ISSN:0003-2654    

DOI:10.1039/AN98611FX009

出版商:RSC    

年代:1986

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN98611BX011

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 265 Determination of Bromide Using A Helium Microwave Induced Plasma with Bromine Generation and Electrothermal Vaporisation for Sample Introduction Mohamed M. Abdillahi and Richard D. Snook* Department of Chemistry, Imperial College, London SW7 2AY, UK A helium microwave induced plasma is utilised for the determination of bromide. The bromide sample solution is either introduced into an oxidation - generation mixture of potassium dichromate - sulphuric acid, or is vaporised electrothermally and then swept into the helium plasma. The microwave induced plasma (MIP) in a TMoto cavity excites bromine and the emission measurements are taken at both the Br II 470.5-nm and 478.6-nm lines. The calibration graphs were linear from 5 ng to 50 pg of bromide using the chemical generation technique at both analytical lines, whereas the calibration data from the graphite rod vaporisation sample introduction technique showed a linearity from 5 ng to 10 pg of bromide.Detection limits for both techniques were 1 ng. Fluorine, chlorine and iodine as well as other common anions and cations do not significantly interfere with the measurements, below 100 times the amount determined. Keywords: Bromide determination; helium microwave induced plasma; bromine generation; electrothermal vaporisation The determination of bromine by an argon microwave induced plasma (MIP) has been recently reported.1 A continuation of the work revealed that a helium plasma sustained in a TMolo cavity at atmospheric pressure provides a more sensitive method for the determination of bromide.The helium MIP has sufficient energy to excite bromine ion emission because of its high ionisation energy of 24.59 eV.2 This is illustrated in this work, by the fact that the most intense bromine lines in the visible spectrum are the ionic lines at 470.5 and 478.6 nm. Argon plasmas, on the other hand, are unable to excite these bromine ion lines in either the MIP or inductively coupled plasma (ICP) used in our laboratory. Beenakker2 has used the TMolo cavity for both argon and helium plasmas and concluded that halogen atomic and ionic emission is more easily induced in a helium microwave plasma than in an argon microwave plasma; he reported better detection limits for the halogens in the helium plasma. Carnahan and Carus03 reported a detection limit of 8 ng of bromine by measuring the emission at the 478.6-nm ion line in a helium MIP by analysing high relative molecular mass halogenated organic compounds.Van Dalen et a1.4 stated that the detection limits for bromine are similar in low and atmospheric pressure microwave induced plasmas and repor- ted detection limits of 0.24 and 0.56 yg ml-l using the 470.5- nm Br I1 line. This paper presents a method with better detection limits for determining bromine. Bromine is generated from a potassium dichromate - sulphuric acid mixture , after micro- litre aliquots of sample solution are added to the generation apparatus. After sufficient time has elapsed to generate bromine, it is then introduced into the plasma. Alternatively, bromide aliquots can be desolvated and vaporised into the microwave induced plasma using an electrothermal vaporisa- tion device.In both sample introduction techniques, the emission intensities of the Br I1 470.5- and 478.6-nm lines were measured. Experimental Reagents and Instrumentation The reagents used for the stock solution and the oxidation - generation mixture were of AnalaR grade (BDH Chemicals Ltd., Poole, Dorset). The oxidation mixture was prepared by dissolving 0.05 g of potassium dichromate in 10 ml of concentrated sulphuric acid. Doubly distilled , de-ionised water (Milli-Q water) was used throughout the experiments for preparing both the stock and working solutions. The helium plasma gas was of high-purity research grade (BOC Ltd., Wembley). The instrumental set-up is shown in Fig.1 and is similar to that used previously' except that the microwave cavity (EMS, Wantage, Berkshire) is a TMolo quarter wave modification of the Beenakker cavity. The helium plasma is supported in a quartz tube (6.4 mm 0.d.; 1.7 mm i.d.) in the microwave cavity, which is viewed axially by the monochromator (Optica Model CF 2768). Sample introduction into the helium MIP was achieved by using the chemical generation cell as described previously,' or a graphite rod electrothermal vaporisation unit .5 Procedure A 10-pl bromide sample solution was added to the generating mixture through the suba-seal septum.' The bromine gener- ated, during 1 min, was swept into the helium plasma and the emission measurements were taken at the 470.5- and 478.6-nm lines.Alternatively, the emission intensity measurements were taken at those wavelengths, after a 10-pl bromide solution was desolvated, and vaporised electrothermally and then swept directly into the microwave induced plasma. Signal regis- tration was achieved using an EM1 9601B nine-stage photo- multiplier tube, the output of which was connected across a 22 kohm load to the input of a JJ Lloyd CR450 chart recorder. TMOIO cavity Monochromator Sample introduction Lens recorder Reflected power meter Flow meter Power supply Microwave power generator Helium d * Present address: Chelsea Instruments Ltd., 5 Epirus Road, London SW6 7UR. Fig. 1. Schematic diagram of the MIP system. A, Signal; B, background266 . _ rn e .- c 60- 50- g 4C- >. w .- ANALYST, MARCH 1986, VOL.111 : 1.0 Results and Discussion c .- C .: 80 .- cn E Lu 40 Wavelength Selection It was found that the Br I1 470.5-nm line is twice as intense as the next brightest Br I1 478.6-nm line, in agreement with other workers.6 - Background - A A . a A A r ' - - Optimisation of the Operating Parameters The microwave forward power, helium flow-rates, entrance and exit slit widths, photomultiplier voltage, applied voltage to the graphite rod atomiser and the generating solution were optimised using the univariate search method. Using the TMolo cavity, the effect of microwave forward power on the emission of Br I1 lines showed an optimum value of 130 W (Fig. 2). In a previous paper we showed that the argon MIP used for determining bromine1 required an optimum forward power of 30 W.This fundamental power difference is inherent in the design of the different cavities employed7 and the nature of the plasma gas used. The physical properties of argon and helium are given in reference 8 (argon: thermal conductivity at 25 "C = 39 x cal s-1 cm-l O C - 1 , specific heat at 25 "C = 0.13 cal 8-1; helium: thermal conductivity at 25 "C = 340 x cal s-l cm-1 OC-1, specific heat at 25 "C = 1.25 cal g-1). The higher thermal conductivity and specific heat mean that helium dissipates more energy and therefore requires higher power to sustain than an argon plasma as was found in the 70 Y At?--.----- 60 70 80 90 100 I I I I I 110 120 130 140 150 Microwave forward powerNV Fig. 2. Effect of microwave forward power on the emission of bromine in a helium MIP.A, Signal; and B, background 1 I I I I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Helium flow-rate/l min-1 Fig. 3. Effect of helium flow-rates on the emission of bromine in a TMolo cavity MIP. A, Signal; and B, background TMolo cavity. Varying the helium flow-rates independently of other variables yielded a maximum signal-to-background ratio at 0.43 1 min-1 (Fig. 3). When the helium plasma gas flow-rate is higher than 0.5 1 min-1, the signal to background ratios were gradually reduced and may be attributed to the decrease in residence time of bromine in the plasma, as the intensity of the signal lowers. It was also observed that at lower helium flow-rates, the analyte was not sufficiently swept into the plasma and the peaks tended to be smaller and broader.We have reported1 that the optimum slit widths for the determination of bromine in an argon plasma using a 314 wave Broida cavity was 100 pm. The wide slit widths were used to isolate the molecular (Br2) band emission, which is much wider than the ionic Br I1 lines. Optimising the slit widths, an optimum of 10 pm was found for both the entrance and exit slit widths using the Br I1 470.5- and 478.6-nm lines. Altering the photomultiplier voltage showed that the best signal-to- background and the highest signal-to-noise ratios were at 1000 V. This differs from the voltage employed in reference 1, because the efficiency of the PMT is worse in the 470-478 nm region than at 291 nm. Optimisation of the generation mixture volume and concentration remain the same as used previously.1 The bromide sample is being oxidised to bromine and then introduced into the plasma where it is subsequently atomised and excited.The voltage applied to the graphite rod was varied to determine the optimum peak height for a 10-yl bromide sample solution. Applied voltages of 1 V (60 s), 3 V (5 s) and 8 V (3 s) were found to be the optimum for evaporation, ashing and vaporisation, respectively, of the bromide sample solution before introduction into the helium microwave induced plasma. These are equivalent to temperatures of 105,300 and 1900 "C, respectively. Table 1 summarises the optimum conditions. Table 1. Optimum conditions for the determination of bromide in a helium MIP Microwave foward power . . . . . . . . Helium flow-rate (generation) .. . . . . Refelected power . . . . . . . . . . Helium flow-rate (ETV) . . . . . . . . Entrance and exit slit widths . . . . . . Photomultiplier (EHT) . . . . . . . . Sample solution volume . . . . . . . . Graphite rod temperatures: Wavelengths selected (Br 11) . . . . . . Ashing . . . . . . . . . . . . . . Drying . . . . . . . . . . . . . . Vaporisation . . . . . . . . . . . . 130 W 0.4 W 0.43 1 min-* 0.8 1 min-1 10 pm 1000 v 10 1.11 470.5 and 478.6 nm 105 "C 300 "C 1900 "C 103 rn C 3 c .- > 102 F 9 s .: 10 c .- C t t c .- 15 Fig. 4. Calibration graphs of Br I1 emission at A, 470.5 nm and B, 478.6 nmANALYST, MARCH 1986, VOL. 111 267 I I I I 102 103 104 0.1 ' 10 Amount of bromidehg Fig. 5. Response data for Br I1 emission at A, 470.5 nm and B, 478.6 nm in an atmospheric helium MIP using ETV Calibration Graphs, Detection Limits and Interferences Double logarithmic calibration graphs (Figs.4 and 5 ) were obtained with respect to bromide in the solution. Fig. 4 illustrates the calibration graphs for Br I1 emission in the helium MIP at A, 470.5 nm and B, 478.6 nm when the analyte is generated chemically as bromine from the potassium dichromate - sulphuric acid oxidation mixture. The detection limit, defined as the sample concentration that produced a signal-to-noise ratio of two, at both the emission lines was 1 ng of bromide and the log - log calibration graphs were linear in the range from 5 ng to 50 yg with a slope of 0.8. Fig. 5 shows the calibration data for the same emission wavelengths when electrothermal vaporisation was used for the sample intro- duction technique.The detection limit was 1 ng and the linear dynamic range was from 5 ng to 10 yg of bromide. Although the 470.5-nm line emission is twice as intense as the 478.6-nm line, the linearity of both lines breaks below 5 ng, hence showing no difference in the detection limits or the linearity of the calibration graphs (Figs. 4 and 5 ) . Possible interference effects of fluorine, chlorine, iodine, sulphate, nitrate, sodium, potassium, iron and mercury were studied. A 1000-fold excess of Na, K, Fe, Hg, SO4 and NO3 did not affect the 10 p.p.m. bromine emission signal. However, it seems that the other halogens have slight interference effects if their concentrations are more than 100-fold greater than the 10 p.p.m. bromide solution. Conclusion These methods present two sample introduction techniques for the helium microwave induced plasma (MIP) and provide a sensitive, rapid and simple way of determining bromide. The two most intense emission lines for bromine in a helium MIP (470.5 and 478.6 nm) have been evaluated and it was found that both lines exhibit similar behaviour, in terms of detection limits and linear dynamic ranges. 1. 2. 3. 4. 5. 6. 7. 8. References Abdillahi, M. M., Tschanen, W., and Snook, R. D., Anal. Chim. Acta, 1985, 172, 139. Bennakker, C. I. M., Spectrochim. Acta, Part B, 1982,32,173. Carnahan, J. W., and Caruso, J. A., Anal. Chim. Acta, 1982, 136,261. Van Dalen, H. P. J., Kuwee, B. G., and De Galan, L., Anal. Chim. Acta, 1982, 142, 159. Long, S. E., Snook, R. D., and Browner, R. F., Spectrochim. Acta, Part B, 1984, 40, 553. Tanabe, K., Haraguchi, H. , and Fuwa, K., Spectrochim. Acta, Part B, 1981, 36, 119. Mulligan, K. J., Hahn, M. H., and Caruso, J. A., Anal. Chem., 1979, 51, 1935. Robin, J. P., Prog. Anal. At. Spectrosc., 1982, 5 , 79. Paper A5J19.5 Received April 31st, 1985 Accepted September 2nd, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100265

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 269 Investigations into the Improvement of the Analytical Application of the Hydride Technique in Atomic Absorption Spectrometry by Matrix Modification apd Graphite Furnace Atomisation Part 1. Analytical Results Klaus Dittrich and Rita Mandry Karl-Marx-University, Chemistry Section, Analytical Centre, Talstrasse 35, 7010 Leipzig, GDR Hydride generation AAS is characterised as a very sensitive analytical technique using the commercial AAS-3 hydride system with quartz tube atomisation. Studies of matrix interferences are reported for the trace elements As, Sb, Se and Te. The hydride-forming elements As, Sb, Bi, Se, Te, Ge, Sn and Pb were used as matrices. Two types of matrix interference can be distinguished: matrix interference in the liquid phase of hydride generation, and matrix interference in the gaseous phase of hydride atomisation.The strong matrix interference in the liquid phase was diminished by matrix modifications and new reagents are characterised. To diminish or avoid matrix interference in the gaseous phase, a new type of graphite tube atomiser was developed. The dimensions of the graphite paper atomiser are comparable to those of commercial quartz tubes, but the heated graphite atomiser volume is very small (comparable to HGA 500 tubes). The analytical applicabilities of both systems were characterised and compared. The absolute detection limits are between 0.1 and 0.5 ng. The relative detection limits in graphite paper atomisers are 2-1000 times better than in quartz tube atomisers.Keywords: Atomic absorption spectrometry; h ydride technique; matrix modification; graphite furnace atomisation In the last 30 years, atomic absorption spectrometry (AAS) has become one of the most commonly used methods in trace analysis. This development was connected with the introduction of some new techniques, especially atomisation, for example AAS with different flames, AAS with electrothermal atomisation in graphite tubes or graphite rods, AAS with evaporation of solid material by lasers, AAS with sputtering and AAS with the generation of volatile hydrides, called hydride AAS for short. The reasons for all these developments have been to improve the analytical criteria of AAS methods, such as detection ability, accuracy, selectivity, reproducibility and applicability. The initial development of hydride AAS was carried out by Holakl in 1969, but in 1955 Erdey et aZ.2 generated volatile hydrides for analytical measurements in d.c.arc atomic emission spectrography . Holak used the well known Marsh reaction in AAS. Arsine (AsH3) was generated by nascent hydrogen (from Zn/H+), transported by an argon stream and collected in a cold trap (liquid nitrogen). After collection, the trap was rapidly heated to room temperature. The volatile arsine was transported by argon into an air - acetylene flame for atomisation. The absorption was measured at 193.7 nm. Up to now three distinct operations have been performed in hydride AAS: hydride generation, hydride transportation and hydride atomisation. The procedure of all steps has been changed and improved in the last decade.Hydride generation. Pollock and West (1972)3 introduced as the reducing agent Mg - Ti3+ - H+ mixtures and Goulden and Brooksbank (1974)4 used A1 - H+ for this purpose. As late as 1972 Braman et aZ.5 introduced BH4- - H+ solutions in atomic emission and Schmidt and Royer6 transferred this hydride generation reaction system to AAS. The latter system is mostly used today, because the homogeneous reduction reaction is faster than the heterogeneous reduction by M - H+ systems. In general, d.5-10% solutions of NaBH4, stabilised by 0.5-270 NaOH, are used. Mostly dilute HCl is used, but HN03, citric acid, oxalic acid, tartaric acid and maleic acid can also be used. Hydride transportation. There are two methods: the direct transfer mode and a collection mode.The latter was necessary for the slowly working M - H+ reduction systems. Since the introduction of the BH4- - H+ reduction, the direct transfer mode has become possible and useful. Nevertheless, in some instances the collection mode was used for enrichment and concentration, but according to Chapman and Dale7 this is useful only for the most stable hydrides, such as AsH3, SbH3, BiH3 and SeH2. Hydride atomisation. The most intense development has taken place in this field. The air - acetylene flame used at first has disadvantages owing to its high background absorption and sensitivity due to plasma dilution. In 1972, Chu et al.8 introduced the electrically heated quartz tube. Knudson and Christian9 also used the graphite tube very early.Special flame techniques, the flame-heated quartz tube6 and the flame-in- tube technique,lo were introduced in 1974. Today the electrically heated quartz tube is most commonly used, because this technique is simple and for many analytical samples the other techniques including the graphite tube do not give advantages. The following advantages and disadvantages of hydride AAS can be summarised: 1. In many instances the traces of the analyte can be separated from the sample, which improves the accuracy. 2. The efficiency of sample introduction into the plasma is very high (nearly loo%), which gives good sensitivity. 3. Large solution volumes can be used, in special instances with the collection mode, which gives good sensitivity. 4. The method can be automated and a flow injection mode of operation is possible.5. Contributions to chemical speciation are possible. l1 For a long time the first point was the major advantage. However, recently it has become more evident that this is also the reason for the main disadvantage, owing to the existence or occurrence of matrix interferences (see also reference 12). In this paper we present proposals for the systematic270 ANALYST, MARCH 1986, VOL. 111 characterisation of matrix interferences, and for avoiding or diminishing such effects. For this purpose we have used the most complicated conditions: the determination of volatile hydride-forming elements (As, Sb, Se and Te) in volatile hydride-forming matrices (As, Sb, Bi, Se, Te, Ge, Sn and Pb). We have used a new AAS-3 hydride system with a quartz tube atomiser (Carl Zeiss, Jena, GDR) and have also developed a new atomisa- tion system, a long-path graphite tube atomiser, based on a new material, on graphite paper. In the following paper, some explanations about the causes of matrix interference are given.Experimental Apparatus An AAS-3 atomic absorption spectrometer (Carl Zeiss), a hydride system for AAS-3 (hydride generation system) and quartz tube atomiser (QT) (controlled by computer) (see Fig. 1) and a graphite paper atomiser (GPA) (laboratory construc- ted) (see Fig. 2) were used. The graphite paper used had a thickness of about 0.1 mm. A rectangular piece was cut out (92 x 33 mm) and a tube of length 92 mm and diameter 9 mm was formed. The overlap was 3-4 mm. The ends of the tube were stabilised by inner graphite rings (0.d.8.8 mm; i.d. 7 mm; length 5 mm) and jammed between two water-cooled brass half-rings. To avoid thermal destruction we used four small outer graphite rings (0.d. 9.5 mm, i.d. 9.1 mm, length 1 mm). The tubes were heated by direct resistance heating (0-20 V; 0-200 A). The pyrometrically measured temperatures are shown in Table 1. The medium lifetime of such a tube at atomisation tempera- tures of about 2000 "C and a heating period of 30 s is about Ar Ar iser NaBH4 Sample A B Fig. 1. AAS-3 hydride device (Carl Zeiss, Jena, GDR) Brass (water-cooled) n Graphite stabilisation Graphite Quartz window f Graphite I I I Quarktube I paper tube, I electrically 1 I heated Ar Hydride shielding Ar, H2 gas Fig. 2. Graphite paper atomiser (GPA) (laboratory-constructed) 70-100 heating cycles.The paper was made by EKL (Berlin, GDR). The shielding gas was argon. The outer flow-rate was 20 1 h-1 and the inner flow-rate was controlled by the hydride system (6-40 1 h-1). For comparison we also used graphite tube cuvettes of the Beckman type (Type 1268) and Carl Zeiss type (AE3). The light sources were electrodeless discharge lamps for As (9 W), Sb (7 W), Se (8 W) and Te (11 W) (Westinghouse, USA). Reagents Reagents were of analytical-reagent grade from Laborchemie Apolda, GDR. Stock solutions of As3+, Sb3+, Se032- and Te032- with concentrations of 1 mg ml-1 were prepared from As203, Sb2O3, Se02 and Te, respectively. Stock solutions for matrix elements were prepared at concentrations of 50-100 mg ml-1 by dissolving the appropriate amounts of As203, Sb203, Se02, Te, Ge02, SnCI2 and Pb(NO& in 1 M hydrochloric or nitric acid.Procedure Vessel A (Fig. 1) contains the alkali stabilised solution of sodium tetrahydroborate(II1) (NaBH4). Vessel B is for the sample and can be changed rapidly. The system is controlled by different Ar flows using the computer (Fig. 3). The acidifed sample is placed in vessel B. After initiation, two gas streams [the first purge stream (36 1 h-1) and the carrier stream (18 1 h-l)] purify the system (about 30 s are necessary). In the reaction phase (reduction by BH4- - H+), the purge stream is closed and the pressure gas valve is opened Gas flow Start Purge phase Reaction phase Purge phase Purge gas 36 I h-' Carrier gas 18 I h-1 Pressure gas 6 I h-1 Purge gas 36 I h-1 Carrier gas 18 I h-1 Measurement 1 I I I Auto-zero Signal integration Computerisation Temperature of quartz tube atorniser constant I I Temperature of graphite paper atomiser I I I I Heating phase Constant-temperature phase Cooling phase Fig.3. Scheme of procedure using hydride device Table 1. Relationship between electrical conditions and the resulting temperatures of the GPA PotentiaUV Primary 110 120 130 140 150 160 170 180 190 200 210 220 230 240 Secondary 9.5 10.2 11 12 13 14 14.8 15.2 16.1 16.9 17.6 18.4 19.1 20.5 Temperature/"C 1450 1600 1700 1800 1900 1980 2100 2120 2200 2280 2310 2400 2460 2560 Heating rate/ "C s-' 100 150 200 300 333 670 750 1000 1000 1300 1500 1700 2000 2300ANALYST, MARCH 1986, VOL. 111 271 (6 1 h-1).Depending on the duration of the reaction phase a variable amount of the reagent is transferred from vessel A into vessel B. After the reaction phase, the second purge (or trans- portation) phase is initiated. The pressure gas supply is closed and the second purge gas supply is opened. During the reaction phase and the second purge phase, the hydride - hydrogen - argon gas mixture is transported to the atomiser. Every time a small gas stream (6 1 h-1) avoids the introduction of air. The first seconds of the reaction phase are used to auto-zero the instrument. After this, the measuring phase is started (peak-height and -area integration modes are possible). If the system is in operation with the quartz tube atomiser, the tube is heated before to the desired temperature (up to 1000 "C) for all procedures.If the system is operated with the graphite paper atomiser, the graphite tube is heated at the end of the first purge period by resistance heating to the desired temperature (up to 2600 "C). The heating period is completed in 2-3 s. The hold time for the selected temperature is given by the total duration of the reaction and the second purge phases. Subsequently the tube is cooled rapidly by water cooling. Results and Discussion Investigations of the Commercial Hydride System (AAS-3, QT) Analyses of pure dilute solutions of As, Sb, Se and Te In order to guarantee high sensitivity, it is necessary to optimise the concentration of the acid used, the amount of NaBH4 and the sample volume. As the sample volume is an independent parameter, two volumes (0.2 and 10 ml) were used for the investigations. It was observed that a concentration of 2 M hydrochloric acid was the best (1 and 3 M HC1 led to signal depressions of about 5-10%).The optimum NaBH4 concentration was 3% and the optimum amounts were 0.8 ml for the smaller and 1.3 ml for the larger sample volume. The possibility of the direct introduction of the NaBH4 (introduction time <0.5 s) was also examined. It appeared that the AA signal height and shape were unchanged, but the reagent volumes required were smaller (0.5 and 1.1 ml, respectively). The analytical results obtained under the optimum condi- tions are shown in Table 2. Table 2 shows that the absolute reciprocal sensitivities (for 1% absorption, to a first approxi- mation this value is similar to a 30 detection limit criterion) are one order of magnitude better for the smaller sample volumes.There are two reasons for this: (1) with small sample volumes the reduction is faster owing to the faster mixing and higher concentration of the reagent; the homogeneity of the solution is also more rapidly achieved; and (2) with smaller volumes the amount of NaBH4 required is smaller, which leads to a smaller dilution of hydride by excess of hydrogen. As the larger volume is more than one order of magnitude greater than the small volume used, Table 2 shows that the large volume gives, as expected, better relative reciprocal sensitiv- ity and therefore also better detection limits. Optimisations of sample volume are possible in both directions, depending on the requirements of specific analy- tical problems.Investigation of matrix interferences of volatile hydride- forming matrices Table 3 shows the systems evaluated and Fig. 4 summarises the results of the investigations. Similarly to our earlier results for the determination of Se and Te,13 we found in all instances more or less strongly decreased signals. From the results in Fig. 4, it can be concluded that trace analysis is impossible in most instances. Classification of matrix interferences Interferences by different matrices have been investigated in hydride generation AAS for many systems. The influence of heavy metals has been frequently described.12 These metal ions only influence the procedure in the liquid phase of hydride generation.In our case it is possible that the matrix interferences could occur in all three steps of the hydride technique. Table 4 indicates the possible causes of interfer- ence. Possibilities of avoidance of matrix interferences in the liquid hydride generation phase Many investigators have described methods for decreasing matrix interferences in the liquid phase of hydride generation. A survey of these papers is given in reference 12. Recent publications by Welz and Melcherl4-16 also deal with this problem and have attempted to offer explanations in some instances. Welz and Melcherl7 concluded that the influence of heavy metals does not consist in the simple formation of soluble selenides, such as CuSe and CoSe, but that the true influence is connected with adsorption and destruction of hydrides on surface-active precipitates of the heavy metals formed by reduction. Procedures recommended for avoiding or decreasing matrix interferences have included change of acidity of the solution, formation of complexes (masking) with the matrix interferents, and formation of precipitates of the matrices.Complexation reagents used include EDTA, KI, citric acid, thiosemicarbazide 1 ,lo-phenanthroline, oxalic acid, thiourea and pyridylaldoxime (for details, see reference Two interesting examples of precipitation reactions should be mentioned. The determination of As in the presence of Se 12). Table 2. Results of hydride AAS using the AAS-3 with hydride system and quartz tube atomiser (QT) Reciprocal sensitivity per 1% A Sample volume 0.2 ml Sample volume 10 ml Relative/ Trace ng ml- element Absolutehg (p.p.b.) As ., . . 0.17 0.9 Sb . . . . 0.11 0.6 Se . . . . 0.2 1.0 Te . . . . 0.19 0.95 Relative/ ng ml-l Absolutehg (p. p. b. ) 1.6 0.16 0.8 0.08 1.8 0.18 1.1 0.11 Table 3. Matrix systems investigated Trace element Matrix substances As . . . . . . Sb, Bi, Se, Te, Ge, Sn, Pb, InSb Sb . . . . . . As, Bi, Se, Te, Ge, Sn, Pb, GaAs, InAs Se . . . . . . As, Sb, Bi, Te, Ge, Sn, Pb, GaAs, InAs, InSb Te . . . . . . As, Sb, Se, Ge, Sn, Pb, GaAs, InAs, InSb Table 4. Possible causes of matrix interferences in hydride AAS Step Interference Hydride generation . . Reduction of matrix (loss of NaBH,); reaction of matrix or reduced matrix with trace amounts of hydride (e.g. precipitation); adsorption of hydride on precipitates Decomposition of unstable hydrides on surface (only for hydride-forming matrices) atomisation mechanism; molecule formation (only for hydride- forming matrices) Hydride transportation .. Hydride atomisation . . Relatively unknown; change in272 ANALYST, MARCH 1986, VOL. 111 1 oc 80 6C 40 20 $ F 0 d - 100 80 60 0 Pb Ge Te I I I I 1 'b) Pb Bi Ge Te Sb Pt: GE As Sn Sb I 0 2 4 6 8 10 0 2 4 6 8 10 Mat rix/pg Fig. 4. used, 10 ng) Interference of hydride-forming matrices on the determination of (a) As, (b) Se, ( c ) Sb and (d) Te, using QT (amount of analyte is actually improved in the presence of Cu2+ ions, because these bind the generated H2Se as soluble CuSe.17J8 Selenium determinations in the presence of Cu2+ ions are improved by the addition of Te02, because the simultaneously generated H2Te forms insoluble CuTe.1'21 Except for the example of the determination of As in the presence of Se, no other literature information was available for reducing the matrix effects of our analytical combinations.We therefore attemp- ted to obtain appropriate improvements by matrix modifica- tion procedures. Investigation of the influence of Cu2+ ions on the determina- tion of As and Sb in the presence of Se and Te. The results of these investigations are illustrated for As in Fig. 5 . The effects observed for Sb were the same and are not shown here. Fig. 5 shows that the influence of the Se matrix is stronger than that of the Te matrix. The general reason for the interference lies in the formation of insoluble compounds such as As2Se3, Sb2Se3, Sb2Se3, As2Se3 and Sb2Tes, and in the adsorption of these compounds on the surface of elemental Se and Te precipitates.The addition of increasing concentrations of Cu2+ ions reduced the depression of both Se and Te on the AA signal. The influence of Cu2+ ions is much stronger for the Te matrix than the Se matrix. We assume that this is due to the smaller solubility constant of CuTe compared with CuSe. Particularly with the Se matrix but also for the Te matrix, it is impossible to avoid completely the matrix interference. From this fact we deduce that there are other matrix interferences than those in solution, e.g., in the gaseous phase. Investigations of the influence of ED TA on the determination of As, Sb, Se and Te in the presence of Bi, Ge, Sn and Pb matrices.The results of these investigations for a trace element to matrix ratio of 1 : 1000 are shown in Table 5. The amount of EDTA was sufficient to complex the metal ions fully. With the Ge and Pb matrices only small improve- ments were observed because Pb2+ ions do not have a strong 0 0.01 0.10 1.00 0 0.01 0.10 1.00 Mat r i xipg Fig. 5. Influence of the matrix modifier Cu2+) on the trace determination of As and Sb in the presence of (A Se and (b) Te matrix (amount of analyte, 10 ng) Table 5. Improvement in the determination of As, Sb, Se and Te in the presence of hydride-forming matrices (Bi, Sn, Pb, Ge; 10 pg) achieved by addition of EDTA Improvement factor Trace element (10 ns) Bi Sn Pb Ge As . . . . . . 7 5* 1.05 1.15 Sb . .. , . . 3.3 loo* 1.05 1.06 Se . . . . . . 1.3 30 * 1.15 1.7 Te . . . . . . - 1.3* 1.7 1.7 * 5 M HCI.ANALYST, MARCH 1986, VOL. 111 273 influence and complex formation between Ge*+ ions and EDTA is not high. A successful outcome was achieved for the complexation of Bi3+ ions, especially for the determination of Sb and As. Te determinations are impossible in the presence of Bi3+ ions, irrespective of the presence or absence of EDTA. Interference by Sn2+ ions, which in most instances is the strongest influence, is diminished by EDTA. The use of a higher concentration of hydrochloric acid ( 5 M) is advan- tageous. Investigation of the influence of citric and tartaric acids on the determination of Se in the presence of Sb and Te matrices. The presence of a 100-fold excess of the complexing reagent (10 pg) improves the analytical results only by a factor of 1.5 (Sb) or 1.4 (Te).Investigations Using the AAS-3 Hydride Generation Systems Combined with the Graphite Paper Atomiser As indicated above, only in some instances is it possible to avoid matrix interference using chemical matrix modification. We therefore assumed that very strong matrix interferences occur in the gaseous phase in addition to the solution phase. In order to evaluate such a hypothesis, it is necessary to provide some variation of the temperature of the atomiser. Because further enhancement of the temperature (> 1000 "C) in quartz tube atomisers is impossible, we investigated the use of a graphite tube atomiser. Characterisation of graphite tubes for hydride atomisation After the first application of graphite tubes in hydride atomisation by Knudson and Christian,g a number of other workers used this technique.12 In most instances small graphite tubes of the HGA 2100 type (Perkin-Elmer) have been used.This graphite tube is not very useful for hydride atomisation, because the inner volume is small (see Table 6) in relation to the large gas volume produced in the generation reaction (argon - hydrogen - hydride mixture up to 80 ml). The analytical sensitivities achieved are therefore relatively poor. We therefore attempted to develop a new graphite tube atomiser, which avoids this disadvantage, and which has dimensions similar to those of the quartz tube atomiser widely Table 6. Dimensions of commercial quartz tube and graphite tube atomisers and of the new developed graphite paper atomiser Graphite tubes HGA 500 Beckman Dimensions QT AE3 1268 GPA Length/mm .. . . . . 145 28 63 92 I.d./mm . . . . . . 18 6 9 9 Internal volume/cm3 . . 37 0.8 4.8 5.9 Heated graphite volume/cm3 . . . . - 0.6 3.6 0.5 used for this technique. Table 6 shows the dimensions of these different types of atomisers. The Beckman Type 1268 atomiser has the longest path length of the commercial graphite tube atomisers. Its disadvantage is the high energy that is needed (10 V, 1000 A = 10 kW), because the mass of graphite to be heated is very large. The graphite paper atomiser developed in this work has a longer path length (150%) compared with the Type 1268 graphite tube, but the heated graphite volume is only 13%.In comparison with the HGA 500 graphite tube type and AE 3 (Carl Zeiss), the enlargement of the inner dimensions is very high, but the heated graphite volume is only 83%. Depending on the temperature required, a power of 3-4 kW is needed. Investigation of pure dilute solutions of volatile hydride- forming elements, As, Sb and Te in graphite tube atomisers The results of these investigations are shown in Table 7. Only with the GPA can the same analytical sensitivities as with the QT be achieved. The results support our assumption that the reason for the relatively infrequent application of graphite tube atomisers in hydride AAS is the poorer analytical sensitivities obtained in normal graphite tube atomisers compared with the QT atomiser.Investigation of the influence of volatile hydride-forming matrices on the determination of As, Sb, Se and Te using the GPA The results of this investigation are shown in Fig. 6. The optimum atomisation temperatures were 1850 "C (Se, Te) and 2000 "C (As, Sb). Fig. 6 shows the improvement of the AA signal depression ( i ) using an enhanced atomisation temperat- ure, (ii) by chemical matrix modification, using the QT atomiser, and (iii) by an enhanced atomisation temperature and chemical matrix modification. It can be seen that the increase in atomisation temperatures available in the GPA gives the best analytical results for the determination of As, Sb and Se in the presence of As, Sb, Bi and Se matrices. Only the effects of a Te matrix on As- and Sb determinations, and of all matrices on Te determinations, are not improved by the use of higher temperatures, In all instances the combination of the Table 7.Results of hydride AAS using different atomisers (reciprocal sensitivity/ng per 1% A ) . Sample volume, 0.2 ml Graphite tube, Trace Quartz Graphite Beckman element tube, AE3 tube, AE3 1268 As . . . . 8.17 0.35 0.20 Sb . . . . 0.11 0.18 0.14 Se . . . . 0.20 0.32 0.24 Te . . . . 0.19 0.29 0.24 Graphite tube, graphite paper 0.16 0.11 0.21 0.20 Table 8. Results of the determination of trace amounts of hydride-forming elements in hydride-forming matrices using GPA at 2000 "C and optimum matrix modification in comparison to QT without matrix modification. Values: reciprocal sensitivities per 0.01 A * Relative detection limits,* p.p.m.with respect to matrix element Trace element Atomiser As Sb Bi Se Te Ge Sn Pb As . . . . . . QRA GPA Sb . . . . . . QRA GPA Se . . . . . . QRA GPA Te . . . . . . QRA GRA - 100 5 180 3 600 600 120 8 1100 2900 230 190 - 400 1 120 3 240 30 1900 1100 2600 260 2 0.7 1800 450 3 2 - 120 8 2900 - - - 140 740 8 45 4 150 40 190 90 1040 30 530 7 600 120 390 290 4 2 0.4 0.4 30 12 290 190 * Because the 0.01 absorbance is in all instances to a first approximation equal to three times the relative standard deviation it is possible to write detection limits.274 ANALYST, MARCH 1986, VOL. 111 1 oc 80 60 40 20 $ E 0 d - 100 80 60 40 20 0 Sb Bi Se Te matrix As 4s Sb Bi Te matrix Se AS Bi Se Te matri) 4s Sb Se matrix Fig. 6. Improvement of AA signal depressions (-) caused by hydride-forming matrices in QT using chemical matrix modification (- - -) (QT) and enhancement of temperature (1900-2000 "C) (GPA) (-.--); sum of improvements (-).Amount of analyte element, 10 ng per 0.2 ml; amount of matrix element, 10 yg per 0.2 ml; EDTA, 1 M; Cu2+, 1 mg per 0.2 ml; citric acid, 5%; and tartaric acid, 5% Table 9. Results of the determination of As, Sb, Se, Te in thin layers of AIIIBV semiconductor materials using hydride AAS with GPA. Conditions: layer separation by chemical etching; dimensions of layer, 10 x 10 x 0.0005 mm; sample, 0.25 mg. Values in parentheses are improvement factors relative to QT Relative detection limit/atoms ~ m - ~ Trace element GaAs InAs InSb - 9 x 1016 As - Sb . . . . . . 2.6 x 1016 5.6 x 1016 Se . . . . . .4.8 (lOo) x 10'8 5.1 x 10'8 Te . . . . . . 5.7 x 1018 . . . . . . (100) 4 x 1017 (100) (100) ( 5 ) ( 5 ) (5) ( 5 ) (10) - 5.4 x 10'8 6.1 x 10'8 Conclusions Application of the graphite paper atomiser , which allows atomisation temperatures up to 2600 "C is very useful for the determination of trace amounts of hydride-forming elements in the presence of hydride-forming matrices. The improvement factors for application of the GPA are 1-3 orders of magnitude. In spite of the strong improvement by high-temperature atomisation, it is necessary to apply matrix modifiers. The improvements that can be obtained by GPA atomisa- tion and matrix modifications show that matrix interferences occur in different ways in the liquid and gaseous phases. To explain the reasons, especially for matrix interferences in the gaseous phase, the results of further experiments are given in the following paper.GPA and chemical matrix modification gave the best results (see Fig. 6 and Table 8). Table 8 shows the results obtained for the determination of AS, Sb, Se and Te in the presence of hydride-forming matrices using the GPA and chemical matrix modification. In all instances significant improvements were achieved, the improvement factors being between 1.5 and 1000. The combination of GPA and chemical matrix modification has been used for the trace determinations of As, Sb, Se and Te in AIIIBV semiconductor materials. As shown in Table 9, considerable analytical improvements were obtained in many instances, Several conclusions can be drawn from these results. In particular, the application of the higher atomisation temperat- ures in the GPA for hydride AAS is very useful, particularly if volatile hydride-forming elements are present as matrices.Strong matrix interference appears to occur in the gaseous phase in addition to the liquid phase of the QT atomiser for this type of matrix. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. References Holak, W., Anal. Chem., 1969, 41, 1712. Erdey, L., Gegus, E., and Koscis, E., Acta Chim. Hung., 1955, 7, 343. Pollock, E. W., and West, S. J., At. Absorpt. Newsl., 1972, 11, 104; 1973, 12, 6. Goulden, P. D., and Brooksbank, P., Anal. Chem., 1974, 46, 1431. Braman, R. S . , Justin, L. L., and Foreback, C. C., Anal. Chem., 1972, 44, 2195. Schmidt, F. J., and Royer, J. L., Anal. Lett., 1973, 6 , 17. Chapman, J . F., and Dale, L. S . , Anal. Chim. Acta, 1979,111, 137. Chu, R. C., Barron, G. P., and Baumgarner, P. A . W., Anal. Chem., 1972, 44, 1476. Knudson, E. J., and Christian, G. D., Anal. Lett., 1973, 6 , 1039. Siemer, D. D., and Hagemann, L., Anal. Lett., 1975, 8, 323. Aggett, J., and Aspell, A. C., Analyst, 1976, 101, 341. Nakahara, T., Prog. Anal. At. Spectrosc., 1983, 6 , 163.ANALYST, MARCH 1986, VOL. 111 275 13. 14. 15. 16. 17. Dittrich, K., Vorberg, B., and Wolthers, H., Tuluntu, 1979,26, 747. 104, 232. Welz, B., and Melcher, M . , Analyst, 1984, 109, 569. Welz, B., and Melcher, M., Analyst, 1984, 109, 573. Welz, B., and Melcher, M. , Analyst, 1984, 109, 577. Welz, B., and Melcher, M., “Wissenchaftliche Beitrage der Karl-Marx Universitat, Analytiktreffen 1982, Atomspektros- kopie,” Karl-Marx-Universitat, Leipzig, 1983, p. 165. Welz, B., and Melcher, M., Anal. Chim. Actu, 1981,131, 131. Kirkbright, G. F., and Taddia, M., At. Absorpt. Newsl., 1979, 18. 68. 20. 21. Azad, J., Kirkbright, G. F., and Snook, R. D., Analyst, 1979, Bye, R., Engrik, L., and Lund, W., 2. Anal. Chem., 1984,318, 25. 18. 19. Paper A5182 Received March 5th, 1985 Accepted September 20th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100269

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 277 Investigations into the Improvement of the Analytical Application of the Hydride Technique in Atomic Absorption Spectrometry by Matrix Modification and Graphite Furnace Atomisation Part II.* Matrix Interferences in the Gaseous Phase of Hydride Atomic Absorption Spectrometry Klaus Dittrich and Rita Mandry Karl-Marx-University, Chemistry Section, Analytical Centre, Talstrasse 35, 70 10 Leipzig, GDR Studies of matrix interferences in hydride AAS were carried out in the presence of hydride-forming matrices. It was found that the main interference in most instances occurs in the gaseous phase of hydride atomisation. Thermodynamic calculations, coupled with spectroscopic and thermal investigations, show that the main cause of matrix interferences in the gaseous phase is the formation of diatomic molecules between the trace and matrix elements (e.g., AsSb).Therefore, significant improvements in accuracy and relative sensitivity can be obtained by using graphite tube atomisers, at temperatures >2000 “C. Keywords : Atomic absorption spectrometry; h ydride technique; matrix modification; graphite furnace atomisation; matrix interferences Most papers dealing with matrix interferences in hydride AAS have concerned matrix interferences that occur in the liquid phase of hydride generation.1 Matrix interferences in the gaseous phase of hydride AAS can only be caused by volatile hydride-forming matrices and are therefore rarely dealt with. Welz and Melcher2 concluded that the reason for the strong matrix interference of volatile hydride-forming elements using quartz tube atomisers (QT) was the absence of H radicals required for the efficient atomisation of hydrides and hydride- forming matrices.As shown in Part I,3 the analytical determinations of “hydride-forming trace elements” in “hydride-forming mat- rices” are improved very strongly by the use of the graphite paper atomiser (GPA). In this paper we report studies that make a contribution to our knowledge of atomisation mechan- isms in hydride AAS and towards the explanation of matrix interferences. Experimental Apparatus An AAS-3 atomic absorption spectrometer (Carl Zeiss, Jena, GDR), a hydride generation system with a quartz tube atomiser (QT) and a graphite paper atomiser (GPA)3 (laboratory constructed) were used.The light sources were As, Sb, Se and Te electrodeless discharge lamps (Westing- house, USA). Reagents Reagents were of analytical-reagent grade from Laborchemie Apolda, GDR. Stock solutions of A++, Sb3+, Se032- and Te032- with concentrations of 1 mg ml-1 were prepared from As2O3, Sb2O3, Se02 and Te, respectively. Stock solutions for matrix elements were prepared at concentrations of 50-100 mg ml-1 by dissolving appropriate amounts of AsZO3, Sb2O3, Se02, Te, Ge02, SnC12 and Pb(N03)2 in 1 M hydrochloric or nitric acid. * For Part I of this series, see p. 269. Procedure The procedure described in Part 13 was followed. Results and Discussion Mechanisms of Atomisation in Hydride AAS Dedina and RubeSka4 and also Welz and Melchers proposed for the mechanism of atomisation in flames and in quartz tubes a hydrogen radical reaction: +H +H +H ASH^ + A S H ~ + H2 + ASH + 2H2 + AS + 3Hz (1) The required H radicals are generated in flames by combus- tion processes.Welz and Melchers proposed that the residual oxygen in the argon and in the sample solution produce H radicals in the hot QT by reaction with the excess of hydrogen generated from the tetrahydroborate(II1) reaction. Further, they showed that at high temperatures (at about 1000 “C) there are reactions between the SiOH groups of the quartz tube surface and the hydrogen, resulting in the release of H radicals. Thermal dissociation was not excluded but it was assumed that this reaction would lead to dimeric and tetrameric As molecules. Alkman et a1.6 proposed that the mechanism of atomisation of AsH3 in graphite tubes involves thermal destruction to solid As as the first step, followed by evaporation and dissociation steps: Destruction Evapo- Disso- Disso- ration ciation ciation ASH3 Assolid I As4- As2 .I AS (2) 1000 “C All these proposals were supported by experimental evidence, but not exactly demonstrated. Equilibria Between Different As Species: Thermodynamic Considerations The exact determination or calculation of the equilibrium composition of species in complex mixtures of vapours in a QT or GPA is very difficult, because the parameters and total concentrations are themselves not well defined.It is possible to assume the following. A total amount of 0.1-10 ng of arsenic should exist in the forms AsH3, AsH2, ASH, As, As2 and As4 in a gaseous volume of 10-100 ml of a hydrogen -278 ANALYST, MARCH 1986, VOL.111 argon mixture, because 1 ml of 3% NaBH4 solution produces in the presence of H+ about 80 ml of hydrogen. The distribution of As species in such a gas mixture is unknown, but it is possible to assume that the total partial pressure of all As species lies between 10-9 and 10-7 atm. In connection with investigations concerning epitaxial processes for the production of AIIIBV- semiconductors (GaAs, Gap) ,7-9 calculations and experiments were carried out for the determination of equilibrium compositions. From these results we can suggest that at a total partial pressure of 0.1 atm for As species and at 827 "C in a hydrogen atmosphere, the equilibrium composition is the following: As 2 x 10-4, AsH2 2 x AsH3 7 x 10-3, As4 42, As3 8 x 10-2 and As2 58%.From these results it can be concluded that (i) thermal dissociation of AsH3 is possible and (ii) the main products at 827 "C are As2 and As4. This is clearly the reason for the assumption by some workers that As atoms do not exist at this temperature. The following equation describes the equilibrium 2As2 G As4 at 1000 "C: (3) where K p = equilibrium constant and P = partial pressure. For total partial pressures of As species of 10-1, 10-5, 10-7, 10-8 and 10-9 atm, it is possible to calculate from the equation 4a2 - 5 2 . . . . . . . . (1 - a2) - P (4) the degrees of dissociation (a) at 1000 "C. The values are 0.5, 80, 99, 99.99, 100 and 100%, respectively. This means that at total pressures of e l O - 3 atm As4 molecules have no practical existence.We have calculated the degree of dissociation of As2 by the dissociation reaction 2As e As2, using the standard values12 for As,, &98 = 68.7 kcal mol-l and $98 = 41.6 cal mol-1 k-1, and for As2, = 45.5 kcal mol-1 and s;98 = 57.6 cal mol-l K-l, the standard reaction enthalpy and entropy as being A@$ = 91.4 kcal mol-1 and AS$! = 25.3 cal mol-1 K-1 and the free enthalpy as being A G k = 83.9 kcal mol-1. Ignoring the dependence of A H on temperature, we calculated the following equilbrium constants for this As2 dissociation: 3 X 10-62(298K), 1.3 x 10-'0(1298K)and7 x lO-4(2298K) Using equation (4), the degrees of dissociation (a) were calculated for the following total pressures (10-1, 10-3, 10-5, 10-7, 10-8 and 10-9 atm) at 1025 "C: 1.8 x lO-3,1.8 X 10-2, 1.8 x 10-1, 1.8, 6 and l8%, respectively; and at 2025 "C: 4.2, 10, 97, 99.9, 100 and loo%, respectively.At 1025 "C at total pressures between 10-7 and 10-9 atm, dissociation is possible and As atoms exist. (5) The radical atomisation mechanism proposed by other workers4>5 is also possible but the following remarks should be made: (a) thermal dissociation is also possible; (b) not all atoms formed react to give molecules (there are also kinetic effects). An increase in temperature to 2025 "C shows that the dissociation to atoms is very high at total pressures up to 10-5 atm. On increasing the temperature from 1000 to about 2000 "C we did not obtain improvements in atomic absorption sensitivity3 for dilute solutions.This means that the atom concentration at both temperatures is the same. In Table 1, dissociation energies for diatomic species of hydride-forming elements are shown. Columns 1 and 2 in Table 1 show that MH and M2 species, that is, As2, Se2, SeH and ASH molecules, are the most stable in comparison with the other molecules. Therefore we can conclude that at 2025 "C all other species are dissociated and at low partial pressures the same situation exists at 1000 "C. Mechanism of Atomisation in Gaseous Phases of Complex Composition The strong depressions of AA signals, which start at 100-fold excesses of typical matrices, were explained by Welz and Melchers by the fact that for the radical mechanism the required H radicals are preferentially consumed by the matrix hydrides.Thus for the trace hydride the degree of atomisation is inefficient or low. Our opinion is that, as shown in the previous section, considerable thermal dissociation of the MH, molecules occur at 1000 "C. Depending on the matrix concentration, the partial pres- sure can be in the range lo-5-10-1 atm. Therefore, it is possible to write the following equations: Mtrace + Mmatrix excess * Mtrace Mmatrix + M2 matrix AS + SbexceSS F AsSb + Sb2 (6) (7) or, e.g., From Table 1, it can be seen that the dissociation energies of such mixed species are of the same order of magnitude as for individual metal molecules. The reaction rate to reach equilibrium should be very high, because the concentration of the matrix is very high. This means that at 1000 "C, most trace species should exist as mixed molecules, because the dissocia- tion energies are similar.We attempted to calculate the degree of dissociation, for such mixed molecules. For the dissociation energy of AsSb we used the arithmetic mean of the dissociation energies of As2 (3.9 eV) and Sb2 (3.1 eV), which is 3.5 eV. As the dissociation energies of the pure substances are very similar to the standard reaction enthalpies for the dissociations, we calculated for the AsSb dissociation energy A@9: = 80.7 kcal mol-1. As the standard entropies for dissociation of the As and Sb molecules are almost identical (As2, Sgg = 25.3 cal mol-1 K-2; Sb2, SgF = 24.7 cal mol-1 K-I), we used the arithmetic mean in this instance also, i.e., 24.9 cal mol-1 K-1 for AS%! (AsSb+ As + Sb).Using these assumptions, we calculated the following Table 1. Dissociation energies (E) of diatomic molecules of volatile hydride-forming elements; literature values'0.11 (calculated values in parentheses) Mole- Mole- Mole- Mole- Mole- Mole- Mole- Mole- Mole- cule EIeV cule EIeV cule EIeV cule EIeV cule Elev cule EIeV cule EIeV cule EIeV cule EIeV ASH 2.8 As, 3.9 AsSb 3.5 AsSe 3.0 AsTe 2.5 AsBi 3.0 AsPb (2.2) (AsSn) - (AsGe) - SbH (2.6) Sb, 3.1 SbBi 2.5 (SbGe) - (SbSn) - SbPb (1.9) SbSe 3.0 SbTe 2.7 BiH 2.5 Bi, 2.0 (BiGe) - (BiSn) - (BiPb) 1.4 BiSe 2.9 BiTe 2.4 GeH 3.2 (Ge,) 2.8 (GeSn) - (GePb) - GeSe 5.0 GeTe 4.2 SnH 2.5 Sn2 2.0 (SnPb) - SnSe 4.1 SnTe 3.6 PbH 1.8 Pb2 1.0 PbSe 3.0 PbTe 2.5 SeH 3.2 Se, 3.4 SeTe 3.0 TeH (2.7) Te, 2.7ANALYST, MARCH 1986, VOL. 111 279 equilibrium constants for the dissociation of AsSb at several temperatures: 298 K, 1.9 x 10-54; 1298 K, 7.8 x 10-9; and 2298 K, 6.4 x 10-3.It is now possible to calculate the ratio of partial pressures, PAslPAsSb, for Sb in a large excess (>loo) for the temperatures 1298 and 2298 K. In Table 2, the ratio of the partial pressures and the degrees of dissociation are shown for different total partial pressures of antimony. At 1025 "C only the mixed diatomic molecules (in addition to Sb2 molecules) exist, and at 2025 "C up to Sb partial pressures of 10-3 atm only the free atoms exist. If we compare the results of these calculations with our previous experimental results (reference 3, Table 8), then the same tendency is evident. It can be concluded that the formation of stable, mixed diatomic molecules is the main reason for the depression of AA signals in the gaseous phase.An exact agreement is impossible, because at such high Sb partial pressures at both temperatures high concentrations of Sb2 molecules are also present. It is clear, however, that, if it was possible to take this factor into account, agreement of the calculated values with the experimental results would be improved. Exact calcula- tions of all the equilibria in this and other analogous systems have not been carried out to date, but it is intended to persue such studies in the future. Dependence of the Determination of As and Sb on the Atomisation Temperature Used in Hydride AAS The results of these investigations are shown in Fig. 1.We investigated the dependence of the AA signals on atomisation temperature in the absence and presence of some matrices. At first in the absence of matrix elements both elements gave, in both the QT and the GPA at very different temperatures, almost the same analytical sensitivity. The small improvement in the GPA is experimentally significant, but is very difficult to explain. One reason could be the different inner diameters of the tubes used.3 There is one difference between the Sb and As determinations: at 1600 "C the As AA signal is significantly reduced. The reason for this is probably the higher stability of the As2 molecule (ED = 3.9 eV) in comparison with the Sb2 molecule (ED = 3.1 eV). At lower temperatures both signals are decreased in the GPA. Because it was impossible to measure the temperature exactly, these results have not been included.This could, however, be explained by the assumption that there are two types of atomisation reaction, radical and thermal. It can be seen in Fig. 1 that in the presence of matrices we have the strongest depression in the QT at 900 "C. Increasing the temperature to 1900-2000 "C decreases the matrix interference in all instances, and in some instances, viz., Sb in As, and As in Sb, Se and Bi, it is removed entirely. At lower temperatures in the GPA a strong matrix interference is again observed. These experimental results support our opinion about matrix interferences in the gaseous phase of hydride AAS and our calculations in the previous section of this paper. Of course, this is only indirect evidence for the existence of mixed diatomic molecules.Spectroscopic Investigations of the Composition of the Vapour in Hydride AAS These experiments were designed to detect spectroscopically the proposed mixed molecules in the hot vapour of the QT in Table 2. Dependence of ratios of partial pressures and degrees of dissociation (a) on Sb partial pressure; PSb/PAs > 100; PAS < 10-7 atm Partial pressure of Sb/atm Ratio a, "/o Temperature/"C 10-5 10-3 lo-' PAslPAsSb . . . . . 1025 7.8 x 10-4 7.8 x 10-6 7.8 X 10-8 a,% . . . . . . 1025 7.8 X lo-* 7.8 x 10-4 7.8 X 10-6 PAslPAsSb . . . . . 2025 6.4 x lo2 6.4 6.4 X 10--2 a, O/O . . . . . . 2025 99.8 86.4 6.0 Without m a 3 As 80 - 5 6 0 - F d 40 4 Se Bi J/ I I 1 I 0 4 I I 900 ' 1600 1800 2000 2200900" QT GPA+ QT GPA-, 1 1600 1800 2000 2200 Tem peratu re/"C Fig.1. Dependence of (a) Sb and ( b ) As determination on atomisation temperature in the presence and absence of hydride-forming matrices. Amount of analyte element, 10 ng; amount of matrix element, 10 pg. QT = Quartz tube atomiser; GPA = graphite paper atomiserANALYST, MARCH 1986, VOL. 11 1 280 0.5 0.4 0.3 0.2 0.1 AS - Sb 366-386 nm 250 300 350 400 Wavelengthhn Fig. 2. Molecular absorption spectra in hydride AA with QT atomisation, using Sb or As alone and in combination. Measurement: point by point. Amounts: X, As, 0.5 mg per point; 0, Sb, 1.0 mg per point; and 0, As - Sb, 0.5 and 1.0 mg per point order to confirm the accuracy of our hypothesis regarding interferences. Unfortunately, the spectroscopic properties of such molecules are unknown. In general, it can be anticipated that the absorption coefficients of such molecules will be small.For these reasons, high concentrations of the elements under investigation, As and Sb, were chosen and a deuterium continuum lamp was used as the light source. The results are shown in Fig. 2. For antimony alone, the spectrum was obtained in the spectral range between 250 and 400 nm. Using the QT as the atomiser we observed absorption bands of the Sb;! molecule. This is also evidence in support of the thermal mechanism, because such high amounts of SbH3 cannot be decomposed by small amounts of hydrogen radicals. On the other hand, this result is also evidence for the very fast recombination of atoms to molecules under these conditions of high concentration.In the range between 360 and 400 nm, the absorption spectra of As alone and As - Sb mixtures was measured. It can be seen that absorption bands of the SbAs molecule are present, consistent with those described in the literature. 10 Similar measurements using the GPA at 2000 “C did not lead to similar characteristic spectra. We therefore believe that this is direct evidence for the existence of such mixed diatomic molecules at 1000 “C in the QT. Conclusions Thermodynamic calculations of the system As - H2 - Ar show that thermal dissociation of AsH3 is in principle possible at temperatures below 1000 “C. Further, the thermodynamic calculations show that mixed molecules of the AsSb type exist at low temperatures and that they can be dissociated at higher temperatures.This means that the main reason for matrix interferences in hydride AAS in the presence of hydride-forming matrices consists in this type of molecule formation. The conclusion in the preceding paragraph was supported by experimental results: measurement of the dependence of the AA signals on the temperature and the spectroscopic detection of AsSb in hydride vapours. From these results and those in Part I,3 the matrix interferences of hydride-forming matrices can be classified as follows. (a) Interferences that involve only molecule formation in the gaseous phase, As in Sb and Sb in As. (b) Interferences that include both molecule formation in the gaseous phase and chemical reaction and adsorption in the liquid hydride generation phase, As, Sb and Se in Bi, Se, Ge, Sn and Pb.(c) Main interferences determined by chemical reaction and adsorption in the liquid phase, Te in As, Sb, Bi, Se, Ge, Sn and Pb and As, Sb and Se in Te. For the analytical determination of hydride-forming ele- ments in the presence of hydride-forming matrices the graphite paper atomiser can be recommended. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. References Nakahara, T., Prog. Anal. At. Spectrosc., 1983, 6, 163. Inui, T., Terada, S., Tamura, H., and Ichinose, N., Fresenius 2. Anal. Chem., 1984, 318, 502. Dittrich, K., and Mandry, R., Analyst, 1986, 111, 269. Dedina, J . , and RubeSka, I., Spectrochim. Acta, Part B , 1980, 35, 119. Welz, B., and Melcher, M., Analyst, 1983, 108, 213. Akman, S . , Genc, O., and Balkis, T., Spectrochim Acta, Part B , 1982,37, 903. Murray, J. J., Papp, C., and Pottie, R. F., J . Chem. Phys., 1973, 58, 2569. Gentner, J. L., Bernardt, C., and Cadoret, R., J. Cryst. Growth, 1982, 56, 332. Ban, V. S., and Ettenberg, M., J. Phys. Chem. Solids, 1973,34, 1119. Rosen, B., Editor, “International Tables of Selected Con- stants, No. 17, Spectroscopic Data Relative to Diatomic Molecules,” Pergamon Press, Oxford, 1970. Krasnova, K. S., “Molekulyarnye Postoianye Regragnitsches- kich Soedinenii,” Khimiya, Leningrad, 1979. “Termitscheskiye Konstanty Weschtschestw,” No. 3, Academy of Sciences of the USSR, Moscow, 1968. Note: Reference 3 is to part I of this series. Paper A51114 Received March 27th, 1985 Accepted September 20th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100277

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 281 Method for Improving the Sensitivity and Reproducibility of Hydride-forming Elements by Atomic Absorption Spectrometry Nicolaos E. Parisis and Aubin Heyndrickx Department of Toxicology, State University of Ghent, Hospitaalstraat 13, 9000 Ghent, Belgium The positive effect of oxygen on the atomisation of hydride-forming elements at temperatures above 800 "C is indicated. Several different materials of construction for gas transport tubing were tested and their influence on the transportation of hydrides and on radical production was studied in order to ascertain their operating efficiency. Silanised glass and FEP tubing gave the highest sensitivity and reproducibility and silicone-rubber and nylon tubing the lowest. The combination of the above, together with the use of 2 M nitric acid as a rinsing agent for the reaction vessel, allows the convenient use of peak areas for the measurement of arsenic, selenium, bismuth, antimony and tin signals at levels of a few nanograms. Keywords: Atomic absorption spectrometry; h ydride generation; oxygen - argon carrier gas; silanised glass tubing The atomisation mechanism for volatile hydride-forming elements in a heated quartz tube was generally thought to involve thermal decomposition.However, in the last few years, some workers have proposed a new mechanism in order to explain the increased sensitivity obtained in their experi- ments. First, Dedina and Rubeskal suggested that hydride atom- isation is caused by free radicals (H-, OH*) generated in the reaction zone of a hydrogen - oxygen flame burning in the T-shaped quartz tube of the system. Welz and Melcher,2 in studies of the selenium interference with trivalent and pentavalent arsenic, found that the only theory that would explain the results was that selenium, which is volatilised earlier than arsenic, increases the deficiency of radicals so that for the arsenic that appears later there remains an insufficient number of radicals to cause atomisation to the same extent as in the absence of selenium.Moroever, arsenic has a consider- ably smaller effect on selenium than vice versa. Recently, the same workers in another study3 concluded that these radicals are formed in a reaction with oxygen at low temperatures (above 600 "C) and in a "clean" environment the concentration of radicals is well above the equilibrium concentration because their formation is a much faster process than the recombination.They found a significant enhancing effect of oxygen on the sensitivity of volatile hydride-forming elements at temperatures around 700-800 "C; they established that this cannot be due to a temperature increase in the gas phase (effective temperature) of the atomiser . In this work, several investigations have been performed with a commercially available hydride system and an electric- ally heated quartz cell atomiser. The influence of different purge gases, gas transfer tubing and reaction flasks on the sensitivity of hydride-forming elements was studied. can be heated to 1000 "C by eight insulated heating wires. This atomisation device, mounted in the sample compartment of the spectrometer, is connected with an electronic system that allows one to select the required duration of purge and reaction, between sample and reagent solution, and controls the temperature to within +20 "C by using an Ni - NiCr thermocouple and a feedback system.Piston stroke pipettes of capacity 10,25 and 50 pl (Assipette No. 100) and pipette tips from Assistent, Sondheim/Rhon, FRG, were used. Tubing Nylon tubing. From Perkin-Elmer (gas supply hose, 079873), 40 cm x 4 mm i.d. Silicone-rubber tubing. From Perkin-Elmer (transfer hose, 094140), 40 cm X 4 mm i.d. Soda-glass tubing. Made in our department from commer- cially available material, 44 cm x 4.5 mm i.d. PTFE tubing. FEP (fluorinated ethylene - propylene) tubing from Rudolf Brand, Wertheim, FRG, 40 cm X 5.7 mm i.d.Procedure Table 1 gives the important operating parameters for the instrument, lamps and hydride system used for the determina- tions. Experimental Apparatus The instrument used was a Perkin-Elmer Model 372 atomic absorption spectrometer, equipped with an external elec- trodeless discharge lamp (EDL) power supply. Perkin-Elmer electrodeless discharge lamps were used for all elements. Results are shown on a four-digit electronic display. A simple diagram of the apparatus is shown in Fig. 1. The hydride-generation device is a Perkin-Elmer MHS-20 mercury - hydride -system. The volatilised hydrides are atomised in an electrically heated quartz tube (165 X 14 mm i.d.) closed at both.ends by quartz windows.The quartz tube Arsine generating chamber W 3% NaBH, in 1% NaOH Fig. 1. Hydride-generation apparatus282 ANALYST, MARCH 1986, VOL. 111 _ _ _ _ ~ ~ ~~~~ - Table 1. Operating parameters MHS-20 EDL Wavelength/ Element power/W nm As(II1) . . . . 8 193.7 Se(IV) . . . . 6 196.0 Bi(V) . . . . 8 223.1 Sb(II1) . . . . 8 217.6 Sn(IV) . . . . 8 224.6 Slit nrn S 0.7 40 0.2 48 0.2 25 0.2 30 0.2 20 width/ Purge I/ Reaction/ Purge II/ 4 20 4 20 4 30 4 50 4 40 S S Other conditions were as follows: measuring mode, inte- grated peak area; reading time, 20 s; background correction, none; quartz cell temperature, 880 k 20 "C; reaction volume, 10 ml; standard dilution liquid, 1.5% mlV H2S04 [except for Sn(IV), 0.75% mlV H2S04]; inlet pressure, 250 kN m-2; and hydride-generating reagent, 3.92 _+ 0.03 ml of 3% NaBH4 in 1% NaOH, 18-22 "C.For all the measurements, 10 ml of 1.5% mlVH2S04 and an appropriate aliquot of the standard solution (10-50 pl) are added to the polypropylene reaction flasks before attachment to the system. When the start button of the control unit is activated, the purge gas flows through the reaction flask for a pre-selected time (purge I) at a flow-rate of 1000 ml min-1, and purges the air from the system. Immediately after, the gas flow-rate is reduced to 400 ml min-1 and a pre-selected amount of tetrahydroborate solution flows through a capillary and the immersion tube into the bottom of the reaction vessel for a pre-selected time (reaction time). At the end of the reaction time, a second selected purge time (purge 11) follows at a flow-rate of 1000 ml min-1, in order to remove all gases from the system.During all the experiments, after each determination of a blank or standard, the reaction vessel was rinsed with 2 M nitric acid (3 x 5 ml) and doubly distilled water (3 X 5 ml). This treatment, as emphasised by Moody and Lindstrom4 and Reamer et a1.,5 is necessary in order to remove all traces of analyte from the walls of the hydride-generation chamber. In this way, the reproducibility of the results and the stability of the base line were greatly improved. Background correction is usually essential for graphite furnace atomic absorption. However, in hydride systems this is generally not so because the analyte is separated from the sample matrix and only very few elements form hydrides and enter the quartz cell atorniser.6 Volatile organic compounds, which could eventually interfere, are decomposed during preliminary ashing of the sample.Background correction could improve the precision of the results for arsenic and selenium owing to the small absorption (0.010 and 0.040 A s for arsenic and selenium, respectively) from the oxygen present in the purge gas. However, this absorption phenome- non is also the same for the blank. This, together with the instability of the deuterium hollow-cathode lamp at wavelengths less than 200 nm, which increases the base-line noise and the detection limit and influences the precision of low-level measurements dramatically, compared with measurements made using the electrodeless discharge lamp alone, does not permit its use.Reagents Sodium tetrahydroborate(III) solution, 3 Yo m1V. Dissolve 30 g of analytical-reagent grade sodium tetrahydroborate(II1) powder (Merck) and 10 g of analytical-reagent grade sodium hydroxide pellets (UCB) in 400 ml of doubly distilled water, dilute to 1 1 and filter. Store in a refrigerator at 4 "C, where it remains stable for at least 1 week. Sulphuric acid working solution, 1.5% mlV. The solution is obtained by diluting the appropriate volume of 96% mlV Suprapur sulphuric acid (Merck) to 1 1. Nitric acid rinsing solution, 2 M. Prepared by diluting the appropriate volume of 65% mlV analytical-reagent grade nitric acid (Merck) to 1 1. Stock standard solutions of As3+, Sb3+ and Bis+, 1000 mg 1-1. Baker atomic spectral standards.Working standard solutions were prepared fresh daily by dilution with doubly distilled water. Stock standard solution of Se4+. Prepared by diluting Titrisol solution (Merck) containing 1000 g of selenium (as SeOz) to 1 1 with doubly distilled water. Aliquots were diluted with 1.5% mlV hydrochloric acid to obtain appropriate working standard solutions. Stock standard solution of Sn4+. Prepared by diluting a Titrisol solution (Merck) containing 1000 g of tin (SnC14) to 1 1 with 10% mlV hydrochloric acid. A 50-pl volume of this solution was further diluted to 50 ml, and 1 ml further to 5 ml, both with 10% mlV hydrochloric acid, to obtain a 0.2 pg ml-l working standard solution. Carrier Gases High-purity argon. High-purity 99% argon + 1% oxygen (Cargal 1). High-purity nitrogen.These gases were used to purge the system and were obtained from L'Air Liquide, Afdeling Precigaz, Liege , Belgium. Results and Discussion Effect of Gas Transport Tubing Different kinds of plastic and glass transfer tubing were tested in order to study their influence on the determination of 10 ng of As(II1) by using the peak area as a measuring mode. In the first part of the experiments, nitrogen was the carrier gas for the hydrides. Silicone-rubber and nylon tubing gave identical results. The reproducibility was very low and the blank values were unstable. This is the main reason why other workers (see Table 4) have found it difficult to use peak areas, especially at analyte levels of 10 ng. The use of glass tubing did improve the sensitivity and reproducibility but the base line remained unstable.A repeated number of blank determinations returned the absorbance values to the zero level. This indicates that a portion of the hydride was not atomised the first time but was absorbed on the surface of the reaction vessel or the gas transfer tubing, or both. This portion of the hydride was atomised when the repeated number of blank determinations was carried out. This is possibly the main reason for the low reproducibility of the standards. Subsequently, the glass tubing was silanised in the following manner. It was thoroughly cleaned, dried and filled with a 5% mlV dimethyldichlorosilane solution in toluene. The organic solvent was allowed to evaporate at room temperature in a fume-cupboard for 2 h. Finally, the tube was emptied, heated for 1 h in an oven at 110 "C and a nitrogen stream was blown through it for 5 min.As is known, dimethyldichlorosilane reacts with the surface hydroxy groups and deactivates the283 ANALYST, MARCH 1986, VOL. 111 Table 2. Precision of replicate analyses of 10 ng of As(II1) Peak area/A s Gas transfer Carrier tubing gas Silicone-rubber* N2 Glass N2 FEP N2 Silanised glass N2 . . . . . . . . . . . . . . . . . . . . Silicone-rubber . . . . Ar FEP . . . . . . . . Ar Silicone-rubber . . . . 99% V/V Ar - 1% v / v o * Glass . . . . . . . . 99% V/V 1% v / v o 2 Silanisedglass . . . . 99% V/V 1% v/v 0 2 FEP . . . . . . . . 99% V/V 1% v / v o 2 Ar - Ar - Ar - * Silicon-rubber and nylon tubing gave similar results. t After 200-220 determinations.3: After 25-30 determinations. Peak height, A Mean F standard (mean k standard deviation/ deviation) A s 0.062 f 0.004 0.265 k 0.029 0.323 4 0.023 0.382 4 0.013 0.385 F 0.013 0.414 4 0.019 0.479 k 0.011 0.063 k 0.003 0.688 k 0.021-t 0.710 k 0.017 0.082 !c 0.002 0.726 k 0.011$ 0.728 !c 0.009$ No. of analyses 10 10 10 10 10 10 10 10 10 10 Relative standard deviation, YO 10.9 7.1 3.4 3.4 4.6 2.3 3.0 2.4 1.5 1.2 glass surface, and this treatment further improved the sensitivity and reproducibility. When not used, the silanised glass tubing was cleaned with 3 x 3 ml of toluene and 3 x 3 ml of methanol, dried with a stream of nitrogen for 5 min and kept in a desiccator; it is well known that moisture destroys the silanised glass surface. After the tubing had been used for more than 500 determinations within a period of 3 weeks, the silanisation process had to be repeated.When argon was used as the carrier gas, all results were higher than those obtained with nitrogen. This difference could be caused by the lower heat capacity of argon compared with nitrogen, resulting in a higher atomisation temperature when using argon. This is supported by the fact that when nitrogen was used, the cell temperature indicator on the front panel of the controller was frequently switched off during the determination, indicating that the feedback circuit was not able to maintain the temperature at the pre-selected value. This was not so when argon or 99% V/V argon - 1% V/V oxygen was used. The results are shown in Table 2. Effect of 99% V/V Argon - 1% V/V Oxygen as a Purge Gas The use of nylon or silicone-rubber tubing together with 99% V/V argon - 1% V/V oxygen as a purge gas slowly and stably increased the sensitivity, which reached its maximum level after about 200 determinations.The reproducibility was low and memory effects made the method time consuming and impractical. Cleaning the nylon and silicone-rubber tubing with methanol and drying with nitrogen destroyed the previously increased sensitivity. As indicated by Reamer et al. 5 with radiotracer techniques for hydride generators, plastic materials initially retain or decompose a considerable portion of the generated hydride, but as subsequent reactions are performed, the amount retained or decomposed decreases and larger portions can enter the atomiser, resulting in an increase in sensitivity.It is possible that the available absorption sites on the walls of the 40 crn long plastic tubing are being filled with the metal or the hydride and therefore deactivating the surface towards further hydride absorption. On washing the tubing with methanol it is possible that the surface reverted to its original high absorption characteristics, resulting in a decrease in the signal for the same amount of generated hydride. With glass tubing, better results were obtained, but the memory effects still remained a serious problem. Silanised glass and FEP tubing gave the most satisfactory results, with a stable base line even if a standard as high as 50 ng of As(II1) was determined. A series of five standards of 10 ng of As(II1) also did not leave any memory effect.The probability of free atom formation from the hydride is proportional to the number of collisions with free radicals. This indicates that the atomisation efficiency increases as the number of radicals increases. This is the reason why, when a quartz cell is used that has been cleaned with hydrofluoric acid for 15 min, the peak-area values of a repeatedly determined standard slowly increase and after about 30 determinations the sensitivity reaches its maximum value. The surface of the quartz cell has an important effect on sensitivity. It probably catalyses the formation of radicals and subsequently the atomisation of hydrides. The quartz cell can be stored in a desiccator and re-used for another day without any decrease in sensitivity.The replacement of 99% V/V argon - 1% V/V oxygen with nitrogen does not destroy the sensitivity immediately. The peak area of the first standard has the same value as when 99% V/V argon - 1% V/V oxygen is used; that for the second standard will be smaller, the third even smaller, and so on. The population of the radicals in the cell was high enough to atomise the first standard completely. However, with the use of nitrogen, the consumption of radicals is greater than its production, with the result that the second standard will not be completely atomised and will give smaller values. As Welz and Melcher3 suggested, when nitrogen or argon is used, the gas bubbles through the sample solution during the purge time and drives the dissolved air out of the solution.If the purge time is less than 60 s, dissolved oxygen still remains in the sample, and this, together with the hydrogen generated from sodium tetrahydroborate(II1) during the reaction time, pro- duces a limited number of radicals in the heated quartz cell. Results obtained with the use of 99% V/V argon - 1% V/V oxygen are given in Table 2. The sensitivities attained for the different hydride-forming elements using 99% V/V argon - 1% V/V oxygen as a purge284 ANALYST, MARCH 1986, VOL. 111 Table 3. Precision of replicate analyses using silanised glass tubing and 99% v/v argon - 1% v/V oxygen Peak area Mean k standard Element deviation/ (10 ns) A s As(1II) . . 0.726+0.011 Se(1V) . . . . 0.469 k 0.007 Sb(II1) . . . . 0.420+0.010 Bi(V) .. . . 0.442+0.008 Sn(1V) . . . . 0.599+0.015 Relative No. of standard analyses deviation, % 10 1.5 10 1.5 10 2.4 10 1.8 10 2.5 Table 4. Calculated sensitivities obtained by different recent methods Sensitivity Element (long) Absorbance Mode Se(1V) . . . . 0.100 Peak area As(II1) . . . . 0.070 Peak height As(II1) . . . . 0.080 Peakheight Sb(II1) . . . . 0.036 Peakheight Bi(II1) . . . , 0.028 Peak height Se(1V) . . . . 0.025 Peak height Sn(1V) . . . . 0.030 Peak height As(II1) . . . . 0.100 Peak height Sb(II1) . . . . 0.001 Peak height Sb(II1) . . . . 0.017 Peakheight Sb(II1) . . . . 0.045 Peak height Bi* . . . . . . 0.015 Peak height Se(1V) . . . . 0.025 Peak height * Not specified. Reference 7 3 8 9 9 9 9 10 11 12 13 14 15 gas, the operating parameters in Table 1 and silanised glass tubing are listed in Table 3.A comparison with recently reported values in the literature is shown in Table 4. Effect of Reaction Flask Material Reamer et aZ.5 found that glass and polypropylene reaction vessels exhibit the greatest absorption of selenium and silanised glass the least. In our experiments with the same materials, statistically no difference was observed in the peak-area values for 10 ng of selenium. The possible reason is that in this system a 100-fold smaller standard of selenium and a much smaller reaction vessel are used. In addition, the sodium tetrahydroborate(II1) solution flows through a capil- lary into the lowest part of the V-shaped reaction flask, which achieves more complete hydride generation. Conclusions Oxygen has an effect on the determination of volatile hydride-forming elements, not only at low quartz cell tem- peratures, as indicated by Welz and Melcher,3 but also at temperatures above 800 "C, possibly by accelerating the production of radicals that may take part in the atomisation mechanism of the hydrides. The use of silanised glass or FEP tubing aids the transportation of gases and the maximum sensitivity can be attained after a few determinations.This method permits the use of peak areas as a measuring mode for the routine determination of hydride-forming elements. One of the advantages of peak-area over peak-height values is the smaller dependence on or independence of fluctuations of parameters such as the valence state of the analyte, reaction speed and time, sodium tetrahydroborate(II1) concentration, acid concentration, temperature changes of the quartz cell and gas flow-rates. 1. 2. 3. 4. 5 . 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. References Dedina, J., and Rubeska, I., Spectrochim. Acta, Part B, 1980, 35, 119. Welz, B., and Melcher, M., Anal. Chim. Acta, 1981, 131, 17. Welz, B., and Melcher, M., Analyst, 1983, 108, 213. Moody, J. R., and Lindstrom, R. M., Anal. Chem., 1977, 49, 14, 2264. Reamer, D. C., Veillon, C., and Tokousbalides, P. T., Anal. Chem., 1981, 53, 245. Dong So0 Lee, Anal. Chem., 1982, 54, 1682. Lloyd, B., Holt, P., and Delves, H. T., Analyst, 1982,107,927. Welz, B., and Melcher, M., Analyst, 1984, 109, 573. Welz, B., and Melcher, M., Spectrochim. Acta, Part B, 1981, 36, 5 , 439. Siemer, D., Koteel, P., and Jariwala, V., Anal. Chem., 1976, 48, 836. AznArez, J., Palacios, F., Ortega, M. S., and Vidal, J. C., Analyst, 1984, 109, 123. Chapman, J . F., and Dale, L. S . , Anal. Chim. Acta, 1979,111, 137. De Doncker, K., Dumarey, R., Dams, R., and Hoste, J., Anal. Chim. Acta, 1983, 153, 33. Terashima, S . , Anal. Chim. Acta, 1984, 156, 301. Verlinden, M., Baart, J., and Deeistra, H., Talanta, 1980, 27, 633. Paper A51232 Received June 27th, 1985 Accepted September 20th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100281

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 285 Studies on the Determination of Cadmium in Blood by Furnace Atomic Non-thermal Excitation Spectrometry Heinz Falk, Erwin Hoffmann and Christian Ludke Central Institute for Optics and Spectroscopy, Academy of Sciences of GDR, Rudower Chaussee 5, 7799 Berlin, GDR and John M. Ottaway and David Littlejohn Department of Pure and Applied Chemistry, University of Strathclyde, Cathedral Street, Glasgow GI IXL, UK Cadmium atomic emission can be detected in a FANES low-pressure Ar discharge at atomiser temperatures as low as 140 "C when the analyte is present as CdCI2. Cadmium chloride molecules vaporised at this temperature are dissociated by electron impact in the discharge, giving a substantial Cd atom concentration before thermal dissociation of the molecules becomes feasible.This results in a 100-fold greater tolerance towards chloride matrix chemical interferences than encountered for Cd in ETA-AAS with tube-wall atomisation. However, the determination of cadmium in deproteinised whole blood by FANES is not totally interference free and a standard additions procedure is required to give an accurate determination. The FANES instrument detection limit for Cd was calculated to be 0.04 pg I-'. For the deproteinisation procedure applied, the detection limit for cadmium in whole blood was 0.2 pg 1-1. Keywords: Atomic emission spectrometry; low-pressure discharge; electrothermal atomisation; electron- impact molecular dissociation; blood matrix interferences The determination of cadmium in biological samples by atomic absorption spectrometry with electrothermal atomis- ation (ETA-AAS) is well established in clinical analysis.However, as with most volatile elements, the measurement of Cd AAS signals is subject to severe chemical and spectral matrix interference effects when conventional tube-wall atomisation procedures are applied. As cadmium has a comparatively low atom appearance temperature, it is not normally possible to ash biological samples at temperatures that allow the complete removal of organic and inorganic matrix constitutents prior to the atomisation stage. Hence, cadmium atom formation tends to be suppressed by the presence of a large excess of chloride salts, and substantial non-specific background absorption invariably occurs. To minimise the influence of these effects in the determination of Cd in whole blood, a variety of regimes has been implemented including the use of matrix matching, matrix modification, platform atomisation and Zeeman-effect background correc- tion.Stoeppler and Brandtl reduced the mass of carbonaceous matter injected into the atomiser tube by deproteinisation of 50-200-yl volumes of whole blood with 1 M HN03. Pleban and Pearson2 simply diluted whole blood with 5% V/V HN03 for the determination of Cd by Zeeman effect - ETA-AAS using a standard additions procedure. Matrix modification with (NH4)2HP04 was applied by Subramanian and Meranger3 to determine Cd directly in blood. A similar procedure was reported by Delves and Woodward,4 but oxygen was added to the atomiser gas flow during the ashing stage to assist removal of the blood matrix.Matrix modification was also used by Hinderberger et al. 5 in conjunction with platform atomisation, and Claeys-Thoread employed platform atomisation and Zeeman-effect background correction for the determination of Cd in blood diluted 1 + 9 with Triton X-100. It is generally accepted that the mechanism of cadmium atomisation depends on the chemical nature of the sample matrix. When present as the chloride, CdC12, cadmium atoms are formed by thermal dissociation of gaseous CdCl molecules produced on vaporisation of CdCI2. In the presence of an excess of chloride salts, however, atom formation is impaired, as reported by Barnard and Fishman7 for solutions containing 1% mlV NaCl, KCI, MgC12 or CaC12. Similar observations for tube-wall atomisation have been reported by other workers .a9 When cadmium is present as an oxy-anion two mechanisms of atom formation are feasible.In an early study, Campbell and Ottawaylo suggested that CdO was reduced on the graphite surface to produce Cd atoms. Salmon and HoIcombell also supported the oxide reduction mechanism and have postu- lated that metallic Cd is formed on the graphite tube. In contrast, L'vov and Ryabchuk12 believe that CdO is dis- sociated in the vapour state. This mechanism was also suggested by Sturgeon and Chakrabarti13 and has been supported by more recent work by Sturgeon and co- workers. 1 4 ~ 5 In this work, the atomisation of cadmium from chloride and nitrate matrices has been studied as part of an investigation aimed at the development of a method for the determination of Cd in whole blood by furnace atomic non-thermal excitation spectrometry (FANES).The characteristics of furnace atomisation with non-thermal excitation have been described in recent publications. 16-19 The technique involves conventional electrothermal atomisation of samples in a tube atomiser in which a low-pressure gas discharge is simul- taneously generated using the graphite tube as the cathode. The system therefore combines the efficiency of electrother- mal vaporisation and atomisation with the high excitation capability of a hollow-cathode type discharge. Although thermal excitation in a conventional electrothermal atomiser allows the measurement of atomic emission signals for many elements with high sensitivity,20>21 energy levels greater than 4 eV are not significantly populated.In contrast, the FANES source is ideally suited to the excitation of metals and non-metals with high excitation potentials (e.g., cadmium, zinc, selenium and halogens). Hence FANES combines many of the attractive features of ETA-AAS and plasma emission spectrometry, not least of which are sub-pg 1-1 detection limits and the ease of operation in a simultaneous multi-element mode. In previous FANES studies, the instrumental features of the system have been described and the analytical characteristics of the source established with respect to analyte detection limits.1619 However, to date, the atomiser has been applied to relatively few analytical problems and a primary aim of this work was to assess the suitability of the FANES method for286 ANALYST, MARCH 1986, VOL.111 the determination of a relatively volatile element such as cadmium in a complex matrix such as whole blood. As the FANES atomiser is operated at low pressure and the sample is vaporised into a low-pressure discharge, it was expected that the analytical behaviour of the source would be different to that of a conventional electrothermal atomiser as applied in atomic absorption spectrometry for this analysis. The influ- ence of alkali and alkaline earth metal salts on the vaporis- ation and atomisation of cadmium in the FANES atomiser has been studied and compared with observations reported in the ETA-AAS literature. The results of the study indicated that the presence of a high-energy discharge assists the dissociation of vapour-phase molecules and for this reason the influence of NaCl, KCl, MgC12 and CaC12 on Cd atom production is less pronounced than in a conventional electrothermal atomiser.Experimental Instrumentation The FANES source, which has been decribed in detail in previous publications,1”19 consisted of a sealed atomiser chamber and power supply unit with separate functions that controlled the heating of the graphite tube and the establish- ment of the discharge. The atomiser was water cooled, and connected to a mechanical pump for evacuation and an argon gas supply system for purging of the tube and formation of the low-pressure Ar discharge. The FANES atomiser tube was similar in dimensions to that used in the Perkin-Elmer HGA-500 graphite furnace.With the exception of the discharge pressure (1-20 Torr) and current (15-60 mA), which were set manually, all parameters were under microprocessor control. A series of up to ten temperature stages could be selected via the instru- ment’s microcomputer. At each step in the temperature programme the operator selected a temperature (up to 3000 “C), a linear ramp rate (0-2000 “C s-1) and a hold time. On the basis of the selected temperature and ramp rate, the microprocessor computed the ramp time. The selection of atomiser gas flow-rate at atmospheric pressure, the initiation of the evacuation stage and the establishment of the discharge were also under computer control and the required conditions were selected when compiling the various steps in the atomiser programme.In this study, the FANES source was operated in conjunc- tion with a laboratory-contructed 1.5-m Rowland circle polychromator equipped with a 2000 grooves mm-1 grating. All cadmium atomic emission measurements were made at the Cd 228.8-nm resonance line wavelength with a spectral band pass of 0.017 nm. Emission signals were recorded on a K 201 Jenoptik chart recorder. For atomic absorption measure- ments, a Cd hollow-cathode lamp was positioned such that the lamp radiation was focused through the FANES atomiser and on to the polychromator entrance slit. FANES Operational Procedure The operation of the FANES source is analogous to that used in conventional atomisation. The main difference in the furnace programme occurs prior to and during the atomisation stage.A sample aliquot (typically 10 or 20 ~ 1 ) was injected into the graphite tube by means of a micropipette and dried and ashed at atmospheric pressure with the injection port lid open and a purge gas flow-rate of 80 1 h-1 to remove matrix vapours from the atomiser. The injection port lid was then closed, the atomiser evacuated to a pressure of about lo-* Torr, the argon discharge gas pressure established (1-20 Torr) and the discharge formed. This sequence was initiated by selection of the appropriate parameters in the programme step prior to the intended atomisation stage. The conditions of pump-down could be arranged to allow a continuation of the ashing step during the evacuation procedure. As in a conventional atomisation regime, the temperature and ramp rate of the atomisation stage were normally selected to give rapid vaporisation and atomisation of the analyte.In the discharge the analyte atoms were excited and the resulting emission signal detected with the polychromator. When atomic absorp- tion measurements were required with the FANES atomiser, the programme was modified to either prevent formation of a discharge or eliminate the evacuation - discharge sequence entirely. The optimum atomiser programme devised for the determi- nation of cadmium in blood is given in Table 1. Modified versions of the programme employed in the chloride salt interference studies are indicated in the appropriate section of the text. Preparation of Whole Blood Samples To minimise the mass of carbonaceous material atomised in the FANES source a deproteinisation procedure similar to that developed by Stoeppler and Brandtl was applied.A 200-y1 volume of whole blood was mixed with 200 yl of water or aqueous calibration solution, 50 yl of concentrated nitric acid were added and the mixture was centrifuged for 7 min to produce a clear supernatant solution over the protein precipi- tate. Volumes of the deproteinised solution (10 or 20 yl) were then injected into the FANES atomiser for the determination of cadmium. Table 1. FANES Step Dry? . . . Ash . . . . . Purge$ . . . Evacuate . . . Pre-atomisation11 Atomise . . . Clean . . . programme for the determination of cadmium in deproteinised blood Temperature/ “C . . . 150 . . . 350 . . . 35 . . . 35 .. 100 . . . 600 . . . 2000 Ramp rate/ “Cs-* 3 100 NPO 0 30 600 1000 Ramp time*/ 45.0 2.0 6.7 0 2.1 0.8 1.9 S Hold time/ 5 28 5 45 90 9 3 S Total step time/ S 50 30 11.7 45 92.1 9.8 4.9 Argon pressure/ Functions Torr 760 760 760 E457 19 19 19 760 Gas flow- rate/ 1 h-I 80 80 80 0 0 0 80 * Calculated from temperature difference and ramp rate selected; for the first step an ambient temperature of 15 “C was assumed. t Conditions for 10-pl injection volumes. $ Additional step used in this application to remove matrix vapour without tube heating. 0 NP implies “no power.” fl When the “E” function is selected evacuation begins at the start of the step, with discharge on after 45 s in this instance; 30 mA current 11 Step used to stabilise discharge before atomisation; could have been redued to 20-30 s.selected; discharge on until end of atornise step.ANALYST, MARCH 1986, VOL. 111 287 Reagents Stock solutions of CdC12, NaCl, KCl, MgC12 and CaC12 were prepared by dissolving salts of the highest available purity in distilled water. High-purity nitric acid was used for the deproteinisation of blood samples and for the addition to CdC12 standard solutions. Results and Discussion Vaporisation Characteristics of Cadmium Salts in the FANES Atomiser It was considered probable that the vaporisation charcteristics of cadmium salts in FANES would be different to those observed in conventional ETA-AAS as atomisation occurs at low pressure and in the environment of a low-pressure discharge. To investigate the influence of both conditions on cadmium atom production in the FANES atomiser, atomic absorption and atomic emission measurements were obtained following vaporisation of cadmium from chloride and nitrate media.In initial experiments, cadmium atomic emission and atomic absorption signals were obtained for the vaporisation of 10-y1 volumes of a 100 pg 1-1 cadmium solution prepared from CdCI2. Emission or absorption signals were obtained for FANES atomisation temperatures in the range 100-1000 "C, at a linear ramp rate of 600 "C s-1. The emission measure- ments were obtained in the presence of an Ar discharge at 17 Torr, and absorption measurements were made without a discharge at 17 Torr or at atmospheric pressure. At an argon pressure of 17 Torr the maximum cadmium atomic emission and atomic absorption signals were obtained at a similar temperature of about 520 "C as measured with a thermo- couple.The first appearance of a cadmium atomic emission signal occurred at a tube temperature of 140 "C. In contrast, however, without the presence of a discharge, atomic absorp- tion signals at 17 Torr were not measured until temperatures above 300 "C. Plots of the relative integrated cadmium atomic emission and atomic absorption signals obtained at 17 Torr for atomisation temperatures in the range 100-1000 "C are illustrated in Fig. 1, together with the corresponding plot for cadmium atomic absorption at atmospheric pressure. The fact that the cadmium emission signal appeared at a considerably lower temperature than the atomic absorption signal at the same pressure suggests that at 17 Torr vaporisation of CdC12 begins at temperatures as low as 140 "C and that dissociation of gaseous CdCl or CdC12 molecules by electron impact in the discharge results in the formation of cadmium atoms at atomiser temperatures much lower than would normally be expected.Without the assistance of the discharge, dissociation of vaporised CdCl - CdC12 molecules i9 a purely thermal process and at 17 Torr did not occur until the atomiser tube had reached a temperature of approximately 300 "C. At atmospheric pressure significant cadmium atom formation did not occur until about 360-380 "C and the maximum AAS signal was obtained at about 680 "C. When an excess of chloride salts is present in solution, the vaporisation characteristics of CdC12 are altered noticeably.Fig. 2 shows the vaporisation curves for cadmium atomic emission obtained at 10 and 17 Torr in the presence of 0.1% mlV NaCl and 0.05% mlV MgC12 when the atomiser tube was heated to temperatures in the range 100-1000 "C at a ramp rate of 600 "C s-1. At 17 Tori the first appearance of cadmium atomic emission occurs at about 320 "C, probably because the evaporation of CdC12 is impaired owing to the occlusion of the cadmium salt in the NaCl - MgC12 matrix. When the discharge pressure is reduced to 10 Torr, evaporation of the chloride salt matrix occurs at a lower temperature and the first appearance of cadmium atomic emission occurs at 200 "C. Fig. 2 also indicates the vaporisation curve for cadmium in the presence of 0.01% V/V HN03. Although the cadmium solution was prepared from CdC12, the presence of an excess of oxy-anion probably ensured the formation of CdO, which has a higher vaporisation temperature than CdC12.A comparison of the vaporisation curves for CdCI2 and "CdO" in Figs. 1 and 2, respectively, (17 Torr) suggests that the dissociation of the gaseous CdO molecules by electron impact in the discharge is less efficient than dissociation of CdCl or CdCI2 by the same process. Influence of Chloride Salts on Cadmium Atomic Emission Intensity in FANES From ETA-AAS studies based on tube-wall atomisation,9 it is known that a matrix of 0.01% mlV NaCl and 0.005% mlV MgC12 causes a 10% depression of the Cd AAS signal. As electron impact in the FANES discharge apparently assists the dissociation of gaseous CdCl - CdCI2 molecules, it was expected that chloride interference effects would be less severe in FANES.Solutions containing 2, 5 , 10 and 20 pg 1-1 of Cd as CdC12 were prepared in aqueous, 0.01% VlV HN03 and 0.1% mlV NaCl - 0.05% mlV MgC12 solutions. For each solution, 10-yl volumes were injected into the FANES atomiser, dried at 150 "C with a ramp rate of 5 "C s-1 and then atomised into the discharge under atomisation conditions of 750 "C and 600 "C s-1. Cadmium atomic emission signals obtained for an Ar pressure of 19 Torr and a discharge current of 30 mA are illustrated in the form of calibration graphs in Fig. 3. No significant difference was observed in the Cd atomic emission intensities obtained for each solution matrix. The slight curvature in the calibration graph observed at 20 pg 1-1 of Cd was probably due to self-absorption of the Cd line profile.As there was no apparent interference of 0.1% mlV NaCl and 0.05% mlV MgC12 on the Cd atomic emission signal, the interferent concentrations were increased to determine the ? 00 200 300 400 500 600 7 Atomiser temperature/"C Fig. 1. Relative integrated atomic emission and atomic absorption signals for 10 p1 of 100 pg 1-1 of Cd at different atomisation temperatures. (A) FANES, 17 Torr Ar, 30 mA; (B) AAS, 17 Torr Ar; (C) AAS, 1 atm Ar. Plotted relative to individual maximum signals obtained at 520 "C for A and B and 680 "C for C 200 300 400 500 600 700 Y - 100 Atomiser temperaturei'c Fig. 2. Relative integrated FANES intensities for 10 p1 of 100 pg I-' Cd in different matrix solutions and at different atomisation tempera- tures.(A) 10 Torr Ar, 30 mA, 0.1% mlV NaCl - 0.05% m/V M CI2; (B) 17 Torr Ar, 30 mA, 0.1% mlV NaCl - 0.05% mlV MgCI,;((!) 17 Torr Ar, 30 mA, 0.01% VlV HN03. Plotted relative to individual maximum signals at 580 "C for A and 650 "C for B and C288 ANALYST, MARCH 1986, VOL. 111 interference-free limit for FANES Cd determinations. Solu- tions containing 10 pg 1-1 of Cd and various concentrations of NaCl, KCI, MgC12 and CaCI2 were prepared, up to levels in excess of the expected concentrations of chloride salts in undiluted whole blood. As shown in Fig. 4, a significant reduction in the Cd atomic emission intensity occurred only at a combined chloride concentration of 1.0% mlV NaCl, 1.0% 80 I 1 Concentration/pg I - Fig.3. FANES calibration raphs for Cd in (0) aqueous solution; ( X ) 0.01% V/VHN03; and (A) 0.1% mlVNaCl- 0.05% mlVMgC1,; conditions as indicated in the text E 50 I .$ 1 C w . + - 0.125 0.25 0.5 1.0 NaCl and KCI 4- plus Z 0.063 0.125 0.25 0.5 MgCI, a n d CaCI2 Matrix concentration, Ol0 m/V Fig. 4. Effect of various combined concentrations of NaCl, KCI, MgCI2 and CaCI, on the FANES atomic emission intensity for 10 yl of 10 pg 1-1 of Cd 50 I I A 40 I / \ C 3 2 30 E e 4- .- 10 0 10 20 30 40 50 60 Ash timeis Fig. 5.. Influence of ashing time on the FANES atomic emission intensity for 10 p1 of 20 pg 1-l Cd at ashing temperatures of (A) 300, (B) 350 and ( C ) 400 "C mlV KCl, 0.5% mlV MgC12 and 0.5% mlV CaC12, about two orders of magnitude higher than the onset of chloride interference in conventional ETA-AAS.9 Determination of Cadmium in Whole Blood Although the deproteinisation procedure described previ- ously was used to minimise the mass of carbonaceous material injected into the atomiser, sufficient of the blood matrix remained to merit the inclusion of an ashing step in the analytical programme.Ashing temperatures of 300, 350 and 400 "C were investigated. The Cd atomic emission signals obtained for 10-pl injection volumes of a 20 pg 1-1 solution after ashing at the above temperatures for 2-60 s are given in Fig. 5. Optimum ashing conditions of 350 "C for 30 s were selected as indicated in Table 1. The cadmium atomic emission vaporisation curve for a deproteinised blood solution containing 10% VlV HN03 was obtained and compared with the corresponding curve for an aqueous Cd solution containing the same HN03 concentra- tion.The cadmium concentration in both solutions was 20 pg 1-1 and separate 10-pl injection volumes were dried, ashed and then atomised at a ramp rate of 600 "C s-1 to various atomisation temperatures in the range 100-700 "C at 100 "C intervals. Although, as indicated in Fig. 6, there appears to be no significant difference in the vaporisation curves obtained for the two solutions, closer inspection in comparison with the vaporisation curves at 17 Torr in Fig. 2 suggests that the chloride salts present in the deproteinised blood solution may still influence the vaporisation of cadmium even though HN03 is present in a large excess. From the results presented in Figs.2-6, it was concluded that the chloride and organic constituents of the deproteinised blood sample were unlikely to exert a severe interference on the determination of cadmium by FANES. However, as indicated in Fig. 7, there was a substantial difference in the slopes of the deproteinised blood standard additions graph and the aqueous cadmium calibration graph when both sets of solutions contained an equal concentration of HN03. For Cd concentrations in the range 2.5-20 pg 1-1, the atomic emission intensity for the deproteinised blood sample was only 20% of the equivalent signal for the HN03 solution. The deproteini- sation process may not have released all the Cd attached to blood protein, which would result in a lower than expected concentration of the metal in the supernatant liquid injected into the FANES atomiser.However, previous experience with the protein precipitation procedure suggests that at a nitric acid concentration of 10% V/V incomplete release of Cd is unlikely. An alternative explanation is that residual organic components and inorganic species, other than chloride salts, I ' I / 1 I 1 4 200 300 400 500 600 700 Atom iser temperatu re/"C Fig. 6. Relative integrated FANES intensities for 10 pl of 20 pg 1- of Cd in (A) deproteinised whole blood and (B) 10% V/V HN03 at different atomisation temperatures. Conditions as in text and Table 1 ; results plotted relative to individual maximum signals at 750 "CANALYST, MARCH 1986, VOL. 111 289 vaporised during the atomisation stage altered the discharge conditions, which may have had an adverse effect on the cadmium atom excitation.As it was not possible to analyse deproteinised blood samples by direct comparison with cadmium standard solu- tions prepared in nitric acid, a standard additions procedure was used to determine the Cd concentration of two whole blood samples supplied by the Biochemistry Department at Glasgow Royal Infirmary. The values obtained are given in Table 2, together with the concentrations determined by ETA-AAS at the hospital. Good agreement was achieved between the two techniques. The Cd detection limit of the FANES standard additions procedure was calculated to be 0.2 yg 1-1 on a 20 basis. For aqueous solutions containing 10% V/V HN03 the Cd detection limit was a factor of 5 lower, 0.04 yg 1-1.The relative standard deviation of the standard additions procedure was about 10% for the two blood samples analysed. This is a precision of at least a factor of two poorer than would normally be expected for conventional ETA-AAS with manual pipetting. The precision was impaired by the spreading of the nitric acid solution droplets in the tube during the drying sequence, and was also influenced by the blank correction required to take account of the cadmium content of the nitric acid available during this study. Conclusions The measurements reported in this work confirmed the widely held opinion that when present as CdCI2, cadmium atom formation proceeds through the dissociation of gaseous CdCl 20 0 5 10 15 Concentrationivg I - Fig. 7. (B) deproteinised whole blood.Conditions as in text and Table 1 FANES calibration graphs for Cd in (A) 10% V/VHN03 and Table 2. Determination of cadmium in deproteinised whole blood by FANES and ETA-AAS Concentrationipg 1- * Sample FANES ETA-AAS" 1 19.5 k 2.0 19.6 2 8.5 t 0.9 8.8 * Samples and values provided by Glasgow Royal Infirmary, Department of Clinical Biochemistry. molecules. The dissociation is assisted in the FANES dis- charge by electron impact and Cd atoms are formed at temperatures as low as 140 "C at 17 Torr. Without the action of the low-pressure discharge, thermal dissociation of CdCl molecules does not occur until around 300 "C at this pressure. Although an excess concentration of alkali and alkaline earth metal chloride salts retards the vaporisation of CdCI2, no significant chemical interferences were encountered until the combined concentrations of NaC1, KCI, etc., had reached 2-3% mlV. In an oxy-anion medium ( e .g . , HN03), it is likely that CdO is formed at some stage in the atomisation process. The vaporisation studies conducted with FANES for Cd in HN03 do not prove conclusively that Cd atoms are formed by the dissociation of gaseous CdO molecules. However, at low pressure the volatility of CdO will undoubtedly increase and it is possible that electron impact in the discharge will assist the thermal dissociation of CdO. Additional measurements of both cadmium atomic absorption and atomic emission signals are required in this instance to give a clear indication of the atomisation mechanism in the FANES atomiser.The chemical interferences encountered in the analysis of deproteinised blood may be due to the action of organic and inorganic constituents (other than chloride salts) on the nature of the discharge. Further fundamental studies of the excitation conditions during atomisation are required in order to assess the influence of the blood matrix in the determination of Cd. Experiments of this nature are in progress. However, the FANES atomiser clearly has potential advantages over conventional ETA-AAS with regard to its tolerance to chloride salt interferences. This work was made possible by the Cultural Exchange Agreement between the Royal Society in the UK and the Academy of Sciences of the GDR. The authors are extremely grateful for the opportunity for collaborative study and for the financial support provided through the Exchange Scheme.Financial support from the Pye Foundation (for D. L.) is also gratefully acknowledged. The authors thank Dr. G. S. Fell and Dr. D. J. Halls, Department of Clinical Biochemistry, Glasgow Royal Infirmary, for the provision of blood samples. 1. 2. 3. 4. 5 . 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. References Stoeppler, M., and Brandt, K., Fresenius 2. Anal. Chem., 1980, 300, 372. Pleban, P. A., and Pearson, K. H., Clin. Chim. Acta, 1979,99, 267. Subramanian, K. S . , and Meranger, J. C., Clin. Chem., 1981, 27, 1866. Delves, H. T., and Woodward, J., At. Spectrosc., 1981, 2, 65. Hinderberger, E. J., Kaiser, M. L., and Koirtyohann, S. R., At. Spectrosc., 1981, 2, 8. Claeys-Thoreau, F., At. Spectrosc., 1982, 3, 188. Barnard, W. M., and Fishman, M. J . , At. Absorpt. Newsl., 1973, 12, 118. Cruz, R. B., and Van Loon, J. C., Anal. Chim. Acta, 1974,72, 231. Campbell, W. C., and Ottaway, J. M., Analyst, 1977,102,495. Campbell, W. C., and Ottaway, J. M., Talanta, 1974,21,837. Salmon, S. G., and Holcombe, J. A., Anal. Chem., 1982, 54, 630. L'vov, B. V., and Ryabchuk, G. N., Spectrochim. Acta, Part B , 1982, 37, 673. Sturgeon, R. E., and Chakrabarti, C. L., Prog. Anal. At. Spectrosc., 1978, 1, 132. Sturgeon, R. E., Siu, K. W. M., and Berman, S . S., Spectrochirn. Acta, Part B, 1984, 39, 213. Sturgeon, R. E., and Berman, S. S . , Anal. Chem., 1985, 57, 1268.290 ANALYST, MARCH 1986, VOL. 111 16. 17. 18. 19. Falk, H . , Hoffmann, E., and Ludke, Ch., Spectrochim. Acta, Part B , 1981, 36,767. Falk, H., Hoffmann, E., and Ludke, Ch., Fresenius 2. Anal. Chem., 1981, 307, 362. Falk, H . , Hoffmann, E., Ludke, Ch., Ottaway, J. M., and Giri, S. K., Analyst, 1983, 108, 1459. Eichardt, K., and Falk, H., Jenaer Rundsch., 1983, 28, 118. 20. 21. Bezur, L., Marshall, J., Ottaway, J. M ., and Fakhrul-Aldeen, R., Analyst, 1983, 108, 553. Giri, S. K., Littlejohn, D., and Ottaway, J. M., Analyst, 1982, 107, 1095. Paper A51271 Received July 23rd, 1985 Accepted October 9th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100285

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 291 Alkyl Cyanide Medium for the Determination of Precious Metals by Atomic Absorption Spectrometry R. Le Houillier and C. De Blois Ministere de I'Energie et des Ressources, Centre de Recherches Minerales, 2700 Rue Einstein, Sainte-Fo y, Quebec, Canada G I P 3 W8 Concentrations ranging from parts per billion levels up to a few parts per million can be adequately determined for platinum, palladium and gold pre-concentrated in a silver bead, when dissolved in an alkaline cyanide solution, by the use of AAS, with a vanadium buffer to correct for precious metal interference. The same medium is also proposed for the determination of silver, platinum, palladium and rhodium pre-concentrated in a gold bead; however, the rhodium recovery is poor. The difficulty in recovering rhodium is not associated with the medium of the final solution but rather with losses produced by the possible formation of lead rhodium oxide during cupellation and with the acid-insoluble flakes containing lead and rhodium.Keywords: Precious metal determination; atomic absorption spectrometry; fire assay pre-concentration; vanadium buffer; alkaline cyanide solution Various methods for the determination of noble metals by atomic absorption spectrometry (AAS) and fire assay have been published and reviewed.l-5 The classical Tire assay is an excellent technique for collecting and concentrating in a bead noble metals from rocks, ores, minerals and other matrices prior to AAS determination. When the beads so produced are dissolved in acid, the contents of noble metals such as silver, gold and palladium can be determined by AAS with good precision.Indeed, Kallman and Hobart6 reported that solu- tions of 100 ng ml-1 of silver and 300 ng ml-1 of gold and palladium can be analysed by AAS with a precision of k 1%. To achieve such results, hydrochloric acid containing a significant amount of gold must be avoided, owing to the possibility of reducing gold in the capillary of the aspirator.6 Moreover, the solubility of silver chloride in a solution containing less than 25% VlV hydrochloric acid is limited. In addition, any precipitation of silver from such a solution containing gold implies a high risk of loss of gold by coprecipitation. Further, silver salts are less soluble in a small volume of hydrochloric acid (25% VIV) than in the same volume of an alkaline cyanide solution.For this reason, occlusion of gold and other precious metals in silver salts may be lost, which is responsible for poor recoveries when a certain amount of a silver salt containing precious metals is not solubilised. However, stable complexes of silver , gold, plati- num, palladium and, to a certain extent, even rhodium are formed in cyanide solutions. The determination of the noble metals by AAS with the use of such a medium is very attractive as dissolution of silver is more rapid and the risk of losses through coprecipitation is avoided. However, interfer- ences between the noble metals in such a medium have not been thoroughly studied. In this paper, an alkaline cyanide medium is proposed for the determination of gold, silver, palladium, platinum and rhodium by AAS in rocks, ores and minerals from parts per billion levels up to 3 p.p.m., after pre-concentration of the noble metals by the fire assay procedure.Interferences are reported and a buffer is proposed. The accuracy and precision obtained for the procedure are presented. Experimental Standard Solutions Standard solutions of precious metals were prepared from Specpure (Johnson Matthey Chemicals) chloroammonium salts of rhodium and palladium, platinum sponge and metallic gold. Silver solutions were prepared from pure silver nitrate (Aldrich). Prepare standard solutions by dissolving weighed amounts of chloroammonium salts of rhodium and palladium in water. Add HCl and dilute with water to obtain a final solution containing 7.5% VlV of HC1.Dissolve in the same medium platinum and gold salts produced by dissolution of the pure metals in aqua regia and evaporate a given volume of this solution to dryness. Dissolve the salts in water and dilute with a solution containing 5% mlV each of KCN and KOH. Add a solution containing 2% mlV of vanadium to obtain a final medium containing 0.5% mlVof KCN and KOH and 1% mlV of vanadium. For silver solutions, dissolve silver nitrate in water, then follow the same procedure as above. The KOH is necessary both as a safety measure, owing to the health hazard with cyanide, and to dissolve any lead remaining in the bead. Buffer Solution To prepare the buffer solution, dissolve 24.0 g of NaV03 overnight, without heating, in 400 ml of demineralised water.After filtration through a Whatman No. 44 filter-paper introduce the solution into a 500-ml calibrated flask and dilute to volume. This solution contains 2% mlV of vanadium. It is important that the demineralised water be low in oxygen in order to avoid the formation of insoluble vanadium oxide. Fire Assay Follow the standard fire assay procedure for collecting and concentrating the noble metals in a bead. Lead was used as the collector throughout the study owing to its technical sim- plicity. The amount of sample used for fire assay was 15 g. For recovery studies, precious metals in solution are added to 15 g of pure quartz and flux mixture and the whole charge is dried before fusion. In general, the lead button containing the precious metals weighs about 25 g. Silver as a Collector As good practice in the fire assay, add silver (10 mg) to collect gold, platinum and palladium in a bead that can be easily dissolved in an acid solution.Conduct the fusion step at 1000 "C but the cupellation must be completed at ca. 940 "C when platinoids are present in order to obtain a bead of low292 ANALYST, MARCH 1986, VOL. 111 lead content. If the cupellation step is not conducted properly, dissolution of the noble salts in alkaline cyanide solution may be a source of problems, because of lead. For the determi- nation of gold, when only gold is present, the silver bead can be produced at a lower cupellation temperature, i.e., about 870 "C. Gold as a Collector Add 5 mg of gold in the fusion step to collect trace amounts of rhodium, platinum and palladium.Perform the fusion at 1000 "C and the cupellation at 940 "C. For silver analysis only, a gold bead obtained after cupellation at 870 "C is adequate. Bead Treatment Treat a silver bead containing gold, platinum and palladium or a gold bead containing platinum, palladium and rhodium in a 30-ml beaker with 5 ml of hot dilute hydrochloric acid YO VlV) to dissolve any gangue left on the surface of the bead after cupellation. Such gangue must be eliminated prior to the dissolution of noble metal salts in alkaline cyanide medium in order to avoid precipitation and thus metal losses.7 Discard the wash solution and treat the bead with 2 ml of hot nitric acid. When no further reaction is observed, add 6 ml of concentrated hydrochloric acid and keep the solution warm for about 10 min, then evaporate the solution to dryness at a low temperature. One advantage of the cyanide dissolution over the use of an acid medium is that if the salts are dried at too high a temperature, precious metal salts may be decom- posed to the metallic state.The cyanide dissolution will still allow a good recovery of those precious metals susceptible to such a reaction. Alkaline Cyanide Dissolution of Noble Metal Salts Add to noble metal salts 1 ml of 5% mlVKCN - 5% mlVKOH solution and dilute the solution to 5 ml with water. Just before the AAS measurements, add 5 ml of sodium metavanadate solution containing 2% mlV of vanadium to eliminate interferences. This addition is carried out just before the AAS measurements because the final solution has a stability of about 4 h.The final volume of solution is 10 ml and it contains 1% mlV of vanadium and 0.5% mlV each of KCN and KOH. Silver and Gold Beads With a silver bead containing only gold, or vice versa, the washing and dissolution procedure is similar to that described above. However, after evaporation to dryness, dissolve the salts in only 0.5 ml of the 5% mlV KCN - KOH solution, then dilute to 5 ml. No buffer for interference suppression is needed for silver and gold determinations only. The final volume of solution is 5 ml and it contains 0.5% mlV each of KCN and KOH. Atomic Absorption Measurements A Varian AA-875 atomic absorption spectrometer, equipped with automatic gas control, an automated background correc- tion system, an air - acetylene burner and an adjustable barrel nebuliser, was used.The wavelengths used to optimise the instrumental parameters with standard solutions are given in Table 1. Table 1 also gives the sensitivities and detection limits (20) obtained for Pt, Pd and Rh dissolved in 0.5% rnlVKCN - KOH - 1% m/V vanadium solution. For gold and silver, the analytical values are valid for vanadium-free solutions. Results and Discussion Interferences In general, the addition of KOH to a cyanide solution enhances the absorbance of the noble metals. The greatest increases are observed with platinum and rhodium in 2% mlV KOH solution. For 8 pg ml-1 of platinum an absorbance increase of 124% is obtained, whereas for 0.8 pg ml-l of rhodium a 40% enhancement is typical.However, at 0.1% mlV KOH, a marked decrease in absorbance is encountered for silver; the decrease is less pronounced for gold. The absorbance of these metals is re-established at a higher concentration of KOH (0.5% mlV). Small silver, platinum and palladium absorbance enhance- ments are observed with increasing concentration of KCN in solution. However, a 54% increase in gold absorbance is obtained in 1% mlV KCN solution. KCN interferes differently with rhodium. A maximum 24% decrease in the absorbance of rhodium occurs at approximately 0.4% mlV KCN. Increasing the KCN concentration to 2% mlV results in a higher absorbance than before the decrease. Table 1. Instrumental parameters and analytical values Lamp Detection limit */ Wavelength/ Slit/ current/ Background Sensitivity/ Element nm nrn mA corrector pg ml-1 ng g-l Au .. . . . . 242.8 1 .o 4 Yes 0.15 15 Ag . . . . . . 328.1 1 .o 3 Yes 0.027 2 Pt 266.0 0.5 10 Yes 0.81 80 Pd . . . . . . 244.8 0.5 5 Yes 0.11 12 Rh 343.5 0.5 5 No 0.065 7 . . . . . . . . . . . . * Detection limits for Au and Ag are valid for 5 ml of final solution compared with 10 ml for all the other elements. These detection limits refer to the original 15-g sample. Table 2. Interferences detected in 0.5% m/V KCN - KOH solution without vanadium buffer Change of analyte absorbance, YO Concentration/ Au 7 Ag Pt , 7 Rh 9 Interferent pg ml-1 l.Opgmi-~ l.Opgml-~ 8.0pgml-1 0.8pgml-1 0.8pgml-l . . . . . . 0 - 34 0 - 32 Au 1500 - Ag . . . . . . 2000 0 0 0 0 Pt . . . . . . 30 0 0 -36 - 73 - 33 Pd .. . . . . 15 0 0 - 40 Rh . . . . . . 15 0 0 - 56 - 22 - - - -ANALYST, MARCH 1986, VOL. 11 1 293 Table 3. Interferent concentrations in 0.5% m/V KCN - KOH - 1% m/V vanadium solution for which vanadium buffer is efficient Interferent concentration/pg ml- 1 Concentration/ Analyte pg ml-1 Ag Au Pt Pd Rh . . . . . . 30 15 15 Au 1 .o 2000 - Ag . . . . . . 1 .o 2000 30 15 15 . . . . . . 50 5 Pt 8.0 1500 500 - 20 Pd . . . . . . 0.8 1500 1000 20 Rh . . . . . . 0.8 1500 200 10 10 - - - Most of the important absorbance changes of the noble metals mentioned above are markedly decreased when a solution containing the same amounts of both KCN and KOH is used. For example, silver, gold and palladium give an almost constant absorbance when solutions containing &lY0 m/V of both KCN and KOH are used.However, rhodium and platinum still show an absorbance increase. Nevertheless, this is not a problem as the final solution of noble metals is always adjusted to contain 0.5% mlV KCN - KOH. Interferences encountered between precious metals dissolved in alkaline cyanide solutions containing 0.5% m/V KCN - KOH are presented in Table 2. No change in analyte absorbance is observed for gold and silver with the concentrations of interferent and analyte reported. The worst situation occurs between platinum and rhodium. It is worth mentioning that, for 1 pg ml-l of lead in solution, no interference with gold, silver, platinum, palladium or rhodium is observed with the concentration of each analyte reported in Table 2. Interferences are eliminated in such alkaline cyanide solutions by the addition of sodium metavanadate.Table 3 reports the analyte and interferent concentrations for which a buffer solution containing 1% mlVof vanadium eliminates the change in analyte absorbance associated with the action of such interferents. It is worth mentioning that 0.15-1% m/V of vanadium in solution increases the absorbance of 1 pg ml-l of rhodium and platinum by 74 and 135%, respectively. However, vanadium has no releasing action on the absorbance of the same concentration of palladium. When only silver and gold are present in a sample, the addition of vanadium can be omitted as there is no significant interference. Procedure Testing Typical fire assay beads resulting from the complete decompo- sition of 15 g of pure quartz, to which a single addition of different amounts of a precious metal were added in solution, were analysed by AAS.Table 4 gives the over-all recovery obtained with a silver bead. No results are given for rhodium as it is known that silver is not a recommended collector for rhodium. Table 5 gives the recoveries of palladium, platinum and rhodium obtained when 5 mg of gold was used as a collector. Palladium gives the best recovery. Gold and platinum have recoveries that tend to be high, between 0.5 and 3 pg (33-200 p.p.b.). Rhodium shows the worst recovery. For a 5-mg gold bead, the amount of gold in solution exceeds the limit for which vanadium buffer is efficient. However, from 200 to 500 yg ml-1 of gold in solution, the decrease in rhodium absorbance is only 4%, which does not explain the decrease in recovery.It is important to note that the recovery reported refers to both fire assay and AAS analyses. The over-all recovery of rhodium is poor and is associated with incomplete dissolution of rhodium from the gold bead. Small flakes recovered from the dissolution of a 5-mg gold bead containing 5 pg of rhodium were analysed and lead and rhodium were found to be the main constituents, as identified by electron probe X-ray microanalysis. A preliminary X-ray diffraction study indicated that the structure of the flake is similar to that Table 4. determination of gold, palladium and platinum in a 10-mg silver bead. Cupellation temperature, 940 "C. Each determination was performed four times Added/ Found/ Recovery, Element Pg Yg YO Au .. . . . . 0.5 0.6 k 0.2 120 1 .o 1.2 f 0.2 120 2.5 2.4 L 0.3 96 5.0 5.5 k 0.5 110 10.0 10.3 k 0.8 103 15.0 15.0 k 0.1 100 Pd . . . . . . 0.5 1 .o 2.5 5.0 10.0 15.0 Pt . . . . . . 1.5 3.0 7.5 10.0 20.0 30.0 0.5 k 0.1 1.1 k 0.1 2.4 k 0.2 5.0 f 0.1 10.0 f 0.3 15.2 f 0.2 1.6 f 0.5 3.8 L 0.5 7.2 k 0.5 9.4 k 0.6 20.0 k 1.4 27.6 f 1.5 100 110 96 100 100 101 106 127 96 94 100 92 Table 5. Determination of palladium, platinum and rhodium in a 5-mg gold bead. Cupellation temperature, 940 "C. Each determination was performed four times Added/ Element Pg Pd . . . . . . 0.5 1 .o 2.5 5.0 10.0 15.0 Pt . . . . . . 1.5 3.0 7.5 12.5 15.0 25.0 Found/ 0.48 f 0.04 1.03 k 0.04 2.35 k 0.18 4.6 k 0.4 10.2 k 0.5 20.0 f 0.5 1.8 f 0.4 3.8 k 0.4 7.3 k 0.4 11.0 k 1.0 14.8 k 2.0 24.7 k 0.5 Recovery, Yo 96 103 94 92 102 100 120 127 97 88 99 99 .. . . . . Rh 0.5 0.43 f 0.05 86 1 .o 0.30 k 0.08 30 2.5 0.73 k 0.05 29 5.0 1.03 k 0.08 21 Table 6. Determination of silver in a 10-mg gold bead. Cupellation temperature, 870 "C. Each determination was performed four times Ag Ag Pg Pg YO found/ Recovery, added/ 2.4 2.1 f 0.5 87.5 4.8 4.2 k 0.5 87.5 15.0 14.3 k 0.6 95.3 of lead rhodium oxide, which would explain why such flakes are not dissolved by acids or alkaline cyanide solutions. A dark grey coating of the gold beads with high rhodium contents was observed. Such a coating is rich in lead and rhodium, as shown by electron probe X-ray microanalysis.ANALYST, MARCH 1986, VOL. 111 294 Table 7. AAS determination of precious metals pre-concentrated in a 5-mg gold bead from blends of standards and a pure quartz.Each determination was performed four times Taken, p.p.b. Found, p.p.b. Blend Ag Pd Pt Rh Agt Pd Pt Rh 3.0 g SARM-7* + 12 g quartz . . 84 306 748 48 - 329 * 16 822 k 31 3 5 t 5 7.5 g SARM-7 + 7.5 gquartz . . 210 765 1870 120 258 ? 61 784 k 24 1747 k 98 99 f 5 5.0 g SARM-7 + 10 g quartz . . 140 510 1247 80 130 5 85 511 k 12 1289 k 38 64 k 8 15.0gSARM-7 . . . . . . 420 1530 3740 240 - 1530 k 90 3640 k 100 170 k 10 * SARM-7, standard prepared by National Institute for Metallurgy, Republic of South Africa. ?- A 10-mg gold bead was produced after cupellation at 870 "C. Table 8. Determination of precious metals in blends of standards and a pure quartz by fire assay and AAS; a 10-mg silver bead was produced after cupellation at 940 "C.Each determination was performed four times Taken, p.p.b. Found, p.p.b. Blend 3.0 g SARM-7 + 12 g quartz 5.0 g SARM-7 + 10 g quartz 7.5 g SARM-7 + 7.5 g quartz 7.5 g SU-lA* + 7.5 g quartz 4.0 g PTA-1* + 11 g quartz 1 .0 g MA* + 14 g quartz . . 15.0gSARM-7 . . . . Au Pd Pt . . 62 306 748 . , 103 510 1247 . . 155 765 1870 . . 310 1530 3740 . . - 185 205 - 813 . . 1186 - . . - - Au Pd Pt 63 * 9 319 k 26 786 k 114 89 k 16 517 k 6 1152 k 156 140 k 15 765 k 13 1818 k 61 310 5 71 1538 k 18 3270 k 160 80+ 18 205 k 12 211 k 50 767 f 183 63 k 24 - - - 1265 -C 84 * SU-lA, PTA-1 and MA, standards prepared by the Canada Centre for Mineral and Energy Technology. From these findings, it appears that the recovery of rhodium by fire assay is a problem that needs more study.During this investigation, it was noticed that when the gold to platinum ratio in a bead is approximately 10, the recovery of rhodium is about 85%, i.e., better than that without platinum. Recovery studies conducted with rhodium already in solution gave excellent results, and indicated that the rhodium recovery problem lies in the fire assay procedure and the acid digestion step. Determination of silver in a gold bead can be conducted without vanadium buffer as precious metals do not interfere. Further, the cupellation temperature must be decreased in order to minimise silver losses, Therefore, silver determina- tions must be conducted on a bead produced at a lower cupellation temperature. Table 6 shows the silver recovery obtained from an analysis performed in a 0.5% mlV KCN - KOH solution. The detection limit of silver determined by AAS is of the order of a few parts per billion.However, silver contamination from the fluxes used in a fire assay often occurs, and it is not recommended to determine silver in the parts per billion range using fire assay as a pre-concentration and separation method. In this work the silver contamination was 1.9 pg (128 ng g-I) and this value was subtracted from the amount found. Standard and Quartz Blend Analysis The whole method applied to blends of standards and a pure quartz give the results presented in Tables 7 and 8. In Table 7, the precious metal content of a 5-mg gold bead, produced at a cupellation temperature of 940 "C, was determined in 0.5% mlV KCN - KOH - 1% rnlV vanadium solution.Silver results obtained from a 10-mg gold bead produced at a cupellation temperature of 870 "C are also given. The silver determination was conducted without vanadium in solution. When only gold, platinum and palladium are to be determined, a silver collection is adequate. Results obtained with a silver bead are given in Table 8 and show that the recovery of precious metals is generally good. However, the platinum recovery decreases at high platinum concentrations in the silver bead, and can be improved by using a gold collector. Conclusion The acid decomposition of precious metal beads produced by fire assay and the dissolution of the noble metal salts in alkaline cyanide solution is an attractive procedure for the determination of gold, silver, palladium, platinum and rho- dium at parts per billion levels in rocks, ores and minerals.Lengthy separation methods are not required as interferencds are corrected by the vanadium buffer added to the cyanide solution. This method offers several advantages over existing methods by eliminating the possible precious metal losses by coprecipitation in HCl solution by formation of stable complexes. Further, it facilitates the rapid dissolution of significant amounts of silver in a small volume of cyanide solution, in contrast to hydrochloric acid. This method is as simple as the others, but safety precau- tions relating to the use of cyanide must be rigorously followed. Finally, the determination of rhodium presents no difficul- ties once it is in solution. However, the recovery of rhodium by fire assay and by acid decomposition of the bead has to be improved. The flakes observed after acid dissolution of a gold bead containing rhodium has not, to our knowledge, been reported elsewhere. The authors thank A. Tremblay, N. Rheaume and P. Plourde for their contributions to the experiments. 1. 2. 3. 4. 5. 6. 7. References Beamish, F. E., and Van Loon, J. C., "Analysis of Noble Metals," Academic Press, New York, 1977. Gupta, J. G., Miner. Sci. Eng., 1973, 5 , 207. Beamish, F. E. , and Van Loon, J. C. , Miner. Sci. Eng., 1972,4, No. 4, 3. Mallett, R. C., Miner. Sci. Eng., 1970, 2, No. 3, 28. Moloughney, P. E., Tuluntu, 1977, 24, 135 Kallm~w, S . , and Hobart, E. W., Tuluntu, 1970, 17, 845. Le Houillier, R., and RhCaume, N., Can. Mefull. Q., 1984,23, 427. Paper A51205 Received June 1 Oth, I985 Accepted September 23rd, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100291

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 295 Interferences of Antimony(V) in the Differentiation of Antimony(ll1) from Antimony(V) by Extraction with Ammonium Tetramethylenedithiocarbamate Using Graphite Furnace Atomic Absorption Spectrometry Etsuro Iwamoto,* Yasu hiko Inoike, and Yuroku Yamamotot Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima 730, Japan and Yasuhisa Hayashi Department of Chemistry, Joetsu University of Education, Joetsu 943, Japan Procedures for the preparation of antimony sample solutions for the differentiation of antimony(ll1) from antimony( V) by extraction with ammonium tetra met hylenedithioca rba mate (am mon iu m pyr rol idi ned it h io- carbamate, APDC) were examined. It was found that, when APDC is added to the antimony(\/) solution of pH less than ca.3, the antimony(\/) - APDC complex is partially co-extracted with antimony(ll1) over the pH range 3 . 5 1 0. Further, the mixing of antimony(ll1) solution with acidic antimony(\/) solution, prepared by oxidising antimony(ll1) potassium tartrate solution, leads to the incomplete extraction of antimony(ll1). A standard procedure for removing the interferences was established. Keywords: Antimon y(lll) determination; antimon y(V) interference; ammonium tetrameth ylene dithiocarba- mate; graphite furnace atomic absorption spectrometry; extraction Although the combination of solvent extraction with atomic absorption spectrometry (AAS) is very effective for the selective determination of mg 1-1 levels of arsenic(II1) and arsenic(V)1-3 or antimony(II1) and antimony(V) ,3-5 much care is needed in the treatment of such small amounts of those elements.Compared with arsenic, antimony is much more subject to the influence of many parameters associated with the extraction of antimony(II1) and antimony(V) because of their hydrolysis reactions. A critical examination of the variables involved in the extraction - spectrophotometric determination of antimony as the ternary chloro complex of Brilliant Green has been reported6: a hydrolysis side-reaction gives products that do not form extractable ion association systems with the dye. Al-Sibaai and Fogg7 used an extraction - spectrophotometric procedure with Brilliant Green and found that dilute standard antimony solutions (4 mg 1-1) prepared by dissolving anti- mony potassium tartrate in water and diluting the solution with water are stable over a period of 50 d, but similar dilute standard antimony solutions containing hydrochloric acid deteriorated rapidly.It was suggested that the effective loss of antimony could be caused by the formation of one or more soluble hydrolysed species but not by adsorption of antimony on the walls of the containers. Ammonium te t rame th ylenedi thiocarbamate (ammonium pyrrolidinedithiocarbamate, APDC) forms complexes with antimony that can be extracted into organic solvents such as isobutyl methyl ketone (IBMK) and nitrobenzene, and these have been used effectively for the selective determination of antimony(II1) and antimony(V) .3-5 However, only a few developments of precise analytical procedures concerning, especially, the preparation of standard sample solutions have been made and some problems concerning the partial extrac- tion of antimony(V) in the pH range 2-10 and interference from antimony(V) for extraction of antimony(II1) have remained.In this work, the extraction behaviour of the APDC - antimony system was studied. Dichloromethane (DCM) and IBMK were selected as solvents in place of nitrobenzene because of their lower boiling-points. * To whom correspondence should be addressed. t Present address: Fukui Institute of Technology, Gakuen 3-618, Fukui 910, Japan. Experimental Reagents All solutions were prepared from analytical-reagent grade chemicals and de-mineralised water, and were stored in polyethylene bottles. Stock antimony(ll1) solution, 10 mg 1-I. Prepared by dissolving 2.742 g of antimony potassium tartrate in water, diluting to 1000 ml with water, taking 10 ml of this stock solution and diluting to 1000 ml with water; no acid being added.Stock antimony(V) solution, 10 mg 1-1. (A) Prepared by oxidising the stock antimony(II1) solution with potassium permanganate, by taking 5 ml of the 1000 mg 1-1 antimony (111) solution, adding about 2 ml of sulphuric acid and 4 ml of 1% potassium permanganate solution and heating at ca. 80 "C for 30 min, adding hydrogen peroxide to remove the excess of permanganate and manganese dioxide produced, and diluting to 500 ml with 4 M hydrochloric acid. (B) Prepared by dissolving 2.778 g of potassium pyroantimonate {K[Sb(OH),].4H20} in water, diluting to 1000 ml with water to give a 1000 mg 1-1 solution and diluting 10 ml of this stock solution to 1000 ml with 4 M hydrochloric acid.APDC solution, 170 mIV. Buffer solution, p H 5.2. Prepared by mixing 1 M acetic acid Sodium tartrate solution, 1 % mIV. and 1 M sodium acetate in suitable proportions. Apparatus Atomic absorption measurements were made with a Nippon Jarrell-Ash Model AA-1 EW atomic absorption spectrometer equipped with a Model FLA-10 electrothermal atorniser and a Model HU-10 furnace. Peak heights were recorded with a Yanaco Model YR-110 chart recorder. A Hamamatsu TV antimony hollow-cathode lamp (L-223) was used as the light source. The background was checked by using a deuterium lamp. Samples were placed in the carbon tube with a Type 4700 Eppendorf pipette.An Iwaki Model KM shaking apparatus was used for solvent extraction.296 ANALYST, MARCH 1986, VOL. 111 General Procedure Preparation of sample solution Based on a critical examination of the experimental paramet- ers, the following procedure is recomended. Place ca. 80 ml of water and an aliquot of sample solution containing anti- mony(II1) and/or antimony(V) in a 100-ml calibrated flask. Add 2 ml of sodium tartrate solution, stir for 30-60 s, add 2-4 drops of methyl orange and then 2 M sodium hydroxide solution to adjust the pH of the sample solution to 4-5 and dilute to 100 ml. Antirnony(II4 determination Place an aliquot of sample solution containing not more than 1 pg of antimony(II1) in a separating funnel. Add 2 ml of APDC solution and 5 ml of acetate buffer solution.Dilute the mixture to 25 ml with water, the pH of the resulting solution being 5-6. Shake the funnel for 5 min with 10 ml of DCM or IBMK, allow it to stand for 30 min and separate the organic phase. Inject 20 yl of the organic phase with a micropipette into the carbon tube. Pass argon through the furnace at a flow-rate of 3 1 min-l, then atomise the sample with the following heating sequence: dry for 30 s at 30 A (ca. 300 "C), ash for 30 s at 70 A (ca. 700 "C) and atomise for 7 s at 230 A (ca. 2300 "C). Record the absorption signal at 217.6 nm. Run a reagent blank using the same instrumental settings and subtract the result from the analytical value. Total antimony determination Place an aliquot of a sample solution containing not more than 1 pg of antimony in a separating funnel and add 2 ml of APDC solution and 3 ml of 1 M hydrochloric acid.Dilute the mixture to 25 ml with water, the pH of the resulting solution being ca. 1. Carry out the extraction and measure the atomic absorption as for antimony( 111). The amount of antimony(V) is calculated from the differ- ence between the total antimony and antimony(II1). Results and Discussion Partial Extraction of Antimony(V) above pH 3.5 The differentiation of antimony(II1) from antimony(V) with APDC extraction is based on the principle that antimony(V) is not extracted above pH 3.5.3-5 However, it was found that when APDC is added to the antimony(V) solution at a pH below 3, followed by addition of the buffer solution, anti- mony(V) is partly extracted above pH 3.5.The effect of the acidity of the 0.1 mg 1-1 antimony(V) solution to which APDC is added on the extraction of antimony(V) for the APDC - DCM system is shown in Fig. 1. The 0.1 mg 1-1 standard solution was prepared by diluting the 10 mg 1-1 stock solution and adjusting the acidity with sodium hydroxide solution. The extraction, in which 10 ml of the standard solution (0.1 mg 1-1) were taken, was carried out at pH 5.2 and antimony was determined in the organic phase. The degree of extraction increased with increasing acidity, although antimony(V) was not extracted when APDC was added above pH 3. The behaviour was independent of the acid (hydrochloric, sulphuric and nitric acid) used to adjust the pH and organic solvents (DCM and IBMK). Stock solution B of antimony(V) gave the same result as stock solution A. Further, in tests lasting up to 12 d, the extractability of antimony(V) from each sample solution prepared from 10 mg 1-1 stock solutions A and B using 4 , l and 0.01 M acids was found to be unchanged, within experimental error.As antimony at low acidity tends to be adsorbed on the walls of glass containers, an attempt was made to check the amount of antimony adsorbed. The relative distribution of anti- mony(V) on extraction at pH 5.2 is given in Table 1 for pH 1.4 and 4 solutions of antimony(V). The percentage values for the O l d 0 PH Fig. 1. Effect of pH of the antimon (V) solution to which APDC is added on extraction at pH 5.2. Sb(V7, 1 pg Table 1. Distribution of antimony(V) in extraction at pH 5.2 Distribution, % pH1.4* pH4* Organic phase .. . . . . . . . . 39 2 Separating funnel . . . . . . . . . . 12 13 Aqueous phase . . . . . . . . . . 49 84 * pH values of the antimony(V) solution (0.1 mg 1-l). separating funnel refer to the amount of antimony adsorbed and present in small amounts of solvents adhering to the walls of the funnel after draining the organic and aqueous phases. Each concentration of antimony(V) was determined accord- ing to the procedure for the total antimony determination. As there is virtually no difference in the percentages for the separating funnel between the pH 1.4 and 4 solutions, it can be concluded that the non-extraction for the pH 4 solution is not due to adsorption. The above observations show clearly that a certain species of antimony(V) forms a complex which is extractable into the organic phase with APDC and the chloride anion is not responsible for its formation.Fig. 2 shows the distribution of species present using 10-5 M antimony(V) at 25 "C.8 It is interesting that the pH dependence of the extractability in Fig. 1 is very similar to that of the formation of Sb(OH)5 in Fig. 2. It seems likely that Sb(OH)5 forms a complex which is extractable in organic solvents with APDC whereas Sb(OH)6- does not, and once the complex has been formed below pH 3 it is stable at pH values higher than this. Interference from Antimony(V) for the Determination of Antimony( 111) In the absence of antimony(V), antimony(fi1) is completely extracted with APDC over a wide acidity range, from 4 M to pH lO.>5 However, it was found that the extraction of antimony(II1) is subject to interference from the presence of antimony(V), depending on the conditions of mixing of the antimony(II1) solution with the antimony(V) solution.Procedures for the preparation of sample solutions contain- ing antimony(II1) and antimony(V) are given in Table 2. The effect on extraction of the concentration of antimony(V) and the acidity of antimony(V) solutions in procedures I and I1 is shown in Fig. 3, where stock solution A was used for antimony(V). Evidently, the extraction of antimony(II1) is subject to interference from antimony(V) in stock solution A and the degree of interference increases with increasing acidity of the antimony(V) solution and the amount of antimony(V) present.In particular, when antimony(II1) was mixed with comparable amounts of antimony(V) in 4 M acid, antimony(II1) was not extracted at all. On the other hand, noANALYST, MARCH 1986, VOL. 111 297 Table 2. Procedures for the preparation of sample solutions Procedure Stock solution (10 mg 1-l) Standard solution (0.1 mg I - I ) d i l u t i o n y * loo m1 mixing- sample solution 1 I . . . . . . Sb(II1) - H20, 1 ml 'I Sb(V) - 4 M HCI, 1 ml-+ neutralisation? - dilution, 100 ml ' . ' ' ~ ~ ~ ~ ~ ~ mixing- neutralisation+ dilution, 100 ml- sample solution I I11 . . . . Sb(II1) - H 2 0 , 1 ml Sb(V)-4 M HCl, 1 ml + dilution (ca. 80 ml mixing + addition (tartrate) + neutralisation > dilution, 100 ml+ sample solution * Dilution with water. t Neutralisation with sodium hydroxide using phenolphthalein.$ Addition of auxiliary complex reagent (sodium tartrate or sodium citrate). I - 0 2 4 6 8 PH Fig. 2. Distribution of species present at 10-5 M Sb(V) in 0.5 M (CH&NCI at 25 "C. Taken from reference 8 80 60 E E .- UJ 40 . Y L a, r Y m t? 20 0 Fig. 3. Effect of amount of antimony(V) and acidity of antimony(V) solution A on the extraction of antimony(II1) with APDC - DCM at pH 5.2. Procedure I, 0; procedure 11, A, 0.01 M HC1; 0 , 1 M HCI; a, 4 M HCI. Sb(III), 1 pg interference was found for antimony(V) in stock solution B. This differs from the behaviour in the partial extraction of antimony(V) discussed in the preceding section. Sulphuric acid produced the same behaviour as hydrochloric acid and the use of methyl orange as an indicator also gave the same results.An increase in the amount of APDC did not increase the degree of extraction of antimony(III), showing that the amount present was sufficient. Although the antimony(II1) standard solution prepared from antimony potassium tartrate is very stable at the 1000 mg 1-1 level in aqueous solution, at low concentrations oxidation of antimony(II1) to antimony(V) may take place.9 Sun et ~ 1 . 1 0 reported that with the APDC - IBMK system no antimony(II1) was extracted at pH 6 in the absence of tartaric acid, which was added as a stabilising agent, but an equivalent Table 3. Recovery tests on antimony(II1) and antimony(\'). Dichloromethane was used as the solvent Antimony addedlpg I-' Antimony found/pg 1-1 Sb(II1) Sb(V) Sb(tota1) Sb(II1) Sb(V) 20 80 106 18 88 30 70 104 31 73 40 60 103 41 62 60 40 101 57 44 70 30 106 77 29 80 20 100 83 17 100 0 94 94 0 *O I 0 1 2 3 4 V/m Fig.4. extraction of Sb(II1) at pH 5.2. Sb(III), 1 pg; Sb(V), 1 pg Effect of volume of 1% sodium tartrate solution on the amount of antimony(V) was found by extraction at pH 1. In these tests lasting up to 6 d at the 1 mg 1-1 level, the dxrease in the degree of extraction at pH 5.2 was less than 10% in the absence of antimony(V). The rate of oxidation appears to depend on the quality of the water used to make the dilutions. Antimony(V) is reported to be hydrolysed rapidly even in 6 M hydrochloric acid.11 It is also speculated that more complex equilibria of hydrolysis would occur for solution A under acidic conditions, and a certain compound of antimony(V) forms a complex with antimony(II1) or adsorbs antimony(III), interfering with the complex formation of antimony(II1) with APDC.An attempt was made to remove this interference by adding auxiliary complexing agents before neutralisation according to procedure I11 in Table 2. It was found that sodium tartrate and sodium citrate are the most effective: antimony(II1) is protected from oxidation or interaction with antimony(V) species by complex formation with tartrate. Fig. 4 shows the effect of the concentration of sodium tartrate solution on the interference of antimony(V) on the extraction of anti- mony(II1) at pH 5.2. At least 2 ml of 1% sodium tartrate are required. The efficiency was independent of standing time for at least 2 h after the addition.EDTA showed no effect.298 ANALYST, MARCH 1986, VOL. 111 Addition of the auxiliary reagent to either antimony solution before mixing is equally effective. In our previous studies,3?4 the antimony(II1) solution was prepared according to proce- dure I. Subramanian and Meranger used ammonium citrate as a buffer solution.5 A recovery test on standard samples was carried out according to the standard procedure established here and satisfactory results were obtained, as shown in Table 3. Although the elucidation of the interference mechanism is beyond our present knowledge, it can be concluded that (i) in order not to extract antimony(V) over the pH range 3.5-10, the pH of the sample solutions must be higher than 3 when APDC is added, and (ii) an auxiliary complexing reagent, tartrate or citrate, is needed to prevent the interference of antimony(V) in the determination of antimony(II1) when using an acidic antimony(V) solution prepared from antimony potassium tartrate. Therefore, care must be taken, especially when using the method of standard additions, as unknown and standard solutions are often acidic. This research was supported in part by a Grant-in-Aid for Scientific Research (Nos. 57470031, 58540363 and 58030061) from the Ministry of Education, Science and Culture, Japan. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. References Karnada, T., Talanta, 1976, 23, 835. Yarnamoto, Y., and Kamada, T., Bunseki Kagaku, 1976, 25, 567. Chung, C. H., Iwamoto, E., Yarnamoto, M., and Yamamoto, Y., Spectrochim. Acta, Part B, 1984, 39, 459. Karnada, T., and Yamamoto, Y., Talanta, 1977, 24, 330. Subramanian, K. S . , and Meranger, J. C., Anal. Chim. Acta, 1981, 124, 131. Burke, R. W., and Menis, O., Anal. Chem., 1966, 38, 1719. Al-Sibaai, A. A., and Fogg, A. G., Analyst, 1973, 98, 732. Baes, C. F., Jr., and Mesmer, R., “The Hydrolysis of Cations,” Wiley, New York, 1976, p. 374. Andreae, M. O., Asmode, J.-F., Foster, P., and Van’t Dack, L., Anal. Chem., 1981, 53, 1776. Sun, H.-W., Shan, X.-Q., and Ni, Z.-M., Talanta, 1982, 29, 589. Newmann, H. M., J . Am. Chem. SOC., 1954, 76,2611. Paper A51314 Received September 9th, 1985 Accepted October 14th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100295

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, MARCH 1986, VOL. 111 Photothermal Deflection Spectroscopy and Studies of P hotoelect roc hem ica I Processes Electrolyte Interfaces Robert E. Wagner, Victor K. T. Wong and Andreas Mandelis 299 Photoconductivity at (0001) n-CdS - Photoacoustic and Photothermal Sciences Laboratory, Department of Mechanical Engineering, University of Toronto, Toronto, Ontario M5S 1A4, Canada Photothermal deflection spectroscopy (PDS) was used to investigate the manner in which the degree of band bending in a photoelectrochemical cell (PEC), consisting of an illuminated n-CdS (0001) single crystal and a polysulphide electrolyte, affects the non-radiative recombination processes in the semiconductor. The photocurrent was also monitored simultaneously as a complementary energy conversion channel.The results show that PDS can be used successfully as an analytical tool for the understanding and interpretation of photoelectrochemical processes a t the photoelectrode - electrolyte interface. Keywords : Photothermal deflection spectroscopy; photoconductivity; photoelectrochemical cell; cadmium sulphide electrode; non-radiative process Several workers have studied cadmium sulphide and cadmium selenide based photoelectrochemical cells (PEC) in the last decade. The important consideration of stable CdS photoelec- trodes during a photoelectrochemical experiment has been successfully addressed by Ellis and co-workers in a series of publications.'-5 The authors have found that sulphide or polysulphides in aqueous solution quench the photoanodic dissolution of the CdS reaction: -A CdS --% Cd2+ (aq.) + S(s) + 2e- (1) (photoanode) The single crystalline CdS photoelectrode has been im- portant technologically, because when used in a PEC, it can form a simple device for sustained conversion of visible optical energy into electricity.The relatively large band gap energy of CdS compared with CdSe and other compound semiconduc- tors renders the former semiconductor more attractive for applications where large open-circuit photovoltages are desired. The analytical methods conventionally used to study photoelectrochemical effects at semiconductor electrode - electrolyte interfaces include photocurrent and photopoten- tial measurements'-5 as a function of the wavelength of the exciting radiation (i.e., photoaction spectra); differential capacitance measurements of the interface2; emission photo- luminescent spectra from the electrode surface following radiative deexcitation of carriers3-5; electroluminescent spec- tra6; voltammetric studies7; and/or combinations of these techniques.l-6 The recent development of photoacoustic spectroscopy (PAS) as an analytical electrochemical tech- niqueg19 has allowed the use of the photoacoustic effect10 in optical spectra acquisition of metal oxide and semiconductor interfaces. The main advantage of PAS over other conven- tional spectroscopies lies in its ability to measure the non- radiative pathway of the deexcitation manifold, i.e., carrier recombination processes and other heat-generating mechan- isms in electrochemical systems. The non-radiative de- excitation component is the main energy loss mechanism in such systems and is detrimental to their quantum and energy efficiency enhancement.11 From the experimental point of view, PAS cannot be used easily to monitor electrode - electrolyte processes in situ owing to the remote positioning of the transducer - detector system. For this reason, photother- ma1 deflection spectroscopy (PDS) has been recently applied to the investigation of electrochemical interface phenom- ena.12J3 PDS is a spectroscopic tool that utilises the mirage effect, the fact that the path of a laser beam, in a given medium, will be bent if the beam encounters a refractive index gradient in its propagation path. This refractive index gradient may be induced by temperature or concentration gradients.This technique therefore can yield solid electrode spectro- scopic information14 through probing the electrolyte portion close to the electrode surface, as well as information concern- ing chemical changes in the electrolyte owing to interfacial chemical reactions. 15 In this work we have exploited the dependence of the PDS signal on the non-radiative quantum efficiency of the CdS photoelectrode deexcitation manifold to monitor in situ the non-radiative mechanism, simultaneously with the photo- generated current at the junction betwen a (0001)-oriented CdS electrode and a stable polysulphide electrolyte. In this fashion, PDS proved to be a valuable channel of information complementary to the conventional analytical methods, aiding in the establishment of a more complete picture of the electrode deexcitation process pathways at the electronic level. Experimental The material used in these experiments was a 1 cm x 1 cm x 0.2 cm low-resistivity (p = 20 ohm cm) n-CdS crystal from Eagle-Picher (Miami, OK), oriented with the optic axis perpendicular to the surface (0001) plane.The crystal was etched prior to mounting in the PEC in a solution consisting of 95% V/V of 3 M HCl and 5% V/V of a 30% hydrogen peroxide solution in water. The crystal was etched for 20 s and then rinsed in distilled water. Subsequently, the crystal had one of its faces coated with a 0.5 mm thick layer of liquid In - Ga amalgam and ohmic contact was assured by allowing diffusion of the In - Ga into the CdS for 3 h at 350 "C. Further, the sample was epoxied on to an acrylic backing, which was chosen for its durability and resistance to electrolyte penetra- tion.The completed working electrode (WE) consisted of the CdS crystal with one face bare and exposed to the electrolyte and the other face contacting a copper lead via the In - Ga amalgam. The metallised face of the WE was insulated from the electrolyte by the acrylic backing and epoxy. In between some of the experiments the WE was immersed in cyclohex- ane, in order to remove any trace amounts of sulphur that may have formed on the surface during experimentation116 Platinum foil was chosen for the counter electrode (CE), covered with Pt black in order to increase the effective surface area available for the cathodic reaction. The CE was300 ANALYST, MARCH 1986, VOL.111 positioned very close to the WE (ca. 5 mm) to decrease the solution resistance. A Fischer saturated calomel electrode (SCE) was employed as the reference electrode (RE). Mott - Schottky plots were obtained from n-CdS in 1 M each of NaOH, Na2S and S. This polysulphide solution was found to optimise the stability and reproducibility of the differential capacitance measurements required for the Mott - Schottky plots. Optical absorption spectra of the polysulphide solution were taken with a Cary 17D spectrophotometer. It was thus found that the 1 + 1 + 1 M solution would absorb most light with energies above the CdS band gap at ca. 510 nm.1717 In order to reduce this absorption a 1 M NaOH - 1 M Na2S - 0.05 M S electrolyte was used for the subsequent photoelectrochem- ical experiments and was found to give satisfactory results.The PEC was made of Teflon and had a screw-on lid, which supported a fused-silica UV grade window for entry of the exciting radiation into the cell. Two more windows made of Crown glass were located on opposite sides of the PEC to allow the passage of the 2-mW, 632.8-nm He - Ne laser probe beam used for PDS measurements. The laser - PEC assembly was mounted on stages, which allowed four degrees of freedom in the beam path movement, two translations and two rotations. The probe beam was further focused with its waist above the CdS electrode surface, using a 15 cm focal length lens. Fig. 1 shows an overview of the experimental apparatus. The source of UV - visible radiation was an Oriel Corp.Model 6141 1000-W Xe arc lamp in series with an Instruments S.A. H-20 monochromator with a concave holographic grating for wavelength selection. The He - Ne probe beam deflection was measured with a United Detector Technology (UDT) Model 431 Position Monitor connected to a UDT SC/25 light position detector. An optical filter with a 5% transmittance for wavelengths below 590 nm was placed over the detector to enhance the signal to noise ratio (SNR). The exciting radiation intensity was modulated by an AMKO OC 4000 mechanical chopper, which also referenced the EG & G Model 5204 lock-in amplifiers used as the PDS and photovoltage signal processors. External d.c. biases were required for Mott - Schottky plots and PDS - photocurrent measurements.A Stonehart and Associates Model BC 1200 potentiostat was used for the purpose of providing a regulated voltage (potentiostatic mode) between the WE and CE. The a.c. ripple voltage required between the WE and CE for Mott - Schottky plots was supplied from a Krohn - Hite Model 500A generator coupled into the potentiostat. The PEC was operated in the conventional three-lead mode (Fig. 1). Data were collected using software programmed into a D.E.C. DPD-11/23 micro- computer via an A/D conversion board. Results Mott - Schottky Analysis In order to calculate the doping density and flat band potential of n-CdS in the 1 M OH- - 1 M S2- - 1 M S electrolyte, the space-charge layer capacitance was measured at the semi- conductor - electrolyte interface. Using the Mott - Schottky model for the junction, the space-charge layer capacitance C,, can be related to the flat band potential VF, by the equationls c-2= ' (V- v,, - kT/q) .. (2) sc q&&&A2 where q is the electronic charge, E is the dielectric constant of the space-charge layer (= 5.2),19 E, is the permittivity of vacuum (= 8.85 pF m-I), ND is the effective donor density, A is the semiconductor electrode area exposed to the electrolyte and V is the applied bias versus SCE. The key assumption to the validity of equation (2) is that the whole potential drop takes place in the space-charge region of the semiconductor. Tomkiewicz20 has shown that this assumption is generally valid at high bias modulation frequencies, where the equi- valent electrical circuit of the electrochemical interface can be represented as a single resistor and a capacitor connected in series.Fig. 2 shows the circuit diagram for the experimental apparatus for the Mott - Schottky plot determination. A d.c. bias was applied across the WE and CE, measured with respect to an SCE. A 5-mV r.m.s. a.c. voltage was super- imposed over V and the PEC reactance was plotted versus the modulation frequency of the a.c. voltage. Following Tom- kiewicz,20 the complex impedance of the cell can be written as 1 Z ( 0 ) = R,(0) + iX(0) = R,(0) - - C S C . . (3) where R,(o) is the resistance of the space-charge layer and X(w) is the cell reactance at angular frequency 03 = 2nf. A plot of log(2nX) versus logfis shown in Fig. 3(a). This graph is a straight line for bias voltages away from the flat band potential.By using the intercepts, s, of curves similar to Fig. 3 with the ordinate for a number of bias voltage values V, the space-charge capacitance was determined for each V from Csc(Vk) = lO-@k). Fig. 3(b) shows the combined experimen- tal and theoretical results for the equivalent resistor - capacitor electrical circuit for several capacitor values. A comparison of Fig. 3(a) and (b) is indicative of the validity of the simple resistor - capacitor representation of the semiconductor - electrolyte interface. Fig. 4 is the Mott - Schottky plot for the interface, from which the VFB value was found to be - 1.15 V versus SCE. Reference ' in - - Lock-in - = ~ N D converter amp I ifier and Position - = . In-phase sensor I - computer - Data storage plotter, Fig.1. Experimental apparatus for PDS and photocurrent measurements. See text for detailsANALYST, MARCH 1986, VOL. 111 + B * c3 f 301 )'A I REF 3 I Lock-in I Lock-in It CE CE REF 4 A B REF I WE 4 Potentiostat . Function A.C. generator 1 - Fig. 2. Experimental apparatus for Mott - Schottky measurements. See text for details '. 0.00 0.70 1.40 2.10 2.80 3.50 4.20 4.90 5.60 Log f Fig. 3. (a) Log - Io plot of the frequency dependence of photoelectrochemical ceflreactance. WE at -0.75 V vs. SCE. (b) Log - log plot of the frequency dependence of the capacitance (C) of a resistor - capacitor simulation circuit: A, C = 10 nF; B, C = 100 nF; and C, C = 1 pF The donor doping density was also calculated in terms of the slope of the Mott - Schottky plot: From this calculation, the doping density was found to be ND = 5.02 x 1014 cm-3.The value of VFB found in this work is in good agreement with the range of values -1.52 to -1.20 V versus SCE for n-CdS of comparable donor densities calcu- lated previously.2 PDS, Photovoltage and Photocurrent Spectra PDS spectra normalised by the Xe lamp spectrum were obtained in situ in the PEC with water and with a polysulphide elecrolyte. The PDS spectrum of CdS in water is shown in Fig. 5. Both the amplitude and the phase indicate an energy band gap at ca. 510 nm, in excellent agreement with previous / / / / / / / / / / / / A A A / lvp; , , n -1.3 -0.5 0.3 1.1 PotentialN vs. SCE Fig. 4. Mott - Schottky plot of n-CdS in 1 M OH- - 1 M S2- - 1 M S electrolyte .? 7.20 > g i m 0.801 7- c' c -200 O -240 2 I -260 a -220 $ E a 0.00 320 400 480 560 640 720 800 Wavelengthhm Fig.5. Photothermal deflection spectrum of n-CdS in water in the open-circuit configuration. Modulation frequency: 25 Hz spectroelectrochemical work.2 PDS signal profiles as a func- tion of beam offset distance from a black absorber surface and as a function of chopping frequency were found to be in general agreement with Murphy and Aamodt's work.21 Some differences in the signal profiles between this work and reference 21 were attributed to our optical versus their302 4 ANALYST, MARCH 1986, VOL. 111 1 0.06 1 I v) ul c c 50 0.05 - - 0.05 ‘5 40 - - Q E 0.04 - - 8 , c 2 0.02 - - 0.02 a Q : 2 , 0.01 v, 10 n a 0.00 0 -1.50 -1.15 -0.80 -0.45 -0.10 0.25 0.60 0.95 1.30 BiasN vs.SCE 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 Log[(lamp power) x 1.6 Wl light radiant flux (white light) S. Modulation frequency: 15 Hz i!r n-CDs in 1 M OH- - 1 M S2- - 0.05 M Fig. PDS and en-circuit photopotential vs. Xe lamp Fig. 9. PDS amplitude and photocurrent - voltage curves for n-CdS irradiated at 505 nm. Modulation frequency: 17 Hz Wavelengthhm Fig. 7. PDS amplitude and photovoltage spectra of n-CdS in 1 M OH- - 1 M S2- - 0.05 M S. Modulation frequency: 17 Hz. Signal drop-off above the band gap is due to electrolyte absorption Wavelengthhm Fig. 8. PDS amplitude and photoaction spectra of n-CdS under different biases vs. SCE. (a) V = -1.5 V; A, PDS; B, photocurrent. (b) V = -0.3 V; A, PDS; B, photocurrent. (c) V = +0.9 V; A, PDS; B, photocurrent.Both signal strengths are in volts measured by the lock-in amplifiers. Modulation frequency: 17 Hz c / ,,? Cd2+(aq.) Conduction band - - - - - - - I n-CdS Fig. 10. Energy level diagram of n-CdS - electrolyte interface showing various possible deexcitation pathways. ED, Activation energy for anodic decomposition; ERedox, energy of electrolyte redox couple; EG, band gap energy of semiconductor; EF, Fermi energy of semiconductor; A, band-to-band transition; B, radiative recombina- tion; C, non-radiative recombination; D, photoanodic decomposi- tion; E, anodic carrier separation; F, anodic electrode dissolution; G, electrolyte redox reaction; f i w , incident photon energy; fiw ’ , lumines- cent photon energy; fiQ, lattice phonon energy; and SCR, space- charge region resistive heating and to their tighter focusing of the probe beam.Fig. 6 shows the measured PDS signal amplitude and open-circuit photopotential for n-CdS in the 1 M OH- - 1 M S2- - 0.05 M S polysulphide electrolyte as a function of incident optical power as determined by an Oriel Model 7090-2 pyroelectric detector. The monochromator was employed with the grating removed from the lamp light beam path (white light) to enhance the SNR, especially of the PDS signal. Electrode illumination with monochromatic light of 510 f 4 nm resolution gave PDS and V,, curves similar to those in Fig. 6, but with a poorer SNR. The linear dependence of V,, on the logarithm of the light irradiance has been observed also by Ellis et aZ.2 and is in agreement with theoretical consider- ations.22 The PDS signal amplitude, however, is linear in the logarithm of the irradiance at low power levels and increases more rapidly at higher power levels.Spectra acquisition in the polysulphide electrolyte was severely hampered at h < 490 nm owing to the onset of strong absorption by the electrolyte. For reasons of SNR optimisa- tion CdS spectra taken in the presence of 1 M NaOH - 1 M Na2S - 0.05 M S electrolyte were not normalised with respect to the absorption spectrum of the electrolyte, as the latter is transparent in the band gap region of interest (A > 500 nm), and the semiconductor spectral features are flat below 500 nm (Fig. 5). PDS and photovoltage spectra were obtained simultaneously. Fig. 7 shows PDS and V,, spectra.The PDSANALYST, MARCH 1986, VOL. 111 303 signal is large even at wavelengths below the band gap and appears to be shifted to the right of the open-circuit photovoltage spectrum. The drop-off of both signals on the high energy side is due to absorption of the incident light by the electrolyte. The sub band gap strength of the PDS signal does not appear in Voc. This feature could either be excitonic in nature23 or surface defect-related.24 It has also been observed in photoacoustic spectra of n-CdS of the same lot obtained in this laboratory.25 PDS and photocurrent spectra were taken under several biases between -1.5 V and +0.9 V versus SCE at 0.3-V increments. The wavelength range was between 470 and 570 nm at 8-nm resolution. Fig. 8 shows results under extreme bias conditions and one intermediate value.A PDS spectral shift to longer wavelengths than the photocurrent spectrum, similar to that in Fig. 7, is apparent at all three bias levels and is characteristic of all other such spectra. For all biases shown, the PDS signal strength changes little compared with varia- tions in the photocurrent, which undergoes a dramatic increase when the applied bias becomes positive to the flat band potential. A substantial PDS signal is present at sub-band gap wavelengths, whereas the photocurrent signal is negligible. Photocurrent - voltage and PDS - voltage measurements were further performed, the WE being illuminated with 505-nm light. The resulting photocurrent and PDS signals were thus monitored as a variable d.c. bias was applied across the WE and CE.Fig. 9 shows the results: the photocurrent increases very rapidly at biases below and around the measured flat band potential, - 1.15 V versus SCE, and tends to saturation at more positive biases. This behaviour is typical of CdS photoresponse.2 The PDS signal amplitude anti- correlates with the photocurrent at biases negative to the flat band potential, exhibits a broad minimum around VFB and increases steadily at more positive biases without signs of saturation. Discussion The major electronic phenomena occuring at the n-type semi- conductor - electrolyte interface on irradiation with band gap or higher energy photons can be summarised as in Fig. 10. The band bending due to the Fermi level mismatch at the interface creates a depletion or space-charge layer, which separates the photocreated electron-hole pairs, thus preventing 100% recombination.For those carriers which eventually recombine (and do not contribute to the anodic photocurrent), both radiative and non-radiative deexcitation processes are the major recombination mechanisms competing for the carrier deactivation. The photoluminescence phenomena observed by Ellis and co-workersl-5 in n-CdS and other 11-VI semicon- ductors were found to be consistent with radiative deexcita- tion processes leading to electron-hole recombination at the interface. Non-radiative recombinations would tend to trans- fer the energy of the photoexcited carriers to lattice phonons resulting in localised heating at the interface. It is expected, therefore, that the PDS signal will be sensitive to the localised surface heating because of the minute variations the latter incurs in the refractive index of the liquid electrolyte adjacent to the heated surface.Assuming an electrochemically stable WE, our photocurrent and PDS results were interpreted as simultaneous monitors of carrier separation and non-radiative recombination mechanisms. The photoluminescent deexcita- tion pathway was not monitored in this work; however, comparisons were made with Ellis and co-workers' results .4 The donor-doping density calculated from the Mott - Schottky plot, ND, can be compared with the theoretical value for an n-type semiconductor: ND=(pePn)-' . . . . . . ( 5 ) 80 60 .- C 3 t E 40 -e Q 4- .- 20 0 Photocu went PDS -1.5 -1.0 -0.5 0 +0.5 +1.0 Applied biasiV vs.SCE Fig. 11. Semi-qualitative curves comparing relative contribution to PDS signal with photocurrent. Photocurrent, anodic carrier separa- tion; NR, interband non-radiative de-excitation; and T, other thermal processes in n-CdS in polysulphide electrolyte (e. g., space-charge layer carrier separation, electron injection into the working electrode and intraband non-radiative de-excitations). The NR curve is in qualitative agreement with Fig. 5a in Streckert et ~ 1 . ~ Any photode- composition of electrode was assumed to be negligible Using p = 20 ohm cm and kn = 300 cm* V-1 s-1,26 the theoretical doping density is found to be ND == 1 x 1015 cm-3, a value within a factor of two of the experimental value. The higher than linear dependence of the PDS signal amplitude on the logarithm of V,, at large radiant fluxes (Fig.6) is consistent with the enhancement of the PDS signal with respect to the open-circuit photovoltage of Fig. 7, at sub-band gap wavelengths, as all spectra were obtained using the highest power rating of the Xe lamp. The shift of the PDS absorption peak to the right in Fig. 7 is most likely due to surface recombination processes, intra-band gap defects and/or sur- face states that would provide efficient non-radiative deexcita- tion pathways22 detectable by the PDS probe. A systematic study of the PDS signal as a function of controlled surface conditioning of the n-CdS crystal will be necessary to elucidate the particular mechanism(s). For energies around and above 510 nm in Fig.7 an anti-correlation is apparent between the V,, and PDS signals. At present, we propose that this is due to an increase in the numbers of efficiently separated electron - hole pairs across the space-charge region and therefore an increased value of VOc, while the number of non-radiatively recombining carriers has accordingly decreased with a sub- sequent decrease in the PDS signal. The maximum value of V,, is seen to occur at ca. 503 nm, in agreement with previously reported results1 within the resolution of our monochromator (8 nm). In Fig. 8, the PDS spectral shifts to longer wavelengths than the photoaction spectra are consistent with the energy balance mechanism proposed above: at super-band gap energies the efficiently photoseparated electron - hole pairs contribute the electron to the anodic current, while the minority carrier participates in a redox reaction at the semiconductor - electrolyte interface2 (process G in Fig 10).This mechanism would tend to pull electrons away from the interface towards the CdS bulk, thus decreasing the probability of non-radiative recombination and would be responsible for the anti- correlation of signal observed in Fig. 8 for photon energies above ca. 510 nm. Another interesting feature of Fig. 8 is the effect of the bias. At applied bias negative to the flat band potential [Fig. 8(a)], band bending is significantly reduced. The decreased electric field across the space-charge layer all but inhibits electron - hole separation, as seen from the greatly decreased photocurrent signal, while the non-radiative deexci- tation pathway remains efficient.As the space-charge layer increases with increasing bias positive to the flat bands, the photoseparation mechanism becomes more efficient and the photocurrent signal increases [Fig 8(b) and (c)]. It is interest- ing that the PDS signal, associated with the non-radiative component, remains approximately as strong in Fig. 8(b) and (c) as in Fig. 8(a). It must be deduced, therefore, that there is a304 ANALYST, MARCH 1986, VOL. 111 significant decrease in the carrier numbers deexciting via pathways other than non-radiative with increasing bias and re-channeling of the excess of carriers to the external circuit. This mechanism is in agreement with the inhibition of the photoluminescent emission observed by Streckert et al.4 at positive biases. An analvsis of Fig. 9 indicates that as the degree of band bending in the CdS space-charge layer is changed, via the applied bias, the predominant heat-generating mechanisms in the WE are altered. For biases at or below -1.2 V, the major source of heat is non-radiative recombination; note that the PDS signal increases and the photocurrent decreases as the voltage decreases from -1.2 V. However, for biases greater than - 1.2 V, non-radiative recombination becomes less important as the carrier-separation efficiency increases. At these positive voltages, where a significant photocurrent is present, the dominant heat sources are: (a) carrier separation in the space-charge layer; (b) electron injection and subse- quent deexcitation into the valence band from the solution redox couple; and (c) non-radiative transitions of hot elec- trons in the conduction band.A semi-qualitative indication as to the relative percentage lcontribution to the PDS signal from the inter-band non-radiative recombination compared with other current - flow related heating processes, such as those discussed above, is shown in Fig. 11. The non-radiative component anti-correlates with the photocurrent, while the other thermal components correlate with the photocurrent, as expected.11 The non-radiative component versus voltage curve in Fig. 11 agrees well with photoluminescent emission intensity versus voltage data from a similar photoelectrochemical experiment with CdS [reference 4, Fig. 5(a)]. This correlation between radiative and non-radiative processes is to be expected as they constitute complementary carrier deexcitation pathways to the current-producing electron - hole separation.Further evidence for assuming that the heat-generation mode in the WE changes nature close to the flat band potential is found when one considers the phase data of the PDS signal for various applied biases. A phase shift of 20” was observed in the region corresponding to the “knee” of the PDS signal versus voltage curve in Fig. 9. This phase shift would suggest a fundamental change in the heat generation processes. The various heat generation processes discussed above are expected to take place at different locations relative to the WE - electrolyte interface; a PDS phase shift would indicate a shift in the thermal source location within the WE.In conclusion, the PDS technique coupled with photoaction spectra has been shown to be capable of monitoring non-radiative recombination and other heat-generating processes at the semiconductor - electrolyte interface and to measure directly the so far little studied non-radiative efficiency of PECs. These observations could be significant in the calculation of PEC efficiency losses and their minimisation through the physical understanding of the interfacial loss mechanisms, for which PDS appears to be a very promising probe. The authors acknowledge the support of the National Sciences and Engineering Research Council of Canada (NSERC) throughout the duration of this project. They are also grateful to the Institute for Hydrogen Systems (IHS), Mississauga, Ontario, for contributing the PDS apparatus towards the completion of this work.Useful initial discussions on some experimental aspects with Drs. S.-M. Park (Department of Chemistry, University of New Mexico) and M. Weber (Department of Chemistry, University of Toronto) are gratefully acknowledged. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. References Ellis, A. B . , Kaiser, S. W., and Wrighton, M. S., J. Am. Chem. SOC., 1976, 98, 6855. Ellis, A. B., Kaiser, S. W., Bolts, J. M., and Wrighton, M. S., J . Am. Chem. SOC., 1977,99,2839. Karas, B. R., and Ellis, A. B., J . Am. Chem. SOC., 1980, 102, 968. Streckert, H. H., Tong, J.-R., Carpenter, M. K., and Ellis, A. B., J . Electrochem. SOC., 1982, 129,772. Streckert, H. H., and Ellis, A. B., J . Phys. Chem., 1982, 86, 4921. Smiley, P. M., Biagioni, R. N., and Ellis, A. B., J. Electro- chem. SOC., 1984, 131, 1068. Wilson, J. R., and Park, S.-M., J. Electrochem. SOC., 1982, 129, 149. Masuda, H., Fujishima, A., and Honda, K., Bull. Chem. SOC. Jpn., 1982, 55, 672. Dohrmann, J. K., and Sander, U . , J. Phys, (Paris), 1983, C6, 281. Bell, A. G., Am. J . Sci., 1880, 20, 305. Fujishima, A., Maeda, Y., Honda, K., Brilmyer, G. H., and Bard, A. J., J . Electrochem. SOC., 1980, 127, 840. Mendoza-Alvarez, J. G., Royce, B. S. H., Sanchez-Sinencio, F., Zelaya-Angel, O., Menezes, C., and Triboulet, R., Thin Solid Films, 1983, 102, 259. Roger, J. P., Fournier, D., and Boccara, A. C., J . Phys. (Paris), 1983, C6, 313. Mandelis, A., J . Appl. Phys., 1983, 54, 3404. Mandelis, A,, and Royce, B. S. H., Appl. Opt., 1984,23,2892. Williams, R., J . Chem. Phys., 1959, 32, 1505. Dutton, D., Phys. Rev., 1958, 112,785. Salvador, P., J . Appl. Phys., 1984, 55, 2977. Van Vechten, J. A., Phys. Rev., 1969, 182, 899. Tomkiewicz, M., J . Electrochem. SOC., 1979, 126, 2220. Murphy, J. C., and Aamodt, L. C., J . Appl. Phys., 1980, 51, 4580. Gerisher, H., in Eyring, H., Henderson, D., and Jost, W., Editors, “Physical Chemistry: An Advanced Treatise,” Volume 9A, Academic Press, New York, 1970, Chapter 5. Thomas, D. G., Hopfield, J. J., and Power, M., Phys. Rev., 1960, 119, 570. Wasa, K., Tsubouchi, K., and Mikoshiba, N., Jpn. J . Appl. Phys., 1980, 19, L475. Siu, E., and Mandelis, A., “Fourth International Topical Meeting on Photoacoustic, Thermal and Related Sciences, Montreal, Canada, 1985,” Technical Digest, WD 11.1, &ole Polytechnique, Montreal. Sze, S. M., “Physics of Semiconductor Devices,” Wiley, New York, 1969, p. 21. Paper A51235 Received July lst, 1985 Accepted November loth, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100299

出版商:RSC    

年代:1986

数据来源: RSC