1). A long ramp time was found to be necessary in order to avoid the build-up of carbonaceous residues. Under these conditions matrix interferences were reduced and a calibration graph obtained with aqueous Ni solutions could be used for quantifi- cation but only when absorbance was measured as peak area. The plots of peak area versus Ni concentration obtained with either aqueous or serum-based standard solutions were parallel (Fig. 2). No significant difference was observed between the concentrations of 38 serum samples within the range 1.5-4.5 pg l-' determined using both aqueous and serum- based calibration standards (paired-data t-test average differ- ence -0.06+0.13 pg 1-l). On the contrary the peak-height response measured for the determination of equal amounts of Ni was higher for serum-based standards than for aqueous solutions (regression line equations aqueous solutions y = 1.3 x 10-3+6.30 x 10-3x r2= 1.000; serum-based standards y = 22.7 x The detection limit (three times the standard deviation of ten replicate measurements of the blank) was 0.15 pg lel equivalent to 0.3 pg I-' in the undiluted sample.The character- + 6.67 x 10-3x r2 = 0.999). 0.22 u) 0.20 E 2 si .$ 0.18 0.16 P X 4 0.14 0.12 I I I I I 40 60 80 100 120 140 Ashing time/s Fig. 1 Background signal observed during the analysis of a serum sample obtained from a haemodialysed patient using increasing total ashing time. All other furnace conditions are as in Table 1 -$ 0.10 ; 0.08 s 0 % 0.06 5 0.04 E 0.02 u 01 - 0 2 4 6 8 1 0 Ni concentration/pg I-' Fig.2 Calibration graphs for A aqueous solutions y = 0.2 x lop3 + 7.783 x 10-3x r2 =0.998; and B serum-based standards y=24.9x 10-3+7.823 x x r2=1.000JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 459 istic mass (mass of analyte in pg that yields a signal of 0.0044 A s) was 14 pg. Precision was evaluated as the relative pooled standard deviation of replicate measurements of serum samples. To determine within-day precision 35 serum samples (average concentration 3.6+ 1.8 pg 1-l) were analysed in duplicate on the same day. Between-day precision was evaluated from the replicate analysis of 14 serum samples (average concentration 3.5 & 1.7 pg 1-') carried out on two different days. The pooled standard deviation (PSD) was calculated in both cases with the following formula i = n where xil and xi2 are the first and second measurement on the ith sample and n is the number of samples.The relative pooled standard deviation was then obtained expressing the calculated PSD as a percentage of the average concentration of the n samples analysed on each occasion. Within-day precision was 5.6% and between-day precision was 7.5%. The average recovery of known amounts of Ni (2.5 5.0 and 10.0 pg 1- ') added to a serum sample was 101 + 4.8% (n = 9). The analysis of Seronorm Trace Element Control Serum (Nycomed AS Diagnostics Oslo Norway 3.2 pg 1-l) and Standard Reference Material (SRM) 8419 Bovine Serum (National Institute of Standards and Technology Gaithersburg MD USA 1.8 pg 1-l) by this method gave average values of 3.21 k0.17 (n= 10) and 0.46k0.05 pg 1-1 (n = 4) respectively.A similar discrepancy with the certified value of the SRM 8419 has been reported by two other groups of (Table 2) who suggested that the recommended value of 1.8 pg 1-1 could be in error. According to the data in Table2 where the performances of this and other ETAAS methods are summarized the pro- posed method compares well with Zeeman ETAAS procedures in terms of characteristic mass precision and accuracy. It shows better precision and detection limit than those previously reported for D,-ETAAS using older instrumentation,6 although Drazniowsky et aL6 reported a much lower value for the characteristic mass. On the other hand the detection limit is higher than those reported for procedures that apply Zeeman- effect background correction and attempts to improve the sensitivity using a multiple injection failed because of the unmanageable increase in background signal.Therefore the normal concentrations of Ni in serum of unexposed subjects cannot be determined by this method. However the measurement of such low levels requires specialized facilities for the control of contamination as described by Nixon et al.,' and is confined to a limited number of research centres. The proposed method using widely available instrumen- tation can be applied by routine laboratories to monitor Ni exposure in clinical and occupational toxicology. Ni Levels in Serum of Haemodialysed Subjects and Controls The average serum Ni concentration measured by this method in 25 patients maintained on haemodialysis was 2.4 & 0.9 pg 1-l (range 0.7-4.0 pg 1-l).In comparison serum Ni levels in eight women who were receiving TPN but had normal renal func- tion ranged between 0.3 and 0.9 pgl-' (meanfSD 0.5 kO.2 pg 1-l). The distribution of the observed values is reported in Fig. 3. Although the control group could in theory be exposed to Ni present as a contaminant in nutrient solutions Berner et ~ 1 . ~ ~ have observed that the daily intake of Ni received by patients maintained on TPN is comparable to the amounts reported to be absorbed through the gastrointestinal tract in healthy subjects. This amount is rapidly eliminated by the kidneys provided that the renal function is not affected.The values observed for serum Ni concentrations in the TPN Serum Ni intervals (upper limit)/pg I -' Fig. 3 Distribution of the serum Ni concentrations in haemodialysed patients (filled bars) and controls (open bars) Table 2 Analytical performance of this method compared with those of previously reported procedures Parameter Background correction Sample pre-treatment Injection Calibration Detection limit*/pg 1-' Characteristic masst/pg Precision Within-day RSD(%)f Between-day RSD(Y0) Seronorm (3.2 pg 1-') SRM 8419 (1.8 pg 1-') Average recovery (YO) Range Accuracy Reference 20 Zeeman Deproteinization Single Aqueous standards 0.05 27.9 3.8 8.1 - 97 _+ 2.7 94-103 6 Deuterium Dilution (1 + 1) Single Serum-based standards 0.9 6.3 6.3 34.9 - - 99+6 86-118 21 Zeeman Dilution (1 + 1) Single Serum- based standards 0.09 13 2.93 & 0.34 0.48 f 0.04 - 8 Zeeman Dilution ( 3 + 1) Multiple Serum-based standards 0.06 11 3.2 - 3.30 k 0.23 0.50 - This method Deuterium Dilution (1 + 1) Single Aqueous standards 0.15 14 5.6 7.5 3.21 k0.17 0.46 + 0.05 101.5k4.8 92-107 ~~ ~~~ ~ ~ ~ ~ ~~ * Three times the standard deviation of the blank value except for ref.10 where the minimum Ni concentration detectable with 95% confidence 7 Mass of Ni which gives a signal of 0.0044 A s. $ RSD relative standard deviation. limits is reported.460 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 3 Reported concentrations of Ni in serum of haemodialysed patients and controls (mean+SD/pg 1-') Reference Controls n Patients n Hopfer et al.ref. 5 1985 0.3 f 0.2 30 5.4k2.1 65 Drazniowsky et al. ref. 6 1985 1.0 (0.6- 1.4)* 71 7.4 (6.0-9.1)* 16 Wills et al. ref. 7 1985 0.44 k 0.1 8 18 3.71 1.54 28 Hopfer et al. ref. 9 1989 0.6 & 0.3 22 7.0 k 2.4 30 Nixon et al. ref. 8 1989 0.14 f 0.09 38 6.38k0.18 27 This work 1993 0.5 f 0.2 8 2.4 k 0.9 25 ~~ ~~~ * Median (lower - upper quartile). Table 4 Reported concentrations of Ni (meanfSD/pg 1-') in serum of haemodialysed patients before and after a dialysis treatment Reference Hopfer et ul. ref. 5 1985 Hopfer et al. ref. 9 1989 This work 1993 Pre-dial ysis Post -dialysis Difference n P* 6.2+ 1.8 7.2 2.2 1.Of 1.1 40 <0.01 3.9 f 2.0 5.2 f 2.5 1.3 f 1.0 9 <0.01 3.0-t 1.3 3.7 & 1.3 0.7 f 0.3 10 <0.01 7.0 f 2.4 8.5 k 2.8 1.5 f 1.3 30 ~ 0 . 0 1 2.2 f 0.9 3.0 & 0.9 0.8 k 0.9 12 <0.01 * Versus pre-dialysis values by paired-data t-test.subjects are comparable to those reported for healthy volun- teers by other workers except for Nixon et aE.* (Table3). However because Ni concentrations lower than 0.3 pg 1-1 are not detectable with this method the average value of Ni levels in the control group could be slightly overestimated. Serum Ni levels did not appear to correlate with the length of dialysis. The average serum Ni concentrations observed in this study for sub-groups of patients maintained on dialysis for less than 10 months (n=9) 10-49 months (n= lo) 50-88 months (n = 5) and 290 months (n = 1) were 2.2 _+ 1.1 2.5 f0.9 2.6f0.3 and 1.9 pg 1-' respectively. Other workers5.* have also reported that in their groups of haemodialysed patients only those who had started the dialysis treatment less than 13 months ago had slightly lower serum Ni concentrations com- pared with the others.The serum Ni levels observed for haemodialysed subjects in this study are lower than those reported by other (Table 3). This could reflect the improvement in the purity of water and electrolyte concentrate solutions now used. The Ni concentration in samples of dialysis fluid collected just before and after the dialyser from five dialysis sets was lower than the detection limit of the proposed method (0.45 pg 1-l for samples diluted 1 + 2). Hopfer et aL5 observed a significant reduction in serum Ni levels of a group of haemodialysed patients six months after the reduction of the Ni content of the dialysis fluid (from 0.82 to 0.53 pg1-l) owing to the introduction of a new reverse osmosis system for purification of the water.The analysis of serum samples obtained on the same day from 12 patients before and after dialysis yielded average Ni concentrations of 2.2 f 0.9 and 3.0 f 0.9 pg l-l respectively ( p < 0.01 paired-data t-test). The average increase in serum Ni concentrations after a single treatment of dialysis was only slightly lower than those reported by Hopfer and co-worker~~.~ (Table 4) despite the lower serum Ni concentrations observed in the present group of subjects. On the contrary the increment of serum albumin in post-dialysis specimens (pre-dialysis value 41 * 3.5 g 1-l; post-dialysis value 45 _+ 5.6 g I-l; average differ- ence 4 * 5 g l-') owing to haemoconcentration was compar- able to the values of 10 and 8% observed by Hopfer and co-worker~.~,~ Although statistically significant the increase in Ni in the present group of subjects was very variable and did not correlate with the increment of serum albumin or with pre- dialysis serum Ni values.In addition two patients had reduced serum Ni levels after dialysis despite the rise in albumin concentrations. A large variability could also be observed from the data reported by Hopfer and c o - ~ o r k e r s ~ . ~ (Table 4). In contrast when individual post-dialysis serum Ni levels were corrected for haemoconcentration using the serum albumin values the average concentration was 2.7 kO.7 pg 1-1 and the increase versus pre-dialysis values (mean f SD 0.5 k 0.8 pg 1-I) was lower and not significant by the paired-data t-test.Despite the improvement in the purity of dialysis fluids hypernickelaemia although moderate still occurred in the present group of haemodialysed patients. Olerud et a2.13 have demonstrated in vitro that Ni can be absorbed into the blood from dialysis fluids against a concen- tration gradient owing to the high affinity of albumin and other plasma proteins for Ni. However the exchange of Ni in vitro during dialysis appears to be variable and dependent on factors that have not yet been completely clarified. Conclusions A method has been described for the determination of Ni in serum by means of D,-ETAAS that can be applied in clinical and occupational toxicology. The levels of Ni in serum of patients maintained on haemodialysis were lower than those previously reported and could reflect the 'clean-up' of haemo- dialysis systems.The authors acknowledge the assistance of the Staff of the Renal Unit Ward 12 Glasgow Royal Infirmary for the collection of blood samples from haemodialysed patients. References Howard J. M. Clin. Chem. 1980 26 1515. Khan S. N. Rahman M. A. and Samad A. Clin. Chem. 1984 30 644. Leach C. N. Jr. Linden J. V. Hopfer S . M. Crisostomo M. C. and Sunderman F. W. Jr. Clin. Chem. 1985 31 556. Milling Pedersen L. and Molin Christensen J. Acta Pharmacol. Toxicol. 1985 59 Suppl. VII 392. Hopfer S . M. Linden J. V. Crisostomo M. C. Catalanatto F. A. Galen M. and Sunderman F. W. Jr Truce Elem. Med. 1985 2 68. Drazniowsky M.Parkinson I. S. Ward M. K. Channon S. M. and Kerr D. N. S. Clin. Chim. Acta 1985 145 219. Wills M. R. Brown C. S. Bertholf R. L. Ross R. and Savory J. Clin. Chim. Acta 1985 145 193. Nixon D. E. Moyer T. P. Squillace D. P. and McCarthy J. T. Analyst 1989 114 1671. Hopfer S. M. Fay W. P. and Sunderman F. W. Jr. Ann. Clin. Lab. Sci. 1989 19 161.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 461 10 11 12 13 14 15 16 Niebor E. Tom R. T. and Sandford W. E. in Nickel and its Role in Biology. Series Metal Ions in Biological Systems eds. Sigel H. and Sigel A. Marcel Dekker New York 1988 vol. 23 Glennan J. D. and Sarkar B. Biochem. J. 1982 19 847. Webster J. D. Parker T. F. Alfrey A. C. Smythe W. R. Kubo H. Neal G. and Hull A. R. Ann. Intern. Med. 1980 92 631. Olerud J. E. Lee M. Y. Uvelli D. A. Goble G. J. and Babb A. L. Arch. Dermatol. 1984 120 1066. Giardini 0 Taccone-Gallucci M. Lubrano R. Ricciardi- Tenore G. Bandino O. Silvi I. Paradisi C. Mannarino O. Citti G. Elli M. and Casciani C. U. Clin. Nephrol. 1984,21 174. Donnelly P. K. Shenton B. K. Alomran A. M. Francis D. M. A. Proud G. and Taylor R. M. R. Proc. Eur. Dial. Transplant. Assoc. Eur. Renal Assoc. 1983 20 297. Graham J. A. Miller F. J. Daniel M. J. Payne E. A. and Gardiner D. E. Enuiron. Res. 1978 16 77. ch. 4 pp. 91-122. 17 Smialowicz R. J. Rogers R. R. Riddle M. M. and Stott G. A. Enuiron. Rex 1984 33 413. 18 Mastrogiacomo I. DeBesi L. Serafini W. Zucchetta P. Romagnoli G. F. Saporiti E. Dean P. Ronco C. and Adami A. Nephron 1984 37 195. 19 Clemons G. K. and Garcia J. F. Toxicol. Appl. Pharmacol. 1981 61 343. 20 Sunderman F. W. Jr. Crisostomo M. C. Reid M. C. Hopfer S. M. and Nomoto S. Ann. Clin. Lab. Sci. 1984 14 232. 21 Andersen J. R. Gammelgaard B. and Reinert S. Analyst 1986 111 721. 22 Berner Y. N. Shuler T. R. Nielsen F. H. Flombaum C. Farkouh S. A. and Shike M. Am. J. Clin. Nutr. 1989 50 1079. Paper 310671 OF Received November 8 1993 Accepted December 28 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900457

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 463 Determination of Silicon in Titanium Dioxide and Zirconium Dioxide by Electrothermal Atomic Absorption Spectrometry Using the Slurry Sampling Technique* Susanne Hauptkorn Germar Schneider and Viliam Krivant Sektion Analytik und Hochsfreinigung der Universitat Ulm AIbert-Einstein-Allee 7 7 0-89069 Ulm Germany A method for the determination of silicon in titanium dioxide and zirconium dioxide powders based on electrothermal atomic absorption spectrometry using the slurry sampling technique has been developed. The experimental conditions with regard to chemical modification the temperature programme the lifetime of the graphite furnace and the slurry concentration were optimized. The behaviour of zirconium and calcium in the graphite tube during the ashing atomization and cleaning steps was investigated using 47Ca and "Zr as radiotracers.Cali bration was performed by the standard additions method using aqueous standards. The results of this technique were compared with those for atomic emission spectrometry. The limits of detection were found to be 7 pg g-' in titanium dioxide and 2 pg g-' in zirconium dioxide respectively. Keywords Silicon determination; titanium dioxide; zirconium dioxide; slurry sampling; electrothermal atomic absorption spectrometry One of the main impurities in ceramic materials is silicon because of its prevalence in the lithosphere. In the production of zirconium dioxide in which zircon (ZrSiO,) is used as a basic material silicon represents a particularly important impurity.' Silicon has adverse effects on the mechanical chemi- cal and electrical properties of many materials because glassy grain boundary phases are formed during the sintering pro- c ~ s s .~ ~ ~ Apart from use in pigments titanium dioxide is also used in vitreous enamels electronic components welding rods synthetic sapphires and rubies. It is also used in capacitors positive temperature coefficient thermistors and piezoelectric materials in electrooptics.* Zirconium dioxide (25-0,) and partially stabilized zirconium dioxide (PSZ) ceramics find applications as cutting tools knife blades milling media pump components machinery wear parts insulation parts and solid electrolytes for fuel cells."" When zirconium dioxide is used as an oxygen sensor the electrical conductivity is influenced by the partial pressure of oxygen which depends on the impurity contents of aluminium and silicon.'2 For the determination of trace elements in ceramic materials mostly solution methods requiring decomposition of the sample and often also separation of the matrix and trace elements have been used.For the determination of silicon titanium dioxide has been decomposed by fusion with ammonium sulfate-sulfuric acid [( NH4)2S04-H2S0,] ,I3 whereas zirconium dioxide has been decomposed by using a mixture of sodium carbonate (Na,CO,) and sodium tetrabo- rate (Na2B207 borax)7 or a mixture of borax boric acid (HBOJ and lithium hydroxide (LiOH).I4 The most important digestion procedures for both matrices are based on utilization of media containing hydrofluoric acid ( HF).12*'5-'7 However using these media no mineralization of yttria stabilized zir- conium dioxide is possible.The limitation of the digestion methods is associated mainly with two problems i.e. with considerable blank values originating from the extraordinarily high over-all concentration of silicon and when using hydro- fluoric acid also with possible losses of silicon. Furthermore all the decomposition procedures are tedious and time consum- ing. Another problem associated with a digestion stage is the introduction of a matrix unsuitable for the subsequent determi- nation step. When hydrofluoric acid is used for the digestion of the samples the formation of silicon tetrafluoride in the * Presented at the XXVIII Colloquium Spectroscopicurn Inter- nationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.t To whom correspondence should be addressed. graphite furnace makes the determination of silicon by electro- thermal atomic absorption spectrometry (ETAAS) impossible. For these reasons direct methods are often preferred for the analysis of ceramics. Titanium dioxide has been analysed for silicon by spark source mass spectrometry,'* atomic emission spectrometry (AES) with spark or d.c. arc excitation,'"*' inductively coupled plasma (ICP) AES with slurry sample introduction2* and X-ray fluorescence ~ p e c t r o m e t r y . ~ ~ ' ~ ~ For the analysis of zirconium dioxide slurry sampling ICP-AES has been ~ s e d . ~ ~ ~ ~ ~ ~ In recent years the slurry sampling technique has increas- ingly been used for the analysis of a wide variety of samples by ETAAS.25,26 However only a few applications to ceramics and related materials have been despite the fact that the samples to be analysed are often in the form of powders with sub-micrometre sized particles which are particu- larly suitable for slurry sampling.In the present work an ETAAS method for the determi- nation of silicon in titanium dioxide and zirconium dioxide based on slurry sampling which avoids the above mentioned disadvantages of the solution techniques has been developed. Experimental Samples Reagents and Radiotracers The particle size of the titanium dioxide sample (Type P25 Degussa Germany) was ~ 0 . 2 pm and that of the investigated zirconium dioxide samples Zr0,- 1 (Dynamit Nobel Troisdorf Germany) Zr02-2 and Zr02-3 (Magnesium Electron Twickenham London UK Type 9066/3 and 1011) was < 2 pm.The commercially available zirconium dioxide powder Z1-02-4 stabilized with 3 mol-% yttria (yttrium oxide) was supplied by Cerasiv (Plochingen Germany) and has a particle size of between 10 and 50 pm. The particle sizes of all samples were determined by scanning electron microscopy. Doubly distilled water was used for preparation of slurries standards and chemical modifier solutions. Calibration stan- dards were prepared by dilution of a stock standard solution (Merck Darmstadt Germany) with a concentration of 1 g 1-'. The hydrochloric acid of pro analysi quality (37% Merck) was purified by sub-boiling distillation.Magnesium nitrate and calcium nitrate were of Suprapur quality (Merck). All other reagents used were of pro analysi quality. The radiotracer experiments were carried out with a com- mercially available calcium radioisotope 47Ca in the chloride form (Amersham-Buchler Braunschweig Germany). The 47Ca tracer had a specific activity of 7.4 MBq pg-' of calcium and464 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 the calcium concentration was 250 pg m1-I. The 97Zr radio- tracer was produced by irradiation of zirconium dioxide for 30 min in the FRM-1 reactor station Garching (Munich Germany) with a thermal neutron flux of 1.3 x loi3 n cm-2 s-'. A specific activity of 0.4 Bq pg-' was obtained. Instrumentation A Perkin-Elmer atomic absorption spectrometer Model 4100 ZL equipped with a THGA graphite furnace an AS-70 autosampler and a USS-100 slurry sampler was used.Background correction was performed using the longitudinal inverse Zeeman effect. For pre-treatment of the suspensions a Sonorex RK 255 H ultrasonic bath (Bandelin Electronic Berlin Germany) was used. Scanning electron micrographs of the titanium dioxide and zirconium dioxide powders were performed on a digital scanning electron microscope Model DSM 962 (Zeiss Oberkochen Germany). The ultrasonic probe Sonoplus HD70 (Bandelin Electronic) was used for producing slurry suspensions of titanium dioxide and zirconium dioxide for the radiotracer experiments. A high-resolution y-ray spectrometer system (EG & G Ortec Munich Germany) consisting of a germanium detector with an efficiency of 44% relative to a 3 x 3 in NaI(T1) detector an energy resolution of 1.72 keV at the 1.332 MeV y-ray of 6oCo was used for counting the 1.297 and 1.148 MeV y-rays of 47Ca and 97Zr respectively.Performance of the Radiotracer Experiments The labelled slurries were prepared by mixing 10mg of the inactive titanium dioxide and 50 mg of the labelled zirconium dioxide respectively with 10 ml of water 80 pg of inactive calcium nitrate modifier and 50 pl of the 47Ca radiotracer in 15 ml polystyrene vessels. The resulting suspension was homo- genized for 20s using an ultrasonic probe and then 20p1 of the slurry were pipetted into the graphite tube. The activity of the graphite tube was counted after the drying ashing atomiz- ation and cleaning steps using the y-ray spectrometer. The accumulation of calcium and zirconium in the tube was estimated by counting the y-rays of 47Ca and 97Zr after five runs were completed.Procedure Slurries of the samples were prepared by mixing 10mg of titanium dioxide or between 10 and 150mg of zirconium dioxide with a solution of 40mg of calcium nitrate in 10ml of doubly distilled water previously checked for the blank value in 15 ml polystyrene vessels by the selected pots pro- Table 1 Temperature programme and instrumental parameters used for slurry ETAAS Temperature programme - Ramp Hold Argon flow/ Step Temperature/"C time/s time/s ml min- Drying 110 1 20 250 130 5 30 250 Charring 1000 10 10 250 Atomization 2400 0 5 0 Cleaning 2600 1 3 250 Instrumental parameters - Wavelength 251.6 nm Sli t-wid t h Source Read 5 s Signal mode Peak area 0.2 nm Hollow cathode lamp 40 mA Sample volume 20 p1 ~ e d u r e .~ ~ ~ ' The suspensions were pre-treated for 10 min in an ultrasonic bath before analysis to disintegrate larger particle agglomerates. The beakers containing the slurries were used directly for autosampling. Before pipetting each aliquot of the slurry using the sampling capillary homogenization was per- formed by ultrasonic agitation with the USS-100 slurry sampler for 30 s at about 4 W. For standardization by the standard additions technique the titanium dioxide and zirconium dioxide slurries were spiked twice in sequence with 1 and 5 pg of silicon respectively. The standard solution containing 100 pg ml-1 of silicon was prepared by diluting the stock standard solution in a poly(propy1ene) calibrated flask and adjusting the pH to 5 with concentrated hydrochloric acid.Temperature programmes and instrumental parameters are summarized in Table 1. Results and Discussion Optimization of the Experimental Conditions When silicon is atomized from both the titanium dioxide and the zirconium dioxide matrix without addition of a chemical modifier the absorption signals have irregular shapes and have poor reproducibility with relative standard deviations up to 50% for integrated absorbance (QA) (n = 5) [see Fig. l(u) and (43. A similar problem was encountered when silicon was atomized from a boron nitride matrix.34 Magnesium nitrate has been reported to be a modifier of universal applicability also suitable for the determination of silicon.35 In the case of the boron nitride matrix the addition of magnesium nitrate as a chemical modifier lead to a significant improvement of both the signal shape and reproducibility while using calcium nitrate this could not be achieved although calcium nitrate has been proposed as an even more efficient modifier than magnesium nitrate for the determination of silicon by ETAAS.36937 In the case of zirconium dioxide the addition of magnesium nitrate leads to a considerable improvement of the signals with respect to the peak shape and reproducibility [see Fig.l(b)]. However when applied to titanium dioxide this modifier caused no improvement in the quality of the signal and it even lead to a reduction in the sensitivity as can be seen in Fig.l(e). For both the titanium dioxide and the zirconium dioxide matrices calcium nitrate proved to be the most suitable chemical modifier. When using this very smooth sharp peaks and an acceptable reproducibility with a relative standard deviation of between 3 and 7% (n=5) are obtained for both matrices [see Fig. l(c) and (f)]. The behaviour of the chemical modifier (calcium) and the matrix element (zirconium) in the graphite tube during the operation of the temperature pro- gramme was studied by using 47Ca and 97Zr as radiotracers. The radiotracer experiments showed that after execution of the whole temperature programme zirconium remained almost quantitatively in the tube (see Fig. 2). This can be explained by the formation of the highly refractory zirconium carbide which melts at 3540°C and volatilizes at 5100"C.38 The behaviour of titanium could not be examined in this way as no suitable radioisotope for this element is available.However a similar behaviour can be expected as titanium carbide with a melting-point of 3140 "C and a boiling-point of 4820 "C also shows refractory behaviour. It is shown in Fig. 3 that calcium also accumulates in the graphite tube with an increasing number of runs. However its behaviour differs from that of zirconium (compare Figs. 2 and 3) as it forms a less stable carbide at temperatures higher than 2000°C.39 While calcium is retained quantitatively in the tube after charring considerable losses occur during atomization and they are further slightly increased during the cleaning step (see Fig.2). For this reason accumulation of calcium deviates from linearity for a higher number of runs whereas zirconium shows an approximately linear behaviour throughout (see Fig. 3).465 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 0.15 0.30 0.15 0 2.5 5.0 0 2.5 5.0 Time/s 0 2.5 5.0 Fig. 1 Influence of the chemical modifier on quality of the absorption signal (solid line) and background signal (broken line) of (a) zirconium dioxide slurry without modifier (integrated absorbance (QA =0.056) (b) zirconium dioxide slurry with 80 pg of magnesium nitrate (QA = 0.1 15) (c) zirconium dioxide slurry with 80 pg of calcium nitrate (QA =0.115) (d) titanium dioxide slurry without modifier (QA = 0.032) (e) titanium dioxide slurry with 80 pg of magnesium nitrate (QA=0.014); and (f) titanium dioxide slurry with 80 pg of calcium nitrate (QA=0.090).In all cases a sample of about 20 pg was applied Charring Atomization Cleaning ' t . B o o ti. Fig. 2 Retention of A 97Zr applied as labelled zirconium dioxide slurry; B 47Ca added as labelled calcium nitrate to zirconium dioxide slurry; and C 47Ca added as labelled calcium nitrate to titanium dioxide slurry I I I I I I I I 1 0 10 20 30 40 50 60 70 80 90 100 No. of heating cycles Although the addition of calcium nitrate as a chemical modifier to the slurry leads to a significant improvement in the quality of the signal for both matrices it does not prevent the build-up of titanium carbide and zirconium carbide resi- dues.To estimate the tube lifetime that is the maximum number of runs with still acceptable signal quality repeated determinations were performed in the same tube. Fig. 4 shows that for both a titanium dioxide and zirconium dioxide matrix peak heights cannot be used for evaluation because of extremely large variations and a rapid decrease in signal especially during the first 50 runs. The integrated signals on the other hand show satisfactory stability for about 300 runs. With further increase in the number of runs the signal quality rapidly decreases and sometimes even breaking of the tubes occurs. In the case of zirconium dioxide a slight decrease in signal with increasing number of runs can be observed during approximately the first 50 runs. However this has no significant Fig.3 Accumulation of A 97Zr applied labelled zirconium dioxide slurry; B 47Ca added as labelled calcium nitrate modifier to zirconium dioxide slurry; and C 47Ca added as labelled calcium nitrate to titanium dioxide slurry influence on the results when the standard additions method is used. Whereas with matrix-free solutions a longer tube lifetime is achievable than with slurries introduction of sample solutions usually containing the concentrated acid mixtures used for decomposition can lead to severe damage to the platform surface. Thus compared with methods requiring decomposition of the sample utilization of slurry samples does not cause shortening of the tube lifetimes. Standardization and Sample Analysis Generally the use of solid materials with certified concen- trations of the elements of interest and matrices corresponding466 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 0.671 0 t lu e g 0.335 2 0 - 0.20 0.15 0.10 $ 0.05 c - ( b ) - I I .y . ~ . I 1 C B A - ; 0' I I I I I J 1.0 b 0.8 pi, *. 0.6 0.4 0.2 I I I 1 I 0 50 100 150 200 250 300 No. of runs Fig. 4 Dependence of the absorbance on the number of runs applying (a) titanium dioxide slurry (0.1 YO m/v); and (b) zirconium dioxide slurry (0.1% m/v); A peak heights; and B integrated absorbance (s) 0 700 900 1100 1300 1500 1700 1900 Tern perat u rePC Fig.5 Charring curves of silicon obtained for A aqueous silicon standard (1 ng of Si); B titanium dioxide slurry (about 1.5 ng of Si); and C zirconium dioxide slurry (about 10 ng of Si).In all cases 80 pg of calcium nitrate were added to those of the samples is considered to be the most accurate standardization method for solid sample ETAAS including the slurry sampling technique. However these materials are costly and are not available for all matrices. The second best choice considering the accuracy achievable is calibration by standard additions using aqueous standard solutions. If accept- able accuracy can thus be achieved standardization by cali- bration curve with aqueous standards is because of greater simplicity and rapidity therefore the preferred method. In this work the last two calibration methods were tested for their accuracy and precision. For both calibration tech- niques accurate standardization requires similar behaviour of the silicon contained in the sample and of the silicon contained in the aqueous standard solution during the charring and the atomization stages. From the charring curves shown in Fig.5 obtained for an aqueous silicon standard solution and for the slurries of titanium dioxide and zirconium dioxide all contain- ing calcium nitrate as a chemical modifier it can be seen that this pre-condition is sufficiently fulfilled for the charring step for all three cases very similar charring curves are obtained. The absorption signals for unspiked and spiked slurries of both titanium dioxide and zirconium dioxide (see Fig. 6 ) show that the atomization behaviour of silicon originating from the sample and from the spiked standard solution is also very similar. Furthermore the characteristic masses of silicon for the aqueous solution titanium dioxide and zirconium dioxide using calcium nitrate as a chemical modifier were calculated in order to ascertain whether standardization by a calibration curve was possible.In this manner characteristic masses for silicon of 105 & 10 pg for the aqueous solution 128 & 11 pg for the titanium dioxide slurry and 420f 130 pg for the zirconium dioxide slurry were obtained. The relatively large standard deviation can be explained by a great increase in the character- istic mass during the first 50 runs. From these results it is evident that standardization by the calibration curve method 1.2 0.60 0 2:5 5.0 Time/s Fig. 6 Absorption signals of silicon for (a) aqueous solution; (b) titanium dioxide slurry A without addition of silicon standard B spiked with 2 ng of silicon and C spiked with 4 ng of silicon and (c) zirconium dioxide slurry A without addition of silicon standard B spiked with 10 ng of silicon and C spiked with 20 ng of silicon is not possible.Therefore the standard additions method was used for calibration. The silicon concentrations determined by this technique in four zirconium dioxide samples and one titanium dioxide samples are summarized in Table 2 along with results obtained by other independent methods. The result obtained for titanium dioxide by this method agrees very well with that obtained byTable 2 Silicon contents (pg g-') determined in titanium dioxide and zirconium dioxide by this method and by AES Slurry Slurry Solution Sample ETAAS * d.c.AESY ICP-AESf ICP-AESg Zr0,- 1 244 f 46 314+ 15 250 _+ 10 255+11 Zr0,-2 166 f 30 90,99 102_+7 105+7 - - Ti02 76f4 75+5 Zr0,-3 130 f 41 81,102 100 + 7 95+7 ZrO2-4 25$-4 - - - * n=7. 7 Ref. 40. f Ref. 15. § Solution ICP-AES involving fusion with NH4HS04.16 d.c. AES both methods producing low standard deviations. In the case of zirconium dioxide the agreement of the results with those obtained by d.c. AES ICP-AES with slurry nebuliz- ation and ICP-AES after decomposition of the sample is within the standard deviations except for the sample Zr0,-2. From a comparison of the results in Table 2 it can also be seen that the slurry ETAAS method provides considerably higher stan- dard deviations than the two ICP-AES techniques. This can be explained by the inhomogeneous distribution of silicon in - 20 pm - 50 pm Fig.7 Scanning electron micrographs of (a) an unstabilized zirconium dioxide sample (ZrO - 1) and (b) yttria stabilized zirconium dioxide JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 467 sample ( ZrOz - 4) 0 0.4 0.8 1.2 1.6 2.0 Slurry concentration (%) Fig. 8 Dependence of the absorbance on the slurry concentration of A titanium dioxide and B zirconium dioxide the zirconium dioxide samples 1-3 and at the same time by the extremely low sample portions (20 pg) taken for the determination by the slurry ETAAS technique. Surprisingly the sample with the lowest silicon content and even with the highest particle size Zr0,-4 gives the best standard deviation and thus has obviously the most homogeneous silicon distri- bution.This sample is an yttria stabilized zirconium dioxide powder which in some respects has different characteristics from the unstabilized zirconium dioxide. For example the yttria stabilized zirconium dioxide powder consists of spherical particles whereas the particles of the unstabilized powders are irregular (see Fig. 7). To establish the linear working range for the proposed method different slurry concentrations were examined and for each slurry concentration the amount of the calcium nitrate modifier was also adjusted accordingly. As is evident from Fig. 8 the maximum slurry concentration applicable for titanium dioxide is about 0.6% m/v which is significantly lower than that for zirconium dioxide (1.25%). This is in accordance with the differences in the characteristic masses of silicon in these two matrices.Because with smaller sample portions the role of the sample homogeneity increases slurry concentrations lower than 0.1 YO were not used. The slurry sampling ETAAS method described enables a detection limit for silicon to be achieved estimated by using the criterion of three standard deviations of the blank of 7 pg g-l for titanium dioxide and 2 pg g-' for zirconium dioxide. For the zirconium dioxide matrix this is one order of magnitude better than the limit of detection achievable by atomic emission methods of 20-30 pg g-'.'5i16 Conclusion Slurry sampling ETAAS has proved to be an advantageous method for the determination of silicon in powdered titanium dioxide and zirconium dioxide samples when calcium nitrate is used as a chemical modifier to overcome otherwise strong matrix interferences. Compared with methods that involve sample decomposition and separation of the silicon this tech- nique leads to lower blanks it is less time consuming and no hazardous acids are needed.A possible drawback might be that contrary to many other applications of the slurry sampling technique the standard additions method must be used for calibration. The authors thank J. Pave1 for making available the d.c. AES results. This work was supported financially by the Bundesministerium fur Forschung und Technologie (G.S. Zr02) and the Deutsche Forschungsgemeinschaft (S.H. TiO,) Bonn. Germanv.468 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 References Ayala J. M. Verdeja L. F. Garcia M. P. Llavona M. A. and Sancho J. P. J. Muter. Sci. 1992 27 458. Kobayashi S. J. Muter. Sci. Lett. 1985 4 266. Jaganathan J. Ewing K. J. Buckley E. A. Peitersen L. and Agganval I. D. Microchem. J. 1990 41 106. MacFarlane D. R. Mincely P. J. and Newman P. J. J. Non- Cryst. Solids 1992 140 335. Jun C. Guojan D. and Chengshan Z. J. Non-Cryst. Solids 1992 140 293. Broekaert J. A. C. Graule T. Jenett H. Tolg G. and Tschopel P. Fresenius 2. Anal. Chem. 1989 332 825. Martinez-Lebrusant C. and Barba F. Analyst 1990 115 1335. Hutton R. C. Anal. Proc. 1984 21 317. Clough D. J. Ceram. Eng. and Sci. Proc. 1985 6 1244. Hirano M. and Inada H. Ceram. Int. 1991 17 359. Stuhrhahn H.H. Dawihl W. and Thamerus G. Ber. Dtsch Keram. Ges. 1975 52 59. Carleer R. Van Poucke L. C. and Francois J.-P. Bull. SOC. Chim. Belg. 1986 95 385. Bastius H. CFI Ceram. Forum Int. 1984 61 140. Kruidhof H. Anal. Chim. Acta 1978 99 193. Lobinski R. Borm W. V. Broekart J. A. C. Tschopel P. and Tolg G. Fresenius’ J. Anal. Chem. 1992 342 563. Lobinski R. Broekart J. A. C. Tschopel P. and Tolg G. Fresenius’ J. Anal. Chem. 1992 342 569. Hiraide M. Namba K. and Kawaguchi H. Anal. Sci. 1990 6 609. Jackson P. F. S. and Whitehead J. Analyst 1966 91 418. Mitzuno T. and Matsumura T. Nippon Kinzoku Gakkaiski 1969,33 344. Rantschke R. and Rehfeld K. H. Spectrochim. Acta Part B 1972 27 211. Kato A. Osumi Y. Nakane M. and Miyake Y. Bunseki Kagaku 1974,23 1036. 22 23 24 25 26 27 ‘2 8 29 30 31 32 33 34 35 36 37 38 39 40 Broekaert J.A. C. Leis F. Raeymaekers B. and Zaray G. Spectrochim. Acta Part B 1988 43 339. Denton C. L. Himsworth G. and Whitehead J. Analyst 1972 97 461. Huang M. and Shen X.-e. Spectrochim. Acta Part B 1989 44 957. Miller-Ihli N. J. Anal. Chem. 1992 64 964A. Bendicho C. and de Loos-Vollebregt M. T. C. J. Anal. At. Spectrom. 1991 6 353. Slovak Z. and Docekal B. Anal. Chim. Acta 1981 129 263. Marecek J. and Synek V. J. Anal. At. Spectrom. 1990 5 385. Docekal B. and Krivan V. J. Anal. At. Spectrom. 1992,7 521. Docekal B. and Krivan V. J. Anal. At. Spectrom. 1993 8 637. Fuller C. W. Analyst 1976 101 961. Karwowska R. and Jackson K. W. Spectrochim. Acta Part B 1986 41 947. Karwowska R. and Jackson K. W. J. Anal. At. Spectrom. 1987 6 273. Hauptkorn S. and Krivan V. Spectrochim. Acta Part B 1994 49 221. Welz B. Schlemmer G. and Mudakavi J. R. J. Anal. At. Spectrom. 1992 7 1257. Gitelman H. J. and Alderman F. R. J. Anal. At. Spectrom. 1990 5 687. Holden A. J. Littlejohn D. and Fell G. S. Anal. Proc. 1992 29 260. Handbook of Chemistry and Physics ed. Weast R. C. CRC Press Boca Raton 74th edn. 1993. Gmelins Handbuch der anorganischen Chemie Calcium Syst. No. 28 Teil B Verlag Chemie Weinheim 8th edn. 1961 pp. 821 and 834. Pavel J. Ciba Geigy Basel personal communication. Paper 3 J04957D Received August 14 1993 Accepted October 20 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900463

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 469 Slurry Sampling for the Determination of Lead in Marine Sediments by Electrothermal Atomic Absorption Spectrometry Using Palladium-Magnesium Nitrate as a Chemical Modifier* Pilar Bermejo-Barrera Carmen Barciela-Alonso Manuel Aboal-Somoza and Adela Bermejo-Barrera Department of Analytical Chemistry Nutrition and Bromatology Faculty of Chemistry University of Santiago de Compostela Spain A method for the determination of lead in slurries of marine sediment using palladium-magnesium nitrate as a chemical modifier has been optimized. To stabilize the marine sediment slurry different thickening agents were studied. The grinding time and the particle size were also studied. To achieve complete pyrolysis of the slurry sample two charring steps were used the first one at a low temperature 480°C and the second at 900 "C.The precision of the method was studied as within-run and within-batch precision; the relative standard deviations obtained in both cases were less than 3%. The relative standard deviation for the repeatability of the over-all procedure was 5.0%. The accuracy of the method was studied using a Certified Reference Material PACS-1 (Marine Sediment) from the National Research Council yf Canada which has a certified lead content of 404 20 mg kg-'; the result obtained was 41 8 & 11 mg kg- . The detection limit for lead was 0.22 pg I-'. Calibrations using aqueous standards and the reference material were compared. This method has been applied to the determination of lead in marine sediment samples from the Galicia Coast and the results were compared with a sample digestion method using nitric and hydrochloric acids in a high pressure poly(tetrafluoroethy1ene) bomb and microwave energy.No significant differences were found between the two procedures. Keywords Lead; sediment; slurry; elecfrothermal atomic absorption spectrometry In recent years in order to avoid problems related to conven- tional wet-oxidation and dry-ashing sample preparation pro- cedures for the determination of metals in sediment samples the direct analysis of solids and slurries by electrothermal atomic absorption spectrometry (ETAAS) has been extensively developed.' The direct analysis of solid samples offers several advantages over conventional procedures including (i) reduced sample preparation time; (ii) decreased possibility of loss of analyte through volatilization prior to analysis; (iii) reduced loss of analyte associated with retention by insoluble residues; and (iv) reduced possibility of sample contamination.Nevertheless there can be problems with the introduction of few milligrams of powdered sample into a graphite tube. The particular specimen selected could not be representative of the total sample the selection of adequate standards for calibration can be difficult and background absorption prob- lems can be present owing to the high levels of matrix present during atomization. Introduction of slurry samples combines the advantages of solid and liquid sampling,'4 and avoids many of the problems associated with direct solid sampling.The slurry technique gives better analytical performance than direct solid sampling because problems associated with weighing the sample and transferring the sample into the graphite tube are avoided. Moreover the concentration of the slurry can easily be changed so that the analyte concentration falls within the range of the calibration graph; however this can present two problems. If the slurry concentration is very high dilution of the slurry can only be carried out within a limited range because the precision is degraded for highly diluted slurries because only a small number of particles will remain in the slurry. If the analyte content in the original sample is low the concentration of the slurry can be increased although the injection precision can deteriorate if slurries are more concentrated.However there are still some problems related to the analysis of sediment samples in the form of slurries. Perhaps the two most important problems are the homogeneity of the slurry * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993. and calibration. Homogenization of the slurry can be achieved by mechanical or ultrasonic agitation of the sample powder in solution or by stabilization of the particles using a thixotropic thickening agent which increases the viscosity of the sus- pension. Different thickening agents have been used in the determination of lead by slurry sampling.Hoening and Van Hoeyweghen' proposed the use of a glycerol medium Littlejohn et aL6 proposed Viscalex HV-30 (acrylic acid polymers) and Bermejo-Barrera et aL7 the use of Triton X-100; Lynch and Littlejohn' also proposed the use of an antifoam agent. To avoid problems with the matrix interferences chemical modification is recommended. In the analysis of solid samples in many instances a chemical modifier added in the form of a liquid is ineffective for stabilization. Thus de Kersabiec and Benedetti' carried out a study of the effect of chemical modifiers in liquid and solid form such as phosphoric acid magnesium nitrate and nickel nitrate on the atomization of geological samples and in both cases double peaks were obtained. The use of chemical modifiers is not a problem for slurry sampling because the interaction between the chemical modifier and the particles of the solid sample is closer than for direct solid sampling.Among the chemical modifiers tried for lead Hinds and co-workerslO.'l proposed the use of palladium alone or a mixture of magnesium and palladium nitrate Lynch and Littlejohn' used only palladium Zongqiang et a1." investigated ammonium dihydrogen phosphate and Ebdon and Le~hotycki'~ proposed the use of ascorbic acid for slurries of various types of samples. In addition Hinds et ~ 2 . ' ~ proposed the use of a fast temperature programme without a chemical modifier. Moreover to avoid problems with the background absorp- tion Ebdon et a1.l' used an oxygen or air ashing step and Hoening et ~ 1 .' ~ proposed pre-digestion of the sediment with nitric acid. In this paper the direct determination of lead in slurries of marine sediment samples is studied using palladium-mag- nesium nitrate as a chemical modifier. Experimental Instrurnenta tion Measurements were carried out using a Perkin-Elmer Model 1 lOOB atomic absorption spectrometer equipped with an470 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 Graphite furnace programme Temperature/ Step "C Dry 1 100 Dry 2 200 Charring 1 480 Charring 2 900 Atomize 2500 Clean 2600 Ramp time/ 10 25 10 10 0 1 S Hold time/ 20 25 10 15 3 3 S Ar flow/ ml min- 300 300 300 300 stop 300 HGA-400 graphite furnace atomizer and an AS-40 auto- sampler. The source of radiation was a lead hollow cathode lamp operated at 10 mA which provided a 283.3 nm line with a spectral bandwidth of 0.7 nm.Deuterium lamp background correction was used. Pyrolytic graphite coated graphite tubes with L'vov platforms were used. For all measurements made during this study integrated absorbance with an integration time of 3 s was used. The temperature and time programmes for the atomizer are shown in Table 1. The volume injected was 20 pl. A Laser Coulter Series LS 100 Fraunhofer optical model from Coulter Electronics Hialeah FL USA was used to check the particle size distribution. An agitator (Vibromatic) and magnetic agitator (Agimatic) from Selecta (Barcelona Spain) were used in the preparation of the slurries. A Portland Model DMR-140 microwave oven and high- pressure poly( tetrafluoroethylene) (PTFE) bombs from Parr Moline IL USA were also used.Reagents Lead nitrate stock standard solution. A 1 mg ml-1 solution of lead (BDH Poole UK). Each test solution was prepared with ultrapure water immediately before use. Triton X-100. Triton X-100 polyethylene glycol mono(p- 1,1,3,3,-tetramethylbutylphenyl) ether for gas chromatogra- phy was obtained from Merck (Darmstadt Germany). Viscalex H V 30. This acrylic copolymer containing carboxyl groups was purchased from Allied Colloids (Bradford Yorkshire UK). Glycerol 99.5%. ACS-reagent grade glycerol was supplied by Sigma Chemicals (St. Louis MO USA). Palladium solution. Prepared by dissolving 300 mg of pal- ladium (99.999%) (Aldrich Chemical Milwaukee WI USA) in 1 ml of concentrated nitric acid and diluting to 100 ml with ultrapure water.If dissolution was incomplete 10 pl of hydro- chloric acid (Suprapur 35% with a maximum lead content of 0.001 mg l-' BDH) were added to the cold nitric acid and heated to gentle boiling in order to volatilize the excess of chloride. Magnesium nitrate. Suprapur Merck. Nitric acid. Suprapur (69.0-70.5%) maximum lead content Hydrochloric acid. Suprapur BDH. Reference material PACS-1 . Marine Sediment National Research Council Canada (NRCC) Ottawa Ontario Canada. Argon. N50 99.999% purity used as a sheath gas for the atomizer and to purge internally. Ultrapure water. Resistivity 18 M a cm-I obtained using a Milli-Q water-purification system (Millipore) All glassware was kept in 10% nitric acid for at least 48 h and washed three times with ultrapure water before use.0.002 mg 1 - ' BDH. Procedure for Slurry Preparation Lyophilized marine sediment samples were ground to reduce them to a particle size of less than 250ym. A portion of the sample 0.25 g was weighed and placed in a polyethylene vial and zirconia beads ( 5 g) and 3 ml of water were added. The vial was agitated in a flask shaker (Vibromatic) for 60 min and then the beads were separated using a sieve funnel (Haldenwanger Technische Keramic Dusseldorf Germany). Finally the slurry was adjusted to its final volume (10 ml) by the addition of water and the amount of Triton X-100 necessary to obtain a concentration of 0.1% Triton X-100. A portion of the slurry with an appropriate amount of chemical modifier was transferred into the autosampler cup and stirred magnetic- ally before being measured.Digestion Procedure For the acid digestion 0.1 g of sediment sample was placed in a PTFE digestion bomb with 2 ml of concentrated nitric acid and 0.5 ml of concentrated hydrochloric acid. The bombs were heated through use of microwave energy for 3 min. When the bombs were cool the samples were made up with ultrapure water to a final volume of 10 ml. Results and Discussion Optimization of the Graphite Furnace Programme Experiments were carried out to determine the optimum temperatures and times for the drying charring and atomiz- ation steps. To obtain an efficient pyrolysis two charring steps were used; the first at 480 "C and the second at 900 "C. These conditions were optimized by means of several measurements for a slurry sediment sample and for aqueous standards both in the presence of palladium-magnesium nitrate.The final ashing curves are shown in Fig. 1 and it can be seen that in both cases 900°C can be used as the optimum charring temperature. A possible problem with the use of slurries in the graphite furnace is incomplete ashing of the organic matrix. To avoid this problem Ebdon et aZ.15 used air or oxygen as an ashing aid thus the normal charring step is converted into an oxidative decomposition process. In previous work on the determination of lead in mussel slurries Bermejo-Barrera et aL7 used an ashing step with air obtaining a significant decrease in the background signal. To establish the effect of ashing sediment slurries in the presence of air or oxygen a study of various air and oxygen flow rates in the first thermal pre-treatment step was performed.The results obtained are shown in Fig. 2. There is no advantage for the background absorption when air or oxygen are used in the thermal pre-treatment step probably owing to the low amount of organic matter present in sediment slurries. For this reason and to avoid rapid degradation of the graphite tube the introduction of air or oxygen during the charring step was not used. Determination of the optimum atomization temperature was i 0.420 0.315 v - 7 0 0 500 700 900 1100 1300 1500 Temperatu re/"C Fig. 1 standard Charring curves for A slurry sediment; and B aqueousJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 47 1 I B I 8 8 c o 0 50 100 150 200 250 300 Air flow rate ml/rnin-' +? s 2 0.16 ( b ) 0.12 0.04 0 I I I I I I 0 50 100 150 200 250 300 Oxygen flow rate/ml min-' Fig.2 Effect of the use of (a) air and (b) oxygen in the first ashing step A background; B signal minus background carried out by studying different atomization temperatures between 1000 and 2600 "C.The choice of this temperature was not made from the atomization curve because double lead absorption peaks were observed at some atomization tempera- tures and hence selection of the optimum temperature was not possible. The effects of atomization temperature on the double peak formation in a sediment sample slurry in the presence of palladium are illustrated in Fig. 3. The double peaks appear at atomization temperatures between 1300 and 2200 "C and at 2800°C; a single absorption peak is observed for atomization temperatures of between 2300 and 2700°C.The optimum atomization temperature chosen was 2500 "C because using this temperature there are no double peaks and the maximum peak area is obtained. This double peak formation by lead in the presence of palladium has been observed previously and the DabekaI7 has explained this behaviour as being due to the formation of refractory lead species in the presence of pal- ladium. However double absorption peaks for lead in the absence of palladium have also been observed,17 and have been attributed to physical effects such as background prob- lems or partial occlusion of lead within the sample matrix or to the presence of two or more lead compounds (Pb PbO) 0 0.5 1.0 1.5 2.0 Time/s i Fig.3 Effect of atomization temperature on double peak formation in the presence of palladium A 2800; B 2700; C 2600; D 2500 E 2400; F 2300; G 2200; H 2100; I 1900; J 1700; K 1500; L 1300; and M 1000°C Time/s Fig.4 Influence of palladium on the absorption profile of a slurry sediment sample A without modifier added integrated absorbance 0.157 s; and B with palladium-magnesium added integrated absorbance 0.175 s having different volatilities. Moreover Qiao and Jackson" have recently observed that the distribution of analyte in the sediment sample contributes to the double peak formation because of two different forms of lead being present in the sediments lead adsorbed on the clay particles and lead on the organic carbon.These workers established that pal- ladium-magnesium nitrate as a chemical modifier was not necessary to determine lead in soil slurries because the slurry particle itself acts as a modifier to stabilize the analyte during pyrolysis. However experience of the present workers has shown that when the modifier was not used a small shoulder was observed (Fig. 4). To compare the atomization of lead from the sample slurry and from an aqueous standard the atomization temperature for lead aqueous standards with added palladium-magnesium nitrate was studied. In this case the double peak was not observed. The absorbance obtained when the atomization temperature was increased from 1100 to 2700°C is constant. Hence the optimum atomization temperature for the sediment slurry 2500"C can be used because at this temperature there are no problems with the aqueous standards.Amount of Chemical Modifier A series of measurements were carried out to determine the optimum concentration of both palladium and magnesium nitrate to be used by adding different amounts of each one of them to a series of slurries in the absence of the other. For the series containing only magnesium nitrate the appearance time is shortened when the amount of magnesium nitrate is increased but the integrated absorbance is very similar in all cases Fig. 5. When only palladium was added the appearance time of the signal was very similar but the peak height increased with the amount of palladium added and the peaks were narrower Fig. 5. The same effects were observed for aqueous standards of lead.The use of palladium with mag- nesium nitrate is recommended because although the peak shown in Fig. 4 (peak B) with palladium-magnesium nitrate and the peak in Fig. 5(a) with palladium only are similar when only palladium was used the absorbance peaks obtained from slurry samples are shifted in time compared with the aqueous standards. This behaviour has been observed previously by Hinds and Jack~on.'~ This shifting effect is avoided when the mixture is used. This is important because aqueous calibrations are to be used. Variations in the integrated absorbance for different amounts of palladium and magnesium nitrate for sediment slurries and aqueous standards are shown in Fig. 6. The optimum amounts chosen were 2mgl-I of palladium and 15mg1-' of mag- nesium nitrate.Grinding Time of the Slurry The optimum grinding time using the zirconia beads was determined by measuring the lead content in several slurries472 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0.18 v) -. 8 0.17 ([I 2 9 0.16 U 4- 2 0.15 4- - 0.14 Y 0 0.5 1 .o 1.5 2.0 Time/s Fig.5 Effect of the amount of modifier on the absorbance peaks of lead. (a) Effect of different amounts of palladium. Integrated absorbances 0.154; 0.162; 0.157; 0.165; 0.162 for A 0; B 3; C 2; D 4; and E 6mg I-' of palladium. (b) Effect of different amounts of magnesium. Integrated absorbances 0.154; 0.154; 0.151; 0.146 and 0.147 s for A 0; B 5; C 10; and D 15 and 20 mg 1- ' of magnesium nitrate of marine sediment sample and a control sediment that had been ground for 30,45,60 and 90 min.The results are presented in Fig. 7 where it can be seen that a grinding time of 60 min is adequate for both types of samples because a longer grinding time does not give a significant improvement in the absorbance signal. Particle Size With the above grinding procedure a particle size of less than 5 pm was achieved which was checked by means of laser diffraction. The particle sizes were studied in a sediment sample and control sediment using different grinding times. The results obtained are given in Table 2. It can be observed that with a grinding time of between 30 and 90 min there is no significant change in the particle size. On the other hand in Fig. 7 can be seen the particle size distribution for both types of samples and these distributions are very similar in all cases 90.00% of the particles have a size of less than 0.5 pm.Effect of Predigestion of Sediment In a study on slurries of solid samples Hoening et all6 used pre-digestion of the sediment in acid medium which then mobilizes several elements into solution after preparation of slurry. Thus only a portion of the trace elements present remained in the solid phase which is nevertheless dispensed together with the enriched solution into the atomizer during sampling. To study the effect of acid pre-digestion on a sediment sample experiments were performed where 0.1 g of sediment sample was treated with 50 ml of concentrated nitric acid and 950 pl of water for different periods of time before the prep- aration of the slurry.For pre-digestion times of 0 5 10 15 and 20 h absorbances of 0.123 0.125 0.123 0.123 and 0.124 respectively were obtained (n=4). The use of the acid pre- digestion does not produce any improvement in the absorbance of lead. 0.19 / P L 0 2 0.17 U r L? 0.16 4- - 0.15 5 Fig.6 Effect of amounts of palladium and magnesium on the inte- grated absorbance signal for (a) slurry sediment and (b) aqueous standard 0.14 0.12 ~ 9 1 I 0.06 ' I I I I I I 30 40 50 60 70 80 90 Timelmi n Fig.7 control; and B sample Effect of grinding time on the absorbance of the slurry A Effect of Different Thickening Agents The effect of thickening agents was studied by preparing slurries that contained Triton X-100 Viscalex HV 30 or glycerol as the stabilizing agent. Several different concen- trations of Triton X-100 between 0.04 and 1 % were evaluatedJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 473 Table 2 Effect of grinding time on particle size Grinding time/ Mean size/ SD*/ Certified Reference Material PACS-1- rnin Pm Pm 30 0.775 0.630 45 0.784 0.603 60 0.768 0.564 90 0.773 0.540 30 0.838 0.808 45 0.843 0.783 60 0.845 0.773 90 0.820 0.704 Sediment sample- 95% Confidence limit/ Pm 0.65 1-0.898 0.666-0.902 0.658-0.879 0.667-0.879 0.680-0.997 0.689-0.996 0.693-0.996 0.68 2-0.958 * SD =standard deviation. to determine which concentration provided the most suitable visual dispersion of particles; 0.04% was found to be sufficient. Also the variation of absorbance of a sediment sample slurry with concentration of Triton X-100 was studied. The results obtained are presented in Table 3 and were blank corrected for each Triton concentration; a concentration of Triton X-100 of 0.1% is more than adequate higher concentrations produc- ing a reduction in absorbance.The same study was carried out with Viscalex HV-30 but it was necessary to increase the hold time during the second drying step for 60 s for this thickening agent. The results obtained blank corrected for each Viscalex concentration are given in Table 3 and again 0.1Y0 was an adequate concentration. Similarly slurries were prepared with glycerol concentrations of 20 30,40 50 and 60% m/v and the results blank corrected for each glycerol concentration obtained are also shown in Table 3. A concentration of 40% glycerol would be the opti- mum concentration but with the use of this thickening agent there were some problems.To dry the sample completely it was necessary to alter the drying steps a first drying step at 150°C with 5 s ramp time and 50 s hold time being necessary and then a second drying step at 350 "C with 80 s ramp time and 40 s hold time. Under these conditions the duration of the graphite furnace programme is considerably increased. Moreover when glycerol was used a heavy smoke appeared on the decomposition of glycerol at about 400°C. The addition of glycerol and Viscalex also caused problems with the reproducibility of the autosampler pipetting because the sample solution adhered to the outside of the autosampler capillary and would sometimes enter the tube through the injection hole impairing precision. This problem has been observed previously by Stephen et aL4 and Miller-Ihli.3 For all these reasons Triton X-100 was selected as the optimum thickening agent.Calibration and Standard Additions Graphs To obtain a calibration graph to standard aqueous solutions containing lead concentrations of between 0 and 7 pg 1-1 were added appropriate volumes of solutions of palladium mag- nesium nitrate and Triton X-100 to give concentrations of 2 mg 1-' 15 mg ml-' and 0.1% respectively. The standard 12 8 4 1 s o 0.5 1.0 2 3 4 6 10 a c 100 700 12 8 4 0 I Table 3 Variation of absorbance with various concentrations of thickening agents Particle diametedpm Fig. 8 Particle size distribution as a result of stirring the slurry with zirconia beads for 60 min (a) marine sediment sample; and (b) sediment control PACS-1 additions method was used over the same range of concen- trations using a sediment sample.The equations obtained were as follows calibration graph &=8.22 X 10-4+6.51 X w 3 C r=0.998 standard additions graphs QA=4.66 x l0-'+6.86~ 1 0 - 3 ~ r=0.996 where QA is integrated absorbance and c is the lead concen- tration in pg I-'. Both graphs are shown in Fig. 8 where they exhibit similar slopes. This means that aqueous calibration is a real possibility and this type of calibration was therefore used. Calibration Using a Certified Reference Material Calibration employing the Certified Reference Material PACS-1 (Marine Sediment) which had a certified lead content of 404f20 mg kg-l was carried out and compared with that using aqueous standards. The equation obtained was as follows calibration with PACS-1 Q ~ = -3.39x 10-3+5.5ox 10-3c This calibration graph is also shown in Fig.8. It was concluded that aqueous calibration is more suitable than calibration with PACS-1 for this analysis. Concentration of Triton X-100 (Yo v/v) Absorbance 0.04 0.122 0.08 0.130 0.10 0.132 0.50 0.110 1 .oo 0.115 Concentration of Viscalex HV 30 Concentration of (Yo v/v) Absorbance glycerol (% m/v) Absorbance 0.04 0.130 20 0.115 0.08 0.133 30 0.119 0.10 0.130 40 0.117 0.50 0.126 50 0.108 1 .oo 0.127 60 0.109474 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0.10 0.08 cu m 2 0.06 2 0.04 d e 0.02 0 1 Fig. 9 Calibration graphs A calibration with aqueous standards; B standard additions; and C calibration with PACS-1 Sensitivity The limit of detection (LOD) the lowest concentration level that can be determined to be statistically different from a blank is defined as LOD=3SD/m (where m is the slope of the calibration graph) corresponding to a 99% confidence level.The limit of quantification (LOQ) is defined as the level above which quantitative results can be obtained with a specified degree of confidence. At the 99% confidence level the value recommended is LOQ=lOSD/m. In both cases SD is the within-run standard deviation of a single blank determination. The values obtained were 0.22pgl-' for the LOD and 0.74 pg 1-l for the LOQ based on ten replicate determinations of the blank. The characteristic mass mo is defined as the mass of analyte in picograms required to give a signal of 0.0044 s for inte- grated absorbance.The characteristic mass obtained was 11.4 & 0.33 pg. The characteristic mass obtained was lower than the literature values as Hinds et a1.I4 found 12.5 pg for lead without a chemical modifier and 18.2pg when they used palladium-magnesium nitrate. In both cases they used a fast furnace programme. Schlemmer and Welz20 found 12pg for 15 pg of palladium+ 1.6 pg of magnesium and Hinds et a2." obtained 14 pg for 0.6 pg of palladium+ 1.0 pg of magnesium. The LOD for the sediment sample using 0.25 g of sample diluting the slurry to 10 ml and taking 25 pl to prepare the final solution with the modifier (1000 pl) was 35 pg kg-'. This limit can be improved by taking a larger volume of the slurry sediment to prepare the final solution.However to obtain the lowest LODs using the slurry procedure it is necessary to increase the concentration of the sample suspended in the slurry. When the slurry concentration was increased an increase in the peak area was observed. This can cause some problems because an increase in sample concentration is related to an increase in viscosity which can impair the precision of the sample introduction. The effect of sample concentration on the precision of the lead signals was studied by preparing slurries that contained 1.0 2.5 5.0 and 10% m/v of sediment. Amounts of aqueous lead solutions were added to each type of slurry to obtain similar absorbances. Seven replicate injections of each slurry were performed and the relative standard deviation (RSD) values calculated.The results obtained are shown in Table 4 and it can be seen that Table 4 Effect of slurry concentration on precision; seven replicates Slurry concentration Coefficient of variation (YO m/v) Mean absorbance (%) 1 2.5 5.0 10.0 0.119 2.6 0.122 2.3 0.126 1.4 0.124 2.2 the slurry concentration does not affect the precision. Moreover the use of higher slurry concentrations does not cause any problems with the background signal as for 1% m/v the background signal was 0.026 s integrated absorbance and for 10% was 0.030s. The LOD using a slurry sediment sample of 10% m/v and taking 25 pl of this solution would be 8.8 pg kg-'. Precision The within-run precision (RSD) of the method (instrumental and matrix factors) obtained for ten replicate analyses of a single sample during the same run was 2.3% (for 2.2 pg I-' of lead).The within-batch precision of the method obtained for ten replicates of three samples with different concentrations of lead added was also investigated. To study the within-batch pre- cision three samples with 3 5 and 7 pg 1-1 of added lead were used and the results were 2.3 1.6 and 2.1% respectively. The repeatability of the over-all procedure was also studied by measuring ten different slurries of the same sample. The RSD obtained was 5.0%. Accuracy To study the accuracy of the method the Certified Reference Material PACS-1 with a certified lead content of 404+20 mg kg-' was used. The reference material and the blank were measured three times.The results obtained expressed as mean & SD was 418 & 11 mg kg-'. The recovery was also calculated and was found to vary between 97.3 94.6 and 99.0% for 3 5 and 7 pg 1-1 of added lead. Applications The method was applied to the determination of lead in marine sediment samples from the Galicia coast (north west Spain). Two sub-samples taken from each sediment sample were prepared in the form of slurries and two sub-samples from each sub-sample were subjected to AAS. The results obtained were compared with those achieved when the samples were digested with nitric and hydrochloric acids in high-pressure bombs using microwave energy. Again two sub-samples from each sediment sample were digested and then two sub-samples from each were subjected to AAS. To compare the results obtained by the two methods the paired t-test was applied.21 The results obtained are given in Table 5.As shown as the calculated t-value is smaller than that obtained from the t-distribution table hence the two methods do not give significantly different values for the mean concentration of lead. Table 5 Comparison of the results obtained in sediment samples from the Galicia coast (Spain) Sample no. 1 2 3 4 5 6 7 Slurry method/ Digestion method/ mg kg-' mg kg-' 8.4 6.2 17.5 17.5 37.8 35.6 46.9 46.9 51.4 49.2 46.9 44.7 78.6 78.6 Mean difference= 1.257 mg kg-' SD of mean difference= 1.176 mg kg-I t = 2.828 Critical value of t (P = 0.01) = 3.71JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 475 Conclusions The use of slurry sample introduction in ETAAS is a convenient method for the determination of lead in marine sediments. The use of palladium-magnesium nitrate as a chemical modifier in atomization of the slurry sediment provides good stabilization of lead up to 900 "C and avoids the formation of double peaks.With the proposed procedure a particle size of less than 5 pm was obtained and with the use of the Triton X-100 as a thickening agent the precision and accuracy of the method are good. Aqueous calibration is considered to be more appro- priate than calibration using a certified reference material thus simplifying the determination. It can be concluded that the slurry sampling procedure is more suited to the determination of lead in marine sediment samples than a wet-digestion procedure because complex dissolution of the marine sediment sample is avoided and thus sample handling is minimal and possible sources of contamination are reduced.References Bendicho C. and de Loos-Vollebregt M. T. C. J. Anal. At. Spectrom. 1991 6 353. Miller-Ihli N. J. Fresenius' J. Anal. Chem. 1990 337 271. Miller-Ihli N. J. J. Anal. At. Spectrom. 1988 3 73. Stephen S. C. Ottaway J. M. and Littlejohn D. Fresenius' 2. Anal. Chem. 1987 328 346. Hoening M. and Van Hoeyweghen P. Anal. Chem. 1986 58 2614. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Littlejohn D. Stephen S. C. and Ottaway J. M. presented at SAC '86-3rd BNASS Bristol UK July 1986 paper number PF13. Bermejo-Barrera P. Aboal-Somoza M. Soto-Ferrero R. and Dominguez-Gonzalez R. Analyst 1993 118 665. Lynch S. and Littlejohn D. J. Anal. At. Spectrom. 1989 4 157. de Kersabiec A. M. and Benedetti M. F. Fresenius' 2. Anal. Chem. 1987 328 342. Hinds M. W. Katyal M. and Jackson K. W. J. Anal. At. Spectrom. 1988 3 83. Hinds M. W. and Jackson K. W. J. Anal. At. Spectrom. 1990 5 199. Yu Z.-q. Vandecasteele C. Desmet B. and Dams R. Microchim. Acta 1990 I 41. Ebdon L. and Lechotycki A. Microchem. J. 1986 34 340. Hinds M. W. Latimer K. E. and Jackson K. W. J. Anal. At. Spectrom. 1991 6 473. Ebdon L. Fisher A. S. Parry H. G. M. and Brown A. A. J. Anal. At. Spectrorn. 1990 5 321. Hoening M. Regnier P. and Wollast R. J. Anal. At. Spectrom. 1989 4 631. Dabeka R. W. Anal. Chem. 1992,64 2419. Qiao H. and Jackson K. W. Spectrochim. Acta Part B 1992 47 1267. Hinds M. W. and Jackson K. W. J. Anal. Atom. Spectrom. 1988 3 997. Schlemmer G. and Welz B. Spectrochim. Acta Part B 1986 41 1157. Miller J. C. and Miller J. N. Statistics for Analytical Chemistry Ellis Horwood Chichester 1986. Paper 3104471 H Received July 27 1993 Accepted October 1 I 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900469

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 477 Direct Determination of Zinc in Sea-water Using Electrothermal Atomic Absorption Spectrometry With Zeeman-effect Background Correction Effects of Chemical and Spectral Interferences" J. Y. Cabon and A. Le Bihan Unite de Recherche Associee au CNRS No. 322 Universite de Brefagne Occidentale 6 Avenue Le Gorgeu 29275 Bfest-Cedex France The determination of zinc in sea-water using an electrothermal atomic absorption spectrometry system with Zeeman-effect background correction is presented. The influence of various chloride and nitrate salts on the atomization signal of zinc was examined. In chloride medium particularly the interference effect induced through losses of zinc chloride by the thermohydrolysis of magnesium chloride and simultaneous generation of HCI during the pyrolysis step is noted.In nitrate medium zinc is more stabilized by Mg>Ca>Na> NH:. The effect of various inorganic and organic acids used as chemical modifiers on the atomization of zinc and background absorption signals in sea-water were examined. In unmodified sea-water a Zeeman interference effect related to the vaporization of the chloride matrix leading to a systematic under- compensation and consequently to erroneous zinc concentration values was observed. In sea-water modified with 1 mol I-' nitric acid a spectral Zeeman interference effect induced by the Zeeman splitting of the absorption bands of NO molecules generated during the decomposition-reduction of nitrate was observed; the induced over-compensation is eliminated by selective pyrolysis at about 850 "C.The chemical interference effect (25%) is related to the simultaneous vaporization of zinc and sodium oxides; the detection limit (3a) being about 80 ng I-' for a 10 pI injected volume of sea-water. In sea-water modified with 0.7 mol I-' oxalic acid there is no significant interference effect and the detection limit in this medium is about 60 ng I-' for a 10 pl injected volume of sea-water. Keywords Atomic absorption spectrometry with Zeeman-effect background correction; zinc determination; sea-water; spectral and chemical interference; nitric and oxalic acid The determination of trace metals in the pgl-' range in sea- water necessitates very stringent precautions against contami- nation.Hence the direct determination of zinc in sea-water by electrothermal atomic absorption spectrometry (ETAAS) with- out a preconcentration step is of interest to the analyst. Owing to the high background absorption signal generated at 213.9 nm by the volatilization of the chloride matrix direct determination of zinc at low-level concentrations in sea-water remains difficult. In previous work zinc has been determined directly in sea-water without a rnodifier,'q2 or by using nitric acid ammonium nitrate4 and citric acid5 as modifiers. All these studies have been performed using deuterium-arc back- ground correction; no platforms; no simultaneous vizualization of specific and background signals leading to a loss of infor- mation; peak height measurements [chart recorder (no digitiz- ation of the signals)]; slow atomization ramps generally under gas flow; and empirical chemical conditions leading to various detection limits (30) of 2.55 (ref.2) 0.6 (ref. 4) and 0.27 pg 1-' (ref. 5 ) . In order to improve the analytical performance of ETAAS for direct determination of zinc in sea-water a more comprehensive study of the atomization of zinc in this medium has been made using a recent ETAAS instrument with a longitudinal-effect background correction system and iso- thermal atomization. In the first part of this work the influence of various chloride (including NaC1 CaC1 and MgC12 naturally present in sea- water) and nitrate salts [including Mg( N03)2 recommended as modifier by the manufacturer] on the atomization signal of zinc in water was studied. In the second part the influence of inorganic and organic acids used as modifiers on the atomization signal of zinc and the sea-water background absorption signal was studied.[Owing to the difficulties in purifying acidic solutions of stabilizing modifiers (Pd and Pt) at the sea-water level concen- * Presented at the XVIII Colloquium Spectroscopicum Inter- nationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectrometry Durham UK July 4-7 1993. trations the influence of such modifiers was not investigated in this study.] The experimental conditions for the determi- nation of zinc at low level concentrations in unmodified sea- water and in the presence of nitric and oxalic acids were optimized. Experimental Reagents Merck suprapure grade.Hydrochloric nitric sulfuric phosphoric and oxalic acids. Sodium magnesium and calcium nitrates. Merck pro analysi. Sodium magnesium and calcium chlorides. Merck pro analysi. Ethylenediaminetetraacetic acid (Na,EDTA) tetrasodium Zinc standard solution 1 g 1-' in 0.5 moll-' HNO,. Merck. Sea-water reference material for zinc determinations. NASS-3 (0.178+0.025 pg 1-' of Zn) and CASS-2 (1.97kO.12 pg I-' of Zn) from the National Research Council Ottawa Canada were used. In this case ultrapure HNO (Merck) was used. Ultrapure water from a Millipore Mro-MQ system was used. Blank determinations on 1 moll-' nitric and 0.7 mol I-' oxalic acids solutions were respectively 0.05 and 0.15 pg 1-' of Zn. salt. Aldrich. Instrumentation A Perkin-Elmer 4100ZL was used for all the atomic absorption measurements.Pyrolytic graphite coated graphite tubes equipped with pyrolytic graphite coated platforms (Perkin- Elmer) were used. Samples and modifier solutions were deliv- ered to the furnace using a Perkin-Elmer AS-70. The light source was a Perkin-Elmer hollow cathode lamp operating at 20 mA. The zinc resonance line at 213.9 nm was used with a 0.7 nm spectral slit-width. The inert gas was argon. Dilutions were carried out with calibrated Gilson Pipetman pneumatic syringes. Typical operating conditions were drying 120 "C t(s) 250 ml min-' argon; pyrolysis T("C) 60 s 250 ml min-' argon; cooling 100 "C 10 s 250 ml min-' argon; atomization,478 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 0.2 T("C) ramp step 0 gas flow interrupted; and cleaning 2500 "C 5 s 250 ml min-' argon.A C - I I A A Results and Discussion Water Medium Fig. 1 shows the evolution of the atomization signal of zinc with temperature in ultrapure water. As it appears the shape of the signal evolves from a single peak shape corresponding to the atomization mechanism:6 ZnO(s)eZnO (g)$Zn( g) + 1/20 to a two peak signal shape corresponding to a modification of the atomization mechanism and probably to a partial reduction of ZnO(s) to Zn by the graphite at the lowest temperature which also could explain the loss of zinc for relatively low calcination temperatures (about 450 "C). The maximum inte- grated absorbance value is reached for an atomization tempera- ture close to 1100°C [rn,=0.5 pg; at 1800 "C in the presence of Mg(N03) the rn reported by the manufacturer is 1.0 pg].The maximum peak height absorbance value is reached for temperatures above 1600 "C (rn = 1.0 pg]; the integration time being 10 times longer at 1100°C than at temperatures above 1600 "C. Theoretical rn values have been calculated from the model of L'vov7 with the use of the software of Berglund and Baxter.' Above 1 lOO"C there is good agreement between experimental and theoretical rn values. So atom formation is quantitative and atom removal is predominantly diffusion controlled. Under these conditions the calculated atomization efficiency &'A is close to 100% and not temperature dependent. Chloride Medium The influence of ammonium sodium magnesium and calcium chlorides at a 0.1 moll-' chloride concentration on the atomiz- ation signal of zinc was examined.Without pyrolysis and with the use of a 0 s atomization ramp step to 1600 "C no significant changes in the shape of the atomization signal of zinc were observed [Fig. 2(a)]. Interference effects on the atomization signal of zinc are not important and are essentially related to the stabilization of ZnC1 by the simultaneous vaporization of the chloride salts (consistent with boiling-point data"); the importance of the depressing effect being related to the amount of chloride in the vapour phase [Fig. 2(a) and (b)]. For this chloride concentration value NaCl and CaC1 have a small interference effect (about 10%); and NH4C1 and MgCl have a depression effect of about 20%. Moreover in the presence of MgC1 at the concentration level of sea-water (about 0.05 mol 1-I) for a slow atomization ramp step as is generally 0-'0° t I 0 co 0.350 2 n 0 2.5 Time/s 5 Fig.1 Influence of temperature on atomization signal of zinc in water ( 5 pl 10 pg 1-' of Zn) A 2000; B 1600; C 1400; D 1300; E 1200; F 1150; and G 1100°C $ 0 4.98 Fig. 2 (a) Zinc atomization signals ( 5 pl 10 pg 1-' of Zn) in the presence of A 0.1 mol I-' NH,Cl; B 0.1 moll-' NaCl; C 0.05 moll-' MgCl,; and D 0.05 moll-' CaCl,. (6) Background absorption signals for 5 pl of A B C and D used for the determination of zinc in sea-water a strong interference effect was observed depending on the heating rate. The influence of the pyrolysis temperature on the atomiz- ation signal of zinc in the presence of these different chlorides was examined.The variations of the integrated absorbance of zinc and of the background absorption signal are presented in Fig. 3(a) and (b). In the presence of MgCl a significant loss of Zn was observed for a pyrolysis temperature of >4OO"C with a corresponding decrease of the MgCl background absorption signal. For temperatures > 400 "C MgC1 is ther- mohydrolysed on the graphite to MgO (non-absorbing species under these experimental conditions) with the generation of hydrogen ~hloride,''-'~ zinc is lost during pyrolysis and as HC1 is simultaneously generated ZnCl is stabilized in the vapour phase owing to the high chloride concentration and blown out of the furnace. For CaC1 and NaCl higher pyrolysis temperatures can be used but Zn is less stabilized by CaC1 than by NaCl despite its higher volatilization temperature 0.20 v) -.8 0.16 n C (D 0.12 v) 0 0.08 4- E 0.04 4- - 0 (b) 0.8 0.6 100 300 500 700 960 1100 1 TemperatureK 1 DO Fig.3 (a) Influence of pyrolysis temperature on (a) integrated absorbance of zinc (5 pl 10 pg 1-' of Zn) in the presence of A 0.1 moll-' NH4C1; B 0.1 moll-' NaC1; C 0.05 MgC1,; and D 0.05moll-' CaC1,; and (b) on background absorption signal of 5 pl of A B C and DJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 479 m 0 f 4.96 0 4.96 V A A n7nn I 0 4.96 0 Timels 5.00 Fig. 4 Zinc atomization (solid line) and background absorption (broken line) signals in the presence of (a) 0.1 moll-' NaNO,; (b) 0.05 moll-' Mg(NO,),; (c) 0.05 moll-' Ca(NO,),; and ( d ) 0.1 moll-' NH,NO ( 5 p1 10 pg 1-' of Zn) [Fig.3 (b)]. This can be explained by a partial thermohydrolysis of CaCl to CaO which induces a loss of Zn as ZnCl species stabilized by the simultaneous generation of HCl. For NaCl the interference effect is not important and could be only due to the simultaneous vaporization of NaC1. In the presence of NH4C1 no loss of Zn was observed through the formation of ZnC1 or the ammonium bonded form during pyr~lysis,'~ but a stabilization of zinc on the graphite furnace was noted. Nitrate Medium The influence of ammonium sodium magnesium and calcium nitrates at a concentration of 0.1 moll-' on the atomization signal of zinc has been examined. As can be seen in Fig.4 without pyrolysis and using a 0 s atomization ramp the shape of the atomization signal of zinc is strongly modified therefore the integrated absorbance values are not very different (about 10%) from those obtained in water.The decomposition- reduction-volatilization step of the nitrate salts generates absorbing species (mainly NO) in the case of NH,NO NaNO Ca( and Mg( NO3) under these experimental conditions which induces an important spectral Zeeman interference leading to an over-compensation at low Zn concentrations.'6 Only sodium oxides resulting from sodium 0.20 VI -. 0.16 m + n 2 0.08 F 2 0.12 m 4- 0.04 e C - 0' I I 1 I 1 100 300 500 700 900 1100 1300 Tern perat u rePC Fig. 5 Influence of pyrolysis temperature on the atomization signal of zinc ( 5 p1 10 pg 1-' of Zn) in the presence of A 0.1 moll-' Mg(NO& B 0.1 moll-' NaN0,; C 0.05 moll-' Mg(N03)2; and D 0.05 moll-' Ca(NO,)* nitrate decomposition induce a background absorption signal simultaneously with the atomization signal of zinc; refractory oxides (CaO and MgO) being not yet volatilized.In Fig. 5 the variations of the integrated absorbance values with the pyrolysis temperature in the presence of the different metallic nitrates are presented. Zinc is stabilized by the nitrate salts which permit a higher pyrolysis temperature according to Mg( NO3) > Ca(N03) > NaN0 > NH,N03. The stabiliz- ation of ZnO is obtained through the adsorption of oxygen or oxygenated compounds on the graphite reductive active sites and through an occlusion process related to the respective vaporization temperatures of the thermally stable oxides.These stabilizing effects permit the elimination of the background absorption signal generated by the decomposition of the nitrate salts by selective pyrolysis and the suppression of the corre- sponding over-compensation. Sea-w a t er Medium In sea-water the atomization signal of zinc and the high background absorption signal generated by the volatilization of sea-water salts are not well separated and the integrated absorbance value is depressed by about 20% as compared with the integrated absorbance value obtained in water. Presence of Inorganic Acids The modification of the chloride matrix to a nitrate sulfate or phosphate matrix by using 1 moll-' HN03 H2S04 or H3PO4I7 does not lead to well separated atomization curves for the zinc and background absorption signals.The atomiz- ation signal of zinc is more complex and a delaying effect has been noted phosphate > sulfate > nitrate >chloride. Under these experimental conditions the integrated absorbance values are depressed by about 25% in a nitrate or sulfate medium and 40% in a phosphate medium. The use of 1 mol l-' HN03 as modifier leads to the smallest background absorption signal decreasing by about 10 times the sea-water background absorp- tion signal. Presence of Organic Acids In Fig. 6 the atomization signal for zinc and the background absorption signal obtained in sea-water in the presence of480 BG 2.000 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 (d 1 Fig. 6 Zinc atomization (solid line) and background absorption (broken line) signals in sea-water in the presence of (a) 0.1 moll-' ascorbic acid; (b) 0.1 moll-' citric acid; (c) 0.1 mol 1-' Na,EDTA; and ( d ) 1 0.1 moll-' oxalic acid and 2 0.7 moll-' oxalic acid ( 5 jd 10 pg 1-I) 0.1 moll-' citric and ascorbic acids and Na,EDTA and 0.1 and 0.7 moll-' oxalic acid are presented.The presence of the different acids leads to a complex atomization signal for zinc but the integrated absorbance values are only slightly smaller than in water (about 10%). The use of the organic acids facilitates the reduction of zinc in sea-water but does not permit a good separation of the atomization signal of zinc and the background absorption signal; zinc being partially seques- tered by the remaining chloride matrix or the corresponding oxides resulting from hydrolysis.It can be noted that for twice the citric acid concentration recommended by Guevremont,' only a partial separation of the atomization signal of zinc and the sea-water matrix absorption signal (which is not signifi- cantly reduced) was observed. Moreover the decompo- sition-volatilization of ascorbic acid citric acid and Na,EDTA generates a simultaneous background absorption signal which cannot be eliminated without losses of zinc. Oxalic acid does not generate any significant background absorption value. At a 0.7 moll-' concentration level by removing chloride as HCl and with conversion of salts into oxides (cf. HN03) it decreases the background absorption signal to a level not very different from that obtained in the presence of 1 moll-' HNO,; the remaining chemical interference effect being below 10%.Optimization of Experimental Conditions If in unmodified sea-water a single peak shape for the atomiz- ation signal of zinc at high concentrations not well separated from an important background signal is observed in the presence of the different inorganic and organic acids (used as modifiers) then the atomization signal of zinc must be complex corresponding to complex atomization mechanisms. The different modifiers studied do not lead to a good separation of the atomization signal of zinc and the background absorp- tion signal. The most important decrease of the background absorption signal is obtained with the use of nitric and oxalic acids so the best experimental conditions for determining zinc at low level concentrations in unmodified sea-water and sea- water modified with nitric or oxalic acid were determined.Chloride Zeeman Interference Effect in Unmodified Sea-water The 'atomization' signals of zinc obtained in a purified sea-water [ammonium pyrrolidin-1-yldithioformate (APDC)- Freon extraction] and with addition of 1 pg 1-1 of Zn are 0 5.00 Time/s Fig. 7 (a) 'Atomization' signal of zinc in A purified sea-water; B spiked (1 pg 1-') unmodified sea-water; and C background ( 5 p1 Tat= 1800°C). (b) Atomization signal of zinc in A 0.5 moll-' NaCl solution; B spiked (1 pg 1-I) 0.5 moll-' NaCl solution; and C back- ground ( 5 PI Kt = 1800 "C) shown in Fig. 7(a). In the purified sea-water a residual 'specific' atomization signal is obtained in the purified sea-water which is delayed as compared with the atomization signal correspond- ing to the addition of 1 pgl-' of Zn; the shape of this signal is very similar to the shape of the background absorption signal.The variation of this residual 'specific' integrated absorbance with the pyrolysis (200-1500 "C) and atomization (850-1800 "C) temperatures is rather small and not analogous to the variation of the integrated absorbance of a spiked sea- water; this signal remains nearly constant as Zn is lost at pyrolysis temperatures of >6OO"C and not atomized at tem- peratures of < 1400 "C. From these observations it appears that this 'specific' atomization signal is not the atomization signal of zinc but a Zeeman interference effect related to the vaporization of the chloride sea-water matrix.This Zeeman interference has also been observed in a 0.5molI-1 NaCl medium [Fig. 7(b)]; a 'specific' atomization signal delayed from the atomization signal of a 1 pg1-l Zn spike being generated during the vaporization of the NaCl matrix. The Zeeman suppressed interference in sea-water appears to be mainly induced by the vaporization of the NaCl salt. In sea- water this Zeeman under-compensation occurring at the end of the Zn atomization signal could be equivalent to theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 48 1 atomization signal of about 1 pg 1-l of Zn at 1800°C and consequently induces an important systematic error (depending on the electrothermal atomization programme) on the direct determination of zinc at low level concentrations in sea-water.Presence of Oxalic and Nitric Acids The maximum integrated absorbance is reached for an atomiz- ation temperature of 1300 "C in the presence of 0.7 mol I-' oxalic acid (m,=0.7 pg) and of only 1400 "C in the presence of 1 moll-' nitric acid (m = 0.9 pg). The chemical interference effect observed in nitric acid being probably due to the production of oxygen in the furnace related mainly to the vaporization of sodium oxide species simultaneously with zinc oxides. This interference is smaller in the presence of oxalic acid because of the lower atomization temperature of zinc in this more reducing medium. In sea-water modified with nitric acid [Fig. 8(a)] the pres- ence of a significant over-compensation is observed just before the appearance of the Zn atomization signal which induces an important systematic error. This Zeeman spectral inter- ference effect observed also in the case of selenium,16 has been attributed to the Zeeman splitting of the absorption bands of NO molecules produced during the decomposition-reduction step of the different metallic nitrates NaNO Mg(NO,) and Ca(NO,),.Therefore owing to the stabilization of ZnO by the metallic oxides mainly MgO (Fig. 5) this spectral inter- ference can be eliminated during pyrolysis at about 850°C without loss of zinc. In 0.7 moll-' oxalic acid [Fig. 8(b)] no systematic over-compensa tion is detected. For two injected sea-water volumes (5 and 10 pl) calibration graphs are linear for an atomization temperature of 1800°C in a 0.7 moll-' oxalic acid medium (pyrolysis 450 "C 60 s) and a 1 mol I-' nitric acid medium (pyrolysis 850 "C 60 s) and pass through the origin indicating no systematic errors.The concentration of Zn was determined in two certified sea-water reference materials NASS-3 and CASS-2 in the presence of 0.7 moll-' oxalic acid and 1 moll-' nitric acid. The values obtained for NASS-3 and CASS-2 (10 measure- ments) are in a good agreement with the respective certified values (Table 1). For a 10 pl injected volume of sea-water the detection limit obtained under these experimental conditions is about 80 ng 1-' in a nitric acid and 60 ng 1-' in an oxalic acid medium. Conclusion The direct determination of zinc in sea-water at the 1 pgl-' level concentration by ETAAS with Zeeman correction is difficult owing to the high background absorption generated $ AA0.050 2 BG 0.600 (b) B .. 0 5.00 Time/s Fig. 8 Atomization and background absorption signals for zinc in sea-water (10 pl 1 pg 1-1 of Zn T 1800°C (a) in the presence of 1 mol I-' HNO A zinc without pyrolysis B background and C zinc with pyrolysis at 850 "C for 60 s; and (b) in the presence of 0.7 moll- ' oxalic acid A zinc and B background Table 1 reference materials in the presence of nitric and oxalic acids; n = 10 Comparison of zinc concentrations obtained using certified Zn concentration/pg 1-' Certified 1 mol I-' 0.7 mol 1-' Sample value/pg 1-' nitric acid oxalic acid NASS-3 0.178 0.025 0.189 & 0.016 0.16 & 0.020 CASS-2 1.97 k0.12 2.05 & 0.10 2.00*0.10 by the vaporization of the chloride matrix which moreover generates a Zeeman interference effect; the induced under- compensation leading to an important systematic error on the determination of low level concentrations of zinc in unmodified sea-water.It appears also that slow atomization ramps lead to loss of ZnC12 through hydrolysis of MgCl and simultaneous generation of HCl. The different inorganic or organic acids used as modifiers do not separate entirely the atomization signal of zinc and the sea-water background absorption signal. Oxalic and nitric acids which significantly reduce the background absorption signal appear to be the most interesting modifiers. In the presence of 1 moll-' HNO the presence of a Zeeman inter- ference effect was noted due to the Zeeman splitting of the absorption bands of the NO molecules generated during the decomposition-reduction step of metallic nitrates in the graph- ite furnace.This interference effect induces an over- compensation which can be eliminated in sea-water without loss of Zn by selective pyrolysis at about 850°C. Therefore it remains an interference effect (25%) related to the simultaneous vaporization of zinc and sodium oxides. In an oxalic acid medium no significant interference effect or systematic over- or under-compensation has been observed. So oxalic acid appears as the most interesting modifier at very low level Zn concentrations. Owing to its commercial availability in a higher purity grade and its higher solubility HNO can be used instead of oxalic acid for practical reasons.Using these experimental conditions the detection limit (30) obtained for an injection of 10 pl of sea-water is about 80 ng 1-l Zn in a nitric acid medium and 60 ngl-' of Zn in an oxalic acid medium leading to a precision of about 10% at the 1 pg 1 - 1 level concentration. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 References Burrell D. C. and Wood G. G. Anal. Chim. Acta 1969 48 45. Campbell W. C. and Ottaway J. M. Analyst 1977 102 495. Le Bihan A. and Courtot-Coupez J. Analusis 1975 3 59. Sturgeon R. E. Bennan S. S. Desaulniers A. and Russell D. S. Anal. Chem. 1979 51 2364. Guevremont R. Anal. Chem. 1981 53 911. L'vov B. V. and Ryabchuk G. N. Spectrochim. Acta Part B 1982 31 673. L'vov B. V. Spectrochim. Acta Part B 1990 45 633. Berglund M. and Baxter D. C. J. Anal. At. Spectrom. 1992 7 461. Frech W. and Baxter D. C. Spectrochim. Acta Part B 1990 45 867. Shekiro J. M. Skogerboe R. K. and Taylor H. E. Anal. Chem. 1988 60 2578. Erspamer J. P. and Niemczyk T. M. Anal. Chem. 1982,54,538. Kantor T. Bezur L. Pungor E. and Winefordner J. D. Spectrochim. Acta Part B 1983 38 581. Byrne J. P. Chakrabarti C. L. Gregoire D. C. Lamoureux M. and Ly T. J. Anal. At. Spectrom. 1992 7 371. Chaudry M. M. Mouillere D. Ottaway B. J. Littlejohn D. and Whitley J. E. J. Anal. At. Spectrom. 1992 7 701. Kantor T. Pungor E. Sztatisz T. and Bezur L. Talanta 1979 26 357. Le Bihan A. Cabon J. Y. and Elleouet C. Analusis 1992,20,601. Cabon J. Y. and Le Bihan A. Anal. Lett. 1986 19 755. Paper 3/0081 OJ Received February 10 1993 Accepted October 7 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900477

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 483 Indirect Determination of Iodide as an Hgxl Complex by Electrothermal Atomic Absorption Spectrometry* P. Bermejo-Barrera A. Moreda-Pi Aeiro M. Aboal-Somoza J. Moreda-Pi Aeiro and A. Bermejo-Barrera Department of Analytical Chemistry Nutrition and Bromatology Faculty of Chemistry University of Santiago de Compostela 75706-Santiago de Compostela Spain A method for the indirect determination of trace amounts of iodide by electrothermal atomic absorption spectrometry through the measurement of the mercury signal generated when small amounts of iodide and mercury are heated in a graphite furnace is described. The measured absorbances are related to the values of the signals from an iodide-mercury complex. The pH required for the formation of the HgJ complex as well as other parameters involved in measurement of the signals are also determined.The limits of detection and quantification obtained were 3.0 and 10.1 pg I-' of iodide respectively and the characteristic mass was 38.8 pg of iodide. The relative standard deviations obtained were from 5.1 -8.9% (n = 7) depending on concentration. In the range 5-20 pg I-' recoveries were 94.8-104.4O/0. The method has been applied to a range of tap waters. Keywords Indirect method; iodide determination; electrothermal atomic absorption spectrometry The determination of anions by atomic absorption spec- trometry (AAS) has usually been performed by indirect methods. This is due to the fact that these species exhibit their main resonance lines in the vacuum ultraviolet region below 190 nm and therefore they cannot be determined directly with conventional instruments.'.' One indirect method is based on a chemical reaction between the anion to be determined and a metal. A reaction such as this is essential for the performance of the method.The absorbance of the metal that has reacted or remains after reaction is measured and related to the concentration of the anion.3 For iodide some indirect methods based on the formation of a metal-iodide complex have been reported."' For instance the reaction of iodide with cadmium as tris( l,lO-phenanthroline)cadmium(11) generates an ion pair that can be extracted before measurement of the cadmium is perf~rmed.~ Silver reacts with iodide to form a silver-iodide complex which when volatilized into a flame allows the measurement of iodide through the signal from atomic ~ilver.~ Iodide reacts with mercury the excess of which is then sorbed onto a cation exchanger the mercury-iodide complex that remains in the solution being measured in this instance.6 Other methods involve the reaction of mercury-iodide complexes with 2,T-dipyridyl which are extracted by ethyl acetate or isobutyl methyl However these indirect methods are often complicated because they involve extractions filtration etc.and hence the possibility of losing the complexes. Thus many workers have determined iodide through a decrease in the metal absorbance in the presence of the iodide. K~ldevere,~ Wifladt et a/.'' and Sun and Julshamn" have determined iodide by measuring the decrease in the mercury signal when a certain concentration of iodide is added. These workers used the cold vapour technique and the mercury-iodide complexes formed have a structure HgI (n = 1,2,3 .. .). In these methods the molar amounts of mercury and iodide used are 1+10. Therefore a high concentration of iodide relative to that of mercury is required to guarantee formation of the complexes. If the concentration of iodide in the sample is low the very small amounts of mercury that can be added give a very small mercury signal. Nomura and Karasawa" have described an indirect method based on the measurement of the mercury-iodide (HgI,) signal * Presented at the XXVIII Colloquium Spectroscopicum Inter- nationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.generated when amounts of mercury and iodide are heated in a graphite furnace. They obtained two peaks for a mercury absorbance signal and related the first of these to the mercury and the second one to the mercury that was contained in the HgI that had formed. Therefore in their method they con- cluded that the molar ratio of iodide to mercury was two. As pointed out by Nomura and Karasawa if thermodynamic factors are considered for this complex to be formed it would be necessary for the molar ratio of iodide to mercury to be six. The aim of the present work was to develop an indirect method for the determination of iodide in the pgl-' range through the formation of an Hg,I complex by electrothermal atomic absorption spectrometry (ETAAS).Experimental Apparatus The absorbance of mercury was measured with a Perkin-Elmer 1100 B atomic absorption spectrophometer equipped with a deuterium lamp for background correction a graphite furnace atomizer Perkin-Elmer HGA-700 and an autosampler Perkin-Elmer AS-70. The radiation source was a mercury electrodeless discharge lamp (connected to its power supply) operated at 4 W which provided a 253.7 nm line. The band- width was 0.7 nm. Pyrolytic graphite coated graphite tubes and pyrolytic graphite (L'vov) platforms were used throughout ( Perkin-Elmer Uberlingen Germany). Reagents All solutions were prepared from analytical-reagent grade chemicals using ultrapure water resistivity 18 Mi2 cm-' which was obtained by means of a Milli-Q water purification system (Millipore).Potassium iodide stock standard solution 1.000 g I-'. Potassium iodide (Merck Darmstadt Germany) 0.327 g was dissolved in water and diluted to 250ml. This solution was diluted to obtain working standard solutions. Mercury(r1) nitrate stock standard solution 1.000 g 1- '. Panreac Barcelona Spain. Nitric acid. A solution containing 2 x moll-' of nitric acid was prepared from Suprapur acid (69-70.5% with a maximum mercury content of 0.001 pg ml-' BDH Chemicals Poole UK) by appropriate dilution with water and was used withou t s tandarization. Palladium stock standard solution 3.000 mg ml - I . Prepared by dissolving 300 mg of palladium (99.999% Aldrich Chemicals Milwaukee WI USA) in 1 ml of concentrated nitric acid and diluting to 100ml with ultrapure water.If the dissolution was incomplete 10 pl of hydrochloric acid484 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 (Suprapur 35.0% with a maximum mercury content of 0.001 pg ml-' BDH Chemicals) were added to the cold nitric acid and heated to gentle boiling in order to volatilize the excess of chloride. Suljde stock standard solution 0.1 g ml-'. Sodium sulfide monohydrate 98.0% ACS-reagent grade (Aldrich Chemie Steinheim Germany) 0.0749 g was dissolved in water and diluted to 100 ml. This solution was diluted to obtain working standard solutions. Argon. N50 purity argon (99.9990%) was used as a sheath gas for the atomizer and to purge internally. Synthetic air and oxygen. Used as gas phase chemical modifiers C45 purity (99.995 YO).Procedure For calibration 20 pl of standard solution containing iodide concentrations of between 0 and 30 pg 1-' in a medium con- taining 300 pg 1-' of mercury and 1 x lop4 mol I-' of nitric acid were injected into the atomizer. The sequential dry-atomi- zation-clean programme (Table 1) of the graphite furnace was run and the integrated absorbances recorded. Results and Discussion When amounts of mercury and iodide are heated in a graphite furnace two peaks are obtained (Fig. 1). An increase in the second peak and a decrease in the first was observed when higher concentrations of iodide were added. Therefore the first peak is related to mercury and appears at about 60"C and the second one at about 630 "C is related to a mercury-iodide complex.The indirect method described below is based on recording the second peak the one related to a mercury-iodide complex.'2 Table 1 Graphite furnace temperature programme and instrumentation Temperature/ Ar flow/ Step "C Ramp/s Hold/s ml min-l 0 (read) 50 30 20 300 Dry Atomization 900 35 15 Clean 2000 13 2 300 Hg electrodeless discharge lamp Wavelength 253.7 nm EDL power 4 W Spectral bandwidth 0.7 nm Read delay 24 s Integration time 10 s Peak-area measurements D lamp background corrector Pyrolytic graphite tubes and platforms (L'vov) Injection volume 20 pl 0 25.0 Timels 50.0 Fig. 1 Effect of I - on the absorbance signal of Hg (300 pg 1-' of Hg) recorded over 50 s A without I-; By with 20 pg 1-' of I-; and C with 40 pg 1-' of I - Optimization of the Graphite Furnace Temperature Programme The proposed programme was optimized by increasing the atomization temperature until the second peak due to the mercury-iodide complex was observed.This begins to appear at a temperature of 630 "C and the peak is recorded completely if the temperature is increased up to 900°C and remains at that temperature for a few seconds. Thus to reach this temperature a ramp time of 35 s and a hold time of 15 s were needed. If the integration is delayed 24s and the integration time is about 12 s the peak due to the mercury-iodide complex can be recorded (Fig. 2). The gas flow during atomization was also optimized; 0 10 and 20 ml min-' were tried using the temperature programme given above. The result was a substantial decrease in the signal (more than 50%) for a gas flow of lOmlmin-' and for 20 ml min-' the signal fell to zero.As the signal obtained for 0-50 pg 1-' of iodide is low (an integrated absorbance of 0.032 for 30 pg 1-I) the use of a gas flow of 10 ml min-' decreases the signal considerably and thus there is no advantage to be gained. In Table 1 the optimized programme and instrumental conditions are shown. Optimization of Amount of Nitric Acid As reported by several workers an acidic medium is required in order for the complexes between the mercury and iodide to be Nitric acid appears to be the most appr~priate.~ However the nitric acid acts as a chemical modifier on the mercury and thus can delay its atomization. Hence in the proposed method an absorbance signal from uncomplexed mercury could occur at the same temperature as the mercury complexed with iodide. A peak could therefore appear even in absence of iodide.To estimate this modifying effect a reagent blank solution of mercury (300 pg 1-') containing different concentrations of nitric acid to give concentrations of about 1 x and 1 x moll-' nitric acid (which is sufficient to guaran- tee formation of the mercury-iodide complexg) were measured. An increase in the signal for greater amounts of acid is observed (Table 2). The lowest integrated absorbance value is related to a concentration of 1 x moll-'. Therefore a concentration of nitric acid of 1 x mol I-' was chosen for which the integrated absorbance of the mercury was negligible (0.002 Table 2). 1 x 24.0 29.0 Time/s 34.0 Fig.2 Effect of I - on the absorbance signal of Hg (300 pg 1-' of Hg] delayed 24 s and recorded over 10 s A without I-; B with 20 pg 1- of I-; and C with 40 pg 1-' of I- Table 2 Absorbance values for a solution containing 300 pg 1-' of mercury and varying concentrations of nitric acid Nitric acid concentration/ mol 1-1 Integrated absorbance/s 1 x 10-4 0.002 1 x 10-3 0.005 1 x lo- 0.014JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 485 Calibration and Standard Additions Graphs Following the procedure described above the calibration graph was found to be linear over the range 0-30 pg 1-' of iodide. The standard additions method was used over the same range of concentrations for a sample of tap water. The equations obtained were as follows Calibration graph QA=0.015+ 1.14 x 10-3[1-] Standard additions graph QA = 0.0047 + 8.022 x [ I-] where QA is the integrated absorbance in s and [I-] the concentration expressed in pg 1-I.The absorbance of the blank was 0.015 s. Both graphs are shown in Fig. 3. As can be seen from the figure the aqueous calibration graph cannot be used to determine the results and calibration by the standard additions method must be used. Precision and Accuracy To study the repeatability of the measurements at different concentrations four solutions containing 0,5 10 and 20 pg 1-' of iodide were prepared and each solution was measured seven times. The relative standard deviations (RSD) obtained are shown in Table 3. The accuracy of the method was studied through the recov- ery. For iodide concentrations of 5 10 and 2Opgl-' the recoveries were 104.4 94.8 and 101.05% respectively.Sensitivity The sensitivity was studied using three parameters the limit of detection (LOD) the limit of quantification (LOQ) and the characteristic mass (mo) which are defined as follows 3 x S D cs 10 x SD cs LOD=- LOQ=- v x c x 0.0044 mo = QA,-QAb 0 4 8 12 16 20 Concentration of l-/pg I-' Fig. 3 A Calibration and B standard additions graphs Table 3 Repeatability of measurements at different iodide concen- trations n = 7 0 5 10 20 where SD is the standard deviation of the measurements of the blank; CS is the slope of the calibration graph; C is sample concentration expressed in pgl-' of iodide; V is the volume of the sample in pl; and QA and QAb are the integrated absorbances of the sample and blank respectively. The results obtained were LOD = 3.0 pg 1-' of iodide; LOQ=10.1 pgl-' of iodide; and mo=38.3 pg of iodide.Effect of Various Chemical Modifiers Although the method offers a low background signal the possibilities of using chemical modifiers such as palladium sulfide synthetic air and oxygen were studied. The results obtained for palladium and sulfide were unsatisfactory. When small amounts of both species were added the mercury-iodide complex signal increased owing to overlap with the signal from the decomposing mercury-palladium or sulfide species. In Figs. 4 and 5 it can be seen that when palladium or sulfide are present the first peak due to mercury(II) decreases and the second one due to the mercury-iodide complex increases.For a palladium concentration of 50 pg 1-' it can be observed that the first peak fell to zero. When sulfide was added similar results were obtained. If a synthetic air or oxygen flow was used an increased signal was observed and the background was completely removed. In order to employ these gas-phase chemical modi- fiers a step was added to the original graphite furnace tempera- ture programme (Table 1). This new step was ashing at 400°C with a ramp time of 20 s (Table 4). Synthetic air and oxygen flows of 50 100 200 and 300 ml min-' were used during this ashing step. The results are shown in the Table 5. As can be observed when synthetic air was used the mercury-iodide complex signal increases and this signal remained constant for flows greater than 200 ml min-'.For oxygen the increase of the signal was lower than for synthetic air and the signal was constant for flow rates greater than 50 ml min-I. Although the new programme has an ashing step (up to 400"C) it does not affect the temperature ramp from 50 to 900 "C so atomization of the mercury-iodide complex does not differ from the pre- vious programme that is the one given in Table 1. To verify 0 25.0 Time/s 50.0 Fig. 4 Effect of Pd on the absorbance signal of 40 pg 1-' of I- A without Pd; B with 30 pg 1-' of Pd; and C with 50 pg I-' of Pd Iodide concentration/pg 1- ' RSD (%) 8.9 7.0 5.8 5.1 25.0 Time/s 50.0 Fig. 5 Effect of S2- on the absorbance signal of 40 pg I-' of I- A without S2-; B with 5 pg 1-' of S2-; and C with 10 pg 1-' of S2-486 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 4 Graphite furnace temperature programme for gas-phase chemical modifiers (oxygen and synthetic air). Other conditions are as shown in Table 1 Temperature/ Ar flow/ Step "C Ramp/s Hold/s ml min-' 50 30 20 300 -* Dry Ash 400 20 0 Atomization 900 15 15 0 (read) Clean 2000 13 2 300 0.15 A I * 200 ml min-' or 50 ml min-' when synthetic air or oxygen were used respectively. n L I Table 5 Absorbance values obtained for gas-phase chemical modifiers (oxygen and synthetic air). With the original programme (Table l) the value obtained was 0.054 Flow/ml min- ' 0 2 4 6 8 10 12 Concentration of anionhg I - ' Fig. 6 Interfering anions in a solution containing 40 pg 1-' of I- A S2-; B CN-; C S2032-; and D SCN- (see text for details) Gas 0 50 100 200 300 0.056 1 Air 0.051 0.059 0.061 0.071 0.069 Oxygen 0.050 0.057 0.059 0.065 0.061 this a standard of 4Opg1-' of iodide was measured using both programmes but without a flow of synthetic air or oxygen.The values obtained by the simpler programme meas- uring the 40 pg 1-' iodide standard twice as can be seen in the Table 5 do not differ significantly. Interferences The effects of foreign ions on the determination of 40 pg I-' of iodide were studied and the results are shown in Figs. 4 and 5. Several cations including Cd2+ Co2+ Ni2+ Pb2+ and Zn2+ interfere with the measured signal. This is attributed to the reaction with the iodide. However cyanide sulfide thio- cyanate and thiosulfate also interfere in the method but these anions increase the signal because they react with the mer- cury(n) forming complexes that are atomized at the same atomization temperature as the mercury-iodide complex.Halide bromide fluoride and chloride ions also interfere in the measurements. The interfering effects of anions such as cyanide sulfide and thiosulfate are greater than those owing to cations and halide anions. For cations such as Cd2+ Co2+ and Ni2+ the interfer- ing effect begins to be important for concentrations of these cations greater than 500 pg 1-' for Pb2+ the interfering effect is observed for concentrations greater than 100 pg 1-' while for Zn2 + the interfering behaviour appears for concentrations lower than 100 pg 1-'. Bromide interferes at concentrations greater than 250 pg 1-' and fluoride and chloride at concen- trations greater than 1500 pg 1-' and 10 mg 1-' respectively. Anions such as cyanide and thiosulfate begin to interfere at concentrations of about 5 pg I-' and about 1 pg 1-l for sulfide.For thiocyanate no interfering behaviour is observed. The interfering effects of the anions and cations mentioned above are shown in Figs. 6 and 7. In both figures the value of integrated absorbance obtained without any interfering ions is shown in addition to the *lo% interval of that value. This interval was used as a limit to consider whether or not an absorbance value for a particular foreign ion was classed as an interference. To conclude some interferences have been observed but since the concentrations of such ions in the samples being analysed (that is the samples for which the method has been developed) are not that high it can be assumed that for many v) -.2 0.042 0 200 400 600 800 1000 Concentration of cation/pg I-' Fig. 7 Ni2+; B Cd2+; C Pb2+; D Zn2+; and E Co2+ (see text for details) Interfering cations in a solution containing 40 pg 1-' of I- A 'Table 6 Iodide levels measured in several tap waters Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 Iodide concentration/pg 1-1 197.8 44.0 141.3 116.2 195.6 179.2 184.3 271.9 280.3 195.3 806.8 110.4 90.8 practical analytical applications there are no interferences in the method. Applications The method was applied to the determination of iodine in tap waters from several towns of Galicia (north-western Spain). A standard additions graph was used to determine the measure- ments. The iodide concentrations obtained (shown in the Table 6) varied between 44.0 and 806.8 pg 1-' of iodide. References 1 Kirkbright G. F. and Johnson H. N. Talanta 1973 21 433. 2 Manfield J. M. West T. S. and Dagnall R. M. Talanta 1974 21 787.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 487 3 Garcia-Vargas M. Milla M. and PCrez-Bustamante J. A. Analyst 1983 108 1417. 4 Kumamaru T. Bull. Chem. SOC. Jpn. 1969 42 956. 5 Fike R. S. and Frank C. W. Anal. Chem. 1978 50 1446. 6 Chuchalina L. S. Yudelvich I. G. and Chinankova A. A. Zh. Anal. Khim. 1981 36 920. 7 Chakraborty D. and Das A. K. At. Spectrosc. 1988 9 189. 8 Chakraborty D. and Das A. K. Talanta 1989 36 669. 9 Kuldvere A. Analyst 1982 107 1343. 10 Wifladt A. M. Lund W. and Bye R. Talanta 1989 36 395. 11 Sun F. and Julshamn K. Spectrochim. Acta Part B 1987,42,889. 12 Nomura T. and Karasawa I. Anal. Chim. Acta 1981 126 241. Paper 31046736 Received August 3rd 1993 Accepted September 20 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900483

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 489 Aboal-Somoza Manuel 469 Absalan G. 45 Adams F. 151 Akman Suleyman 333 Alves Luis C. 399 Amarasiriwardena Dula 199 Anderson David R. 67 Anderson S. E. 263 Anzano Jesus M. 125 Argentine Mark D. 199 Arnold J. T. 263 Arriagada Lorna 93 Back M. H. 45 Barciela-Alonso Carmen 469 Barnes Ramon M. 199 Barshick Christopher M. 83 Baxter Douglas C. 297 Bayne Charles K. 83 Begley Ian S. 171 Belarra Miguel A. 125 BeneS Petr 303 Bermejo-Barrera Adela 469 483 Bermejo-Barrera Pilar 469 483 Bernasconi G. 151 Berndt Harald 39 193 Betti Maria 385 Biffi Claudio 443 Blanco Gonzalez E. 281 Boge Edward M. 369 Branch Simon 33 Briand Alain 17 Brown Nicole V. 363 Bruno SCrgio N. F. 341 BudiE Bojan 53 Burakov V. S. 307 Cabon J. Y. 477 Camara Carmen 291 Campbell Michael 187 Campos Reinaldo C.341 Carrion Nereida 205 217 Caruso Joseph A. 145 Castillo Juan R. 125 311 Chakrabarti C. L. 45 Chartier FrCdCric 17 Cheam Venghout 315 Chirinos JosC 237 Cimadevilla Enrique Alvarez- Cabal 117 Cobo I. G. 223 Coedo A. G. 223 Cooper 111 C. B. 263 Cornejo Silva G. 93 cserfalvi Tamas 345 Cujes Ksenija 285 Curtius Adilson J. 341 Dadfarnia Shayessteh 7 Dahl Kari 1 Dams Richard 23 177 187 483 CUMULATIVE AUTHOR INDEX JANUARY-MARCH 1994 Deruaz D. 61 Desrosiers Roland 3 15 Doner Giileren 333 Dorado M. T. 223 Durrant Steven F. 199 Ebdon Les 33 Eljuri Elias 205 Emteborg Hikan 297 Epler Katherine S. 79 Fell Gordon S. 457 Fernandez de la Campa M. R. Fernandez Alberto 205 217 Fischer W. 257 375 Florian K. 257 Fonesca Rodney W. 167 Foster Robert D.273 Geertsen Christian 17 GomiSEek Sergej 285 Goode Scott R. 73 Goossens Jan 177 187 Gower Stephen A. 363 369 Gregoire D. Conrad 393 Hadgu Negassi 297 Harnly James M. 419 Hauptkorn Susanne 463 Heitmann U. 437 Hese A. 437 Hiernaut Tania 385 Hinds Michael W. 451 HlavaEek I. 245 251 HlavaEkovi I. 245,251 Holcombe James A. 167 415 Houk R. S. 399 Hoult Gavin 7 Howe Alan M. 273 Hu Yanping 213 Huang Zhuoer 11 Hudnik Vida 53 Hutton J. C. 45 Hutton Robert C. 385 Isaevich A. V. 307 Itriago Ana 205 Jackson Jason G. 167 Jakubowski Norbert 193 Janssens K. 151 Jepkens Brigitte 193 Jones Delwyn G. 369 Katskov Dmitry A. 321 431 Kimber Graham M. 267 Kmetov Veselin 443 Koch Lothar 385 Kogan Valentina V. 451 Kolihova Dana 303 Kratzer Karel 303 Krieger Brian L.267 Krivan Viliam 463 Krushevska Antoaneta 199 Kubova Jana 241 231 Kumamaru Takahiro 89 Lacour Jean-Luc 17 Lazik C. 45 Le Bihan A. 477 Lechner Josef 315 Lee Julian 393 Lile E. S. 263 Lbpez-Gonzalvez M. Angeles Luecke Werner 105 Manickum Colin K. 227 Manninen Pentti K. G. 209 Manzoori Jamshid L. 337 Marais Pieter J. J. G. 321 431 Marchante Gaybn Juan Manuel 117 Marcus R.K. 45 Marin Sergio R. 93 Martinez-Garbayo Maria Paz Martinsen Ivar 1 Mauchien Patrick 17 McAllister Trevor 427 McCrindle Robert I. 321 431 McLeod Cameron W. 67 Mermet Jean-Michel 17 61 Mezei Pal 345 Milagros Gomez M. 291 Milton Dafydd M.P. 385 Minnich Michael G. 399 Misakov P. Ya. 307 Moens Luc 177 187 Moreda-Pifieiro A 483 Moreda-Piiieiro J. 483 Mori Toshio 159 Moulton Gary P. 419 Murillo Miguel 205 217 237 Nakahara Taketoshi 159 Naoumidis A.375 Naumenkov P. A. 307 Nevoral Vladislav 241 Nickel H. 257 375 O’Haver Thomas C. 79 419 Okamoto Yasuaki 89 O’Neill Peter 33 Outred Michael 381 Palacios M. Antonia 291 Patriarca Marina 457 Payling Richard 363 369 Peachey Russell M. 267 Perez-Arantegui J. 31 1 Perez Parajbn Juan M. 111 Petrucci G. A. 131 Petty John D. 267 Poussel E. 61 Quentmeier Alfred 355 QuerrC G. 311 Radziuk Bernard 1 29 1 125 217 Raikov S. N. 307 Rasmussen Gert 385 Romon-Guesnier Sabine 199 RonEeviC Sanda 99 Rubio J. 151 Rummeli Mark H. 381 Salbu Brit 1 Saleemi Abdollah 337 Sanz-Medel Alfredo 11 1 117 Schaldach Gerhard 39 Scheie Andrew J. 415 Schneider Germar 463 Schoknecht G. 437 Schwarzer Rudolph 43 1 Sekerka Ivan 315 Selby Mark 267 Sharp Barry L.171 Sheppard Brenda S. 145 Shtepan Aleksander M. 321 Siroki Marija 99 Sjostrom Sten 17 Smith B. W. 131 Smith Clare M. M. 419 Smith David H. 83 Smith Fraser O. 267 Smith Trevor A. 67 SpevaEkova Vera 303 Steers Edward B. M. 381 Stevenson C. L. 131 StreSko Vladimir 241 Stuewer Dietmar 193 Sugawa Kazumitsu 89 Sy T. 437 Thomas Christopher L. 73 Thomassen Yngvar 1 Thompson K. Clive 7 Tittarelli Paolo 443 Tsalev Dimiter L. 405 Turak Elvan E. 267 Turk Gregory C. 79 Valdes-Hevia y Temprano Vanhaecke Frank 187 Vanhoe Hans 23 177 187 Veber Marjan 285 Verbeek Alistair A. 227 Versieck Jacques 23 Vincze L. 151 Wade Jeffery W. 83 Webb C. 263 Weiss Zdenek 351 Wiederin Daniel R. 399 Winefordner J. D. 131 Wrobel Katarzyna 117 281 Zander A. T. 263 Zhang Zhanxia 213 Zheng Jianguo 213 Zilkova Jana 303 231 281 M.C. 231Ramon M. Barnes Editor Department of Chemistry LGRC Towers University of Massachusetts Amherst MA 01 003-0035 Telephone (413) 545-2294 fax 545-4490 0 b jective The ICP INFORMATION NEWSLETTER is a monthly journal published by the Plasma Research Group at the University of Massachusetts and is devoted exclusively to the rapid and Impartial dissemination of news and literature information re- lated to the development and applications of plasma sources for spectrochemical analysis. Background ICP stands for inductively coupled plasma discharge which during the past decade has become the leading spectrochemi- cal excitation source for atomic emission spectroscopy. ICP discharges also are applied commercially as an ion source for mass spectrometry and as an atom and ion cell in atomic fluo- rescence spectrometry.The popularity of this source and the need to collect in a single literature reference all of the pertinent data on ICP stimulated the publication of the ICP INFOR- MAT/ON NE WSLE77€R h 1975. Other popular plasma sources h. microwave induced plasmas direct current plasmas and glow discharges also are included in the scope of the ICP IN- FORMATION NEWSLETTER. Scope As the only authoritative monthly journal of its type the ICP INFORMATION NEWSLETTER is read in more than 40 coun- tries by scientists actively applying or planning to use the ICP or other types of plasma spectroscopy. For the novice in the field the ICP /NFORMATION NEWSLETTER provides a conuse and systematic source of information and background material needed for the selection of instrumentation or the development of methodology. For the experienced scientist it offers a sin- gle-source reference to current developments and literature.Editorial The ICP INFORMAT/ON NEWSLETT€R is edited by Dr. Ramon M. Barnes Professor of Chemistry University of Mas- sachusetts at Amherst with the assistance of a 20-member Board of National Correspondents composed of leading plasma spectroscopists. The Board members from around the world report news viewpoints and developments. Dr. Barnes has been conducting plasma research on ICP and other dis- charges since 1968. He also serves as chairman of the Winter Conference on Plasma Spectrochemistry sponsored by the ICP INFORMATION NEWSLETTER.Regular Features *Original submitted and invited research articles by ICP and Complete bibliography of all major ICP publications. *Abstracts of all ICP papers presented at major US and inter- .First-hand accounts of world-wide ICP developments. .Special reports on dcp microwave glow discharge and other Calendar and advanced programs of plasma meetings. *Technical translations and reprints of critical foreign-lan- guage ICP papers. Critical reviews of plasma-related books and software. Conference Activities The ICP INFORMATION NEWSLETTER has sponsored seven international meetings on developments in atomic plasma spectrochemical analysis since 1980 in San Juan Orlando San Diego St. Petersburg and Kailua-Kona. Meeting pro- ceedings have appeared as Developments in Atomic Plasma Spectrochemical Analysis (Wiley) Plasma Spectrochemistry and Plasma Spectrochemistry Il-1V (Pergamon Press) as well as in special issues of Spectrochimica Acta Part B and Journal of Analytical Atomic Spectrometry.The 1994 Winter Confer- ence on Plasma Spectrochemistry will be held in San Diego California January 10 - 15 1994; its proceedings will be published by Fall 1994. Subscription Information Subscriptions are available for 1 2 issues on either an annual or volume basis. The first issue of each volume begins in June and the last issue is published in May. For example Volume 18 runsfrom June 1992 through May 1993. Backissues beginning with Volume 1 May 1975 also are available. To begin a subscription complete the form below and submit it with prepayment or purchase information.For additional informa- tion please call (41 3) 545-2294 fax (41 3) 545-4490 or contact the Editor. Credit cards accepted. plasma experts. national meetings. plasma progress. To order complete this section and send it to ICP Information Newsletter %Dr. Ramon M. Barnes Depart- ment of Chemistry Lederle GRC Towers University of Massachusetts Am herst MA 01 003-0035 USA. Start a subscription for the following issue 0 Volume(s)- (June 19- - May 19- ) or Q 19 (January - December). Enclosed IY Prepayment 0 Check or money order OVlSA 0 MasterCard Account No. (All 13 or 16 digits) ) or 0 Send invoice. Date Card holder Name Expiration date Cardholder Signature . Amount Due $ Mail to; N- Organization Cl Purchase order (No. City State/Cou ntry ZI P/Postalcode Telephone Te lex/f ax Note For each credit-card transaction a 4 % service charge will be added reflecting our bank charges.Current subscription rates are $60 (North America) $85 (Europe South America) or $94 (Africa Asia Indian/Pacific Ocean Areas Middle East and Russia). Back issue rates available on request. All payments should be made with US dollars by draft on a US bank by international money order or by credit card. Foreign bank checks are not accepted.Ramon M. Barnes Editor Department of Chemistry LGRC Towers University of Massachusetts Amherst MA 01 003-0035 Telephone (413) 545-2294 fax 545-4490 0 b jective The ICP INFORMATION NEWSLETTER is a monthly journal published by the Plasma Research Group at the University of Massachusetts and is devoted exclusively to the rapid and Impartial dissemination of news and literature information re- lated to the development and applications of plasma sources for spectrochemical analysis.Background ICP stands for inductively coupled plasma discharge which during the past decade has become the leading spectrochemi- cal excitation source for atomic emission spectroscopy. ICP discharges also are applied commercially as an ion source for mass spectrometry and as an atom and ion cell in atomic fluo- rescence spectrometry. The popularity of this source and the need to collect in a single literature reference all of the pertinent data on ICP stimulated the publication of the ICP INFOR- MAT/ON NE WSLE77€R h 1975. Other popular plasma sources h. microwave induced plasmas direct current plasmas and glow discharges also are included in the scope of the ICP IN- FORMATION NEWSLETTER.Scope As the only authoritative monthly journal of its type the ICP INFORMATION NEWSLETTER is read in more than 40 coun- tries by scientists actively applying or planning to use the ICP or other types of plasma spectroscopy. For the novice in the field the ICP /NFORMATION NEWSLETTER provides a conuse and systematic source of information and background material needed for the selection of instrumentation or the development of methodology. For the experienced scientist it offers a sin- gle-source reference to current developments and literature. Editorial The ICP INFORMAT/ON NEWSLETT€R is edited by Dr. Ramon M. Barnes Professor of Chemistry University of Mas- sachusetts at Amherst with the assistance of a 20-member Board of National Correspondents composed of leading plasma spectroscopists.The Board members from around the world report news viewpoints and developments. Dr. Barnes has been conducting plasma research on ICP and other dis- charges since 1968. He also serves as chairman of the Winter Conference on Plasma Spectrochemistry sponsored by the ICP INFORMATION NEWSLETTER. Regular Features *Original submitted and invited research articles by ICP and Complete bibliography of all major ICP publications. *Abstracts of all ICP papers presented at major US and inter- .First-hand accounts of world-wide ICP developments. .Special reports on dcp microwave glow discharge and other Calendar and advanced programs of plasma meetings.*Technical translations and reprints of critical foreign-lan- guage ICP papers. Critical reviews of plasma-related books and software. Conference Activities The ICP INFORMATION NEWSLETTER has sponsored seven international meetings on developments in atomic plasma spectrochemical analysis since 1980 in San Juan Orlando San Diego St. Petersburg and Kailua-Kona. Meeting pro- ceedings have appeared as Developments in Atomic Plasma Spectrochemical Analysis (Wiley) Plasma Spectrochemistry and Plasma Spectrochemistry Il-1V (Pergamon Press) as well as in special issues of Spectrochimica Acta Part B and Journal of Analytical Atomic Spectrometry. The 1994 Winter Confer- ence on Plasma Spectrochemistry will be held in San Diego California January 10 - 15 1994; its proceedings will be published by Fall 1994.Subscription Information Subscriptions are available for 1 2 issues on either an annual or volume basis. The first issue of each volume begins in June and the last issue is published in May. For example Volume 18 runsfrom June 1992 through May 1993. Backissues beginning with Volume 1 May 1975 also are available. To begin a subscription complete the form below and submit it with prepayment or purchase information. For additional informa- tion please call (41 3) 545-2294 fax (41 3) 545-4490 or contact the Editor. Credit cards accepted. plasma experts. national meetings. plasma progress. To order complete this section and send it to ICP Information Newsletter %Dr. Ramon M. Barnes Depart- ment of Chemistry Lederle GRC Towers University of Massachusetts Am herst MA 01 003-0035 USA.Start a subscription for the following issue 0 Volume(s)- (June 19- - May 19- ) or Q 19 (January - December). Enclosed IY Prepayment 0 Check or money order OVlSA 0 MasterCard Account No. (All 13 or 16 digits) ) or 0 Send invoice. Date Card holder Name Expiration date Cardholder Signature . Amount Due $ Mail to; N- Organization Cl Purchase order (No. City State/Cou ntry ZI P/Postalcode Telephone Te lex/f ax Note For each credit-card transaction a 4 % service charge will be added reflecting our bank charges. Current subscription rates are $60 (North America) $85 (Europe South America) or $94 (Africa Asia Indian/Pacific Ocean Areas Middle East and Russia). Back issue rates available on request.All payments should be made with US dollars by draft on a US bank by international money order or by credit card. Foreign bank checks are not accepted.Ramon M. Barnes Editor Department of Chemistry LGRC Towers University of Massachusetts Amherst MA 01 003-0035 Telephone (413) 545-2294 fax 545-4490 0 b jective The ICP INFORMATION NEWSLETTER is a monthly journal published by the Plasma Research Group at the University of Massachusetts and is devoted exclusively to the rapid and Impartial dissemination of news and literature information re- lated to the development and applications of plasma sources for spectrochemical analysis. Background ICP stands for inductively coupled plasma discharge which during the past decade has become the leading spectrochemi- cal excitation source for atomic emission spectroscopy. ICP discharges also are applied commercially as an ion source for mass spectrometry and as an atom and ion cell in atomic fluo- rescence spectrometry. The popularity of this source and the need to collect in a single literature reference all of the pertinent data on ICP stimulated the publication of the ICP INFOR- MAT/ON NE WSLE77€R h 1975.Other popular plasma sources h. microwave induced plasmas direct current plasmas and glow discharges also are included in the scope of the ICP IN- FORMATION NEWSLETTER. Scope As the only authoritative monthly journal of its type the ICP INFORMATION NEWSLETTER is read in more than 40 coun- tries by scientists actively applying or planning to use the ICP or other types of plasma spectroscopy.For the novice in the field the ICP /NFORMATION NEWSLETTER provides a conuse and systematic source of information and background material needed for the selection of instrumentation or the development of methodology. For the experienced scientist it offers a sin- gle-source reference to current developments and literature. Editorial The ICP INFORMAT/ON NEWSLETT€R is edited by Dr. Ramon M. Barnes Professor of Chemistry University of Mas- sachusetts at Amherst with the assistance of a 20-member Board of National Correspondents composed of leading plasma spectroscopists. The Board members from around the world report news viewpoints and developments. Dr. Barnes has been conducting plasma research on ICP and other dis- charges since 1968.He also serves as chairman of the Winter Conference on Plasma Spectrochemistry sponsored by the ICP INFORMATION NEWSLETTER. Regular Features *Original submitted and invited research articles by ICP and Complete bibliography of all major ICP publications. *Abstracts of all ICP papers presented at major US and inter- .First-hand accounts of world-wide ICP developments. .Special reports on dcp microwave glow discharge and other Calendar and advanced programs of plasma meetings. *Technical translations and reprints of critical foreign-lan- guage ICP papers. Critical reviews of plasma-related books and software. Conference Activities The ICP INFORMATION NEWSLETTER has sponsored seven international meetings on developments in atomic plasma spectrochemical analysis since 1980 in San Juan Orlando San Diego St.Petersburg and Kailua-Kona. Meeting pro- ceedings have appeared as Developments in Atomic Plasma Spectrochemical Analysis (Wiley) Plasma Spectrochemistry and Plasma Spectrochemistry Il-1V (Pergamon Press) as well as in special issues of Spectrochimica Acta Part B and Journal of Analytical Atomic Spectrometry. The 1994 Winter Confer- ence on Plasma Spectrochemistry will be held in San Diego California January 10 - 15 1994; its proceedings will be published by Fall 1994. Subscription Information Subscriptions are available for 1 2 issues on either an annual or volume basis. The first issue of each volume begins in June and the last issue is published in May. For example Volume 18 runsfrom June 1992 through May 1993. Backissues beginning with Volume 1 May 1975 also are available.To begin a subscription complete the form below and submit it with prepayment or purchase information. For additional informa- tion please call (41 3) 545-2294 fax (41 3) 545-4490 or contact the Editor. Credit cards accepted. plasma experts. national meetings. plasma progress. To order complete this section and send it to ICP Information Newsletter %Dr. Ramon M. Barnes Depart- ment of Chemistry Lederle GRC Towers University of Massachusetts Am herst MA 01 003-0035 USA. Start a subscription for the following issue 0 Volume(s)- (June 19- - May 19- ) or Q 19 (January - December). Enclosed IY Prepayment 0 Check or money order OVlSA 0 MasterCard Account No. (All 13 or 16 digits) ) or 0 Send invoice. Date Card holder Name Expiration date Cardholder Signature . Amount Due $ Mail to; N- Organization Cl Purchase order (No. City State/Cou ntry ZI P/Postalcode Telephone Te lex/f ax Note For each credit-card transaction a 4 % service charge will be added reflecting our bank charges. Current subscription rates are $60 (North America) $85 (Europe South America) or $94 (Africa Asia Indian/Pacific Ocean Areas Middle East and Russia). Back issue rates available on request. All payments should be made with US dollars by draft on a US bank by international money order or by credit card. Foreign bank checks are not accepted.

ISSN:0267-9477    

DOI:10.1039/JA9940900489

出版商:RSC    

年代:1994

数据来源: RSC