摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 381 Microwave Boosted Glow Discharge Source Using a Slab-line Cavity* Michael Outred Mark H. Rummeli and Edward B. M. Steers SECEAP University of North London Holloway Road London UK N78DB The microwave boosted glow discharge source (GDS) developed by Leis et a/. gives improved analytical performance compared with a Grimm GDS but is somewhat inflexible and unsuitable for more detailed studies of the effects produced by the supplementary discharge. A simple and flexible form of microwave boosted GDS employing a slab-line microwave cavity is described. The source was designed for investigations on fundamental excitation processes in unboosted and microwave boosted GDS. The preliminary results show that the electrical and spectral properties of this source exhibit similar general trends to those of the ‘Leis’ source but the more flexible structure will allow more detailed studies of these properties.Keywords Glow discharge source; microwave boosted discharge; slab-line cavity; charge exchange Direct current glow discharge sources (GDS) offer advantages for bulk and surface analysis of metallic samples but in many cases improved limits of detection are desirable. In a recent review Leis and Steers’ have surveyed the various forms of supplementary discharge that have been used to give significant improvement in detection limits. One of the most convenient methods is that used by Leis et u E . ~ in which a supplementary microwave discharge is generated in a Beenakker coaxial TM, mode cavity3 which forms an integral part of a GDS based on the Grimm s o ~ r c e .~ ~ The intensities of sample resonance lines are greatly increased even though sputter rates are reduced; the background radiation increases by a smaller factor so that improvements in limits of detection by a factor of about 10 can be achieved. In all cases the changes in the spectral output are very complex; they can conveniently be described by the enhance- ment factors F of the various lines where F is defined as the ratio of the intensity with the supplementary discharge to that without the supplementary discharge with constant current and pressure. Table 1 shows typical values for F.’ Thus in addition to the analytical applications of the boosted GDS a study of the spectral changes can aid the study of the excitation processes occurring in the discharge.Table 1 General effects of the supplementary microwave discharge on various groups of lines. (After Leis and Steers’) Typical Emitting species Group of lines values of F Inert gas atoms All lines Inert gas ions All lines Atoms sputtered from the cathode levels Lines from low lying upper Series of upper levels with same electron configuration nl ionization (above or below) Upper levels close to Lines excited directly by Lines excited by cascade from CT excited levels Lines not excited directly or indirectly by CT (upper level too high or too low or CT prevented by selection rules) Ions of cathode material charge transfer (CT) 2-5 0.2-0.5 20-50 Falls with increasing n 100-200 0.1 0.4 3 The structure of the ‘Leis’ source utilizing the Beenakker cavity is somewhat inflexible and the dimensional parameters cannot easily be changed.Moreover side-on observation is difficult. The use of a slab-line cavity6 provides a much more flexible system for studies on discharge processes; various forms of simple d.c. discharge tubes have been constructed located in the slab-line cavity. One typical source (GDS2) is shown in Fig. l(u); in this case a uniform piece of fused silica tubing was used so that the tube could be moved relative to the electrodes and could easily be cleaned or replaced when it became coated with sputtered material. The cavity could be placed at various points along the tube [i.e. the distance D in Fig. l(a) could be varied]. Various cathode sizes have been used with fused silica protective shields to avoid stray dis- charges where necessary so that the discharge always occurs at the end face of the cathode rod. Plane or hollow cathodes can be used.The open structure of the source allows side-on observations to be made though these may be limited by sputtered deposits on the walls. Side-on absorption measure- ments can be carried out in addition to axial measurements made using electrodes with axial holes. There is no significant stray microwave radiation and the plasma impedance can be determined from microwave meas~rernents.~ In an earlier version (GDS1) a shouldered tube [Fig. l(b)] was used with (a) To microwave generator t To vacuum system INlI - Coaxial line Window I 1’ u/O-‘ring Cathbde Water cooling v Anode Parallel rectangular Fused silica tube side plates ( b ) f Cathode face Fig.1 Example of a microwave boosted GDS (GDS2) using the slab-line cavity (see text for details); and (b) sketch of ‘shouldered’ * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium on Analytical Applications of Glow Discharges in Optical and Mass Spectrometry York UK July 4-7 1993. discharge tube382 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Microtron 3 - different end blocks but this was more difficult to prepare and proved to be less flexible in use. In this paper preliminary measurements are reported which show that the supplementary microwave discharge affects the electrical and spectral characteristics in the same way as in the Leis source particularly when the latter is used with a ceramic restrictor tube.In the slab-line boosted GDS the microwave discharge is located between the anode and cathode of the d.c. discharge. By contrast in the normal Leis source the micro- wave discharge takes place on the opposite side of the anode tube to the cathode (Fig. 2); if the anode tube is replaced by a ceramic restrictor tube then the surface indicated in Fig. 2 acts as the anode and the microwave discharge lies between the anode and cathode. In the former case the d.c. potential falls with increasing microwave power until a voltage plateau is reached and the intensities also reach a stable value; with the ceramic restrictor tube the voltage can fall to a very low value at high microwave powers so that eventually the lines of the cathode material fall in intensity as the number of sputtered atoms is drastically reduced.This type of behaviour is observed with the slab-line boosted GDS described here. T ~ ~ ~ - T - 20 dB directional- 20 dB directional Slab-line coupler _L c o u p i e r L - cavity Experimental An EMS Microtron 3 was used as the microwave power generator; it has meters that indicate forward and reflected microwave power but the values given are only approximate. The forward power reading is derived from the magnetron current but the efficiency of the magnetron is dependent on the magnitude and phase of the reflected power; the reflected power measurement relies on an uncalibrated crystal detector. The arrangement shown in Fig.3 was therefore used to allow accurate measurements of the net microwave power with tuning stubs to reduce the power reflected back to the Microtron 3. Further details of the commercial equipment used is given in Table 2. The source is connected to a vacuum system with a diffusion pump and liquid nitrogen cooled traps so that the source can Anodehestrictor tube ____._________..__-_.__________.__.______ Anode potential Cathode potential Fig.2 Details of the location of the anode/restrictor tube and elec- trode surfaces in the ‘Leis’ microwave boosted GDS. Using a ceramic restrictor tube the surface marked with a dotted line (and areas further left) acts as the anode Table 2 Equipment used for experimental work ~ ~~ ~~ ~ ~~ ~~ ~- D.c. power supply Microwave power supply Microwave power measurement KSM HVI 2200 (constant current with 10 kQ ballast resistor) E.M.S.Microtron 3 (2.45 GHz 200 W maximum stabilized) MI-Sanders TFT power heads Type 6463 used with 20 dB directional couplers and Type 6460 power meters Kodak Plus X film or E720 photoelectric scanning attachment Spectrographlspectrometer Hilger Medium Quartz with be evacuated to hPa and filled with high-purity neon or argon. The pressure of the filling gas is measured with an MKS Baratron gauge. Plane and hollow cathodes of a h - minium and copper have been employed. Photographic recording of the spectrum was used for overall surveys of the changes produced by the microwave discharge. The plateholder of the medium quartz spectrograph was replaced by the E720 scanning attachment for intensity measurements on individual lines.Results and Discussion The overall effect of the supplementary discharge is apparent in Fig. 4 which shows medium quartz spectrograms for an aluminium cathode in GDS2 with neon as the carrier gas; the same exposure time was used for boosted and unboosted conditions and for the microwave discharge on its own. A number of typical lines have been marked; lines from low- lying A1 I levels (e.g. 257.5 256.7 265.2 266.0 308.2 and 309.3 nm) show a great increase in intensity with the supple- mentary discharge. Those A1 I1 lines which are directly excited by charge transfer (e.g. 263.2nm) fall greatly in intensity whilst the intensity of the 281.6 nm line falls by a smaller amount. The upper level of this line can be populated by a cascade process from charge transfer excited levels and the fact that its intensity falls by a smaller amount than that of lines excited directly by charge transfer suggests that other excitation processes (most probably direct electronic collisional excitation) are also significant for this line.The overall d.c. voltage between anode and cathode affects the energy of the ions and atoms bombarding the cathode (and hence the amount of sputtering) and also the energy acquired by charged particles between collisions (and hence the spectrum). The changes in the overall voltage caused by the microwave discharge are therefore very significant. The variation of enhancement factor with power depends greatly on the line studied. In general lower microwave powers were needed for the smaller diameter discharge tube (GDSl) but the general form of the variations for a particular line were similar in both discharge tubes.Some examples of the variation with microwave power of the voltage and the enhancement factor (derived from photoelectric intensity measurements) for individual lines are given in Figs. 5-8; more detailed results and discussions will be published later. Power Power 1 head I I head I Fig. 3 Schematic arrangement of the microwave measuring equipmentJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 :<- -*. 'X x\ 383 I A1 I1 281.6 Fig. 4 Comparison of d.c. boosted and microwave only discharges GDS2 plane aluminium cathode 10 mm diameter; neon pressure 1.7 hPa; current 8 mA (d.c.and boosted discharges); voltage d.c. 1040 V boosted 610 V net microwave power 7 W (boosted and microwave discharges); exposure time 3 min The exact form of the variations with microwave power depends on the form of GDS used the position of the cavity relative to the discharge (Le. distance D for GDS2) the cathode geometry the filling gas its purity and pressure. In some cases there is a significant plateau region at some point on the voltage versus microwave power characteristic though further power increases cause a marked fall in voltage; in other cases this plateau region is not observed. With hollow cathodes where the production of charge carriers takes place mainly within the cathode the microwave discharge has less effect on the electrical characteristics than for a plane cathode except at low pressures.The form of the intensity versus microwave power character- istic depends to some extent on the voltage uersus power I/ I 0 10 20 30 40 50 Net microwave power/W 1200 1100 3 8 IJ ro 1000 900 800 Fig. 5 Variation of enhancement factor (Cu I 296.1 nm) and voltage with microwave power for the GDS2 plane copper cathode 16 mm diameter; neon pressure 2.0 hPa; current 10 mA - 1 7 0 0 & 0.2 0.1 c I I I I 600 0 10 20 30 40 Net microwave power/W Fig. 6 Variation of enhancement factor (Cu I1 252.7 nm) and voltage with microwave power for GDS2 plane copper cathode 16mm diameter; neon pressure 2.7 hPa; current 16 mA relationship but also on the particular transition studied. Some typical results obtained with a copper cathode in GDS2 with neon as the carrier gas are shown in Figs.5 and 6 together with the variation of discharge voltage with power (cathode diameter 16 mm). For the Cu I 296.1 nm line (Fig. 5 ) there is initially a rapid increase in intensity with net microwave power followed by an extended plateau region (enhancement factor ~ 3 5 ) . On the other hand for the Cu I1 252.7 nm line whose upper level is excited by charge transfer the enhance- ment factor is always less than 1 and falls steadily with increasing microwave power (Fig. 6). Results for the Cu I 327.4nm line show the same trends with GDSl and GDS2 but for lower microwave powers in GDS1. Figs. 7 and 8 show results obtained using an aluminium 0 I 1 2 3 4 5 6 7 Net microwave power/W Fig. 7 Variation of enhancement factor (A1 I 396.2 nm) and voltage with microwave power for GDS1 plane aluminium cathode 10mm diameter; neon pressure 1.3 hPa; current 3 mA 7 300 0 1 2 3 4 5 6 7 Net microwave power/\/\/ Fig.8 Variation of enhancement factor (A1 I 396.2 nm) and voltage with microwave power for GDS1 plane aluminium cathode 10mm diameter; neon pressure 2.6 hPa; current 8 mA384 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 cathode with neon using GDS1. In both cases the intensity of the A1 I 396.2 nm line initially rises with increasing micro- wave power but then falls again as the sputtering rate falls consequent on the reduced overall voltage. At a pressure of 1.3 hPa (Fig. 7) a very high cathode fall is needed to provide enough charge carriers to maintain a current of 3 mA.These charge carriers are readily produced by a very low microwave power so there is initially a sharp fall in the overall voltage. The different forms of the variation of enhancement factor with power in the two cases probably reflect differing contri- butions to the sputtering process from argon ions argon atoms and aluminium ions under the various experimental conditions. The results presented give some examples of the enhance- ment factor versus microwave power relationships and show some effects of the overall voltage. Much more detailed investi- gations are needed; these are in progress and more detailed results including spatial studies will be published in due course. Although the source in its present form is not suitable for analytical applications the flexible structure the ability to apply the microwave field at various positions the potential for making absorption measurements and the wide range of inert gases that can be used in a static system (the source section is sealed during operation) makes it a very useful tool for investigations on discharge processes and for studying the effects of the supplementary discharge. M. H. R. wishes to thank the Science and Engineering Research Council and FI Elemental Winsford Cheshire UK for finan- cial support under an SERC CASE studentship. References 1 Leis F. and Steers E. B. M. Spectrochim Acta Part B 1994 49 289. 2 Leis F. Broekaert J. A. C. and Laqua K. Spectrochim. Acta Part B 1987 42 1169. .3 Beenakker C. I. M. Spectrochim Acta Part B 1976 31 483. 3 Grimm W. Naturwissenschaften 1967 54 586. 5 Grimm W. Spectrochim Acta Part B 1968 23 413. 6 Outred M. and Hammond C. B. Physica Scripta 1976 14 81. 7 Outred M. and Hammond C. B. J. Phys. D. 1980 13 1069. Paper 31050551; Received August 20 1993 Accepted November 5 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900381

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 385 Adaptation of a Glow Discharge Mass Spectrometer in a Glove-box for the Analysis of Nuclear Materials* Maria Betti Gert Rasmussen Tania Hiernaut and Lothar Koch Commission of the European Communities Joint Research Centre Institute for Transuranium Elements Postfach 2340 76125 Karlsruhe Germany Dafydd M.P. Milton and Robert C. Hutton Fisons Instruments VG Elemental Ion Path Road Three Winsford Cheshire UK C W7 3BX A VG9000 glow discharge mass spectrometer has been modified for the direct analysis of solid nuclear samples within a glove-box environment. Because containment is needed for the analysis of this kind of material the glove-box encloses all parts of the instrument that come into contact with the sample namely the ion source chamber sample interlock and associated pumping system. External modifications eliminate outside contamination by the fitting of absolute filters on all source supplies.Internally the design of the ion source has been altered to minimize the number of operations performed inside the glove-box thereby simplifing operation and routine maintenance. These modifications retain the ion extraction and focusing properties of the instrument. The data presented show that there is no compromise in the analytical performance of the instrument when placed in the glove-box. Data representative of nuclear materials is also shown. Keywords GIow discharge mass spectrometry; glove-box; nuclear material Glow discharge (GD) sources have a long history in analytical chemistry principally as sources for optical emission spec- tr~metry.’-~ Their advantages are related to direct solid sam- pling stable output and possibilities for depth profile and thin layer analysis.More recently GD sources have been coupled with mass spectrometry (GDMS) and the combination of these two established methods have made available to the analytical chemist a new exciting technique for the analysis of solid samples.46 The advantages of MS over conventional optical techniques lie in the increased sensitivity obtained by direct sampling of the ions and a much simpler spectrum which makes possible the determination of a wider range of elements. This has also been accompanied by much simpler quantification. Plasma sources coupled with mass spectrometers have thus dramati- cally extended both elemental coverage and detection limits in quantitative trace element analysis.This is true both of GDMS and the inductively coupled plasma (ICP) MS techniques. Glow discharge mass spectrometry has found widespread use in the determination of trace elements in a wide range of inorganic materials.”” Conducting samples such as metals alloys and semiconductors can be analysed directly by a d.c. GD. Insulating samples require indirect means to convert them into a conducting f0rm~9~ or the use of an r.f. powered GD”l’ for direct analysis. A priori the GD mass spectrometer is an analytical tool which would be ideally suited for the analysis of solid samples of nuclear origin requiring minimum chemical treatment. The elemental and isotopic capabilities of GDMS could be fully applied to materials having non-natural isotopes and/or non- natural isotopic abundances.Other advantages of GDMS are ( i ) virtually all elements can be determined; ( i i ) the wide dynamic range of the detector allows the determination of both major components and trace constituents within the same analytical cycle; and ( i i i ) decoupling of the atomization and ionization processes results in uniform sensitivities for many elements and also minimum matrix effects. For the analysis of nuclear materials difficulties arising from * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) Post-Symposium on Analytical Applications of Glow Discharge in Optical and Mass Spectrometry York UK July 4-7 1993.the radioactive nature of the sample have to be overcome. Firstly the operator has to be protected from the material which means that the use of glove-boxes (alpha beta protec- tion) and/or hot cells (alpha beta and gamma protection) with master-slave manipulators is a necessity. Secondly in order to avoid contamination of the working area the analytical instruments have to be modified in order that containment is assured and no radioactive material leaks either into the laboratory or into the environment. Complete instruments cannot be introduced into a glove-box because electronics are very sensitive to radiation. In practice electronics and parts that might need special maintenance are kept outside the glove-box and only samples and the corresponding sampling stage are contained in the box.From the experience gained from other instrumentation,12 used for GDMS measurements the glove-box should enclose the ion source chamber sample interlock and associated pumping system. All supplies to the ion source (argon discharge support gas and liquid nitrogen for cryogenic cooling of the discharge cell) and pumping ports should be fitted with absolute filters to eliminate any external contamination. The ion source itself has been designed to minimize the number of operations and to simplify routine maintenance inside the glove-box area. This has been achieved through the use of simple plug-in components and by reducing the number of screws. In this paper a modified GDMS instrument in a glove-box for the analysis of radioactive materials of nuclear origin is described and some preliminary results are reported.Experimental Instrumentation Mass Spectrometer The VG 9000 glow discharge mass spectrometer has been described in detail previo~sly.’~ The instrument consists of a GD ion source coupled to a double-focusing mass spectrometer of reverse (Nier-Johnson) geometry. This provides high trans- mission and sensitivity whilst operating at high resolving power (typically 5000 10% valley definition > 75% trasmission). Ion detection is accomplished by means of a dual detection system comprised of a Faraday cup for the measurement of large386 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Detectors Water cooling system Fig. 1 Schematic diagram of the installation of the discharge source housing in the glove-box (typically > Daly detector14 for the detection of lower signals.A) ion currents and a transverse mounted Source housing For the analysis of nuclear samples the GD ion-source housing and the associated pumps are placed within a glove-box. The glove-box is provided with an extraction system and filters for all gas lines. Absolute filters (> 99.99% efficiency Solfiltra Lagarenne France) are situated both inside and outside the glove-box which is kept at a lower pressure than the exterior (20mm water column). In Fig. 1 a schematic diagram of the installation of the GD source housing in a glove-box is given showing all connections to the glove-box. Stainless-steel filters ( 3 pm porosity) are installed in the argon compressed air and liquid nitrogen inlets and absolute filters (0.3 pm porosity) in the vacuum line to prevent any radioactive contamination.The cryogenic pump installed in the glove-box is shown in Fig. 2(a) and the absolute filters connected with the pump in Fig. 2(b). In Fig. 3(a) the instrument is shown in the Institute’s workshop after the first stage of its installation in the glove- box and in Fig. 3(b) in the hot lab after the closing of the glove-box for the handling of radioactive material. fb) Ion source The probe and sample interlock have been shortened to give easier access to the ion source within the glove-box. The source chamber door opens on a slider mechanism to give easier access to the ion source. The Wilson-seal can assembly provides a vacuum interlock region so that sample change over can be performed with the ion source maintained at high vacuum (Fig.4). The ion source itself has been designed to minimize the number of operations performed within the glove-box area and t o simplify routine maintenance. This has been achieved by utilizing a ‘universal’ cell for the analysis of both Pin and flat samples and a ‘plug-in’ focus stack. The source itself Fig.2 (a) Cryogenic pump installed in glove-box under the GD source housing. (b) Absolute filters installed for the containment of contamination right side inside glove-box; left side outside glove-boxJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 (4 387 Fig. 4 Modified probe and sample interlock Fig. 5 Modified source and its housing chamber Fig.3 (a) Complete view of the instrument in glove-box during the installation in the workshop.(b) Final view of the instrument installed in glove-box after closing the glove-box for nuclear material handling (Fig. 5) is split up into various components comprising a mounting plate with removable cell and focus stack assemblies. The source mounting position remains affixed to the back wall of the source housing chamber. The focus stack then plugs into a recess in the mounting plate and is held in place by four fixing rods. Electrical contact to the plates of the focus stack is made by a series of copper-beryllium contacts. This eliminates the need to disconnect any wires when removing the focus stack thus simplifying its removal. The contact assembly is connected to the high tension feed-through by kapton-coated wire (Dupont France).The focus stack assembly (Fig. 6) consists of a series of tantalum plates separated by PEEK spacers mounted onto a base containing the source-defining slit for the mass spec- trometer. The focus stack provides deflection and focusing of the ion beam in the y- and z-directions to give the best object on the source defining slit. The plates are shaped so that when Fig. 6 Modified focus stack assembly the focus stack is in position they make electrical contact with the appropriate connector on the contact assembly. The focus stack assembly also contains a mounting bracket for location of the cell and sample holder. The ‘universal’ cell has been designed to accommodate a range of pin and flat samples.The cell itself consists of a universal body that plugs into the focus stack. This cell body based around the existing flat geometry,” then remains located388 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 in position. The ‘universal pin holder’ can be used for the analysis of a wide range of pin samples. In Fig. 7(a) and (b) a sample pin assembled in the holder and the different parts used for the assembling respectively are shown. The use of a combination of cathode plates (A) anode chamber bodies (B) and sample chucks (C) allows analysis of pin and rod samples (D) up to 7 mm in diameter. With the appropriate sample holder combination the sample is held in position in the chuck with a screw. The chuck is then placed in the sample holder with the tapered end mating with the appropriate cathode plate.The spring ensures that a tight reproducible gas seal is maintained. Electrical insulation between the anode and cathode components is achieved by using a ceramic ring and nylon screws (F) are used to provide a gas-tight seal. The analysis of flat samples is performed using the flat sample holder as described previo~s1y.l~ In Fig. 7(c) the differ- ent parts used for assembling the flat sample in the holder are shown in detail. The sample (A) is held in the holder against an insulating disk (B) to provide electrical isolation from the front plate (C) of the sample holder which is at anode potential. Sample size can vary from approximately 10 to 38 mm in diameter and from wafer thin to a thickness of 20mm.To cope with different sample sizes a range of front plates and insulating discs are available. In Fig. 7(c) for sample (A) the points where the sputtering took place can also be clearly seen. Changing the sample geometry from pin to flat simply requires changing to the appropriate sample holder when loading the sample. It is no longer necessary to break vacuum 1.0 do this thus reducing the number of operations required inside the glove-box. Materials The argon discharge gas (BOC 99.9999%) enters the discharge cell via a heated getter inlet system (SAES GP50). The pressure is regulated using a leak valve (Fisons Cheshire UK) and monitored using an ion gauge situated above the cryogenic pump [Edwards Coolstar 1500 (Edwards Crawley Sussex UK)] serving the source housing. The discharge cell is cooled using a flow of liquid nitrogen to reduce background gases such as water vapour.A certified reference material CRM 115 depleted uranium metal (uranium and uranium-235 standard) obtained from New Brunswick Laboratory (US Department of Energy) was used. Once optimization of the discharge parameters had been per- formed a discharge voltage of 0.9 kV with a corresponding cur- rent of 0.6 mA was used throughout unless otherwise specified. Fig. 7 (a) Pin sample mounted in the proper holder; and (b) the different parts used for the assembling A cathode plate; B anode chamber body; C sample chuck; D pin or rod sample; E ceramic ring; F nylon screws; G spring; and H assembly holder (c) Flat samples together with the proper holder A samples; B insulating disk; C front plate of the sample holder (anode); and D loaded sample fixationJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 389 Table 1 Comparison of GDMS instrumental specifications at various stages of its installation in a glove-box A before modifications and installation in glove-box; B after modifications and before installation in glove-box; and C after modifications and after installation in glove-box Specification Analyser vacuum/10-8 mbar Mass resolution A1 cu Au Matrix current/lO-" A Transmission (%) Stability (10 min) (RSD Yo) Stability (30 min) (RSD Yo) Peak position stability (ppm) Mass calibration (mmu)* Mass marking? (ppm) Ion counting efficiency (Yo) Abundance sensitivity (ppm) Reproducibility (RSD Yo) Major Minor Traces Major Minor Traces N C 0 Accuracy (RSD YO) Gas background (ppm) A 3.0 5384 4562 4139 70 4.9 2.94 4.22 9 7 234 76.0 0.43 0.8 1.2 3.5 1 .o 4.0 3.9 0.5 0.3 0.5 B 1.8 6940 6626 5466 75 6.2 2.52 3.87 13 8 100 80.7 0.38 1.6 2.6 1.8 2.5 2.7 2.1 4.3 3.6 0.7 C 2.4 6174 5624 6744 67 8.4 1.32 4.68 24 5 180 76 0.2 1 1 .o 3.7 5.2 1.5 4.4 2.8 0.4 0.9 2.4 * mmu = millimass unit.t Precision of repeat calibrations. The samples were pre-sputtered prior to analysis for 2 min with a 1 mA current. Results and Discussion GDMS Performance at Various Stages of Its Installation in the Glove-box As described above for the installation of the GDMS in a glove-box many modifications of the source housing and of the ion source were necessary. Particular attention was given to ensure that the instrumental specifications would not be affected and in the case that some of the specifications changed it was important to know to what extent the modifications influenced the instrumental performances.From this point of view the instrumental specifications of the GD mass spectrometer were checked throughout the technical modifications required for its installation in the glove- box and the performance of the spectrometer was found to be unaffected. In Table 1 the instrumental specifications tested at various stages are summarized. None of the parameters checked appeared to vary to a significant extent in the three stages tested particularly the specifications that might be expected to be directly affected by the modifications like source vacuum mass resolution matrix current transmission and gas background.Isotope Abundance Studies in Uranium Metal The isotopic analysis of uranium is important to establish the enrichment of the sample and thus its potential use as a nuclear fuel. Of primary interest is the 235U:238U ratio. In this investigation isotopic abundances were measured in uranium metal samples. A certified reference material (CRM 115) uranium metal depleted in 235U was measured. Three samples of this standard were analysed. The results of these measurements are shown in Table 2 where the signal intensities (expressed in amperes) measured for the three isotopes of uranium 234U 235U and 238U are reported. Each result represents the mean of ten measurements with each measurement lasting 5 s.The grand mean (in amperes and in concentration units) RSD CRM values and bias are shown at the base of each isotope column. The internal precision [% standard error (SE)] reported in parentheses for each isotope in each sample describes the reproducibility of each measurement of the individual isotopes tested for a single sample and is defined as %SE(l~)=lOO/xm{[ i = l n (xm-xi)2]1'2}/n(n-1) where x is the mean value and n is the number of measure- ments performed. The RSD values (external precision) reflect the reproduc- ibility of the isotopic concentration measurements from differ- ent specimens of the same original sample. This reproducibility is typically 0.2% or better even for 234U at a measured concentration of 8.26 ppm. This suggests that GDMS could be used to measure isotopic abundances at trace levels.In Table 3 results obtained for another uranium metal sample are shown. The abundance value measured for 235U in CRM 115 was used to produce a mass bias correction factor. This factor was used to correct the 235U abundance in a uranium sample with unknown isotopic composition. This correction gives an abundance which is accurate to within 0.12% of the natural abundance. No such correction was performed for the 234U isotope. From the data presented in Tables 2 and 3 it is clear however that CRM 115 is also depleted in 234U. Both CRM 115 depleted uranium and the natural uranium metal samples were analysed in the flat sample form. The RSDs obtained for both samples over 10 and 5 runs respectively indicate very good stability of the discharge and homogeneity of the sample surface.Comparison With Other Techniques The depleted uranium sample (CRM 115) was also analysed using isotope dilution analysis mass spectrometry (IDMS) and ICP-MS. Table 2 Analysis of CRM 115 depleted uranium reference sample. Signal intensities in amperes; values in parentheses are the SE (YO) Sample 1 2 Mean A Mean (concentration) CRM 115 value (%) Bias (YO) RSD (Yo) 234u 4.626 x (1.8) 4.638 x (1.6) 4.630 x (1.7) 4.6314 x 0.132 8.26 ppm Not certified Not applicable 235u 1.072 x (1.23) 1.075 x (0.88) 1.070 x (1.0) 1.0724 x 0.235 O.1973Yo 0.2008 (7) - 1.7 2 3 8 ~ 5.253 x lo-'' (0.7) 5.273 x lo-'' (0.4) 5.273 x lo-'' (0.6) 5.266 x lo-'' 0.219 99.770% 99.7762 -0.006214390 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 - 2.1 ~ 0 1 c a 7 1.5 tA 1.2 2 0.9 - -. +-' .- . - - 0.6 0.3 - - Table 3 Analysis of uranium metal sample. Isotope values expressed in concentration units \ Measurement Mean SD RSD (%) Literature datai6 Bias (%) 234u (PPm) 57.950 59.090 59.150 59.330 59.33 1 59.9702 ppm 0.5803 ppm 0.98 9 55 PPm 235U (mass-%) 0.703 1 0.7064 0.7070 0.7145 0.7122 0.7086% 0.005 Yo 0.65 0.72% - 1.6 238U (mass-%) 99.2730 99.2745 99.2736 99.2729 99.2740 99.2736% 0.0007% 0.0007 99.2745% - 0.0009 When analysing CRM 115 by IDMS an accuracy of 0.1% was achieved. This represents an improvement in accuracy over GDMS mainly owing to the fact that an internal standard is used. The accuracy of GDMS is improved when the mass bias is accounted for.When this correction is made the accuracy obtained by GDMS for the natural sample is 0.12%. Without this correction the precision and accuracy of GDMS is comparable to that measured using ICP-MS. The major advantage of GDMS is the very quick and straightforward sample preparation. Both IDMS and ICP-MS require sample dissolution dilution and for IDMS further preparation of spiked aliquots. The GDMS method simply requires a flat surface of metal. As for handling of radioactive samples all techniques require the use of a glove-box. Consequently the ability of GDMS to perform direct analy- sis upon uranium provides an accurate and rapid isotopic screening process in preliminary investigations. Trace Element Determination in Uranium Metal The VG9000 is capable of analysis over the whole mass range and can thus be used to determine the concentrations of many other elements such as the transition metals and rare earth elements.Trace element analysis in uranium metal is important in the specification of the material. In Table 4 the elements observed in CRM 115 are listed. After the rapid complete survey the sample was analysed only for the trace elements observed. Among the elements listed in Table 4 some of particular importance are those elements with high neutron capture Table4 Trace element analysis of CRM 115 uranium metal. Concentrations expressed in ppm Element B Na Mg A1 Si Cr Mn Fe Ni c u Zr Mo Ru Ag I Pb Th Measured concentration 0.021 0.064 11.0 19.6 43.3 12.4 73.8 15.0 6.5 0.070 0.56 0.11 0.85 0.044 0.062 0.17 17.7 Standard deviation 0.001 0.003 0.3 0.5 1 .o 0.007 0.4 1.2 0.4 0.3 0.003 0.02 0.006 0.03 0.008 0.2 0.002 RSD (Yo) (n = 5 ) 5.9 4.0 3.1 2.7 2.2 3.7 3.1 1.6 2.6 4.7 4.3 3.5 5.5 3.8 17.4 1.3 3.5 2.4 (a) I -.10.008 10.010 10.012 10.014 10.016 10.018 1 4A v) c .- 11.004 11.006 11.008 11.010 11.012 11.014 11.016 m/z Fig. 8 GDMS spectra for (a) loB and (b) "B isotooes obtained in the analysis of an uianium metal sample.' honcentraiion of boron 20 PPb was 51.92 51.94 51.96 d Z Fig. 9 GDMS spectrum obtained when analysing 52Cr (170 ppb) in a uranium metal sample in the presence of carbon (540 ppm). The peak on the right is due to 40Ar'2C+; the mass resolution necessary to resolve "Cr from 40Ar'2C+ is 2400 cross-sections such as lithium boron cadmium etc. for which a precise and accurate determination is required.Boron for instance cannot be easily measured in diluted solutions at ppb levels (which correspond to ppm levels in the solid) by ICP-MS because of low ionization efficiency low sensitivity and blank contamination. In GDMS boron can easily be detected at low concentration levels directly in the solid sample. In Fig. 8 the peaks of loB [8(a)] and llB [8(b)] isotopes are shown measured on the same sample at a concentration level of 20 ppb in uranium matrix. Another example showing the power of GDMS for trace detection is reported in Fig. 9. Here the peak of "Cr which is often requested to be analysed as an impurity in uranium isJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 391 very well separated from the 40Ar'2C+ peak.The sample contained 540ppm of C and 52Cr has been determined at a level of 170 ppb. For the determination of trace elements in uranium metal five different runs were performed. In Table 4 the mean concen- tration values and the internal precision obtained over the five runs are reported for all elements found. The precision in the analysis is typically 5% RSD or less even at the ppb level. The exception is iodine which may be inhomogeneously distributed in the sample. Detection Limits In GDMS the detection limit depends on the number of points acquired the integration time and the resolution used. The data relevant to the elements of Table 4 were collected by acquiring 200 points for an integration time of 100ms for the Daly detector and using high resolution (5000) to overcome potential molecular and isobaric interferences.This resulted in typical detection limits of 3-10 ppb for most elements. Using low (1500) resolution acquiring the same number of points and for the same integration time as for high resolution acquisition the detection limits can be improved. The detection limit for thorium in uranium determined in this way was 0.2 ppb. Conclusions The modifications made to the GD source of a VG9000 GD mass spectrometer for its installation in a glove-box did not alter the instrumental specifications significantly. Elemental and isotopic capabilities of GDMS can be successfully applied to the characterization of samples of unknown isotopic com- position such as uranium metal. The GDMS technique has a precision and accuracy comparable to ICP-MS and for preliminary investigation the GDMS technique can give information on the isotopic abundance composition faster than IDMS.UP 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 The authors acknowledge warmly the workshop staff of the Transuranium Institute and Mr. Dockendorf Mr. Schrodt and Mr. Ougier for their fruitful technical support in setting _ - the instrumentation in the glove-box. - References Denoyer E. Van Grieken R. Adams F. and Natusch D. F. S. Anal. Chem. 1982 54 26A. Koch K. H. Spectrochim. Acta Part B 1984 39 1067. KO J. B. Spectrochim. Acta Part B 1984 39 1405. Fang D. and Seegopaul P. J. Anal. At. Spectrom. 1992 7 959. Jakubowski N. and Stuewer D. J. Anal. At. Spectrom. 1992 7 951. Shimamura T. Takahashi T. Honda M. and Nagai H. J . Anal. At. Spectrom. 1993 8 453. King F. L. and Harrison W. W. Mass Spectrom. Rev. 1990 9 285. Harrison W. W. in Inorganic Mass Spectrometry eds. Adams F. Gijbels R. and van Grieken R. Wiley New York 1988. Coburn J. W. and Kay E. Appl. Phys. Lett. 1971 18 435. Donohue D. L. and Harrison W. W. Anal. Chem. 1975,47,1528. Duckworth D. C. and Marcus R. K. Anal. Chem. 1989,61,1879. Garcia Alonso J. I. Thoby-Schultzendorff D. Giovannone B. and Koch L. J. Anal. At. Spectrom. 1993 8 673. Cantle J. E. Hall E. F. Shaw C. J. and Turner P. J. Int. J. Mass Spectrom. lon Process. 1983 46 11. Daly N. R. Reu. Sci. lnstrum. 1960 31 264. Milton D. M. P. Hutton R. C. and Ronan G. A. Fresenius' J. Anal. Chem. 1992 343 773. De Bievre P. and Barnes I. L. Int. J. Mass Spec. Ion Proc. 1985 65 211. Paper 3/05230C Received August 31 1993 Accepted November 2 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900385

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 393 Determination of Cadmium and Zinc Isotope Ratios in Sheep's Blood and Organ Tissue by Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry* Invited Lecture D. Conrad Gregoire Geological Survey of Canada 607 Booth Street Ottawa Canada KIA OE8 Julian Lee New Zealand Pastoral Agricultural Research Institute Ltd. Fitzherbert West Private Bag 7 7008 Palmerston North New Zealand A method is described for the determination of Cd and Zn isotope ratios in sheep's blood and organ tissue. Samples were digested with nitric acid using a microwave oven. Cadmium and Zn were separated from matrix components using adsorption chromatography prior to isotope ratio measurement by electrothermal vaporization inductively coupled plasma mass spectrometry.A concentration factor of 35 was achieved. Limits of detection for the determination of Cd and Zn in blood were 0.34 and 0.40 pg g-' respectively. Cadmium isotope ratios ("'Cd '06Cd; "'Cd '"Cd) were determined with a precision of 2-3% for both peak height and area count measurements. Zinc isotope ratios (68Zn 67Zn; 68Zn 66Zn) were determined with a precision of 2% for peak height measurements and 1% for peak area count measurements. Keywords Inductively coupled plasma mass spectrometry; electrothermal vaporization; isotope ratios; blood liver and kidneys A characteristic feature of Cd metabolism in animals is its poor homeostatic control and accumulation in the kidneys and liver. This may pose a potential problem since these organs cannot be exported when their Cd concentration exceeds 1 mg kg-' of fresh tissue.The Cd concentration in New Zealand soils and pastures (which are naturally low) has gradually increased as a result of the regular and extended use of imported phosphatic fertilizers containing varying trace amounts of naturally occurring Cd. Cadmium is a non-essential metal which because of its toxic properties has been widely studied in small animals and humans. However the few studies reported for ruminants are based on the intake of Cd at concentrations much higher than those encountered by grazing animals. The main impediment to the completion of more realistic studies involving whole body infusion of enriched stable isotopes has been the lack of adequate analytical techniques capable of measuring Cd iso- tope ratios in small samples at concentration levels below 0.2ng g-'.Pool sizes of Cd in most tissues especially blood are low and therefore it is desirable that additions of enriched isotopes in this study lo6Cd be kept at low levels to minimize changes to physiological concentrations. This imposes further constraints on analysis. In recent years inductively coupled plasma mass spec- trometry (ICP-MS) has been applied to human nutrition and metabolic studies. Serfass et a1.4 reported that Zn isotope ratios in human faecal material could be determined with a precision of better than 1 %. Janghorbani and co-worker~~-'~ have reported extensively on the determination of isotope ratios of several elements in biological materials by ICP-MS including blood and faeces.Isotope ratios of Fe,5-8 Cu,6v8 Zn,6*8 Br9 and Lila were determined with a precision ranging from 1 to 2%. Delves and Campbell" have reported a precision of better than 0.5% for the determination of major isotope ratios of Pb in human whole blood. More recently Viczian et a1.12 deter- mined Pb isotope ratios in blood and environmental materials * Presented in part at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium 5th Surrey Conference on Plasma Source Mass Spectrometry Durham UK July 4-6 1993. GSC publication No. 17793. to identify potential environmental sources of childhood lead poisoning. Smith et determined B isotope ratios in a variety of biological samples with a precision ranging from 0.4 to 1.5% depending on B concentrations.In ruminants Cu and Se metabolism has been recently studied using stable isotope tracer methodology and ICP-MS.14*15 Solution nebulization (SN) as a means of sample introduc- tion in ICP-MS requires from 5 to 15 ml of sample solution for the accurate and precise determination of isotope ratios. Although the use of a direct injection neb~lizer'~ significantly reduces the volume of sample solution required (0.5-1 ml) these volumes can still be too great for applications involving scarce samples such as blood and/or analytes such as Cd which occur at ppb to sub-ppb concentration levels in blood and tissue. Electrothermal vaporization (ETV) requires only microlitre volumes of sample solution and allows for the determination of analytes at parts per trillion (ppt) concentration I e ~ e l s .~ ~ ~ ' ' This high sensitivity is essential to the successful determination of Cd isotope ratios an element whose concentration is in the ppt range in blood. Microlitre sample volumes enable precon- centration techniques to be used even on millilitre volumes of blood or gram amounts of tissue. This paper reports on the application of ETV-ICP-MS to the determination of Cd and Zn isotope ratios in blood and tissue samples. Samples were obtained from several sheep which were part of a lo6Cd and 67Zn whole-body infusion study." Sheep were continuously infused for several days with enriched stable isotopes at the rate of 1.5 pg h-' for lo6Cd and 30pg h-l for 67Zn. At regular intervals blood samples were withdrawn by syringe for analysis. Although Zn occurs in blood at ppm concentration levels and its isotope ratios have been successfully determined using SN-ICP-MS this element was included in this study in order to compare the performance of ETV- and SN-ICP-MS for the determination of Zn iso- tope ratios.Experimental Sample Collection and Preparation Sheep aged 6-8 months were housed indoors in metabolism crates and fed cut ryegrass from overhead feeders at hourly394 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 intervals. After a period of adaption to the diets stable isotopes [ 80.8 Yo Io6Cd and 9 1.1 YO 67Zn (Isotec Miamisburg OH USA)] in sterile physiological saline were infused into one side of a bilateral catheter. At regular intervals blood samples were withdrawn by syringe from the other side of the catheter into chilled polypropylene tubes containing 0.1 ml of 1667 nkat ml-' of heparin (checked for Cd and Zn impurities).Whole blood samples chilled to 4 "C were centrifuged for 15 min at 2000g to separate plasma from red cells. Tissue samples were obtained following the slaughter of the animals and freeze-dried prior to analysis. All samples were stored at - 10 "C prior to final processing. Plasma or red cells (7 g) were wet digested with a minimum volume (10 ml) of concentrated HNO (Aristar). Digestions were performed in a Milestone MLS 1200 microwave oven system. The digest was evaporated to near dryness on a hot- plate and the sample was then made up to 10 ml with 1% HNO,. Liver and kidney samples were prepared in an ana- logous manner using 0.4 g of freeze-dried sample.Prior to the determination of Cd and Zn isotope ratios by ETV-ICP-MS both analytes were separated from matrix com- ponents using off-line adsorption chromatography. The acidity of the prepared blood or organ tissue digest was adjusted to a pH of 6.0 with ammonia solution (Aristar) before being passed through a 1 ml column containing silica-immobilized 8-hydro~yquinoline.'~ The resin was eluted with 10 ml of 1 moll-' HNO,-O.l moll-' HCl and evaporated to dryness in the microwave oven. The residue was dissolved in 0.2 ml of 1% HNO and divided into two 0.1 ml portions. The first portion was used for the determination of Cd isotope ratios and the second was diluted to 5 ml (or more) for the determi- nation of Zn isotope ratios.For the determination of Cd isotope ratios a preconcentration factor of 35 was achieved increasing the concentration of Cd in solution from approxi- mately 0.2 ngg-' in plasma to 7 ngg-' in the analytical sample. The much higher concentration of Zn in plasma (0.7-1.0 pg g-') and in whole blood (4-5 pg g-') required dilution of the concentrates before measurement of the isotope ratios was possible. Both Cd and Zn levels in organ tissues are at much higher concentrations compared with blood levels and these samples also required dilution prior to analysis by either ETV- (Cd Zn) or SN- (Zn) ICP-MS. Reagent blanks were treated as samples and taken through the entire digestion and preconcentration steps. Recoveries of Cd and Zn for the overall procedure including digestion and adsorption chroma- tography (each column separately assessed) steps were deter- mined using spikes (1 ng ml-' of Cd and 2 pg ml-' of Zn) prepared from stock standard solutions (Spectrosol Merck Poole Dorset UK).Recoveries were 111 & 18% for Cd and 95 f 10% for Zn (n = 30) for inter-batch analyses. The ICP-MS signal intensities for Cd and Zn in reagent blanks were low accounting for about 1% of the sample signal intensity. Blank intensities for analyte isotopes were subtracted from sample intensities prior to calculation of isotope ratios. Instrumentation A Perkin-Elmer SCIEX Elan 5000 ICP mass spectrometer equipped with an HGA-600MS electrothermal vaporizer and Model AS-60 autosampler was used for multi-isotope mass spectral analysis.The experimental conditions for both the Elan 5000 and the HGA-600MS are given in Table 1. Optimization of plasma and mass spectrometer conditions was accomplished using solution nebulization sample introduc- tion. The HGA-6OOMS was interfaced to the argon plasma via a 0.8m length of 6mm (i.d.) Teflon tubing. Operation of the HGA-600MS was completely computer controlled. During the drying and charring stages of the temperature programme opposing flows of argon gas (300 ml min-') originating from both ends of the graphite tube removed water and other vapours through the dosing hole. During the high temperature Table 1 Instrumental operating conditions and data acquisition parameters ICP mass spectrometer- R.f. power/W 1000 Outer argon flow rate/l min-' Intermediate argon flow rate/ml min - ' Carrier argon flow rate/ml min-' Sampler/skimmer Nickel 15.0 850 900 HGA-600MS electrothermal vaporizer- Sample volume/pl 20 Drying stage (10 s ramp) 90°C for 40 s 300 Charring stage (3 s ramp) 300°C for 10 s 300 Vaporization temperaturerc 2300 Internal argon flow rate/ml min-' Internal argon flow rate/ml min-' Heating rate/"C s-' 2000 Time at maximum temperature/s 5 Data acquisition- Dwell time (ETV)/ms Dwell time (SN)/ms Measurements per isotope (SN) Scan mode Points per spectral peak Isotopes monitored per measurement cycle Signal measurement mode Cd isotope ratios Zn isotope ratios 20 400 100 1 2 Peak-hopping Intensity maximum Integrated signal pulse or vaporization step a graphite probe was pneumatically activated to seal the dosing hole.Once sealed a valve located at one end of the HGA workhead directed the carrier argon flow originating from the opposite end of the tube directly to the argon plasma at a flow rate of 900ml min-'. Pyrolytic graphite coated graphite tubes were used throughout. Procedure for Measurement of Isotope Ratios Although the isotope ratios of both Cd and Zn could be measured during the same vaporization cycle the isotope ratio for each element was determined separately. This was done to reduce the cycle time between the intensity measurements of the two isotopes for each analyte. The fast transient nature of the ETV-ICP-MS signal requires a relatively short duty cycle to ensure good precision of the isotope ratio measurement.Fig. 1 illustrates the transient nature of the ETV-ICP-MS signal obtained for the vaporization of 200 pg of Cd and 50 pg of Zn. Using a dwell time of 20 ms (Table 1) approximately 40 intensity readings were obtained for each isotope during the high temperature vaporization step. Isotope ratios could 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Time/s Fig. 1 ETV-ICP-MS signal pulses for 200 pg of Cd and 50 pg of ZnJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 395 be calculated using either peak height (maximum intensity) or peak area (integrated) counts. Except where otherwise noted isotope ratios are reported as the mean of five separate measurements. For ETV-ICP-MS determinations the addition 10 pl of NASS-3 (diluted 1 in 500) as chemical modifier was added to both standard and sample solutions.The reference material NASS-3 Open Ocean Seawater is available from the National Research Council of Canada (Ottawa). The addition of this solution provides 0.7 ng of salt containing Na Cl Ca Mg and Sr which act as a physical carrier,l7 ensuring efficient transport of vaporized analyte from the graphite tube to the argon plasma. Prior to dilution and use the NASS-3 sea-water was purified of any traces of Cd and Zn by adsorption chromatography .Ig For SN-ICP-MS isotope ion intensities for three Zn isotopes were monitored during the same measurement cycle. A measurement cycle consisted of 100 separate sequential measurements (dwell time = 400 ms) for each analyte isotope. Isotope ratios were calculated from the mean of the intensities obtained from five such measurement cycles.Instrumental mass discrimination for both ETV and SN work was generally less than the precision of the isotope ratio measurement itself. Correction for mass discrimination was accomplished using as a reference the mean of ten separate ETV-ICP-MS isotope ratio measurements for a pure aqueous standard of Cd or Zn (High-Purity Standards Charleston NC USA). Comparison of this value with the accepted value for the natural abundance ratio2' was used to calculate the appropriate correction factor. Corrections for mass discrimi- nation for SN-ICP-MS measurements were carried out in an analogous manner using intensities measured during ten separ- ate measurement cycles. Results and Discussion Selection of Isotopes and Analytical Figures of Merit Cadmium has eight stable isotopes ranging from m/z 106 to 116.Of these isotopes only "'Cd (natural abundance 12.86%) is free of isobaric interferences from neighbouring isotopes of other elements and was therefore selected as the reference isotope for Cd isotope ratio determinations. The Io6Cd isotope (1.22%) is isobaric with lo6Pd (27.10%) lo8Cd (0.89%) is isobaric with Io8Pd (26.7%) and "'Cd (12.43%) is isobaric with 'IoPd (13.5%). The '12Cd isotope (23.79%) is isobaric with '12Sn (0.95%) '13Cd (12.34%) is isobaric with '131n (4.16%) '14Cd (28.81 %) is isobaric with '14Sn (0.65%) and '16Cd (7.66%) is isobacric with 'I6Sn (14.24%). For the spike isotope either Io6Cd or Io8Cd was suitable as both have low natural abundances. The Io6Cd isotope was selected as the spike isotope purely on the basis of cost of the enriched stable isotope.Zinc has five stable isotopes ranging from m/z 64 to 70. Of these isotopes 66Zn (27.81%) 67Zn (4.11%) and 68Zn (18.56%) are free of isobaric interferences from isotopes of other elements. The major isotopes of zinc 64Zn (48.9%) is isobaric with 64 Ni (1.16%) and 70Zn (0.63%) is isobaric with 70Ge (20.52%). Interference from the molecular ion 40Ar14N14N+ at m/z 6821 was not observed. Because of its low relative abundance and freedom from is0 baric interferences 67Zn was selected for use as the spike isotope. Although 70Zn has a much lower natural abundance than 67Zn and would therefore be better than 67Zn as a spike isotope the use of this isotope is prohibitively expensive considering the pool size for Zn of an animal with a mass of 50 kg.Table 2 summarizes the analytical figures of merit for the determination of Cd and Zn by ETV-ICP-MS. The blank matrix used to measure the background for each element was 10 pl of diluted purified NASS-3 solution. The relatively high blank obtained for Zn was attributed to Zn contamination Table 2 Analytical figures of merit for the determination of Cd and Zn by ETV-ICP-MS Parameter "'Cd "Zn NASS-3 background (n = 10) 185 3162 Standard deviation 32 191 RSD (Yo) 17.3 6.0 Integrated counts per 100 pg/s 42 0oO 214 000 RSD (Yo n=10) 2.9 1.9 LOD ( 3 4 absolute/pg 0.23 0.27 LOD ( 3 4 relative (20 pl)/pg ml-' 12 14 originating partly from the NASS-3 solution and partly from the graphite tube.This background diminished only slightly during the course of the experiments. The relative standard deviation (RSD) for the measurement of peak area counts for both elements was of the order of 2-3%. The absolute limit of detection for Cd and Zn was 0.23 and 0.27 pg respectively. For a 20 pl sample volume relative limits of detection of 12 and 14pgml-1 were obtained for Cd and Zn respectively. Taking into account a preconcentration factor of 35 the limit of detection in blood and tissue was 0.34pg g-' for Cd and 0.40pg g-' for Zn. For Cd this concentration is about 600 times below natural levels of Cd in sheep's plasma. Measurement of Isotope Ratios Tables 3 and 4 summarize the analytical results for the determination of Cd and Zn isotope ratios in a standard reference solution.Isotope ratios were determined on 200 pg of Cd and 50pg of Zn as nitrate in the presence of NASS-3. Table 3 Precision of Cd isotope ratio measurement (corrected for mass discrimination) by ETV-ICP-MS Run 1 2 3 4 5 6 7 8 9 10 Mean SD RSD (Yo) Integrated Peak height 10.33 10.56 10.75 10.45 10.90 10.27 10.07 10.58 10.94 10.55 10.37 10.06 10.97 10.64 10.69 10.94 9.84 10.38 11.03 10.53 10.54 10.54 0.27 0.39 2.61 3.71 Integrated Peak height 1.013 1.038 1.034 1.033 1.019 1.01 1 1.053 1.057 1.060 1.030 1.007 1.027 1.049 1.012 1.007 1.044 1.05 1 1.065 1.05 1 1.033 1.035 1.035 0.017 0.02 1 1.67 2.03 Table 4 Precision of Zn isotope ratio measurement (corrected for mass discrimination) by ETV-ICP-M S Run 68Zn "Zn 68Zn "Zn 1 2 3 4 5 6 7 8 9 10 Mean SD RSD (Yo) Integrated Peak height 4.527 4.527 4.450 4.541 4.638 4.453 4.483 4.530 4.448 4.565 4.372 4.388 4.462 4.490 4.735 4.425 4.614 4.580 4.509 4.581 4.516 4.516 0.060 0.113 1.33 2.50 Integrated Peak height 0.664 0.665 0.666 0.665 0.663 0.668 0.667 0.669 0.677 0.672 0.658 0.642 0.667 0.667 0.658 0.675 0.678 0.671 0.677 0.673 0.667 0.667 0.006 0.01 1 0.87 1.72396 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Isotope ratios were corrected for mass discrimination and calculated using both peak height intensities and peak area counts. Isotope pairs were selected to include both the reference and spike isotope used in the actual study. A third isotope for each element was also selected such that a comparison could be made of the precision of measurement for isotope ratios ranging from unity to some higher value.For Cd and Zn isotope ratios peak height measurements gave poorer precision than ratios calculated using peak area counts. This is probably due to the uncertainty introduced into the peak height measurement from the sequential measure- ment of isotope intensities. A reduction in the dwell time could provide more readings per signal pulse but the shorter measurement time would degrade the counting statistics. Both elements exhibited the same trend with respect to the measure- ment precision of ratios ranging from 1 to 10. The isotope ratio for ' W d "'Cd (1.035) was measured with a precision that was about 1% better than the precision of the "'Cd lo6Cd (10.54) isotope ratio. For zinc the measurement precision for the 68Zn:66Zn (0.667) ratio was about 0.5% better than the precision of the 68Zn 67Zn (4.516) isotope ratio measurement.Comparison of Tables 3 and 4 shows that in general the Zn isotope ratio can be determined with a better precision than the Cd isotope ratio. Other studies completed in this laboratory17 have shown that the vaporization and transport of Cd is complicated by some factor related to the operation of the electrothermal vaporizer or the vaporization/transport process. Further studies are underway to investigate this problem in detail and to determine if this is the source of signal instability. Tables 5 and 6 summarize results for the determination of 111Cd:106Cd and 68Zn:67Zn isotope ratios in a number of representative plasma red cell and organ tissue samples not necessarily obtained from the same sheep.The determination of the ll'Cd lo6Cd isotope ratio was complicated by a Pd contamination resulting in an isobaric interference from lo6Pd (27.10%). The source of the Pd has not been determined but Table 5 Cd isotope ratios in sheep's blood and organ tissue Sample Plasma (2 h) Plasma (4 h) Plasma (8 h) Plasma (17 h) Plasma (24 h) Plasma (50 hj Plasma (56 hj Plasma ( 120 h) Red cells Kidney Liver 111Cd. 106cd 1.531 f0.028 1.263 & 0.045 0.987 f 0.014 0.758 k 0.045 0.69 1 k 0.003 0.624 f 0.01 8 0.621 0.030 0.61 5 k 0.01 7 1.639 f 0.033 0.313 k0.003 0.070 & 0.001 Table 6 Comparison of Zn isotope ratios in sheep's blood and organ tissue determined by ETV- and SN-ICP-MS Sample 68Zn %n Plasma (2 h) Plasma (4 h) Plasma (8 h) Plasma (17 h) Plasma (24 h) Plasma (42 h) Plasma (66 h) Plasma (72 h) Red cells Kidney Liver ETV 4.000 f 0.076 3.984 _+ 0.056 3.937 & 0.059 3.817 f 0.069 3.650 f 0.073 3.289 f 0.022 3.096 -t 0.062 2.985 -+_ 0.045 4.5 19 f 0.036 3.51 1 f0.046 3.379 f 0.022 SN 4.132 f 0.066 4.049 f 0.065 3.937 k0.020 3.876 0.041 3.636 f 0.028 3.145 f0.033 3.075 fO.010 2.857 k 0.03 1 4.510f0.018 3.533 f 0.026 3.349 k0.014 probably originated from acids used in the sample prep- aration step.Fig. 2 illustrates the vaporization characteristics of Pd rela- tive to that of two Cd isotopes. Clearly Pd is less volatile relative to Cd and is vaporized at a time corresponding to the decaying portion of the Cd analyte signal pulse. For this reason it was decided to calculate Cd isotope ratios for the blood and tissue samples using peak height measurements when only a small correction for lo6Pd isobaric interference was necessary. The correction to the Io6Cd ion count rate corresponded to the intensity of the lo5Pd ( x 1.199 to convert to lo6Pd) ion at the time corresponding to the maximum (or peak time) of the Cd signal pulse.Table 5 shows that the ' W d lo6Cd isotope ratio in blood and organ tissues varied from 0.070 to 1.531. For plasma samples the Cd isotope ratio decreased by 60% during the course of the infusion study. The use of these data to calculate enrichment factors and other indices to assess adsorption tissue entry rates and excretion of Cd and Zn are in progress. A measurement precision of 2% (peak height) for the "'Cd lo6Cd isotope ratio was more than adequate to accu- rately monitor the change in this ratio with time.Table 6 shows that the 68Zn:67Zn isotope ratio in blood and organ tissue varied from 2.985 to 4.519. For plasma samples the Zn isotope ratio decreased by 34% during the course of the infusion study. Although the change in the Zn isotope ratio was much smaller than for Cd a measurement precision of 0.87% (peak area counts) was sufficient to success- fully carry out metabolic studies. Following the measurement of 68Zn 67Zn by ETV-ICP-MS an aliquot of the remaining sample was diluted with distilled deionized water for isotope measurements using SN-ICP-MS. The results given in Table 6 show good agreement between ETV and SN isotope ratio measurements.The precision of the SN-ICP-MS isotope ratio measurement was about 0.6% and was limited by the amount of sample solution available ( 5 ml). Conclusion This work has shown that ETV-ICP-MS can successfully measure isotope ratios of Cd and Zn in blood and tissue samples at naturally occurring concentrations for these elements. The precision obtainable for isotope ratio measure- ments was of the order of 30 to 150 times smaller than the variation in isotope ratios observed during the course of these studies. Allowing 2 min per isotope ratio determination and five replicates per sample approximately 50 samples could be analysed by ETV-ICP-MS during an 8 hour day. The graphite tube could be used for approximately 250 firings before requir- ing replacement.0 0.5 1 .o 1.5 2.0 Timels Fig.2 Comparison of vaporization characteristics for Pd and Cd isotopes in spiked blood sampleJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 397 The loan of the HGA-600MS from the Perkin-Elmer Corporation is gratefully acknowledged. References Smith R. M. Griel L. C. Muller L. D. Leach R. M. and Baker D. E. J. Anim. Sci. 1991 69 4078. Van Der Veen H. G. and Vreman K. Netherlands J. Agric. Sci. 1986 34 145. Wentink G. H. Wensing T. Baars A. J. Van Beek H. and Zeeywen A Bull. Environ. Contam. Toxicol. 1987 39 131. Serfass R. E. Thompson J. J. and Houk R. S. Anal. Chim. Acta 1986 188 73. Janghorbani M. Ting B. T. G. and Fomon S . J. Am. J. Hematol. 1986 21 277. Ting B. T. G. and Janghorbani M. Anal. Chem. 1986,58 1334. Ting B. T. G. and Janghorbani M. Spectrochim. Acta. Part B 1987 42 21. Ting B. T. G. and Janghorbani M. J. Anal. At. Spectrom. 1988 3 325. Janghorbani M. Davis T. and Ting B. T. G. Analyst 1988 113 405. 10 11 12 13 14 15 16 17 18 19 20 21 Sun X. F. Ting B. T. G. Zeisel S . H. and Janghorbani M. Analyst 1987 112 1223. Delves H. T. and Campbell M. J. J. Anal. At. Spectrom. 1988 3 343. Viczian M. Lasztity A. and Barnes R. J. Anal. At. Spectrom. 1990 5 293. Smith F. G. Wiederin D. R. Houk R. S. Egan C. and Serfass R. E. Anal. Chim. Acta 1991 248 229. Koening K. M. Buckley W. T. and Shelford J. A. Can. J. Anim. Sci. 1991 71 175. Buckley W. T. Can. J. Anim. Sci. 1991 71 155. Gregoire D. C. Lamoureux M. Chakrabarti C . L. and Byrne J. J. Anal. At. Spectrom. 1992 7 579. Sturgeon R. E. Willie S. N. Zheng J. Kudo A. and GrCgoire D. C. J. Anal. At. Spectrom. 1993 8 1053. Lee J. unpublished data. Sturgeon R. E. Berman S . S. Willie S. N. and Desaulniers J. A. H. Anal. Chem. 1981 53 2337. De Bievre P. Gallet M. Holden N. E. and Barnes I. L. J. Phys. Chem. Ref. Data 1984 12 809. Tan S. H. and Horlick G. Appl. Spectrosc. 1986 40 445. Paper 310404 7 J Received July 12 1993 Accepted September 15 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900393

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

399 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Removal of Organic Solvents by Cryogenic Desolvation in inductively Coupled Plasma Mass Spectrometry* Invited Lecture Luis C. Alvest Michael G. Minnich Daniel R. Wiederin* and R. S. Houk6 Ames Laboratory US Department of Energy Department of Chemistry low&tate University Ames IA 5001 1 USA Methanol ethanol acetone or acetonitrile were nebulized continuously with an ultrasonic nebulizer. The solvent was removed from the aerosol stream by repetitive heating at approximately 100 "C and cooling in a set of cryogenic loops at -80 "C. The resulting aerosol was then introduced into an inductively coupled plasma mass spectrometer. Ethanol was the only solvent that required a continuous dose of additional 0 ( 1 4 % ) in the aerosol gas to prevent deposition of carbon on the sampler.Oxide ratios for LaO+:La+ and UO+:U + were 0.03-0.1 YO. Cryogenic desolvation attenuated but did not eliminate the usual carbon-containing polyatomic ions (e.g. CO' C02+ Arc+ and ArCO+). Analyte sensitivities from metal nitrate salts in methanol were comparable to the sensitivities from aqueous metal solutions. Substantial memory effects were observed from several metal complexes. Keywords Organic solvents; inductively coupled plasma mass spectrometry; desolvation; polyatomic ions The introduction of organic solvents into an inductively coupled plasma (ICP) is beset with Generally volatile organic solvents overload the plasma so that a high forward power is necessary to stabilize the di~charge.~ Analysis of organic solvents by ICP mass spectrometry (ICP-MS) faces several additional complications.Carbon deposits on the interface numerous polyatomic ions such as CO+ C02+ and Arc' are observed and the sensitivity and detection limits for analyte ions are usually poorer than those obtained when aqueous solutions are nebulized.l0?'l These problems are so severe that many analysts prefer to simply digest organic samples and introduce them as aqueous solutions. When organic solvents cannot be avoided most analysts use only relatively involatile ones such as xylene or isobutyl methyl ketone. In ICP-MS a small dose of O2 is often added to the aerosol gas to prevent carbon deposition1*l2. As might be expected this remedy increases spectral overlap problems from metal oxide ions (MO+).Removal of the solvent after nebulization is one general solution to these problems. Maessen et d5 have examined this remedy thoroughly for ICP emission spectrometry. Cryogenic desolvation at -77 "C was used by Wiederin et ~ 1 . ' ~ for the analysis of organic solvents by ICP atomic emission spec- trometry. Essentially the ICP operated stably at forward power levels of only about 1.0 kW when cryogenic desolvation was employed. Analyte emission sensitivity was similar to that obtained from aqueous solutions and interferences from mol- ecular bands were minimal.13 Cryogenic desolvation has proven very valuable for removal of water and HCl from aqueous samples in ICP-MS.14,15 The current paper extends this work to organic solvents. Hill et di2 have recently described a condensation system based on Peltier coolers for analysis of organic solvents by ICP-MS.They did not cool the aerosol below -4O"C and they noted the probable advantages of removing more solvent by cooling the aerosol at still lower temperatures. The solvents studied in the present work are relatively volatile and are usually considered * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium 5th Surrey Conference on Plasma Source Mass Spectrometry Durham UK July 4-6 1993. t Present address G. D. Searle Searle R&D Building P 4701 Searle Parkway Skokie IL 60077 USA. $ Present address Cetac Technologes Incorporated 5600 S. 42nd Street Omaha NE 68107 USA. 9 To whom correspondence should be addressed.among the more 'difficult' solvents used for introduction into the ICP.16 The behaviour of simple inorganic salts is also compared with that of relatively volatile metal complexes because loss of volatile species or memory effects are possible problems when aerosols are heated. Experimental A Perkin-Elmer Sciex ELAN Model 250 ICP mass spec- trometer with upgraded ion optics and software was used. Typical operating conditions are listed in Table 1. Note that an ultrasonic n e b ~ l i z e r ' ~ ~ ~ ~ was used. Operating conditions were optimized daily to maximize the signal for La+ in methanol. The ion lens voltages required to maximize the analyte ion signals were the same when either the various organic solvents or aqueous solution^'^^^^ were nebulized. The liquid flow rate was 1.5 ml min-'.The cryogenic desolvation system is shown in Fig. 1. This apparatus is similar to the standard condenser provided with the nebulizer. The bulk of the organic solvent became con- densed in the first loops. This condensed solvent was still liquid and was drained off periodically through Tygon tubing under the loops. The aerosol was then heated and cooled repeatedly in a set of copper loops similar to those used for aqueous ~olvents.'~ This process removed much of the remain- ing solvent from the aerosol. Without the heating steps solvent tended to condense back from the vapour phase onto the sample particles in the cold loops; these undesirable wet droplets were then transported readily to the plasma as noted previously by Maessen et aL5 and Wiederin et ~ 1 .' ~ Peak hopping data were acquired in the multi-element mode at low resolution setting ( 1 u width at 10% valley) with three measurements per peak 20 ms dwell time and 1 s measurement time. Spectra were acquired in the sequential mode with ten measurements per peak and a 1 s measurement time. Count rates were not corrected for isotopic overlaps. Chemicals and Standard Solutions High-performance liquid chromatography grade organic sol- vents were used for all experiments (Fisher Scientific Pittsburgh PA USA). Standard solutions were prepared by diluting aliquots from 1000 mg 1-1 aqueous standards (Plasma Chem Farmingdale NJ USA) with methanol. The aqueous standards were supplied as nitrate salts. For comparative purposes stock solutions of metal complexes400 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Table 1 Typical operating conditions _____~ ~ Ultrasonic Nebulizer Current setting (arbitrary units) Desolvation heater temperature Desolvation condenser temperature/"C Cryocooler ICP torch Plasma forward power/kW Argon flow ratefl min-' Outer Intermediate Aerosol Sampling position Sampler Skimmer Ion lens settingsp Bessel box stop Bessel box barrel Bessel box plate Einzel 1 and 3 Einzel 2 Electron multiplier voltagem Cetac Technologies (Omaha NE USA) Model U-5000 6 Dependent on solvent - 10 Cryocool CC-10011 at - 80 "C; Neslab (Portsmouth NH USA) Ames Laboratory designI7; outer tube extended 30 mm from inner tubes 1.25-1.5 12 0.3 1.6 20 mm above load coil on centre Copper 1.1 mm diameter orifice Nickel 0.9 mm diameter orifice - 5.9 + 5.4 - 11.0 - 19.8 - 130.0 - 4000 Heating/cooling coils (CU loops) Kept at \fl? heater temperature Dry -7 + Y Drain Fig.1 Diagram of cryogenic desolvation device -80 "C .(absolute ethanol) with various ligands were also prepared. These analyte com- pounds were selected to represent a range of melting- and boiling-points. Metal acetate stock solutions were prepared by dissolving analytical-reagent grade acetate salts of Zn Co and Mn (Fisher Scientific) at a concentration of 1 mg 1-l in meth- anol. The Co(CO),NO Y(acac) (acac = acetylacetonate) and Nb(OC,H,) (Strem Chemicals Newburyport MA USA) were weighed and dissolved completely in methanol to give solutions that contained 1 rng1-l of the metal. Analyte solutions were then prepared by further dilution of these stock solutions.Results and Discussion General Observations The solvents and analyte compounds studied and their melting- and boiling-points are presented in Table 2. As noted pre- vi~usly'~ the ultrasonic nebulizer produces an extremely intense aerosol from these solvents. Nevertheless the plasma operates stably at moderate forward powers of 1.25-1.5 kW. After ignition the axial channel is punched by simply turning on the aerosol gas flow during nebulization of the solvent The heater temperature was selected to be 40°C above the boiling- point of the solvent as noted previ0us1y.l~ Methanol and acetonitrile could be nebulized indefinitely without noticeable green C2 emission or deposition of carbon on the sampler.Substantial carbon deposition and C emission are observed when ethanol is nebulized. In this case oxygen is added to the aerosol flow through the side arm of a small T-junction ( 5 mm id.) at the base of the torch. The argon carrying the aerosol passes straight through the T-junction into the injector tube of the torch. The additional oxygen burns off the carbon deposited on the sampling cone. Oxygen is bled in gradually until the O2 flow rate is just high enough to remove the green C2 emission. This 0 flow is typically 1-5% of the aerosol gas flow. When acetone was nebulized a small amount of carbon was deposited on the cone. This carbon deposition caused the analyte signal to drift down by roughly 10% per hour which is worse than the drift normally seen.Adding a brief burst of O2 into the aerosol gas flow for approximately 30 s every 2 h removes this deposit. After the 0 burst the analyte signal recovers to its original value within the usual relative precision interval of 1-2%. With cryogenic desolvation continuous addition of 0 was not necessary for the analysis of acetone. The different behaviour seen for these four solvents can be explained based upon their melting-points and on the ratio of carbon to oxygen in each solvent molecule. The cold loops are at - 80 "C some 34 "C below the melting-point of acetonitrile (Table 2). Thus the vapour pressure of acetonitrile at the exit of the cryocondenser is very low. The other three solvents have higher vapour pressures because the loop temperature is above their melting-points. Methanol and acetone have almost the same melting-points (about - 95 "C) but acetone causes more C emission and carbon deposition because it has a 2:l ratio of C atoms relative to 0.Ethanol has a still lower melting- point and a 2:l ratio of C:O. Thus the solvent load out of the condenser is greatest for ethanol and the stoichiometry of ethanol also favours carbon deposition and C2 emission. Background Spectra Generally the mass spectra from an ICP containing organic solvents show the usual major ions (e.g. Ar+ ArH' Ar2+ 0' and H20f) as well as substantial levels of additional polyatomic ions from the constituents of the solvent.l0?l1 The count rates observed for four of the more troublesome polya- tomic ions from each solvent are shown in Table 3.As expected the count rates for these background ions vary with the solvent used since a fixed loop temperature yields different solvent loads. No Cu' from the loops is observed when organic solvents are nebulized. Acetonitrile was unexpectedly found to give the highest level of ArO' even though there is no oxygen in acetonitrile. This was possibly due to the presence of a substantial concentration of Fe' (up to 3 pg l-') which contributes to the background at m/z=56. The background at m/z= 56 in Table 3 is also substantially higher than that seen during cryogenic desolv-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 401 Table 2 Physical data for solvents and solutes and heater temperatures used Solvent Methanol Ethanol Acetone Acetonitrile Solute* Mn( CH3C00)2 * 4H20 Co( CH3COOj2*4H20 Zn( CH3COO) - 2H20 Y (acac) Co(CO),NO Nb(OC3H5)5 Melting-point/"C Boiling-point/"C Heater temperature/"C - 93.9 65 105 - 117.3 78.5 119 - 95.35 56.2 96 - 45.7 81.6 122 > 300 > 270 248 139 6 - ~ ~~~~ * These solutes were introduced as methanol solutions; the heater temperature was 105 "C in each case.Table 3 Count rates for polyatomic ions observed for various organic solvents Count rate/counts s-' Solvent CO + (m/z = 28) CO (m/z=44) Arc+ (m/z=52) ArO+ (m/z=56) Methanol 1.7 x lo6 11 x 103 5.4 x 103 25 x 10' Acetone 3.0 x lo6 48 103 24 x 103 30 x 10' Ethanol 1.5 x lo6 29 x 103 15 x 103 17 x 10' Ethanol + 0 3.7 x lo6 64 x 103 83 x 103 64 x lo2 Acetoni trile 0.12 x lo6 24 x 103 15 x 103 66 x lo2 ation of aqueous aerosol^.^^*^^ When ethanol is nebulized the addition of 0 enhances the signals from all four of these ions even from Arc' which contains no oxygen.Naturally CO' C02' and Arc' are much more intense when organic solvents are nebulized than is the case with aqueous solutions. Background equivalent concentration (BEC) values for '*Si' 44Ca+ 52Cr+ and 56Fe+ are presented in Table4 for methanol as solvent. For each case the BEC is better (ie. a lower value) in the present work than in two earlier attempts at ICP-MS of organic even though methanol is more volatile and thus a more 'difficult' solvent than the xylene and white spirit used in previous studies.l07l1 Despite this improvement the four polyatomic ions CO' CO,' Arc' and ArO' are still abundant enough to cause problems even with cryogenic desolvation at - 80 "C.Analyte Sensitivity and Oxide Formation Solutions containing La and U at 5OOpg1-' in different solvents were nebulized to determine if analyte sensitivities varied among solvents. These two elements form refractory oxide ions so the abundance of metal oxide ions was also measured relative to that for metal ions (M') to evaluate Table 4 Background equivalent concentrations for analytes at m/z values corresponding to CO+ CO,+ Arc' and ArO+ BEC values/pg 1-'* Reference Si + Ca + Cr + Fe+ (solvent) (m/z = 28) (m/z = 44) (m/z = 52 j (m/z = 56) Methanol 8 50 240 3.0 1.2 Xylene'O 12x 103 5oox 103 400 40 White spirit'' -? 8.4x 103 1.2x 103 20 * BEC=solution concentration of analyte required to give a net signal equal to the signal for the polyatomic ion. Concentrations refer to the total amount of the element required to provide the necessary signal at the particular m/z shown i.e.they have been corrected for isotopic abundance. t Signal for CO' was too high to measure in this case. possible problems from spectral interferences caused by MO +. A summary of the data obtained is presented in Table 5. Sensitivities for La and U vary slightly among the solvents with the exception of ethanol where the analyte sensitivities are poorer by a factor of 5. The addition of 1-2% 0 to the central channel while nebulizing ethanol boosts the signal for La' and U+ without a prohibitive increase in the abundance of L a o + and UO'. As expected acetonitrile gives the lowest MO+:M + ratios because it lacks oxygen.Metal carbides (MC') are not observed for either La (< 10 counts s-l net) or U (<20 counts s-l net) even when the methanol sample contains La and U at 100mgl-l. The analyte sensitivities obtained from organic solvents are also similar to those observed from aqueous samples with this particular ICP-MS instrument. 1 4 9 1 5 Organic versus Inorganic Metal Standards The objectives of this experiment were to determine (i) if inorganic- and organic-bound metals had different sensitivities and (ii) if volatile and low-melting organometallic complexes would be lost in the cryogenic loops. Solutions of inorganic Co Zn and Mn ions [e.g. Zn(NO,),] at 500 pgl-' were prepared in methanol and a second set of solutions of Co Zn and Mn acetates at 500 pg 1-1 were also prepared in methanol.The melting-points for the acetates were measured and are shown in Table 2. The measured sensitivities i.e. count rate per unit concen- tration are presented in Table 6. For each metal the sensitivity from the acetate complex is essentially the same as that from the inorganic nitrate. Therefore little or no analyte is lost from the acetate complexes during the desolvation process. These acetate complexes have melting-points of 248 "C or higher. They neither melt nor boil in the heaters which are at 105 "C. Thus the acetate complexes are not volatilized and pass through the desolvation device readily as solid aerosol particles. Three other metal complexes were tested in much the same fashion. Solutions of Co(C03)N0 Y (acac) and Nb(OC,H,) were prepared in methanol with each metal present at a concentration of 100 pg 1- '.Reference solutions were prepared by dissolving Co Y and Nb nitrates in methanol at concen-402 ...,,. JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 5 Signals for La+ U+ and metal oxides (MO') in different solvents* Count rate (counts s-') or oxide ratio (YO) Species 1 3 9 ~ ~ + 2 3 8 ~ + La0 + Lao+ :La+ uo+ uo+ :u+ Methanol Acetonitrile Acetone Ethanol Ethanol + O,? 1.8 x lo6 1.9 x lo6 2.6 x lo6 3.8 x 105 2.1 x 106 6.1 105 8.1 x 105 1.4 x lo6 3.2 x 105 9.6 x 105 766 655 1349 203 1759 583 367 1300 250 733 0.04 0.03 0.05 0.05 0.08 0.1 0.05 0.09 0.08 0.08 * Organic solutions contained 500 pg 1-' of La and U. t Oxygen was added to the central channel at about 2% of the total aerosol gas flow.Table 6 Sensitivity for analyte elements as acetate complexes and as inorganic nitrate salts in methanol Sensitivity/I06 counts s-' per mg I-' Analyte "Mn+ 'j4Zn + 59c0 + Acetate complex Nitrate salt 1.6 1.3 1.1 1.7 1.1 1.4 trations of 100 pg1-I. As shown in Table 7 the sensitivity fo-r Nb in the low-melting Nb(OC2H5) is similar to that for the Nb nitrate salt. The sensitivities for Co and Y in the complex Co(CO),NO and Y(acac) are somewhat higher than the sensitivities for Co and Y as nitrate salts. The Co(C0,)NO boils at 50 "C (Table 2) some 55 "C below the temperature of the heated loops. Likewise Nb(OC,H,) should melt in the heated loops yet the Nb+ signal shown in Table 6 for this complex indicates that it passes through the loops readily.Apparently boiling or melting of the species in these aerosols does not cause a drastic loss of analyte during the desolvation process. Detection limits for Co Y and Nb in either complex or nitrate form are 80 20 and 30 ng l-' respectively. Memory Effects Substantial memory effects are seen from some of the com- plexes. The worst such problem is illustrated by the rinse-out curves in Fig. 2. The Nb' signal when the analyte is present as Nb(NO,) decays to 0.1% of the steady-state level about 60s after the sample is removed as noted previously for various elemenfs.l4 In contrast the Nb+ signal from Nb(OC,H,) never decays to the 0.1% level but stops at a level corresponding to about 2% of the steady-state signal. Similar though less severe memory effects are seen for Y (acac) and Co(CO),NO whose rinse-out curves level off at 0.2 and 0.3% of the steady-state signals respectively.Such memory effects are occasionally seen when solutions containing volatile species are dispersed into finely-divided aerosols. Memory problems can also be exacerbated when the aerosols are heated. For example inorganic forms of Hg B Table 7 Sensitivity for analyte elements as complexes and as inorganic nitrate salts in methanol Sensitivity/106 counts s-' per mg 1-' Analyte 5 9 c ~ + 89y + 93Nb+ Complex 2.8* 3.8t 1.8$ Nitrate salt 1.7 2.4 1.7 * Co present as Co(CO),NO. -f Y present as Y(acac),. $ Nb present as Nb(OC,H,),. I I I I 0 120 240 360 480 Time/s Fig. 2 Rinse out curves for Nb' in methanol. A Nb at 100 pg 1-' as Nb(N03),; and B Nb at 300 pg 1-' as Nb(OC2H5)5.In each case the Nb sample was removed at the time indicated by the vertical broken line and 0 s cause similar problems in conventional desolvation The precise causes of memory effects are often obscure or not easily attributable to simple chemical reasoning and the present work is no exception. For example Co(CO),NO boils at 50°C and should be readily volatilized in the heaters which are at 105 "C. The Nb(OC,H,) (b.p. 142 "C) should melt in the heaters but not boil yet Nb(OC,H,) shows a much worse memory problem than Co(CO),NO. At any rate the chemical form of the analyte element does influence the sample throughput and rinse-out procedures required as discussed previously by Van Heuzen.16 Conclusion Cryogenic desolvation allows continuous analysis of difficult solvents that would otherwise plug the sampler or extinguish the plasma.Polyatomic ions are less abundant than is usually the case with organic solvents although some important analytes are still obscured particularly 28Si 44Ca 52Cr and 56Fe. This desolvation method should prove valuable for the measurement of trace amounts of inorganic ions in highly purified organic solvents used in the semiconductor industry and in materials sciences. However the analyst should investi- gate possible memory effects that depend on the chemical form and physical properties of the analyte particularly when the analyte is present as neutral or volatile complexes. Also the sensitivity may depend somewhat on the chemical form of the analyte for reasons that are unclear at this time.In the future the use of still lower temperatures for the cold loops could prove advantageous as none of the solvents were frozen into solids in the present work. Caution Since the heater temperature (100-140 "C) may be higher than the flash point of the solvent highly flammable vapours and aerosols produced from organic solvents and organometallic compounds must be kept within the inert gas stream present inside the nebulizer and cryocondensers. TheJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 403 aerosol or vapour must not come in contact with the heating elements on the exterior of the heater. Care should also be observed when handling oxygen. An arrestor valve was installed on the oxygen cylinder.The loan of an ultrasonic nebulizer from Cetac Technologies Incorporated is gratefully acknowledged. Ames Laboratory is operated by Iowa State University for the US Department of Energy under Contract No. W-7405-Eng-82. This research was supported by the Office of Basic Energy Sciences Division of Chemical Sciences. References Boorn A. W. and Browner R. F. Anal. Chem. 1982 54 1402. Hausler D. W. and Taylor L. T. Anal. Chem. 1981 53 1223. Hausler D. W. and Taylor L. T. Anal. Chem. 1981 53 1227. Kreuning G. and Maessen F. J. M. Spectrochim. Acta Part B 1989 44 367. Maessen F. J. M. J. Kreuning G. and Balke J. Spectrochim. Acta Part B 1986 41 3. Maessen F. J. M. J. Seeverens P. J. H. and Kreuning G. Spectrochim. Acta Part B 1984 39 1171. Brotherton T. Barnes B. Vela N. and Caruso J. J. Anal. At. Spectrom. 1987 2 389. 8 9 10 11 12 13 14 15 16 17 18 19 Barrett P. and Pruszkowska E. Anal. Chem. 1984 56 1927. Blades M. W. and Caughlin B. L. Spectrochim. Acta Part B 1985 40 579. Hausler D. Spectrochim. Acta Part B 1987 42 63. Hutton R. C. J. Anal. At. Spectrom. 1986 1 259. Hill S. J. Hartley J. and Ebdon L. J Anal. At. Spectrom. 1992 7 23 895. Wiederin D. R. Houk R. S. Winge R. K. and D’Silva A. P. Anal. Chem. 1990 62 1155. Alves L. C. Wiederin D. R. and Houk R. S. Anal. Chem. 1992 64 1 164. Alves L. C. Allen L. A. and Houk R. S. Anal. Chem. 1993 65 2468. Van Heuzen A. A. 4th Surrey Conference on Plasma Source Mass Spectrometry Guildford Surrey UK July 1991. Scott R. H. Fassel V. A. Kniseley R. N. and Nixon D. E. Anal. Chem. 1974 46 75. Fassel V. A. and Bear B. R. Spectrochim. Acta Part B. 1986 41 1089. Olson K. W. Haas W. J. Jr. and Fassel V. A. Anal. Chem. 1977 46 632. Paper 310465 1 F Received August 3 1993 Accepted November 29 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900399

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

405 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Electrothermal Atomic Absorption Spectrometry in Occupational and Environmental Health Practice-a Decade of Progress and Establishment* A Review invited Lecture Dimiter L. Tsalev Faculty of Chemistry University of Sofia Sofia 1126 Bulgaria Electrothermal atomic absorption spectrometry (ETAAS) is today an established technique in the vast application area of occupational and environmental health practice. Up to 52 elements have so far been determined in biological matrices such as body fluids and tissues food and related samples with a view to the assessment of occupational and environmental exposure. For at least a third of these analytes ETAAS is the current technique of choice. The progress in this field over the last decade is critically reviewed and discussed.Emphasis is given to the development of direct procedures and simple pre-treatment techniques the selection of suitable chemical modifiers and appropriate reaction media some practical problems as well as the rational combination of the graphite atomizer with preconcentration and speciation techniques. Keywords Nectrothermal atomic absorption spectrometry; biological sample ; clinical sample ; trace element determination ; review Aims and Scope The decade after the publication of our two-volume monograph on atomic absorption spectrometry (AAS) in occupational and environmental health has brought substantial pro- gress in instrumentation and methodology in the vast appli- cation area of biological trace element research thus justifying the preparation of an updated text.3 The aim of the present overview is to summarize the information on the progress achieved and the problems faced.The ten year period of 1983-1992 is covered the determination of up to 52 analyte elements in biological matrices such as body fluids and tissues food and related samples is considered with a view to the assessment of occupational and environmental exposure (Table 1). Since the number of relevant AAS papers well exceedes 2000,3 only keynote references are given here. Certain related matrices such as plant tissues water and air are not considered unless important improvements in methodology or speciation approaches have been involved in analytical procedures. Important selected sources of information in this field are the recent book chapters" and monographs.'"18 Position of AAS Among Current Techniques Despite the large selection of sensitive analytical techniques for trace element determinations in biological samples (Table 2) AAS still occupies the place of a routine analytical technique in this field for up to 30 analyte elements.In fact only a few of the tabulated relevant techniques (Table 2) are real alternatives to AAS in clinical/biochemical laboratories e.g. ion-selective electrodes (ISEs) for F- and Li+ molecular absorption spectrometry for phosphate and Fe fluorimetry for Se and flame emission spectrometry for Li. Obviously the economic considerations still weigh against the important methodological assets of the alternative techniques such as the better or competitive sensitivity [inductively coupled plasma mass spectrometry (ICP-MS) neutron activation analyses (NAA) and differential pulse anodic stripping voltammetry * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.(DPASV)] or multi-element performance [inductively coupled plasma atomic emission spectrometry (ICP-AES) ICP-MS instrumental NAA (INAA)]. Thus AAS is now an established reliable and cost-effective technique owing to the unique combination of the following advantages high sensitivity limits of detection (LODs) down to the ng and pg range; relatively good accuracy; adequate precision for most trace element determinations; large elemental coverage; small sample size requirements; relatively simple sample preparation; high [for flame AAS (FAAS)] to moderate (for ETAAS) sample throughput rate; applicability to solid or slurried microsamples; moderately priced equipment; reliability of apparatus simple operation and easy maintenance; automation; and well estab- lished methodology.Some intrinsic limitations and drawbacks of the AAS tech- nique should however be taken into consideration (i) limited potential for simultaneous multi-element analysis [although two and four channel instruments are commercially available the typical AAS approach relies on (automated) one element at a time measurements]; (ii) dissolution is typically required with solid samples; (iii) fairly narrow dynamic range of rectilin- ear calibration; (iu) non-applicability to several biologically/environmentally important elements (Br C1 F I and S); and (u) very poor sensitivity for certain analytes (e.g.B P Ti W and U). Up to 52 analytes have been determined so far in biological samples by means of AAS as depicted in Fig. 1. The distri- bution of papers published during the decade 1983-1992 between the four AAS techniques [FAAS ETAAS hydride generation AAS (HGAAS) and cold vapour AAS (CVAAS)] is given in Fig. 2. Whenever possible preference is given to FAAS procedures which are sufficiently sensitive to permit direct determinations of the electrolytes (K Na Ca and Mg) therapeutic levels of Li (14 papers) and Au (4 papers) and physiological levels of Cu (128) Fe (83) and Zn (146) in biological fluids (the numbers of FAAS papers for the period are given in parentheses).Furthermore FA AS procedures have been documented for several other analytes Pb (74) Cd (72) Mn (52) Cr (23) Co (17) Ni (16) Sn (mainly in canned food 14) A1 (11) and several other analytes (with < 10 papers). Important progress in FAAS methodology has been achieved406 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Table 1 Interest in trace element analysis of biomedical samples Area of interest Occupational exposure- Primary interest Subsidiary interest Environmental exposure- Nutritionlbalance studies- Deficiency Excessive levels Inert markers Clinical importance- Primary interest Subsidiary interest Forensic interest- Analyte As Be Cd Co Cr F Hg Mn Ni Pb Sb V Ag Al Ba Bi Cu Mo Te Ti T1 Se Si Sn W Zn Al As Ba Cd Cr F Hg Ni Pb Se Sn V Co Cr Cu F Fe Mn Se Zn Al As B Ba Cd Co Cr Cu F Fe Hg Mn Ni Pb Se Sn T1 Co Cr Yb Al* Au* Cu Fe Li* Mn P Pb Pt* Se Zn B Ba Bi* Co Cr F Ga* Mo Rb* Sb* V Generally hard to predict e.g.As Au Ba Hg Sb T1 * Denotes interest in therapeutic concentration levels. Table 2 Other relevant analytical techniques Technique Neutron activation analysis ICP-AES ICP-MS Flame emission photometry Differential pulse anodic stripping voltammetry Potentiometric stripping analysis Adsorptive differential pulse voltammetry Ion-selective electrodes Fluorimetry Molecular spectrometry Catalytic method Isotope dilution mass spectrometry Element Ag Al As Au €3 Co Cr Cs Fe Ga Hg In La and lanthanides Mn Mo Pd Sb Se Sn Te Ti T1 V W Zn Al Au* B Ba Re Cu Fe Li*,Mo P Pt* Rb Si Sr Zn Al Au B Ba Co Cs Cu Fe La and lanthanides Mn Mo Ni P Pt* Rb Sr TI Zn Cs Li Rb Sr Bi Cd Cu In Pb T1 Sb Zn Bi Cd Co Cu Ni Pb T1 Sn Zn Ni Pt F Li Be Ga lanthanides Se Cu Fe P Zn Cr Cu Li Pb T1 Se Ag v * Denotes interest in therapeutic concentration levels.(TI Na Mg K Ca [ R b l ( p q Y Cs Ba La B W Pr Gd Tb Dy Er Yb Fig. 1 Applications of AAS to biological samples italics FAAS preferable; underlining HGAAS or CVAAS (Hg) alternative to ETAAS; single lined box ETAAS relevant technique; and double lined box ETAAS technique of choice owing to the miniaturization and automation of procedures by pulse nebulization and flow injection'' as well as by increasing sensitivity for volatile analytes (Cd Pb Zn etc.) by means of the 'slotted tube atom trap' (STAT).2oi21 However FAAS often lacks sensitivity and thus relies on efficient pre- Several important analytes are usually determined by vapour generation techniques mercury by CVAAS (153 papers from the total number of 185) and As Bi Ge Se Sn and Te by HGAAS (with 150 9 4 141 44 and 11 papers respectively). Despite the need for a more thorough sample pre-treatment stage the vapour generation techniques provide better relative LODs (typically around and below 1 Pg I-') and offer valuable speciation capabilities.22 The HGAAS technique is a definite ETAAS 57% concentration.1172 390 Fig. 2 Distribution of relevant AAS papers (1983-1992) between the AAS techniquesJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 407 competitor for ETAAS as far as difficult volatile analytes such as As Bi Ge Sn and Te are concerned. Noteworthy are the recent developments in HGAAS methodology for Pb22 (includ- ing plumbane generation from slurried biological s a m p l e ~ ~ ~ ~ ~ ) the on-line pre-treatment of biological liquids in a microwave d i g e ~ t o r ~ ~ ~ ~ and the dramatic improvements of LODs by in situ collection of hydrides in graphite tube atomizer^.^^.^^.^^ ETAAS as the Current Technique of Choice for up to 20 Analytes With the advent of the electrothermal atomization technique hundreds and thousands of analytical laboratories gained access to an extremely sensitive and readily acquired analytical tool. Many of these laboratories happened to be newcomers to the field of biological trace element research or ultratrace analysis by ETAAS or both; thus some period was needed to learn by experience that this precious technique can produce both highly accurate results and artefacts! This situation can best be illustrated by observing the trends of so-called 'normal' or 'reference values' for the trace element contents of human body fluids and t i s s ~ e s .~ ~ ~ ~ ~ ~ ~ These values have been steadily dropping over the years during the last two decades and have decreased by several-fold to orders of magnitude just owing to the better control of exogenous contamination during specimen collection and sample pre-treatment. Therefore essential components of the progress in the 1980s were not only the hardware support and methodological improvements but also the better philosophy organization education train- ing and quality assurance (QA) in ultratrace analysis.16 Crucial Role of Pre-Analytical Stages Although the analyst has no legal responsibility for the quality of samples received in the laboratory the best and most meaningful results in biological trace element studies are Table 3 Summary of the relevant biological specimens obtained (i) by interdisciplinary teams; (ii) after careful plan- ning of the multi-stage studies of which the ETAAS analysis is only an integral part; (iii) by taking into account all essential pre-sampling considerations age sex health status nutritional and smoking habits past exposures diurnal rhythm and seasonal variations medical and cosmetic treatments etc.; (iv) on selection of meaningful specimens with a diagnostic value (see for example Table 3); (v) after proper consideration of the methodological and technical reliability of specimen collection devices and sampling protocols; and (vi) upon suitable preservation and storage of samples (see for example the monographs in refs.1 2 16 and 30). Therefore most ordinary clinical laboratories are able to handle without serious problems assays such as Cu and Fe in serum Zn in plasma Pb and Cd in blood and Cu in urine but fail in sub- pg 1 - ' assays. Special (plastic) sampling appliances rather than stainless-steel needles are required for collecting non- contaminated blood samples for determination of Cr Mn Ni and V at sub-ygl-' levels. Moreover scrupulous cleaning of the sampling site flushing of sampling catheter with a few ml of blood thorough shielding from airborne dust and carefully selected storage containers should be ensured.33 Contamination (Table 4) and losses (Table 5) are the principle sources of errors that accompany all subsequent stages of assays but are particu- larly difficult to identify and control during the pre-analytical stages.An indication of the severity of airborne contamination is given by Fig. 3 wherein the ratio of the mean soil content to the median plasma or serum level of the most ubiquitous elements are plotted. Note the 1 x 105-1 x 107-fold excess for the first several analytes (Al V Si Mn and Cr) which can cause gross errors unless clean-room or laminar-flow box facilities are available. The trace element content of hair or nail samples typically exceeds by several orders of magnitude the corresponding levels of these analytes in serum or plasma (Fig.4). These specimens offer several important advantages such as easy Sample Whole blood Plasma or serum Urine Bile Bone Cerebrospinal fluid Erythrocytes Exhaled air Hair/nail Milk Saliva Soft tissues Sputum and nasal secretions Sweat Teeth Element As Be C d t Co Hg Pb Sb Se Te V T1 Al Au* Be Bi* Co Cr Cu Fe Ga* Li* Mo Ni P Pt* Se Si V Z n Ag Al As Au* B Ba Be Bi* Cd Co Cr Cu F Fe Ga* Hg In Li* Mn Mo Ni P Pb Pt* Rb* Sb Se Si Sn Sr Te TI I/ Zn Cr Mn Pd Al F Pb Sr Al Cu Fe Mn Zn Cr Fe Li Se Te Ag Au As Ba Cd Co Cr Cu F Fe Hg Mn Mo Pb Sb Se Sr Te Tl V Zn As Be Cd Co Cr Cu F Fe Hg Mn Mo Ni P Pb Se Sn V Zn Ag Au* Li* Hg Sr Zn Al Cd Cu Fe Ga* Hg Li* Mn Ni Pb Pt* Rb Se Si Zn Be Ni Si Hg Mn Se Te F Pb Sr * Denotes assays at therapeutic levels.t Analytes in italics are of primary importance. Table 4 Some sources of contamination Source Airborne Tobacco smoke Cosmetics and jewellery Sweat Sampling devices Storage vessels/stoppers/preservatives Paper Reagents Haemol ysis Contaminant Al Cr Fe Hg La Mn Pb Si Ti V Zn Al Be Cd Mn Zn Ag Al Au Bi Al Cr Mn Pb Co Cr Fe Mn Mo Ni V Al Cd Co Cr Mn Pb Si Zn Al Mn Zn Al As B Bi Cr Cu Mn Ni Pb Se Si Sn Zn Cs Fe Mn Pb Rb Zn408 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 At V Si Mn Cr Ga Ba La Co Fe Pb Ni Sr Sn As F T Element Fig. 3 Expectations of aiborne contamination. The ratio of mean soil contents [soil] to the median plasma ( P ) or serum ( S ) concentrations given on a logarithmic scale; arrows indicate that the corresponding bars may prove higher than depicted since the concentration levels for some ultra-trace elements in serum or plasma are reported as ‘below the LOD 100000 1 ’ Pb Cr Ag La Ga Mo V Ni Sb TI As Fe Cs Al Mn Cd Hg Sn Co Si Zn Au Yb Be Ba Cu Element Fig.4 Concentration ratios in hair to those in serum or in plasma (median values); see Fig.3 for explanation of use of arrows painless and non-invasive sampling stability on storage easy decomposition and very favourable analyte-to-matrix ratios. Thus hair and nails have been studied extensively with a view to environmental and (less so) occupational exposure monitor- ing (Table 3).’” Unfortunately these specimens are subject to large biological variability and are difficult to clean from exogenous contamination factors which limit the usefulness of these assays to biological monitoring on a group basis as well as to some medical and forensic tests.Nevertheless toe-nail specimens have proved much easier to decontaminate and a n a l y ~ e ~ ~ and have provided useful information on exposure to Mn35 and Cr.36 Progress in Instrumentation The role of hardware support is decisive in analyses of complex biological matrices at ng g-’ levels; the most important recent achievements are listed and briefly commented on below. Table 5 Common causes for analyte losses during sample preparation ( i ) Background correction facilities have become more efficient and reliable (see the reviews in refs.37 and 38). Instruments with Zeeman-effect background correction are offered by several manufacturers and the Smith-Hieftje correc- tion technique3’ has also been successfully commercialized. Attention has been paid to the correction of fast transient signals by means of proper algorithm^.^^^^' Zeeman-effect background correction has proved particularly useful in deter- minations of the most volatile and short-wavelength analytes (As Cd Hg Sb Se and Te); in analyses of urine at low dilution factors 1-3-fold); in the presence of phosphate-containing modifiers4’ and matrices (bone teeth); and in most analyses of solid microsamples and slurries (see the reviews in refs. 14 43 and 44). ( i i ) Faster electronics of modern instruments is essential for the accurate and precise acquisition and processing of transient peak signals (e.g.50 points per s). (iii) Video display of both AA and background signals with options for overlaying signals from samples standards and standard additions provides helpful information for developing procedures and quality control (QC). (iu) Faster heating rates and temperature-controlled heating are becoming standard and are an integral part of the stabilized temperature platform furnace (STPF) concept.45 More uniform heating and better spatial isothermality are achieved using an integrated-contact transverse-heated platform-equipped graphite a t ~ m i z e r . ~ ~ * ~ ~ (u) The L‘vov has been widely adopted; mechanical stability and precision are further improved by using ‘forked’ platforms placed in ‘partitioned‘ tubes5’ and atomizers equipped with integrated platforms.47 (ui) Both integrated absorbance (QA) and peak height (A,) signals are processed simultaneously thus allowing better flexibility .(uii) Versatile autosamplers perform dilutions addition of modifiers blending of mixed or composite modifiers running QC sample^,^' performing ‘hot injection^'^'-^^ and ‘multiple injection^',^^,^^ standard additions re-calibration and re-zeroing etc. (uii) Automated/overnight operation and successive multi- analyte capabilities help to improve the sample throughput rate by up to 30-40%. (ix) Alternate gases (‘gaseous chemical modifiers’ see for example the reviews in refs. 42 and 58) such as 0’ or air as an in situ ashing aid and H2+Ar as a reductant for noble metal modifiers are now handled in a safer and more con- venient way although still with caution(!) and at the expense of the lifetime of the graphite tubes and p1atf0rms.l~ (x) Flow injection facilities for on-line sample pre-treatment on-line precon~entration~’~~ and ~peciation~’“~ have become available and promise unlimited potential for the future (see the reviews in refs.64 and 65). ( x i ) Solid/slurry sampling devices have been automated6668 but still need to be perfected and better adapted to automated unattended performance (see the reviews in refs. 14 44 69 and 70). (xii) Graphite probe atomization7’ has become commer- cially available and a ~ t o m a t e d ~ ’ ~ ~ ~ and has exhibited good performance with biological l i q ~ i d s ~ ~ ~ ~ and digests,74 slurries75 and solid micros ample^.^^ Cause Volatilization H ydrolysis/adsorption Undissolved residues Incomplete release upon protein precipitation Undissolved fat fraction Analyte As B Cd Cr F Ge Hg Pb Sb Se Sn Te T1 Zn Ag Al Bi Hg Pb Sb Si Sn Ti W Ag Al Au Ba Be Cr Ir Mo P Pd Pt Sb Si Sn Sr Ti V As Au Co Mn Ni Se e.g.Co and Ni in milkTable 6 Availability of ETAAS reports on individual elements and biological matrices (total number of references estimated for the decade 1983-1992) Sample matrix Analyte Ag A1 As Au B Ba 3 12 5 4 - - 4 87 3 4 - - - 2 - 3 - - 5 1 9 1 7 l - - - 7 6 - - - - 11 - l - - 2 2 1 - - - 2 2 1 l - - 5 2 5 1 0 4 - - - - I - - - 5 9 1 6 1 3 - 4 4 1 7 - - 1 2 2 4 1 5 2 4 8 15 142 74 13 6 5 - 6 1 4 2 - - - - - - - - Be Bi Cd Co Cr 3 35 8 10 4 13 13 21 I - - 1 5 42 18 41 - 8 6 12 - 6 1 - - l - - - 21 3 11 - 2 2 8 - 61 14 14 - l - - - 47 10 16 1 28 6 10 3 7 2 1 6 10 229 60 125 - 3 - 21 c s c u 10 29 6 14 7 2 15 10 34 29 16 NE-F 119 6 - - Ga Gd Ge 1 1 - 1 - 1 - - - - - - Hg 3 2 5 5 - In Ir La Whole blood Plasma or serum Other blood fractions Urine Hair or nails Bone Teeth Milk and dairy products Miscellaneous body fluids Soft tissues Faeces Food/feed and beverages Marine food Water Number of ETAAS papers Number of ETAAS speciation papers - 1 1 2 - - 1 - 2 3 8 3 7 5 30 3 - - 2 2 1 1 4 - - 1 5 - - Li Mn 3 16 12 26 1 - 5 8 1 10 1 2 1 9 1 7 2 21 2 19 1 9 1 NE 19 96 1 - - - - - Mo 2 10 Ni P 6 - 14 - 22 - 6 - - - - - - - 8 2 2 - 14 2 18 9 9 3 3 3 78 12 - - Pb 98 12 3 37 9 9 4 33 1 53 70 34 NE 302 3 - Pd Pr Pt 2 8 2 8 - - - - Rb Ru Sb Se Si Sn 3 32 - 1 1 34 4 1 - 2 - 1 7 15 2 1 3 4 - 1 - l - - - 8 - - - 2 1 - 2 22 - 3 2 1 3 1 9 12 - 9 8 10 2 10 17 123 6 29 1 7 - 12 - - - - - - - - - Sr Tb Te Ti T1 V 5 1 - 3 - 2 9 - 1 - 1 1 - 3 - 16 16 1 - - - - - - - Yb Zn 6 - 24 5 1 2 - - - - Whole blood Plasma or serum Other blood fractions Urine Hair or nails Bone Teeth Milk and dairy products Miscellaneous body fluids Soft tissues Faeces Food/feed and beverages Marine food Water Number of ETAAS papers Number of ETAAS speciation papers - 1 1 - Y io m W 1 1 1 6 1 9 1 7 7 36 - - - - - 8 5 15 1 - 18 7 - NE 1 62 12 - - - - - - - NE 4 - * Fluorine determined by ETA molecular absorption spectrometry of AlF.7 NE not evaluated.410 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 0.01 Progress in Appliances and Related Materials The role in QA of as a minimum the following technical tools and relevant materials cannot be over emphasized. (i) Newer more reliable blood collection devices and storage containers are now a ~ a i l a b l e . ' ~ . ~ ~ ~ ~ ~ ~ ~ (ii) Special instruments for contamination-free sampling quartz and titanium knives plastic scissors TiN coated surgical instruments,80 e t ~ . ~ ~ ~ ~ (iii) Laminar-flow clean air benches. (iv) Fast and efficient microwave sample preparation (see the monograph in ref. 81 and the reviews in refs. 82 and 83) but these are still subject to improvements in view of safety and on-line p e r f o r m a n ~ e . ~ ~ ~ ~ . ~ ~ ~ ~ ~ (v) Mechanized and automated temperature-controlled wet chemical decomposition^.^^^^^ (vi) Bomb d e c o r n p o ~ i t i o n s ~ ~ ~ ~ ~ where there is still serious concern about safety and incompleteness of decomposition.(vii) Biological reference materials (see the reviews in refs. 16,78,88-93). -* +R I I I I t Progress in Analytical Methodology The distribution of the 1172 published reports on ETAAS between the 46 analytes and the most typical biological matrices is presented in Table 6. The table is intended to give an idea of the availability of ETAAS procedures and therefore only the number of papers for the decade being considered is included while selected procedures and references can be traced in ref. 3. Note that information on water analysis is not treated exhaustively and also that fluorine is in fact determined by ETAAS equipment but with electrothermal gas-phase mol- ecular absorption spectrometry of A1F94-97 or elsewhere by molecular fluorescence spectrometry of MgF.98 There are two different approaches to the development of analytical procedures which can conditionally be designated as 'more instrumental' and 'more chemical'.The first one is based on direct injection of untreated or simply diluted biological fluids and would be the favoured approach in a busy clinical laboratory. Thus contamination control is greatly facilitated sample processing errors are eliminated reagent consumption and manpower expenses are reduced. On the other hand however this approach relies on more versatile (expensive) apparatus and (typically) on longer instrumental time so as to ensure adequate in situ sample treatment; it also entails more complicated calibration procedures.This philosophy has proved very useful in determinations of ubiquitous elements at pg1-l levels in blood serum and urine (Al Cr Mn and Si) as well as in routine large-scale assays for priority analytes such as blood Pb and urinary Cd. Direct procedures have been developed for most analytes in biological fluids that are sufficiently sensitive by ETAAS cJ Fig. 5 for median serum or plasma levels of trace elements versus the characteristic concentration in Zeeman-effect ETAAS with 10 pl sample aliquots (characteristic concen- tration Co data taken from ref. 99). The 'more chemical' philosophy is viable enough although not so attractive most biological tissues and food samples are still analysed after decomposition or solubilization; protein precipitation can prove cost effective for some determinations on blood or serum/plasma; while preconcentration is often inevitable for many analytes at endogenous (normal/deficiency) sub-pg 1-1 levels i.e.Ag As Au B Be Bi Co Cs Ga Ge Hg In Ir Mo Ni Pd Pt Ru Sb Sn Te Ti T1 and V with As Be Bi Cs and Ni being determined with borderline sensitivity. (Table 7). Successful procedures result from rational combinations and compromises within the frames of these two extreme approaches. Below some recent methodological achievements are summarized and briefly discussed. + + Zn Fe Cu Rb+ + Si I loo0l 100 + + Se 8.t.' + Ba Sr + lo t Li Ag Yb.L +Pb .Au 'Ga Cd+ + + + Te + Hg + v Fig. 5 Median serum or plasma levels uersus the characteristic con- centrations for Zeeman ETAAS (10 pl aliquot C data from ref. 99) Suitable diluents and reaction media Dilute aqueous solutions of some reagents can be considered as media compatible both with the electrothermal atomizer and biological liquids or digests aqueous Triton X-100 (e.g. 0.05-0.25% v/v) a very efficient non-ionic surfactant for haemolysing whole blood samples and facilitating their smooth drying-ashing in the electrothermal atomizer; ammonia solu- tion (e.g. 1% v/v15) also an efficient solubilizer; NH; salts; strong organic bases serving as solubilizers such as tetra- alkylammonium hydroxides (TAAH alkyl = methyl or ethy1),34,100,101 hyamine h y d r o ~ i d e ' ~ ' ~ ~ ~ dieth~lenetriamine;"~ dilute HNO (e.g.from 10mmoll-l for blood/serum up to 1 moll- for urine digests and deproteinized blood/~erum;~,'~~ NH4N03; dilute aqueous H,Oz (e.g. 1-3% v/v); EDTA [pref- erably as the (NH4)4 or (NH4),H2 salt]; ascorbic acid and many other organic acids; SH containing ligands such as cysteine (for Au106) and dithiocarbamates; some organic sol- vents (but not chlorinated) e.g. methanol ethanol 5% v/v butan01,'~~ 10% v/v ethylene glycol,'o8 octanol (anti- f ~ a m ) . ' ~ ~ * l ' ~ See also the reviews on chemical modification in refs. 42 and 58 and organic additives in ref. 11 1. More efSicient chemical modijiers The classic nickel- and phosphate-based modifiers are steadily being replaced by more efficient and universal noble metal modifiers.42 The 'reduced Pd'l12 and Pd + Mg(N03)2113*114 modifiers have been widely acknowledged; thus up to 21 analytes of high and moderate volatility have been covered in the recent extensive study on the latter mixed modifier by Welz et al.'14 Noteworthy is the synergistic combination of Pd with a l b ~ m i n " ~ * ' ~ ~ and of Pd with MoV1 Vv and Mg(NO,) which may prove rather useful since some of these components could be present at mg levels in sample digests when added as catalysts in wet-chemical digestions kn.. MoV1 (ref.117) and Vv] or as ashing aids [e.g. following drfashing in the presence of Mg(N03)2].118 Some other noble metals have shown better performance than Pd in individual cases thus justifying further evaluation e.g. Ru+ascorbic acid for As In P Te and Tl;'19 Rh for Bi in blood and serum;'20 Pt for Te;'03 and Pd + Rh + Ru + Pt + ascorbic acid for Sb in serum and urine.lZ1 Practical considerations such as an impairment of modifier effectiveness by excess of chloride (Ru > Pd)1199'22 and nitricJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 41 1 Table 7 Selected reagents for liquid-liquid extraction from biological digests Reagent Acetylacet one Cupferron and analogous reagents Dimethylglyoxime Dithiocarbamates Di thizone Halides 8-hydroxyquinoline Ion-association complexes Liquid ion exchangers o-Diamines Toluene-3,4-dithiol Xanthates Miscellaneous ~~ ~~ ~ ~~~~~~~ Analyte Be Cr Mo Be Bi Ga In Mo Sb Sn Ti T1 V Ni Pd Ag As Au Bi Co Cr Ga Hg In Ni Pb Pd Pt Sb Se Sn Te TI V Ag Bi Cd Hg Ni Pb T1 As Au Bi Ga Ge Hg In Pb Sb Sn Te T1 Mo V B Co Cr Ga Hg Pt TI Be Cr Ga Mo Se Mo Bi Co Ni As Au B Co Ge Li Mo Ni Sn acid,123 blank contributions excessive background (phos- p h a t e ~ ~ ~ ~ ~ ~ ~ ) and cross-c~ntamination~~J~~ play a decisive role in the final selection of a modifier.The severe attack of the graphite surface by NH,NO La"' and phosphate based modifiers cannot be overlooked either. The t h e ~ r e t i c a l ~ ' * ~ ~ " ' ~ ~ and practical aspects4' of selecting appropriate modifiers have been discussed in detail elsewhere and charts plotted of the relationships and analogies between numerous potential modi- fiers and analytes.126,128 Carbide coated tubeslplatforms Although still far from being used in routine applications and not yet commercially available carbide-coated atomizers have shown several useful effects thermal stabilization of volatile analytes analogous to the action of corresponding modifiers (Zr W V and Mo) and better resistance to corrosion (W Nb and Ta) in particular with organic solvents extracts (W129 and Ta13') and chromatographic effluents,13' fats and oils (Nb and Ta',') biological liquids with added phosphate133 or NH,NO modifier^,'^^.^^^ digests with high content of ( HN03) acid (TaC136) as well as in procedures involving in situ ashing in air or ~xygen.'~'.'~~ Ma jor improvements in performance have been achieved with difficult analytes such as P and Sn (see the reviews in refs.138 and 139). Fast temperature programmes Whenever possible speeding up determinations by using rapid prograrnme~'~' with very steep drying-ashing steps or 'hot i n j e c t i ~ n ' ~ ~ - ~ ~ is a very attractive a p p r ~ a c h .~ ~ ~ ' ~ ~ These methods could improve sensitivity and precision when working with organic solvents/extracts5' and effluents and yet more importantly they could double the sample throughput rate while cutting the total cycle times (heating + autosampling) to less than 1 min.145 Successful applications to urine,53-55,141-143,14S milks5 and biological digests55*140*144~145 are documented but more viscous samples such as blood or serum54~140*141 would typically need large dilution factors and slower drying ramps. In situ oxygenlair ashing This approach is particularly efficient with simply diluted or slurried samples of blood and serum and plasma (even at low d i l ~ t i o n s ) ' ~ ~ ' ~ ~ ~ ' ~ ~ fats and and food and milk powder.68 The lifespan of the graphite tubes and platforms will however be reduced by several fold for example to between 50 and 250 firings only depending on the pyrolysis temperature (e.g.450-550 "C) pre-treatment temperature for oxygen desorption and removal from the atomizer (up to 900-950 "C) atomization temperature and other factors. SimpliJcation/rationalization of the sample decomposition step Inasmuch as the decomposition of the organic matrix is the major contributor to analytical inaccuracy there is a trend to keep pre-instrumental treatment as simple as possible. In fact biological fluids are best analysed either directly after dilution or after a simple acid pre-treatment such as protein precipi- tation with HNO,.Thus the decomposition of organic matter and removal of the bulk matrix components are performed in situ aided by the versatile heat profiling and by the reactive constituents of diluent and modifier(s) and eventually by the carbide-coated graphite surface and the alternative active gas if any. Protein precipitation has been applied to many analytes (Al Bi Cd Cr Fe Li Mn Ni Pb and T1) but lower results have been recorded for some protein-bound analytes (e.g. for Mn in whole blood"' but not in serum or plasma,148 for Col4' and Ni in blood). Liberation from proteins is improved at higher temperatures and at higher HN03 levels (up to about 1 moll-')1o5 and in the presence of Triton X-100105 as well as after a mild enzymic pre-digestion stage.'" Biological tissues and foodstuffs also do not necessarily need complete decompo- sition; therefore wet-chemical digestions with HNO or with HNO and H202 (which can be hazardous) in open or pressur- ized vessels are given preference in most cases.The concen- tration of HNO should better be kept to < 1% v/v in the final digests and should by no means exceed 5-10% v/v in order to protect the graphite surface from intensive corrosion and to ensure adequate effectiveness of the Pd modifier; if needed bulk acid may be evaporated or neutralised with ammonia solution (giving an NH,N03 r n ~ d i f i e r ) . ' ' ~ * ~ ~ ~ * ~ Some lipophilic species of the analyte can be found in the undissolved fat fraction (e.g.Co and Ni in milk powderlS3 but not A1 in brain tissuelS4 neither Mn in liver kidney or muscle tissue155). Solubilization of small samples of soft tissues,'00~102*103 with strong bases has proved very convenient in ETAAS determinations of numerous analytes and only for safety considerations (noxious vapours of tetra- methyl ammonium or tetraethylammonium hydroxides) does this technique seem to be neglected in practical work. Enzymic digestion^'^'*'^^'^^ would appear most promising in speciation studieslS8 and with large samples containing low levels of ubiquitous analytes (owing t o their low blanks) The carbonization technique,lS9 originally developed for X-ray fluorescence transforms samples of plant and animal tissues on calcination (off-line) at 300 "C into fine-powdered residues (about 50% mass reduction) with losses of <5% for many analytes Al Cd Cr Co Cu Mn Ni Rb V and Zn.lS9 The method has been adapted for determinations of Ti,160 Cd16' and Cr16' by ETAAS in vegetal food with atomization of the carbonized slurry.and412 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Flow injection on-line preconcentration The determination of many elements still relies on preconcen- tration because of the low levels encountered and inadequate sensitivity of the direct ETAAS technique (Fig. 5). A typical current technique for preconcentration is liquid-liquid extrac- tion from biological digests (Table 7). Complete decomposition of the organic matter and transforming the analyte to a definite oxidation state are essential in this case.Off-line preconcen- trations by co-precipitation of dithiocarbamates'62 and oxin- ate^'^^ and by ion exchange (Ba Be Ir Mo Pd Pt Sb Sr and T1) are also known. Only recently have on-line flow injection procedures for environmental waters6w2 and biologi- cal digest^,^',^^ based on solid-phase sorbent extraction of dithiocarbamate complexes on CI8-microcolumns been reported. These techniques would appear to be very promising in speciation studies at analyte levels in the pg range but this work has so far only been carried out with water samples.61i62 Another recent technique with very low LODs (<0.1 pg 1-I) and even better speciation capabilities is HGAAS with in situ collection of the h y d r i d e ~ ~ ~ ~ ' ? ' ~ ~ which has been extensively discussed elsewhere.22 Speciation Electrothermal AAS by itself provides very limited potential for speciation as reflected in the appearance of only a few publications.In situ pre-treatment of spiked samples of urine so as to volatilize Cr"' differentially during the pyrolysis step in the presence of volatilizers while delaying CrV' in the atomizer has been elab~rated.'~'-'~~ A 1 aboratory-designed two-stage at~mizer'~'*'~~ for differential atomization of Hg compounds in urine and sweat has been r e ~ 0 r t e d . l ~ ~ Interfacing ETAAS with chromatographic technique^,^^^^^^'^^ and hydride generation22 would call for further research and hardware developments before such techniques could be used in routine laboratories. However ICP-MS is a good rival for speciation work as are the related AAS techniques for on-line speciation FAAS (for Cd Cu Fe Pb and Zn) CVAAS (Hg) and hydride derivatization-AAS (As Sb Se and Sn).22,170 Hence most speciation studies are currently performed by off-line separ- ations by liquid chromatography (Al Au Be Cd Cu Fe Fe Pb Pt Se Sn and Zn) extraction [As"'-AsV Cr"'-Crv' Pt (cis-trans) Sb"'-Sbv Serv-Sevl) Sn(inorganic-organic) and by co-precipitation of Cr'"-Crv'].1 2 3 4 5 6 7 8 9 10 11 12 References Tsalev D. L. and Zaprianov Z. K. Atomic Absorption Spectrometry in Occupational and Environmental Health Practice Volume I Analytical Aspects and Health Significance CRC Press Boca Raton FL 1983. Tsalev D. L. Atomic Absorption Spectrometry in Occupational and Environmental Health Practice Volume 11 Determination of Individual Elements CRC Press Boca Raton FL 1984.Tsalev D. L. 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ISSN:0267-9477    

DOI:10.1039/JA9940900405

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 415 Multi-element Detection Using Second Surface Trapping With Electrothermal Vaporization Mass Spectrometry* Invited Lecture Andrew J. Scheie and James A. Holcombet Department of Chemistry and Biochemistry The University of Texas at Austin Austin TX 78772 USA A new technique for electrothermal vaporization mass spectrometry using second surface trapping is presented. The method involves atmospheric pressure vaporization of the analyte from a graphite cup and condensation of the vapour onto a cooled tantalum surface. The tantalum surface is introduced into a vacuum chamber through a series of differentially pumped seals and is positioned under a quadrupole mass analyser and radiatively heated using a 250 W filament. An electron impact ionizer (70 eV 3 mA) is employed for ionization.The system allows thermal pre-treatment of the sample in the cup prior to vaporization and trapping thereby enabling the analytes studied (Ag Cd and Pb) to be trapped and simultaneously detected in their elemental forms. Detection limits for the current system for the elements studied are in the low nanogram range. Keywords Electrothermal vaporization mass spectrometry; second surface trapping; silver; cadmium; lead Inductively coupled plasma combined with detection by mass spectrometry (ICP-MS) relies upon solution nebulization for sample introduction and often requires several millilitres of sample for analysis. While electrothermal vaporizers have been interfaced to mass spectrometer^'-^ and commercial units are now available studies have shown that interferences exist even with simple alkali metal salt mat rice^.^.^ Both instruments use the relatively expensive ICP as an ionization source and because of the attendant need of atmospheric pressure sampling through sampling and skimmer cones they also require sub- stantial pumping stations.Coupling the electrothermal vaporizer to the mass spec- trometer without the ICP ionizer could retain the advantage of handling microsamples while minimizing instrument cost and operating expenses. Also if properly designed it could minimize analyte transport problems and reduce pumping speed require- ments. Several studies”16 have used an electrothermal vaporizer heated within a vacuum under a quadrupole as a diagnostic tool for studying atomization processes in electrothermal atom- izers typically used for electrothermal atomic absorption spec- trometry (ETAAS).Likewise Styris and Redfield17y18 have interfaced an electrothermal vaporizer operated at 1 atm ( 1 atm = 101.325 kPa) to a mass spectrometer through skimmer cones to produce molecular beam sample introduction. For diagnostic purposes both approaches have been very successful. However direct vaporization in vacuum fails to take advantage of the excellent atomization efficiencies shown for ETAAS because it lacks the high temperature collision gas that ensures formation of analyte in the elemental form. Even the molecular beam sampling displays a number of analyte- containing molecular fragment^.^',^^ Hence a variety of mol- ecular species are typically detected whose signal intensities are strongly matrix dependent which is detrimental when considering this approach as a quantitative analytical tool.Nonetheless quantitative results are possible for simple ~amp1es.l~ With the molecular beam approach the skimmer cone samples only a small fraction of the sample vaporized from the surface and even this amount is mixed with a much larger amount of sheath gas; therefore sensitivity may not be optimal. A means of vaporizing the sample directly under the entrance to the mass spectrometer without an atmosphere-to- vacuum interface is desirable. * Presented at the XXVTII Colloquium Spectroscopicum Inter- nationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.To whom correspondence should be addressed. Falklg addressed the possibility of quantitative analysis using atomic beam time-of-flight mass spectrometry (AB-TOFMS). While Falk’s research presented preliminary results with no verification of quantitative results it succinctly discussed the possibilities of improving detection limits by minimizing losses in each stage. It was not suggested however that AB-TOFMS can provide the reproducible atomization capabilities demon- strated for more conventional ETAAS or that it can minimize or eliminate the possibility of interferences or matrix effects which depending on the sample analysed produce variations in the species vaporized. Holcombe and c o - w ~ r k e r s ~ ~ ~ ~ have shown that the use of a second surface atomizer (with AA detection) has the capability of efficiently trapping the analyte on a cooled insert placed within the furnace and that this trapped material could be easily released with a very high atomization efficiency upon re-heating of this surface inside a pre-heated furnace operated at 1 atm.A modification of this trapping method has also been successfully employed by H o c q ~ e l l e t . ~ ~ This general trapping approach also has the benefit of eliminating chemical and spectroscopic interferences since many of the gaseous matrix components and decomposition products of common samples are not significantly trapped on the probe. Furthermore it has been shown that the trapping of a more volatile but condensable matrix component could be prevented2’ and that the analyte captured is dispersed on the second surface21 rather than being present as large crystals or droplets that can be found after simple desolvation of a sample placed on a graphite surface (see for example ref.26). This preliminary decomposition and ana- lyte dispersal should eliminate many of the gaseous products observed which were a result of sample decomposition in earlier electrothermal atomization M S studies. A system has been constructed which incorporates electro- thermal vaporization (ETV) MS and second surface trapping. It contains a rapid atmosphere-to-vacuum sample introduction system a software program which will permit data acquisition under two operational modes (full mass scan and ion hopping) a trapping surface which can be cooled efficiently for trapping and heated rapidly for vacuum desorption and a means of heating the trapping surface within the vacuum.Experimental Apparatus The apparatus used for these experiments is designed to provide sample vaporization in graphite cups at 1 atm conden-416 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 G Tan ta I u m second Controlled temperature power supply Fig. 1 Schematic diagram of tantalum second surface trap positioned for analyte trapping sation of the analyte onto a cooled tantalum second surface rapid introduction of the trapped analyte into high vacuum and thermal desorption of the trapped analyte under a quadru- pole mass analyser for mass spectral analysis. Details of the 1 atm vaporization and trapping are shown in Fig.1. With the tantalum trap still in vacuum the sample solution is deposited into the graphite cup within the atomiz- ation chamber. Sample drying and thermal pre-treatment occurs without the second surface over the furnace. The translational rod is then retracted and the furnace is raised to meet the trapping surface made from 99.95% pure tantalum foil (0.127 mm thick Johnson Matthey Seabrook NH USA). The cooling probe delivers N gas at ambient temperature to the inside of the trap at 17 1 min-l to ensure efficient conden- sation of the analyte vapour onto the tantalum trapping surface while the furnace is at vaporization temperature. A Varian (Springvale Australia) CRA-90 is used to supply power to the graphite cup (2.9 mm id.) in a modified CRA-90 workhead which is in a 1 atm Ar environment.After the sample is trapped the N coolant gas is turned off the furnace is lowered and the trapped sample is then introduced directly into vacuum without exposure to air. Details of the vacuum chamber are shown in Fig.2. The tantalum trap on the translational rod is inserted into the lo-* Torr (1 Torr= 133.322 Pa) main chamber via two diflerentially pumped chambers sealed by fluorocarbon O-ring mechanical seals at 10-3-10-7 Torr respectively. The analyte- containing surface is positioned directly beneath the electron- impact ionizer of a quadrupole mass filter (Model C50 Extrel Pittsburgh PA USA) with a channeltron electron multiplier (Galileo Electro-Optics Sturbridge MA USA). A 250 W tungsten filament radiatively heats the surface to desorb the analyte thermally for filtering and detection.Quadrupole mass spectrometer v > Heater '*\kw filament Fig. 2 Schematic diagram of tantalum second surface trap positioned within the ultra-high vacuum chamber for analyte detection by MS The quadrupole was operated in pulse-counting mode using an F-100T Amplifier-Discriminator (Modern Instrumentation Technology Boulder CO USA) and Keithley Series 500 data acquisition hardware (Keithley MetraByte Taunton MD USA). The quadrupole and Series 500 were controlled by a 386 microcomputer using a data acquisition and analysis program written in ASYST (Asyst Technologies Rochester NY USA). The data were finally manipulated and plotted using Quattro Pro 4.0 (Borland International Scotts Valley CA USA).Solutions All metal solutions (Ag Cd and Pb) were prepared from 1000 ppm stock solutions by dissolution of the appropriate nitrate salt in distilled de-ionized water. Multi-element solu- tions were also prepared directly from stock solutions; 2-4 pl aliquots were used for analyses. Procedure The experimental run consisted of several steps. First the graphite cup was cleaned at a high temperature (>2500 K) under 1 atm of Ar in the atomization chamber with the tantalum trap still in the vacuum chamber. The sample was then deposited into the cup followed by a gentle drying step and charring at approximately 600K. The rod was then retracted the cup raised to meet the trap and the cooling probe lowered into the rod and trap. The N2 cooling gas (17 1 min-') was turned on and the furnace was heated at 800 K s-' to 1500 K (held for 1 s).Immediately after this the furnace was lowered the coolant gas turned off the N2 probe removed and the rod translated into high vacuum. When the tantalum surface was directly beneath the quadru- pole the heating filament was raised into position underneath the tubular trap. The filament was first pulse-heated (approxi- mately 50% power for 1 s) to desorb any residual gases that might have adsorbed onto the filament and then held at full power to heat the tantalum radiatively and desorb the sample. Throughout the heating pulse-counting data were collected for each mass by the acquisition hardware and software. Analysis time is approximately 10 min per sample.Results and Discussion Multi-element Solution Several masses were monitored for separate samples containing 80 ng of each of the three metals studied in order to determine the chemical form of the analyte leaving the surface. For the elements studied the M+ dominated with no detectable signal noted for species such as MO+. This is in contrast to earlier electrothermal atomization MS mechanistic studies where MO + was commonly observed during decomposition. For example when a Cd sample solution was deposited directly on tantalum dried and charred both Cd+ and CdO+ were detected (Fig. 3). This suggests that graphite cup vaporization produces the free metal as in conventional ETAAS which then condenses onto the tantalum surface. While preliminary results are encouraging this may not always be the case depending on the complexity of the matrix.As more interferents are present a greater possibility exists that the analyte could be trapped and re-released in more than one chemical form. However with the knowledge of available chemical modifiers as well as the flexibility provided by the trapping and heating cycles it could be possible to atomize in the cup before trapping. Thus the elemental form would be trapped thereby simplifying detection and quantification. A profile of the time-dependent signals for multi-element determination of Ag Cd and Pb is shown in Fig.4. With a data collection rate of 0.6Hz for each element and three elements monitored the profiles shown consist of approxi-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 v) w c 4.0 0 3 3.5 c.’ .- v) 5 3.0 Y .- - w 2.5 J 417 - - A g - Cd Pb - I 1 1 2 3 4 5 6 Time/s Fig.3 Mass spectrometer signals for 2 p1 of a 4.9 ppm Cd solution directly deposited on a Ta surface and desorbed in vacuo 3000 I v) 2500 1 cdh~g 2 2000 3 8 1500 \ + > .- gJ 1000 e 4- - 500 0 6 12 18 24 30 36 42 Time/s Fig. 4 Sequential multi-element ETV-MS with trapping for a 4 p1 aliquot of 20 ppm Ag Cd and Pb 5.0 I 7 4.5 1 2.0 ’ I I I 1 .o 1.5 2.0 2.5 3.0 Log (amounthg) Fig.5 Calibration curves for mixed element standards (Ag Cd and Pb) mately 20 points defining the shape. The metals range in volatility from high to moderate yet they were all successfully atomized trapped and re-desorbed in the same run. While raising the graphite cup to a temperature sufficient to vaporize the most refractory of the analytes the tantalum must remain cool enough to prevent loss of the most volatile analyte when trapping is occurring. This was successfully accomplished with these metals and N2 cooling yet more effective cooling may be required as the range of vaporization temperatures increases. Also the indirect heating in vacuum of this sample with a 250 W filament was adequate but modifications (such as electrothermal heating of the trap) are required to ensure thermal desorption of the more refractory metals.With the Ag+ signal monitored at m/z 107 Cd+ at m/z 114 and Pb’ at m/z 208 a multi-element working curve was generated (Fig. 5). of detection were extrapolated. They were 4.2 2.4 and 5.3 ng respectively assuming a signal-to-noise ratio of 3 where the noise is counting uncertainty in the background.The inherent background noise with the electron impact ionizer off is less than 1 count s-l but system background gases with the ionizer on increases this value significantly. The dynamic range is presently limited on the high end by pulse coincidence and on the low end by the background noise. For the current system the graphite cup vaporization and trapping process is efficient (approximately 90”/0) so mass transport to the ionizer region in the vacuum system more efficient ionization and reduction of the background are likely areas on which to focus future studies. Conclusions Preliminary results have shown that ETV-MS using second surface trapping is not only capable of sequential multi- elemental detection but that the trapping step permits vaporiz- ation of the analytes in one chemical form; for Ag Cd and Pb this form is the gaseous metal.Improvements on the current system include the need for higher thermal desorption tempera- tures in vacuum for analysis of the refractory metals as well as the previously mentioned possibilities for increasing the signal-to-noise ratio. This research was supported in part by The National Science Foundation grant No. CHE902059 1. We also acknowledge Grady Rollins for his help in construction of the system. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 References Gregoire D. C. J. Anal. At. Spectrom. 1988 3 309. Park C. J. and Hall G. E. M. J. Anal. At. Spectrom.1988,3,355. Newman R. A. Osborn S. and Siddik Z. H. Clin. Chim. Acta 1989 179 1991. Park C. J. and Hall G. E. M. J. Anal. At. Spectrom. 1987,2,473. Ediger R. D. and Beres S . A. Spectrochim. Acta Part B 1992 47 907. Wang P. Majidi V. and Holcombe J. A. Anal. Chem. 1989 61 2652. Bass D. A. and Holcombe J. A. Anal. Chem. 1987 59 974. Bass D. A. and Holcombe J. A. Anal. Chem. 1988 60 578. Droessler M. S. and Holcombe J. A. Spectrochim. Acta Part B 1987 42 981. Styris D. L. Prell L. J. Redfield D. A. Holcombe J. A. Bass D. A. and Majidi V. Anal. Chem. 1991 63 508. Styris D. L. and Kaye J. H. Spectrochim. Acta Part B 1981 36 41. Styris D. L. and Kaye J. A. Anal. Chem. 1982 54 864. Styris D. L. Anal. Chem. 1984 56 1070. Styris D. L. Fresenius’ 2. Anal. Chem. 1986 323 710. Sturgeon R. E. Mitchell D. F. and Berman S . S. Anal. Chem. 1983 55 1059. Ham N. S. and McAllister T. Spectrochim. Acta Part B 1988 43 789. Styris D. L. and Redfield D. A. Anal. Chem. 1987 59 2891. Styris D. L. and Redfield D. A. Anal. Chem. 1987 59 2897. Falk H. J. Anal. At. Spectrom. 1992 7 255. Holcombe J. A. and Sheehan M. T. Appl. Spectrosc. 1982 36 631. Rettberg T. M. and Holcombe J. A. Spectrochim. Acta Part B 1984 39 249. Rettberg T. M. and Holcombe J. A. Spectrochim. Acta Part B 1986 41 377. Rettberg T. M. and Holcombe J. A. Anal. Chem. 1986,58 1462. Rettberg T. M. and Holcombe J. A. Anal. Chem. 1988 60 600. Hocquellet P. Spectrochim. Acta Part B 1992 47 719. Churella D. J. and Copeland R. T. Anal. Chem. 1978 62 309. Paper 3/058401 Received September 28 1993 Accepted November 26 1993 Limits of Detection Using single-ion monitoring for the signals for Ag Cd and Pb and the background counts over the same time period limits

ISSN:0267-9477    

DOI:10.1039/JA9940900415

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 419 High Current Pulsing of a Xenon Arc Lamp for Electrothermal Atomic Absorption Spectrometry Using a Linear Photodiode Array* Clare M. M. Smith and James M. Harnlyt USDA ARS BHNRC NCL Bldg. 161 BARC-East Beltsville MD 20705 USA Gary P. Moulton* and Thomas C. O’Haver Department of Chemistry University of Maryland College Park MD 20743 USA A series of 300 and 500 W xenon arc lamps normally operated at 20 and 35 A respectively have been pulsed as high as 300 A to achieve higher intensities in combination with a linear photodiode array detector. Initial tests without pulsing showed that the 500 W lamps are generally more intense but the 300 W lamps were more intense at 200 nm. With 300 A pulses both lamps showed a factor of 500 increase in the pulse intensity over the simmer intensity.With a 0.5 ms pulse and a 3.75% duty cycle pulsing at 300 A provided a factor of 18 increase in the integrated intensity over normal d.c. operation. The increase in integrated intensity can result in a comparable improvement in detection limits since the instrument is detector noise limited. Both the 300 and 500 W lamps exhibited failure after the equivalent of 200 atomizations at 200 A. With 100 A pulses the 300 W lamp was still operating after the equivalent of 800 atomizations. At both pulse levels the decrease in intensity with time was accelerated as compared with d.c. operation. It was concluded that an improved lamp design is necessary to make pulsed operation economically attractive.Keywords Continuum source atomic absorption spectrometry; linear photodiode array detection; xenon arc lamp; pulsing. In recent literature continuum source atomic absorption spec- trometry (CSAAS) has been described where a linear photodi- ode array (LPDA) detector’-’ is employed with pulsing of the xenon arc continuum source.1’2 The most significant results to date have arisen from the use of the LPDA.3*5 With d.c. operation of the continuum source and LPDA detection detection limits have been achieved which are comparable to conventional line source atomic absorption spectrometry (AAS) even in the far ultraviolet (UV) region for arsenic (193.7 nm) and selenium (196.0 nm). These improved detection limits are a result of the greater quantum efficiency and the multiplex advantage of the LPDA as compared with detection using a photomultiplier tube (PMT). Initial results suggested that further improvements in the detection limits can be achieved by pulsing the xenon arc continuum source.’?’ Improvements in the detection limits should be proportional to the increase in intensity since the limiting noise is the LPDA read noise.The use of an LPDA detector makes pulsing of the xenon arc source feasible. With a continuum source the optimum signal-to-noise ratio is obtained by ratioing intensities on and off the analytical line. With PMT detection this is accomplished by mechanical wavelength modulation using a quartz refractor plate on a galvanometer.6 The fastest wave- length modulation frequencies (200 Hz) are not compatible with a lamp pulse of 1 ms or less.In the research instrument used in this study,’-’ a short LPDA (256 pixels) is mounted in the focal plane of a high-resolution echelle spectrometer and dedicated to the measurement of the spectral intensities around a single absorption line typically a spectral range of less than 1 nm. At periodic intervals (> 50 Hz) the LPDA is read (the accumulated charge of each pixel is read giving an intensity for each pixel) and an absorbance is computed by ratioing intensities on and off the analytical line.3 Pulses of the lamp can be readily inserted between the LPDA reads. Because intensities from a single read are used to compute absorbance variations between lamp pulse intensities have no effect on the * Presented at the XXVIII Colloquium Spectroscopicum Inter- nationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.t To whom correspondence should be addressed. 1 Present address Abbot Laboratories Chicago IL USA. variance of the signals. Since the wavelength region is moni- tored simultaneously the lamp pulse widths can be extremely short i.e. fractions of a millisecond. Pulsing is most efficiently accomplished using two power supplies connected in parallel to the lamp. The d.c. or simmer supply furnishes the simmer current while the pulse supply furnishes short high-current pulses and operates with a low (< 5%) duty cycle. The advantage of pulsing over d.c. operation is manifested in two ways. Firstly the maximum power level of the lamp can be exceeded during pulsing using the ‘burst’ mode of operation.Secondly the emitted intensity of the lamp in the UV increases exponentially with the pulse current. The ‘burst’ mode of operation is defined as the use of a short period of pulse-mode operation followed by a lengthy period of d.c. operation during which the lamp is allowed to ~ 0 0 1 . ~ Electrothermal atomization is ideally suited to the burst mode of operation. During this short burst or period of pulsed operation the power level of the lamp can be exceeded as pulsed operation need only last for the duration of the atomiz- ation cycle 1-5 s depending on the element being determined. Between atomizations 1-3 min of simmer are possible while the furnace cools the next sample is deposited dried and charred.With a 1 min simmer period and a 5 s burst the effective power rating of the lamp can almost be d o ~ b l e d . ~ ICL Technology the manufacturers of the Cermax xenon arc lamps (300 and 500 W) used in this study has shown that the relationship between the lamp intensity and the current is given by Ip/Is = (i,/i,)”.” (1) where I is the pulse intensity I is the simmer intensity i is the pulse current and is is the simmer current.8 It can be seen that with respect to the emitted intensity there is an exponential advantage to directing the lamp’s power into a high-current pulse rather than into d.c. (constant current) operation. For a fair comparison of d.c. and pulsed operation however the gain in intensity must be considered over the entire pulse cycle (pulse plus simmer).Thus the effective increase in integrated intensity viewed by the LPDA over the pulse cycle will depend on the pulse current and width and the duty cycle. These three factors are interactive and are limited by the effective power rating of the lamp. All the power cannot be directed into the pulse since the lamp cannot be420 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 turned off between pulses. A minimum simmer current of 14 A is required to prevent damage to the cathode from arc wander. High-current pulsing of the lamp is deleterious to lifetime of the lamp life and to inten~ity.~ The extent of the effect is dependent primarily on the pulse current. Repeated pulsing of a 300 W xenon arc lamp to 100 A for 0.5 ms produced a 65% decrease in intensity after 20 000 These data were obtained with a 0.4% duty cycle one pulse every 2min.Normal operation of the lamp is of course accompanied by a decrease in intensity. A loss of 35% in intensity is expected for 1000 h of ~peration.~ The many possible variations of the pulsing operation make it difficult to compare the number of lamp pulses to hours of lamp lifetime. In terms of the number of analytical experiments for which the lamp can be used however pulsing can be expected to have a deleterious effect. Only limited pulsing data have been reported previously. ‘y2 Initial pulsing results suffered from the lack of a separate power supply’ and true pulsing *as not possible. Instead a square-wave modulated peak was superimposed on the simmer intensity by supplying a reference frequency to the power supply.In this manner the lamp current was modulated between 20 A and 30-40 A at 60 Hz with a 50% duty cycle resulting in a doubling of the integrated intensity. The upper current limit was determined by the limit of the power supply. In the second study only preliminary data were available for the custom built power supply.2 In this study the intensities of a series of 300 and 500 W xenon arc lamps were compared for dx. and pulsed operation. Peak and integrated intensities were measured as a function of the pulse current. The detection limits obtainable with lamp pulsing were projected for arsenic as a function of the pulse intensity. Integrated intensities were measured as a function of the total number of pulses.The lifetimes of the lamps pulsed at 100 and 200A were compared with normal d.c. operation in terms of intensity and the length of lamp lifetime. Experimental Instrumentation The laboratory constructed CSAAS has been previously de~cribed.3.~ A schematic diagram of the system used is given in Fig. 1. Either a 300 or 500 W Cermax lamp (Models LX300-UV and LX5OO-UV respectively ILC Technology Sunnyvale CA USA) was used. A pulse power supply was custom built for this project by the Electronic Development Group of the Department of Physics of the University of Maryland. This power supply was designed to deliver 200 A for 1.5 ms across 8 i2 with a 20% duty cycle but was found to be capable of delivering 280 A under these conditions. The current level and peak width of the pulse power supply were set manually.The start of each pulse was initiated by a digital signal from the computer. The pulse power supply was connec- ted to the lamps in parallel with the 300 and 500 W Cermax power supplies (Models PS300-1 and PS5OOSW-1 respectively ILC Technology). The 300 W supply was modified by placing a blocking diode in the cathode line to prevent the high current pulse from causing damage. A blocking diode was already in place in the 500 W power supply. During lamp ignition the pulse supply was disconnected from the lamp by means of a mechanical switch to prevent the r.f. ignition pulse from damaging the pulse power supply. An electronic shutter and driver/timer [Uniblitz Models LS6Z (normally closed) and T132 respectively Vincent Associates Rochester NY USA] was used to gate the exposure of the LPDA.For these studies a commercial furnace and power supply (HGA-500 Perkin- Elmer Norwalk CT USA) were used. Lamp Pulsing The 300 W lamps were normally operated at 20 A in the d.c. mode while the 500 W lamps were normally operated at 35 A. With pulsing both lamps were operated at a 20A simmer current. Current pulses of 20-300 A were superimposed on the simmer current. Data were acquired using two pulsing regimes. tn both cases the lamps were operated in the pulsed mode for 5 s with 120 s of simmer between bursts a timing scheme compatible with a normal electrothermal atomization (ETA) cycle. In the first pulsing regime a 3.75% duty cycle was used with a 0.5 ms wide pulse at 75 Hz (13.3 ms cycle).The majority of the results were obtained with the second pulsing regime of a 5% duty cycle with a 1 ms pulse width at 50 Hz (20 ms cycle time). In both cases with LPDA detection a 500 x 500 pm entrance slit was used. Intensity Measurements D.c. operation A series of 300 and 500 W lamps were compared in the d.c. mode of operation at eight different wavelengths between 201.1 and 402.2nm. These measurements were made at the centre of each order using PMT detection. The signal from the PMT was connected directly to an oscilloscope. Measurements were made with matching slit settings of 50 pm wide and 500 pm high and a PMT setting of 550 V. The 300 W lamps were operated at 20 A while the 500 W lamps were operated at 35 A. At all wavelengths intensity measurements were made on and between orders.The between-order measurements provided far stray light intensity which was used to correct the on-order intensities. The light measured between the orders of the echelle spectrometer is reflected light from the visible region and is a measure of the unabsorbable light on the order. The stray light was as large as 40% at 201.1 nm. Pulsing operation One set of experiments employed the PMT detector for pulsed operation. In this experiment the LPDA was removed from the spectrometer but was connected to the computer in the normal manner except for the ‘data out’ line. The PMT was installed in its usual position in the spectrometer and the analogue data line was hooked to the computer in place of the LPDA ‘data out’ line.Timing of the ‘read trigger’ was shifted so that the reading of 256 pixels corresponded to 256 analogue PMT measurements that started with the start of the lamp pulse. In this manner the intensity of the lamp and the following simmer intensity levels were measured at 30 ps intervals. The PMT measurements were made at 240.7 nm the cobalt wavelength. Intensity measurements made with the LPDA were made on and off the order to correct for far stray light. Measurements were also made with and without gating. For studies of the lamp intensity as a function of pulse current d.c. lamp measure- ments on and off the order were made between sets of pulsed lamp measurements. For lamp lifetime studies measurements were made continuously on-order with off-order measure- ments being made every 5000 pulses. Measurements were made in the d.c.mode at the start and end of each day. Between 10 000 and 15 000 pulses were made per day. Results and Discussion Comparison of Lamp Intensities for D.c. Operation The 300 W xenon arc Cermax lamps have been used almost exclusively for CSAAS. In this study both the 300 and 500 W lamp were used. The higher power of the 500 W lamp was attractive since higher pulse currents could be used and a longer lifetime was anticipated.” The 500 W lamps however are more expensive than the lower power lamps ($800 uersus $500). Initially a series of new 300 and 500 W lamps were compared for operation in the d.c. mode (see Experimental) at eightJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Simmer power supply 42 1 -- - _ _ _ _ _ _ _ _ _ _ _ - - - - ‘2 D Pulse power A supply I -L Video signal L\ I Trigger ADC I Master start Lamp pulse trigger Shutter trigger Clock DAC Computer Experiment start Int Fig.1 Schematic diagram of the CSAAS system wavelengths between 200 and 400 nm at operating currents of 20 and 35 A respectively. The intensities in Table 1 reflect the change in lamp intensity as well as the Cchelle spectrometer transmission efficiency as a function of wavelength. Zander et d.ll have reported almost a factor of ten decrease in the transmission efficiency of the Cchelle between 250 and 200 nm. Thus the majority of the loss below 250 nm can be attributed to the spectrometer. It can be seen that on average the 500 W lamps are more intense at most wavelengths.At 215 nm however the sources are comparable and at 201.1 nm the 300 W lamps are actually more intense. It is generally expected that the intensity ratio of the lamps is roughly proportional to the power ratio. The greater intensity of the 300 W lamp at 200 nm however is not all that surprising. Although the larger lamps generate more power the arc gap is larger and the ‘brightness’ (lumens per steradian) may not be as high as it is for lower power lamps. This ‘brightness’ is the most critical parameter of the lamp since the effective lamp intensity is based on the radiation that can be focused through a 500 x 500 pm entrance aperture. Intensity Versus Pulse Amplitude The intensity increases obtained by pulsing the lamps are shown in Figs. 2 and 3.A new 300 and a 500 W lamp (both number 1 lamps from Table 1) were pulsed at 75 Hz with a 0.5ms pulse width (3.75% duty cycle) for a series of pulse currents up to 300 A. The simmer current for both lamps was 20 A and each lamp was operated in the pulse mode for 5 s. All measurements were made at 240 nm. The current-voltage characteristics of the 300 and 500 W lamps are almost the same. The maximum current levels for both lamps (20 and 35 A respectively) assumes a 14 V drop across the electrodes. The power levels shown in Table2 were computed from current-voltage data from Chinnock’ for the 300 W lamp. In Fig. 2 the predicted and experimentally determined peak- to-simmer intensity ratios of the 500 W lamp are compared. Experimental results were obtained using PMT measurements (see Experimental) and the predicted values were computed from the pulse and simmer currents using eqn.(1). Fig. 2 shows Table 1 Lamp intensities (in arbitrary units) as a function of wavelength Wavelength/ nm 201.1 214.5 225.2 250.2 274.7 300.3 352.0 402.2 500 W lamps* 300 W lamps? 1 0.066 0.15 0.38 1.1 2.7 6.2 17.0 22.5 2 0.062 0.13 0.44 1.4 3.2 7.2 18.5 22.5 3 0.060 0.12 0.34 1.1 3.0 8.4 20.0 23.0 1 0.075 0.16 0.30 0.94 2.3 5.4 15.0 21.5 2 0.062 0.13 0.25 0.75 2.0 5.5 15.8 28.0 3 0.070 0.15 0.28 0.8 1 2.0 5.0 14.0 21.0 r 600 2 500 400 4d .- cn .- L E .- 300 c .- v1 5 200 r .- Q) 7 Q 2 100 0 Pulse current/A Fig.2 Peak intensity ratios as a function of pulse current for A 500 W lamp with PMT detection; and B predicted ratios from eqn. (1) * Operated at 35 A.t Operated at 20 A.422 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 I 1 150 i- PC Pulse current/A Fig.3 Integrated intensity ratios as a function of pulse current for A a 300 W lamp and LPDA detection; B a 500 W lamp with LPDA detection; and C predicted ratios Table 2 Lamp operating power Power/W Simmer Pulse current/A current/A 20 0 50 - 100 150 - 200 - 250 - 300 - - 0.5 ms* 75 Hz 280 328 378 434 495 558 619 1.0 ms* 50 Hz 280 343 41 1 48 5 567 651 732 * For 0.5 and 1.0 ms pulse widths the duty cycles were 3.75 and 5% respectively. The total cycle times were 13.3 and 20 ms respectively. that the predicted and experimental intensity ratios compare very well. Substituting the experimental data yields an average exponent of 2.2.Thus the intensity enhancement predicted by eqn. ( 1 ) was verified. The pulse power supply was only specified and calibrated for pulse currents as high as 200A. The agreement between the predicted values and the experimental results in Fig. 2 suggests that under the specific pulse parameters of the experi- ment the pulse power supply is still calibrated and capable of delivering current at 300 A. In Fig. 3 the ratios of the integrated intensities for pulsed and d.c. operation for the 300 W and 500 W lamp and the predicted values using an LPDA detector are compared. Pulsing conditions were identical to those for Fig. 2 except that the pulse currents were raised to 300 A in a different series of steps. Integration of the signal was accomplished with an LPDA (see Experimental). The predicted values were com- puted from eqn.(1) based on the simmer and pulse current and the duty cycle. It can be seen that the effective intensity increase at pulse currents below 200A is almost identical for both lamps and agrees well with the predicted values. With a 200A pulse the increase in integrated intensity is a factor of 70 and 64 respectively for the 500 and 300 W lamps as compared with the predicted ratio of 70. At pulse currents above 200 A both lamps show a reduced response. It was noticed when using the 300 W power supply that the simmer current did not return to its original level after a pulse but fell to a lower level. This effect was accompanied by a visible drop in the current meter of the simmer supply. The result was a decrease in the integrated intensities.This most probably accounted for the poor performance of the 300 W lamp above 200A. A subsequent study showed that this decrease was produced by the blocking diode which had been inserted in the line to the 300 W supply and was not owing to the lamp. No such effect was observed with the 500 W lamp and power supply or when running the 300 W lamp on the 500 W supply. The reduced response of the 500 W lamp above 200A could be due to the power limit of the pulse power All the results presented to this point were obtained without gating the LPDA exposure with the electronic shutter. Gating has been shown to be essential to accurate background correc- tion and to eliminate flicker noise when the array is read in a sequential manner.3 In addition gating minimizes the contri- bution of the simmer intensity and provides better signal-to- noise ratios when measuring integrated intensity at low pulse currents. Consequently the rest of the data were obtained with gated exposure of the LPDA.supply Detection Limits Versus Pulse Amplitude Integrated intensity measurements as a function of the pulse amplitude were repeated at the arsenic wavelength of 193.7 nm using a 300 W lamp a 50 Hz pulse frequency a 1 ms pulse width (5% duty cycle) and gating which permitted trans- mission of approximately 18% of the total integrated intensity. Thus the gate was open for approximately 3.6 ms of the 20 ms cycle time. At 193.7 nm it was necessary to correct for the far stray light of the 6chelle. This was achieved by making intensity measurements on and off the order.The increase in the integrated intensity produces a pro- portional decrease in the absorbance noise with the LPDA d e t e ~ t o r . ~ This is a result of the detector noise being limiting. The decrease of the experimentally measured absorbance noise with increasing pulse current and increasing integrated inten- sity is clearly shown in Table 3. As a consequence of the reduced absorbance noise the detection limits will improve proportionally since the characteristic mass is unaffected by intensity and remains constant. The predicted detection limits for arsenic as a function of pulse current are presented in Table 3. The detection limit with no pulsing was experimentally determined.5 Gating of the LPDA exposure decreased the integrated intensity by a factor of five and caused a decrease in the detection limits by a comparable factor.With increasing pulse current the predicted detection limit decreased as expected. The improvement in the detection limit with a 300 A pulse was a factor of 20 compared with d.c. operation with 18% gating. The use of an 18% gate is extreme. In general a 75% gate is used in routine operation; the shutter is closed for approxi- mately 5 ms the time necessary to read 64 pixels at a 26 kHz pixel read frequency. A 75% gate produces a detection limit of 33 pg for arsenic and pulsing of the lamp to 300 A results in a detection limit of approximately 0.3. A faster analogue- to-digital converter can be used to increase the gate trans- mission further and reduce the detection limit. Ideally a solid- state detector capable of simultaneously shifting all pixels into Table 3 Detection limits for arsenic as a function of pulse current Detection limit/pg Pulse current/A 0 0 40 80 120 160 200 240 270 300 Gated* No Yes Yes Yes Yes Yes Yes Yes Yes Yes Noise? 0.01 30 0.0662 0.0120 0.0095 0.0032 0.0021 0.001 1 0.0009 0.0008 0.0007 18% Gatet 25 125 22 18 6 4 2 2 2 1.3 75% Gate$ 25 33 6 5 2 1 0.5 0.4 0.3 0.3 * 18Y0 Transmission gate.t Experimentally determined. $ Computed from duty cycle.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 423 read registers could be used which would result in no deterior- ation of the detection limit and would still provide the accurate background correction. The use of a 300 A pulse on a routine basis is not practical for a 300 W lamp.At 300 A the lamp is operated at a power level of 732 W. This is 20% higher than the factor of two increase that is tolerable with burst-mode operation7 The factor of two is obviously conservative or else lamp failure would have occurred. The 300 A pulse current is still below the factor of two for the 500 W lamp. Realistically however pulse currents in the 100-200A range should prove more useful. Pulsing in this range would result in improvement of the detection limit of arsenic from 25 to between 20 and 0.5 pg (Table 3). Lamp Lifetime It is clear that there is a substantial detection limit advantage to be obtained by operating the xenon arc lamp at high pulse currents. However there is a major trade-off involved in terms of lamp lifetime. The detection limit values given in Table 3 were obtained with a new lamp and the measurements were made within the first few hours of operation.With continual pulsing at a level which exceeds the power rating of the lamp a significant reduction in lamp lifetime can be expected. The longevity of a new 500 W lamp pulsed to 200 A at 50Hz with a 5% duty cycle and a simmer current of 20A was studied. Intensity measurements were again made at 193.7 nm and were corrected for far stray light (see Experimental). The plot of relative output versus number of pulses is shown in Fig. 4. It can be seen that initially the integrated intensity decreased extremely rapidly with the number of pulses. After 5000 pulses the integrated intensity decreased by approximately 95% of its original value.Between 5000 and 50000 pulses the pulsed intensity of the lamp remained fairly constant at 5% of the original value. During this interval the variation between pulses was approximately 2%. Although the integrated intensity with the 200A pulse decreased dramatically the ratio of the pulsed to d.c. operation was still approximately the same; that is pulsing is still providing the same improvement in lamp intensity as com- pared with d.c. operation as it did when the lamp was new. Thus the d.c. lamp intensity had also decreased by 95% of its original value. The 500 W lamp was still operational after 75000 pulses. After 50 000 pulses however the integrated intensity became extremely erratic. The LPDA was successful in removing flicker noise from the absorbance calculation but the variation between pulses was extreme.Integrated intensity uersus the number of 200 A pulses for a loo 6 1 b 20dOO 4odoo 60d00 80d00 / d o 0 0 No. of pulses Fig. 4 Relative intensity as a function of number of pulses for A 300 W lamp with 200 A pulses; B 300 W lamp with I00 A pulses; and C 500 W lamp with 200 A pulses new 300 W lamp is also shown in Fig. 4. This study employed conditions identical with those for the 500 W lamp. Initially the behaviour of the 300 W lamp was very similar to that of the 500 W lamp i.e. a rapid decrease in the integrated intensity was observed in the first 6000 pulses followed by stable operation for the next 50 000 pulses. Variation between pulses was again around 2%.The integrated intensity of the 300 W lamp decreased by only 90% of its original intensity compared with 95% for the 500 W lamp. After approximately 60000 pulses the lamp failed to ignite. This was preceded by frequent incidents when the lamp extinguished in the course of an experiment. Finally a new 300 W lamp was pulsed under conditions identical with those used previously using a pulse current of 100 A. The same dramatic decrease (to approximately 10%) in the integrated intensity is observed in the first 6000 pulses. As with the 300 W lamp with a 200 A pulse very stable operation was observed beyond 6000 pulses with little variation between pulses. At 100 A however the lamp is still operational after 240000 pulses with a variation between pulses of less than 2%.A 500 W lamp was not tested with 100 A pulses. It is reasonable to assume however that the 500 W lamp would exhibit the same extended lifetime (as compared with 200A pulsing) as the 300 W lamp. During the period when the output of the lamps was relatively stable (from around 20000 to 50000 pulses) the integrated intensities of the 300 and 500 W lamps operated at 200 A were approximately equal to the 300 W lamps operated at 100 A delivering approximately one third that intensity. The 90% loss in intensity observed with a 100 A pulsing of the 300 W lamp is considerably greater than that reported by the manufacturer who reports a 35% loss in intensity after 20 000 pulses. In this study however longer pulses ( 1 ms versus 0.5 ms) and a higher duty cycle (5% versus 0.4% or less) were used.It must be stressed that the original reference intensities in this study were determined as the average intensity for the first 360 pulses. A delay of several thousand pulses in determin- ing the reference intensity would make a considerable difference in the shape of the curves in Fig. 4. Pulsing in Perspective It is not possible to consider the merits of pulsing of the Cermax lamps without considering the behaviour of the lamps in the d.c. mode of operation. In Fig. 5 integrated intensity is plotted as a function of d.c. hours of operation for five 300 W lamps one 500 W lamp (none of which were pulsed) and the manufacturer’s predicted behaviour. The broken lines represent the mean and the extremes of lamp intensity specified by the manufacturer.The termination of an experimental lamp trace 120 1 0 ’ I 1 1 10 100 Period of operatiodh Fig.5 Corrected intensity as a function of hours of operation for 300 W (A-E) and 500 W (F) lamps operated in d.c. mode. The dotted lines represent the mean and extremes of lamp intensity specified by the manufacturer424 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 in Fig. 5 does not mean that the lamp failed it represents the point at which the lamp was removed from the study. It can be seen that the experimental plots of the lamps all fall close to or below the lower extreme predicted by the manufacturer. After 10-40 h of operation the intensity of three of the lamps had fallen to 50% of their original intensity. The 500 W lamp lost intensity at a faster rate than the 300 W lamps but it is difficult to reach any conclusions with such limited data.During the course of uninterrupted lamp oper- ation there was some decrease in intensity depending on the length of operation. The largest drops in intensity were observed between the time the lamps were shut off for the night and re-ignited the next day. Attempts to re-optimize the position of the lamp to achieve the intensity of the previous day were always futile. A main source of error in comparing the experimental data and the manufacturers data is the determination of the initial intensity. In this study the initial intensity of the lamps were made within minutes of the first ignition. As mentioned pre- viously a delay in acquiring the intensity to be used as the reference would considerably change the shape of the curves in Fig.5. The observation that most of the lamps tested in this study fall close to the lower limit suggests that the manufac- turer's reference point represents the intensity averaged over a longer period of time than employed in this study. Using arsenic as a reference element detection limits obtain- able at various stages in the lifetime of the 300 W lamp in d.c. and pulsed mode (using 100 and 200 A pulses) were computed (Table 4). For these computations it was assumed that pre- atomization and atomization cycle data acquisition required 5 s and the entire ETA cycle required 3 min. It was further assumed that a 75% gate was used and the pulsing frequency was 60 Hz thus 300 pulses were required per atomization. The detection limits for the pulsing of the 300 W lamp are based on the experimental results in this study.The detection limits for d.c. operation were based on the manufacturers predicted behaviour and the data in Fig. 5. As expected pulsing at either current and with either a 18 or 75% gate offers significant improvement in the detection limit for the first 20 atomizations (the first hour of operation or 6000 pulses). With the 18% gate after the first 20 atomiza- tions d.c. operation and pulsing at 200A are comparable. After 200 atomizations at 200 A the 300 W lamp fails and d.c. operation offers detection limits a factor of two better than those achieved with pulsing at 100A with an 18% gating. After 1000 atomizations d.c.operation and pulsing the lamp at 100A with 75% gating provides the same detection limit. This comparability should last until the lamp fails since the intensity of the lamp is stable in both modes of operation. An alternative means of evaluating the lamp lifetime is to consider the number of experiments that can be run with the 300 and 500 W lamps with 100 and 200 A pulses. Both lamps proved to have pulsed lifetimes of 60000 pulses for a 200 A pulse. Based on a 60 Hz pulsing frequency 5 s of data acqui- sition per atomization and 50 atomizations per experiment (five standards in triplicate five samples in triplicate and 20 blanks) one experiment requires 15 000 pulses. This means Table 4 Detection limits (pg) for arsenic as a function of lamp lifetime that with this pulsing regime both Cermax lamps pulsed at 200A could only be used for four experiments.Pulsed at 100 A the 300 W lamp and probably the 500 W lamp could be used for 16 experiments and most probably more. Physical Changes in the Lamps In both d.c. and pulsed operation it appears that physical changes in the position of the cathode relative to the anode dictate the effective lifetime of the lamps. It is well documented that in d.c. operation the intensity of the xenon arc lamps decreases asymptotically to a level that remains steady for the duration of the lifetime of the lamp. This corresponds to an initial melting-back of the pointed tip of the cathode until it reaches a blunt shape that is relatively impervious to further change in shape. Eventually the continued heating of the three struts that support the cathode produces metal fatigue and results in a shift in the position of the cathode such that the arc gap is too large to permit ignition of the lamp. In the d.c.mode this event usually comes after more than 1000 h of operation. When pulsed at 200 A both the 300 and 500 W lamps experience an acceleration of both processes the melting of the tip and the eventual extreme shift of the cathode away from the anode. The melting of the cathode tip increases the arc gap and results in a larger more diffuse arc. The more diffuse arc is less bright in the UV and is less than optimum for focusing through a 500pm square aperture. Consequently a loss in intensity in the UV accompanies the ageing of the lamp and this loss is more pronounced than at longer wavelengths.It appears that pulsing at currents as high as 200A not only accelerates the cathode melting process but exceeds the limits reached with d.c. operation. After 6000 pulses at 200A the d.c. intensities of the 300 and 500 W lamps were far less than predicted for normal deterioration. Examination of the failed 300 W lamp revealed a separation of the cathode structure of the lamp. This was not true of the 500 W lamp supporting the manufacturers statement that the 500 W lamp was more suited for the high current pulses. Conclusions For d.c. operation the 500 W xenon arc lamp is in general a factor of 1.5 more intense except below 210nm where the :300 W lamp is more intense. Pulsing at high currents provides a significant increase in the integrated intensity of the LPDA and can result in a factor of up to 80 improvement in the detection limit for a new lamp.Pulsing at 200A induces premature failure of both the 300 and 500 W lamps after 60 000 pulses. The cost of the lamps make such limited lamp lifetime intolerable. Pulsing at 100 A is less deleterious but also provides less significant improvements in detection limits. After the first .SO h a 300 W lamp pulsed at 100 A is at best comparable to d.c. operation. While the intensity increase obtained with high current pulsing of new lamps will continue to be attractive the practical use is very limited. An improved lamp design is Number of atomizations 1 50 100 150 200 500 1000 Time/h 0.05 2.5 5.0 7.5 10 25 50 Number of pulses 300 15 000 30 000 45 OOO 60 000 150 000 300 000 D.c. operation Pulsed operation ( 18% transmission gate) Pulsed operation (75% transmission gate) ILC This study 25 25 26 26 27 29 28 30 30 32 32 37 33 52 100 A 200 A 9 3 70 22 75 25 75 25 80 30 80 - 80 - 100 A 200 A 3 0.5 22 4.7 25 4.5 25 4 30 5 30 30 - -JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 425 necessary before pulsing of xenon short arc lamps will be feasible for CSAAS. References 1 Moulton G. P. O’Haver T. C. and Harnly J. M. J. Anal. At. Spectrom. 1989 4 673. 2 Moulton G. P. O’Haver T. C. and Harnly J. M. J. Anal. At. Spectrom. 1990 5 145. 3 Harnly J. M. J. Anal. At. Spectrom. 1993 8 317. 4 Smith C. M. M. Nichol R. and Littlejohn D. J . Anal. At,. Spectrom. 1993,8 989. 5 Smith C. M. M. and Harnly J. M. Spectrochim. Acta Part B in the press. 6 Harnly J. M. Anal. Chem. 1986 58 933A. 7 Chinnock R. ICL Engineering Note No. 152 Use of X e Short Arcs as Pulsed Light Source ILC Technology Sunnyvale CA 1982. 8 Naval Research Laboratory Ultraviolet Output from Pulsed Short Arcs Washington D.C. Memorandum Report 2427 April 1971. 9 ILC Technology Sunnyvale CA product literature. 10 ILC Technology Sunnyvale CA personal communication. 11 Zander A. T. Miller M. H. Hendrick M. S. and Eastwood D. Appl. Spectrosc. 1985 39 1. Paper 3/04066F Received July 12 1993 Accepted October 1 I 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900419

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 427 Equilibrium and Mass Spectrometry of Nitrate Decomposition in Electrothermal Atomic Absorption Spectrometry* Trevor M c Al I iste r CSlRO Materials Science and Technology Locked Bag 33 Clayton Victoria 3 768 Australia The decomposition of metal nitrates to oxides is assumed to occur in electrothermal atomic absorption spectrometry during the pre-atomization drying or thermal pre-treatment of nitrate samples. The mechanism of thermal decomposition of nitrates to solid oxides and gaseous NO is well established but some evidence from recent electrothermal mass spectrometry (ET-MS) experiments has been used to support an alternative mechanism that of gaseous metal oxide formation e.g. MNO,-+MO(g) + NO,(g) The ET-MS of nickel cobalt copper magnesium and lead nitrate samples have been re-examined using a quadrupole mass spectrometer with cross-beam rather than axial sampling of gas from the furnace.With this new sampling geometry no gaseous metal oxides were detected over a range of heating rates from slow drying to rapid atomization. These results are in keeping with thermochemical equilibrium calculations as is the observation of gaseous phosphorus oxides in the thermal decomposition of NH,H,PO,. Earlier observations of gaseous metal oxides of the above elements by ET-MS are attributed to the formation of molecular clusters of oxides and/or nitrates during rapid decomposition of the metal nitrates. Keywords Electrothermal atomic absorption Spectrometry; mass spectrometry; equilibrium; nitrate decomposition; gaseous oxide It has generally been assumed in electrothermal atomic absorp- tion spectrometry (ETAAS) that in the analysis of nitric acid matrices or nitrates of metals the metal nitrate precipitated in the furnace by drying will decompose on further heating to give the solid metal oxide and gaseous nitrogen dioxide and oxygen M( NO~)~+MO(S) + 2NO,(g) + O.5O2(g) (1) The production of gaseous metal atoms from nitrates in ETAAS should then occur by the reduction of MO(s) by graphite at higher temperatures or via gas-phase decomposition of a stable gaseous oxide MO(g).The mechanism of the isothermal decomposition of solid metal nitrates has been well established' and more recent has tended to confirm the accepted features of nucleation and growth of oxide in the nitrate and the diffusion of the NO,(g) produced to the solid-gas interface.In ETAAS N02(g) has been observed by mass spectrometry (MS) during pre-atomization heating of nitrates of c o ~ p e r ~ n i ~ k e l ~ lead6*'*' and ~ o b a l t . ~ Also reported in these studies were MO+ ions having been observed during rapid pre- atomization heating and attributed to oxide species in the gas phase arising from the rapid decomposition of nitrate.7 L'vov'' has suggested that the precursors of the MO+ ions are genuine gaseous diatomic oxides MO(g) and that a different mechan- ism of oxide formation is operating in the rapidly heated furnace of the ETAAS system to that in the isothermal kinetic studies M(NO,),(s)~MO(g)+2NO,(g)+0.502(g) (2) or M(N03)2(s)-,M0(g)+2N0(g)f 1.502(g) (3) Although these reactions are not thermochemically favoured compared with reaction (1) in the temperature region 400-700 K L'vov has argued that in ETAAS the kinetic rates of reactions (2) or (3) are substantially faster than that of ( 1 ) owing to solid-state factors such as the rate of diffusion of NO,(g) or NO(g) through the solid reactant and pro- duct.L'vov proposed that the product MO(g) condenses in the l3TAAS furnace at 1 atm (1 atm = 101 325 Pa) so that * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993. reduction of MO(s) can then proceed without loss of analyte. Electrothermal mass spectrometry (ET-MS) experiments con- ducted in ZIUCUO would however detect the product MO(g) because the gaseous oxide should not condense at the low pressures of those experiments.As this novel mechanism contravenes the accepted view of solid nitrate decomposition and relies on incidental reports of MO' ions in work which had other objectives and therefore neglected the phenomenon the experiments on rapid pre- atomization heating of cobalt copper nickel and lead nitrates have been repeated using a recently modified ET-MS instru- ment of cross-beam geometry," and the thermochemical equi- librium in the decomposition of nitrate was calculated by the free energy minimization m e t h ~ d . ' ~ . ' ~ Experimental Several designs of ET-MS apparatus have been reported. Bass and Holcombe7 used a graphite rod atomizer whereas Styris,14 Sturgeon et aL6 and Ham and McAllisterg used tubular graph- ite furnace atomizers.All systems used quadrupole mass spec- trometry (QPMS) instruments and a variety of sample loading methods were employed involving loading the furnace at 1 atm and transferring it to a pumping region or injecting a sample into the furnace in UQCUO through a carefully aligned capillary. The design of Styris14 was elaborated to permit the sampling of the furnace gases during atomization from 1 atm through a skimmer cone into the QPMS instrument at low pressure. The common feature of all these designs was the placement of the atomizer in line with the axis of the QPMS instrument. The design of Ham and McAllisterg has been modified recently'' to permit cross-beam sampling of the furnace gases with the atomizer situated off the axis of the QPMS system (see Fig.1). Such a geometry is commonly used in MS to prevent particles generated in experiments being translated down the axis of the QPMS instrument and creating inter- ference in the electron multiplier detector. This design has the additional benefit of enabling the furnace temperature to be measured by an infrared IR pyrometer (Ircon Modline 11) and was used recently to determine the appearance temperatures of GeO(g) from alkaline and acid matrices of Ge.I5 The sensitivities of the ET-MS systems designed so far are low especially when QPMS is being used to scan over a mass spectrum rather than monitoring a single peak. A typical sample in ET-MS is therefore of the order of several hundred428 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 Ion source Viewing Graphite furnace Twin Cu rod feedthrough Fig. 1 Schematic diagram of the cross beam sampling geometry of the ET-MS instrumentation viewed from above nanograms. In this work 5 pl of 0.1 g 1-l solutions of the metal nitrates in de-ionized water were used. Thermochemical equilibrium in the thermal decomposition of the metal nitrates was calculated as p r e v i o ~ s l y ~ * ~ ~ using the CSIRO Thermochemistry System of Turnbull and Wadsley,12 which employs the free energy minimization technique of Eri ksson. l3 Equilibrium Calculations L'vov calculated the equilibrium partial pressure of MO(g) in reaction (2) for magnesium cobalt and copper nitrate hexahy- drates for reaction (3) for anhydrous nickel nitrate and for reactions (2) and (3) for anhydrous lead nitrate.In view of the uncertainty in the degree of hydration of the deposited salts in vacuum ET-MS and of the lack of reliable thermochemical data especially entropies and specific heats of the nitrates and their hydrates such calculations can only be taken as a very approximate indication of the equilibrium in thermal decompo- sition of these nitrates. When matched with experimental observations of appearance temperatures which are also very approximate only the most tentative conclusions can be drawn. Therefore unhydrated nickel and lead nitrates were taken as examples as used in L'vov's calculations," for the estimation of equilibrium by free-energy minimization allowing for both MO(s) and MO(g) as possible products along with N02(g) NO(g) and 02(g).For Ni(N03)2 the following values were used AH,(298 K)= -415 kJ mol-I from Wagman et ~ 1 ; ' ~ S0(298 K)= 183 J mo1-l K-l calculated by the method of Mills and Latimer in the CSIRO Thermochemistry System;I2 and C,= 13.9+0.067T J mol-' K-l by analogy with the data for Mg(N03)2.12 For Pb(N03)2 the values used were AHf( 298 K) = - 45 1.9 kJ mol from Wagman et ~ 1 . ; ' ~ So = 220 J rno1-l K-l again calculated by the method of Mills and Latimer;12 and C as for Ni(N03)2. The data for NiO(g) and PbO(g) came from the CSIRO Thermochemistry System database.12 The results show that either NiO(s) or PbO(s) should be the overwhelmingly dominant metal oxide product of the thermal decomposition of the anhydrous metal nitrates between 400 and 700 K and that the gas phase is composed of NO2 and O as in reaction (1).The over-all pressures in both systems are given in Table 1. By repeating L'vov's method," but noting that the exper- imental data2-g indicated that NO,(g) rather than NO(g) is produced in the decomposition a restricted equilibrium based on reaction (2) only can be calculated. The partial pressures found for gaseous nickel and lead oxides shown in Table 2 vary from 1 x atm between 300 and 700 K. to 1 x Table 1 Variation with temperature of equilibrium pressure of YO2( g j + O,( g j in the reactions ! i ) Ni(N03),(s)=NiO(s)+ 2NO,(g) +O.5O2(g) (ii) Pb(NO,),(s)=PbO(s)+2NO,(g)+0.5O2(g) ((iiij pressure of N02(g)+02(gj produced in 1 x the rate data of Criado et aL3 for reaction (i) s according to Temperature/K 300 350 400 450 500 550 600 650 700 p(i)/atm* 4 x 9 x lo+ 6 x 1 x lo- 0.18 1.4 7.9 32.7 - p (ii)/atm 2 x lo-'* 2 x 10-9 4 x 10-7 2 x 10-5 4 x 10-4 6 x 5 x 0.3 1.3 p(iii)/atm I 10-19 1 10-15 2 10-9 4 10-7 1 10-5 4 10-4 7 10-3 1 x 10-l' - * 1 atm= 101 325 Pa.Table 2 Variation with temperature of pressure of MO(g) in the reactions (i) Ni( N03),(s) =NiO(gj + 2N02(g)+0.50,(g) (ii) Pb(NO,),(s)=PbO(g)+ 2NO,(g)+0.50,(g) Temperature/K 300 3 50 400 450 500 550 600 650 700 p NiO (g)/atm* 1 x 10-39 1 x 10-25 1 x 10-15 1 x 10-13 1 x 1 x 10-l8 1 x lo-" 1 x 10-'O 1 x P PbO(g)/atm 1 x 10-17 1 10-14 2 10-9 2 10-5 1 10-4 1 x 1 x lo-" 7 x 1 x lo+ * 1 atm= 101 325 Pa.L'vovfo defined the detection limit for the ET-MS as z 1 mPa or 1 x lo-* atm. In comparing experimental observations of appearance temperatures of MO' a lower level probably 1 x atm is more realistic in the ET-MS system used in the present experiments as this tends to be the background pressure in the apparatus. This pressure should be reached by NiO(g) between 650 and 700 K and by PbO(g) at ~ 5 0 0 K. These calculations do correlate approximately with the experimental observations. However at these temperatures the pressure of N02(g) and 02(g) from reaction (1) would be much higher than that of MO(g) from reaction (2). It could be expected that the pressure of these gases expanding in the vacuum of the ET-MS appar- atus would break up the sample crystals unless reaction (1) is retarded for reasons undiscovered by previous investigat~rs.l-~ Indeed in the case of nickel the kinetic data for the thermal decomposition of Ni(N03)2 can be used to estimate the pressure of NO2 and O2 gases generated from the decompo- sition of Ni(N03)2 in 1 x s which is an appropriate time scale for earlier experimental observations.' The results of this calculation using the data of Criado et u Z .~ for a simple zero- order mechanism with A (frequency factor) = 1 x lo5 s-' and E (activation energy) = 84 kJ mol-' are given in the third column of Table 1 [P (iii)]. The data of Mu and Perlmutter2 yielded higher pressures at every temperature in this calcu- lation. The results still suggest substantial pressure of these gases above the background in the vacuum of the ET-MS instrument at = 600 K.Moreover in the case of hydrated salts dehydration reactions2 will yield substantial pressures of H20(g) which will disrupt the sample at temperatures even lower than that of the nitrate decomposition. It is noteworthy that in all cases specified by L'vov as hydrated salts the appearance temperature of the MO' ions was lower than with the unhydrated salts.429 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Results A common feature of the reports on MO(g) in ET-MS is the use of a high heating rate (z 500 K s-I). Nickel can be taken as a typical example of this. In our earliest experiments on ET-MS it was found that an atomization heating pulse applied to the furnace containing a nitric acid solution of nickel without previous drying or thermal pre-treatment gave a transient signal due to NiO' at the beginning of the heating pulse (see Fig.2).17 In subsequent work on cobalt,' the practice of pre-atomization treatment in uucuo at very low heating rates was adopted which eliminated the COO' signal from the early part of the atomization heating cycle. Droessler and Holcombe' observed NiO+ in rapid heating to 625 K accompanied by strong signals for NO2+ and 02+ and a much weaker signal for Ni(N03),' from 500 ng of Ni(N03)2. A repeat of the nickel experiment in the modified cross- beam ET-MS instrument with the ionizing energy at 15eV and using rapid heating (500 K s-') without pre-atomization treatment showed no NiO' although NO,' and 02+ were observed. An important consideration in comparing this experi- ment with the earlier work is the ionizing energy used.Since the earliest work the practice of using low ionizing energies ( z 15 eV) has been adopted in order to emphasize the signals owing to the ionization of atoms or to molecular ions over those of fragment ions from molecules and also to suppress background signals owing to the mass spectrum of the diffusion pump oil. At 15 eV any significant amount of NiO(g) emitted from the furnace should be detected as has been seen in other cases such as that of GeO(g) from Ge.I5 On the other hand at the ionizing energies used in our early work on nickel (50eV) and in that of Droessler and Holcombe' (70eV) substantial fragmentation of molecules such as Ni(NO,),(g) should occur and the weak Ni(NO,),+ observed by Droessler and Holcombe could be the parent ion in a 70eV mass spectrum containing a prominent fragment NiO+.In the present work Ni(N03)2+ at 15 eV was not observed. In view of the simultaneous detection of NO,(g) and 02(g) the earlier work could be interpreted as having detected not NiO(g) but a nickel nitrate species [which will be called NiO(s) for convenience] introduced into the vacuum during the decompo- sition Ni(N03)2-,NiO(s)+ 2N02(g)+0.50,(g) The most investigated example of this metal oxide-metal nitrate phenomenon is that of lead,6y7g8 where PbO' was observed at z 600 K during rapid heating. Bass and Holcombe7 Ni' also observed that no PbO+ was produced when PbO(s) was heated in the furnace. They concluded therefore that the PbO' was associated with the nitrate decomposition and was due to PbO being released from the surface during the associated crystal rearrangement and generation of gaseous products.They did not specify the state of the PbO thus released. In a similar experiment on a lead nitrate sample in the cross-beam ET-MS system used in the present work at 15 eV ionizing energy no PbO+ was observed. Wang et ul. made very similar observations for Cu(N03) and in addition noted that the CuO + signal was substantially reduced by charring the deposited nitrate at 523 K under 1 atm pressure. This last observation is in keeping with the result of the effect of a low pre-atomization heating on the appearance of COO +.' In the cross-beam ET-MS instrument when rapid heating of samples of CU(NO,)~ and Co(N03) was carried out without pre-atomization treatment neither CuO + or Cu,O + nor COO+ were detected at 15 eV ionizing energy in contrast to the earlier work4y9 from apparatus in which the atomizer was situated on the axis of the QPMS system.The non- detection of these oxide ions in cross-beam ET-MS stands in contrast to the case of ions from volatile oxides of known thermochemical stability such as Ga,O(g) In,O(g) and As,O,(g) which have been detected in the 'on-axis' ET-MS instrument.18 A repeat of these earlier experiments on gallium indium and arsenic using the modified cross-beam ET-MS system found all the oxide species associated with these systems which were predicted by free energy minimization calculations. Experiments were also carried out in the cross-beam ET-MS instrument on the remaining two systems specified by L'vov Mg(N0,)2 and NH,H,PO,.In the case of magnesium no MgO+ or Mg(OH),+ ions were detected. This is however a less relevant case than the other nitrates as the original experiments of Styris and Redfieldlg were carried out on their atmospheric pressure sampling ET-MS instrument and clus- tering of species could have occurred in the expansion inside the sampling cones Mg(OH),+ for example could be an ionized cluster MgO-H20. The case of NH,H,PO is particu- larly interesting because it is the only species investigated by L'vov for which a straightforward calculation of equilibrium composition without restriction of products predicts the gener- ation of a substantial amount of gaseous oxide P4010(g) during thermal decomposition.The original work of Bass and Holcombe,' however found not P4010+ but PO2+ and a range of unspecified heavier oxyphosphorus ions. The mass spectrum of vapours from phosphorus oxides is known to be complex." NiO+ Fig. 2 of Ni(NO,) deposited in a graphite furnace Mass spectrum howing Ni' and NiO+ peaks at m/z 58 60 74 and 76 taken at 50 eV ionizing energy during rapid heating of a sample430 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 In the cross-beam ET-MS system for NH4H2P04 PO2+ was detected at the beginning of heating and P2+ at ~1300°C much later in the heating pulse. This latter ion is presumably due to P2(g) or P,(g) from reduction of the phosphorus oxides by C analogous to the As2+ observed in the ET-MS of arsenic." Discussion The new evidence from the cross-beam ET-MS experiments casts serious doubt on the proposition that reaction (2) is occuring in ETAAS of nitrate solutions.Genuine gaseous diatomic oxides such as NiO(g) CoO(g) CuO(g) or PbO(g) should be detected by the cross-beam apparatus as have been GeO(g),15 the triatomic oxides Ga20(g) and In20(g) and the polyatomic oxide As,06(g).18 What then is the source of the MO+ ions observed in the experiments with axial placement of the atomizer? In the case of nickel the work of Droessler and Holcombe' suggested that the precursor could be a nickel nitrate species but if it is Ni(NO,),(g) the polyatomic ion Ni(N03)2f must be relatively unstable as it was not detected by cross-beam ET-MS.In their work on lead Bass and Holcombe suggested that PbO (in an unspecified state) was released from the surface of the atomizer during the nitrate decomposition whereas heating a sample of PbO(s) in the atomizer showed no PbO'. It could be that disruption of the crystal structure of Pb(N03)2 by the rapid evolution of NO,(g) during rapid heating of the atomizer is spraying PbO off the atomizer surface in the form of large clusters. At a pressure of 1 atm in ETAAS these would rapidly condense again on the atomizer surface but in the vacuum of the axial ET-MS system they might be projected into the ion source by the force of the expanding NO,(g) and having trajectories close to the axis of the QPMS instrument their product ions would also move along the axis of the QPMS and be detected.The observation by Wang et aL4 and in earlier work in this laboratoryg that pre-treatment by low heating rates reduces or eliminates the oxide signal suggests that such pre-treatment gives a less violent decomposition of the nitrate in which substantially less oxide is sprayed into the vacuum of the ET-MS system. In cross-beam ET-MS the clusters produced in the rapid decomposition that entered the ion source would have been accelerated at right angles to the axis of the QPMS instrument by the force of exothermic decomposition of the nitrate. The velocity of the product NiO' ion again at right angles to the quadrupole axis would be substantially higher than that of an ion originating from a gaseous oxide molecule with a normal gas kinetic velocity produced by an equilibrium process at 500-600 K.Once this velocity reaches about 1 x 10' cm s-' it is of the same order as that required by an ion of rn/z=74 moving in an arc of radius 0.3 cm under the influence of a draw-out field of lOVcm-' values which are reasonable criteria for the successful extraction of an ion into the quadru- pole analyser in this apparatus. These clusters need not be solely of oxide molecules but could be a mixture of reactant nitrate and product oxide for which the dominant ion in the ET-MS instrument is MO'. There is scope here for further experiments with more sophisticated ion sources offering higher draw-out potentials than were available here in order to detect these product MO + ions and estimate their velocities.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 References Stern K. H. J. Phys. Chem. Ref. Data 1972 1 747. Mu J. and Perlmutter D. D. Thermochim. Acta 1982 56 253. Criado J. M. Ortega A. and Real C. React. Solids 1987 4 93. Wang P. Majidi V. and Holcombe J. A. Anal. Chem. 1989 59 974. Droessler M. S. and Holcombe J. A. Spectrochim. Acta Part B 1987 42 981. Sturgeon R. E. Mitchell D. F. and Berman S. S. Anal. Chem. 1983,55 1059. Bass D. A. and Holcombe J. A. Anal. Chem. 1987 59 974. Bass D. A. and Holcombe J. A Anal. Chem. 1988 60 578. Ham N. S. and McAllister T. Spectrochim. Acta Part B 1988 43 789. L'vov B. V. Mikrochim. Acta 1991 11 299. McAllister T. Int. J. Mass Spectrom. Ion Proc. 1990 101 127. Turnbull A. G. and Wadsley M. W. The CSIRO Thermochemistry System Version V CSIRO Division of Mineral Products Port Melbourne 1986. Eriksson G. Chem. Scr. 1975 8 100. Styris D. L. Fresenius' 2. Anal. Chem. 1986 323 710. Doidge P. S. and McAllister T. J. Anal. At. Spectrom. 1993 8 409. Wagman D. D. Evans W. H. Parker V. B. Halow I. Bailey S. M. and Schumm R. H. NBS Tech. Note 270-3 and 270-4 US Department of Commerce 1968 and 1969. Ham N. S. and McAllister T. unpublished data. McAllister T. J. Anal. At. Spectrom. 1990 5 171. Styris D. L. and Redfield D. A. Anal. Chem. 1987 59 2891. Muenow D. W. Uy 0. M. and Margrave J. L. J. Inorg. Nucl. Chem. 1970,32 3459. Paper 3/04059C Received July 5 1993 Accepted September 28 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900427

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

43 1 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Use of a Furnace With a Graphite Filter for Electrothermal Atomic Absorption Spectrometry* Dmitry A. Katskovt Rudolph Schwarzer Pieter J. J. G. Marais and Robert 1. McCrindle Centre for Applied Chemistry Technikon Pretoria Private Bag X680 Pretoria South Africa The development of an atomization technique for electrothermal atomic absorption spectrometric (ETAAS) analysis based on the filtration of the analyte vapour through heated graphite is presented in its historical context. Initially the method was applied to the analysis of solids. Despite the effectiveness of atomization the time taken to prepare each solid sample and graphite tube arrangement made single-element determi- nations impractical using the ETAAS technique.The proposed new version of the furnace with a graphite filter was designed for the analysis of liquids using commercial ETAAS instruments equipped with an autosampler programmable power supply and a system for background correction. The main advantages of the atomizer were discovered in the course of the determination of Al Bi Cd and Cu for different sample volumes and Cd Pb and Bi in the presence of NaCl and CuCI matrices. These advantages include a 1.6-2.8-fold increase in sensitivity; the possibility of increasing the volume of doped solutions up to 100 pl and at the same time reducing the drying period to 15 s; and obtainment of a lower level of spectral background and chemical interferences without chemical modification. The accuracy of the method was verified by the determination of Cd and Pb in whole blood and steel. Keywords Electrothermal atomic absorption spectrometry; furnace with graphite filter; matrix interference; analysis of blood and steel The effect of the enhancement of the analytical signal in electrothermal atomic absorption spectrometry (ETAAS) when a furnace made from porous graphite was replaced by a furnace lined with Ta foil has been described by L'vov.' The results obtained led to the conclusion that at high temperatures ordinary (not pyrolytic) graphite becomes transparent to the vapours of some metals.The subsequent interest displayed in the transportation of a metal vapour through graphite resulted because of some practical problems in the ETAAS analysis of solids.' When certain powdered chemicals or organic materials were analysed using the boat technique a lengthy procedure was required for the decomposition and pyrolysis of the matrix and the removal of adsorbed gases.Rapid heating of the atomizer caused pulsed ejection of sample powder or soot particles into the analytical zone together with the atomic vapour. The corresponding background signals restricted the limit of detec- tion of the analyte elements. The effect of transportation of metal vapours through graph- ite has been used in different atomizerss7 to prevent the ejection of analyte material into the analytical zone. The atomizer 'furnace with ring ~avity'~ consisted of two coaxial graphite tubes situated between removable graphite washers of special shape [Fig.l(u)]. The external tube was made of more dense graphite than the internal tube. The powder to be analysed was sampled into the ring space between these two tubes. When both tubes were heated electrically via the washers the vapour of the sample passed into the analytical zone of the central cavity through the graphite. About 100-150mg of a powdered mixture of the solid sample with graphite could be sampled. In spite of very low limits of detection for some highly-volatile elements ( z 1 x this atomizer has not really been used in practice because of the complexity of construction and poor reproducibility of analyt- ical results owing to the lack of control of the temperature and heating rate of the internal tube. The idea of the 'capsule-in-flame' atomizer [Fig.1 (b)) * Presented at the XXVIII Colloquium Spectroscopicurn Inter- nationale (CSI) York UK June 29-July 4 1993 and the CSI Post Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993. t On leave from the State Institute for Applied Chemistry Dobrolyobova pr. 14 St. Petersburg 197198 Russia. (b) 4 6 t- 5 / 9 4 4 Fig. 1 Atomizers with the sample vapour filtration (a) furnace with ring cavity; (b) capsule-in-flame; (c) capsule-in-furnace; ( d ) ring chamber tube; (e) furnace with graphite insert. 1 and 2 Internal and external tubes; 3 washers; 4 powdered sample; 5 graphite contact; 6 capsule; 7 burner; 8 tube furnace; 9 graphite plug; 10 graphite insert; 11 graphite thread; and 12 dosing hole appeared to be more promising than its predecessor.In this m e t h ~ d ~ the powdered sample mixed with graphite was placed in a graphite cylinder (capsule) of about 20mm in length and with an external diameter of 3.5 111111 having an internal cavity of 1.5-2 mm in diameter and a length of about 18 mm. About 30mg of powder could be dosed into the capsule cavity. The capsule was held between two water-cooled stands which served as electric contacts. The burner for acetylene-air or acetylene-dinitrogen oxide flames was situated beneath the capsule. When the capsule was heated by an electric current the sample vapour was introduced into the flame which maintained a constant and sufficiently high temperature in the analytical zone.432 JOURNAL OF ANA An analytical practice described by Kruglikova6 made it possible to determine the main advantages of the filtration of the sample vapour through the heated graphite partition and also the disadvantages of the atomizer used.It appeared that a substantial portion of high- and even low-volatility carbide forming elements such as Ti and V could be transported rapidly through the heated graphite. Any interferences resulting from light scattering were absent; the chemical and spectral interferences were decreased substantially. When refractory metals were being determined some draw- backs of the atomizer4 were discovered. A longitudinal tem- perature gradient due to the cooling of the ends of the capsule caused some of the vapour to travel along the capsule cavity. This vapour then condensed at the ends of the capsule in a low-temperature region.This led to the evaporation process being hindered. Attempts to increase the evaporation rate by means of increasing the electrical power caused damage to the capsule. An attempt to eliminate this limitation and also to increase the sensitivity of determination led to the next design of atomizer the 'capsule-in-f~rnace',~ [Fig. 1 (c)]. A plugged cap- sule filled with a sample of powdered graphite was inserted into the cavity of the graphite furnace which was then heated through its contacts following a pre-set programme. It was discovered that with this atomizer the same range of elements as with the capsule-in-flame could be determined but with an approximately 50-fold increase in sensitivity. No traces of the destruction of the capsule which was made of carbon rod for atomic emission spectral analysis could be found when the capsule was used for the determination of Ti and V.The principal advantages of vapour filtration such as the suppression of background and interferences were demon- strated by the direct determination of Ag Cu and Ni in solid materials using a specially designed 'ring chamber tube'7 [Fig. l(d)]. The results obtained from different calibration methods using standard solutions (i.e. calibration curve addition of a standard solution to a known mass of solid sample of a standard solid reference material) were in a good agreement with certified values. When the developed techniqueM was applied to routine determinations the limitations of conventional AAS as a single-element method of analysis of solids were highlighted the labour and time consumed in making a capsule or another special atomizer which would have only a short lifespan; and dosing of the sample into the capsule or handling the powder or granulated sample in the furnace appeared to be too expensive for the determination of only one element.Apparently the atomizers developed based on the designs in Fig. 1 would be much better utilized in any versions of atomic absorption or atomic emission that are multi-element techniques. The next stage in the development of the method was to use the advantages of vapour filtration for the analysis of liquids. A laboratory-made cylindrical pyrolytic graphite coated graph- ite furnace was used in the experiments.**' An insert made from porous graphite in the form of a spool was placed in the central part of the furnace [Fig.1 (e)]. The liquid to be analysed was placed into the ring space between the furnace and insert walls by using the dosing hole. It was then dried and ashed in accordance with the pre-set programme. To prevent wetting of the graphite insert and penetration of liquid into the body of the furnace coils of graphite thread were rolled round the graphite insert. This served as a sample collector. At the atomization stage evaporation of the sample distributed on the collector was assumed to be delayed relative to the temperature of the insert. The effect of this delay resulted in the vapour entering the analytical zone at a higher temperature. This caused the thread to behave in a similar fashion to a L'vov platform.The following characteristics of the atomizer were estab- lished:8-'0 the range of elements that could be determined ,YTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 included the high- and medium-volatility metals; the sensitivity of determination for most elements was practically the same as for the ordinary commercial furnace when the integration mode of registration was used; the relative limit of detection was improved because of the large volume of analyte (100-150 pl); and the most important advantage of the atom- izer was the significant decrease in spectral and chemical interferences. The main drawbacks of the design in Fig. l(e) were disco- vered when the graphite insert was placed into an electrother- rnal atomizer of an Hitachi 2-9000 AA instrument equipped with autosampler and programmable power supply instead of the laboratory-made furnace." It appeared that the accuracy of the determinations was inferior to that obtained with the commercial furnace.The graphite insert was heated not only by radiation but partly by the electrical current. The resistance of the contacts between the insert and the furnace changed after each firing and after touching the insert with the sampler nipple. As a result the heating rate of the insert varied and was impossible to control. This drawback displayed itself most often when the heating programme of maximum power was used to increase the delay between the heating of the thread and that of the furnace and insert. The object of the following investigation was to optimize the atomizer design so that it could be used in commercial instrumentation and to highlight the advantages of the method in the determination of elements in everyday materials.In accordance with this aim the design of the atomizer was refined'' and some of the analytical characteristics for the determinations of Cd Pb Bi Cu and A1 were investigated and compared with those found using a furnace equipped with a platform.l2 The properties of the atomizer were also investi- gated by carrying out the following determinations Cd Bi and Cu in the presence of a large excess of NaCl as the matrix; Cd and Pb in the presence of CuC1,; and Cd and Pb in whole blood; and Pb in steel. Experimental Instrumentation Atomic absorption measurements were made using a Perkin- Elmer Model 5000 spectrometer with deuterium lamp back- ground correction and HGA-500 accessories an AS-40 auto- sampler a Perkin-Elmer 56 recorder and hollow cathode lamps operated at their recommended currents.A Keller micro-pyrometer Model PB06 was used for measurement of the stable temperature of the furnace and the filter either for visual observations or for determining their heating kinetics. Commercial atomizer furnace with a platform (FP) Pyrolytic graphite coated graphite tube atomizers (Perkin- Elmer) with a solid pyrolytic graphite platform were used in comparative experiments. Laboratory-made atomizer furnace with filter (FF) The design of the atomizer is shown in Fig. 2. The filter (1) was made from a carbon rod used for spectral emission analysis and was inserted into a pyrolytic graphite coated graphite tube furnace (2a and 2b).Graphite thread (3) was placed in the ring cavity between the furnace and the filter. Two types of commercially available graphite furnaces [ Perkin-Elmer and Pyrocarbo (South Africa)] of length 28 mm were used with the standard set of graphite accessories (4). The only feature which distinguished these furnaces from the originals were four 1 mm wide slits (5) of 4 mm length on both ends cut equidistant from each other. The filter having the same length as the furnace had the shape of a spool with bulges at both ends. The externalJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 433 . 4 4 Fig. 2 Furnace with graphite filter 1 graphite filter; 2a and 2b pyro- lytic graphite coated furnaces; 3 graphite thread; 4 electrical contacts; 5 cuttings; 6 dosing hole; 7 opening in the contact; 8 auto-sampler tip; and AA and BB sectional views diameter of the bulges was exactly the same as the internal diameter of furnace cavity (about 6mm) which provided a tight contact between the furnace ends and the filter.The external and internal diameters of the central part of the filter were 4 and 2.5 mm respectively. The length of the central part was 13 mm. To provide maximum light flux through the filter cavity the internal diameter of the filter in the area of the bulges was increased to 4.5 mm. The graphite thread of about 50mm in length was rolled onto the central part of the filter before being inserted into the furnace or pushed into the ring cavity through the dosing hole (6) after the furnace and the filter had been joined.Rejnement of atomizer and position of tip of autosampler After assembling the atomizer (Fig. 2) it was placed in its normal working position between the electrical contacts (4) with the dosing hole (6) against the hole in the contact (7). The positions of the dosing hole and the tip of the auto- sampler (8) was adjusted to provide for the sample liquid to be injected into the ring cavity. (If a drop of liquid landed near the dosing hole after the sample had been inserted it could act as a plug and prevent steam being emitted during the drying step. This would result in the sample bubbling out.) To prevent the sample being placed on the filter surface near the dosing hole the tip of the autosampler was slightly bent as is shown in Fig.2. Preparation of atomizer The atomizer was fired with a step-by-step increase in tempera- ture until 2900 "C was reached. Each step was 400-500 "C with a manual hold time of 4-5 s. There were two reasons for this conditioning firstly to clean the atomizer and secondly to ensure that a reliable electrical contact between the furnace and the filter had been achieved. At high temperature under the longitudinal pressure from the cone contacts the slotted ends of the furnace were pressed firmly onto the ends of the filter. After preparation the atomizer was ready for use. For a well prepared atomizer at any step in the temperature pro- gramme both the filter and the furnace were heated at the same rate.When the maximum power was applied for rapid heating of the atomizer a small delay in the temperature of the thread relative to that of the furnace and the filter was observed. The five atomizers prepared were examined and used at different stages of the research. The stabilized temperature of the external wall was approximately 100-150°C higher than indicated by the power supply. The temperature of the filter was also higher at about 1100 as against 1000°C and 2500 as against 2300 "C for the furnace. Reagents The samples of Al Bi Cd Cu and Pb were made by diluting standard solutions (Titrisol from Merck). Nitric acid (1-2%) was added to each solution. Solutions of NaCl and CuCl of different concentrations were prepared from the solid salts of analytical-reagent grade.The certified reference material of whole blood (Seronorm N 010010 Nycomed) was used without dilution. The certified reference material of solid steel (BCS N335) was dissolved in a mixture of nitric and hydro- chloric acids (0.4 and 4%) and then the solution was diluted up to 0.7% by mass. General Procedure The analytical characteristics of the newly designed atomizer (FF) were compared with those found when using a furnace with a platform (FP) in two sequential sets of experiments. Analytical lines the currents of the hollow cathode lamp and the monochromator slit-widths were the same as recommended in the manufacturer's man~a1.l~ For both atomizers the pro- gramme of maximum power was used in the atomization step. For the FP the temperature and the drying charring atomiz- ation and cleaning times as defined by Slavin et all4 were used.For the FF the optimal conditions were also determined in sequential experiments. These are summarized in Table 1. It should also be noted that the role and value of gas flow through the cavity of the atomizer remained unclear. There was no definite relationship between the gas flow rate as indicated on the power supply and the magnitude or shape of the analytical signal. Consequently the gas flow rates given in Table 1 were not critical. All the final dilutions of the analyte additions of matrix or reference solutions were performed directly in the atomizer by means of two-step dosing with the aid of the autosampler. For each analytical signal the shape was recorded and integrated absorbance was registered.Results and Discussion Dosing Volume and Drying Time For the stabilized temperature platform furnace (STPF) in the conventional mode of operation an analyte volume of about 20pl and a drying time for this volume of about 40 s at a temperature of 250 "C is re~0mrnended.l~ An increase above this optimum in the volume dosed led to the platform being overwhelmed and the liquid spreading over the surface of the furnace. Fast drying at a temperature higher than rec- ommended caused the analyte solution to boil and sputter. As a result when the conditions as given were not applied the shape of the analytical signals was distorted and the analysis became inaccurate. To illustrate this inaccuracy some traces of the atomic absorption signals from 0.1 ng of Cd in an FP are given in Fig.3(a). Different volumes of water (10-50 pl) were added to 10 p1 of analyte in sequential experiments. For all the samples the drying conditions as given above were used despite the fact that the sample volumes exceeded those recommended by a factor 2-3. It is clear from Fig. 3(a) that an increase in sample volume was accompanied by the appearance of some additional atomic absorption signals preceding the analytical signal. An accept- able volume for a single dosing was confirmed to be less than 20-30 pl. The results of the same experiment with the FF are given in Fig. 3(b). The range of the volumes of water added was434 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 Experimental conditions as used for the determination of metals in the furnace with graphite filter Furnace heating programme Drying Charring - Internal Dosed volume/ Temperature/ Rate/ Hold time/ Temperature/ Rate/ Hold time/ gas flow/ P1 "C "C s-l S "C "C s-l S ml min- Metal Cd Pb Bi A1 c u 5-100 250 5 10 - - - - - 5-20 5-20 - 500 5 15* 50t 500 5 15* 500 5 15 700 5 15 700 5 15 - - - - Atomization Cleaning Dosed Internal volume/ Temperature/ Rate/ Hold time/ Temperature/ Rate/ Hold time/ gas flow/ Metal PI "C "C s-l S "C "C s-l S ml min-' Cd 5-100 1000 0 5 Pb 1200 0 5 Bi - 1200 0 5 A1 5-20 2400 0 5 c u 5-20 2400 0 5 - 1500 1 3 5ot 17001 1 3 1700 1 3 2800 1 3 2800 1 5 - - - - * In the analysis of whole blood the charring time used was 60 s.t Gas stopped at the atomization step.$ For the analysis of steel the cleaning temperature was 2700 "C. 10 20 30 10 s H :I I 30 40 50 60 70 80 90 Time - Fig.3 Atomic absorption signals for the determination of Cd after the addition of different volumes of water (in p1) to lop1 of sample containing 0.1 ng of Cd. (a) In the furnace with a platform; and (b) in the furnace with a graphite filter increased from 10 to 100 pl and the drying time was reduced to 15 s for all the volumes dosed. In this experiment all the signals had the same shape and magnitude. In a series of ten determinations of Cd in the samples containing equal amounts of analyte but whose volume varied by a factor ten the relative standard deviation (RSD) for the integrated absorbance was only 1.7%. It appeared that in spite of the thick stream of steam from the dosing hole during the drying period in the case of large volumes there were no significant losses of analyte.This might be explained by the large adsorbing surface in the ring cavity provided by the graphite thread. Some experiments performed to compare the atomizers with and without graphite thread confirmed this theory. The reproducibility of the atomic absorption signals in the case of large dosing volumes was much poorer for the atomizer without the thread. Sensitivity and Detection Limit The integrated absorbances for Cd Pb Bi Cu and A1 were compared for the same amounts of analyte in the FP and FF. The results show an increase in absolute sensitivity for determi- nations in the FF relative to FP by a factor of 2.8 for Cd 2.2 for Pb 1.6 for Bi 2.6 for A1 and 1.6 for Cu.These data made it possible to estimate the atomic vapour losses which occurred through the dosing hole. Taking into account that the absorb- ing layer lengths are approximately the same for the FF and the FP and the volume of their analytical zone is in a ratio of 1:3 it is clear that theoretically in the absence of vapour losses through the dosing hole the gain in absolute sensitivity for the FF should be a factor of about 3. In practice more than 7% of the vapour was lost during the determination of Cd and 45% for Cu. Apparently the value of this loss depends on the transportation rate of the metal vapour through the graphite. This estimation made clear the importance of finding a material for the filter with a high diffusion permeability. The different increases in sensitivities of the elements investigated indicated that the rate of diffusion of the metal vapour through graphite depends on the chemical interaction between the element and graphite at high temperatures. Taking into account both the increase in dosing volume of the liquid and analytical signal for the FF it appeared that the relative sensitivity of the determination could be improved on average about 10-fold.It should be noted that the gain in sensitivity cannot be completely accompanied by a similar improvement in detection limit because of some decrease in the light flux through the atomizer. For the configuration of the FF the light flux through the analytical zone was 1.6 times less than through the FP.Consequently the magnitude of shot noise had to increase ( 1.6)O.' = 1.25 times. Supposing the limit of detection to be determined by the relationship of the magnitudes of signal to shot noise it was improved in the FF by an average 10/1.25=8 times. This estimate could be correct if there is no light coming into the spectral instrument from the end of heated filter. It might not be valid in the determination of refractory metals. The problem relating to the general improve- ment in the limits of detection must be solved on the basis of the optimization of the filter configuration taking into account the original geometry of the light beam length and diameterJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VO1 of the analytical zone thickness of the walls and volume of the ring cavity space.Matrix Interferences The results of the determination of Cd Bi and Cu in the presence of increasing amounts of NaCl when background correction was applied are shown in Fig. 4 for FF and FP. The dependence of the magnitude of the background on the analytical lines of Cd and Bi is also plotted against the matrix concentration in Fig. 4(a) and (b). For all three elements the extension of the linear parts of the plots which characterize freedom from spectral inter- ferences is much wider in the case of the FF than in the case of the FP. To resolve this problem of the behaviour of the plots the Cd signals in the presence of large amounts of NaCl were recorded without background correction for both fur- naces (Fig.5). In Fig. 5 for the FP the analytical signal for 0.1 ng of Cd almost overlapped with the background of the molecular absorbance of 2 pg of NaCI. It is clear from Fig. 4 that in the case of the FP the magnitude of the background signals grew proportionally with the mass of the sample matrix. If an arbitrary assumption that the background correction system of the instrument functions efficiently if the magnitude of background signals does not exceed the same of atomic signal is made it is possible to determine the spectral inter- ference free limit (SIFL). In accordance with the plots in Fig. 4 for the FP this limit for an NaCl matrix in the determination of 0.05 ng of Cd was about 2 pg. For the determinations of Cd in the FP with chemical modification as described in the literature,14 the SIFL for NaCl was found to be about 200 pg.In the FF the background signal was distributed in time relative to the atomic absorbance signal of Cd across a wider region [Fig. 5(b)]. At the instant that the Cd signal appeared O . t t 0 Amount of NaCl added/vg Fig. 4 Integrated atomic absorbance (solid lines) and magnitude of background signals (broken lines) for the determination of (a) 0.05 ng of Cd (b) 10 ng of Bi and (c) 0.3 ng of Cu in the presence of increasing amounts of NaCl with the furnace equipped with a filter (F) and with a platform (P). The arrows mark the SIFL as determined in the text for an FP 0.6 a 0.4 e 0 a 0 m L) 0.2 0 9 43 5 - 0 5 (b) ltomization Cleaning 0 5 10 Jirne/s Fig.5 Appearance of the peak when determining without back- ground correction 0.1 ng of Cd in the FP in the presence of (a) 2 pg of NaCl and (b) in the FF in the presence of 400 pg of NaCl the magnitude of background was much less than the atomic absorption in spite of a 200-fold increase in the amount of matrix when compared with Fig.5(a) (400 pg). For both Cd and Bi the magnitude of the background at the instant when the atomic signal appeared grew non-proportionally with the amount of matrix [Fig. 4(a) and (b)]. An increase in back- ground occurred only at the stage when the furnace was cleaned i.e. when the temperature was increased. It is thus possible to assume that when the filter was used the SIFL was at least 200 times larger than for the FP without chemical modification and according to data in the literat~re,'~ at least twice as large as those obtained with chemical modification. The results given in Fig.4 for Cd Bi and Cu led to the conclusion that vapour of NaCl passed through the graphite at a slower rate than did the atomic vapour. This peculiarity in the behaviour of atomic and molecular species made it possible to use the background correction for a wide range of matrix concentrations. The increase in the SIFL in the case of the determination of Pb in CuCl (Table 2) was evidence that the nature of the effect is characteristic for molecular vapour. Determination of Cd and Pb in Reference Materials To highlight the problems that could arise in practice the determination of Cd and Pb in whole blood and Pb in steel was performed without any special pre-treatment or chemical modifiers." In the determination of Cd and Pb in whole blood 10-20 pl samples were dosed into the FF.To destroy the organic matrix and remove the smoke from the FF a charring temperature of 500°C was required for about 1 min. Calibration curve and standard additions methods were used. In the case of Pb the results obtained by using a calibration Table 2 Spectral interferences in the furnaces investigated Interference free limit/pg Analyte Matrix Platform* Platform? Filter Cd NaCl 200 60 > 400 - 90 > 1000 Bi cu NaCl 20 20 >400 Cd CuCl - > 100 > 100 Pb CuCl - 10 > 1000 NaCl * From the literature ref. 14. This work.436 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 3 Determination of Pb and Cd in reference materials Results Certified Con tent Reference content/ determined/ Method of Metal material pg I-' pg 1-' determination Cd Blood* 2.7 2.5 Standard additions (2.4-3.0) 2.5 Calibration curve Pb Blood* 42 41.8 Standard additions (3 7-46) 41.8 Calibration curve Pb Steel? 100 105 Standard additions * Senonorm N 010010. t A 0.7% solution of steel BCS M335 in HCI (4%) and HNO ( 0.4 yo).curve and those from the standard additions methods were practically the same in spite of the high temperature of charring. These results are within the specifications of the certificate (Table 3). The following peculiarities were discovered when Cd was determined. In each of a sequential series of experiments when whole blood and a reference solution of Cd in 0.3% solution of nitric acid in water was dosed the magnitude and area of the signals changed in sequential firings. The Cd signals from the reference solution decreased when it was dosed after the blood and in contrast the signals of Cd from blood increased when the blood followed the reference solution.Both the decreasing and increasing magnitudes reached their limit after 4-5 firings. The difference between the initial and final signals was 15-20% of the magnitude and 5-7% of the area. Neither the introduction into the atomizer of a 5% solution of nitric acid in between dosing the series of blood and reference solutions nor a 10 s exposure of the atomizer to a temperature of 2500°C at the cleaning step after each atomization period could eliminate the short-term memory effect.An investigation of the nature of the effect is of considerable interest but is not within the scope of this paper. The only suggestions that might be made is that the effect could have been due to adsorption of anions onto the ash particles or the cooling of the ends of the furnace where the vapour condensed. The results of analysis calculated from the data obtained for the last five signals in a series of ten firings for blood and reference solution coincided with the certified concentration of Cd in the material under investigation. Exactly the same result was obtained when the method of standard additions was used (Table 3). In the determination of Pb in steel some peculiarities were also noted. Most of the Pb contained in the sample was atomized at the low temperature as shown in Table 1 without any particular problems.To remove the residues of Pb an unexpectable high furnace temperature was required. This high temperature was apparently associated with the formation at the high temperature of Pb compounds or solid solutions with steel and graphite as the components. For this removal the cleaning temperature was increased to 2700 "C and the method of standard additions was used for the determinations. The results obtained are in good agreement with the data of the certificate (Table 3). gation than the standard furnace with platform when used in a commercial atomic absorption instrument with background correction autosampler and programmable power supply. The advantages found were an increase in absolute sensitivity of determination and volume of analyte dosed; a decrease in the determination time when considering the reduction of the drying period; and an extension of the interference free limit.The results of the analysis of some reference materials demon- strated the accuracy of the determinations without the use of chemical modification. There remain some problems to be solved and the technique needs to be refined. For further improvement of the analytical characteristics of the atomization method discussed a complete study is suggested which would include the following items investigation of metal and molecular vapour diffusion through graphite of different density porosity and chemical activity to find the best material for the filter; modelling of the vapour transport processes in the atomizer to find the optimal con- figuration of the filter; transformation of the general design to prevent storage of matrix material at the ends of the furnace; clarification of the function of the graphite thread and finding the optimal length of the threads; investigation of long-term stability of the results in the presence of chemically active matrices; and determination of refractive and carbide forming elements.Even considering these issues the results obtained are very promising for analytical applications. These and other problems need to be clarified and the vapour filtration method could be used as the basis not only for AA analyses but with other related technique inductively coupled plasma (ICP) electrothermal atomization (ETA) ICP mass spectrometry ETA furnace atomic non-thermal emission spectrometry etc. when electrothermal evaporation of the samples can be applied. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 References L'vov B. V. Atomic Spectrochemical Analysis Nauka Moscow 1966 and Hilger London 1970. Langmyhr F. J. Talanta 1977 24 277. Katskov D. A. Kruglikova L. P. L'vov B. V. and Polzik L. K. Zh. Appl. Spectrosc. 1974 20 739. Katskov D. A. Kruglikova L. P. and L'vov B. V. Zh. Anal. Chem. 1975 30 238. Katskov D. A. Grinshtein I. L. and Kruglikova L. P. Zh. Appl. Spectrosc. 1980 32 536. Kruglikova L. P. Ph.D. Thesis State Institute for Applied Chemistry Leningrad 1975. Shmidt K. R. and Falk H. Spectrochim. Acta Part B 1987,42,3. USSR Patent No. 1448251 priority 1.1.1987. Katskov D. A. Vasil'eva L. A. and Grinshtein I. L. Abstracts of X I CANAS Moscow 1990 p. 48. Vasil'eva L. A. Grinshtein I. L. and Katskov D. A. Zh. Appl. Spectrosc. 1993 48 1345. Provisional R.S.A. patent No. 93/4657 29.6,1993. Katskov D. A. Marais P. J. J. G. McCrindle R. I. and Schwarzer R. XXVIII Colloquium Spectroscopicurn Internationale York UK June 29-July 4 1993. Perkin-Elmer Analytical Methods for Atomic Absorption Spectrophotometry Norwalk CT USA 1976. Slavin W. Manning D. C. and Carnrick G. R. At. Spectrosc. 1981 2 5. Tzalev D. L. Slaveykova V. I. and Manjukov P. B. Spectrochim. Acta Rev. 1990 13 225. Conclusion The electrothermal atomizer developed yielded better analyt- ical characteristics for the elements and matrices under investi- Paper 3 f06416F Received October 22 1993 Accepted December 20 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900431

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

437 JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 High-sensitivity Detection of Selenium and Arsenic by Laser-excited Atomic Fluorescence Spectrometry Using Electrothermal Atomization* U. Heitmann T. Sy and A. Hese lnstitut fur Strahlungs- und Kernphysik Technische Universitat Berlin HardenbergstraRe 36 D- 70623 Berlin Germany G. Schoknecht Bundesgesundheitsamt lnstitut fur Sozialmedizin und Epidemiologie General-Pape-StraRe 62 0-72107 Berlin Germany High-sensitivity detection of the trace elements selenium and arsenic is reported. The method applied is laser-excited atomic fluorescence spectrometry using electrothermal atomization within a graphite furnace atomizer. For the production of tunable laser radiation in the vacuum ultraviolet (VUV) spectral region a laser system was developed that consists of two laboratory-built dye lasers pumped by a Nd:YAG laser.The laser radiations are subsequently frequency doubled and sum frequency mixed by non-linear optical KDP or BBO crystals respectively. The system works with a repetition rate of 20 Hz and provides output energies of up to 100 pJ in the VUV at a pulse duration of 5 ns. The investigations focused on the detection of selenium and arsenic in aqueous solutions and in samples of human whole blood. From measurements on aqueous standards detection limits of 1.5 ng I-' for selenium and 5.4 ng I-' for arsenic were obtained with corre- sponding absolute detected masses of only 15 or 54 fg respectively. The linear dynamic ranges spanned six orders of magnitude and good precision was achieved.In the case of human whole blood samples the recovery was found to be within the range 96-104O/0. The determination of the selenium content yielded medians of 119.5k17.3 pg I-' for 200 frozen blood samples taken in 1988 and 109.1 k15.6 pg I-' for 103 fresh blood samples. Keywords Laser-excited atomic fluorescence spectrometry; electrothermal atomization; detection of selenium and arsenic; vacuum ultraviolet spectral region; human whole blood Within recent years enormous interest has arisen in high- sensitivity detection methods in the medical and biological sector. In particular the investigation of trace elements and their influence on the human organism has attained increasing importance.',' Many methods for example atomic absorption (AAS) or fluorescence spectrometry (AFS) inductively coupled plasma (ICP) techniques neutron activity analysis (NAA) and related techniques have become established in the analytical They make the detection of numerous elements poss- ible but some difficulties often occur when biological samples have to be analysed.This is mostly caused by spectral or chemical interferences during the meas~rements.l'-'~ The introduction of a laser as the excitation source in contrast to conventional discharge lamps leads to almost complete elimination of such problems. This technique is called laser-excited atomic fluorescence spectrometry ( It offers double selectivity because of the efficient excitation of specific atomic transitions with spectrally narrow laser radi- ation followed by the spectrally selective detection of the fluorescence of the element.Therefore even in the most complex samples background-free analytical signals can be obtained without supplementary compensation being neces- sary. Moreover the high laser intensities of pulsed systems lead to saturation of the fluorescence of the element which reduces the influence of laser fluctuations and implies lower detection limits. The combination of LEAFS with the well studied electrother- mal atomization within a graphite furnace (ET-LEAFS) yields further improvements because previous experience with regard to sample preparation and handling can be u ~ e d . ' ~ ' ~ In addition only low sample volumes are required typically 10 pl are used in the present experiments. A recent overview on the * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.topic of ET-LEAFS and the superiority compared with conven- tional analytical methods has been given by Sjo~trom,'~ and in an article by Smith et a1.,20 who summarized the detection limits for several elements obtained by different methods. The best detection limit reported of 0.005 ng I-' was achieved for thallium which corresponds to a mass of 0.1 fg.21 The present investigations were focused on the high- sensitivity detection of the trace elements selenium and arsenic. Selenium is an essential trace element for humans. It is responsible for circulatory disturbances and diseases of the heart,22-24 and received worldwide attention in connection with Keshan disease that appeared in a province of China in 1979 and which was explained by selenium defi~iency.~',~~ Former investigations have also shown a protective role of selenium against c a n ~ e r ~ ~ ' ~ ~ and it also has some de-toxifying effects on heavy metals.29 On the other hand in high concentrations selenium shows toxic properties.The average selenium content in human blood of German citizens is normally in the region of about 100 pg 1-'. Although numerous investigations have been made in the past the influence of selenium on the human organism has not been completely clarified. Arsenic is a non-essential trace element but it is also of enormous interest in medi~ine,~'.~~ for example high concen- trations are related to a chronic liver disease.32 An important factor is the high toxicity of arsenic even in moderate concen- trations because it is used extensively in agriculture and industry.33 The extremely short excitation wavelengths of selenium and arsenic in the vacuum ultraviolet (VUV) spectral region are a real challenge for an analytical detection system.The lines most often used for these elements are 196.0 and 193.7 nm respectively. Therefore the aim of the work was the develop- ment of a laser system that enables the efficient generation of tunable radiation in the VUV and the application of the system to the high-sensitivity detection of trace elements by ET-LEAFS. The laser radiation is produced by sum frequency438 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 mixing (SFM) in a non-linear BBO crystal. An alternative method is stimulated Raman shifting (SRS),34,35 but the appli- cation of SFM yields higher conversion efficiencies in this spectral region.36 For wavelengths below 190 nm where BBO shows an increasing absorption the situation is changed and SRS would be the preferred method. In the present paper results obtained for the detection of selenium and arsenic in aqueous solution are presented. The selenium contents in 303 samples of human whole blood were determined in co-operation with the German Bundes- gesundheitsamt (BGA) in Berlin who study the effects of trace elements on the human organism. To our knowledge this is the first application of ET-LEAFS to the detection of both of these elements.Experimental VUV Laser System For the production of tunable VUV laser radiation the system which is depicted schematically in Fig. 1 was developed. It consists of two laboratory-built and symmetrically arranged dye lasers; each laser has one oscillator and one amplifier stage. For the oscillators grazing incidence arrangements with retroreflectors and double prism beam expansions were chosen. This is the best compromise to get high output energies in combination with small bandwidths. A commercial Nd:YAG laser (Lumonics HY 500 special version) was used as the pump source for the two sychronously pumped dye lasers. This is internally frequency doubled and provides output energies of up to 100 mJ with a pulse duration of 8 ns at a repetition rate of 20 Hz.The system also enables repetition rates of 50 and 100Hz. One dye laser operates in the red spectral region around 641 nm and the other runs in the yellow region at 565 nm. The laser radiation of the second laser is subsequently frequency doubled within a KDP crystal. The crystal is followed by a quartz compensator which rotates in the opposite direction and eliminates the beam shift during angle tuning to obtain optimal type-I phase matching. The laser beams at 641 and 283nm are then combined and sum frequency mixed within a BBO crystal. This yields the required VUV laser radiation at 196 nm which is finally separated from the remaining fundamental waves by a Pellin-Broca prism. The laser radiation in the VUV is guided under an atmosphere of pure nitrogen because of the absorption of the oxygen present in the laboratory air which is about 5% per meter.The laser system produces output energies of up to 100 pJ at a pulse duration of 5 ns in the VUV which means a peak power of 20kW. The corresponding spectral bandwidth is 13 GHz and the relative standard deviation (RSD) of the pulse fluctuations was determined to be 25%. A wavelength scan is achieved under control of a personal computer which scans both dye lasers by stepping motor drives and also rotates the KDP crystal. The BBO crystal is not moved. To obtain long- term stability of the laser system both dye laser oscillators as well as the non-linear optical crystals are temperature stabilized to within k0.05 "C at about 33 "C. In this way stable con- ditions over several days are achieved.196 nm A 641 nrn Dye laser 532 nm .I- ll Fig. 1 Schematic arrangement of the tunable VUV laser system A detailed description of the VUV laser system with corre- sponding system performances can be found in ref. 36. ET-LEAFS System 'The analytical arrangement for the ET-LEAFS measurements is depicted schematically in Fig. 2. The VUV laser radiation is focused through a pierced hole in the mirror into a commercial graphite furnace atomizer ( Perkin-Elmer HGA-500) equipped with an autosampler (Perkin-Elmer AS-40). Pyrolytic graphite coated graphite tubes with L'vov platforms were used. The analytical signal is observed as the backward fluorescence (i.e. backward in relation to the direction of the laser beam used for excitation) of the excited atoms collected via the mirror and focused by a lens onto the entrance slit of a monochroma- tor (B & M Spektronik d=0.1 mm f=25 cm).For signal detection a solar blind photomultiplier (Hamamatsu R 166 UH) is used to avoid background signals caused by thermal radiation of the atomizer. The signal is subsequently preampli- fied (Ortec 9301) and worked up by a fast boxcar integrator (Stanford Research SR 250). A personal computer finally handles the signal recording and also controls the laser system and the atomizer. Sample Preparation Aqueous solutions are obtained by dilution of standard solu- tions of the elements Titrisol (Merck 1 g 1-I) with multiply distilled and de-ionized water (Millipore). The acid level is adjusted to be 0.2% v/v in nitric acid.Because of the high volatility of selenium and arsenic a commercial palladium nitrate modifier (Merck 10 g 1-I) is used for chemical modifi- cation Depending on the element the modifier is further diluted to concentrations of 0.2-0.5 g 1-1 of palladium. For blood samples a reference blood standard Seronorm (Nycomed Batch 010010) with a certified selenium content of 93 & 4 pg l-' was used. Real blood samples from the BGA are transferred directly into ethylenediaminetetraacetic acid (H,EDTA) coated tubes. Owing to the high sensitivity of the system the blood samples were further diluted normally by a factor of 10 or 20. The samples should then contain 0.2% v/v nitric acid and 0.01% v/v Triton X-100. The addition of Triton X-100 causes a reduction in the surface tension enabling better sample deposition onto the L'vov platform and also reduces residues within the vessel of the autosampler.During an analytical cycle 10 p1 of the sample are first transferred into the graphite tube followed by the same amount of modifier. Thermal treatment then takes place using a standard graphite furnace programme as first proposed by Welz et a1.37 and only slightly modified for the present experi- ments. The ashing temperature is raised to 1000°C and an atomization temperature of 2200 "C was chosen. This pro- cedure enables about 250 successive measurements to be made before the graphite tube has to be changed. Results Aqueous Selenium Samples The first investigations focused on the detection of selenium.A simplified energy-level scheme for selenium is given in Fig. 3. As already mentioned the main absorption line lies in the VUV spectral range at 196nm. From the excited level there are three possibilities for observation of fluorescence either resonant detection of the transition back to the ground state or of the cross transitions at 204 or 206 nm. The last two cases have the advantage of being free from laser stray light. To test the spectral behaviour of selenium the exit slit of the monochromator and the photomultiplier was replaced by an optical multichannel analyser (0-SMA Princeton Instruments). As seen from the spectrally and time resolved fluorescence after excitation at 196 nm (cJ Fig. 4) only the\ Boxcar 4 integrator __ I 2 Signal Preamplifier Printer Computer Photomultiplier controller ;” \ \ filter Trigger \ \ High vo I tag e Control unit u \ Fig. 2 Schematic set-up of the ET-LEAFS system Nd YAG pump laser Fig.3 Simplified energy-level scheme of selenium \ Dye lasersT - andSFM three expected fluorescence lines at 196 204 and 206 nm appear. No secondary transitions or spectral interferences of other elements can be observed. Only some laser stray light can be seen in front of and behind the main peak. However the 0-SMA system used was not optimized for applications in the VUV spectral range. This explains the high sample concentration used in this example. For high-sensitivity measurements the photomultiplier is therefore used which provides a gain of about 500. A further interesting point is the dependence of the fluor- escence signal on the excitation energy.With increasing laser energy the element fluorescence shows a saturation effect. This can be seen in Fig. 5 for excitation at 196 nm and observation on the cross transition at 204nm. The two curves represent different optical adjustments on different days. Nevertheless both curves show similar behaviour and yield the same satu- ration energy of 1.0f0.2 pJ in the presence of the palladium modifier. With a cross-section of the laser beam within the atomizer of 0.2 mm’ this corresponds to a saturation intensity 439 190 195 200 205 21 0 Wavelengthhm Fig.4 Time and spectrally resolved fluorescence of an aqueous selenium sample over a time period of 3.2 s; excitation at 196 nm of 100+20 kW cm-’. The saturation level is defined as the level where half of the maximum excitable fluorescence is obtained.Some measurements were also made without the presence of the modifier and even this led to a higher saturation energy of 2.1 f 0.4 @. These results were considered again on different days with modified optical parameters (e.g. beam cross-section). The calibration curve for aqueous selenium samples that is given in Fig. 6 shows excellent linear behaviour. The detection limit follows from extrapolation of the calibration curve for three times the standard deviation (SD) of a blank signal to 1.5ng1-I. With a sample volume of lop1 this leads to an absolute detected mass of only 15 fg which means an enormous increase in sensitivity compared with conventional systems. From this detection limit a linear dynamic range over six440 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 80 500 9 *O - 0 2 4 6 8 10 12 14 16 18 E nergytpJ Fig.5 Dependence of the fluorescence signal on the laser energy (with Pd modifier); excitation at 196 nm; and observation at 204 nm. See text for explanation of the two curves 100000 1 1 L c. .- f 1000 - same blood sample led to slightly higher values of 6.5% for the RSD and 1.1% for the precision of the mean. As a first practical application a co-operative study with the BGA was undertaken. During an epidemiological study some thousands of whole blood samples were taken from the population of Berlin and the selenium content was deter- mined.38 Firstly 200 blood samples that were taken in 1988 and subsequently frozen were analysed.This yielded the distribution of the selenium content given in Fig. 7. The mean value was found to be 122.0 pg 1-I. From the Gaussian fit a median of 119.5 pg 1-1 was obtained with a corresponding SD of 17.3 pg 1-l. In Fig. 8 where the selenium content is depicted as a function of the age of the test person no tendency can be seen either with regard to the age or to the sex of the test person. Some extremely high values of a few samples can only be explained by an additional intake of selenium preparations. In addition to the measurements mentioned above 103 fresh whole blood samples were also investigated which led to a mean value of 114.2 pg 1-I. The median was determined to be 109.1 pg l-' with a corresponding SD of 15.6 pg 1-l.Within statistical uncertainties this is the same result as was obtained from the frozen samples. The comparative investigation of analytical samples with different detection methods was an important aspect. An overview of the expected or certified values is given in Table 1 and the measured values obtained with the present system and with a flow injection atomic spectrometry system (Perkin- Elmer FIAS-200) at the Freie Universitat (FU) of Berlin. Investigations with neutron activitation analysis (NAA) at the Hahn-Meitner-Institut (HMI) in Berlin were also proposed. I0 ' I I I I I 0.1 1 10 100 1000 10000 Selenium concentratiordpg I -' 40 I Fig. 6 Calibration graph for aqueous selenium samples detection limit 1.5 ng 1-' orders of magnitude is obtained. The calibration curve bends over with concentrations above 5000 pg 1-1 and unfortunately concentrations above 1000 pg 1-1 cause contamination of the graphite tube.Therefore measurements were restricted up to this practical limit. Normally an unknown sample is analysed with three success- ive measurements and the mean value taken. In this way standard deviations of between 2 and 3% were obtained. The long-term behaviour of the system was investigated by 34 successive measurements of the same 5 pg 1-1 aqueous selenium sample which represents a time period of nearly 2 h. An RSD for an individual measurement of 5.1 % was achieved which resulted in a precision of 0.9% for the mean value. Whole Blood Samples To calibrate the system for use with whole blood samples the reference blood standard Seronorm was used.When determin- ing the selenium concentrations in unknown samples a cali- bration curve was first recorded using five standard solutions. Each individual sample was then analysed three times and the mean value was compared with the calibration curve to give the real selenium content. In all cases the area of the fluor- escence signal i.e. integration of the fluorescence of the element at the observation wavelength over time was evaluated. This enables the alternative procedure of aqueous selenium stan- dards to be used because equal element concentrations yield equal signal areas independent of the sample matrix. From investigations with the standard additions method the recovery of a known selenium blood concentration was determined to be within the range 96-104%.Owing to the more complex sample matrix successive measurements of the 70 84 98 112 126 140 154 168 182 196 Selenium contenttpg I - ' Fig.7 Distribution of the selenium content of frozen whole blood samples mean 122.0; median 119.5; and SD 17.3 pg 1-' 200 C cn Q Q) 100 a 0 .O 0. 0 Fig. 8 Selenium content of whole blood samples as a function of the age of the test subjects 0 female 121 subjects; and 0 male 79 subjectsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 441 250 I v) C 3 - .- 200 f? e +- .- 150 - Table 1 Comparison of results obtained with different analytical methods - - - Sample Seronorm Human blood - A B C Sheep blood - A B C D Range expected/pg I - ' 93 +4 M 100 x 200 M 400 x 300 ET-LEAFS,* FIAS,? found/pg 1 - found/pg I - 92+3 95f 10 83+2 92+_11 105 f 9 88+2 96+7 104 + 2 105_+3 98+4 258 & 5 288 2 16 334$5 331 & 13 300+4 321 + 14 * Present group. 7 Schafer et al.FU Berlin.39 Unfortunately the reactor only started operation fairly recently after service work so the first results are not yet available. For these measurements the whole blood standard three human whole blood samples and four blood samples of sheep who had been fed on different diets and had been given different selenium additives in their food were selected. On the whole good agreement of the results for these samples was obtained with only some larger differences for two sheep samples. This can be explained by dilution uncertainties during sample preparation with the FIAS system.Aqueous Arsenic Samples The second trace element investigated with the system was arsenic. This is a more complicated situation because of the short excitation wavelength and more complex energy-level scheme (cf. Fig. 9). Two possibilities exist for excitation from the ground state. The most frequent transition is that at 193.7nm followed by two cross transitions at 243.7 and 245.7 nm for non-resonant observation. The other transition is at 197.3 nm with a cross transition at 249.3 nm. Owing to the high temperature within the graphite furnace during sample atomization some collision-induced transitions into upper energy levels also exist with corresponding fluorescence lines. This can be seen in the spectrally resolved fluorescence spectrum of arsenic obtained with the 0-SMA system used (cf.Fig. 10). After excitation at 193.7 nm only the resonance fluorescence and the two cross transitions at 243.7 and 245.7 nm are expected. Nevertheless the other lines mentioned also appear in particular the anti-Stokes line at 189.0 nm. The calibration curve for aqueous arsenic samples with excitation at 193.7 nm and observation at 245.7 nm shows a similar behaviour to the one obtained for selenium. The I 189.0 nm 1 I I I I l193.7 nm 197.3 nm I I I I r Fig. 9 Simplified energy-level scheme of arsenic 300 0 180 190 200 210 220 230 240 250 260 Wavelengthln m Fig 10 Spectrally resolved fluorescence of an aqueous arsenic sample (400 pg I-'); excitation at 193.7 nm detection limit was 5.4 ng 1-' which is slightly worse than the one for selenium but in comparison with other methods it is still an excellent result.Again from the detection limit a linear dynamic range over nearly six orders of magnitude is obtained. Different combinations of wavelengths for excitation and observation were tested for arsenic. The results for the two best combinations are as follows. The first was excitation at 197.3 nm in combination with the unusual observation on the anti-Stokes line at 193.7 nm. Even in this case a fairly good detection limit of 500 ng 1-' was found which corresponds to an absolute detected mass of 5 pg. The RSD of a single measurement was found to be 5.9% and the precision was 1.0%. Owing to the special type of transition a high saturation energy of 15 pJ was obtained which means a saturation intensity of 1.5 MW cm-2.The best results were achieved with excitation at 193.7 nm followed by observation at 245.7 nm. As already mentioned the detection limit was determined to be 5.4 ng l-l which corresponds to an absolute detected mass of only 54fg with an RSD of 6.9% and a precision of 1.2% for the mean. The saturation energy was found to be 0.8 pJ which implies a saturation intensity of 80 kW cm-2. Conclusions The ET-LEAFS technique was applied to the determination of the trace elements selenium and arsenic in aqueous solution and in samples of human whole blood. The precision accuracy and sensitivity of the method are good and are comparable with or better than those of conventional methods. Because of the double selectivity of the method only simple sample preparation even in the case of whole blood samples is required and no supplementary compensation for the back- ground is necessary.Also the system operates under the full control of a computer enabling easy wavelength scanning for example between selenium and arsenic. The application of the system within an epidemiological study was shown to operate without any problems and promises good results in the future. To our knowledge this is the first time ET-LEAFS has been applied to the detection of the trace elements selenium and arsenic. One reason for this could be the experimental diffi- culties associated with achieving tunable laser radiation in the VUV. Furthermore the best detection limits reported to date for both elements were obtained. With some alterations to the experimentation further improvements are expected.For example lower detection limits should be obtainable by a reduction of the pulse fluctuations of the Nd:YAG laser and also by improved optics for fluorescence detection. An extension of the investigations to other trace elements or the simultaneous detection of elements using an optical442 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 multichannel analyser in combination with several laser sources is intended. Owing to the complexity and the fairly high price of the system it should primarily be applied to the detection of several elements such as antimony and chromium where other systems have problems at low concentrations or with spectral interferences. Therefore ET-LEAFS should be seen as a comp- lementary technique to conventional methods.We thank Dr. K. Schafer and Mrs. K. Meyer for their helpful cooperation and comparative measurements with the FIAS system. This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 References Tolg G. in Trace Element Analytical Chemistry in Medicine and Biology eds. Bratter P. and Schramel P. Walter de Gruyter Berlin 1988 vol. 5 p. 1. Fishbein L. Int. J. Environ. Anal. Chem. 1984 17 113. Holcombe J. and Hassell D. Anal. Chem. 1990 62 169R. Bozsai G. Schlemmer G. and Grobenski Z. Talanta 1990 37 545. Pyen G. and Browner R. Appl. Spectrosc. 1988 42 262. Vien S. and Fry R. Anal. Chem. 1988 60 465. DUlivo A. FUOCO R. and Papoff P.Talanta 1985 32 103. Blekastad V. Jonsen J. Steinnes E. and Helgeland K. Acta Med. Scand. 1984 216 25. Irons R. Schenk E. and Giauque R. Clin. Chem. 1976,22,2018. Parsons M. Major S. and Forster A. Appl. Spectrosc. 1983 37 411. Saeed K. and Thomassen Y. Anal. Chim. Acta 1981 130 281. Welz B. and Schubert-Jacobs M. J. Anal. At. Spectrom. 1986 1 23. Aller A. J. and Garcia-Olalla C. J. Anal. At. Spectrom. 1992 7 753. 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Phys. B 1992 55 419. Welz B. Schlemmer G. and Mudakavi J. R. J. Anal. At. Spectrom. 1988 3 695. Schoknecht G. SozEp. Hefte 1985 1 1. Schafer K. personal communication. Paper 3 J04639G Received August I 1993 Accepted September 28 1993

ISSN:0267-9477    

DOI:10.1039/JA9940900437

出版商:RSC    

年代:1994

数据来源: RSC