摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 505 Shadow Spectral Filming A Method of Investigating Electrothermal Atomization Part 1. Dynamics of Formation and Structure of the Absorption Layer of Thallium Indium Gallium and Aluminium Atoms Albert Kh. Gilmutdinov Yu. A. Zakharov V. P. lvanov and A. V. Voloshin Department of Physics University of Kazan 18 Lenin Str. Kazan 420008 USSR A technique for investigating electrothermal atomization using shadow spectral filming is presented. This consists of obtaining an image of the interior of the furnace by backlighting with the analytical line of the element under study and recording the image of the shadow produced by the absorbing vapour by use of a cine camera. It is then possible to visualize the dynamics of the atomic vapour filling the furnace and simultaneously giving information on the temperature change of the furnace walls and platform.The method is used to investigate the dynamics of formation and the structure of the absorption layers of TI In Ga and Al atoms in different atomization regimes. The results obtained show strong inhomogeneity of the distribution of atoms within a graphite furnace. The extent of non-uniformity is different for different elements and is largely determined by the atomizer platform. A number of anomalous effects have been established 'inverse' atomization of the elements (the formation of atoms begins not near the platform where the sample is deposited but near the opposite wall); the anisotropy of the velocity of longitudinal and transverse propagation of the atomic vapours within a furnace; and the localization of Al under the platform.Keywords Electrothermal atomic absorption spectrometry; visualization of atomic vapour distribution; shadow spectral filming All versions of atomic spectrometry emission fluorescence absorption and photoionization require prior transforma- tion of the substance from a condensed state into an atomic vapour i.e. atomization of a sample. High-temperature flames inductively coupled plasmas various types of electric discharges and laser emission are normally used as atomizers. One of the most common techniques is elec- trothermal atomization (ETA) of a sample in a graphite furnace. Although the proposed method was originally suggested for use with atomic absorption spectrometry (AAS),' the technique owing to its simplicity and versatility has also found wide application in emission2y3 and atomic fluores- ~ e n c e ~ - ~ spectrometry and laser photoioni~ation.~J' Hence it can be said that the problems presented by ETA are characteristic of atomic spectrometry as a whole.The urgency of the investigation has considerably increased in recent years because workers in this area have come close to achieving controlled9 atomization. Whereas originally ETA was passive i.e. after a sample had been deposited in the atomizer the analyst had virtually no influence on the process of atomization more recently persistent attempts have been made to optimize this process in order to achieve predictable and controllable atomization.Numerous approaches to the solution of this problem have been proposed optimal temperature pro- grammes of atomizer heating;1° introduction of a plat- form;' ' construction of an isothermal atomizer;12 atomiza- tion in an atmosphere which is variable in its compo- sition13 or pressure;14 atomizer surface treatment;lS and finally the most radical intervention in the ETA process chemical modification of a sample.16 Despite the numerous inve~tigationsl~ of each of these techniques it would still be premature to refer to fully controlled atomization. Realiza- tion of this is certainly impossible without detailed studies of atom formation processes. In the 1980s) ETA studies reached a qualitatively new stage that of fundamental investigations characterized by attempts to describe the atomization process at a micro- scopic level.Such an approach became possible due to the use of various independent including non-optical methods these included electronic microscopy X-ray diffraction mass spectrometry and Auger and photoelectron spectros- copy among others. Nevertheless the most common and most appropriate for use in the analysis of real samples is AAS.18 However this technique confronts the user with two problems. Firstly the absorption signal gives information on the atomization which has been distorted to a certain extent by the recording system of the spectr~meter'~ and by atomic transfer in the atomizer gas phase.2o Secondly and more importantly the recorded signal is a complex integration of the atom distribution volume of the furnace being viewed.In order to improve the quality and amount of information obtained using AAS a recording technique that simultane- ously registers the signal from nine zones has been pro- posed.21 This technique was subsequently used for studying the atomization dynamics for a number of elements,21-26 and for the study of stationary absorbing layers.27 It was established as a result of these experiments that contrary to earlier ideas the distribution of atoms across the atomizer might not be homogeneous and the spatial gradient depends on the element under study the wall material and the composition of the atmosphere within the atomizer. The proposed method however has one limita- tion the information is supplied only from a narrow region along the diameter of the furnace which is seen through the slit of the monochromator.This drawback was overcome in other s t ~ d i e s ~ ~ ~ ~ where the data were obtained on the distribution of Na atoms over the entire cross-sections of the CRA-90 and HGA-2100 atomizers. In one instance a resonance Schlieren method was used,28 while in another recording of the attenuation of laser emission by a vidicon recorder with subsequent computer processing of the signal was employed.29 The results of these experiments show that the distribution of Na vapour across the furnace is definitely non-homogeneous with a pronounced decrease in concentration near to the graphite walls. In order to optimize the utility of these approaches these techniques require the use of tunable lasers in the ultraviolet (UV)506 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 region where most of the analytical lines are located. Another drawback of these techniques is the requirement of using amounts of metal that exceed those employed in real analyses 200 ng28 and 53 ng29 respectively. Thus the methods described make it impossible to investigate the atomic absorption layers of a wide range of elements. In the present paper a method known as shadow spectral filming (SSF) is proposed.30 It consists of obtaining the image of the atomizer volume by backlighting with the analytical line of the element of interest and recording by use of a cine-camera the shadow produced by the absorbing vapour. The possibilities of the method are illustrated by an investigation of the dynamics of the formation of the absorption layer of the Group I11 elements (Tl In Ga and Al) .The atomization of these elements exhibits a number of anomalies the nature of which is subject to different interpretations. When atomizing salts containing Al double peaks3' and anomalously high residence times of the atoms within the graphite furnace have been recorded.32 Mass ~pectrometry~~ has shown the complex composition of the gas phase where aluminium sub-oxides and carbides are present along with the atomic species. Finally at low rates of heating for microgram amounts of A1 explosive bursts of atomic absorption were o b ~ e r v e d ~ ~ ? ~ ~ against the back- ground of a smooth increase in the signal. The atomization of In Ga and TI in a graphite furnace is also complicated and not fully understood. It is for example accompanied by the formation of molecules in the gas phase which have been classed as hitherto unknown carbides.36 On the other hand mass spectrometric investi- gations of the atomization of In and Ga3' showed an important role for the sub-oxides.The depressive effect of certain halides on the absorption of these elements is well known.38 A characteristic feature of the absorption signals of the elements is their complex profile consisting of two peaks.39 The dynamics of formation and the structure of the atomic layers of these elements is investigated in the present work. The results of a study of the distribution of Al In and Ga molecules across the graphite furnace will be presented in Part 2 of this Experimental SSF Unit Image formation A schematic diagram of an SSF unit is shown in Fig.1. The main purpose is to obtain a monochrome image of the interior of a tube atomizer (3) in the cine-camera frame (13). The principal optical component is a fast MDR-2 monochromator (LOMO USSR) assembled according to the Czerny-Turner design. Two spherical mirrors (7) and (9) with focal lengths of 400 mm and two flat rotating mirrors (6) and (10) serve as the objectives. The monochro- mator is equipped with three 140 x 150 mm replaceable diffraction gratings (8) used in the first order. The gratings have 300 lines mm-l for the 1-2.5 pm spectral region 600 lines mm-l for 0.5-1.25 pm and 1200 lines mm-l for 200-600 nm. In the experiments described a slitless monochromator was used the entrance and exit slits were removed and the light passed through a 30 mm aperture in the casing.The linear spectrum light source (l) the emission from which illuminated the atomizer cavity (3) was placed at the focal point of the lens (2). Numerical estimates of the unit show that the light passing into the monochromator through the diaphragm (4) and the lens (5) forms an image of the tube cavity on the diffraction grating surface with an enlargement factor of 1.14. Subsequently the dispersed emission was focused by the lens (1 1) with an 1 . 2 3 Fig. 1 Schematic diagram of the shadow spectral filming unit. Lower set of figures are distances in mm between the elements of the unit. Focal lengths for lenses 2 5 and 11 are 60 614 and 112 mm respectively.(For ,details see text) aperture diaphragm (1 2) onto the cine-camera frame (1 3). By rotating the grating the image of the furnace cavity can be obtained from the light of the required spectral line. Thus at the distances indicated in Fig. 1 the monochrome image of the tube cavity (reduced 3.5 times) is obtained on the film. The cine-camera was used without its lens. In this situation the entire optical system of the unit (4)-(12) served as an objective. Lenses (2) ( 5 ) and (1 1) were made of quartz which made filming in the UV region possible. As the unit is designed for use over a wide spectral range of extended objects all types of optical aberrations are characteristic. According to estimate^,^' when the wave- length of light in the UV range is changed by 100 nm the focal length of the lens changes by 0.5 cm owing to chromatic aberration.In the present experiments when filming at different wavelengths this type of distortion could be compensated for by a corresponding displacement of the camera along the optical axis. The role of astigmatism is even more significant. calculation^^^ have shown that in the Czerny-Turner system even a point light source is repre- sented by two 0.3 mm long sections separated by a distance of 2.8 mm. The simplest way of reducing these distortions is the use of paraxial beams obtained by decreasing the diameter of the 'pencil' of rays. In an SSF unit this can be achieved by introducing a 3 mm diaphragm (1 2) into the system the aperture angle in object space (relative to the atomizer) becomes 4.8 x rad whereas in image space it is 16 x rad.This technique also decreases spherical aberration. The use of a smaller aperture however results in a decrease in the sensitivity of the unit. The resolution of the diffraction grating in this sytem is low owing to the small operational area thus with a 1200 lines mm-' grating the resolution is only 8200. This nevertheless is acceptable when electrodeless discharge lamps (EDLs) are used as the prime sources as they emit a relatively small number of atomic lines the intensities of which are sufficient for obtaining a bright monochrome image. In the experiments described below conventional electrodeless VSB-2 lamps were used. A typical picture obtained by the unit is presented in Fig.2. Frame (a) shows the interior of a cold furnace from the light of the Ga resonance line; the same picture but with the atomizer heated to 2000 "C is shown in frame (b). It can be seen that the luminous furnace walls produce a bright halo due to the light of the hot central part. The intermedi- ate dark ring in Fig. 2(b) is an image of the cool far end of the furnace and the holder. The near end of the tube also has a lower temperature and therefore remains invisible.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 507 Fig. 2 Image of the inner tube of the furnace in the light of the Ga 4 17.2 nm line. (a) Cold furnace; (b) furnace temperature 2000 "C and; (c) as for (6) but without the prime source The image of a furnace heated to the same temperature but without the translucent emission from the prime source is shown in Fig.2(c). It can be seen that the light from the furnace walls does not introduce any distortions into the image of the interior of the furnace. Spectral spatial and temporal resolution The spectral resolution of the unit is defined as its ability to produce an image of the atomizer cavity from the light at the required wavelength. This depends on the disper- sion of the spectrometer and the diaphragm of the field of sight (4). All the experiments in this work were carried out using a 1200 lines mm-I grating providing an angu- lar dispersion of 0.044 degrees nm-I. Calculation of the spectral resolution of the unit at the diaphragm (4) with a diameter of 6 mm yielded 39.5 nm. With a 1 mm diaphragm the resolution is 12.5 nm.Further decrease of the diameter leads to vignetting of the atomizer cavity image. The effect of the size of the diameter entrance diaphragm (4) is illustrated in Fig. 3 with an example of the resolution of the Ga lines at 392.4 403.3 and 4 17.2 nm. The light flux passing through the unit was recorded by a photomultiplier which was installed in place of the cine-camera. It is evident from curve A that using a diaphragm (4) with a diameter of 6 mm these spectral lines are not resolved. The use of a 2 mm diameter diaphragm results in an almost complete separation of the light fluxes (curve B) their magnitude decreasing insignificantly. Further decreasing the diameter of the diaphragm (4) to 1 mm somewhat improves the spectral resolution (curve C) however this is accompanied by a considerable loss in image brightness.Therefore in the experiments described below the diameter of the dia- phragm (4) is 2 mm which ensured a spectral resolution of about 15 nm. The location of the spectral lines of the elements under study at a distance of < 15 nm is generally not convenient for SSF as it results in the superimposition of images. However when the difference in line sensitivity is small the proximity of the spectral lines can be used to advantage. For instance the 394.4 and 396.1 nm A1 doublet cannot be resolved by the unit and a sufficiently sharp image of the furnace cavity is obtained simultaneously in the light of these two wavelengths. As they differ in sensitivity by a factor of only 1.5 the absorption picture is qualitatively retained the gain in brightness making filming at a higher speed possible.Images of the furnace cavity obtained at characteristic wavelengths of the Ga spectrum are presented in Fig. 4. The cross-hairs are formed by 0.4 mm W wires (numbers 1 and 3 are located 1 mm from the ends of the furnace number 2 is located at the centre). It can be seen from Fig. 4 (frames 2 and 4) that when the spectral lines of Ga are isolated the image of the entire furnace volume is sufficiently sharp and undistorted. In order to evaluate the spatial resolution of the unit the thickness of the wires in the pictures was " 390 400 410 420 430 Wavelengthlnm Fig. 3 Dependence of intensity of the light beam striking the recording system on the wavelength.Diameter of the diaphragm (Fig. 1,4) A 6 B 2; and C 1 mm. Diaphragm (Fig. 1 7) diameter equals 3 mm measured and compared with their true thickness taking into account the magnification of the photograph. These evaluations have shown that the spatial resolution was approximately equal to 0.15 mm which was sufficient for investigation of the 6 mm furnace. Similar filming of the cross-hairs was carried out with a furnace heated to 2000 "C. No image distortions were detected. This suggests that a change in the gas refraction coefficient due to heating has little effect on the formation of the images. Temporal resolution of the unit is determined by the speed of filming. Filming was performed with a 16 mm cine-camera at speeds of 12 16 and 24 frames s-* half the time being taken by exposure the other half by frame transport.Thus the minimum temporal resolution is equal to 0.04 0.03 and 0.02 s respectively. Recording and information treatment In order to record non-stationary absorbing layers A-2 negative cine-film (TASMA USSR) with a spectral sensitiv- ity range of from 230 to 650 nm was used. The speed was 450 ASA and the contrast coefficient 1.5. Density S produced on the film by light that had passed through the absorbing layer was measured by a microphoto- meter. A calibration graph of A=f(S) where f is a mathematical function was plotted in order to translate these densities into more informative absorbance values (A). For this purpose the neutral filters accounting for certain absorbance values were filmed before the experi- ment.As an example Fig. 5(a) presents the results of filter filming on the T1 377.5 nm resonance line. A current of 140 mA was supplied to the EDL which produces a moderate brightness of the line. The figures over the frames are the photometric values of the film densities the lower figures being the absorbance produced by the filter. The calibration graph plotted on the basis of these values is given in Fig. 5(b) curve A. It can be seen from the figures presented that the density can be visualized roughly within the range 0.2-1.1. As follows from curve A Fig. 5(b) these densities correspond to the absorbance range 0.2<A< 1.2. Absor- bance variations beyond this range are virtually indistin- guishable visually. This visual range may be considerably broadened however if the fact that densities depend not only on the absorbance of the absorbing layer but also the brightness of the prime source is taken into account.Fig. 5(b) curve B shows a calibration graph obtained under the same conditions as for curve A but with a brighter light source (the current supplied was 175 mA). Much larger absorbance values correspond to the visual range508 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 Fig. 4 Images of furnace interior and W cross-hairs obtained at various wavelengths of the Ga spectrum. Wire 2 is located in the centre of the furnace wires 1 and 3 are located 1 mm from the ends of the furnace L 0 1 2 Density Fig. 5 (a) Filming of neutral filters at the T1 377.5 nm line and (b) calibration graph of A=f(S) plotted from the results obtained from (a).Current supplied to EDL A 140 mA; and B 175 mA 0.2 <S< 1.1 in this instance. Absorbing layers producing absorbances of less than 0.8 are practically indistinguish- able against the background of bright penetrating light and only optically thick layers can be observed on the film. Thus by varying the brightness of the light source on the SSF unit the absorbing layers can be seen'roughly within the range 0.2-2 A (Fig. 5). (Note that photometry also provides reliable information in the range of densities that are visually indistinguishable up to A = 0.03. This mini- mum value is determined by the non-uniformity of furnace lighting and inhomogeneity of the film emulsion.) When considering the features of SSF it should be noted that the correct data on atomic distribution are obtained only in the absence of non-selective absorption.This limits the possibilities of the method for the investigation of the atomization of samples with complex matrices. Thus before using SSF it is necessary to ensure that there is no background scatter or molecular absorption; in the present study such .measurements were made using a standard spectrometer with background correction. Temperature measurement The temperatures of the wall and platform were measured by using a photodiode which had been calibrated prior to use using a disappearing filament pyrometer (OPPIR- 17 USSR). The emission from the platform was recorded through the dosing hole. The contribution of reflection from the hotter tube was taken into account by a procedure described previo~sly.~~ The wall temperature was estimated by recording the emission from the outside of the tube near the dosing hole.This simplified procedure is not very reliable but the accuracy (-t 50 "C) is sufficient for subse- quent qualitative analysis. Procedures A 10 pl volume of the solution was injected into the furnace with a microlitre pipette. The instrumental parameters of atomizer operation when the sample was deposited onto the wall and the platform are presented in Tables 1 and 2 respectively. It can be seen from the tables that compara- tively low heating rates were used in order to separate in time the different atomization mechanisms. The following lines were used for SSF T1 377.5 nm In 410.1 and 451.1 nm Ga 403.3 nm A1394.4 and 396.1 nm (superimposed).The choice of these lines makes it possible to demonstrate the operation of the unit over a wide range of the spectrum. The selected lines do not coincide with the molecular absorption spectra formed in the UV region during the atomization of these elements.36 Reagents and Materials All of the experiments in this paper were carried out using the atomizing system Graphite- 1 ,44 supplied with aJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 509 Table 1 Instrumental parameters for the Graphite-1 system with samples injected on the wall Time/s Internal gas flow/ Step TemperaturePC Ramp Hold ml min-l 1 90 5 50 150 2 300 (Tl) 1 20 150 1000 (Al) 10 10 150 1700 (Al)* 20 1 150 3 1800 (Tl) 5 5 0 2400 (Al) 4 4 0 2300 (AI)* 30 0 150 * Step for explosive atomization.Table 2 Instrumental parameters for the Graphite-1 system with samples injected on the platform Time/s Internal gas flow/ Step Temperature/"C Ramp Hold ml min-l 1 260 1 60 150 2 400 (Tl In Ga) 1 20 150 1300 (Al) 1 20 15 3 1800 (Tl) 5 5 0 2000 (In Ga) 5 5 0 2400 (Al) 4 4 0 Saturn-3 spectrometer (OKBA Severodonezk USSR). The atomizer was mounted in the optical train without any change in the design of the unit. Perkin-Elmer pyrolytic graphite coated graphite tubes (28 mm in length 8 mm o.d. 6 mm id.) and platforms were used in this study. Stock solutions at a concentration of 1000 mg 1-1 were prepared by dissolving TlN03 A1(N03)3 A12(SOJ3. 1 8H20 and In(N03)-3H20 in doubly distilled water.The stock solution of Ga with a concentration of 50 mg 1-l was prepared by dissolving 3.36 mg of Ga203 in 1 ml of concentrated HCl and subsequently diluting with water to a volume of 50 ml. A suspension of Ga203 was prepared by carefully mixing finely dispersed Ga203 powder with water in order to obtain a metal concentration of 50 mg 1-l. Test solutions were prepared by dilution of the stock solutions. All of the reagents were of analytical-reagent grade. Argon gas of the highest quality was used as the internal purge gas and as the sheath gas. Results and Discussion Thallium Owing to the high volatility of T1 the use of a platform and low ashing temperatures not exceeding 400 "C are rec- ommendedl7 for the determination of T1 in a graphite furnace.However under these conditions the atomization impulse has a complex shape especially noticeable at comparatively low rates of furnace heating. In Fig. 6(a) curve A represents the absorbance profile for 10 ng of T1 at a heating rate of about 200 "C s-' (lines T and Tp are images of the wall and platform respectively). This consists of two peaks which demonstrates the complexity of the atomiza- tion mechanism. In order to clarify the situation the same process was recorded by using the SSF method. The visual record of the dynamics of the formation of the T1 absorbing layer is given in Fig. 6(b). Recording began simultaneously with the atomization cycle and thus the first frame in the figure refers to a furnace temperature of 400 "C (see Table 2). Filming was carried out at 24 frame s-l with a moderately bright EDL which is described by curve A in Fig.5(b). As under these conditions absorbance values of less than 0.1 are visually indistinguishable only the second (high-temperature) peak of T1 atomization can be seen on the film. The atoms vaporizing directly from the platform subsequently fill the entire cavity of the furnace. Initially (frames 1 63- 172) weak non-uniformities are observed in the distribution of atoms across the furnace with a greater localization occurring near the platform. Subsequently (frames 175-1 95) these non-uniformities level out. In order to image a weaker low-temperature peak it is necessary to increase the amount of metal deposited. Fig. 7(a) shows the onset of the atomization of 500 ng of T1 which is the appearance of the first peak of atomization. [Note that the peak appears earlier for such an excessive amount of the metal; see Fig.8(6).] The results exhibit a clear anomaly the atoms enter the gas phase not from the platform where the sample was deposited but from the upper furnace wall. This leads to an inverse distribution of T1 concentration in the cross-section of the furnace with a maximum not above the atom source (the platform) but at the opposite wall. A possible explanation of this phenome- non is that the sample first vaporized as molecules which are invisible in the light of the T1 377.5 nm atomic line. Subsequently the molecules dissociate (or are reduced) on the higher temperature graphite wall and re-enter the furnace as atoms.Fig. 7(a) might be the image of this secondary process. In order to avoid the occasional deposi- tion of the sample in the region of the dosing hole when the pipette is inserted these experiments were repeated with the sample being deposited and dried on the platform outside the furnace. The results coincided completely with those presented in Fig. 7(a). The assumption regarding the molecular vaporization of T1 is also confirmed by the following experiment. A sample containing 500 ng of T1 was deposited on the platform and was ashed for 20 s at a temperature of 500 "C. Later the platform was removed while the furnace was re-heated following the same pro- gramme as before (Table 1). The absorbance profile appearing in this instance is presented in Fig. 6(a) curve B.This secondary signal increases with an increase in temper- ature and ashing time. This confirms the assumption that the sample first vaporizes as molecules which are subse- quently atomized on the furnace wall. The use of excessive amounts of metal leads to rapid saturation of density therefore the second peak of T1 atomization in Fig. 7(a) (final frames) is impossible to distinguish. In order to be able to observe this peak it is necessary to use a brighter light source. As shown by Fig. 5(b) an increase in the power supply to the EDL to 175 mA makes it possible to see the absorption layer in the region 0.3tAt1.7 (curve B). The results of filming the same atomization of 500 ng of T1 in this regime are shown in Fig. 7(b). It is a direct continuation of Fig.7(a) and it shows the high-temperature peak of TI atomization. It can be seen that in this instance atomic vapour forms directly at the platform and this process repeats all the features presented in Fig. 6(b). In addition to the initial inverse atomization the results given in Figs. 6 and 7 exhibit two further pronounced features. (2) It can be seen from Fig. 7 that no T1 atoms are present under the platform long after the complete filling of the upper half of the furnace (frames 76-94). This means that in the initial stages the length of the absorbing layer does not exceed that of the platform (1 5 mm). Thus at the beginning of atomization the distribution of T1 atoms is definitely non-homogeneous not only in the transverse but also in the longitudinal direction.In other words there is a pronounced anisotropy in the rate of propagation of T1510 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 1600 1400 800 3 5 7 9 1 1 Time/s Fig. 6 Absorbance versus time profile for the atomization of 10 ng of T1 curve A. Curve B is a result of the secondary atomization of T1 from the wall (see text). Curves T and T present the image of wall and platform temperatures respectively. (b) Imaging the process by SSF. Figures over the frames are their numbers beginning at the onset of atomization. The images were recorded at 24 frames s-* vapour in the graphite furnace i.e. the atoms propagate much faster in the transverse than in the longitudinal direction. (ii) Frames 67-79 in Fig. 7(a) clearly show a sharp decrease in the concentration of Tl atoms on the surface of the platform.As the temperature of the plat- form at this time is approximately 300 "C lower than that of the walls [see Fig. 6(a)] it can be assumed that the T1 vapour is adsorbed (or condensed) intensely at the mark- edly cooler platform. Subsequently as the platform is heated they re-enter the gas phase in the furnace. Thus there are at least three ways in which free T1 atoms can be formed as a result of dissociation (or reduction) of molecules at the wall; by direct atomization from the platform; and through the secondary desorption of atoms from the platform with heating. When the absorbance is recorded using a conventional spectrometer [curve A Fig. 6(a)] the monochromator entrance slit generally isolates a narrow zone along the diameter of the furnace.Therefore the signal A(?) (where ? represents time) is largely formed by the atoms located in this zone. As can be seen from Figs. 6 and 7 the distribution of the TI atoms across the furnace is non-homogeneous. At the same time as has been shown by calculation^,^^ the magnitude of measured absorbance depends not only on the number of absorhlng atoms but also to a great extent on their distribution along the diameter of the furnace. In order to obtain quantitative information on the non- uniformities of the atomic layer photometric measure- ments of the pictures should be carried out. An example of such photometry is given in Fig. 8 for the atomization of 500 ng of TI as described above. The probing beam of the microphotometer had a section with a 0.5 mm side.Data presentation is similar to that described previously:21-26 the absorbance vemm height profiles at time intervals of 0. I7 s (starting at 2.4 s) prior to peak absorbance is shown in Fig. 8(b) and a similar representation for times after peak absorbance is shown in Fig. 8(c). Fig. 8(d) shows the cross- section of the furnace and the region subjected to photome- try (shaded area). (Note that photometry considerably widens the range of densities that can be distinguished.) The microphotometer can reliably record non-uniformities of density within the absorbance limits of from 0.05 [the onset of atomization in Fig. 8(b)] to 1.5. The figure confirms all of the conditions for Tl atomization formulated earlier initially atoms enter the gas phase from the upper wall of the furnace leading to the inverse distribution of concentration [lower curves in Fig.8(b)]; later the flow of atoms from the platform begins to dominate which results in a normal distribution of atomic concentration with a maximum near the platform (upper curves); and finally at atomization end [Fig. 8(c)] the distribution of atoms proves to be uniform across the entire furnace. This shows that at this stage T1 vapours diffuse through the open ends of the furnace without noticeable interaction with the graphite wall. The results given above refer to platform atomization. Fig. 9(4 presents the dynamics of the formation of the absorption layer of the TI atoms for 20 ng of the metal using atomization from the wall. In this instance there are no anomalies in the distribution of the atoms.Initially [frames 44-52 and Fig. 9(b)] the distribution of atoms is as expected with a pronounced concentration maximum near the bottom of the furnace where the sample was deposited. Subsequently as the sample volatilizes the atomic distribu- tion becomes practically uniform [Fig. 9(4 frame 72 and Fig. 9(c)] which verifies the absence of any appreciable interaction between the T1 vapour and the wall. Such behaviour of the atoms being atomized from the wall is in complete agreement with the earlier investigations by Holcombe and co-workers.21-2* Indium Fig. 10(a) presents a record of the atomization of 100 ng of In from the platform. Recording was carried out at theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL.6 51 1 Fig. 7 Dynamics of the formation of the absorption layer of the T1 atom using atomization from the platform for 500 ng of the metal (24 frames s-l). Fig. (b) is a continuation of (a) 1.2 1 .o 0.8 0.6 0.4 0.2 0 0 3 6 2 6 10 0 3 6 Height /rn m Time/s Heig ht/mm Fig. 8 (a) Absorbance versus time profile for 500 ng of TI using atomization from the platform. Absorbance versus height contours at 0.17 s time intervals ( h ) prior to and (c) after the peak absorbance. ( d ) Cross-section of the atomizer. The shaded area in ( h ) and (c) indicates the platform 4 10.18 nm line as an intensive band of absorption by the In molecule is superimposed on the normally recommended 303.9 nm line.36 It can be seen that in spite of the atomization of a large amount of metal the distribution of the atoms across the furnace is fairly non-uniform.The main features of the process however are generally the same as for T1 as described above. In fact initially (frames 97-104) there is an inverse distribution of atoms with a sharp decrease in concentration near the platform. This effect is more evident during recording at the non- resonance wavelength of 45 1.1 nm. In this instance Fig. 10(b) gives an image of the behaviour of excited In atoms. The increase in the non-uniformity might be owing to the greater sensitivity of the excited atoms to temperature gradients. As can be seen from frames 76-82 Fig. 10(b) the temperature difference between the platform and the upper wall is 250 "C. It can be easily estimated that as the temperature increases from 1200 to 1 500 "C the concentra- tion of In atoms at the excited 2P3,2 level from which a transition with R=451.1 nm begins increases by about 20%.The dynamics of the formation of the absorbing layer of In atoms during sample atomization from the furnace wall512 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 0 3 6 Heig ht/m m 2 4 6 Timeis 1400 9 F +- 1200 g k I- F 'c--i 1000 0 3 6 HeighVmm Fig. 9 (a) Absorbance versus time profile for 20 ng of TI using atomization from the wall (broken line indicates change of wall temperature). Absorbance versus height contours at time intervals of 0.12 s (b) prior to and (c) after the peak absorbance. ( d ) Imaging of the process by SSF (1 6 frames s-') is virtually the same as for a similar situation using T1 (Fig.9). At the start of atomization there are weakly defined non-uniformities with a greater localization of the atoms near the bottom of the furnace where the sample was deposited. At the point of decay of the impulse the absorbing layer is virtually homogeneous. Gallium Atomization of 50 ng of Ga from the graphite platform recorded at the resonance wavelength of 403.3 nm is shown in Fig. 1 l(a). It can be seen that the formation of the absorbing layer proceeds in the same way as with the atomization of TI (Fig. 7). In this experiment hydrochloric acid was used to prepare the sample. As it is known that chlorine has a strong effect on the formation of the Ga signal,17 the atomization of a suspension of Ga203 was recorded.The results given in Fig. 1 l(b) almost coincide with the preceding record Fig. 1 l(a). This means that the effect of chlorine on the atom distributions presented can be ignored. It is noteworthy that frames 104-1 10 Fig. 1 l(b) indicate a sharp decrease in the concentration of Ga atoms near the lower surface of the platform. This shows that adsorption (or condensation) occurs only on the upper surface of the platform (frames 99-104) but also in the lower part. Thus it may be assumed that the considerably cooler platform acts in the graphite furnace as a 'sponge' first absorbing the atoms and then returning them to the gas phase in the course of subsequent heating. The imaging of Ga atomization from the furnace wall again yields a record that almost coincides with Fig.9 for a similar situation using T1 therefore it is not presented here. In summary it can be said that the atomization of T1 In and Ga proceeds via a fairly similar mechanism. The differences in the dynamics of the formation of their absorbing layers is of a quantitative rather than a funda- mental nature. A vital role is played by the molecules of these elements particularly in the initial stages of the atomization. The formation of molecular layers will be dealt with in Part 2 of this which gives a more detailed description of the atomization mechanism of these elements. Another feature common to all of the elements in question is a pronounced anisotropy of the propagation of the vapours in the furnace volume. The most likely cause for this is the considerable longitudinal non-isothermality of the graphite tube.Indeed the temperature at the ends of a 28 mm HGA-type furnace does not exceed 1000 "C when the centre of the furnace is at 2000 0C.46 Thus longitudinal atomic diffusion occurs in a medium with a rapidly decreasing temperature. This leads to absorption and/or vapour condensation at the cooler ends of the furnace. Another cause for the anisotropy could be the thermal expansion of the inert gas within the furnace as a result of its being heated. As the furnace has open ends and a dosing hole in the centre a picture of the gas flow during itsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 5135 14 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 515 expansion will be very complicated. For the extreme situation in the absence of*€he dosing hole simple expres- sions for the velocity of gas expansion through the ends of the furnace have been ~btained.~’~~* However under gas stop conditions the ends of an HGA-type furnace are to a certain extent closed to the expanding gas. Therefore it will partially escape through the sample aperture. The velocity of this flow v can be estimated on the basis of another idealized assumption i.e. that the ends of the furnace are completely closed (Fig. 12). The mass of gas inside the furnace will diminish with heating by the law rn(t)=p(t)V where pmconstantlir is the gas density and V the furnace volume varying in reverse proportion to the temperature V= nD2 L/4.Hence the rate of decrease in the mass equals 7TD2 constant dT X L X - X- V x - =- dm dt dt 4 P dt dP -= On the other hand the mass of gas dm escaping during the time dt through the dosing hole of diameter d is dm = v dt nd2p(T)/4 Assuming that in the course of the expansion all of the gas escapes through the sample aperture these values may be equated. This yields an expression for the velocity of gas flow through the dosing hole L dT (s T x d t v = - x - For HGA-type furnaces L=2.8 cm D=0.6 cm d=O.1 cm. As can be estimated from the temperature data presented in Figs. 6 10 and 11 for the onset of atomization of the elements being investigated (l/T) x (dT/dt)=O. 15 s-l which gives ve= 17 cm s-l. At the same time the velocity (vd) of longitudinal diffusion of vapour may be estimated by the formula vd%L/rd where 7 d = L 2 / 8 D * is the ‘diffusion’ residence time of atoms.The diffusion coefficients D in argon for the atoms of the elements in question are listed in ref. 49. Numerical estimates give v,=5-6 cm s-l. Under real conditions the gas will also escape through the ends of the furnace. However even if only 20% of the argon escapes through the dosing hole the values v and v prove to be comparable. The relative role of the thermal expansion is likely to decrease sharply as furnace temperature increases. How- ever at the commencement of atomization of elements of low and medium volatility such as TI In and Ga the contribution of this effect to the formation of the absorbing layer may be considerable.Aluminium Atomization in the analytical regime presupposes con- tinued heating of the atomizer to a given temperature when the internal sheath gas has been stopped. Fig. 13 shows film images of vapour filling the graphite furnace volume during Fig. 12 Idealized pattern of the thermal expansion of inert gas within the furnace. (See text for details) the atomization process. The images were recorded at 16 frames s-l. Atomization of this amount of metal (10 ng) produced sufficiently high densities of atomic vapour for direct observation of the process. It can be seen from the film that despite the absence of an internal argon flow there are pronounced gradients of A1 atom concentration in the cross-section of the atomizer with the atoms being local- ized near the walls of the graphite furnace. The sharp decrease in vapour density near the sample aperture is also noteworthy. This loss may be due to diffusion and thermal expansion in addition to the formation of stable oxides and cyanides resulting from the diffusion of air through the dosing hole.50 In this instance atomization occurs against the back- ground of a rapid rise in the temperature of the furnace.This is indicated on the film by the expanding light ring which is due to emission by the central part of the atomizer walls. The inner dark ring shows the more distant cooler end of the furnace which testifies to the presence of considerable longitudinal temperature gradients during heating of the furnace. Fig. 14(a) shows the record of atomization of 5 ng of A1 under conditions similar to that for stabilized temperature platform furnace (STPF) (Table 2).It can be seen that with atomization from the platform the general tendencies in the distribution of A1 vapour are the same as in the preceding example there are pronounced concentration gradients with a greater concentration of atoms being located near the furnace walls. In order to obtain quantitative data from the film photometric measurements were carried out using a microphotometer which enabled the construction of absor- bance contours by means of a calibration graph such as that shown in Fig. 5(b). The results are presented in Fig. 14(b) for four characteristic stages of atomization i. e. onset (frame 73) maximum (78) and drop (102 and 118) in the vaporization pulse.Again the concentration distributions prove to be in- verse with the maximum amount of the A1 atoms being located not above the platform but a considerable distance from it. This effect is not as pronounced as in the preceding example of TI and Ga but it is fairly distinguishable. This testifies to the considerable role of the A1 molecular species in the formation of atoms. Fig. 14 also shows an interesting vapour re-distribution during sample atomization at the Fig. 13 Dynamics of the formation of the absorption layer of the A1 atom using atomization from the wall for 10 ng of the metal. The images were recorded at 16 frames s-’516 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 73 78 102 118 Fig. 14 (a) Dynamics of the formation of absorption layer of the A1 atom using atomization from the platform for 5 ng of the metal.The images were recorded at 24 frames s-'. (b) Structure of the absorption layer at characteristic moments. The figures indicate absorbance values onset [frame 73 Fig. 14(b)] most of the A1 atoms are distributed in the furnace volume above the platform whereas they are largely localized under the platform at the end of the cycle (frames 102 and 1 18). For comparison recording of the atomization impulse in question by the traditional photoelectric method is shown 0.9 Q) 0.6 u m G s n a 0.3 0 3 5 7 Ti m e/s 2500 u 9 2 w 2000 g P I- 1500 1000 Fig. 15 Absorbance versus time profile for 5 ng of A1 using atomization from the platform. Curves T and Tp indicate the change of wall and platform temperature respectively in Fig.15 in which the stages represented in Fig. 14(b) are indicated. Besides imaging the dynamics of the formation of the atomic absorption layer the data presented afford addi- tional information on the character of platform heating. The images in Fig. 14(a) vividly show the lag in heating of the platform relative to that of the walls of the atom- izer when sample evaporation commences the temperature of the wall of the furnace will have almost reached the maximum value which is demonstrated by the bright halo that does not change with time (see also curve T in Fig. 15). In contrast the platform temperature Tp is constantly rising to reach its maximum value only at the end of the atomization cycle (frames 98 and 118).A marked inhomo- geneity in platform luminescence in the cross-section can be seen the emission of the surface layers of the platform is noticeably brighter than that of the interior (frames 83-87). This might be because of the reflection of light emitted by the walls but might also indicate non-uniformity in platform heating in the cross-section. Spikes Imaging This effect first described by L'vov and c o - w ~ r k e r s ~ ~ ~ ~ ~ has been the subject of numerous investigations and reviews on the subject have been p ~ b l i s h e d . ~ ~ - ~ ~ The effect consists of a rapid release of A1 against the background of a slow increase in the signal [Fig. 16(a)]. Spikes are regularly observed at slow heating (5-20 "C s-l) of microgram amounts of aluminium oxide.The SSF method gives a unique opportu-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 2 0 10 20 Time/s 2300 1700 517 Fig. 16 (a) Explosive atomization of 1 pg of A1 and ( h ) imaging of a single spike by the SSF method. The images were recorded at 24 frames s-l 0.5 al c m e 5 3 0.25 2 6 10 14 Ti me/s 2000 9 f! 2 \ 4- i! 1500 ? 0 Fig. 17 (a) Impulse atomization of 8 ng of A1 with an imposed explosive effect and (b) its imaging with the use of SSF. The images were recorded at 24 frames s-'518 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1 99 1 VOL. 6 nity to record the dynamics of development of each individual spike. As an example Fig. 16(b) shows an image of the second impulse of Fig. 16(a) occurring at 19 10 "C. It can be seen that the A1 atoms completely fill the cross- section of the atomizer in less than 0.04 s (frames 2 and 3) i e .the atomization has an explosive character. Subsequent dissipation of the vapour occurs near the location of the dosing hole within about 0.2 s (frames 4- 10). The explosive process in question is initiated and terminated at the side wall of the atomizer (frames 2 and 8) rather than at the bottom of the atomizer where the sample was initially deposited. It is evident that the distribution of A1 in the cross-section of the furnace is distinctly non-uniform at any moment in time. However in this instance the distribution is largely affected by internal gas flow (150 ml min-l). The spikes may also occur with the atomization of A1 using a more typical heating rate and amounts of sample.Under these conditions the spikes are randomly superim- posed on the principal absorption impulse thus introducing large errors in the results. A similar situation arose in the determination under STPF conditions of Vs4 and P.ss Fig. 17(a) shows the absorption signal obtained when atomizing 8 ng of A1 with a superimposed spike. The relevant record of the image is presented in Fig. 17(b) where the second frame shows an explosive impulse. It can be seen that the signal is initiated at the furnace walls rather than at the platform where the sample was deposited. This confirms the earlier assumptions2 that the explosive effect is due to the secondary atomization of A1 vapours adsorbed at the walls of the graphite furnace. It is interesting to note that in this instance also a picture similar to that shown in Fig.14 is observed by the end of the atomization cycle the atomic vapour is localized under the platform (frames 70-72). As this phenomenon is observed under gas stop conditions the following qualitative explanation can be suggested. The platform divides the atomizer into two parts unequal not only in size but also in terms of the composition of the gas phase. Evidently the concentration of oxygen under the platform is minimal whereas the concentration of mole- cules containing carbon is likely to be at a maximum. Thus at the bottom of the furnace conditions are created that are considerably more reducing than those above the platform. This should bring about an increase in the dissociation of the A1 molecules.40 The results of the investigation of different atomization regimes for A1 have been presented. A common feature of all the examples considered is a marked inhomogeneity of the distribution of atoms across the furnace with their pronounced localization near the furnace walls.In order to make unequivocal conclusions concerning the atomization mechanisms of the elements involved the distribution of the molecules in the furnace volume must be investigated. These results will be presented in Part 2 of this The authors are grateful to V. L. Makarov for useful comments on the results obtained in this work. References L'vov B. V. Atomic Absorption Spectrochemical Anaiysis Adam Hilger London 1970. Littlejohn D. and Ottaway J. M. Analyst 1979 104 1 138.Baxter D. C. Frech W. and Lundberg E. Analyst 1985 110,475. Dittrich K. and Stark H.-J. J. Anal. At. Spectrom. 1986,1,37. Dittrich. K. and Stark H.-J. J. Anal. At. Spectrom. 1987 2 63. Bolshov M. A. Zybin A. V. Koloshnikov V. G. and Smirenkina I. I. Spectrochim. Acta Part B 1988 43 519. 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Bekov G. I. Radaev V. N. andLetokhov V. S. Spectrochim. Acta Part B 1988 43 491. Laser Analytical Spectroscopy ed. Letokhov V. S. Nauka Moscow 1986. Gilmutdinov A. Kh. and Zakharov Yu. A. Izvestiya Akad. Nauk SSSR Ser. Fiz. 1989 53 1820. Slavin W. Carnrick G. R. Manning D. C. and Pruszkovska E. At. Spectrosc. 1983 4 69. L'vov B. V. Pelieva L. A.and Sharnopolskii A. M. Zh. Prikl. Spektrosk. 1977 27 395. Frech W. Baxter D. C. and Hutsch B. Anal. Chem. 1986 58 1973. Frech W. Lundberg E. and Cedergren A. Prog. Anal. At. Spectrosc. 1985 8 257. Hassel D. C. Rettberg T. M. Fort F. A. and Holcombe J. A. Anal. Chem. 1988 60 2680. Ortner H. M. and Kantusher E. Talanta 1975 22 581. Ediger R. D. At. Absorpt. Newsl. 1975 14 127. Slavin W. Graphite Furnace AAS. A Source Book Perkin- Elmer Norwalk CT USA 1984. Katskov D. A. Zh. Prikl. Spektrosk. 1983 38 181. Lundberg E. and Frech W. Anal. Chem. 1981 53 1437. Gilmutdinov A. Kh. and Shlyakhtina 0. M. Spectrochim. Acta Part B 1991 46 1121. Holcombe J. A. Rayson G. D. and Akerlind N. Jr. Spectrochim. Acta Part B 1982 37 3 19. Rayson G. D. and Holcombe J. A. Spectrochim.Acta Part B 1983 38 987. Holcombe J. A. and Rayson G. D. Prog. Anal. At. Spectrosc. 1983 6 225. McNally J. and Holcombe J. A. Anal. Chem. 1987,59,1105. Droessler M. S. and Holcombe J. A. J. Anal. At. Spectrom. 1987 2 785. Wang P. X. Majidi V. and Holcombe J. A. Anal. Chem. 1989,61 2652. Katskov D. A. Savel'eva G. O. Kopeikin V. A. and Grinshtein I. L. Zh. Prikl. Spektrosk. 1988 49 7. Stafford D. J. and Holcombe J. A. J. Anal. At. Spectrom. 1988 3 35. Huie C. W. and Curran C. J. Appl. Spectrosc. 1988 42 1307. USSR Pat. 1330520 MKI G 01 N 21/67. Katskov D. A. and Grinshtein I. L. Zh. Prikl. Spektrosk. 1980,33 1004. Zheng Y. Woodriff R. and Nichols J. A. Anal. Chem. 1984 56 1388. Styris D. L. and Redfield D. A. Anal. Chem. 1987,59,2891. L'vov B. V. and Savin A. S. Zh. Anal. Khim. 1982,37,2110. L'vov B. V. Dokl. Akad. Nauk. SSSR 1983 271 119. L'vov B. V. Norman E. A. and Polzik L. K. Zh. Prikl. Spektrosk. 1987 47 711. McAllister T. J. Anal. At. Spectrom. 1990 5 17 1. Dittrich K. Schneider S. Spiwakow B. Ya. Suchowejewa L. N. and Zolotov Yu. A. Spectrochim. Acta Part B 1979 34 257. Katskov D. A. Grinshtein I. L. and Krug!ikova L. P. Zh. Prikl. Spektrosk. 1980 33 804. Gilmutdinov A. K. Zakharov Yu. A. Ivanov V. P. and Voloshin A. V. J. Anal. At. Spectrom. submitted for publica- tion (0/4000B). Salmon S. G. and Holcombe J. A. Anal. Chem. 1978 50 1714. Goldstein S. A. and Walters J. P. Spectrochim. Acta Part B 1976 31 295. Welz B. Sperling M. and Schlemmer G. Spectrochim. Acta Part B 1988 43 1187. Burakov V. S. Esilevski B. A. Misakov P. Ya. and Pelieva L. H. Zh. Prikl. Spektrosk. 1985 42 336. Gilmutdinov A. Kh. Abdullina T. M. Gorbachev S. F. and Markarov V. L. Zh. Anal. Khim. 1991 46 1481. Wu. S. Chakrabarti C. L. and Rogers J. T. Prog. Anal. Spectrosc. 1987 10 11 5. Holcombe J. A. Spectrochim. Acta Part B 1983 38 609. Gilmutdinov A. Kh. and Fishman I. S. Spectrochim. Acta Part B 1984 39 171. L'vov B. V. Spectrochim. Acta Part B 1990 45 633. Sturgeon R. E. and Falk H. Spectrochim. Acta Part B 1988 43 421.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 519 5 1 L'vov B. V. Spectrochim. Acta Part B. 1989 44 1257. 52 Gilmutdinov A. Kh. Zakharov Yu. A. and Ivanov V. P. Zavod. Lab. 1989. 11 31. 53 Bendicho C. and de Loos-Vollebregt M. T. C.. Spectrochirn. Acta Part B 1990 45 547. 54 Manning D. C. and Slavin W. Spectrochim. .4cta Part B 1985 40 461. 55 Welz. B. Curtius A. T. Schlemmer G . Ortner H. M. and Birzer W. Spectrochim. Acta Part B 1986 41 1175. Paper 0/0289 7E Received J m e 2 7th I990 Accepted May 17th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600505

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 52 1 Tungsten-tube Electrothermal Atomizer WETA-90 Part 1. Design and Performance of the Atomizer Vaclav Sychra JiFi Doleial Robert HlavaC Libor PgtroS Olga VyskoCilova and Dana Kolihova Institute of Chemical Technology Technicka 5 166 28 Prague 6 Czechoslovakia Petr Puschel institute for Brown Coal 434 37 Most Czechoslovakia A new version of a transversely heated tungsten-tube electrothermal atomizer (denoted WETA-90) has been developed and tested for a number of elements. The performance of two types of tungsten tubes is compared. The capability of this system to reach stabilized temperature atomization is discussed. Characteristic masses for 23 elements are listed and compared with those obtained with graphite furnaces (heated graphite atomizers) following the original or modified stabilized temperature platform furnace concept.The WETA-90 atomizer features unique analytical parameters particularly for analytes which react with graphite and form analyte compounds of low volatility. Keywords Tungsten-tube atomizer; electrothermal atomization from tungsten surface; atomic absorption spectrometry; determination of refractory elements In a previous paper' a simple tungsten-tube atomizer was described consisting of two profiled tungsten strips forming a cylindrical cavity which could be accommodated in the workhead of a commercial Varian carbon rod atomizer (CRA-63 or CRA-90) and operated as an alternative to the graphite tubes and cups. The process of atom formation was also studied in this atomizer and it was demonstrated that solid phase or vapour phase thermal dissociation of analyte species is the most probable mechanism of atomization.2 Two years later further improvements were announced covering a new design of workhead and voltage and optical feedback circuits in the power supply.This resulted in the construction of an independent unit denoted as the WETA- 82 that was compatible with most existing atomic absorp- tion spectrometer^.^ This system featured an absence of carbide formation and memory effects homogeneous tem- perature distribution along the tube and a rapid and controlled heating resulting in high sensitivity and analytes of medium and low volatility were atomized under virtually isothermal condition^.^-^ Chakrabarti and co-~orkers*~~ studied both theoretically and practically spatial and temporal temperature distribu- tion on the tungsten-tube surface and in the gas phase in the tube.They reported a significant temperature gradient over the circumference of the tube and suggested possible improvements in the design of the atomizer with a view to making the temperature distribution more uniform. They also showed that the experimentally determined gradient between the temperature of the gas and the tube surface was smaller than that predicted. The WETA-82 atomizer was manufactured commercially in Czechoslovakia (Laboratory Instruments Prague) in 1984-1985. Despite the fact that almost 100 units have been sold and used routinely few papers dealing with analytical applications of this atomizer have been pub- l i ~ h e d .~ J ~ - l ~ Most of these applications cover carbide- forming elements and other analytes of low volatility such as Ba," rare earth e l e m e n t ~ ~ J ~ J ~ P5 and V.5 The intention of the present paper is to introduce a new model of transversely heated tungsten-tube electrothermal atomizer (denoted WETA-90). Compared with the WETA- 82 all parts of the system e.g. power supply workhead and the tube itself have been altered significantly taking into account the latest electrothermal atomizer technology and nearly ten years of experience with the operation of the original system. Experimental Instruments All measurements were carried out with a Varian Techtron AA 775 ABQ double-beam atomic absorption spectro- meter.The wavelengths of the analyte lines and lamp currents were as recommended by the manufacturer. A spectral bandwidth of 0.2 nm was used throughout. The atomizer surface temperatures were measured with a dual-wavelength pyrometer (Quotienten Pyrometer QP3 1 Leybold-Heraeus Hanau Germany). Dynamic tempera- ture measurements were made with a calibrated Ge photodiode. Absorbance-time and temperature-time pro- files were measured with a six-channel storage oscilloscope (Tesla OPD 600 ValaSskC MezifiEi Czechoslovakia). Sampling was performed manually with an adjustable 15 pl syringe (SGE Melbourne Victoria Australia). Reagents Specpure metals or compounds (Johnson Matthey Roys- ton Hertfordshire UK) were used for the preparation of stock solutions of the metals at a concentration of 1000 pg ml-l.All working solutions were prepared immediately before use by stepwise dilution of the stock solutions with doubly distilled de-ionized water and were acidified with nitric acid (except for Sb Sn and Zr where hydrochloric acid was used) to a final concentration of 1% v/v. Tungsten-tube Atomizer WETA-90 The WETA-90 consists of a power supply and control unit a workhead with the tungsten tube and a computer (Fig. 1). The workhead is permanently connected to the power supply and control unit by a supply cord which carries all the gas water power cables fibre optics and other electrical supplies. The power supply and control unit includes a three- phase transformer all power circuits supplying the power to the tungsten tube a computer-controlled gas control unit power supply for the microcomputer and part of the electronics including a 12 bit analogue-to-digital (ND) converter.The microcomputer (conventional 8 bit labora- tory-built based on an 8080A microprocessor) works in an operational system CP/M and has an internal RAM memory of 128 kbytes and an external memory consisting522 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 Power supply - 14 V-2500 A L I P a Atomizer workhead Constant current power supply 1 1 i A py 3 I L P a v) 4nalogue electronics 12 bit A/D converter 1 P - - * " - l l - - 1 0 n 4 0 \ I LUIILIUII~I \OV401 I 4 I Microcomputer (8080A) Fig. 1 Schematic diagram of a WETA-90 electrothermal atomizer of two 5 i in floppy-disk drives with a capacity of 2 x 360 kbytes.It also contains a single-chip microcomputer (con- troller) based on an 8048 microprocessor which is used solely to control the temperature of the tungsten tube. Instead of a voltage feedback as applied in the WETA-82 atomizer a resistance feedback is used to control the temperature of the tube in the range 40-1260 "C; the temperature in the range 1260-3200 "C is controlled with an optical feedback circuit incorporating a sensitive silicon photodiode specially made for this purpose. The perform- ance of the resistance feedback is illustrated in Fig. 2. Calibration dependences R= f(T) and U= f(T) (where R is the resistance of the tungsten tube Uis the output voltage of an amplifier of the optical feedback circuit and T is the temperature of the tungsten tube) for the resistance feed- 500 5 I + t K i I I 0 1 2 3 3.33 rlms Fig.2 Dependence of tungsten-tube current versus time -4 communication between microprocessors; B start of A/D conver- sion measured value is the zero line for correction of an amplifier offset; C switching on of a constant current (1 00 A) power supply; D start of A/D conversion measured value is voltage correspond- ing to a workhead temperature; E start of AID conversion. measured value is atomizer voltage corresponding to the resistance of a tungsten tube; F. switching off of the constant current ( 100 A) power supply; G calculation of time needed for supplying power to the tungsten tube H switching on of a power supply ( 14 V-2500 A) for time calculated in step G; I. calculation of real temperature values from measured data communication between microproces- sors; J switching off of the power supply ( 1 4 V-2500 A); and K.end of the control cycle back and the optical feedback respectively are stored in the memory of the microcomputer. At the beginning of each temperature cycle the resistance of the tungsten tube and the temperature of the workhead (including the tempera- ture of the tube) are first measured; from the data obtained the dependence R =f(v is recalculated for this particular temperature cycle. During temperature regulation of the tungsten tube by means of the optical feedback the temperature of the tube is measured at intervals of 560 ps; if it is higher (or lower) than the required temperature the power supply (1 4 V-2500 A) is either switched on or off.An automatic recalibration of the optical feedback circuit can be readily realized. During this recalibration the micro- computer gradually sets two different temperature values within the range of optical feedback but utilizes the resistance feedback for the temperature control. The tem- perature values obtained are then used for the correction of the dependence U= f(v. Both feedbacks feature high stability and reproducibility of temperature settings 2 4 K at 11O"C k 2 0 K a t 1OOO"Cand +40Kat300OoC.The accuracy of the tube temperature when different tubes are used is within f 5 K (t- 2% above 250 "C) .of the pre-set temperature value provided that the mass ratio of the upper and lower strips of the tube is within k 1% tolerance. The computer software allows 15 steps of the tempera- ture programme to be written by the operator.In each step the operator can select final temperature time (or heating rate) necessary to reach the pre-set temperature hold time mode of introduction of the sheath gas the composition of the sheath atmosphere and three independent commands for external instruments and accessories. The heating rate of the tube can be set continuously in the range 0.01-30 K ms-l. The ratio of argon to hydrogen in the sheath gas may be altered from 0 to 0.75 in 0.05 increments. Each programme can be arbitrarily modified and stored on the floppy disk. The tungsten tube is located in a water-cooled and gas- tight workhead which is shown schematically in Fig. 3. The tube is heated transversely to its longitudinal axis.The workhead is opened by swinging away the right-hand sidewall by means of a cam disc. Unlike the WETA-82 workhead this workhead features so called 'free clamping' of the tungsten tube laid in the recess of the contactJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 523 electrodes which prevents rapid deformation of the tube owing to dilation of the material. In order to decrease the temperature gradient over the circumference of the tube to a minimum value the distance between the contact workhead electrodes was increased compared with the WETA-82 workhead. Thus the cooling action of the water- cooled contact electrodes on the wings of the tube was diminished. Two iris diaphragms mounted at the sidewalls of the workhead and fitted with circular quartz windows serve for rapid and precise alignment of the optical beam through the tungsten tube; the diaphragm close to the entrance slit of the monochromator can efficiently prevent the excess of radiation from the hot tube from falling on the entrance slit.b Fig. 3 WETA-90 tungsten-tube workhead The workhead enables two modes of introduction of the sheath atmosphere into the compartment where the tube is placed. The sheath gas can flow from the bottom part of the workhead external flow and/or from cones directed to both ends of the tube internal flow. The role of the internal gas flow is to help to remove products of drying and pyrolysis from the inner volume of the tube. Contact sensors measuring the resistance of the tube for the resistance feedback are carefully insulated from the other parts of the workhead in order to overcome the distortion of measured resistance values due to contact resistance between the contact area on the workhead electrode and the tube itself. The important parts of the workhead are also a sensor measuring the instantaneous temperature of the workhead and a fibre optic monitoring the radiation from the end of the lower part of the tungsten tube.The tungsten tube itself consists of two profiled tungsten strips 0.127 mm thick and 20 mm wide manufactured by highly sophisticated technology in Metallwerk Plansee (Reutte Tirol Austria).14 The strips form a tube 20 x 6 mm i d . A sampling hole is drilled in the centre of the upper strip. A sampling microboat pressed into the bottom part of the lower strip can accommodate up to 25 pl of the sample solution. A standard tube is shown in Fig.4(a). The physical dimensions of the tungsten tube are very similar to those of most commercial graphite furnaces. The WETA-90 workhead can also accommodate modi- fied tungsten tubes which consist of the standard upper part (a) Fig. 4 Different designs of tungsten tubes (a) standard tube; ( b and c) modified lower strips of the tube Fig. 5 Tungsten tube modified lower part with sampling micro- boat and overflow rims and a modified lower strip [see Fig. 4(b) and (c) and Fig. 5].15 Because of the change of the effective cross-section for current in the modified wings of the lower strip these tubes are expected to exhibit the so called ‘autoplatform effect’ i.e. a delay in heating of the central part (microboat) of the lower strip.A similar effect was achieved by Lawson et all6 by ‘end heating’ (heating the forked supports) of an 18 mm CRA atomizer. The performance of these tubes is discussed under Results and Discussion. Results and Discussion Measurement of Surface Temperature Distribution in Modi- fied Tubes In order to verify the predicted performance of the modified tungsten tubes time-temperature dependences at5 24 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 five different test points (see Fig. 6) on the surface of the modified tungsten strip located for the purpose of these measurements in the upper part of the tube were investi- gated. Because of the response curve of the photodiode used for the dynamic temperature measurements these depen- dences could be measured with reasonable precision only at temperatures above 1400 K (Fig.7). From this figure it can be seen that throughout most of the heating cycle the temperature in the middle part of the strip where the sample is deposited test points 2 and 3 lags behind the temperature of the end of the strip test point 1. This temperature difference during the initial part of heating is typically 50-250 K the corresponding delay in time being typically 50-1 50 ms. The temperature at the test points 1,2 and 3 is relatively quickly balanced when the final pre-set tube temperature is reached and the power is cut off. The 4. fin 1 Fig. 6 Temperature test points on the modified lower tungsten strip I 1 2600 1 I 2 400 1600 0 I 0.2 0.4 0.6 0.8 1.0 1.2 t / S Fig.7 Time-temperature variation at different test points on the modified lower tungsten strip. Test points refer to those given in Fig. 6 3000 2500 0 0.2 0.4 0.6 0.8 1.0 tls Fig. 8 Temperature difference between test points 1 and 3 as a function of time and heating rate. Solid lines without points represent temperature variation at various heating rates and the solid lines with points represent temperature difference between test points 1 and 3 at various heating rates. A 1.5; B 5; and C 15 K ms-' 1 1 .o 0 C 2 u) 2 0 J3000 2000 1000 I I 1 10 0 1 2 3 tls Fig. 9 Copper (75 pg) atomization from A standard. tungsten tube; and B tungsten tube with modified lower strip (c). Line C represents the tube temperature variation fact that the ends of the tube are heated slightly faster than the middle part of the lower strip should prevent condensa- tion of the analyte at the ends of the tube and diminish the loss of analyte due to expulsion.Slight overheating at test point 4 as expected is due to an increase in the local thermal and/or electrical resistance at that point. The temperature differences between test points 4 and 5 and test points 1-3 indicate that there is no significant temperature gradient over the circumference of the tube. Fig. 8 shows the temperature difference between test points 1 and 3 as a function of time and heating rate. The higher the heating rate the higher the temperature differ- ence between test points 1 and 3 in the initial part of the heating cycle.The temperature distribution measured in the modified tungsten tubes should result in a shift in the time of appearance of the analyte i.e. in a shift of the analyte peak towards the region of stabilized temperature atomization as compared with the standard tubes. This is shown in Fig. 9 where the peaks for Cu obtained using the standard tube and from the modified tube are compared. The modified tube is recommended for the determination of analytes of high and medium volatility. Since the 0.300 0) C ; 0.200 2 a 0 0.100 n " 2400 2600 2800 3000 3200 7°C Fig. 10 Change in absorbance signal at the W 255.14 nm line with the atomizer temperature and hydrogen flow rate A 100; B 400; C 800; and D 1450 ml min-l. Flow rate of argon 2 1 min-'; heating rate 5 K ms-l; 20 ,ul H,O added prior to the atomizationJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 525 Table 1 Typical experimental characteristic masses of elements Atomization Characteristic mass/pg temperature/ Element Wavelength/nm "C WETA-90* HGA-STPF method? Ag A1 As Ba Bi Cd Cr c u DY Er Ga Mn Ni Pb Sb sc Sr Sn Tm V Y Yb Zr 328.1 308.2 193.7 553.6 306.8 228.8 357.9 324.8 42 1.2 400.8 287.4 279.5 232.0 283.3 217.6 39 1.2 460.7 224.6 37 1.8 3 18.5 410.2 398.8 360.1 1800 2700 2400 2600 2200 1800 2600 2400 3000 3000 2400 2300 2600 1800 2500 3000 2600 2400 2800 3000 3100 2800 3200 0.8 9.7 1.5 7.4 0.2 0.7 0.8 5.5 6.2 0.7 2.8 3.1 4.4 0.14 2.5 1 1 1 1 19 11 23 40 600 0.6 1.2 11.8 15 22 4.3$ 0.4 2.4 2.5 6.5$ 17$ 1.7 7.5 7.7 8.7$ 0.8$ 3.1$ 12 22 20 24$ 38$ 1.7 - *Values based on pe?k height measurement under optimized heating rate and hydrogen flow rate conditions.?Peak area values. $Values obtained from ref. 17; all other values from refs. 1 8 and 19. modified tubes exhibit significantly shorter lifetimes com- pared with the standard tubes even when the use of very high heating rates and very high atomization temperatures are prevented for some complex analytical applications a rapid heating of the standard tube combined with chemical modification of the analyte is preferable to provide a shift in the analyte pulse. Determination of Tungsten in the Gas Phase of the Tube Recently a paper has been published13 by one user of the original system WETA-82 indicating that there is too much atomic tungsten in the vapour phase of the tube at a relatively low temperature which would cause spectral interferences.Such observations were not made in the present study as can be seen from Fig. 10. A relatively small absorption signal for tungsten was observed at the most sensitive tungsten line at 255.14 nm above 2800 "C. No non-specific absorption as a result of the presence of tungsten species was observed. Since the main precursor of the tungsten atoms and other tungsten species is believed to be tungsten trioxide the discrepancy in the paper mentionedI3 could be explained by the superior performance of the WETA-90 workhead i.e. by lower oxygen partial pressure in the sheath gas as compared with the WETA-82. Figures of Merit Since the application of high heating rates together with the non-porous nature of the tungsten surface results in the generation of absorbance peaks with half-widths which are very often less than 100 ms the ratio of peak height to peak area is much higher for the WETA-90 than for graphite furnaces and peak height is usually used for signal evalua- tion.Characteristic masses based on the peak height evaluation and atomization temperatures for 23 elements are listed in Table 1 and compared with those obtained with graphite furnaces (heated graphite atomizers HGA) utiliz- ing the original or modified stabilized temperature platform furnace (STPF) concept.17J8 The results for the WETA-90 are in most instances better (results are comparable for Dy and Y) than those for the HGA furnaces. When comparing the results for some carbide-forming elements it should be taken into account that the modified STPF concept assumes that the analyte is atomized from a tantalum platform inserted into a tanta- lum lined graphite furnace which is a very complicated and impractical procedure.Lifetime of the Tungsten Tube The lifetime of the tungsten tubes depends of the oxygen and nitrogen content in the protective atmosphere acidity of the solutions analysed sample matrix and the tempera- ture programme used. At 2500 "C the tube can be re-used 200-350 times provided that hydrogen ( 10% v/v) is added to the argon purge gas. Despite the improved clamping of the tube in the workhead slight distortion of the tube occurs after 200-300 hundred firings. The lifetime of the tungsten tubes is significantly reduced when samples with complex organic matrices e g .petro- leum samples that leave an appreciable amount of reactive carbon in the tube are analysed. Tungsten carbide formed in the furnace at high temperature rapidly changes the chemical and mechanical properties of the tube. These problems could be partly overcome by analysing such samples with a tungsten platform inserted into the tungsten tube. Problems encountered during the analysis of organic samples in the WETA-90 will be the subject of a separate paper. Conclusions The new design of the tungsten-tube furnace workhead and power supply and the addition of a microcomputer incor-526 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 porated in the WETA-90 electrothermal atomizer signifi- cantly improved the performance and ease of manipulation of the system.It seems to be one of the few tube atomizers to date which features simultaneously homogeneous surface temperature distribution along the tube and over the circumference of the tube spatial and temporal isothermal- ity almost total absence of memory effects for carbide- forming elements high sensitivity and considerable free- dom from matrix interferences. The real value of this system will probably be recognized when electrothermal atomic absorption spectrometric assemblies in which tung- sten and graphite tubes are easily interchangeable become available and instruments for use with atomic absorption with a fast response have been developed. Two prototypes of the WETA-90 are currently being tested one in this laboratory and the other in the applica- tion laboratory of Bodenseewerk Perkin-Elmer Uberlingen Germany.The capability of the WETA-90 to solve practical analytical problems will be discussed in future papers. References Sychra V. Kolihova D. VyskoEilova O. HlavaE R.. and Puschei P. Anal Chim. Acta 1979 105 263. VyskoEilova O. Sychra V. Kolihova D. and Puschel P. .4nal. Chim. Acta 1979 105 271. Puschel P. Formanek Z. HlavaE R. Kolihova D. and Sychra V. Anal. Chim. Acta 1981 127 109. Sychra V. Kolihova D. HlavaE R. Doleial J. Piischel P. and Formanek Z. in Wissenschaftliche Beitrage 'Analytiktrqf- .fen 1982' K.M.U. Leipzig 1983 p. 154. HlavaE R. Ph.D. Thesis Prague Institute of Chemical Technology 1986. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Ortner H.M. Birzer W. Welz B. Schlemmer G. Curtius A. J. Wegscheider W. and Sychra V. Fresenius 2. Anal. Chem. 1986,323 68 1. Sychra V. Kolihova D. HlavaE R. Doleial J. VyskoEilova O. Puschel P. Formanek Z. and Ortner H. M. paper presented to the X Conference on Analytical Atomic Spectros- copy Torufi Poland 1988. Chakrabarti C. L. Delgado A. H. Chang S. B. Falk H. Huton T. J. Runde G. Sychra V. and Doleial J. Spectro- chim. Acta Part B 1986 41 1075. Chakrabarti C. L. Delgado A. H. Chang S. B. Falk H. Sychra V. and Doleial J. Spectrochim. Acta Part B 1989 44 209. Komarek J. and Ganoszy M. Collect. Czech. Chem. Com- mun. 1991 56 764. Koiuinikova J. Chem. Listy 1984 78 1209. Zemberyova M. Ph.D. Thesis Comenius University Bratis- lava Czechoslovakia 1985. Krakovska E. J. Anal. At. Spectrom. 1990 5 205. Puschel P. Patent No. B1 174 728 Czechoslovakia 1978. Ortner H. M. Wilhartitz P. Doleial J. HlavaE R. Sychra V. and Puschel P. Patent Application No. PV-75-89 Czecho- slovakia 1989. Lawson S. R. Dewalt F. G. and Woodriff R. Prog. Anal. At. Spectrosc. 1983 6 1. L'vov B. V. J. Anal. At. Spectrom. 1988 3 9. L'vov B. V. Nikolaev V. G. Norman E. A. Polzik L. K. and Mojica M. Spectrochim. Acta Part B 1986 41 1043. Grobenski Z. paper presented at the X Conference on Analytical Atomic Spectroscopy Toruii Poland 1988. Paper 0/05615D Received December I4th I990 Accepted May 16th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600521

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 527 Electrothermal Vaporization Sample Introduction System for the Analysis of Pelletized Solids by Inductively Coupled Plasma Atomic Emission Spectrometry Vassili Karanassios,* J. M. Ren and Eric D. Salint Department of Chemistry McGill University 80 1 Sherbrooke St. West Montreal Quebec H3A 2K6 Canada A new approach for solid sample introduction into a furnace for use with an inductively coupled plasma has been developed and tested with atomic emission spectrometry. A powdered sample is mixed with graphite and pressed into a pellet. The pellet is placed between the electrodes of a modified electrothermal vaporization device. A current causes rapid ohmic heating of the pellet and results in analyte vaporization. The vapour is swept into the plasma by an Ar carrier gas stream.In this preliminary report system characterization with single element oxide standards and testing with powdered botanical samples are described. The system shows considerable promise for rapid screening of botanical samples of environmental concern. Detection limits for Cd Zn and Pb of 1 3 and 0.6 ppb (300 800 and 150 pg) were obtained using the oxide standards. Keywords Electrothermal vaporization; sample introduction; inductively coupled plasma atomic emission spectrometry; powders; solids analysis Several approaches aimed at extending the analytical capability and utility of the inductively coupled plasma (ICP) by employing methods for the direct analysis (e.g. with little or no pre-treatment) of solid samples are currently being investigated.'-* Included in these are elec- trothermal vaporization (ETV)9-46 and direct sample inser- tion (DSI)47-s2 devices arc and spark v a p o r i ~ a t i o n ~ ~ - ~ ~ laser a b l a t i ~ n ~ ~ - ~ ~ electrical vaporization of thin filrnP and p o ~ d e r ~ ~ - ~ ~ and s l ~ r r y ~ ~ - ~ ~ ~ sample introduction systems.Of these ETV devices are most commonly used. Com- mercial and laboratory constructed ETV devices have been used with ICP atomic emission spectrometry (ETV-ICP- AES)9-33 and ICP mass spectrometry (ETV-ICP-MS)34-46 for the analysis of l i q ~ i d ~ - ~ ~ * ~ ~ - ~ ~ and solid sample^.^^-^^ In solid sampling ETV the material to be analysed is typically placed into a sample holder (z.e. a graphite cup or rod) in the form of a powder.In ICP spectrometry the role of an ETV sample introduc- tion system is to dry and/or ash (char) a sample and to generate analyte vapour. The vapour is swept by a carrier gas stream (typically Ar) into the ICP (Fig. 1) for further atomization and excitation/ionization. This is in marked contrast to electrothermal atomic absorption spectrometry (ETAAS) using a graphite furnace in which the ETV device (ie. the furnace) must generate an atomic population as a precursor to atomic absorption. Three key advantages are realized by coupling ETV devices to ICP atomic emission spectrometers. Firstly the matrix effects in ETV-ICP-AES are not as significant as in ETV-AAS because the ETV device simply vaporizes the analyte species whereas in ETV-AAS it must generate free analyte atoms.Secondly vaporization (ETV device) is separate from atomization and excitation/ionization (ICP). The separate control of the ETV and the ICP facilitates independent optimization and results in a system with an overall improved analytical capability. Thirdly when coupled to a polychromator ICP- AES system simultaneous multi-element determinztions can be made. There are also advantages to using ETV devices with ICP-MS. For example the lack of continuous sample aspiration into the plasma eliminates water vapour and results in 'dry' p1asmas.35~42.50~51 As a consequence reductions in spectrosc~pic~~ (i. e. spectral overlaps arising from polyatomic oxide and hydroxide species) and non- * Present address Department of Chemistry University of t To whom correspondence should be addressed.Waterloo Waterloo Ontario N2L 3G1 Canada. Entrance Refractor Concave slit plate grating Plasma box \ / \ Data control I :! transDort - Graphite pellet electronics Sample space Fig. 1 Pellet-ETV-ICP-AES system spectroscopics1 (e.g. matrix induced signal changes) inter- ferences are observed. In spite of the advantages direct solid analysis by ETV is not without shortcomings. These are partly a result of the inherent difficulties associated with finding appropriate calibration standards and with sample handling weighing and the assurance of homogeneity. The last point is an important consideration in the analysis of solid .samples. For example the 0.1-5 mg sample mass typically used with solid sample ETV deviceslo1J02 is difficult to handle and because of ~ampling'~~JO~ and homogeneitylo3-lo6 considera- tions might not be representative of the composition of the bulk of the sample.This last issue is addressed in this work by mixing 25 mg of a powdered sample with 225 mg of graphite and forming a 0.25 g pellet. Work in this laboratory with a pellet-DSI device,52 in which a pellet is directly inserted into the ICP demonstrated that the homogeneity problems were reduced and the detection limits significantly improved by the larger sample mass allowed by this system. The larger sample mass is also easier to handle. These pellet-DSI system concepts are directly transferable to the pellet-ETV system used in this work.528 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL.6 In the proposed system a pellet is placed between two electrodes (Fig. 1). The current applied between them causes ohmic heating of the pellet and results in analyte vaporization. Because the pellet also serves as a sample container the need for a sample holder is eliminated. In addition the direct contact of the pellet with the electrodes and the intimate contact of the heated graphite with the sample allows large amounts of sample to be heated rapidly. In this first report a pellet-ETV sample introduction system for ICP-AES is described. This unoptimized proto- type sample introduction system shows considerable prom- ise for the analysis of inorganic powders and for the rapid screening of botanical samples of environmental concern. Experimental Instrumentation The basic components of the ETV-ICP-AES system devel- oped in this work are shown in block diagram form in Fig.1. A list of instrumentation and materials suppliers is provided in Table 1. Basic instrument specifications and typical operating conditions are given in Table 2. Table 1 List of suppliers of instrumentation and materials ICP Spectrometer Readout electronics Acquisition and control ETV and software microcomputer Mixedmill Pellet press Graphite powder Oxide standards Botanical samples Plasma-Them Kresson NJ USA Jarrell-Ash Model 90750 Thermo Jarrell-Ash Franklin MA USA Technical Service Laboratories Missisauga Ontario Canada AST X-former AT 10 MHz 80286 CPU AST Research Irvine CA USA Model HGA 2200 Perkin-Elmer Norwalk CT USA Model 5 100 Spex Industries Metuchen NJ USA Parr Instrument Moline IL USA Bay Carbon Bay City MI USA J.T. Baker Phillipsburg NJ USA Fisher Scientific Fair Lawn NJ USA Aldrich Milwaukee WI USA Ontario Ministry of the Environment Inorganic Trace Contaminants Section Rexdale Ontario Canada Table 2 Instrument specifications and typical operating conditions ICP R.f. generator Frequency Maximum power output Torch Typical power Outer (coolant) gas Intermediate (auxiliary) gas Central (nebulizer) gas Observation height Pol ychromator Focal length Grating Slits Detector Plasma Therm Model 2500 27.12 MHz crystal controlled 2.00 kW Fassel-t ype 1.00 kW forward < 10 W reflected Ar 14 1 min-I Ar 0.5 1 min-' Ar varied from 0.2 to 1.0 1 min-I 15 mm above the load coil Jarrell-Ash Model 90750 0.75 m 2400 grooves mm-I 25 ,um entrance 50 and 100 ,urn exit PMT Readout electronics Technical Service Laboratories (Table 1 ) Observation time 16 s Number of data points 200 Temperature*/"C Ramp/s Hold/s ETV device- Drying cycle Oxide standards 100 tl 2 Botanical samples 300 t l 60 Oxide standards 200 tl 1 Botanical samples 400 1 120 Oxide standards 1250 1 5 Botanical samples 1250 1 5 Charring cycle Vaporization cycle * Nominal values as shown on the Perkin-Elmer controller not measured.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 529 0 2 4 6 8 10 12 14 16 Time/s Fig. 2 Typical analyte emission temporal behaviour of 250 ng of Pb (obtained running a single element oxide standard) Electrothermal vaporization produces a plug of analyte vapour which when introduced into the plasma generates a transient emission signal.A typical example of analyte emission temporal behaviour is shown in Fig. 2 for 250 ng of Pb. The transient nature of the emission signals (Fig. 2) necessitates the use of a direct reading spectrometer for simultaneous multi-element analyses. This type of signal also dictates the nature of the analogue electronics as the analyte emission temporal behaviour must be monitored as a function of time. However the integrating readout electronics found in most commercially available direct reading ICP spectrometers are sub-optimal for the acquisi- tion of transient signal^.^^-^^ Furthermore because digitized analyte emission temporal behaviour must be examined by the operator during experimentation it determines the software requirements for data acquisition and for signal display and pro~essing.~~ The multichannel photomultiplier tube (PMT) based direct reading spectrometer (Table 2) used for this work was modified by Technical Service Laboratories in order to handle transient signals. 107~108 Briefly the current output of each PMT channel is integrated (for a programmed period of time typically a few ms) using a high-speed operational amplifier with a 100 pF capacitor in its feedback loop.The resultant voltage is digitized using a 12-bit analogue-to- digital converter interfacedlo7 to an IBM PC compatible microcomputer.109 The software allows one channel to be interrogated repeatedly with a minimum conversion time of 50 ps per data point. A number of converted values are then summed to build the dynamic range.The software also permits repeated sequential interrogation of up to 50 channels with a minimum conversion rate of 2 ms per data point per channel. The spectrometer was a direct reading instrument origi- nally designed for an ax. spark source. The direct reader was modifiedlo7 by adding a computer-controlled galvani- cally driven refractor plate behind the entrance slit (Fig. 1). This modification allows rapid measurement (i.e. 10 ms per data point per channel) of the emission signal on and off the spectral peak. In this way a quasi-simultaneous observation of analyte emission temporal behaviour (e.g. on-peak measurement) and background (off-peak measure- ment) can be obtained. This is an important consideration especially for samples with complex matrices.52 The heart of the sample introduction system shown in Fig.1 is the ETV device. This is a commercially available ETAAS system (Table l) which has been slightly modified to accommodate rapid interconversion between an ETAAS configuration and a pellet-ETV-ICP system. The furnace was modified (Fig. 3) by removing the graphite tube and by replacing the graphite contact rings and the left and right observation windows with machined brass blocks (identi- Water k 3 . 0 c m i Water Base Fig. 3 Cross-sectional view of the pellet-ETV device (illustration to scale) Fig. 4 Sample holder (dimensions in cm illustration to scale) fied as brass electrodes in Fig. 3). The modified furnace assembly is enclosed within a Pyrex chamber. The chamber is a hollow cylinder (3.4 cm in length and 2.7 cm in diameter) with two tube connections.One tube serves as a carrier gas inlet and the other as an outlet. Vaporized samples are routed into the plasma using Tygon tubing ( ~ 4 5 cm in length = 5 mm i.d. and x 7 mm 0.d.). As this pilot study was aimed at testing the validity of the pellet-ETV concept the design parameters of the ETV device (i.e. chamber volume and geometry and tube length) were not optimized. Central to the ETV device is the sample holder (Fig. 4). This is a notched coarse-threaded brass screw. By using a pair of tweezers a pelletized sample is placed at the notch located at one end of the sample holder (Fig. 4). At the other end there is an O-ring to prevent vapour leakage.The sample holder is positioned inside the brass electrode as shown in Fig. 3. Electrical contact between the electrodes is achieved through the pellet (Fig. 3). Drying charring and vaporization are obtained by programming the time and the intensity of the current applied to the pellet. During vaporization the pelletized sample is heated to incandescence. Method Sample preparation consisted of grinding botanical samples manually in a mortar for about 15 min (the average particle size was about 160 pm) and mixing the powdered sample with spectroscopically pure graphite (1 + 9). Powdered oxide standards were prepared by dilution with spectros- copically pure graphite according to a procedure described previou~ly.~~ A typical sample preparation time of 15-20 min was required for batch preparation of 5 pellets.An accurately weighed portion of this mixture (250 mg) was hand pressed using a Parr press (Table 1) to form a pellet which is 9 mm long and 4 mm in diameter. Pellets placed in5 30 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 the ETV device were ramped through a short drying cycle followed by a charring cycle. The temperature and time for these cycles were sample- type dependent (Table 2). These steps were necessary even for the oxide standards as a rapid release of gases often resulted in cracking of the pellets. This in turn gave rise to irregular peak shapes and double or split peaks. With the botanical samples higher temperatures and longer charring times were necessary (Table 2) otherwise the cracks in the pellets were more profound (verified by visual inspection) and highly irregular peaks were observed.Some pellets would even break causing the runs to be aborted. At the end of the charring cycle the temperature was rapidly raised to 1250 "C. Although direct comparisons with other ETV sample introduction systems might be misleading owing to differences in sample-holder design (2. e. graphite-cup -rod and -tube) the vaporization tem- peratures used with these systems are typically higher (ie. 1600-2400 0C,23 2400 0C,10J7 2500 0C,33 2600 0C,11329 2700-3000 0C,22 and 3000 0C.32 At 1250 "C very little graphite was released from the pellet and the ETV device could be used continuously for about half a day without any noticeable deposits or memory effects.The use of higher vaporization temperatures was also tested and resulted in sharper peaks. However at about 1400 "C some graphite was also released this was visually con- firmed by the observation of a weak carbon emission in the plasma and a carbon deposit in the chamber and the Tygon tube. As the temperature was raised from 1400 to 1800 "C a progressively more intense carbon emission and a larger deposit were observed. Larger deposits caused increasingly persistent memory effects thus requiring a more frequent cleaning of the chamber and the tube. This disturbed the operation and increased the analysis time. At about 1800 "C the chamber and the tube had to be cleaned after every run. Although such temperatures can be advantageous in overcoming matrix effects they also give rise to a pressure pulse (as explained later).Single channel data acquisition parameters were chosen so that 200 points were acquired using a data acquisition rate of about 13 data points per second. From the results shown in Fig. 2 it can be concluded that adequate data were acquired to give a suitable peak shape. Data were manipu- lated on the IBM PC compatible microcomputer109 using a spreadsheet and were transferred to an Apple Macintosh microcomputer for further processing display and presentation. Results and Discussion Analyte Emission Temporal Behaviour In order to establish the nature of the signals generated by this system analyte emission temporal behaviour was recorded for several elements. Results obtained by running single element oxide standards are shown in Fig.5 for 250 ng of As Cd Pb and Zn and 25 ng of Hg and Mn. A peak appearance time (defined as the time between the start of the vaporization cycle and the peak maximum) of less than 10 s was observed for all of the elements tested. Typical examples of the sequence in which the analytes volatilized from the pellet (ie. a 'volatility sequence') are shown in Fig. 6. These were obtained using single element oxide standards (Table 1) and the conditions listed in Table 2. In general peak widths (at half-height) ranged between 1 and 2.5 s (Figs. 5 and 6) and the post-vaporization level was of approximately equal magnitude to that of pre-vaporization. The data also show that relatively high concentrations of elemental impurities are present in commercially available spectroscopic graphite (Fig.5 lower trace). The absence of a pressure pulse of the type observed 150000 100000 50000 0 h . 25000 7 2oooo f 10000 .= 15000 x c .- cn 5000 a Y - c o 30000 20000 10000 0' ' ' ' ' ' ' ' 1 60000 50000 40000 30000 20000 10000 0 4000 3000 - 2000 - 1000 - - - 0 2 4 6 8 I01214 16 0 2 4 6 8 10121416 Timels Fig. 5 Typical analyte emission temporal behaviour (obtained running single element oxide standards). Upper trace analyte emission (250 ng) of As Cd Pb and Zn; and analyte emission (25 ng) of Hg and Mn. Lower trace graphite blank 100 1 25 - t= I.. I Mn Zn Pb 0 2 4 6 8 10 12 14 16 Time/s Fig. 6 Volatility sequence for Hg Mn Zn and Pb (see text for discussion) during the high temperature vaporization stage in other ETV s ~ s ~ ~ ~ s ~ ~ ~ J ~ J ~ is noteworthy.The pulse mani- fests itself as a decrease in the intensity of the plasma background and has been attributed to the rapid heating of the carrier gas by the sample holder. The rapid heating induces gas expansion which creates a momentary increase in the carrier gas flow rate and results in a corresponding decrease in plasma continuum emission. An increase in the length of the Tygon tube,I0 sample carrier gas flow rate13 and observation height,13J5 a reduction in the volume of the ETV chamber1* and a double wall glass chamber15 have been identified as means by which the adverse effects of background signal depression are reduced. The lack of an intense pressure pulse with this system was attributed to the use of low vaporization temperatures and the small surface area of the pellet. However with higher vaporization temperatures a pressure pulse became noticeable with this system.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1.VOL. 6 53 1 0 0.2 0.4 0.6 0.8 1 .o 1.2 Carrier gas flow rate/! min-’ Fig. 7 Effect of central tube flow rate on signal-to-background ratios (obtained running single element oxide standards) of 250 ng of Pb and Zn; and 25 ng of Mn 2 .- 2 - 6000 5000 ‘ 4000 3000 2000 1000 0 v) - .4 /I I /I f? 0 2 4 0 2 4 0 2 4 Time/s Fig. 8 Reproducibility (obtained running single element oxide standards) for 250 ng of Pb and Zn (see text for discussion) Operating Conditions Although the data shown in Figs. 5 and 6 illustrate the basic temporal characteristics of the ETV-ICP-AES signals the exact nature depends on a number of system parameters.These include vaporization temperature and duration tube length chamber volume and geometry plasma power and gas flow rates. As a proof-of-concept approach was adopted in this study no attempt was made to examine the effect of all of these parameters on the analyte emission temporal behaviour. Only the effect of the gas flow rate in the central tube (which is equivalent to the nebulizer flow rate of pneumatic nebulizer systems) was studied. The gas flow rate in the central tube was varied from 0.2 to 1.0 1 min-l. The results (reported as peak heights) obtained by running the single element oxide standards (250 ng of Pb and Zn and 25 ng of Mn) are shown in Fig.7. It is apparent that this gas flow rate is a key parameter and that compromise conditions are important with this system. Although not shown ti 2 gas flow rate in the central tube also had an effect on peak shapes. In general higher flow rates produced sharper peaks as expected. Basic Analytical Performance Characteristics In order to obtain an indication of the potential analytical performance characteristics of this system the precision was measured the detection limits were determined and calibration graphs constructed. Unless otherwise stated the parameters listed in Table 2 and pellets containing single element oxide standards were used. Fig. 8 shows the signals obtained for three successive runs of 250 ng of Pb and Zn. In general peak heights and peak shapes were reproducible. Similar results were obtained for Cd and Mn.Average relative standard deviations deter- mined from six replicate runs were 4.8% (peak height) and 7.0% (peak area). These results compare favourably with those reported by other workers for solid-sample-ETV-ICP systems2-6 but are much better than those for a pellet-DSI- ICP In contrast to the reproducible signals obtained when running pellets containing the same amount of analyte the shape of the emission signal changes considerably as a function of the amount of analyte in a pellet. For example signals for 2500 and 25 ng of Cd are shown in Fig. 9. Similar changes in analyte emission temporal behaviour have been reported by other w o r k e r ~ . ~ ~ J ~ ~ These are analogous to the ‘concentration effects’ observed in ETAAS.111J12 It could be suggested that the changes are due to thermal vaporization effects to diffusion and transport effects (Cd transport efficiency has been found to increase with sample mad2) and to changes in plasma characteristics. However more work remains to be done so as to establish the influence these parameters exert on peak shapes.Detection limits (Table 3) determined using peak height measurements are three times the standard deviation of the background while running 25 ng of a single element oxide standard. Also included in Table 3 are the detection limits obtained when running liquid samples using a conventional pneumatic nebulizer and the detection limits reportedS2 using a pellet-DSI device. The experimental configuration used for the determination of the detection limits remained unchanged with the exception of the sample introduction system.In general the detection limits obtained using a pellet-ETV system are 6-10 times better than those 300000 r 1 200000 100000 h c v1 c 3 .- .- c. g o n v 2 2000 0 2 4 6 8 10 12 14 16 Ti m e/s Fig. 9 Effect of concentration on peak shapes for (a) 2500 ng of Cd and (b) 25 ng of Cd. Lower trace graphite blank532 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 Table 3 Detection limits Pellet-ETV Line/ Pellet-DSI* Liquid Element nm ppb pg (PPb) (PPb) C d I 228.8 1 300 10 26 Pb I 220.3 0.6 150 70 28 Zn I1 213.8 3 800 20 82 *See reference 52. Table 4 Analysis of MOE botanical samples (concentration in PPm) Pb Zn Sample type Found MOE Found MOE V85-1 l o + 1 19k2.0 370k 12 140-r- 11 White birch 3.2 20.2 4 370k5 200 Norway maple 39 2 1 95 39k2 39 reported for a pellet-DSI-ICP-AES systems2 and 30-50 times better than those obtained when running liquid samples.Linear calibration graphs for Cd Pb and Zn covering a concentration range of 2-3 orders of magnitude were established using peak heights and single element oxide standards. Non-linear calibration graphs were obtained for As Hg and Mn. No explanation can be offered at this time. In order to evaluate the analytical performance of the pellet-ETV device further and to test the feasibility for the direct analysis of ‘real’ samples botanical samples provided by the Ontario Ministry of the Environment (MOE) were used. Included in these are sample types designated as V85- 1 Norway maple and White birch by the MOE.Certified values and with the exception of V85-1 statistical data are not available to us. Elemental concentrations quoted by the MOE and reported in Table 4 are the average of a large number of analyses of these sample types. According to the MOE these concentrations were obtained after a 3 h open- vessel hot-plate digestion and analysis by ICP-AES. In this laboratory sample preparation consisted of mixing 0.300 g of a ground botanical reference sample with 2.700 g of spectroscopic graphite as previously described for batch preparation of 12 pellets. The results obtained using the calibration graphs constructed from the single element oxide standards are shown in Table 4 and are encouraging considering the sample size and the fact that there was neither matrix matching nor sample pre-treatment.Conclusion The results of this preliminary investigation show that a pellet-ETV-ICP-AES system is a promising method for elemental determinations with powdered samples. This system is suitable for the rapid screening of powdered botanical samples of environmental concern. Improve- ments in the analytical performance characteristics of this system are expected by designing an optimized pellet-ETV device and by using a spectrometer configured for the ICP. Further improvements are expected by coupling a pellet- ETV device to an ICP mass spectrometer. For example if the improvement in detection limits is similar to that observed for the pellet-DSI-ICP-MS,s2 detection limits for the pellet-ETV-ICP-MS should also improve by as much as three orders of magnitude.In addition owing to dry sample introduction afforded by ETV devices significant reduc- tions in spectroscopic and non-spectroscopic effects are expected (similar to DSI-ICP-MSSo~sl). Financial assistance from the Ontario Ministry of the Environment (Projects 414G and 270R) and L. Blain’s assistance 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 27 28 29 30 31 32 33 34 35 36 37 38 39 References Kantor T. Spectrochim. Acta Part B 1983 38 1483. Ng K. C. and Caruso J. A. Appl. Spectrosc. 1 985 39 7 19. Matusiewicz H. J. Anal. At. Spectrom. 1986 1 171. Broekaert J. A. C. and Boumans P. W. J. M. in Inductively Coupled Plasma Emission Spectrometry ed.Boumans P. W. J. M. Wiley New York 1987 part I ch. 6 p. 296. Van Loon J. C. 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ISSN:0267-9477    

DOI:10.1039/JA9910600527

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 Determination of Minor and Trace Elements in Ferrochromium and Ferromanganese by Inductively Coupled Plasma Atomic Emission Spectrometry* 535 Ivan HlavaCek and lrena HlavaCkova Chemical Laboratories of Poldi United Steelworks CS-272 62 Kladno Czechoslovakia A procedure using inductively coupled plasma atomic emission spectrometry (ICP-AES) has been developed for the analysis of ferrochromium and ferromanganese i.e. for the determination of aluminium cobalt chromium copper manganese molybdenum nickel silicon titanium and vanadium. The sample is dissolved with a mixture of phosphoric and sulphuric acids in a polytetrafluoroethylene vessel without the application of hydrofluoric acid. In this way both ferrochromium and ferromanganese samples are quantitatively converted into a soluble form. No separation of the analyte elements from the matrix is required.The sample dissolution procedure the instrumental conditions for ICP-AES some important spectral interferences the precision of determination and detection and determination limits are given. The detection limits were 12 ng ml-I for Co Cu and V; 60 ng ml-l for Mn Mo Ni Si and Ti; 120 ng ml-I for Al and Cr. The precision of determination characterized by ten standard deviations was 20-40 ng ml-l for Co Cu and V; 200-400 ng ml-l for Al Cr Mn Mo Ni Si and Ti. The sample dissolution was also performed in a microwave oven. The analytical procedures were verified by means of Czechoslovak British and German ferrochromium and ferromanganese certified reference materials.The silicon content in the ferroalloys was also verified by a gravimetric method For comparison the samples were analysed by flame atomic absorption spectrometry. Keywords Inductively coupled plasma atomic emission spectrometry; ferrochromium and ferromanganese; multi-element analysis; phosphoric acid and silicon; microwave digestion The steel producing industry requires rapid and reliable analysis of ferroalloys such as ferrochromium and ferro- manganese. Inductively coupled plasma atomic emission spectrometry (ICP-AES) enables complex routine chemical analyses to be performed which were previously carried out by a combination of classical and instrumental analytical procedures. The main interest is focused on the multi- element analysis of ferrochromium (carbon content 0.0 1 - 1 0%) and ferromanganese by ICP-AES.Scott et a!. analysed ferromanganese for aluminium boron cobalt chromium copper molybdenum nickel titanium and vanadium in the presence of the matrix elements e.g. iron and manganese. They found that the effects of the matrix elements on the detection limits were insignificant. The effect of the ferromanganese matrix on the intensities of the spectral lines for the analyte elements was negligible. Kanaev and Trofimov2 have reviewed the flame atomic absorption spectrometry (FAAS) procedures used for the analysis of various ferroalloys including ferrochromium and ferromanganese. Foster and Garden3 determined the silicon content in ferromanganese samples after dissolution with hydrofluoric hydrochloric and nitric acids.For ferrotungsten analysis using ICP-AES the solid sample (including silicon) was dissolved using phosphoric acid4 in a polytetrafluoroethylene (PTFE) vessel provided that the silicon is bonded as silicotungstic acid. The sample decomposition with phosphoric acid was used for both low- and high-alloy steels e.g. corrosion-resistant and high- speed and nickel-base alloys (Nim~nic).~i~ Experimental Instrumentation All measurements were made by means of an ARL 33000 LA sequential emission spectrometer with an inductively coupled argon plasma. The spectrometer is equipped with a Commodore 64 computer connected on-line. A Henry *Presented in part at the VII Polish Spectroanalytical Confer- ence and X CANAS (Conference on Analytical Spectroscopy) Torun Poland 1988.radiofrequency generator with an operating power of 1250 W an operating frequency of 27.12 MHz and a maximum reflected power of 10 W was employed. A Fassel-type quartz plasma torch was used. The pneu- matic concentric glass nebulizer (Meinhard type) operated under a pressure of 0.3 MPa. The argon carrier gas of flow rate 1.15 1 min-l passed through the nebulizer after being moistened by means of a bubbler. The sample uptake was by means of the Venturi effect without the use of a peristaltic pump. The sample uptake rate was 1.5-1.8 ml min-l for water and depended on the nebulizer used. The spectrometer consisted of a monochromator with a 1 m radius concave grating ( 1440 lines mm-l) in a Paschen- Runge mounting with fixed secondary slits which were sequentially opened and shut as required.The reciprocal dispersion was 0.695 nm mm-l in the first order. The observation height was 14 18 22 or 26 mm. A Perkin-Elmer 503 atomic absorption spectrometer equipped with an electrodeless discharge lamp source was used for comparative analysis by FAAS. A commerical domestic microwave oven Philips M 704 with a timer and variable power settings equivalent to outputs of 210 330 450 and 600 W was used. Reagents and Solutions All of the chemicals used were of analytical-reagent grade and solutions were prepared with de-ionized water. All stock solutions of metals were prepared from high-purity metals (99.9% or higher) and stored in polyethylene bottles. The detailed preparation of the element stock solutions has been described previ~usly.~ Hydrochloric acid (36% m/m) nitric acid (65% m/m) sulphuric acid (98% m/m) phosphoric acid (85% m/m) and hydrogen peroxide (30% m/m) were also used.Instrumental Conditions for ICP-AES The instrumental conditions for ICP-AES are given in Table 1. All of the measurements were performed using the spectral lines recommended by the manufacturer except for536 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 Table 1 Analyte elements and instrumental conditions for ICP-AES Secondary Observation Element Wavelength/ slit-width/ height/ and line nm Pm mm A1 I c o I Cr I c u I Mn I1 Mo I Ni I1 Si I Ti I v I1 394.401 350.228 360.533 324.754 257.610 3 17.035 23 1.604 251.61 1 363.546 31 1.071 75 75 75 75 75 75 75 75 75 75 22 26 22 22 18 22 18 18 22 18 cobalt where the Co I 350.228 nm line was used instead of the original Co I1 238.892 nm line.The original cobalt line at 238.892 nm was severely affected by the iron spectra. The choice of observation height for the analyte elements had been made previ~usly.~ Table 2 shows a summary of spectral interferences of accompanying elements when measuring at the instru- mental conditions chosen. The linearity of the measure- ments was confirmed by analyses of prepared sample solutions; the analyte elements were added within the expected concentration range of the real ferroalloy samples. Instrumental Conditions for FAAS For verification of the accuracy of the results the determi- nation of the elements was carried out by FAAS using the same sample solutions as for ICP-AES and from a sulphuric acid medium for ferrochromium samples and a hydrochloric and nitric acid medium for ferromanganese samples.For the determination of elements in sample solutions containing phosphoric acid depression of the signals was found. The effect of silicon on the determina- tion of manganese' in ferrochromium was almost elimi- nated using a dinitrogen oxide-acetylene flame. Sample Preparation ICP-AES Ferrochromiurn. Weigh 0.500 g of sample in the fine powder form into a PTFE beaker (it is important for the so-called 'hard ferrochromium' i. e. ferrochromium con- taining more than 2% of carbon to be ground to a particle size of less than 0.15 mm without separating the various size fractions.) Add 10 ml of sulphuric acid (1 + 1) and 25 ml of concentrated phosphoric acid.Dissolve under a PTFE deckel (the water must not be evaporated from the acid mixture too quickly) by heating at about 150 "C. After sample dissolution remove the deckel and then evaporate the solution until white fumes of sulphur trioxide appear at a temperature of between 180 and 220 "C for about 10 min cool and dilute immediately to 250 ml with water in a glass calibrated flask. Ferrornanganese. Weigh 0.500 g of the sample in a fine powder form into a PTFE beaker. Add 10 ml of sulphuric acid (1 + l) 25 ml of concentrated phosphoric acid and 10 ml of concentrated nitric acid. Dissolve under a PTFE deckel by heating at about 100 "C. After sample dissolution remove the deckel and then evaporate the solution until white fumes of sulphur trioxide appear at a temperature of between 180 and 220 "C for about 10 min cool add a few drops of hydrogen peroxide until the solution is colourless and dilute immediately to 250 ml with water in a glass calibrated flask.The sample solutions obtained by this process were clear and stable for long periods and accurate analytical results were obtained with the same sample solutions 3 months later. Microwave digestion For sample preparation using the microwave oven weigh 0.500 g of sample (ferrochromium or ferromanganese) into a PTFE beaker. Add the same amounts of acids as before (see procedure for ferrochromium or ferromanganese) to the samples. Leave the ferromanganese samples for 3 min. Place the PTFE beaker with sample in the microwave oven heat for about 10 min at 330 W cool and dilute imme- diately (after adding a few drops of hydrogen peroxide for ferromanganese) to 250 ml in a glass calibrated flask.FAAS Ferrochromium. Weigh 0.500 g of sample into a glass beaker add 20 ml of sulphuric acid (1 + 1) and dissolve by heating at about 100 "C. After sample dissolution evapo- rate the solution until white fumes of sulphur trioxide appear cool and dilute to approximately 50 ml. Add 5 ml of concentrated nitric acid heat to boiling cool dilute to 100 ml with water in a glass calibrated flask and filter. When determining silicon it is necessary to use the sample decomposition procedure as described under ICP-AES. Ferromanganese. Weigh 0.500 g of sample into a glass beaker add 10 ml of nitric acid (1 + 1) and 10 ml of hydrochloric acid (1 + 1) and dissolve by heating at about Table 2 Interferences from added elements of 1 % concentration (ICP-AES) Background equivalent concentration (Oh) Interfering element A1 c o Cr c u Mn Mo Ni Si Al c o Cr c u Fe Mn Mo Ni Si Ti V - t O .O O 1 0.0002 <0.0001 <0.0001 0.0001 0.0003 0.001 1 0.0002 0.000 1 5 <0.0001 <o.oooo 1 0.0001 3 0.0000 1 0.00004 0.0000 1 0.0004 0.0004 0.0001 0.00025 0.000 1 2 - tO.OOO 1 0.025 (0.00 1 0.0045 <0.00001 0.000 1 0.00004 0.0007 0.0002 0.0003 - <0.0000 1 < 0.00002 0.00003 <0.00001 tO.OOOO 1 -0.00035 <0.00001 <o.ooo 1 0.00006 <0.00001 - <0.00002 <o.oooo 1 t0.00002 (0.001 t0.00005 - <0.00003 <0.00001 <o.ooo 1 <0.00005 0.00005 0.00005 0.0003 <0.0002 <0.0001 0.001 0.0002 t0.00005 <0.0001 0.00 12 (0.001 - t0.00005 (0.005 t 0.0005 t0.00 1 0.000 1 0.0002 0.0002 <o.ooo 1 0.0002 0.00005 - t 0.00005 <0.0002 0.000 1 t0.00003 <0.0005 <0.0004 0.0075 <0.00006 <0.0003 t0.0003 - Ti 0.000 1 0.0006 0.0005 0.0004 0.0005 <0.0002 0.08 0.0001 5 0.001 0.0007 - V < 0.00005 ~0.00015 0.0003 0.00005 0.00006 t0.0005 0.00 1 5 <0.00001 <0.0001 0.01 1JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL.6 537 100 "C. After sample dissolution heat to boiling cool then dilute to 100 ml in a glass calibrated flask and filter. When determining chromium it is necessary to use the sample decomposition procedure described under ICP-AES because chromium might not otherwise be quantitatively converted into a soluble form.A11 of the sample solutions analysed by ICP-AES or FAAS contained the same amounts of acid so that undesirable effects of for example viscosity and density could be avoided. A blank and synthetic samples were used for calibration and were prepared by the same procedure as for the real ferrochromium or ferromanganese samples. The matrix element composition was simulated using high-purity metals. For example both the blank and synthetic calibra- tion samples contained 0.15 g of iron metal and 0.35 g of chromium metal for the ferrochromium samples containing 30 k 5% of iron and 70 f 5% of chromium. Similarly the blank sample and synthetic calibration samples contained 0.05 g of iron metal and 0.40 g of manganese metal for the ferromanganese samples containing 10 -t 5% of iron and 80 t 5% of manganese.The synthetic calibration samples also contained known additions of the analyte elements in the concentration range to be considered. For practical purposes the matrix composition of the calibration samples can also be timulated using the Czecho- slovakian Analytical Normal (CSAN) Certified Reference Materials (CRM) 4-2-0 1 ferrochromium and CSAN CRM 4-3-0 1 ferromanganese with known additions of some analyte elements. Results and Discussion The dissolution of the ferrochromium and ferromanganese samples using hydrochloric nitric or sulphuric acid is sometimes unsuitable because of the precipitation of silicic acid from sample solutions of high silicon content. Further- more some analyte elements can be adsorbed by the precipitated silicic acid causing a loss of analyte elements from the sample solution after filtration.In addition sample dissolution can often be incomplete. Dissolution of ferrochromium especially of 'hard ferrochromium' is 100 ' 50 .$ 0 I C 0) w .- PB - A' / 50 /+ - /* 2 0 - * /* c 0) 0 2 4 6 8 1 0 0 2 4 6 8 1 0 0 .- Q) tT ; 100 - v - 50 - 0 - + 0 2 4 6 8 1 0 t 1 I " " 0 2 4 6 8 1 0 very difficult; it is possible with repeated applications of sulphuric acid but it is time consuming. Chromium(mr) sulphate precipitates from ferrochromium sample solutions in the presence of sulphuric acid particularly after evapora- ting to white fumes of sulphur trioxide. Application of hydrofluoric acid is also unsuitable because the nebulizing system consists of a glass nebulizer a glass chamber and a quartz plasma t o r ~ h .~ Unfortunately in this work the spectrometer was not equipped with an HF resistant nebulizing system. Furthermore the sample dissolution with hydrofluoric acid or various acid mixtures was also incomplete. Fusion of the ferrochromium samples with sodium peroxide contaminates the final sample solutions with alkali salts and crucible material e.g. iron and nickel. Also nebulizer clogging ionization interferences and sample contamination by analyte elements can occur. For this reason an alternative procedure for sample dissolution was investigated. The potential problems were solved by the application of phosphoric acid which gave good results. For example a ferrochromium sample con- taining up to about 10% of carbon was dissolved and analysed without any problems.It has been found that phosphoric acid with the potential addition of sulphuric acid can dissolve all analyte and matrix elements including silicon. However it is necessary to perform the sample dissolution in PTFE vessels because both glass and quartz vessels are corroded during the procedure thereby contami- nating the sample solutions with silicon. It is presumed that SiP207 is formed during the dissolu- tion process.8 Silicon is determined together with the other analyte elements thereby eliminating the need for a separate gravimetric determination of silicon. The use of a microwave oven for sample dissolution was also examined using the same procedure as in the PTFE beakers under atmospheric pressure.The digestion time was reduced from 2-3 h to about 10 min for both ferrochromium and ferromanganese samples. The spectral interferences (ICP-AES) of the matrix elements on the intensities of the spectral lines of the analyte elements were investigated. Table 2 shows the effects of the interfering elements for the chosen instrumen- tal conditions. A substantial interference effect was found 0 1 2 3 4 5 Mo /' +/+ 1 1 1 1 1 0 2 4 6 8 1 0 0 4 8 12 16 20 0 4 8 12 16 20 Concentration/pg mi-' Fig. 1 Calibration graphs (background corrected) for A pure aqueous acid solutions; and B synthetic ferrochromium solutions538 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 ~~ Table 3 Comparison of results (O/O) for ferrochromium reference materials CSAN CRMs 4-2-01 and 4-2-02 British Chemical Standard (BCS) CRM 203/2 and Bundensanstalt fur Materialforschung und -prufurg (BAM) Euronorm CRM (ECRM) 533-1 are low carbon content; CSAN CRM 4-2-03 is of mid-range carbon content; and CSAN CRM 4-2-04 BCS-CRM 204/1 and BAM ECRM 530-1 are high carbon content Sample Method/source A1 Co c u Mn Mo Ni Si Ti V &AN CRM 4-2-01 (0.073% C 70.36% Cr) CSAN CRM 4-2-02 (0.01 1% c 84.70% Cr) (1.27% C 66.86% Cr) (6.18% C 70.27% Cr) (0.027% C 7 1.7% Cr) (4.56% C 66.3% Cr) (6.46% C 64.9 Cr) (0.008~/0 c CSAN CRM 4-2-03 CSAN CRM 4-2-04 BCS-CRM 203/2 BCS-CRM 2041 1 BAM ECRM 530-1 BAM ECRM 533-1 66.2% Cr) * Informative value.Certificate FAAS Certificate FAAS Certificate FAAS Certificate FAAS Certificate FAAS Certificate FAAS Certificate ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES Certificate ICP-AES - (0.02 (0.02 (0.02 < 0.02 (0.02 (0.02 (0.02 (0.02 - - - - - (0.02 (0.02 (0.02 0.56 0.615 0.008 - (0.02 - 0.04 0.040 0.015 0.013 0.04 0.040 0.03 0.033 0.044 0.045 0.045 0.055 0.053 0.038 0.036 0.053 0.053 - - - - - 0.016 0.0 1 5 0.014 0.013 0.022 0.02 1 0.015 0.0 12 0.010 0.010 0.013 0.01 1 0.007 0.006 - - - - - - 0.009 - 0.30 0.31 0.27 0.275 0.27 0.27 0.08 0.07 0.30 0.315 0.19* 0.165 0.16 0.16 0.155 0.18 0.165 - - - - - (0.0 1 (0.01 (0.01 (0.01 (0.0 1 (0.01 (0.01 (0.0 1 (0.01 (0.01 (0.01 (0.01 (0.01 - - - - - - - (0.01 - 0.33 0.34 0.105 0.1 1 0.36 0.37 0.28 0.27 0.22 0.24 0.40* 0.38 0.40 0.19 0.19 0.31 0.305 - - - - 1.96 2.00 1.98 0.20 0.18 1.58 1.65 1.59 0.73 0.75 0.72 0.67 0.67 1.53* 1.55 1.52 0.49 0.52 0.09 0.09 5 - - - t0.005 (0.01 - - (0.01 - - tO.O1 - - (0.01 (0.005 (0.01 - - - 0.015 0.05 0.045 - t o .0 1 - 0.088 0.089 0.04 1 0.042 0.064 0.064 0.12 0.115 0.180 0.178 0.185 0.105 0.103 0.23 0.24 0.023 0.023 - - - - Table 4 Comparison of results (O/O) for real ferrochromium samples Sample (7.1 7% c 69.10% Cr) (6.18% C 70.55% Cr) (0.145% C 70.00% Cr) (0.067% C 70.00% Cr) (0.25°/o C 70.50% Cr) (1.66% C 68.65% Cr) (0.42% C 70.00% Cr) (6.92% C 69.65% Cr) A1 A2 A3 A4 A5 A6 A7 A8 Method/source Gravimetric FAAS Gravimetric FAAS Gravimetric FAAS Gravimetric FAAS Gravimetric FAAS Gravimetric FAAS Gravimetric FAAS Gravime tric FAAS ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES c o 0.04 I 0.04 1 0.036 0.038 0.04 1 0.040 0.040 0.038 0.040 0.039 0.04 1 0.040 0.040 0.041 0.035 0.035 - - - - - - - - at the Cr I 360.533 nm spectral line for samples containing cobalt and high iron contents.The interfering spectral lines are Co I 360.536 nm and Fe 1360.546 nm. In practice the spectral matrix interference was eliminated using a blank and calibration samples that contained almost iden- tical amounts of the matrix elements such as iron and chromium for ferrochromium or iron and manganese for ferromanganese (described under Sample Preparation). The possible interferences (see Table 2) were corrected if necessary for the increase or decrease in the spectral background due to differences in the sample matrix c u 0.027 0.026 0.029 0.029 0.026 0.027 0.032 0.033 0.022 0.023 0.032 0.032 0.026 0.027 0.035 0.034 - - - - - - - - Mn Ni 0.47 0.38 0.45 0.36 0.86 0.34 0.84 0.32 0.2 1 0.33 0.20 0.32 0.2 1 0.31 0.2 1 0.30 0.18 0.35 0.195 0.33 0.40 0.36 0.4 1 0.35 0.38 0.35 0.4 1 0.36 0.47 0.31 0.47 0.32 - - - - - - - - - - - - - - - - Si 0.90 0.9 1 1.41 1.40 1.34 1.34 1 .oo 0.98 0.345 0.34 0.63 0.62 1.45 1.44 1.05 1.05 - - - - - - - - composition.(The chromium or manganese contents of the matrix in real ferrochromium and ferromanganese samples were determined by titrimetric methods because of the greater precision of determination.) The influence of the sample matrix on the slopes of the calibration graphs for the analyte elements was also investigated. The calibration graphs for some analyte elements in ferrochromium are shown in Fig. 1.These graphs were measured for both pure aqueous acid solutions of the elements and synthetic ferrochromium solutions containing 0.6 mg ml-l of iron and 1.4 mg ml-l ofJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 539 Table 5 Comparison of results (Yo) for reference materials ferromanganese. CSAN CRM 4-3-01 and BCS-CRM 280/1 are of low carbon content; and CSAN CRM 4-3-02 and BCS-CRM 208/1 of high carbon content Sample (1.03% C 90.62% Mn) CSAN CRM 4-3-01 CSAN CRM 4-3-02 (6.74% C 77.56% Mn) Methodlsource Certificate NAA FAAS Certificate NAA FAAS ICP-AES ICP-AES ICP-AES Certificate FAAS ICP-AES ICP-AES Certificate FAAS ICP-AES ICP-AES A1 c o 0.038 0.034 0.035 0.066 0.072 0.070 - - Cr 0.39 0.35 0.37 0.053 0.06 0.06 0.16* 0.155 0.56* 0.53 0.55 - - - - - - cu 0.060 0.066 0.063 0.I 1 0.155 0.150 0.04* 0.041 0.038 0.04* 0.039 0.037 - - - - - Mo 0.01 3 0.01 5 0.01 5 0.027 0.03 0.025 - - Ni Si 0.69 Ti V - (0.005 (0.02 - 0.023 0.023 - - 0.042 0.043 - - 0.046 0.043 - - 0.045 0.044 - 0.05 0.05 0.005 (0.01 - 0.70 0.03* - 0.005 (0.01 - 0.08* 0.06 0.07 - - 0.01 0.01 5 - (0.005 (0.02 0.14 0.145 - 0.02 0.01 0.98 0.89 0.07 2.01 - Insoluble residue (6.80% C 76.4% Mn) Insoluble residue (0.47% C 80.4% Mn) Insoluble residue BCS-CRM 208/1 BCS-CRM 280 0.04 0.05 0.036 0.036 0.01 5 0.01 5 0.075 0.07 - 0.008 (0.02 - - 0.04 5 0.045 - - 0.02 5 0.02 - - 0.1 1 0.10 - 1.97 0.025 * Informative value not certified. Table 6 Comparison of results (To) for real ferromanganese samples Sample Method/source Co c u Ni Si - 0.535 0.16 - 0.165 0.535 - 0.47 0.165 0.49 - 0.50 0.16 - 0.17 0.525 - 2.50 0.04 - 0.035 2.46 - 0.13 0.12 - 0.12 0.13 - 2.47 0.04 - 0.04 2.42 0.155 - B1 (6.45% C 79.55% Mn) (6.64% C 79.30% Mn) (6.54% C 80.50% Mn) (5.68% C 77.50% Mn) (6.50% C 79.25% Mn) (5.67% C 69.25% Mn) B2 B3 B4 B5 B6 Gravimetric FAAS Gravimetric FAAS Gravimetric FAAS Gravime tric FAAS Gravime tric FAAS Gravime tric FAAS ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES ICP-AES - 0.099 0.098 0.099 0.097 0.099 0.099 0.018 0.015 0.077 0.078 - - - - - 0.09 0.09 0.09 0.085 0.09 0.09 0.02 0.015 0.06 0.06 - - - - 0.024 0.020 0.05 0.04 Table 7 Precision of determination of analyte elements (Yo) Sample Parameter A1 c o Cr Cu Mn Mo Ni Si Ti V &AN CRM 4-2-0 1 Average ( n = 1 2) ferrochromium t0.02 0.040 - 0.015 0.31 (0.01 0.34 1.98 t0.01 0.089 0.0023 SD* - 0.0007 - 0.0013 0.008 - 0.016 0.022 - RSDT - 1.9 - 8.7 2.5 - 4.7 1.1 - 2.6 Average ( n = 6) SD - 0.0013 0.012 0.0010 - 0.005 0.008 0.018 - 0.0010 RSD - 3.6 3.1 1.5 - 33.3 15.7 2.5 - 4.3 &AN CRM 4-3-0 1 ferromanganese t 0 .0 2 0.035 0.37 0.063 - 0.015 0.05 0.70 (0.01 0.023 * SD Standard deviation. t RSD Relative standard deviation. chromium. Corrections (see Table 2) were applied for the increase in the spectral background caused by the presence of the sample matrix elements. The slopes of the two calibration graphs for every analyte element were almost identical the relative differences were found to be <lo/o. No significant matrix effect occurred for these elements. A small but detectable difference in the slopes of the calibra- tion graphs for aluminium was observed. Similar results were obtained for ferromanganese in both the pure aqueous acid solutions of the analyte elements and the synthetic ferromanganese solutions containing 0.2 mg ml-I of iron and 1.6 mg ml-l of manganese.The matrix effect was eliminated by simulating the matrix composition in both the blank and calibration samples. The effect of sodium salts which contaminated the synthetic calibration samples taken from the silicon stock540 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 solution has been described previ~usly.~ By using suitable standard ferrochromium and ferromanganese samples for calibration i.e. CRMs the sodium effect was eliminated. The accuracy of ICP-AES was verified by means of the Czechoslovak British and German reference materials and real ferrochromium and ferromanganese samples.For comparison some samples were analysed by FAAS. In some instances the silicon content was also verified by a gravimet ric met hod. The certified values and the analytical results for the reference materials and real ferrochromium samples obtained using FAAS and ICP-AES are summarized in Tables 3 (reference materials) and 4 (real samples) includ- ing the matrix composition. For silicon the results of the gravimetric analysis are given in Table 4. Special emphasis was given to the silicon determination and therefore the correlation of results obtained using ICP-AES and a gravimetric method was studied over a long period. Altogether 100 ‘soft’ (<2% of carbon) and 100 ‘hard‘ (2- 10% of carbon) ferrochromium samples were analysed by both methods.An evaluation of the results obtained was carried out by the method of linear regression. The correlation coefficients for soft and hard ferrochromium were found to be 0.997 and 0.988 respectively. The certified values and results for the reference ma- terials and real ferromanganese samples obtained using FAAS and ICP-AES are summarized in Tables 5 (reference materials) and 6 (real samples) including _the matrix composition For the ferromanganese samples CSAN CRM 4-3-01 and CSAN CRM 4-3-02 the results obtained using neutron activation analysis (NAA) are also shown in Table 5 . The results of gravimetric analysis for silicon are given in Table 6 . A difference between the certified value and the value obtained using ICP-AES for Si in the ferromanganese sample BCS-CRM 208/1 was also found (Table 5).The insoluble non-metallic residue that remained after sample dissolution was analysed using ICP-AES after separation by membrane filtration and fusion with a mixture of sodium carbonate and sodium tetraborate (1+1) in a platinum crucible. The residue contained nearly 100% silicon dioxide. The same analysis was carried oyt for the ferromanganese reference materials samples CSAN CRM 4-3-02 and BCS-CRM 280. The results obtained for insoluble silicon are also included in Table 5. Both the ferrochromium and the ferromanganese samples are quantitatively dissolved using the described decomposi- tion procedures except when aluminium and silicon (bonded as oxides) are pre~ent.~ If the ferroalloy sample contains a non-metallic component e.g.as in a slag it is necessary to analyse an insoluble residue after fusing. Therefore it is necessary to check visually the final sample solutions. The precision cf the analytical method using ICP-AES yas studied for CSAN CRM 4-2-01 ferrochromium and CSAN CRM 4-3-0 1 ferromanganese. The statistical data are presented in Table 7. The limits of detection defined as three times the standard deviation of the background noise and determi- nation defined as ten times the standard deviation of the Table 8 Limits of detection and determination Limit of detection Limit of determination Element O/o ng ml-I Oh ng ml-‘ A1 0.006 Co 0.0006 Cr 0.006 Cu 0.0006 Mn 0.003 Mo 0.003 Ni 0.003 Si 0.003 Ti 0.003 V 0.0006 20 12 20 12 60 60 60 60 60 12 0.02 0.002 0.02 0.002 0.0 1 0.0 1 0.0 1 0.0 1 0.0 1 0.002 400 40 400 40 200 200 200 200 200 40 background noise for all the analyte elements in the presence of the matrix elements are reported in Table 8 for the wavelengths listed in Table 1.Conclusion The ICP atomic emission spectrometric procedure can be applied effectively to the multi-element analysis of ferro- chromium and ferromanganese. The described procedure is reliable and relatively simple. Silicon can be determined together with the other analyte elements. The application of hydrofluoric acid is not required and therefore a quartz plasma torch and a glass nebulizer can be used because they are not corroded. The sample solutions obtained are clear and stable for long periods. If a microwave oven is used for the sample dissolution the digestion time is substantially reduced. The results obtained using ICP-AES are in good agreement with the certified FAAS and gravimetric values. The proposed dissolution procedure with phosphoric acid is widely applicable and has already been used for various other types of alloys and steels. References Scott R. H. Strasheim A. and Oakes A. R. ZCP Inf Newsl. 1978 3 448. Kanaev N. A. and Trofimov N. V. Atomno-Absorbcionnyi i Plamennofotometricheskii Analizy Splavov Metallurgiya Mos- cow 1983 pp. 101 and 146. Foster P. and Garden J. Analusis 1973-1974 2 675. HlavaEek I. and HlavaEkova I. J. Anal. At. Spectrom. 1986 1 331. HlavaEkova I. and HlavaEek I. Hutn. Listy 1984 39 890. HlavaEek I. and HlavaEkova I. Czechoslovakian Patent No. A 0 231 522 1985. Begak 0. Yu. Zh. Anal. Khim. 1975 30 2269. Philbrick F. A. Holmyard E. J. and Palmer W. G. A . Text Book of Theoretical and Inorganic Chemistry J. M. Dent and Sons London 1949 p. 587. Talvitie N. A. Anal. Chem. 1951 23 623. Paper 0/010 74J Received March 12th I990 Accepted May 9th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600535

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 54 1 Laser Ablation in a Liquid Medium as a Technique for Solid Sampling Yasuo lida Akira Tsuge Yoshinori Uwamino Hisashi Morikawa and Toshio lshizuka Government Industrial Research Institute Nagoya 7 Hirate-cho Kita Nagoya 462 Japan A new technique for solid sampling laser ablation in a liquid medium has been developed and evaluated. A solid sample was held in a liquid medium contained in a flat-bottomed beaker. Laser pulses from a Q-switched Nd:YAG laser were introduced from the bottom of the beaker and were focused on the surface of the solid sample. Both the vapour and the particles produced by the laser ablation were trapped directly by the surrounding liquid and a suspension was formed. The technique offers both time and spatial separation of the sampling and introduction and excitation processes in the laser ablation.The morphological observation of trapped particles the evaluation of trapping efficiency and fractional ablation and the direct introduction of the suspension into an inductively coupled argon plasma were carried out. Nearly 100% trapping efficiency and the absence of fractional ablation were obtained. Keywords Laser ablation; solid sampling; suspension; metal and ceramic samples; liquid medium Laser ablation has been widely applied to the direct sampling and introduction of solid samples into inductively coupled plasmas (ICPS),'-~ microwave-induced plasma^,^^^ glow discharges8v9 and various other plasma sources for atomic spectrometry. In most instances an inert gas stream has been used as the carrier for the sample vapour and particles.However loss of analyte during transport between the ablation site and the excitation source has been discussed as a cause for the decrease in the accuracy and precision of this t e ~ h n i q u e . ~ ~ ~ ~ ~ ~ ~ Arrowsmith and Hughes'O have investigated the entrainment and transport of ablated particles in the gas flow to a secondary excitation source. Furthermore from the pulse-like evolution of the same vapour either a high-speed scanning or multichannel detection system is inevitably needed for multi-element quantification and background correction. In this study a mode of laser ablation for solid sampling is proposed i.e. laser ablation in a liquid medium (LALM). The technique provides separation between the sampling and introduction processes and the following merits emerge (1) temporal separation ie.the necessity of high- speed scanning or a multichannel detection system can be precluded; (2) spatial separation i.e. the laser can be positioned separately from the massive excitation and detection instruments; (3) solid standard samples become unnecessary if the direct introduction of a laser-generated suspension into an excitation plasma or the dissolution of the suspension is quantitatively achieved; (4) the loss of analyte on transport between the ablation cell and the plasma source can be eliminated; and ( 5 ) some insights concerning the laser ablation process such as fractional ablation (vaporization) can be obtained.Supposed demerits of LALM are the dilution effects by a liquid medium and the possibility of contamination in the sampling process as compared with the direct sampling and introduction technique. Two-step methods where the laser sampling and the sample introduction are separated by using graphite collectors have been reported by other groups.11.12 However in these approaches the trapping efficiency of the laser-ablated particles has not been sufficient and the graphite can be a source of contamination in ultratrace analysis. The merits of the proposed technique LALM have been ascertained in combination with ICP atomic emission spectrometry (ICP-AES) and some of the results viz. observation of the trapped particles by scanning electron microscopy (SEM) trapping efficiency and the direct introduction of the suspension into the ICP will be discussed later from an analytical point of view.Experimental Apparatus The specifications of the apparatus and the operating conditions are summarized in Table 1. The assembly for LALM is schematically represented in Fig. 1. A sample was positioned in a liquid medium (10 ml) contained in a flat-bottomed beaker (25 ml). Two methods of sample holding were employed firstly sticking the sample to the flat top of a quartz rod with double-sided adhesive tape; and secondly holding the sample at the top of a stainless- steel rod by suction through a bore with an O-ring seal. A Quick Connects vacuum-tight connector (Swagelok Part No. SS-QC4-B-400) was assembled at the other side of the rod in order to maintain a reduced pressure without suction.The latter holding mode was used in the sample Table 1 Apparatus and operating conditions Q-switched Nd:YAG laser- Supplier Model Energy Pulse width Repetition rate Wavelength Electronic rn icro balance- Quantel International 150 mJ per pulse 10 ns 10 Hz 1064 nm Y G-580A Supplier Mettler ME 30 Model Weighing range (electrical) 0-30 mg Capacity 1020 mg Reproducibility +. 1 Pg ICP atomic emission spectrometer- Supplier Model Monochrometer R.f. incident power Ar flow rate Nebulizer Scanning electron microscope- Seiko Instruments 100 cm focal length JY-38P I1 3600 grooves mm-l holographic grating 1.3 kW Coolant gas 16 1 min-l auxiliary gas 0.2 1 min-' carrier gas 0.4 1 min-' Glass concentric Supplier JEOL Accelerating voltage 7 kV Working distance 1 5 mm Model JSM-F7542 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 Fig. 1 Schematic diagram of the laser ablation assembly. A Flat- bottomed beaker; B sample; C quartz (or stainless-steel) rod for holding the sample which can be rotated by the action of a stepping motor; D single lens of 150 mm focal length; and E 45" prism mass measurement. The sample could be rotated at a speed of 50 rev min-l by a stepping motor to prevent the creation of a deep crater on the sample surface. The laser beam was focused with a single lens of 150 mm focal length on to the sample surface through the base of the flat-bottomed beaker. The spot size of the laser beam was adjusted to 1 mm. The introduction of the laser beam from the bottom of the beaker prevents the deleterious effects of surface waves which stem from the shock of laser ablation.Samples A brass sheet (Nilaco 62.3% Cu; and 37.7% Zn) of 1 mm thickness was cut into squares of 10 x 10 mm and polished with No. 800 silicon carbide paper (Refine Tec). Sintered zirconia (Tosoh Zr02 with 4% Y) was obtained as sheets of 10 x 10 x 1 mm. The zirconia sheets were washed with dilute hydrochloric acid solution for 1 h at room tempera- ture. Both samples were washed with distilled water and ethanol and dried with hot air. Elemental Analysis The elemental contents of the samples and suspensions were determined by ICP-AES. The brass sample (0.3 g) was dissolved in 10 ml of nitric acid (1 + 1) by heating on a hot-plate and then diluted.The brass suspension formed by the 2000 laser pulses taken into 10 ml of distilled water was dissolved by the addition of 1 ml of nitric acid (1 + l) on a hot-plate. The zirconia suspension formed in 10 ml of distilled water was trans- ferred into a poly(tetrafluoroethy1ene) vessel and evapor- ated to near dryness. After the addition of 10 ml of sulphuric acid (1 + 2) the vessel was set in a pressure bomb (San-ai Model NT-25) and heated at 230 "C for 24 h.13 SEM Samples A 5 pl volume of settled suspension was dried on a glass plate (5 x 5 x 1 mm) at room temperature. The plate was fixed on to a sample stage using a silver paste (Fujikura Chemical Dotite) and was then coated with gold. Results and Discussion SEM Observations Fig. 2 shows the trapped products of LALM of a brass sample in water.These are fine spherical particles typically less than 1 ,urn together with a small amount of amorphous aggregates. Thompson et al. l4 observed the substances ablated in a glass chamber with gas flow and subsequently collected on a filter with a 0.4 pm pore size. They reported that the particles collected on the filter represented 20-30% (by mass) of the ablated material and that there were a large number of spherical particles ranging in size from 10 pm to less than 1 pm. On comparing the results the smaller size of the particles and the large amount of amorphous material obtained by LALM is significant. It suggests that in LALM the ablated materials in the form of liquid or vapour have cooled more rapidly and been trapped by the surrounding liquid medium.The ablated material from zirconia placed in water is shown in Fig. 3. Distorted spheres of around 1 pm or less and crooked needles are observed. Less amorphous ma- terial is found in the ablated material from zirconia than that from brass. A spherical particle with a tail has also been observed by Thompson et all4 The morphological differ- ence between the ablated material from the metal brass and that from the ceramic material zirconia stems from the difference in the melting-p~intsl~ (brass 932; Cu 1083; Zn 420; and Zr02 about 2700 "C) and b~iling-pointsl~ (Cu 2567; Zn 907; and ZrOz about 5000 "C). Namely the amorphous materials are condensed directly from the sample vapour and the needles are solidified from the splashing of viscous drops of ablated material.* Fig. 2 Scanning electron microscope photograph of spherical particles and amorphous material produced by the laser ablation of brass in water 1 prn H Fig. 3 produced by the laser ablation of sintered zirconia in water Scanning electron microscope photograph of materialJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 543 Table 2 Trapping efficiency of brass samples ablated in water Amount trapped/mg Amount Sample ablated*/mg Cu Zn Total 1 0.273 0.163 0.099 0.267 2 0.243 0.151 0.090 0.241 3 0.264 0.162 0.098 0.260 4 0.257 0.158 0.095 0.253 5 0.253 0.155 0.093 0.248 6 0.267 0.164 0.099 0.263 Average 0.260 0.160 0.096 0.255 SD$ 0.01 1 0.006 0.004 0.010 * 2000 laser shots. t Cu:Zn in target brass samples were 1.65 f 0.02. $ Standard deviation.Recovery (O/o) 97.8 99.2 98.5 98.4 98.0 98.5 98.4 0.5 Cu:Znt 1.65 1.68 1.65 1.66 1.67 1.66 1.66 0.0 1 Trapping Efficiency and Fractional Ablation The trapping efficiency of LALM was tested with the brass samples. The efficiency was calculated from the mass lost on ablation by weighing the sample directly with the electronic microbalance and the mass trapped in the suspension by analysing the elemental content with ICP- AES after the dissolbtion with nitric acid. The results are shown in Table 2. A trapping efficiency of nearly 100% was achieved for LALM. Although the formation of fine bubbles were occasionally observed by video monitoring of the LALM process the results indicate that sufficient cooling and condensation of vapour were attained.The fractional vaporization occurring in the laser abla- tion process has been discussed previously.2J6 The Cu:Zn ratios in the suspensions were also determined and are shown in Table 2. The Cu:Zn ratio in the suspension agreed with that of the target brass sample and the existence of fractional ablation can be denied. Baldwin16 has collected the laser-ablated material from a brass sample fixed on a film and suspended 0.5 mm above the sample surface. The Cu:Zn ratio in the material collected (2.0) was significantly smaller than that in the target material (2.7). Baldwin16 also used Q-switched lasers hence the discrepancy between the results might indicate that elemental redistribution occurs among the vapour and particles according to the particle sizes and the vapour pressures of the elements.However the nearly 100% collection efficiency of ablated materials achieved with LALM offers a reliable method of sampling the target material whether an elemental redistribution exists or not. Direct Introduction of the Suspension into the ICP As the suspension obtained by using LALM consists of fine particles of around 1 pm or less and is stable over several hours it can be introduced directly into the ICP via the usual concentric nebulizer without any additional operations. The brass suspension used was dark brown and could be instantaneously changed into a clear solution by the addition of 2 p1 of nitric acid. The emission intensities of the Cu I (324.75 nm) and Cu I1 (224.70 nm) and Zn I (213.86 nm) and Zn I1 (206.19 nm) lines were compared between the suspensions and the acidified solutions.All of the lines showed a similar tendency in that the intensities of the suspensions were decreased to 60-70% of those of the solution. Because replacement of the nebulized sample of a suspension by dilute nitric acid solutions brought about a rapid rise and fall of the emission intensity the causes for the difference in the intensities were considered to be (i) absorption of suspended materials on the way to the nebulizer; and (ii) the difference in uptake efficiency in the spray chamber and plasma between the droplets formed from a solution and the particles from a suspension. The use of a specified nebulization system for slurry introduc- tion17J8 would minimize the difference in the intensities.Ebdon et al.17 reported that an alumina slurry of very fine particle size 100% of the particles less than 5 pm which can be easily obtained by LALM gave the same emission intensity as compared with solution introduction. The direct introduction of zirconia suspensions obtained by LALM was also investigated. The emission intensities of the Zr I1 line at 343.82 nm of the suspension were approximately 35% of those of the dissolved solution of the suspension. This decrease is more striking than that of the brass suspension. Similar results have been reported in the direct slurry introduction study. Long and Brennerl* showed that the refractory materials were difficult to atomize directly even in an ICP. Unlike the brass suspen- sions the zirconia suspensions were difficult to dissolve.The dissolved portion of the zirconia which was measured by filtering the suspension with a disposable syringe filter of 0.45 pm pore size were t l % both for the suspension obtained by LALM in a sulphuric acid (1 +2) medium and for the suspension with the addition of sulphuric acid after LALM in water. By using LALM in sulphuric acid (1 + 2) a decrease of about 40% in the amounts ablated versus that obtained by the LALM in water were brought about. This could be because of the difference in the viscosities of the media which suppress the expansion of the laser induced plasma into the liquid medium. From a practical point of view there exist some limita- tions for the combination of LALM and the direct introduc- tion of a suspension for ICP-AES because of the insufficient efficiency of the nebulization introduction step in an ICP which is usually 1% efficient or less.Compared with the gas- phase transfer in the usual laser ablation process aqueous sample introduction might cause solvent loading in the ICP and in ICP mass spectrometry higher intensities of molecular oxide species. Therefore other types of sample introduction techniques such as electrothermal vaporiza- tion or glow discharge sputtering,19 should be more advan- tageous for trace element determinations by LALM. Conclusions The merits of LALM have been experimentally ascertained i.e. the temporal and spatial separation of the sampling and introduction and excitation processes and the almost 100% trapping efficiency. The LALM process produces a suspen- sion consisting of fine particles of around 1 pm or less,544 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL.6 which can be directly introduced into an ICP. Once the suspension is transformed into a solution the usual concentration or separation techniques can be adopted such as extraction ion exchange precipitation etc. which are impossible for gas-phase laser ablation. The demerit of LALM i.e. the dilution by the liquid medium can be overcome by the above mentioned tech- niques and/or by the use of highly sensitive instruments e.g. an ICP mass spectrometer. However at this stage the advantage of the spatial resolution in normal laser ablation is lost by the accumulated sampling in LALM which is needed in practice owing to the lower concentration level of the suspension obtained. The quantitative analysis of solid samples without solid standards is one of the merits of LALM but this objective has not been fully accomplished yet especially for ceramic materials.The techniques and devices developed for the slurry injection will improve the situation for ICP-AES with direct introduction of the suspension. Also the dissolution procedure if required should be easier for a suspension consisting of fine particles than for solid sample materials. References 1 Thompson M. Goulter J. E. and Sieper F. Analyst 1981 106 32. 2 Kawaguchi H. Xu J. Tanaka T. and Mizuike A. Bunseki Kagaku 1982,31 E185. 3 Ishizuka T. and Uwamino Y. Spectrochim. Acta Part B 1983 38 519. 4 5 6 7 8 9 10 11 16 17 18 19 Gray A. L. Analyst 1985 110 551. Arrowsmith P. Anal. Chem. 1987 59 1437. Leis F. and Laqua K. Spectrochim. Acta Part B 1978 33 727. Ishizuka T. and Uwamino Y. Anal. Chem. 1980 52 125. Iida Y. Spectrochim. Acta Part B 1990 45 427. Barshick C. M. and Harrison W. W. Microkim. Acta 1989 111 169. Arrowsmith P. and Hughes S. K. Appl. Spectrosc. 1988,42 1231. Rudnevsky N. K. Tumanova A. N. and Maximova E. V. Spectrochim. Acta Part B 1984 39 5. Wennrich R. and Dittrich K. Spectrochim. Acta Part B 1987 42 995. Ishizuka T. Uwamino Y. and Tsuge A. Bunseki Kagaku 1985 34 487. Thompson M. Chenery S. and Brett L. J. Anal. At. Spectrom. 1990 5 49. CRC Handbook of Chemistry and Physics ed. Weast R. C. CRC Press Boco Raton 1983. Baldwin J. M. Appl. Spectrosc. 1970 24 429 Ebdon L. Foulkes M. E. and Hill S. J. Anal. At. Spectrum. 1990 5 67. Long G. L. and Brenner I. B. J. Anal. At. Spectrom. 1990,5 495. Kitagawa K. Kanoh S. Ohta K. and Yanagisawa M. Anal. Sci. 1988 4 153. Paper 1/00 784J Received February 19th 1991 Accepted May 29th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600541

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 545 Preliminary Investigations of a Helium Alternating Current Plasma for the Determination of Metals by Atomic Emission Spectrometry Luis A. Colon* and Eugene F. Barry Department of Chemistry University of Lowell One University A venue Lowell MA 01 854 USA The construction and operation of an alternating current plasma are described. The plasma is generated across two copper electrodes utilizing helium as the plasma and nebulizer gas. The plasma operates at a frequency of 60 Hz. Aqueous solutions containing elements of interest are introduced into the plasma by two different devices a glass frit nebulizer and a thermospray interface. Improved detection limits are associated with the latter. Analytical characteristics for 14 elements are reported including detection limits (30) for the metals investigated at the ppb level.Linear ranges of 2-4 orders of magnitude are observed. Signal precision at the level of 10 times the detection limit ranges from 1.9 to 10% relative standard deviation. Keywords Alternating current plasma; glass frit nebulizer; thermospray nebulizer; atomic emission spectro- metry; aqueous metal determination Atomic emission spectrometry (AES) using plasmas as excitation sources has become one of the most commonly used analytical techniques for trace element determination. The widely used emission sources are inductively coupled plasma (ICP); direct current plasma; and microwave- induced plasma (MIP). All systems offer low detection limits reliable precision and a large linear dynamic range for many elements the ICP being the most widely investi- gated and used.'** Unfortunately the original investment and the cost of operation can be considered as limiting factors.Possible approaches to minimizing these factors have been ~ t u d i e d . ~ ? ~ The MIP is the least expensive and least complicated of the three emission source^,^-^ particularly when low power (approximately 200 W) is used with minimal gas consump- tion. It is common practice to introduce the sample into the MIP in the vapour phase making this type of plasma source a suitable detector with gas chromatography (GC).8>9 With the development of the Beenakker cavity,lOJ1 liquid aero- sols can be introduced at atmospheric pressure although this process requires large power levels and a high gas ons sump ti on;^^-^^ these requirements may lead to complex- ity and an increase in operating cost.Few reports have appeared on the use of low-powered MIPS for the direct introduction of an aqueous solution.1sJ6 The detection limits found are considerably higher (ppm level) in compar- ison with other plasma systems. Lower detection limits have been reported with the use of a low power high efficiency MIP." A helium alternating current plasma (ACP) developed in this laboratory is described in this paper as an inexpensive alternative for analysis by AES. The ACP is based on the same principle as the micro-arc originally developed as a sample introduction device for the MIP.'* The micro-arc has also been employed as an emission source in atomic spectroscopy for samples in the gaseous phase.19920 Never- theless analysis by AES of direct liquid samples introduced via nebulization into the micro-arc has not been reported. The ACP is generated by a high voltage low a.c. step- up transformer. The plasma is not extinguished by the introduction of liquid sample aerosols and utilizes helium as the plasma support gas simultaneously serving as the nebulizer gas. The helium species in the plasma produce sufficient energy to excite other atomic species producing characteristic elemental emission. The ACP is probably the simplest and most inexpensive system to construct and operate that can be used as an emission source. The ACP has been successfully employed as a specific element detector for GC2'J2 and high-performance liquid chromatography paper.23 Thus this paper describes the construction operation and analytical performance of the ACP for the detection of 14 elements.Liquid aerosols generated via a glass frit nebulizer (GFN) are directly Table 1 Instrumental components Component Power supply (ax.) Monochromator (slit-width 30 pm and slit-height 5 mm) PMT power supply PMT (1000 V) Picoammeter Nebulizers Lens Discharge tube Data acquisition Computer Support gas Low pass filter Model/T ype Webster ignition EU-700 0.35 m transformer 12-8AB7 7640 R446 4 14-s Glass frit Thermospray Fused silica biconvex 1 mm id. quartz LabCalc AT compatible Helium ultra-high purity Time constant 0.2 s 25.4 mm diameter 101 mm focal length Manufacturer STA-Rite Frankfort KY USA GCA McPherson Acton MA USA McPherson Acton MA USA Hamamatsu Middlesex NJ USA Keithley Instruments Cleveland OH USA Laboratory constructed Laboratory constructed Oriel Stratford CT USA Laboratory constructed Galactic Industries Salem NH USA Zenith Data Systems St.Joseph MI USA Northeast Airgas Manchester NH USA Laboratory constructed * Present address Department of Chemistry Stanford University Stanford CA 94305-5080 USA.546 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 introduced into the plasma. The results are compared with those given by a thermospray sample introduction system. Linear responses detection limits and precision are discussed. Experimental Instrumentation The instrumental components utilized in the construction of the ACP emission system are listed in Table 1.A diagram of the ACP is shown in Fig. 1. The a.c. discharge was generated across two copper or tungsten electrodes (3 mm 0.d.) and maintained in a controlled helium atmosphere by means of the quartz discharge assembly illustrated in Fig. 2. The airtight assembly was constructed in the laboratory using two pieces of quartz tubing (4 mm i d . x 6 mm o.d. 5 D 0 r' Fig. 1 Schematic representation of the ACP A a.c. power supply; B PMT power supply; C quartz discharge assembly (see Fig. 2); D focusing lens; E monochromator; F PMT in housing; G picoam- meter; H sample introduction system; I flow meter controller; J helium supply; and K data acquisition system * 0 10 E Fig. 2 Schematic representation of the quartz discharge assembly with thc clcctrodes in place A copper electrodes B 2 cm long ( 1 mm i.d.x 6 mm 0.d.) discharge tube C transfer and electrode container tubing ( 5 cm long 4 mm i.d. x 6 mm 0.d.); D electrode holder; and E sample from nebulizer cm long) attached to a piece of quartz capillary tubing (1 mm i.d. x 6 mm o.d. 2 cm long). The discharge was constrained into the capillary tube of the discharge assembly. One of the electrodes was kept inside the horizontal arm of the assembly (see Fig. 2) while the second electrode was positioned outside of the assembly at the exit end of the capillary tube. The sample was introduced into the plasma through the arm that is perpendicular to the capillary tubing of the discharge assembly. The furnace ignition transformer used as the ax.power supply was operated at its maximum capability (14000 V 20 mA) with the aid of a Powerstat variable autotransformer which was fed with a line supply of 120 V at a frequency of 60 Hz. The ignition transformer was water-cooled to avoid over- heating. Sample Introduction The GFN was constructed from a modified Pyrex sintered glass filter-funnel (15 ml) and has been described else- where.24 An Ismatec peristaltic pump (Model 76 14-30 Cole-Parmer Chicago IL USA) delivered the sample of interest to the GFN at a rate of 0.5 ml min-l. The helium supply was maintained at a pressure of 60 psi. The thermospray interface was utilized in the flow injection mode. The thermospray probe was constructed of stainless-steel capillary tubing (k in 0.d. x 0.004 in id.30 cm long) (Alltech Associates Deerfield IL USA). The probe was introduced into a flexible metal tube (4 in id.) wrapped with heating tape and glass wool then with aluminium foil. Chromel-ahmel thermocouples attached to the flexible metal tube were employed in order to Capillary probe (in Swagelok fitting) Coolant out t Helium Liebig I Coolant in Waste condenser 35 cm r Fig. 3 Spray chamber-desolvation system .............. .... I t Helium ........ ............. \ S P W chamber Fig. 4 Tip of the capillary probe in place with a Swagelok fittingJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 547 Table 2 Detection limits (cL) and linear response for Cd and Zn using two plasma viewing positions Element Viewing position cL (ppb) Linearity (order of magnitude) Cd End-on 24 Side-on 46 Zn End-on 5 Side-on 29 2 3 2 3.5 monitor the temperature of the interface.A heated spray chamber (1 5 cm long 5 cm 0.d. x 4.6 cm id.) connected to a water-cooled Liebig condenser provided partial desolvation and solvent removal respectively. The system resembles one which has been used in combination with the ICP.25 The spray chamber-desolvating system is depicted in Fig. 3 and the tip of the capillary held in place with a Swagelok fitting is illustrated in Fig. 4. Condensation in the transfer tube (1 5 cm long 4 mm i.d. x 6 mm o.d.) from the condenser to the plasma was prevented by maintaining the temperature above 70 "C with heating tape. A Spectra-Physics solvent delivery system (Model SP8700 San Jose CA USA) delivered the sample carrier stream (water) at a rate of 1 ml min-l to a Rheodyne-type injector (Cotati CA USA) equipped with a sample loop of 200 pl which was connected to the capillary probe.A silica-based column (150~4.6 mm i.d. 5 pm C8 Alltech Associates) was placed immediately before the sample injector in order to establish the required minimum pump pressure of 600 psi for proper flow regulation. Reagents Reference solutions of all metals were prepared from 1000 ppm stock solutions (Fisher Scientific Fair Lawn NJ USA) using distilled water and dilute hydrochloric or nitric acid (1%) (Ultrex J. T. Baker Phillipsburg NJ USA) matching the matrix of the original stock solution as required. Procedure The nebulized solutions were transported to the plasma by using helium as the carrier gas which at the same time served as the plasma supporting gas. Optimization was performed by means of univariate searches using a signal- to-background noise ratio as the optimization criterion.The thermospray probe and spray chamber were optimized for Cr and Zn metals and the resulting optima were maintained throughout. For the comparative line intensity experiments solutions of 100-200 ppm of the elements studied were nebulized into the plasma. The plasma was focused at the entrance slit of the monochromator with a magnification factor of 1.5 throughout. The net analyte emission resulted from the difference between the apparent analyte emission and the blank emission. Data collection was achieved by means of the data acquisition system at a sampling rate of 3 Hz (Le.three data points were sampled every second). Random noise was reduced by using the Savitzky-Golay algorithm (a moving-window smoothing function),26 yielding improved signal-to-noise (S/N) ratios. The reported results were obtained by measuring the peak height of the response. Detection limits were calculated according to the method recommended by IUPAC (30 n = 201.27 Results and Discussion The response of the ACP is dependent on the ax. voltage output with the maximum signal response at the upper limit of the power supply. Thus the power supply was operated at its maximum capability producing an output 7 10 15 20 25 Distance/mm Fig. 5 Effect of the electrode distance on the ACP response A Zn response side-on viewing using GFN; and B Cr response end-on viewing using a thermospray interface voltage of approximately 14000 V a.c.(20 mA). The distance between the electrodes also has a pronounced effect on the plasma response as shown in Fig. 5 for Cr and Zn. Although the measurements were obtained at two plasma viewing positions (Table 2) with two sample introduction devices the optimum signal and S/N ratio is associated with a 20 mm gap in each instance. The design of the discharge tube shown in Fig. 2 allows two possible views of the plasma; side-on (transversal) and end-on (axial). In the transversal confi- guration the plasma is positioned vertically and viewed through the walls of the discharge tube. In the end-on position the centre of the fireball of the horizontally positioned plasma is observed.The differences in plasma response between the axial and transversal viewing in terms of detection limits (c,) and linearity for Zn and Cd are compiled in Table 2. The lower c in the axial position can be attributed to the fact that the measure- ment of emitted radiation is performed on the total analyte species emitting along the plasma axis at a parti- cular time interval. Alternatively when the plasma is observed in the transversal position the measurement is obtained for the analyte at a particular point of the plasma channel. Thus the observed signal in the axial mode may be expected to be higher than the signal observed in the transversal position owing to a summa- tion behaviour of the signal along the plasma axis. How- ever the longer emitting path offered by the end-on approach permits increased analyte residence times where self-absorption is more likely to occur as the concentration of the emitting species increases thereby causing non-linearity in profiles of response versus con- centration.Evidently the lower cL for the end-on approach com- promises the linear response of the ACP. On the other hand the discharge tube which may endure approximately548 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 20 h of use is continually undergoing changes owing to the deposition of the metallic copper sputtered from the electrodes and other metallic oxides from the analytes. In addition as observed with MIP source^,^ devitrification is likely to occur generating a reproducibility problem. Therefore the end-on approach was maintained for the remaining studies.Thermospray Optimization The temperature of the thermospray probe and the spray chamber has an influence on the analyte response. The probe temperature affects the degree of vaporization of the carrier liquid and is dependent on its composition and flow rate;28~29 therefore optimum temperatures may vary with the nature of the vaporizer.30 The effect of the probe and spray chamber temperature on the S/N ratio was systemati- cally studied for sample solutions containing 2 ppm of Cr and 0.5 ppm of Zn. The optimum temperatures were found to be 245 and 240 "C for the probe and spray chamber respectively. These values were maintained for the remain- ing thermospray-ACP studies. Plasma Characteristics The excitation of the analyte species can be attributed to highly energetic helium species present in the plasma.A characteristic pink-purple colour was observed in the well defined and stable plasma. A characteristic background spectrum from the helium ACP with de-ionized water being nebulized into the plasma (GFN 0.5 1 min-I) is presented in Fig. 6. Molecular emission from the OH NH and N2 bands is shown. Other elemental species present in the plasma are also shown (H He 0). Although it is not shown in Fig. 6 emission from the electrodes was observed but at a relatively low intensity. However Si emission from the discharge tube was not observed and can be attributed to the fact that the plasma arc was concentrated in the centre of the capillary tube as a thin plasma jet being more diffuse towards the edges.Apparently the edges of the plasma do not have sufficient energy to atomize and excite Si from the discharge tube. An analyte can undergo various excitation processes depending on the plasma and its characteristic^.^^ This situation may produce different relative intensities for a given element depending upon the excitation source. Results from a comparative line intensity study for the most intense lines of the elements are given in Table 3. All the measurements were performed using a slit- width and height of 30 pm and 5 mm respectively. The H H H OH (second order) \ 200 400 600 Wavelengt h/n m I Fig. 6 Characteristic background spectrum of the helium ACP K Li Mn Na Ni Pb Table 3 Comparison of line intensity for several elements Element Wavelength (and line)/nm Relative intensity Ba 455.40 (11) 1 .oo 493.41 (11) 0.64 553.55 (I) 0.079 Ca 393.37 (11) 0.24 396.85 (11) 0.17 422.67 (I) 1 .oo Cd 214.44 (11) 0.12 226.50 (11) 0.28 228.80 (I) 1 .oo c o 240.72 (I) 1 .oo 241.16 (I) 0.44 242.49 (I) 0.78 345.35 (I) 0.53 359.35 (I) 0.70 360.53 (I) 0.49 248.33 (I) 1 .oo 371.99 (I) 0.2 1 766.49 (I) 1 .oo 670.70 (I) 1 .oo 257.61 (11) 0.99 259.37 (11) 1 .oo 260.57 (11) 0.86 403.08 (I) 0.73 330.14 (11) 0.28 589.00 (I) 1 .oo 589.59 (I) 0.49 341.48 (I) 1 .oo 346.17 (I) 0.60 351.51 (I) 0.53 352.46 (I) 0.99 283.31 (I) 0.93 363.57 (I) 0.56 368.35 (I) 1 .oo 405.78 (I) 0.94 Sr 407.77 (11) 1 .oo 421.55 (11) 0.77 460.77 (I) 0.84 Cr 357.87 (11) 1 .oo Fe 238.20 (11) 0.084 248.81 (I) 0.58 769.90 (I) 0.76 Zn 213.86 (I) 1 .oo centre of the fireball of the axially viewed plasma was focused on the entrance slit of the monochromator for the atomic and ionic emission lines.At this stage of the investigation no experiments were conducted in order to observe if the intensity of the analyte emission line varied with the spatial region of the source. The line intensities were normalized to the most intense line of the element of interest to which a value of 1.00 was arbitrarily assigned. The strongest line was selected for the determination of a particular element. Analytical Performance The performance of the ACP was evaluated for 14 elements in aqueous solutions. The helium flow rate was optimized for each individual element studied using both sample introduction devices.Maximum S/N ratios were obtained by employing helium flow rates in the range from 0.5 to 1.8 1 min-l for the GFN and 0.8 to 1.2 1 min-l for the thermospray interface. The determinations of the elements under study were performed using the appropriate opti-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 549 Table 4 Comparison of the detection limits (ppb) with other plasma sources ACP MIP Element GFN Thermospray Ar-LP* He-HE? NMIPS Ba 220 Ca 7.0 Cd 10 c o 54 Cr - Fe 130 K 18 Li 1.2 Mn 67 Na 0.74 Ni 180 Pb 120 Sr 29 Zn 29 * Low power Ar MIP ref. 16. t Helium high efficiency MIP ref. 17. 4 Nitrogen MIP ref. 34. 9 Ref. 35. 7 Ref. 36. 99 8.0 5.0 36 51 68 - 0.40 13 13 12 10 24 - 2 70 - 60 7.5 - - 2 700 75 12000 105 975 45 36 - 64 - 10 350 16.5 3 1.5 - - - - 37 17 675 12 15 28 1.2 - - - 5.4 0.22 0.29 - - - 16 120 IcPg 1.3 0.19 2.5 6.0 6.1 4.6 7.51 4.21 1.4 29 10 42 0.42 1.8 mum helium flow rates for the GFN and the thermospray nebulizer.By using the thermospray at a fixed helium flow rate (1 1 min-l) for all elements the results obtained at the optimum helium flow rates did not change significantly. However the GFN could not be operated at a fixed helium flow rate without having a significant impact on the signal response. This behaviour suggests that multi-element deter- minations are possible with one set of conditions when using the ACP-thermospray interface but are unlikely to be obtained with the ACP-GFN interface for the elements that require different helium flow rates. Although the GFN generates a very fine mist with a very small droplet size di~tribution,~~ which greatly enhances efficient sample introduction into the plasma,33 the thermo- spray nebulizer showed lower detection limits than the GFN by factors of 2-10 (see Table 4). It was believed that this performance might be attributed to the heated spray chamber and the partial removal of the solvent from the system permitting a highly desolvated analyte to reach the plasma instead of a wet aerosol.Thus in order to eliminate a possible ‘cooling’ effect of the aerosol on the plasma which may decrease the energy available for the excitation process a suitable desolvation system similar to that used with the thermospray interface was connected to the GFN. In this fashion partial desolvation of the analyte was facilitated; however the cL was improved for only a few elements (Co Ni Pb) and by no more than a factor of 2.This trend suggests that the application of heat to the expansion chamber further desolvates a fraction of the hot aerosol leaving the capillary probe since the analyte released by the thermospray is partially de~olvated.~~ This is supported by the fact that signal enhancement is observed when the expansion chamber is heated until an optimum is reached. On the contrary a desolvation chamber for the GFN might not achieve analyte desolvation to the same extent as the thermospray and the heated chamber because the aerosol leaving the GFN is not partially desolvated; therefore the only desolvation performed is by means of the heated chamber. Calibration graphs were constructed for analyte concen- trations ranging from near the cL to at least 100 ppm.Linear dynamic ranges were observed to be between 2 and 4 orders of magnitude. Detection limits acquired with the ACP for the metals studied and a comparison with data reported with other plasma sources are shown in Table 4. The detection limits obtained with other systems were not necessarily defined by the IUPAC ( 3 0 ) method.27 Thus in order to compare the ACP with the more established systems all detection limits (c,) shown in Table 4 were converted into the 30 criterion by multiplying the original values by the appropriate conversion factor where neces- sary. In this standardized approach the ACP results are comparable or superior to those reported for the MIP sources.Nevertheless a superior cL is achieved by the ICP which can be operated at one set of conditions for multi- element determinations. The feasibility of the ACP for the determination of halogens in an aqueous solution was studied. Unfortunately the results were not as encouraging as expected; thus further studies were not pursued with the present system. The repeatability of the peak height measurements was determined with solutions of the elements under considera- tion in this study. At least six repetitive measurements were performed for each analyte at a level of one order of magnitude higher than the estimated cL. The relative standard deviation (RSD) was <10% for both sample introduction approaches with average values of 5.4 and 7.0% for the GFN and the thermospray interface respectively.Interference Studies Interference studies were conducted by using two classical interference systems the Ca-phosphate system to study the depression of the Ca atom emission signal when refractory compounds are likely to be formed; and the Ca-Na system to observe the effect of an easily ionizable element on the Ca response. The Ca atomic signal was monitored at 422.7 nm. Solutions for each system were prepared containing 10 ppm of Ca and different concentra- tions of the corresponding interferent (phosphate as H3P04 and Na as NaC1). The effect of increasing the H3P04 and Na concentration on the Ca emission signal is shown in Figs. 7 and 8 respectively. The interference produced by increas- ing the H3P04 concentration becomes severe at an H3P04:Ca molar ratio above 0.12; at a molar ratio of 1.2 the signal is reduced to about lo% decreasing rapidly to zero above that molar ratio.On the other hand the interference produced by the addition of an easily ionizable element (Na) is associated with an enhancement of the Ca5 50 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 I % 0 2 0.6 a .s 0.4 m - 2 0.2 0 0.012 0.12 1.2 12 H,PO,:Ca molar ratio Fig. 7 Effect of increasing the H3P04 concentration on the Ca emission signal 1 I & 0.8 . al 2 0.6 . (d Q) U 2 - 0.4 0.2 I I 1 100 0 0.1 1 .o 10 “a1 ( ~ P m f Fig. 8 Effect of increasing the Na concentration on the Ca emission signal atomic signal for concentrations of up to 10 ppm of Na. A further increase of the Na concentration did not enhance the Ca emission response. Excitation Temperature The excitation temperature was determined from the spectral emission intensities of ten Fe atomic lines by using the slope method.37 The thermometric species was intro- duced into the ACP by means of the GFN with the monochromator slit-widths adjusted to 20 pm.Relative transition probabilities tabulated by Reips and Bridges and K ~ r n b l i t h ~ ~ and normalized to the Fe I 371.994 nm line were employed in the temperature calculations. Excita- tion temperatures obtained with the transition probabili- ties of Reif,38 5900 K k 7% yielded a slightly better correlation coefficient (ie. 0.98 in comparison with 0.97) and less relative error than those tabulated by Bridges and K ~ r n b l i t h ~ ~ 5640 K k 9%.Nevertheless no statistical dif- ference was found between the two results at the 95% confidence level. It should be noted that temperature gradients which may exist within the source have not been considered in these experiments. As observed by Vogel and K o l a ~ i n s k i ~ ~ in spectroscopic measurements of temperature distributions in short arcs higher temperature values observed for the ACP in these preliminary studies might correspond to a spatial region close to the electrodes. Although these values seem to be sufficient for excitation of the analytes and are comparable to other plasma sources,14J7941*42 an examination of the c and the inter- ference studies strongly suggests that the ACP is a ‘cooler’ and less robust plasma source than the ICP.Conclusion Although the ACP is not as powerful and does not have the capabilities of the ICP for example it has been demon- strated that the helium ACP is a feasible inexpensive alternative for spectrochemical determinations at high ppb and low ppm levels. The ACP can easily be implemented in the laboratory at a relatively low cost. External initiation of the plasma is not required because the voltage used is above the breakdown voltage for striking the a.c. arc and direct nebulization of the aqueous solution does not extinguish the plasma. The authors acknowledge the Seed Money Research Program at the University of Lowell for providing financial support and L. A. C. thanks the Graduate School at the University of Lowell for providing a summer research fellowship in 1990.Also the authors are deeply apprecia- tive for the data acquisition system donated by Galactic Industries Corporation (Salem NH USA) and the capillary probe very generously donated by Alltech Associates (Deerfield IL USA). 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 References Inductively Coupled Plasma in Analytical Atomic Spectroscopy eds. Montaser A. and Golightly D. W. VCH New York 1987. Inductively Coupled Plasma Emission Spectroscopy ed. Boum- ans P. W. J. M. Wiley New York 1987 parts I and 11. Hieftje G. M. Spectrochim. Acta Part B 1983 38 1465. Boumans P. W. J. M. and Hieftje G. M. in Inductively Coupled Plasma Emission Spectroscopy ed. Boumans P. W. J. M. Wiley New York 1987 part I ch. 5. Skogerboe R. K. and Coleman G.N. Anal. Chem. 1976,48 61 1A. Zander A. T. and Hieftje G. M. Appl. Spectrosc. 1981 35 357. Goode S. R. and Baughman K. W. Appl. Spectrosc. 1984,38 755. McCormack A. J. Tong S. C. and Cooke W. D. Anal. Chem. 1965,37 1470. Estes S. A. Uden P. C. and Barnes R. M. Anal. Chem. 1981 53 1829. Beenakker C. I. M. Spectrochim. Acta Part B 1976 31 483. Beenakker C. I. M. Bosman B. and Boumans P. W. J. M. Spectrochim. Acta Part B 1978 33 373. Michlewicz K. G. and Carnahan J. W. Anal. Chem. 1985 57 1092. Hass D. L. and Caruso J. A. Anal. Chem. 1984 56 2014. Urh J. J. and Carnahan J. W. Anal. Chem. 1985 57 1253. Ng K. C. and Shen W. Anal. Chem. 1986 58 2084. Long G. L. and Perkins L. D. Appl. Spectrosc. 1987,41,980. Long G. L. and Perkins L. D. Appl. Spectrosc. 1989,43,499. Layman L.and Hieftje G. M. Anal. Chem. 1975 47 194. Churchwell M. E. Messman J. D. and Green R. B. Spectrosc. Lett. 1985 18 679. Green R. B. and Williams R. R. Anal. Chim. Acta 1986 187 301. Costanzo R. B. and Barry E. F. Anal. Chem. 1988,60 826. Costanzo R. B. and Barry E. F. J. Chromatogr. 1989 467 373. Costanzo R. B. and Barry E. F. J. High Resolut. Chromatogr. Chromatogr. Commun. 1989 12 522. Colon L. A. and Barry E. F. J. Chromatogr. 1990,513 159. Koropchak J. A. and Winn D. H. Anal. Chem. 1986 58 2258.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 5 5 1 26 27 28 29 30 31 32 33 34 35 Savitzky A. and Golay M. J. E. Anal. Chem. 1964,36 1627. 36 Nomenclature Symbols Units and their Usage in Spectro- chemical Analysis 11 Data Interpretation Spectrochim. Acta 37 Part B 1978 33 241. Vestal M. L. and Fergusson G. J. Anal. Chem. 1985 57 38 2373. 39 Koropchak J. A. and Winn D. H. Appl. Spectrosc. 1987 41 131 1. 40 Roychowdhury S. B. and Koropchak J. A. Anal. Chem. 41 1990 62 484. Ingle J. D. Jr. and Crouch S. R. Spectrochemical Analysis 42 Prentice-Hall Englewood Cliffs NJ 1988 chs. 7 and 8. Layman L. R. and Lichte F. E. Anal. Chem. 1982 54 634. Nisamaneepong W. Hass D. L. and Caruso J. A. Spectro- chim. Acta Part B 1985 40 3. Deutsch R. D. Keilsonhn J. P. and Hieftje G. M. Appl. Spectrosc. 1985 39 531. Winge R. K. Peterson V. J. and Fassel V. A. Appl. Spectrosc. 1979 33 206. Fraley D. M. Yates D. and Manahan S. E. Anal. Chem. 1979 51,2225. Kalnicky D. J. Fassel V. A. and Kniseley R. N. Appl. Spectrosc. 1977 31 137. Reif I. Ph.D. Thesis Iowa State University 197 1. Bridges J. M. and Kornblith R. L. Astrophys. J. 1974 192 793. Vogel N. and Kolacinski Z. J. Phys. D 1987 20 545. Golightly D. W. Porrzapf A. F. and Thomas C. P. Spectrochim. Acta Part B 1977 32 3 13. Faires L. M. Palmer B. A. and Engleman R. Jr. Spectro- chim. Acta Part B 1984 39 819. Paper 1/01612A Received April 8th I991 Accepted June 20th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600545

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 553 Study of Internal Standardization for Analysis of Powdered Samples Using a Theta Pinch Discharge Zuwei Wang* and Alexander Scheeline School of Chemical Sciences University of Illinois 1209 W. California Street Urbana lL 61801 USA Theta pinch discharges are capable of sampling ceramics and other refractory substances for atomic emission spectrometry. Previously only qualitative information has been obtained. Research on the quantitative aspects of this technique has become feasible owing to improvements in the detection system (a charge coupled array detector and echelle spectrometer) and completion of preliminary parametric studies. This initial quantitative study of the direct sampling of powdered samples using single theta pinch discharges reports the shot-to-shot reproducibility [relative standard deviation (RSD) 20-30%] internal standard correlation (RSD 10-1 5%) and sampling curves for several elements.Keywords Theta pinch discharge; atomic emission spectrometry; internal standard; direct solid sampling Considerable problems remain for the elemental analysis of ceramics and other refractory solids. Most methods require lengthy dissolution fusion or dilution pre-treatment. Con- tamination and loss of volatile species can occur during these procedures in spite of precautions. Developments in the field of ceramics require analysts to research into reliable direct methods of elemental analysis further and also to study suitable certified reference materials for calibration.' Many publications have appeared on elemental analysis of ceramics.Some direct methods have been reviewed.lJ Most research on the direct analysis of ceramics and other refractory materials by plasma spectrometry has focused on techniques such as spark ablati0n,~9~ laser ablati~n,~ nebu- lization of ~lurries,~?~ direct i n ~ e r t i o n ~ ~ ~ - ~ and electrothermal atomi~ation.~ These techniques have advantages in terms of speed ease of sample preparation and low detection limits (in some instances) compared with wet methods but still need further improvement to meet the requirement of fast routine analysis for quality control with adequate accuracy independent of the form particle size melting-point thermal history and concomitant composition of the ana- lyte material. The theta pinch discharge has been developed as an additional technique for direct sampling of ceramics and other refractory materials because of its high sampling energy.Previously only qualitative information could be confidently obtained in this laboratory owing to deficien- cies in the available detection technology; qualitative performance was however promising. Research on the quantitative aspects of this technique has become feasible because a charge coupled array detector and Cchelle spectrometer have been used for detection17J8 and pre- liminary parametric studies have been completed. l4 Initial quantitative studies of spectrochemical analysis of powdered samples by single theta pinch discharges with investigations of the shot-to-shot reproducibility internal standard correlation and sampling curves for several elements are presented. Experimental Low-pressure Argon Plasma According to earlier work reported by White,16 low-pressure argon is a satisfactory discharge gas for the theta pinch.The optimum argon pressure is element dependent but a pressure of 20 Torr (2.666 x lo3 Pa) gives a satisfactory sampling efficiency and signal-to-noise ratio for the elements determined in this work. * On leave from the Dalian Institute of Chemical Physics Dalian China. Mirrored Coil for Energy Transfer A mirrored coil described previously,lOJ1 was used for the analysis of powdered samples. It has a satisfactory energy transfer efficiency for the direct sampling of solids in the theta pinch discharge. A comparison of this design with other types of coil is currently being made.Polymer Cup for Sample Introduction Samples were held 10 mm from the axis of the discharge tube in a holder made of poly(methy1 methacrylate). This material was chosen because polycarbonate holders decom- pose giving rise to a carbon coating on the analyte powders. Typically a pellet specimen with dimensions 2.25 mm deep and 5.25 mm in diameter was employed. The cup was held on a mandrel of length 155 mm which was held by one of the pinch chamber end caps. In order to prevent contamina- tion a clean sample cup was used for each specimen. Sample Powders and Sample Preparation Analytical-reagent grade powders of the various oxides were used as received. Mixtures were prepared by weighing appropriate amounts on an analytical balance (total mass approximately 500 mg) then hand grinding in an agate mortar for 30 min.Approximately 30 mg of the mixture were then compressed into a pellet in the sample cup. Although a pellet press was also used it was found that placing the powder in the cup and packing it manually was more convenient and generated larger signals because of the ease with which the loose surface powder can be sampled. Particle size was generally 10 pm or less as measured by optical microscopy. Detailed studies of particle size effects are planned. For all experiments the diluent powder was A1203. Impurity concentrations as measured in an assay of a portion of the powder using inductively coupled plasma atomic emission spectrometry are listed in Table 1.In order to determine these concentrations the A1203 was fused with LiB02 dissolved in HN03 and 0.6 mol dm-3 0.12 mol dmP3 HC1 and then aspirated. A 100 mg portion of the sample was ultimately diluted to 50 ml of solution. Reagent blanks were subtracted from all measured concen- trations. Background equivalent concentrations (BEC) in the plasma were no higher than 2.5 ppm (equivalent in the undigested oxide powder) for the elements listed in Table 1 except for potassium which had a BEC of 1 ppm in the plasma or equivalently 500 ppm in the oxide powder. Binary or ternary mixtures of oxide powders were employed to obtain working curves. For experiments on554 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 Table 1 Impurities in the A1203 matrix Element Concentration (ppm) Na Ti Ca Fe K Mg 2300 1030 420 345 <loo 23 measurement precision the mixture compositions given in Tables 3 and 4 were used.Charge Coupled Detector and Echelle Spectrometer Previous work16 has demonstrated that it is not possible to use the theta pinch for quantitative analysis unless internal standardization is routinely employed and real-time diag- nosis of self-reversal is possible which necessitates the use of multichannel detection. An kchelle spectrometer de- signed and constructed in the laborato~y,~~J* with a charge coupled array detector (CCD) (Thomson-CSF 7882 CCD Photometrics Tucson AZ USA) cooled by liquid nitrogen was employed. A wide range of wavelengths was accessible allowing simultaneous monitoring of many elements.All the data presented here were obtained from individual pinch firings. While much of this work could have been accomplished using a gated photomultiplier tube (PMT) and a direct To vacuum I Discharge vessel .Sample holder r - - I/ Controller w Power Grounded wall ichelle spectrometer IE EE488 I I Fig. 1 Experimental set-up. Discharge vessel shown both in the top and end-on view. Mirrors Mpl and Mp2 are off-axis paraboloids Table 2 Experimental parameters Parameter Value Voltage 26 kV Current (peak) 85 kA Capacitance 6.05 pF Argon pressure 19.5 Torr Sample position (distance off axis of discharge vessel) (2.600 x lo3 Pa) 10 mm 1 Exposure duration (No. of discharges) reading spectrometer two characteristics of the pinch make the use of the CCD-Cchelle spectrometer combination particularly effective.Firstly the linewidths and shapes appear to be irreproducible from shot to shot but are consistent between elements on individual firings. Thus the ability to select an effective linewidth on each firing improves the precision. Furthermore any unanticipated interferences may be more readily seen with full spectral coverage (CCD or photographic plate) than with isolated line measurements (PMT). Secondly the pinch produces a significant amount of radiofrequency interference (RFI). While use of fibre optics for digital signal transmission and a Faraday cage around the discharge limits the difficulties analogue measurement during a discharge is nevertheless difficult. By recording optical signals on the CCD during the discharge and reading the data when the RFI has subsided the RFI ceases to be a problem.This is not possible using gated integration with PMTs. For all measurements the background was subtracted by interpolating a linear baseline between points adjacent to the line of interest. Lines were sufficiently broad that wings of nearby lines might have led to overestimation of the continuum with a consequent underestimation of line intensity. Integrated intensities rather than peak heights were employed for all lines. The equipment used is shown in Fig. 1 and some important experimental parameters are shown in Table 2. According to the spatially resolved emission experiments the best sampling position is about 10 mm off the axis of the discharge ve~se1.l~ The CCD shutter was opened for 2 s encompassing the moment of discharge firing resulting in collection of time-integrated data.Results and Discussion Discharge Reproducibility and Internal Standardization Although a precedent from other discharge systems sug- gested that internal standardization would probably im- prove reproducibility adequate characterization of internal standard signals in the pinch has not previously been reported. The shot-to-shot reproducibility of A1 I at 396.2 nm and Sr I1 at 42 1.6 nm in the theta pinch is shown in Fig. 2. A total of 20 single theta pinch discharges were used. For ease of visual comparison the raw A1 I signal was multiplied by a scale factor of 2.5. The intensity fluctua- tions for the individual lines are pronounced. The ratio of Sr I1 to A1 I emission is also shown and has a lower relative standard deviation (RSD) (10%) and range (20% of the mean) than the raw emission data (RSD 20%; range 7000 I 1 200 A v) I-’ .- 5 6000 2 5 5000 f 5 4000 .- >.v) I-’ .- 3000 4- c .- p 2000 g 1000 4d I I-’ - 0 2 4 6 8 10 12 14 16 18 20 Pinch number Fig. 2 Shot-to-shot reproducibility and internal standard correla- tion. A Sr I1 42 1.6 nm emission; B scaled A1 I 396.2 nm emission; and C ratio of Sr 1I:Al I emissionJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 555 Table 3 Improvement of reproducibility by using several elements as internal standards (20 replicates per sample) RSD (O/O) Element Internal standard Without With Sample and line/nm and line/nm internal standard internal standard A* A1 I 396.2 Sr I1 421.6 B* Mg I1 279.5 Sr I1 421.6 C t Mg I1 280.3 Ca I1 317.9 D$ Ti I1 376.5 Ti I1 379.5 * Composition (Yo m/m) MgO 5 ; MnO 5; SrO 5; and A1203 85.t Composition (Oh m/m) MgO 5 ; MnO 5; CaO 5; and A1203 85. $ Composition (Yo m/m) TiO 20; and A1203 80. 22 19 23 18 10 10 13 5 Table 4 Improvement of reproducibility by using the matrix element Al I as an internal standard (20 replicates per sample) RSD (O/O) Element Internal standard Without With Sample and line/nm and line/nm internal standard internal standard E* Mg I1 280.3 A1 I 396.2 Ft Ti I1 376.1 A1 I 396.2 G$ Ti I1 368.5 A1 I 396.2 H* Ca I1 317.9 A1 I 396.2 23 21 30 29 15 8 9 13 * Composition (Yo m/m) MgO 5; MnO 5; CaO 5; and A1203 85. t Composition (Yo m/m) MgO 5; Ti02 5; and A1203 90.$ Composition (Oh m/m) MnO 5; Crz03 5; TiOz 5 ; and A1,03 85. 80% of the mean). Sample transport rather than photon shot noise is the cause of the signal fluctuations. The transport varies between discharges for a number of reasons including differences in the packing of sample particulates plasma fluid dynamics and energy transfer into the plasma and from the plasma to the sample. Very little of the sample is consumed when the pinch is fired; the top layer of powder is vaporized. While the change in the surface is evident the thickness of the pellet does not appear to change to the naked eye. Erosion depth is certainly less than 0.5 mm and might be substantially less. Clearly intensity reproducibility of 20-30% RSD does not meet the analytical requirements for fast routine quality control analysis for the production of ceramics.Fig. 2 suggests however that internal standardization is a viable alternative for general analysis. Several elements were used as internal standards. Im- provement in the RSD was different for each element (shown in Table 3 for some elements). The last line pair (Table 3) is an example of self-correlation between two Ti I1 lines giving an RSD of about ~O/O which is the best of any of the examples. Apparently much of the uncertainty in emission intensity is due to sample transport but a residual uncertainty is due to other factors which might include a small contribution from shot noise in the photometric signal readout and digitization noise in the CCD excita- tion fluctuations between discharges or in different spatial regions of a single discharge and background fluctuations.In all these examples A1 was vaporized from A1203 the major matrix substance. Other elements were present in the amount of approximately 5% m/m in the form of the oxide. The major matrix element Al was also used as an internal standard in order to avoid the need for mixing a separate internal standard with each specimen. The shot-to- shot reproducibility was improved (Table 4). By using A1 I at 396.2 nm as an internal standard the reproducibility of Ti I1 is improved by a factor of 3 compared with the use of no internal standard and is nearly as good as the self- correlation (Table 3). Plausible reasons for the good correlations include their similar boiling-points and wave- lengths.Apparently whether the emitting species is an atom or ion is not a main consideration for internal standard selection with this discharge. Ti Sampling From TiOz Titanium is an important trace element in several types of ceramics. The variation of concentration of this element can affect some qualities of the ceramics such as electrical conductivity. When the 396.2 nm neutral emission line of A1 (the major matrix element other than oxygen) is used as an internal standard for Ti 11 both the reproducibility and linearity are satisfactory as shown in Fig. 3. A typical concentration of Ti in the ceramics studied is about 0.5Oh m/m which is in the linear region of the working curve. Thus direct quantitative determination of minor elements such as Ti in ceramics is possible using a theta pinch discharge. I A [Ti] ("A m/m) Fig.3 Working curve for Ti as Ti02 in A1203 with Al as an internal standard. Abscissa concentraton (O/o d m ) includes the amount of Ti known to be in the blank556 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 03 1 I I I Mg Sampling From MgO Another important element present in ceramics is Mg. While Mg is a minor or trace element in some ceramics it was investigated here as a major species. Data are shown for Mg sampling from MgO in Fig. 4. The intensities were measured at three different wavelengths 279.5 nm (reso- nance line) 279.1 nm and 279.8 nm (non-resonance lines). All three lines give rise to non-linear working curves in the absence of internal standardization. Generally in emission sources the resonance lines of Mg I1 are reversed when the concentration of Mg is high.This has been confirmed both in the literature19 and by spark experiments using this detection system.20 Thus one would expect to be able to detect reversal in the pinch. Initially dips in the centre of the 279.5 nm line were interpreted as evidence for reversal. Later investigation showed that the line shape is irreprodu- cible from one firing to the next. While self-reversal is the likely cause of the central dip the extent of a central dip varies substantially from shot to shot rendering definitive experiments difficult. Furthermore line broadening is sufficient that a significant portion of the light falls outside most conceivable Doppler absorption linewidths.The use of Mn I1 at 294.9 nm proved to be acceptable as an internal standard for Mg. Linear working curves as seen in Fig. 4(b) were obtained. There appears to be a reduction in the total sample uptake as the concentration of MgO is increased (Fig. 5). The Mn emission decreases as the Mg concentration increases although the Yo m/m of Mn is constant and the at.-% of Mn changes by less than 3 parts per thousand. Magnesium oxide has a higher melting- and boiling-point than either of the other oxides. For MgO to alter the signal from Mn there must either be a change in x 1200 I- 1 I ( b ) 4t 0 3 6 9 12 [MgO] (% m/m) Fig. 4 Sampling of Mg in theta pinch. (a) No internal standardiza- tion and (b) with Mn I1 294.9 nm used for internal standardization. Intensities measured at A 279.1 nm; B 279.5 nm (resonance line); and C 279.8 nm.Concentration in O/o m/m; three replicate measurements per point 600 5 500 0 400 0 > 300 cn w 8 9 4- In .- c 200 - 100 I 0 I 1 0 3 6 9 12 [MgO] (“A m/m) Fig. 5 Intensity of Mn I1 as a function of [Mg]; [Mn] = 6% m/m in an A1203 matrix. Concentration in O/o m/m; three replicate measurements per point excitation conditions as the concentration of Mg changes or there must be a change in the uptake of all species despite the fact that the specimens are mixtures of pure powders. Although there is no evidence of changes in the excitation of the support gas with the introduction of the analyte there is weak evidence that sampling is changed proportionally for all analyte species even when the melting-point of a component of the sample changes.An alternative explanation is that particle size and hardness are critical experimental variables and MgO causes clumping of the mixed powder samples. Larger particles give smaller signals for all species. While the linearity seen in the data presented here is promising examination of the effects of particle size and analyte speciation must be pursued. The effects of changes in melting-point and hardness must be isolated from the effects of particle size alone. Conclusion Sample transport variability is the major noise source in theta pinch emission spectrometry. Thus internal standardization is a useful means of improving the reprodu- cibility of analytical measurements. Not only is the signal- to-noise ratio improved but the working curve linearity improves also.While much work must still be carried out to develop the theta pinch as a routine analytical source the feasibility of obtaining quantitative measurements has now been demonstrated. While we are aware of the Generalized Internal Standard method21*22 and n o r m a l i z a t i ~ n ~ ~ ~ ~ ~ techniques which could benefit from the multiplicity of available analytical lines an attempt to characterize the use of such methods together with the pinch has not yet been made. They are a logical means of using a large amount of data that has been obtained but not yet processed. This work was supported by the Office of Basic Energy Sciences US Department of Energy (Grant DE-FG02 84ER 132 18). The technical assistance of Duane L.Miller is greatly appreciated. References 1 Broekaert J. A. C. Graule T. Jenett H. Tolg G. and Tschopel P. Fresenius 2. Anal. Chem. 1989 332 825. 2 Ishizuka N. and Morikawa H. Bunseki 1986 471; Anal. Abstr. 1987 49 4B187. 3 Broekaert J. A. C. Leis F. Raeymaekers B. and Zaray Gy. Spectrochim. Acta Part B 1988 43 339.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 557 4 Aziz A. Broekaert J. A. C. Laqua K. and Leis F. Spectrochim. Acta Part B 1984 39 109 1. 5 Technical Information PQ 709 A VG Elemental Winsford Cheshire 1987. 6 Raeymaekers B. Graule T. Breokaert J. A. C. Adams. F. and Tschopel P. Spectrochim. Acta Part B 1988 43 923. 7 Reisch M. Nickel H. and Mazurkiewicz M. Spectrochim. Acta Part B 1989 44 307. 8 Karanassios V. and Horlick G.Spectrochim. Acta Part B 1990,45 85 119 and 129. 9 Kirkbright G. F. and Snook R. D. Anal. Chem. 1979 51 1938. 10 Kamla G. J. and Scheeline A. Anal. Chem. 1986 58 923. 11 Kamla G. J. and Scheeline A. Anal. Chem. 1986 58 932. 12 White J. S. and Scheeline A. Anal. Chem. 1987 59 305. 13 Scheeline A. and White J. S. Spectrochim. Acta Part B 1988 43 55 1. 14 White J. S. Lee G. H. and Scheeline A. Appl. Spectrosc.. 1989 43 99 1. 15 Scheeline A Lee G. H. and White J. S. Appl. Spectrosc. 1990,44 108 1. 16 White J. S. Ph.D. Thesis University of Illinois 1988. 17 Scheeline A. Bye C. A. Rynders S. W. and Miller D. L. in Optical and Spectroscopic Techniques for the 19903 SPIE Proceedings V. eds. Learner J.M. and McNamara B. J. SPIE Bellingham WA 1990 vol. 1318 p. 44. 18 Scheeline A. Bye C. A. Rynders S. W. and Miller D. L. Appl. Spectrosc. 1991 45 334. 19 Coleman D. M. and Walters J. P. J. Appl. Phys. 1977 48 3297. 20 Bye C. A. personal communication 1990. 21 Lorber A. and Goldbart Z. Anal. Chem. 1984 56 37. 22 Lorber A. Goldbart Z. and Eldan M. Anal. Chem. 1984 56 43. 23 Beaty J. S. and Belmore R. J. J. Test. Eval. 1984 12 212. 24 Practice for Fundamental Calculations to Convert Intensities into Concentration in Optical Emission Spectrochemical Analy- sis Annual Book of ASTM Standards 3.06 E l 58-86 American Society for Testing and Materials Philadelphia PA 1990. Paper I /00023C Received February Ist 1991 Accepted May 30th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600553

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 559 Accuracy of Multi-element Analysis of Human Tissue Obtained at Autopsy Using Inductively Coupled Plasma -Mass Spectrometry* Thomas D. B. Lyon and Gordon S. Fell Trace Element Unit Institute of Biochemistry Royal Infirmary Glasgow G4 OSF UK Keith McKay and Roger D. Scott Scottish Universities Research and Reactor Centre National Engineering Laboratory East Kilbride UK The accuracy of inductively coupled plasma mass spectrometry (ICP-MS) for the multi-element analysis of human tissue has been assessed by analysis of National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1577a Bovine Liver International Atomic Energy (IAEA) Certified Reference Material (CRM) H 8 Kidney and IAEA CRM H 4 Animal Muscle for All Mg Mn Fe Co Zn Cu Mo Rb Sr and Cd.Twenty three out of 24 determinations agreed with certified values. An intercomparison was also made between ICP-MS and the well established technique of atomic absorption spectrometry (AAS) for Al Mg Mn Fe Cu Zn and Cd in human tissue samples which covered a much wider range of analyte concentrations than the reference materials. Drift in the ICP-MS instrument was studied for 3 h and all but the lighter elements Al and Mg were found to be acceptably controlled by an In internal standard. The equivalence of the ICP-MS and AAS data was compared using the usual statistics of correlation and regression. In addition comparisons were made using the statistical analysis advocated by Bland and Altman.In spite of the problems found ICP-MS is capable of accurate and rapid multi-element analysis of human autopsy tissue. Keywords Inductively coupled plasma mass spectrometry; multi-element analysis; accuracy; autopsy tissue; reference materials Several have appeared recently which discuss the determination of trace elements in biological samples by inductively coupled plasma mass spectrometry (ICP-MS). The purpose of this paper was to investigate the suitability of ICP-MS as a routine method for multi-element analysis of human autopsy material. The accuracy of this technique was examined by the analysis of National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1577a Bovine Liver International Atomic Energy Agency (IAEA) Certified Reference Material (CRM) H 8 Kidney and IAEA CRM H 4 Animal Muscle and by an intercomparison with standard methods of atomic absorption spectrometry (AAS) e.g.flame AAS (FAAS) and electrothermal AAS (ETAAS) using a selection of tissue samples takenat autopsy covering the wide concentration range found in both healthy and diseased tissues. Experimental Instrumentation The ICP-MS results were obtained using a PlasmaQuad (V.G. Elemental Winsford Cheshire UK). Details of the instrumental operating conditions used are given in Table 1. The applied dead time was set at the unusually low value of 3 ns. Choice of applied dead time will be discussed in a future publication. The flame AAS results for Mg (285.2 nm) Zn (213.9 nm) Fe (248.3 nm) and Cd (228.8 nm) were obtained using a Perkin-Elmer Model 3030 spectrometer while a Perkin-Elmer Model 2280 spectrometer with an HGA-500 graphite furnace and an AS-1 autosampler was used for the Al Mn and Cu study.Duplicate injections of 20 ,ul were used in the furnace work. The temperature programmes were as follows A1 (309.3 nm) ambientkamp 10 s to 1400 "Champ 2 s to atomize at 2700 "C hold 10 s; Mn (279.5 nm) ambienthamp 1 s to 140 "C and 7 s/ramp 2 s to 1100 "C hold 30 s/ramp 2 s to 2700 "C hold 5 s; and Cu (325 nm) ambienthamp 1 s to 140 "C hold 7 s/ramp 7 s to 900 "C hold 23 s/ramp 2 s to 2700 "C hold 10 s. * Presented at the Second International Conference on Plasma Source Mass Spectrometry University of Durham UK September 24th-28th 1990. Table 1 Plasma operating details Plasma- R.f.power Forward Reflected Gas controls Auxiliary Coolant Nebulizer Nebulizer Spray chamber Ion sampling- Sampling cone Skimmer cone Sampling distance Lens settings- 1.37 kW t 1 0 w 0.6 1 min-I 13 1 min-I 0.7 1 min-' Babington V-groove type high solids Scott-type double bypass water cooled pumped at 0.7 ml min-' (ambient) Nickel sampler (Nicone) with 1.0 mm Nickel (001 Type) with 0.75 mm orifice 10 mm from load coil orifice L1 -36V L2 -52V L3 -1.8 V L4 -62V Extraction lens -200 V Optimization- The lenses were adjusted to maximize the llsIn signal Vacuum- Expansion stage Intermediate Analyser Data acquisition- Peak jumping Sweevs 2.4 mbar < l x mbar 3 x 10+-4 x 1 0-6 mbar 10 Channels per peak 3 Dwell time 50 000 pus Sample Preparation Five replicate samples of the reference materials and single freeze-dried autopsy samples of heart@) muscle(m) liver(1)560 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 and kidney(k) were weighed (0.500 g) into perfluoroalkoxy resin ‘bombs’ (60 ml Savillex Minnesota USA) and 1 ml of HN03 [Aristar grade Merck (formerly BDH)] was added together with 1 ml of In as internal standard (1.25 mg 1-I). The In spike was added at this point ratherthan immediately before mass spectrometric analysis in order to compensate for possible aerosol losses on opening the bomb or for incomplete transfer of the digest. The bombs were placed in a domestic microwave oven and heated for 2 min at full power (600 W). After cooling the bombs were opened and the contents washed into an acid-washed plasticvial and made up to volume (25 ml). This digestion procedure does not oxidize all the fat contained in some tissue samples and any undigested globules of fat were removed with a disposable plastic pipette.The incomplete destruction of any fat haslittle or no effect on the accuracy of the results. This can be seen from the good agreement of the values determined for the reference materials. Earlier attempts to dissolve the fat by including hydrogen peroxide in the digestion were successful but were abandoned after the failure of several bombs during the digestion process. Blanks carried through the digestion procedure gave a negligible response and no correction was made to the results. Inductively coupled plasma mass spectrometry The digests were nebulized directly into the ICP mass spectrometer without further dilution.Quantification of all reference materials and samples was made using external multi-element standard solutions made up in 4% HN03 and containing In as the internal standard. The top standard contained 5000 pg 1-1 of Mg Fe Zn and Cd 1000 pg 1-’ of Cu and 100 pg 1-l of Al Mn Co Mo Rb and Sr. A low standard was prepared by making a 10-fold dilution of the top standard. The isotopes used for the determinations were selected on the basis of freedom from polyatomic isobaric species. Interference from polyatomic species in the biological matrix has been discussed elsewhere.’ The isotopes chosen are shown in Table 2. Atomic absorption spectrometry The digests were manually diluted with water as required for each element of interest and analysed sequentially against simple single element standards.All digests for the magnesium assay were diluted 50-fold with 0.1% m/v lanthanum chloride solution and all digests for the determi- nation of iron containing more than 8 pg 1-* were diluted 1 0-fold with water. Statistical Interpretation The results of the intercomparisons are plotted in Figs. 2-8 (coincident data points are represented by a number). The coefficient of correlation r and the equation oftheregression line aregiven. Uncertainties are quotedat the 95% probability level. Agreement between two methods is usually considered satisfactory ifrand the slope ofthe regression equation(rn) are both close to unity and the intercept of the regression equation is zero.However it has been suggested that neither r nor the regression equation measure agreement between methods. Bland and Altman5 have described a statistical analysis which is specifically designed to assess agreement between measure- ment techniques. In this test the difference (A) between the two methods AAS and ICP-MS for each sample is plotted against its mean value. Ideally the data should lie within a normal distribution about the horizontal line through the zero difference. The 95% normal range for the differences defines the ‘limit of agreement’ while the mean difference can be thought of as the ‘bias’. Results and Discussion Reference Materials The results for the three reference materials are shown in Table 2 and are satisfactory for most practical purposes.~ Table 2 Results obtained for reference materials using ICP-MS. Errors are given at the 95% confidence interval (n=5) IAEA H 4 IAEA H 8 NIST SRM 1577a Animal Musclelpg g-I Kidneylpg g-’ Bovine Liver/pg g-l Element Isotope Found Certified Found Certified Found Certified 25 1081* 1050k59 782 f 112 818 f 7 5 589 f 43 600+ 15 A1 27 0.45k0.7 -7 0.98 * 0.6 - 0.95 f 0.17 - Mg Mn 55 0.52 + 0.04 0.52 20.04 6.1 k 0.5 5.7 f 0.3 10k0.6 9.9 k 0.8 Fe 57 46k8 4 9 f 2 297 f 20 265 k 15 187+ 14 194220 co 59 - (0.004) 0.1 3 f 0.0 1 (0.13) 0.24 f 0.02 c u 65 4.1 k0.5 4.0k0.3 33.5 f 1.6 31.3 k 1.8 149+ 15 158k7 Zn 66 90k6 86 k 3.5 205 f 23 193 2 6 1 2 4 t 14 19f1.5 21.4k3 22.2 f 0.8 11.3f0.6 12.5fO.l Rb 85 21.8f2 Sr 88 0.06f0.02 - Mo 98 0.041 f0.008 (0.05) 2.4 f 0.1 2.2 k 0.3 3.8f0.12 3.5 f 0.5 Cd 111 Cd 106 - - l8Ok 13 189f5 - * Single determination. j- Not determined or not available.0.21 f 0.05 123k8 1.2f0.06 (1.1) 0.135 f 0.005 0.138 f 0.003 0.40 f 0.03 0.44 f 0.06 - - - - - Table 3 Change in concentration of the top standard measured over 150 min period. The results after In compensation are expressed as % change of the actual concentration Elapsed In/ 1 O3 counts time/min S-’ Mg A1 Mn Fe Co Cu Zn Rb Sr Mo Cd Ba 0 279 0 0 0 0 0 0 0 0 0 0 0 0 36 219 + 5.4 + 1.9 + 1.2 -1.4 -5.3 -6.9 0.0 -1.6 0.0 -4.3 -0.8 -0.4 66 178 +14.6 + 8.2 + 6.5 +3.8 -1.7 -4.3 +5.2 +1.6 +3.8 -3.2 -0.4 0.0 96 170 +12.4 + 7.5 + 6.8 +3.4 -2.1 -4.6 +3.5 +2.6 +5.6 -1.1 +0.8 +0.8 126 145 +10.5 + 4.7 + 3.8 0.0 -5.5 -8.2 +2.2 +0.9 +4.4 -2.6 -1.4 +0.3 150 130 +21 +15 +10 +6.1 -0.2 -3.3 +6.5 +4.9 +7.9 +0.3 +0.3 +0.8JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 56 1 0) 800 Is) s - a 600 s ti 0 400 I Drift and Internal Standardization The internal standard used was In. This is added primarily to compensate for changes in the sensitivity of the ICP-MS system. The changes in sensitivity can be time dependent i.e. drift or can be a function of the sample i.e. a matrix effect. The efficacy of the single In internal standard to control drift and matrix effects was tested over a 3 h period by presenting a standard as an unknown after every fifth sample. The results are shown in Table 3. The In- compensated results for the heavier elements Cd and Ba are excellent and the elements from Mn to Mo (mass 55-98) are adequately compensated for with mean drifts between -5.5 and +6%.The signals from the lighter elements A1 and Mg increase during the course of the run to such an extent that the data may be greater than 10% in error and regular recalibration is indicated for these analytes. The In count rate decayed by about 50% during the course of the run. This decay in sensitivity is usually 0 - 0.0 2 0 0 2 02 0 . O H - 0 0 0 0 - 0 . 0 0. 2. - 00 I 1 I attributed to the progressive clogging of the sample orifice and to the defocusing of the ion beam as the run progresses. The decay is doubly exponential and is shown in Fig. 1. This figure also shows that the In internal standard in the tissue digests behaves in the same way as the In in the standard and implies that there is no matrix effect for In in tissue digests.Igarashi et a1.6 have investigated the properties of In and 0 . I 360 480 600 720 840 960 FAAS result/pg g-’ ( b ) 122 0 0. 0 0 0 0 0 0 0.0 0 0 . 0 0 0 0. 0 . 0 0 2 80 I Is) D O a‘ -80 = I -1 14 0 50 100 150 Ti me el a psed/mi n I 1 I 0 I 360 480 600 720 840 960 Mean resultlpg g-’ Fig. 1 Decay of In signal with elapsed time. Closed squares are the top standard repeated at intervals and open squares are samples Fig. 3 (a) Magnesium regression plot (38 samples 17 1; 12 k; 7 m; and 2 h). r=0.91; m=0.92 (CI=O.78-1.05); and (b) Bland and Altman plot for Mg (a) 0 7 120 Is) Is) p 80 e ii 0 ’ 40 0 10.5 0 0 =i 7.0 v) 2 a u v) 4 3.5 0 0. 0 0 . .. -H 1 I I 1 I I 0 25 50 75 100 125 ETAAS resuIt/pg g-’ 202 63 I I 1 0 2.0 4.0 6.0 8.0 10.0 ETAAS result/pg g-’ 0.85 2 2 0 .2 2 2. 0 0 0 2 7 0 m cn 3. -0.3 2 4 2 I cn EI 0.0 a“ 2 2. 0 0 2 . 2 or 2 0 0 0 0 0 0 ,-a- 0. -0.6 1 0 0 0. 0 1 0 I 0 2 4 6 8 10 Mean result/pg g-’ 0.0 0.6 1.2 1.8 2.4 3.0 Mean result/pg g-’ Fig. 2 (a) Aluminium regression plot. The square points represent 20 samples (48 samples 27 1; 12 k; 7 m; and 2 h. See text for identities of 1 k m and h). r=0.998,; rn=1.01 (CI=O.99-1.03); and (b) Bland and Altman plot for A1<3 pg g-I Fig. 4 (a) Manganese regression plot (68 samples 30 1; 20 k; 11 m; and 7 h). r=0.98; rn=0.97 (CI=O.93-1.01); and (6) Bland and Altman plot for Mn562 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 -160 other internal standards for the purpose of determining U and Th in lung tissue. These workers found an increase in the In signal over the duration of the first five measure- ments and thereafter a decay to about 50% of the initial value.The In in the standard behaved similarly to that in the digests but the heavy elements U and Th drifted by 10- 1 5% towards higher concentrations over the duration of eight consecutive measurements. The explanation of the initial increase found by Igarashi et a1.6 (but not observed in our own study) is not known but may lie in the fact that they used a concentric type nebulizer with an uptake rate of 1.7 ml min-I compared with our use of a Babington nebulizer with an uptake of 0.7 ml min-l. 0 - 0 I I 1 I Comparison of Atomic Absorption and Plasma Mass Spectrometry Aluminium The correlation coefficient r=0.998 equation is Inductively Coupled and the regression Al(ICP-MS)= 1.01( & 0.0 16)Al(ETAAS)+ 0.3( k 0.3) The slope m is not significantly different from unity nor is the intercept significantly different from zero.Fig. 2(a) shows that much of the data are concentrated in a group at a low concentration and this is not ideal for regression analysis which requires an even spread of the data. The regression is heavily influenced by the point at 150 pg g-l. In Fig. 2(b) the distribution of the data has no deleterious effect on the comparison. The mean difference or bias is 0.01 5 pg g-l [confidence interval (CI) from -0.07 to 0.0981 and is not significant. The limit of agreement is from -0.36 to 0.39 pg g-l. Magnesium The correlation coefficient r=0.9 1 and the regression equation is The slope m is not different from unity nor is the intercept Mg(ICP-MS)=0.92( f 0.14)Mg(FAAS)+46( -t 86) significantly different from zero [Fig.3(a)]. Fig. 3(b) shows the mean difference is 4.4 pg g-l (CI from - 15 to 24) i.e. there is no significant bias. The limit of agreement is from - 114 to 122 pg g-l. The poorer level of agreement found for Mg may be attributed at least in part to a combination of drift during the ICP-MS measurement (see Table 3) and the large dilution step ( x 50) necessary for FAAS. Manganese The correlation coefficient r= 0.98 and the regression equation is Mn(1CP-MS)=0.97( f 0.04)Mn(ETAAS)+O. 19( f 0.22) The slope rn is not significantly different from unity and the line passes through the origin [Fig. 4(a)]. Fig. 4(b) shows that the mean difference is - 0.07 pg g-l (CI from - 0.18 to 0.045) i.e.there is no significant bias. The limit of agreement is from -0.98 to 0.85 pg g-l. Iron The correlation coefficient r= 0.99 and the regression equation is Fe(1CP-MS)=0.965( f 0.028)Fe(FAAS)+ 6.9( k 26). The 95% CI for the slope is 0.94-0.99. As the CI does not include 1 m is significantly different from unity (p<0.05) [Fig. 5(a)] and a systematic error in one or other of the methods is indicated. This however is not true. Examina- tion of Fig. 5(b) shows that the differences fall into two populations thus invalidating simple regression analysis. Up to a mean value of about 400 pg g-l [Fig. 5(a)] there is very close agreement between the two methods while above 400 pg g-l the differences suddenly increase [Fig.5(6)]. This is attributed to the fact that dilutions become necessary for FAAS at about this point. Below 400 pg g-l the mean difference is -3.7 pg g-l (CI from - 12 to 4.6) i.e. there is no significant bias. The limit of agreement is from -26 to 19 pg g-*. Above 400 pg g-l there is a significant bias of 33.7 pg g-' (CI 6-61) and a limit of 2400 - w w z 1600 In 2 v) 7 800 a 0 0 (a) 0 0 0 0 0 3 0 0 . 0 . 0 3... 0.34 0.2 2. 2 3 0 07 20 4763 0 500 1000 1500 2000 2500 FAAS resuIt/pg g-' 50 I (c) 1 I 0 19 I 3 2j -50 -1 00 70 140 210 280 350 420 Mean resuIt/pg g-' 160 - 0 0 0 a' 0 0 0 0 0 0. 0. 0 0.0 0 - 0 2 . 0 0 . 0 0 0 0.0. 0 0 - 486250 00 00 0 . 0 0 0 160 c 0) 0 s o 400 800 1200 1600 2000 2400 Mean result/pg g-' Fig. 5 (a) Iron regression plot (75 samples 30 1; 27 k; 1 1 m; and 7 h).r=0.99; m=0.96 (CI=O.94-0.99); (b) Bland and Altman plot for all Fe data; (c) Bland and Altman plot for Fet400 pg g-'; and (d) Bland and Altman plot for Fe>400 p g g-'JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 563 agreement of from - 146 to 21 3 pg g-l. This level of bias is unimportant for the work envisaged. Copper The correlation coefficient r= 0.99 and the regression equation is The slope is not significantly different from unity and the line goes through the origin [Fig. 6(a)]. Fig. 6(b) shows that the mean difference is -0.053 pg g-l (CI from -0.34 to 0.22) i.e. there is no significant bias. The limit of agreement is from -2.2 to 2.1 pg g-l. Cu(ICP-MS) = 1 .O 1 ( +- O.O3)Cu( FAAS) - 0.08( k 0.5 6) 0 45 t (a’ 0 3 - 30 3 E d.0 15 0 0. 0 0 0 - 0 3 0 00 0 02.2.. 2 232 0 240 2 0 4 70 1 7 0.0 0 0 0 0. o r n o 0 0 . 0 2 0 0 . 0 - 032 20 0 0 2 0 0 2 ....2.... 0 0 O H 0 0 0 0 0 Fig. 6 (a) Copper regression plot (62 samples 17 1; 28 k; 12 m; and 5 h). r=0.99; m=l.Ol (CI=O.98-1.04); and (b) Bland and Altman plot for Cu (a) 0 0 0 0 0 0 0 2 0 . 0. 002 aa 0 . 2 2. 0 . 100 200 300 400 500 FAAS result/pg g-‘ 35 I 1 -35 t 0 0 1 1 I I I 100 150 200 250 300 Mean result/pg g-’ Fig. 7 (a) Zinc regression plot (3 1 samples 13 1; 12 k; 4 m; and 2 h). r=0.98; m= 1.01 (CI=0.95-1.07); and (b) Bland and Altman plot for Zn 240 1 (a) 0 0 0 0 0 0 0. 0.0 0 0 0. a.3. 32. 0 1 1 1 0 50 100 150 200 250 FAAS result/pg g-’ l2 t (b’ 1 1 1 1 1 1 200 250 0 50 100 150 Mean result/pg g-’ Fig.8 (a) Cadmium regression plot (29 kidney samples). r= 1 .OO; m= 1.00 (CI=O.98-1.01); and (b) Bland and Altman plot for Cd Zinc The correlation coefficient r= 0.98 and the regression equation is Zn(1CP-MS)= 1.01( *0.06)Zn(FAAS)+7.5( f 12) showing that m is not significantly different from unity and that the line passes through the origin [Fig. 7(a)]. Fig. 7(b) shows the mean difference to be -7.9 pg g-l (CI from - 13 to -2.7) i.e. there is a positive bias towards ICP-MS. An explanation of the positive bias towards ICP-MS could be the presence of the polyatomic species 34S02+ arising from the 1% sulphur content of dry tissue. This explanation was tested by analysing a solution that matched the tissue digests in sulphur content (8 mmol dm-3 sulphuric acid).The 34SOtf signal at m/z 66 was negligible and cannot explain the bias. The limit of agreement is from -20 to 10 Pug g-l. Cadmium The correlation coefficient r= 1 .OO and the regression equation is indicating that m is not different from unity and that the intercept is zero [Fig. 8(a)]. Fig. 8(b) shows that the mean difference is 1.1 pg g-’ (CI from -0.06 to 2.1) and the limit of agreement from - 1.7 to 3.9. Cd(1CP-MS)= 1.00( f 0.016)Cd(FAAS)-0.80( * 1.8) Conclusion The statistical test of Bland and Altman5 is superior to regression analysis and revealed small biases for Fe (at high concentration) and Zn. These constant differences between the two methods are small but nevertheless irritating. The ICP-MS technique has been shown to give results for Al Mg Mn Fe Cu Zn and Cd that are generally in good agreement with reference materials and with standard methods of AAS.Analysis of reference materials IAEA H 4 IAEA H 8 and NIST SRM 1577a indicate that the ICP-MS564 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 technique described here is also applicable to Co Mo Rb and Sr. The ICP-MS method is much faster and is comparable in accuracy to AAS. The increased dynamic range of the ICP-MS method is also advantageous as dilution errors are eliminated. The main problem that has to be guarded against is drift. In this work drift was controlled by a single internal standard and regular re- calibration. However other methods of control perhaps using multiple internal standards or isotope dilution should be considered. The authors thank Dr. Gordon D. Murray Medical Statistics Unit Department of Surgery Western Infirmary Glasgow for his assistance with the statistical aspects of this paper. References 1 2 3 4 5 6 Lyon T. D. B. Fell G. S. Hutton R. C. and Eaton A. N. J. Anal. At. Spectrom. 1988 3 265. Beauchemin D. McLaren J. W. and Berman S. S. J. Anal. At. Spectrom. 1988 3 775. Ridout P. S. Jones H. R. and Williams J. G. Analyst 1988 113 1383. Friel J. K. Skinner C. S. Jackson S. E. and Longench H. P. Analyst 1990 115 269. Bland J. M. and Altman D. G. Lancet 1986 i 307. Igarashi Y. Kawamura H. Shiraishi K. and Takaku Y. J. Anal. At. Spectrom. 1989 4 571. Paper 0/05 5 3 3 F Received December 1 Oth 1990 Accepted June 7th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600559

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 565 Interference of Copper Silver and Gold in the Determination of Selenium by Hydride Generation Atomic Fluorescence Spectrometry an Approach to the Studies of Transition Metal Interferences Alessandro D'Ulivo and Leonard0 Lampugnani Consiglio Nazionale delle Ricerche Istituto di Chimica Analitica Strumentale Via Risorgimento 35 56 I00 Pisa Italy Roberto Zamboni Dipartimento di Chimica e Chimica Industriale Universita di Pisa Via Risorgimento 35 56100 Pisa Italy An attempt has been made to elucidate the mechanism of liquid-phase interferences caused by Cull Ag' and AulIl on the evolution of hydrogen selenide. An empirical equation able to describe accurately the interference plots obtained by systematic studies has been found.The equation has also been derived using a physical model for the interferences based on the capture of the hydrogen selenide by the active surface of metal colloid aggregates growing in the solution and having a fractal structure. Keywords Hydride generation; non-dispersive atomic fluorescence spectrometry; selenium determination; interference The hydride generation (HG) technique combined with atomic spectrometry as a detection system can be consi- dered at present one of the most powerful analytical tools for the determination of many elements such as As Sb Bi Se Te Ge Sn and Pb.1-4 The main limitation for such a technique stems from interferences arising when some inorganic ions are present in the reaction matrix. Since the first systematic study by Smith5 of the interfer- ence from 48 foreign elements on the determination of As Sb Bi Se Te and Ge many other interesting studies have appeared.6-21 Moreover most of the papers concerning the analytical use of HG coupled with atomic spectrometry have included interference figures from a large number of foreign ions.A careful survey of the above literature shows wide discrepancies in the magnitude of the interference effects reported by different workers. Most of this non-homogene- ity appears to be due to the complex mechanism of action of the interfering species so that many experimental details may play an important role in the determination of the interference figures for a given HG apparatus. It is recognized that the magnitude of a given interference effect depends upon the type of HG system (batch continuous or flow injection) ernployed,l0J the chemical conditions em- ployed in the reduction ~ t e p ~ J ~ J ~ q ~ ~ the mixing sequence of the reagents9.I8 and the type of atomizer.I0J2J1 In order to elucidate the complex mechanism of action of interferences occurring in the liquid phase and caused by the transition metal ions several workers have made some interesting studies using specifically designed experiments.Kirkbright and Taddia,6 reporting on the interference of Cull Nil1 PtlV and PdIr on arsine generation observed the formation of a finely dispersed black precipitate. They proposed that the interference was caused by the reduction of the interfering ions to the metal by sodium tetrahydro- borate.The finely dispersed metals were then thought to adsorb and decompose the arsine on their surface also supposed by Smith.5 In order to support the hypothesis they performed an experiment in which the addition of Ni powder to the reaction solution caused the complete suppression of the As signal. Welz and Melcher13 proposed that metals in the ionic form hardly contributed to the interference effects in the determination of Se. They bubbled hydrogen selenide evolved from a batch HG system through a wash bottle containing the metal ions in acid solution. The resulting measured interferences were several orders of magnitude lower than those observed when the metal ions were present in the reaction vessel. Bax et a/.'* studied the interference of W1 NilI and Cot1 on the determination of Se using a continuous HG system.They employed a specially designed mixing apparatus in order to obtain five different mixing sequences of the analyte interferent and reagents. They showed evidence of the possibility of several interference mechanisms. In particular the elimination of the generated hydrogen selenide by the products (metal and/or metal boride) of the reaction between the interferent ions and sodium tetrahydroborate and the catalytic decomposition of sodium tetrahydroborate by the metal ions or by their reaction products appeared to play the major role. They concluded that the reaction of the metal ion with the hydrogen selenide did not contribute (Nil1 and Co") or hardly contributed (Cull) to the interferences. The main problem of the results described above is their applicability to a real analytical system.As they were obtained by means of indirect experiments some perturba- tions are always present with respect to the original analytical system. The present work is based on the idea that the results of systematic studies in which the depletion of the analytical signal caused by a given concentration of interferent is used to characterize the interference figures for the analytical system should contain information on the mechanisms at the origin of the interferences. For this purpose the depressive effects of Cu Ag and Au on the determination of Se by HG with non-dispersive atomic fluorescence spectrometry (NDAFS) were measured for two different concentrations of Se. A wide range of interferent concentra- tions was investigated in order to obtain well-defined interference plots in the range 0-100% depression of the signal.The peak height peak area and peak shape of the AF signal were considered in order to collect more complete information about the interference phenomena. In this paper it is assumed that the interference plots can be accurately represented by an empirical equation which can be justified by the assumption of a physical model for the interference mechanism. The present study while suggest- ing an alternative method to report interference data by means of their analytical description represents a new approach to gaining direct information on the nature of the liquid-phase interference in the HG technique.566 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL.6 Experimental Apparatus The batch HG vessel was made of borosilicate glass (Fig. 1). The atomizer was a miniature argon-hydrogen flame supported on a borosilicate glass tube similar to one used previously.22 The flame was obtained with flow rates of 0.30 1 min-' for hydrogen and 0.85 1 min-' for argon the latter also serving as the carrier gas passing through the HG vessel and sweeping the generated hydride to the atomizer. The HG vessel was connected to the atomizer by means of a 30 cm long borosilicate transfer line (6 mm o.d. 2 mm The NDAF spectrometer was the same as described previou~ly.~~ Reagents All reagents were Suprapure grade (Merck) unless otherwise specified. Standard stock solutions (1000 pg rn1-I) of the elements were prepared by using Titrisol solutions (Merck).The Au standard stock solution was prepared from the pure metal (Johnson Matthey) dissolved in hot aqua vegia [hydro- chloric acid-nitric acid (3 + 1 )] and appropriately diluted with hydrochloric acid. A 5% sodium tetrahydroborate solution was prepared by dissolving the reagent (SpectrosoL Merck) in 0.1 mol dm-3 sodium hydroxide solution and filtering through a 0.45 pm membrane. This solution was kept refrigerated and diluted to the required concentration just before use. Milli-Q water (Millipore) was used throughout. Procedure A 5 ml volume of 0.5 mol dm-3 hydrochloric acid was placed in the reaction vessel and spiked with microlitre volumes of a standard solution of the analyte and of the interfering element according to the measurement to be performed. A pipette containing 1 ml of 1% sodium tetrahydroborate solution was inserted in the injection port.Argon was then allowed to flow through the vessel while the sample was subjected to vigorous magnetic stirring. Fig. 1 Borosilicate reaction vessel used in the present work. 1 Ar carrier gas inlet; 2 Ar carrier gas outlet; 3 disposable micropipette tip for the NaBH injection; and 4 poly(tetrafluorethy1ene) (PTFE)-coated magnet fitted with PTFE rings Table 1 Summary of the experimental parameters Mean power for EDL* Power modulation for EDL* Photomultiplier Photomultiplier voltage RC time constant Atomizer Burner Observation height Argon flow rate Hydrogen flow rate Reaction vessel Transfer line Sample volume and acidity Reducing agent Hydride generation 7 w 7031 Hz square wave 100% amplitude modulation 0.5 duty cycle R759 Hamamatsu 750 V 0.5 s Miniature argon-hydrogen flame Borosilicate glass tube 9 mm i.d.8 mm above the burner top 0.85 1 min-' 0.3 1 min-' See Fig. 1 Borosilicate glass 30 cm long 6 mm o.d. 2 mm i.d. 5 ml 0.5 mol dm-3 HC1 1 ml of 1% NaBH solution Batch mode manual NaBH injection * EDL= Electrodeless discharge lamp. When the baseline had stabilized the sodium tetrahydro- borate solution was injected and the peak recorded. The gases evolved from the reaction solution were directly continuously supplied to the atomizer. A summary of the experimental parameters is reported in Table 1. Since the metals Cu Ag and Au are strong interfering elements they generate a memory interference effect also observed by Meyer et aL7 In order to avoid erroneous evaluation of the signal variation after each interference measurement the bottom part of the vessel was washed with nitric acid (1 + I) then several analyte blanks were run until the signal was restored to the original level.In many instances poisoning of the cell occurred then the bottom part of the vessel had to be replaced with a clean one. Overnight cleaning with aqua regia was found to be effective in recovering the poisoned cells. Results and Discussion General Consideration Dedina,12 in a classification of the interferences occurring in the HG technique divided them into liquid-phase interfer- ences and gas-phase interferences. For transition metals according to current literature,12-16J8 the interference ef- fects occur in the liquid phase.Therefore the discussion contained in the present paper is based on the assumption that the metals considered as interferents give rise only to liquid-phase interferences. The HG-NDAFS apparatus employed in the present study has a concentration detection limit (3a of the blank) of 0.006 ng ml-1 with a dynamic range extending up to 80 ng m1-1.22 Both the selenium concentrations 0.5 and 50 ng ml-l chosen to perform interference experi- ments were within the dynamic range and gave rise to signal levels well above the 0.02 ng ml-l blank level. Interference Measurement and Data Presentation The depressive effects of Cu Ag and Au on the AF signal obtained by the reduction of 0.5 and 50 ng ml-1 of SeIV are reported in Figs.2 3 and 4 respectively.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER I99 1 VOL. 6 567 10 1 o2 1 o3 1 o4 1 o5 Cu"/ng mi-' Fig. 2 Effect of Curl concentration on the AF signal depression of Se. Peak height measurements for A 0.5; and B 50 ng ml-1 S P . Peak area measurements for C 0.5; and D 50 ng mi-' SeiV 100 8? 1 2 50 0 Fig. 3 I # 1 1 I 1 10 1 o2 1 o3 10' 1 o5 Ag'lng ml-' Effect of AR' concentration on the AF signal depression of Sey Peak height measurements for A 0.5; and B 50 ngml-I SerV. Peak area measurements for C 0.5; and D 50 ng ml-1 Se'" Only for Cu was the magnitude of the interference independent of the Se concentration and therefore the threshold above which the interferent effect starts was controlled essentially by the Cu concentration.However in all instances two distinct interference plots were obtained depending on whether peak height area measurements were performed. This discrepancy is correlated to the modifica- tion of the peak shape consequent upon the increasing interference effect as shown in Figs. 5 6 and 7. The Cu interference began its depressive effect at the tail of the peak and then its action extended progressively to the 100 - v a 50 0 1 10 1 o2 1 o3 1 o4 1 o! Au"'/ng ml-' Fig. 4 Effect of Aul" concentration on the AF signal depression of Se. Peak height measurements for A 0.5; and B 50 ng ml-' SeV. Peak area measurements for C 0.5; and D 50 ng ml-1 SeV A 1 A 0 15 30 Time/s Fig. 5 Signal shapes obtained by the reduction of (a) 0.5; and (b) 50 ng ml-1 of Setv in the presence of various concentrations of Cu" A=O; B=O.l; C=0.2; and D=0.5 ,ug ml-1 Cur'.The NaBH solution was injected at t=O s 0 15 30 45 Time/s Fig. 6 Signal shapes obtained by the reduction of 50 ng ml-L of SeIV in the presence of various concentrations of Au? A=O; B=0.5; C=2.0; and D=5.0,ug ml-I Au'l'. The NaBH solution was injected at t=O s whole peak (Fig. 5). The modification of the peak shape with the increasing interference effect was essentially the same at low and high concentrations of Se. At low concentrations of Se the interference behaviour of Ag and Au was similar to that of Cu in terms of peak shape modification. However Ag and Au began to interfere at a lower concentration than Cu. The interference plots at low concentrations of Se show that the peak area depression is never less than the peak height depression.This reflects the sequence of peak shape modifications described above. At high concentrations of Se the interference behaviour568 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 of Ag and Au was different from that observed at low concentrations of Se. The peak shape modification due to the interferences involved a more pronounced tailing of the peak with respect to the peak without interference (Figs. 6 and 7). This behaviour was reflected in the interference plots where at a given interference concentration the peak area depression was never larger than the peak height depression. For Ag the singular inversion in area depres- sion shown by the interference plot was a consequence of the signal splitting into two peaks (Fig.7). A first consideration of the present systematic studies led to the conclusion that the assumption of Meyer et al.' that the magnitude of interferences does not depend on analyte concentration cannot be generally valid. It is also note- worthy that the signal peak height and signal peak area measurements may lead to a different numerical evaluation of the same interference effect. If the role played by these parameters is ignored different interference figures can be obtained by different operators using the same HG appara- tus under the same experimental conditions. However the interference plots shown in Figs. 2 3 and 4 indicate the difficulty in reporting complete and concise information about the interference behaviour using a given HG apparatus.The set of experimental points contained in the plots are tedious to present in a report either in table form or in a pattern image. Therefore an attempt has been made to give a mathemati- cal description of interference plots by fitting the experi- mental data with an equation describing the well-known saturation function C" Ba+cu p=- where [ 100 p=AS(o/o)] is the relative AF signal depression measured experimentally c is the interferent concentration expressed in ng ml-l and B and a are the equation parameters optimized by the fitting procedure. In particular B represents the interference concentration giving a 50% signal depression while a is related to the slope of the rising portion of the interference plot.The results of the non-linear least squares fits for all six sets of the experimental data are presented in Table 2. The quality of the fits on the bases of both the sum of residuals and the coefficient of correlation can be considered more than satisfactory. Figs. 8 and 9 show the worst and the best of such fits respectively. The variable p (O<p=Gl) of eqn. (1) is obtained by experimental measurement of the AF signal F. If Fo is the t - (0 C 0 In LL .- a 60 s H B Time - Fig. 7 Signal shapes obtained by the reduction of 50 ng ml-l SeIV in the presence of various concentrations of Ag! A=O; B=O.l; C=0.2; D=0.3; E=0.5; F=0.7; and G=1.0 pug ml-I Ag'. The NaBH solution was injected at t=O s 0.4 0.2 0 I 10 lo2 lo3 lo4 lo5 lo6 Ag'/ng mlF' Fig. 8 Experimental data (0) and fitted curve (-) obtained for peak area measurements for Ag interference on the reduction of 50 ng ml-l SeIV 1 .o 0.8 0.6 a 0.4 0.2 0 I 1 10 lo2 lo3 lo4 105 10' Au"'/ng ml-' Fig.9 Experimental data (0) and fitted curve (-) obtained for peak height measurements for Au interference on the reduction of 50 ng ml-' SelV AF signal in the absence of interference and F is the AF signal in the presence of interference then If So is the total Se concentration S the concentration of Se leaving the solution in the presence of the interferent and b the slope of the calibration graph then Fo= bSo and F= bS. If s* is the concentration of Se remaining in the solution which is unable to give a signal because of the interference process then p also represents the fraction of Se retained in the solution following the interference p=s*ls (3) Eqn.(1) should possess some physical significance related to the nature of the interference phenomena but for the moment it can be considered as an empirical function giving concise and complete information on the interfer- ence figures using a given HG apparatus with the aim of reducing the fragmentation and discrepancies in the litera- ture data. Assumption of a Physical Model for the Interferences In order to discuss the physical interpretation of the interference plots some assumptions based on the experi- mental evidence or literature data about the nature of the interfering agent and the interference mechanism must be made.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 569 Table 2 Values of parameters obtained in the non-linear least squares fit Peak area- Sell/ Interferent ng ml-I B a CU" 0.5 168k13 1.3k0.11 Ag' 50 293k45 0.77k0.10 AulI1 0.5 55 k 5.6 0.95 k0.08 CU" 50 165i 13 1.5 k0.17 Ag' 0.5 40k3.6 1.6 k0.19 Au"' 50 3500k 138 1.31 50.06 Peak height- SeIV/ Interferent ng ml-I B a CU" 0.5 430k32 1.4 k 0.13 CU" 50 350k33 1.4 k0.16 Ag' 0.5 86k7.3 1.2k0.12 Ag' 50 160k 17 1.3-tO.16 AuI" 50 2050k94 0.48 k 0.02 Au'" 0.5 1 2 1 k l l 1.3-tO.13 z (Error)2 162 200 222 58 1 207 45 c (Error)* 168 27 1 160 47 1 235 43 r2 0.9975 0.9970 0.9960 0.9873 0.9965 0.9979 r2 0.9969 0.995 1 0.9964 0.99 17 0.9953 0.9988 Chemical and physical nature of the interferent The interfering species are supposedly formed by the reaction between the metal ions and tetrahydroborate to give presumably the metal^*^*^^ in the form of particles capable of capturing hydrogen selenide by means of active sites exposed on their ~urface.~.~ The interaction between the surface and the adsorbed molecule can be assumed to be an equilibrium reaction characterized by K,.The microscopic structure of the interferent particles can be supposed to be highly porous with highly irregular boundaries. This assumption is based on evidence from several experiments on metal colloid aggregation in solu- Weitz and co-workers26,28 reported several transmission electron microscopic images of Au colloid aggregates obtained from NaAuCl reduction with triso- dium citrate. The aggregates show a highly disordered structure typical of fractal o b j e ~ t s ~ ' ? ~ ~ and characterized by a mass fractal dimension D ( 1 d X 3 ) obtained by light scattering methods.The fractal dimension of the aggregates is scale invariant i.e. the aggregates grow in a self-similar mode. The dynamics of the aggregation process is depen- dent on the experimental conditions used; fast and slow aggregation processes develop aggregates with different fractal dimensions.26728 The fast aggregation process which seems to be the one that takes place under our experimental conditions is obtained in the presence of a high concentration of electrolytes in solution. In the course of fast aggregation the mean cluster radius R depends both on the initial metal concentration c and time t according R a (ct) lID (4) Considering that m the mass of a cluster is proportional to the r a d i u ~ ~ ~ .~ ~ at a fixed time then the number of the clusters per unit volume of solution is conserved independently of the initial metal concentration c . ~ O maRD ( 5 ) Interference mechanism As shown previously the peak shape changes with the interferent concentration. This explains why the ratio of the peak area to peak height also changes with the interferent concentration. It is also apparent that the analysis of the peak shape modification can contribute to a better insight of the interferent processes and to a better definition of the physical model. Before discussing the phenomena that modify the peak shape it must be realized which processes control the peak shape in the absence of the interferent.The signal shape in the absence of an interferent (Figs. 5 and 6 curve A) can be thought of as being due to the combination of several different processes namely (i) the formation of hydrogen selenide (ii) the transfer of the hydrogen selenide from the liquid to the gaseous phase (ziz) the broadening and delay of the peak due to the dead volume and (iv) the broadening and delay of the peak due to anomalous interactions of the hydrogen selenide31 with the glass surface. Process (i) according to literature data,32 should be very fast compared with the peak lifetime. Considering that the addition of sodium tetrahydroborate is completed in less than 0.5 s this process should not contribute significantly to the peak shape. The contribution of process (iv) is difficult to evaluate.Considering that the cumulative dead volume of the reaction vessel the transfer line and the atomizer was about 90 ml the appearance of a peak maximum should be expected after about 6.5 s instead of the 5-6 s observed experimentally. This small difference can be explained by the fact that during the injection of sodium tetrahydro- borate the argon flow rate was suddenly augmented by the hydrogen evolved from the solution. This phenomenon was also recognizable on the signal as a small shoulder appear- ing just prior to the peak maximum. No anomalous delay in peak appearance as reported by D e d i ~ ~ a ~ l was observed in the batch HG system reported here. So if any anomalous delay occurs it should be of little relevance using the present apparatus.Therefore it can be assumed that the peak shape of the AF signal is due to a combination of processes (ii) and (iii). In the present HG setup any modification of the signal shape caused by the interferent can be correlated to the circumstance that the interference phenomena occurs in whole or in part during the release of the hydride from the reaction solution. The peak shape can therefore be considered to be the result of two dynamic processes having the same starting time the release of hydrogen selenide from the solution and the formation of active sites able to trap the generated hydrogen selenide.570 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 It can be seen from Fig 5 that the rising portion of the peak remains practically the same in the presence of 0.1 pg ml-l of Cu. During this time hydrogen selenide has already formed and starts to leave the solution without any apparent interference.After this early period the formation of the active sites and the capture of the hydride begins. This evidence seems to exclude the possibility of catalytic decomposition of the sodium tetrahydroboratel* with the consequent depletion in the hydride reaction yield. Considering that the same behaviour is observed inde- pendently of the Se concentration used it is reasonable to suppose that the interference is controlled also by the rate of formation of the active sites. The same conclusion can be made for Ag and Au at low Se concentration levels. For Ag and Au at high Se concentration levels the hydrogen selenide seems at first to be captured and then subsequently partially released in a slow process.This experimental evidence on the basis of the hypothetical model can be explained by the progressive decrease of the active surface consequent upon the growth of aggregates with time and the destruction of some of the active sites available. In conclusion the total number of hydride molecules retained in solution should be considered proportional to the cumulative number of active sites present at the end of the aggregation process of the colloid. For copper it can be supposed that the number of active sites always remains high or that the process of capture leads to the chemical decomposition of the adsorbed hydride. Mathematical description of the model In order to describe quantitatively the model just outlined it is assumed that at a given time after the beginning of the cluster growth the concentration c of the interfering metal ion produces a number n of clusters per unit volume of solution.Each of the clusters possess a mean diameter 2R mean surface area a and a mean mass m. If the clusters are considered to be fractal objects they possess fractal dimen- sions 0 and D for the active surface area and the mass respectively. In the particular instance where the clusters can be considered to be Euclidean objects then D,=2 and D=3. The dependence of the surface area a and mass rn on the radius R of a single cluster will be a = constJPa and m = constJP where const and const are dimensionless constants de- pending on the fractal geometry of the clusters.The dependence of the surface area on the mass will be a = const,m' (6) where const,= const const,-a a= D,/D If c the mass per unit volume of solution of the interfering metal ion generates a mass per unit volume of solution of the interferent c it can be said that c,=h,c where h is a constant related to the type of chemical reaction and its yield. From eqns. (4) and ( 5 ) an increase in c would produce an increase in m maintaining the same number n of clusters per unit volume of solution. Considering the mass of a single cluster m= h,c/n the total surface area per unit volume of solution A = n,a will be (7) and A = (const,n,(l -")h,")c * If it can be supposed that the fractal structure of the aggregates remain unchanged at least in the range of interferent concentrations used in this study then the term in parentheses is a constant and A = const 1P (8) If there are d active sites per unit of surface area the number of active sites M per unit volume of solution will be M,,=d,A and where const2 = constld,.through the following process M = const2P ( 9 ) If the hydrogen selenide is captured by the active sites H,Se+Q=Q* where Q is a free active site and Q* is the active site after the capture of hydrogen selenide the equilibrium constant of the process will be Ki = W/(SM) where W and Mare the number of occupied and free active sites present in the unit volume of solution. Knowing that W=F and assuming W<<M then M=Mo and eqn. (10) becomes PIS= KiMo (1 1) Considering the mass balance So=S+F and the rela- tionship in eqn.(3) eqn. (1 1) becomes p=Mo/(Mo+ l/Ki) Using eqn. (9) and postulating that (const2Ki)-' =Ba then eqn. (1) is obtained. Physical meaning of B and a According to the above mathematical description the parameter B the characteristic concentration of the inter- ferent giving a 50% signal depression is given by B = (const,n,(l -a)h,*d,Ki)- (13) It is thus dependent on chemical and physical parameters strictly related to the nature both of the analyte and of the interfering species the adsorption energy of the analyte on the particle surface the surface structure of the particles generated their number and so on. As B is greatly dependent on experimental conditions eqn. (1 3) justifies the discrepancies existing in the literature about the magnitude of the interference effects reported.The parameter a is related to the fractal structure of the particle as it is 'felt' by the fractal interrogator in this instance the hydrogen selenide molecule and also to the nature of the interaction between the interrogator and the cluster surface. The experimental values of D reported in the litera- t ~ r e ~ ~ are in the ranges 1.95<0,<3.04 for physisorption 1 .6<0,<2. 13 for chemisorption on a dispersed metal catalyst and 0.7 1 <D,<5.8 for heterogeneous catalysis on dispersed metals. In the present study the peak height measurements give a values more homogeneous than those given by peak area measurements. This is probably because the peak height measurements are performed at a time (after the start of the reaction and corresponding to the peak maximum) when the 'ripening' of the interfering clusters gives a better agreement with the assumptions made in the mathematical description.For peak area measurements a represents a mean value evaluated on the whole peak lifetime and might be affected by changes in the morphology of the interfering particles that are likely to occur during a longer time span. Even if no specific data relative to the hydrogen selenideJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 57 1 interaction with colloids of Group IB metals were found in the literature the values of CY reported in Table’2 seem consistent with its physical meaning and with the values of D reported above. Conclusion For liquid-phase interferences caused by Group IB transi- tion metals on the generation of hydrogen selenide in a batch HG system the magnitude of the interference effects as a function of the interferent concentration is related to the mechanism of the interference effect.A relatively simple physical model for the interferences based on the trapping of the hydrogen selenide at the surface of the interferent particles supposedly grown in the solution and having a highly folded surface that is fractal in nature seems to be in good agreement with the experimen- tal results. In fact the relevant equation describes accurately the interference plots and justifies in terms of chemical-physi- cal parameters the different behaviour of the metals considered and also the discrepancies among the interfer- ence data reported in the literature.Considering that the dynamics of aggregation for colloi- dal particles is a complex process dependent on many chemical and physical parameters it is reasonable to suppose that the experimental conditions play a role in determining the magnitude of the interferences not only through the control of the chemical reaction responsible for the production of the interfering species but also through the control of the dynamics of their aggregation into colloidal particles and of their resulting morphology. References 1 Robbins W. B. and Caruso J. A. Anal. Chem. 1979 51 889A. 2 Godden R. G. and Thomerson D. R. Analyst. 1980 105 1137. 3 Nakahara T. Prog. Anal. At. Spectrosc. 1983 6 163. 4 Dedina J. Prog. Anal. At.Spectrosc. 1988 11 251. 5 Smith A. E. Analyst 1975 100 300. 6 Kirkbright G. F. and Taddia M. Anal. Chim. Acta 1978 100 145. 7 Meyer A. Hofer Ch. Tolg G. Raptis S. and Knapp G. Fresenius Z. Anal. Chem. 1979 296 337. 8 Verlinden M. and Deelstra H. Fresenius 2. Anal. Chem. 1979 296 253. 9 Pierce F. D. and Brown H. R. Anal. Chem. 1976 48 693. 10 Pierce F. D. and Brown H. R. Anal. Chem. 1977 49 1417. 11 Hershey J. W. and Keliher P. N. Spectrochim. Acta Part B 1986 41 713. 12 Dedina J. Anal. Chem. 1982 54 2097. 13 Welz B. and Melcher M. Analyst 1984 109 569. 14 Welz B. and Melcher M. Analyst 1984 109 573. 15 Welz B. and Melcher M. Analyst 1984 109 577. 16 Welz B. and Shubert-Jacobs M. J. Anal. At. Spectrorn. 1986 1 23. 17 Petrick K. and Krivan V. Fresenius Z. Anal. Chem. 1987 327 338. 18 Bax D. Agterdenbos J. Worrel E. and Kolmer J. B. Spectrochim. Acta Part B 1988 43 1349. 19 Yamamoto M. Yamamoto Y. and Yamashige T. Analyst 1984 109 1461. 20 Welz B. and Melcher M. Spectrochim. Acta Part B 198 1,36 439. 21 Dittrich K. and Mandry R. Analyst 1986 111 277. 22 DUlivo A. J. Anal. At. Spectrom. 1989 4 67. 23 DUlivo A. Festa C. and Papoff P. Talanta 1983 30 907. 24 Creighton J. A. Blatchford C. G. and Albrecht M. G. J. Chem. SOC. Faraday Trans. 2 1979 75 790. 25 Creighton J. A. Alvarez M. S. Weitz D. A. andGaroff S. J. Phys. Chem. 1983 87 4793. 26 Weitz D. A. and Oliveria M. Phys. Rev. Lett. 1984 52 1433. 27 Matsushita M. in The Fractal Approach to Heterogeneous Chemistry Surfaces Colloids Polymers ed. Avnir D. Wiley Chichester 1989 p. 161. 28 Weitz D. A. Huang J. S. Lin M. Y. and Sung J. Phys. Rev. Lett. 1985 54 1416. 29 Pfeifer P. and Avnir D. J. Chem. Phys. 1983 79 3558. 30 Weitz D. A. Huang J. S. Lin M. Y. and Sung J. Phys. Rev. Lett. 1984 53 1657. 31 Dedina J. Fresenius Z. Anal. Chem. 1986 323 771. 32 Agterdenbos J. and Bax D. Anal. Chim. Acta 1986 188 127. 33 Farin D. and Avnir D. in The Fractal Approach to Hetero- geneous Chemistry Surfaces Colloids Polymers ed. Avnir D. Wiley Chichester 1989 p. 27 1. Paper 0/05149G Received November 16thl 1990 Accepted May 29thl 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600565

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 573 Influence of Solvent Physical Properties on Drop Size Distribution Transport and Sensitivity in Flame Atomic Absorption Spectrometry With Pneumatic Nebulization* Juan Mora Vicente Hernandis and Antonio Canals Division of Analytical Chemistry University of Alicante 03080 Alicante Spain The aim of this paper has been to study the influence that the main physical properties of the solvent (surface tension viscosity and volatility) exert on the magnitudes of the following parameters the drop size distribution of the primary aerosol; the transport efficiencies for analyte and solvent; and the sensitivity in flame atomic absorption spectrometry with pneumatic nebulization. Two series of experiments have been carried out one with pure solvents and another with methanol-water mixtures of variable composition always using 2 pg ml-' Mn as the analyte.The results show that surface tension and to a lesser extent viscosity determine the mean size of the distribution. The span on the other hand does not appear to follow any simple relationship with the physical properties of the solvent. The analyte transport rate is improved by a high solvent volatility andalso by a distribution with a small mean drop size and a high span. Absorbance values are essentially consistent with analyte transport rates this tendency being modified by the diluting effect of the solvent carried to the flame. Keywords Flame atomic absorption spectrometry; pneumatic nebulization; drop-size distribution; transport efficiency The most common method for sample introduction in flame atomic absorption spectrometry (FAAS) is through the pneumatic nebulization of solutions. In the nebuliza- tion step a spray (primary aerosol) is generated the characteristics of which have a great influence on the transport parameters (analyte transport rate en and solvent transport rate EJ.In turn these parameters together with the characteristics of the tertiary aerosol i.e. the aerosol that enters the flame base will determine the analytical signal. Usually the equation of Nukiyama and Tanasawa' has been used to describe the features of the primary aerosols generated using pneumatic nebulization. Several experimental studies have been accomplished recently on the applicability of this equation to the primary aerosols generated in atomic spectrometry under normal operating condition^.*-^ According to these studies the predictions of the equation differ noticeably from the experimental results.This applies to both inductively coupled plasma atomic emission spectrometry (ICP-AES)2-4 and FAAS.5 Owing to this discrepancy empirical equations3 or mathe- matical models6 have been proposed for the description of the primary aerosols generated by concentric nebulizers. Even though the number of basic experimental studies on aerosol generation in the field of atomic spectroscopy is not very great it is beyond doubt that the physical properties of the solvent have a great influence on the droplet formation ~ t e p ~ - ~ and also on the aerosol transport through the spray chamber to the atomization ell.^-^ Organic solvents or mixtures of water and organic solvents are currently used in atomic spectrometry (i) to separate interfering elements; (ii) to concentrate the ana- lyte; and (iii) to improve sensitivity.1° Several workers have tried to correlate the sensitivity improvements with solvent physical properties.l-l However these correlations appear to be fairly artificial in some instances as they do not try to understand and hence to explain the nebulization and transport processes as a whole. In order to do that it is necessary to perform a detailed individual study on each of the processes. By taking into account previous works on the nebuliza- tion step it appears that the physical properties of the ~~~ ~~~ * Presented at the Fifth Biennial National Atomic Spectroscopy Symposium (BNASS) Loughborough UK 18th-20th July 1990.solvent that show a greater influence on the characteristics of the pneumatically generated aerosols are surface tension viscosity and v ~ l a t i l i t y . ~ * ~ J ~ J ~ The mean drop size of the resulting aerosol decreases with decreasing surface tension (the main factor) and viscosity and also with increasing volatility. The surface tension of the solvent can be modified by using (a) aqueous solutions of surfactants; (b) mixtures of water and a compatible organic solvent; and (c) pure organic solvents. The effect of the forces of surface tension in situation (a) is the cause of a controversy frequently found in the literature. In a previous study by the same workers16 a possible explanation for the action of these forces in aqueous solutions of surfactants has been given.Following this explanation the forces require a certain time to exert their influence. This would be the time necessary for the surfactant molecules to migrate from the bulk of liquid to the surface being generated so that they can exhibit their surfactant action. Because this time is longer than that necessary for the nebulization step at least with long chain surfactants the resulting aerosols show the same characteristics as those obtained without the addition of surfactants i e . with the solvent alone.2 This study deals with the effect of surface tension and other physical properties of the solvent on the nebulization transport and atomization processes in situations (b) and (c) mentioned above.To this end drop size distributions of the primary aerosols transport efficiency for both analyte and solvent and absorbance have been sequentially measured under identical experimental conditions for a series of water-methanol mixtures and for a series of pure solvents with different values of their physical properties. Experimental All of the reagents employed were of analytical-reagent grade. The water used was distilled and de-ionized. The solvents employed together with their main physical properties are listed in Table 1. All of the solutions contained 2 pg ml-1 Mn as the analyte and were prepared in the same way as the standards from a stock solution of 2000 pg ml-1 Mn in water to which a small amount of concentrated HC1 was added (1% v/v) and diluted with the appropriate solvent to the working concentration.574 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL.6 The nebulizer the systems used for the control of the gas and liquid flows and the system used to measure (by an indirect method) the transport efficiencies have been described previously.16 All of the experiments were per- formed with a Perkin-Elmer Model 373 spectrometer under the experimental conditions shown in Table 2. Drop size distributions of the primary aerosols were measured at a distance of 28 mm from the nebulizer tip by means of a particle sizer based on laser Fraunhofer Table 1 Physical properties of solvents at 20 "C Pure solvents- Solvent Water Benzaldeh yde Formic acid Acetic acid Propan- 1-01 Methanol d r x lo3/ J t x lo3/ Nm-' N smm2 RVS 72.6 1 .oo 0.08 40.0 1.52 0.0 I 37.6 1.85 0.30 27.8 1.32 0.20 23.8 2.2 1 0.27 22.7 0.60 1 .oo Methanol-water ( v h ) mixtures- O X 103/ J X 1031 Methanol + water Nm-I N ~ r n - ~ RV o+ 100 10+ 90 25+ 75 50+ 50 60+ 40 80+ 20 90+ 10 loo+ 0 72.6 59.0 46.4 35.3 33.0 27.3 25.4 22.7 1 .oo 1.22 1.56 1.76 1.63 1.25 0.93 0.60 0.08 0.14 0.23 0.39 0.48 0.66 0.80 1 .oo * 0 =Surface tension.t J =Viscosity. $ RV=Relative volatility ratio of liquid volume of solvent to liquid volume of methanol necessary to saturate a given empty volume. Table 2 FAAS operating conditions Wavelength 279.5 nm Slit-width 0.2 nm Lamp intensity 35 mA Observation height 8.0 mm CZH2 flow 2.7 1 rnin-' Total air flow 19.6 1 min-' Nebulizer air flow (Q,) 5.6 1 min-' Liquid flow (Q,) 4.5 ml min-I Integration time 5 s diffraction (Malvern Instruments 2600~).All of the measurements were made with a lens of 100 mm focal length which encompasses a droplet diameter range of between 1.9 and 188 pm. The software employed was version M5.4 and the calculations were made in 'model independent' mode which does not presume any particular function for the distribution as has been recommended by Jackson and Sam~elson,~~ and is therefore the most appropriate mode for multimodal distributions or for those containing a large percentage of fine particles. The system used for drop size distribution determination has the inherent advantage of being non-intrusive to the aerosol. The accuracy is kd0/o for volume median diameter as reported by the manufacturer.'* Results and Discussion Pure Solvent Drop size distribution of the primary aerosol The results obtained for the drop size distributions of the six solvents are shown in Table 3.The description of a distribution assumed to be approximately log-normal requires two parameters at least one for the location of the centre of the distribution and another for the characteriza- tion of its width. In this study Dso (the volume median diameter) and the span have been employed to this end. The parameter of DsO was chosen for the central tendency because it corresponds to the maximum of the log-normal volume distribution curve. The first point to be recalled is that volume concentration (VC) values are not the same for all solvents even though Q (liquid flow) is the same (4.5 ml min-l).There are two reasons for this. The first is solvent evaporation which allows a small amount of liquid volume to be lost in the measurement area. The second is the presence of droplets smaller than 1.9 pm which is a smaller diameter than can be measured by the particle sizer as the photons diffracted by these droplets fall out of the detector. As the liquid fraction contributed by these droplets is always relatively small and can be estimated by the particle sizer through the obscuration (OB) value (see Table 3) it seems clear that the differences in the VC values fairly noticeable in some instances are mainly due to evaporation. Closely related to this the sequence of the VC values is exactly opposite to that of the relative volatility (RV) values viz.VC Benzaldehyde> water> acetic acid-propan- l-ol RV Benzaldehydetwater<acetic acid=propan- l-ol =formic acid>methanol -formic acid(methano1 Table 3 Distribution parameters for pure solvents v c * DSO$l V,.~II V1.9fl SSAII/ Solvent (O/O) OBt pm Spang (%) (Oh) m2 ml-3 Water 0.0034 0.1464 16.9 3.7 7.0 4.1 0.7105 Benzaldehyde 0.0038 0.2072 12.1 3.1 9.6 4.8 0.9127 Formic acid 0.0026 0.1578 12.9 3.8 12.3 7.1 1.0145 Acetic acid 0.0026 0.1693 9.8 3.4 12.5 6.5 1.0916 Propan- 1-01 0.0028 0.1761 10.0 2.2 10.6 5.4 1.0272 Methanol 0.0018 0.1607 7.7 3.0 20.1 11.1 1.4744 * VC=Volume concentration. This value indicates the ratio of total volume of liquid aerosol contained in the measurement volume to t OB=Obscuration.This value indicates the proportion of incident light that is being scattered out of the beam by the aerosol. $ DSo=Droplet distribution diameter below which 50% of the cumulative aerosol volume is found. Hence D90 and DlO=90 and lo% 8 Distribution span [(D90-Dlo)/Dso]. A measurement of the distribution spread. 1 V3.0 and V,.,=Percentage of volume found in droplets smaller than 3.0 and 1.9 pm respectively. 11 SSA = Specific surface area of the aerosol. the total measurement volume (1 0 mm diameter x 14.3 mm beam length active). ~espectivel y .JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 575 It should be noted that solvent evaporation proceeds at a particularly rapid rate during the first moments in the life of the aerosol9 for several reasons (i) the liquid surface is enormous; (ii) the air layer surrounding a given droplet is not saturated and is being continuously renewed as the droplets decelerate more slowly than the gas at the beginning of their path because of their inertia; and (iii) the vapour pressure of a given liquid increases with decreasing curvature radius of the surface.'O The differences in VC values from one solvent to another have an immediate effect on the OB values.If two solvents of equal VC value are compared the OB values should increase as D50 decreases because two droplets intercept a greater fraction of the laser beam than does a single droplet the volume of which is the sum of the volumes of the first two droplets. Likewise for two aerosols of equal distribu- tion the one with the higher VC value will show a higher OB value.In good agreement with this benzaldehyde is the solvent with the highest OB value whereas water in spite of its relatively high VC value shows the lowest OB value because the distribution is the coarsest (a higher DsO value i.e. 16.9 pm). Formic acid acetic acid and propan-1-01 have similar OB values as their distributions and their VC values are also similar. The OB value for methanol is similar to those of the last three solvents because the VC is fairly low while the distribution is finer (a lower Dso value i.e. 7.7 pm). The characteristics of the drop size distribution of the primary aerosol and hence the Ds0 and span values are mainly determined by the surface tension (a) and viscosity (J) of the l i q ~ i d ~ - ~ but solvent evaporation and droplet coalescence tend to modify them from the very beginning evaporation in terms of diminishing the Ds0 and increasing the span and coalescence in terms of producing the opposite results9 The graphs of the experimental results reveal this situation effectively.Water which is the solvent with the highest c value by far appears to be the one with the highest Ds0 and lowest specific surface area of the aerosol (SSA) even though its viscosity is fairly low. The influence of viscosity becomes clearer on comparing methanol and propan-1-01 shown in Table 3 as their a values are similar while their J values are clearly different The D50 value for methanol is 7.7 pm whereas for propan-1-01 is 10.0 pm.A comparison between benzaldehyde and acetic acid reveals the influence of surface tension as the viscosities are similar. Increasing solvent viscosity enhances the capability of the solvent to damp the oscillations that appear on the surface during nebulization leading in turn to a lengthening of the liquid vein before the collapse of the droplets. This effect results in an increase in the mean drop size of the spray.2 The influence of the surface tension on the nebulization can be described as follows:19 the energy available for the pneumatic generation of an aerosol comes solely from the kinetic energies of the gas (and liquid) streams. This energy is partly employed in the formation of the new surface in addition to being used in the acceleration of the liquid stream and the resulting droplets.The energy employed for the formation of the new surface is proportional to the aerosol surface area and also to the surface tension of the solvent. In a series of experiments where the kinetic energies of the gas and liquid streams remain roughly unchanged as in this instance the energy available for surface formation should be also unchanged. Thus solvents with the lowest values for surface tension should generate more surface. This is accomplished through the production of a finer aerosol. From Table 3 it can be seen that a discussion of some of the discrepancies that arise on comparison of the values of Djo and SSA is necessary. Two distributions of equal span but different Ds0 should have different SSA the smaller the value for D the greater the SSA (e.g.water and formic acid). The situation is not so obvious if the span values are not the same. It might then be that a distribution with a smaller D50 shows a lower SSA (e.g. benzaldehyde and formic acid). Formic acid provides a higher fraction of the total volume enclosed in small droplets than does benzal- dehyde in spite of its larger D50 value as its span is also higher [see Fig. l(a)]. Something similar happens between formic acid and propan- 1-01 [see Fig. 1 (b)]. Whereas the D50 value is larger for formic acid the SSA values are similar because of the noticeable differences between their span values. As the D50 values for each of the solvents studied depend on o and J simultaneously it is not unlikely that Dso could be linearly related to a combination of both physical magnitudes. There is good correlation between Dso and the magnitude x = s x a+ J where s is a parameter the value of which is around 0.1 (provided that a is written in N m-' x lo3 and J i n N s m-2 x lo3) as shown in Fig.2. The correlation between D and x is much better than with CT or J separately. Transport eficiency If the spray chamber. is considered as a type of filter with a given cut-off diameter (dc) droplets smaller than d will succeed in reaching the flame whereas larger droplets will drain away. Obviously this description of the chamber action is simple and the tertiary aerosol will always contain some droplets larger than d and some droplets smaller than d will have drained away before reaching the flame.6 However this simplified mechanism can be retained as it is useful for the following discussion on the relationship between the drop size distribution of the primary aerosol and transport rate.The amount of analyte and solvent transported to the flame ( W, and S,, respectively) along the spray chamber is likely to depend on the characteristics of the primary aerosol (mainly D50 and span) and on solvent volatility. Among the factors that modify the aerosol distribution 100 (a) dip m Fig. 1 and B benzaldehyde; and (b) A formic acid; and B propan-1-01 Cumulative drop size distribution for (a) A formic acid;576 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 Table 4 Transport and signal results for pure solvents &I*/ ESt ~ 1 0 N &"§ Solvent ml min-I (%o) pg min-I (Yo) A,! A,," Water 0.4 1 9.1 0.40 4.5 1.00 2.5 Benzaldeh yde 0.43 9.6 0.44 4.9 1.01 2.3 Formic acid 1.42 31.6 1.61 17.9 2.64 1.6 Acetic acid 1.52 33.8 1.85 20.6 3.91 2.1 Propan- 1-01 1.28 28.4 1.75 19.4 3.49 2.0 Methanol 2.22 49.3 2.79 31.0 4.67 1.7 * S, = Solvent transport rate. t &,= Solvent transport efficiency.$ W, = Analyte transport rate. 9 E = Analyte transport efficiency. fi A,= Relative absorbance (A,/AHBtcr). 11 A,=Ratio between relative absorbance and analyte transport rate (Ar/ WloJ. 5 L " " " ' 2 3 4 5 6 7 8 9 X Fig. 2 Variation of D verms x (a linear combination of surface tension and viscosity) for pure solvents. x=O.l xa+J a in N m-'x lo3 and J in N s m-2x lo3. Correlation line D,,=(2.6 +_(IS)+( 1.74&00.10) X; r2=0.987 along its way (impaction settling coalescence solvent volatility etc.) volatility shows a peculiar aspect in that it modifies the analyte and the solvent distribution curve in different ways (Fig.3). Thus the analyte concentration gradients being a function of the droplet size arise as a result of solvent evaporation i.e. after a given period of time the analyte concentration in a droplet will be higher as the droplet is smaller. As the relative rate of solvent loss increases as the droplet size decreases the effect of evaporation on the DsO value is to increase it in the solvent distribution curve and to diminish it in the analyte distribution curve. On the other hand the effect on the span values is to reduce them in the solvent distribution curve and to increase them in the analyte distribution curve.Hence assuming that only V3.0 for instance will reach the flame (in other words the dc of the spray chamber is 3.0 pm) then two solvents of equal initial size distribution (equal D50 and span) will transport different amounts of analyte to the flame if their RV values are different. The fraction of analyte transported to the flame will always be greater than the fraction of liquid volume transferred into the flame (see Fig. 3). The difference between these two fractions increases on increasing solvent volatility. The value of W, will also be enhanced on increasing solvent volatility as shown in Fig. 3. This is approximately the situation between benzaldehyde and formic acid (see Table 4). The comparison is somewhat different for solvents with different distributions.If two solvents show similar values for Dso and RV then that with the greater span will have a higher transport rate for both solvent and analyte. This is Log d- Fig. 3 Ideal distribution curves (a) for solvent; and (b) for analyte. A Initial distribution curve before evaporation; B distribution curve after evaporation. For (a) the area under curve A corresponds to 100% of the solvent. The area between curves A and B corresponds to the amount of solvent evaporated. For (b) the area under curve A=area under curve B= 100% ofthe analyte. The cut- off diameter of the spray chamber 3 pm is indicated the situation between propan- 1-01 and acetic acid. Acetic acid because of its greater span contributes a larger V3.0 as shown in Fig. 4.Consequently higher W, and S, values can be expected for acetic acid than for propan-1-01. For two solvents with equal RV and span values and different Dso values the solvent with the smaller D50 value will be the one with higher W, and S, values not only because it contains a greater fraction of liquid volume below the d but also because its evaporation is more rapid. Some predictions about the transport of analyte and solvent can be made on the basis of the above discussion and the distributions obtained for pure solvents (Table 3). Methanol is likely to be the most efficient solvent in terms of transport as it shows the highest values of V and RV (this is why it shows the lowest VC value). Water and benzaldehyde should be placed at the other end as they show the smallest values in these magnitudes.Water shows the lowest V3.0 followed by benzaldehyde which is the least volatile followed by water (this explains the fact thatJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 577 100 - 5 50 3 0 > - a b " 1 10 100 d h m 1000 Fig. 4 Cumulative drop size distribution for A acetic acid and B propan-1-01 showing the effect of their different span values on V3.0; a= 12.5% b= 10.6% benzaldehyde is the solvent with the higher VC followed by water). The rest of the solvents will probably lie in the intermediate zone relative to their transport efficiency. It can be seen in Table 4 that these predictions hold reasonably well in practice for all of the solvents investi- gated. Only W, for formic acid appears to be somewhat lower than expected (but not the S,,,).However there is a certain lack of precision associated with the indirect methods of measuring transport efficiency as quoted by Smith and Browner.2o The analyte concentration in the drained away fractions increases on increasing solvent RV from approximately 2.1 pg ml-1 for benzaldehyde to 2.7 pg ml-l for methanol. A mass balance for analyte and solvent allows the estimation with the uncertainty associated with these calculations that the solvent load to the flame in vapour form (S,) varies from 0.2 ml min-l for water and benzaldehyde to 1.2 ml min-' for methanol these volumes being expressed as liquid volumes. Analytical signal The analytical signal depends mainly on the analyte transport rate and the atomization efficiency. The analyte transport rate is determined by the drop size distribution of the primary aerosol and the transport phenomena along the spray chamber.The atomization efficiency depends on the size distribution of the tertiary aerosol the drop nature and composition of the flame and also on the element to be determined. A controversy exists in the literature on the changes induced in the flame and in the atomization efficiency when the analyte is introduced using an organic solvent. Some workers14y21722 allude to variations (increases and decreases) in the flame temperature whereas others indicate variations in the composition of the flame23 or in its dimen~ions.~~ All of these factors will affect the atomi- zation efficiency and hence analytical signal on changing the solvent nature.However W, will be the dominant influence on the analytical signal as Mn is relatively insensitive to variations in the nature and composition of the flame. A clear parallelism between the W, values and the absorbance values can be seen in Table 4. Nevertheless the atomization efficiency decreases with increasing transport rate. Thus on going from water to methanol W, increases by a factor of 7.0 whereas the signal increases by only 4.7. As Mn is only slightly sensitive to variations in the temperature and reducing character of the flame and the drop size distributions of tertiary aerosols are expected to be ~imilar,~ the decrease in the atomization efficiency might come from two possible sources ( i ) a decrease in the slope of the analytical curve due to the increase of the analyte concentration in the flame; and (ii) a dilution of the analyte in the flame as a result of variations in the geometry as organic solvents can cause flame enlargements.This second explanation seems reasonable. If 7.50 9.40 and 10.70 cm2 are used for the cross-sectional areas of the flame with water propan- 1-01 and methanol24 as solvents respectively the resulting values for the analyte transport rate per unit of cross-sectional area (qi) are 0.0533 0.186 and 0.261 and the relative values (qiJ 1.00 3.49 and 4.90 respectively. These values are in better agreement with the relative absorbances (1 .OO 3.49 and 4.67) than those of relative W, (1.00 4.38 and 6.97). In order to observe this behaviour it is necessary that the hollow cathode lamp beam width at the analyte wavelength be smaller than the flame width.The cross-sectional area of the flame using methanol as the solvent is taken as 10.70 cm2 which is the value corre- sponding to the use of ethanol as reported by Sziv6s et al.,24 taking into account that the behaviour is very similar and that they did not investigate methanol. Methanol-Water Mixtures Drop size distribution of the primary aerosol Table 5 summarizes the most relevant parameters of the drop size distributions of the methanol-water mixtures. Firstly it can be noticed that although the and VC values for water in Table 3 (pure solvents) agree with those in Table 5 (mixtures) this situation does not hold for methanol the values being higher in Table 5 than in Table 3.Secondly the span values are generally higher in Table 5 than in Table 3. The discrepancies between the two distributions for methanol may be assigned to several causes; the most significant being the period of time that elapsed (about two months from September to December) between the series of measurements. During this time room tempera- ture undoubtedly dropped (this variable was not moni- tored) and the nebulizing air temperature and solutions temperature probably dropped. This could cause the mean drop size and VC of the first series (higher tempera- ture) to be lower than those of the second series (these differences increase with increasing solvent RV). The increase of the span value may also be related to this temperature difference but it is difficult for an adjustable nebulizer to be in the same geometrical placement exactly for such long periods of time.It is not surprising there- fore that different results were obtained for methanoL It can be stated that the results obtained within a given series of measurements are strictly comparable. For dif- ferent series of measurements the tendencies are compar- able but not the absolute values. In Table 5 it can be seen that the VC values diminish at a steady rate as the proportion of methanol increases owing to the enhancement of the solvent volatility. The Dso value also decreases with increasing methanol content. These two factors size decrease and RV increase have an opposite influence on the OB value. The experimental results show that the decrease in drop size is the dominant effect and so the OB increases until a maximum is reached for the water-methanol mixture (90+ lo) followed by a slight decrease for pure methanol ( 100%).Also in this series the D50 value is controlled by surface tension and viscosity. Surface tension decreases markedly with increasing methanol content whereas viscosity in- creases until it reaches a maximum for the water-ethanol mixture (50+ 50) and then decreases steeply. Once again in this series the Ds0 values correlate much better with a linear combination of surface tension and viscosity than with578 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 1991 VOL. 6 Table 5 Distribution parameters for methanol-water (vh) mixtures Solvent vc Methanol (O/O) Water (O/O) (O/o) OB 0 10 25 50 60 80 90 100 100 90 75 50 40 20 10 0 0.0036 0.0038 0.0039 0.0037 0.0036 0.0034 0.0034 0.0030 0.1434 0.1687 0.1854 0.1971 0.2000 0.2056 0.2 190 0.2094 D5d v3.0 vl.9 pm Span (Yo) (To) 17.0 5.8 7.0 4.6 16.5 5.9 7.8 4.9 15.5 6.4 8.6 5.3 13.8 5.3 10.4 6.3 13.2 5.3 11.1 6.8 11.9 4.9 12.7 7.7 10.9 4.9 13.6 7.7 9.7 5.1 15.4 8.5 SSAI m2 ml-3 0.7092 0.7503 0.8020 0.91 11 0.9538 1.0519 1.1013 1.1966 Table 6 Transport and signal results for methanol-water (v/v) mixtures Solvent &Ot/ Es Methanol (O/O) Water (O/O) ml min-' (O/o) 0 10 25 50 60 80 90 100 100 90 75 50 40 20 10 0 0.4 1 0.52 0.64 1.06 1.42 1.77 2.00 2.26 9.1 11.2 14.1 23.0 28.7 36.3 40.9 50.2 W,O,/ pgmin-1 (2) A A 0.4 1 4.5 1.00 2.4 0.48 5.1 1.35 2.8 0.57 6.2 1.78 3.1 0.89 9.5 2.21 2.5 1.56 15.7 2.57 1.7 2.24 23.0 3.32 1.5 2.56 26.1 3.97 1.6 2.82 31.3 4.87 1.7 either of them individually (Fig.5). The discussion of drop size distribution of the primary aerosol under Pure Solvent is also valid for solvent mixtures. The span values in the series using methanol-water mixtures are higher than in the series using pure solvents as has already been mentioned but the variations are not significant. Transport eflciency As the D50 value for the aerosol decreases and the volatility increases the loss of analyte in the spray chamber will decrease with increasing methanol content of the mixture. Table 6 shows that both the W, and S, values increase with increasing methanol content in addition to the transport efficiencies E and e,. However the increase in the transport efficiency is slower at the beginning of the series [from water to mixture (50+50)] than at the end [from mixture (50+ 50) to methanol] probably because the RV 5 15 -.0 a" 10 2 3 4 5 6 7 8 9 X Fig. 5 Variation of Dz0 versus x (a linear combination of surface tension and viscosity) for methanol-water (v/v) mixtures. x=O. 1 x a+J a in N m-l x lo3 and J in N s m-2 x lo3. Correlation line D5,=(6.1 ?0.6)+(1.41 kO.10) x; 9=0.971 increases in a similar way. It is evident that the increase in transport efficiency from the beginning to the end of the series is far greater than the increase in V3.0 values. This behaviour can be justified by taking into account that V3.0 values were measured close to the nebulizer tip i.e. a short time after the droplets were generated whereas the effici- ency measurement includes the much larger evaporation time associated with the passage of droplets through the spray chamber.When using solvent mixtures the vapour phase should become enriched in the more volatile component i.e. methanol. This makes the composition and density of the drained fraction of the solvent dissimilar to the fraction fed to the flame except for pure solvents e.g. methanol (100%) and water (1 00%). The composition and density of both fractions will not be the same as that of the initial mixture. These circumstances that were not taken into considera- tion on making the calculations will certainly introduce a given level of inaccuracy in the S, values,20 which were determined by an indirect method.For the same reasons the solvent load to the flame in vapour form (S,) and the solvent load to the flame in liquid form (S,) will not have the same composition either. Both fractions should increase with increasing proportions of methanol; S because of the RV increase and S because of the decrease in D50 values. Analytical signa I The values given in Table 6 show that the absorbance increases with increasing W,,,. However the increase in relative absorbance (about %fold) is lower than the corresponding relative transport increase (about 7-fold). Once again this divergence is more pronounced along the second part of the series (increased methanol) when Stot becomes important making W, less efficient for the production of the signal as seen in the last column of Table 6.The widening of the flame because of the solvent load might be one of the reasons for this behaviour as discussed for analytical signal under Pure Solvent. This explanation agrees with that of Gustavsson2s concerning the effect ofJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY OCTOBER 199 1 VOL. 6 579 analyte transport on inductively coupled plasma emission intensity. Conclusions The organic solvents used in this work both in pure form and in mixtures with water result in a sensitivity enhance- ment compared with the use of water alone. This enhance- ment is due to an improvement in the analyte transport which in turn is the result of a finer primary aerosol and also higher solvent volatility. Surface tension together with viscosity determine the volume median diameter of the primary aerosol formed but the span is not easily correlated with the solvent physical properties.Both distribution parameters are im- portant for transport processes. The spray chamber can be considered as a sort of ‘filter’ with a given cut-off diameter for a given set of gas and liquid flows. Solvent volatility greatly alters the initial drop size distribution thus allowing an increased fraction of the analyte to be carried to the flame. However the flame widening caused by the high solvent load does not increase the absorbance as much as expected from transport enhancement. This fact counter- balances a significant part of the beneficial effect of the improved analyte transport on the sensitivity. Nevertheless the effect of the solvent load in FAAS seems not to be so critical as in ICP-AES.In order to achieve a higher sensitivity in FAAS solvents with low surface tension and viscosity and high volatility should be used. A desolvation system would also be advisable to avoid the-effect of solvent load although this point has not been tested. The Comisidn Interministerial de Ciencia y Tecnologia (CICYT Spain) is acknowledged for financial support (Grant No. PB88-0288). J. M. also expresses his apprecia- tion to the Instituto de Estudios Juan Gil Albert (Diputa- cion Provincial de Alicante Spain) for a scholarship. References 1 Nukiyama S. and Tanasawa Y. in Experiments on the Atomization of Liquids in an Air Stream tr. Hope E. Defense Research Board Department of National Defense Ottawa Ontario Canada 1950.2 Sharp B. L. J. Anal. At. Spectrom. 1988 3 613. 3 Canals A. Wagner J. Browner R. F. and Hemandis V. Spectrochim. Acta Part B 1988 43 132 1. 4 Canals A. Hernandis V. and Browner R. F. J. Anal. At. Spectrom. 1990 5 61. 5 Robles C. Mora J. and Canals A. unpublished work. 6 Gustavsson A. Anal. Chem. 1983 55 94. 7 Gustavsson A. Anal. Chem. 1984 56 815. 8 Sharp B. L. J. Anal. At. Spectrom. 1988 3 939. 9 Canals A. Hemandis V. and Browner R. F. Spectrochim. Acta Part B 1990 45 591. 10 Cresser M. S. Prog. Anal. At. Spectrosc. 1982 5 35. 11 Feldman F. J. Bosshart R. E. and Christian G. D. Anal. Chem. 1967 39 1175. 12 Lemonds A. J. and McClellan B. E. Anal. Chem. 1973 45 1455. 13 Attiyat A. S. Microchem. J. 1987 36 228. 14 Allan J. E. Spectrochim. Acta 1961 17 467. 15 Farino J. and Brbwner R. F. Anal. Chem. 1984 56 2709. 16 Mora J. Canals A. and Hemandis V. J. Anal. At. Spectrom. 1991 6 139. 17 Jackson T. A. and Samuelsen G. S. Proc. Photo-Opt. Znstrum. Eng. 1985 573 73. 18 Malvern Instruments Particle Sizer Reference Manual. Ver- sion 5.4. October 4th 1987. 19 Faske A. J. Ph.D. Thesis Georgia Institute of Technology Atlanta GA USA 1986 p. 84. 20 Smith D. D. and Browner R. F. Anal. Chem. 1982,54,533. 21 Robinson J. W. Anal. Chim. Acta 1960 23 479. 22 Avni R. and Alkemade C. Th. J. Mikrochim. Acta 1960 3 460. 23 Harrison W. W. and Juliano P. O. Anal. Chem. 1969 41 1016. 24 Szivos K. Pungor E. and Kiss L. Talanta 1979 26 849. 25 Gustavsson A. in Inductively Coupled Plasmas in Analytical Atomic Spectrometry ed. Montaser A. and Golightly D. W. VCH Weinheim 1987 p. 4 19. Paper 1 /006 15K Received February I1 th 1991 Accepted May 23rd I991

ISSN:0267-9477    

DOI:10.1039/JA9910600573

出版商:RSC    

年代:1991

数据来源: RSC